A family of α-amino acid salicylaldiminates incorporating the binuclear V₂O₃³⁺ core: electrosynthesis, structure, and metal valence

Abstract

The green title complexes of the type Et₄N[V₂O₃(l-Asal)₂] have been synthesized in nearly quantitative yields by one-electron electroreduction (at 0.2 V vs SCE) of V₂O₃(l-Asal)₂ in dichloromethane solution containing tetraethylammonium perchlorate. Here l-Asal^2- is the deprotonated salicylaldimine of l-alanine (A = al), l-valine (A = va), or l-phenylalanine (A = pa). Electronic and IR spectra as well as metal reduction potentials (E_1/2 ~0.4 V vs SCE) of the complexes are reported. In Et₄N[V₂O₃(l-alsal)₂]·MeCN both the metal atoms have distorted square pyramidal geometry but the metrical differences between the two are considerable. The relative disposition of the two terminal VO groups in the complex is intermediate between cis and trans. The V-O-V angle and V···V distance are 113.2(3)° and 3.067(3) Å, respectively. The two V-O lengths in the V-O-V bridge are very unequal, 1.768(6) and 1.905(7) Å, corresponding to a V^V-O-V^IV description. Valence localization is consistent with the ⁵¹V hyperfine structure of the axial EPR spectra (3d_xy1 ground state) of the whole family of solid complexes: s = 1/2; g|| 1.95, g_⊥ ~1.98, A||~ 180 G, and A_⊥ ~ 67 G at 300 K. The spectra in frozen (77 K) dichloromethane solution are essentially the same. On the other hand, isotropic room temperature solution spectra of the family have 15 hyperfine lines (g_iso ~1.97; A_iso ~51 G) revealing that the unpaired electron and hence the metal valence are delocalized over both metal atoms on the X-band EPR time scale. The present results are compared with those of the few other V₂O₃³⁺ complexes known revealing certain patterns. Crystal data for the Et₄N[V₂O₃(l-alsal)₂]·MeCN complex are as follows: chemical formula, C₃₀H₄₁N₄O₉V₂; crystal system, orthorhombic; space group, P2₁2₁2₁; a = 9.797(5) Å, b = 12.854(6) Å, c = 27.550(11) Å; Z = 4.