Whereas the stereochemical rigidity of the coordination sphere of boxmi/Cu^II catalysts is key to achieving high enantioselectivity in the electrophilic alkylation of β-ketoesters, this pathway is outperformed by a radical process for the corresponding catalytic transformation of oxindoles, giving rise to racemic products. For the corresponding Zn^II catalysts, the selectivity in the latter process is outstanding despite the greater plasticity of the coordination shell. This reaction was thus developed into a highly useful synthetic method, which enabled the conversion of wide range of substrates with high yields and enantioselectivities.

Whereas the stereochemical rigidity of the coordination sphere of boxmi/Cu^II catalysts is key to achieving high enantioselectivity in the electrophilic alkylation of β-ketoesters, this pathway is outperformed by a radical process for the corresponding catalytic transformation of oxindoles, giving rise to racemic products. For the corresponding Zn^II catalysts, the selectivity in the latter process is outstanding despite the greater plasticity of the coordination shell. This reaction was thus developed into a highly useful synthetic method, which enabled the conversion of wide range of substrates with high yields and enantioselectivities.

Pincer-stabilized nickel(I) complexes readily react with molecular oxygen to form dinuclear 1,2-μ-peroxo-bridged nickel(II) complexes, which are the major components of a dynamic equilibrium with the corresponding mononuclear superoxo species. The peroxo complexes further react with hydrogen peroxide to give the corresponding nickel(II) hydroperoxides. One of these hitherto elusive species was characterized by X-ray diffraction for the first time [O–O bond length: 1.492(2) Å].

Pincer-stabilized nickel(I) complexes readily react with molecular oxygen to form dinuclear 1,2-μ-peroxo-bridged nickel(II) complexes, which are the major components of a dynamic equilibrium with the corresponding mononuclear superoxo species. The peroxo complexes further react with hydrogen peroxide to give the corresponding nickel(II) hydroperoxides. One of these hitherto elusive species was characterized by X-ray diffraction for the first time [O–O bond length: 1.492(2) Å].

A detailed study on the effects of core halogenation of tetraazaperopyrene (TAPP) derivatives is presented. Its impact on the solid structure, as well as the photophysical and electrochemical properties, has been probed by the means of X-ray crystallography, UV/Vis and fluorescence spectroscopy, high-resolution electron energy loss spectroscopy (HREELS), cyclic voltammetry (CV), and DFT modeling. The aim was to assess the potential of this approach as a construction principle for organic electron-conducting materials of the type studied in this work. Although halogenation leads to a stabilization of the LUMOs compared to the unsubstituted parent compound, the nature of the halide barely affects the LUMO energy while strongly influencing the HOMO energies. In terms of band-gap engineering, it was demonstrated that the HOMO–LUMO gap is decreased by substitution of the TAPP core with halides, the effect being found to be most pronounced for the iodinated derivative. The performance of the recently reported core-fluorinated and core-iodinated TAPP derivatives in organic thin-film transistors (TFTs) was investigated on both a glass substrate, as well as on a flexible plastic substrate (PEN). Field-effect mobilities of up to 0.17 cm² Vs⁻¹ and on/off current ratio of >10⁶ were established.

Invited for the cover of this issue is the group of Professor L. H. Gade at the University of Heidelberg, Germany. The cover image shows the representation of a 1,3-bis(2-pyridylimino)isoindoline (BPI) compound along with its C₂-symmetric derivatives. These ligands were first reported in the early 1950s and have found widespread application in organic, inorganic and materials chemistry.

Since the first report in the early 1950s, 1,3-bis(2-pyridylimino)isoindolines (BPIs) have found widespread applications in organic, inorganic and materials chemistry. This microreview focuses on recent progress towards chiral BPI derivatives as ligands for enantioselective catalysis as well as developments in the use of BPI complexes in materials science, focusing on luminescent and birefringent materials.

