Cooperative Bimetallic Effects on New Iridium(III) Pyrazolate Complexes:  Hydrogen−Hydrogen, Carbon−Hydrogen, and Carbon−Chlorine Bond Activations

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-d8 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 26, the signal of the bridging hydride ligand shows two very different JHP couplings; in contrast, for the chloride complexes 7 and 8, two equal JHP couplings are observed. NOE and T1 measurements lead to the conclusion that in complexes 26 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)2H32-H2)(PiPr3)2] (9). Under a deficiency of H2, in toluene-d8 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.