Tetradentate N₂S₂Vanadyl(IV) Coordination Complexes: Synthesis, Characterization, and Reactivity Studies

Abstract

The versatile N(2)S(2) tetradentate ligands (bme-daco)(2-), (bme-dach)(2-), 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 N(2)S(2)M 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 N(2)S(2)Ni complexes have been prepared as mimics of possible N(2)S(2)(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 [Et(4)N](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 N(2)S(2) ligand, is given.