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

Zero-Field Splitting in Pseudotetrahedral Co(II) Complexes: a Magnetic, High-Frequency and -Field EPR, and Computational Study

Six pseudotetrahedral cobalt­(II) complexes of the type [CoL₂Cl₂], with L = heterocyclic N-donor ligand, have been studied in parallel by magnetometry, and high-frequency and -field electron paramagnetic resonance (HFEPR). HFEPR powder spectra were recorded in a 50 GHz < ν < 700 GHz range in a 17 T superconducting and 25 T resistive magnet, which allowed constructing of resonance field vs frequency diagrams from which the fitting procedure yielded the <i>S</i> = 3/2 spin ground state Hamiltonian parameters. The sign of the axial anisotropy parameter <i>D</i> was determined unambiguously; the values range between −8 and +11 cm^–1 for the given series of complexes. These data agree well with magnetometric analysis. Finally, quantum chemical <i>ab initio</i> calculations were performed on the whole series of complexes to probe the relationship between the magnetic anisotropy, electronic, and geometric structure

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)

High-Frequency/High-Field Electron Paramagnetic Resonance and Theoretical Studies of Tryptophan-Based Radicals

Tryptophan-based free radicals have been implicated in a myriad of catalytic and electron transfer reactions in biology. However, very few of them have been trapped so that biophysical characterizations can be performed in a high-precision context. In this work, tryptophan derivative-based radicals were studied by high-frequency/high-field electron paramagnetic resonance (HFEPR) and quantum chemical calculations. Radicals were generated at liquid nitrogen temperature with a photocatalyst, sacrificial oxidant, and violet laser. The precise <i>g</i>-anisotropies of l- and d-tryptophan, 5-hydroxytryptophan, 5-methoxytryptophan, 5-fluorotryptophan, and 7-hydroxytryptophan were measured directly by HFEPR. Quantum chemical calculations were conducted to predict both neutral and cationic radical spectra for comparison with the experimental data. The results indicate that under the experimental conditions, all radicals formed were cationic. Spin densities of the radicals were also calculated. The various line patterns and <i>g</i>-anisotropies observed by HFEPR can be understood in terms of spin-density populations and the positioning of oxygen atom substitution on the tryptophan ring. The results are considered in the light of the tryptophan and 7-hydroxytryptophan diradical found in the biosynthesis of the tryptophan tryptophylquinone cofactor of methylamine dehydrogenase

Magnetic Properties of a Dinuclear Nickel(II) Complex with 2,6-Bis[(2-hydroxyethyl)methylaminomethyl]-4-methylphenolate

Magnetic properties of dinuclear nickel­(II) complex [Ni₂(<i>sym</i>-hmp)₂]­(BPh₄)₂·3.5DMF·0.5­(2-PrOH) (<b>1</b>), where (<i>sym</i>-hmp)⁻ is 2,6-bis­[(2-hydroxyethyl)­methylaminomethyl]-4-methylphenolate anion and DMF indicates dimethylformamide, were investigated using high-frequency and -field electron paramagnetic resonance (HFEPR). To magnetically characterize the mononuclear nickel­(II) species forming the dimer, its two dinuclear zinc­(II) analogues, [Zn₂(<i>sym</i>-hmp)₂]­(BPh₄)₂·3.5DMF·0.5­(2-PrOH) (<b>2</b>) and [Zn₂(<i>sym</i>-hmp)₂]­(BPh₄)₂·2acetone·2H₂O (<b>2′</b>), were prepared. One of them (<b>2′</b>) was structurally characterized by X-ray diffractometry and doped with 5% mol nickel­(II) ions to prepare a mixed crystal <b>3</b>. From the HFEPR results on complex <b>1</b> obtained at 40 K, the spin Hamiltonian parameters of the first excited spin state (<i>S</i> = 1) of the dimer were accurately determined as |<i>D</i>₁| = 9.99(2) cm^–1, |<i>E</i>₁| = 1.62(1) cm^–1, and <i>g</i>₁ = [2.25(1), 2.19(2), 2.27(2)], and for the second excited spin state (<i>S</i> = 2) at 150 K estimated as |<i>D</i>₂| ≈ 3.5 cm^–1. From these numbers, the single-ion zero-field splitting (ZFS) parameter of the Ni­(II) ions forming the dimer was estimated as |<i>D</i>_Ni| ≈ 10–10.5 cm^–1. The HFEPR spectra of <b>3</b> yielded directly the single-ion parameters for <i>D</i>_Ni = +10.1 cm^–1, |<i>E</i>_Ni| = 3.1 cm^–1, and <i>g</i>_iso = 2.2. On the basis of the HFEPR results, the previously obtained magnetic data (Sakiyama, H.; Tone, K.; Yamasaki, M.; Mikuriya, M. <i>Inorg. Chim. Acta</i> <b>2011</b>, <i>365</i>, 183) were reanalyzed, and the isotropic interaction parameter between the Ni­(II) ions was determined as <i>J</i> = −70 cm^–1 (<b>H</b>_ex = −<i>J <b>S</b></i>_A·<i><b>S</b></i>_B). Finally, density functional theory calculations yielded the <i>J</i> value of −90 cm^–1 in a qualitative agreement with the experiment

