Co-reporter:Van Ha Nguyen, Mansur B. Ibrahim, Waseem W. Mansour, Bassam M. El Ali, and Han Vinh Huynh
Organometallics June 26, 2017 Volume 36(Issue 12) pp:2345-2345
Publication Date(Web):June 13, 2017
DOI:10.1021/acs.organomet.7b00329
Charge-tagged bis(1,2,4-triazolin-5-ylidene)palladium(II) complexes have been successfully synthesized via a postmodification strategy. Reacting PdBr2 with bromo-functionalized 1,2,4-triazolium salts A·HBr and B·HBr in the presence of silver oxide afforded the bis(carbene)palladium(II) complexes trans-[PdBr2(A)2] (1a) and trans-[PdBr2(B)2] (1b), which contain tethered bromoalkyl chains. Subsequent postcoordinative nucleophilic substitution converted the bromo into ammonium groups, producing the water-soluble complexes trans-[PdBr2(C)2]Br2 (2a) and trans-[PdBr2(D)2]Br2 (2b), while attempts to prepare ammonium-functionalized triazolium salts for direct metalation were futile. All four complexes were fully characterized by means of multinuclear NMR spectroscopy, ESI mass spectrometry, elemental analysis, and X-ray diffraction analysis. The presence of trans-anti and trans-syn rotameric complexes in solution was elucidated by 1H and 13C NMR spectroscopy and theoretical calculations. Additionally, the two charge-tagged complexes, 2a,b, were found to be highly active precatalysts for the Suzuki–Miyaura and Mizoroki-Heck reactions in iPrOH/H2O and molten TBAB as an ionic liquid.
Co-reporter:Wenqin Wu, Qiaoqiao Teng, Yi-Yuan Chua, Han Vinh Huynh, and Hung A. Duong
Organometallics June 26, 2017 Volume 36(Issue 12) pp:2293-2293
Publication Date(Web):April 26, 2017
DOI:10.1021/acs.organomet.7b00180
A systematic evaluation of N-heterocyclic carbene ligands in the iron-catalyzed cross-coupling reactions of aryl chlorides and arylmagnesium reagents is performed. There is no clear correlation between the donor strength of the N-heterocyclic carbene and the reaction outcome. Instead, the highest yields of the desired biaryl product are obtained with sterically demanding ligands possessing large %Vbur values. Through this study, SIPrNap has been identified as an efficient and general ligand for the coupling of both aryl chlorides and tosylates.
Co-reporter:Han Vinh Huynh;Christine Hui Min Koh;Van Ha Nguyen
Dalton Transactions 2017 vol. 46(Issue 34) pp:11318-11326
Publication Date(Web):2017/08/29
DOI:10.1039/C7DT01912B
A series of rare [NiX2(MeCCprop)] complexes bearing the cis-chelating benzimidazole-derived dicarbene ligand MeCCprop and varying anionic coligands (2, X = N3; 3, X = NCS; 4, X = I; 5, X = O2CCF3) have been prepared and coligand dependent structural and spectroscopic features have been evaluated. This study also revealed an unusual ‘reverse’ carbene transfer reaction from nickel to silver giving the disilver species [Ag2X2(μ–κ2-MeCCprop)] (6, X = OAc; 7, X = O2CCF3). A preliminary catalytic study of two representative NiII diNHC complexes in the aqueous and phosphine-free Suzuki–Miyaura coupling reaction of aryl halides is reported as well. These reactions provide good yields of coupling products, but do not require inert conditions.
Co-reporter:Qiaoqiao Teng
Dalton Transactions 2017 vol. 46(Issue 3) pp:614-627
Publication Date(Web):2017/01/17
DOI:10.1039/C6DT04222H
The properties and reactivities of transition metal complexes are rooted in the stereoelectronic properties of their ligands. While the bulk of a ligand can be easily evaluated and compared by the drawing of its Lewis structure, prediction on the electronic contributions is often less straightforward. Thus, several electronic parameters have been developed for the experimental evaluation of ligands throughout the years. This article accounts for the most recent one developed by the Huynh group, which employs 13C NMR spectroscopy to determine ligand donor strengths using N-heterocyclic carbene complexes. This parameter not only proves to be safer, more convenient and accurate in comparison to existing methodologies, but it also provides, in certain cases, more intuitive and reliable results. Furthermore, it is currently the only one that allows the direct comparison of various Werner-type and organometallic ligands on a unified scale.
Co-reporter:Boon Ying Tay, Cun Wang, Pim Huat Phua, Ludger Paul Stubbs and Han Vinh Huynh
Dalton Transactions 2016 vol. 45(Issue 8) pp:3558-3563
Publication Date(Web):2016/01/14
DOI:10.1039/C5DT03366G
Hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) was studied by using mono- and bidentate p-cymene ruthenium(II) N-heterocyclic carbene (NHC) complexes as catalyst precursors. In water, all complexes were found to be reduced in situ to form ruthenium nanoparticles (RuNPs) with a high hydrogenation activity. In organic solvents, complexes with monodentate NHC ligands also formed nanoparticles, while complexes with bidentate ligands gave rise to stable homogeneous catalysts with moderate hydrogenation activities.
Co-reporter:Xiaoke Xie and Han Vinh Huynh
ACS Catalysis 2015 Volume 5(Issue 7) pp:4143
Publication Date(Web):May 28, 2015
DOI:10.1021/acscatal.5b00588
Mixed N-heterocyclic carbene (NHC)/phosphine complexes of the type [RuCl(p-cymene)(bimy)(PPh3)]PF6 (bimy = benzimidazolin-2-ylidene) have been synthesized and fully characterized. Complex 1 bearing the 1,3-dibenzylbenzimidazolin-2-ylidene ligand is able to selectively catalyze both dehydrogenative amidation, mono-, and diamination (N-alkylation) through coupling of simple alcohols with amines effectively yielding a range of amides and secondary and tertiary amines. Selectivity is achieved by controlling the fate of the common hemiaminal intermediate, which in turn can be simply influenced by the choice of base and solvent.Keywords: acceptorless dehydrogenation; borrowing hydrogen; hemiaminal; N-heterocyclic carbene (NHC); ruthenium; selectivity
Co-reporter:Qiaoqiao Teng and Han Vinh Huynh
Chemical Communications 2015 vol. 51(Issue 7) pp:1248-1251
Publication Date(Web):01 Dec 2014
DOI:10.1039/C4CC08270B
Metallation of a pentadentate diNHC proligand bearing a dipropyl-pyridine-2,6-dicarboxamide with silver and gold affords mono- and dinuclear, double-stranded bis(NHC) complexes as useful building blocks for metallo-supramolecules. The digold(I) complex acts as a metallo-bis(pincer) ligand to furnish the first example of an organometallic NHC-helicate upon coordination to cobalt.
Co-reporter:Shuai Guo, Jan Christopher Bernhammer and Han Vinh Huynh
Dalton Transactions 2015 vol. 44(Issue 34) pp:15157-15165
Publication Date(Web):20 Jan 2015
DOI:10.1039/C4DT03201B
Ligand redistribution reactions are well documented for silver(I) N-heterocyclic carbene (NHC) complexes of the type [AgX(NHC)] (X = halido ligand), but only two reports have been described in the literature for gold analogues of the general formula [AuX(NHC)]. In both cases, the NHCs in question were exceptionally strong donors. To probe the dependence of ligand redistribution processes on NHC donor strength, a model study was conducted using a weakly donating 1,2,4-triazolin-5-ylidene (tazy) ligand and different halido coligands. For [AuX(tazy)] (X = Cl, Br, OAc, tazy = 4-benzyl-1-methyl-1,2,4-triazolin-5-ylidene), no ligand redistribution was found, while a reversible disproportionation between [AuI(tazy)] in solution and [Au(tazy)2][AuI2] in the solid state was observed and studied by means of X-ray crystallography, NMR and UV-Vis spectroscopy, as well as DFT calculations.
Co-reporter:Qiaoqiao Teng and Han Vinh Huynh
Inorganic Chemistry 2014 Volume 53(Issue 20) pp:10964-10973
Publication Date(Web):September 29, 2014
DOI:10.1021/ic501325j
A series of 15 mononuclear complexes [PdBr(iPr2-bimy)(L2)]PF6 (1–15) (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene, L2 = aromatic 1,2-diimines, diazabutadienes, or methylene-, ethylene- and propylene-bridged di-N-heterocyclic carbenes) and two dicarbene-bridged, dinuclear complexes [Pd2Br4(iPr2-bimy)2(diNHC)] (16 and 17) were synthesized and characterized by multinuclear NMR spectroscopy, electrospray ionization mass spectrometry, and in some cases X-ray diffraction analysis. The influence of the 15 bidentate ligands L2 on the 13Ccarbene signals of the iPr2-bimy reporter ligand in the chelate complexes was studied, on the basis of which a facile methodology for the donor strength determination of bidentate ligands was developed.
Co-reporter:Jan Christopher Bernhammer, Gilles Frison and Han Vinh Huynh
Dalton Transactions 2014 vol. 43(Issue 23) pp:8591-8594
Publication Date(Web):14 Apr 2014
DOI:10.1039/C4DT01047G
In NHC pincer complexes incorporating a hemilabile donor site, there exists an equilibrium between the true pincer form and a pseudopincer coordination isomer. The influence of the NHC moieties on this isomerism has been studied by DFT calculations.
Co-reporter:Jan C. Bernhammer and Han Vinh Huynh
Organometallics 2014 Volume 33(Issue 20) pp:5845-5851
Publication Date(Web):July 18, 2014
DOI:10.1021/om500484q
Four bis(benzimidazolin-2-ylidene) nickel(II) complexes featuring thioether moieties in the side chain have been synthesized by reactions of the respective benzimidazolium salts with nickel(II) acetate in molten tetrabutylammonium bromide as an ionic liquid. All complexes were obtained as inseparable mixtures of trans-syn and trans-anti rotamers, as evidenced by NMR spectroscopy. For one of the complexes, X-ray diffraction confirmed the square-planar coordination geometry. The catalytic activity of all complexes for Suzuki–Miyaura cross-coupling was examined. Under the optimized conditions, both aryl bromides and aryl chlorides were successfully coupled in the presence of triphenylphosphine as additive. Yields ranged from good to moderate for electron-deficient aryl halides, while electron-rich aryl halides were found to be unreactive.
Co-reporter:Jan C. Bernhammer and Han Vinh Huynh
Organometallics 2014 Volume 33(Issue 5) pp:1266-1275
Publication Date(Web):February 24, 2014
DOI:10.1021/om500083r
Six benzimidazolin-2-ylidene palladium(II) complexes with an alkyl–alkyl thioether moiety in the side chain have been synthesized. Due to the hemilabile metal–sulfur bond, the complexes exhibit a marked fluxionality, as evidenced by NMR studies. The thioether moiety is readily displaced by pyridine as well as by another NHC ligand. Hetero(bis)-NHC complexes formally derived from all six chelating mono-NHC complexes have been synthesized as well. For both series of complexes, the catalytic activity has been explored, and they were found to be active catalysts for the intermolecular hydroamination reaction between a sterically hindered aniline and an alkyne in the presence of triflic acid. Furthermore, the complexes catalyze the direct arylation of 1-methylpyrrole.
