The new oxovanadium(V) complex, [VO(acac)(dametsc)] (1) (acac = acetylacetonate (-1), H2dametsc = diacetylmonoxime-4-ethylthiosemicarbazone), has been prepared and characterized by studying its physicochemical properties. The X-ray crystal structure of the complex (1) has been determined and showed the presence of vanadium(V) atom in a unique heptacoordination state with distorted pentagonal bipyramidal geometry. The oximato ligand in the pentagonal plane is bonded to the central vanadium atom in dihapto manner with the formation of three membered ring, while the other three coordination sites in the plane are occupied by hydrazinic imine nitrogen, thiolate sulfur, and one of the acac oxygen atoms. The axial position sites are defined by the other acac oxygen and the trans oxo-atom. The supramolecular structure of the complex is exclusively constructed by intermolecular interactions, N–HO and C–HO.

“This paper is dedicated to Professor William (Bill) P. Griffith, Imperial College, London, for his many contributions in coordination chemistry and their X-ray crystal structure determinations.”

1. Introduction

Although heptacoordinate pentagonal bipyramidal vanadium compounds are still rare with conventional ligands, peroxo (η2-O, O) and oximato (η2-O, N) vanadium(V) complexes usually adopt this structure [1, 2]. Iron(III) and nickel(II) complexes of meridional bis-diacetylmonoxime Schiff base of S-methyl/S-benzyldithiocarbazate (H2damsm/H2damsb), [Fe(Hdamsm)2]+ (2), and [Ni(Hdamsb)2] (3), each complex had a distorted octahedral geometry in which the ligand behaves as a monoanionic NNS-tridentate and coordinates via oxime nitrogen, hydrazinic imine nitrogen, and thiolate sulfur with silent hydroxo oxime group [3, 4]. There is interest in the coordination chemistry of oximes as they have served as models for biological systems such as vitamin B12 and myocardial perfusion imaging agents [5]. Oxovanadium(V) complexes associated with one acetylacetonato group have not been isolated so far, though their effectiveness as active intermediate in peroxide oxidation of thioether to sulfoxide [6]. We report here the synthesis and X-ray crystal structure of the new oxovanadium(V) complex (1) in its unusual heptacoordination geometry containing both acetylacetonato and oximato ligands.

2. Experimental

2.1. Synthesis

The ligand of diacetylmonoxime-4-ethylthiosemicarbazone (H2dametsc) is prepared by using a similar method to that previously reported in [3]. To a solution of H2dametsc (0.108 g, 0.5 mmol) in hot methanol (12 cm3), [VO(acac)2] [7] (0.133 g, 0.5 mmol) was added. The mixture was then stirred and refluxed on steam bath for 1 h; reddish brown solution was obtained and left for slow evaporation at room temperature. Red crystals so formed were suitable for X-ray diffraction; they were filtrated off, washed carefully with little by methanol, followed by ether, and dried in vacuo. Yield, 0.13 g (70%); Anal. Calc. for C12H19N4O4SV: C, 39.3; H, 5.2; N, 15.3%. Found: C, 39.1; H, 5.1; N, 15.2%. IR (KBr disc): 3241 m [ν(N–H)]; 1566 νs, 1379 s [ν(C=O), acac]; 943 νs [ν(V=O)]. UV-Vis. (CH2Cl2); 325 nm (ε, 11350 M−1 cm−1). Molar conductivity (Ω−1 cm2 mol−1), 6 (MeOH); = 0 BM.

2.2. X-Ray Crystallographic Study

Red monoclinic crystals of [VO(acac)(dametsc)] (1) having appropriate dimensions were measured on Bruker Kappa CCD diffract meter equipped with graphite-monochromated MoKα radiation (λ = 0.71073 Å); the unit cell dimensions and intensity data were measured at 298 K. The crystal data were collected up to 54.97° in 2θ and the structure was solved by least squares fit of the angular setting of strong reflection based on F2. The program used to solve structure was SIR92 [8], while the program used to refine structure was maXus [9]. Integration and scaling of the reflections were performed with the HKL Denzo-Scalepack system of programs [10]. The nonhydrogen atoms were refined with anisotropic thermal parameters. Crystallographic data for (1) are summarized in Table 1.

CCDC795958 contains the supplementary crystallographic data for the paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre.

