Unexpected Coordination Complexes of the Metal Tris-silylamides M{N(SiMe3)2}3 (M = Ti, V).

Abstract

The synthesis, molecular structures, and spectroscopic details of a series of isocyanide and nitrile complexes of the early first-row transition-metal tris(silyl)amides M{N(SiMe3)2}3 (M = Ti, V) are reported. Previously, first-row transition-metal tris(silyl)amides were generally thought to be incapable of forming complexes with Lewis bases due to their excessive steric crowding. However, it is now shown that simple treatment of the base-free trisamides with 2 equiv of an isocyanide or nitrile base at room temperature results in the formation of the trigonal bipyramidal complexes Ti{N(SiMe3)2}3(1-AdNC)2 (1), Ti{N(SiMe3)2}3(CyNC)2 (2), Ti{N(SiMe3)2}3(ButNC)2 (3), Ti{N(SiMe3)2}3(PhCN)2 (4), V{N(SiMe3)2}3(1-AdNC)2 (5), V{N(SiMe3)2}3(CyNC)2 (6), V{N(SiMe3)2}3(ButNC)2 (7), and V{N(SiMe3)2}3(PhCN)2 (8), which incorporate two donor ligands (1-AdNC = 1-adamantyl isocyanide, CyNC = cyclohexyl isocyanide, ButNC = tert-butyl isocyanide, PhCN = benzonitrile). All complexes display a characteristic increase in the frequency of the multiple bonded C-N stretching mode which is observed to be in the range of 2170-2190 cm-1 for the isocyanide complexes 1-3 and 5-7 and at 2250 cm-1 for the nitrile complex 8. This effect was not observed for the titanium nitrile complex 4, suggesting weak binding of the donor to titanium. Paramagnetic 1H NMR studies showed these complexes to have detectable, though extremely broadened, signals attributable to the trimethylsilyl groups of the amide ligands (δ = ca. 2.8 ppm for titanium isocyanide complexes, ca. 4.5-4.7 ppm for vanadium isocyanide complexes). A variable-temperature 1H NMR study showed that in solution these complexes exist as mixtures of the five-coordinate species and a putative four-coordinate species coordinating a single Lewis basic ligand. Electronic spectroscopy indicated that the vanadium complexes 5-8 bind the Lewis bases more strongly than the corresponding titanium complexes, where the spectra of complexes 1-4 are essentially identical to the base-free Ti{N(SiMe3)2}3 at the temperatures and concentrations studied. In contrast to these results, no corresponding complexes were detected for the metal silylamides M{N(SiMe3)2}3 (M = Cr, Mn, Fe, or Co) when treated with the isocyanide or nitrile bases.