Vanadium(V) complexes of mandelic acid

Abstract

New vanadium complexes of mandelic acid (NMe₄)₄[V₂O₄((R)-mand)₂][V₂O₄((S)-mand)₂] (1), (NMe₄)₂[V₂O₄((S)-mand)₂]·H₂O (2), (NEt₄)₂[V₂O₄((S)-mand)₂]·H₂O (3), (PPh₄)₂[V₂O₄((R)-mand)((S)-mand)]·2H₃CCOCH₃·2H₂O (4), and (NH₄)_2.5(NEt₄)_0.5[V₃O₇((R)-mand)((S)-mand)]·2H₂O (5) (mand²⁻ = 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 [V₂O₄(mand)₂]²⁻ (V₂L₂) anions, the structure of [V₃O₇((R)-mand)((S)-mand)]³⁻ (V₃L₂) in 5 is unique and represents a new structural type in vanadium(V) chemistry. Solution studies of vanadate with mandelic acid by ⁵¹V NMR revealed the presence of two dominant species V₂L₂ and V₃L₂ in aqueous solutions and increasing fraction of V₃L₂ species at slightly acidic pH (pH ≈ 6).

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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 α-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 α-hydroxycarboxylic acids the mutual interaction of these groups of compounds was intensively studied.^3–551V 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–α-hydroxycarboxylic acid system, apart from common vanadates, are the dinuclear and trinuclear complexes of the composition V₂L₂ and V₃L₂ (L = anion of the α-hydroxycarboxylic acid) accompanied by a mononuclear complex VL.⁷ Several dinuclear vanadium(V) complexes with α-hydroxycarboxylato ligands were also reported in the solid state: (NH₄)₂[V₂O₄(glyc)₂],^8,9 Rb₂[V₂O₄(glyc)₂],¹⁰ Na₂[V₂O₄(mlact)₂]·7H₂O,¹¹ (NBu₄)₂[V₂O₄(mlact)₂]·2H₂O,¹² (NH₄)₂[V₂O₄(ehba)₂]·H₂O,¹³ and Cs₂[V₂O₄((S)-lact)₂]·2H₂O¹⁰‡ and a richer series of vanadium complexes with α-hydrocarboxylic acids with more than one carboxyl group such as malic,¹⁴ citric,^15–23 homocitric²⁴ and tartaric acid.^25–28 The crystal structures confirmed the composition and geometry as were proposed by ⁵¹V NMR spectroscopy; however, the V₃L₂ 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 α-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 α-hydroxycarboxylic acids by mandelato complexes. We present the synthesis and characterization of dinuclear V₂L₂ complexes and a trinuclear V₃L₂ complex which revealed a surprising structure not identical to the previously proposed geometry.

Scheme 1 Stereoisomers of mandelic acid (R)-(−)-H₂mand = D-H₂mand and (S)-(−)-H₂mand = L-H₂mand.

Experimental

Materials and methods

All chemicals were of analytical grade and used as received. Stock aqueous solutions of (NMe₄)VO₃ and (NEt₄)VO₃ (1 mol dm⁻³) were prepared by dissolution of V₂O₅ in conc. (NMe₄)OH and (NEt₄)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 V₂O₅. 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 ⁵¹V NMR spectra were recorded at 278 K on a Varian Mercury Plus 600 MHz spectrometer operating at 157.88 MHz for ⁵¹V in 5 mm tubes. Chemical shifts (δ) are given in ppm relative to VOCl₃ as an external standard (δ = 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 Kα radiation (λ = 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 F² (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†).

Calculation of infrared spectra

The molecular structure of [V₂O₄((S)-mand)₂]²⁻ from compound 2 has been optimized at the DFT level with the BP86 functional^32,33 using tight convergence criteria and an ultrafine integration grid in vacuo. We have employed the all-electron Wachsters+f (14s11p6d3f)/[8s6p4d1f] basis set^34,35 for the vanadium atom and the 6-31++G(d,p) basis set^36,37 for the remaining atoms. Afterwards, harmonic vibrational frequencies were calculated. Quantum chemical calculations were performed using the Gaussian 09 Revision D.01 program³⁸ and for subsequent potential energy distribution (PED) analysis the VEDA4 program was utilized.³⁹

Syntheses

(NMe₄)₄[V₂O₄((R)-mand)₂][V₂O₄((S)-mand)₂] (1). To 9.4 mL of acetone, 0.4 mL of aqueous rac-H₂mand (0.5 mol dm⁻³; 0.2 mmol) and 0.2 mL of NMe₄VO₃ (1 mol dm⁻³; 0.2 mmol) were added. The solution was kept at −20 °C. After three weeks, yellow needle-like crystals of 1 were isolated. Yield: 50 mg (40% based on V). Analytical data for C₂₄H₃₆N₂O₁₀V₂ in % (calc. %): C 46.05 (46.91), H 6.24 (5.91), N 4.58 (4.56), V 16.3 (16.6).

