Effect of Lewis Acids on the Structure and Reactivity of a Mononuclear Hydroxomanganese(III) Complex.

The addition of Sc(OTf)3 and Al(OTf)3 to the mononuclear MnIII-hydroxo complex [MnIII(OH)(dpaq)]+ (1) gives rise to new intermediates with spectroscopic properties and chemical reactivity distinct from those of [MnIII(OH)(dpaq)]+. The electronic absorption spectra of [MnIII(OH)(dpaq)]+ in the presence of Sc(OTf)3 (1-ScIII) and Al(OTf)3 (1-AlIII) show modest perturbations in electronic transition energies, consistent with moderate changes in the MnIII geometry. A comparison of 1H NMR data for 1 and 1-ScIII confirm this conclusion, as the 1H NMR spectrum of 1-ScIII shows the same number of hyperfine-shifted peaks as the 1H NMR spectrum of 1. These 1H NMR spectra, and that of 1-AlIII, share a similar chemical-shift pattern, providing firm evidence that these Lewis acids do not cause gross distortions to the structure of 1. Mn K-edge X-ray absorption data for 1-ScIII provide evidence of elongation of the axial Mn-OH and Mn-N(amide) bonds relative to those of 1. In contrast to these modest spectroscopic perturbations, 1-ScIII and 1-AlIII show greatly enhanced reactivity toward hydrocarbons. While 1 is unreactive toward 9,10-dihydroanthracene (DHA), 1-ScIII and 1-AlIII react rapidly with DHA (k2 = 0.16(1) and 0.25(2) M-1 s-1 at 50 °C, respectively). The 1-ScIII species is capable of attacking the much stronger C-H bond of ethylbenzene. The basis for these perturbations to the spectroscopic properties and reactivity of 1 in the presence of these Lewis acids was elucidated by comparing properties of 1-ScIII and 1-AlIII with the recently reported MnIII-aqua complex [MnIII(OH2)(dpaq)]2+ ( J. Am. Chem. Soc. 2018, 140, 12695-12699). Because 1-ScIII and 1-AlIII show 1H NMR spectra essentially identical to that of [MnIII(OH2)(dpaq)]2+, the primary effect of these Lewis acids on 1 is protonation of the hydroxo ligand caused by an increase in the Brønsted acidity of the solution.