Oxo- and hydroxomanganese(IV) adducts: a comparative spectroscopic and computational study

Earth Abundant Oxidation Catalysts for Removal of Contaminants of Emerging Concern from Wastewater: Homogeneous Catalytic Screening of Monomeric Complexes

Twenty novel Mn, Fe, and Cu complexes of ethylene cross-bridged tetraazamacrocycles with potentially copolymerizable allyl and benzyl pendant arms were synthesized and characterized. Multiple X-ray crystal structures demonstrate the cis-folded pseudo-octahedral geometry forced by the rigidifying ethylene cross-bridge and show that two cis coordination cites are available for interaction with substrate and oxidant. The Cu complexes were used to determine kinetic stability under harsh acidic and high-temperature conditions, which revealed that the cyclam-based ligands provide superior stabilization with half-lives of many minutes or even hours in 5 M HCl at 50-90 °C. Cyclic voltammetry studies of the Fe and Mn complexes reveal reversible redox processes indicating stabilization of Fe2+/Fe3+ and Mn2+/Mn3+/Mn4+ oxidation states, indicating the likelihood of catalytic oxidation for these complexes. Finally, dye-bleaching experiments with methylene blue, methyl orange, and rhodamine B demonstrate efficient catalytic decolorization and allow selection of the most successful monomeric catalysts for copolymerization to produce future heterogeneous water purification materials.

Synthesis and Characterization of Late Transition Metal Complexes of Mono-Acetate Pendant Armed Ethylene Cross-Bridged Tetraazamacrocycles with Promise as Oxidation Catalysts for Dye Bleaching

Ethylene cross-bridged tetraazamacrocycles are known to produce kinetically stable transition metal complexes that can act as robust oxidation catalysts under harsh aqueous conditions. We have synthesized ligand analogs with single acetate pendant arms that act as pentadentate ligands to Mn, Fe, Co, Ni, Cu, and Zn. These complexes have been synthesized and characterized, including the structural characterization of four Co and Cu complexes. Cyclic voltammetry demonstrates that multiple oxidation states are stabilized by these rigid, bicyclic ligands. Yet, redox potentials of the metal complexes are modified compared to the "parent" ligands due to the pendant acetate arm. Similarly, gains in kinetic stability under harsh acidic conditions, compared to parent complexes without the pendant acetate arm, were demonstrated by a half-life seven times longer for the cyclam copper complex. Due to the reversible, high oxidation states available for the Mn and Fe complexes, the Mn and Fe complexes were examined as catalysts for the bleaching of three commonly used pollutant model dyes (methylene blue, methyl orange, and Rhodamine B) in water with hydrogen peroxide as oxidant. The efficient bleaching of these dyes was observed.

An ethylene cross-bridged pentaazamacrocycle and its Cu2+ complex: constrained ligand topology and excellent kinetic stability

Rigid and topologically constrained ethylene cross-bridged tetraazamacrocycles have been increasingly utilised for thirty years as they form remarkably stable transition metal complexes for catalysis, biomedical imaging, and inorganic drug molecule applications. Extending these benefits to pentaazamacrocycles has been achieved and a first transition metal complex prepared and structurally characterized.

A new synthesis of porphyrins via a putative trans-manganese(iv)-dihydroxide intermediate

A new method for the synthesis of meso-substituted porphyrins was developed. In this two-step methodology, the first step involves the condensation of pyrroles/dipyrromethanes with aldehydes in a water-methanol mixture under acidic conditions. The second step involves manganese induced cyclization followed by oxidation via PhIO/O2. This methodology has been useful for the synthesis of a wide range of trans-A2B2 porphyrins and also symmetric porphyrins in moderate to good yields. A detailed investigation of the manganese induced cyclization reaction has allowed us to characterize a Mn-porphyrinogen complex. A series of analytical and spectroscopic techniques and DFT calculations have led us to the conclusion that the putative intermediate species are trans-manganese(iv)-dihydroxide complexes. EPR and magnetic susceptibility measurements helped us to assign the oxidation state of the manganese complexes in their native state. The assumption of trans-manganese(iv)-dihydroxide as the true intermediate for this porphyrin synthesis has been authenticated via in situ UV-Vis experiments. This new methodology is certainly different from other previously reported methodologies in many aspects and most importantly these reactions can be easily performed on a gram scale for the synthesis of porphyrins.

