Synthesis, Characterization, and Physicochemical Properties of Manganese(III) and Manganese(V)−Oxo Corrolazines

Abstract

The structural and physicochemical properties of the manganese-corrolazine (Cz) complexes (TBP8Cz)Mn(V)O (1) and (TBP8Cz)Mn(III) (2) (TBP = p-tert-butylphenyl) have been determined. Recrystallization of 2 from toluene/MeOH resulted in the crystal structure of (TBP8Cz)Mn(III)(CH3OH) (2 x MeOH). The packing diagram of 2 x MeOH reveals hydrogen bonds between MeOH axial ligands and meso N atoms of adjacent molecules. Solution binding studies of 2 with different axial ligands (Cl-, Et3PO, and Ph3PO) reveal strong binding, corroborating the preference of the Mn(III) ion for a five-coordinate environment. High-frequency and field electron paramagnetic resonance (HFEPR) spectroscopy of solid 2 x MeOH shows that 2 x MeOH is best described as a high-spin (S = 2) Mn(III) complex with zero-field splitting parameters typical of corroles. Structural information on 1 was obtained through an X-ray absorption near-edge structure (XANES)/extended X-ray absorption fine structure (EXAFS) study and compared to XANES/EXAFS data for 2 x MeOH. The XANES data for 1 shows an intense pre-edge transition characteristic of a high-valent metal-oxo species, and a best fit of the EXAFS data gives a short Mn-O bond distance of 1.56 A, confirming the structure of the metal-oxo unit in 1. Detailed spectroelectrochemical studies of 1 and 2 were performed revealing multiple reversible redox processes for both complexes, including a relatively low potential for the Mn(V) --> Mn(IV) process in 1 (near 0.0 V vs saturated calomel reference electrode). Chemical reduction of 1 results in the formation of a Mn(III)Mn(IV)(mu-O) dimer as characterized by electron paramagnetic resonance spectroscopy.

High-spin Mn–oxo complexes and their relevance to the oxygen-evolving complex within photosystem II

“…The colored traces are experimental data and the black traces are simulation of the corresponding spectra using the parameters given in Table 2. Inset shows the broadening of a hyperfine line at 68 mT due to 17 (Fig. 2, Inset) showed that all six hyperfine lines of the signal at g = 8.…”

“…The red traces are data and the black traces are simulations. Inset shows the broadening of a region of hyperfine lines due to 17 O enrichment. Experimental conditions: temperature 11 K, frequency 9.332 GHz, and power 20 mW.…”

“…An alternative approach to exploring possible intermediates in O−O bond formation is to develop synthetic systems with Mn-oxo species and investigate spectroscopically whether they support either a Mn V -oxo state or a Mn IV -oxyl state (16)(17)(18). We have thus prepared a series of monomeric Mn-oxo complexes [Mn n H 3 buea(O)] m (n = 3+ to 5+; m = 2− to 0) in which [H 3 buea] 3− provides a strong anionic ligand field while maintaining local C 3 symmetry around the manganese centers ( Fig.…”

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

“…In the absence of either reductant or acid, no 16 is observed, demonstrating the necessity of both components to forge the O-H bond and the accuracy of the bracketed BDFE. The pK a of 16 is estimated to be approximately 22, which is thought to contribute most of the energy to the effective O-H BDFE of 16 given the low potential of the Mn IV /Mn V couple (−0.05 V v. SCE) [100][101]. Moreover, the basicity of 14 is largely credited for the discrepancy between the O-H bond strengths of 16 and 17 given the small difference in M V /M IV potential of Mn and Cr (ΔE = −100 mV, where chromium features a more positive potential; ca.…”

Biomimetic Epoxidation of Olefins Catalyzed by Metalloporphyrins with Molecular Oxygen

“…Figure 7a shows the initial EPR spectrum of the Mn(TPP)Cl and the silent spectrum is characteristic of Mn(III) species from integer-spin (non-Kramers) Mn(III) S=2 porphyrin compound. [79][80][81] When isobutyraldehyde was added into Mn(TPP)Cl solution, a broad EPR signal of Mn(II) was observed (Figure 7b). [82] The change of EPR signal was attributed to the reaction of Mn (III) porphyrin with aldehyde, from which an acyl radical was generated and the Mn (III) porphyrin was converted to Mn(II) species.…”