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

Treatment
of (iPrPDI)­Fe­(N2)2 (iPrPDI, 2,6-(2,6-iPr2C6H3NCMe)2C5H3N) with CO2 and ethylene
resulted in the formation of a homologous series of saturated and
unsaturated iron carboxylate products, (iPrPDI)­Fe­(O2CR), the distribution of which depends on the ratio of the
reagents. The solid-state and electronic structures of a saturated
product, (iPrPDI)­Fe­(O2CC2H5), 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, CO2 insertion, or reaction with H2 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 (iPrPDI)­Fe­(N2)2, supporting C–O
oxidative addition and C–C bond formation through metallacycle
intermediates.