Structural Characterization of a Series of N5-Ligated MnIV -Oxo Species

Analysis of extended X-ray absorption fine structure (EXAFS) data for the MnIV -oxo complexes [MnIV (O)(DMM N4py)]2+ , [MnIV (O)(2pyN2B)]2+ , and [MnIV (O)(2pyN2Q)]2+ (DMM 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 [MnIV (O)(DMM N4py)]2+ and [MnIV (O)(2pyN2B)]2+ are 1.72 and 1.70 Å, respectively. In contrast, the Mn=O distance for [MnIV (O)(2pyN2Q)]2+ was significantly longer (1.76 Å). We attribute this long distance to sample heterogeneity, which is reasonable given the reduced stability of [MnIV (O)(2pyN2Q)]2+ . The Mn=O distances for [MnIV (O)(DMM N4py)]2+ and [MnIV (O)(2pyN2B)]2+ could only be well-reproduced using DFT-derived models that included strong hydrogen-bonds between second-sphere solvent 2,2,2-trifluoroethanol molecules and the oxo ligand. These results suggest an important role for the 2,2,2-trifluoroethanol solvent in stabilizing MnIV -oxo adducts. The DFT methods were extended to investigate the structure of the putative [MnIV (O)(N4py)]2+ ⋅(HOTf)2 adduct. These computations suggest that a MnIV -hydroxo species is most consistent with the available experimental data.

How Metal Ion Lewis Acidity and Steric Properties Influence the Barrier to Dioxygen Binding, Peroxo O-O Bond Cleavage, and Reactivity

Herein we quantitatively investigate how metal ion Lewis acidity and steric properties influence the kinetics and thermodynamics of dioxygen binding versus release from structurally analogous Mn-O₂ complexes, as well as the barrier to Mn peroxo O-O bond cleavage, and the reactivity of Mn oxo intermediates. Previously we demonstrated that the steric and electronic properties of Mn^III-OOR complexes containing N-heterocyclic (N^Ar) ligand scaffolds can have a dramatic influence on alkylperoxo O-O bond lengths and the barrier to alkylperoxo O-O bond cleavage. Herein, we examine the dioxygen reactivity of a new Mn^II complex containing a more electron-rich, less sterically demanding N^Ar ligand scaffold, and compare it with previously reported Mn^II complexes. Dioxygen binding is shown to be reversible with complexes containing the more electron-rich metal ions. The kinetic barrier to O₂ binding and peroxo O-O bond cleavage is shown to correlate with redox potentials, as well as the steric properties of the supporting N^Ar ligands. The reaction landscape for the dioxygen chemistry of the more electron-rich complexes is shown to be relatively flat. A total of four intermediates, including a superoxo and peroxo species, are observed with the most electron-rich complex. Two new intermediates are shown to form following the peroxo, which are capable of cleaving strong X-H bonds. In the absence of a sacrificial H atom donor, solvent, or ligand, serves as a source of H atoms. With TEMPOH as sacrificial H atom donor, a deuterium isotope effect is observed (k_H/k_D = 3.5), implicating a hydrogen atom transfer (HAT) mechanism. With 1,4-cyclohexadiene, 0.5 equiv of benzene is produced prior to the formation of an EPR detected Mn^IIIMn^IV bimetallic species, and 0.5 equiv after its formation.

