Tetradentate N2S2 Vanadyl(IV) Coordination Complexes: Synthesis, Characterization, and Reactivity Studies

The versatile N2S2 tetradentate ligands (bme-daco)2−, (bme-dach)2−, and (ema)4− are known to accommodate many divalent transition-metal ions (M = NiII, PdII, PtII, PbII, ZnII, CdII, CuII, and FeII) while maintaining reactivity at the S-thiolate sites of the respective N2S2M complexes. The vanadyl ion, of interest for its pharmacological possibilities and its spin-label reporter properties for bioinorganic studies, also shows an affinity for such mixed nitrogen/sulfur-donor environments. Thus, (VO)2+ analogues of a well-characterized series of N2S2Ni complexes have been prepared as mimics of possible N2S2(VO) formed from in vivo binding sites of the tripeptide motif, Cys−X−Cys. The nucleophilicity of the S-thiolate in these systems is explored with alkylating agents. IR [ν(VO)], electronic spectral, and electron paramagnetic resonance measurements are presented. X-ray diffraction studies of (bme-daco)(VO), (bme-dach)(VO), and [Et4N]2[(ema)(VO)] further characterize the vanadyl complexes. A comparison of the spectral properties with the product of vanadyl interaction with the CGC tripeptide, the biological analogue of the tetraanionic N2S2 ligand, is given.