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

The versatile N2S2 tetradentate ligands (bme-daco)(2-), (bme-dach)(4-), and (ema)(4-) are known to accommodate many divalent transition-metal ions (M = Ni-II, Pd-II, Pt-II, Pb-II, Zn-II, Cd-II, Cu-II, and Fe-II) 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/sulfurdonor 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 [gamma(VO)], electronic spectral, and electron paramagnetic resonance measurements are presented. X-ray diffraction studies of (bme-daco)(V 0), (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.