Reductive Cyclopentamerization of Carbon Monoxide to the Elusive Croconate Radical Trianion

Coupling reactions of carbon monoxide represent an ideal synthetic route to cyclic oxocarbons. Whereas oxocarbon anions based on rings of three, four and six carbon atoms have been synthesized from CO coupling, cyclic five-membered croconate anions have evaded capture by this method. Here, we show that CO coupling initiated by rare-earth dinitrogen complexes in concert with molybdenum hexacarbonyl yields rare-earth complexes of the exotic croconate radical trianion, [C₅O₅]^3-, a species only observed previously under cryogenic irradiation. Contrasting reactions of the rare-earth dinitrogen complexes with CO alone produce ketene-carboxylate anions, highlighting the crucial templating role of molybdenum in forming croconate. Spectroscopic and computational analyses establish the radical nature of the croconate anion and reveal a heterobimetallic reaction pathway for its assembly with multielectron transfer. This discovery surmounts a long-standing challenge in oxocarbon chemistry, illustrating how strongly reducing rare-earth compounds and mixed-metal cooperativity can enable the isolation of sought-after organic radicals.