The mononuclear Mn(IV)-oxo complex [MnIV(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 [MnIV(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 [MnIV(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 [MnIV(O)(N4py)]2+ as the 4E excited state predicted to be involved in hydrogen-atom-transfer reactivity. The CASSCF calculations predict enhanced MnIII-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.

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 MnIV species with well-defined spectroscopic properties, which provides an opportunity to identify mononuclear MnIV products from O–O bond activation of the corresponding MnIII–peroxo adduct. The peroxomanganese(III) intermediate, [MnIII(O2)(Me2EBC)]+, was prepared at low-temperature by the addition of KO2 to [MnII(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 [MnIII(O2)(Me2EBC)]+ intermediate was examined by density functional theory (DFT) and time-dependent (TD) DFT calculations. Detailed spectroscopic investigations of the decay products of [MnIII(O2)(Me2EBC)]+ revealed the presence of mononuclear MnIII–hydroxo species or a mixture of mononuclear MnIV and MnIII–hydroxo species. The nature of the observed decay products depended on the amount of KO2 used to generate [MnIII(O2)(Me2EBC)]+. The MnIII–hydroxo product was characterized by Mn K-edge XAS, and shifts in the pre-edge transition energies and intensities relative to [MnIII(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 MnIV center upon decay of a nonporphyrinoid MnIII–peroxo center.

A mononuclear hydroxomanganese(III) complex was synthesized utilizing the N5 amide-containing ligand 2-[bis(pyridin-2-ylmethyl)]amino-N-2-methyl-quinolin-8-yl-acetamidate (dpaq2Me ). This complex is similar to previously reported [MnIII(OH)(dpaqH)]+ [Inorg. Chem. 2014, 53, 7622–7634] but contains a methyl group adjacent to the hydroxo moiety. This α-methylquinoline group in [MnIII(OH)(dpaq2Me)]+ gives rise to a 0.1 Å elongation in the Mn–N(quinoline) distance relative to [MnIII(OH)(dpaqH)]+. Similar bond elongation is observed in the corresponding Mn(II) complex. In MeCN, [MnIII(OH)(dpaq2Me)]+ reacts rapidly with 2,2′,6,6′-tetramethylpiperidine-1-ol (TEMPOH) at −35 °C by a concerted proton–electron transfer (CPET) mechanism (second-order rate constant k2 of 3.9(3) M–1 s–1). Using enthalpies and entropies of activation from variable-temperature studies of TEMPOH oxidation by [MnIII(OH)(dpaq2Me)]+ (ΔH‡ = 5.7(3) kcal–1 M–1; ΔS‡ = −41(1) cal M–1 K–1), it was determined that [MnIII(OH)(dpaq2Me)]+ oxidizes TEMPOH ∼240 times faster than [MnIII(OH)(dpaqH)]+. The [MnIII(OH)(dpaq2Me)]+ complex is also capable of oxidizing the stronger O–H and C–H bonds of 2,4,6-tri-tert-butylphenol and xanthene, respectively. However, for these reactions [MnIII(OH)(dpaq2Me)]+ displays, at best, modest rate enhancement relative to [MnIII(OH)(dpaqH)]+. A combination of density function theory (DFT) and cyclic voltammetry studies establish an increase in the MnIII/MnII reduction potential of [MnIII(OH)(dpaq2Me)]+ relative to [MnIII(OH)(dpaqH)]+, which gives rise to a larger driving force for CPET for the former complex. Thus, more favorable thermodynamics for [MnIII(OH)(dpaq2Me)]+ can account for the dramatic increase in rate with TEMPOH. For the more sterically encumbered substrates, DFT computations suggest that this effect is mitigated by unfavorable steric interactions between the substrate and the α-methylquinoline group of the dpaq2Me ligand. The DFT calculations, which reproduce the experimental activation free energies quite well, provide the first examination of the transition-state structure of mononuclear MnIII(OH) species during a CPET reaction.

X-band electron paramagnetic resonance (EPR) spectroscopy was used to probe the ground-state electronic structures of mononuclear MnIV complexes [MnIV(OH)2(Me2EBC)]2+ and [MnIV(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 MnIV-hydroxo versus MnIV-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 MnIV-oxo complex [MnIV(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 MnIV-hydroxo analogue [MnIV(OH)2(Me2EBC)]2+ (|D| = 0.75(0.25) cm–1; E/D = 0.15(2)), although the complexes have similar 55Mn values (a = 7.7 and 7.5 mT, respectively). The ZFS parameters for [MnIV(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 MnIV 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 MnIV 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 MnIV ground-state properties and provides an initial assessment for calculating MnIV ZFS parameters with quantum chemical methods.

