Characterization of Monomeric Mn^II/III/IV–Hydroxo Complexes from X- and Q‑Band Dual Mode Electron Paramagnetic Resonance (EPR) Spectroscopy

Manganese–hydroxo species have been implicated in C–H bond activation performed by metalloenzymes, but the electronic properties of many of these intermediates are not well characterized. The present work presents a detailed characterization of three Mn^<i>n</i>–OH complexes (where <i>n</i> = II, III, and IV) of the tris­[(<i>N</i>′-<i>tert</i>-butylureaylato)-<i>N</i>-ethylene]­aminato ([H₃buea]^3–) ligand using X- and Q-band dual mode electron paramagnetic resonance (EPR). Quantitative simulations for the [Mn^IIH₃buea­(OH)]^2– complex demonstrated the ability to characterize similar Mn^II species commonly present in the resting states of manganese-containing enzymes. The spin states of the Mn^III and Mn^IV complexes determined from EPR spectroscopy are <i>S</i> = 2 and 3/2, respectively, as expected for the <i>C</i>₃ symmetry imposed by the [H₃buea]^3– ligand. Simulations of the spectra indicated the constant presence of two Mn^IV species in solutions of [Mn^IVH₃buea­(OH)] complex. The simulations of perpendicular- and parallel-mode EPR spectra allow determination of zero-field splitting and hyperfine parameters for all complexes. For the Mn^III and Mn^IV complexes, density functional theory calculations are used to determine the isotropic Mn hyperfine values, to compare the excited electronic state energies, and to give theoretical estimates of the zero-field energy

Synthesis and Reactivity of Tripodal Complexes Containing Pendant Bases

The synthesis of a new tripodal ligand family that contains tertiary amine groups in the second-coordination sphere is reported. The ligands are tris­(amido)­amine derivatives, with the pendant amines attached via a peptide coupling strategy. They were designed to function as new molecular catalysts for the oxygen reduction reaction (ORR), in which the pendant acid/base group could improve the catalyst performance. Two members of the ligand family were each metalated with cobalt­(II) and zinc­(II) to afford trigonal-monopyramidal complexes. The reaction of the cobalt complexes <b>[Co­(L)]</b>^<b>–</b> with dioxygen reversibly generates a small amount of a cobalt­(III) superoxo species, which was characterized by electron paramagnetic resonance (EPR) spectroscopy. Protonation of the zinc complex Zn­[N­{CH₂CH₂NC­(O)­CH₂N­(CH₂Ph)₂}₃)]⁻ (<b>[Zn­(TN</b>^<b>Bn</b><b>)]</b>^<b>–</b>) with 1 equiv of acid occurs at a primary-coordination-sphere amide moiety rather than at a pendant basic site. The addition of excess acid to any of the complexes <b>[M­(L)]</b>^<b>–</b> results in complete proteolysis and formation of the ligands <b>H</b>_<b>3</b><b>L</b>. These undesired reactions limit the use of these complexes as catalysts for the ORR. An alternative ligand with two pyridyl arms was also prepared but could not be metalated. These studies highlight the importance of the stability of the primary-coordination sphere of ORR electrocatalysts to both oxidative <i>and</i> acidic conditions

L‑Edge X‑ray Absorption Spectroscopy of Dilute Systems Relevant to Metalloproteins Using an X‑ray Free-Electron Laser

L-edge spectroscopy of 3d transition metals provides important electronic structure information and has been used in many fields. However, the use of this method for studying dilute aqueous systems, such as metalloenzymes, has not been prevalent because of severe radiation damage and the lack of suitable detection systems. Here we present spectra from a dilute Mn aqueous solution using a high-transmission zone-plate spectrometer at the Linac Coherent Light Source (LCLS). The spectrometer has been optimized for discriminating the Mn L-edge signal from the overwhelming O K-edge background that arises from water and protein itself, and the ultrashort LCLS X-ray pulses can outrun X-ray induced damage. We show that the deviations of the partial-fluorescence yield-detected spectra from the true absorption can be well modeled using the state-dependence of the fluorescence yield, and discuss implications for the application of our concept to biological samples