Toward an Understanding of the Ambiguous Electron Paramagnetic Resonance Spectra of the Iminoxy Radical from <i>o</i>‑Fluorobenzaldehyde Oxime: Density Functional Theory and <i>ab Initio</i> Studies

Iminoxy radicals (R₁R₂CNO^•) possess an inherent ability to exist as <i>E</i> and <i>Z</i> isomers. Although isotropic hyperfine couplings for the species with R₁ = H allow one to distinguish between <i>E</i> and <i>Z</i>, unequivocal assignment of the parameters observed in the EPR spectra of the radicals without the hydrogen atom at the azomethine carbon to the right isomer is not a simple task. The iminoxyl derived from <i>o</i>-fluoroacetophenone oxime (R₁ = CH₃ and R₂ = <i>o</i>-FC₆H₅) appears to be a case in point. Moreover, for its two isomers the rotation of the <i>o</i>-FC₆H₅ group brings into existence the <i>syn</i> and <i>anti</i> conformers, depending on the mutual orientation of the F atom and CNO^• group, making a description of hyperfine couplings to structure even more challenging. To accomplish this, a vast array of theoretical methods (DFT, OO-SCS-MP2, QCISD) was used to calculate the isotropic hyperfine couplings. The comparison between experimental and theoretical values revealed that the <i>E</i> isomer is the dominant radical form, for which a fast interconversion between <i>anti</i> and <i>syn</i> conformers is expected. In addition, the origin of the significant <i>A</i>_F increase with solvent polarity was analyzed

Oxidation of 1‑Methyl-1-phenylhydrazine with Oxidovanadium(V)–Salan Complexes: Insight into the Pathway to the Formation of Hydrazine by Vanadium Nitrogenase

A series of oxidovanadium­(V) complexes [VO­(L-κ⁴O,N,N,O)­(OR)] (<b>1a</b>, R = Et, L = L¹; <b>1b</b>, R = Me, L = L¹; <b>2</b>, R = Me, L = L²; <b>3</b>, R = Me, L = L³) were synthesized by the σ-bond metathesis reaction between [VO­(OR)₃] and the linear diaminebis­(phenol) derivatives H₂L (salans) containing different para-substituents on the phenoxo group [CMe₃CH₂CMe₂, L¹; Me, L²; Cl, L³]. As shown by X-ray crystallography complexes <b>1a</b>, <b>1b</b>, and <b>2</b> exhibit cis-α geometry, do have a stereogenic vanadium center, and exist as a racemic mixture of the Δ cis-α and Λ cis-α enantiomers. In solution, as demonstrated by ¹H and ⁵¹V NMR investigations, the structures of complexes <b>1</b>–<b>3</b> are consistent with their solid state. The reactions of <b>1b</b>, <b>2</b>, and <b>3</b> with NH₂NMePh in <i>n</i>-hexane afforded the oxidovanadium­(IV) [VO­(L-κ⁴O,N,N,O)] (<b>4</b>, L¹; <b>5</b>, L²; <b>6</b>, L³) and 1,4-dimethyl-1,4-diphenyl-2-tetrazene (Me₂Ph₂N₄) (<b>7</b>) as the main products together with a small amount of hydrazido­(2-) vanadium­(V) [V­(L³-κ⁴O,N,N,O)­(NNMePh)­(OMe)] (<b>8</b>). Proposed reaction course for the oxidation of NH₂NMePh by <b>1b</b>–<b>3</b> is discussed. Compounds <b>4</b>–<b>8</b> were characterized by chemical and physical techniques including the X-ray crystallography for <b>4</b>, <b>7</b>, and <b>8</b>. The solid-state (powder) electron paramagnetic resonance spectra and magnetic features strongly indicate that complexes <b>4</b>–<b>6</b> are formed as a mixture of a mononuclear (<i>S</i> = 1/2) and a dinuclear (<i>S</i> = 1) species

Can Carbamates Undergo Radical Oxidation in the Soil Environment? A Case Study on Carbaryl and Carbofuran

Radical oxidation of carbamate insecticides, namely carbaryl and carbofuran, was investigated with spectroscopic (electron paramagnetic resonance [EPR] and UV–vis) and theoretical (density functional theory [DFT] and ab initio orbital-optimized spin-component scaled MP2 [OO-SCS-MP2]) methods. The two carbamates were subjected to reaction with ^•OH, persistent DPPH^• and galvinoxyl radical, as well as indigenous radicals of humic acids. The influence of fulvic acids on carbamate oxidation was also tested. The results obtained with EPR and UV–vis spectroscopy indicate that carbamates can undergo direct reactions with various radical species, oxidizing themselves into radicals in the process. Hence, they are prone to participate in the prolongation step of the radical chain reactions occurring in the soil environment. Theoretical calculations revealed that from the thermodynamic point of view hydrogen atom transfer is the preferred mechanism in the reactions of the two carbamates with the radicals. The activity of carbofuran was determined experimentally (using pseudo-first-order kinetics) and theoretically to be noticeably higher in comparison with carbaryl and comparable with gallic acid. The findings of this study suggest that the radicals present in soil can play an important role in natural remediation mechanisms of carbamates