A facile and high-yielding protocol to the known Ti(II) complex trans-[(py)4TiCl2] (py = pyridine) has been developed. Its electronic structure has been probed experimentally using magnetic susceptibility, magnetic circular dichroism, and high-frequency and high-field electron paramagnetic resonance spectroscopies in conjunction with ligand-field theory and computational methods (density functional theory and ab initio methods). These studies demonstrated that trans-[(py)4TiCl2] has a 3Eg ground state (dxy1dxz,yz1 orbital occupancy), which, as a result of spin–orbit coupling, yields a ground-state spinor doublet that is EPR active, a first excited-state doublet at ∼60 cm–1, and two next excited states at ∼120 cm–1. Reactivity studies with various unsaturated substrates are also presented in this study, which show that the Ti(II) center allows oxidative addition likely via formation of [Ti(η2-R2E2)Cl2(py)n] E = C, N intermediates. A new Ti(IV) compound, mer-[(py)3(η2-Ph2C2)TiCl2], was prepared by reaction with Ph2C2, along with the previously reported complex trans-(py)3Ti═NPh(Cl)2, from reaction with Ph2N2. Reaction with Ph2CN2 also yielded a new dinuclear Ti(IV) complex, [(py)2(Cl)2Ti(μ2:η2-N2CPh2)2Ti(Cl)2], in which the two Ti(IV) ions are inequivalently coordinated. Reaction with cyclooctatetraene (COT) yielded a new Ti(III) complex, [(py)2Ti(η8-COT)Cl], which is a rare example of a mononuclear “piano-stool” titanium complex. The complex trans-[(py)4TiCl2] has thus been shown to be synthetically accessible, have an interesting electronic structure, and be reactive toward oxidation chemistry.

Manganese-containing, mid-valent oxidants (MnIII–OR) that mediate proton-coupled electron-transfer (PCET) reactions are central to a variety of crucial enzymatic processes. The Mn-dependent enzyme lipoxygenase is such an example, where a MnIII–OH unit activates fatty acid substrates for peroxidation by an initial PCET. This present work describes the quantitative generation of the MnIII–OMe complex, [MnIII(OMe)(dpaq)]+ (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) via dioxygen activation by [MnII(dpaq)]+ in methanol at 25 °C. The X-ray diffraction structure of [MnIII(OMe)(dpaq)]+ exhibits a Mn–OMe group, with a Mn–O distance of 1.825(4) Å, that is trans to the amide functionality of the dpaq ligand. The [MnIII(OMe)(dpaq)]+ complex is quite stable in solution, with a half-life of 26 days in MeCN at 25 °C. [MnIII(OMe)(dpaq)]+ can activate phenolic O–H bonds with bond dissociation free energies (BDFEs) of less than 79 kcal mol−1 and reacts with the weak O–H bond of TEMPOH (TEMPOH = 2,2′-6,6′-tetramethylpiperidine-1-ol) with a hydrogen/deuterium kinetic isotope effect (H/D KIE) of 1.8 in MeCN at 25 °C. This isotope effect, together with other experimental evidence, is suggestive of a concerted proton-electron transfer (CPET) mechanism for O–H bond oxidation by [MnIII(OMe)(dpaq)]+. A kinetic and thermodynamic comparison of the O–H bond oxidation reactivity of [MnIII(OMe)(dpaq)]+ to other MIII–OR oxidants is presented as an aid to gain more insight into the PCET reactivity of mid-valent oxidants. In contrast to high-valent counterparts, the limited examples of MIII–OR oxidants exhibit smaller H/D KIEs and show weaker dependence of their oxidation rates on the driving force of the PCET reaction with O–H bonds.

