1. Chemical context

Vanadium, a biologically important trace element with the +V oxidation state, has received considerable attention among the three (viz., +III, +IV and +V) physiologically important oxidation states. In this oxidation state, vanadium exists in three different motifs, viz., VO³⁺, V₂O₃⁴⁺ and VO₂⁺. The formation and stability of these three motifs depends upon the nature of the solvent, the pH of the reaction medium and basicity of the donor atoms of the ligand(s), with a preference for N, O-donor ligands because of the hard acidic nature of V^V. It is evident from the literature (Mondal et al., 2010, 2008) that the vanadium complexes containing VO₂⁺ motifs are formed in basic media. Vanadium compounds show the catalytic cycle of haloperoxidase activity has been suggested to proceed through hydrogen-bonding inter­actions (Colpas et al., 1996; Messerschimdt & Wever, 1996; Weyand et al., 1999; Isupov et al., 2000). In the presence of appropriate hydrogen-bond donors, hydrogen bonding is a general feature of vanadium(IV) and vanadium(V) complexes (Mondal et al., 2010; Plass, 1997, 1998; Plass & Yozgatli, 2003; Pohlmann & Plass, 2001; Pohlmann et al., 2005; Vergopoulos et al., 1993; Sutradhar et al., 2006). In general, these examples lead to the formation of hydrogen-bonded mol­ecular assemblies ranging from simple dimers to three-dimensional networks.

In this work, an ionic compound of dioxidovanadium(V) containing a symmetric N-(pyridine-2-ylcarbamo­yl)picolinamide (Shao et al., 1999) ligand (H₂L) bound to vanadium through NNO in an asymmetric fashion, was synthesized in the presence of di­ethyl­amine in good yield and characterized by X-ray crystallography. The title compound may be used for anti­diabetic drug development (Jia et al., 2017).

2. Structural commentary

The solid-state mol­ecular structure was confirmed by single-crystal X-ray characterization. The title compound crystallizes in the triclinic crystal system, space group P. The asymmetric unit (Fig. 1a) comprises a di­ethyl­ammonium cation, a complex dioxidovanadium(V) and a water mol­ecule, which is inter­linked between the two ionic parts of the compound through hydrogen bonding (Fig. 1b). The anionic part of the compound consists of one crystallographically independent V⁵⁺ ion, two oxido ligands and one NNO donor ligand with coordination sphere of the VO₃N₂ type (Fig. 1a). The V⁵⁺ ion is coordinated by two oxygen (O1 and O2) atoms (oxido ligands), one nitro­gen (N1) atom of the pyridine ring, one deprotonated amide nitro­gen (N2) atom and a deprotonated amide-oxygen (O3) through enolization (Fig. 2) of the ligand. The five-coordinate V⁵⁺ ion has a distorted square-pyramidal geometry with one of the two oxido oxygen atoms (O1) at the apex. The extent of distortion from a perfect square-pyramidal geometry can be qu­anti­fied by the structural index parameter (τ = 0.35), as determined from the equation τ = (β - α)/60 (where β and α are the two largest L—M—L angles), which is 0 for an ideal­ized square pyramid and 1 for a trigonal bipyramid (Nair et al., 2018; Ghosh et al., 2022). The square plane consists of one nitro­gen atom from the pyridine ring (N1), one deprotonated amide nitro­gen (N2), one enolate oxygen (O3) and one oxido oxygen (O2) atom of the ligands. The vanadium atom is located 0.555 (4) Å above the equatorial plane and displaced towards the axial O1 atom. Selected bonds involving the V atom are given in Table 1. The V—O1 bond is longer than V—O2, probably due to the involvement of O1 in a hydrogen bond with the water hydrogen atom H5C (Table 2). Among the three V—O bonds, the longest is the V—O3 bond length due to the absence of a V—O π-bond (Mondal et al., 2010; Jia et al., 2017). In the absence of di­ethyl­amine, the formation of neutral dioxido complex has been reported in which the uncoordinated pyridine atom N4 is protonated (Jia et al., 2017), but in this case the protonation of the di­ethyl­amine moiety (pK_a = 10.98) is probably due to its higher basicity than pyridine (pK_a = 5.23).

Table 2 Hydrogen-bond geometry (Å, °) D—H⋯A D—H H⋯A D⋯A D—H⋯A N5—H5A⋯O5 0.95 (3) 1.92 (3) 2.848 (3) 165 (3) N5—H5B⋯N3i 0.85 (3) 2.08 (3) 2.918 (3) 171 (3) O5—H5C⋯O1 0.85 1.94 2.776 (3) 169 O5—H5D⋯O4ii 0.85 1.99 2.838 (2) 178 C2—H2⋯O2iii 0.93 2.48 3.277 (3) 143 C14—H14A⋯O4i 0.97 2.57 3.172 (3) 121 C15A—H15A⋯N4i 0.97 2.59 3.233 (5) 124 Symmetry codes: (i) ; (ii) ; (iii) .