We herein report the catalytic enantioselective hydrodehalogenation based on the interplay of a chiral molecular nickel(I)/nickel(II)hydride system. Prochiral geminal dihalogenides are dehalogenated via a secondary configurationally unstable, potentially metal-stabilized radical intermediate. In a subsequent step, the liberated radical is then trapped by the nickel(II) hydrido complex, present in a large excess under the catalytic conditions, which in turn induces the enantioselectivity during the hydrogen atom transfer onto the radical intermediate. These new chiral nickel(I) complexes were found to catalyze the asymmetric hydrodehalogenation of geminal dihalogenides with moderate to good enantiomeric excess values using LiEt₃BH as reductant. The main side product generally observed is the dehalogenated alkene, whereas the hydrodehalogenation of the chiral monohalogen compound occurred much more slowly despite the large excess of reductant.

The enantioselective trifluoromethylthiolation of β-ketoesters using chiral copper–boxmi complexes as catalysts is reported. A number of α-SCF₃-substituted β-ketoesters have been obtained with up to >99 % enantiomeric excess (ee), and the trifluoromethylthiolated products were then transformed diastereoselectively to α-SCF₃-β-hydroxyesters with two adjacent quaternary stereocenters.

Organic thin-film transistors (TFTs) are prepared by vacuum deposition and solution shearing of 2,9-bis(perfluoroalkyl)-substituted tetraazaperopyrenes (TAPPs) with bromine substituents at the aromatic core. The TAPP derivatives are synthesized by reacting known unsubstituted TAPPs with bromine in fuming sulphuric acid, and their electrochemical properties are studied in detail by cyclic voltammetry and modelled with density functional theory (DFT) methods. Lowest unoccupied molecular orbital (LUMO) energies and electron affinities indicate that the core-brominated TAPPs should exhibit n-channel semiconducting properties. Current-voltage characteristics of the TFTs established electron mobilities of up to μ_n = 0.032 cm² V⁻¹ s⁻¹ for a derivative which was subsequently processed in the fabrication of a complementary ring oscillator on a flexible plastic substrate (PEN).

The coordination chemistry of the monoanionic PNP pincer ligand 3,6-di-tert-butyl-1,8-bis[(diphenylphosphanyl)methyl]carbazole (Cbzdiphos) was investigated for different iridium(I) and iridium(III) complexes. Reaction of the protio ligand with [Ir(acac)(cod)] (acac = acetylacetonate; cod = 1,5-cyclooctadienyl) gave [(Cbzdiphos)Ir(cod)] (1). Upon hydrogenation the dihydrido complex [(Cbzdiphos)IrH₂] (2) was obtained, and its reactivity in different organometallic transformations was examined. Treating 2 with ammonia afforded [(Cbzdiphos)IrH₂(NH₃)] (3), whereas treatment with diphenylacetylene (dpa) yielded the Ir^I compound [(Cbzdiphos)Ir(dpa)] (4) and trans-stilbene. Furthermore, the conversion with norbornene led to a reactive species, which activated C–H and C–Cl bonds. The reaction with THF gave a Fischer carbene [(Cbzdiphos)Ir{=C(OC₃H₆)}] (8) after double C–H bond activation, whereas treatment with benzene or chlorobenzene afforded the oxidative addition products [(Cbzdiphos)Ir(Ph)H] (5) and [(Cbzdiphos)Ir(Ph)Cl] (7).

High-spin Fe^II–alkyl complexes with bis(pyridylimino)isoindolato ligands were synthesized and their paramagnetic ¹H and ¹³C NMR spectra were analyzed comprehensively. The experimental ¹³C—¹H coupling values are temperature (T⁻¹)- as well as magnetic-field (B²)-dependent and deviate considerably from typical scalar ¹J_CH couplings constants. This deviation is attributed to residual dipolar couplings (RDCs), which arise from partial alignment of the complexes in the presence of a strong magnetic field. The analysis of the experimental RDCs allows an unambiguous assignment of all ¹³C NMR resonances and, additionally, a structural refinement of the conformation of the complexes in solution. Moreover the RDCs can be used for the analysis of the alignment tensor and hence the tensor of the anisotropy of the magnetic susceptibility.

