Cooperative Activation of CO 2 and Epoxide by a Heterobinuclear Al-Fe Complex via Radical Pair Mechanisms.

Activation of inert molecules like CO 2 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 2 and other substrates through cooperative behavior of two radical intermediates. The complex L dipp (Me)AlFp ( 2 , L dipp = HC{(CMe)(2,6- i Pr 2 C 6 H 3 N)} 2 , Fp = FeCp(CO) 2 , Cp = η 5 -C 5 H 5 ) was found to insert CO 2 and cyclohexene oxide, producing L dipp Al(Me)(μ:κ 2 -O 2 C)Fp ( 3 ) and L dipp Al(Me)(μ-OC 6 H 10 )Fp ( 4 ), respectively. Detailed mechanistic studies indicate unusual pathways in which (i) the Al-Fe bond dissociates homolytically to generate formally Al II and Fe I 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 dipp that stabilizes the formally Al II intermediates, instead giving them predominantly Al III -like physical character. The redox noninnocent nature of the radical intermediates was elucidated through direct observation of L dipp Al(Me)(OCPh 2 ) ( 22 ), a metalloradical species generated by addition of benzophenone to 2 . Complex 22 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 2 activation.