Increase of Direct C-C Coupling Reaction Yield by Identifying Structural and Electronic Properties of High-Spin Iron Tetra-azamacrocyclic Complexes

Macrocyclic ligands have been explored extensively as scaffolds for transition metal catalysts for oxygen and hydrogen atom transfer reactions. C-C reactions facilitated using earth abundant metals bound to macrocyclic ligands have not been well-understood but could be a green alternative to replacing the current expensive and toxic precious metal systems most commonly used for these processes. Therefore, the yields from direct Suzuki-Miyaura C-C coupling of phenylboronic acid and pyrrole to produce 2-phenylpyrrole facilitated by eight high-spin iron complexes ([Fe3+L1(Cl)2]+, [Fe3+L4(Cl)2]+, [Fe2+L5(Cl)]+, [Fe2+L6(Cl)2], [Fe3+L7(Cl)2]+, [Fe3+L8(Cl)2]+, [Fe2+L9(Cl)]+, and [Fe2+L10(Cl)]+) were compared to identify the effect of structural and electronic properties on catalytic efficiency. Specifically, catalyst complexes were compared to evaluate the effect of five properties on catalyst reaction yields: (1) the coordination requirements of the catalyst, (2) redox half-potential of each complex, (3) topological constraint/rigidity, (4) N atom modification(s) increasing oxidative stability of the complex, and (5) geometric parameters. The need for two labile cis-coordination sites was confirmed based on a 42% decrease in catalytic reaction yield observed when complexes containing pentadentate ligands were used in place of complexes with tetradentate ligands. A strong correlation between iron(III/II) redox potential and catalytic reaction yields was also observed, with [Fe2+L6(Cl)2] providing the highest yield (81%, -405 mV). A Lorentzian fitting of redox potential versus yields predicts that these catalysts can undergo more fine-tuning to further increase yields. Interestingly, the remaining properties explored did not show a direct, strong relationship to catalytic reaction yields. Altogether, these results show that modifications to the ligand scaffold using fundamental concepts of inorganic coordination chemistry can be used to control the catalytic activity of macrocyclic iron complexes by controlling redox chemistry of the iron center. Furthermore, the data provide direction for the design of improved catalysts for this reaction and strategies to understand the impact of a ligand scaffold on catalytic activity of other reactions.

Manganese-Oxygen Intermediates in O-O Bond Activation and Hydrogen-Atom Transfer Reactions

Biological systems capitalize on the redox versatility of manganese to perform reactions involving dioxygen and its derivatives superoxide, hydrogen peroxide, and water. The reactions of manganese enzymes influence both human health and the global energy cycle. Important examples include the detoxification of reactive oxygen species by manganese superoxide dismutase, biosynthesis by manganese ribonucleotide reductase and manganese lipoxygenase, and water splitting by the oxygen-evolving complex of photosystem II. Although these enzymes perform very different reactions and employ structurally distinct active sites, manganese intermediates with peroxo, hydroxo, and oxo ligation are commonly proposed in catalytic mechanisms. These intermediates are also postulated in mechanisms of synthetic manganese oxidation catalysts, which are of interest due to the earth abundance of manganese. In this Account, we describe our recent efforts toward understanding O-O bond activation pathways of Mn^III-peroxo adducts and hydrogen-atom transfer reactivity of Mn^IV-oxo and Mn^III-hydroxo complexes. In biological and synthetic catalysts, peroxomanganese intermediates are commonly proposed to decay by either Mn-O or O-O cleavage pathways, although it is often unclear how the local coordination environment influences the decay mechanism. To address this matter, we generated a variety of Mn^III-peroxo adducts with varied ligand environments. Using parallel-mode EPR and Mn K-edge X-ray absorption techniques, the decay pathway of one Mn^III-peroxo complex bearing a bulky macrocylic ligand was investigated. Unlike many Mn^III-peroxo model complexes that decay to oxo-bridged-Mn^IIIMn^IV dimers, decay of this Mn^III-peroxo adduct yielded mononuclear Mn^III-hydroxo and Mn^IV-oxo products, potentially resulting from O-O bond activation of the Mn^III-peroxo unit. These results highlight the role of ligand sterics in promoting the formation of mononuclear products and mark an important step in designing Mn^III-peroxo complexes that convert cleanly to high-valent Mn-oxo species. Although some synthetic Mn^IV-oxo complexes show great potential for oxidizing substrates with strong C-H bonds, most Mn^IV-oxo species are sluggish oxidants. Both two-state reactivity and thermodynamic arguments have been put forth to explain these observations. To address these issues, we generated a series of Mn^IV-oxo complexes supported by neutral, pentadentate ligands with systematically perturbed equatorial donation. Kinetic investigations of these complexes revealed a correlation between equatorial ligand-field strength and hydrogen-atom and oxygen-atom transfer reactivity. While this trend can be understood on the basis of the two-state reactivity model, the reactivity trend also correlates with variations in Mn^III/IV reduction potential caused by changes in the ligand field. This work demonstrates the dramatic influence simple ligand perturbations can have on reactivity but also illustrates the difficulties in understanding the precise basis for a change in reactivity. In the enzyme manganese lipoxygenase, an active-site Mn^III-hydroxo adduct initiates substrate oxidation by abstracting a hydrogen atom from a C-H bond. Precedent for this chemistry from synthetic Mn^III-hydroxo centers is rare. To better understand hydrogen-atom transfer by Mn^III centers, we developed a pair of Mn^III-hydroxo complexes, formed in high yield from dioxygen oxidation of Mn^II precursors, capable of attacking weak O-H and C-H bonds. Kinetic and computational studies show a delicate interplay between thermodynamic and steric influences in hydrogen-atom transfer reactivity, underscoring the potential of Mn^III-hydroxo units as mild oxidants.

