Synthesis, structure, and reactivity of an unusual platinum olefin carbene complex, [(η4-cod)Pt{=C(Ph2P=NSiMe3)2-κC,κN}]

The organolithium dimer [Li2C(Ph2P=NSiMe3)(2)](2) ([Li-2-1](2)) reacts with 2 equiv of [PtCl2-(cod)] (cod = 1,5-cyclooctadiene) in Et2O solution to give the novel Pt-carbene complex [eta(4)-cod)Pt{=CPh2P=NSiMe3)(2)-kappaC,kappaN}] (2), which is characterized by a relatively long Pt-C(carbene) bond and relatively short Pt-C(olefin) interactions indicative of an electron-rich Pt center. Under oxygen- and moisture-free conditions, 2 is remarkably inert; the cod ligand is not displaced by either monodentate or bidentate phosphines, phosphine sulfide, phosphite, or pyridine at room temperature, nor is the four-membered C,N-chelate ring opened by these reagents. However, 2 reacts with the electrophiles MeOTf (OTf = trifluoromethanesulfonate) and CO2. In the former case, the uncoordinated N atom of 2 is methylated to form [eta(4)-cod)Pt{=C(Ph2P=NSiMe3)(Ph2P=N(Me)SiMe3-kappaC,kappaN}][OTf] (3). In the latter, initial nucleophilic attack by the uncoordinated N atom on CO2, followed by trimethylsilyl group migration from N to O, gives [eta(4) -cod)Pt{=C(Ph2P=NSiMe3)(Ph2P= NC(O)OSiMe3-kappaC,kappaN}] (4). Heating a benzene solution of 2 to 100 degreesC in a sealed tube for several hours, or to 60 degreesC for 10 min in the presence of H2O, yields the ortho-metalated complex [eta(4)-cod)Pt{CH(Ph(C6H4)P=NSiMe3)(Ph2P=NSiMe3)-kappaC,kappaC'}] (5), which is characterized by Pt-C distances that are in the typical range for Pt-C(alkyl) and Pt-C(arene) interactions. The diastereoselectivity of the ortho metalation is consistent with an oxidative addition (nucleophilic) reaction mechanism for the ortho metalation. In contrast to 2, 5 displays typical cod substitution reactivity with chelating phosphines. The reaction between [Li-2-1](2) and [PtCl2(cod)] in C6H6 solution proceeds only very slowly and gives [Li][Pt{CH(Ph(C6H4)P=NSiMe3)(Ph2P=NSiMe3)-kappaC,kappaC'} {CH(Ph2P=NSiMe3)(2)-kappaC,kappaN}] (7) in low yield.