A Series of [Co(Mabiq)Cl_2–<i>n</i>] (<i>n</i> = 0, 1, 2) Compounds and Evidence for the Elusive Bimetallic Form

The synthesis and characterization of a series of cobalt compounds, coordinated by the redox-active macrocyclic biquinazoline ligand, Mabiq [2–4:6–8-bis­(3,3,4,4-tetramethyldihydropyrrolo)-10–15-(2,2′-biquinazolino)-[15]-1,3,5,8,10,14-hexaene-1,3,7,9,11,14-N₆], is presented. The series includes the monometallic Co­(Mabiq)­Cl₂ (<b>1</b>), Co­(Mabiq)Cl (<b>2</b>), and Co­(Mabiq) (<b>4</b>), with formal metal oxidation states of 3+ → 1+. A binuclear cobaltous compound, Co₂(Mabiq)­Cl₃ (<b>3</b>), also was obtained, providing the first evidence for the ability of the Mabiq ligand to coordinate two metal ions. The electronic structures of the paramagnetic <b>2</b> and <b>3</b> were examined by electron paramagnetic resonance spectroscopy and magnetic susceptibility studies. The Co^II ion that resides in the N₄-macrocylic cavity of <b>2</b> and <b>3</b> adopts a low-spin <i>S</i> = ¹/₂ configuration. The bypirimidine functionality in <b>3</b> additionally coordinates a high-spin <i>S</i> = ³/₂ cobaltous ion in a tetrahedral environment. The two metal ions in <b>3</b> are weakly coupled by magnetometry. The square-planar, low-valent <b>4</b> offers one of a limited number of examples of structurally characterized N₄-macrocyclic Co^I compounds. Spectroscopic and density functional theory computational data suggest that a Co^II(Mabiq^•) description may be a reasonable alternative to the Co^I formalism for this compound