| dc.description.abstract | A wide range of MnIII intermediates were generated and studied using various spectroscopic techniques, including electronic absorption, magnetic circular dichroism (MCD), variable-temperature, variable-field (VTVH) MCD, electron paramagnetic resonance (EPR), and X-ray absorption. Computational methods, such as density functional theory (DFT), time-dependent DFT (TD-DFT), and multi-reference ab initio calculations, were used to complement and aid in the interpretation of the spectroscopic data. Specifically, this combined spectroscopic/computational approach was employed to systematically investigate the geometric and electronic structure of a series of peroxomanganese(III) (MnIII-O2) intermediates supported by tridentate, tetradentate, and pentadentate ligands. This work has afforded new insight into the steric and electronic effects of the supporting ligand on the physical properties and reactivity of manganese species. The most significant results made in these studies are summarized below. MnIII-O2 adducts have been postulated as important intermediates in small molecule oxidation catalysts and manganese containing enzymes. A series of tetradentate dipyridyl- (or diquinolinyl-) diazacycloalkane ligands were synthesized to systematically probe the effects of steric and electronic perturbations on the properties of MnII and MnIII-O2complexes. In the initial study, a set of four [MnII(L7py2R)]2+ complexes, supported by the tetradentate 1,4-bis(2-pyridylmethyl)-1,4-diazepane ligand and derivatives with pyridine substituents in the five (R = 5-Br) and six-positions (R = 6-Me and 6-MeO), were synthesized. Treatment of these MnII precursors with H2O2 or KO2 at -40 oC resulted in the formation of MnIII-O2 complexes [MnIII(O2(L7py2R)]+ differing only in the identity of the pyridine ring substituent. The electronic structures of two of these complexes, [MnIII(O2(L7py2R)]+ and [MnIII(O2(L7py26-Me)]+, were examined with our combined spectroscopic/computational approach. While these complexes exhibit similar ground state parameters, their low-temperature MCD spectra revealed significant shifts in electronic transition energies that are correlated to differences in Mn-O2 interactions among these complexes. These results indicate that the [MnIII(O2(L7py2H)]+ complex exhibits symmetric Mn-Operoxo bond lengths, consistent with a side-on bound peroxo ligand. In contrast, the peroxo ligand of the [MnIII(O2(L7py26-Me)]+ complex is bound in a more end-on fashion, with asymmetric Mn-Operoxo distances. This difference in binding mode can be rationalized either in terms of the greater electron-donating abilities of the methyl-appended pyridines or by steric interactions between the peroxo ligand with the substituent at the six position. To distinguish between steric and electronic effects, additional MnII complexes were generated with pyridine derivatives with substituents in the four-position (R = 4-Me and 4-Cl), with different numbers of atoms in the diazacycloalkane backbone (n = 6, 7, 8), and with the type of N-ligand donor changed from a pyridine to a quinoline ligand. Spectroscopic analysis of these MnIII-O2 intermediates revealed a significant shift in the lowest-energy electronic transition when a bulky functional group is on the six position of the pyridine rings. These results indicate that the MnIII-O2 complexes lacking substituents at the six positions of the pyridine rings (L7py2H, L8py2H, L7py24-Me and L7isoq2) exhibit symmetric Mn-Operoxo bond lengths, consistent with a side-on bound peroxo ligand. The peroxo ligand of the complexes with a substituent at the six position (L7py26-Me, L7q2, and L8py26-Me) exhibit asymmetric Mn-Operoxo bond lengths, consistent with a more end-on bound peroxo ligand. This difference in binding mode can be rationalized by steric clash of the peroxo ligand with the 6-substituent at the six positions. To determine the effects of small ligand perturbations on the reactivity of the peroxo group, the more thermally stable MnIII-O2 complexes were reacted with cyclohexanecarboxaldehyde, a model substrate used to probe the relative nucleophilicities of peroxo metal species. Surprisingly, the rate of deformylation did not correlate with the expected nucleophilicity of the MnIII-O2 unit, as the inclusion of methyl substituents on the pyridines affords slower deformylation rates. It was proposed that adding methyl-substituents to the pyridines, or increasing the number of carbons on the diazacycloalkane backbone, sterically hinders nucleophilic attack of the peroxo ligand on the carbonyl carbon of the aldehyde. With small ligand perturbations causing large modulations of the Mn-Operoxo bond lengths, additional MnIII-O2 complexes, supported by aminopyridyl pentadentate ligands, were evaluated in collaboration with Dr. Elodie Anxolabéhère-Mallart (Laboratoire d'Electrochimie Moléculaire, Université Paris). Two MnIII-O2 complexes supported by pentadentate ligands, N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine (mL52) and N-methyl-N,N',N'-tris((1-methylimidazolyl)methyl)ethane-1,2-diamine (imL52) were examined with the spectroscopic/computational method to probe the geometric and the electronic structure. The electronic absorption spectra of these MnIII-O2 complexes (intense broad feature in the range of 15 000 cm-1 to 18 000 cm-1 with a weak shoulder at ~23 000 cm-1) are uniquely different compared to analogous tetradentate ligands (weak broad feature ~ 15 500 cm-1 with an intense absorption feature at ~ 22 000 cm-1). The combined spectroscopic/computational approach was allowed for unambiguous assignment of the structure of these complexes because, although several MnIII-O2 binding modes are possible, each mode should have a unique set of spectral properties. The possible modes are either a seven-coordinate, side-on MnIII-O2species; a six-coordinate, end on MnIII-O2 species; or a six-coordinate, side-on MnIII-O2 adduct with a dissociated N donor from the ligand. This spectroscopic/computational approach revealed that MnIII-O2 complexes supported by pentadentate ligands are stabilized by a six-coordinate environment, with one of the pyridine/imidazole groups dissociated and with a side-on peroxo binding mode. With the observed trend that MnIII-O2 adducts are stabilized by a six-coordinate environment, an additional MnIII-O2 complex supported by a tridentate ligand was synthesized to evaluate its coordination environment and its electronic structure. A bulky scorpionate-based ligand, tris(3,5-diphenylpyrazolyl)borate (TpPh2) was designed to stabilize a MnIII-O2 complex at room temperature. We proposed that the bulky phenyl groups should shield the peroxo ligand and may form a weak hydrogen bonding network, thereby stabilizing a MnIII-O2 intermediate, allowing for characterization by XRD, which the instability of the other intermediates had precluded. The MnII XRD structure revealed a six coordinate complex with the TpPh2 ligand in a facially coordinating geometry, with three solvent molecules completing the coordination sphere. Treatment of this MnII complex with H2O2 or KO2 at room temperature resulted in the formation of a MnIII-O2 complex that is stable for several days at ambient conditions. The increase stability allowed for the isolation of single needle crystals for XRD analysis. The XRD structure confirmed the assignment of this species as a MnIII-O2 adduct (Mn-O and O-O of 1.86 and 1.43 Å) and revealed a distorted six-coordinate octahedral complex with one solvent molecule completing the coordination sphere. The geometric and electronic structure of this MnIII-O2 complex was characterized by using the combined spectroscopic/computational approach. | |
