Manganese–peroxos are proposed as key intermediates in a number of important biochemical and synthetic transformations. Our understanding of the structural, spectroscopic, and reactivity properties of these metastable species is limited, however, and correlations between these properties have yet to be established experimentally. Herein we report the crystallographic structures of a series of structurally related metastable Mn(III)–OOR compounds, and examine their spectroscopic and reactivity properties. The four reported Mn(III)–OOR compounds extend the number of known end-on Mn(III)–(η1-peroxos) to six. The ligand backbone is shown to alter the metal–ligand distances and modulate the electronic properties key to bonding and activation of the peroxo. The mechanism of thermal decay of these metastable species is examined via variable-temperature kinetics. Strong correlations between structural (O–O and Mn⋯Npy,quin distances), spectroscopic (E(πv*(O–O) → Mn CT band), νO–O), and kinetic (ΔH‡ and ΔS‡) parameters for these complexes provide compelling evidence for rate-limiting O–O bond cleavage. Products identified in the final reaction mixtures of Mn(III)–OOR decay are consistent with homolytic O–O bond scission. The N-heterocyclic amines and ligand backbone (Et vs Pr) are found to modulate structural and reactivity properties, and O–O bond activation is shown, both experimentally and theoretically, to track with metal ion Lewis acidity. The peroxo O–O bond is shown to gradually become more activated as the N-heterocyclic amines move closer to the metal ion causing a decrease in π-donation from the peroxo πv*(O–O) orbital. The reported work represents one of very few examples of experimentally verified relationships between structure and function.

Helicobacter pylori, a pathogen that colonizes the human stomach, requires the nickel-containing metalloenzymes urease and NiFe-hydrogenase to survive this low pH environment. The maturation of both enzymes depends on the metallochaperone, HypA. HypA contains two metal sites, an intrinsic zinc site and a low-affinity nickel binding site. X-ray absorption spectroscopy (XAS) shows that the structure of the intrinsic zinc site of HypA is dynamic, and able to sense both nickel loading and pH changes. At pH 6.3, an internal pH that occurs during acid shock, the zinc site undergoes unprecedented ligand substitutions to convert from a Zn(Cys) 4 site to a Zn(His) 2 (Cys) 2 site. NMR spectroscopy shows that binding of Ni(II) to HypA results in paramagnetic broadening of resonances near the N-terminus. NOEs between the β-CH 2 protons of Zn cysteinyl ligands are consistent with a strand-swapped HypA dimer. Addition of nickel causes resonances from zinc binding motif and other regions to double, indicating more than one conformation can exist in solution. Although the structure of the high-spin, 5-6 coordinate Ni(II) site is relatively unaffected by pH, the nickel binding stoichiometry is decreased from one per monomer to one per dimer at pH = 6.3. Mutation of any cysteine residue in the zinc binding motif results in a zinc site structure similar to that found for holo-WT-HypA at low pH and is unperturbed by the addition of nickel. Mutation of the histidines that flank the CXXC motifs results in a zinc site structure that is similar to holo-WT-HypA at neutral pH (Zn(Cys) 4 ) and is no longer responsive to nickel binding or pH changes. Using an in vitro urease activity assay, it is shown that the recombinant protein is sufficient for recovery of urease activity in cell lysate from a HypA deletion mutant, and that mutations in the zinc-binding motif result in a decrease in recovered urease activity. The results are interpreted in terms of a model wherein HypA controls the flow of nickel traffic in the cell in response to nickel availability and pH. KeywordsHelicobacter pylori; XAS; HypA; metallochaperone; zinc; nickel; ITC; NMR mmaroney@chemistry.umass.edu. Supporting Information Available: Figures of CD spectra for zinc-site cysteine mutants of HypA, Thermal melts of WT-and zinc-site cysteine and histidine mutants, molecular weight determinations by size-exclusion chromatography, ITC thermograms for zinc-site cysteine and histidine mutants, raw ITC titration data, zinc K-edge XANES and EXAFS data and fits for Cys → Asp and His95A mutations, nickel K-edge XANES and EXAFS data and fits for Cys → Asp zinc-site mutations, and UV-vis spectra of HypA with nickel bound. Tables of mutagenic primers, best EXAFS fits to Zn K-edge data for Cys → Asp mutations, best EXAFS fits to Ni Kedge data for zinc-site Cys → Asp mutations, alternate fits for zinc and nickel K-edge EXAFS (39 pages). This information is available free of charge via the Internet at

