Introduction

Mandelic acid (Scheme 1) is a chiral alpha hydroxy carboxylic acid known for two hundred years [1]. It exists in the form of two enantiomers and thus is a useful precursor for the synthesis of drugs [2, 3], while the acid itself has significant antibacterial properties [4, 5]. Vanadium has already been known to be able to form some types of complexes in a stereospecific manner, focusing specifically to carboxylic acids, a stereospecific discrimination was observed for complexes employing tartrate [6], 2-amino-3-methylpentanoate (isoleucine) [7], and indeed mandelate [8]. Whilst combining these two key parameters of mandelic acid and vanadium, in this work, we inspect the interaction of vanadium mandelato complexes with Δ- and Λ-[Fe(bpy)3]2+.

Scheme 1
scheme 1

(R)-( −)- and (S)-( +)-mandelic acid

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Our previous investigations of vanadium(V) complexes incorporating mandelic acid established the existence of two types of solid vanadium mandelato complexes: MI2[V2O4(mand)2] (V2L2-type, mand=C8H6O32–) and MI3[V3O7(mand)2] (V3L2-type) [9, 11]. Actually, vanadium has been known for a long time to form complexes of the composition M2[V2O4(ligand)2] (M2V2L2) (ligand=alpha hydroxy monocarboxylic acid), however, the existence of a V3L2 complex was proposed in the past only based on solution speciation studies [10], until the discussion about its possible existence was finally closed by an X-ray structure determination of (NH4)2.5(NEt4)0.5[V3O7(R-mand)(S-mand)] in 2019 [11]. Moreover, it was observed that the V3L23− anion was only isolable in the presence of the racemic mandelic acid but not in an enantiopure environment. On the other hand, the system has shown a significant tendency towards a redox process (reduction of vanadium(V) with mandelic acid to vanadium(IV)) resulting in the impossibility to observe timelapse reactions in solutions. We have observed in this work that in the reaction system FeSO4—bpy—KVO3—mand—H2O—CH3CN there is no reduction of vanadium(V) and, advantageously, the presence of the chiral [Fe(bpy)3]2+ cation allows monitoring interactions between chiral ions.

In addition, heterometallic compounds often surmount homometallic compounds due to the presence of cooperative intermetallic effects [12, 13]. Vanadium heterometallic compounds, in particular, mediate in dinitrogen and nitric oxide activation [14], or exhibit significant antiproliferative and high cytotoxic activity [15, 16]; giving the research presented herein a non-negligible application potential in bioinorganic chemistry.

Experimental

Materials and methods

All chemicals were of analytical grade and were used as received. KVO3 was prepared from purified NH4VO3 as described previously [6]. Elemental analyses C, H, N were determined on a Vario MIKRO cube (Elementar). The vanadium content was determined using an ICP MS Thermo Scientific iCap-Q; the iron content was determined using an AAS Perkin-Elmer Model 1100. Solid-state IR spectra were recorded on a Thermo Scientific Nicolet 6700 FTIR spectrometer in nujol mulls, KBr pellets and by employing the ATR technique. UV–vis spectra and CD spectra were recorded on a JASCO J-815 CD spectrometer in 1.0 cm quartz cuvettes at room temperature using water as solvent. 57Fe Mössbauer spectroscopy was performed using a Wissel spectrometer in transition arrangement at room temperature. α-Fe was used for calibration for fitting in the program NORMOS. The spectrum was recorded by the scintillation detector ND-220-M (NaI:Tl+). 51 V NMR spectra were recorded at 278 K on a VNMRS 600 MHz spectrometer (157.88 MHz for 51 V) in 5 mm tubes. Chemical shifts (δ) are given in ppm relative to VOCl3 as an external standard (δ = 0 ppm). Data for X-ray single crystal diffraction were collected using a Bruker D8 VENTURE diffractometer (compound 1) and a Nonius KappaCCD diffractometer with Bruker APEXII detector (compound 2). The phase problem was solved by direct methods and structure model were refined using software SHELXT 2018/2 [17] and SHELXL 2018/3 [18]. Geometric data were obtained using Platon [19]. Graphics were obtained with Diamond [20].