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 N,N-bis(2-pyridylmethyl)-N-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 C-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.

Magnetic Circular Dichroism Evidence for an Unusual Electronic Structure of a Tetracarbene-Oxoiron(IV) Complex

In biology, high valent oxo-iron(IV) species have been shown to be pivotal intermediates for functionalization of C-H bonds in the catalytic cycles of a range of O₂-activating iron enzymes. This work details an electronic-structure investigation of [Fe^IV(O)(L^NHC)(NCMe)]²⁺ (L^NHC = 3,9,14,20-tetraaza-1,6,12,17-tetraazoniapenta-cyclohexacosane-1(23),4,6(26),10,12(25),15,17(24),21-octaene, complex 1) using helium tagging infrared photodissociation (IRPD), absorption, and magnetic circular dichroism (MCD) spectroscopy, coupled with DFT and highly correlated wave function based multireference calculations. The IRPD spectrum of complex 1 reveals the Fe-O stretching vibration at 832 ± 3 cm^-1. By analyzing the Franck-Condon progression, we can determine the same vibration occurring at 616 ± 10 cm^-1 in the E(d_xy → d_xz,yz) excited state. Both values are similar to those measured for [Fe^IV(O)(TMC)(NCMe)]²⁺ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). The low-temperature MCD spectra of complex 1 exhibit three pseudo A-term signals around 12 500, 17 000, and 24 300 cm^-1. We can unequivocally assign them to the ligand field transitions of d_xy → d_xz,yz, d_xz,yz → d_z2, and d_xz,yz → d_x2-y2, respectively, through direct calculations of MCD spectra and independent determination of the MCD C-term signs from the corresponding electron donating and accepting orbitals. In comparison with the corresponding transitions observed for [Fe^IV(O) (SR-TPA)(NCMe)]²⁺ (SR-TPA = tris(3,5-dimethyl-4-methoxypyridyl-2-methy)amine), the excitations within the (FeO)²⁺ core of complex 1 have similar transition energies, whereas the excitation energy for d_xz,yz → d_x2-y2 is significantly higher (∼12 000 cm^-1 for [Fe^IV(O)(SR-TPA)(NCMe)]²⁺). Our results thus substantiate that the tetracarbene ligand (L^NHC) of complex 1 does not significantly affect the bonding in the (FeO)²⁺ unit but strongly destabilizes the d_x2-y2 orbital to eventually lift it above d_z2. As a consequence, this unusual electron configuration leads to an unprecedentedly larger quintet-triplet energy separation for complex 1, which largely rules out the possibility that the H atom transfer reaction may take place on the quintet surface and hence quenches two-state reactivity. The resulting mechanistic implications are discussed.

