Synthesis, Structure, and Reactivity of an Unusual Platinum Olefin Carbene Complex, [(η4-cod)Pt{C(Ph2PNSiMe3)2-κC,κN}

Abstract

The organolithium dimer [Li2C(Ph2PNSiMe3)2]2 ([Li2-1]2) reacts with 2 equiv of [PtCl2(cod)] (cod = 1,5-cyclooctadiene) in Et2O solution to give the novel Pt−carbene complex [(η4-cod)Pt{C(Ph2PNSiMe3)2-κC,κN}] (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 [(η4-cod)Pt{C(Ph2PNSiMe3)(Ph2PN(Me)SiMe3-κC,κN}][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 [(η4-cod)Pt{C(Ph2PNSiMe3)(Ph2PNC(O)OSiMe3-κC,κN}] (4). Heating a benzene solution of 2 to 100 °C in a sealed tube for several hours, or to 60 °C for 10 min in the presence of H2O, yields the ortho-metalated complex [(η4-cod)Pt{CH(Ph(C6H4)PNSiMe3)(Ph2PNSiMe3)-κC,κC‘}] (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 [Li2-1]2 and [PtCl2(cod)] in C6H6 solution proceeds only very slowly and gives [Li][Pt{CH(Ph(C6H4)PNSiMe3)(Ph2PNSiMe3)-κC,κC‘}{CH(Ph2PNSiMe3)2-κC,κN}] (7) in low yield.