Mixed ligand carbonyl complexes of rhodium(I) and rhodium(III)

Abstract

Evidence is presented which suggests that the compounds trans-[RhX(CO)₂PPh₃](Ia)(X = Cl, Br, I), made from [RhCl(CO)₂]₂ and triphenylphosphine (1 : 2 molar ratio), should be reformulated as the dimeric complexes trans-[RhXCOPPh₃]₂(Ib). Similar compounds with X = SCN,OCOMe are also described. Detailed i.r. solution studies of the preparative reaction for X = Cl are interpreted in terms of the initial formation of a labile intermediate cis-[RhCl(CO)₂PPh₃]_n(III)(n probably 2), followed by partial decarbonylation and isolation of (Ib) and a small amount of cis-[RhCl(CO)PPh₃]₂(Ic). The compounds (Ib) undergo bridge cleavage reactions with various Lewis bases, giving [RhX(CO)(PPh₃)L](L = PPh₃,PMePh₂, P(C₆H₁₁)₃, AsPh₃, SbPh₃, C₅H₅N, Me₂S, etc.). With SbPh₃, the five-co-ordinate complex [RhCl(CO)PPh₃(SbPh₃)₂] is also formed. The ¹H n.m.r. of [RhCl(CO)(PPh₃)(PMePh₂)] shows only a singlet for the methyl resonance, indicative of easy phosphine exchange in a strongly coupled PP′ complex. As expected, [RhCl(CO)(PPh₃)(AsPh₃)] readily undergoes oxidative addition reactions, giving [RhClYZCO(PPh₃)(AsPh₃)](YZ = Cl₂, I₂, CH₃l). The latter product is in equilibrium with its acyl isomer [RhClI(COCH₃)(PPh₃)(AsPh₃)]0·5CH₃l. However, pure [RhClI(CH₃)(CO)(PPh₃)(AsPh₃)] can be synthesised by reaction of the acyl–alkyl mixture [RhClI(CH₃)(CO)PPh₃]₂(IV) and [RhClI(COCH₃)PPh₃]₂(V)(previously formulated as the single compound [RhClI(CO)(COCH₃)PPh₃]) with AsPh₃. Similarly, [RhClI(CH₃)(CO)PPh₃L](L = PPh₃ or SbPh₃) can be obtained. These compounds readily undergo isomerisation to the more stable acyl isomer. Finally, [RhCl₃(CO)PPh₃(AsPh₃)] can also be prepared by reaction of trans-[RhCl₃(CO)PPh₃]₂(originally formulated as trans-[RhCl₃(CO)₂PPh₃]) with AsPh₃.