VANADIUM COMPLEXES OF THE TRIDENTATE SCHIFF-BASE LIGAND N-SALICYLIDENE-N'-(2-HYDROXYETHYL)ETHYLENEDIAMINE - ACID-BASE AND REDOX CONVERSION BETWEEN VANADIUM(IV) AND VANADIUM(V) IMINO PHENOLATES

Abstract

The reaction of vanadyl sulfate or vanadyl acetonylacetate (VO(ACAC)2) with N-salicylidene-N′-(2-hydroxyethel)ethylenediamine (H2SHED) affords a wide range of vanadium(IV) and vanadium(V) complexes. The structure of VIVO(HSHED)(ACAC) was determined by an X-ray analysis of red crystals having the following crystallographic parameters: space group Pbca (orthorhombic); a = 11,607 (5), b = 25.960 (2), c = 11.701 (5) A; V = 3526 (2) A3; Z = 8. The final R indices were R = 0.051 and Rw = 0.040 for 1485 observed data. The vanadium is six-coordinate with the phenolate, imine, and amine donors of HSHED- oriented in a meridional geometry. The hydroxyl oxygen remains unbound. The ACAC moiety is a bidentate chelate with one oxygen atom (V1-O5, 2.194 (5) A) trans to the short V=O bond (V1-O1, 1.596 (4) A). When the complex is dissolved in methanol, ACAC dissociates from a small fraction of VIVO(HSHED)(ACAC), yielding air-sensitive VIVO(SHED) in which an alkoxide oxygen atom of the ligand has been inserted in the coordination sphere. If this green material is left in air for several days, orange blocks and yellow plates are deposited. X-ray analysis of both crystal types reveals that the two materials are weakly associated solid-state dimers of the pervanadyl-HSHED- complex VO2(HSHED), which result from different hydrogen bonding in the crystals. Once again, HSHED- is a meridional, tridentate chelate with amine, imine, and phenolate ligation. The orange material shows a vanadium-vanadium separation of 3.103 (1) A and a long V1-O1′ distance of 2.298 (2) A. In contrast, the yellow crystals reveal a more weakly associated structure with V1-V1′ of 3.251 (1) A and V1-O1′ equal to 2.455 (2) A. Acidification of an acetonitrile solution of either the yellow or orange compounds results in the formation of a stable oxo-hydroxo-VVHSHED complex, which is red. Further addition of acid generates the deep blue, highly unstable monoxo-VVHSHED complex. A comparison to related V(IV) and V(V) complexes and the potential biological relevance are discussed. Crystallographic parameters for [VO2(HSHED)]2 (yellow, 1): space group P21/c (monoclinic); α = 11.246 (3), b = 6.842 (2), c = 17.022 (5) A: β = 114.38 (6)°; V = 1193.0 (6) A3; Z = 4. The final R indices were R = 0.027 and Rw = 0.027 for 1729 observed data. Crystallographic parameters for [VO2(HSHED)]2 (orange, 2): space group P21/c (monoclinic); a = 10.249 (2), b = 10.999 (2), c = 10.578 (2) A; β = 119.16 (4)°; V = 1191.6 (4) A3; Z = 4. The final R indices were R = 0.029 and Rw = 0.029 for 1265 observed data.