M06-L-based quantum chemical calculations were performed to examine two key elementary steps in rhodium (Rh)-xantphos-catalyzed hydroformylation: carbonyl ligand (CO) dissociation and the olefin insertion into the Rh–H bond. For the resting state of the Rh-xantphos catalyst, HRh(xantphos)(CO)2, our M06-L calculations were able to qualitatively reproduce the correct ordering of the equatorial–equatorial (ee) and equatorial–axial (ea) conformers of the phosphorus ligands for 16 derivatives of the xantphos ligand, implying that the method is sufficiently accurate for capturing the subtle energy differences associated with various conformers involved in Rh-catalyzed hydroformylation. The calculated CO dissociation energy from the ea conformer (ΔE = 21–25 kcal/mol) was 10–12 kcal/mol lower than that from the ee conformer (ΔE = 31–34 kcal/mol), which is consistent with prior experimental and theoretical studies. The calculated regioselectivities for propene insertion into the Rh–H bond of the ee-HRh(xantphos)(propene)(CO) complexes were in good agreement with the experimental l:b ratios. The comparative analysis of the regioselectivities for the pathways originating from the ee-HRh(xantphos)(propene)(CO) complexes with and without diphenyl substituents yielded useful mechanistic insight into the interactions that play a key role in regioselectivity. Complementary computations featuring xantphos ligands lacking diphenyl substituents implied that the long-range noncovalent ligand–ligand and ligand–substrate interactions, but not the bite angles per se, control the regioselectivity of Rh-diphosphine-catalyzed hydroformylation of simple terminal olefins for the ee isomer. Additional calculations with longer chain olefins and the simplified structural models, in which the phenyl rings of the xantphos ligands were selectively removed to eliminate either substrate–ligand or ligand–ligand noncovalent interactions, suggested that ligand–substrate π-HC interactions play a more dominant role in the regioselectivity of Rh-catalyzed hydroformylation than ligand–ligand π–π interactions. The present calculations may provide foundational knowledge for the rational design of ligands aimed at optimizing hydroformylation regioselectivity.

The mononuclear hydroxomanganese(III) complex, [MnIII(OH)(dpaq)]+, which is supported by the amide-containing N5 ligand dpaq (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) was generated by treatment of the manganese(II) species, [MnII(dpaq)](OTf), with dioxygen in acetonitrile solution at 25 °C. This oxygenation reaction proceeds with essentially quantitative yield (greater than 98% isolated yield) and represents a rare example of an O2-mediated oxidation of a manganese(II) complex to generate a single product. The X-ray diffraction structure of [MnIII(OH)(dpaq)]+ reveals a short Mn–OH distance of 1.806(13) Å, with the hydroxo moiety trans to the amide function of the dpaq ligand. No shielding of the hydroxo group is observed in the solid-state structure. Nonetheless, [MnIII(OH)(dpaq)]+ is remarkably stable, decreasing in concentration by only 10% when stored in MeCN at 25 °C for 1 week. The [MnIII(OH)(dpaq)]+ complex participates in proton-coupled electron transfer reactions with substrates with relatively weak O–H and C–H bonds. For example, [MnIII(OH)(dpaq)]+ oxidizes TEMPOH (TEMPOH = 2,2′-6,6′-tetramethylpiperidine-1-ol), which has a bond dissociation free energy (BDFE) of 66.5 kcal/mol, in MeCN at 25 °C. The hydrogen/deuterium kinetic isotope effect of 1.8 observed for this reaction implies a concerted proton–electron transfer pathway. The [MnIII(OH)(dpaq)]+ complex also oxidizes xanthene (C–H BDFE of 73.3 kcal/mol in dimethylsulfoxide) and phenols, such as 2,4,6-tri-t-butylphenol, with BDFEs of less than 79 kcal/mol. Saturation kinetics were observed for phenol oxidation, implying an initial equilibrium prior to the rate-determining step. On the basis of a collective body of evidence, the equilibrium step is attributed to the formation of a hydrogen-bonding complex between [MnIII(OH)(dpaq)]+ and the phenol substrates.

Mn K-edge X-ray absorption spectroscopy (XAS) was used to gain insights into the geometric and electronic structures of [MnII(Cl)2(Me2EBC)], [MnIV(OH)2(Me2EBC)]2+, and [MnIV(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 [MnIV(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 [MnII(Cl)2(Me2EBC)] and [MnIV(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 MnIV═O and MnIV–OH complexes, but this trend was strongly modulated by the MnIV 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 MnIV═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 3dx2–y2 and 3dxy MOs because of enhanced 3d-4px,y mixing. This gives rise to a broader pre-edge feature for trigonal MnIV═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 MnV═O adduct; these findings are discussed within the context of previous examinations of oxomanganese(V) complexes.

