Strong Antiferromagnetic Coupling of Spins in the (MDABCO + )(C 60 .− ) Salt with 3D Close Packing of the C 60 .− 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 100 K showed 3D close packing of fullerenes with eight fullerene neighbors for each C 60 .− . These neighbors are located at 10.01–10.11 Å center‐to‐center distances (250 K) and van der Waals interfullerene C⋅⋅⋅C contacts are formed with four fullerene neighbors arranged in the bc plane. Fullerene ordering observed below 160 K 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 100 K. As a result, fullerenes are located in the three‐dimensional packing at 9.91–10.12 Å center‐to‐center distances and 18 short interfullerene C⋅⋅⋅C contacts are formed for each C 60 .− . Although they are closed packed, fullerenes are not dimerized down to 1.9 K. Magnetic data indicate strong antiferromagnetic coupling of spins in the 70–300 K range with a Weiss temperature of Θ =−118 K. Magnetic susceptibility shows a round maximum at 46 K. 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 =−25.3 K. This magnetic coupling is one of the strongest observed for C 60 .− salts.