{'fraction_non_alphanumeric': 0.0464011691633175, 'fraction_numerical': 0.031640482279868466, 'mean_word_length': 4.321150855365475, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Vanadium( V ) complexes of mandelic acid †

and (NH 4 ) )(( )- mandelato ligand) have been synthesized and characterized by single crystal X-ray diﬀraction and FT-IR spectroscopy. The band assignment in the IR spectra was corroborated by DFT calculations. While the structures of 1–4 comprise the expected dinuclear [V 2 O 4 ( mand ) 2 ] 2 (cid:3) (V 2 L 2 ) anions, the structure of [V 3 O 7 (( R )- mand )(( S )- mand )] 3 (cid:3) (V 3 L 2 ) in 5 is unique and represents a new structural type in vanadium( V ) chemistry. Solution studies of vanadate with mandelic acid by 51 V NMR revealed the presence of two dominant species V 2 L 2 and V 3 L 2 in aqueous solutions and increasing fraction of V 3 L 2 species at slightly acidic pH (pH E 6).


Introduction
Vanadium participates in an essential way in several biochemical processes. This element was found at the active centre of some enzymes, including vanadium dependent nitrogenases or vanadium haloperoxidases which enable halogenation of organic compounds. 1 On the other hand a-hydrocarboxylic acids are also involved in many basic biochemical processes (such as the Krebs cycle, Cori cycle, photorespiration and others). 2 Due to the importance of both vanadates and a-hydroxycarboxylic acids the mutual interaction of these groups of compounds was intensively studied. 3-5 51 V NMR studies revealed complex equilibria involving several species of different nuclearities; however, solid state investigations confirmed with certainty only dinuclear complexes. 6 The dominant components in the vanadate-a-hydroxycarboxylic acid system, apart from common vanadates, are the dinuclear and trinuclear complexes of the composition V 2 L 2 and V 3 L 2 (L = anion of the a-hydroxycarboxylic acid) accompanied by a mononuclear complex VL. 7 Several dinuclear vanadium(V) complexes with a-hydroxycarboxylato ligands were also reported in the solid state: (NH 4 ) 2 [V 2 O 4 (glyc) 2 ], 8 13 and Cs 2 [V 2 O 4 ((S)-lact) 2 ]Á2H 2 O 10 ‡ and a richer series of vanadium complexes with a-hydrocarboxylic acids with more than one carboxyl group such as malic, 14 citric, [15][16][17][18][19][20][21][22][23] homocitric 24 and tartaric acid. [25][26][27][28] The crystal structures confirmed the composition and geometry as were proposed by 51 V NMR spectroscopy; however, the V 3 L 2 complexes do not have a model compound with a solved crystal structure. In this list, mandelato complexes are obviously missing. Mandelic acid is a common chiral aromatic a-hydroxycarboxylic acid (Scheme 1). Isolation of non-peroxido vanadium(V) complexes of mandelic acid was, so far, not possible mainly due to its strong reduction potential towards V V . In this work we augment the family of vanadium(V) complexes of a-hydroxycarboxylic acids by mandelato complexes. We present the synthesis and characterization of dinuclear V 2 L 2 complexes and a trinuclear V 3 L 2 complex which revealed a surprising structure not identical to the previously proposed geometry.

Materials and methods
All chemicals were of analytical grade and used as received. Stock aqueous solutions of (NMe 4 )VO 3 and (NEt 4 )VO 3 (1 mol dm À3 ) were prepared by dissolution of V 2 O 5 in conc. (NMe 4 )OH and (NEt 4 )OH, adjustment of the pH to 8.0 and dilution. Elemental analyses (C, H, and N) were performed on a Vario MIKRO cube (Elementar). The analysis of hydrogen might be influenced by hygroscopic properties of the samples. Vanadium was determined gravimetrically as V 2 O 5 . Solid state IR spectra were recorded on a Thermo Scientific Nicolet 6700 FT IR spectrometer in Nujol mulls, KBr pellets and by the ATR technique. The 51 V NMR spectra were recorded at 278 K on a Varian Mercury Plus 600 MHz spectrometer operating at 157.88 MHz for 51 V in 5 mm tubes. Chemical shifts (d) are given in ppm relative to VOCl 3 as an external standard (d = 0 ppm).

