Theoretical analysis of the [Mn2(μ-oxo)2(μ-carboxylato)2]+ core

The first example of a dinuclear manganese complex containing two oxo and two carboxylate bridges, [Mn-2 (mu-O)(2)(mu-O2CArTol)(2)(bpy)(2)](+) (where bpy = 2,2'-bipyridine, and (ArCO2-)-C-Tol = 2,6-di(p-tolyl)benzoate), was reported recently (J. Ani. Chem. Soc. 2003, 125, 13010). X-ray crystallographic analysis performed on this complex reveals a trapped mix-valence species as evidenced, for example, by very different metal-ligand bond distances at the Mn-III and Mn-IV centers. The fact that there are rather bulky bridging carboxylate ligands present in this recently reported dinuclear species raises the question as to whether they affect the extent of valence trapping and the metrical parameters in general. Specifically, it was thought that intramolecular nonbonded contacts could play an important role. In the work reported here, density functional theory calculations were used to address this issue. Structural parameters obtained from calculations on a model compound bearing sterically small bridging carboxylates, [Mn-2(mu-O)(2)(mu-O2CH)(2)(bpy)(2)](+), are in good agreement with the experimentally determined single crystal X-ray structure. Thus, the sterically large carboxylate bridges in [Mn-2(mu-O)(2)(mu-O2CArTol)(2)(bpy)(2)](+) appear not to have a significant effect on the metal-ligand bond distances and angles. There is calculated to be minimal Mn...Mn bonding despite contraction of the Mn...Mn distance relative to related complexes. In addition to calculations on the mixed-valence (MnMnIV)-Mn-III complex, various electronic configurations of the corresponding (MnMnIII)-Mn-III and (MnMnIV)-Mn-IV complexes are explored. Although our calculations support assignment of [Mn-2(mu-O)(2)(mu-O2CH)(2)(bpy)(2)](+) as a valence-trapped (MnMnIV)-Mn-III configuration involving high-spin Mn-III, a delocalized configuration arising from low-spin Mn-III is calculated to lie very close in energy. The energetic proximity of the delocalized configuration is attributed to an effective crossed-exchange mechanism, which permits mixing of an e(g)-based orbital (nominally on high-spin Mn-III) with a vacant t(2g)-based orbital (nominally on Mn-IV).