Abstract
We report the synthesis, characterization, and reactivity of a Cr(II) complex, [Cr(H2O)(LISQ)2] (1) [(LISQ)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-CrIII species, [Cr(O2)(OH)(LISQ)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-CrIII 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)(LISQ)2]− which undergoes disproportionation producing an unstable O=Cr(V) species, [O=Cr(OH)(LISQ)2] and a stable Cr(III) compound, [Cr(OH)(DMF)(LISQ)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.
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.