We present a comparison of experimental and simulated frequency- and field-domain electron paramagnetic resonance (EPR) spectra of integer and half-integer high-spin transition-metal ion complexes. ...For the simulation of EPR spectra a new tool within the EPR simulation software EasySpin is introduced, which allows for field- and frequency-domain EPR simulations with the same theoretical model and the same set of spin Hamiltonian parameters. The utility of this approach is demonstrated on the integer-spin complexes NiBr2(PPh3)2 and Tp2MnSbF6 (both S = 1) and the half-integer-spin Fe(III) porphyrins, hemin (Fe(PPIX)Cl) and Fe(TPP)Cl (both S = 5/2). We demonstrate that the combination of field- and frequency-domain EPR techniques allows the determination of spin Hamiltonian parameters, in particular large zero-field splittings, with high accuracy.
Herein, we report a radical borylation of aromatic amines through a homolytic C(sp2)−N bond cleavage. This method capitalizes on a simple and mild activation via a pyrylium reagent (ScPyry‐OTf) thus ...priming the amino group for reactivity. The combination of terpyridine and a diboron reagent triggers a radical reaction which cleaves the C(sp2)−N bond and forges a new C(sp2)−B bond. The unique non‐planar structure of the pyridinium intermediate, provides the necessary driving force for the aryl radical formation. The method permits borylation of a wide variety of aromatic amines indistinctively of the electronic environment.
A radical borylation of aromatic amines through a homolytic C(sp2)−N bond cleavage is presented. This method capitalizes on a simple and mild activation via a pyrylium reagent (ScPyry‐OTf) that primes the amino group for reactivity. The combination of terpyridine and a diboron reagent triggers a radical reaction which cleaves the C(sp2)−N bond and forges a new C(sp2)−B bond. The non‐planar structure of the pyridinium, provides the necessary driving force for the challenging aryl radical formation.
Nanoparticulate manganese oxides, formed in Nafion polymer from a series of molecular manganese complexes of varying nuclearity and metal oxidation state, are shown to effectively catalyze water ...oxidation under neutral pH conditions with the onset of electrocatalysis occurring at an overpotential of only 150 mV. Although XAS experiments indicate that each complex generates the same material in Nafion, the catalytic activity varied substantially with the manganese precursor and did not correlate with the amount of MnO x present in the films. The XAS and EPR studies indicated that the formation of the nanoparticulate oxide involves the dissociation of the complex into Mn(II) species followed by oxidation on application of an external bias. TEM studies of the most active films, derived from Mn(Me3TACN)(OMe)3+ and (Me3TACN)2MnIII 2(μ-O)(μ-CH3COO)22+ (Me3TACN = N,N′,N″-trimethyl-1,4,7-triazacyclononane), revealed that highly dispersed MnO x nanoparticles (10–20 nm and 6–10 nm, respectively) were generated in the Nafion film. In contrast, the use of Mn(OH2)62+ resulted in both a higher manganese oxide loading and aggregated nanoparticles with 30–100 nm approximate size, which were less effective water oxidation catalysts. Much higher turnover frequencies (TOFs) were observed for films derived from the two complexes, viz., ∼20 molecules of O2 per Mn per hour in dark and 40 molecules of O2 per Mn per hour under illumination at an overpotential of 350 mV, when compared with MnO x films made with Mn(OH2)62+. This corresponds to a TOF > 100 molecules of O2 per Mn per second for a 10 nm MnO x nanoparticle. Thus, the catalytic activity is dependent on the ability to generate well-defined, dispersed nanoparticles. Electrochemical and spectroscopic methods have been used to follow the conversion of the molecular precursors into MnO x and to further evaluate the origin of differences in catalytic activity.
