Cooperative Activation of CO and Epoxide by a Heterobinuclear Al-Fe Complex via Radical Pair Mechanisms.

Activation of inert molecules like CO is often mediated by cooperative chemistry between two reactive sites within a catalytic 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 CO and other substrates through cooperative behavior of two radical intermediates. The complex L(Me)AlFp (, L = HC{(CMe)(2,6-PrCHN)}, Fp = FeCp(CO), Cp = η-CH) was found to insert CO and cyclohexene oxide, producing LAl(Me)(μ:κ-OC)Fp () and LAl(Me)(μ-OCH)Fp (), respectively. Detailed mechanistic studies indicate unusual pathways in which (i) the Al-Fe bond dissociates homolytically to generate formally Al and Fe 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 noninnocent nature of L that stabilizes the formally Al intermediates, instead giving them predominantly Al-like physical character. The redox noninnocent nature of the radical intermediates was elucidated through direct observation of LAl(Me)(OCPh) (), a metalloradical species generated by addition of benzophenone to . Complex was characterized by X-band EPR, Q-band EPR, and ENDOR spectroscopies as well as computational modeling. The "radical pair" pathway represents an unprecedented mechanism for CO activation.