Multinuclear nuclear magnetic resonance studies of dirhodium and platinum–rhodium ‘A-frame’ complexes of bis(diphenylphosphino)methane

Abstract

The ³¹P-{¹H} NMR spectra of the single A-frame complexes [Rh₂X₂(µ-Y)(µ-dppm)₂]ⁿ⁺(dppM = Ph₂PCH₂PPh₂; n= 0, Y = CO, X = Cl, Br or I; n= 0, Y = SO₂, X = CI; n= 0, Y = S, X = CO; n= 1, Y = Cl, X = CO) and the double A-frame complexes [Rh₂X₂(µ-Y)(µ-CO)(µ-dppm)₂]⁺(Y = H or Cl, X = CO) have been successfully analysed as [[A]₂X]₂ spin systems allowing the |J(RhRh)| and ²J(PC) coupling constants to be determined. For the single A-frame complexes |J(RhRh)| was found to correlate well with ²J(PCP); J(RhRh)= 0 for both of the double A-frame complexes. In those cases where crystal-structure data are available, ²J(PCP) correlates well with the Rh-Rh separation for both single and double A frames. It is concluded that ²J(PCP) is a reliable indicator of the presence or absence of a metal-metal bond in these compounds. The cationic complexes [(PhCC)Pt(µ-SH)(µ-dppm)₂Rh(CO)][BF₄] and [(PhCC)Pt(µ-SR)(µ-dppm)₂Rh(CO)][PF₆](R = Me or CH₂Ph) have been prepared from [CIPt(µ-CCPh)(µ-dppm)₂Rh(CO)][PF₆]via[(PhCC)Pt(µ-S)(µ-dppm)₂Rh(CO)]. When static, these complexes possess no symmetry and thus two values of ²J(PCP) are obtained for each molecule. These couplings are comparable in size only for [(PhCC)Pt(µ-SH)(µ-dppm)₂Rh(CO)]⁺; in the case of both [(PhCC)Pt(µ-SR)(µ-dppm)₂Rh(CO)]⁺(R = Me or CH₂Ph). ²J(PCP) is much larger across one dppm ligand in the molecule than across the other. This finding is attributed to steric crowding between the methylene protons of the dppm and the µ-SR ligands and is consistent with the data obtained from the ³¹P-{¹H} NMR spectrum of [Rh₂(CO)₂(µ-SCH₂Ph)(µ-dppm)₂⁺, which has been successfully analysed as an [ABX]₂ spin system. Measurement and analysis of the ¹⁰³Rh-{¹H} NMR spectrum of [Rh₂Br₂(µ-CO)(µ-dppm)₂] has demonstrated that J(RhRh) has the same relative sign as ²J(PRhP) in this system and must therefore have a positive sign. The ¹⁰³Rh NMR chemical shift data for the dirhodium A-frame complexes are also given.