Investigating the Formation of Different (NH₄)₂[M(H₂O)₅(NH₃CH₂CH₂COO)]₂[V₁₀O₂₈]·nH₂O (M = Co^II, Ni^II, Zn^II, n = 4; M = Cd^II, Mn^II, n = 2) Crystallohydrates

Abstract

Three hybrid compounds based on decavanadates, i.e., (NH₄)₂[Co(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·4H₂O (1), (NH₄)₂[Ni(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·4H₂O (2), and (NH₄)₂[Cd(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·2H₂O (3), (where β-Hala = zwitterionic form of β-alanine) were prepared by reactions in mildly acidic conditions (pH ~ 4) at room temperature. These compounds crystallise in two structure types, both crystallising in monoclinic P2₁/n space group but with dissimilar cell packing, i.e., as tetrahydrates (1 and 2) and as a dihydrate (3). An influence of crystal radii and spin state of the central atom in [M(H₂O)₅(β-HAla)]²⁺ complex cations on the crystal packing leading to the formation of different crystallohydrate forms was investigated together with previously prepared (NH₄)₂[Zn(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·4H₂O (4) and (NH₄)₂[Mn(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·2H₂O (5) and spin states of [M(H₂O)₅(β-HAla)]²⁺ (M = Co²⁺, Ni²⁺, and Mn²⁺) cations in solution were confirmed by ¹H-NMR paramagnetic effects. FT-IR and FT-Raman spectra for 1–5 are in agreement with the X-ray structure analysis results.

1. Introduction

Decavanadate anion is the predominant and most stable V(V) oxo-anion found in the acidic aqueous solutions [1]. It is known in different protonation states, [H_nV₁₀O₂₈]^(6–n)– (n = 0–4) [2,3,4].

The decavanadate anion consists of ten edge-sharing VO₆ octahedra. Six of them form a rectangular 2 × 3 arrangement, with two VO₆ octahedra joining the arrangement from the top and two from the bottom via edge sharing with the six octahedra lying in the central plane. The ideal symmetry of [V₁₀O₂₈]^6– anion is given by the D_2h point group symmetry. In most crystal structures, [V₁₀O₂₈]^6– anion usually occupies special positions, mostly inversion centres, and its symmetry is changed correspondingly; however, its geometry remains close to the ideal D_2h symmetry.

In crystal structures, decavanadates act as acceptors of protons in supramolecular arrangements; protonated decavanadate species can also act as proton donors. Typically, if decavanadate anion is not protonated on mutually centrosymmetrically arranged sites, such decavanadate anions will likely form a centrosymmetric dimer interconnected via anion-anion hydrogen bonds [5] or via hydrogen bonding to counter-cations lying on a centre of symmetry [6]. Considering the sterical effect of the decavanadate anion and the influence of size, shape, and nature of counter-cations, a substantial part of decavanadates include water molecules as a stabilising element of the crystal structure. Formation of different crystallohydrates depends on reaction conditions [7] or on temperature changes, even if the reaction conditions remain the same [8]. As a result, decavanadates are known to form a large variety of supramolecular assemblies [9].

The oxovanadates (V), including decavanadates and peroxovanadium compounds, are of great interest in bioinorganic chemistry and biochemistry because of their antidiabetic, antibacterial, antiprotozoal, antiviral, and anticancer properties [10,11]. Studying of non-covalent interactions based on electrostatic attractive forces between decavanadate anion and appropriate molecules (organic cations, hybrid inorganic–organic complex cations, and biomacromolecules) could provide important information on the cooperative effects of decavanadate ions in biological systems [12,13,14].

Apart from the biological importance of decavanadates, there are other emerging application possibilities. Some decavanadates exhibit water oxidation activity [15] or heterogeneous bifunctional catalytic properties in the selective oxidation of sulphides and Mannich reaction [16]. Decavanadates with superior proton conductivity [17] and photoluminescent sensing properties for the detection of Zn²⁺ and Co²⁺ [18] were also prepared. Ammonium decavanadate nanodots—holey reduced graphene oxide nanoribbons [19] and α-Co(OH)₂ nanoplates with decavanadate anion [20] can be used as electrodes for supercapacitors.

