Redox Cycling, pH Dependence, and Ligand Effects of Mn(III) in Oxalate Decarboxylase from <i>Bacillus subtilis</i>

This contribution describes electron paramagnetic resonance (EPR) experiments on Mn­(III) in oxalate decarboxylase of <i>Bacillus subtilis</i>, an interesting enzyme that catalyzes the redox-neutral dissociation of oxalate into formate and carbon dioxide. Chemical redox cycling provides strong evidence that both Mn centers can be oxidized, although the N-terminal Mn­(II) appears to have the lower reduction potential and is most likely the carrier of the +3 oxidation state under moderate oxidative conditions, in agreement with the general view that it represents the active site. Significantly, Mn­(III) was observed in untreated OxDC in succinate and acetate buffers, while it could not be directly observed in citrate buffer. Quantitative analysis showed that up to 16% of the EPR-visible Mn is in the +3 oxidation state at low pH in the presence of succinate buffer. The fine structure and hyperfine structure parameters of Mn­(III) are affected by small carboxylate ligands that can enter the active site and have been recorded for formate, acetate, and succinate. The results from a previous report [Zhu, W., et al. (2016) <i>Biochemistry</i> <i>55</i>, 429–434] could therefore be reinterpreted as evidence of formate-bound Mn­(III) after the enzyme is allowed to turn over oxalate. The pH dependence of the Mn­(III) EPR signal compares very well with that of enzymatic activity, providing strong evidence that the catalytic reaction of oxalate decarboxylase is driven by Mn­(III), which is generated in the presence of dioxygen

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)

NMR Investigations of Dinuclear, Single-Anion Bridged Copper(II) Metallacycles: Structure and Antiferromagnetic Behavior in Solution

The nuclear magnetic resonance (NMR) spectra of single-anion bridged, dinuclear copper­(II) metallacycles [Cu₂(μ-X)­(μ-<b>L</b>)₂]­(A)₃ (<b>L</b>_{<i><b>m</b></i>} = <i>m</i>-bis­[bis­(1-pyrazolyl)­methyl]­benzene: X = F^–, A = BF₄^–; X = Cl^–, OH^–, A = ClO₄^–; <b>L</b>_{<i><b>m</b></i>}<b>*</b> = <i>m</i>-bis­[bis­(3,5-dimethyl-1-pyrazolyl)­methyl]­benzene: X = CN^–, F^–, Cl^–, OH^–, Br^–, A = ClO₄^–) have relatively sharp ¹H and ¹³C NMR resonances with small hyperfine shifts due to the strong antiferromagnetic superexchange interactions between the two <i>S</i> = ¹/₂ metal centers. The complete assignments of these spectra, except X = CN^–, have been made through a series of NMR experiments: ¹H–¹H COSY, ¹H–¹³C HSQC, ¹H–¹³C HMBC, <i>T</i>₁ measurements and variable-temperature ¹H NMR. The <i>T</i>₁ measurements accurately determine the Cu···H distances in these molecules. In solution, the temperature dependence of the chemical shifts correlate with the population of the paramagnetic triplet (<i>S</i> = 1) and diamagnetic singlet (<i>S</i> = 0) states. This correlation allows the determination of antiferromagnetic exchange coupling constants, −<i>J</i> (<b>Ĥ</b> = −<i>J</i><b>Ŝ</b>₁<b>Ŝ</b>₂), in solution for the <b>L</b>_{<i><b>m</b></i>} compounds 338­(F^–), 460­(Cl^–), 542­(OH^–), for the <b>L</b>_{<i><b>m</b></i>}* compounds 128­(CN^–), 329­(F^–), 717­(Cl^–), 823­(OH^–), and 944­(Br^–) cm^–1, respectively. These values are of similar magnitudes to those previously measured in the solid state (−<i>J</i>_solid = 365, 536, 555, 160, 340, 720, 808, and 945 cm^–1, respectively). This method of using NMR to determine −<i>J</i> values in solution is an accurate and convenient method for complexes with strong antiferromagnetic superexchange interactions. In addition, the similarity between the solution and solid-state −<i>J</i> values of these complexes confirms the information gained from the <i>T</i>₁ measurements: the structures are similar in the two states

