Effect of nuclearity and reduction state of the central ligand on magnetic properties of hexaazatrinaphthylene-based cobalt(ii) and iron(ii) complexes: from extremely weak to record-breaking antiferromagnetic exchange interaction†

A series of hexaazatrinaphthylene (HATNA) and hexaazatrianthracene (HATA) complexes with high-spin four-coordinate cobalt(II) (S = 3/2) and iron(II) (S = 2) ions were synthesized. These compounds vary in charge of the central hexaazatriphenylene (HAT) ligand (L) and the number of coordinated metal atoms. The occupancy of the three coordinating sites on the HAT ligands greatly influenced the crystal packing of the complexes. While trinuclear complexes typically pack as isolated monomers or dimers, the complexes [(Co^III₂)(HATNA)]⁰·C₆H₄Cl₂ or {(K⁺)(crypt)}{(Co^III₂)₂(HATA)}⁻·C₆H₄Cl₂ (5) (crypt is cryptand[2.2.2]), containing one or two metal atoms, respectively, form stacks with extensive π–π-interactions. {(K⁺)(crypt)}{(Co^III₂)₂(HATNA)}⁻·2.75C₆H₄Cl₂ (2), {(K⁺)(crypt)}{(Fe^III₂)₂(HATNA)}⁻·3C₆H₄Cl₂ (4), complex 5, and {(K⁺)(crypt)}₂{(Fe^IICl₂)₂(HATNA)}²⁻·3C₆H₄Cl₂ (8) are rare examples of complexes which contain less number of metal ions than that of coordination sites. Interestingly, the antiferromagnetic Fe–Fe superexchange value in complex 8, with a diamagnetic dianion ligand, was two orders of magnitude lower than that observed in similar trinuclear Fe(II) complexes. In contrast, compounds 2, 4 and 5, which contain a radical ligand, exhibit extremely strong direct metal–radical exchange interactions (−J > 600 cm⁻¹), maintaining a giant spin state at room temperature. To the best of our knowledge, this is the largest magnetic coupling value ever recorded among Co(II)-containing complexes. The trinuclear radical complexes {(K⁺)(crypt)}{(Fe^IICl₂)₃(HATNA)}⁻·C₆H₁₄ and {(K⁺)(crypt)}{(Co^IICl₂)₃(HATNA)}⁻ exhibit considerable metal–ligand coupling, leading to high-spin systems at low temperatures. Trinuclear complexes {(K⁺)(crypt)}₂{(Co^III₂)₃(HATA)}²⁻·4C₆H₄Cl₂ and (CV⁺)₂{(Co^IICl₂)₃(HATA)}²⁻·4C₆H₄Cl₂ (CV⁺ is crystal violet cation) with diamagnetic ligands demonstrate only Co–Co exchange, which is notably stronger than previously reported Fe–Fe and Mn–Mn couplings. In {(K⁺)(crypt)}₃{(Co^III₂)₃(HATNA)}³⁻·3C₆H₄Cl₂·3C₆H₁₄, the HATNA trianion forms short Co–N bonds, enhancing both Co–radical and Co–Co antiferromagnetic couplings. The latter one gets strong enough to lead to an antiparallel arrangement of Co spins, and, hence, to the supressed magnetic moment.