Core substitution of tetraazaperopyrenes (TAPPs) has been achieved, and with it, considerable variation of their photo- and redox-chemical properties. Through Suzuki cross coupling starting from the fourfold core-brominated tetraazaperopyrene, aryl-substituted TAPPs were synthesized, which displayed very high fluorescence quantum yields (up to 100 %) in solution. Besides the Suzuki reactions, Stille and Sonogashira cross-couplings were also found to be suitable methods for core derivatization, as demonstrated in the syntheses of alkynyl-substituted tetraazaperopyrene congeners. Furthermore, TAPPs incorporating intramolecular donor–acceptor combinations of aromatic units (8, 9) were accessible by coupling the electron-poor peropyrene core with electron-rich aromatic units, which act as strong electron donors. Finally, C-heteroatom coupling (O, S, N) gave rise to novel TAPP derivatives with strongly modified redox-chemical behaviour and photophysics in the solid state as well in solution. In particular, TAPP derivatives displaying red fluorescence were obtained for the first time.

A series of dichlorocyclophosphazanes [{ClP(μ-NR)}₂] containing chiral and achiral R groups was obtained from simple commercially available amines and PCl₃. Their condensation reactions with axially chiral biaryl diols yielded ansa-bridged chiral cyclophosphazane (CycloP) ligands. This highly modular methodology allows extensive elaboration of the ligand set, in which the chirality can be introduced at the diol bridge and/or the amido R group. This provides the possibility to observe match and mismatch effects in catalysis. A series of twenty CycloP ligands was synthesized and characterized by multinuclear NMR spectroscopy, HRMS, elemental analysis, and in selected cases, single-crystal X-ray diffraction. These studies show that all of the ditopic CycloP ligands are C₂ symmetric, rendering their metal coordination sites symmetry equivalent. Two well-established enantioselective reactions were explored by using late-transition metal CycloP complexes as catalysts; the gold-catalyzed hydroamination of γ-allenyl sulfonamides and the asymmetric nickel-catalyzed three-component coupling of a diene and an aldehyde. The steric demands of the CycloP ligands have a subtle influence on the reactivity and selectivity observed in both reactions. Good enantiomeric ratios (e.r.) as high as 89:11 in the gold-catalyzed reaction and 92:8 in the nickel-catalyzed bis-homoallylation reaction were observed.

Reaction of [Zr{(NAr)₂N_py}(NMe₂)₂] (Ar=3,5-xylyl: 2 a, mesityl: 2 b) with one or two molar equivalents of 1,1-diphenylhydrazine gave the mixed amido/hydrazido(1−) complex [Zr{(NMes)₂N_py}(HNNPh₂)(NMe₂)] (3), the bis-hydrazido complex [Zr{(NMes)₂N_py}(HNNPh₂)₂] (4), and, in the presence of excess 4-dimethylaminopyridine (DMAP), hexacoordinate hydrazinediidozirconium complexes [Zr{(NXyl)₂N_py}(NN(Me)Ph)(dmap)₂] (5) and [Zr{(NXyl)₂N_py}(NNPh₂)(dmap)₂] (6). The reaction of one equivalent of the zirconium–hydrazinediide [Zr{(NTBS)₂N_py}(NNPh₂)(py)] (1) with disubstituted alkynes at RT for 16 h led to the formation of seven-membered diazazirconacycles 7 a–7 e in high yields. Similar reactivity was observed by reacting bis-amido complex 2 b with one molar equivalent of the corresponding alkyne and diphenylhydrazine. The formation of the seven-membered zirconacycles implied a key coupling step that involved the alkyne and one of the aryl rings of the diphenylhydrazinediido ligand. In some cases, such as the reaction with 2-butyne, the corresponding metallacycle was only obtained in modest yields (45 % for the reaction with 2-butyne) and a second major product, vinylimido complex 9, was formed in almost equal amounts (42 %) by 1,2-amination (formal insertion of the alkyne). The formation of compounds 7 a and 9 followed in part the same sequence of reaction steps and a key intermediate, an azirinido complex, represented a “bifurcation point” in the reaction network. Reaction of 1.2 equivalents of several diarylhydrazines and various substituted alkynes (1 equiv) at ambient temperature (or at 80 °C) in the presence of 10 mol % [Zr{(NXyl)₂N_py}(NMe₂)₂] (2 a) gave the corresponding indole derivatives. On the other hand, the replacement of 1,1-diarylhydrazines by 1-methyl-1-phenyl hydrazine led to head-to-head cis-1,3-enynes in good yields.

