Unusual Seven Coordination of Oxovanadium(V) Oximato Complex: Synthesis and X-Ray Crystal Structure

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–H O and C–H？O. “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