Structure and Reactivity of (μ-Oxo)dimanganese(III,III) and Mononuclear Hydroxomanganese(III) Adducts Supported by Derivatives of an Amide-Containing Pentadentate Ligand

Mononuclear Mn-III-hydroxo and dinuclear (mu-oxo)dimanganese(III,III) complexes were prepared using derivatives of the pentadentate, amide-containing dpaq ligand (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate). Each of these ligand derivatives (referred to as dpaq(5R)) contained a substituent R (where R = OMe, Cl, and NO2) at the 5-position of the quinolinyl group. Generation of the Mn-III complexes was achieved by either O-2 oxidation of Mn-II precursors (for [Mn-II(dpaq(5OMe))](+) and [Mn-II(dpaq(5Cl))](+) or PhIO oxidation (for [Mn-II(dpaq(5NO2))](+)). For each oxidized complex, H-1 NMR experiments provided evidence of a water-dependent equilibrium between paramagnetic [Mn-III(OH)(dpaq(5R))](+) and an antiferromagnetically coupled [(MnMnIII)-Mn-III(mu-O)(dpaq(5R))(2)](2+) species in acetonitrile, with the addition of water favoring the Mn-III-hydroxo species. This conversion could also be monitored by electronic absorption spectroscopy. Solid-state X-ray crystal structures for each [(MnMnIII)-Mn-III(mu-O)(dpaq(5R))(2)](OTf)(2) complex revealed a nearly linear Mn-O-Mn core (angle of ca. 177 degrees), with short Mn-O distances near 1.79 angstrom, and a Mn center dot center dot center dot Mn separation of 3.58 angstrom. X-ray crystallographic information was also obtained for the mononuclear [Mn-III(OH)(dpaq(5Cl))](OTf) complex, which has a short Mn-O(H) distance of 1.810(2) angstrom. The influence of the 5-substituted quinolinyl moiety on the electronic properties of the [Mn-III(OH)(dpaq(5R))](+) complexes was demonstrated through shifts in a number of H-1 NMR resonances, as well as a steady increase in the Mn-III/II cyclic voltammetry peak potential in the order [Mn-III(OH)(dpaq(5OMe))](+) < [Mn-III(OH)(dpaq)(+) < [Mn-III(OH)(dpaq(5Cl))](+) < [Mn-III(OH)(dpaq(5NO2))](+). These changes in oxidizing power of the Mn-III-hydroxo adducts translated to only modest rate enhancements for TEMPOH oxidation by the [Mn-III(OH)(dpaq(5R))](+) complexes, with the most reactive [Mn-III(OH)(dpaq(5NO2))](+) complex showing a second-order rate constant only 9-fold larger than that of the least reactive [Mn-III(OH)(dpaq(5OMe))](+) complex. These modest rate changes were understood on the basis of density functional theory (DFT)-computed pK(a) values for the corresponding [Mn-II(OH2)-(dpaq(5R))](+) complexes. Collectively, the experimental and DFT results reveal that the 5-substituted quinolinyl groups have an inverse influence on electron and proton affinity for the Mn-III-hydroxo unit.