Tuning the magnetic properties of dinuclear cobalt-tetraoxolene compounds: from valence tautomerism to ferromagnetic coupling

The reaction of a non-innocent ligand 2,7-di-tert-butyl-pyrene-4,5,9,10-tetraone (pyrene^Q-Q, Q = quinone), Co₂(CO)₈, and tris(2-pyridylmethyl)amine derivatives (Me_ntpa) gave rise to a series of dinuclear cobalt-tetraoxolene compounds formulated as {[Co(Me_ntpa)]₂(pyrene^Sq-Sq)}[Co(CO)₄]₂·S (Sq = semiquinone; n = 0, S = 2C₆H₅CH₃ for 3; n = 1 for 4; n = 2, S = 2CHCl₃ for 5). Magnetic measurements and single-crystal X-ray diffraction studies revealed that compound 3 underwent incomplete valence tautomeric transition, while compounds 4 and 5 maintained in the steady electronic structure of {Co^II-HS-pyrene^Sq-Sq-Co^II-HS} in the whole temperature range. Remarkably, strong ferromagnetic interactions in both 4 and 5 were confirmed by experimental and theoretical studies. Theoretical calculations showed that the successive introduction of methyl groups enhanced the steric rigidity of tpa, and hence the {Co^II-HS-pyrene^Sq-Sq-Co^II-HS} state was stabilized sterically and electronically, which in turn altered the energetic order and gap among the different electronic states, which resulted in distinct magnetic properties.