MODELING VANADIUM BROMOPEROXIDASE - SYNTHESIS, STRUCTURE, AND SPECTRAL PROPERTIES OF VANADIUM(IV) COMPLEXES WITH COORDINATED IMIDAZOLE

Abstract

A series of vanadium(IV)-SALIMH complexes [HSALIMH = 4-(2-(salicylideneamino)ethyl)imidazole] have been synthesized and characterized by X-ray crystallography, electron paramagnetic resonance, infrared, and visible spectroscopy, and mass spectrometry. The compounds VO(SALIMH)ACAC·MeOH (1), VO(SALIMH)SAL·MeOH (SAL is the anion of salicyl-aldehyde) (2), and VO(SALIMH)2·3EtOH (3) represent the first fully characterized vanadium(IV) compounds with imidazole in their coordination sphere. Compounds 1-3 react reversibly with acid to form complexes in which either the bound imidazole has been protonated (1′ and 3′) or the bidentate ligand has been protonated and subsequently displaced by solvent (4). Addition of excess acid yields the solvated vanadyl ion VO(SOL)52+ (5) (SOL = solvent). Protonation and subsequent ligand displacement by solvent induce an increase in the EPR hyperfine coupling constants A∥ and A⊥. The implications of this work toward understanding the reduced catalytically inactive form of vanadium bromoperoxidase are discussed. X-ray parameters are as follows: 1, C18H23N3O5V1, 412.34 g/mol, crystal system, orthorhombic (Pbca), a = 20.673 (4) A, b = 21.335 (4) A, c = 9.078 (1) A, V = 4404 (1) A3, Z = 8, 2629 data collected with 5° < 2θ < 45°, 1807 data collected with Fo > 6σ(F), R = 0.039, Rw = 0.038; 2, C20H21N3O5V1, 434.34 g/mol, crystal system, orthorhombic (Pbca), a = 12.336 (5) A, b = 14.928 (6) A, c = 22.03 (1) A, V = 4067 (3) A3, Z = 8, 3588 data collected with 5° < 2θ < 50°, 2723 data collected with Fo > 2.0σ(F), R = 0.0799, Rw = 0.0479; 3, C30H42N6O6V1, 633.64 g/mol, crystal system, monoclinic (P21/c), a = 13.763 (7) A, b = 17.589 (4) A, c = 14.701 (2) A, β = 108.97 (2)°, V= 3365 (1) A3, Z = 4,4398 data collected with 5° < 2θ < 45°, 3059 data collected with Fo > 5σ(F) R = 0.0697, Rw = 0.0949.