Magnetic Excitations in Metalloporphyrins by Inelastic Neutron Scattering: Determination of Zero-Field Splittings in Iron, Manganese, and Chromium Complexes

Zero field splitting (ZFS) parameters of several nondeuterated metalloporphyrins [M­(TPP)­Cl] and [Mn­(TPP)] (H₂TPP = tetraphenylporphyrin) have been directly determined by inelastic neutron scattering (INS). The ZFS values are the following: <i>D</i> = 6.33(8) cm^–1 for [Fe­(TPP)­Cl], −2.24(3) cm^–1 for [Mn­(TPP)­Cl], 0.79(2) cm^–1 for [Mn­(TPP)], and |<i>D</i>|= 0.234(12) cm^–1 for [Cr­(TPP)­Cl]. The work shows that compounds with magnetic excitations below ∼30 cm^–1 could be determined using nondeuterated samples

Syntheses and Characterization of Tantalum Alkyl Imides and Amide Imides. DFT Studies of Unusual α‑SiMe₃ Abstraction by an Amide Ligand

Reaction of TaCl₂(NSiMe₃)­[N­(SiMe₃)₂] (<b>1</b>) with alkylating reagents form the alkyl amide imide complexes TaR₂(NSiMe₃)­[N­(SiMe₃)₂] (R = Me (<b>2</b>), CH₂Ph (<b>3</b>)) and mixed amide imide compounds Ta­(NR′₂)₂(NSiMe₃)­[N­(SiMe₃)₂] (R′ = Me (<b>4</b>), Et (<b>5</b>)). The reaction of <b>2</b> and 0.5 equiv of O₂ leads to preferential oxygen insertion into one Ta–Me bond, yielding the alkoxy-bridged alkyl dimer Ta₂(μ-OMe)₂Me₂(NSiMe₃)₂[N­(SiMe₃)₂]₂ (<b>6</b>) as cis and trans isomers. Crystallization of the <i><b>cis</b></i><b>-6</b> and <i><b>trans</b></i><b>-6</b> mixture gave only crystals of <i><b>trans</b></i><b>-6</b>. When the crystals of <i><b>trans</b></i><b>-6</b> were dissolved in benzene-<i>d</i>₆, conversion of <i><b>trans</b></i><b>-6</b> to <i><b>cis</b></i><b>-6</b> occurred until the <i><b>trans</b></i><b>-6</b> ⇌ <i><b>cis</b></i><b>-6</b> equilibrium was reached with <i>K</i>_eq = 0.79(0.02) at 25.0(0.1) °C. Kinetic studies of the exchange gave the rate constants <i>k</i> = 0.018(0.002) min^–1 for the <i><b>trans</b></i><b>-6</b> → <i><b>cis</b></i><b>-6</b> conversion and <i>k</i>′ = 0.022(0.002) min^–1 for the reverse <i><b>cis</b></i><b>-6</b> → <i><b>trans</b></i><b>-6</b> conversion at 25.0(0.1) °C. Complex <b>6</b> reacts with additional O₂, forming the dialkoxy dimer Ta₂(μ-OMe)₂(OMe)₂(NSiMe₃)₂[N­(SiMe₃)₂]₂ (<b>7</b>) as cis and trans isomers. Solid-state structures of <b>3</b> and <i><b>trans</b></i><b>-6</b> have been determined by X-ray diffraction analyses. The mixed amide imide compounds Ta­(NR′₂)₂(NSiMe₃)­[N­(SiMe₃)₂] (R′ = Me (<b>4</b>), Et (<b>5</b>)) have also been prepared by salt metathesis reactions employing TaCl₃[N­(SiMe₃)₂]₂ (<b>8</b>). The pathway from <b>8</b> to <b>4</b> and <b>5</b> eliminates Me₃Si–NR′₂ (R′ = Me, Et), converting the amide N­(SiMe₃)₂ ligand to the imide NSiMe₃ ligand. Such intramolecular imidation is rare. The mechanism of this process has been computationally probed, and α-elimination involving the mixed amide species TaCl₂(NMe₂)­[N­(SiMe₃)₂]₂ (<b>9</b>) is discussed. Diffusion-ordered spectroscopy (DOSY) studies of <b>1</b>–<b>6</b> and <b>8</b> show that only the alkoxy-bridged <i><b>cis-</b></i><b>6</b> and <i><b>trans</b></i><b>-6</b> are dimers in benzene-<i>d</i>₆ solution at 25 °C

Magnetic Transitions in Iron Porphyrin Halides by Inelastic Neutron Scattering and Ab Initio Studies of Zero-Field Splittings

