Bimetallic systems. Part 12. Mixed rhodium(I)–platinum(II) acetylide complexes containing bridging Ph2PCH2PPh2. Crystal structures of [(MeCC)Pt(µ-dppm)2(σ,η-CCMe)Rh(CO)]PF6 and of [ClPt(µ-dppm)2(σ,η-CCMe)Rh(CO)]PF6

Abstract

Treatment of [Pt(CCMe)₂(dppm-P)₂] with [Rh₂Cl₂(CO)₄] gave [(MeCC)Pt(µ-dppm)₂(σ,η-CCMe)Rh(CO)]Cl readily converted into the corresponding PF₆^– salt (1b) the crystal structure of which was determined. Other complexes of the type [(RCC)Pt(µ-dppm)₂(σ,η-CCR)Rh(CO)]Cl were made similarly; with R = Ph, p-tolyl, CH₂CH₂Ph, or C(Me)CH₂. The complexes are fluxional with the low-temperature limiting ¹H-{³¹P} n.m.r. spectrum showing non-equivalent pseudoequatorial and pseudo-axial CH₂ protons, H_e coupled to ¹⁹⁵Pt and H_a not. The fluxional process corresponds to interchange of H_e and H_a and interchange of terminal and bridging CCR. When heated in toluene for 3 h, [(RCC)Pt(µ-dppm)₂(σ,η-CCR)Rh(CO)]Cl (R =p-tolyl or Ph) was converted into [(RCC)Pt(µ-dppm)₂(σ,η-CCR) RhCl]. With CO, [(p-MeC₆H₄CC)Pt(µ-dppm)₂(σ,η-CCC₆H₄Me-p)RhCl] rapidly gave back [(p-MeC₆H₄CC)Pt(µ-dppm)₂(σ,η-CCC₆H₄Me-p)Rh(CO)]Cl. Treatment of [Pt(CCR)₂(dppm-P)₂] with [Rh₂Cl₂(C₈H₁₄)₄](C₈H₁₄= cyclo-octene) also gave [(RCC)Pt(µ-dppm)₂(σ,η-CCR)RhCl](R =p-tolyl or Ph) but the complexes were not isolated pure. Treatment of [(PhCC)₂Pt(µ-dppm)₂HgCl₂] with [Rh₂Cl₂(CO)₄] caused rapid and complete displacement of HgCl₂, giving [(PhCC)Pt(µ-dppm)₂(σ,η-CCPh)Rh(CO)]⁺; similarly treatment of [(PhCC)₂Pt(µ-dppm)₂AgCl], [(PhCC)₂Pt(µ-dppm)₂Cul], or [(PhCC)₂Pt(µ-dppm)₂Au]Cl with [Rh₂Cl₂(CO)₄] gave [(PhCC)Pt(µ-dppm)₂(σ,η-CCPh)Rh(CO)]⁺. Treatment of [Cl(RCC)Pt(µ-dppm)₂AgCl] with [Rh₂Cl₂(CO)₄] gave [ClPt(µ-dppm)₂(σ,η-CCR)Rh(CO)]⁺(R = Me, Ph, or p-tolyl) isolated as PF₆^– or AgCl₂^– salts. These complexes could also be made in ‘one-pot’ syntheses, viz. successive treatment of [Pt(dppm-PP′)₂]Cl₂ with AgO₂CMe–PhCCH followed by treatment with [Rh₂Cl₂(CO)₄], without isolation of the intermediate platinum–silver complex. The crystal structures of [(MeCC)Pt(µ-dppm)₂(σ,η-CCMe)Rh(CO)]PF₆(1b) as the dichloromethane solvate and of [ClPt(µ-dppm)₂(σ,η-CCMe)Rh(CO)]PF₆(5a) were determined. Crystals of (1b) are orthorhombic, space group Pbca, a= 19.212(7), b= 27.364(6), c= 21.468(5)Å, and Z= 8; those of (5a) are orthorhombic, space group Pn2₁a, a= 43.39(1), b= 25.178(9), c= 10.164(6)Å, and Z= 8. Final R factors were 0.088 for 4 500 and 0.058 for 6 320 observed reflections, respectively. In each complex cation the two metal centres [Pt ⋯ Rh 3.099(2) for (1b) 3.066(2) and 3.086(2)Å for (5a)] are bridged by a methylacetylide group σ-bonded to Pt and π-bonded in an unsymmetrical side-on fashion to Rh [mean Rh–CPt 2.24(2), mean Rh–CMe 2.44(2)Å], giving rise to an A-frame structure.