A Series of [Co(Mabiq)Cl_2-n] (n = 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 (1), Co(Mabiq)Cl (2), and Co(Mabiq) (4), with formal metal oxidation states of 3+ → 1+. A binuclear cobaltous compound, Co₂(Mabiq)Cl₃ (3), 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 2 and 3 were examined by electron paramagnetic resonance spectroscopy and magnetic susceptibility studies. The Co^II ion that resides in the N₄-macrocylic cavity of 2 and 3 adopts a low-spin S = ¹/₂ configuration. The bypirimidine functionality in 3 additionally coordinates a high-spin S = ³/₂ cobaltous ion in a tetrahedral environment. The two metal ions in 3 are weakly coupled by magnetometry. The square-planar, low-valent 4 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.