Magnetic Properties and Electronic Structure of Manganese-Based Blue Pigments: A High-Frequency and -Field EPR Study

A variety of new oxide-based materials based on hexagonal phase of YInO₃ have been recently described. In some of these materials, the In­(III) ions are substituted by Mn­(III), which finds itself in a trigonal-bipyramidal (TBP) coordination environment. While YInO₃ is colorless and YMnO₃ is black, mixed systems YIn_1–<i>x</i>Mn_<i>x</i>O₃ (0.02 < <i>x</i> < 0.25) display intense blue color and have been proposed as novel blue pigments. Since the Mn­(III) ion is paramagnetic, its presence imparts distinct magnetic properties to the whole class of materials. These properties were investigated by electron paramagnetic resonance (EPR) in its high-frequency and -field version (HFEPR), a technique ideally suited for transition metal ions such as Mn­(III) that, in contrast to, for example, Mn­(II), are difficult to study by EPR at (conventional) low frequency and field. YIn_1–<i>x</i>Mn_<i>x</i>O₃ with 0.02 < <i>x</i> < 0.2 exhibited high-quality HFEPR spectra up to room temperature that could be interpreted as arising from isolated <i>S</i> = 2 paramagnets. A simple ligand-field model, based on the structure and optical spectra, explains the spin Hamiltonian parameters provided by HFEPR, which were <i>D</i> = +3.0 cm^–1, <i>E</i> = 0; <i>g</i>_⊥ = 1.99, <i>g</i>_∥ = 2.0. This study demonstrates the general applicability of a combined spectroscopic and classical theoretical approach to understanding the electronic structure of novel materials containing paramagnetic dopants. Moreover, HFEPR complements optical and other experimental methods as being a sensitive probe of dopant level

Observation of a Photogenerated Rh₂ Nitrenoid Intermediate in C–H Amination

Rh₂-catalyzed C–H amination is a powerful method for nitrogenating organic molecules. While Rh₂ nitrenoids are often invoked as reactive intermediates in these reactions, the exquisite reactivity and fleeting lifetime of these species has precluded their observation. Here, we report the photogeneration of a transient Rh₂ nitrenoid that participates in C–H amination. The developed approach to Rh₂ nitrenoids, based on photochemical cleavage of N–Cl bonds in <i>N</i>-chloroamido ligands, has enabled characterization of a reactive Rh₂ nitrenoid by mass spectrometry and transient absorption spectroscopy. We anticipate that photogeneration of metal nitrenoids will contribute to the development of C–H amination catalysis by providing tools to directly study the structures of these critical intermediates

High Spin Co(I): High-Frequency and -Field EPR Spectroscopy of CoX(PPh₃)₃ (X = Cl, Br)

The previously reported pseudotetrahedral Co­(I) complexes, CoX­(PR₃)₃, where R = Me, Ph, and chelating analogues, and X = Cl, Br, I exhibit a spin triplet ground state, which is uncommon for Co­(I), although expected for this geometry. Described here are studies using electronic absorption and high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy on two members of this class of complexes: CoX­(PR₃)₃, where R = Ph and X = Cl and Br. In both cases, well-defined spectra corresponding to axial spin triplets were observed, with signals assignable to three distinct triplet species, and with perfectly axial zero-field splitting (zfs) given by the parameter <i>D</i> = +4.46, +5.52, +8.04 cm^–1, respectively, for CoCl­(PPh₃)₃. The crystal structure reported for CoCl­(PPh₃)₃ shows crystallographic 3-fold symmetry, but with three structurally distinct molecules per unit cell. Both of these facts thus correlate with the HFEPR data. The investigated complexes, along with a number of structurally characterized Co­(I) trisphosphine analogues, were analyzed by quantum chemistry calculations (both density functional theory (DFT) and unrestricted Hartree–Fock (UHF) methods). These methods, along with ligand-field theory (LFT) analysis of CoCl­(PPh₃)₃, give reasonable agreement with the salient features of the electronic structure of these complexes. A spin triplet ground state is strongly favored over a singlet state and a positive, axial <i>D</i> value is predicted, in agreement with experiment. Quantitative agreement between theory and experiment is less than ideal with LFT overestimating the zfs, while DFT underestimates these effects. Despite these shortcomings, this study demonstrates the ability of advanced paramagnetic resonance techniques, in combination with other experimental techniques, and with theory, to shed light on the electronic structure of an unusual transition metal ion, paramagnetic Co­(I)