Co-reporter:Dan Yuan and Han Vinh Huynh
Organometallics 2014 Volume 33(Issue 21) pp:6033-6043
Publication Date(Web):October 9, 2014
DOI:10.1021/om500659v
A series of Pd(II) dibromido complexes 2–6 bearing cis-chelating hetero-dicarbenes, which contain two different types of NHCs linked by a propylene chain, have been synthesized. In most cases, the N-methylbenzimidazolin-2-ylidene moiety was kept as one NHC donor, while the other one varies with different heterocyclic backbones. As an exception, the hetero-diNHC in complex 8 is derived by combining 1,2,4-triazole and indazole precursors instead. Analogous complexes 9–17, carrying more labile CF3CO2– or CH3CN ligands, were synthesized by reacting the aforementioned bromido complexes with AgO2CCF3 or AgOTf in CH3CN. A systematic catalytic comparison of 9–17 in the direct arylation of pentafluorobenzene with 4-chlorobromobenzene was carried out, and complexes that contain bulkier and less electron-donating ligands were found to be more active. Complex 12, carrying the mesitylimidazolin-2-ylidene unit, proved to be the most efficient, and its activity was also tested in the direct arylation of tetrafluorobenzenes.
Co-reporter:Jan C. Bernhammer and Han Vinh Huynh
Organometallics 2014 Volume 33(Issue 1) pp:172-180
Publication Date(Web):December 11, 2013
DOI:10.1021/om400929t
A series of six benzimidazolium salts with an alkyl–alkyl thioether moiety in the side chain has been synthesized. While it was impossible to obtain the platinum(II) complexes by direct reaction between ligand precursors and basic platinum salts, the mild silver carbene transfer reaction gave the desired complexes in all cases. X-ray crystallography confirmed the expected κ2C,S coordination mode of the benzimidazolin-2-ylidene ligands, with a cis arrangement of the carbene and the hemilabile thioether moieties in all complexes. Preliminary studies of the catalytic activity of these complexes showed them to be active catalysts for the intermolecular hydroamination of alkynes with sterically hindered anilines in conjunction with silver triflate. Additionally, the complexes catalyzed the hydrosilylation of alkenes with excellent yields and good regioselectivity.
Co-reporter:Dan Yuan, Qiaoqiao Teng, and Han Vinh Huynh
Organometallics 2014 Volume 33(Issue 7) pp:1794-1800
Publication Date(Web):March 21, 2014
DOI:10.1021/om500140g
Oxidation of easily accessible thiolato-functionalized dinuclear Pd(II) NHC complexes 1–3 by Oxone gave rise to sulfonate-NHC complexes 4–6. This represents the first template-directed approach to NHC complexes bearing sulfonate functions, where the sulfur atoms undergo a six-electron oxidation, changing their oxidation states from −II to +IV. The catalytic activities of water-soluble 4–6 were also tested in aqueous Mizoroki–Heck reactions.
Co-reporter:Shuai Guo and Han Vinh Huynh
Organometallics 2014 Volume 33(Issue 8) pp:2004-2011
Publication Date(Web):April 10, 2014
DOI:10.1021/om500139b
The dipalladium triazolidine-diylidene complex all-trans-[PdBr2(CH3CN)]2(μ-ditz) (1) (ditz = 1,2,4-trimethyltriazolidine-3,5-diylidene) was synthesized via in situ deprotonation of the precursor salt with a basic metal precursor. Ligand replacements of all-trans-1 with monodentate or chelating phosphines afforded the dicarbene-bridged complexes all-cis-[PdBr2(PPh3)]2(μ-ditz) (2) and [PdBr(DPPP)]2(μ-ditz)Br2 (3), respectively. Bromido substitution of all-cis-2 gave tetra-acetato complex all-cis-[Pd(CH3COO)2(PPh3)]2(μ-ditz) (4) with retention of the configuration as the predominant product. In addition, monopalladium triazolin-5-ylidene complexes trans-[PdBr2(CH3CN)(tazy)] (6, tazy =1,4-dimethyltriazolin-5-ylidene), cis-[PdBr2(PPh3)(tazy)] (7), [PdBr(DPPP)(tazy)]Br (8), and cis-[Pd(CH3COO)2(PPh3)(tazy)] (9) were also synthesized as the respective mononuclear equivalents for comparison. A comparative catalytic study revealed the general superiority of dinuclear complexes 1–4 over their respective mononuclear counterparts 6–9 in the direct C5-arylation reaction of 1-methylimidazoles. Overall, mixed dicarbene/diphosphine complex 3 showed the best catalytic performance.
Co-reporter:Jan C. Bernhammer, Ning Xi Chong, Ramasamy Jothibasu, Binbin Zhou, and Han Vinh Huynh
Organometallics 2014 Volume 33(Issue 13) pp:3607-3617
Publication Date(Web):June 27, 2014
DOI:10.1021/om500566n
Indazolin-3-ylidenes (indy) are among the most strongly donating N-heterocyclic carbenes, but the structural diversity of their complexes is still limited. Two dimeric palladium(II) complexes, [PdBr2(indy-5)]2 (2a) and [PdBr2(indy-6)]2 (2b) (indy-5 = 2,3-dihydro-1H-pyrazolo[1,2-a]indazolin-3-ylidene, indy-6 = 6,7,8,9-tetrahydropyridazino[1,2-a]indazolin-3-ylidene], bearing indazolin-3-ylidene ligands with different sizes of the fused aliphatic ring can be obtained by silver carbene transfer. The reaction of these dimers with pyridine yielded trans-[PdBr2(indy)(pyridine)] complexes (3a,b), while the poorly soluble monophosphine complexes cis-[PdBr2(indy)(PPh3)] (4a,b) were obtained by reaction with triphenylphosphine. Ligand substitution of the latter with silver trifluoroacetate afforded cis-[Pd(O2CCF3)2(indy)(PPh3)] complexes (5a,b) with improved solubilities, allowing for their detailed characterizations. In the presence of sodium tetrafluoroborate, cationic bis(phosphine) complexes trans-[PdBr(PPh3)2][BF4] (6a,b) could be obtained. Similarly, cis-[PdBr(dppe)][BF4] (7a,b) and cis-[PdBr(dppp)][BF4] (8a,b) were obtained (dppe = bis(diphenylphosphino)ethane; dppp = bis(diphenylphosphino)propane) with the respective chelating diphosphines. A preliminary catalytic study revealed that the complexes incorporating monodentate phosphine ligands are good catalysts for the Sonogashira cross-coupling, while moderate to good yields were achieved with all complexes for the hydroamination of carbon–carbon triple bonds.
Co-reporter:Qiaoqiao Teng, Daniel Upmann, Sheena Ai Zi Ng Wijaya, and Han Vinh Huynh
Organometallics 2014 Volume 33(Issue 13) pp:3373-3384
Publication Date(Web):June 30, 2014
DOI:10.1021/om500274c
A series of 20 bis(functionalized NHC) Pd(II) complexes have been conveniently synthesized through postmodification reactions of the common parent NHC complexes trans-[PdBr2(C3Br-bimy)2] (1a) and trans-[PdBr2(C3Br-imy)2] (1b) (C3Br-bimy = 1-benzyl-3-(3-bromopropyl)benzimidazolin-2-ylidene and C3Br-imy = 1-benzyl-3-(3-bromopropyl)imidazolin-2-ylidene) with an N-C3Br tether. Depending on the nature of the nucleophiles added, competing bromido ligand displacements also occurred. In comparison to the conventional access to functionalized NHC complexes, which involves metalation of individually prefunctionalized azolium salts, this highly modular method proves more effective in cost and time savings. The tetraalkylammonium-functionalized complex trans-[PdBr2(C3NEt3-bimy)2]Br2 (14) as a product of the second-generation postmodification of complex 1a was also tested for its catalytic activity in Mizoroki–Heck coupling reactions to study and model the effect of a catalytic amount of ammonium salt additive in Pd-catalyzed C–C coupling reactions. The ammonium functionalities in this complex exhibit a positive effect on the catalysis in comparison to its parent complexes.
Co-reporter:Jan C. Bernhammer, Harvenjit Singh, and Han Vinh Huynh
Organometallics 2014 Volume 33(Issue 16) pp:4295-4301
Publication Date(Web):August 12, 2014
DOI:10.1021/om5006275
A series of five trans-[PdBr2(amine)(indy)] complexes (amine = diethylamine, dipropylamine, dibutylamine, diisobutylamine, morpholine; indy = indazolin-3-ylidene) with pendant teriary amine functionalities in the side chain of the NHC ligand has been prepared by postcoordinative modification of a single bromoalkyl-functionalized precursor complex. This approach allows for a synthesis of functionalized N-heterocyclic carbene complexes more efficient than the metalation of prefunctionalized azolium salts. All complexes have been fully characterized, and the molecular structures of three complexes are reported. A correlation exists between the 13C NMR shift of Ccarbene and the pKb values of the coordinated amines. Furthermore, all complexes were found to be active catalysts for the direct arylation of 1-methylpyrrole with good to excellent yields.
Co-reporter:Han Vinh Huynh and Qiaoqiao Teng
Chemical Communications 2013 vol. 49(Issue 39) pp:4244-4246
Publication Date(Web):19 Jul 2012
DOI:10.1039/C2CC33789D
The synthesis of a bromopropyl-substituted NHC–Pd(II) complex, which can undergo exemplary and versatile 2nd and 3rd generation post-modifications easily affording 7 new functionalised NHC complexes, is demonstrated.
Co-reporter:Dan Yuan and Han Vinh Huynh
Inorganic Chemistry 2013 Volume 52(Issue 11) pp:6627-6634
Publication Date(Web):May 16, 2013
DOI:10.1021/ic400672z
Dimeric thiolato-bridged Ni(II) and Pt(II) NHC complexes 2 and 4 have been synthesized from ligand precursor A through a combined and in situ deprotonation/hydrolysis protocol of a thioester-functionalized benzimidazolium salt in the presence of the respective metal salts. Reactivity studies of 2 and 4, and their previously reported Pd(II) analogue 1a toward either Me3OBF4, NaOH, or Na2S·9H2O revealed clear differences. Complex 2 decomposed when treated with Me3OBF4. On the other hand, its reaction with aqueous NaOH solution in the presence of NaBF4 yielded trinuclear [Ni3S3O] complex 6, which possesses an interesting [Ni3S3] triangle with a capping μ3-oxido ligand. Pt(II) analogue 4 was converted to the tetranuclear [Pt4S4] macrocycle 5 when treated with Me3OBF4, in analogy to the result from 1a, while no defined products could be isolated when 4 was treated with either NaOH or Na2S·9H2O. Pd(II) analogue 1a reacted with Na2S·9H2O to give the tripalladium [Pd3S3S] complex 7 bearing a capping μ3-sulfido ligand.