Magnetic Properties of a Dinuclear Nickel(II) Complex with 2,6-Bis[(2-hydroxyethyl)methylaminomethyl]-4-methylphenolate

Magnetic properties of dinuclear nickel­(II) complex [Ni₂(<i>sym</i>-hmp)₂]­(BPh₄)₂·3.5DMF·0.5­(2-PrOH) (<b>1</b>), where (<i>sym</i>-hmp)⁻ is 2,6-bis­[(2-hydroxyethyl)­methylaminomethyl]-4-methylphenolate anion and DMF indicates dimethylformamide, were investigated using high-frequency and -field electron paramagnetic resonance (HFEPR). To magnetically characterize the mononuclear nickel­(II) species forming the dimer, its two dinuclear zinc­(II) analogues, [Zn₂(<i>sym</i>-hmp)₂]­(BPh₄)₂·3.5DMF·0.5­(2-PrOH) (<b>2</b>) and [Zn₂(<i>sym</i>-hmp)₂]­(BPh₄)₂·2acetone·2H₂O (<b>2′</b>), were prepared. One of them (<b>2′</b>) was structurally characterized by X-ray diffractometry and doped with 5% mol nickel­(II) ions to prepare a mixed crystal <b>3</b>. From the HFEPR results on complex <b>1</b> obtained at 40 K, the spin Hamiltonian parameters of the first excited spin state (<i>S</i> = 1) of the dimer were accurately determined as |<i>D</i>₁| = 9.99(2) cm^–1, |<i>E</i>₁| = 1.62(1) cm^–1, and <i>g</i>₁ = [2.25(1), 2.19(2), 2.27(2)], and for the second excited spin state (<i>S</i> = 2) at 150 K estimated as |<i>D</i>₂| ≈ 3.5 cm^–1. From these numbers, the single-ion zero-field splitting (ZFS) parameter of the Ni­(II) ions forming the dimer was estimated as |<i>D</i>_Ni| ≈ 10–10.5 cm^–1. The HFEPR spectra of <b>3</b> yielded directly the single-ion parameters for <i>D</i>_Ni = +10.1 cm^–1, |<i>E</i>_Ni| = 3.1 cm^–1, and <i>g</i>_iso = 2.2. On the basis of the HFEPR results, the previously obtained magnetic data (Sakiyama, H.; Tone, K.; Yamasaki, M.; Mikuriya, M. <i>Inorg. Chim. Acta</i> <b>2011</b>, <i>365</i>, 183) were reanalyzed, and the isotropic interaction parameter between the Ni­(II) ions was determined as <i>J</i> = −70 cm^–1 (<b>H</b>_ex = −<i>J <b>S</b></i>_A·<i><b>S</b></i>_B). Finally, density functional theory calculations yielded the <i>J</i> value of −90 cm^–1 in a qualitative agreement with the experiment

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

Vanadocene <i>de Novo</i>: Spectroscopic and Computational Analysis of Bis(η⁵‑cyclopentadienyl)vanadium(II)

The magnetic and electronic properties of the long-known organometallic complex vanadocene (VCp₂), which has an <i>S</i> = 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: <i>D</i> = +2.836(2) cm^–1, <i>g</i>_⊥ = 1.991(2), <i>g</i>_∥ = 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 ⁴A_2g ground state to the ⁴E_1g, ⁴E_2g, and ⁴E_1g excited states, using symmetry labels from the <i>D</i>_5<i>d</i> point group (i.e., staggered VCp₂). Contributions to the <i>D</i> 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 <i>D</i> values for all models considered were obtained from DFT calculations (<i>D</i> = 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, <i>D</i> values were also computed using the SORCI (<i>s</i>pectroscopically <i>or</i>iented <i>c</i>onfiguration <i>i</i>nteraction) method, which provides multireference descriptions of ground and excited states. SORCI calculations gave accurate <i>D</i> values (2.86–2.90 cm^–1), where the dominant (∼80%) contribution to <i>D</i> arises from spin–orbit coupling between ligand-field states, with the largest contribution from a low-lying ²A_1g state. In contrast, the <i>D</i> 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 <i>ab initio</i> methods

Cytosine Nucleobase Ligand: A Suitable Choice for Modulating Magnetic Anisotropy in Tetrahedrally Coordinated Mononuclear Co^II Compounds

A family of tetrahedral mononuclear Co^II complexes with the cytosine nucleobase ligand is used as the playground for an in-depth study of the effects that the nature of the ligand, as well as their noninnocent distortions on the Co­(II) environment, may have on the slow magnetic relaxation effects. Hence, those compounds with greater distortion from the ideal tetrahedral geometry showed a larger-magnitude axial magnetic anisotropy (<i>D</i>) together with a high rhombicity factor (<i>E</i>/<i>D</i>), and thus, slow magnetic relaxation effects also appear. In turn, the more symmetric compound possesses a much smaller value of the <i>D</i> parameter and, consequently, lacks single-ion magnet behavior

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