INTRODUCTION

Anions of carboxylic acids are capable of manifesting diverse coordination modes and, hence, they are convenient for the design of polynuclear transition metal complexes with the magnetic [1–7] and/or luminescence properties [8–14]. They also exhibit catalytic [15, 16] and biological activities [17]. Bridging anions of dicarboxylic acids find wide use in the construction of porous metal-organic frameworks capable of selective sorption of small molecules and ions [18–26]. Substituted malonic acids (H2R2mal) play a special role among the representatives of this large class of ligands, since their anions are capable of forming six-membered chelate cycles with 3d-metal cations and, correspondingly, can form stable mononuclear bis-chelate anionic complexes [M(R2mal)2]2– (M = VO2+, Co2+, Ni2+, Cu2+, Zn2+) containing free for coordination oxygen atoms of carboxyl groups. These complexes are promising as metal-containing blocks for assembling polynuclear structures and coordination polymers containing metal atoms of various types [27, 28]. The nature of substituents (R) in a malonate anion affects not only the solubility of the reaction product in a specific reaction medium but its crystal structure. In addition, cations of alkaline (M') and alkaline-earth (M") metals introduced into the reaction as a salt of substituted malonic acid play the structure forming role in the malonate systems. In the most part of cases, the radius of these cations in combination with the size of the substituent of the malonate ligand determines the dimensionality of the formed polymeric structures [\({\text{M}}_{2}^{''}\)M(R2mal)2]n and [M"M(R2mal)2]n (M is a 3d-metal cation) [29]. A doubtless advantage of these compounds is the possibility of substituting alkaline and alkaline-earth metal cations by double-charged cations of magnesium and various d metals, resulting in the formation of new heterometallic compounds with different compositions and structures. For example, the reactions of compounds {[Ba(VO)-(Me2mal)2(H2O)] · H2O}n and {[Ba3(VO)3(Bumal)6-(H2O)13] · 4H2O}n with manganese(II) sulfate in aqueous solutions led to the complete substitution of the barium(II) cations by manganese(II) cations to form 1D polymers {[Mn(VO)(R2mal)2(H2O)5] · H2O}n (R2 is Me2 or Bu, respectively) [30]. In the case of the heterometallic K–M complexes, both the complete and partial substitution of potassium ions is possible. The partial substitution of potassium cations by the corresponding metal cations was demonstrated for the reactions of [K4(VO)2(Сbdc)4(H2O)4]n with magnesium nitrate and [K8Cu4(Me2mal)8(H2O)8]n with magnesium, nickel(II), or cobalt(II) nitrates [31, 32], whereas the reaction of [K8Cu4(Me2mal)8(H2O)8]n with cadmium nitrate afforded the product of complete substitution of the potassium cations by cadmium [33].

The possibilities of the substitution of K+ cations by cations of Mg2+ and d metals (Mn2+, Cd2+) in the reaction of vanadyl sulfate and butylmalonic acid potassium salt in a ratio of 1 : 2 were studied in this work. Two new oxovanadium(IV) compounds were isolated as single crystals and studied by X-ray diffraction analysis: the product of the partial substitution of potassium cations by magnesium, 2D polymer {[K2Mg2(VO)3(Bumal)6(H2O)15] · 2H2O}n (I) with the composition and structure differed from those of the earlier described complex {[KMg0.5(VO)(Cbdc)2-(H2O)5.5] · 2.5H2O}n with cyclobutane-1,1-dicarboxylate anions [31] and the product of the complete substitution of potassium cations by cadmium, 1D polymer {[Cd(VO)(Bumal)2(H2O)5] · H2O}n (II) similar in structure to the previously described complex {[Mn(VO)(Bumal)2(H2O)5] · H2O}n [30].

EXPERIMENTAL

New complexes were synthesized in air using distilled water, ethanol (95%), VOSO4 · 3H2O (99+%), KОН (99+%), Mg(NO3)2 · 6H2O (98+%), Mn(NO3)2 · xH2O (98%, Sigma Aldrich), Cd(NO3)2 · 4H2O (99+%, Acros Organics), and H2Bumal (99%, Sigma-Aldrich). IR spectra were recorded in a frequency range of 4000–400 cm–1 on a Spectrum 65 FT-IR spectrometer (PerkinElmer) equipped with a Quest ATR Accessory attachment (Specac). Elemental analysis was carried out on a EuroЕА 3000 CHNS analyzer (EuroVector).