Copper(II) Carboxylate Dimers Prepared from Ligands Designed to Form a Robust π···π Stacking Synthon: Supramolecular Structures and Molecular Properties

The reactions of bifunctional carboxylate ligands (1,8-naphthalimido)­propanoate, (<b>L</b>_<b>C2</b>^<b>–</b>), (1,8-naphthalimido)­ethanoate, (<b>L</b>_<b>C1</b>^<b>–</b>), and (1,8-naphthalimido)­benzoate, (<b>L_C4^–)</b> with Cu₂(O₂CCH₃)₄(H₂O)₂ in methanol or ethanol at room temperature lead to the formation of novel dimeric [Cu₂(<b>L</b>_<b>C2</b>)₄(MeOH)₂] (<b>1</b>), [Cu₂(<b>L</b>_<b>C1</b>)₄(MeOH)₂]·2­(CH₂Cl₂) (<b>2</b>), [Cu₂(<b>L</b>_<b>C4</b>)₄(EtOH)₂]·2­(CH₂Cl₂) (<b>3</b>) complexes. When the reaction of <b>L</b>_<b>C1</b>^<b>–</b> with Cu₂(O₂CCH₃)₄(H₂O)₂ was carried out at −20 °C in the presence of pyridine, [Cu₂(<b>L</b>_<b>C1</b>)₄(py)₄]·2­(CH₂Cl₂) (<b>4</b>) was produced. At the core of complexes <b>1</b>–<b>3</b> lies the square Cu₂(O₂CR)₄ “paddlewheel” secondary building unit, where the two copper centers have a nearly square pyramidal geometry with methanol or ethanol occupying the axial coordination sites. Complex <b>4</b> contains a different type of dimeric core generated by two κ¹-bridging carboxylate ligands. Additionally, two terminal carboxylates and four trans situated pyridine molecules complete the coordination environment of the five-coordinate copper­(II) centers. In all four compounds, robust π···π stacking interactions of the naphthalimide rings organize the dimeric units into two-dimensional sheets. These two-dimensional networks are organized into a three-dimensional architecture by two different noncovalent interactions: strong π···π stacking of the naphthalimide rings (also the pyridine rings for <b>4</b>) in <b>1</b>, <b>3</b>, and <b>4</b>, and intermolecular hydrogen bonding of the coordinated methanol or ethanol molecules in <b>1</b>–<b>3</b>. Magnetic measurements show that the copper ions in the paddlewheel complexes <b>1</b>–<b>3</b> are strongly antiferromagnetically coupled with –<i>J</i> values ranging from 255 to 325 cm^–1, whereas the copper ions in <b>4</b> are only weakly antiferromagnetically coupled. Typical values of the zero-field splitting parameter <i>D</i> were found from EPR studies of <b>1</b>–<b>3</b> and the related known complexes [Cu₂(<b>L</b>_<b>C2</b>)₄(py)₂]<b>·</b>2­(CH₂Cl₂)<b>·</b>(CH₃OH), [Cu₂(<b>L</b>_<b>C3</b>)₄(py)₂]<b>·</b>2­(CH₂Cl₂) and [Cu₂(<b>L</b>_<b>C3</b>)₄(bipy)]<b>·</b>(CH₃OH)₂<b>·</b>(CH₂Cl₂)_3.37 (<b>L</b>_<b>C3</b>^<b>–</b> = (1,8-naphthalimido)­butanoate)), while its abnormal magnitude in [Cu₂(<b>L</b>_<b>C2</b>)₄(bipy)] was qualitatively rationalized by structural analysis and DFT calculations

Halide and Hydroxide Linearly Bridged Bimetallic Copper(II) Complexes: Trends in Strong Antiferromagnetic Superexchange Interactions