Co-reporter:Han Vinh Huynh and Chen-Shiang Lee
Dalton Transactions 2013 vol. 42(Issue 19) pp:6803-6809
Publication Date(Web):21 Feb 2013
DOI:10.1039/C3DT50237F
Pd(II) complexes [PdBr(A-κ3CNC)]Br (1) and [PdBr(B-κ3CNC)]Br (2) with new CNC pincer-type ligands derived from 1,2,4-triazolin-5-ylidenes have been synthesized and characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis. The more soluble complex 1 proved to be an efficient pre-catalyst in copper- and amine-free Sonogashira reaction with high turnover numbers. The potential for recycling of the catalyst was also demonstrated.
Co-reporter:Haresh Sivaram, Jackie Tan and Han Vinh Huynh
Dalton Transactions 2013 vol. 42(Issue 34) pp:12421-12428
Publication Date(Web):17 Jun 2013
DOI:10.1039/C3DT51071A
A series of cationic gold(I) heteroleptic complexes bearing the pyrazole-derived N-heterocyclic carbene (NHC) FPyr (1,2,3,4,6,7,8,9-octahydropyridazino[1,2-a]indazolin-11-ylidene), and either a 1,3-disubstituted benzimidazole-derived NHC of the type RR′-bimy (3: R = R′ = CHPh2; 4: R = CHPh2, R′ = iPr; 5: R = R′ = CH2Ph; 6: R = R′ = iBu; 7: R = R′ = n-Pr; 8: R = R′ = Et; 9: R = R′ = 2-propenyl) or a non-NHC co-ligand L (10: L = PPh3; 11: L = P(OPh)3; 12: L = DMAP) (DMAP = 4-dimethylaminopyridine) have been synthesized from [AuCl(FPyr)] (1). Complexes 3–12 have been characterized using multinuclei NMR spectroscopies, ESI mass spectrometry, and elemental analysis. X-ray diffraction analyses have been performed on complexes 5, 6, and 9–11. To the best of our knowledge, 11 represents the first gold–NHC complex to bear the P(OPh)3 ligand. The cytotoxic activities of complexes 3–12 have been studied in vitro with the NCI-H1666 non-small cell lung cancer cell line.
Co-reporter:Han Vinh Huynh;Hong Lee Ong;Jan C. Bernhammer;Gilles Frison
European Journal of Inorganic Chemistry 2013 Volume 2013( Issue 26) pp:4654-4661
Publication Date(Web):
DOI:10.1002/ejic.201300334
Abstract
The oxidative addition of palladium(0) to the C2-protected 5-bromo-2-methylbenzimidazolium tetrafluoroborate 3 affords the complex trans-[PdBr(bimy)(PPh3)2]BF4 (4), which is the first complex of an unprecedented benzimidazole-derived ligand (bimy) with a potentially “mesoionic and remote” character. DFT calculations revealed that the bimy ligand bridges the gap between the mesoionic pyrazolin-4-ylidene and the anionic phenyl ligand. A preliminary catalytic study revealed the promising activity of 4 in the Sonogashira coupling of aryl bromides with phenylacetylene.
Co-reporter:Yu Liu;Dr. Rakesh Ganguly; Han Vinh Huynh; Weng Kee Leong
Angewandte Chemie International Edition 2013 Volume 52( Issue 46) pp:12110-12113
Publication Date(Web):
DOI:10.1002/anie.201307102
Co-reporter:Yu Liu;Dr. Rakesh Ganguly; Han Vinh Huynh; Weng Kee Leong
Angewandte Chemie 2013 Volume 125( Issue 46) pp:12332-12335
Publication Date(Web):
DOI:10.1002/ange.201307102
Co-reporter:Dipl.-Chem. Jan C. Bernhammer;Dr. Gilles Frison; Han Vinh Huynh
Chemistry - A European Journal 2013 Volume 19( Issue 38) pp:12892-12905
Publication Date(Web):
DOI:10.1002/chem.201301093
Abstract
Carbenes derived from five-membered heterocycles with different numbers of nitrogen atoms ranging from two to four lead formally either to normal N-heterocyclic or mesoionic carbenes with, in some cases, the same skeletal structure. The electronic structures of fourteen of these compounds were examined by means of DFT calculations at the B3LYP/aug-cc-pVTZ level. The examined parameters include the energies of the σ-lone pair at Ccarbene and the π-HOMO of the protonated form, which are correlated to the first and second proton affinities. The singlet–triplet energy gap was used as a measure of the stability of the N-heterocyclic carbene (NHC) towards dimerisation. Natural population analysis provided insight into the variation of the pπ population and the natural charge at Ccarbene with NHC structure. Additionally, the transition metalNHC bond in L-AuCl and L-TiCl4 and the nature of the orbital interactions between the NHC and the transition-metal fragment were analysed in detail by the extended transition state–natural orbitals for chemical valence (ETS–NOCV) approach at the BP86/TZ2P level. Similarities and differences between the NHCgold and the NHCtitanium bond are discussed, and trends in key bonding properties can be traced back to the variation of the electronic parameters of the NHC.
Co-reporter:Shuai Guo, Haresh Sivaram, Dan Yuan, and Han Vinh Huynh
Organometallics 2013 Volume 32(Issue 13) pp:3685-3696
Publication Date(Web):June 25, 2013
DOI:10.1021/om400313r
A series of new Au(I) hetero-bis-NHC complexes [Au(iPr2-bimy)(NHC)]X (X = BF4, PF6, 2–6) and the hetero-tetrakis-NHC complex [Au2(iPr2-bimy)2(μ-ditz)](BF4)2 (7) have been synthesized using the Au(I) acetato complex [Au(O2CCH3)(iPr2-bimy)] (C) as a basic metal precursor (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene, ditz = 1,2,4-triazolidine-3,5-diylidene). The Au(III) hetero-bis-NHC complex trans-[AuCl2(iPr2-bimy)(Bn2-bimy)]BF4 (12; Bn2-bimy = 1,3-dibenzylbenzimidazolin-2-ylidene) and the hetero-tetrakis-NHC complex all-trans-[Au2Cl4(iPr2-bimy)2(μ-ditz)](BF4)2 (13) were obtained by oxidation of their corresponding Au(I) hetero-NHC precursors. For all Au(I) hetero-NHC complexes, the 13C carbene signals of the constant iPr2-bimy ligand are found to be highly correlated with those in Pd(II) analogues of the type trans-[PdBr2(iPr2-bimy)(NHC)], which could be applied to detect the σ-donating ability of the trans-standing NHC. In addition, an interesting ligand redistribution process was observed for some of the Au(I) hetero-bis-NHC complexes.
Co-reporter:Han Vinh Huynh, Shuai Guo, and Wenqin Wu
Organometallics 2013 Volume 32(Issue 16) pp:4591-4600
Publication Date(Web):August 13, 2013
DOI:10.1021/om400563e
A complete series of ten Au(I) and Au(III) NHC complexes of the general formulas [AuIX(iPr2-bimy)] (1–3, X = Cl, Br, and I), [AuI(iPr2-bimy)2]BF4 (4), [AuIIIX3(iPr2-bimy)] (5–7, X = Cl, Br, and I) and trans-[AuIIIX2(iPr2-bimy)]BF4 (8–10, X = Cl, Br, and I) bearing the iPr2-bimy ligand, and all three common halido ligands have been synthesized and fully characterized. Detailed trends in their NMR and UV–Vis spectroscopic properties have been studied, and their electrochemical behavior have been probed by cyclic voltammetry. The solid state molecular structures of all new complexes determined by single crystal X-ray diffraction are also described.
Co-reporter:Shuai Guo, Miao Hui Lim, and Han Vinh Huynh
Organometallics 2013 Volume 32(Issue 23) pp:7225-7233
Publication Date(Web):November 14, 2013
DOI:10.1021/om400911u
A series of 2-coordinate heteroleptic Cu(I) complexes of the general formula [Cu(IPr)(L)]PF6 (2–5, L = NHC or phosphine) have been synthesized via either (i) chlorido substitution by phosphine or in situ generated free NHC or (ii) the Ag–NHC transfer protocol using [CuCl(IPr)] (1) as a precursor (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene). The reactions of precursor 1 with diphosphine ligands afforded 3-coordinate heteroleptic Cu(I) complexes of the type [Cu(IPr)(L2)]PF6 (6 and 7, L2 = diphosphine). Complexes 1–7 have been subjected to a catalytic one-pot sequential CuAAC study, in which aromatic amines serve as the precursors to aryl azides. Hetero-bis(NHC) complexes 2–4 proved to be generally superior compared to their mixed NHC/phosphine counterparts 5–7. Overall, complex [Cu(Bn2-imy)(IPr)]PF6 (2), bearing the Bn2-imy (Bn2-imy = 1,3-dibenzyl-imidazolin-2-ylidene) coligand, showed the best catalytic performance.
Co-reporter:Jan Christopher Bernhammer and Han Vinh Huynh
Dalton Transactions 2012 vol. 41(Issue 28) pp:8600-8608
Publication Date(Web):04 May 2012
DOI:10.1039/C2DT30526G
The relative ligand donor strengths of 10 pyrazole-derived ligands has been determined with great accuracy, making use of the interdependence between the donor strength of the co-ligand and the 13C NMR chemical shift of the iPr2-bimy carbene signal in trans-[PdBr2(iPr2-bimy)L] complexes (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene; L = pyrazole-derived ligand). Even subtle variations in the substitution pattern of the pyrazole backbone up to three bonds away from the coordinating nitrogen could be detected reliably using this methodology. Alkylation experiments conducted on the pyrazoles using electrophiles of varied reactivity (ethyl bromide, ethyl iodide, and trimethyloxonium tetrafluoroborate) served as a benchmark to rank the pyrazoles in three groups of gradually increasing nucleophilicity, which correlated well with their determined donor strength.
Co-reporter:Dan Yuan and Han Vinh Huynh
Organometallics 2012 Volume 31(Issue 1) pp:405-412
Publication Date(Web):December 23, 2011
DOI:10.1021/om2010029
A series of hetero-bis(carbene) complexes trans-[PdBr2(iPr2-bimy)(trz)] 1–4 (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene; trz = 1,2,3-triazolin-5-ylidene) bearing the constant iPr2-bimy and varying mesoionic 1,2,3-triazolin-5-ylidenes with different N-substituents has been synthesized as complex probes. Their 13C NMR spectroscopic evaluation shows that mesoionic 1,2,3-triazolin-5-ylidenes are, in general, stronger donors than classical NHCs, while weaker than some nonclassical NHCs, such as pyrazolin-3-ylidenes and mesoionic imidazolin-4-ylidenes. More importantly and for the first time, this methodology proves useful in establishing substituent effects in the donating abilities of 1,2,3-triazolin-5-ylidenes on a finer level. In addition, the trifluoroacetato analogues [Pd(O2CCF3)2(iPr2-bimy)(trz)] 5–7 have been synthesized through salt metathesis of 1, 2, and 4 with AgO2CCF3. The catalytic activities of complexes 1, 2, and 4–7 were examined in the direct arylation of pentafluorobenzene. Complexes bearing less donating trz ligands perform better in this catalysis, and trifluoroacetato complexes outperformed their bromido analogues.
