English abstract
Several variously substituted tridentate thiosemicarbazones possessing phenolic, naphtholic or pyridyl moieties were synthesized by stepwise or template reactions. The chemical compositions of these Schiff bases were ascertained by elemental analyses. Vibrational spectroscopy demonstrated that these ligands have a preference for the thione tautomer in the solid state. This observation was further corroborated by single-crystal X-ray structure determination of the representative pyridyl thiosemicarbazone Hbpetsc and the phenolic thiosemicarbazone H2(3,5-'Buz)-saletsc. However, these ligands differed in the conformations they adopted.
Treatment of each of the thiosemicarbazones with the precursor complex [VO(acac)2] or the salt VOSO4-XH20 under different reaction conditions afforded an array of high valent vanadium complexes. The thiosemicarbazones exhibited remarkable coordination versatility as they readily underwent base- or metal-assisted tautomerisation. Likewise, vanadium demonstrated its peculiar tendency to exist in different cationic forms as demanded by the Schiff-base ligand: non-oxo V4* or VO2+ in the tetravalent state and Vo* or VO2* in the pentavalent state. These high-valent complexes were formulated in accordance with microanalyses, IR spectroscopy and representative X-ray structure determinations.
The novel non-oxo vanadium(IV) complex (V{(3,5-'Buz)-saldmtsc}2] was obtained from the reaction of stoichiometric amounts of H2(3,5-Buz)-saldmtsc with [VO(acac)2] in basic medium by spontaneous deoxygenation of the vanadyl moiety occurred. The oxovanadium(IV) complexes [VO{(3,5-Buz)-salatsc} (phen)].MeOH, [VO{(3,5-'Buz)-saldmtsc) (phen)] and [VO{(3,5-'Buz)-saldmtsc)(4,4-Mez-bipy)] are the first examples of crystallographically elucidated ternary oxovanadium(IV) complexes of thiosemicarbazones. The dioxovanadium(V) complex (VO2(dpkdmtsc) is one of a very few structurally characterized thiosemicarbazone complexes of this type. The [VO2]* core shows cis coordination of the oxo atoms in a distorted square pyramidal structure, with the V=Oap shorter than the V=Ocg For the VO" cation, only one compound was formed, namely the five-coordinate complex [VO{(3,5-'Buz)-saldmtsc}(OM)]. However, this cation was stabilized easily by (tsc)(hq)] (Hatsc = phenolic or naphtholic thiosemicarbazone).
Variable-temperature magnetic susceptibility measurements proved that the oxovanadium(IV) and non-oxo vanadium(IV) complexes are paramagnetic with one unpaired electron. The axial X-band ESR spectra showed that the single electron of vanadium(IV) in these complexes resides in the dxy orbitai. Electronic absorption spectroscopy revealed a broad absorption band indicative of an overlap of three expected ligand field absorptions for these complexes. On the other hand, the [VO2(tsc)] or (VO2(Htsc)] exhibited only an LMCT band in the visible spectrum consistent with the absence of d electrons. Finally, the complexes [VO{(3,5-'Buz) saldmtsc) (phen)], [VO{(3,5-Buz)-salatsc) (phen)] and [VO{(3,5-'Buz) saldmtsc}(4,4-Mez-bipy)] displayed quasi-reversible cyclic voltammetric response associated with the VOH/VO2+ redox couple.