Centrosymmetric [Cu₂(μ-X)­(μ-<b>L</b>_{<i><b>m</b></i>}*)₂]­(ClO₄)₃ (X = F^–, Cl^–, Br^–, OH^–, <b>L</b>_{<i><b>m</b></i>}* = <i>m</i>-bis­[bis­(3,5-dimethyl-1-pyrazolyl)­methyl]­benzene)], the first example of a series of bimetallic copper­(II) complexes linked by a linearly bridging mononuclear anion, have been prepared and structurally characterized. Very strong antiferromagnetic exchange coupling between the copper­(II) ions increases along the series F^– < Cl^– < OH^– < Br^–, where −<i>J</i> = 340, 720, 808, and 945 cm^–1. DFT calculations explain this trend by an increase in the energy along this series of the antibonding antisymmetric combination of the p orbital of the bridging anion interacting with the copper­(II) d_<i>z</i>² orbital

Dinuclear Complexes Containing Linear M–F–M [M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II)] Bridges: Trends in Structures, Antiferromagnetic Superexchange Interactions, and Spectroscopic Properties

The reaction of M­(BF₄)₂·<i>x</i>H₂O, where M is Fe­(II), Co­(II), Ni­(II), Cu­(II), Zn­(II), and Cd­(II), with the new ditopic ligand <i>m</i>-bis­[bis­(3,5-dimethyl-1-pyrazolyl)­methyl]­benzene (<b>L_<i>m</i>*</b>) leads to the formation of monofluoride-bridged dinuclear metallacycles of the formula [M₂(μ-F)­(μ-<b>L_<i>m</i>*</b>)₂]­(BF₄)₃. The analogous manganese­(II) species, [Mn₂(μ-F)­(μ-<b>L_<i>m</i>*</b>)₂]­(ClO₄)₃, was isolated starting with Mn­(ClO₄)₂·6H₂O using NaBF₄ as the source of the bridging fluoride. In all of these complexes, the geometry around the metal centers is trigonal bipyramidal, and the fluoride bridges are linear. The ¹H, ¹³C, and ¹⁹F NMR spectra of the zinc­(II) and cadmium­(II) compounds and the ¹¹³Cd NMR of the cadmium­(II) compound indicate that the metallacycles retain their structure in acetonitrile and acetone solution. The compounds with M = Mn­(II), Fe­(II), Co­(II), Ni­(II), and Cu­(II) are antiferromagnetically coupled, although the magnitude of the coupling increases dramatically with the metal as one moves to the right across the periodic table: Mn­(II) (−6.7 cm^–1) < Fe­(II) (−16.3 cm^–1) < Co­(II) (−24.1 cm^–1) < Ni­(II) (−39.0 cm^–1) ≪ Cu­(II) (−322 cm^–1). High-field EPR spectra of the copper­(II) complexes were interpreted using the coupled-spin Hamiltonian with <i>g</i>_<i>x</i> = 2.150, <i>g</i>_<i>y</i> = 2.329, <i>g</i>_<i>z</i> = 2.010, <i>D</i> = 0.173 cm^–1, and <i>E</i> = 0.089 cm^–1. Interpretation of the EPR spectra of the iron­(II) and manganese­(II) complexes required the spin Hamiltonian using the noncoupled spin operators of two metal ions. The values <i>g</i>_<i>x</i> = 2.26, <i>g</i>_<i>y</i> = 2.29, <i>g</i>_<i>z</i> = 1.99, <i>J</i> = −16.0 cm^–1, <i>D</i>₁ = −9.89 cm^–1, and <i>D</i>₁₂ = −0.065 cm^–1 were obtained for the iron­(II) complex and <i>g</i>_<i>x</i> = <i>g</i>_<i>y</i> = <i>g</i>_<i>z</i> = 2.00, <i>D</i>₁ = −0.3254 cm^–1, <i>E</i>₁ = −0.0153, <i>J</i> = −6.7 cm^–1, and <i>D</i>₁₂ = 0.0302 cm^–1 were found for the manganese­(II) complex. Density functional theory (DFT) calculations of the exchange integrals and the zero-field splitting on manganese­(II) and iron­(II) ions were performed using the hybrid B3LYP functional in association with the TZVPP basis set, resulting in reasonable agreement with experiment

Halide and Hydroxide Linearly Bridged Bimetallic Copper(II) Complexes: Trends in Strong Antiferromagnetic Superexchange Interactions

