Synthesis, characterisation and biological activities of mixed ligand oxovanadium (IV) complexes derived from N,N-diethyl-N′-para-substituted-benzoylthiourea and hydrotris(3,5-dimethylpyrazolyl)borate

A series of oxovanadium (IV) complexes namely, [Tp*VOL¹], [Tp*VOL²], [Tp*VOL³], [Tp*VOL⁴] and [Tp*VOL⁵] (where Tp = hydrotris(3,5-dimethylpyrazolyl)borate, HL¹ = 1-benzoyl-3,3-diethylthiourea, HL² = 1-(4-chlorobenzoyl)-3,3-diethylthiourea, HL³ = 1,1-diethyl-3-(4-methoxybenzoyl)thiourea, HL⁴ = 1,1-diethyl-3-(4-nitrobenzoyl)thiourea, HL⁵ = N-(diethylcarbamothioyl)biphenyl-4-carboxamide) were prepared and characterised on the basis of mass spectrometry, elemental analysis and spectroscopy techniques (IR, UV–Vis and electron paramagnetic resonance, EPR) and mass spectrometry. The crystal structure of [Tp*VOL³] was elucidated from single crystal X-ray diffraction study and the crystals adopted a triclinic system with a P-1 (Z = 2) space group and unit cell parameters of a = 10.4953(4), b = 11.5290(5) and c = 15.0656(7) Å. The cyclic voltammetry showed a reversible oxidation of V (IV)/V(V) at about 0.50 V (vs Fc⁺/Fc). The HOMO and LUMO of benzoylthiourea were established by theoretical calculation based on DFT level using B3LYP method and a double numeric plus polarisation (DNP) basis set. The in vitro tests for antimicrobial activity showed that the complexes have a moderate inhibitory effect. The complex [Tp*VOL¹] exhibited cytotoxic activity against the human hepatocellular carcinoma cell line (HepG2) at the concentration of 20 mg ml⁻¹. However, no cytotoxic activity was observed against the Chang liver cells.