In this work, we report the synthesis, crystal structure determination, and properties of three decavanadates, i.e., (NH₄)₂[Co(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·4H₂O (1), (NH₄)₂[Ni(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·4H₂O (2), and (NH₄)₂[Cd(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·2H₂O (3), prepared by the same procedure we used previously for the preparation of two decavanadates containing the same [M(H₂O)₅(β-HAla)]²⁺ cations in two different crystallohydrate forms, i.e., tetrahydrate (NH₄)₂[Zn(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·4H₂O (4) and dihydrate (NH₄)₂[Mn(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·2H₂O (5) [21]. The compounds 1 and 2 are isostructural with 4, while 3 is isostructural with 5. In [22], compound 2 was prepared by using an alternative hydrothermal “Direct Synthesis” approach by the reaction of metal powder in anion-deficient conditions with V₂O₅ in an aqueous solution of β-alanine and ammonium acetate. Subsequently, compound 2 was used as a precursor for the preparation of V₂O₅/MnV₂O₆ mixed oxide, and its ability to act as a water oxidation catalyst for oxygen production was studied. Later, 1 was prepared by the same “Direct Synthesis” method and used as a precursor for the preparation of solid mixed oxides CoV₂O₆/V₂O₅ and Co₂V₂O₇/V₂O₅ by thermal decomposition; both these mixed oxides and 1 can act as catalysts for photoinduced water oxidation [23]. The aim of the present study is to obtain more complete insight into the formation of different crystallohydrates with the general formula (NH₄)₂[M(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·nH₂O, where M is a divalent metal ion, from the same reaction conditions.

2. Materials and Methods

2.1. General

All chemicals were of reagent or better grade, obtained from commercial sources (Mikrochem (Pezinok, Slovakia), Sigma-Aldrich (Saint Louis, MI, USA), Slavus (Donja Stubica, Croatia), Lachema (Brno, Czech Republic)). Infrared spectra were obtained from KBr pellets on a Thermo Scientific Nicolet 6700 FTIR spectrometer in the 400–4000 cm⁻¹ range (Waltham, MA, USA). The Raman spectra were registered with a Raman micro-spectrometer Senterra (Bruker Optik, Ettlingen, Germany), using a laser with the wavelength 532 nm, with a maximum power of 2 mW, in the range 64–4467 cm⁻¹, with 10× objective lenses, and 2 scans for 10 s each. ¹H-NMR spectra were recorded on a Bruker AVANCE Neo 400 MHz (operating at 9.37 T, 400 MHz) using D₂O as a reference. Chemical shifts are reported in Hz. Vanadium (V) was determined volumetrically by titration with FeSO₄ (c = 0.1 m) using diphenylamine as the indicator.

2.2. Synthesis and Crystallisation

2.2.1. Synthesis of (NH₄)₂[Co(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·4H₂O (1)

CoSO₄.7H₂O (1.4055 g; 5 mmol) was added to a solution of β-alanine (0.89 g; 10 mmol) in water (20 mL). After stirring for 15 min, a solution of NH₄VO₃ (1.170 g; 10 mmol) in water (40 mL) was added under the immediate formation of a fine precipitate. The solution with precipitate was stirred for 30 min and filtered. The pH of the filtrate was adjusted to 4.0 with 2M H₂SO₄. To the reddish-brown solution obtained, ethanol (10 mL) was added. Reddish brown crystals were isolated after standing for 22 days at 4 °C in the refrigerator.

Yield: 0.8 g/52.1% (calc. for vanadium). Anal. Calc. for C₆H₅₀N₄O₄₆Co₂V₁₀ (MW = 1541.76 g/mol) V, 34.10%. Found: V, 33.96%.

2.2.2. Synthesis of (NH₄)₂[Ni(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·4H₂O (2)

NiCl₂.6H₂O (1.188 g; 5 mmol) was added to a solution of β-alanine (0.89 g; 10 mmol) in water (20 mL). After stirring for 15 min, a solution of NH₄VO₃ (1.170 g; 10 mmol) in water (60 mL) was added, followed by the immediate formation of a fine precipitate. The solution with precipitate was stirred for 30 min and filtered. The pH of the filtrate was adjusted to 4.0 with 4M HCl. To the yellow solution obtained, ethanol (10 mL) was added. Dark orange crystals with an intense green tone were isolated after standing for 20 days at 4 °C in the refrigerator.

