A series of oxoperoxovanadium(V) complexes (ligands:  H3nta = nitrilotriacetic acid, H3heida = N-(2-hydroxyethyl)iminodiacetic acid, H2ada = N-(2-amidomethyl)iminodiacetic acid, Hbpg = N,N-bis(2-pyridylmethyl)glycine, and tpa = N,N,N-tris(2-pyridylmethyl)amine) were characterized as functional models for the vanadium haloperoxidase enzymes. The crystal structures of K[VO(O2)Hheida], K[VO(O2)ada], [VO(O2)bpg], and H[VO(O2)bpg]2(ClO4) were obtained. These complexes all possess a distorted pentagonal bipyramidal coordination sphere containing a side-on bound peroxide. In the presence of sufficient acid equivalents these complexes catalyze the two-electron oxidation of bromide or iodide by peroxide. Halogenation of an organic substrate was demonstrated by following the visible conversion of Phenol Red to Bromophenol Blue. In the absence of substrate, dioxygen can be generated by the halide-assisted disproportionation of hydrogen peroxide. In addition, some of these complexes can efficiently catalyze the peroxidative halogenation reaction, performing multiple turnovers in minutes. The kinetic analysis of the halide oxidation reaction indicates a mechanism which is first order in protonated peroxovanadium complex and halide. The bimolecular rate constants for both bromide and iodide oxidation were determined, with the iodide rates being approximately 5−6 times faster than the bromide rates. The rate constants obtained for bromide oxidation range from a maximum of 280 M-1 s-1 for the Hheida complex to a minimum of 21 M-1 s-1 for the Hbpg complex. The pK a of activation for each complex in acetonitrile was determined to range from 5.4 to 6.0. On the basis of the chemistry observed for these model compounds, a mechanism of halide oxidation and a detailed catalytic cycle are proposed for the vanadium haloperoxidase enzyme.

The dioxovanadium(V) complexes VO2(bpg) (1), [VO2(pmida)]- (2), and [VO2(ada)]- (3) have been synthesized and characterized as models for the vanadium haloperoxidases. These compounds react with hydrogen peroxide in acetonitrile to form the corresponding peroxovanadium(V) complexes that have been previously studied by stopped-flow spectrophotometry. 1H and 51V NMR spectra of the VO2 + complexes in aqueous solution provide a clear picture of the solution structure of each complex. The results of these kinetic studies suggest an associative mechanism in which peroxide binds to a protonated form of the vanadium complex, followed by loss of a bound hydroxide or water molecule in the rate-determining step of the reaction and rapid rearrangement to the final product. The addition of acid to the reaction mixture results in rapid increases in the rate of peroxide binding by vanadium as a result of increased protonation of the complex. As in previous studies of similar reactions in aqueous solution, the reaction is first order in [H+] for substoichiometric amounts of acid, but when acid is present in excess, the dependence on [H+] becomes more complex, implicating the presence of hydroxide- and water-ligated intermediates. Under conditions in which no acid is added to the reaction mixture, the rate constants for formation of the peroxovanadium complex from the vanadium−peroxide adduct are 0.12 ± 0.04 s-1 for 1, 0.33 ± 0.03 s-1 for 2, and 0.29 ± 0.06 s-1 for 3. The implications of this study with respect to catalysis by the vanadium-dependent haloperoxidase enzymes are discussed.

