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-hydroxo and dinuclear (μ-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) contained a substituent R (where R = OMe, Cl, and NO) at the 5-position of the quinolinyl group. Generation of the Mn complexes was achieved by either O oxidation of Mn precursors (for [Mn(dpaq)] and [Mn(dpaq)] or PhIO oxidation (for [Mn(dpaq)]). For each oxidized complex, H NMR experiments provided evidence of a water-dependent equilibrium between paramagnetic [Mn(OH)(dpaq)] and an antiferromagnetically coupled [MnMn(μ-O)(dpaq)] species in acetonitrile, with the addition of water favoring the Mn-hydroxo species. This conversion could also be monitored by electronic absorption spectroscopy. Solid-state X-ray crystal structures for each [MnMn(μ-O)(dpaq)](OTf) complex revealed a nearly linear Mn-O-Mn core (angle of ca. 177°), with short Mn-O distances near 1.79 Å, and a Mn···Mn separation of 3.58 Å. X-ray crystallographic information was also obtained for the mononuclear [Mn(OH)(dpaq)](OTf) complex, which has a short Mn-O(H) distance of 1.810(2) Å. The influence of the 5-substituted quinolinyl moiety on the electronic properties of the [Mn(OH)(dpaq)] complexes was demonstrated through shifts in a number of H NMR resonances, as well as a steady increase in the Mn cyclic voltammetry peak potential in the order [Mn(OH)(dpaq)] < [Mn(OH)(dpaq)] < [Mn(OH)(dpaq)] < [Mn(OH)(dpaq)]. These changes in oxidizing power of the Mn-hydroxo adducts translated to only modest rate enhancements for TEMPOH oxidation by the [Mn(OH)(dpaq)] complexes, with the most reactive [Mn(OH)(dpaq)] complex showing a second-order rate constant only 9-fold larger than that of the least reactive [Mn(OH)(dpaq)] complex. These modest rate changes were understood on the basis of density functional theory (DFT)-computed p K values for the corresponding [Mn(OH)(dpaq)] 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-hydroxo unit.