(NMe₄)₂[V₂O₄((S)-mand)₂]·H₂O (2). To 9.5 mL of acetone, 0.3 mL of aqueous (S)–H₂mand (0.5 mol dm⁻³; 0.15 mmol) and 0.2 mL of NMe₄VO₃ (1 mol dm⁻³; 0.2 mmol) were added. The solution was kept at −20 °C. After six weeks, yellow needle-like crystals of 2 were isolated. Yield: 25 mg (20% based on V). Analytical data for C₂₄H₃₈N₂O₁₁V₂ in % (calc. %): C 45.10 (45.58), H 6.05 (6.06), N 4.59 (4.43).

(NEt₄)₂[V₂O₄((S)-mand)₂]·H₂O (3). To 100 mL of acetone, 5 mL of NEt₄VO₃ (1 mol dm⁻³; 5 mmol) and 5 mL of (S)–H₂mand (1 mol dm⁻³ in EtOH; 5 mmol) were added. The solution was kept at −20 °C. After one day, yellow plate-like crystals of 3 were isolated. Yield: 69 mg (20% based on V). Analytical data for C₃₂H₅₄N₂O₁₁V₂ in % (calc. %): C 50.49 (51.61), H 7.40 (7.31), N 3.73 (3.76).

(PPh₄)₂[V₂O₄((R)-mand)((S)-mand)]·2H₃CCOCH₃·2H₂O (4). To 9.4 mL of acetone, 0.4 mL of aqueous rac-H₂mand (0.5 mol dm⁻³; 0.2 mmol), 0.2 mL of NMe₄VO₃ (1 mol dm⁻³; 0.2 mmol) and 0.225 g of PPh₄Cl (0.6 mmol) were added. The solution was kept at −20 °C. After one month yellow needle-like crystals of 4 were isolated. Yield: 44 mg (17% based on V). Analytical data for V₂P₂C₇₀H₆₈O₁₄ in % (calc. %): C 63.77 (64.82), H 4.92 (5.28), V 8.5 (7.9).

(NH₄)_2.5(NEt₄)_0.5[V₃O₇((R)-mand)((S)-mand)]·2H₂O (5). The pH of an aqueous solution of NEt₄VO₃ (1.5 mL, c = 1 mol dm⁻³, 1.5 mmol) was adjusted to 9.0 with concentrated NH₃. Afterwards, the solution was mixed with 1.2 mL of aqueous solution of rac-H₂mand (1.2 mmol; c = 1 mol dm⁻³). To the obtained solution acetone (35 mL) was added. The resulting solution was allowed to crystallize at −20 °C. After 3 weeks light yellow crystals in the form of rectangular plates were isolated. Yield: 117 mg (11% based on V). Analytical data for C₂₀H₃₆N₃O₁₅V₃ in % (calc. %) C 33.35 (33.77), H 6.46 (5.10), N 6.02 (5.91), V 21.89 (21.48).

The (NEt₄)₃[{VO₂(mlact)}₂VO₂(OH)₂] (type V₃L₂) complex was obtained in the mixture with (NEt₄)₂[VO₂(mlact)₂] (type V₂L₂).⁷ Attempts to isolate the V₃L₂ complex in a form suitable for X-ray studies failed. The proposed structure of this complex we discussed in the text.

Results and discussion

Crystal structures

The prepared dinuclear mandelato complexes can be divided into two groups. For compounds 1–3 we observed that the central {V₂O₄} unit binds always to the mandelato anion of the same configuration. Thus, racemic compound 1 contains both enantiomers [V₂O₄((S)-mand)₂]²⁻ and [V₂O₄((R)-mand)₂]²⁻ related in the crystal packing by the center of symmetry (Fig. 1), while compounds 2 and 3 contain only the [V₂O₄((S)-mand)₂]²⁻ anion. The [V₂O₄((R)-mand)((S)-mand)]²⁻ anion in 4 contains both enantiomers of the racemic mandelic acid (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 τ 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.

Fig. 1 Molecular structures of anions [V₂O₄((R)-mand)₂]²⁻ (upper, from 1) and [V₂O₄((S)-mand)₂]²⁻ (lower, from 2) showing atom labeling in the central {V₂O₈} group. Colour code: V orange, O red, C black, and H grey. Displacement ellipsoids are displayed at a 30% probability level.