A range of 2,9-perfluoroalkyl-substituted tetraazaperopyrene (TAPP) derivatives (1–5) was synthesised by reacting 4,9-diamino-3,10-perylenequinone diimine (DPDI) with the corresponding carboxylic acid chloride or anhydride in the presence of a base. The reaction of compounds 1–4 with dichloroisocyanuric acid (DIC) in concentrated sulphuric acid resulted in the fourfold substitution of the tetraazaperopyrene core, yielding the 2,9-bisperfluoroalkyl-4,7,11,14-tetrachloro-1,3,8,10-tetraazaperopyrenes 6–9, respectively. The optical and electrochemical data demonstrate the drastic influence of the core substitution on the properties. All compounds are highly luminescent (fluorescence quantum yields of up to Φ=0.8). The LUMO energies of the tetrachlorinated TAPP derivatives (determined by cyclic voltammetry and computed by DFT calulations) were found to be below −4 eV. In the course of this work the performance of TAPP derivatives in organic thin-film transistors (TFTs) was investigated, and their n-channel characteristics with field-effect mobilities of up to 0.14 cm² V⁻¹ s⁻¹ and an on/off current ratio of >10⁶ were confirmed. Long-term stabilities of 3–4 months under ambient conditions of the devices were established. Complementary inverters and ring oscillators with n-channel TFTs based on compound 8 and p-channel TFTs based on dinaphtho-[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) were fabricated on a glass substrate.

A series of chiral mono-, di-, and trinuclear gold(I) complexes have been prepared and used as precatalysts in the asymmetric cyclohydroamination of N-protected γ-allenyl sulfonamides. The stereodirecting ligands were mono-, di-, and tridentate 2,5-diphenylphospholanes, which possessed C₁, C₂, and C₃ symmetry, respectively, thereby rendering the catalytic sites in the di- and trinuclear complexes symmetry equivalent. The C₃-symmetric trinuclear complex displayed the highest activity and enantioselectivity (up to 95 % ee), whilst its mono- and dinuclear counterparts exhibited considerably lower enantioselectivities and activities. A similar trend was observed in a series of mono-, di-, and trinuclear 2,5-dimethylphospholane gold(I) complexes. Aurophilic interactions were established from the solid-state structures of the trinuclear gold(I) complexes, thereby raising the question as to whether these secondary forces were responsible for the different catalytic behavior observed.

A new class of chiral tridentate N-donor pincer ligands, bis(oxazolinylmethylidene)isoindolines (boxmi), was synthesized in three steps starting from readily available phthalimides. Their reaction with ethyl (triphenylphosphoranylidene)acetate by means of a key-step Wittig reaction gave the ligand backbones, which were condensed with amino alcohols and then cyclized to obtain the corresponding ligands. These ligands were subsequently applied in the nickel(II)-catalyzed enantioselective fluorination of oxindoles and β-ketoesters to obtain the corresponding products with enantioselectivities of up to >99 % ee and high yields. Application of the chiral pincer ligands in the chromium-catalyzed enantioselective Nozaki–Hiyama–Kishi reaction of aldehydes gave the corresponding alcohols with an optimal enantioselectivity of 93 %.

