Reductive elimination of biaryl from diarylbis(phosphine)platinum(II) complexes in solution: kinetics and mechanism

Abstract

The thermal decompositions of complexes cis-[PtR₂L₂][R = Ph or C₆H₄Me-4; L₂=(PPh₃)₂, {P(C₆H₄Me-4)₃}₂, (PMePh₂)₂, Ph₂PCH₂PPh₂, Ph₂PCH₂CH₂PPh₂, or Me₂PCH₂CH₂PMe₂] in toluene solution have been investigated. Their stability to thermolysis varies markedly with L₂, and only complexes of monotertiary phosphines are labile at 60 °C. Reaction occurs via primary concerted unimolecular reductive elimination of biaryl, and conforms to a first-order kinetic rate law. Concurrent but independent secondary decomposition of [PtL₂] generates both arene and biaryl as minor products. The elimination of 4,4′-bitolyl from cis-[Pt(C₆H₄Me-4)₂(PPh₃)₂] exhibits a pronounced negative entropy of activation, suggestive of a conformationally restricted transition state. The inclusion of free PPh₃ suppresses secondary processes and accelerates primary decay. Although itself inert under these conditions, [PtR₂(Ph₂PCH₂PPh₂)] also eliminates biaryl in a concerted and kinetically first-order manner when allowed to react in the presence of 10 equivs. of Ph₂PCH₂PPh₂. These observations provide further evidence for the facilitation of reductive elimination by added nucleophiles.