The reaction of [(PyNMe)Fe(CFSO)], 1, with excess peracetic acid at -40 °C generates a highly reactive intermediate, 2b(PAA), that has the fastest rate to date for oxidizing cyclohexane by a nonheme iron species. It exhibits an intense 490 nm chromophore associated with an S = 1/2 EPR signal having g-values at 2.07, 2.01, and 1.94. This species was shown to be in a fast equilibrium with a second S = 1/2 species, 2a(PAA), assigned to a low-spin acylperoxoiron(III) center. Unfortunately, contaminants accompanying the 2(PAA) samples prevented determination of the iron oxidation state by Mössbauer spectroscopy. Use of MeO-PyNMe (an electron-enriched version of PyNMe) and cyclohexyl peroxycarboxylic acid as oxidant affords intermediate 3b(CPCA) with a Mössbauer isomer shift δ = -0.08 mm/s that indicates an iron(V) oxidation state. Analysis of the Mössbauer and EPR spectra, combined with DFT studies, demonstrates that the electronic ground state of 3b(CPCA) is best described as a quantum mechanical mixture of [(MeO-PyNMe)Fe(O)(OC(O)R)] (∼75%) with some Fe(O)(OC(O)R) and Fe(OOC(O)R) character. DFT studies of 3b(CPCA) reveal that the unbound oxygen of the carboxylate ligand, O2, is only 2.04 Å away from the oxo group, O1, corresponding to a Wiberg bond order for the O1-O2 bond of 0.35. This unusual geometry facilitates reversible O1-O2 bond formation and cleavage and accounts for the high reactivity of the intermediate when compared to the rates of hydrogen atom transfer and oxygen atom transfer reactions of Fe(OC(O)R) ferric acyl peroxides and Fe(O) complexes. The interaction of O2 with O1 leads to a significant downshift of the Fe-O1 Raman frequency (815 cm) relative to the 903 cm value predicted for the hypothetical [(MeO-PyNMe)Fe(O)(NCMe)] complex.

Mechanistic understanding of electro-and photocatalytic CO2 reduction is crucial to develop strategies to overcome catalytic bottlenecks. In this regard, herein it is presented for a new CO2-to-CO reduction cobalt aminopyridine catalyst, a detailed experimental and theoretical mechanistic study toward the identification of bottlenecks and potential strategies to alleviate them. The combination of electrochemistry and in-situ spectroelectrochemistry together with spectroscopic techniques led us to identify elusive key electrocatalytic intermediates derived from complex [L N4 Co(OTf)2] (1) (L N4 =1-[2-pyridylmethyl]-4,7-dimethyl-1,4,7triazacyclononane) such as a highly reactive cobalt (I) (1 (I)) and cobalt (I) carbonyl (1 (I)-CO) species. The combination of spectroelectrochemical studies under CO2, 13 CO2 and CO with DFT disclosed that 1 (I) reacts with CO2 to form the pivotal 1 (I)-CO intermediate at the 1 (II/I) redox potential. However, at this reduction potential, the formation of 1 (I)-CO restricts the electrocatalysis due to the endergonicity of the CO release step. In agreement with the experimentally observed CO2-to-CO electrocatalysis at the Co I/0 redox potential, computational studies suggested that the electrocatalytic cycle involves striking metal carbonyls. In contrast, under photochemical conditions, the catalysis smoothly proceeds at the 1 (II/I) redox potential. Under the latter conditions, it is proposed that the electron transfer to form 1 (I)-CO from 1 (II)-CO is under diffusion control. Then, the CO release from 1 (II)-CO is kinetically favored, facilitating the catalysis. Finally, we have found that visible-light irradiation has a positive impact under electrocatalytic conditions. We envision that light-irradiation can serve as an effective strategy to circumvent the CO poisoning and improve the performance of CO2 reduction molecular catalysts. 1750 1950 2050 2150 Wavenumber (cm-1) Co I-CO Co 0-CO Co II-CO Theoretical 43 cm-1 Experimental under CO 2 Experimental under CO 1910 cm-1 44 cm-1 [L N4 Co I-CO] +