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

Abstract

Irradiation of a THF solution containing Mn2(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 d4 Mn(III). trans-MnIII(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 Mn2(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) A), and without the structural distortion that might be indicative of Mn(III). This result, with the spectral and magn...