Oxidovanadium complexes with N-donor heterocyclic chelates.
The growing significance of vanadium in medicinal inorganic chemistry is due to the diverse biological activities of its metal complexes, as elaborated in Chapter 1. These biological activities stem from the fact that vanadium is an essential trace element as well as its ability to form active pro-drugs under physiological conditions. To improve the bio-availability of these potential metallopharmaceuticals, the use of biologically relevant ligand systems such as heterocyclic ligands were considered. These chelators should provide the stability and the ability to promote absorption through cell-membranes. The techniques as described in Chapter 2 were employed to analyze and characterize the formulated heterocyclic ligands and their metal complexes. The attained research findings are mainly divided into two studies which involve the explorative coordination chemistry of two classes of ligands: 2-pyridylbenzimidazole (see Chapter 3) and 2-phenylsubstituted heterocyclics (see Chapter 4). An additional brief study is described in Chapter 5 which discusses the attempted coordination of a uracil Schiff base ligand. In Chapter 3, the coordination behaviour of Hpybz (2-pyridylbenzimidazole) towards vanadium in various oxidation states (i.e. +III/IV/V) was explored. The six-coordinate complex cis- [VVO2(Hpybz)(pybz)] (1) was isolated as the CH3OH.(H2O)2 hydrate from the reaction of NH4VO3 and Hpybz in aqueous methanol. The crystal structure shows that the vanadium is bonded to two cis-oxido ligands, and to the two bidentate ligands pybz and Hpybz. This combination of ligands confers six-coordination on the metal centre, which is a rare coordination number for a mononuclear dioxido complex of vanadium(V). From the reaction between Hpybz and VCl3 the cationic complex salt cis-[VIII(OH)2(Hpybz)2]Cl (2) was formed. The ligands in cis- [VIII(OH)2(Hpybz)2]Cl exhibits the same coordination behaviour as in 1, but instead of the dioxido moiety present in 1, two hydroxyl co-ligands are coordinated to the metal centre, with both chelator ligands neutral. Conductivity measurements in DMF affirmed that the compound is a 1:1 electrolyte. A novel binuclear mixed-valence oxidovanadium compound, (μ- O)[VVO(pybz)2.VIVO(Hpybz)(acac)] (3), was obtained from the reaction of Hpybz with VO(acac)2. ESR analysis illustrates paramagnetic behaviour typical of a type I dimer. The metal compound, VO(Hpbyz)2SO4 (4).H2O was isolated in a good yield from the reaction of two equivalents of Hpybz with vanadyl sulfate. Chapter 4 reports the isolation of oxidovanadium compounds with 2-phenylsubstituted benz(imidazole/othiazole/oxazole) chelators. The 2:1 molar reaction between NH4VO3 and 2- hydroxyphenylbenzothiazole (Hobs) led to the formation of a polynuclear vanadium(IV) complex, [VO(obs)2]n (1). The atmospheric oxygen-induced oxidation reaction of VCl3 and 2- hydroxyphenyl-1H-benzoxazole (Hobo) afforded a similar oxidovanadium compound, [VO(obo)2]n (2). A characteristic eight-line isotropic signal was observed in the ESR spectrum of 2 in DMF while, due to the poor solubility of 1, a singlet was attained upon analysis of the single crystals. A diamagnetic dioxidovanadium(V) complex, cis-[VO2(obz)py] (3) (Hobz = 2- hydroxyphenyl-1H-benzimidazole) was isolated from the reaction of NH4VO3 and Hobz in a methanolic solution. A broad singlet is found in the 51V NMR spectrum at -520.7 ppm for the d0-vanadium centre. The intra-ligand (π-π*) relaxations [466 nm for 1, 376 nm for 2 and 469 nm for 3] could be observed in the emission spectra which were obtained in anhydrous DMF. In an effort to synthesize a coordination compound of vanadium, the reaction of a heterocyclic ligand, 2-mercaptophenyl-1H-benzimidazole (Hsbz) with vanadyl sulfate resulted in an unexpected reaction product, [C26H20N4S2].[SO4].4H2O (4). In Chapter 5, the metal-induced cyclization of 5-amino-6-[(Z)-(2-hydroxybenzylidene)amino]- 1,3-dimethylpyrimidine-2,4-(1H, 3H)-dione (H3duo) by NH4VO3 resulted in the formation of a cyclized benzimidazole derivative, 8-(2-hydroxyphenyl)-1,3-dimethyl-1H-purine-2,6-(3H, 7H)- dione (1). The IR spectra of H3duo and its cyclized form are nearly identical where only minor shifts in the significant bands are observed. The molecular transformation was more evident when comparing the 1H NMR spectra of H3duo and 1.