Electronic Structures of Tris(dioxolene)chromium and Tris(dithiolene)chromium Complexes of the Electron-Transfer Series [Cr(dioxolene)3]z and [Cr(dithiolene)3]z (z = 0, 1−, 2−, 3−). A Combined Experimental and Density Functional Theoretical Study

From the reaction mixture of 3,6-di-tert-butylcatechol, H2[3,6Lcat], [CrCl3(thf)3], and NEt3 in CH3CN in the presence of air, the neutral complex [CrIII(3,6Lsq)3] (S = 0) (1) was isolated. Reduction of 1 with [Co(Cp)2] in CH2Cl2 yielded microcrystals of [Co(Cp)2][CrIII(3,6Lsq)2(3,6Lcat)] (S = 1/2) (2) where (3,6Lsq)1- is the π-radical monoanionic o-semiquinonate of the catecholate dianion (3,6Lcat)2-. Electrochemistry demonstrated that both species are members of the electron-transfer series [Cr(3,6LO,O)]z (z = 0, 1−, 2−, 3−). The corresponding tris(benzo-1,2-dithiolato)chromium complex [N(n-Bu)4][CrIII(3,5LS,S)2(3,5LS,S)] (S = 1/2) (3) has also been isolated; (3,5LS,S)2- represents the closed-shell dianion 3,5-di-tert-butylbenzene-1,2-dithiolate(2−), and (3,5LS,S)1- is its monoanionic π radical. Complex 3 is a member of the electron-transfer series [Cr(3,5LS,S)3]z (z = 0, 1−, 2−, 3−). It is shown by Cr K-edge and S K-edge X-ray absorption, UV−vis, and EPR spectroscopies, as well as X-ray crystallography, of 1 and 3 that the oxidation state of the central Cr ion in each member of both electron-transfer series remains the same (+III) and that all redox processes are ligand-based. These experimental results have been corroborated by broken symmetry density functional theoretical calculations by using the B3LYP functional.