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 (55)Mn 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 D (D = +1.2(0.4) cm(-1)) and rhombicity (E/D = 0.22(1)) perturbed relative to the Mn(IV)-hydroxo analogue [Mn(IV)(OH)2(Me2EBC)](2+) (|D| = 0.75(0.25) cm(-1); E/D = 0.15(2)), although the complexes have similar (55)Mn values (a = 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 D, the E/D 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 n-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 D, whereas the CASSCF/NEVPT2 method fared worse, considerably overestimating the magnitude of D in several cases. Both methods were poor in reproducing experimental E/D 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.

Redox inactive metal ion triggered N-dealkylation by an iron catalyst with dioxygen activation: a lesson from lipoxygenases

Utilization of dioxygen as the terminal oxidant at ambient temperature is always a challenge in redox chemistry, because it is hard to oxidize a stable redox metal ion like iron(III) to its high oxidation state to initialize the catalytic cycle. Inspired by the dioxygenation and co-oxidase activity of lipoxygenases, herein, we introduce an alternative protocol to activate the sluggish iron(III) species with non-redox metal ions, which can promote its oxidizing power to facilitate substrate oxidation with dioxygen, thus initializing the catalytic cycle. In oxidations of N,N-dimethylaniline and its analogues, adding Zn(OTf)2 to the [Fe(TPA)Cl2]Cl catalyst can trigger the amine oxidation with dioxygen, whereas [Fe(TPA)Cl2]Cl alone is very sluggish. In stoichiometric oxidations, it has also been confirmed that the presence of Zn(OTf)2 can apparently improve the electron transfer capability of the [Fe(TPA)Cl2]Cl complex. Experiments using different types of substrates as trapping reagents disclosed that the iron(IV) species does not occur in the catalytic cycle, suggesting that oxidation of amines is initialized by electron transfer rather than hydrogen abstraction. Combined experiments from UV-Vis, high resolution mass spectrometry, electrochemistry, EPR and oxidation kinetics support that the improved electron transfer ability of iron(III) species originates from its interaction with added Lewis acids like Zn(2+) through a plausible chloride or OTf(-) bridge, which has promoted the redox potential of iron(III) species. The amine oxidation mechanism was also discussed based on the available data, which resembles the co-oxidase activity of lipoxygenases in oxidative dealkylation of xenobiotic metabolisms where an external electron donor is not essential for dioxygen activation.

Formation, Characterization, and O-O Bond Activation of a Peroxomanganese(III) Complex Supported by a Cross-Clamped Cyclam Ligand

Although there have been reports describing the nucleophilic reactivity of peroxomanganese(III) intermediates, as well as their conversion to high-valent oxo-bridged dimers, it remains a challenge to activate peroxomanganese(III) species for conversion to high-valent, mononuclear manganese complexes. Herein, we report the generation, characterization, and activation of a peroxomanganese(III) adduct supported by the cross-clamped, macrocyclic Me2EBC ligand (4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). This ligand is known to support high-valent, mononuclear Mn(IV) species with well-defined spectroscopic properties, which provides an opportunity to identify mononuclear Mn(IV) products from O-O bond activation of the corresponding Mn(III)-peroxo adduct. The peroxomanganese(III) intermediate, [Mn(III)(O2)(Me2EBC)](+), was prepared at low-temperature by the addition of KO2 to [Mn(II)(Cl)2(Me2EBC)] in CH2Cl2, and this complex was characterized by electronic absorption, electron paramagnetic resonance (EPR), and Mn K-edge X-ray absorption (XAS) spectroscopies. The electronic structure of the [Mn(III)(O2)(Me2EBC)](+) intermediate was examined by density functional theory (DFT) and time-dependent (TD) DFT calculations. Detailed spectroscopic investigations of the decay products of [Mn(III)(O2)(Me2EBC)](+) revealed the presence of mononuclear Mn(III)-hydroxo species or a mixture of mononuclear Mn(IV) and Mn(III)-hydroxo species. The nature of the observed decay products depended on the amount of KO2 used to generate [Mn(III)(O2)(Me2EBC)](+). The Mn(III)-hydroxo product was characterized by Mn K-edge XAS, and shifts in the pre-edge transition energies and intensities relative to [Mn(III)(O2)(Me2EBC)](+) provide a marker for differences in covalency between peroxo and nonperoxo ligands. To the best of our knowledge, this work represents the first observation of a mononuclear Mn(IV) center upon decay of a nonporphyrinoid Mn(III)-peroxo center.

