Biomimetic Peroxo- and Oxo-manganese Complexes: Insights into Structure and Reactivity through Kinetic, Spectroscopic, and Computational Studies

Manganese centers that react with O2 and its reduced derivatives mediate a diverse array of biologically important reactions including the detoxification of superoxide, the conversion of nucleotides to deoxynucleotides, and the generation of O2 from H2O. Peroxo-, oxo-, and hydroxo-manganese motifs are frequently invoked in the catalytic cycles of Mn enzymes. To that end, biomimetic complexes featuring peroxo-, oxo-, and hydroxo-manganese adducts were synthesized and studied using spectroscopic techniques, including variable-temperature electronic absorption, electron paramagnetic resonance (EPR), X-ray absorption (XAS), and magnetic circular dichroism (MCD) spectroscopies along with computational methods, such as density functional theory (DFT) and time-dependent DFT. The structural and spectroscopic properties of these species were investigated in order to better understand how the geometric and electronic structure of these complexes affects reactivity

Reaction Landscape of a Pentadentate N5-Ligated MnII Complex with O2•− and H2O2 Includes Conversion of a Peroxomanganese(III) Adduct to a Bis(μ-oxo)dimanganese(III,IV) Species

Herein we describe the chemical reactivity of the mononuclear [MnII(N4py)(OTf)](OTf) (1) complex with hydrogen peroxide and superoxide. Treatment of 1 with one equivalent superoxide at −40 °C in MeCN formed the peroxomanganese(III) adduct, [MnIII(O2)(N4py)]+ (2) in ~30% yield. Complex 2 decayed over time and the formation of the bis(μ-oxo)dimanganese(III,IV) complex, [MnIIIMnIV(μ-O)2(N4py)2]3+ (3) was observed. When 2 was formed in higher yields (~60%) using excess superoxide, the [MnIII(O2)(N4py)]+ species thermally decayed to MnII species and 3 was formed in no greater than 10% yield. Treatment of [MnIII(O2)(N4py)]+ with 1 resulted in the formation of 3 in ~90% yield, relative to the concentration of [MnIII(O2)(N4py)]+. This reaction mimics the observed chemistry of Mn-ribonucleotide reductase, as it features the conversion of two MnII species to an oxo-bridged MnIIIMnIV compound using O2− as oxidant. Complex 3 was independently prepared through treatment of 1 with H2O2 and base at −40 °C. The geometric and electronic structures of 3 were probed using electronic absorption, electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD), variable-temperature, variable-field MCD (VTVH-MCD), and X-ray absorption (XAS) spectroscopies. Complex 3 was structurally characterized by X-ray diffraction (XRD), which revealed the N4py ligand bound in an unusual tetradentate fashion

Mn K‑Edge X‑ray Absorption Studies of Oxo- and Hydroxo-manganese(IV) Complexes: Experimental and Theoretical Insights into Pre-Edge Properties

Mn K-edge X-ray absorption spectroscopy (XAS) was used to gain insights into the geometric and electronic structures of [Mn^II(Cl)₂­(Me₂EBC)], [Mn^IV(OH)₂­(Me₂EBC)]²⁺, and [Mn^IV(O)­(OH)­(Me₂EBC)]⁺, which are all supported by the tetradentate, macrocyclic Me₂EBC ligand (Me₂EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]­hexadecane). Analysis of extended X-ray absorption fine structure (EXAFS) data for [Mn^IV(O)­(OH)­(Me₂EBC)]⁺ revealed Mn–O scatterers at 1.71 and 1.84 Å and Mn–N scatterers at 2.11 Å, providing the first unambiguous support for the formulation of this species as an oxohydroxomanganese­(IV) adduct. EXAFS-determined structural parameters for [Mn^II(Cl)₂­(Me₂EBC)] and [Mn^IV(OH)₂­(Me₂EBC)]²⁺ are consistent with previously reported crystal structures. The Mn pre-edge energies and intensities of these complexes were examined within the context of data for other oxo- and hydroxomanganese­(IV) adducts, and time-dependent density functional theory (TD-DFT) computations were used to predict pre-edge properties for all compounds considered. This combined experimental and computational analysis revealed a correlation between the Mn–O­(H) distances and pre-edge peak areas of Mn^IVO and Mn^IV–OH complexes, but this trend was strongly modulated by the Mn^IV coordination geometry. Mn 3d-4p mixing, which primarily accounts for the pre-edge intensities, is not solely a function of the Mn–O­(H) bond length; the coordination geometry also has a large effect on the distribution of pre-edge intensity. For tetragonal Mn^IVO centers, more than 90% of the pre-edge intensity comes from excitations to the MnO σ* MO. Trigonal bipyramidal oxomanganese­(IV) centers likewise feature excitations to the MnO σ* molecular orbital (MO) but also show intense transitions to 3d_<i>x</i>²_{–<i>y</i>²} and 3d_<i>xy</i> MOs because of enhanced 3d-4p_x,y mixing. This gives rise to a broader pre-edge feature for trigonal Mn^IVO adducts. These results underscore the importance of reporting experimental pre-edge areas rather than peak heights. Finally, the TD-DFT method was applied to understand the pre-edge properties of a recently reported <i>S</i> = 1 Mn^VO adduct; these findings are discussed within the context of previous examinations of oxomanganese­(V) complexes