Synthesis, Characterization, and Electrochemical Analyses of Vanadocene Tetrametaphosphate and Phosphinate Derivatives

The synthesis and characterization of a series of new vanadocene-derived tetrametaphosphate or diphenylphosphinate complexes, [PPN]₂[CpV­(P₄O₁₂)] (<b>1</b>), [Cp₂VOP­(O)­Ph₂] (<b>2</b>), [CpV­(μ²-O₂PPh₂)₄VCp] (<b>3</b>), and [Cp₂V­(μ²-O₂PPh₂)₂VCp₂]­[PF₆]₂ (<b>4</b>), is reported ([PPN]⁺ = bis­(triphenylphosphoranylidene)­ammonium; Cp = η⁵-cyclopentadienyl). The complexes were synthesized from the methyl vanadocene (Cp₂VMe) precursor, through protonation of the Me or Cp linkages using the tetrametaphosphate or phosphinate acid precursors [PPN]₂[P₄O₁₂H₂] (for <b>1</b>) and Ph₂P­(O)­OH (for <b>2</b> and <b>3</b>). Oxidation of <b>2</b> with [Fc]­[PF₆] resulted in dimerization, yielding the bimetallic <b>4</b> ([Fc]⁺ = ferrocenium). Electrochemical analysis of this reaction revealed a possible ECE mechanism that includes prior and subsequent electron transfer to this dimerization. The electronic structure of the dimer <b>4</b> was probed by SQUID magnetommetry and X-band EPR spectroscopy (100 and 4 K). The dimer was found to contain two antiferromagnetically coupled V^IV centers, as well as a small portion of monomeric V^IV species, <b>2</b>^<b>+</b>. In contrast to <b>2</b>, oxidation of <b>1</b> resulted in an EC mechanism, the product of which remains unknown. Preliminary reactions with compounds <b>1</b> and <b>2</b> bearing free PO groups were performed using H atom donors to test their ability to undergo H atom transfer in the context of the proposed “reduction-coupled oxo activation” mechanism; however, no clear reaction pathway supporting this mechanism has yet been observed

Secondary Coordination Sphere Effects in Ruthenium(III) Tetraammine Complexes: Role of the Coordinated Water Molecule

The complexes <i>trans</i>-[Ru^III(NH₃)₄(4-pic)­(H₂O)]­(CF₃SO₃)₃ (<b>1</b>) and [Ru^III(NH₃)₅(4-pic)]­(CF₃SO₃)₃ (<b>2</b>) were isolated and studied experimentally by electron paramagnetic resonance (EPR) and UV–vis spectroscopies, cyclic voltammetry, and X-ray crystallography and theoretically by ligand-field theory (LFT) and density functional theory (DFT) calculations. Complex <b>1</b> is reported in two different crystal forms, <b>1a</b> (100 K) and <b>1b</b> (room temperature). EPR and UV–vis spectroscopies suggest that aqua ligand interaction in this low-spin ruthenium­(III) complex changes as a function of hydrogen bonding with solvent molecules. This explicit water solvent effect was explained theoretically by DFT calculations, which demonstrated the effect of rotation of the aqua ligand about the N_pic–Ru–O_aq axis. The UV–vis spectrum of <b>1</b> shows in an aqueous acid solution a broad- and low-intensity absorption band around 28 500 cm^–1 (ε ≈ 500 M^–1 cm^–1) that is assigned mainly to a charge-transfer (CT) transition from the equatorial ligands to the Ru β-4d_<i>xy</i> orbital (β-LUMO) using DFT calculations. The electronic reflectance spectrum of <b>1</b> shows a broad and intense absorption band around 25 500 cm^–1 that is assigned to a CT transition from 4-picoline to the Ru β-4d_<i>xz</i> orbital (β-LUMO) using DFT calculations. The t_2g⁵ set of orbitals had its energy splitting investigated by LFT. LFT analysis shows that a rhombic component arises from <i>C</i>_2<i>v</i> symmetry by a simple π-bonding ligand (H₂O in our case) twisting about the trans (<i>C</i>₂) axis. This twist was manifested in the EPR spectra, which were recorded for <b>1</b> as a function of the solvent in comparison with [Ru­(NH₃)₅(4-pic)]³⁺ and [Ru­(NH₃)₅(H₂O)]³⁺. Only <b>1</b> shows an evident change in the <b>g</b>-tensor values, wherein an increased rhombic component is correlated with a higher nucleophilicity (donor) solvent feature, as parametrized by the Abraham system