X-ray diffraction

Crystal data, data collection and structure refinement details are summarized in Table 1. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were refined isotropically and were treated by a mixture of independent and constrained refinement. For compound 1, the solvent molecules CH3CN and H2O present in the structure acquired a mutually disordered arrangement and the solvent molecules were refined with their s.o.f.’s summing to unity. The solvent present in compound 2 could not be modelled with satisfactory results and its contribution to the diffraction data was subtracted by using the SQUEEZE procedure implemented in Platon. The selected bond parameters are listed in Table 2.

Table 1 Crystal structure solution and refinement data
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Table 2 Selected geometrical parameters (bond lengths and δ in Å) for symmetrically independent part of central {V2O8} group in the crystal structure of 1 (for comparison also in (NMe4)4[V2O4((R)-mand)2][V2O4((S)-mand)2], 1*), as well as {V3O11} group in the crystal structure of 2 (for comparison also in (NH4)2.5(NEt4)0.5[V3O7((R)-mand)((S)-mand)]·2H2O, 5*)
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Syntheses

[Fe(bpy)3][V2O4(rac-mand)2]·4.9H2O·0.1CH3CN (1, FeV2L2).

Solution A: FeSO4·7H2O (69.5 mg, 0.250 mmol) was dissolved in H2O (2 mL), 1.5 mL of the solution 2,2’-bipyridine in CH3CN (0.50 mol/L, 0.75 mmol) and 2.5 mL of CH3CN were added. Solution B: To 1.0 mL of aqueous rac-mandelic acid (0.50 mol/L, 0.50 mmol) 1.0 mL of KVO3 solution (0.50 mol/L, 0.50 mmol) and 32 mL of water were added. Solution A and B were mixed, stirred for 15 min and filtered. The final solution was kept at 5 °C in a beaker covered with Petri dish. After three weeks, dark red crystals of 1 were isolated. Yield: 0.065 g. Analytical data for C46.20H46.10FeN6.10O14.90V2 in % (calcd. %): C 50.96 (51.24), H4.13 (4.29), N 7.65 (7.89). IR (cm–1): 3419 m, 1641sh, 1633vs, 1602 s, 1456 s, 1445 s, 1344 s, 1313 m, 1282w, 1272w, 1242 s, 1194w, 1169w, 1161w, 1123w, 1088 s, 1063 s, 966vs, 954vs, 916vs, 903w, 804 m, 773vs, 791vs, 733 s, 699 m, 688w, 659w, 649w, 621w, 602 m, 573w, 530 m, 483w, 441 s.

(H3O)[Fe(bpy)3]4[V3O7(S-mand)2]3·28H2O (2, FeV3L2).

Solution A: FeSO4·7H2O (139 mg, 0.500 mmol) was dissolved in H2O (3 mL), 3.0 mL of the solution 2,2’-bipyridine in CH3CN (0.50 mol/L, 1.5 mmol) and 5 mL of CH3CN were added. Solution B: To 2.0 mL of aqueous (S)-mandelic acid (0.50 mol/L, 1.0 mmol) 3.0 mL of KVO3 solution (0.50 mol/L, 1.5 mmol) and 5 mL of water were added. Solution A and B were mixed, stirred for 15 min and filtered. The final solution was kept at 5 °C in a beaker covered with Petri dish. After three weeks, dark red crystals of 2, usually grown together into round shape similar to sea urchin, were isolated. Yield: 0,070 g. Analytical data for C168H191Fe4N24O68V9 in % (calcd. %): C 46.30 (46.74), H 4.03 (4.46), N 7.43 (7.79), Fe 5.53 (5.18), V 10.46 (10.62). Release of water of crystallization and H3O+ by isothermal heating until the constant weight at 116 °C: experimental weight loss (calculated): 12.99% (12.13%). IR (cm–1): 3422vs, 1643vs, 1603 s, 1467 s, 1444 s, 1427 m, 1337 s, 1315 s, 1273w, 1243w, 1194w, 1171w, 1159w, 1096 m, 1067w, 1028w, 925vs, 806 m, 774vs, 736vs, 721vs, 694 m, 659w, 620w, 603 m, 578w, 528w, 478w, 443 m.

In the case that further precipitate is formed during the standing of preparative solutions for synthesis of 2, it is recommended to filtrate the solutions again. Nevertheless, by a longer standing of the solutions recrystallization of precipitate to crystalline product occurred (Figure S1, Supporting Information).