A five-coordinate Mn(iv) intermediate in biological water oxidation: spectroscopic signature and a pivot mechanism for water binding

Among the four photo-driven transitions of the water-oxidizing tetramanganese-calcium cofactor of biological photosynthesis, the second-last step of the catalytic cycle, that is the S₂ to S₃ state transition, is the crucial step that poises the catalyst for the final O-O bond formation. This transition, whose intermediates are not yet fully understood, is a multi-step process that involves the redox-active tyrosine residue and includes oxidation and deprotonation of the catalytic cluster, as well as the binding of a water molecule. Spectroscopic data has the potential to shed light on the sequence of events that comprise this catalytic step, which still lacks a structural interpretation. In this work the S₂-S₃ state transition is studied and a key intermediate species is characterized: it contains a Mn₃O₄Ca cubane subunit linked to a five-coordinate Mn(iv) ion that adopts an approximately trigonal bipyramidal ligand field. It is shown using high-level density functional and multireference wave function calculations that this species accounts for the near-infrared absorption and electron paramagnetic resonance observations on metastable S₂-S₃ intermediates. The results confirm that deprotonation and Mn oxidation of the cofactor must precede the coordination of a water molecule, and lead to identification of a novel low-energy water binding mode that has important implications for the identity of the substrates in the mechanism of biological water oxidation.

Synthesis, structural studies, and oxidation catalysis of the manganese(II), iron(II), and copper(II) complexes of a 2-pyridylmethyl pendant armed side-bridged cyclam

The first 2-pyridylmethyl pendant armed structurally reinforced cyclam ligand has been synthesized and successfully complexed to Mn²⁺, Fe²⁺, and Cu²⁺ cations. X-ray crystal structures were obtained for the diprotonated ligand and its Cu²⁺ complex demonstrating pentadentate binding of the ligand with trans-II configuration of the side-bridged cyclam ring, leaving a potential labile binding site cis to the pyridine donor for interaction of the complex with oxidants and/or substrates. The electronic properties of these complexes were determined by means of solid state magnetic moment, with a low value of μ = 3.10 μ_B for the Fe²⁺ complex suggesting it has a trigonal bipyramidal coordination geometry, matching the crystal structure of the Cu²⁺ complex, while the μ = 5.52 μ_B value for the Mn²⁺ complex suggests it is high spin octahedral. Cyclic voltammetry in acetonitrile revealed reversible redox processes in all three complexes, suggesting catalytic reactivity involving electron transfer processes are possible for these complexes. Screening for oxidation catalysis using hydrogen peroxide as the terminal oxidant identified the Fe²⁺ complex as the oxidation catalysts most worthy of continued development.

Reactivity and O2 Formation by Mn(IV)- and Mn(V)-Hydroxo Species Stabilized within a Polyfluoroxometalate Framework

