A Family of α-Amino Acid Salicylaldiminates Incorporating the Binuclear V2O33+Core: Electrosynthesis, Structure, and Metal Valence

The green title complexes of the type Et 4 N[V 2 O 3 ( l -Asal) 2 ] have been synthesized in nearly quantitative yields by one-electron electroreduction (at 0.2 V vs SCE) of V 2 O 3 ( 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 Et 4 N[V 2 O 3 ( 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 V O 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 51 V hyperfine structure of the axial EPR spectra (3d x y 1 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 V 2 O 3 3+ complexes known revealing certain patterns. Crystal data for the Et 4 N[V 2 O 3 ( l -alsal) 2 ]·MeCN complex are as follows: chemical formula, C 30 H 41 N 4 O 9 V 2; crystal system, orthorhombic; space group, P 2 1 2 1 2 1; a = 9.797(5) Å, b = 12.854(6) Å, c = 27.550(11) Å; Z = 4.