A monomeric MnII complex has been prepared with the facially-coordinating TpPh2 ligand, (TpPh2 = hydrotris(3,5-diphenylpyrazol-1-yl)borate). The X-ray crystal structure shows three coordinating solvent molecules resulting in a six-coordinate complex with Mn–ligand bond lengths that are consistent with a high-spin MnII ion. Treatment of this MnII complex with excess KO2 at room temperature resulted in the formation of a MnIII–O2 complex that is stable for several days at ambient conditions, allowing for the determination of the X-ray crystal structure of this intermediate. The electronic structure of this peroxomanganese(III) adduct was examined by using electronic absorption, electron paramagnetic resonance (EPR), low-temperature magnetic circular dichroism (MCD), and variable-temperature variable-field (VTVH) MCD spectroscopies. Density functional theory (DFT), time-dependent (TD)-DFT, and multireference ab initio CASSCF/NEVPT2 calculations were used to assign the electronic transitions and further investigate the electronic structure of the peroxomanganese(III) species. The lowest ligand-field transition in the electronic absorption spectrum of the MnIII–O2 complex exhibits a blue shift in energy compared to other previously characterized peroxomanganese(III) complexes that results from a large axial bond elongation, reducing the metal–ligand covalency and stabilizing the σ-antibonding Mn dz2 MO that is the donor MO for this transition.

Density functional theory calculations have been performed to gain insight into the origin of ligand effects in rhodium (Rh)-catalyzed hydroformylation of olefins. In particular, the olefin insertion step of the Wilkinson catalytic cycle, which is commonly invoked as the regioselectivity-determining step, has been examined by considering a large variety of density functionals (e.g., B3LYP, M06-L); a range of substrates, including simple terminal (e.g., hexene, octene), heteroatom-containing (e.g., vinyl acetate), and aromatic-substituted (e.g., styrene) alkenes, and different ligand structures (e.g., monodentate PPh3 ligands and bidentate ligands such as DIOP, DIPHOS). The calculations indicate that the M06-L functional reproduces the experimental regioselectivities with a reasonable degree of accuracy, while the commonly employed B3LYP functional fails to do so when the equatorial–equatorial arrangement of phosphine ligands around the Rh center is considered. The different behavior of the two functionals is attributed to the fact that the transition states leading to the Rh–alkyl intermediates along the pathways to isomeric aldehydes are stabilized by the medium-range correlation containing π–π (ligand–ligand) and π–CH (ligand–substrate) interactions that cannot be handled properly by the B3LYP functional due to its inability to describe nonlocal interactions. This conclusion is further validated using the B3LYP functional with Grimme’s empirical dispersion correction term: i.e., B3LYP-D3. The calculations also suggest that transition states leading to the linear Rh–alkyl intermediates are selectively stabilized by these noncovalent interactions, which gives rise to the high regioselectivities. In the cases of heteroatom- or aromatic-substituted olefins, substrate electronic effects determine the regioselectivity; however, these calculations suggest that the π–π and π–CH interactions also make an appreciable contribution. Overall, these computations show that the steric crowding-induced ligand–ligand and ligand–substrate interactions, but not intraligand interactions, influence the regioselectivity in Rh-catalyzed hydroformylation when the phosphine ligands are present in an equatorial–equatorial configuration in the Rh catalyst.

Over the past 7 years, there have been a significant number of studies describing the structural and electronic properties, as well as the chemical reactivity, of synthetic peroxomanganese adducts. Many redox-active manganese enzymes, including manganese-containing superoxide dismutases, extradiol catechol dioxygenases, and ribonucleotide reductases, are proposed to feature peroxomanganese intermediates in their catalytic cycles. The recent efforts to model these intermediates using synthetic complexes have thus provided a strong complement to mechanistic studies of the enzymes. This review provides both a summary and a perspective of work in this area, with an emphasis on the relationship between geometric and electronic structure and chemical reactivity for η2-peroxomanganese(III) and η1-alkylperoxomanganese(III) adducts.

A non-porphyrinic, mononuclear oxomanganese(IV) complex was generated at room temperature and characterized by spectroscopic methods. The MnIVO adduct is capable of activating C–H bonds by a H-atom transfer mechanism and is more reactive in this regard than most MnIVO species.

A novel and efficient method for preparing [MnIII(O2)(L)]+ complexes using electrochemically generated superoxide is reported, with the reaction probed by low temperature electronic absorption and electron paramagnetic resonance spectroscopic techniques.

