Synthesis, characterization, DFT calculations and bromoperoxidase activity of binuclear oxidovanadium complexes containing vitamin B6

Abstract Two binucelar oxidovanadium complexes, [(VO2)2(µ-Hpydn)2] (1) and [(VO)2(µ-pydn)2(bpy)2]·5H2O (2), where pydn = pyridoxine and bpy = 2,2’-bipyridine, were synthesized and characterized by elemental analysis and infrared (IR), electron paramagnetic resonance (EPR), and electronic (UV-vis) spectroscopies. The structure of 2 was determined by single-crystal X-ray diffraction analysis. It is comprised of two distorted octahedral oxidovanadium(IV) centers with a terminal bpy ligand and bridged by the methoxido group of the pydn ligands, forming the {VIVO(µ-OR)2}2+ core with anti-orthogonal configuration. Density functional theory (DFT) calculations at the B3LYP/LANL2DZ level were used to obtain the optimized geometries and to study the electronic structure of 1 and 2. For 2, EPR analysis in the solid state, magnetic susceptibility measurements and the theoretical magnetic exchange coupling constant (J) of 20.1 cm−1 indicate a weak ferromagnetic coupling. 51V NMR and EPR spectroscopies suggest partial breakage of both binuclear structures in solution. Both complexes act as pre-catalysts for the bromination of phenol red to phenol blue in the presence of KBr, H2O2 and perchloric acid. Graphical Abstract