Yield: 1.343 g/87.5% (calc. for vanadium). Anal. Calc. for C₆H₅₀N₄O₄₆Ni₂V₁₀ (MW = 1541.32 g/mol) V, 34.11%. Found: V, 33.69%.

2.2.3. Synthesis of (NH₄)₂[Cd(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·2H₂O (3)

Cd(NO₃)₂.4H₂O (1.542 g; 5 mmol) was added to a solution of β-alanine (0.89 g; 10 mmol) in water (20 mL). After stirring for 15 min, a solution of NH₄VO₃ (1.170 g; 10 mmol) in water (40 mL) was added, followed by the immediate formation of a fine precipitate. The solution with precipitate was stirred for 30 min and filtered. The pH of the filtrate was adjusted to 4.25 with 4M HNO₃. To the yellow solution obtained, ethanol (10 mL) was added. Orange crystals were isolated after standing for 15 days at 4 °C in the refrigerator.

Yield: 0.702 g/43.7% (calc. for vanadium). Anal. Calc. for C₆H₄₆N₄O₄₄Cd₂V₁₀ (MW = 1612.67 g/mol) V, 31.59%. Found: V, 31.44%.

(NH₄)₂[Zn(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·4H₂O (4) and (NH₄)₂[Mn(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·2H₂O (5) samples were prepared for physical measurements according to [22].

2.3. X-ray Data Collection and Structure Determination

Intensity data for the compounds 1–3 were collected on a Kuma KM–4 CCD diffractometer using graphite monochromated MoKα radiation (0.71073 Å) by the ω- and φ-scan techniques at room temperature. Data collection, data reduction, and finalisation were carried out using CrysAlis Pro-Version 1.171.43.128a software [24]. Intensity data for 4 and 5 were reprocessed from original images collected under the conditions in [21] by using the above-mentioned up-to-date version of CrysAlis Pro software to take advantage of improved data processing, especially regarding twin data reduction handling.

The structures were solved by SHELXT [25] and refined by the full matrix least-squares method on all F² data using SHELXL-2018/3 [26]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms of methylene groups were refined using a riding model, and the ammonium group was refined as a free rotor, with C–H and N–H distances free to refine. Hydrogen atoms of water molecules were refined with all O–H and all H…H distances restrained to be equal; similarly, the tetrahedral shape of ammonium cation was retained by restraining all N–H and all H…H distances to be equal. Thermal parameters of the H atoms were constrained to U_iso(H) = 1.2U_eq(C) and U_iso(H) = 1.5U_eq(N, O).

Both compounds 3 and 5 crystallise as non-merohedral twins. Two domains related to the twofold rotation about the c-axis were found using Ewald Explorer in the CrysAlis Pro software, and data were refinalised using internal programme features. The twin domain volume ratios were refined to 0.6515(7):0.3485(7) for 3 and 0.8436(4):0.1564(4) for 5.

Geometrical calculations were performed using SHELXL-2018/3, Olex2 1.5 [27], and PARST [28]. Olex2 1.5 and DIAMOND [29] were used for molecular graphics. Octahedral distortion parameters ζ [30], Δ [31], Σ [32], and Θ [33] were calculated using OctaDist 3.1.0 [34]. Hydrogen bonding geometries were normalised to neutron distances following a literature procedure [35,36].

Crystal data, conditions of data collection, and refinement results for the compounds 1–5 are reported in Table S1.

2.4. Paramagnetic ¹H-NMR Measurements

For the Evans method [37,38,39], approx. 2 mM solutions of 1, 2, 4, and 5 using 3% (v/v) t-BuOH solution in H₂O were prepared. The inner n.m.r. tube (o.d. 2 mm) was loaded with 3% (v/v) t-BuOH solution in D₂O as a reference. The outer n.m.r. tube (o.d. 5 mm) was filled with 500 μL of the sample solution.

