Synthesis and dynamic behaviour of bis(ethylene)(tertiary phosphine)-platinum complexes

Abstract

Reactions of ethylene-saturated solutions of bis(cyclo-octa-1,5-diene)platinum in light petroleum with tertiary phosphines or triphenylarsine afford white crystalline bis(ethylene)platinum complexes [Pt(C₂H₄)₂L][L = PMe₃, PMe₂, Ph, PMePh₂, PPh₃, P(C₆H₁₁)₃ or AsPh₃]. Bis(ethylene)(tricyclohexyIphosphine)platinum reacts with tetrafluoroethylene to give the mixed-olefin complex [Pt(C₂H₄)(C₂F₄){P(C₆H₁₁)₃}]. These 16-electron platinum(o) complexes show dynamic behaviour in solution with rotation of the ethylene ligands. N.m.r. studies (¹H and ¹³C) at various temperatures establish an apparent equivalence of methylene-group sites at ambient temperatures, but at low temperatures there are two CH₂, group signals in agreement with a molecular structure in which the ligand atom L, the platinum atom, and the carbon atoms of the ethylene molecules are coplanar. Activation energies for ethylene rotation in the complexes [Pt(C₂H₄)₂L] fall in the range (10.2–13.0)± 0.4 kcal mol^–1, Substitution in [Pt(C₂H₄)₂{P(C₆H₁₁)₃] of C₂H₄ by C₂F₄ results in a 3 kcal mol^–1 lowering of the barrier to rotation of the remaining ethylene molecule.