Introduction
Discovery of the biological activity of vanadium compounds has boosted an intense development in the bioinorganic chemistry of this element. Vanadium compounds may have the metal in oxidation states ranging from þ II to þ V, but vanadium(IV) and (V) predominate in vivo under physiological conditions [1]. In the highest oxidation state, the element acts as catalyst for the oxidation of organic substrates and interacts with a variety of biological targets, many of them of medical interest [2,3]. For instance, voluminous reports are available for the synthesis, characterization and biological activity of vanadium-based coordination compounds with small blood serum bioligands as lactate and citrate [4], oxalate [5] and phosphate ions [6,7], amino acids [8,9], vitamins [10,11] and their derivatives.
Pyridoxine is the most stable form of vitamin B6 [12], also found in its pre-active form as pyridoxal and pyridoxamine. In the digestive tract, pyridoxine is converted into its phosphorylated active form, pyridoxal 5 0 -phosphate, and functions as a coenzyme to nearly 60 enzymes, most of them involved in the metabolism of proteins and amino acids [13]. One of the final products synthesized with the aid of vitamin B6 is acetyl coenzyme A, important in energy production and synthesis of proteins, lipids, and acetylcholine [14]. Neither pyridoxine nor any other form of vitamin B6 is naturally produced by animals, meaning that their only source of the vitamin is through dietary intake [15].
Vanadium complexes mostly portray Schiff base derivatives of vitamin B6, generally condensed with amino-acid [16], amine [17,18], or hydrazide moieties [19][20][21][22]. These complexes have been studied for a variety of purposes, including insulin-enhancing activity [17], antitumor applications [23], and in mimicking vanadium-dependent bromoperoxidases [19,20,24]. In particular, [V V O 2 (sox-pydx)] À , where soxH-pydxH was obtained through condensation of oxamohydrazide and pyridoxal, was active in the bromination of benzene and salicylaldehyde, and exceptionally efficient in the bromination of phenol red to phenol blue [19,20]. Moreover, a binuclear vanadium(V) complex with a Schiff base ligand made up of pyridoxal and succinohydrazide has been described [22]. In contrast, studies employing forms of vitamin B6 itself as direct ligands to vanadium are less explored. The few reports focus on potentiometric [25], thermogravimetric and spectroscopic investigations [26]. One speciation study carried out in solution of oxidovanadium(2þ) (VO 2þ ) complexes containing pyridoxine, pyridoxal, and pyridoxamine as ligands pointed out the formation of mononuclear complexes with the 2:1 ligand-to-metal ratio, with the ligands chelating the octahedral metal center in a bidentate manner [25]. Another study reported a vanadium(III) complex bearing octahedral geometry with one chelating pyridoxine moiety [26]. Interestingly, different from what is observed in the present work, the great majority of vanadium complexesin oxidation states from þ III to þ V, containing vitamin B6 itself or its Schiff base derivatives as ligandsare found as mononuclear compounds.
The present study involves the synthesis and characterization of two binuclear vanadium complexes with f(V V O 2 )(m-OR)g 2 2À and fV IV O(m-OR)g 2 2þ cores comprising the 4-methoxido arm of pyridoxine (pydn) as bridging ligand. Crystallographic data for [(VO 2 ) 2 (m-Hpydn) 2 ], hereinafter named complex 1, has already been reported [27]; nonetheless, this work describes a new methodology to obtain 1 in good yield and its further characterization by elemental analysis and spectroscopic methods. To our knowledge, this is the first study reporting the synthesis and characterization of a complex containing pyridoxine and 2,2 0 -bipyridine (bpy) as co-ligand, [(VO) 2 (mpydn) 2 [28].
Elemental analysis (C, H, and N) was performed using a Perkin Elmer 2400 Series elemental analyzer. Infrared (IR) spectra were recorded from KBr pellets on a FTIR MB-BOMEN spectrophotometer, from 400 to 4000 cm À1 , employing spectral resolution of 4.0 cm À1 . Electronic spectra were recorded in the solid state and in solution on a Perkin Elmer LAMBDA 1050 UV/Vis/NIR spectrophotometer equipped with a PMT/ InGaAs/PbS three-detector setup. Solution and powder electron paramagnetic resonance (EPR) spectra were recorded at room temperature and 77 K on an X-band (9.5 GHz) Bruker EMX-Micro spectrometer. Spectra were simulated using the EasySpin package [29] for the Matlab software. The 51 V NMR spectrum was registered using a Bruker 400 MHz Avance III spectrometer (9.4 T) equipped with a multinuclear direct detection probe (5 mm), calibrated 90 pulses, 51200 scans, recycling delay of 0.100 s, acquisition times of 0.218 s, and spectral width of 714 ppm. The 51 V nucleus was detected at 105.2 MHz, and spectral intensities were normalized by comparison with the reference VOCl 3 (neat, capillary) signal (0.00 ppm). Magnetic susceptibilities in the solid state at 297 K were measured using a modified Gouy method [30,31] with a Johnson-Matthey MKII magnetic susceptibility balance. (NH 4 ) 2 Fe(SO 4 ) 2 Á12H 2 O and ultrapure water were used as calibration standards. Pascal constants were employed to correct the diamagnetism of the ligands [32] (v DIA for C 36 H 42 N 6 O 12 V 2 ¼ À505.5 Â 10 À6 cm 3 mol À1 ). Thermogravimetric (TG) analyses were performed on a TGA Q500 TA instrument. Samples were heated in aluminum pans from 25 to 900 C at a heating rate of 10 C min À1 using N 2 /O 2 as carrier gas.

Synthesis of [(VO 2 ) 2 (m-Hpydn) 2 ] (1)
A suspension of V 2 O 5 (0.455 g, 2.50 mmol) in 50 mL of an aqueous solution of tris (0.363 g, 3.00 mmol) was stirred at room temperature for 48 h to give an orange solution that received the slow addition of an aqueous solution of pyridoxine hydrochloride (0.618 g, 3.00 mmol) in 10 mL of water. The reaction mixture was then allowed to stand, producing pale greenish-yellow crystals in a few minutes. The crystals were filtered off, washed with a small amount of water and dried in air. The product was soluble in dmf. Yield 0.703 g, 56% based on vanadium. Anal calcd. for C 16

