Cooperative C-O Bond Activation by a Heterobinuclear Al-Fe Complex Operating by a Radical Pair Mechanism

Activation of inert molecules like CO2 is often mediated by cooperative chemistry between two reactive sites within a cata-lytic assembly, the most common form of which is Lewis acid/base bifunctionality observed in both natural metalloenzymes and synthetic systems. Here, we disclose a heterobinuclear complex with an Al-Fe bond that instead activates CO2 and other substrates through cooperative behavior of two radical intermediates. The complex L(Me)Al-Fp (2, L = HC{(CMe)(2,6-iPr2C6H3N)}2, Fp = FeCp(CO)2) was found to insert CO2 and cyclohexene oxide, producing LAl(Me)(µ:κ2-O2C)Fp (3) and LAl(Me)(µ-OC6H10)Fp (4), respectively. Further atom transfer, decarbonylation, and isomerization reactivity was also ob-served. Detailed mechanistic studies on the CO2 and epoxide insertion reactions indicate an unusual mechanism in which (i) the Al-Fe bond dissociates homolytically to generate formally AlII and FeI metalloradicals, then (ii) the metalloradicals add to substrate in a pairwise fashion initiated by O-coordination to Al. The accessibility of this unusual mechanism is aided, in part, by the redox non-innocent nature of L that stabilizes formally AlII intermediates with predominantly AlIII-like character. This “radical pair” pathway represents an unprecedented mechanism for CO2 activation.