Co-reporter:Haresh Sivaram, Ramasamy Jothibasu, and Han Vinh Huynh
Organometallics 2012 Volume 31(Issue 3) pp:1195-1203
Publication Date(Web):January 30, 2012
DOI:10.1021/om201268m
A gold(I) NHC complex bearing a heteroalicyclic indazolin-3-ylidene ligand, [AuCl(Indy)] (1) (Indy = 6,7,8,9-tetrahydropyridazino[1,2-a]indazolin-3-ylidene), has been synthesized via the silver–carbene transfer method. Conversion of complex 1 to its heavier halido analogues [AuBr(Indy)] (2) and [AuI(Indy)] (3) was achieved by metathesis reactions involving LiBr and NaI in acetone, respectively. In contrast to 1 and 2, complex 3 undergoes ligand disproportionation/autoionization upon crystallization, forming the solid complex salts [Au3I2(Indy)4][Au3I4(Indy)2] (3′) or [Au(Indy)2][AuI2] (3″) depending on the solvent used. This reversible process assisted by aurophilic interactions, and only occurring in the iodido complex 3, has been studied further by spectroscopic comparison with [Au(Indy)2]BF4 (4) and selective conversion of 3 to the gold(III) species [AuI3(Indy)] (5). All complexes 1–5 have been fully characterized using multinuclei NMR spectroscopies, ESI mass spectrometry, and X-ray diffraction analysis.
Co-reporter:Shuai Guo and Han Vinh Huynh
Organometallics 2012 Volume 31(Issue 12) pp:4565-4573
Publication Date(Web):May 31, 2012
DOI:10.1021/om3003625
The 1,2,4-trimethyltriazolidin-3,5-diylidene (ditz) bridged dipalladium heterotetracarbene complex [PdBr2(iPr2-bimy)]2(μ-ditz) (3) (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) was prepared by Ag–carbene transfer involving the 1,2,4-trimethyltriazolium dication (C), Ag2O, and the precursor complex (iPr2-bimyH)[PdBr3(iPr2-bimy)] (2). Bromido substitution of 3 with AgO2CCH3 and AgO2CCF3afforded the carboxylato complexes [Pd(O2CCH3)2(iPr2-bimy)]2(μ-ditz) (4) and [Pd(O2CCF3)2(iPr2-bimy)]2(μ-ditz) (5). Multinuclei NMR spectroscopies and X-ray diffraction analyses showed that the all-trans isomers of 3, 4, and 5 are the predominant products in all three cases. In addition, the decomposition product of complex 4, trans-[Pd(O2CCH3)2(iPr2-bimy)2] (trans-6) was structurally determined by X-ray crystallography. A comparative catalytic study revealed the superiority of complexes 3, 4, and 5 over the previously reported mononuclear bis(benzimidazolin-2-ylidine) complexes without ditz bridges in the Mizoroki–Heck coupling reaction. Overall, complex 4 bearing acetato coligands showed the best catalytic performance.
Co-reporter:Jan C. Bernhammer and Han Vinh Huynh
Organometallics 2012 Volume 31(Issue 14) pp:5121-5130
Publication Date(Web):June 29, 2012
DOI:10.1021/om300464b
Ten palladium(II) complexes bearing a pyrazolin-5-ylidene ligand have been synthesized by oxidative addition and silver carbene transfer pathways. The weakly bound acetonitrile ligand in the initially obtained trans-[PdBr2(MeCN)(Pyry)] complex (6, Pyry = 1-phenyl-2,3-dimethylpyrazolin-5-ylidene) could be replaced by other donor ligands, and additional NHC ligands were introduced either by silver carbene transfer reactions or via reaction with in situ generated free carbenes. Using our previously reported 13C NMR-based electronic parameter, the pyrazolin-5-ylidene ligand is estimated to be among the most strongly donating ligands on our scale so far. The complexes obtained were employed as catalysts for the direct arylation of pentafluorobenzene with moderate to good yields under optimized conditions.
Co-reporter:Haresh Sivaram, Jackie Tan, and Han Vinh Huynh
Organometallics 2012 Volume 31(Issue 16) pp:5875-5883
Publication Date(Web):July 2, 2012
DOI:10.1021/om300444c
A series of Au(I) and Au(III) mono-, homobis-, and heterobis(carbene) complexes, [AuCl(FPyr)] (2), [Au(iPr2-bimy)2]PF6 (3), [Au(FPyr)2]PF6 (4), [Au(FPyr)(iPr2-bimy)]PF6 (5), [AuCl3(iPr2-bimy)] (6), [AuCl3(FPyr)] (7), [AuCl2(iPr2-bimy)2]PF6 (8), [AuCl2(FPyr)2]PF6 (9), and [AuCl2(FPyr)(iPr2-bimy)]PF6 (10), bearing the benzimidazole-derived iPr2-bimy (1,3-diisopropylbenzimidazolin-2-ylidene) and/or the pyrazole-derived FPyr (1,2,3,4,6,7,8,9-octahydropyridazino[1,2-a]indazolin-11-ylidene) N-heterocyclic carbene (NHC) ligands have been synthesized. Complexes 2–10 have been fully characterized using multinuclei NMR spectroscopy, ESI mass spectrometry, and elemental analysis. X-ray diffraction analyses have been performed on 2, 3, 5, 6, and 8. Together with the previously reported [AuCl(iPr2-bimy)] (1), the cytotoxic activities of all 10 complexes have been studied in vitro with the NCI-H1666 non-small cell lung cancer cell line. The cationic bis(carbene) complexes 3–5 and 8–10 show better cytotoxicity in comparison to cisplatin. In particular, the heterobis(carbene) complexes 5 and 10 have superior activity, with IC50 values of around 0.2 μM.
Co-reporter:Yuan Han and Han Vinh Huynh
Dalton Transactions 2011 vol. 40(Issue 10) pp:2141-2147
Publication Date(Web):06 Dec 2010
DOI:10.1039/C0DT01037E
The immense success of N-heterocyclic carbenes in recent years has initiated the search for even stronger ligands leading to the discovery of abnormal and remote NHCs. This article reflects our particular interest in the coordination chemistry of pyrazolin-4-ylidenes as a contribution to this field. A modular approach to 4-iodopyrazolium salts with different substitution patterns is described, which upon oxidative addition to Pd0 gave rise to a library of new Pd(II) pyrazolin-4-ylidene complexes. A preliminary study showed that selected complexes are active precatalysts in Suzuki–Miyaura and Mizoroki–Heck coupling reactions. The 3,5-substituents of pyrazolin-4-ylidenes are found to have significant effects on complexation and catalytic activities. Finally, the nature of this new class of ligands is discussed, and a future direction for further explorations in this exciting field is envisioned.
Co-reporter:Han Vinh Huynh, Weiheng Sim and Chee Fei Chin
Dalton Transactions 2011 vol. 40(Issue 44) pp:11690-11692
Publication Date(Web):10 Oct 2011
DOI:10.1039/C1DT11472G
Bridge cleavage reactions of the dimeric monocarbene complex [PdBr2(iPr2-bimy)]2 can be effectively used to end-cap pyridine containing pseudorotaxanes affording stable [2]rotaxanes.
Co-reporter:Dan Yuan and Han Vinh Huynh
Dalton Transactions 2011 vol. 40(Issue 44) pp:11698-11703
Publication Date(Web):11 Aug 2011
DOI:10.1039/C1DT10789E
The thiolato-bridged dimeric Pd(II) NHC complex 1 has been synthesized from the reaction of thioester-functionalized imidazolium salt B and Pd(OAc)2. The isolation of its interesting constitutional isomer 2 bearing both classical C(2)-bound and mesoionic C(4)-bound ligands coordinating to two different metal centers in the same complex allowed for a direct comparison of these isomeric carbenes. Reactivity studies of 1 with NaSCH(CH3)2 and NaBF4 afforded the tetranuclear compound 3 with a [Pd4S4] macrocycle. All complexes have been fully characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis.
Co-reporter:Liqin Xue, Lijun Shi, Yuan Han, Chungu Xia, Han Vinh Huynh and Fuwei Li
Dalton Transactions 2011 vol. 40(Issue 29) pp:7632-7638
Publication Date(Web):15 Jun 2011
DOI:10.1039/C1DT10433K
A series of carbene complexes [PdBr2(iPr2-bimy)L] (C2–C13) with different types of co-ligands (L) have been tested for their catalytic activities in the carbonylative annulation of 2-iodophenol with phenylacetylene in DMF to afford the respective flavone 2a. Complex C12 with an N-phenylimidazole co-ligand showed the best activity and also afforded high yields when the substrate scope was extended to other aryl or pyridyl acetylenes. In addition, catalyst C12 was also efficient in the carbonylative annulation of 2-iodoaniline with acid chlorides giving the desirable 2-substituted 4H-3,1-benzoxazin-4-ones (4) in good yields. Additionally, this Pd–NHC complex also proved to be a very efficient catalyst for the hydroxycarbonylation of iodobenzene derivatives at low catalyst loading and under low CO pressure. These results demonstrate the versatility and efficiency of this phosphine-free Pd(II)–NHC complex in different types of carbonylations of aryl iodides under mild conditions.
Co-reporter:Dan Yuan, Haoyun Tang, Linfei Xiao and Han Vinh Huynh
Dalton Transactions 2011 vol. 40(Issue 35) pp:8788-8795
Publication Date(Web):27 Apr 2011
DOI:10.1039/C1DT10269A
Three thioether bridged diimidazolium dibromides with different steric and electronic properties have been synthesized as precursors to carbene-based CSC pincer ligands. Palladation afforded CSC Pd(II) pincer complexes for bulky and electron rich ligand systems, whereas the least donating ligand led to the formation of a pseudo-pincer complex. All complexes have been fully characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis. The catalytic activities of pincer versus pseudo-pincer complexes have been compared in the intermolecular hydroamination of alkynes with anilines as well.
Co-reporter:Han Vinh Huynh
Applied Organometallic Chemistry 2011 Volume 25( Issue 7) pp:
Publication Date(Web):
DOI:10.1002/aoc.1768
No abstract is available for this article.
