NEW SEMIQUINONE CATECHOLATE COMPLEXES THAT EXHIBIT VALENCE TAUTOMERISM - SYNTHESIS AND CHARACTERIZATION OF MN-III(THF)(2)(3,6-DBSQ)(3,6-DBCAT) AND OBSERVATIONS ON THE MN-IV(3,6-DBSQ)(2)(3,6-DBCAT)/MN-III(3,6-DBSQ)(3) EQUILIBRIUM IN THE SOLID-STATE/

Irradiation of a THF solution containing Mn-2(CO)(10)and 3,6-di-tert-b utyl-1,2-benzoquinone (3,6-DBBQ) have dark green crystals of Mn(thf)(2 )(3,6-DBQ)(2). Structural characterization has shown that the inner co ordination geometry about the metal has the axially elongated trans st ructure that appears characteristically for d4 Mn(III). trans-Mn-III(t hf)(2)(3,6-DBSQ)(3,6-DBCat) has a solid-state electronic spectrum that resembles spectra of related Mc(III) complexes containing nitrogen-do nor ligands. Crystallographically imposed mmm symmetry averages struct ural differences between the SQ (semioquinoate) and Cat (catecholate) ligands. When the related reaction betwenn Mn-2(CO)(10) and 3,6-DBBQ w as carried out in the presence of 2,2′-bithiophene, Mc(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 ind icative of Mn(III). This result, with the spectral and magnetic proper ties of the complex, points to the Mn-IV(3,6-DBSQ)(2)(3,6-DBCat) charg e distribution at room temperature in the solid state. An intense tran sition at 2300 nm has been assigned as an intervalence transfer (IT) b and between SQ and Cat ligands of the Mn(IV) redox isomer. This band d ecreases in intensity with increasing temperature. Similar behavior ha s been observed for Mn-IV(N-N)(DBCat)(2) species as an indication of v alence 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.