VALENCE TAUTOMERISM WITHIN A LINEAR POLYMER CONSISTING OF PYRAZINE-BRIDGED MANGANESE-QUINONE SUBUNITS - SYNTHESIS AND CHARACTERIZATION OF [MN-III-(MU-PYZ)(3,6-DBSQ)(3,6-DBCAT)](N)

Irradiation of a hexane/THF solution of Mn-2(CO)(10), 3,6-di-tert-buty l-1,2-benzoquinone, and pyrazine has been used to prepare the linear [ trans-Mn(mu-pyz)(3,6-DBQ)(2)](n) polymer. Structural characterization [monoclinic, C2/m, Z = 2, a = 20.148(5) Angstrom, b = 7.547(1) Angstro m, c = 14.156(2) Angstrom, beta = 112.23(2)degrees, V = 1992.5(6) Angs trom(3), R = 0.067] has shown that complex subunits have an axially el ongated structure that appears characteristically for d(4) Mn(II). In the solid state at room temperature, [trans-Mn-III(mu-pyz)(3,6-DBSQ)(3 ,6-DBCat)](n) shows an electronic spectrum consisting of strong absorp tions 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 (mu-pyz)(3,6-DBSQ)(2)](n). Valence tautomerism between Mn(II) and Mn(I II) redox isomers is responsible for changes in equatorial Mn-O bond l engths, but with little variation in the axial Mn-N length. This is co mpared with the related equilibrium between Co(II) and Co(III) redox i somers of [trans-Co(mu-pyz)(3,6-DBQ)(2)](n), that results in large cha nges for both Co-O and Co-N lengths, contributing to a mechanical effe ct.