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

The reactions of newly designed lithiated triamidoamines Li₃L^R (R = iPr, Pen, and Cy₂) with VCl₃(THF)₃ under N₂ yielded dinitrogen-divanadium complexes with a μ-N₂ between vanadium atoms [{V(L^R)}₂(μ-N₂)] (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 = Cy₂ (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(¹⁴N-¹⁴N) 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 N₂ to give a hetero-dinuclear vanadium complex with μ-N₂ between vanadium and potassium, [VK(L^Cy2)(μ-N₂)(18-crown-6)] (4). The N-N distance and ν(¹⁴N-¹⁴N) 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[C₁₀H₈] to give NH₃ and N₂H₄. The yields of NH₃ and N₂H₄ (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.