Results and discussion

Synthesis

Upon at our numerous attempts at synthesizing vanadium mandelato complexes with [Fe(bpy)3]2+ as counterion, we observed some noteworthy phenomena:

  1. (a)

    Although the V2L2 complex with (S)-mandelic acid exists in solution, we never obtained this complex in a pure crystalline form. In contrast, the V2L2 complex with racemic mandelic acid easily provided crystals from solutions with n(V)/n(mand) ≥ 2 (compound 1).

  2. (b)

    Exactly opposite is the situation with the V3L2 complex. The crystals of the FeV3L2 complex can be readily prepared under proper conditions using (S)-mandelic acid (compound 2), but we never succeeded in synthesising the FeV3L2 complex using racemic mandelic acid (Scheme 2).

  3. (c)

    There is a distinct dependence of the reaction products on the mixed solvent CH3CN—H2O composition for complex 2. The empirically ascertained optimum volume ratio is approximately 40/60. Low contents of either water or acetonitrile are unhelpful in providing the desired compound in its crystalline form.

  4. (d)

    For the preparation of 1, there is no strong dependence on the V(CH3CN)/V(H2O) ratio, but low contents of acetonitrile seem to be preferable.

  5. (e)

    The mixing of solutions of FeSO4, 2,2’-bipyridine, KVO3 and mandelic acid always leads to the formation of some precipitates. Further portions of precipitate appeared sometimes while the solutions were left to stand. The initial precipitate is X-ray amorphous and possesses the composition approximately corresponding to the FeV2L2 complex (the determined ratio n(Fe)/n(V) was approx. ½). Removing the precipitates facilitates the crystallization of pure compounds. Considering the above-mentioned phenomena, we decided to have a closer look on the preparative solutions by means of 51 V NMR spectroscopy.

Scheme 2
scheme 2

Reaction pathways leading to products 1 and 2

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NMR spectra

The 51 V NMR spectrum of a solution, whose composition corresponds to the conditions of synthesis of 2 is shown in Fig. 1. The spectrum is in principle identical with the spectrum of aqueous solution, upon considering shifts caused by changing the solvents [11]. Besides the signals of vanadates [21,22,23], the spectrum exhibits the dominant signal of V2L2 (−525 ppm) and the weak signal of V3L2 (one V atom: the central vanadium atom V1 in Fig. 1). The signal of V3L2 (two V atoms: the two equivalent vanadium atoms V2, V2’ in Fig. 1) is overlapped by the strong signal of V2L2 and can be seen as an upfield shoulder. The only difference in comparison with the spectrum of aqueous solution is that the V3L2 (2 V) signal in water is seen as downfield shoulder of the dominant signal.

Fig. 1
figure 1

51 V NMR spectrum of the crystallization solution of 2. Conditions: c(V) = 75 mmol/dm3, V(CH3CN): V(H2O) = 40: 60, n(Fe): n(bpy): n(V): n(S-mand) = 1: 3: 3: 2, spectrum collected immediately upon mixing the solutions. Major components: V2L2 − 525 ppm, V3L2 ≈ − 530 ppm (shoulder), − 542 ppm. Minor vanadates: V1 = HxVO4(3–x)– (− 561 ppm), V2 = HyV2O7(4–y)– (− 575 ppm), V4 = V4O124– (− 579 ppm), V10 = HzV10O28(6–z.)– (− 421 ppm, − 498 ppm, − 513 ppm)

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The comparison of the time dependence of the 51 V NMR spectra for systems with (S)-mandelic acid and racemic mandelic acid is shown in Fig. 2. These solutions were standing in beakers and were covered with Petri dishes at 5 °C. The solutions had the same composition, except the isomeric composition of mandelic acid. While standing, some products (precipitates) were formed (vide ultra). Roughly until the 15th day of standing, the spectra were very much alike, and the obtained solid products exhibited very similar IR spectra both for (S)-mandelic and rac-mandelic acid. After this period, a relatively fast differentiation of the 51 V NMR spectra has begun. The spectra of solutions containing (S)-mandelic acid exhibited strong peaks corresponding to V2L2 + V3L2 (2 V) and medium peaks of V3L2 (1 V). It must be noted that under conditions of the experiment, nearly all CH3CN was evaporated from the solution, what is documented also by the position of the V2L2 + V3L2 (2 V) peak (−532 ppm) that corresponds to the value for aqueous solution. In contrast to the spectrum of the system with (S)-mandelic acid, the spectrum of the solution with rac-mandelic acid after 35 days of standing exhibited only very a weak signal of V2L2 + V3L2 (2 V), while the V3L2 (1 V) signal was completely absent.

