Reversible Redox, Spin Crossover, and Superexchange Coupling in 3d Transition-Metal Complexes of Bis-azinyl Analogues of 2,2':6',2"-Terpyridine.

The terpyridine-inspired tridentate ligand, 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)-pyridine (BTP), was synthesized and utilized to isolate five [MII(BTP)2]2+ (M = Fe and Co) transition-metal complexes of various anions (BF4-, ClO4-, and NCS-). Notably, when metal-halide precursors are employed, 1:1 M/L products [Co2II(μ-X)2(BTP)2X2] (X = Cl or Br) are obtained, exhibiting μ-halide bridges. All complexes were structurally characterized through single-crystal X-ray diffraction and their electrochemical and magnetic properties were investigated. Electrochemical studies reveal that the free BTP ligand has a single irreversible reduction, however; upon coordination to a metal center, stabilization of the reduced BTP ligand occurs and up to four reductive processes associated with the molecule are observed within the solvent window. The metal-centered redox processes were either reversible or partially reversible, and fall within the range of 0.13 < E1/2 < 0.90 V vs. Fc/Fc+. Although there was minimal effect on the redox properties of the metal centers, there is strong dependence on the spin state. SQUID magnetometry elucidated a low spin state for the FeII complexes at room temperature, revealing a diamagnetic electronic structure. On the other hand, the cobalt monometallic complexes, [CoII(BTP)2]2+, showed gradual spin crossover properties between 1.8-370 K, displaying a minor dependence on the spin crossover behavior based on the respective anion. Additionally, ferromagnetic exchange interactions of J = +2.57 and +2.98 cm-1 were obtained using the -2J formalism for the dinuclear CoII complexes, [Co2 II(μ-X)2(BTP)2X2] for X = Cl and Br, respectively. [ABSTRACT FROM AUTHOR]