STRUCTURAL AND SPECTROSCOPIC EVIDENCE FOR MULTIPLE ALPHA-AGOSTIC INTERACTIONS IN DIALKYL COMPLEXES OF NIOBIUM

Mono- and dineopentyl complexes of niobium containing ancillary imido and cyclopentadienyl ligands have been prepared via treatment of (C5H5 )Nb(NR)Cl2 (R = t-Bu, 2,6-C6H3-i-Pr2) With 1 or 2 equiv of neopentylma gnesium chloride, respectively. The crystal structure of (C5H5)Nb(N-2, 6-C6H3-i-Pr2)(CH2CMe3)2 (C27H43NNb, M(r) = 474.6, monoclinic, P2(1)/n, a = 9.272(1) angstrom, b = 17.237(2) angstrom, c = 17.020(2) angstrom , beta = 95.02(1)degrees, V = 2709.7 angstrom3, Z = 4, D(x) = 1.163 g cm-3, lambda(Mo Kalpha) = 0.710 73 angstrom, mu = 0.44 mm-1, F(000) = 1012, T = 295 K) shows that a alpha-hydrogen on each neopentyl methyle ne lies within close contact (av 2.36 angstrom) of the metal center wi th Nb-C-H(alpha) bond angles of 87 and 89-degrees. The presence of alp ha-agostic interactions is confirmed by NMR studies on the partially d euterated derivatives (C5H5)Nb(NR)(CHDCMe3) (R = t-Bu, 2,6-C6H3-i-Pr2) .