•Transition metal complexes containing the bulky SnBut3 group have been synthesized.•The pendant groups on tin control both the geometry and reactivity.•Electronically unsaturated metal complexes containing SnBut3 can also be prepared.•The new complexes are able to function as viable homogeneous catalysts.Transition metal complexes containing the sterically encumbered SnBut3 group are the focus of this brief review. The ability of tin compounds to modify both heterogeneous and homogeneous catalysts has been known for some time. Our recent efforts center on utilizing the reagent But3SnH to investigate the influence of this ligand, which combines a steric profile similar to that of PBut3, with functional reactivity at the Sn-H bond. These properties of the SnBut3 ligand allow preparation of new complexes and comparison of their structures and reactivities with less encumbered trialkyl stannanes. This has allowed fine-tuning of the strained molecular geometry at the coordinatively unsaturated site and study of how that influences small molecule activation. Reversible H2 binding and activation, H2–D2 scrambling to form HD, C-H activation of bound ligands and solvents, catalytic hydrogenation, and catalytic hydrostannylation have all been observed for select complexes. In addition, a range of metal cluster complexes incorporating stannane moieties have been prepared and structurally characterized. The But3SnH ligand provides a new dimension for preparation of new molecular architectures with potential applications in homogeneous and heterogeneous catalysis.Download high-res image (132KB)Download full-size image

•Selective hydrostannylation catalysis by a platinum complex.•Rare structure of an Intermediate complex.•Catalytically active platinum complex containing two different sterically demanding ligands.The complex Pt(IPr)(SnBut3)(H), 1 [IPr=N-heterocylic carbene ligand N,N’-bis-(2,6-(diisopropyl)phenyl)imidazol-2-ylidene] reacts with trimethylsilylacetylene to yield the C-H activation product Pt(IPr)(SnBut3)(κ1-C≡CSiMe3)(H)2, 2. Complex 2 isomerizes to form the unique complex Pt(IPr)[η2-E-(SnBut3)(H)C=C(SiMe3)(H)], 3. This demonstrates that in addition to functioning as a bulky ligand the SnBut3 group can also be actively involved in catalysis. Compound 1 catalyzes the reaction of trimethylsilylacetylene and tri-t-butylstannane at room temperature to afford the selective hydrostannylated product E-(SnBut3)(H)C=C(SiMe3)(H), 4. Structures of all compounds have been determined by single crystal x-ray diffraction analyses.The complex Pt(IPr)(SnBut3)(H) is an active and selective hydrostannylation catalyst for the production of E-(SnBut3)(H)CC(SiMe3)(H). A unique intermediate complex Pt(IPr)[η2-E-(SnBut3)(H)CC(SiMe3)(H)], was isolated and characterized crystallographically.Download high-res image (116KB)Download full-size image

The complex Pt(IPr)(SnBut3)(H) (1) was obtained from the reaction of Pt(COD)2 with But3SnH and IPr [IPr = N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]. Complex 1 undergoes exchange reactions with deuterated solvents (C6D6, toluene-d8, and CD2Cl2), where the hydride ligand and the methyl hydrogen atoms on the isopropyl group of the IPr ligand have been replaced by deuterium atoms. Complex 1 reacts with H2 gas reversibly at room temperature to yield the complex Pt(IPr)(SnBut3)(H)3 (2). Complex 2 also undergoes exchange reactions with deuterated solvents as in 1 to deuterate the hydride ligands and the methyl hydrogen atoms on the isopropyl group of the IPr ligand. Complex 1 catalyzes the hydrogenation of styrene to ethylbenzene at room temperature. The reaction of 1 with 1 equiv of styrene at −20 °C yields the η2-coordinated product Pt(IPr)(SnBut3)(η2-CH2CHPh)(H) (3), and with 2 equiv of styrene, it forms Pt(IPr)(η2-CH2CHPh)2 (4).