Figure 1 (a) The mol­ecular structure with the atom-numbering scheme and ellipsoids drawn at the 50% probability level and (b) the intra­molecular hydrogen bond.

Figure 2 The keto–enol tautomeric forms of the ligand.

3. Supra­molecular features

The oxygen (O5) atom of water acts as a hydrogen-bond donor with an acceptor oxido group (O1) of the dioxidovan­adium(V) complex in the same asymmetric unit (O5—H5C⋯O1) and a symmetry-related amide oxygen (O4) atom in a neighbouring asymmetric unit (O5—H5D⋯O4) (Table 2). The O5 atom also acts as a hydrogen-bond acceptor for the amine H5A atom (N5—H5A⋯O5). Another amine hydrogen (H5B) is hydrogen bonded with the N3 atom (N5—H5B⋯N3) of an adjacent complex. Two C—H⋯O and one C—H⋯N inter­actions are also observed. These hydrogen bonds within the same and different asymmetric units enhance the crystal packing of the compounds (Figs. 1b and 3). The mono-periodic constructs are packed perpendicular to the bc plane, giving rise to an overall three-dimensional packing arrangement (Fig. 4).

Figure 3 The hydrogen bonding in adjacent asymmetric units.

Figure 4 The three-dimensional packing arrangement of the components of the title compound.

4. Database survey

Mondal et al. (2010) reported numerous ionic dioxidovanadium(V) compounds with O,N,O donor ligands in presence of different types of bases. Jia et al. (2017) also reported a neutral dioxidovanadium(V) compound with the same ligand.

5. Synthesis and crystallization

To a solution of picolinohydrazide (0.137 g, 1 mmol) in methanol (25 ml) was added ethyl picolinate (0.151 g, 1 mmol). The solution was heated under reflux for 3 h. The reaction mixture was cooled to room temperature and a methano­lic solution (20 ml) of [V^IVO(acac)₂] (0.265 g, 1 mmol) was added with stirring. After stirring for 2 h, a methano­lic solution (10 ml) of di­ethyl­amine (1 ml) was added with continuous stirring. The solution immediately turned yellow and the reaction mixture was then refluxed for 1 h. The reaction mixture was then kept for slow evaporation at room temperature. A yellow X-ray quality crystalline compound was obtained, which was filtered, washed with methanol and dried over silica gel (fused). Yield: 0.34 g (82%). Crystals of the complex were obtained after 4-days on slow evaporation at room temperature.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3. N-bound H atoms were refined with U_iso(H) = 1.2U_eq(N). C-bound H atoms and water H atoms were placed at calculated positions (C—H = 0.93–0.97 Å, O—H = 0.85 Å) and refined as riding with U_iso(H) = 1.2U_eq(C) or 1.5U_eq(C-methyl,O). Initially, residual electron density ws noted near to C15. The part command was used to locate the two positions of C15 (i.e., C15A, in PART 1; and C15B, in PART 2). The site occupancie are 0.7 and 0.3, respectively. Subsequently, an isotropic refinement was done and finally, an anisotropic refinement is performed.

Table 3 Experimental details Crystal data Chemical formula (C4H12N)[V(C12H8N4O2)O2]·H2O Mr 415.32 Crystal system, space group Triclinic, P Temperature (K) 298 a, b, c (Å) 7.6850 (4), 9.4135 (4), 13.9147 (7) α, β, γ (°) 105.609 (2), 101.103 (2), 96.253 (2) V (Å3) 937.50 (8) Z 2 Radiation type Mo Kα μ (mm−1) 0.57 Crystal size (mm) 0.32 × 0.18 × 0.03   Data collection Diffractometer Bruker D8 Quest with Photon II area detector Absorption correction Multi-scan (SADABS; Krause et al., 2015) Tmin, Tmax 0.670, 0.747 No. of measured, independent and observed [I > 2σ(I)] reflections 61285, 5048, 3852 Rint 0.081 (sin θ/λ)max (Å−1) 0.685   Refinement R[F2 > 2σ(F2)], wR(F2), S 0.038, 0.110, 1.06 No. of reflections 5048 No. of parameters 274 H-atom treatment H atoms treated by a mixture of independent and constrained refinement Δρmax, Δρmin (e Å−3) 0.43, −0.39 Computer programs: APEX2 and SAINT (Bruker, 2016), SHELXT2018/2 (Sheldrick, 2015a), SHELXL (Sheldrick, 2015b) and OLEX2 (Dolomanov et al., 2009).