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

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-) pi-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(O-2)(OH)(L-ISQ)(2)](-) is generated initially in DMF solution of 1 in the presence of air owing to its immediate deprotonation followed by O-2 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(O-2)(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.