The reaction of Pt(COD)2 with one equivalent of tri-tert-butylstannane, But3SnH, at room temperature yields Pt(SnBut3)(COD)(H)(3) in quantitative yield. In the presence of excess But3SnH, the reaction goes further, yielding the dinuclear bridging stannylene complex [Pt(SnBut3)(μ-SnBut2)(H)2]2 (4). The dinuclear complex 4 reacts rapidly and reversibly with CO to furnish [Pt(SnBut3)(μ-SnBut2)(CO)(H)2]2 (5). Complex 3 reacts with N,N′-di-tert-butylimidazol-2-ylidene, IBut, at room temperature to give the dinuclear bridging hydride complex [Pt(SnBut3)(IBut)(μ-H)]2 (6). Complex 6 reacts with CO, C2H4, and H2 to give the corresponding mononuclear Pt complexes Pt(SnBut3)(IBut)(CO)(H)(7), Pt(SnBut3)(IBut)(C2H4)(H)(8), and Pt(SnBut3)(IBut)(H)3 (9), respectively. The reaction of IBut with the complex Pt(SnBut3)2(CO)2 (10) yielded an abnormal Pt-carbene complex Pt(SnBut3)2(aIBut)(CO) (11). DFT computational studies of the dimeric complexes [Pt(SnR3)(NHC)(μ-H)]2, the potentially more reactive monomeric complexes Pt(SnR3)(NHC)(H) and the trihydride species Pt(SnBut3)(IBut)(H)3 have been performed, for NHC = IMe and R = Me and for NHC = IBut and R = But. The structures of complexes 3–8 and 11 have been determined by X-ray crystallography and are reported.

The kinetics of the reaction of Ph3SnH with excess •Cr(CO)3C5Me5 = •Cr, producing HCr and Ph3Sn–Cr, was studied in toluene solution under 2–3 atm CO pressure in the temperature range of 17–43.5 °C. It was found to obey the rate equation d[Ph3Sn–Cr]/dt = k[Ph3SnH][•Cr] and exhibit a normal kinetic isotope effect (kH/kD = 1.12 ± 0.04). Variable-temperature studies yielded ΔH‡ = 15.7 ± 1.5 kcal/mol and ΔS‡ = −11 ± 5 cal/(mol·K) for the reaction. These data are interpreted in terms of a two-step mechanism involving a thermodynamically uphill hydrogen atom transfer (HAT) producing Ph3Sn• and HCr, followed by rapid trapping of Ph3Sn• by excess •Cr to produce Ph3Sn–Cr. Assuming an overbarrier of 2 ± 1 kcal/mol in the HAT step leads to a derived value of 76.0 ± 3.0 kcal/mol for the Ph3Sn–H bond dissociation enthalpy (BDE) in toluene solution. The reaction enthalpy of Ph3SnH with excess •Cr was measured by reaction calorimetry in toluene solution, and a value of the Sn–Cr BDE in Ph3Sn-Cr of 50.4 ± 3.5 kcal/mol was derived. Qualitative studies of the reactions of other R3SnH compounds with •Cr are described for R = nBu, tBu, and Cy. The dehydrogenation reaction of 2Ph3SnH → H2 + Ph3SnSnPh3 was found to be rapid and quantitative in the presence of catalytic amounts of the complex Pd(IPr)(P(p-tolyl)3). The thermochemistry of this process was also studied in toluene solution using varying amounts of the Pd(0) catalyst. The value of ΔH = −15.8 ± 2.2 kcal/mol yields a value of the Sn–Sn BDE in Ph3SnSnPh3 of 63.8 ± 3.7 kcal/mol. Computational studies of the Sn–H, Sn–Sn, and Sn–Cr BDEs are in good agreement with experimental data and provide additional insight into factors controlling reactivity in these systems. The structures of Ph3Sn–Cr and Cy3Sn–Cr were determined by X-ray crystallography and are reported. Mechanistic aspects of oxidative addition reactions in this system are discussed.