Synthesis of [(VO) 2 (m-pydn) 2 (bpy) 2 ]Á5H 2 O (2)
A mixture of 2,2 0 -bipyridine (0.156 g, 1.00 mmol) and pyridoxine hydrochloride (0.206 g, 1.00 mmol) in 20.0 mL of a 50% aqueous methanol solution received the addition of triethylamine (419 mL, 3.00 mmol) and was heated to 60 C. After the addition of solid VOSO 4 ÁxH 2 O (0.168 g, 1.00 mmol) under continuous stirring, the solution became reddish-brown after 30 min. Crystals of the same color were obtained after keeping the solution at room temperature for 48 h. The crystals were isolated by filtration, washed with 10 mL of cold water and dried in air. The product was soluble in dmf, dmso, and in the alcohols methanol, ethanol and propan-2-ol. Yield 0.275 g, 65% based on vanadium. Anal calcd. for C 36

2.4.
Single-crystal X-ray diffraction analysis (SC-XRD) of 2 Data were collected on a Bruker D8 Venture diffractometer equipped with a Photon 100 CMOS area detector, Mo-Ka radiation (0.71073 Å), a graphite monochromator and temperature control system Cryoflex II. A suitable crystal of 2 was mounted on a MicroMount TM /mesh and examined at room temperature (298 K). Data were processed using the APEX3 program [33]. The structure was determined by the intrinsic phasing methods in the SHELXT program [34] and refined by full-matrix least-squares methods, on F 2 's, in SHELXL-2018/3 [34]. Computer programs used were run through WinGX [35]. Scattering factors for neutral atoms were taken from reference [36]. The nonhydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms attached to carbon atoms were included in idealized positions with U(iso)'s set at 1.2 U(eq) or, for the methyl group hydrogen atoms, 1.5 U(eq) of the parent carbon atoms; the hydrogen atoms of the hydroxyl moiety and water molecules of crystallization were located in the difference map and were refined freely using isotropic displacement, except for H1A and H1B, for which DFIX distance restraints were applied. Drawings were made with ORTEP-3 [35], Mercury [37] or Diamond [38]. Table 1 depicts the main collection and refinement data for 2.

Hirshfeld surface and 2D fingerprint plot analyses
The Hirshfeld surfaces (HS) were computed using the Crystal Explorer 21.5 software [39]. HS and 2D fingerprint plots drawn using d norm (normalized distance), with the nearest nucleus internal (di) and external (de) distances to the surface [40], were used to explore the intermolecular interactions in the crystal packing of 1 and 2.

DFT calculations
All DFT calculations were carried out with the corrected hybrid density functional B3LYP [41] and the LANL2DZ basis set [42] available in the Gaussian16 software package [43]. The optimized geometries of both vanadium complexes were calculated in vacuum using the crystallographic coordinates as starting point. For 2, DRX data were used as initial input, omitting the two lattice water molecules. Calculated vibrational spectra show good correlation with those recorded experimentally, and the scale factor of 0.9600 was employed to correct the DFT-calculated frequencies [44]. The absence of imaginary frequencies indicates the optimized geometries in vacuum were successfully obtained. The frontier orbitals were rendered by Chemcraft 1.8 [45]. The broken symmetry (BS) approach [46] was used in Gaussian16 to calculate the magnetic exchange coupling constant (J) involving the two metal ions in 2.

Primary studies of phenol red bromination
The bromination of phenol red to bromophenol blue was evaluated adapting a methodology described elsewhere [47,48].

