Ethylene Substitution in a Bis-Ethylene Complex of Rh/Os and Unusual Brønsted−Lowry Basicity of an N-Heterocyclic Carbene

Reactions of the bis-ethylene complex [RhOs(η2-C2H4)2(μ-CO)2(dppm)2][CF3SO3], with CO, MeCN, PMe3, IMe4, and [PPN][Cl] (IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene; PPN+ = bis(triphenylphosphine)iminium) yield the monoethylene products [RhOsL(C2H4)(μ-CO)2(dppm)2][CF3SO3] (L = CO (2), MeCN (3), PMe3 (4), IMe4 (5)) and [RhOsCl(C2H4)(μ-CO)2(dppm)2] (6), respectively, by substitution of the Rh-bound ethylene ligand. In the case of L = CO, carbon monoxide was added at −78 °C, since at higher temperatures substitution of both ethylene ligands occurred to give the known product [RhOs(CO)4(dppm)2][CF3SO3]. For L = IMe4, the stoichiometry had to be carefully controlled, since in the presence of more than 1 equiv of IMe4 deprotonation of a dppm methylene group by IMe4 occurred to yield [RhOs(IMe4)(C2H4)(μ-CO)2(Ph2PCHPPh2)(dppm)]. The IMe4 product (5) could also be obtained in THF solution by substitution of the acetonitrile ligand in compound 3. However, when this reaction was carried out in acetonitrile, deprotonation of the acetonitrile ligand by IMe4 occurred to give the acetonitrilide intermediate [RhOs(NCCH2)(C2H4)(μ-CO)2(dppm)2], which rapidly coupled with another 1 equiv of 3 to yield the enamino−nitrile-bridged product [(RhOs(C2H4)(μ-CO)2(dppm)2)2(μ-NHC(CH3)CHCN)][CF3SO3].