The unique metal abstracting peptide asparagine-cysteine-cysteine (NCC) binds nickel in a square planar 2N:2S geometry and acts as a mimic of the enzyme nickel superoxide dismutase (Ni-SOD). The Ni-NCC tripeptide complex undergoes rapid, site-specific chiral inversion to dld-NCC in the presence of oxygen. Superoxide scavenging activity increases proportionally with the degree of chiral inversion. Characterization of the NCC sequence within longer peptides with absorption, circular dichroism (CD), and magnetic CD (MCD) spectroscopies and mass spectrometry (MS) shows that the geometry of metal coordination is maintained, though the electronic properties of the complex are varied to a small extent because of bis-amide, rather than amine/amide, coordination. In addition, both Ni–tripeptide and Ni–pentapeptide complexes have charges of −2. This study demonstrates that the chiral inversion chemistry does not occur when NCC is embedded in a longer polypeptide sequence. Nonetheless, the superoxide scavenging reactivity of the embedded Ni-NCC module is similar to that of the chirally inverted tripeptide complex, which is consistent with a minor change in the reduction potential for the Ni–pentapeptide complex. Together, this suggests that the charge of the complex could affect the SOD activity as much as a change in the primary coordination sphere. In Ni-NCC and other Ni-SOD mimics, changes in chirality, superoxide scavenging activity, and oxidation of the peptide itself all depend on the presence of dioxygen or its reduced derivatives (e.g., superoxide), and the extent to which each of these distinct reactions occurs is ruled by electronic and steric effects that emenate from the organization of ligands around the metal center.

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.

Synthetically generated metallopeptides have the potential to serve a variety of roles in biotechnology applications, but the use of such systems is often hampered by the inability to control secondary reactions. We have previously reported that the NiII complex of the tripeptide lll-asparagine-cysteine-cysteine, lll-NiII-NCC, undergoes metal-facilitated chiral inversion to dld-NiII-NCC, which increases the observed superoxide scavenging activity. However, the mechanism for this process remained unexplored. Electronic absorption and circular dichroism studies of the chiral inversion reaction of NiII-NCC reveal a unique dependence on dioxygen. Specifically, in the absence of dioxygen, the chiral inversion is not observed, even at elevated pH, whereas the addition of O2 initiates this reactivity and concomitantly generates superoxide. Scavenging experiments using acetaldehyde are indicative of the formation of carbanion intermediates, demonstrating that inversion takes place by deprotonation of the alpha carbons of Asn1 and Cys3. Together, these data are consistent with the chiral inversion being dependent on the formation of a NiIII-NCC intermediate from NiII-NCC and O2. The data further suggest that the anionic thiolate and amide ligands in NiII-NCC inhibit Cα–H deprotonation for the NiII oxidation state, leading to a stable complex in the absence of O2. Together, these results offer insights into the factors controlling reactivity in synthetic metallopeptides.

The magnetic and electronic properties of the long-known organometallic complex vanadocene (VCp2), which has an S = 3/2 ground state, were investigated using conventional (X-band) electron paramagnetic resonance (EPR) and high-frequency and -field EPR (HFEPR), electronic absorption, and variable-temperature magnetic circular dichroism (VT-MCD) spectroscopies. Frozen toluene solution X-band EPR spectra were well resolved, yielding the 51V hyperfine coupling constants, while HFEPR were also of outstanding quality and allowed ready determination of the rigorously axial zero-field splitting of the spin quartet ground state of VCp2: D = +2.836(2) cm–1, g⊥ = 1.991(2), g∥ = 2.001(2). Electronic absorption and VT-MCD studies on VCp2 support earlier assignments that the absorption signals at 17 000, 19 860, and 24 580 cm–1 are due to ligand-field transitions from the 4A2g ground state to the 4E1g, 4E2g, and 4E1g excited states, using symmetry labels from the D5d point group (i.e., staggered VCp2). Contributions to the D parameter in VCp2 and further insights into electronic structure were obtained from both density functional theory (DFT) and multireference SORCI computations using X-ray diffraction structures and DFT-energy-minimized structures of VCp2. Accurate D values for all models considered were obtained from DFT calculations (D = 2.85–2.96 cm–1), which was initially surprising, because the orbitally degenerate excited states of VCp2 cannot be properly treated by DFT methods, as they require a multideterminant description. Therefore, D values were also computed using the SORCI (spectroscopically oriented configuration interaction) method, which provides multireference descriptions of ground and excited states. SORCI calculations gave accurate D values (2.86–2.90 cm–1), where the dominant (∼80%) contribution to D arises from spin–orbit coupling between ligand-field states, with the largest contribution from a low-lying 2A1g state. In contrast, the D value obtained by the DFT method is achieved only fortuitously, through cancellation of errors. Furthermore, the SORCI calculations predict ligand-field excited-state energies within 1300 cm–1 of the experimental values, whereas the corresponding time-dependent DFT calculations fail to reproduce the proper ordering of excited states. Moreover, classical ligand-field theory was validated and expanded in the present study. Thus older theory still has a place in the analysis of paramagnetic organometallic complexes, along with the latest ab initio methods.