Synthetic approach
Aiming to explore the coordination chemistry of vitamin B6 with vanadium(V) and vanadium(IV) in aqueous medium, we carried out reactions using NaVO 3 , V 2 O 5 and [VO(acac) 2 ] as starting materials. All attempts to conduct direct reactions at room temperature and/or under reflux produced the binuclear complex [(VO 2 ) 2 (m-Hpydn) 2 ] (1); however, despite all efforts to optimize the reaction conditions, it was isolated in low yields and/or among a mixture of other by-products. The synthesis of 1 was first reported by Sabirov et al. [27] from a reaction of VOSO 4 with pydn in water, but without mentioning the yield or purity of the product and no further characterizations besides SC-XRD were described. Unfortunately, we failed to isolate a pure product using a similar synthetic approach. A synthetic approach that our research group recently explored is based on the solubilization of V 2 O 5 in an aqueous solution of the common buffer tris(hydroxymethyl)aminomethane (tris) at room temperature. The solubility of the oxide in the tris aqueous solution is slow and progressive, being twice more effective than in the ionic liquid choline chloride-urea [49] and 24 times more soluble than in water [50]. The addition of the pydn to the resulting orange solution led to 1 in 56% yield and high Scheme 1. Schematic syntheses of products 1 and 2 purity (Eq. 1 in Scheme 1). The structure of 1 was confirmed by SC-XRD (Supplementary Data, Figures S1 and S2, and Tables S1-S3).
In an attempt to synthesize an oxidovanadium(IV) complex, the reaction of pydn with VOSO 4 was carried out in the presence of bpy as co-ligand and in a binary solvent system consisting of MeOH/water (1:1 ratio) to overcome the poor solubility of bpy in water. The reaction resulted in brownish-red crystals, [(VO) 2 (mpydn) 2 (bpy) 2 ]Á5H 2 O (2), suitable for SC-XRD (Eq. 2). Products 1 and 2 were also analyzed by powder X-ray diffraction. The powder diffractograms showed good correlation with the corresponding simulated single-crystal diffraction data ( Figure S3).  2), with the atom numbering scheme and thermal ellipsoids drawn at 50% probability. All hydrogen atoms were omitted for clarity. Symmetry code: i: 1-x, 1-y, 1-z. Differences in intensities and peak positions may be due to the preferred orientation of the crystallites in the powdered samples.

Hirshfeld surface analyses
The Hirshfeld surfaces (HS) (Figure 3a) and 2D fingerprint plots ( Figures S6 and S7) were generated to explore the intermolecular interactions in the crystal packing of 1 and 2. The red spots over the HS indicate the presence of strong hydrogen bonds, white spots represent weak hydrogen bonds, and the blue region displays an area without or with very weak intermolecular contributions on the surface. For 1, the strongest hydrogen bonds (O À HÁÁÁO ¼ V, N À HÁÁÁO ¼ V) occur between two dimers, while for 2, only hydrogen bonds involving the binuclear complex and water molecules (O water ÀHÁÁÁO, C À HÁÁÁO water , and O water ÀHÁÁÁN) were found. In the HS shape index plots, Figure 3b, the red triangular regions over the aromatic rings of the pydn ligands could be correlated with the pÁÁÁp stacking interactions that contribute to the stabilization of the crystal packing. The quantization of the interatomic contacts obtained by the 2D fingerprint plots evidence the presence of long and thin peaks in the lower left side of each graph, indicating the presence of strong interactions as HÁÁÁO/OÁÁÁH for 1 ( Figure S6a) and HÁÁÁO/OÁÁÁH and NÁÁÁH/HÁÁÁN for 2 ( Figure S7a). Moreover, the central region of sky-blue color on the 2D plots for the overall interactions suggests the presence of pÁÁÁp stacking interactions, as observed in similar complexes [59][60][61][62].
The percentages of all contacts on the surface, including reciprocal interactions, are distinct and depicted in Figure 3c. For 1, the HÁÁÁO/OÁÁÁH contacts represent the highest contribution (50%). Other contacts, in order of contribution, are HÁÁÁH, CÁÁÁH/HÁÁÁC,   (Table S2). Average percentage errors [63] for bond lengths and angles of 1.6% and 1.7%, and 1.9% and 1.6%, were respectively found for 1 and 2.  between the two vanadium centers (Figure 4a). While the vanadium(IV) centers in 2 bear an octahedral geometry (Figure 4b), in 1 the vanadium(V) centers present a highly distorted square pyramidal geometry, with a s parameter of 0.40 (experimental) and 0.38 (calculated) [64]. A marked difference between both complexes is the protonation of pydn N pyridyl in 1, whereas in 2 the pyridyl nitrogen remains deprotonated. The reaction mixture that gave 1 started at pH 8 and, after addition of pydn, the pH lowered to 4, a value below the pKa of pydn (5.0) [65,66]. In the case of 2, the reaction started at pH 9 and finished at pH 7, preventing protonation at the pyridine ring.
The experimental and calculated infrared spectra of both complexes show good correlation, the largest difference between the experimental and calculated bands (Tables S5 and S6) occurring at the (OÀH) and (NÀH) modes close to 3500 cm À1 , since the calculations were carried out at the single-molecule level, excluding intermolecular interactions such as hydrogen bonds.

