Electronic Communication between S=1/2 Spins in Negatively-charged Double-caged Fullerene C Derivative Bonded by Two Single Bonds and Pyrrolizidine Bridge.

Radical anion salt {cryptand[2.2.2] (K )} (bispheroid) ⋅3.5C H Cl (1) of the double-caged fullerene C derivative, in which fullerene cages are linked by a cyclobutane bridging cycle and additionally by a pyrrolizidine moiety, was obtained. Each fullerene cage in this derivative accepts one electron on reduction, thus forming the (bispheroid) dianions with two interacting S=1/2 spins on the neighboring cages. Low-temperature magnetic measurements reveal a singlet ground state of the bispheroid dianions whereas triplet contributions prevail at increased temperature. An estimated exchange interaction between two spins J/k =-78 K in 1 indicates strong magnetic coupling between them, nearly two times higher than that (J/k =-44.7 K) in previously studied (C ) dimers linked via a cyclobutane bridge only. The enhancement of magnetic coupling in 1 can be explained by a shorter distance between the fullerene cages and, possibly, an additional channel for the magnetic exchange provided by a pyrrolizidine bridge. Quantum-chemical calculations of the lowest electronic state of the dianions by means of multi-configuration quasi-degenerate perturbation theory support the experimental findings.