A journey into metal–carbon bond homolysis

Abstract

This article surveys the current knowledge in metal alkyl complexes with homolytically weak metal-carbon bonds, therefore prone to thermally produce alkyl radicals. It outlines the role of a metal complex as a moderator to control the radical reactivity ("persistent radical effect"). It describes the methods that have been used so far (as well as others that are potentially available) to investigate the metal-carbon bond cleavage thermodynamic and kinetic parameters, including their caveats and limitations. A number of systems scrutinized in the author's own laboratory and in those of collaborators are presented and discussed. These investigations have combined metal complexes and alkyl radicals, with guidance and understanding provided by DFT calculations, to achieve higher performance in the controlled radical polymerization of challenging monomers (vinyl acetate, vinylidene fluoride) and in olefin radical cross-coupling, and have brought to light mechanistic questions of more general relevance.

Cobalt–Carbon Bonding in a Salen-Supported Cobalt(IV) Alkyl Complex Postulated in Oxidative MHAT Catalysis

“…In the cobalt(IV) species 3, the BDFE was calculated to be 22 kcal/mol at −65 °C, which is stronger than the calculated cobalt(III)−carbon bond in the neutral analogue 2 (20 kcal/ mol). 17 This opposes the well-known trend that BDFE values decrease with increasing oxidation state, 39,40 which is expected since the homolysis of a metal−ligand bond formally reduces the metal by one electron. To test the idea that the Co IV −C bond in 3 is stronger than the Co III −C bond in 2, we used the thermochemical cycle in Scheme 3, which demonstrates that the difference between these BDFE values is equivalent to the energetic difference between the (salen Ph )Co(iPr) +/0 and (salen Ph )Co +/0 redox potentials.…”

“…1,3 This radical can bind reversibly to form cobalt−carbon bonds (Scheme 1), which lowers the concentration of the radical by an amount related to the homolytic bond dissociation free energy (BDFE) of the cobalt−alkyl complex. 16,17 The reversible homolysis of Co−C bonds has been established for cobalt(III) alkyl species, 18 biochemistry (methylcobalamin) and radical polymerizations. 17 However, most of the newly developed MHAT alkene reactions have two important differences from the previously studied systems: they utilize a strong oxidizing agent, most often fluorocollidinium (N-fluoro-2,4,6-trimethylpyridinium, ColF + ), and they enable the coupling of the alkyl fragment with nucleophiles.…”

“…The most popular catalysts for MHAT alkene reactions are salen-supported cobalt complexes, which catalyze alkene isomerization, hydroarylation, , hydroalkoxylation, , and epoxidation. , In these reactions, the alkene is treated with stoichiometric silane and oxidant in the presence of a salen–cobalt­(II) precatalyst. − Reaction of the cobalt species with silane presumably gives an unobserved cobalt­(III) hydride complex, which transfers an H atom to the alkene to form a cobalt­(II) complex and an alkyl radical. , This radical can bind reversibly to form cobalt–carbon bonds (Scheme ), which lowers the concentration of the radical by an amount related to the homolytic bond dissociation free energy (BDFE) of the cobalt–alkyl complex. , …”

Atom Transfer Radical Polymerization: A Mechanistic Perspective

“…Transition metal complexes can be employed to control the propagation of radicals via formation/dissociation of organometallic species, as in organometallic mediated radical polymerization (OMRP) . Cobalt complexes are the most employed in OMRP .…”

A Hydrometalation Initiation Mechanism via a Discrete Cobalt-Hydride for a Rapid and Controlled Radical Polymerization

“…Reversible deactivation radical polymerization (RDRP) techniques allow access to novel polymers by design and precise control of their molecular weight, dispersity, and other architectural properties. − In RDRP, reversible termination mechanisms include using persistent radicals such as alkoxyamines , or metal complexes , to mediate polymer chain growth. In particular, cobalt-mediated radical polymerizations (CMRP) use cobalt­(II) complexes as persistent radicals to deactivate propagating radicals and assert control over polymerization. − In CMRP, dormant polymer chains contain organocobalt end groups, which can reversibly dissociate to generate propagating radicals and Co II complexes as polymerization deactivators.…”