Synthesis of {[K2Mg2(VO)3(Bumal)6(H2O)15] · 2H2O}n (I). A solution of K2Bumal, which was prepared by the neutralization of H2Bumal (0.221 g, 1.38 mmol) with KOH (0.154 g, 2.76 mmol) in water (15 mL), was added to a solution of VOSO4 · 3H2O (0.15 g, 0.69 mmol) in distilled water (10 mL). The reaction mixture was stirred at 40°С for 10 min, Mg(NO3)2 · 6H2O (0.354 g, 1.38 mmol) was added to the obtained blue solution, and the stirring was continued for 10 min. The resulting solution was kept at 22°С. Blue crystals suitable for X-ray diffraction formed in 2 months were separated from the mother liquor by filtration , washed with cold water (Т ≈ 5°C) and ethanol (Т ≈ 22°C), and dried in air. The yield of compound I was 0.211 g (57.9% based on VOSO4 · 3H2O).

For C42H94O44Mg2K2V3

Anal. calcd., %

С, 31.87

Н, 5.99

Found, %

C, 32.51

H, 5.75

IR (ATR; ν, cm–1): 3184 vw ν(O–H), 2958 w ν(C–H), 2935 w ν(C–H), 2873 w ν(C–H), 1638 s νas(COO–), 1583 s νas(COO–), 1400 s νs(COO–), 1348 s, 1326 s, 1304 m, 1272 m, 1245 w, 1199 m, 1116 w, 1103 s, 1006 s ν(V=O), 952 s, 892 s, 808 s δ(OCO), 774 s, 745 s, 700 vs, 613 s, 526 s, 496 s, 454 s [34, 35].

Synthesis of {[Cd(VO)(Bumal)2(H2O)5] · H2O}n (II). A solution of K2Bumal, which was prepared by the neutralization of H2Bumal (0.148 g, 0.92 mmol) with KOH (0.103 g, 1.84 mmol) in water (15 mL), was added to a solution of VOSO4 · 3H2O (0.1 g, 0.46 mmol) in distilled water (10 mL). The reaction mixture was stirred at 40°С for 10 min, Cd(NO3)2 · 4H2O (0.284 g, 0.92 mmol) was added to the obtained blue solution, and the stirring was continued for 10 min. The formed blue crystalline precipitate was filtered off, washed with water (Т ≈ 22°C), and dried in air. The crystals suitable for X-ray diffraction were obtained by the slow evaporation of the mother liquor by filtration at 22°C for 7 days. The crystals were separated from the mother liquor by filtration, washed with water (Т ≈ 22°C), and dried in air. The yield of compound II was 0.124 g (44.5% based on VOSO4 · 3H2O).

For C14H32O15CdV

Anal. calcd., %

С, 27.85

Н, 5.45

Found, %

C, 27.96

H, 5.53

IR (ATR; ν, cm–1): 3652 vw, 3497 w.br ν(O–H), 3371 w.br ν(O–H), 3222 w.br ν(O–H), 2960 w ν(C–H), 2930 w ν(C–H), 2870 vw ν(C–H), 1549 s νas(COO–), 1418 s νs(COO–), 1354 m, 1341 m, 1313 m, 1290 m, 1232 w, 1202 w, 1190 w, 1139 vw, 1103 w, 1071 vw, 987 s ν(V=O), 963 m, 912 w, 865 vw, 807 m δ(OCO), 772 m, 715 s, 669 m, 511 s, 486 s, 456 s, 440 s, 428 s, 420 s [34, 35].

Single crystal X-ray diffraction study of compounds I and II were carried out on a Bruker SMART APEX II diffractometer equipped with a CCD detector (MoKα, λ = 0.71073 Å, graphite monochromator) [36]. A semiempirical absorption correction was applied [37]. The structures were solved by direct methods and refined by full-matrix least squares in the anisotropic approximation for all non-hydrogen atoms. Hydrogen atoms were generated geometrically and refined using the riding model. The calculations were performed using the SHELX-97 and SHELX-2014 program packages [38]. The crystallographic parameters and structure refinement details for compounds I and II are presented in Table 1.

Table 1.   Crystallographic data and structure refinement details for compounds I and II
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The structural data for compounds I and II were deposited with the Cambridge Crystallographic Data Centre (CIF files CCDC nos. 1911220 and 1033008 for I and II, respectively); deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk/data_request/cif).

RESULTS AND DISCUSSION

The reaction of VOSO4 · 3H2O, K2Bumal, and Mg(NO3)2 · 6H2O in a molar ratio of 1 : 2 : 2 afforded the 2D coordination polymer {[K2Mg2-(VO)3(Bumal)6(H2O)15] · 2H2O}n (I). A similar reactions using Cd(NO3)2 · 4H2O and Mn(NO3)2 · xH2O instead of Mg(NO3)2 · 6H2O taken in the same amounts give the 1D polymers {[Cd(VO)-(Bumal)2(H2O)5] · H2O}n (II) and {[Mn(VO)-(Bumal)2(H2O)5] · H2O}n (the latter was synthesized earlier using other procedure, see [30]), which have the same structures and contain no potassium cations (Scheme 1).

Scheme 1.