Three peroxomanganese(III) complexes [MnIII(O2)(mL52)]+, [MnIII(O2)(imL52)]+, and [MnIII(O2)(N4py)]+ supported by pentadentate ligands (mL52 = N-methyl-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine, imL52 = N-methyl-N,N′,N′-tris((1-methyl-4-imidazolyl)methyl)ethane-1,2-diamine, and N4py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) were generated by treating Mn(II) precursors with H2O2 or KO2. Electronic absorption, magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD data demonstrate that these complexes have very similar electronic transition energies and ground-state zero-field splitting parameters, indicative of nearly identical coordination geometries. Because of uncertainty in peroxo (side-on η2 versus end-on η1) and ligand (pentadentate versus tetradentate) binding modes, density functional theory (DFT) computations were used to distinguish between three possible structures: pentadentate ligand binding with (i) a side-on peroxo and (ii) an end-on peroxo, and (iii) tetradentate ligand binding with a side-on peroxo. Regardless of the supporting ligand, isomers with a side-on peroxo and the supporting ligand bound in a tetradentate fashion were identified as most stable by >20 kcal/mol. Spectroscopic parameters computed by time-dependent (TD) DFT and multireference SORCI methods provided validation of these isomers on the basis of experimental data. Hexacoordination is thus strongly preferred for peroxomanganese(III) adducts, and dissociation of a pyridine (mL52 and N4py) or imidazole (imL52) arm is thermodynamically favored. In contrast, DFT computations for models of [FeIII(O2)(mL52)]+ demonstrate that pyridine dissociation is not favorable; instead a seven-coordinate ferric center is preferred. These different results are attributed to the electronic configurations of the metal centers (high spin d5 and d4 for FeIII and MnIII, respectively), which results in population of a metal-peroxo σ-antibonding molecular orbital and, consequently, longer M–Operoxo bonds for peroxoiron(III) species.

The metal abstraction peptide (MAP) tag is a tripeptide sequence capable of abstracting a metal ion from a chelator and binding it with extremely high affinity at neutral pH. Initial studies on the nickel-bound form of the complex demonstrate that the tripeptide asparagine-cysteine-cysteine (NCC) binds metal with 2N:2S, square planar geometry and behaves as both a structural and functional mimic of Ni superoxide dismutase (Ni-SOD). Electronic absorption, circular dichroism (CD), and magnetic CD (MCD) data collected for Ni-NCC are consistent with a diamagnetic NiII center. It is apparent from the CD signal of Ni-NCC that the optical activity of the complex changes over time. Mass spectrometry data show that the mass of the complex is unchanged. Combined with the CD data, this suggests that chiral rearrangement of the complex occurs. Following incubation of the nickel-containing peptide in D2O and back-exchange into H2O, incorporation of deuterium into non-exchangeable positions is observed, indicating chiral inversion occurs at two of the α carbon atoms in the peptide. Control peptides were used to further characterize the chirality of the final nickel-peptide complex, and density functional theory (DFT) calculations were performed to validate the hypothesized position of the chiral inversions. In total, these data indicate Ni-SOD activity is increased proportionally to the degree of structural change in the complex over time. Specifically, the relationship between the change in CD signal and change in SOD activity is linear.

