Synthetic and Computational Studies on the Thermal and Photochemical Reactions of [NPN]TaMe3 (NPN = PhP(CH2SiMe2NPh)2) and [MesNPN]TaMe3 (MesNPN = PhP(CH2SiMe2N(2,4,6-Me3C6H2))2)

Abstract

The thermolysis of [PhP(CH2SiMe2NPh)2]TaMe3 leads to the elimination of methane and the formation of cyclometalated derivative [PhP(CH2SiMe2NPh)(CH2SiMe2N-o-C6H4)]TaMe2, which was characterized by NMR spectroscopy and single crystal X-ray analysis. Computational studies confirm the expected four-membered transition state involving an ortho-N-phenyl-C–H bond and a Ta-methyl unit. The photolysis of [PhP(CH2SiMe2NPh)2]TaMe3 takes a different course; loss of methane also occurs but results in the formation the methylidene complex, [PhP(CH2SiMe2NPh)2]Ta═CH2(Me), which was characterized by NMR spectroscopy. Attempts to block the cyclometalation process by replacement of the N-phenyl substituent with N-Mesityl (Mesityl = 2,4,6-Me3C6H2) is also reported. With this bulkier ancillary ligand, the reactions are more complicated with multiple products being observed in an overall slow process. The reactions of the trimethyl, the cyclometalated product and the methylidene with H2 were also investigated and found to exhibit different rates of hydrogenolysis. This has implications for some of the steps in the reaction of [PhP(CH2SiMe2NPh)2]TaMe3 with H2 to generate dinuclear tetrahydride ([PhP(CH2SiMe2NPh)2]Ta)2(μ-H)4.