Cationic methyl complexes of rhodium(III): synthesis, structure, and some reactions

Cationic methyl complex of rhodium(III), trans-[Rh(Acac)(PPh3)2(CH3)(CH3CN)][BPh4] (1) is prepared by interaction of trans-[Rh(Acac)(PPh3)2(CH3)I] with AgBPh4 in acetonitrile. Cationic methyl complexes of rhodium(III), cis-[Rh(Acac)(PPh3)2 (CH3)(CH3CN)][BPh4] (2) and cis-[Rh(BA)(PPh3)2(CH3)(CH3CN)][BPh4] (3) (Acac, BA are acetylacetonate and benzoylacetonate, respectively), are obtained by CH3I oxidative addition to rhodium(I) complexes [Rh(Acac)(PPh3)2] and [Rh(BA)(PPh3)2] in acetonitrile in the presence of NaBPh4. Complexes 2 and 3 react readily with NH3 at room temperature to form cis-[Rh(Acac)(PPh3)2(CH3)(NH3)][BPh4] (4) and cis-[Rh(BA)(PPh3)2(CH3)(NH3)][BPh4] (5), respectively. Complexes 1–5 were characterized by elemental analysis, 1H and 31P{1H} NMR spectra. Complexes 1, 2, 3 and 4 were characterized by X-ray diffraction analysis. Complexes 2 and 3 in solutions (CH2Cl2, CHCl3) are presented as mixtures of cis-(PPh3)2 isomers involved into a fluxional process. Complex 2 on heating in acetonitrile is converted into trans-isomer 1. In parallel with that isomerization, reductive elimination of methyl group with formation of [CH3PPh3][BPh4] takes place. Replacement of CH3CN in complexes 1 and 2 by anion I− yields in both cases the neutral complex trans-[Rh(Acac)(PPh3)2(CH3)I]. Strong trans influence of CH3 ligand manifests itself in the elongation (in solid) and labilization (in solution) of rhodium–acetonitrile nitrogen bond.