With three examples, we have established that cis-cinnamic acids can dimerize via a diradical intermediate in the crystalline state provided that the intermolecular distance is less than 4.2 Å. In the excited state of cis-isomers, C–C bond formation with an adjacent molecule competes with geometric isomerization.

The reaction of Pt(IPr)(SnBu3t)(H), 1 [IPr = N-heterocylic carbene ligand N,N ′-bis-(2,6-(diisopropyl) phenyl)imidazol-2-ylidene] with Ru5(μ5-C)(CO)15, 2, in 1.2:1 (and 2.2:1) ratio in benzene solvent at refluxing temperature afforded the octahedral monoplatinum–pentaruthenium cluster complexes PtRu5(IPr)(CO)15(μ6-C), 3 in 54 % (10 %) yield, PtRu5(IPr)(CO)14(H)2(μ6-C), 4 in 6 % (10 %) yield and the diplatinum–pentaruthenium cluster complex Pt2Ru5(IPr)2(CO)15(μ6-C), 5 in 2 % (36 %)yield. Complex 3 readily reacts with H2 at room temperature to give complex 4. Compound 5 exhibits dynamical activity in solution where the two Pt(IPr) groups are exchanging rapidly. All three compounds were structurally characterized by single-crystal X-ray diffraction analyses.

Cis-trans isomerization is one of the most common and well-investigated photoreaction of olefins in solution. This occurs via 180° rotation of one of the substituents on CC bond requiring large space around the double bond. One would expect that in crystals the neighboring molecules would prohibit such a process. In spite of this in early 1960s Schmidt and co-workers reported occurrence of such a process in crystals via 2 + 2 ‘meta cyclobutane’ formation. In this study by examining the photochemistry of four cis-stilbazolium salts we show that the photoconversion to the corresponding trans isomers does not occur via 2 + 2 addition. Large separation that ensures considerable space between neighboring olefins favors cis-trans isomerization in crystals. An aspect that is puzzling is that the product trans isomer phase separates and recrystallizes within the parent cis crystals and photodimerizes to the cyclobutane dimer. Details of this phenomenon are to be understood.•Cis to trans isomerization of stilbazolium salts occurs in crystalline state.•Metastable intermediate is not involved in the photoisomerization process.•Free space around the rotating group is essential.•The final product cyclobutane is not involved in the isomerization process.

The complex Pt(SnBut3)2(CNBut)2(H)2, 1, was obtained from the reaction of Pt(COD)2 and But3SnH, followed by addition of CNBut. The two hydride ligands in 1 can be eliminated, both in solution and in the solid state, to yield Pt(SnBut3)2(CNBut)2, 2. Addition of hydrogen to 2 at room temperature in solution and in the solid state regenerates 1. Complex 2 catalyzes H2−D2 exchange in solution to give HD. The proposed mechanism of exchange involves reductive elimination of But3SnH from 1 to afford vacant sites on the Pt center, thus facilitating the exchange process. This is supported by isolation and characterization of Pt(SnMes3)(SnBut3)(CNBut)2, 3, when the addition of H2 to 2 was carried out in the presence of free ligand Mes3SnH (Mes = 2,4,6-Me3C6H2). Complex Pt(SnMes3)2(CNBut)2, 5, can be prepared from the reaction of Pt(COD)2 with Mes3SnH and CNBut. The exchange reaction of 2 with Ph3SnH gave Pt(SnPh3)3(CNBut)2(H), 6, wherein both SnBut3 ligands are replaced by SnPh3. Complex 6 decomposes in air to form square planar Pt(SnPh3)2(CNBut)2, 7. The complex Pt(SnPri3)2(CNBut)2, 8, was also prepared. Out of the four analogous complexes Pt(SnR3)2(CNBut)2 (R = But, Mes, Ph, or Pri), only the But analogue does both H2 activation and H2−D2 exchange. This is due to steric effects imparted by the bulky But groups that distort the geometry of the complex considerably from planarity. The reaction of Pt(COD)2 with But3SnH and CO gas afforded trans-Pt(SnBut3)2(CO)2, 9. Compound 9 can be converted to 2 by replacement of the CO ligands with CNBut via the intermediate Pt(SnBut3)2(CNBut)2(CO), 10.