Spectroscopic and Computational Investigations of a Mononuclear Manganese(IV)-Oxo Complex Reveal Electronic Structure Contributions to Reactivity

The mononuclear Mn­(IV)-oxo complex [Mn^IV(O)­(N4py)]²⁺, where N4py is the pentadentate ligand <i>N</i>,<i>N</i>-bis­(2-pyridylmethyl)-<i>N</i>-bis­(2-pyridyl)­methylamine, has been proposed to attack C–H bonds by an excited-state reactivity pattern [Cho, K.-B.; Shaik, S.; Nam, W. J. Phys. Chem. Lett. 2012, 3, 2851−2856 (DOI: 10.1021/jz301241z)]. In this model, a ⁴E excited state is utilized to provide a lower-energy barrier for hydrogen-atom transfer. This proposal is intriguing, as it offers both a rationale for the relatively high hydrogen-atom-transfer reactivity of [Mn^IV(O)­(N4py)]²⁺ and a guideline for creating more reactive complexes through ligand modification. Here we employ a combination of electronic absorption and variable-temperature magnetic circular dichroism (MCD) spectroscopy to experimentally evaluate this excited-state reactivity model. Using these spectroscopic methods, in conjunction with time-dependent density functional theory (TD-DFT) and complete-active space self-consistent-field calculations (CASSCF), we define the ligand-field and charge-transfer excited states of [Mn^IV(O)­(N4py)]²⁺. Through a graphical analysis of the signs of the experimental <i>C</i>-term MCD signals, we unambiguously assign a low-energy MCD feature of [Mn^IV(O)­(N4py)]²⁺ as the ⁴E excited state predicted to be involved in hydrogen-atom-transfer reactivity. The CASSCF calculations predict enhanced Mn^III-oxyl character on the excited-state ⁴E surface, consistent with previous DFT calculations. Potential-energy surfaces, developed using the CASSCF methods, are used to determine how the energies and wave functions of the ground and excited states evolved as a function of MnO distance. The unique insights into ground- and excited-state electronic structure offered by these spectroscopic and computational studies are harmonized with a thermodynamic model of hydrogen-atom-transfer reactivity, which predicts a correlation between transition-state barriers and driving force

X‑Band Electron Paramagnetic Resonance Comparison of Mononuclear Mn^IV-oxo and Mn^IV-hydroxo Complexes and Quantum Chemical Investigation of Mn^IV Zero-Field Splitting

