Access to stable metalloradical cations with unsupported and isomeric metal-metal hemi-bonds
Metalloradical species [Co2Fv(CO)4].+ (1.+, Fv=fulvalenediyl) and [Co2Cp2(CO)4].+ (2.+, Cp=η5-C5H5), formed by one-electron oxidations of piano-stool cobalt carbonyl complexes, can be stabilized with weakly coordinating polyfluoroaluminate anions in the solid state. They feature a supported and an unsupported (i.e. unbridged) cobalt-cobalt three-electron σ bond, respectively, each with a formal bond order of 0.5 (hemi-bond). When Cp is replaced by bulkier Cp∗ (Cp∗=η5-C5Me5), an interchange between an unsupported radical [Co2Cp∗2(CO)4].+ (anti-3.+) and a supported radical [Co2Cp∗2(μ-CO)2(CO)2].+ (trans-3.+) is observed in solution, which cocrystallize and exist in the crystal phase. 2.+ and anti-3.+ are the first stable thus isolable examples that feature an unsupported metal-metal hemi-bond, and the coexistence of anti-3.+ and trans-3.+ in one crystal is unprecedented in the field of dinuclear metalloradical chemistry. The work suggests that more stable metalloradicals of metal-metal hemi-bonds may be accessible by using metal carbonyls together with large and weakly coordinating polyfluoroaluminate anions.
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