Co-reporter:Han Vinh Huynh, Ramasamy Jothibasu
Journal of Organometallic Chemistry 2011 696(21) pp: 3369-3375
Publication Date(Web):
DOI:10.1016/j.jorganchem.2011.07.018
Co-reporter:Yuan Han, Dan Yuan, Qiaoqiao Teng, and Han Vinh Huynh
Organometallics 2011 Volume 30(Issue 5) pp:1224-1230
Publication Date(Web):February 10, 2011
DOI:10.1021/om101169x
Pd(II) dimers [PdI2(rNHC)]2 (1a,b) of pyrazole-based remote N-heterocyclic carbenes (rNHCs) have been synthesized through oxidative addition of 4-iodopyrazolium iodides (A, B) to [Pd2(dba)3]. Reaction of 1a with aromatic (CN-Xyl) or aliphatic (CN-Cy) isocyanides led to the template-assisted formation of novel Pd(II) dimers 8 and 9 bearing betainic C-imino ligands via isocyanide insertion into Pd−CrNHC bonds and subsequent dimerization. In contrast, both isocyanides reacted with the dimers [PdI2(Me2-indy)]2 (2) and [PdI2(Me2-bimy)]2 (3) bearing indazolin-3-ylidenes and benzimidazolin-2-ylidenes with formation of mononuclear mixed carbene/isocyanide complexes 4−7. Notably, only dimer 8 underwent further bridge cleavage with excess isocyanide, yielding the mixed C-imino/CN-Xyl complex 10, while dimer 9 remained intact. These results highlight the uniquely different reactivities of complexes with carbenes having no α-nitrogen versus those with one or two α-nitrogen atoms, as a result of their decreasing donor abilities.
Co-reporter:Ramasamy Jothibasu and Han Vinh Huynh
Chemical Communications 2010 vol. 46(Issue 17) pp:2986-2988
Publication Date(Web):05 Mar 2010
DOI:10.1039/B925977E
The versatile coordination chemistry of strongly donating indazolin-3-ylidene ligands as new members of the NHC family is demonstrated on 12 new PdII, AuI and RhI complexes, which were fully characterized by multinuclear NMR spectroscopies, ESI mass spectrometry and X-ray diffraction studies.
Co-reporter:Han Vinh Huynh, Ying Xia Chew
Inorganica Chimica Acta 2010 Volume 363(Issue 9) pp:1979-1983
Publication Date(Web):10 June 2010
DOI:10.1016/j.ica.2009.02.035
A new thiophene-functionalized benzimidazolium salt (2) has been prepared by reacting N-methylbenzimidazole with 2-bromomethylthiophene (1), which in turn was obtained by bromination of 2-thiophenemethanol with PBr3. Subsequent reaction of salt 2 with Pd(OAc)2 afforded the cis-configured bis(carbene) Pd(II) complex (cis-3), which in solution exists as an inseparable mixture of cis–anti and cis–syn-rotamers in a 3.5:1 ratio. All new compounds have been fully characterized by spectroscopic and spectrometric methods. A preliminary catalytic study shows that cis-3 is highly active in the Suzuki–Miyaura coupling of aryl bromides with phenylboronic acid in/on water as environmentally benign reaction media.Reaction of the new 1-(2-thienylmethyl)-3-methylbenzimidazolium salt 2 with Pd(OAc)2 afforded the first thiophene-functionalized bis(benzimidazolin-2-ylidene) Pd(II) complex (cis-3) as a mixture of rare cis–syn and cis–anti rotamers. A preliminary catalytic study reveals that cis-3 is highly active in the Suzuki–Miyaura coupling of aryl bromides with phenylboronic acid in water as environmentally benign media.
Co-reporter:Han Vinh Huynh, Chun Hui Yeo and Ying Xia Chew
Organometallics 2010 Volume 29(Issue 6) pp:1479-1486
Publication Date(Web):February 18, 2010
DOI:10.1021/om9010966
Four imidazolium (5a/b) and benzimidazolium (6a/b) salts with hemilabile alkyl-aryl thioether functions have been prepared via a straightforward and modular pathway in order to compare their reactivities toward palladation. Reaction of 5a/b with Pd(OAc)2 gave complex product mixtures, whereas 6a/b afforded the desired bis(benzimidazolin-2-ylidene) complexes 8a/b in good yields. The difference in reactivities of benzimidazole versus imidazole derivatives was attributed to the presence of additional acidic protons at C4/5 positions of the imidazolium ring, leading to competing and unselective deprotonation reactions. The milder Ag-NHC transfer reaction, on the other hand, provided either mono- or bis(imidazolin-2-ylidene) complexes (9 or 7a/b) in good yields depending on the ligand:metal ratio. The interesting hemilability in monocarbene complex 9 was investigated by spectroscopy and thioether displacement reaction with PPh3, yielding the mixed NHC-PPh3 complex 10 in high yields. An initial comparative catalytic study also reveals that the mixed-donor complex 10 exhibits the highest activity among the complexes tested.
Co-reporter:Ramasamy Jothibasu, Kuo-Wei Huang and Han Vinh Huynh
Organometallics 2010 Volume 29(Issue 17) pp:3746-3752
Publication Date(Web):August 3, 2010
DOI:10.1021/om100241v
Metathetical reaction of AgSCN with a series of trans-dihalido-bis(carbene) nickel(II) complexes in CH3CN readily afforded the novel diisothiocyanato-bis(carbene) complexes [Ni(NCS)2(NHC)2] (trans-2a, NHC = 1,3-diisopropylbenzimidazolin-2-ylidene; trans-2b, NHC = 1,3-diisobutylbenzimidazolin-2-ylidene; trans-2c, NHC = 1,3-dibenzylbenzimidazolin-2-ylidene; cis-2d, NHC = 1,3-di(2-propenyl)benzimidazolin-2-ylidene; cis-2e, NHC = 1-propyl-3-methylbenzimidazolin-2-ylidene) as greenish-yellow powders in moderate to good yields. While dihalido-bis(carbene) Ni(II) complexes exclusively form trans-complexes, a trans−cis isomerization occurs upon halido−isothiocyanato exchange with complexes bearing less bulky carbene ligands, i.e., cis-2d/e. DFT calculations indicated that this isomerization can be attributed to a reduced energy difference between trans- and cis-isomers of diisothiocyanato complexes. All complexes have been characterized by multinuclear NMR spectroscopy, ESI mass spectrometry, and X-ray diffraction analysis. A catalytic study revealed that cis-complexes generally exhibit greater activities in the Kumada−Corriu coupling reaction.
Co-reporter:Dan Yuan and Han Vinh Huynh
Organometallics 2010 Volume 29(Issue 22) pp:6020-6027
Publication Date(Web):October 13, 2010
DOI:10.1021/om1008023
The thiolato-bridged dimeric Pd(II) benzimidazolin-2-ylidene complex 1, with a [Pd2S2] core, was conveniently prepared by the reaction of the thiol-functionalized benzimidazolium salt C and Pd(OAc)2. More straightforwardly, 1 can also be synthesized by direct treatment of the thioester-functionalized benzimidazolium salt B with Pd(OAc)2 in wet DMSO under in situ hydrolysis of the thioester function. A subsequent salt metathesis reaction of 1 with AgO2CCF3 afforded the mixed dicarboxylato/NHC analogue 2 in quantitative yield, leaving the sulfur bridges of the [Pd2S2] core unaffected despite the use of the soft Ag(I) ions. Treatment of 1 with Me3OBF4 resulted in an unexpected bromido abstraction of 1 leading to an unusual rearrangement/dimerization reaction to give the tetranuclear NHC complex 3, which features a [Pd4S4] macrocylic square with sulfur corners. These reactions demonstrate the structural diversity of the thiolato-functionalized N-heterocyclic carbene complexes and may offer access to metallo-NHC-based supramolecular architectures. A comparative catalytic study revealed the superiority of NHC/thiolato complex 2 over complexes 1 and 3 in aqueous Suzuki−Miyaura couplings at very low catalyst loading.
Co-reporter:Yuan Han Dr.;Li Juan Lee Dr.
Chemistry - A European Journal 2010 Volume 16( Issue 3) pp:771-773
Publication Date(Web):
DOI:10.1002/chem.200902737
Co-reporter:Yuan Han and Han Vinh Huynh
Dalton Transactions 2009 (Issue 12) pp:2201-2209
Publication Date(Web):05 Feb 2009
DOI:10.1039/B816471A
Reaction of the bromo-bridged dimeric monocarbene complex [PdBr2(iPr2-bimy)]2 (1) (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) with various isocyanides afforded a series of mixed carbene–isocyanide complexes [PdBr2(iPr2-bimy)(CN-R)] (2a: R = Cy; 2b: R = nBu; 2c: R = Xyl) as inseparable mixtures of trans- and cis-isomers. The reactivity of 2c towards nucleophiles was studied. A new mixed NHC–ADC Pd(II) complex (4) was obtained in low yield when 2c was reacted with 2,6-dimethylaniline. Reaction of 2c with hydrazine yielded a hydrazine-bridged complex (6) via ligand substitution. In addition, salt metathesis of 2c with AgO2C2F3 afforded a cis arranged complex [Pd(O2CCF3)2(iPr2-bimy)(CN-Xyl)] (cis-7). Most complexes were fully characterized by multinuclei NMR spectroscopies, ESI or FAB mass spectrometry and X-ray diffraction analysis.
Co-reporter:Han Vinh Huynh, Dan Yuan and Yuan Han
Dalton Transactions 2009 (Issue 35) pp:7262-7268
Publication Date(Web):31 Jul 2009
DOI:10.1039/B907887H
Three new dibenzimidazolium salts bearing thioether (B·2HBr, B·2HNO3) and sulfoxide (C·2HBr) containing bridges have been synthesized as sulfur-functionalized dicarbene precursors. Palladation of B·2HBr and C·2HBr afforded two new pseudo-pincer complexes cis-[PdBr2(B-κ2C)] (1) and cis-[PdBr2(C-κ2C)] (2), in which the sulfur-donor remains pendant. Reaction of precursor B·2HNO3 in the presence of 1 equiv of KBr, on the other hand, yields the first CSC-Pd(II) pincer complex [PdBr(B-κ3CSC)]NO3 (3) bearing two carbene moieties. All three complexes have been fully characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis. Their catalytic activities in the Mizoroki–Heck reaction have been evaluated as well.
