Crystal structures of a series of bis(acetylacetonato)oxovanadium(IV) complexes containing N-donor pyridyl ligands

Three six-coordinate complexes of bis(acetylacetonato)oxovanadium(IV) containing N-donating pyridyl ligands are reported. Both cis and trans isomers were isolated and characterized from these systems.


Chemical context
Oxovanadium(IV) complexes have been cited as having numerous practical pharmacological applications ranging from anticancer agents to anti-fungal agents and, more recently, as an insulin mimetic (Singh et al., 2014;Abakumova et al., 2012;Amin et al., 2000). Currently investigations are underway to further understand how the oxovanadium complexes perform this wide array of tasks. As an insulin mimetic, it is postulated that oxovanadium complexes interact with multiple points of the cell signaling pathway associated with the insulin hormone (Amin et al., 2000;Srivastava & Mehdi, 2005). Alternatively, studies have shown that it interacts directly with glucose transporters found on the cellular surface (Hiromura et al. 2007;Makinen & Brady, 2002). Furthermore, vanadium has been found to have important interactions in DNA repair systems, which have made it a lucrative target for much oncological/pharmacological research (Abakumova et al., 2012;Kostova, 2009).
Oxovanadium complexes chelated by two acetylacetonate ligands form a five-coordinate bonding system that can act as a Lewis acid (Nenashev et al. 2015;Ugone et al., 2019;Costa Pessoa, 2015;Correia et al. 2017). This system can undergo a reaction with a Lewis base to increase its coordination ISSN 2056-9890 bonding number to six. Of the extensive studies regarding the properties and applications of such complexes, relatively few single-crystal structures have been reported. For instance, five compounds containing N-donor ligands, a focus of this work, have been characterized by single-crystal diffraction (Meicheng et al., 1983(Meicheng et al., , 1984Silva et al., 2013;Kadirova et al., 2009;da Silva et al. 2007;Caira et al., 1972). Given the structural dependence on functions and application, a deeper study of the molecular structure of such complexes is warranted. In this work, we describe the structures of VO(C 5 H 7 O 2 ) 2 L, where L = pyridine (1), 4-cyano-pyridine (2), and 4-methoxypyridine (3), and the isolation of different isomeric forms. The complexes were synthesized rapidly in an Anton Paar Monowave 50 synthesis reactor in 5 minutes at 323 K and crystallized upon cooling the mother liquor.

Structural commentary
Figs. 1-3 illustrate the molecular structures of compounds 1-3. Compounds 1 and 2 crystallize in the monoclinic space group C2/c. In both complexes, a twofold axis runs along the O-V-N bonding axis, leading to an asymmetric unit that consists of half of the molecular structure. Upon symmetry expansion, both 1 and 2 adopt distorted octahedral geometries around the vanadium metal center with the oxo and pyridyl ligands trans to one another. Each acetylacetonate ligand chelates the vanadium center through two oxygen atoms to form a five-membered ring. In 1 and 2, the equatorial plane consisting of the vanadium center and four acetylacetonate oxygen atoms distorts away from the V O double bond. In 1, the O oxo -V-O acac bond angles are 98.05 (3) and 99.84 (3) and in 2 are 98.42 (4) and 98.91 (3) .
Compound 3 exists as a different isomeric form, with the oxo and 4-methoxypyridine ligand being cis to one another. This removes the twofold symmetry seen in compounds 1 and 2 and compound 3 crystallizes in the space group P2 1 /n. Similarly to 1 and 2, compound 3 adopts a distorted octahedral geometry upon chelation by two bidentate acetylacetonate ligands.

Figure 2
A view of compound 2, showing the atom labeling. Displacement ellipsoids are at the 50% probability level and H atoms have been omitted for clarity. [Symmetry code (i) Àx + 1, y, Àz + 3 2 ].

Figure 3
A view of compound 3, showing the atom labeling. Displacement ellipsoids are at the 50% probability level and H atoms have been omitted for clarity.

Figure 1
A view of compound 1, showing the atom labeling. Displacement ellipsoids are at the 50% probability level and H atoms have been omitted for clarity. [Symmetry code (i) Àx + 1, y, Àz + 3 2 ].
The V-O and V O bond lengths for 1-3 are are similar to those observed in related complexes (Singh et al., 2014;Abakumova et al. 2012;Meicheng et al., 1983;Silva et al., 2013;Kadirova et al., 2009). Most notable are variances in the V-N bond lengths in the complexes. In 1 and 2, the V-N bond lengths are of similar nature at 2.3861 (16) and 2.4022 (15) Å , respectively. However in 3, the V-N bond length is much shorter at 2.1140 (12) Å , likely from a combination of the cisisomeric structure in 3 and the electron-donating methoxy group of the 4-methoxypyridine ligand.

Figure 4
Crystal packing diagram of compound 1 with non-covalent interactions shown with dotted orange lines.

Figure 5
Crystal packing diagram of compound 2 with non-covalent interactions shown with dotted orange lines.

Synthesis and crystallization
Bis(acetylacetonato)oxovanadium(IV) (VO(acac) 2 ) and the N-donor ligands pyridine, 4-cyanopyridine, and 4-methoxypyridine were purchased and used without further purification. To an Anton Paar Monowave synthesis reactor vial, a 1:1 molar ratio of VO(acac) 2 and an N-donor ligand (0.75 mmol scale) was added and dissolved into 5 mL of dichloromethane. Once dissolved completely, each solution was reacted in an Anton Paar Monowave 50 synthesis reactor at 323 K for 5 min. Following all of the reactions, a slight precipitate was filtered and the resulting filtrate was allowed to slowly evaporate to produce single crystals suitable for X-ray diffraction studies. In addition to characterization by single crystal X-ray diffraction, each complex was characterized by

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. Single crystals were examined under Infineum V8512 oil. The datum crystal was affixed to a MiTeGen loop and transferred to the cold nitrogen stream of a Bruker APEXII diffractometer equipped with an Oxford Cryosystems 700 low-temperature apparatus. Unit-cell parameters were determined using reflections harvested from three sets of 12 0.5 ! scans scans. An optimal data-collection strategy was determined for an arbitrary hemisphere of data to 99.8% completeness to a resolution of 0.8 Å . (Bruker, 2015) Unit-cell parameters were refined using reflections harvested research communications from the data collection with I ! 10(I). All data were corrected for Lorentz and polarization effects, and runs were scaled using SADABS (Krause et al., 2015). The structures were solved using the Autostructure option within APEX3. This option employs an iterative application of the direct methods, Patterson synthesis, and dual-space routines of SHELXT (Sheldrick, 2015a). The models were refined routinely (SHELXL; Sheldrick, 2015b). Hydrogen atoms were placed at calculated geometries and allowed to ride on the position of the parent atom. Methyl H atoms were allowed to rotate but not to tip to best fit the experimental electron density. Hydrogen displacement parameters were set to 1.5U eq (C) for methyl and 1.2U eq (C) for all other hydrogen atoms.

Data collection
Bruker APEXII diffractometer Radiation source: fine-focus sealed tube Detector resolution: 8.33 pixels mm -1 combination of ω and φ-scans Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )

Bis(acetylacetonato-κ 2 O,O′)oxido(pyridine-4-carbonitrile-κN)vanadium(IV) (compound2)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.