Labile Hydrido Complexes of Iridium(III):  Synthesis, Dynamic Behavior in Solution, and Reactivity toward Alkenes

The trisacetonitrile complexes [IrClH(P i Pr 3 )(NCCH 3 ) 3 ]BF 4 ( 1 ) and [IrH 2 (P i Pr 3 )(NCCH 3 ) 3 ]BF 4 ( 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 [HP i Pr 3 ]BF 4 . 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 PMe 3 . 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)(P i Pr 3 )(NCCH 3 ) 3 ]BF 4 ( 8 ), which has been used for the preparation of the stable ethyliridium(III) complexes [IrCl(Et)(P i Pr 3 )(Py) 2 (NCCH 3 )]BF 4 ( 9 ) and [Ir(η 2 -O 2 CCH 3 )Cl(Et)(P i Pr 3 )(NCCH 3 ) 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 [IrH 2 (η 2 -C 2 H 4 )(P i Pr 3 )(NCCH 3 ) 2 ]BF 4 ( 11 ) and the diethyl derivative [Ir(Et) 2 (P i Pr 3 )(NCCH 3 ) 3 ]BF 4 ( 14 ). At room temperature in solution, 14 undergoes reductive elimination of ethane to form the iridium(I) species [Ir(P i Pr 3 )(NCCH 3 ) 3 ]BF 4 ( 15 ) and [Ir(P i Pr 3 )(η 2 -C 2 H 4 )(NCCH 3 ) 2 ]BF 4 ( 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(η 3 -C 3 H 5 )(P i Pr 3 )(NCCH 3 ) 2 ]BF 4 ( 19 ) and [IrH(η 3 -C 3 H 5 )(η 2 -C 3 H 6 )(P i Pr 3 )(NCCH 3 )]BF 4 ( 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 .