Reactivity of cationic agostic and carbene structures derived from platinum(II) metallacycles

Abstract

This paper describes the formation of new platinacyclic complexes derived from the phosphine ligands PiPr2Xyl, PMeXyl2, and PMe2Arinline image (Xyl=2,6-Me2C6H3 and Arinline image=2,6-(2,6-Me2C6H3)2-C6H3) as well as reactivity studies of the trans-[Pt(C^P)2] bis-metallacyclic complex 1 a derived from PiPr2Xyl. Protonation of compound 1 a with [H(OEt2)2][BArF] (BArF=B[3,5-(CF3)2C6H3]4) forms a cationic δ-agostic structure 4 a, whereas α-hydride abstraction employing [Ph3C][PF6] produces a cationic platinum carbene trans-[Pt{PiPr2(2,6-CH(Me)C6H3}{PiPr2(2,6-CH2(Me)C6H3}][PF6] (8). Compounds 4 a and 8 react with H2 to yield the same 1:3 equilibrium mixture of 4 a and trans-[PtH(PiPr2Xyl)2][BArF] (6), in which one of the phosphine ligands participates in a δ-agostic interaction. DFT calculations reveal that H2 activation by 8 occurs at the highly electrophilic alkylidene terminus with no participation of the metal. The two compounds 4 a and 8 experience C–C coupling reactions of a different nature. Thus, 4 a gives rise to complex trans-[PtH{(E)-1,2-bis(2-(PiPr2)-3-MeC6H3)CH[DOUBLE BOND]CH}] (7) that contains a tridentate diphosphine–alkene ligand, through agostic C[BOND]H oxidative cleavage and C–C reductive coupling steps, whereas the C–C coupling reaction in 8 involves classical migratory insertion of its [Pt[DOUBLE BOND]CH] and [Pt[BOND]CH2] bonds promoted by platinum coordination of CO or CNXyl. The mechanisms of the C[BOND]C bond-forming reactions have also been investigated by computational methods.