The reaction of Ru3(CO)12 with Pt(IMes)2 in benzene solvent at room temperature afforded the monoplatinum–triruthenium cluster complex Ru3Pt(IMes)2(CO)11, 1, in 21% yield and the trigonal bipyramidal cluster complex Ru3Pt2(IMes)2(CO)12, 2, in 26% yield. The reaction of Ru(CO)5 with Pt(IMes)2 in benzene solvent at 0 °C yielded two trinuclear cluster complexes, the monoplatinum–diruthenium Ru2Pt(IMes)(CO)9, 3, and the monoruthenium–diplatinum cluster complex RuPt2(IMes)2(CO)6, 4. The reaction of 2 with hydrogen at 80 °C afforded the tetrahydrido–tetraruthenium complex Ru4(IMes)(CO)11(μ-H)4, 5, and the dihydrido–diruthenium–diplatinum complex Ru2Pt2(IMes)2(CO)8(μ-H)2, 6. All six compounds were structurally characterized by single-crystal X-ray diffraction analyses.

The enthalpy of oxygen atom transfer (OAT) to V[(Me3SiNCH2CH2)3N], 1, forming OV[(Me3SiNCH2CH2)3N], 1–O, and the enthalpies of sulfur atom transfer (SAT) to 1 and V(N[t-Bu]Ar)3, 2 (Ar = 3,5-C6H3Me2), forming the corresponding sulfides SV[(Me3SiNCH2CH2)3N], 1–S, and SV(N[t-Bu]Ar)3, 2–S, have been measured by solution calorimetry in toluene solution using dbabhNO (dbabhNO = 7-nitroso-2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene) and Ph3SbS as chalcogen atom transfer reagents. The V–O BDE in 1–O is 6.3 ± 3.2 kcal·mol–1 lower than the previously reported value for 2–O and the V–S BDE in 1–S is 3.3 ± 3.1 kcal·mol–1 lower than that in 2–S. These differences are attributed primarily to a weakening of the V–Naxial bond present in complexes of 1 upon oxidation. The rate of reaction of 1 with dbabhNO has been studied by low temperature stopped-flow kinetics. Rate constants for OAT are over 20 times greater than those reported for 2. Adamantyl isonitrile (AdNC) binds rapidly and quantitatively to both 1 and 2 forming high spin adducts of V(III). The enthalpies of ligand addition to 1 and 2 in toluene solution are −19.9 ± 0.6 and −17.1 ± 0.7 kcal·mol–1, respectively. The more exothermic ligand addition to 1 as compared to 2 is opposite to what was observed for OAT and SAT. This is attributed to less weakening of the V–Naxial bond in ligand binding as opposed to chalcogen atom transfer and is in keeping with structural data and computations. The structures of 1, 1–O, 1–S, 1–CNAd, and 2–CNAd have been determined by X-ray crystallography and are reported.

•Six new bimetallic ruthenium–germanium carbonyl cluster complexes are reported.•Compound 1, shows the addition of GetBu groups to the intact starting Ru3 cluster.•Compound 2 provides evidence for cluster fragmentation in the reaction system.•Higher nuclearity ruthenium–germanium complexes 3–6 with varying Ru–Ge ratios are obtained as a result of cluster condensation.In heptane solvent, Ru3(CO)12 reacts with tertiary butyl germane at reflux, to afford six new bimetallic ruthenium–germanium carbonyl cluster complexes Ru3(CO)9(μ3-GetBu)2, 1, Ru2(CO)6(μ-GetBuH)3, 2, Ru4(CO)10(μ4-Ge2tBu2)(μ-GetBuH)2, 3, Ru4(CO)8(μ4-Ge2tBu2)(μ-GetBuH)2(μ3-GetBu)(H), 4, Ru5(CO)12(μ3-GetBu)2(μ4-GetBu)(H), 5, Ru6(CO)12(μ3-GetBu)4(H)2, 6. Complex 2 was obtained as a result of cluster fragmentation while cluster condensation provided higher nuclearity ruthenium–germanium complexes 3–6 with varying Ru–Ge ratios. All six compounds were structurally characterized by a combination of IR, 1H NMR, mass spectrometry and single-crystal X-ray diffraction analyses.The reaction of Ru3(CO)12 with tertiary butyl germane has yielded six new ruthenium–germanium carbonyl cluster complexes with a variety of Ru–Ge ratios. All six compounds were structurally characterized by single-crystal X-ray diffraction analyses.