Co-reporter:Han Vinh Huynh ;Ramasamy Jothibasu
European Journal of Inorganic Chemistry 2009 Volume 2009( Issue 13) pp:1926-1931
Publication Date(Web):
DOI:10.1002/ejic.200801149
Abstract
The formation of mono- versus bis(chelate) NiII complexes bearing N-heterocyclic dicarbene ligands can be controlled by the flexibility of the ligand bridge. A short methylene spacer exclusively gives rise to a dicationic bis(chelate) complex [Ni(MeCCmeth)2]Br2 (1), whereas a more flexible propylene spacer affords a neutral monochelate complex [NiBr2(MeCCprop)] (2). Complex 2 was found to autoionize very slowly to the corresponding dicationic bis(chelate) over ca. 45 d in [D6]dmso. The formation of the bis- versus monochelate complex can be attributed to the different stabilities of the resulting metallacycles. The catalytic activity of monochelate 2 was tested in the Kumada–Corriu coupling of aryl halides with arylmagnesium reagents at ambient temperature. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
Co-reporter:Han Vinh Huynh, Jiang Wu
Journal of Organometallic Chemistry 2009 694(3) pp: 323-331
Publication Date(Web):
DOI:10.1016/j.jorganchem.2008.10.044
Co-reporter:Ramasamy Jothibasu and Han Vinh Huynh
Organometallics 2009 Volume 28(Issue 8) pp:2505-2511
Publication Date(Web):March 27, 2009
DOI:10.1021/om900140e
Salt metathesis reaction of the dihalo-bis(carbene) complex trans-[NiBr2(NHC)2] (1, NHC = 1,3-diisopropylbenzimidazolin-2-ylidene) with NaN3 in DMF at elevated temperature afforded the diazido-bis(carbene) complex trans-[Ni(N3)2(NHC)2] (2) as a red crystalline solid in a yield of 78%. Complex 2 served as a metal-template for the 1,3-dipolar cycloaddition of 2,6-dimethylphenylisocyanide (CN-Xyl) to the azido ligands to yield the mixed tetrazolato-carbodiimido complex trans-[Ni(CN4-Xyl)(NCN-Xyl)(NHC)2] (3) at ambient temperature and the dicarbodiimido complex trans-[Ni(NCN-Xyl)2(NHC)2] (4) at 70 °C. Reaction of alkyl isocyanides with 2 at ambient temperature gave the ditetrazolato complexes trans-[Ni(CN4-R)2(NHC)2] (5, R = tert-butyl; 6, R = cyclohexyl) in good yields. The novel cationic “abnormal” tetrazolin-5-ylidene complex trans-[Ni(CN4-tBu,Me)2(NHC)2](BF4)2 (7, tBu = tert-butyl) was synthesized by direct methylation of 5 with [Me3O]BF4. All compounds have been fully characterized by multinuclear NMR spectroscopies and ESI mass spectrometry. The solid state molecular structures of complexes 2, 4, 5, and 7·2CH2Cl2 have also been confirmed by X-ray diffraction studies.
Co-reporter:Yuan Han, Li Juan Lee and Han Vinh Huynh
Organometallics 2009 Volume 28(Issue 9) pp:2778-2786
Publication Date(Web):April 8, 2009
DOI:10.1021/om8010849
Three 4-iodopyrazolium salts with 3,5-dimethyl (3a), 3,5-diphenyl (3b), and 3,5-diisopropyl (3c) substituents, respectively, were synthesized using a modular approach. The oxidative addition of 3a−c to Pd2(dba)3/PPh3 afforded products (trans-4a, cis-/trans-4b and 4c) of different geometries or connectivities, indicating a dramatic substituent effect on the formation of pyrazolin-4-ylidene complexes. In addition, the reactions of 3a−c with Pd2(dba)3 in the presence of pyridine yielded new mixed pyrazolin-4-ylidene/pyridine complexes (5a−c). All complexes have been fully characterized by multinuclear NMR spectroscopies, ESI mass spectrometry, and X-ray diffraction analyses. Furthermore, an initial catalytic study on Suzuki−Miyaura and Mizoroki−Heck cross-coupling reactions also reveals a significant substituent effect on catalytic activities.
Co-reporter:Han Vinh Huynh, Yuan Han, Ramasamy Jothibasu, and Jie An Yang
Organometallics 2009 Volume 28(Issue 18) pp:5395-5404
Publication Date(Web):September 3, 2009
DOI:10.1021/om900667d
The electronic parameters of 25 Werner-type and organometallic ligands have been experimentally determined and ranked on a unprecedented unified 13C NMR scale using safe and easily obtainable complexes of the type trans-[PdBr2(iPr2-bimy)L]n− (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene; L = ligand in question) as spectroscopic probes. The methodology is based on the sensitivity of the constant iPr2-bimy carbene signal to the donor strengths of the varying co-ligands, which even allows detection of backbone and substituent effects more accurately than previous carbonyl-based systems. For the evaluation of N-heterocyclic carbenes (NHCs), a one-pot approach to novel hetero-bis(carbene) complexes bearing two different NHCs is introduced. Furthermore, the first complex of a strongly donating indazolin-3-ylidene ligand is presented. The molecular structures of 10 complex probes have been characterized by single-crystal X-ray diffraction analyses.
Co-reporter:Swee Kuan Yen, Lip Lin Koh, Han Vinh Huynh and T. S. Andy Hor
Dalton Transactions 2008 (Issue 5) pp:699-706
Publication Date(Web):29 Nov 2007
DOI:10.1039/B713152F
Novel Pd(II) mixed N,S-heterocyclic carbene (NSHC)-phosphine complexes of the general formula [PdBr2(NSHC)(PR3)] were obtained from bridge cleavage of dinuclear NSHC complexes of type [PdBr2(NSHC)]2 [NSHC = 3-benzylbenzothiazolin-2-ylidene and 3-propylbenzothiazolin-2-ylidene] with triphenylphosphine, tricyclohexylphosphine and 2-diphenylphosphanyl-pyridine. All complexes have been fully characterized by 1H and 13C NMR spectroscopy, ESI mass spectrometry and elemental analysis. The X-ray crystal structures of complexes 3–8 are reported. The complexes exhibit moderate to good catalytic activity in the Suzuki–Miyaura coupling reaction of aryl bromides and chlorides.
Co-reporter:Han Vinh Huynh, Ling Rong Wong and Pearly Shuyi Ng
Organometallics 2008 Volume 27(Issue 10) pp:2231-2237
Publication Date(Web):April 24, 2008
DOI:10.1021/om800004j
Nickel(II) bis(benzimidazolin-2-ylidene) complexes of the general formula [NiBr2(NHC)2] (NHC = 1,3-dibenzylbenzimidazolin-2-ylidene, 7; NHC = 1,3-diisopropylbenzimidazolin-2-ylidene, 8; NHC = 1,3-dibenzhydrylbenzimidazolin-2-ylidene, 9; NHC = 1,3-diisobutylbenzimidazolin-2-ylidene, 10; NHC = 1-isopropyl-3-benzylbenzimidazolin-2-ylidene, 11; NHC = 1-benzhydryl-3-benzylbenzimidazolin-2-ylidene, 12) have been prepared and fully characterized by spectroscopic methods and single-crystal X-ray structure analyses. All complexes adopt a square-planar geometry with nickel as the crystallographic inversion center and a trans arrangement of the carbene ligands. For complexes 11 and 12, bearing unsymmetrically substituted ligands, only the trans-anti configuration was found in the solid state. In addition, the structures of 8, 9, 11, and 12 reveal a fixed orientation of the N-isopropyl and N-benzhydryl substituents with the C−H groups pointing to the nickel(II) center to maximize rare intramolecular C−H···Ni anagostic or preagostic interactions. The large downfield shift of these C−H protons in the 1H NMR spectrum compared to their precursor salts indicates that these interactions are retained in solution. Preliminary catalytic studies show that complexes 7−12 are active in the Ullmann coupling of bromobenzene and 4-bromoanisole. In particular, complexes 8, 9, and 12, with sterically more demanding ligands, exhibit the best catalytic activities. The coupling reaction was found to be successful when carried out in neat [Bu4N]Br as ionic liquid, but not in dry DMF or in DMF with [Bu4N]Br as an additive.
Co-reporter:Yuan Han and Han Vinh Huynh
Chemical Communications 2007 (Issue 10) pp:1089-1091
Publication Date(Web):05 Jan 2007
DOI:10.1039/B615441G
The first pyrazolin-4-ylidene complexes of palladium(II) have been synthesized by oxidative addition of 4-iodopyrazolium salts to Pd2(dba)3/PPh3 and were fully characterized by multinuclear NMR spectroscopies, ESI mass spectrometry and X-ray diffraction studies.
Co-reporter:Swee Kuan Yen, Lip Lin Koh, Han Vinh Huynh and T. S. Andy Hor
Dalton Transactions 2007 (Issue 35) pp:3952-3958
Publication Date(Web):27 Jul 2007
DOI:10.1039/B706968E
3-(2-Propenyl)benzothiazolium bromide (A) provides a direct and simple entry to Pd(II) complexes with N,S-heterocyclic carbene (NSHC) ligands functionalized with an allyl pendant with hemilabile potential. Addition of salt A to Pd(OAc)2 eliminates HOAc and affords the bis(carbene) complexes cis-[PdBr2(NHSC)2] (cis-1, NSHC = 3-(2-propenyl)benzothiazolin-2-ylidene) and trans-[PdBr2(NHSC)2] (trans-1) along with the monocarbene complexes [PdBr2(NSHC)] (2) and trans-[PdBr2(benzothiazole-κN)(NSHC)] (3) as minor side products. Salt-metathesis of cis-1 with AgO2CCF3 yields the mixed dicarboxylato-bis(carbene) complex cis-[Pd(O2CCF3)2(NSHC)2] (4). Complexes cis-1, trans-1 and 4 were characterized by multinuclear NMR spectroscopies, ESI mass spectrometry and elemental analysis. The molecular structures of complexes cis-1, 2 and 3 have been determined by X-ray single crystal diffraction. Complexes cis-1 and 4 as well as an in situ mixture of Pd(OAc)2 and salt A are active toward Suzuki–Miyaura coupling of aryl bromides and activated aryl chlorides giving good conversions.
Co-reporter:Yuan Han, Han Vinh Huynh and Geok Kheng Tan
Organometallics 2007 Volume 26(Issue 26) pp:6581-6585
Publication Date(Web):November 15, 2007
DOI:10.1021/om7009107
Two monocationic complexes of pyrazole-derived remote carbene ligands of the type trans-[PdI(rNHC)(PPh3)2]+OTf– {rNHC = 2-ethyl-3,5-dimethyl-1-phenylpyrazolin-4-ylidene (6a), 1-ethyl-2,3,5-trimethylpyrazolin-4-ylidene (6b)} were prepared by oxidative addition of 4-iodo-1,2,3,5-tetrasubstituted pyrazolium triflate salts to [Pd2(dba)3]/PPh3 in good yields. Both compounds were fully characterized by multinuclei NMR spectroscopies, electrospray ionization mass spectrometry, and X-ray diffraction analysis. A comparative study on the aqueous Suzuki−Miyaura catalytic activities of these cationic complexes with their previously reported neutral counterparts reveals the superiority of the former.
