Dioxygen binding and activation by a highly reactive Cr(II) compound containing S,N-donors derived from o-aminothiophenol

Abstract

We report the synthesis, characterization, and reactivity of a Cr(II) complex, [Cr(H₂O)(L^ISQ)₂] (1) [(L^ISQ)¹⁻ is o-iminothionebenzosemiquinonate(1−) π-radical], that is highly stable in solid state in the presence of air but undergoes spontaneous change in solution, both in the presence and absence of air. Physicochemical studies in solution show that a superoxo-Cr^III species, [Cr(O₂)(OH)(L^ISQ)₂]⁻ is generated initially in DMF solution of 1 in the presence of air owing to its immediate deprotonation followed by O₂ binding to the deprotonated species. The formation of this superoxo-Cr^III species is prominent and gradual in the presence of CH₃OH, a scavenger of CrO²⁺ species. This Cr(O₂)²⁺ species in turn is converted to another highly reactive O=Cr(IV) intermediate [O=Cr(OH)(L^ISQ)₂]⁻ which undergoes disproportionation producing an unstable O=Cr(V) species, [O=Cr(OH)(L^ISQ)₂] and a stable Cr(III) compound, [Cr(OH)(DMF)(L^ISQ)₂] (2). The rate of this disproportionation is enhanced in the presence of MnCl₂, [N(n-Bu)₄]PF₆ and KSCN. The generated O=Cr(IV) species interacts with DNA with complete cleavage. The O=Cr(V) species slowly disappears from solution as revealed from EPR studies.

Keywords:

• Superoxo-Cr(III) complex
• Oxo-chromium(IV) complex
• Oxo-chromium(V) complex
• EPR and electronic spectra
• DNA interaction

Acknowledgments

M.K. thanks the Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, for the help with the elemental analysis and IR spectrum. We thank the SAIF, IIT-Madras for EPR and magnetic moment measurements and CLRI for providing EPR facilities. A.P.K. thanks Dr C.S. Gopinath, Catalysis Division, National Chemical Laboratory, Pune, for his help with the ESCA studies. P.T.M. thanks the DST, Govt. of India for Ramanna Fellowship (SR/S1/RFIC-02/2006) and also the INSA for its Senior Scientistship. We thank the Reviewers for their valuable suggestions to improve the manuscript.