Fig. 2
figure 2

Time-lapse 51 V NMR spectra of solutions prepared under following conditions: c(V) = 84 mmol/dm3, V(CH3CN): V(H2O) = 40: 60, n(Fe): n(bpy): n(V): n(S- or rac-mand) = 1: 3: 3.35: 2. Precipitates were removed by filtration before measurement

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The 51 V NMR spectra are fully in accordance with the results of syntheses. After the preliminary period (approximately 0–15 days) and eventual filtration of the subsequent precipitate, 2 (FeV3L2) was crystallizing. From the solutions with rac-mandelic acid, 1 (FeV2L2) was crystallizing after the preliminary period. The synthesis of this compound is not dependent on the conditions of preparation and can be obtained for various V(CH3CN)/V(H2O) ratios and for n(V)/n(mand) between 2/2 and 3.35/2. The complicated course of the reactions preceding preparations of 1 and 2, which are moreover accompanied by the changes in composition of solvents (the preferential evaporation of acetonitrile) and in the concentrations of reacting ions due to the formation of precipitate, we are unable to explain fully. Nevertheless, the 51 V NMR spectroscopy proved to be very useful means, which helps to reveal the changes occurred in solution.

X-ray structures

The X-ray structure analysis revealed that 1 is racemic compound (space group P21/n), which (besides the solvents) consists of both enantiomers of cation—Δ-[Fe(bpy)3]2+ and Λ-[Fe(bpy)3]2+, and both enantiomers of anion—[V2O4(R-mand)2]2– (Fig. 3) and [V2O4(S-mand)2]2–. Bond lengths (Table S1) are comparable with the bond lengths of the anionic part of compound (NMe4)4[V2O4((S)-mand)2][V2O4((R)-mand)2] [11].

Fig. 3
figure 3

Molecular structure of the anion [V2O4(R-mand)2].2– found in 1

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Composition of 2 deserves further discussion: the compound formula [Fe(bpy)3]4[V3O7(S-mand)2]3·28H2O exhibits a charge disbalance; if the oxidation and charge numbers are as follow [FeII(bpy0)3]2+ and [VV3O–II7(S-mand2–)2]3–, then a cation with 1 + charge number is missing. Excluding some possibilities, e. g. one atom Fe(III) out of four iron atoms (ruled out by Mössbauer spectroscopy, see below) or the protonation of one oxygen atom out of twenty one oxygen atoms, which we hold for improbable, we consider the presence of one H3O+ ion in formula unit of compound 2. The hydronium ion appears on the right side of the equation of synthesis of 2:

$${\text{4Fe}}^{{{\text{2}} + }} + {\text{ 12}}bpy + {\text{ 9H}}_{{\text{2}}} {\text{VO}}_{{\text{4}}} ^{{-}} + {\text{ 7H}}_{{\text{2}}} mand \to {\text{ }}\left\{ {\left[ {{\text{Fe}}\left( {bpy} \right)_{{\text{3}}} } \right]_{{\text{4}}} \left[ {{\text{V}}_{{\text{3}}} {\text{O}}_{{\text{7}}} \left( {mand} \right)_{{\text{2}}} } \right]_{{\text{3}}} } \right\}^{{-}} + {\text{ H}}_{{\text{3}}} {\text{O}}^{ + } + {\text{ H}}mand^{{-}} + {\text{ 14H}}_{{\text{2}}} {\text{O}}$$

