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 .