Co-reporter:Han Vinh Huynh, Ramasamy Jothibasu and Lip Lin Koh
Organometallics 2007 Volume 26(Issue 27) pp:6852-6856
Publication Date(Web):November 22, 2007
DOI:10.1021/om700867e
The reaction of Pd(OAc)2 with 1,3-dibenzylbenzimidazolium bromide (A) and 1-propyl-3-methylbenzimidazolium iodide (B) afforded the dihalo-bis(carbene) complexes cis-[PdBr2(Bz2-bimy)2] (1) and cis-[PdI2(Pr,Me-bimy)2] (2), respectively. Halide substitution of 1 and 2 with AgO2CCH3 gave the mixed diacetato-bis(carbene) complexes cis-[Pd(O2CCH3)2(Bz2-bimy)2] (3) and cis-[Pd(O2CCH3)2(Pr,Me-bimy)2] (4). In situ deprotonation of isopropylthiol with the mixed carbene-carboxylato complexes 3 and 4 yielded the novel dipalladium complexes [Pd2(μ-iPr-S)2(Bz2-bimy)4](BF4)2 (5) and [Pd2(μ-iPr-S)2(Pr,Me-bimy)4](BF4)2 (6) with a [Pd2S2] core solely supported by N-heterocyclic carbenes. All compounds have been fully characterized by multinuclei NMR spectroscopies and ESI mass spectrometry. The solid state molecular structures of complexes 2, 3, 5, and 6 have also been confirmed by X-ray diffraction studies.
Co-reporter:Han Vinh Huynh, Chun Hui Yeo and Geok Kheng Tan
Chemical Communications 2006 (Issue 36) pp:3833-3835
Publication Date(Web):03 Aug 2006
DOI:10.1039/B608325K
The truely hemilabile nature of a novel thioether-functionalized N-heterocyclic carbene ligand is demonstrated in a range of Pd(II) complexes.
Co-reporter:Han Vinh Huynh, Joanne Hui Hui Ho, Tiong Cheng Neo, Lip Lin Koh
Journal of Organometallic Chemistry 2005 Volume 690(Issue 16) pp:3854-3860
Publication Date(Web):15 August 2005
DOI:10.1016/j.jorganchem.2005.04.053
Reaction of N,N′-dimethylbenzimidazolyl iodide (A) with Pd(OAc)2 in DMSO gives selectively trans-bis(N,N′-dimethylbenzimidazoline-2-ylidene) palladium(II) diiodide (trans-2) in 77% yield. The selective formation of the trans-coordination isomer and thus the cis–trans rearrangement is driven by the insolubility of trans-2 in DMSO. X-ray single-crystal diffraction analysis and 13C NMR spectroscopy confirm the trans-geometry of the square planar Pd(II) complex. Catalytic studies show that cis-1 and trans-2 are highly efficient in the Mizoroki–Heck coupling reaction of aryl bromides and activated aryl chlorides both in DMF and [N(n-C4H9)4]Br as ionic liquid. The catalytic activities of Pd(II) complexes with N-heterocyclic carbene ligands derived from benzimidazole are comparable to their imidazole-derived analogues.A trans-configured benzimidazole-2-ylidene complex of palladium(II) (trans-2) was selectively obtained from the reaction of precursor A with Pd(OAc)2 in DMSO and structurally characterized. The novel complex and its cis-isomer show high catalytic activities in the Mizoroki–Heck reaction comparable to imidazole-derived carbene complexes.
Co-reporter:Han Vinh Huynh, Duc Le Van, F. Ekkehardt Hahn, T.S.Andy Hor
Journal of Organometallic Chemistry 2004 Volume 689(Issue 10) pp:1766-1770
Publication Date(Web):15 May 2004
DOI:10.1016/j.jorganchem.2004.02.033
Mixed carbene-carboxylate complexes of Palladium(II) have been prepared by reacting {1,1′-dimethyl-3,3′-methylenediimidazoline-2,2′-diylidene} palladium(II) diiodide (1) [Angew. Chem. 107 (1995) 2602; Angew. Chem. Int. Ed. Engl. 34 (1995) 2371; J. Organomet. Chem. 557 (1998) 93] with AgO2CR, where R=CF3, CF2CF3 and CF2CF2CF3. In this manner, {1,1′-dimethyl-3,3′-methylenediimidazoline-2,2′-diylidene} palladium(II) bis(trifluo-roacetate) (2), {1,1′-dimethyl-3,3′-methylenediimidazoline-2,2′-diylidene} palladium(II) bis(pentafluoropropionate) (3) and {1,1′-dimethyl-3,3′-methylenediimidazoline-2,2′-diylidene} palladium(II) bis(heptafluorobutyrate) (4) were obtained. All three complexes were fully characterized by 1H-, 13C- and 19F NMR spectroscopy as well as ESI mass spectrometry. X-ray crystal structure analyses of complexes 3 and 4 reveal mononuclear species with a square planar metal center coordinated by a cis-chelating dicarbene and two monodentate carboxylate ligands. The results show that the introduction of a cis-chelating N,N-heterocyclic carbene ligand stabilizes the palladium-carboxylate moiety effectively.Mixed carbene-carboxylate complexes of palladium were obtained from the reaction of the dicarbene dihalide precursor 1 with AgO2CR, where R=CF3, CF2CF3 and CF2CF2CF3. In all three novel complexes (2–4), the palladium center is coordinated by a cis-chelating dicarbene and two monodentate fluorocarboxylate ligands in a square planar fashion.
Co-reporter:Han Vinh Huynh, Dan Yuan and Yuan Han
Dalton Transactions 2009(Issue 35) pp:NaN7268-7268
Publication Date(Web):2009/07/31
DOI:10.1039/B907887H
Three new dibenzimidazolium salts bearing thioether (B·2HBr, B·2HNO3) and sulfoxide (C·2HBr) containing bridges have been synthesized as sulfur-functionalized dicarbene precursors. Palladation of B·2HBr and C·2HBr afforded two new pseudo-pincer complexes cis-[PdBr2(B-κ2C)] (1) and cis-[PdBr2(C-κ2C)] (2), in which the sulfur-donor remains pendant. Reaction of precursor B·2HNO3 in the presence of 1 equiv of KBr, on the other hand, yields the first CSC-Pd(II) pincer complex [PdBr(B-κ3CSC)]NO3 (3) bearing two carbene moieties. All three complexes have been fully characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis. Their catalytic activities in the Mizoroki–Heck reaction have been evaluated as well.
Co-reporter:Yuan Han and Han Vinh Huynh
Dalton Transactions 2009(Issue 12) pp:NaN2209-2209
Publication Date(Web):2009/02/05
DOI:10.1039/B816471A
Reaction of the bromo-bridged dimeric monocarbene complex [PdBr2(iPr2-bimy)]2 (1) (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) with various isocyanides afforded a series of mixed carbene–isocyanide complexes [PdBr2(iPr2-bimy)(CN-R)] (2a: R = Cy; 2b: R = nBu; 2c: R = Xyl) as inseparable mixtures of trans- and cis-isomers. The reactivity of 2c towards nucleophiles was studied. A new mixed NHC–ADC Pd(II) complex (4) was obtained in low yield when 2c was reacted with 2,6-dimethylaniline. Reaction of 2c with hydrazine yielded a hydrazine-bridged complex (6) via ligand substitution. In addition, salt metathesis of 2c with AgO2C2F3 afforded a cis arranged complex [Pd(O2CCF3)2(iPr2-bimy)(CN-Xyl)] (cis-7). Most complexes were fully characterized by multinuclei NMR spectroscopies, ESI or FAB mass spectrometry and X-ray diffraction analysis.
Co-reporter:Dan Yuan and Han Vinh Huynh
Dalton Transactions 2011 - vol. 40(Issue 44) pp:NaN11703-11703
Publication Date(Web):2011/08/11
DOI:10.1039/C1DT10789E
The thiolato-bridged dimeric Pd(II) NHC complex 1 has been synthesized from the reaction of thioester-functionalized imidazolium salt B and Pd(OAc)2. The isolation of its interesting constitutional isomer 2 bearing both classical C(2)-bound and mesoionic C(4)-bound ligands coordinating to two different metal centers in the same complex allowed for a direct comparison of these isomeric carbenes. Reactivity studies of 1 with NaSCH(CH3)2 and NaBF4 afforded the tetranuclear compound 3 with a [Pd4S4] macrocycle. All complexes have been fully characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis.
Co-reporter:Han Vinh Huynh, Weiheng Sim and Chee Fei Chin
Dalton Transactions 2011 - vol. 40(Issue 44) pp:NaN11692-11692
Publication Date(Web):2011/10/10
DOI:10.1039/C1DT11472G
Bridge cleavage reactions of the dimeric monocarbene complex [PdBr2(iPr2-bimy)]2 can be effectively used to end-cap pyridine containing pseudorotaxanes affording stable [2]rotaxanes.
Co-reporter:Boon Ying Tay, Cun Wang, Pim Huat Phua, Ludger Paul Stubbs and Han Vinh Huynh
Dalton Transactions 2016 - vol. 45(Issue 8) pp:NaN3563-3563
Publication Date(Web):2016/01/14
DOI:10.1039/C5DT03366G
Hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) was studied by using mono- and bidentate p-cymene ruthenium(II) N-heterocyclic carbene (NHC) complexes as catalyst precursors. In water, all complexes were found to be reduced in situ to form ruthenium nanoparticles (RuNPs) with a high hydrogenation activity. In organic solvents, complexes with monodentate NHC ligands also formed nanoparticles, while complexes with bidentate ligands gave rise to stable homogeneous catalysts with moderate hydrogenation activities.
Co-reporter:Qiaoqiao Teng and Han Vinh Huynh
Dalton Transactions 2017 - vol. 46(Issue 3) pp:NaN627-627
Publication Date(Web):2016/11/24
DOI:10.1039/C6DT04222H
The properties and reactivities of transition metal complexes are rooted in the stereoelectronic properties of their ligands. While the bulk of a ligand can be easily evaluated and compared by the drawing of its Lewis structure, prediction on the electronic contributions is often less straightforward. Thus, several electronic parameters have been developed for the experimental evaluation of ligands throughout the years. This article accounts for the most recent one developed by the Huynh group, which employs 13C NMR spectroscopy to determine ligand donor strengths using N-heterocyclic carbene complexes. This parameter not only proves to be safer, more convenient and accurate in comparison to existing methodologies, but it also provides, in certain cases, more intuitive and reliable results. Furthermore, it is currently the only one that allows the direct comparison of various Werner-type and organometallic ligands on a unified scale.
Co-reporter:Shuai Guo, Jan Christopher Bernhammer and Han Vinh Huynh
Dalton Transactions 2015 - vol. 44(Issue 34) pp:NaN15165-15165
Publication Date(Web):2015/01/20
DOI:10.1039/C4DT03201B
Ligand redistribution reactions are well documented for silver(I) N-heterocyclic carbene (NHC) complexes of the type [AgX(NHC)] (X = halido ligand), but only two reports have been described in the literature for gold analogues of the general formula [AuX(NHC)]. In both cases, the NHCs in question were exceptionally strong donors. To probe the dependence of ligand redistribution processes on NHC donor strength, a model study was conducted using a weakly donating 1,2,4-triazolin-5-ylidene (tazy) ligand and different halido coligands. For [AuX(tazy)] (X = Cl, Br, OAc, tazy = 4-benzyl-1-methyl-1,2,4-triazolin-5-ylidene), no ligand redistribution was found, while a reversible disproportionation between [AuI(tazy)] in solution and [Au(tazy)2][AuI2] in the solid state was observed and studied by means of X-ray crystallography, NMR and UV-Vis spectroscopy, as well as DFT calculations.
