The structure, coordination properties, insertion processes, and dynamic behavior in solution of the five-coordinate complexes [IrXH(biPSi)] (biPSi = kappa-P,P,Si-Si(Me){(CH(2))(3)PPh(2)}(2); X = Cl (1), Br (2), or I (3)) have been investigated. The compounds are formed as mixtures of two isomers, anti and syn, in slow equilibrium in solution. The equilibrium position depends on the halogen and the solvent. Both isomers display distorted square-based pyramidal structures in which the vacant position sits trans to silicon. The equatorial plane of the syn isomer is closer to the T structure due to distortions of steric origin. The small structural differences between the isomers trigger remarkable differences in reactivity. The syn isomers form six-coordinate adducts with chlorinated solvents, CO, P(OMe)(3), or NCMe, always after ligand coordination trans to silicon. The anti isomers do not form detectable adducts with chlorinated solvents and coordinate CO or P(OMe)(3) either trans to silicon (kinetic) or trans to hydride (thermodynamic). NCMe coordinates the anti isomers exclusively at the position trans to hydride. Qualitative and quantitative details (equilibrium constants, enthalpies, entropies, etc.) on these coordination processes are given and discussed. As a result of the different coordination properties, insertion reagents such as acetylene, diphenylacetylene, or the alkylidene resulting from the decomposition of ethyl diazoacetate selectively insert into the Ir-H bond of 1-syn, not into that of 1-anti. These reactions give five-coordinate syn alkenyl or alkyl compounds in which the vacancy also sits trans to silicon. Acetylene is polymerized in the coordination sphere of 1. The nonreactive isomer 1-anti also evolves into the syn insertion products via anti<-->syn isomerizations, the rates of which are notably dependent on the nature of the insertion reactants. H(2) renders anti<-->syn isomerization rates of the same order as the NMR time scale. The reactions are second order (k(obs) = k(anti<-->syn)[H(2)]) and do not involve H(2)/IrH hydrogen atom scrambling. A possible isomerization mechanism, supported by MP2 calculations and compatible with the various experimental observations, is described. It involves Ir(V) intermediates and a key sigma Ir-(eta(2)-SiH) agostic transition state. A similar transition state could also explain the anti<-->syn isomerizations in the absence of oxidative addition reactants, although at the expense of high kinetic barriers strongly dependent on the presence of potential ligands and their nature.

The reactions of [Ir(μ-OMe)(cod)]2 with the N-allyl-substituted benzimidazolium salts 1-methyl-3-(2-propenyl)benzimidazolium iodide (2) and 1,3-di(2-propenyl)benzimidazolium bromide (3) have been found to afford the five-coordinated Ir(I) complexes [IrX(cod)(η2-C-NHC)] (X = I, NHC = 1-methyl-3-(2-propenyl)benzimidazol-2-ylidene, 4; X = Br, NHC = 1,3-di(2-propenyl)benzimidazol-2-ylidene, 5), respectively. The cationic derivative [Ir(cod)(η2:η2-C-NHC)]BF4 (NHC = 1,3-di(2-propenyl)benzimidazol-2-ylidene, 6), which contains an iridium center exclusively coordinated by sp2 carbon atoms, has been prepared by treatment of 5 with AgBF4. The reaction between [Ir(μ-Cl)(cod)]2 and 3 in ethanol, in the presence of excess NaOEt, has allowed the synthesis of the four-coordinate complex [IrBr(cod)(C-NHC)] (NHC = 1,3-di(propyl)benzimidazol-2-ylidene, 7) after deprotonation of 3 and hydrogenation of both N-allyl substituents. The compounds 5, 6, and 7 have been characterized by X-ray diffraction. The neutral complexes 4, 5, and 7 have been tested as catalysts in the transfer hydrogenation of cyclohexanone using 2-propanol as hydrogen source. The catalytic reactions using 4 and 5 have been observed to progress without hydrogenation of the allyl substituents.

In the presence of reactants such as acetonitrile, trimethylphosphine, and diphenylacetylene, the 1,5-cyclooctadiene iridium(I) complex [Ir(1,2,5,6-η-C 8 H 12 )(NCCH 3 )(PMe 3 )]BF 4 (1) has been found to transform into compounds containing cyclooctadiene or cyclooctadienyl ligands in η 3 ,η 2 -; κ,η 3 -; κ 2 ,η 2 -; and η 3 -coordination modes. All these reactions are initiated by an intramolecular C-H activation of the COD ligand and followed by either inter-or intramolecular insertion, or reductive elimination and further C-H activation elementary steps. Compound 1 has also been observed to undergo facile intermolecular oxidative additions of dihydrogen, hydrosilanes, and phenylacetylene to afford iridium(III) hydride complexes. Evidence for the insertion of COD into the Ir-H bonds of these new complexes has been obtained from the isolation of a monohydride complex containing a κ,η 2 -cyclooctenyl ligand, from the isomerization of a silyl derivative into analogues containing 1,4-and 1,3cycloctadiene ligands, and from the occurrence of H/D scrambling among Ir-H and COD C-H sites in the product of DCtCPh oxidative addition. Si-Si coupling reactions to give disilanes and C-C coupling reactions to give an iridacyclopentadiene complex and 1,2,4triphenylbenzene have also been observed in silane and phenylacetylene excess, respectively. Competition of all these intra-and intermolecular reactions under the conditions of phenylacetylene hydrosilylation has been found to result in catalytic reactions, the selectivity of which depends on the presence of introduced acetonitrile and its concentration.