The base structural fragment of compound I (Fig. 1) is formed by three bis-chelate dianions [VO(Bumal)2(H2O)]2– containing crystallographically nonequivalent vanadium atoms (V(1), V(2), and V(3)), two magnesium atoms (Mg(1) and Mg(2)), two potassium atoms (K(1) and K(2)), and fifteen water molecules coordinated to the metal atoms. A similar structure of the elementary unit (three bis-chelate dianions with nonequivalent vanadium atoms) was observed earlier in the structures of the Ba–V [30], Sr–V [39], and Na–V [40] compounds with Bumal2– anions . The vanadium atom of each [VO(Bumal)2(H2O)]2– dianion is in the distorted octahedral coordination environment of six O atoms. The equatorial plane of the polyhedron is occupied by the O atoms of the carboxyl groups of two butylmalonate anions coordinated via the chelate mode, and the O atom of the vanadyl group and the O atom of the water molecule are located in the axial positions. The bond lengths and bond angles characterizing the coordination polyhedra of vanadium lie in the ranges observed for other known substituted oxovanadium(IV) malonates (Table 2) [30, 31, 39–45]. Within one elementary unit, the metal fragments [V(3)O(Bumal)2(H2O)]2– and [V(2)O(Bumal)2-(H2O)]2– bind the Mg(1) and Mg(2) magnesium atoms due to the coordination of the carboxyl groups of the butylmalonate anions, whereas the K(1) atom links the Mg(1) atom with the bis-chelate dianions [V(1)O(Bumal)2(H2O)]2– and [V(2)O(Bumal)2-(H2O)]2– due to the coordination of two bridging water molecules and four O atoms of the carboxyl groups of the vanadium-containing fragments. Each of the Mg(2), K(1), and K(2) atoms also coordinates one carboxylate O atom of the metal fragments [V(1)O(Bumal)2(H2O)]2–, [V(2)O(Bumal)2-(H2O)]2–, and [V(3)O(Bumal)2(H2O)]2–, respectively, belonging to the adjacent units and thus links the elementary structural fragments into the layered polymeic structure (Fig. 2), which is additionally stabilized due to hydrogen bonds (Table 3). The distances between the vanadium atoms V(1)···V(2), V(2)···V(3), and V(3)···V(1) in the elementary structural fragment are 7.882, 7.652, and 11.583 Å, respectively. The Mg(1) atom is in the octahedral coordination environment: it coordinates five water molecules along with one carboxylate O atom of the [V(3)O(Bumal)2(H2O)]2– dianion. Two water molecules (O(9w) and O(10w)) are bridging and bind the Mg(1) atom to the K(1) atom, whereas three water molecules (O(1w), O(3w), and O(11w)) are coordinated in the monodentate mode. The octahedral environment of the Mg(2) atom is formed by two carboxylate O atoms of the metal fragments [V(1)O(Bumal)2(H2O)]2– and [V(2)O(Bumal)2(H2O)]2– belonging to the adjacent structural units and by four water molecules, one of which is bridging (O(15w)) and binds the Mg(2) atom to the K(2) atom, whereas three water molecules (O(13w), O(14w), and O(16w)) are coordinated in the monodentate mode. The values of the main angles in the Mg polyhedra are presented in Table 2. The K(2) atom additionally coordinates three water molecules (O(4w), O(6w), and O(7w)) in the monodentate mode.

Fig. 1.
figure 1

Base fragment of the 2D polymeric structure of compound I. Butyl substituents of the acid anions and hydrogen atoms are omitted.

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Table 2. Selected bond lengths (d, Å) and bond angles (ω, deg) characterizing the coordination polyhedra of vanadium (CPV), magnesium and cadmium (CPMg, Cd) in compounds I and II
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Fig. 2.
figure 2

Structure of the polymeric layer in compound I. Blue lines connect the V(1), V(2), and V(3) atoms of the elementary units. Butyl substituents and hydrogen atoms are omitted.

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Table 3.   Geometric parameters of hydrogen bonds in the structure of compound I*
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In spite of similar procedures for the synthesis of the earlier obtained 1D polymer {[KMg0.5-(VO)(Cbdc)2(H2O)5.5] · 2.5H2O}n and new 2D polymer {[K2Mg2(VO)3(Bumal)6(H2O)15] · 2H2O}n (I), their compositions and structures are absolutely different, which can be due to both the nature of the carboxylate anion (steric effect of the substituent in the malonate ligand) and the ratio of metals in the structure. The K : Mg : V ratio in the elementary fragment of the compound with the Cbdc2– anions is 1 : 0.5 : 1, and that in compound I is 2 : 2 : 3. Thus, the content of Mg in the elementary unit of compound I is by four times higher than that in cyclobutane-1,1-dicarboxylate, whereas the vanadium content is by three times higher than that in cyclobutane-1,1-dicarboxylate, which favored, most likely, the formation of the structure of a higher dimensionality in spite of the presence of the bulky butyl substituent in the malonate anion.