Manganese(IV,V)-hydroxo and oxo complexes are often implicated in both catalytic oxygenation and water oxidation reactions. Much of the research in this area is designed to structurally and/or functionally mimic enzymes. On the other hand, the tendency of such mimics to decompose under strong oxidizing conditions makes the use of molecular inorganic oxide clusters an enticing alternative for practical applications. In this context it is important to understand the reactivity of conceivable reactive intermediates in such an oxide-based chemical environment. Herein, a polyfluoroxometalate (PFOM) monosubstituted with manganese, [NaH2(Mn-L)W17F6O55](q-), has allowed the isolation of a series of compounds, Mn(II, III, IV and V), within the PFOM framework. Magnetic susceptibility measurements show that all the compounds are high spin. XPS and XANES measurements confirmed the assigned oxidation states. EXAFS measurements indicate that Mn(II)PFOM and Mn(III)PFOM have terminal aqua ligands and Mn(V)PFOM has a terminal hydroxo ligand. The data are more ambiguous for Mn(IV)PFOM where both terminal aqua and hydroxo ligands can be rationalized, but the reactivity observed more likely supports a formulation of Mn(IV)PFOM as having a terminal hydroxo ligand. Reactivity studies in water showed unexpectedly that both Mn(IV)-OH-PFOM and Mn(V)-OH-PFOM are very poor oxygen-atom donors; however, both are highly reactive in electron transfer oxidations such as the oxidation of 3-mercaptopropionic acid to the corresponding disulfide. The Mn(IV)-OH-PFOM compound reacted in water to form O2, while Mn(V)-OH-PFOM was surprisingly indefinitely stable. It was observed that addition of alkali cations (K(+), Rb(+), and Cs(+)) led to the aggregation of Mn(IV)-OH-PFOM as analyzed by electron microscopy and DOSY NMR, while addition of Li(+) and Na(+) did not lead to aggregates. Aggregation leads to a lowering of the entropic barrier of the reaction without changing the free energy barrier. The observation that O2 formation is fastest in the presence of Cs(+) and ∼fourth order in Mn(IV)-OH-PFOM supports a notion of a tetramolecular Mn(IV)-hydroxo intermediate that is viable for O2 formation in an oxide-based chemical environment. A bimolecular reaction mechanism involving a Mn(IV)-hydroxo based intermediate appears to be slower for O2 formation.

Magnetic circular dichroism and computational study of mononuclear and dinuclear iron(IV) complexes

High-valent iron(IV)-oxo species are key intermediates in the catalytic cycles of a range of O2-activating iron enzymes. This work presents a detailed study of the electronic structures of mononuclear ([FeIV(O)(L)(NCMe)]2+, 1, L = tris(3,5-dimethyl-4-methoxylpyridyl-2-methyl)amine) and dinuclear ([(L)FeIV(O)(μ-O)FeIV(OH)(L)]3+, 2) iron(IV) complexes using absorption (ABS), magnetic circular dichroism (MCD) spectroscopy and wave-function-based quantum chemical calculations. For complex 1, the experimental MCD spectra at 2-10 K are dominated by a broad positive C-term band between 12000 and 18000 cm-1. As the temperature increases up to ~20 K, this feature is gradually replaced by a derivative-shaped signal. The computed MCD spectra are in excellent agreement with experiment, which reproduce not only the excitation energies and the MCD signs of key transitions but also their temperature-dependent intensity variations. To further corroborate the assignments suggested by the calculations, the individual MCD sign for each transition is independently determined from the corresponding electron donating and accepting orbitals. Thus, unambiguous assignments can be made for the observed transitions in 1. The ABS/MCD data of complex 2 exhibit ten features that are assigned as ligand-field transitions or oxo- or hydroxo-to-metal charge transfer bands, based on MCD/ABS intensity ratios, calculated excitation energies, polarizations, and MCD signs. In comparison with complex 1, the electronic structure of the FeIV=O site is not significantly perturbed by the binding to another iron(IV) center. This may explain the experimental finding that complexes 1 and 2 have similar reactivities toward C-H bond activation and O-atom transfer.

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_x²–y² and 3d_xy 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 S = 1 Mn^V=O adduct; these findings are discussed within the context of previous examinations of oxomanganese(V) complexes.

Experimental and theoretical studies were performed to correlate the Mn pre-K-edge data for Mn^IV=O and Mn^IV−OH adducts with geometric and electronic structure. Mn 3d-4p mixing, which primarily accounts for the pre-edge intensities, is not solely a function of the Mn−O(H) bond length but is modulated by the coordination geometry. Thus, depending on the specifics of the ligand field, Mn^IV−OH, Mn^IV=O, and even Mn^V=O species can show pre-edge peaks of comparable area and height.