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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.

The reaction of fac-[IrH2(NCCH3)3(PiPr3)]BF4 (1) with potassium pyrazolate gave the binuclear 34-electron complex [Ir2(μ-H)(μ-Pz)2H3(NCCH3)(PiPr3)2] (2). The structure of 2 was determined by X-ray diffraction. An electrostatic potential calculation located three terminal hydride ligands and one hydride bridging both iridium centers. The feasibility of this arrangement was studied by EHMO calculations. The spectroscopic data for 2 show that the complex is rigid in solution on the NMR time scale. In solution, the acetonitrile ligand of 2 dissociates. The activation parameters for this dissociation process in toluene-d 8 are ΔH ⧧ = 20.9 ± 0.6 kcal mol-1 and ΔS ⧧ = 2.5 ± 1.3 e.u. Reaction of 2 with various Lewis bases (L) gives the substitution products [Ir2(μ-H)(μ-Pz)2H3(L)(PiPr3)2] (L = C2H4 (3), CO (4), HPz (5)). The reaction of complex 5 with C2H4 yields the ethyl derivative [Ir2(μ-H)(μ-Pz)2(C2H5)H2(HPz)(PiPr3)2] (6); this reaction is reversible. Complexes 2 and 3 react with CHCl3 to give CH2Cl2 and the compounds [Ir2(μ-H)(μ-Pz)2H2(Cl)(L)(PiPr3)2] (L = NCCH3 (7), C2H4 (8)). In the 1H NMR spectra of 2 − 6, the signal of the bridging hydride ligand shows two very different J HP couplings; in contrast, for the chloride complexes 7 and 8, two equal J HP couplings are observed. NOE and T 1 measurements lead to the conclusion that in complexes 2 − 6 the hydride bridges the iridium centers in a nonsymmetric fashion, whereas for 7 and 8 the bridge is symmetrical. This structural feature largely influences the reactivity. Compounds 2 and 3 undergo H/D exchange under a D2 atmosphere. Analysis of the isotopomeric mixtures of 2 reveals downfield isotopic shifts in the 31P{1H} NMR spectrum. Downfield as well as high-field shifts are found for the hydride signals in the 1H NMR spectrum of partially deuterated 2. Further reaction of 3 with H2 gave ethane and the dihydrogen complex [Ir2(μ-H)(μ-Pz)2H3(η2-H2)(PiPr3)2] (9). Under a deficiency of H2, in toluene-d 8 solution, 9 undergoes H/D scrambling with the participation of the solvent. It has also been found that under H2 complex 3 catalyzes the hydrogenation of cyclohexene.

The methoxycarbonylation of alkenes catalyzed by palladium(II) complexes with P,N-donor ligands, 2-(diphenylphosphinoamino)pyridine (Ph2PNHpy), 2-[(diphenylphosphino)methyl]pyridine (Ph2PCH2py), and 2-(diphenylphosphino)quinoline (Ph2Pqn) has been investigated. The results show that the complex [PdCl(PPh3)(Ph2PNHpy)]Cl or an equimolar mixture of [PdCl2(Ph2PNHpy)] and PPh3, in the presence of p-toluensulfonic acid (TsOH), is an efficient catalyst for this reaction. This catalytic system promotes the conversion of styrene into methyl 2-phenylpropanoate and methyl 3-phenylpropanoate with nearly complete chemoselectivity, 98% regioselectivity in the branched isomer, and high turnover frequency, even at alkene/Pd molar ratios of 1000. Best results were obtained in toluene-MeOH (3 : 1) solvent. The Pd/Ph2PNHpy catalyst is also efficient in the methoxycarbonylation of cyclohexene and 1-hexene, although with lower rates than with styrene. Related palladium complexes [PdCl(PPh3)L]Cl (L = Ph2PCH2py and Ph2Pqn) show lower activity in the methoxycarbonylation of styrene than that of the 2-(diphenylphosphinoamino)pyridine ligand. Replacement of the last ligand by (diphenylphosphino)phenylamine (Ph2PNHPh) or 2-(diphenylphosphinoaminomethyl)pyridine (Ph2PNMepy) also reduces significantly the activity of the catalyst, indicating that both the presence of the pyridine fragment as well as the NH group, are required to achieve a high performing catalyst. Isotopic labeling experiments using MeOD are consistent with a hydride mechanism for the [PdCl(PPh3)(Ph2PNHpy)]Cl catalyst.