The modular one-pot synthesis of a large family of bi- and tridentate 2,5-dimethyl- and 2,5-diphenyl-substituted phospholanes employs air-stable chiral phospholanium chloride salts and primary amines or NH₄Cl as starting materials. These were transformed into the C₂-symmetric dimethyl- and diphenylphospholane ligands, which reacted with [Rh(cod)₂]BF₄ (cod=1,5-cyclooctadiene) to yield the rhodium complexes [Rh(L)(cod)]BF₄ (L=bisphospholane ligands). The corresponding trisphospholane complexes, 11 and 12, were obtained in high yields (81 and 92 %, respectively), and fully characterised by NMR spectroscopy, mass spectrometry and elemental analysis. Whilst in the C₃-symmetric complex 11, containing the tridentate 2,5-dimethylphospholane, the ligand is bound symmetrically, different coordination behaviour was found for the diphenyl-substituted complex 12, in which the coordination of only two of the three phospholane moieties to the metal centre was observed. A DFT study at the B3PW91 level established minimum energy structures consistent with experimental findings in solution and in the solid state. The non-coordinated phospholane unit present in 12 allowed further modification of the complex through the coordination of Au^IX (X=Cl, C₆F₅ and tris(trifluoromethyl)phenyl (^FMes)) fragments to the pendant phosphane. To investigate the potential of the new ligands, the enantioselective hydrogenation of a series of prochiral olefins as benchmark substrates, using isolated Rh complexes as catalysts, was studied. The substrates included methyl esters of three dehydro-α-acetamido acids and two itaconic acid derivatives. In general good to excellent enantioselectivities (of up to >99 % ee) were observed. Ligand backbone modification by coordination of bulky AuX substituents to the free phospholane unit in complex 12 led to an outstanding enhancement of the catalyst performance and there was a clear correlation between the properties of the complex periphery and the enantioselectivity.

The structural chemistry and reactivity of 1,3,8,10-tetraazaperopyrene (TAPP) on Cu(111) under ultra-high-vacuum (UHV) conditions has been studied by a combination of experimental techniques (scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy, XPS) and DFT calculations. Depending on the deposition conditions, TAPP forms three main assemblies, which result from initial submonolayer coverages based on different intermolecular interactions: a close-packed assembly similar to a projection of the bulk structure of TAPP, in which the molecules interact mainly through van der Waals (vDW) forces and weak hydrogen bonds; a porous copper surface coordination network; and covalently linked molecular chains. The Cu substrate is of crucial importance in determining the structures of the aggregates and available reaction channels on the surface, both in the formation of the porous network for which it provides the Cu atoms for surface metal coordination and in the covalent coupling of the TAPP molecules at elevated temperature. Apart from their role in the kinetics of surface transformations, the available metal adatoms may also profoundly influence the thermodynamics of transformations by coordination to the reaction product, as shown in this work for the case of the Cu-decorated covalent poly(TAPPCu) chains.

