From spin-crossover to single molecule magnetism: tuning magnetic properties of Co(ii) bis-ferrocenylterpy cations via supramolecular interactions with organocyanide radical anions

Abstract

TCNQ (7,7,8,8-tetracyanoquinodimethane) anion-radical derivatives were used to fine tune the magnetic properties of the [Co^II(Fctp)₂]²⁺ (Fctp = 4′-(2-ferrocenyl)-2,2′:6′2′′-terpyridine) cation in the solid state. The cocrystallization of [Co^II(Fctp)₂]²⁺ with TCNQ˙⁻ yielded the two pseudo-polymorphic products [Co^II(Fctp)₂] (TCNQ)₂ (1) and [Co^II(Fctp)₂] (TCNQ)₂·MeCN (2) whereas the analogous reaction with TCNQF˙⁻ (TCNQF = 2-fluoro-7,7,8,8-tetracyanoquinodimethane) exclusively yielded [Co^II(Fctp)₂] (TCNQF)₂·MeCN (3). Compound 1 exhibits slow relaxation of magnetization under an applied DC field with U_eff = 19.1 K and τ₀ = 9.8 × 10⁻⁶ s. Compounds 2 and 3 are isostructural but exhibit different spin-crossover behavior with transition temperatures of T_1/2 = 336 K and 226 K, respectively. Investigations of the solid state structures by DFT calculations indicate that the differences in magnetic properties of the cationic moiety, [Co^II(Fctp)₂]²⁺, are induced by supramolecular interactions between [Co^II(Fctp)₂]²⁺ and tunable TCNQ˙⁻/TCNQF˙⁻ anion-radical derivatives.