Centrosymmetric [Cu₂(μ-X)­(μ-<b>L</b>_{<i><b>m</b></i>}*)₂]­(ClO₄)₃ (X = F^–, Cl^–, Br^–, OH^–, <b>L</b>_{<i><b>m</b></i>}* = <i>m</i>-bis­[bis­(3,5-dimethyl-1-pyrazolyl)­methyl]­benzene)], the first example of a series of bimetallic copper­(II) complexes linked by a linearly bridging mononuclear anion, have been prepared and structurally characterized. Very strong antiferromagnetic exchange coupling between the copper­(II) ions increases along the series F^– < Cl^– < OH^– < Br^–, where −<i>J</i> = 340, 720, 808, and 945 cm^–1. DFT calculations explain this trend by an increase in the energy along this series of the antibonding antisymmetric combination of the p orbital of the bridging anion interacting with the copper­(II) d_<i>z</i>² orbital

Solid State Collapse of a High-Spin Square-Planar Fe(II) Complex, Solution Phase Dynamics, and Electronic Structure Characterization of an Fe(II)₂ Dimer

Square-planar high-spin Fe­(II) molecular compounds are rare, and until recently, the only four examples of non-macrocyclic or sterically driven molecular compounds of this kind shared a common FeO₄ core. The trianionic pincer-type ligand [CF₃-ONO]­H₃ (<b>1</b>) supports the high-spin square-planar Fe­(II) complex {[CF₃-ONO]­FeCl}­{Li­(Sv)₂}₂ (<b>2</b>). In the solid state, <b>2</b> forms the dimer complex {[CF₃-ONO]­Fe}₂­{(μ-Cl)₂­(μ-LiTHF)₄} (<b>3</b>) in 96% yield by simply applying a vacuum or stirring it with pentane for 2 h. A detailed high-frequency electron paramagnetic resonance and field-dependent ⁵⁷Fe Mössbauer investigation of <b>3</b> revealed a weak antiferromagnetic exchange interaction between the local iron spins which exhibit a zero-field splitting tensor characterized by negative <i>D</i> parameter. In solution, <b>2</b> is in equilibrium with the solvento complex {[CF₃-ONO]­FeCl­(THF)}­{Li₂(Sv)₄} (<b>2·Sv</b>) and the dimer <b>3</b>. A combination of frozen solution ⁵⁷Fe Mössbauer spectroscopy and single crystal X-ray crystallography helped elucidate the solvent dependent equilibrium between these three species. The oxidation chemistry of <b>2·Sv</b> was investigated. Complex <b>2</b> reacts readily with the one-electron oxidizing agent CuCl₂ to give the Fe­(III) complex {[CF₃-ONO]­FeCl₂}­{Li­(THF)₂}₂ (<b>4</b>). Also, <b>2·Sv</b> reacts with 2 equiv of TlPF₆ to form the Fe­(III) complex [CF₃-ONO]­Fe­(THF)₃ (<b>5</b>)

Syntheses, Structural, Magnetic, and Electron Paramagnetic Resonance Studies of Monobridged Cyanide and Azide Dinuclear Copper(II) Complexes: Antiferromagnetic Superexchange Interactions