3. Results and Discussion

Our oxovanadium(V) complex (1) was prepared under aerobic conditions in MeOH, by exchange of one acac in with the tetradentate ligand in its di-deprotonated form (dametsc2−). A perspective view of the complex (1) is illustrated in Figure 1. As indicated by the bond angles in Table 2, the vanadium atom has the pentagonal bipyramid geometry. In particular, the sum of angles of the pentagonal plane (~357.4°), which is bite less than the ideal value of 360°, the axial, O7–V–O3 has the angle 177.37 (8)° (near 180°), similarly typical to that found for [(teg)(Br)2]Br (teg = pentadentate tetraglyme ligand, CH3O(CH2CH2O)4CH3) [11]. The axial V=O7 [1.586 (2) Å] is arranged symmetrically above the approximate pentagonal plane in which two V–O6,8 bonds, two V–N4,9 bonds, and one V–S2 bond all make angles ranging from 94.52 (9)° to 98.82 (9)°. The oximato ligand is bonded in a dihapto (η2-N, O) manner with the formation of a three membered ring that has an angle, O8–V–N9 equal to 37.28 (8)°, similar to those found for other η2-NO-oximato vanadium(V) complexes [2].

The longer bond length C11–S2 [1.721 (3) Å] is close to that found for similar related complexes (2) and (3) which had N,N,S-donor ligands chelated in their thiolate form, in contrast to the shorter lengths for the C=S (~1.68 Å) double bond (thione form) found in the free ligand and its nickel(II) complex [Ni(H2damtsc)]Cl2 [12]. This thiol form is also identified by the fact that the hydrazinic nitrogen N4 is not bonded to any hydrogen atom and has the distance, C11–N4 [1.338 (3) Å], that is in the range for C=N (~1.30 Å) double bond [13]. Similar bond lengths for N4–N5 [1.367(3) Å] were observed for the complexes (2) and (3) [3, 4].

The trans-influence of oximato oxygen (O8) and oxo ligand (O7) on S2 and O3 atoms, respectively, manifests itself in elongation of V–S2 [2.4899 (9) Å], compared to a short bond length in related N, N, S-thiolato oxime metal complexes (2) and (3) [M–S, ~2.22–2.30 Å], and also the distance V–O3 [2.159 (2) Å] is longer than V–O6 [1.964 (2) Å] for the chelated O3, O6 (in acac ligand) to vanadium atom.

In structure 1, the coordinated heteroligands, acac and dametsc2−, are both anionic; however, they differ in the charge. The doubly charged ligand is preferentially bound to vanadium atom in the equatorial plane, while the monocharged bidentate ligand is chelated in both the equatorial and axial positions. This is similarly observed for O, O and N, O-bidentate peroxo oxalate (ox2−) and picolinato (pic) ligands in the pentagonal bipyramid complex, [(O2)(pic)(ox)] [14].

Figure 2 shows the intermolecular hydrogen bonding (for HO interactions) characterized in the lattice for complex (1). Each pair of (1) molecules is interconnected via hydrogen bonds; oxo ligand (O7) and oxygen atom (O6) of acac ligand from one molecule are attached with the other neighboring molecule through hydrogen atoms in N10–H10 and C22–H22B, respectively. Strong interaction has been found for N10–H10O7 [dH(H10O7) = 2.040 Å] compared to C22–H22BO6 [dH(H22BO6) = 2.676 (2) Å] (Table 3). However, the latter C–HO interaction is somewhat scarce in coordination compounds; it has been recognized to play an important role in protein structure and stability [15]. The theoretical calculations showed that the C–HO association energy, −2.1 kcalmol−1, essentially contributes to the structure stabilization [16].

4. Conclusion

The ligand, diacetylmonoxime-4-ethylthiosemicarbazone (H2dametsc), reacts with [VO(acac)2] in aerobic conditions to form the new oxovanadium(V) complex, [VO(acac)(dametsc)]. The X-ray crystal structure of the complex has been determined and shows that it has a distorted pentagonal bipyramid geometry. The intermolecular interactions, N–HO and C–HO, stabilize the supramolecular structure of the complex.

Conflict of Interests

The authors declare that they do not have conflict of interests regarding the publication of this paper.


The authors thank National Research Centre in Cairo for the X-ray crystallographic facility.