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

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

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

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

Hydroxide-Bridged Cubane Complexes of Nickel(II) and Cadmium(II): Magnetic, EPR, and Unusual Dynamic Properties

The reactions of M­(ClO₄)₂·<i>x</i>H₂O (M = Ni­(II) or Cd­(II)) and <i>m</i>-bis­[bis­(1-pyrazolyl)­methyl]­benzene (<b>L</b>_<b>m</b>) in the presence of triethylamine lead to the formation of hydroxide-bridged cubane compounds of the formula [M₄(μ₃-OH)₄(μ-<b>L</b>_<b>m</b>)₂(solvent)₄]­(ClO₄)₄, where solvent = dimethylformamide, water, acetone. In the solid state the metal centers are in an octahedral coordination environment, two sites are occupied by pyrazolyl nitrogens from <b>L</b>_<b>m</b>, three sites are occupied by bridging hydroxides, and one site contains a weakly coordinated solvent molecule. A series of multinuclear, two-dimensional and variable-temperature NMR experiments showed that the cadmium­(II) compound in acetonitrile-<i>d</i>₃ has <i>C</i>₂ symmetry and undergoes an unusual dynamic process at higher temperatures (Δ<i>G</i>_Lm^‡ = 15.8 ± 0.8 kcal/mol at 25 °C) that equilibrates the pyrazolyl rings, the hydroxide hydrogens, and cadmium­(II) centers. The proposed mechanism for this process combines two motions in the semirigid <b>L</b>_<b>m</b> ligand termed the “Columbia Twist and Flip:” twisting of the pyrazolyl rings along the C_pz–C_methine bond and 180° ring flip of the phenylene spacer along the C_Ph–C_methine bond. This dynamic process was also followed using the spin saturation method, as was the exchange of the hydroxide hydrogens with the trace water present in acetonitrile-<i>d</i>₃. The nickel­(II) analogue, as shown by magnetic susceptibility and electron paramagnetic resonance measurements, has an <i>S</i> = 4 ground state, and the nickel­(II) centers are ferromagnetically coupled with strongly nonaxial zero-field splitting parameters. Depending on the Ni–O–Ni angles two types of interactions are observed: <i>J</i>₁ = 9.1 cm^–1 (97.9 to 99.5°) and <i>J</i>₂ = 2.1 cm^–1 (from 100.3 to 101.5°). “Broken symmetry” density functional theory calculations performed on a model of the nickel­(II) compound support these observations

Dinuclear Metallacycles with Single M–O(H)–M Bridges [M = Fe(II), Co(II), Ni(II), Cu(II)]: Effects of Large Bridging Angles on Structure and Antiferromagnetic Superexchange Interactions

The reactions of M­(ClO₄)₂·<i>x</i>H₂O 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>}*) in the presence of triethylamine lead to the formation of monohydroxide-bridged, dinuclear metallacycles of the formula [M₂(μ-OH)­(μ-<b>L</b>_{<i><b>m</b></i>})₂]­(ClO₄)₃ (M = Fe­(II), Co­(II), Cu­(II)) or [M₂(μ-OH)­(μ-<b>L</b>_{<i><b>m</b></i>}*)₂]­(ClO₄)₃ (M = Co­(II), Ni­(II), Cu­(II)). With the exception of the complexes where the ligand is <b>L</b>_{<i><b>m</b></i>} and the metal is copper­(II), all of these complexes have distorted trigonal bipyramidal geometry around the metal centers and unusual linear (<b>L</b>_{<i><b>m</b></i>}*) or nearly linear (<b>L</b>_{<i><b>m</b></i>}) M–O–M angles. For the two solvates of [Cu₂(μ-OH)­(μ-<b>L</b>_{<i><b>m</b></i>})₂]­(ClO₄)₃, the Cu–O–Cu angles are significantly bent and the geometry about the metal is distorted square pyramidal. All of the copper­(II) complexes have structural distortions expected for the pseudo-Jahn–Teller effect. The two cobalt­(II) complexes show moderate antiferromagnetic coupling, −<i>J</i> = 48–56 cm^–1, whereas the copper­(II) complexes show very strong antiferromagnetic coupling, −<i>J</i> = 555–808 cm^–1. The largest coupling is observed for [Cu₂(μ-OH)­(μ-<b>L</b>_{<i><b>m</b></i>}*)₂]­(ClO₄)₃, the complex with a Cu–O–Cu angle of 180°, such that the exchange interaction is transmitted through the d_<i>z</i>² and the oxygen s and p_<i>x</i> orbitals. The interaction decreases, but it is still significant, as the Cu–O–Cu angle decreases and the character of the metal orbital becomes increasingly d_{<i>x</i>²–<i>y</i>²}. These intermediate geometries and magnetic interactions lead to spin Hamiltonian parameters for the copper­(II) complexes in the EPR spectra that have large <i>E</i>/<i>D</i> ratios and one <i>g</i> matrix component very close to 2. Density functional theory calculations were performed using the hybrid B3LYP functional in association with the TZVPP basis set, resulting in reasonable agreement with the experiments