Secondary Coordination Sphere Effects in Ruthenium(III) Tetraammine Complexes: Role of the Coordinated Water Molecule

The complexes <i>trans</i>-[Ru^III(NH₃)₄(4-pic)­(H₂O)]­(CF₃SO₃)₃ (<b>1</b>) and [Ru^III(NH₃)₅(4-pic)]­(CF₃SO₃)₃ (<b>2</b>) were isolated and studied experimentally by electron paramagnetic resonance (EPR) and UV–vis spectroscopies, cyclic voltammetry, and X-ray crystallography and theoretically by ligand-field theory (LFT) and density functional theory (DFT) calculations. Complex <b>1</b> is reported in two different crystal forms, <b>1a</b> (100 K) and <b>1b</b> (room temperature). EPR and UV–vis spectroscopies suggest that aqua ligand interaction in this low-spin ruthenium­(III) complex changes as a function of hydrogen bonding with solvent molecules. This explicit water solvent effect was explained theoretically by DFT calculations, which demonstrated the effect of rotation of the aqua ligand about the N_pic–Ru–O_aq axis. The UV–vis spectrum of <b>1</b> shows in an aqueous acid solution a broad- and low-intensity absorption band around 28 500 cm^–1 (ε ≈ 500 M^–1 cm^–1) that is assigned mainly to a charge-transfer (CT) transition from the equatorial ligands to the Ru β-4d_<i>xy</i> orbital (β-LUMO) using DFT calculations. The electronic reflectance spectrum of <b>1</b> shows a broad and intense absorption band around 25 500 cm^–1 that is assigned to a CT transition from 4-picoline to the Ru β-4d_<i>xz</i> orbital (β-LUMO) using DFT calculations. The t_2g⁵ set of orbitals had its energy splitting investigated by LFT. LFT analysis shows that a rhombic component arises from <i>C</i>_2<i>v</i> symmetry by a simple π-bonding ligand (H₂O in our case) twisting about the trans (<i>C</i>₂) axis. This twist was manifested in the EPR spectra, which were recorded for <b>1</b> as a function of the solvent in comparison with [Ru­(NH₃)₅(4-pic)]³⁺ and [Ru­(NH₃)₅(H₂O)]³⁺. Only <b>1</b> shows an evident change in the <b>g</b>-tensor values, wherein an increased rhombic component is correlated with a higher nucleophilicity (donor) solvent feature, as parametrized by the Abraham system

Mechanistic Studies of the Spore Photoproduct Lyase via a Single Cysteine Mutation

5-Thyminyl-5,6-dihydrothymine (also called spore photoproduct or SP) is the exclusive DNA photodamage product in bacterial endospores. It is repaired by a radical SAM (<i>S</i>-adenosylmethionine) enzyme, the spore photoproduct lyase (SPL), at the bacterial early germination phase. Our previous studies proved that SPL utilizes the 5′-dA• generated by the SAM cleavage reaction to abstract the H_6pro<i>R</i> atom to initiate the SP repair process. The resulting thymine allylic radical was suggested to take an H atom from an unknown protein source, most likely cysteine 141. Here we show that C141 can be readily alkylated in the native SPL by an iodoacetamide treatment, suggesting that it is accessible to the TpT radical. SP repair by the SPL C141A mutant yields TpTSO₂^– and TpT simultaneously from the very beginning of the reaction; no lag phase is observed for TpTSO₂^– formation. Should any other protein residue serve as the H donor, its presence would result in TpT being the major product at least for the first enzyme turnover. These observations provide strong evidence to support C141 as the direct H atom donor. Moreover, because of the lack of this intrinsic H donor, the C141A mutant produces TpT via an unprecedented thymine cation radical reduction (proton-coupled electron transfer) process, contrasting to the H atom transfer mechanism in the wild-type (WT) SPL reaction. The C141A mutant repairs SP at a rate that is ∼3-fold slower than that of the WT enzyme. Formation of TpTSO₂^– and TpT exhibits a <i>V</i>_max deuterium kinetic isotope effect (KIE) of 1.7 ± 0.2, which is smaller than the ^D<i>V</i>_max KIE of 2.8 ± 0.3 determined for the WT SPL reaction. These findings suggest that removing the intrinsic H atom donor disturbs the rate-limiting process during enzyme catalysis. As expected, the prereduced C141A mutant supports only ∼0.4 turnover, which is in sharp contrast to the >5 turnovers exhibited by the WT SPL reaction, suggesting that the enzyme catalytic cycle (SAM regeneration) is disrupted by this single mutation

