Realization of Organocerium-Based Fullerene Molecular Materials Showing Mott Insulator-Type Behavior

Electron-rich organocerium complexes (C5Me4H)3Ce and [(C5Me5)2Ce(ortho-oxa)], with redox potentials E1/2 = −0.82 V and E1/2 =
−0.86 V versus Fc/Fc+, respectively, were reacted
with fullerene (C60) in different stoichiometries to obtain
molecular materials. Structurally characterized cocrystals: [(C5Me4H)3Ce]2·C60 (1) and [(C5Me5)2Ce(ortho-oxa)]3·C60 (2) of C60 with cerium-based, molecular rare earth precursors
are reported for the first time. The extent of charge transfer in 1 and 2 was evaluated using a series of physical
measurements: FT-IR, Raman, solid-state UV–vis–NIR spectroscopy,
X-ray absorption near-edge structure (XANES) spectroscopy, and magnetic
susceptibility measurements. The physical measurements indicate that 1 and 2 comprise the cerium(III) oxidation state,
with formally neutral C60 as a cocrystal in both cases.
Pressure-dependent periodic density functional theory calculations
were performed to study the electronic structure of 1. Inclusion of a Hubbard-U parameter removes Ce f states from the
Fermi level, opens up a band gap, and stabilizes FM/AFM magnetic solutions
that are isoenergetic because of the large distances between the Ce(III)
cations. The electronic structure of this strongly correlated Mott
insulator-type system is reminiscent of the well-studied Ce2O3.