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The complexes [VO(H(2)O)ada] (1), [VO(H(2)O)Hheida] (2), and [VO(H(2)O)aeida] (3) (H(2)ada, N-(carbamoylmethyl)iminodiacetic acid; H(3)heida, N-(2-hydroxyethyl)iminodiacetic acid; H(2)aeida, N-(2-aminoethyl)iminodiacetic acid) were synthesized and crystallographically characterized. Crystallographic parameters for 1.2H(2)O: monoclinic, space group P2(1)/c (No. 14), a = 7.327(2) Å, b = 23.386(7) Å, c = 7.258(3) Å, alpha = 90 degrees, beta = 110.95(2) degrees, gamma = 90 degrees, V = 1204.6(7) Å(3), Z = 4, R1 = 0.0353, and wR(2)() = 0.0848. Crystallographic parameters for 2.H(2)O: orthorhombic, space group Pbca (No. 61), a = 10.512(2) Å, b = 11.727(2) Å, c = 16.719(5) Å, alpha = 90 degrees, beta = 90 degrees, gamma = 90 degrees, V = 2060.6(8) Å(3), Z = 8, R1 = 0.0297, and wR(2)() = 0.0758. Crystallographic parameters for 3: monoclinic, space group P2(1)/c (No. 14), a = 6.785(1) Å, b = 9.714(2) Å, c = 14.959(2) Å, alpha = 90 degrees, beta = 95.12(1) degrees, gamma = 90 degrees, V = 982.2(3) Å(3), Z = 4, R1 = 0.0298, and wR(2)() = 0.0762. In each structure, the tetradentate ligand is disposed so that the tertiary nitrogen is bound trans to the vanadyl oxo, and the rest of the donors occupy equatorial coordination positions. In solution, the structural integrity of these compounds is maintained as observed by UV/visible and EPR spectroscopies, and axial ligation by nitrogen is inferred on the basis of ESEEM spectroscopy. The implications of this study with respect to understanding the coordination environment of VO(2+) in the reduced, inactive form of vanadium bromoperoxidase (VBrPO) are discussed, and it is proposed that significant changes in the coordination environment of vanadium in VBrPO occur upon its reduction, which may provide a plausible explanation for its irreversible inactivation.

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ESEEM (electron spin echo envelope modulation) spectra of a series of vanadyl complexes have for the first time revealed the spectral features that arise from amine nitrogen donors trans to the vanadyl oxo moiety. The complexes [VO(H2O)ada] (1) and [VO(H2O)Hheida] (2) (H2ada, N-(2-acetamido)iminodiacetic acid; H3heida, N-(2-hydroxyethyl)iminodiacetic acid) have recently been synthesized and crystallographically characterized (Hamstra, B. J.; Houseman, A. L. P.; Colpas, G. J.; Kampf, J. W.; LoBrutto, R.; Frasch, W. D.; Pecoraro, V. L. Inorg. Chem. 1997, 36, 4866−4874). In each structure, the ligand is disposed so that the tertiary nitrogen is bound trans to the vanadyl oxo. In solution, the structural integrity of these compounds is maintained as observed by UV/vis and EPR spectroscopies. ESEEM spectra of 1, 2, and [VO(H2O)nta]- (3) (H3nta, nitrilotriacetic acid) reveal that the superhyperfine coupling constants for axial amines are ≅1.3 MHz as opposed to the ≅5.0 MHz superhyperfine couplings typically observed for equatorially coordinated amines. The data indicate that not only can axial nitrogen donors to VO2+ be detected, but they may also be distinguished from nitrogen donors in the equatorial plane of the vanadyl ion. The implications of this study with respect to understanding the coordination environment of VO2+ in the vanadyl-substituted chloroplast F1-ATPase (CF1) and the reduced, inactive form of vanadium bromoperoxidase (VBrPO) are discussed.

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The technique of 51V electron spin echo−electron nuclear double resonance (ESE-ENDOR) has been used to investigate a structurally diverse series of oxovanadium(IV) complexes. From ENDOR spectra, 51V nuclear quadrupole coupling constants, which reflect the electric field gradient established at the vanadium nucleus, are estimated using a perturbation theory approach. The magnitude of the nuclear quadrupole interaction is reduced by ligands binding trans to the oxygen of the oxovandium moiety due to a reduction of the electric field gradient that is established primarily by the oxo bond. Variation of ligands containing nitrogen and oxygen donor atoms oriented cis to the oxo bond gives rise to minimal changes in the quadrupole coupling constant. The nuclear quadrupole coupling constant is thus shown to be sensitive to ligands binding trans to the oxygen of the oxo bond. ENDOR can thusly serve as a new spectroscopic probe in the identification of weakly coordinating donors in the axial position such as may be found in vanadyl-substituted proteins.