Large separation of magnetic levels and slow relaxation in metal complexes are desirable properties of single‐molecule magnets (SMMs). Spin‐phonon coupling (interactions of magnetic levels with ...phonons) is ubiquitous, leading to magnetic relaxation and loss of memory in SMMs and quantum coherence in qubits. Direct observation of magnetic transitions and spin‐phonon coupling in molecules is challenging. We have found that far‐IR magnetic spectra (FIRMS) of Co(PPh3)2X2 (Co‐X; X=Cl, Br, I) reveal rarely observed spin‐phonon coupling as avoided crossings between magnetic and u‐symmetry phonon transitions. Inelastic neutron scattering (INS) gives phonon spectra. Calculations using VASP and phonopy programs gave phonon symmetries and movies. Magnetic transitions among zero‐field split (ZFS) levels of the S=3/2 electronic ground state were probed by INS, high‐frequency and ‐field EPR (HFEPR), FIRMS, and frequency‐domain FT terahertz EPR (FD‐FT THz‐EPR), giving magnetic excitation spectra and determining ZFS parameters (D, E) and g values. Ligand‐field theory (LFT) was used to analyze earlier electronic absorption spectra and give calculated ZFS parameters matching those from the experiments. DFT calculations also gave spin densities in Co‐X, showing that the larger Co(II) spin density in a molecule, the larger its ZFS magnitude. The current work reveals dynamics of magnetic and phonon excitations in SMMs. Studies of such couplings in the future would help to understand how spin‐phonon coupling may lead to magnetic relaxation and develop guidance to control such coupling.
Direct determination of magnetic excited levels and spin‐phonon couplings in the compounds in three SMMs, leading to magnetic relaxation, has been achieved by the combined use of inelastic neutron scattering (INS), far‐IR magnetic spectroscopy (FIRMS), high‐frequency and ‐field EPR, frequency‐domain FT terahertz EPR, ligand‐field theory (LFT) analysis, and DFT calculations of phonons (i. e., both molecular and lattice vibrations). Calculated spin densities on Co(II) ions correlate with magnetic separations in the compounds.
Here we report an in situ electron paramagnetic resonance (EPR) study of a low-cost, high-stability cobalt oxide electrodeposited material (Co-Pi) that oxidizes water at neutral pH and low ...over-potential, representing a promising system for future large-scale water splitting applications. Using CW X-band EPR we can follow the film formation from a Co(NO3)2 solution in phosphate buffer and quantify Co uptake into the catalytic film. As deposited, the film shows predominantly a Co(II) EPR signal, which converts into a Co(IV) signal as the electrode potential is increased. A purpose-built spectroelectrochemical cell allowed us to quantify the extent of Co(II) to Co(IV) conversion as a function of potential bias under operating conditions. Consistent with its role as an intermediate, Co(IV) is formed at potentials commensurate with electrocatalytic O2 evolution (+1.2 V, vs. SHE). The EPR resonance position of the Co(IV) species shifts to higher fields as the potential is increased above 1.2 V. Such a shift of the Co(IV) signal may be assigned to changes in the local Co structure, displaying a more distorted ligand field or more ligand radical character, suggesting it is this subset of sites that represents the catalytically ‘active’ component. The described spectroelectrochemical approach provides new information on catalyst function and reaction pathways of water oxidation.
We have investigated the single‐molecule magnets MnIII2(5‐Brsalen)2(MeOH)2MIII(CN)6NEt4 (M=Os (1) and Ru (2); 5‐Brsalen=N,N′‐ethylenebis(5‐bromosalicylidene)iminate) by frequency‐domain ...Fourier‐transform terahertz electron paramagnetic resonance (THz‐EPR), inelastic neutron scattering, and superconducting quantum interference device (SQUID) magnetometry. The combination of all three techniques allows for the unambiguous experimental determination of the three‐axis anisotropic magnetic exchange coupling between MnIII and RuIII or OsIII ions, respectively. Analysis by means of a spin‐Hamiltonian parameterization yields excellent agreement with all experimental data. Furthermore, analytical calculations show that the observed exchange anisotropy is due to the bent geometry encountered in both 1 and 2, whereas a linear geometry would lead to an Ising‐type exchange coupling.
Intriguing coupling: A three‐axis anisotropic exchange coupling is revealed in the isostructural single‐molecule magnets Mn‐Os‐Mn and Mn‐Ru‐Mn by terahertz frequency‐domain electron paramagnetic resonance, inelastic neutron scattering, and magnetic measurements (see picture). This peculiar behavior is explained by the presence of orbitally dependent exchange.
