Synthesis of a new potentially hemilabile ligand: 1-[2-(diphenylphosphanyl)ethyl]-3,5-dimethylpyrazole, and comparison of its bonding properties with the related 1-[2-(ethylamino)ethyl]-3,5-dimethylpyrazole ligand toward Rh^I

The new ligand 1-[2-(diphenylphosphanyl)ethyl]-3,5-dimethylpyrazole (2) has been prepared by the reaction of 1-(chloroethyl)-3,5-dimethylpyrazole and PPh2Li. The bidentate N,N ligand 1-[2-(ethylamino)ethyl]-3,5-dimethylpyrazole (1) and 2 react with [Rh(COD)(THF)2][BF4] to give [Rh-(COD)(1)][BF4] (3) and [Rh(COD)(2)][BF4] (4), respectively. Substitution of 1,5-cyclooctadiene with carbon monoxide in the latter complexes generates [Rh(CO)2(1)][BF4] (5) and [Rh(CO)2(2)][BF4] (6), respectively. Treatment of Rh(COD)(THF)2][BF4] with two equivalent amounts of 2 results in the complex [Rhf2)2][BF4] (7), which converts to [Rh(CO)(2)2][BF4] (8) on reaction with carbon monoxide. Ligand 2 in complex 8 exhibits a hemilabile character. The single-crystal X-ray structures of 3, 4, and 7 are reported. © Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.