Labile hydride complexes of iridium(III):: Synthesis, dynamic behavior in solution, and reactivity toward alkenes

The trisacetonitrile complexes [IrClH((PPr3)-Pr-i)(NCCH3)(3)]BF4 (1) and [IrH2((PPr3)-Pr-i)(NCCH3)(3)]BF4 (2) have been prepared in one-pot reactions with high yields by reaction of the iridium(I) dimers [Ir(mu-Cl)(coe)(2)](2) and [Ir(mu-OMe)(cod)(2)](2) with the phosphonium salt [(HPPr3)-Pr-i]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)((PPr3)-Pr-i)(NCCH3)(3)]BF4 (8), which has been used for the preparation of the stable ethyliridium(III) complexes [IrCl(Et)((PPr3)-Pr-i)(Py)(2)(NCCH3)]BF4 (9) and [Ir(eta(2)-O2CCH3)Cl(Et)((PPr3)-Pr-i)(NCCH3)(3)] (10), respectively. The molecular structure of 10 has been determined by X-ray crystalIography, The reaction of 2 with ethene, at low temperature, results in the sequential formation of the ethene complex [IrH2(eta(2)-C2H4)((PPr3)-Pr-i)(NCCH3)(2)]BF4 (11) and the diethyl derivative [Ir(Et)(2)((PPr3)-Pr-i)(NCCH3)(3)]BF4 (14) At room temperature in solution, 14 undergoes reductive elimination of ethane to form the iridium(I) species [Ir((PPr3)-Pr-i)(NCCH3)(3)]BF4 (15) and [Ir((PPr3)-Pr-i)(eta(2)-C2H4)(NCCH3)(2)]BF4 (16). These cations readily react with H-2 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(eta(3)-C3H5)((PPr3)-Pr-i)(NCCH3)(2)]BF4 (19) and [IrH(eta(3)-C3H5)(eta(2)-C3H5)(eta(2)-C3H6)((PPr3)-Pr-i)(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 H-2 to give propene and reform the starting complex 2.