Hydronium ion is also supposed to be formed during the recrystallization of the preliminary precipitate:

$${\text{4}}\left[ {{\text{Fe}}\left( {bpy} \right)_{{\text{3}}} } \right]\left[ {{\text{V}}_{{\text{2}}} {\text{O}}_{{\text{4}}} \left( {S - mand} \right)_{{\text{2}}} } \right]{\text{ }} + {\text{ HVO}}_{{\text{4}}} ^{{{\text{2}}{-}}} + {\text{ 3}}0{\text{H}}_{{\text{2}}} {\text{O }} \to {\text{ }}\left( {{\text{H}}_{{\text{3}}} {\text{O}}} \right)\left[ {{\text{Fe}}\left( {bpy} \right)_{{\text{3}}} } \right]_{{\text{4}}} \left[ {{\text{V}}_{{\text{3}}} {\text{O}}_{{\text{7}}} \left( {S - mand} \right)_{{\text{2}}} } \right]_{{\text{3}}} \cdot{\text{28H}}_{{\text{2}}} {\text{O }} + {\text{ 2H}}mand^{{-}}$$

The [V3O7(S-mand)2]3– anion (C2 symmetry) in crystal structure of 2 is diastereomer of [V3O7(R-mand)(S-mand)]3– anion (Cs symmetry), which was described previously [11] (Fig. 4). Thus, 2 is a chiral non-racemic compound (space group P21212, Sohncke group) which structure contains Δ-[Fe(bpy)3]2+ cations and [V3O7(S-mand)2]3– anions (Fig. 4, left). Bond lengths of [V3O7(S-mand)2]3– anion are comparable with [V3O7(R-mand)(S-mand)]3– one (Tab. S1), while the tetragonal pyramid {VO4} based on V1 atom in 2 is more distorted (34%) compared to (NH4)2.5(NEt4)0.5[V3O7((R)-mand)((S)-mand)]·2H2O, compound 5 in [11] (17%).

Fig. 4
figure 4

a Molecular structure of the anion [V3O7(S-mand)2]3– found in 2 (left) showing labelling in the central {V3O11} group (symmetrically independent part) and asymmetric carbon atom of the mand2– ligand. The C2 axis is also shown. b Molecular structure of the anion [V3O7(R-mand)(S-mand)]3– found in compound (NH4)2.5(NEt4)0.5[V3O7(R-mand)(S-mand)]·2H2O (right) [11] showing labelling in the central {V3O11} group (symmetrically independent part) and asymmetric carbon atom of the mand2– ligand. The symmetry plane is also shown

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Infrared spectroscopy

The selected characteristic IR bands of the prepared compounds are summarized in Table S2. Both compounds exhibit typical strong bands corresponding to vibrations of the coordinated carboxylate ligands and to the V=O stretching vibrations (Figure S3 and Figure S4 in Supporting Information).

We tried to prove the hypothesis of the presence of a hydronium ion by the IR spectrum. The pyramidal H3O+ ion should give rise to four IR bands due to the active vibrations ν1(A1) (νs(OH3)), ν3(E) (νas(OH3)), ν2(A1) (δas(OH3)) and ν4(E) (δs(OH3)) [24]. Unfortunately, the expected IR bands in the regions 2800–3200 cm–1 (ν1, ν3 bands), 1550–1650 cm–1 (δas(OH3)) and 1000–1100 cm–1 (δs(OH3)) [25, 26] fall in the range where the intensive bands of organic components and water of crystallization of 2 occur. Despite our concentrated effort, our attempts to identify hydronium ion by the IR spectra failed.

UV–vis and CD spectra

The characteristic bands in the UV–vis spectrum of 2 in water appearing at 495 and 525 nm correspond to the MLCT transitions (Figure S2). The CD spectrum of 2 (Fig. 5) in aqueous solution exhibits well-distinguished bands. The spectrum fully corresponds to the of Δ-[Fe(bpy)3]2+ [27] a contribution originating in the chiral anion was not observed. The Δ-configuration of [Fe(bpy)3]2+ is thus in aqueous solution preserved (at least for some time).

