Strong Antiferromagnetic Coupling of Spins in the (MDABCO+)(C60•-) Salt with 3D Close Packing of the C60•- Radical Anions (MDABCO+: N-Methyldiazabicyclooctanium Cation)

A new salt, (MDABCO(+))(C-60(.-)) (1; MDABCO(+)=N-methyldiazabicyclooctanium cation), was obtained as single crystals. The crystal structure of 1 determined at 250 and 100K showed 3D close packing of fullerenes with eight fullerene neighbors for each C-60(.-). These neighbors are located at 10.01-10.11 angstrom center-to-center distances (250K) and van der Waals interfullerene CC contacts are formed with four fullerene neighbors arranged in the bc plane. Fullerene ordering observed below 160K is accompanied by the appearance of one and a half independent C-60(.-) and trebling of the unit cell along the b axis. Fullerenes are packed closer to each other at 100K. As a result, fullerenes are located in the three-dimensional packing at 9.91-10.12 angstrom center-to-center distances and 18 short interfullerene CC contacts are formed for each C-60(.-). Although they are closed packed, fullerenes are not dimerized down to 1.9K. Magnetic data indicate strong antiferromagnetic coupling of spins in the 70-300K range with a Weiss temperature of =-118K. Magnetic susceptibility shows a round maximum at 46K. Such behavior can be described well by the Heisenberg model for square two-dimensional antiferromagnetic coupling of spins with an exchange interaction of J/k(B)=-25.3K. This magnetic coupling is one of the strongest observed for C-60(.-) salts.