The reaction of Ru5(CO)15(μ5-C) with Ni(COD)2 in acetonitrile at 80 °C affords the bimetallic octahedral ruthenium–nickel cluster complex Ru5Ni(NCMe)(CO)15(μ6-C), 3. The acetonitrile ligand in 3 can be replaced by CO and NH3 to yield Ru5Ni(CO)16(μ6-C), 4, and Ru5Ni(NH3)(CO)15(μ6-C), 5, respectively. Photolysis of compound 3 in benzene and toluene solvent yielded the η6-coordinated benzene and toluene Ru5Ni carbido cluster complexes Ru5Ni(CO)13(η6-C6H6)(μ6-C), 6, and Ru5Ni(CO)13(η6-C7H8)(μ6-C), 7, respectively. All five new compounds were structurally characterized by single-crystal X-ray diffraction analyses.

The reaction of Fe2(CO)9 and Bu3tSnH yielded the bimetallic cluster complexes Fe2(μ-SnBu2t)2(CO)8, 1, and Fe4(μ4-Sn)(μ-SnBu2t)2(CO)16, 3. Compound 3 contains two Fe2(CO)8(μ-SnBu2t) groups held together by a central quadruply bridging tin atom, giving an overall bow-tie structure for the one tin and four iron atoms. Refluxing compound 1 in toluene solvent affords the complex Fe2[μ-SnBut(CH2Ph)]2(CO)8, 4, where two of the But groups in 1 have been replaced with benzyl groups, as a result of selective benzylic C–H bond activation of solvent toluene. Similarly refluxing compound 1 in ortho-, meta- and para-xylene solvents gives the complexes where two, three and four of the But groups in 1 have been replaced by the respective xylyl groups. Compound 1 also reacts with ethylbenzene to furnish the complex Fe2[μ-SnBut(MeCHPh)]2(CO)8, 14, where two of the But groups in 1 have been replaced as a result of the benzylic C–H activation of ethylbenzene. A mechanism based on a radical pathway is proposed for the selective C–H bond activation by 1.

The reaction of Ni(COD)2 with two equivalents of the TEMPO radical at 68 °C affords the 16 e– “bow-tie” complex Ni(η2-TEMPO)2, 1, in 78% yield. Compound 1 reacts with tert-butyl isocyanide and phenylacetylene at room temperature to yield the 16 e– distorted square planar nickel complexes Ni(η2-TEMPO)(η1-TEMPO)(CNtBu), 2, and Ni(η2-TEMPO)(η1-TEMPOH)(CCPh), 4, respectively. The facile reactivity of 1 is aided by the transition of the TEMPO ligand from an η2 to η1 binding mode. Complex 4 is an unusual example of hydrogen atom transfer from phenylacetylene to a coordinated TEMPO ligand.

The new platinum complex Pt(SntBu3)2(CNtBu)2(H)2, 1, was obtained in 32% yield from the reaction of Pt(COD)2 with tBu3SnH and CNtBu at room temperature. Compound 1 is a mononuclear 18 electron platinum complex in an octahedral geometry which contains two SntBu3's, two CNtBu's, and two hydride ligands. The two hydride ligands in 1 can be eliminated, both in solution and in the solid state, to yield the 16 electron complex Pt(SntBu3)2(CNtBu)2, 2. Compound 2 reacts with hydrogen at room temperature in solution and in the solid state to regenerate 1.