Frontier molecular orbitals and electronic spectra
The calculated contour surfaces for selected frontier orbitals of 1 and 2 are shown in Figure 5. For 1, there is significant contribution of the oxygens of the VO 2þ group for HOMOÀ1 and HOMO (Highest Occupied Molecular Orbital), while the Hpydn ligands and vanadium centers participate in the LUMO (Lowest Unoccupied Molecular Orbital) and LUMOþ1. For 2, d xy from the vanadium(IV) centers constitute the frontier orbitals HOMOÀ3 and HOMO-2, whereas HOMO-1 and HOMO are mainly formed by pydn ligands, and LUMOþ1 is mainly built up of CC-p Ã orbitals from the bpy moiety.
The profiles of the calculated electronic spectra of 1 and 2 are in good agreement with the UV-visible reflectance spectra obtained experimentally ( Figure S10 and Figure 6, respectively), with displacement to lower wavelengths, a common feature in DFT calculations of vanadium complexes (Table 3) [69]. For both, the bands below 350 nm were assigned to p!p Ã transitions of the aromatic rings of pydn, and a more intense band at higher energy is attributed to VÀO phenolate ligand-to-metal chargetransfer transitions (LMCT p(O)!d(V)). Moreover, in the same region, the spectrum of 2 presents bands of the intraligand p!p Ã transitions of bpy, and metal-to-ligand charge-transfer transitions (MLCT d(V)!p Ã bpy ) were predicted at 484 nm. The three dd transitions expected for vanadium(IV) complexes in octahedral environment [19] were predicted at 713, 642 and 450 nm and attributed to d xy !(d xz , d yz ), d xy !d x 2 -y 2 and d xy !d z 2 , respectively. Tables S7 and S8 show the contour surfaces for selected orbitals that participate in the electronic transitions.

Solid-state X-band CW-EPR spectroscopy for 2
The EPR spectrum registered at 77 K of a powdered sample of 2 exhibits features similar to those of other binuclear oxidovanadium(IV) complexes with intramolecular Table 5. Calculated exchange coupling constants (J) obtained by B3LYP/LANL2DZ level theory and structural parameters for binuclear octahedral oxidovanadium(IV) compounds.
Compound Formula   ferromagnetic exchange-coupled d 1 -d 1 systems (V IV O 2þ ÀV IV O 2þ ), consistent with a triplet state S ¼ 1 ( Figure S11). The broad line centered at 3380 G (g ¼ 1.978, D p-p ¼ 234 G, shoulders at 2808 and 4010 G) is probably generated by the allowed DM S ± 1 transition, while the weak broad signal at 1650 G results from the forbidden DM S ± 2 triplet À singlet transition [70]. The value of the axial zero-field splitting parameter (D) of 1723 MHz from the solidsample spectrum agrees well with the crystallographic VÁÁÁV distance of 3.13 Å. The difference between the value of D recorded from the powdered sample and the one found in solution (2020 MHz) is within an acceptable range, since, in the solid state, the movement of the molecules is constrained by packing and, in solution, their movement has more degrees of freedom [70,71].