The ∆f value, defined as the difference between the chemical shift of the ¹H-NMR t-Bu signal in the sample solution and that of the t-BuOH reference solution, was used to calculate the molar susceptibility of the complex using the following equation:

$${\chi }_{\mathrm{m}}={\displaystyle \frac{3\Delta f}{4\pi cf}}$$

(1)

where c is the concentration of the paramagnetic complex in the solution in mmol.L⁻¹ and f is the spectrometer frequency (400 MHz). Considering there are 2 [M(H₂O)₅(β-HAla)]²⁺ cations per formula unit, the value of c is twice the concentration of the compound in the solution.

The χ_p values [cm³mol⁻¹] are molar susceptibilities corrected for diamagnetic contribution to the susceptibility in the sample.

$${\chi }_{\mathrm{p}}={\chi }_{\mathrm{m}}-{\chi }_{\mathrm{d}}$$

(2)

where χ_d represents diamagnetic contributions from the ligands, ions, inner-core electrons, etc. [40]. The following values were used to calculate the effective magnetic moment:

$${\mu }_{eff}=\sqrt{{\displaystyle \frac{3kT{\chi }_{\mathrm{p}}}{N_{\mathrm{A}}{\mu }_{\mathrm{B}}^2}}}$$

(3)

where k is the Boltzmann constant, T is temperature [K], N_A is Avogadro’s number, and μ_B is the Bohr magneton. Effective magnetic moment is compared to the calculated value for the central atom in question as follows [41]:

$${\mu }_{cal}=2\sqrt{S\left(S+1\right)}{\mu }_{\mathrm{B}}$$

(4)

3. Results and Discussion

Decavanadates with complex cations, i.e., (NH₄)₂[Co(H₂O)₅(β-HAla)]₂[V₁₀O₂₈] ·4H₂O (1), (NH₄)₂[Ni(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·4H₂O (2), and (NH₄)₂[Cd(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·2H₂O (3), were obtained by crystallisation from the β-alanine—CoSO₄—NH₄VO₃—H₂SO₄—H₂O—ethanol (1), β-alanine—NiCl₂—NH₄VO₃—HCl—H₂O—ethanol (2), and β-alanine—Cd(NO₃)₂—NH₄VO₃—HNO₃—H₂O—ethanol (3) reaction systems in mildly acidic (pH~4) conditions. Attempts to prepare compounds of Mg²⁺, Sr²⁺, Ba²⁺, Pb²⁺, and Hg²⁺ from the same reaction system were unsuccessful, obtaining ammonium decavanadate as a product.

The preparation of compound 1 by the hydrothermal reaction of cobalt powder with V₂O₅ in an aqueous solution containing β-alanine and ammonium acetate was reported earlier [23]. However, from the described green colour of the solution and products obtained by hydrothermal synthesis when compared with the orange product we obtained, it can be inferred that a partial reduction of V(V) to V(IV) has taken place. This partial reduction can be avoided by using our preparation method. Although the synthesis was targeted towards using the prepared compound as a precursor for the preparation of mixed Co/V oxides by thermal decomposition, to avoid the presence of lower oxidation states of vanadium in the products, not only during the reactions in solutions but even after thermal decomposition in the air atmosphere, it is a good practise to use purified V(V) precursors [42].

3.1. Crystallographic Characterisation of Prepared Compounds

All prepared compounds belong to one of two monoclinic (space group P2₁/n) structure types with substantially different cell parameters and cell packing—tetrahydrates (1, 2, and 4) (Figure 1a) and dihydrates (3 and 5) (Figure 1b).

The asymmetric unit of all prepared dihydrates consists of one half of the [V₁₀O₂₈]^6– anion with C_i symmetry lying on an inversion centre, one [M(H₂O)₅(β-HAla)]²⁺ cation, one NH₄⁺ cation, and two water molecules in general positions (Figure 2a).

The asymmetric unit of tetrahydrates consists of one half of the [V₁₀O₂₈]^6– anion with C_i symmetry lying on an inversion centre, one [M(H₂O)₅(β-HAla)]²⁺ cation, one NH₄⁺ cation, and one water molecule in general positions (Figure 2b).

