Acid Catalysis in the Oxidation of Substrates by Mononuclear Manganese(III)–Aqua Complexes

Abstract

Acids are known to enhance the reactivities of metal–oxygen intermediates, such as metal–oxo, −hydroperoxo, −peroxo, and −superoxo complexes, in biomimetic oxidation reactions. Although metal–aqua (and metal–hydroxo) complexes have been shown to be potent oxidants in oxidation reactions, acid effects on the reactivities of metal–aqua complexes have never been investigated previously. In this study, a mononuclear manganese­(III)–aqua complex, [(dpaq5NO2)­MnIII(OH2)]2+ (1; dpaq5NO2 = 2-[bis­(pyridin-2-ylmethyl)]­amino-N-quinolin-8-ylacetamidate with an NO2 substituent at the 5 position), which is relatively stable in the presence of triflic acid (HOTf), is used in the investigation of acid-catalyzed oxidation reactions by metal–aqua complexes. As a result, we report a remarkable acid catalysis in the six-electron oxidation of anthracene by 1 in the presence of HOTf; anthraquinone is formed as the product. In the HOTf-catalyzed six-electron oxidation of anthracene by 1, the rate constant increases linearly with an increase of the HOTf concentration. Combined with the observed one-electron oxidation product, anthracene (derivative) radical cation, and the substitution effect at the 5 position of the dpaq ligand in 1 on the rate constants of the oxidation of anthracene, it is concluded that the oxidation of anthracene occurs via an acid-promoted electron transfer (APET) from anthracene to 1. The dependence of the rate constants of the APET from electron donors, including anthracene derivatives, to 1 on the driving force of electron transfer is also shown to be well fitted by the Marcus equation of outer-sphere electron transfer. To the best of our knowledge, this is the first example showing acid catalysis in the oxidation of substrates by metal­(III)–aqua complexes.

Oxidative versus basic asynchronous hydrogen atom transfer reactions of Mn(iii)-hydroxo and Mn(iii)-aqua complexes

“…In the previous study, the one-electron reduction peak potentials ( E pc,red vs. SCE at a scan rate of 100 mV s −1 ) of [(dpaq 5R )Mn III (OH)] + were reported to be −0.16 V, −0.24 V, −0.28 V and −0.30 V for R = NO 2 , Cl, H and Me, respectively. 76 Thus, the ET from Me 10 Fc ( E ox vs. SCE = −0.08 V; E ox represents the one-electron oxidation potential of the electron donor) 77 to [(dpaq 5R )Mn III (OH)] + was energetically feasible, but the ET from 4-MeO-2,6- t Bu 2 C 6 H 2 OH ( E ox vs. SCE = 0.90 V) 78 to [(dpaq 5R )Mn III (OH)] + was energetically unfeasible, and it underwent the CPET as the rate-determining step to surmount the uphill ET process.…”

“…of trifluoromethanesulfonic acid (HOTf ) to the MeCN solution of [(dpaq 5R )Mn III (OH)](OTf ). 76,78,79 Instrumentation UV-vis spectra were recorded on a Hewlett Packard 8453 diode array spectrophotometer equipped with a UNISOKU Scientific Instruments Cryostat USP-203A for low-temperature experiments or on a UNISOKU RSP-601 stopped-flow spectrometer equipped with a MOS-type highly sensitive photodiode array. 1 H NMR spectra were measured with a Bruker model digital AVANCE III 400 FT-NMR spectrometer.…”

“…of trifluoromethanesulfonic acid (HOTf) to the MeCN solution of [(dpaq 5R )Mn III (OH)](OTf). 76,78,79…”

“…76,78,79 The X-ray crystal structures of [(dpaq 5R ) Mn III (OH 2 )] 2+ (R = H, Cl and NO 2 ) were previously reported as a mononuclear Mn III center in a distorted octahedral environment, with an aqua ligand bound to trans to the amide group of the dpaq 5R ligand. 76,78 In this study, an X-ray crystal structure of [(dpaq 5Me )Mn III (OH 2 )] 2+ was determined as shown in Fig. 6, which also revealed a mononuclear Mn III center in a distorted octahedral environment.…”

“…In the previous study, the one-electron reduction potentials (E red vs. SCE) of [(dpaq 5R )Mn III (OH 2 )] 2+ were reported to be 1.04 V, 0.99 V, 0.95 V and 0.93 V for R = NO 2 , Cl, H and Me, respectively. 76 The one-electron reduction potentials (E red vs. SCE) of [(dpaq 5R )Mn III (OH 2 )] 2+ complexes are approximately 1.0 V higher than those of [(dpaq 5R )Mn III (OH)] + complexes, resulting in that the reactivities of [(dpaq 5R )Mn III (OH 2 )] 2+ complexes are much higher than those of [(dpaq 5R )Mn III (OH)] + complexes. The largest and smallest k 2 values obtained for HAT by [(dpaq 5R ) Mn III (OH 2 )] 2+ are with the most electron-donating substituent (i.e., OMe) and the most electron-withdrawing substituent (i.e., NO 2 ), respectively.…”