In acetonitrile solvent, Fe5(CO)15(μ5-C), 1, reacts with Ni(COD)2 at room temperature to afford the iron−nickel complex Fe5Ni(NCMe)(CO)15(μ6-C), 3. The acetonitrile ligand in 3 can be replaced by CO and NH3 to yield Fe5Ni(CO)16(μ6-C), 4, and Fe5Ni(NH3)(CO)15(μ6-C), 6, respectively. When refluxed in acetonitrile solvent, compound 3 loses a vertex to form the square pyramidal Fe4Ni complex Fe4Ni(NCMe)2(CO)12(μ5-C), 7. Compound 7 readily converts to Fe4Ni(NCMe)(CO)13(μ5-C), 8, by losing one of its acetonitrile ligands. Addition of acetonitrile to 8 gives compound 7. When heated to 110 °C under an atmosphere of CO, both compounds 7 and 8 furnish the octahedral Fe4Ni2 complex Fe4Ni2(CO)15(μ6-C), 9. All six compounds were structurally characterized by single-crystal X-ray diffraction analyses.

The bimetallic NiSn2 complex Ni(SnBu3t)2(CO)3,1, was obtained from the reaction of Ni(COD)2 and Bu3tSnH and CO. The reaction of Co2(CO)8 and Bu3tSnH afforded the bimetallic Co–Sn complex Co(SnBu3t)(CO)4, 3. Compound 3 was also obtained from the reaction of Co4(CO)12 and Bu3tSnH but in a lower yield. Both compounds 1 and 3 were characterized by single crystal X-ray diffraction, and possess trigonal bipyramidal geometries around the transition metal centre with two and one stannyl ligands, respectively.

The new bimetallic Fe–Ni carbide containing cluster complex, Fe4Ni(Cp)2(CO)10(μ5-C), 4 was afforded by metal–metal exchange and metal cluster rearrangement processes in the reaction of Fe5(μ5-C)(CO)15, 1, with NiCp2 at 80 °C. A minor product Fe5(Cp)2(CO)10(μ5-C), 5, was also obtained from this reaction which contains no nickel and is a di-Cp substituted derivative of 1. Both compounds were characterized by a combination of IR, 1H NMR, mass spectrometry, elemental and single crystal X-ray diffraction analyses. Compound 4 consists of an open Fe4Ni(μ5-C) cluster framework with a carbide ligand where one iron atom bridges a butterfly arrangement of one nickel and three iron atoms.

In the current investigation, reactions of the “bow-tie” Ni(η2-TEMPO)2 complex with an assortment of donor ligands have been characterized experimentally and computationally. While the Ni(η2-TEMPO)2 complex has trans-disposed TEMPO ligands, proton transfer from the C–H bond of alkyne substrates (phenylacetylene, acetylene, trimethylsilyl acetylene, and 1,4-diethynylbenzene) produce cis-disposed ligands of the form Ni(η2-TEMPO)(κ1-TEMPOH)(κ1-R). In the case of 1,4-diethynylbenzene, a two-stage reaction occurs. The initial product Ni(η2-TEMPO)(κ1-TEMPOH)[κ1-CC(C6H4)CCH] is formed first but can react further with another equivalent of Ni(η2-TEMPO)2 to form the bridged complex Ni(η2-TEMPO)(κ1-TEMPOH)[κ1-κ1-CC(C6H4)CC]Ni(η2-TEMPO)(κ1-TEMPOH). The corresponding reaction with acetylene, which could conceivably also yield a bridging complex, does not occur. Via density functional theory (DFT), addition mechanisms are proposed in order to rationalize thermodynamic and kinetic selectivity. Computations have also been used to probe the relative thermodynamic stabilities of the cis and trans addition products and are in accord with experimental results. Based upon the computational results and the geometry of the experimentally observed product, a trans–cis isomerization must occur.