Magnetic properties of 2
The magnetic behavior of binuclear vanadium(IV) complexes with a fVO(m-OR) 2 VOg 2þ core is sensitive to the geometry, V-O bridging distance, VÁÁÁV distance, and V-O-V and O-V-O angles [72]. In view of the anti-orthogonal configuration of the fVO(m-OMeR) 2 VOg 2þ core, a possible interaction between the single electron of the V IV centers, S ¼ 1 = 2 , was investigated by DFT calculations. The spin densities found for the high-symmetry state (triplet) and for the low-symmetry state (singlet) are shown in Figure 7. Noteworthy, the interaction between the d xy orbital from vanadium atoms and the p x and p y orbitals from the methoxido bridge oxygens are present in both states. Table 4 presents the exchange coupling constant (J) values calculated both from crystallographic and optimized geometry data. The Bleaney-Bowers equation (Eq. 3) was used to calculate the molar susceptibility (v M ) values displayed in Table 4 from the J values obtained from DFT, where N av is Avogadro's number, g is the Land e factor, k B is the Boltzmann constant, l B is the Bohr magneton, T is the temperature and J is the exchange coupling constant. The theoretical v M is close to the experimental value of 0.00239 emu K mol À1 , determined from a powdered sample of 2 using the modified Gouy method at 297 K. The experimental v M along with the DFT calculations suggest a weak ferromagnetic behavior for 2, in good agreement with the discreet decrease in intensity of the EPR signal registered at 77 K as compared to that at room temperature ( Figure S11) [73]. Based on the experimental results, the magnetic properties of 2 were compared to other complexes in an anti-orthogonal configuration with the fVO(m-OMeR) 2 VOg 2þ core [51][52][53][54]. For those complexes, only [Et 3 NH] 2 [(VO) 2 (bbac)] 2 Á4CH 2 Cl 2 [51] displays experimental magnetic studies, presenting a strong antiferromagnetic coupling of À167.9 cm À1 . The data in Table 5 refer to the structural parameters obtained from the crystallographic data, and the exchange coupling constants (J) calculated by the broken-symmetry approach using the B3LYP/LANL2DZ level theory. Despite the few compounds available, there seems to be a tendency between the J values and the structural parameters ( Figure S12

Thermogravimetric analysis (TGA) of 1 and 2
The thermogravimetric profile of 1 ( Figure S13a) presented only one event (determined value ¼ 66.50%; calculated value ¼ 66.97%) associated with the loss of the two pydn. For 2, there were two weight losses ( Figure S13b): the first one represents the loss of five water molecules (determined value ¼ 10.28%; calculated value ¼ 10.34%), and the second one is associated with the loss of the sum of bpy and pydn ligands (determined value ¼ 74.31%; calculated value ¼ 74.28%). Although the single-crystal X-ray diffraction analysis of 2 presented four crystallization water molecules, both elemental analysis and TGA support the formula [(VO) 2 (C 8 H 9 NO 3 ) 2 (C 10 H 8 N 2 ) 2 ]Á5H 2 O, used in the catalytic studies.

Solution stability studies of 1 and 2
The stability of the binuclear oxidovanadium complexes in solution has generally been addressed by EPR and 51 V NMR spectroscopies, which detect V IV and V V ions, respectively. Both complexes showed breakage of the binuclear structure to some extension, revealing an equilibrium between mono and binuclear species. The 51 V NMR spectrum registered for 1 in dmf/D 2 O solution at 1.0 mmol L À1 showed three broad signals at À527, À547 and À574 ppm (Figure 8a). The first, with the highest intensity, falls in the range reported for other oxidovanadium(V) complexes [74]. The other two signals at À547 ppm and À574 ppm could be generated by coordination of solvents to the vacant pentacoordinate vanadium site, or products of the partial breakage of the binuclear structure to give mononuclear species, exemplified by [(VO 2 )(Hpydn)L n ], where L ¼ dmf or water. Such breakage or interconversion between bi-and mononuclear vanadium complexes, especially those containing alkoxide ligands, is quite common in the solution chemistry of vanadium compounds [74][75][76].
The EPR spectrum registered for 2 from a 2.0 mmol L À1 dmso solution at 77 K presented a typical hyperfine pattern of a mononuclear vanadium(IV) species (3d 1 , I ¼ 7/2) superimposed to the broad line generated by the binuclear component (Figure 8b, Table 6). The half-field transition appears at 1640 G, suggesting the maintenance of the spin-spin interaction of the binuclear component in solution. The 63% prevalence of the binuclear species with respect to the mononuclear one evidences partial breakage of the dimeric structure into two equivalent mononuclear species. In addition, the spectrum registered from a 10.0 mmol L À1 dmf solution at 77 K ( Figure S14) presented a typical hyperfine pattern of a mononuclear oxidovanadium(IV) species with g iso ¼ 1.9667 (g jj ¼ 1.9424, g ? ¼ 1.9789) and A iso ¼ 277.5 MHz (A jj ¼ 489.3 MHz, A ? ¼ 171.6 MHz) as shown for the solution in dmso; however, without the presence of the broad central lines of the binuclear species or the half-field transition, indicating full breakdown of the binuclear structure in dmf solution. The EPR parameters of the mononuclear component are quite similar in both solvents, with g jj < g ? < 2.0023 and A jj ) A ? , pointing to an axial symmetry with a d xy ground state, characteristic of a six-coordinate oxidovanadium(IV) species [77], comparable to the results predicted by DFT (see Section 3.4.3).

