Binuclear and trinuclear 3d-metal complexes of HATNA and HAT(CN)6 with unique magnetic behavior: from extremely strong antiferromagnetic coupling to opposite sign of zero-field splitting in the same complex

Abstract

A series of anionic bi- and trinuclear complexes of Co^II, Fe^II and Mn^II with hexaazatrinaphthylene (HATNA) and hexaazatriphenylenehexacarbonitrile (HAT(CN)₆), incorporating paramagnetic Cp*₂Fe⁺ cations (S = 1/2), were obtained. It was demonstrated that even a mild Cp*₂Fe reductant can be used to reduce metal complexes of HAT-type ligands. Metal iodides were transferred from the pristine powder to the complexes in a non-coordinating solvent via complex formation with spiropyran or tetrabutylammonium iodide. The isostructural complexes (Cp*₂Fe⁺)[(M^III₂)₂·HATNA]⁻·C₆H₄Cl₂·0.5C₆H₁₄ (M = Co (1), Fe (2) and Mn (3)) contain monoanionic [(M^III₂)₂·HATNA]⁻ units, which alternate in π-stacks with Cp*₂Fe⁺ cations and also form π-stacked [(M^III₂)₂·HATNA⁻]₂ dimers. Metal ion coordination is asymmetric, featuring one short and one long M–N bond. The short M–N bonds are 1.973(6) Å for Co^II (1), 2.039(4) Å for Fe^II (2), and 2.115(4) Å for Mn^II (3); these are the shortest bonds reported to date among studied HAT-based complexes for Co^II and Fe^II. These complexes exhibit very strong antiferromagnetic metal–radical coupling, which isolates their high-spin ground states at high temperatures. To the best of our knowledge, the exchange coupling value of −300 cm⁻¹ observed for 3 is the highest among Mn^II-containing complexes based on a radical ligand with delocalized spin. Compound (Cp*₂Fe⁺)[(Fe^III₂)₃·HATNA]⁻·C₆H₄Cl₂ (4) is unique due to its magnetic behavior, which is severely different from that previously observed for similar trinuclear radical Fe^II-containing HAT-based complexes. Instead of a smooth increase in the χ_MT value with a temperature decrease, attributed to the formation of a high-spin system, a rapid increase below 30 K is observed. Trinuclear trianionic species [(Fe^III₂)₃·HAT(CN)₆]³⁻ are isolated in (Cp*₂Fe⁺)₃[(Fe^III₂)₃·HAT(CN)₆]³⁻·4C₆H₄Cl₂ (5) and do not participate in intermolecular exchange coupling. The unusual magnetic behavior of 5 can only be described by a model implying different signs of D for metal ions in the complex. Such a feature arises from the substantial difference in the coordination geometry of one Fe₁ ion compared to the other two Fe₂ ions.