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), was 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(II)I2)2.HATNA](-).C6H4Cl2.0.5C6H14 (M = Co (1), Fe (2) and Mn (3)) contain monoanionic [(M(II)I2)2.HATNA] (-) units, which alternate in π-stacks with Cp*2Fe+ cations and also form π-stacked [(M(II)I2)2.HATNA(-)]2 dimers. Metal ions 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(II)I2)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 rise of χMT 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(II)I2)3.HAT(CN)6](3-) are isolated in (Cp*2Fe+)3[(Fe(II)I2)3.HAT(CN)6](3-).4C6H4Cl2 (5) and do not participate in intermolecular exchange coupling. An unusual magnetic behavior of 5 can only be described by a model implying different sign of D for metal ions in the complex. Such feature arises from the substantial difference in the coordination geometry of one Fe1 ion compared to the two other Fe2 ions.