Primary studies of phenol red bromination
The bromoperoxidase activity of 1 and 2 was assessed by converting phenol red (HPhR) to bromophenol blue (BrPhB), a classical substrate often used to evaluate the activity of vanadium-dependent bromoperoxidases [78] and model complexes [79,80]. The reactive process is shown in Scheme 2 and results in the visible color change of the solution from yellow to blue. The course of the reaction was monitored by electronic absorption spectroscopy, following the decrease of the absorption band at 443 nm and the appearance of a new peak at 592 nm, presenting an isosbestic point at 490 nm [81,82]. Negative control reactions using KBr and H 2 O 2 without the addition of the vanadium complexes and reactions adding freshly prepared solutions of 1 and 2 did not show any conversion to BrPhB. Screening experiments were carried out with 0.5, 1, 2, 4 and 8 equivalents of HClO 4 per mol of the complexes. Optimized conditions were found when one equivalent of HClO 4 was added to the dmf solution of the vanadium complexes. Attempts to use a large excess of acid (4 and 8 equivalents) caused decomposition of the complexes and the catalytic mixture was inactivated. Significant catalytic activity was observed for both complexes at concentrations above 0.4 Â 10 À4 mol L À1 and became ca. 3 times more effective when the highest concentrations were used ( Figure S15 and Figure 9). Similar activities have been found for 1 and 2, possibly related to the structural features of the complexes (Figure 10). The slight advantage observed for 2 may be due to partial breakage in solution to form [VO(bpy)(pydn)] mononuclear complexes, which can form the active peroxidovanadium(V) species upon reaction with H 2 O 2 [81,82]. Despite all the efforts dedicated to attempts to run kinetic studies, we were not capable to get the constant rates given the narrow range of concentrations wherein both complexes were catalytically active in the reaction mixtures.

Conclusion
We have synthesized two binuclear oxidovanadium complexes with vitamin B6 ligands and the vanadium centers in two different oxidation states: [(V V O 2 ) 2 (m-Hpydn) 2 ] (1) and [(V IV O) 2 (m-pydn) 2 (bpy) 2 ]Á5H 2 O (2). The former was obtained in high purity by a soft synthetic route using V 2 O 5 as starting material and in the presence of the common biological buffer tris. Both complexes were extensively characterized in the solid state, and DFT calculations were used to study magnetic and electronic properties. Our results pointed to a mild ferromagnetic interaction between the vanadium(IV) centers of 2, with a J value close to those found for other complexes with analogous antiorthogonal fV IV O(m-OMeR) 2 V IV Og 2þ cores. Spectroscopic studies in solution by 51 V NMR and EPR suggested that the binuclear structure of both complexes suffers total or at least partial breakage in solution to form mononuclear species, depending on the solvent. Finally, similar catalytic activities were found for both compounds, assessed through the oxidative bromination of phenol red to bromophenol blue.

Supplementary information
The supplementary material associated with this article can be found in the online version. Crystallographic data for both products have been deposited as CIF files at the Cambridge Crystallographic Data Center with deposition numbers 2129316 for 1 and 2129317 for 2. The files can be obtained free at http://www.ccdc.cam.ac.uk/structures. Ribeiro for discussions and helpful suggestions. J.M.M., R.C.R.B., G.B.B., F.S.S., D.S., E.L.S and G.G.N. thank CNPq, CAPES-PrInt (Finance Code 001) and pesquisa/PRPPG/UFPR for research grants and scholarships.

Disclosure statement
No potential conflict of interest was reported by the author(s).