Valence Tautomerism within a Linear Polymer Consisting of Pyrazine-Bridged Manganese−Quinone Subunits. Synthesis and Characterization of [MnIII(μ-pyz)(3,6-DBSQ)(3,6-DBCat)]n

Irradiation of a hexane/THF solution of Mn 2 (CO) 10, 3,6-di- tert -butyl-1,2-benzoquinone, and pyrazine has been used to prepare the linear [ trans -Mn(μ-pyz)(3,6-DBQ) 2 ] n polymer. Structural characterization [monoclinic, C 2/ m, Z = 2, a = 20.148(5) Å, b = 7.547(1) Å, c = 14.156(2) Å, β = 112.23(2)°, V = 1992.5(6) Å 3, R = 0.067] has shown that complex subunits have an axially elongated structure that appears characteristically for d 4 Mn(III). In the solid state at room temperature, [ trans -Mn III (μ-pyz)(3,6-DBSQ)(3,6-DBCat)] n shows an electronic spectrum consisting of strong absorptions at 880 and 2090 nm, bands associated with Mn(III) redox isomers in earlier studies. Upon heating, band intensity changes in a way that indicates a reversible shift to the Mn(II) redox isomer, [ trans -Mn II (μ-pyz)(3,6-DBSQ) 2 ] n . Valence tautomerism between Mn(II) and Mn(III) redox isomers is responsible for changes in equatorial Mn−O bond lengths, but with little variation in the axial Mn−N length. This is compared with the related equilibrium between Co(II) and Co(III) redox isomers of [ trans -Co(μ-pyz)(3,6-DBQ) 2 ] n that results in large changes for both Co−O and Co−N lengths, contributing to a mechanical effect.