VALENCE TAUTOMERISM FOR CATECHOL SEMIQUINONE COMPLEXES OF THE TRANS-M(BUPY)(2)(3,6-DBQ), (M=MN, FE, CO) SERIES

Studies directed at the investigation of metal-quinone electron transfer have been carried out for members of the M(Bupy)(2)(3,6-DBQ)(2) series where Bupy is 4-tert-butylpyridine, 3,6-DBQ is the 3,6-di-tert-butylbenzoquinone ligand in either its reduced semiquinonate or catecholate form, and M is Mn, Fe or Co. Structural characterization on Mn(Bupy)(2)(3,6-D BCat), and Fe(Bupy)(2)(3,6-DBSQ)(3,6-DBCat) (Mn(Bupy)(2)(3,6-DBCat)(2): triclinic, P (1) over bar, a = 10.294(15), b = 10.632(8), c = 11.085(5) Angstrom, alpha = 93.00(3), beta = 90.78(3), gamma = 112.12(3)degrees, V = 1122(1) Angstrom(3) and Z = 1; Fe(Bupy)(2)(3,6-DBSQ)(3,6-DBCat): triclinic, P (1) over bar, a = 10.326(2), b = 10.646(2), c = 11.192(2) Angstrom, alpha = 92.40(2), beta = 92.17(1), gamma = 111.87(1)degrees, V = 1138.9(4) Angstrom(3) and Z = 1) has shown that the molecules are in the trans isomeric form. Features of the inner coordination spheres about both metal ions are in accord with the charge distributions indicated. Crystallographically imposed inversion symmetry disorders the Cat and SQ ligands of the iron complex. Both Co(Bupy)(2)(3,6-DBSQ)(3,6-DBCat) and Mn(Bupy)(2)(3,6-DBSQ)(3,6-DBCat) show low-energy charge transfer transitions in the region between 3000 and 5000 cm(-1), and exhibit valence tautomeric equilibria in solution and in the solid state. Equilibria for the cobalt complex involve shifts between Co-III(Bupy)(2)(3,6-DBSQ)(3,6-DBCat) and Co-II(Bupy)(2)(3,6-DBSQ), charge distributions in separate electron transfer and metal spin transition steps. Solid state equilibria for the manganese complex appear to occur between Mn-III(Bupy)(2)(3,6-DBSQ)(3,6-DBCat) and Mn-IV(Bupy)(2)(3,6-DBCat)2 charge distributions, with an additional shift to the Mn-II(Bupy)(2)(3,6-DBSQ)(2) form in toluene solution. Fe(Bupy)(2)(3,6-DBSQ)(3,6-DBCat) shows a low intensity transition at 2150 nm, but fails to show evidence for a tautomeric equilibrium.