The synthesis of the three ligands employed in this study is based on the condensation of two molar equivalents of (S)-valinol with the diester precursors pyrrole-2,5-bis(ethyl)acetate (2a), furan-2,5-bis(ethyl)acetate (2b) and thiophene-2,5-bis(ethyl)acetate (2c). This gave the corresponding bis(oxazolinylmethyl)pyrrole (iPrL_NH, 4a), bis(oxazolinylmethyl)furan (iPrL_O, 4b) and bis(oxazolinylmethyl)thiophene (iPrL_S, 4c) ligands. Stirring 4a in MeOD at ambient temperature in the presence of a catalytic amount of acetic acid (1 mol-%) led to complete hydrogen/deuterium exchange in the two bridging methylene groups of the ligand. This behaviour is explained by an acetate-mediated reversible proton transfer between the oxazoline N atom and the methylene bridge, a conjecture which was supported by a DFT study of the process. Deprotonation of iPrL_NH (4a) with tBuLi at –78 °C and subsequent stirring with NiCl₂ yielded the square planar nickel(II) complex [Ni(iPrL_N)Cl] (5). However, on stirring iPrL_NH (4a) with nickel acetate in methanol, a deep red nickel acetato complex [Ni(iso-iPrL_N)(OAc)] (6) bearing the isomerized tridentate pincer ligand was obtained. Reaction of acetato complex 6 with Me₃SiCl in dichloromethane cleanly gave the corresponding chlorido complex [Ni(iso-iPrL_N)Cl] (7), which is the isomer of compound 5. The intraligand rearrangement was explained by acetate-mediated proton transfer between the methylene bridges and the 3/4-positions of the pyrrole ring, the computed thermodynamic driving force being ΔG = –9.8 kcal mol^–1. Neither thiophene derivative 4c nor furan-derived ligand 4b gave robust, isolable complexes with nickel(II). However, reaction of iPrL_O (4b) with [CrCl₃(thf)₃] in thf yielded the yellow-green complex [CrCl₃(iPrL_O)] (8), whereas no complexation occurred with the analogous thiophene-derived bisoxazoline 4c. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The C₃-symmetric trisoxazoline-supported scandium complex [Sc(iPr-trisox)(CH₂SiMe₃)₃] (2) is highly active in the stereospecific polymerization of propene, 1-butene, 1-pentene, 1-hexene and 1-heptene, when activated with two equivalents of [Ph₃C][B(C₆F₅)₄]. The polymers thus produced were found to possess narrow molecular weight distributions and high levels of tacticity control (up to 99 % mmmm). Some insight into the nature of the active species was obtained by ¹H, ¹³C and ²⁹Si NMR experiments. In particular, the formation of two equivalents of Ph₃CCH₂SiMe₃ at ambient temperature was observed alongside a C₃-symmetric scandium complex tentatively assigned as the dication [Sc(iPr-trisox)(CH₂SiMe₃)]²⁺.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

A series of bidentate oxazoline-NHC ligands has been synthesized in which the two heterocycles are connected by a CH₂ linker. The corresponding rhodium(I) complexes were prepared by direct deprotonation of the imidazolium halide salts followed by the addition of a solution of [Rh(nbd)Cl]₂ at low temperature. The cationic square planar rhodium complexes were generated by halide abstraction via addition of an excess of KPF₆ in a CH₂Cl₂/water solvent system. Alternatively, the deprotonation of the imidazolium hexafluorophosphates and reaction with [Rh(nbd)Cl]₂ directly gave the complex cations. These as well as oxazoline-NHC systems, in which the two heterocycles are directly connected or through a CMe₂ bridge, were investigated in the rhodium-catalyzed hydrosilylation of acetophenone. The comparison of the three ligand families showed that the catalysts obtained by direct coupling of oxazolines and N-heterocyclic carbenes, generating highly rigid chelate ligands, remain the most efficient systems giving the secondary alcohols in high enantioselectivity.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

A detailed density functional theory (DFT) computational study (using the BP86/SV(P) and B3LYP/TZVP//BP86/SV(P) level of theory) of the rhodium-catalyzed hydrosilylation of ketones has shown three mechanistic pathways to be viable. They all involve the generation of a cationic complex [L_nRh^I]⁺ stabilized by the coordination of two ketone molecules and the subsequent oxidative addition of the silane, which results in the Rh–silyl intermediates [L_nRh^III(H)SiHMe₂]⁺. However, they differ in the following reaction steps: in two of them, insertion of the ketone into the RhSi bond occurs, as previously proposed by Ojima et al., or into the SiH bond, as proposed by Chan et al. for dihydrosilanes. The latter in particular is characterized by a very high activation barrier associated with the insertion of the ketone into the SiH bond, thereby making a new, third mechanistic pathway that involves the formation of a silylene intermediate more likely. This “silylene mechanism” was found to have the lowest activation barrier for the rate-determining step, the migration of a rhodium-bonded hydride to the ketone that is coordinated to the silylene ligand. This explains the previously reported rate enhancement for R₂SiH₂ compared to R₃SiH as well as the inverse kinetic isotope effect (KIE) observed experimentally for the overall catalytic cycle because deuterium prefers to be located in the stronger bond, that is, CD versus MD.

