Cooperative Activation of CO₂ and Epoxide by a Heterobinuclear Al–Fe Complex via Radical Pair Mechanisms

Abstract

Activation of inert molecules like CO2 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 CO2 and other substrates through cooperative behavior of two radical intermediates. The complex Ldipp(Me)­AlFp (2, Ldipp = HC­{(CMe)­(2,6- i Pr2C6H3N)}2, Fp = FeCp­(CO)2, Cp = η5-C5H5) was found to insert CO2 and cyclohexene oxide, producing LdippAl­(Me)­(μ:κ2-O2C)­Fp (3) and LdippAl­(Me)­(μ-OC6H10)­Fp (4), respectively. Detailed mechanistic studies indicate unusual pathways 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 noninnocent nature of Ldipp that stabilizes the formally AlII intermediates, instead giving them predominantly AlIII-like physical character. The redox noninnocent nature of the radical intermediates was elucidated through direct observation of LdippAl­(Me)­(OCPh2) (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 CO2 activation.

Divergent CO2 Activation by Tuning the Lewis Acid in Iron‐Based Bimetallic Systems

“…Monitoring the reaction by 31 P{ 1 H} NMR we observed the complete consumption of the starting iron species (87.0, 79.0, 70.0 and 62.6 ppm) and the appearance of a new set of three broad signals (85.1, 80.4 and 70.4 ppm, with relative intensities 1 : 1 : 2 in benzene-d 6 ). The corresponding 11 B-{ 1 H} NMR spectrum exhibited a broad signal at À 1.6 ppm similar to the chemical shift observed for the somewhat related [(depe) 2 Fe(N 2 )B(C 6 F 5 ) 3 ] product (À 6.3 ppm) from N 2 -activation. [18] IR spectroscopy provided relevant information, as two stretch bands were found at 1641 cm À 1 and 1251 cm À 1 indicative of a CO 2 unit.…”

Recent developments on the transformation of CO2utilising ligand cooperation and related strategies

“…178,179 In one such example, the reaction with CO 2 (1 atm) resulted in its insertion into the Al–Au bond to form the gold( i ) metalla-carboxylate Al(μ-O 2 C)Au, where the μ-κ 1 - C ,κ 2 - O , O bridge was supported by cooperative Au–η 1 - C and Lewis acidic Al–κ 2 - O , O bonds. 180 Similarly, a number of other heterobimetallic complexes containing Cu–Al, 181 Ir–Al, 182 and Fe–Al 183 bonds were reported. The examples discussed above highlight the cooperation between nucleophilic and Lewis acidic metal centres.…”

Recent advances in cooperative activation of CO2and N2O by bimetallic coordination complexes or binuclear reaction pathways

“…Camp and co-workers described a unique example where an alkylaluminum Lewis acid induced umpolung character to Very recently our group reported cooperative activation of CO 2 by a heterobimetallic Al-Fe complex. 49…”

“…Very recently our group reported cooperative activation of CO 2 by a heterobimetallic Al–Fe complex. 49 The complex, L(Me)Al–Fp ( 19a ) (L = HC{(CMe)(2,6- i Pr 2 C 6 H 3 N)} 2 , Fp = FeCp(CO) 2 ), was found to react with CO 2 cleanly at room temperature and 1 atmosphere pressure. The CO 2 inserted product L(Me)Al(O 2 C)Fp ( 19b ) was isolated as a white solid in excellent yield (Scheme 19).…”