IMPLICATIONS FOR THE SPECTROSCOPIC ASSIGNMENT OF VANADIUM BIOMOLECULES - STRUCTURAL AND SPECTROSCOPIC CHARACTERIZATION OF MONOOXOVANADIUM(V) COMPLEXES CONTAINING CATECHOLATE AND HYDROXIMATE BASED NONINNOCENT LIGANDS

Forty-one compounds of the general formula V(V)OLL', where L is a tridentate ligand (HSALIMH = [4-(2-salicylideneaminato)ethyl]imidazole; H2SHED = N-(salicylideneaminato)-N'-(2-hydroxyethyl)ethylenediamine; HENSAL = N-(salicylideneaminato)ethylenediamine) and L' is a bidentate, noninnocent ligand (e.g., catechol (H-CAT), pyrogallol (H2PYR), or salicylhydroxamic acid (H3SHI)) have been prepared and spectroscopically characterized. Three of these novel complexes have been structurally characterized by X-ray crystallography [V(V)O(HSHED)(CAT), 1, V(V)O(SALIMH)(CAT), 3, V(V)O(HSHED)(SHI), 35] which allows for a direct comparison of the coordination environments of (V(IV)O)2+ and (V(V)O)3+ with nearly identical ligand sets. The complexes VO(SALIMH)L' provide a direct test for a model for the enzyme vanadium bromoperoxidase that has coordinated imidazole, a single, terminal oxo ligand and oxygen donors that may be part of a noninnocent ligand. Furthermore, one can directly ascertain the effects of substituting primary or secondary amines for imidazole in an isostructural environment by comparing the properties of VO(HSHED)L' and VO(ENSAL)L'. Previously reported V(O)O3N2 compounds have shown chemical shifts in a narrow range between 300 and 600 ppm upfield of VOCl3. In contrast, these V(V)OLL' complexes cover nearly 1200 ppm and can have resonances as much as 600 ppm downfield of VOCl3. This unprecedented shift range and location is a direct consequence of the coordinated noninnocent ligands. The observed chemical shifts linearly correlate with the inverse energies of the ligand-to-metal charge-transfer bands in the visible and near infrared spectrum, and, therefore, a modification of Ramsey theory has been applied to extract the absolute shielding of these compounds. By extension, the absolute shielding of vanadium in the bromoperoxidase and vanadium transferrin can be inferred. This shielding scale indicates that for vanadium bromoperoxidase the temperature independent shielding factor sigma(p) is very small relative to most V(V) complexes implying that noninnocent ligands which have a large sigma(p) are almost certainly not involved in the coordination sphere of the metal in this enzyme. Crystal data: 1, monoclinic, P2(1)/n, a = 13.660 (9) angstrom; b = 6.483 (4) angstrom; c = 19.18 (1) angstrom; beta = 96.69 (6)-degrees; V = 1681 (2) angstrom3; Z = 4. For 3527 data collected between 5 less-than-or-equal-to 2theta less-than-or-equal-to 50-degrees and 2569 data > 0.6sigma-(F) the structure refined to R1 = 0.063 (R2 = 0.059); 3, triclinic, P1BAR, a = 6.773 (1) angstrom, b = 10.717 (2) angstrom; c = 12.150 (4) angstrom; alpha = 98.40 (2)-degrees; beta = 92.13 (2)-degrees; gamma = 94.05 (2)-degrees; V = 862.8 (4) angstrom3, Z = 2. For 3427 data collected between 5 less-than-or-equal-to 2theta less-than-or-equal-to 50-degrees and 2860 data > 0.6sigma(I) the structure refined to R1 = 0.042 (R2 = 0.057); 35, monoclinic, P2(1)/n, a = 10.569 (3) angstrom; b = 11.698 (4) angstrom; c = 15.498 (4) angstrom; beta = 98.85 (2)-degrees; V = 1882.6 (8) angstrom3, Z = 4. For 3840 data collected between 5 less-than-or-equal-to 2theta less-than-or-equal-to 50-degrees and 2764 data > 0.6sigma(F) the structure refined to R1 = 0.068 (R2 = 0.058).