Structural Characterization of [VO(salicylhydroximate)(CH30H)] 3: Applications to the Biological Chemistry of Vanadium(V)

The biological chemistry of vanadium has garnered increased attention due to the recent isolation of vanadium enzymes capable of nitrogenase [l] and bromoperoxidase [2,3] activities. The latter enzymes, isolated from marine algae, contain a mononuclear V(V) active site [4] that does not appear to require redox chemistry in the catalytic mechanism. It is well established that bacteria and algae often produce low molecular weight chelating agents (siderophores) which will sequester metal ions and facilitate the transport of these nutrients into the cell [5]. The two major classes of siderophores are based on catecholate and hydroxamate metal ligands [6]. Because vanadium bromoperoxidase is isolated from algae and bacteria, we felt that it would be interesting to examine the coordination chemistry of vanadium with hydroxamate ligands that might form the basis of vanadium sequestering agents for these organisms. In the process of our studies, we have isolated an intriguing trinuclear cluster composed of V03+ and salicylhydroxamic acid and report herein the structure and solution chemistry of this molecule. [VO(salicylhydroximate)(CHsOH)] a (1) can be prepared in 70% yield by the room temperature reaction of one equivalent of VO(acetylacetonate)z or VCla with salicylhydroxamic acid and three equivalents of NaOCHs or NaOH in methanol. Air acts as the oxidant to form deep blue solutions of 1. Slow evaporation of a methanol solution gave deep blue blocks which were suitable for X-ray crystallographic analysis. Crystallographic data for 1: monoclinic, space group P2Jc, a = 11.006(3), b = 15.722(7), c = 2 1.240(6) A; fi = 111.40(2)‘; v = 3422(2) A3, Z = 4; l(Mo Kor) =0.7107 A; crystal dimensions 0.42 X 0.49 X 0.52 mm3; p = 8.19 cm-l. The intensities of 4504 reflections were measured at room temperature (0 G26 < 453 on a P2r diffractometer using MO Ka radiation. The structure was solved using the SHELX-86 program. All nonhydrogen atoms were refined using anisotropic thermal parameters. Hydrogen atoms were located, but not refined, and placed at fixed distances (0.95 A) from bonded carbon atoms. All calculations were carried out using the SHELX-86 program. For Fig. 1. An ORTEP diagram of 1 with thermal ellipsoids at 30% probability. Selected mean bond lengths (A) and angles (“) for chemically equivalent bonds with range in parentheses: V=O, 1.59 (1.582-1.602); V-O,, 1.85 (1.8421.861); v-o,, 2.08 (2.061-2.096); V-O,., 2.12 (2.0902.143); V-On, 1.86 (1.858-1.868); V-N, 2.02 (2.0142.030); On-N, 1.37 (1.362-1.376); Cn-N, 1.33 (1.3181.339); Cn-0, 1.26 (1.260-1.275); V-V, 4.66 (4.6524.671); O=V-0,, 91 (88.1-92.7); O=V-O,, 108 (107.5108.6); O=V-On, 94 (93.8-94.7); O=V-N, 98 (97.098.1); O=V-0,, 165 (163.1-166.7); V-N-On, 120 (119.5-120.6); N-0,-V, 120 (119.4-120.2).