Counterions manage metal-to-metal electron transfer: the role of intermolecular interactions in MMET-active [Fe4Co4] cubes

Abstract

Unveiling the effect of intermolecular interactions on metal-to-metal electron transfer (MMET) is challenging, but preferentially essential for the engineering of magnetically switchable crystalline materials and devices. Herein, we reported a family of [Fe₄Co₄] cubes sharing the identically cyanide-bridged cubic octanuclear architecture {[Fe(pzTp)(CN)₃]₄[Co(ddpd)]₄}⁴⁺ (pzTp⁻ = tetrakis(pyrazolyl)borate, ddpd = N,N’-dimethyl-N,N’-dipyridin-2-ylpyridine-2,6-diamine) but different counterions with tunable sizes, including BF₄⁻ (1·BF₄), ClO₄⁻ (2·ClO₄), PF₆⁻ (3·PF₆), AsF₆⁻ (4·AsF₆) and SbF₆⁻ (5·SbF₆). All compounds were isomorphic and crystallized in the tetragonal P2₁c space group. Magnetic susceptibility measurements revealed that compound 1·BF₄ with the smallest counterion exhibited the most pronounced MMET behavior, while 5·SbF₆ with the largest counterion completely lost the MMET behavior. By employing both experimental and computational approaches, we demonstrated that the number and strength of intermolecular interactions depend highly on the counterion size. The enhanced interactions impose a strong limiting effect on the octahedral coordination geometry of the cobalt centers, which consequently have a non-negligible influence on the orbital overlap, electrostatic potential, and redox potentials, thereby modulating MMET behavior.