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

Abstract

A new salt, (MDABCO(+))(C60(·-)) (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 C60(·-). 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 C60(·-) 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 C60(·-). 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/kB = -25.3 K. This magnetic coupling is one of the strongest observed for C60(·-) salts.

Synthesis, Structure, and Properties of the Fullerene C60 Salt of Crystal Violet, (CV+)(C60.−)⋅0.5 C6H4Cl2, which Contained Closely Packed Zigzagged C60.− Chains

“…The UV/Vis–NIR spectra of starting salt (CV + )(Cl − ), salt 1 , and reference salt (MDABCO + )(C 60 .− ) are shown in Figure . The CV + cation showed an intense absorption band within the visible‐light range, with a maximum at 16 950 cm −1 (590 nm) and a weaker band at 32 570 cm −1 (307 nm).…”

“…Fullerenes have shown promising conducting and magnetic properties in the radical anion state . For example, monoanions in (M + )(C 60 − ) (M=K, Rb, Cs) form 1D polymers with metallic properties .…”

“…Several compounds with quasi‐1D/‐2D metallic conductivity have been isolated as crystals from solution: (Na + ) x (THF) y C 60 ( x ≈0.4, y ≈2.2), (K + )(THF) 5 (C 60 .− ) ⋅ 2 THF, (MDABCO + )(C 60 .− ) ⋅ TPC (MDABCO= N ‐methyldiazabicyclooctanium cation; TPC=triptycene), and [cryptand{2,2,2}(Na + )](C 60 ) 3 − ⋅ 2 C 6 H 4 Cl 2 . Fullerene salts with monomeric C 60 .− have also shown strong magnetic spin coupling with ferro‐ or antiferromagnetic ordering. For example, phase transition into a weak ferromagnet was detected at 16 K in (TDAE + )(C 60 .− ) (TDAE=tetrakis(dimethylamino)ethylene), whilst magnetic susceptibility measurements showed a maximum at 46 K, with an exchange interaction of J / k B =−25.4 K in (MDABCO + )(C 60 .− ) .…”

“…Fullerene salts with monomeric C 60 .− have also shown strong magnetic spin coupling with ferro‐ or antiferromagnetic ordering. For example, phase transition into a weak ferromagnet was detected at 16 K in (TDAE + )(C 60 .− ) (TDAE=tetrakis(dimethylamino)ethylene), whilst magnetic susceptibility measurements showed a maximum at 46 K, with an exchange interaction of J / k B =−25.4 K in (MDABCO + )(C 60 .− ) . However, in some cases, strong magnetic coupling could not order the spins, even down to 1.9 K, most probably owing to spin frustration, as in (Ph 3 MeP + )(C 60 .− ) …”

“…Cations or neutral components with bulky substituents are used to prevent the dimerization of C 60 .− through close packing. In this case, the fullerene monoanions can manifest promising physical properties even down to liquid helium temperatures . Another opportunity is the preparation of fullerene complexes with partial charge transfer, which would not show dimerization, even in the close‐packed structures …”