Mingling Light, Oxygen, and Organometallics to Form Cobalt-Carbon Bonds in the Confines of a Metal-Organic Nanocage

Abstract

First-row transition-metal complexes often show a propensity for forming reactive radical species, such as superoxide complexes (M–O2•) generated by the binding of O2 to the metal, or free alkyl-radicals formed via M–C homolysis. Such radicals are important intermediates in reactions catalyzed by synthetic metal complexes and metalloenzymes, but their high reactivity can lead to undesired side reactions such as quenching by solvent, oxygen, or other radicals. In this work, we show that confinement of a CoII porphyrin complex in a large porphyrin-walled M8L6 nanocage allows for the taming of radical reactivity to enable clean oxidative alkylation of the cobalt center with tetraalkyltin reagents via an unexpected process mediated by O2 and light, which usually promote homolytic decomposition of porphyrin-supported CoIII–alkyl bonds. Indeed, analogous CoIII–alkyl complexes in free solution degrade too quickly under the alkylating conditions to enable their clean formation. The nanocage also acts as a size-selective barrier for alkylating agents, allowing CoIII–alkyl formation using SnMe4 and SnEt4 but not SnBu4. Likewise, Co–C homolysis is facilitated by the persist radical reagent TEMPO but not by a bulky derivative of TEMPO. These results show that nanoconfinement is a promising strategy for guiding radical-based organometallic reactivity under otherwise prohibitive conditions.