The reactions of Cu­(ClO₄)₂ with NaCN and the ditopic ligands <i>m</i>-bis­[bis­(1-pyrazolyl)­methyl]­benzene (<b>L</b>_{<i><b>m</b></i>}) or <i>m</i>-bis­[bis­(3,5-dimethyl-1-pyrazolyl)­methyl]­benzene (<b>L</b>_{<i><b>m</b></i>}*) yield [Cu₂(μ-CN)­(μ-<b>L</b>_{<i><b>m</b></i>})₂]­(ClO₄)₃ (<b>1</b>) and [Cu₂(μ-CN)­(μ-<b>L</b>_{<i><b>m</b></i>}<b>*</b>)₂]­(ClO₄)₃ (<b>3</b>). In both, the cyanide ligand is linearly bridged (μ-1,2) leading to a separation of the two copper­(II) ions of ca. 5 Å. The geometry around copper­(II) in these complexes is distorted trigonal bipyramidal with the cyanide group in an equatorial position. The reaction of [Cu₂(μ-F)­(μ-<b>L</b>_{<i><b>m</b></i>})₂]­(ClO₄)₃ and (CH₃)₃SiN₃ yields [Cu₂(<i>μ-</i>N₃)­(<i>μ-</i><b>L</b>_{<i><b>m</b></i>})₂]­(ClO₄)₃ (<b>2</b>), where the azide adopts end-on (μ-1,1) coordination with a Cu–N–Cu angle of 138.0° and a distorted square pyramidal geometry about the copper­(II) ions. Similar chemistry in the more sterically hindered <b>L</b>_{<i><b>m</b></i>}* system yielded only the coordination polymer [Cu₂(<i>μ-</i><b>L</b>_{<i><b>m</b></i>}*)­(<i>μ-</i>N₃)₂­(N₃)₂]. Attempts to prepare a dinuclear complex with a bridging iodide yield the copper­(I) complex [Cu₅(<i>μ-</i>I₄)­(μ-<b>L</b>_{<i><b>m</b></i>}*)₂]­I₃. The complexes <b>1</b> and <b>3</b> show strong antiferromagnetic coupling, −<i>J</i> = 135 and 161 cm^–1, respectively. Electron paramagnetic resonance (EPR) studies coupled with density functional theory (DFT) calculations show that the exchange interaction is transmitted through the d_<i>z</i>² and the bridging ligand s and p_<i>x</i> orbitals. High field EPR studies confirmed the d_<i>z</i>² ground state of the copper­(II) ions. Single-crystal high-field EPR has been able to definitively show that the signs of <i>D</i> and <i>E</i> are positive. The zero-field splitting is dominated by the anisotropic exchange interactions. Complex <b>2</b> has −<i>J</i> = 223 cm^–1 and DFT calculations indicate a predominantly d_{<i>x</i>²–y²} ground state

A Tale of Two Metals: New Cerium Iron Borocarbide Intermetallics Grown from Rare-Earth/Transition Metal Eutectic Fluxes

R₃₃Fe_{14–<i>x</i>}Al_{<i>x</i>+<i>y</i>}B_{25–<i>y</i>}C₃₄ (R = La or Ce; <i>x</i> ≤ 0.9; <i>y</i> ≤ 0.2) and R₃₃Fe_{13–<i>x</i>}Al_<i>x</i>B₁₈C₃₄ (R = Ce or Pr; <i>x</i> < 0.1) were synthesized from reactions of iron with boron, carbon, and aluminum in R–T eutectic fluxes (T = Fe, Co, or Ni). These phases crystallize in the cubic space group <i>Im</i>3̅<i>m</i> (<i>a</i> = 14.617(1) Å, <i>Z</i> = 2, <i>R</i>₁ = 0.0155 for Ce₃₃Fe_13.1Al_1.1B_24.8C₃₄, and <i>a</i> = 14.246(8) Å, <i>Z</i> = 2, <i>R</i>₁ = 0.0142 for Ce₃₃Fe₁₃B₁₈C₃₄). Their structures can be described as body-centered cubic arrays of large Fe₁₃ or Fe₁₄ clusters which are capped by borocarbide chains and surrounded by rare earth cations. The magnetic behavior of the cerium-containing analogs is complicated by the possibility of two valence states for cerium and possible presence of magnetic moments on the iron sites. Temperature-dependent magnetic susceptibility measurements and Mössbauer data show that the boron-centered Fe₁₄ clusters in Ce₃₃Fe_{14–<i>x</i>}Al_{<i>x</i>+<i>y</i>}B_{25–<i>y</i>}C₃₄ are not magnetic. X-ray photoelectron spectroscopy data indicate that the cerium is trivalent at room temperature, but the temperature dependence of the resistivity and the magnetic susceptibility data suggest Ce^3+/4+ valence fluctuation beginning at 120 K. Bond length analysis and XPS studies of Ce₃₃Fe₁₃B₁₈C₃₄ indicate the cerium in this phase is tetravalent, and the observed magnetic ordering at <i>T</i>_C = 180 K is due to magnetic moments on the Fe₁₃ clusters