The quality of X-ray diffraction data allowed us to find all hydrogen atoms from the ifference electron density map and refine them semi-freely using a set of suitable restraints [43]. As a result, the accurate orientations of water molecules, –NH₃⁺ groups, and NH₄⁺ cations were successfully determined.

The following four types of oxygen atoms can be distinguished in the decavanadate anions: terminal O_T bonded to only one vanadium atom, and O(μ₂), O(μ₃), and O(μ₆) bridging atoms, connecting 2, 3, and 6 vanadium atoms, respectively. Table S2 contains V–O and M–O bond lengths and bond valences/bond valence sums (BVS) calculated using the following equation [44,45]:

$$s=\exp \left({\displaystyle \frac{R_0-R}{B}}\right)$$

(5)

where R is bond length and R₀ and B are empirical parameters for given bond type, to confirm protonation state of the decavanadate anion, oxidation number of V, and central atoms of [M(H₂O)₅(β-HAla)]²⁺ complex cations. For V^V–O bonds, the values R₀ = 1.803 Å and B = 0.37 were used [46]. The BVS values obtained are in the range of 1.61–1.76 for V = O_T bonds, 1.764–1.893 for V–O(μ₂) bonds, 1.88–1.92 for V–O(μ₃) bonds, and 1.975–1.998 for V–O(μ₆) bonds. A comparison of BVS values obtained for particular bonds also confirms the rigidity of the [V₁₀O₂₈]^6– anion, only marginally influenced by adjacent molecules. For protonated O atoms, expected BVS values are Σs < 1.5; thus, in all prepared compounds, the presence of an unprotonated [V₁₀O₂₈]^6– anion was confirmed. BVS values for all V atoms are in the range of 5.02–5.11, confirming oxidation state V(V).

For the BVS calculations for central atoms M in [M(H₂O)₅(β-HAla)]²⁺ cations, the following values of the bond parameters were used: R₀ = 1.685 Å and B = 0.37 for Co²⁺ in 1 [47], R₀ = 1.675 Å and B = 0.37 for Ni²⁺ in 2 [45], R₀ = 1.904 Å and B = 0.37 for Cd²⁺ in 3 [46], R₀ = 1.704 Å and B = 0.37 for Zn²⁺ in 4 [46], and R₀ = 1.762 Å and B = 0.4 for Mn²⁺ in 5 [48]. In all [M(H₂O)₅(β-HAla)]²⁺ cations in the compounds 1–5, oxidation state II for the central atom was confirmed.

All the molecules present in both dihydrates and tetrahydrates are involved in the creation of an extensive hydrogen bonding network (Table S3). Most of these interactions are medium hydrogen bonds with bond distances d(H…A) between 1.5 to 2.2 Å and bond angles belonging to the range 130–180° after normalisation to neutron distances [49].

The values of average M–O distances $\overline{d}$(M–O) in [M(H₂O)₅(β-HAla)]²⁺ complex cations of the compounds 1–5 (

Table 1. Average M–O distances, octahedral distortion parameters, and octahedron volumes in [M(H₂O)₅(β-HAla)]²⁺ complex cations of the compounds 1–5.

Compound	$\overline{\mathit{d}}$(M–O) [Å]	ζ	Δ	Σ	Θ	V [Å3]
1	2.090(15)	0.065267	5.1 × 10−5	21.9313	51.4217	12.14
2	2.055(11)	0.050418	2.7 × 10−5	23.2468	50.2841	11.54
3	2.26(4)	0.195609	3.10 × 10−4	53.3874	144.5566	15.30
4	2.090(15)	0.064040	4.8 × 10−5	28.7336	66.9865	12.14
5	2.17(4)	0.198905	3.38 × 10−4	43.3304	113.7109	13.47

) are in good agreement with the values of the crystal radii for r(Co²⁺ hs) = 85.5 pm, r(Ni²⁺) = 83 pm, r(Cd²⁺) = 109 pm, r(Zn²⁺) = 88 pm, r(Mn²⁺ hs) = 97 pm, and r(O^2–) = 121 pm, respectively; crystal radii corresponding with low spin states are significantly smaller (r(Co²⁺ ls) = 79 pm, r(Mn²⁺ ls) = 81 pm) [50]. The M–O_water distances are in the range 2.070(2)–2.1195(19) Å for 1, 2.038(2)–2.0729(19) Å for 2, 2.232(3)–2.327(3) Å for 3, 2.0665(11)–2.1152(10) Å for 4, and 2.1369(19)–2.2197(18) for 5. The M–O_β-HAla distances d(M–O91) (Table S2) are in the range 2.0614(18)–2.200(3) Å in the order of increasing crystal radii. Octahedral distortion parameters in [M(H₂O)₅(β-HAla)]²⁺ complex cations (Table 1) also show significant differences between particular values for tetrahydrates and for dihydrates.