Peroxomanganese(III) adducts have been postulated as important intermediates in manganese-containing enzymes and small molecule oxidation catalysts. Synthetic peroxomanganese(III) complexes are known to be nucleophilic and facilitate aldehyde deformylation, offering a convenient way to compare relative reactivities of complexes supported by different ligands. In this work, tetradentate dipyridyldiazacycloalkane ligands with systematically perturbed steric and electronic properties were used to generate a series of manganese(II) and peroxomanganese(III) complexes. X-Ray crystal structures of five manganese(II) complexes all show the ligands bound to give trans complexes. Treatment of these MnII precursors with H2O2 and Et3N in MeCN at −40 °C results in the formation of peroxomanganese(III) complexes that differ only in the identity of the pyridine ring substituent and/or the number of carbons in the diazacycloalkane backbone. To determine the effects of small ligand perturbations on the reactivity of the peroxo group, the more thermally stable peroxomanganese(III) complexes were reacted with cyclohexanecarboxaldehyde. For these complexes, the rate of deformylation does not correlate with the expected nucleophilicity of the peroxomanganese(III) unit, as the inclusion of methyl substituents on the pyridines affords slower deformylation rates. It is proposed that adding methyl-substituents to the pyridines, or increasing the number of carbons on the diazacycloalkane, sterically hinders nucleophilic attack of the peroxo ligand on the carbonyl carbon of the aldehyde.

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 5 (R = Br) and 6 positions (R = Me and MeO), are reported. X-ray crystal structures of these complexes all show the L7py2R ligands bound to give a trans complex. Treatment of these MnII precursors with either H2O2/Et3N or KO2 in MeCN at −40 °C results in the formation of peroxomanganese complexes [MnIII(O2)(L7py2R)]+ differing only in the identity of the pyridine ring substituent. The electronic structures of two of these complexes, [MnIII(O2)(L7py2H)]+ and [MnIII(O2)(L7py2Me)]+, were examined in detail using electronic absorption, low-temperature magnetic circular dichroism (MCD) and variable-temperature variable-field (VTVH) MCD spectroscopies to determine ground-state zero-field splitting (ZFS) parameters and electronic transition energies, intensities, and polarizations. DFT and TD-DFT computations were used to validate the structures of [MnIII(O2)(L7py2H)]+ and [MnIII(O2)(L7py2Me)]+, further corroborating their assignment as peroxomanganese(III) species. While these complexes exhibit similar ZFS parameters, their low-temperature MCD spectra reveal significant shifts in electronic transition energies that are correlated to differences in Mn−O2 interactions among these complexes. Taken together, these results indicate that, while the [MnIII(O2)(L7py2H)]+ complex exhibits symmetric Mn−Operoxo bond lengths, consistent with a side-on bound peroxo ligand, the peroxo ligand of the [MnIII(O2)(L7py2Me)]+ complex is bound in a more end-on fashion, with asymmetric Mn−Operoxo distances. This difference in binding mode is rationalized in terms of the greater electron-donating abilities of the methyl-appended pyridines and suggests a simple way to modulate MnIII−O2 bonding through ligand perturbations.

The electronic structures of the bis(hydroxo)manganese(IV) and oxohydroxomanganese(IV) complexes [MnIV(OH)2(Me2EBC)]2+ and [MnIV(O)(OH)(Me2EBC)]+ were probed using electronic absorption, magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD spectroscopies. The d−d transitions of [MnIV(OH)2(Me2EBC)]2+ were assigned using a group theory analysis coupled with the results of time-dependent density functional theory computations. These assignments permit the development of an experimentally validated description for the π and σ interactions in this complex. A similar analysis performed for [MnIV(O)(OH)(Me2EBC)]+ reveals that there is a significant increase in the ligand character in the Mn π* orbitals for the MnIV═O complex relative to the bis(hydroxo)manganese(IV) complex, whereas the compositions of the Mn σ* orbitals are less affected. Because of the steric features of the Me2EBC ligand, we propose that H-atom transfer by these reagents proceeds via the σ* orbitals, which, because of their similar compositions among these two compounds, leads to modest rate enhancements for the MnIV═O versus MnIVOH species.

A non-porphyrinic, mononuclear oxomanganese(IV) complex was generated at room temperature and characterized by spectroscopic methods. The MnIVO adduct is capable of activating C–H bonds by a H-atom transfer mechanism and is more reactive in this regard than most MnIVO species.

A novel and efficient method for preparing [MnIII(O2)(L)]+ complexes using electrochemically generated superoxide is reported, with the reaction probed by low temperature electronic absorption and electron paramagnetic resonance spectroscopic techniques.

