Ceric Ammonium Nitrate Oxidation of a Mn Complex Generates a Bis(μ-oxo)dimanganese(IV,IV) Product via a Mn-Oxo Intermediate.

The oxidation of manganese complexes using ceric ammonium nitrate (CAN) is often complicated by the fact that cerium(IV) can serve as both an oxidant and a Lewis acid. In this work, we explore the reaction of CAN with the Mn complex [Mn(OTf)(N4py)](OTf) (N4py = ,-bis(4-methoxy-3,5-dimethyl-2-pyridylmethyl)--bis(2-pyridyl)methylamine). We chose this complex as multiple oxidation products, including oxomanganese(IV) and bis(μ-oxo)dimanganese(III,IV) complexes, have previously been reported.

C-H Bond Oxidation by Mn-Oxo Complexes: Hydrogen-Atom Tunneling and Multistate Reactivity.

The reactivity of six Mn-oxo complexes in C-H bond oxidation has been examined using a combination of kinetic experiments and computational methods. Variable-temperature studies of the oxidation of 9,10-dihydroanthracene (DHA) and ethylbenzene by these Mn-oxo complexes yielded activation parameters suitable for evaluating electronic structure computations. Complementary kinetic experiments of the oxidation of deuterated DHA provided evidence for hydrogen-atom tunneling in C-H bond oxidation for all Mn-oxo complexes.

Enhanced Understanding of Structure-Function Relationships for Oxomanganese(IV) Complexes.

A series of manganese(II) and oxomanganese(IV) complexes supported by neutral, pentadentate ligands with varied equatorial ligand-field strength (N3pyQ, N2py2I, and N4py) were synthesized and then characterized using structural and spectroscopic methods. On the basis of electronic absorption spectroscopy, the [Mn(O)(N4py)] complex has the weakest equatorial ligand field among a set of similar Mn-oxo species. In contrast, [Mn(O)(N2py2I)] shows the strongest equatorial ligand-field strength for this same series.

Evidence for the Chemical Mechanism of RibB (3,4-Dihydroxy-2-butanone 4-phosphate Synthase) of Riboflavin Biosynthesis.

RibB (3,4-dihydroxy-2-butanone 4-phosphate synthase) is a magnesium-dependent enzyme that excises the C4 of d-ribulose-5-phosphate (d-Ru5P) as formate. RibB generates the four-carbon substrate for lumazine synthase that is incorporated into the xylene moiety of lumazine and ultimately the riboflavin isoalloxazine. The reaction was first identified by Bacher and co-workers in the 1990s, and their chemical mechanism hypothesis became canonical despite minimal direct evidence.

Differences in chemoselectivity in olefin oxidation by a series of non-porphyrin manganese(IV)-oxo complexes.

High valent metal-oxo intermediates are versatile oxidants known to facilitate both oxygen atom transfer (OAT) and hydrogen atom transfer (HAT) reactions in nature. In addition to performing essential yet challenging biological reactions, these intermediates are known for their selectivity in favoring the formation of one oxidation product. To understand the basis for this selectivity, we explore the role of equatorial ligand field perturbations in Mn-oxo complexes on chemoselectivity in cyclohexene oxidation.

Mechanistic insight into oxygen atom transfer reactions by mononuclear manganese(IV)-oxo adducts.

High-valent metal-oxo intermediates are well known to facilitate oxygen-atom transfer (OAT) reactions both in biological and synthetic systems. These reactions can occur by a single-step OAT mechanism or by a stepwise process initiated by rate-limiting electron transfer between the substrate and the metal-oxo unit. Several recent reports have demonstrated that changes in the metal reduction potential, caused by the addition of Brønsted or Lewis acids, cause a change in sulfoxidation mechanism of MnIV-oxo complexes from single-step OAT to the multistep process.

Structural Characterization of a Series of N5-Ligated Mn -Oxo Species.

Analysis of extended X-ray absorption fine structure (EXAFS) data for the Mn -oxo complexes [Mn (O)( N4py)] , [Mn (O)(2pyN2B)] , and [Mn (O)(2pyN2Q)] ( N4py=N,N-bis(4-methoxy-3,5-dimethyl-2-pyridylmethyl)-N-bis(2-pyridyl)methylamine; 2pyN2B=(N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine, and 2pyN2Q=N,N-bis(2-pyridyl)-N,N-bis(2-quinolylmethyl)methanamine) afforded Mn=O and Mn-N bond lengths. The Mn=O distances for [Mn (O)( N4py)] and [Mn (O)(2pyN2B)] are 1.72 and 1.

Mn-Oxo complex of a bis(benzimidazolyl)-containing N5 ligand reveals different reactivity trends for Mn-oxo than Fe-oxo species.

Using the pentadentate ligand (N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine, 2pyN2B), presenting two pyridyl and two (N-methyl)benzimidazolyl donor moieties in addition to a central tertiary amine, new MnII and MnIV-oxo complexes were generated and characterized. The [MnIV(O)(2pyN2B)]2+ complex showed spectroscopic signatures (i.e.

Mn-Peroxo adduct supported by a new tetradentate ligand shows acid-sensitive aldehyde deformylation reactivity.

The new tetradentate L7BQ ligand (L7BQ = 1,4-di(quinoline-8-yl)-1,4-diazepane) has been synthesized and shown to support MnII and MnIII-peroxo complexes. X-ray crystallography of the [MnII(L7BQ)(OTf)2] complex shows a monomeric MnII center with the L7BQ ligand providing four donor nitrogen atoms in the equatorial field, with two triflate ions bound in the axial positions. When this species is treated with H2O2 and Et3N at -40 °C, a MnIII-peroxo adduct, [MnIII(O2)(L7BQ)]+ is formed.

Equatorial Ligand Perturbations Influence the Reactivity of Manganese(IV)-Oxo Complexes.

Manganese(IV)-oxo complexes are often invoked as intermediates in Mn-catalyzed C-H bond activation reactions. While many synthetic Mn -oxo species are mild oxidants, other members of this class can attack strong C-H bonds. The basis for these reactivity differences is not well understood.