Fig. 2 Molecular structure of the anion [V₂O₄((R)-mand)((S)-mand)]²⁻ found in compound 4 showing atom labeling in the central {V₂O₈} group (symmetrically independent part). Colour code: V orange, O red, C black, and H grey. Displacement ellipsoids are displayed at a 30% probability level.

Table 1 Selected geometrical parameters of compounds 1–4

		Compound bond lengths and δ in Å
		1	2	3	4*
Oh – oxygen atom from the original alcoholic group, Oc – oxygen atom from the original carboxylic acid group. τ – parameter calculated from two bond angles in the tetragonal pseudoplane as (α − β)/60; τ = 0 tetragonal pyramidal geometry, τ = 1 trigonal bipyramidal geometry. δ – displacement of the V atom from the ideal plane calculated from the positions of four atoms forming the tetragonal pseudoplane. * – only symmetrically independent bond parameters of the anion are given.
V=O	V1–O1	1.6206(12)	1.621(4)	1.622(3)	1.6213(9)
	V2–O2	1.6213(13)	1.613(4)	1.617(3)	1.6248(8)
	V1–O3	1.6305(12)	1.617(4)	1.628(3)	—
	V2–O4	1.6308(13)	1.617(4)	1.636(3)	—
V–Oh	V1–O5	1.9730(10)	1.957(3)	1.954(3)	V1–O3′ 1.9636(8)
	V2–O5	2.0438(11)	2.017(3)	2.033(3)	V1–O3 2.0319(8)
	V1–O8	2.0280(11)	2.015(3)	2.032(3)	1.9637(8)
	V2–O8	1.9690(11)	1.969(3)	1.980(3)	—
V–Oc	V1–O9	1.9714(11)	1.960(4)	1.965(3)	V1–O4 1.9775(9)
	V2–O6	1.9711(12)	1.989(4)	1.957(3)	—
τ V1		0.2	0.38	0.11	0.15
δ V1		0.527	0.533	0.487	0.513
τ V2		0.04	0.15	0.18	—
δ V2		0.565	0.555	0.533	—

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Compound 5 is the first isolated vanadium(V) complex of an α-hydroxycarboxylic with the stoichiometry V₃L₂. Previous attempts to clarify the structure of such complexes relied mostly on ⁵¹V 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 [V₃O₇((R)-mand)((S)-mand)₂]³⁻ 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 V₃L₂ complexes. The bridging μ₃-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 atom. Key structural parameters of [V₃O₇((R)-mand)((S)-mand)]³⁻ are summarized in Table 2.

Scheme 2 Structure of V₃L₂ type complexes as proposed by Tracey et al.⁷

Fig. 3 Molecular structure of the anion [V₃O₇((R)-mand)((S)-mand)]³⁻ found in 5 showing atom labeling in the central {V₃O₁₂} group. Colour code: V orange, O red, C black, and H grey. Displacement ellipsoids are displayed at a 30% probability level.

Table 2 Selected geometrical parameters of compound 5

Bond lengths and δ in Å*
Oh – oxygen atom from the original alcoholic group, Oc – oxygen atom from the original carboxylic acid group. τ – parameter calculated from the two bond angles in the tetragonal pseudoplane as (α − β)/60; τ = 0 tetragonal pyramidal geometry, τ = 1 trigonal bipyramidal geometry. δ – displacement of the V atom from the ideal plane calculated from the positions of four atoms forming the tetragonal pseudoplane.
V=O	V2–O5	1.6157(17)
	V2–O6	1.6407(18)
	V1–O7	1.6419(18)
	V1–O8	1.6217(19)
V–Oh	V1–O2	1.9634(12)
	V2–O2	2.0183(13)
V–Oc	V2–O3	1.9775(15)
V–(μ3-O)	V2–O1	1.9061(7)
	V1–O1	1.9712(18)

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	Structural parameters
	Torsion angle (O8–V1⋯V2–O6)	23.930°
	τ V1	0.168
	δ V1	0.462
	τ V2	0.169
	δ V2	0.521

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The triangular center [V₃(μ₃-O)] has been known in several vanadium(II, III, IV) complexes. However, most of these compounds exhibit D_3h symmetry; or at least approximate C₃ axis passing through the (μ₃-O) atom.⁴⁰ In the case of anion [V₃O₇((R)-mand)((S)-mand)]³⁻ 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 [V₃O₇((R)-mand)((S)-mand)]³⁻ differs from the proposed one⁷ in: (1) the absence of bridging OH⁻ groups between V1 and V2 atoms, (2) the presence of a bridging (μ₃-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 ν(V=O) vibrations in the region around 930 cm⁻¹ as well as to ν(C–O)_h, ν(C–O)_co and ν(C–O)_un (for symbols see Table 2) vibrations of the coordinated carboxylato ligand.