The polymeric chains of compound II are formed by the {VO(Bumal)2(H2O)} fragments similar in structure to the bis-chelate blocks of compound I (Fig. 3, Table 2), and all vanadium atoms in the structure are crystallographically equivalent. The metal fragments {VO(Bumal)2(H2O)} are bound into polymeric chains by the cadmium atoms, each of which coordinates two carboxylate O atoms of the ligand of the adjacent bis-chelate vanadium-containing fragments. Unlike the previously obtained compound {[Cd(VO)-(Сbdc)2(H2O)5] · H2O}n [44] in which the dicarboxylic acid anions of the {VO(Сbdc)2(H2O)} fragment manifest different coordination modes (µ3,κ2-chelate bridging, and κ2-chelate), both acid anions of the {VO(Bumal)2(H2O)} fragment in compound II demonstrate the µ,κ2 coordination mode. The interatomic distances V(1)···Cd(1) and V(1)···Cd(2) in the polymeric chain are not equal being 5.155 and 5.089 Å, respectively, whereas the distances between the adjacent cadmium atoms are the same and equal to 9.235 Å, which is by ~1.34 Å longer than that in {[Cd(VO)(Cbdc)2(H2O)5] · H2O}n. In addition, unlike V–Cd cyclobutane-1,1-dicarboxylate, the V···V distances in compound II are different and elongated by ~2.4 Å (10.178 and 10.311 Å). The crystal structure of compound II is additionally stabilized due to hydrogen bonds, whose formation involves the solvate and coordinated water molecules and the O atoms of the carboxyl groups (Table 4). The structure of compound II contains crystallographically nonequivalent cadmium atoms of two types, Cd(1) and Cd(2), with different characteristics of the octahedral coordination polyhedra. The equatorial plane of the Cd(1) polyhedron is formed by four O atoms of the water molecules coordinated in the monodentate mode (two O(3w) and two O(4w)), and the axial positions are occupied by the O(7) atoms of the carboxyl groups of the butylmalonate anions of the {VO(Bumal)2(H2O)} bis-chelate fragments. In the Cd(2) polyhedron, the carboxylate O(5) atoms of the bis-chelate fragments {VO(Bumal)2(H2O)} lie in the equatorial plane together with two O atoms of the water molecules coordinated in the monodentate mode (two O(2w)), and two other O atoms of the water molecules (O(5w)) are located in the axial positions. The values of the main angles in the Cd polyhedra are presented in Table 2.

Fig. 3.
figure 3

Structure of the polymeric chain in complex II. Butyl substituents of the acid anions and hydrogen atoms are omitted.

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Table 4.   Parameters of hydrogen bonds in the structure of compound II*
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The consideration of the crystal packing of the polymeric chains shows that the distances between the Cd atoms of the adjacent chains are shorter than the Cd···Cd distances within one chain by 3.29 Å (5.949 Å), whereas the corresponding interatomic V···V distances are shorter by 2.9 Å (7.403 and 7.436 Å).

The structure and packing of the polymeric chains in the crystal of compound II are similar to the structure and packing of the chains in the crystal of its manganese-containing analog {[Mn(VO)(Bumal)2-(H2O)5] · H2O}n synthesized by the substitution of the barium(II) cations (see [30]) (Fig. 4). This indicates that the radius of the d-metal cation exerts no effect on the structure of the heterometallic compound formed by the bis-chelate vanadyl fragments as it was observed for the compounds with cyclobutane-1,1-dicarboxylate anions (see [44]).

Fig. 4.
figure 4

Comparison of the structures of the polymeric chains in compounds (a) {[Mn(VO)(Bumal)2(H2O)5] · H2O}n and (b) II. Butyl substituents of the acid anions and hydrogen atoms are omitted.

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Thus, like for copper(II) malonates, for the systems with oxidovanadium(IV), only the partial substitution of potassium cations by magnesium is shown to occur, regardless of the nature of the substituent in the malonate anion [28]. The {[K2Mg2(VO)3(Bumal)6-(H2O)15] · 2H2O}n complex (I) with butylmalonate anions containing the bulky substituent has a higher dimensionality (2D) than the corresponding compound with cyclobutane-1,1-dicarboxylate anions {[KMg0.5(VO)(Cbdc)2(H2O)5.5] · 2.5H2O}n having the 1D structure. This effect is most likely can be explained by the fact that the elementary fragment of compound I contains more metall centers capable of binding the bis-chelate vanadium-containing fragments between each other. At the same time, the potassium cations are completely substituted upon the interaction with magnesium and cadmium in spite of a substantial difference in their ion radii. The formed 1D polymeric complexes of the formula {[M(VO)(Bumal)2(H2O)5] · H2O}n (M = Mn, Cd) have the same structures.