X-band electron paramagnetic resonance (EPR) spectroscopy was used to probe the ground-state electronic structures of mononuclear Mn^IV complexes [Mn^IV(OH)₂­(Me₂EBC)]²⁺ and [Mn^IV(O)­(OH)­(Me₂EBC)]⁺. These compounds are known to effect C–H bond oxidation reactions by a hydrogen-atom transfer mechanism. They provide an ideal system for comparing Mn^IV-hydroxo versus Mn^IV-oxo motifs, as they differ by only a proton. Simulations of 5 K EPR data, along with analysis of variable-temperature EPR signal intensities, allowed for the estimation of ground-state zero-field splitting (ZFS) and ⁵⁵Mn hyperfine parameters for both complexes. From this analysis, it was concluded that the Mn^IV-oxo complex [Mn^IV(O)­(OH)­(Me₂EBC)]⁺ has an axial ZFS parameter <i>D</i> (<i>D</i> = +1.2(0.4) cm^–1) and rhombicity (<i>E</i>/<i>D</i> = 0.22(1)) perturbed relative to the Mn^IV-hydroxo analogue [Mn^IV(OH)₂(Me₂EBC)]²⁺ (|<i>D</i>| = 0.75(0.25) cm^–1; <i>E</i>/<i>D</i> = 0.15(2)), although the complexes have similar ⁵⁵Mn values (<i>a</i> = 7.7 and 7.5 mT, respectively). The ZFS parameters for [Mn^IV(OH)₂(Me₂EBC)]²⁺ were compared with values obtained previously through variable-temperature, variable-field magnetic circular dichroism (VTVH MCD) experiments. While the VTVH MCD analysis can provide a reasonable estimate of the magnitude of <i>D</i>, the <i>E</i>/<i>D</i> values were poorly defined. Using the ZFS parameters reported for these complexes and five other mononuclear Mn^IV complexes, we employed coupled-perturbed density functional theory (CP-DFT) and complete active space self-consistent field (CASSCF) calculations with second-order <i>n</i>-electron valence-state perturbation theory (NEVPT2) correction, to compare the ability of these two quantum chemical methods for reproducing experimental ZFS parameters for Mn^IV centers. The CP-DFT approach was found to provide reasonably acceptable values for <i>D</i>, whereas the CASSCF/NEVPT2 method fared worse, considerably overestimating the magnitude of <i>D</i> in several cases. Both methods were poor in reproducing experimental <i>E</i>/<i>D</i> values. Overall, this work adds to the limited investigations of Mn^IV ground-state properties and provides an initial assessment for calculating Mn^IV ZFS parameters with quantum chemical methods

Ligand-Exchange-Induced Growth of an Atomically Precise Cu₂₉ Nanocluster from a Smaller Cluster

The copper hydride nanocluster (NC) [Cu₂₉Cl₄H₂₂­(Ph₂phen)₁₂]Cl (<b>2</b>; Ph₂phen = 4,7-diphenyl-1,10-phenanthroline) was isolated cleanly, and in good yields, by controlled growth from the smaller NC, [Cu₂₅H₂₂(PPh₃)₁₂]­Cl (<b>1</b>), in the presence of Ph₂phen and a chloride source at room temperature. Complex <b>2</b> was fully characterized by single-crystal X-ray diffraction, XANES, and XPS, and represents a rare example of an <i>N*</i> = 2 superatom. Its formation from <b>1</b> demonstrates that atomically precise copper clusters can be used as templates to generate larger NCs that retain the fundamental electronic and bonding properties of the original cluster. A time-resolved kinetic evaluation of the formation of <b>2</b> reveals that the mechanism of cluster growth is initiated by rapid ligand exchange. The slower extrusion of CuCl monomer, its transport, and subsequent capture by intact clusters resemble elementary steps in the reactant-assisted Ostwald ripening of metal nanoparticles

Ligand-Exchange-Induced Growth of an Atomically Precise Cu₂₉ Nanocluster from a Smaller Cluster

The copper hydride nanocluster (NC) [Cu₂₉Cl₄H₂₂­(Ph₂phen)₁₂]Cl (<b>2</b>; Ph₂phen = 4,7-diphenyl-1,10-phenanthroline) was isolated cleanly, and in good yields, by controlled growth from the smaller NC, [Cu₂₅H₂₂(PPh₃)₁₂]­Cl (<b>1</b>), in the presence of Ph₂phen and a chloride source at room temperature. Complex <b>2</b> was fully characterized by single-crystal X-ray diffraction, XANES, and XPS, and represents a rare example of an <i>N*</i> = 2 superatom. Its formation from <b>1</b> demonstrates that atomically precise copper clusters can be used as templates to generate larger NCs that retain the fundamental electronic and bonding properties of the original cluster. A time-resolved kinetic evaluation of the formation of <b>2</b> reveals that the mechanism of cluster growth is initiated by rapid ligand exchange. The slower extrusion of CuCl monomer, its transport, and subsequent capture by intact clusters resemble elementary steps in the reactant-assisted Ostwald ripening of metal nanoparticles