Table 3 Selected bands in the IR spectra of the prepared compounds and their assignment

Characteristic group	Wavenumbers in cm^−1
	1	2	3	4	5
(C–O)un – uncoordinated (C–O) groups, (C–O)co – coordinated (C–O) groups and (C–O)h – deprotonated alcoholic groups.
ν(V=O)	921 (s)	928 (vs)	935 (s)	925 (s)	910 (s)
	942 (s)			936 (m)	924 (vs)
ν(C–O)h	1048 (m)	1059 (s)	1061 (m)	1063 (m)	1072 (m)
		1084 (m)
ν(C–O)co	1315 (m)	1323 (s)	1304 (m)		1298 (s)
	1332 (m)		1329 (m)		1308 (s)
ν(C–O)un	1654 (vs)	1655 (vs)	1655 (v)	1651 (s)	1672 (vs)

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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 [V₂O₄((S)-mand)₂]²⁻ 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.^41–43

Table 4 Selected infrared bands for [V₂O₄((S)-mand)₂]²⁻

Calculated wavenumbers with rel. intensity in parentheses	Experimental wavenumbers	Band assignment with PED
Relative intensities are given on the scale of 0–100, the bands with intensities <5 were not included. ar – aromatic circle, un – uncoordinated C=O group, co – coordinated C–O group, Cc – carbon atom from the carboxylic acid group, Ch – carbon atom from the original alcoholic group.
3096 (9)	2962 m	ν(C–H)ar 92%
3080 (11)	2888 m	ν(C–H)ar 92%
1653 (9)		ν(C=O)un 83%
1651 (100)	1659 vs	ν(C=O)un 83%
1299 (12)	1336 m-s	δ(HCC)ar 64%
1293 (52)	1323 s	ν(C=O)co 66%
1082 (11)	1084 m	ν(C–O)h 37% δ(HCC)ar 21%
1051 (15)	1059 s	ν(C–O)h 37% δ(HCC)ar 21%
968 (17), 964 (10), 953 (16), 951 (21)	928 vs	ν(V=O) 91% (mean)
768 (8)	765 s	δ(OCO) 26%
705 (11)	696 m	tors(HCCC)ar 36%, tors(CCCC)ar 18%
675 (5)	622 w	tors(HCCC)ar 44%, tors(CCCC)ar 39%
556 (6)	537 w	δ(OChCc)ar 23%, ν(Cc–Ch) 12%
420 (15)	444 s	δ(OChCc)ar 18%

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Solution studies of vanadate with mandelic acid

The ⁵¹V NMR spectra have been measured for a number of nonperoxido vanadium(V) α-hydroxycarboxylato complexes.^7,13,44–47 The only published data on nonperoxido vanadium(V) complexes with mandelic acid⁴⁴ supposed the existence of complexes, “a” of the n(V) : n(L) = 1 : 1 type with a signal at −532 ppm and, “b” with unknown stoichiometry, the signal for which appeared at −548 ppm. Studied α-hydroxycarboxylic acids can be divided into two groups: The first one consists of the α-hydroxycarboxylic acids containing one carboxylic acid group and one alcoholic group, e.g. glycolic, lactic and mandelic acids. The second group involves α-hydroxycarboxylic acids with more than one carboxyl and hydroxyl groups; such as citric,⁴⁵ glyceric or malic acids.⁴⁴ Accordingly, the anions of α-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 α-hydroxycarboxylic acids of the first group suggested the existence of two dominant complexes with V₂L₂²⁻ and V₃L₂³⁻ stoichiometries (L represents the α-hydroxycarboxylato(2−) ligand). While the V₂L₂²⁻ complex gives rise to one ⁵¹V NMR signal, the trinuclear complex exhibits different peaks for the central and “peripheral” vanadium atoms. Besides the signals of V₂L₂²⁻ and V₃L₂³⁻, very weak signals of other species (VL⁻, VL²⁻, VL³⁻, and V₄L₂⁴⁻) have also been observed. The ⁵¹V NMR investigation we performed in the H₂VO₄⁻/rac-H₂mand/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.

