New Semiquinone/Catecholate Complexes That Exhibit Valence Tautomerism. Synthesis and Characterization of MnIII(thf)2(3,6-DBSQ)(3,6-DBCat) and Observations on the MnIV(3,6-DBSQ)2(3,6-DBCat)/MnIII(3,6-DBSQ)3 Equilibrium in the Solid State

Irradiation of a THF solution containing Mn 2 (CO) 10 and 3,6-di- tert -butyl-1,2-benzoquinone (3,6-DBBQ) gave dark green crystals of Mn(thf) 2 (3,6-DBQ) 2 . Structural characterization has shown that the inner coordination geometry about the metal has the axially elongated trans structure that appears characteristically for d 4 Mn(III). trans -Mn III (thf) 2 (3,6-DBSQ)(3,6-DBCat) has a solid-state electronic spectrum that resembles spectra of related Mn(III) complexes containing nitrogen-donor ligands. Crystallographically imposed mmm symmetry averages structural differences between the SQ (semiquinonate) and Cat (catecholate) ligands. When the related reaction between Mn 2 (CO) 10 and 3,6-DBBQ was carried out in the presence of 2,2‘-bithiophene, Mn(3,6-DBQ) 3 was obtained. Crystallographic characterization has shown that the molecule is octahedral with relatively short Mn−O bond lengths (1.863(5)−1.927(5) Å), and without the structural distortion that might be indicative of Mn(III). This result, with the spectral and magnetic properties of the complex, points to the Mn IV (3,6-DBSQ) 2 (3,6-DBCat) charge distribution at room temperature in the solid state. An intense transition at 2300 nm has been assigned as an intervalence transfer (IT) band between SQ and Cat ligands of the Mn(IV) redox isomer. This band decreases in intensity with increasing temperature. Similar behavior has been observed for Mn IV (N-N)(DBCat) 2 species as an indication of valence tautomerism. In this case the equilibrium is between Mn IV (3,6-DBSQ) 2 (3,6-DBCat) and Mn III (3,6-DBSQ) 3 redox isomers.