X-ray diffraction
Diffraction data were collected using a Kappa Apex II (Bruker) diffractometer equipped with a Cryostream Cooler (Oxford Cryosystems) and D8 VENTURE Kappa Duo (Bruker) using detector PHOTON 100 CMOS. Data were collected using graphite-monochromated Mo Ka radiation (l = 0.71073 Å) and were corrected for radiation absorption and polarization effects by methods incorporated in the diffractometer software. The phase problem was solved by direct methods (SHELXS) 29 and refined by full-matrix least-squares based on F 2 (SHELXL). 30 All non-hydrogen atoms were refined with no restraints and with anisotropic displacement parameters. Hydrogen atoms were included in idealized positions and refined as riding atoms. Graphics were obtained with DIAMOND. 31 In the case of compound 5 we were forced to use SQUEEZE to expel cations and molecules of the solvent because of heavy disorder. We therefore present only the structure of the anionic component of the compound. Crystal structure data and refinement details are summarized in Table S1 (see ESI †).
( ( The pH of an aqueous solution of NEt 4 VO 3 (1.5 mL, c = 1 mol dm À3 , 1.5 mmol) was adjusted to 9.0 with concentrated NH 3 . Afterwards, the solution was mixed with 1.2 mL of aqueous solution of rac-H 2 mand (1.2 mmol; c = 1 mol dm À3 ). To the obtained solution acetone (35 mL) was added. The resulting solution was allowed to crystallize at À20 1C. After 3 weeks light yellow crystals in the form of rectangular plates were isolated. Yield: 117 mg (11% based on V). Analytical data for C 20  The (NEt 4 ) 3 [{VO 2 (mlact)} 2 VO 2 (OH) 2 ] (type V 3 L 2 ) complex was obtained in the mixture with (NEt 4 ) 2 [VO 2 (mlact) 2 ] (type V 2 L 2 ). 7 Attempts to isolate the V 3 L 2 complex in a form suitable for X-ray studies failed. The proposed structure of this complex we discussed in the text.

Crystal structures
The prepared dinuclear mandelato complexes can be divided into two groups.  (Fig. 2). Because of the C i symmetry it is a meso-anion. It can be concluded from the stereochemical point of view that the formation of vanadium(V) mandelato complexes is not stereospecific, because it is possible to obtain the chiral anion (1) and the achiral compound in complex (4) starting from a racemic precursor of rac-mandelic acid. Table 1 summarizes geometrical data for the anions in all compounds. As confirmed by the calculation of t parameters in pentacoordinated systems, the vanadium atoms V1 and V2 form slightly distorted tetragonal pyramidal coordination geometry and are displaced above the calculated plane by about 0.5 Å. Axial positions are occupied by oxido ligands O1 and O2, respectively. The tetragonal pseudoplane is completed by one oxido ligand (O3, and O4, resp.), and one oxygen atom from the carboxylate anion (O6, and O9, resp.) and the two vanadium atoms are connected by hydroxylate oxygen atoms O5 and O8.
Compound 5 is the first isolated vanadium(V) complex of an a-hydroxycarboxylic with the stoichiometry V 3 L 2 . Previous attempts to clarify the structure of such complexes relied mostly on 51 V NMR spectroscopy. 7 The proposed structure is shown in Scheme 2. However, the X-ray structure analysis of 5 revealed a different structure (Fig. 3). The [V 3 O 7 ((R)-mand)((S)-mand) 2 ] 3À anion contains both enantiomers of rac-mandelic acid. Unlike in Scheme 2, all vanadium atoms are pentacoordinated. Two oxido ligands always occupy the axial positions and one equatorial position. The central vanadium atom V1 is further coordinated by two oxygen atoms coming from the hydroxylate group and the oxido ligand, O1. The oxygen atom, O1, represents an unexpected feature of the anion as it was not expected in V 3 L 2 complexes. The bridging m 3 -O1 ligand is connecting all three vanadium atoms of the anion. The coordination of the V2 atom is completed by the chelating mandelato ligand that coordinates through one hydroxylato (O2) and one carboxylate (O3) oxygen     Table 2.
The triangular center [V 3 (m 3 -O)] has been known in several vanadium(II, III, IV) complexes. However, most of these compounds exhibit D 3h symmetry; or at least approximate C 3 axis passing through the (m 3 -O) atom. 40 In the case of anion [V 3 O 7 ((R)-mand)-((S)-mand)] 3À the geometry around the O atom is different and it lies in the symmetry plane of the C S symmetry.
In conclusion, the molecular structure of [V 3 O 7 ((R)-mand)-((S)-mand)] 3À differs from the proposed one 7 in: (1) the absence of bridging OH À groups between V1 and V2 atoms, (2) the presence of a bridging (m 3 -O) ligand connecting all vanadium atoms, (3) the coordination number of the central vanadium atom; and (4) the overall symmetry C S .