Table 5 ⁵¹V NMR chemical shifts of α-hydroxycarboxylato vanadium(V) complexes in aqueous solutions, and ionic strength 1.0 mol dm⁻³ NaCl. Chemical shifts are given in ppm

	V2L2^2−	V3L2^2−		Ref.
		Central V	Peripheral V
a Ionic strength of 0.15 mol dm^−3 NaCl. b Without adjustment of the ionic strength.
L-Lactic acid	−533	−550	−533	7
L-Lactic acid^a	−533	−551	−532	46
α-Hydroxyisobutyric acid	−549	−569	−538	7
2-Ethyl-2-hydroxybutyric acid	−554	−578	−538	7
rac-Mandelic acid^b	−533	−551	∼−530	This work

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In the ⁵¹V NMR spectrum of the NMe₄VO₃–rac-H₂mand–H₂O system (Fig. 4), it is possible to identify two groups of species. Besides various vanadates (V₁: H₂VO₄⁻, V₂: H₂V₂O₇²⁻, V₄: V₄O₁₂⁴⁻, V₅: V₅O₁₅⁵⁻, and V₁₀: H_xV₁₀O₂₈^(6−x)−),⁴⁸ two broader peaks are present. The signal at −533 ppm corresponds mainly to the V₂L₂²⁻ complex and the signal at −551 ppm arises from the central vanadium atom of the V₃L₂³⁻ complex. The common signal of the two peripheral vanadium atoms of the V₃L₂²⁻ complex is evidently overlapped by the signal at −533 ppm. This overlapped signal is sometimes observed as a downfield shoulder of the −533 ppm signal (Fig. 4). After subtraction of the doubled intensity of the −551 signal from the intensity of the −533 ppm signal, we can estimate the ratio of concentrations of the V₂L₂²⁻ and V₃L₂³⁻ 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 V₃L₂³⁻ complex is supported by higher pH values (≈6) in agreement with the literature data (Fig. 5).^7,46,47

Fig. 4 The ⁵¹V NMR spectrum of the NMe₄VO₃–rac-H₂mand–H₂O system, c(V) = 0.03 mol dm⁻³, c[rac-H₂mand] = 0.02 mol dm⁻³, pH = 5.72. Assignment of the chemical shifts: V₂L₂ −533 ppm, V₃L² −551 ppm, V₁ −561 ppm, V₂ −574 ppm, V₄ −579 ppm, and V₁₀ −517 ppm, −502 ppm and −425 ppm.

Fig. 5 The ⁵¹V NMR spectra of the NMe₄VO₃–NMe₄OH–rac-H₂mand–H₂O system. c(V) = 0.02 mol dm⁻³, and c(rac-H₂mand) = 0.02 mol dm⁻³. The assignment of chemical shifts: V₂L₂ −534 ppm (pH = 4.64), −533 ppm (pH = 5.37), and −532 ppm (pH = 5.85 and 6.23); V₃L₂ −551 ppm (pH = 5.37); V₁ −561 ppm (pH = 5.37, 5.85 and 6.23); and V₂ −573 ppm (pH = 5.85 and 6.23).

As the complexes were isolated from mixed acetone–H₂O 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 −551 ppm peak with the −533 ppm peak. On the other hand, the addition of acetone was necessary for the isolation of solid compounds from the solution.

Conclusions

In spite of the fact that the aqueous mandelic acid–vanadate system is prone to redox reactions, we succeeded in the synthesis of new vanadium(V) mandelato complexes by crystallization at −20 °C in acetone–water solvent. Dinuclear chiral [V₂O₄(mand)₂]²⁻ (V₂L₂) 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 [V₂O₄((R)-mand)((S)-mand)]²⁻ anion in 4 comprises both enantiomers of the mandelato ligand. The [V₃O₇((R)-mand)((S)-mand)] (V₃L₂) 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 {V₃-μ₃O} 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 α-hydroxycarboxylic acids.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Scientific Grant Agency of the Ministry of Education of Slovak Republic and Slovak Academy of Sciences VEGA project no. 1/0507/17, and by the Slovak Research and Development Agency (APVV-17-0324). LK acknowledges financial support from the Austrian Science Fund (FWF), project no. M2200; and the University of Vienna. RG acknowledges support from Charles University Centre of Advanced Materials (CUCAM) (OP VVV Excellent Research Teams) CZ.02.1.01/0.0/0.0/15_003/0000417.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 1911693–1911697. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9nj02275a

‡ Abbreviations of ligands: glyc²⁻ – glycolato, lact²⁻ – lactato, mlact²⁻ – methyllactato, ehba²⁻ – 2-ethyl-2-hydroxybutanoato.

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