The trisacetonitrile complexes [IrClH(PiPr3)(NCCH3)3]BF4 (1) and [IrH2(PiPr3)(NCCH3)3]BF4 (2) have been prepared in one-pot reactions with high yields by reaction of the iridium(I) dimers [Ir(μ-Cl)(coe)2]2 and [Ir(μ-OMe)(cod)2]2 with the phosphonium salt [HPiPr3]BF4. The rates of exchange between free acetonitrile and the labile acetonitrile ligands of complexes 1 and 2 have been measured by NMR spectroscopy. This kinetic study has shown that both complexes readily dissociate one acetonitrile ligand trans to hydride, giving rise to fluxional five-coordinate intermediates. Substitution products 3−7 have been obtained by treatment of complexes 1 and 2 with CO and PMe3. The structures determined for 3−7 can be rationalized on the basis of the steric requirements of the ligands, indicating that the products are formed by thermodynamic control. Ethene inserts reversibly into the Ir−H bond of 1 to give the compound [IrCl(Et)(PiPr3)(NCCH3)3]BF4 (8), which has been used for the preparation of the stable ethyliridium(III) complexes [IrCl(Et)(PiPr3)(Py)2(NCCH3)]BF4 (9) and [Ir(η2-O2CCH3)Cl(Et)(PiPr3)(NCCH3)3] (10), respectively. The molecular structure of 10 has been determined by X-ray crystallography. The reaction of 2 with ethene, at low temperature, results in the sequential formation of the ethene complex [IrH2(η2-C2H4)(PiPr3)(NCCH3)2]BF4 (11) and the diethyl derivative [Ir(Et)2(PiPr3)(NCCH3)3]BF4 (14). At room temperature in solution, 14 undergoes reductive elimination of ethane to form the iridium(I) species [Ir(PiPr3)(NCCH3)3]BF4 (15) and [Ir(PiPr3)(η2-C2H4)(NCCH3)2]BF4 (16). These cations readily react with H2 to regenerate 2, closing a cycle for ethene hydrogenation in which several participating species have been identified. The reaction of 2 with propene in solution also allows the characterization of products of propene coordination (17) and insertion (18). In this case, the species obtained after elimination of propane are products of allylic C−H activation:  [IrH(η3-C3H5)(PiPr3)(NCCH3)2]BF4 (19) and [IrH(η3-C3H5)(η2-C3H6)(PiPr3)(NCCH3)]BF4 (20). The structure of complex 19 has been determined by X-ray diffraction, and the kinetics of dissociation of its two labile acetonitrile ligands have been studied by NMR spectroscopy. Complex 19 undergoes electrophilic activation of H2 to give propene and reform the starting complex 2.

The arene complex [IrH2(η6-C6H6)(IMes)]PF6 (3) has been prepared in a one-pot synthesis from conventional starting materials. The facile substitution of its benzene ligand can be exploited in the preparation of many other NHC Ir(III) dihydrides, for which the solvent complexes [IrH2(L)3(IMes)]PF6 (L = acetone-d 6, 4; NCMe, 5), the NHC-phosphine compound [IrH2(NCMe)2(IMes)(PiPr3)]BF4 (6), and the water-soluble analogue [IrH2(NCMe)2(IMes)(TPPTS)]BF4 (7) constitute representative examples. The reactions of dihydrides 5 and 6 with hydrogen acceptors such as ethylene, propylene, and diphenylacetylene have been examined. Depending on the dihydride and the acceptor, they have led to different Ir(III) complexes with a cyclometalated IMes moiety and hydride, alkyl, or alkenyl ligands: [Ir(R)(IMes′)(NCMe)2(L)]PF6 {R = H, L = NCMe (8), PiPr3 (9); L = NCMe, R = Et (10); nPr (11); Z-C(Ph)CHPh (12)}. Because of their six-membered rings, such cyclometalated compounds have been found to adopt two possible conformations in equilibrium. The rates of exchange between conformers are of the same order as the NMR time scale, and the equilibrium position is governed by steric factors. By reaction with phenylacetylene, compounds 6 and 9 have afforded a common hydride alkynyl complex [IrH(CCPh)(NCMe)2(IMes)(PiPr3)]PF6 (14). Its selective formation as a deuteride isotopomer from the reaction between 9 and PhCCD has proven that Ir(I) species, although nonobserved, are accessible.