Two series of peripherally functionalised dendrimers were synthesised based on poly(ethyleneimine) and poly(amidoamine) (PAMAM) dendrimers by reaction of (R)-6-N-[(γ-carboxyl)butanoyl]-aminomethyl-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl (“Glutaroyl-Aminap”) with first to fifth generation PPI and zeroth to fourth generation PAMAM dendrimers using ethyl-N,N-dimethylcarbodiimide (EDC)/1-hydroxybenzotriazole as coupling reagents. All dendritic ligands were characterised by NMR spectroscopy, elemental analysis and, for generations G0–G3, MALDI-TOF mass spectrometry. The relationship between the size/generation of the dendritic ligands and their catalytic properties was established in the asymmetric hydrosilylation of acetophenone. It could be shown that (R)-6-aminomethyl-2,2′-bis(diphenylphosphino)-1,1′-binaphthyl (“H-Aminap”) can be attached to the dendrimers without any significant loss of catalytic selectivity compared to the monomeric ligand Benzoyl-Aminap. The selectivities observed in these reactions were higher than those obtained using unfunctionalised BINAP as ligand.

Reaction of the triamidostannates(II) MeSi{SiMe₂N(p-tol)}₃SnLi(OEt₂) (1a) and MeSi{SiMe₂N(3,5-xyl)}₃SnLi(OEt₂) (1b) with 1/2 molar equiv. of [RhCl(diolefin)]₂ (diolefin = COD, NBD) and phosphanes, phosphites or isonitriles (“L”) gave the square-planar complexes [MeSi{SiMe₂NAryl}₃SnRh(L)(diolefin)] (Aryl = 3,5-xyl, p-tol) in which thestannates are directly bonded to rhodium through Rh–Sn bonds. In contrast, the analogous transformation of 1b with 1/2 equiv. of [RhCl(C₂H₄)₂]₂ and PiPr₃ in toluene did not lead to a square-planar bisethylene complex [MeSi{SiMe₂N(3,5-xyl)}₃SnRh(C₂H₄)₂(PiPr₃)] but the brown 18e π-arene complex [MeSi{SiMe₂N(3,5-xyl)}₃SnRh(PiPr₃)(η⁶-toluene)] (6). Equilibria of the square planar diolefin complexes with π-arene systems such as 6 were observed upon their dissolution in toluene.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

A new type of mediator for cobalt(II)-mediated radical polymerization is reported which is based on 1,3-bis(2-pyridylimino)isoindolate (bpi) as ancillary ligand. The modular synthesis of the bis(pyridylimino)isoindoles (bpiH) employed in this work is based on the condensation of 2-aminopyridines with phthalodinitriles. Reaction of the bpiH protio-ligands with a twofold excess of cobalt(II) acetate or cobalt(II) acetylacetonate in methanol gave [Co(bpi)(OAc)], which crystallize as coordination polymers, and a series of [Co(acac)(bpi)(MeOH)], which are mononuclear octahedral complexes. Upon heating the [Co(acac)(bpi)(MeOH)] compounds to 100 °C under high vacuum, the coordinated methanol was removed to give the five-coordinate complexes [Co(acac)(bpi)]. The polymerization of methyl acrylate at 60 °C was investigated by using one molar equivalent of the relatively short-lived radical source 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70) as initiator (monomer/catalyst/V-70: 600:1:1). The low solubility of the acetato complexes inhibits their significant activity as mediators in this reaction, whereas the acetylacetonate complexes control the radical polymerization of methyl acrylate more effectively. The radical polymerizations of the hexacoordinate complexes did not show a linear increase in number-average molecular weight (M_n) with conversion; however, the polydispersities were relatively low (PDI=1.12–1.40). By using the pentacoordinate complexes [Co(acac)(bpi)] as mediators, a linear increase in M_n values with conversion, which were very close to the theoretical values for living systems, and very low polydispersities (PDI<1.13) were obtained. This was also achieved in the block copolymerization of methyl acrylate and n-butyl acrylate. The intermediates with the growing acrylate polymer radical (^.PA) were identified by liquid injection field desorption/ionization mass spectrometry as following the general formula [Co(acac)(4-methoxy-bpi)-(MA)_n-R] (MA: methyl acrylate; R: C(CH₃)(CH₂C(CH₃)₂OCH₃)CN), a notion also confirmed by NMR end-group analysis.