High-oxidation-state metal complexes with multiply bonded ligands are of great interest for both their reactivity as well as their fundamental bonding properties. This paper reports a combined ...spectroscopic and theoretical investigation into the effect of the apical multiply bonded ligand on the spin-state preferences of threefold symmetric iron(IV) complexes with tris(carbene) donor ligands. Specifically, singlet (S = 0) nitrido {PhB(ImR)3}FeN, R = tBu (1), Mes (mesityl, 2) and the related triplet (S = 1) imido complexes, {PhB(ImR)3}Fe(NR′)+, R = Mes, R′ = 1-adamantyl (3), tBu (4), were investigated by electronic absorption and Mössbauer effect spectroscopies. For comparison, two other Fe(IV) nitrido complexes, (TIMENAr)FeN+ (TIMENAr = t ris2-(3-aryl- im idazol-2-ylidene) e thylami n e; Ar = Xyl (xylyl), Mes), were investigated by 57Fe Mössbauer spectroscopy, including applied-field measurements. The paramagnetic imido complexes 3 and 4 were also studied by magnetic susceptibility measurements (for 3) and paramagnetic resonance spectroscopy: high-frequency and -field electron paramagnetic resonance (for 3 and 4) and frequency-domain Fourier-transform (FD-FT) terahertz electron paramagnetic resonance (for 3), which reveal their zero-field splitting parameters. Experimentally correlated theoretical studies comprising ligand-field theory and quantum chemical theory, the latter including both density functional theory and ab initio methods, reveal the key role played by the Fe 3d z 2 (a1) orbital in these systems: the nature of its interaction with the nitrido or imido ligand dictates the spin-state preference of the complex. The ability to tune the spin state through the energy and nature of a single orbital has general relevance to the factors controlling spin states in complexes with applicability as single molecule devices.
We have investigated the novel single‐molecule magnet (NEt4)Mn2(5‐Brsalen)2(MeOH)2Cr(CN)6 (1; 5‐Brsalen=N,N′‐ethylenebis(5‐bromosalicylidene)iminato anion) using spectroscopic as well as ...magnetization and susceptibility measurements. Frequency‐domain Fourier‐transform terahertz electron paramagnetic resonance (FDFT THz‐EPR) based on the generation of THz radiation from a synchrotron in combination with inelastic neutron scattering (INS) allows for the discrimination between intermultiplet and intramultiplet transitions. Together with ac/dc magnetic susceptibility measurements the obtained set of data provides a complete characterization of the lowest energetic magnetic excitations. We find that the new compound 1 exhibits much weaker intermolecular interactions than found in the closely related compound: KMn2(5‐Brsalen)2(H2O)2Cr(CN)6 (2). Furthermore, two phonon lines in the vicinity of the magnetic excitations are detected.
Main attraction: We have investigated the single‐molecule magnet (NEt4)Mn2(5‐Brsalen)2(MeOH)2Cr(CN)6 (1; 5‐Brsalen=N,N′‐ethylenebis(5‐bromosalicylidene)iminato anion) by using spectroscopic as well as magnetization and susceptibility measurements. The used methods allow for a complete characterization of the lowest‐energetic magnetic excitations.
Electron paramagnetic resonance (EPR) spectroscopy on protein single crystals is the ultimate method for determining the electronic structure of paramagnetic intermediates at the active site of an ...enzyme and relating the magnetic tensor to a molecular structure. However, crystals of dimensions typical for protein crystallography (0.05 to 0.3mm) provide insufficient signal intensity. In this work, we present a microwave self-resonant microhelix for nanoliter samples that can be implemented in a commercial X-band (9.5 GHz) EPR spectrometer. The self-resonant microhelix provides a measured signal-to-noise improvement up to a factor of 28 with respect to commercial EPR resonators. This work opens up the possibility to use advanced EPR techniques for studying protein single crystals of dimensions typical for x-ray crystallography. The technique is demonstrated by EPR experiments on single crystal FeFe-hydrogenase (
; CpI) with dimensions of 0.3 mm by 0.1 mm by 0.1 mm, yielding a proposed
-tensor orientation of the H
state.
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