Infrared spectroscopy
The selected characteristic bands in the IR spectra of the prepared mandelato complexes and their assignment are summarized in Table 3 (the spectra are shown in Fig. S1-S10 (ESI †) and bands of the cations and solvents are tabulated in Table S2, (ESI †)). All compounds exhibit typical strong bands corresponding to n(VQO) vibrations in the region around 930 cm À1 as well as to n(C-O) h , n(C-O) co and n(C-O) un (for symbols see Table 2) vibrations of the coordinated carboxylato ligand.
Theoretical calculations are very helpful for more precise band assignment using PED analysis that allows decomposing complicated vibrational movement of the molecule into several components. Thus, we employed this method for band assignment of the model [V 2 O 4 ((S)-mand) 2 ] 2À anion from compound 2.
The theoretical values are in good agreement with experimental ones (Table 4) and enable reliable band assignment as shown in Table 3. We recently employed DFT calculations of IR spectra for several chiral and non-chiral vanadium(V) complexes using similar basis sets and methods demonstrating that the interpretation of IR spectra of distinct geometries may be performed reliably using theoretical calculations. [41][42][43] Solution studies of vanadate with mandelic acid The 51 V NMR spectra have been measured for a number of nonperoxido vanadium(V) a-hydroxycarboxylato complexes. 7,13,[44][45][46][47] The only published data on nonperoxido vanadium(V) complexes with mandelic acid 44 supposed the existence of complexes, ''a'' of the n(V) : n(L) = 1 : 1 type with a signal at À532 ppm and, ''b'' with unknown stoichiometry, the signal for which appeared at À548 ppm. Studied a-hydroxycarboxylic acids can be divided into two groups: The first one consists of the a-hydroxycarboxylic acids containing one carboxylic acid group and one alcoholic group, e.g. glycolic, lactic and mandelic acids. The second group involves a-hydroxycarboxylic acids with more than one carboxyl and hydroxyl groups; such as citric, 45 glyceric or malic acids. 44 Accordingly, the anions of a-hydroxycarboxylic acids of the second group can act as flexi-dentate ligands and have greater protonation variability in complexes. The most thorough studies 7,46 devoted to a-hydroxycarboxylic acids of the first group suggested the existence of two dominant complexes with V 2 L 2 2À and V 3 L 2 3À stoichiometries (L represents the a-hydroxycarboxylato-(2À) ligand). While the V 2 L 2 2À complex gives rise to one 51 V NMR signal, the trinuclear complex exhibits different peaks for the central and ''peripheral'' vanadium atoms. Besides the signals of V 2 L 2 2À and V 3 L 2 3À , very weak signals of other species (VL À , VL 2À , VL 3À , and V 4 L 2 4À ) have also been observed. The 51 V NMR investigation we performed in the H 2 VO 4 À /rac-H 2 mand/H + system agrees with the general picture of the composition of complexes obtained by Tracey et al. 7,47 and Pettersson et al. 46 Summarized data from so far studied systems are given in Table 5.
In the 51 V NMR spectrum of the NMe 4 VO 3 -rac-H 2 mand-H 2 O system (Fig. 4), it is possible to identify two groups of species. complex is evidently overlapped by the signal at À533 ppm. This overlapped signal is sometimes observed as a downfield   shoulder of the À533 ppm signal (Fig. 4). After subtraction of the doubled intensity of the À551 signal from the intensity of the À533 ppm signal, we can estimate the ratio of concentrations of the V 2 L 2 2À and V 3 L 2 3À complexes. This ratio decreases from pH = 4.65 (4.1) through pH = 5.37 (2.3), pH = 5.85 (1.4) and pH = 6.23 (1.2). Thus, the V 3 L 2 3À complex is supported by higher pH values (E6) in agreement with the literature data (Fig. 5). 7,46,47 As the complexes were isolated from mixed acetone-H 2 O solutions, we looked at the speciation also in mixed solvents (Fig. S11, ESI †). The addition of acetone to the aqueous solution results in the broadening of the peaks, raising the value of chemical shift and coalescence of the À551 ppm peak with the À533 ppm peak. On the other hand, the addition of acetone was necessary for the isolation of solid compounds from the solution.
À20 1C in acetone-water solvent. Dinuclear chiral [V 2 O 4 (mand) 2 ] 2À (V 2 L 2 ) complexes in 1, 2 and 3 contain only one enantiomer of the mandelate anion regardless of whether chiral or racemic mandelic acid was used in the synthesis. On the other hand, the dinuclear achiral [V 2 O 4 ((R)-mand)((S)-mand)] 2À anion in 4 comprises both enantiomers of the mandelato ligand. The [V 3 O 7 ((R)mand)((S)-mand)] (V 3 L 2 ) anion in 5 possesses several unique structural features. To the best of our knowledge, it is the first vanadium(V) trinuclear complex with a triangular {V 3 -m 3 O} centre. The symmetry of the anion is C S and it contains both enantiomers of mandelate. The solution studies confirmed the expected similarity of the mandelic acid-vanadate system in acid aqueous solutions to systems with other a-hydroxycarboxylic acids.

Conflicts of interest
There are no conflicts to declare.