Mononuclear Manganese(III) Superoxo Complexes: Synthesis, Characterization, and Reactivity

Abstract

Metal–superoxo species are typically proposed as key intermediates in the catalytic cycle of dioxygen activation by metalloenzymes involving different transition metal cofactors. In this regard, while a series of Fe–, Co–, and Ni–superoxo complexes have been reported to date, well-defined Mn–superoxo complexes remain rather rare. Herein, we report two mononuclear Mn III –superoxo species, Mn(BDPP)(O 2 •– ) ( 2 , H 2 BDPP = 2,6-bis((2-( S )-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Mn(BDP Br P)(O 2 •– ) ( 2′ , H 2 BDP Br P = 2,6-bis((2-( S )-di(4-bromo)phenylhydroxyl-methyl-1-pyrrolidinyl)methyl)pyridine), synthesized by bubbling O 2 into solutions of their Mn II precursors, Mn(BDPP) ( 1 ) and Mn(BDP Br P) ( 1′ ), at −80 °C. A combined spectroscopic (resonance Raman and electron paramagnetic resonance (EPR) spectroscopy) and computational study evidence that both complexes contain a high-spin Mn III center ( S Mn = 2) antiferromagnetically coupled to a superoxo radical ligand ( S OO• = 1/2), yielding an overall S = 3/2 ground state. Complexes 2 and 2′ were shown to be capable of abstracting a H atom from 2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPO-H) to form Mn III –hydroperoxo species, Mn(BDPP)(OOH) ( 5 ) and Mn(BDP Br P)(OOH) ( 5′ ). Complexes 5 and 5′ can be independently prepared by the reactions of the isolated Mn III -aqua complexes, [Mn(BDPP)(H 2 O)]OTf ( 6 ) and [Mn(BDP Br P)(H 2 O)]OTf ( 6′ ), with H 2 O 2 in the presence of NEt 3 . The parallel-mode EPR measurements established a high-spin S = 2 ground state for 5 and 5′ .

Magnetic Properties and Electronic Structure of the S = 2 Complex [MnIII{(OPPh2)2N}3] Showing Field-Induced Slow Magnetization Relaxation

“…The spectrum of the complex 1 Mn was simulated (Figure A) by assuming A z = 155 MHz and B 4 4 = 4.2(2) × 10 –3 cm –1 (Table S4). The value of B 4 4 agrees well with that determined (albeit less precisely) by HFEPR and is of the same order of magnitude as that observed for other mononuclear S = 2 Mn­(III) complexes. ,,,,, Similarly, the A z value is consistent with a tetragonally elongated Mn­(III) octahedral coordination sphere exhibiting a 5 B 1g electronic ground state (vide infra). ,− …”

“…The spectrum comprises impressively narrow lines (full width at half-maximum, ∼0.6 mT) superimposed on a broad background. Narrow lines in parallel-mode X-band EPR spectra of frozen solutions of Mn­(III) complexes are not unprecedented in the literature, ,,− but in the present case, the narrowness of the lines allows for the resolution of additional subtler features. Apart from the dominating strong sextet, additional lines can be readily recognized.…”

Structural and Spectroscopic Evidence for a Side-on Fe(III)–Superoxo Complex Featuring Discrete O–O Bond Distances

“…In particular, considering that DFT computations often slightly overestimate O–O stretching frequencies, as exemplified by related Mn–superoxo and hydroperoxo complexes. 65 , 66 All findings thus lend credence to the computed electronic structure of 1 ( Figure S8 ).…”

Heme-FeIII Superoxide, Peroxide and Hydroperoxide Thermodynamic Relationships: FeIII-O2•– Complex H-Atom Abstraction Reactivity

“…However, second-order rate constants ( k 2 ) for non-heme metal-superoxide (i.e., M n + -(O 2 •– )) complexes (M = Cu, Mn, Co, and Cr) involved in HAT reactions with TEMPO-H have been reported in a number of cases (see Table S5). ,,− …”