Formation of Singly Bonded {CpCo(CO)-η ² -C ₆₀ ^– } ₂ Dimers upon the Dimerization of the CpCo(CO)-η ² -C ₆₀ ^•– Radical Anions

Interaction of {Cryptand(K+)}(C60•–) with CpCo(CO)2 yields the air-sensitive crystalline salt {Cryptand(K+)}{CpCo(CO)-η2-C60•–} (1), in which C60•– replaces carbonyl group in CpCo(CO)2. Two Co–C bonds of 2.028(9) and 2.057(8)Å are formed. The magnetic moment of 1 is 1.72 μB, indicating a doublet S = 1/2 spin state of {CpCo(CO)-η2-C60•–} at 300 K. The compound shows a broad EPR signal at g = 2.0000 with a line width of 3.93 mT (295 K), characteristic of C60•–, confirming spin and electron density localization on the fullerene cage. This is the first coordination complex of a transition metal with C60•–. Below 220 K, {CpCo(CO)-η2-C60•–} dimerizes to form diamagnetic singly bonded {CpCo(CO)-η2-C60–}2 dimers. Carbon atoms of the [6,6,5] junctions form an intercage single C–C bond of 1.619(18)Å. Dimerization elongates the Co–C bonds to 2.098(13) and 2.106(13)Å. Approximately 20% of S = 1/2 spins remain after dimerization indicating partial preservation of the paramagnetic monomeric phase. Coordination of C60•– to CpCoCO becomes asymmetric in this phase, with shorter and longer Co–C bonds of 2.012(14) and 2.262(14) Å, converting the η2-type to η -type coordination. Dimerization is partially suppressed owing to the formation of densely packed C60•– chains, where radical anions align hexagon-over-hexagon in parallel with close interplanar distances.