Fig. 5
figure 5

The CD spectrum of saturated aqueous solution of 2

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Mössbauer spectroscopy

To exclude the option that a mixture of Fe(II) + Fe(III) is present in 2 and thus the charge is balanced with [Fe(bpy)3]3+ rather than H3O+, we collected a Mössbauer spectrum of 2 (Fig. 6). At first glance, the spectrum looks like a simple doublet from one kind of Fe atom in the sample, which corresponds to a magnetically disordered structure (paramagnetic or diamagnetic state). However, this doublet spectrum is slightly asymmetric (ratio D21 = D2: D1 = 0.93), which may be caused by non-equivalent vibrations in individual lattice axes or the presence of another type of Fe atom. From the previous knowledge of the structure of the given sample, the presence of two iron atoms in the structure can be assumed, and therefore the asymmetry of the doublet was attributed to two types of Fe atoms with different environments around them. The Mössbauer spectrum was described by two subspectra with a Lorentzian profile, which correspond to two Fe atoms with different surroundings. The shape of the spectrum already indicates the distribution of electric charge around the Fe core present in the sample. The doublet subspectrum corresponds to an inhomogeneous distribution of electric charge around the Fe nucleus, whereas the singlet corresponds to a more homogeneous distribution around the Fe nucleus. The obtained values of isomer shift, δ = 0.296(4) mm/s, and quadrupole splitting, ΔEQ = 0.328(9) mm/s are typical for the oxidation state of iron Fe2+ in a low-spin complex with octahedral coordination [28, 29]. The singlet subspectrum is characterized by an isomeric shift, δ = 0.32(2) mm/s, also corresponding to the iron 2 + oxidation state atom. From the depth of both sub-spectra (surface under the curve), the percentage representation of both Fe atoms in the sample can be attributed, which corresponds to 82.2 and 17.8%, for Fe2+ with an inhomogeneous environment and Fe2+ with a homogeneous environment, respectively.

Fig. 6
figure 6

Full Mössbauer spectrum of 2 (left) and a magnified part of the spectrum (right), where the red points, black and blue lines, and green and purple filled surfaces correspond to the measured data, the overall fit and the difference between the measured values and the fit, and the subspectra from individual iron atoms, respectively. (Color figure online)

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As a result, the data obtained match with the crystal structure containing the [Fe(bpy)3]2+ cation [30,31,32,33,34,35] occupying a disordered position (Fig. 7). Based on the structure refinement results, one of the bpy ligands occupy two positions with occupancies 72% and 28% corresponding to an irregular and regular octahedron, respectively.

Fig. 7
figure 7

Ball and stick model of the [Fe(bpy)3]2+ cation present in 2. The atoms of the darker bpy ligand exhibit occupancies roughly 28%, while the atoms of the bpy ligand in a more disordered position exhibit occupancy of roughly 72%

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Conclusions

The MVO3 (M=Na, K, NMe4, NEt4)—mandelic acid—H2O—(co-solvent) systems are prone to undergo redox processes by the formation of vanadium(IV) compounds. The synthesis and storage of the vanadium(V)-mandelato complexes including these cations necessitate the employment of temperature about −20 °C. In contrast, similar systems containing [Fe(bpy)3]2+ are stable against redox reactions. Consequently, although compounds 1 and 2 have been prepared at 5 °C in order to obtain better crystals, these compounds can actually be synthesized and stored at room temperature.

The peculiarity of synthesizing 2 stems from the fact that the V3L2 complex is always a minor species in solution; only the proper interplay of experimental factors (molar ratio and concentration of reacting compounds, duration of the reactions, the composition of solvent, access to surroundings atmosphere etc.) can lead to successful syntheses. We carried out numerous attempts to obtain single crystals of 2 by liquid-to-liquid diffusion strategies in closed vials, but all these attempts ended with failure. Only attempts with conducting reactions in covered beakers and accompanied by changing the solvent composition (preferential evaporation of CH3CN) and by the formation of precipitates led to positive results.

Compound 2 is the second example of a vanadium(V)-mandelato complex of the V3L2 type. Contrarily to our previous results using NH4+ and NEt4+ as the counter ions, where we prepared the V3L2 complex only with racemic mandelic acid, the presence of the [Fe(bpy)3]3+ ions enabled the synthesis of the V3L2 complex only with (S)-mandelic acid. Due to the complexity of reaction systems mentioned above, we were unable to disclose the reasons for such a stereospecific behaviour. In addition, to the differences in reaction solutions revealed by NMR spectroscopy, we expect that also the crystallochemical properties of crystalline phases contribute to the stereospecific formation of the vanadium(V) mandelato complexes.