HFEPR and Computational Studies on the Electronic Structure of a High-Spin Oxidoiron(IV) Complex in Solution

Nonheme iron enzymes perform diverse and important functions in biochemistry. The active form of these enzymes comprises the ferryl, oxidoiron­(IV), [FeO]²⁺ unit. In enzymes, this unit is in the high-spin, quintet, <i>S</i> = 2, ground state, while many synthetic model compounds exist in the spin triplet, <i>S</i> = 1, ground state. Recently, however, Que and co-workers reported an oxidoiron­(IV) complex with a quintet ground state, [FeO­(TMG₃tren)]­(OTf)₂, where TMG₃tren = 1,1,1-tris­{2-[<i>N</i>2-(1,1,3,3-tetramethylguanidino)]­ethyl}­amine and OTf = CF₃SO₃^–. The trigonal geometry imposed by this ligand, as opposed to the tetragonal geometry of earlier model complexes, favors the high-spin ground state. Although [FeO­(TMG₃tren)]²⁺ has been earlier probed by magnetic circular dichroism (MCD) and Mössbauer spectroscopies, the technique of high-frequency and -field electron paramagnetic resonance (HFEPR) is superior for describing the electronic structure of the iron­(IV) center because of its ability to establish directly the spin-Hamiltonian parameters of high-spin metal centers with high precision. Herein we describe HFEPR studies on [FeO­(TMG₃tren)]­(OTf)₂ generated in situ and confirm the <i>S</i> = 2 ground state with the following parameters: <i>D</i> = +4.940(5) cm^–1, <i>E</i> = 0.000(5), <i>B</i>₄⁰ = −14(1) × 10^–4 cm^–1, <i>g</i>_⊥ = 2.006(2), and <i>g</i>_∥ = 2.03(2). Extraction of a fourth-order spin-Hamiltonian parameter is unusual for HFEPR and impossible by other techniques. These experimental results are combined with state-of-the-art computational studies along with previous structural and spectroscopic results to provide a complete picture of the electronic structure of this biomimetic complex. Specifically, the calculations reproduce well the spin-Hamiltonian parameters of the complex, provide a satisfying geometrical picture of the <i>S</i> = 2 oxidoiron­(IV) moiety, and demonstrate that the TMG₃tren is an “innocent” ligand

A Neutrally Charged Trimethylmanganese(III) Complex: Synthesis, Characterization, and Disproportionation Chemistry

The synthesis and properties of an unusual, neutrally charged and volatile <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethylethylenediamine trimethyl manganese­(III) complex, (TMEDA)­MnMe₃, are described, along with its facile disproportionation to the corresponding Mn­(II) and Mn­(IV) complexes. Characterization by single-crystal XRD, UV–vis spectroscopy, high-frequency and -field EPR (HFEPR), magnetic susceptibility, and density functional theory (DFT) computations indicate that the (TMEDA)­MnMe₃ electronic structure can be described as largely square pyramidal Mn­(III) centered. The paucity of manganese­(III) polyalkyls and the simplicity and reactivity of this compound implicate it as a potentially useful synthetic building block

Ligand Substituent Effects in Manganese Pyridinophane Complexes: Implications for Oxygen-Evolving Catalysis

A series of Mn­(II) complexes of differently substituted pyridinophane ligands, (Py₂NR₂)­MnCl₂ (R = <i>ⁱ</i>Pr, Cy) and [(Py₂NR₂)­MnF₂]­(PF₆) (R = <i>ⁱ</i>Pr, Cy, <i>^t</i>Bu) are synthesized and characterized. The electrochemical properties of these complexes are investigated by cyclic voltammetry, along with those of previously reported (Py₂NMe₂)­MnCl₂ and the Mn­(III) complex [(Py₂NMe₂)­MnF₂]­(PF₆). The electronic structure of this and other Mn­(III) complexes is probed experimentally and theoretically, via high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy ab initio quantum chemical theory (QCT), respectively. These studies show that the complexes contain relatively typical six-coordinate Mn­(III). The catalytic activity of these complexes toward both H₂O₂ disproportionation and H₂O oxidation has also been investigated. The rate of H₂O₂ disproportionation decreases with increasing substituent size. Some of these complexes are active for electrocatalytic H₂O oxidation; however this activity cannot be rationalized in terms of simple electronic or steric effects