Elucidating the Protonation Site of Vanadium Peroxide Complexes and the Implications for Biomimetic Catalysis

Abstract

Coordination complexes of vanadium(5+) played a key role in understanding the structure and mechanism of vanadium-dependent haloperoxidases, particularly the effects of protonation on peroxide coordination to dioxovanadium(5+) species, and in the activation of the peroxo-oxovanadium(5+) complex for substrate oxidation. There has been no spectroscopic evidence that could test the presence of a hydroxo intermediate in a catalytically active oxovanadium(5+) complex. Herein we report the use of the pre-edge transition in X-ray absorption spectroscopy as a spectroscopic signature for V=O bonding. Displacement of oxo donors with hydrogen peroxide or chloride donors dramatically decreases the pre-edge intensity, confirming that the source of the intense pre-edge feature is closely related to the -bonding associated with the V=O. Protonation of a catalytically active tripodal amine oxovanadium(5+) complex has no affect on the pre-edge intensity and, therefore, rules out the possibility of a hydroxo intermediate in the catalytic cycle.

Salophen and salen oxo vanadium complexes as catalysts of sulfides oxidation with H2O2: Mechanistic insights

“…It is interesting to notice that the calculated pKa compare reasonably with the values of 5.4-5.9 observed for the vanadium-dependent haloperoxidases enzymes [50,51]. Nevertheless, deprotonated peroxides such as LV V (O)(OO) − are known to be poorly active in oxygen transfer [45,46,49], therefore, even if they could actually be formed in solution according to the pKa values discussed above, they are not the reasonable catalytically active species in the present system. Therefore, the presence of the LV V (O)(OOH) peroxo complexes in solution is the most reliable scenario.…”

“…Indeed, LV V (OH)(OO) species are higher in energy by 12.5 and 4.1 kcal mol −1 for salen and salophen, respectively, while a H-LV V (O)(OO) derivative (where the proton is located at the oxygen atom of the L ligand) is 2.2-8.7 kcal mol −1 higher when L = salophen (the same structure with L = salen failed to converge). Protonation of the peroxide species, with no involvement of the oxo group, was recently demonstrated by XANES spectroscopy for a [HheidaV(O)(OO)] − complex (heida N-(2-hydroxyethyl)iminodiacetic acid) to be the key step to activate the species for oxygen transfer catalysis [49].…”

A density functional theory investigation of the bromide oxidation mechanism by a vanadium bromoperoxidase model complex

“…Experimental and theoretical data suggest that the vanadium(V) oxo complex, as an active catalyst, has a structure containing one peroxide unit and one Schiff base ligand [3,4,8,15,17]. It is not known whether the ligand containing one peroxide unit is of a hydroperoxo (OOH) or peroxo (O 2 ) nature.…”

“…2) leads to a significant polarisation of the O 2 -O 3 bond (Dq = 0.11) in comparison with the unprotonated complex 1 (Dq = 0.02, Table 1). The peroxide unit in the active catalyst is a requirement for the oxidation process [17]. This group acts in the catalytic reactions by a protonation process [8,15].…”

“…Some computational investigations of VHPo and VHPo model complexes have been reported [15][16][17][18][19]. In this work, we conducted a computational study with complex 1 as a particular case of the catalytic cycle mechanism (Scheme 1).…”