Co-reporter:Jan Christopher Bernhammer, Gilles Frison and Han Vinh Huynh
Dalton Transactions 2014 - vol. 43(Issue 23) pp:NaN8594-8594
Publication Date(Web):2014/04/14
DOI:10.1039/C4DT01047G
In NHC pincer complexes incorporating a hemilabile donor site, there exists an equilibrium between the true pincer form and a pseudopincer coordination isomer. The influence of the NHC moieties on this isomerism has been studied by DFT calculations.
Co-reporter:Xiaoke Xie and Han Vinh Huynh
Inorganic Chemistry Frontiers 2015 - vol. 2(Issue 12) pp:NaN1603-1603
Publication Date(Web):2015/10/12
DOI:10.1039/C5QO00292C
A series of six [RuX(2-thienyl–NHC)(p-cymene)] complexes (9a–c and 10–14; X = halido) have been prepared via NHC directed C–H activation. All complexes have been fully characterized, and the molecular structures of four complexes are reported. The rotational barrier of the N-benzyl substituent in the bromido complex 9b has been measured by variable temperature 1H NMR spectroscopy. Preliminary reactivity studies of complex 12 with alkynes provided four new annulated aza-heterocyclic thiophenes.
Co-reporter:Liqin Xue, Lijun Shi, Yuan Han, Chungu Xia, Han Vinh Huynh and Fuwei Li
Dalton Transactions 2011 - vol. 40(Issue 29) pp:NaN7638-7638
Publication Date(Web):2011/06/15
DOI:10.1039/C1DT10433K
A series of carbene complexes [PdBr2(iPr2-bimy)L] (C2–C13) with different types of co-ligands (L) have been tested for their catalytic activities in the carbonylative annulation of 2-iodophenol with phenylacetylene in DMF to afford the respective flavone 2a. Complex C12 with an N-phenylimidazole co-ligand showed the best activity and also afforded high yields when the substrate scope was extended to other aryl or pyridyl acetylenes. In addition, catalyst C12 was also efficient in the carbonylative annulation of 2-iodoaniline with acid chlorides giving the desirable 2-substituted 4H-3,1-benzoxazin-4-ones (4) in good yields. Additionally, this Pd–NHC complex also proved to be a very efficient catalyst for the hydroxycarbonylation of iodobenzene derivatives at low catalyst loading and under low CO pressure. These results demonstrate the versatility and efficiency of this phosphine-free Pd(II)–NHC complex in different types of carbonylations of aryl iodides under mild conditions.
Co-reporter:Dan Yuan, Haoyun Tang, Linfei Xiao and Han Vinh Huynh
Dalton Transactions 2011 - vol. 40(Issue 35) pp:NaN8795-8795
Publication Date(Web):2011/04/27
DOI:10.1039/C1DT10269A
Three thioether bridged diimidazolium dibromides with different steric and electronic properties have been synthesized as precursors to carbene-based CSC pincer ligands. Palladation afforded CSC Pd(II) pincer complexes for bulky and electron rich ligand systems, whereas the least donating ligand led to the formation of a pseudo-pincer complex. All complexes have been fully characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis. The catalytic activities of pincer versus pseudo-pincer complexes have been compared in the intermolecular hydroamination of alkynes with anilines as well.
Co-reporter:Jan Christopher Bernhammer and Han Vinh Huynh
Dalton Transactions 2012 - vol. 41(Issue 28) pp:NaN8608-8608
Publication Date(Web):2012/05/04
DOI:10.1039/C2DT30526G
The relative ligand donor strengths of 10 pyrazole-derived ligands has been determined with great accuracy, making use of the interdependence between the donor strength of the co-ligand and the 13C NMR chemical shift of the iPr2-bimy carbene signal in trans-[PdBr2(iPr2-bimy)L] complexes (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene; L = pyrazole-derived ligand). Even subtle variations in the substitution pattern of the pyrazole backbone up to three bonds away from the coordinating nitrogen could be detected reliably using this methodology. Alkylation experiments conducted on the pyrazoles using electrophiles of varied reactivity (ethyl bromide, ethyl iodide, and trimethyloxonium tetrafluoroborate) served as a benchmark to rank the pyrazoles in three groups of gradually increasing nucleophilicity, which correlated well with their determined donor strength.
Co-reporter:Yuan Han and Han Vinh Huynh
Chemical Communications 2007(Issue 10) pp:NaN1091-1091
Publication Date(Web):2007/01/05
DOI:10.1039/B615441G
The first pyrazolin-4-ylidene complexes of palladium(II) have been synthesized by oxidative addition of 4-iodopyrazolium salts to Pd2(dba)3/PPh3 and were fully characterized by multinuclear NMR spectroscopies, ESI mass spectrometry and X-ray diffraction studies.
Co-reporter:Swee Kuan Yen, Lip Lin Koh, Han Vinh Huynh and T. S. Andy Hor
Dalton Transactions 2008(Issue 5) pp:NaN706-706
Publication Date(Web):2007/11/29
DOI:10.1039/B713152F
Novel Pd(II) mixed N,S-heterocyclic carbene (NSHC)-phosphine complexes of the general formula [PdBr2(NSHC)(PR3)] were obtained from bridge cleavage of dinuclear NSHC complexes of type [PdBr2(NSHC)]2 [NSHC = 3-benzylbenzothiazolin-2-ylidene and 3-propylbenzothiazolin-2-ylidene] with triphenylphosphine, tricyclohexylphosphine and 2-diphenylphosphanyl-pyridine. All complexes have been fully characterized by 1H and 13C NMR spectroscopy, ESI mass spectrometry and elemental analysis. The X-ray crystal structures of complexes 3–8 are reported. The complexes exhibit moderate to good catalytic activity in the Suzuki–Miyaura coupling reaction of aryl bromides and chlorides.
Co-reporter:Han Vinh Huynh and Chen-Shiang Lee
Dalton Transactions 2013 - vol. 42(Issue 19) pp:NaN6809-6809
Publication Date(Web):2013/02/21
DOI:10.1039/C3DT50237F
Pd(II) complexes [PdBr(A-κ3CNC)]Br (1) and [PdBr(B-κ3CNC)]Br (2) with new CNC pincer-type ligands derived from 1,2,4-triazolin-5-ylidenes have been synthesized and characterized by multinuclei NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis. The more soluble complex 1 proved to be an efficient pre-catalyst in copper- and amine-free Sonogashira reaction with high turnover numbers. The potential for recycling of the catalyst was also demonstrated.
Co-reporter:Qiaoqiao Teng and Han Vinh Huynh
Chemical Communications 2015 - vol. 51(Issue 7) pp:NaN1251-1251
Publication Date(Web):2014/12/01
DOI:10.1039/C4CC08270B
Metallation of a pentadentate diNHC proligand bearing a dipropyl-pyridine-2,6-dicarboxamide with silver and gold affords mono- and dinuclear, double-stranded bis(NHC) complexes as useful building blocks for metallo-supramolecules. The digold(I) complex acts as a metallo-bis(pincer) ligand to furnish the first example of an organometallic NHC-helicate upon coordination to cobalt.
Co-reporter:Ramasamy Jothibasu and Han Vinh Huynh
Chemical Communications 2010 - vol. 46(Issue 17) pp:NaN2988-2988
Publication Date(Web):2010/03/05
DOI:10.1039/B925977E
The versatile coordination chemistry of strongly donating indazolin-3-ylidene ligands as new members of the NHC family is demonstrated on 12 new PdII, AuI and RhI complexes, which were fully characterized by multinuclear NMR spectroscopies, ESI mass spectrometry and X-ray diffraction studies.
Co-reporter:Haresh Sivaram, Jackie Tan and Han Vinh Huynh
Dalton Transactions 2013 - vol. 42(Issue 34) pp:NaN12428-12428
Publication Date(Web):2013/06/17
DOI:10.1039/C3DT51071A
A series of cationic gold(I) heteroleptic complexes bearing the pyrazole-derived N-heterocyclic carbene (NHC) FPyr (1,2,3,4,6,7,8,9-octahydropyridazino[1,2-a]indazolin-11-ylidene), and either a 1,3-disubstituted benzimidazole-derived NHC of the type RR′-bimy (3: R = R′ = CHPh2; 4: R = CHPh2, R′ = iPr; 5: R = R′ = CH2Ph; 6: R = R′ = iBu; 7: R = R′ = n-Pr; 8: R = R′ = Et; 9: R = R′ = 2-propenyl) or a non-NHC co-ligand L (10: L = PPh3; 11: L = P(OPh)3; 12: L = DMAP) (DMAP = 4-dimethylaminopyridine) have been synthesized from [AuCl(FPyr)] (1). Complexes 3–12 have been characterized using multinuclei NMR spectroscopies, ESI mass spectrometry, and elemental analysis. X-ray diffraction analyses have been performed on complexes 5, 6, and 9–11. To the best of our knowledge, 11 represents the first gold–NHC complex to bear the P(OPh)3 ligand. The cytotoxic activities of complexes 3–12 have been studied in vitro with the NCI-H1666 non-small cell lung cancer cell line.
Co-reporter:Yuan Han and Han Vinh Huynh
Dalton Transactions 2011 - vol. 40(Issue 10) pp:NaN2147-2147
Publication Date(Web):2010/12/06
DOI:10.1039/C0DT01037E
The immense success of N-heterocyclic carbenes in recent years has initiated the search for even stronger ligands leading to the discovery of abnormal and remote NHCs. This article reflects our particular interest in the coordination chemistry of pyrazolin-4-ylidenes as a contribution to this field. A modular approach to 4-iodopyrazolium salts with different substitution patterns is described, which upon oxidative addition to Pd0 gave rise to a library of new Pd(II) pyrazolin-4-ylidene complexes. A preliminary study showed that selected complexes are active precatalysts in Suzuki–Miyaura and Mizoroki–Heck coupling reactions. The 3,5-substituents of pyrazolin-4-ylidenes are found to have significant effects on complexation and catalytic activities. Finally, the nature of this new class of ligands is discussed, and a future direction for further explorations in this exciting field is envisioned.
Co-reporter:Han Vinh Huynh and Qiaoqiao Teng
Chemical Communications 2013 - vol. 49(Issue 39) pp:NaN4246-4246
Publication Date(Web):2012/07/19
DOI:10.1039/C2CC33789D
The synthesis of a bromopropyl-substituted NHC–Pd(II) complex, which can undergo exemplary and versatile 2nd and 3rd generation post-modifications easily affording 7 new functionalised NHC complexes, is demonstrated.
Co-reporter:Swee Kuan Yen, Lip Lin Koh, Han Vinh Huynh and T. S. Andy Hor
Dalton Transactions 2007(Issue 35) pp:
Publication Date(Web):
DOI:10.1039/B706968E