The zwitterionic form of β-alanine is characterised by the formation of an intramolecular salt bridge between the ammonium group and the carboxylic group, forming a six-membered S(6) ring [51,52]. In tetrahydrates (Figure 3a), smaller Co²⁺, Ni²⁺, and Zn²⁺ ions prefer the coordination of the less bulky oxygen atom of the carboxylate group not involved in the intramolecular hydrogen bond, while larger Mn²⁺ and Cd²⁺ cations in dihydrates (Figure 3b) prefer the coordination of the oxygen atom involved in the formation of the intramolecular hydrogen bond. This different behaviour influences not only the strength of the intermolecular hydrogen bond in β-alanine but also the entire hydrogen bond network. As a result, the [M(H₂O)₅(β-HAla)]²⁺ cation in tetrahydrates contains two intramolecular hydrogen bonds, i.e., the above-mentioned salt bridge N1–H1…O92 between –NH₃⁺ hydrogen and non-coordinated carboxylate oxygen, and O55–H55B…O92 hydrogen bond between coordinated water molecules and the same non-coordinated carboxylate oxygen, thus forming another S(6) supramolecular ring. In dihydrates, the [M(H₂O)₅(β-HAla)]²⁺ cation contains one three-centred chelated intramolecular hydrogen bond, i.e., N1–H1B…O53 and N1–H1B…O91, where H1B belongs to the –NH₃⁺ group, O53 to the coordinated water molecule, and O91 carboxylate oxygen coordinated to the metal atom. Alongside the intramolecular S(6) ring formed by β-alanine, the S(4) ring involving the coordinated oxygen atom of the carboxylate group, the hydrogen atom of the ammonium group, and the oxygen atom of the coordinated water molecule is formed. These changes are also reflected as a change in corresponding torsion angles (

Table 2. Selected torsion angles in [M(H₂O)₅(β-HAla)]²⁺ complex cations of the compounds 1–5.

	1	2	3	4	5
M–O91–C1–O92	3.8(4)	0.8(4)	51.7(7)	4.3(2)	59.8(4)
M–O91–C1–C2	−176.41(18)	−179.03(17)	−128.4(4)	−175.70(9)	−119.9(3)
O91–C1–C2–C3	174.9(2)	173.3(2)	−36.8(5)	174.61(12)	−39.3(3)
O92–C1–C2–C3	−5.3(4)	−6.5(4)	143.1(4)	−5.41(18)	141.0(2)
C1–C2–C3–N1	70.5(3)	71.4(3)	59.0(5)	70.03(16)	60.9(3)

) and in the presence of different numbers of cocrystallised water molecules in both crystal forms.

3.2. Molar Susceptibility Determination of [M(H₂O)₅(β-HAla)]²⁺ Ions in Solution by Paramagnetic ¹H-NMR

To determine the spin state of paramagnetic central atoms in the [M(H₂O)₅(β-HAla)]²⁺ (M = Co^II, Ni^II, Mn^II) complex cations, molar susceptibilities of the complex cations in the solutions of 1, 2, and 5 were determined based on the paramagnetic shift of the ¹H-NMR signal of the t-Bu group in the paramagnetic solution against the diamagnetic reference (3% v/v t-BuOH solution in D₂O). For the assessment of diamagnetic contribution, the ¹H-NMR chemical shifts of the signal belonging to the t-Bu group in the solution of 4 and in a blank sample containing t-BuOH in D₂O were also recorded (Figure 4).

