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

Abstract

The addition of Sc(OTf) 3 and Al(OTf) 3 to the mononuclear Mn III -hydroxo complex [Mn III (OH)(dpaq)] + (1) gives rise to new intermediates with spectroscopic properties and chemical reactivity distinct from those of [Mn III (OH)(dpaq)] + . The electronic absorption spectra of [Mn III (OH)(dpaq)] + in the presence of Sc(OTf) 3 (1-Sc III ) and Al(OTf) 3 (1-Al III ) show modest perturbations in electronic transition energies, consistent with moderate changes in the Mn III geometry. A comparison of 1 H NMR data for 1 and 1-Sc III confirm this conclusion, as the 1 H NMR spectrum of 1-Sc III shows the same number of hyperfine-shifted peaks as the 1 H NMR spectrum of 1. These 1 H NMR spectra, and that of 1-Al III , 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-Sc III 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-Sc III and 1-Al III show greatly enhanced reactivity toward hydrocarbons. While 1 is unreactive toward 9,10dihydroanthracene (DHA), 1-Sc III and 1-Al III react rapidly with DHA (k 2 = 0.16(1) and 0.25(2) M −1 s −1 at 50 °C, respectively). The 1-Sc III 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-Sc III and 1-Al III with the recently reported Mn III −aqua complex [Mn III (OH 2 )(dpaq)] 2+ (J. Am. Chem. Soc. 2018, 140, 12695−12699). Because 1-Sc III and 1-Al III show 1 H NMR spectra essentially identical to that of [Mn III (OH 2 )(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.

Deeper Understanding of Mononuclear Manganese(IV)–Oxo Binding Brønsted and Lewis Acids and the Manganese(IV)–Hydroxide Complex

“…Very recently, however, Paine, Paul, Biswas, and co-workers reported a quite contradictory observation, in which they claimed that a [(dpaq)­Mn IV (O)] + complex, synthesized by reacting 2 with terminal oxidants, such as m -chloroperbenzoic acid ( m -CPBA), PhIO, CAN, and [Ru III (bpy) 3 ] 3+ , in the presence of protic acids (e.g., HOTf and HClO 4 ) and a redox-inactive metal ion (e.g., Sc 3+ ), does not bind metal ions (or proton) . In addition, binding of the metal ions to the metal–oxo moiety in metal–oxo complexes has been questioned in several cases . As mentioned above, metal ions and protons play a significant role(s) in modulating the chemical properties of metal–oxo species in biological and abiological reactions.…”

Effects of Noncovalent Interactions on High-Spin Fe(IV)–Oxido Complexes

“…Another popular approach to modulating microenvironments is to change the electrostatic properties around M–oxido species by treating them with redox-inactive Lewis acids (LAs) such as Group II metal ions and Sc 3+ ions. − The premise that these auxiliary metal ions affect function has been previously substantiated in synthetic systems. For instance, work by Agapie examined the effects of LAs on the properties of manganese-oxido clusters. − Complementary work by Lau showed that in the presence of a LA, the rate of alkane oxidation by metal–oxido systems increased .…”

How does Lewis acid affect the reactivity of mononuclear high‐valent chromium–oxo species? A theoretical study

“…Addition of Sc(OTf) 3 to 1 . Previous studies revealed that the adding of Lewis acid (i.e., Sc 3+ ion) not only stabilize the high‐valent metal–oxo species, but also affects their oxidizing capabilities 18–29 . Experimental results are seemingly simple; species 1 , which is unreactive, is still unreactive when adding Sc(OTf) 3 ( 1‐Sc ).…”

“…The chemistry of these species has therefore been of great interest to the bioinorganic chemistry community during the past decades. Especially, redox‐inactive metal ions functioning as Lewis acids have pivotal roles in regulating the oxidizing capabilities of such metal–oxo species in many enzymatic and chemical transformations 18–29 . Previous studies have revealed that binding of a redox‐inactive metal ion can not only markedly change the reactivity, but also tune the valence tautomerization of the metal–oxo species 18,30–32 .…”

“…Despite these none‐to‐minor differences upon Sc 3+ ion binding, we know by experience that Sc 3+ ion binding may enhance the reactivities of the species 18–29 . With all theoretical characterization results for these four species in hand, we can now investigate their reactivities toward both CH bond activation and sulfoxidation reactions.…”