Iron-Mediated Coupling of Carbon Dioxide and Ethylene: Macrocyclic Metallalactones Enable Access to Various Carboxylates.

Treatment of (PDI)Fe(N) (PDI, 2,6-(2,6-iPrCHN═CMe)CHN) with CO and ethylene resulted in the formation of a homologous series of saturated and unsaturated iron carboxylate products, (PDI)Fe(OCR), the distribution of which depends on the ratio of the reagents. The solid-state and electronic structures of a saturated product, (PDI)Fe(OCCH), were elucidated. Product distributions, deuterium labeling studies, and stoichiometric experiments support initial formation of a five-membered metallalactone intermediate, which undergoes subsequent ethylene insertions to generate macrocyclic metallalactones. Competitive β-hydride elimination, CO insertion, or reaction with H determines the fate of the metallalactone, the latter accounting for formation of iron complexes with saturated carboxylates. Similar reactivity was observed upon addition of propiolactone and ethylene to (PDI)Fe(N), supporting C-O oxidative addition and C-C bond formation through metallacycle intermediates.