Manganese-containing, mid-valent oxidants (MnIII–OR) that mediate proton-coupled electron-transfer (PCET) reactions are central to a variety of crucial enzymatic processes. The Mn-dependent enzyme lipoxygenase is such an example, where a MnIII–OH unit activates fatty acid substrates for peroxidation by an initial PCET. This present work describes the quantitative generation of the MnIII–OMe complex, [MnIII(OMe)(dpaq)]+ (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) via dioxygen activation by [MnII(dpaq)]+ in methanol at 25 °C. The X-ray diffraction structure of [MnIII(OMe)(dpaq)]+ exhibits a Mn–OMe group, with a Mn–O distance of 1.825(4) Å, that is trans to the amide functionality of the dpaq ligand. The [MnIII(OMe)(dpaq)]+ complex is quite stable in solution, with a half-life of 26 days in MeCN at 25 °C. [MnIII(OMe)(dpaq)]+ can activate phenolic O–H bonds with bond dissociation free energies (BDFEs) of less than 79 kcal mol−1 and reacts with the weak O–H bond of TEMPOH (TEMPOH = 2,2′-6,6′-tetramethylpiperidine-1-ol) with a hydrogen/deuterium kinetic isotope effect (H/D KIE) of 1.8 in MeCN at 25 °C. This isotope effect, together with other experimental evidence, is suggestive of a concerted proton-electron transfer (CPET) mechanism for O–H bond oxidation by [MnIII(OMe)(dpaq)]+. A kinetic and thermodynamic comparison of the O–H bond oxidation reactivity of [MnIII(OMe)(dpaq)]+ to other MIII–OR oxidants is presented as an aid to gain more insight into the PCET reactivity of mid-valent oxidants. In contrast to high-valent counterparts, the limited examples of MIII–OR oxidants exhibit smaller H/D KIEs and show weaker dependence of their oxidation rates on the driving force of the PCET reaction with O–H bonds.

A monomeric MnII complex has been prepared with the facially-coordinating TpPh2 ligand, (TpPh2 = hydrotris(3,5-diphenylpyrazol-1-yl)borate). The X-ray crystal structure shows three coordinating solvent molecules resulting in a six-coordinate complex with Mn–ligand bond lengths that are consistent with a high-spin MnII ion. Treatment of this MnII complex with excess KO2 at room temperature resulted in the formation of a MnIII–O2 complex that is stable for several days at ambient conditions, allowing for the determination of the X-ray crystal structure of this intermediate. The electronic structure of this peroxomanganese(III) adduct was examined by using electronic absorption, electron paramagnetic resonance (EPR), low-temperature magnetic circular dichroism (MCD), and variable-temperature variable-field (VTVH) MCD spectroscopies. Density functional theory (DFT), time-dependent (TD)-DFT, and multireference ab initio CASSCF/NEVPT2 calculations were used to assign the electronic transitions and further investigate the electronic structure of the peroxomanganese(III) species. The lowest ligand-field transition in the electronic absorption spectrum of the MnIII–O2 complex exhibits a blue shift in energy compared to other previously characterized peroxomanganese(III) complexes that results from a large axial bond elongation, reducing the metal–ligand covalency and stabilizing the σ-antibonding Mn dz2 MO that is the donor MO for this transition.

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.

Peroxomanganese(III) adducts have been postulated as important intermediates in manganese-containing enzymes and small molecule oxidation catalysts. Synthetic peroxomanganese(III) complexes are known to be nucleophilic and facilitate aldehyde deformylation, offering a convenient way to compare relative reactivities of complexes supported by different ligands. In this work, tetradentate dipyridyldiazacycloalkane ligands with systematically perturbed steric and electronic properties were used to generate a series of manganese(II) and peroxomanganese(III) complexes. X-Ray crystal structures of five manganese(II) complexes all show the ligands bound to give trans complexes. Treatment of these MnII precursors with H2O2 and Et3N in MeCN at −40 °C results in the formation of peroxomanganese(III) complexes that differ only in the identity of the pyridine ring substituent and/or the number of carbons in the diazacycloalkane backbone. To determine the effects of small ligand perturbations on the reactivity of the peroxo group, the more thermally stable peroxomanganese(III) complexes were reacted with cyclohexanecarboxaldehyde. For these complexes, the rate of deformylation does not correlate with the expected nucleophilicity of the peroxomanganese(III) unit, as the inclusion of methyl substituents on the pyridines affords slower deformylation rates. It is proposed that adding methyl-substituents to the pyridines, or increasing the number of carbons on the diazacycloalkane, sterically hinders nucleophilic attack of the peroxo ligand on the carbonyl carbon of the aldehyde.