A 31P nuclear magnetic resonance investigation of the structure, equilibria, and kinetics of [Pt(PR3)n] in solution

Abstract

The compounds [PtL₄](L = PMe₃, PMe₂Ph, PMePh₂, PEt₃, or PBuⁿ₃), [PtL₃][L = PEt₃, PBuⁿ₃, P(p-tolyl)₃, P(CH₂Ph)₃, PPrⁱ₃, or P(C₆H₁₁)₃], and [PtL₂][L = PPrⁱ₃, P(C₆H₁₁)₃, or PBu^t₂Ph] have been prepared and shown to exist in solution. In the cases of L = PMePh₂, PEt₃, PBuⁿ₃, n= 3 and L = PPrⁱ₃, P(C₆H₁₁)₃, n= 2 it has proved possible to measure the equilibrium constant, ΔH^⊖, and ΔS^⊖ for [PtL_n+1]⇌[PtL_n]+ L. For [Pt(PMe₃)₄], [Pt(PMe₂Ph)₄], [Pt(PMePh₂)₄], [Pt(PEt₃)₄], [Pt(PBuⁿ₃)₄], [Pt(PPrⁱ₃)₃], and [Pt{P(C₆H₁₁)₃}₃], the tertiary phosphine exchange is dissociative, while for [Pt(PPrⁱ₃)₂] and [Pt{P(C₆H₁₁)₃}₂] associative exchange occurs. In the case of [Pt(PEt₃)₃] and [Pt(PBuⁿ₃)₃] both associative and dissociative exchange occur, the relative rates of which depend on the temperature and concentration of free tertiary phosphine. In the cases of [Pt(PPrⁱ₃)₂] and [Pt{P(C₆H₁₁)₃}₂] a marked solvent effect has been found and attributed to possible toluene co-ordination producing [Pt(η-toluene)(PR₃)₂] in equilibrium with [Pt(PR₃)₂]. No evidence is found for steric crowding affecting ΔH^⊖ or ΔH^‡, but ΔS^⊖ and ΔS^‡ are affected due to interpenetration of the ligands and resulting loss of motional freedom, while previously it has been assumed that interligand steric crowding destabilizes the molecule via the enthalpy term.