Reaction of the dichloro complexes [M(N₂^TBSN_py)Cl₂] (M=Zr: 1, Hf: 2; TBS: tBuMe₂Si; py: pyridine) with one molar equivalent of LiNHNPh₂ gave mixtures of the two diastereomeric chlorohydrazido(1−) complexes [M(N₂^TBSN_py)(NHNPh₂)Cl] (M=Zr: 3 a,b, Hf: 4 a,b) in which the diphenylhydrazido(1−) ligand adopts a bent κ¹ coordination. This mixture of isomers could be cleanly converted into the deep green diphenylhydrazido(2−) complexes [Zr(N₂^TBSN_py)(NNPh₂)(py)] (5) and [Hf(N₂^TBSN_py)(NNPh₂)(py)] (6), respectively, by dehydrohalogenation with lithium hexamethyldisilazide (LiHMDS) in the presence of one molar equivalent of pyridine. Both complexes contain a linearly coordinated hydrazinediide for which a DFT-based frontier orbital analysis established bonding through one σ and two π orbitals. A high polarity of the MN bond was found, in accordance with the description of hydrazinediide(2−) acting as a six-electron donor ligand. The pyridine ligand in [M(N₂^TBSN_py)(NNPh₂)(py)] (M=Zr: 5, Hf: 6) is substitutionally labile as established by line-shape analysis of the dynamic spectra (ΔG^≠=19 kcal mol⁻¹). A change in denticity of the hydrazido unit from κ¹ to κ² was studied by DFT methods. Both forms are calculated to be very close in energy and are only separated by shallow activation barriers, which supports the notion of a rapid κ¹ to κ² interconversion. This process is believed to happen early on in the NN scission in the presence of coupling reagents. Frontier orbital and natural population analyses suggest that a primarily charge-controlled nucleophilic attack at N_α is unlikely whereas interaction with an electrophile could play an important role. This hypothesis was tested by the reaction of 5 and 6 with one molar equivalent of B(C₆F₅)₃ to give [Zr(N₂^TBSN_py)(NNPh₂){B(C₆F₅)₃}] (7) and [Hf(N₂^TBSN_py)(NNPh₂){B(C₆F₅)₃}] (8). In these products, B(C₆F₅)₃ becomes attached to the N_α atom of the side-on bound hydrazinediide and there is an additional interaction of an ortho-F atom of a C₆F₅ ring with the metal centre.

Rotational molecular symmetry, modularity and other aspects of ligand design have played a role in the development of a new class of stereodirecting ligands. The use of highly symmetrical, stereodirecting ligands may reduce the number of transition states and diastereomeric reaction intermediates and, in favourable cases, this degeneration of alternative reaction pathways may lead to high stereoselectivity in catalytic reactions and greatly simplifies the analysis of such transformations. In this concept article, we describe the way in which these considerations have played a role in the development of a new class of stereodirecting ligands. Tris(oxazolinyl)ethanes (“trisox”) have proved to be versatile ligand systems for the development of enantioselective catalysts of the d- and f-block metals employed in a wide range of catalytic conversions. These include Lewis acid catalysed transesterifications, CC and CN coupling reactions, the catalytic polymerisation of α-olefins as well as Pd-catalysed allylic alkylations. An overview of the current state of this field is given and the potential for further development will be highlighted.