Reactivity of Cationic Agostic and Carbene Structures Derived from Platinum(II) Metallacycles

This paper describes the formation of new platinacyclic complexes derived from the phosphine ligands PiPr(2)Xyl, PMeXyl(2), and PMe(2)ArXyl2 (Xyl=2,6-Me2C6H3 and ArXyl2=2,6-(2,6-Me2C6H3)(2)-C6H3) as well as reactivity studies of the trans-[Pt(C boolean AND P)(2)] bis-metallacyclic complex 1a derived from PiPr(2)Xyl. Protonation of compound 1a with [H(OEt2)(2)][BArF] (BArF=B[3,5-(CF3)(2)C6H3](4)) forms a cationic -agostic structure 4a, whereas -hydride abstraction employing [Ph3C][PF6] produces a cationic platinum carbene trans-[Pt{PiPr(2)(2,6-CH(Me)C6H3}{PiPr(2)(2,6-CH2(Me)C6H3}][PF6] (8). Compounds 4a and 8 react with H-2 to yield the same 1:3 equilibrium mixture of 4a and trans-[PtH(PiPr(2)Xyl)(2)][BArF] (6), in which one of the phosphine ligands participates in a -agostic interaction. DFT calculations reveal that H-2 activation by 8 occurs at the highly electrophilic alkylidene terminus with no participation of the metal. The two compounds 4a and 8 experience C-C coupling reactions of a different nature. Thus, 4a gives rise to complex trans-[PtH{(E)-1,2-bis(2-(PiPr(2))-3-MeC6H3)CHCH}] (7) that contains a tridentate diphosphine-alkene ligand, through agostic CH oxidative cleavage and C-C reductive coupling steps, whereas the C-C coupling reaction in 8 involves classical migratory insertion of its [PtCH] and [PtCH2] bonds promoted by platinum coordination of CO or CNXyl. The mechanisms of the CC bond-forming reactions have also been investigated by computational methods.