A Family of α-Amino Acid Salicylaldiminates Incorporating the Binuclear V2O33+ Core: Electrosynthesis, Structure, and Metal Valence
journal contributionposted on 01.01.1997, 00:00 by Sujit Mondal, Prasanta Ghosh, Animesh Chakravorty
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-Asal2- 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 (E1/2 ∼ 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 VV−O−VIV description. Valence localization is consistent with the 51V hyperfine structure of the axial EPR spectra (3dxy1 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 (giso ∼ 1.97; Aiso ∼ 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.