At the concentrations given, there was no significant diamagnetic contribution χ_d from the ligands and other ions in the solution based on a comparison of 4 and blank t-BuOH ¹H-NMR spectra, thus μ_eff values were calculated directly from χ_m values (

Table 3. Experimental data, molar magnetic susceptibilities, and effective magnetic moments of the [M(H₂O)₅(β-HAla)]²⁺ complex cations in the compounds 1, 2, and 5.

Compound	Δf [Hz]	c [mmol.L−1]	T [K]	χm [cm3mol−1]	μeff [μB]
1	67.576	4.011	302.35	0.010052	4.95
2	30.2619	4.004	302.25	0.004509	3.32
5	95.534	4.000	300.85	0.014235	5.88

).

The results are in good agreement with the calculated μ_cal values by using Equation (4) for three unpaired electrons of Co²⁺(hs) in 1, two unpaired electrons of Ni²⁺ in 2, and five unpaired electrons of Mn²⁺(hs) in 5. These findings agree with the assignment of spin states based on average $\overline{d}$(M–O) distances.

3.3. Vibrational Spectroscopy

The FT-IR (Figure 5a) and FT-Raman (Figure 5b) spectra of crystalline 1–5 are quite similar because of the same chemical components present. The FT-IR and FT-Raman spectra of the individual compounds are mutually compared on Figure S1a–e. The assignments of the bands are summarised in Table S4 [7,21,53].

The common features of decavanadate IR spectra involve the bands with highest intensity in the range 920–1000 cm⁻¹ corresponding to valence vibrations of terminal V=O bonds and two broad series of bands related to asymmetric and symmetric bridging O–V₂ vibration modes in the range of 843–748 cm⁻¹ (ν_as, IR)/866–823 cm⁻¹ (ν_as, Raman) and 596–524 cm⁻¹ (ν_s, IR)/591–533 cm⁻¹ (ν_s, Raman), respectively.

The assignment of the bands corresponding to δ_d(NH₃⁺), δ_d(NH₄⁺), and both ν_s and ν_as(COO⁻), is based on a comparison of partially deuterated samples [21].

To distinguish between binding modes of carboxylate group in complexes, it is possible to use the differences between frequencies of asymmetric and symmetric stretching vibrations, ∆ = ν_as(COO⁻)—ν_s(COO⁻). Carboxylato complexes exhibiting ∆ values that are significantly greater than ionic values, typically with ∆ ≥ 200 cm⁻¹ (cf. 164 and 184 cm⁻¹ for zwitterionic β-HAla [53]), have usually unidentate coordination [54]. The ∆ values of studied compounds (∆ = 230 for 1, 2, and 3; ∆ = 233 for 4; and ∆ = 199 for 5) are typical for monodentate mode of carboxylate group bonding, thus confirming the results of X-ray structure analysis.

The presence of water molecules is confirmed by the bands in the region 3484–3380 cm⁻¹. These bands usually appear in the 3600–3400 cm⁻¹ range, and their shift towards lower wavenumbers is caused by the occurrence of O–H…O hydrogen bonds.

4. Conclusions

We prepared (NH₄)₂[Co(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·4H₂O, (NH₄)₂[Ni(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·4H₂O, and (NH₄)₂[Cd(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·2H₂O by synthesising them in a mildly acidic aqueous solution, which, in comparison with hydrothermal direct synthesis from metallic powder and V₂O₅, prevented partial reduction of V(V) to V(IV) during the preparation of (NH₄)₂[Co(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·4H₂O. The prepared compounds were characterised by X-ray structure analysis and vibration spectroscopy. The FT-IR and FT-Raman spectra confirmed the presence of the [V₁₀O₂₈]^6– anion and monodentate coordination mode of β-HAla in complex cation in accordance with crystallographic findings. To confirm the spin state of central atoms in [M(H₂O)₅(β-HAla)]²⁺ cations, including previously prepared (NH₄)₂[Mn(H₂O)₅(β-HAla)]₂[V₁₀O₂₈]·2H₂O, the Evans method was used to confirm the assignment of spin states based on crystallographic data. The increasing crystal radius of central atoms and their different preferences for donor atoms resulting in rebuilding of the hydrogen bonding network is the main cause of the existence of two different crystallohydrate forms.