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

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