Abstract
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.