The Steric Effect in Preparations of Vanadium(II)/(III) Dinitrogen Complexes of Triamidoamine Ligands Bearing Bulky Substituents

The reactions of newly designed lithiated triamidoamines Li(3)L(R) (R = iPr, Pen, and Cy(2)) with VCl(3)(THF)(3) under N(2) yielded dinitrogen–divanadium complexes with a μ-N(2) between vanadium atoms [{V(L(R))}(2)(μ-N(2))] (R = iPr (1) and Pen (2)) for the former two, while not dinitrogen–divanadium complexes but a mononuclear vanadium complex with a vacant site, [V(L(Cy2))] (R = Cy(2) (3)), were obtained for the third ligand. The V–N(N2) and N–N distances were 1.7655(18) and 1.219(4) Å for 1 and 1.7935(14) and 1.226(3) Å for 2, respectively. The ν((14)N–(14)N) stretching vibrations of 1 and 2, as measured using resonance Raman spectroscopy, were detected at 1436 and 1412 cm(–1), respectively. Complex 3 reacted with potassium metal in the presence of 18-crown-6-ether under N(2) to give a hetero-dinuclear vanadium complex with μ-N(2) between vanadium and potassium, [VK(L(Cy2))(μ-N(2))(18-crown-6)] (4). The N–N distance and ν((14)N–(14)N) stretching for 4 were 1.152(3) Å and 1818 cm(−1), respectively, suggesting that 4 is more activated than complexes 1 and 2. The complexes 1, 2, 3, and 4 reacted with HOTf and K[C(10)H(8)] to give NH(3) and N(2)H(4). The yields of NH(3) and N(2)H(4) (per V atom) were 47 and 11% for 1, 38 and 16% for 2, 77 and 7% for 3, and 80 and 5% for 4, respectively, and 3 and 4, which have a ligand L(Cy2), showed higher reactivity than 1 and 2.