Zero-field splitting (ZFS) parameters of nondeuterated metalloporphyrins [Fe­(TPP)­X] (X = F, Br, I; H₂TPP = tetraphenylporphyrin) have been directly determined by inelastic neutron scattering (INS). The ZFS values are <i>D</i> = 4.49(9) cm^–1 for tetragonal polycrystalline [Fe­(TPP)­F], and <i>D</i> = 8.8(2) cm^–1, <i>E</i> = 0.1(2) cm^–1 and <i>D</i> = 13.4(6) cm^–1, <i>E</i> = 0.3(6) cm^–1 for monoclinic polycrystalline [Fe­(TPP)­Br] and [Fe­(TPP)­I], respectively. Along with our recent report of the ZFS value of <i>D</i> = 6.33(8) cm^–1 for tetragonal polycrystalline [Fe­(TPP)­Cl], these data provide a rare, complete determination of ZFS parameters in a metalloporphyrin halide series. The electronic structure of [Fe­(TPP)­X] (X = F, Cl, Br, I) has been studied by multireference ab initio methods: the complete active space self-consistent field (CASSCF) and the N-electron valence perturbation theory (NEVPT2) with the aim of exploring the origin of the large and positive zero-field splitting <i>D</i> of the ⁶A₁ ground state. <i>D</i> was calculated from wave functions of the electronic multiplets spanned by the d⁵ configuration of Fe­(III) along with spin–orbit coupling accounted for by quasi degenerate perturbation theory. Results reproduce trends of <i>D</i> from inelastic neutron scattering data increasing in the order from F, Cl, Br, to I. A mapping of energy eigenvalues and eigenfunctions of the <i>S</i> = 3/2 excited states on ligand field theory was used to characterize the σ- and π-antibonding effects decreasing from F to I. This is in agreement with similar results deduced from ab initio calculations on CrX₆^3– complexes and also with the spectrochemical series showing a decrease of the ligand field in the same directions. A correlation is found between the increase of <i>D</i> and decrease of the π- and σ-antibonding energies <i>e</i>_λ^X (λ = σ, π) in the series from X = F to I. Analysis of this correlation using second-order perturbation theory expressions in terms of angular overlap parameters rationalizes the experimentally deduced trend. <i>D</i> parameters from CASSCF and NEVPT2 results have been calibrated against those from the INS data, yielding a predictive power of these approaches. Methods to improve the quantitative agreement between ab initio calculated and experimental <i>D</i> and spectroscopic transitions for high-spin Fe­(III) complexes are proposed

Slow Magnetic Relaxations in Cobalt(II) Tetranitrate Complexes. Studies of Magnetic Anisotropy by Inelastic Neutron Scattering and High-Frequency and High-Field EPR Spectroscopy

Three mononuclear cobalt­(II) tetranitrate complexes (A)₂[Co­(NO₃)₄] with different countercations, Ph₄P⁺ (<b>1</b>), MePh₃P⁺ (<b>2</b>), and Ph₄As⁺ (<b>3</b>), have been synthesized and studied by X-ray single-crystal diffraction, magnetic measurements, inelastic neutron scattering (INS), high-frequency and high-field EPR (HF-EPR) spectroscopy, and theoretical calculations. The X-ray diffraction studies reveal that the structure of the tetranitrate cobalt anion varies with the countercation. <b>1</b> and <b>2</b> exhibit highly irregular seven-coordinate geometries, while the central Co­(II) ion of <b>3</b> is in a distorted-dodecahedral configuration. The sole magnetic transition observed in the INS spectroscopy of <b>1</b>–<b>3</b> corresponds to the zero-field splitting (2­(<i>D</i>² + 3<i>E</i>²)^1/2) from 22.5(2) cm^–1 in <b>1</b> to 26.6(3) cm^–1 in <b>2</b> and 11.1(5) cm^–1 in <b>3</b>. The positive sign of the <i>D</i> value, and hence the easy-plane magnetic anisotropy, was demonstrated for <b>1</b> by INS studies under magnetic fields and HF-EPR spectroscopy. The combined analyses of INS and HF-EPR data yield the <i>D</i> values as +10.90(3), +12.74(3), and +4.50(3) cm^–1 for <b>1</b>–<b>3</b>, respectively. Frequency- and temperature-dependent alternating-current magnetic susceptibility measurements reveal the slow magnetization relaxation in <b>1</b> and <b>2</b> at an applied dc field of 600 Oe, which is a characteristic of field-induced single-molecule magnets (SMMs). The electronic structures and the origin of magnetic anisotropy of <b>1</b>–<b>3</b> were revealed by calculations at the CASPT2/NEVPT2 level

Slow Magnetic Relaxations in Cobalt(II) Tetranitrate Complexes. Studies of Magnetic Anisotropy by Inelastic Neutron Scattering and High-Frequency and High-Field EPR Spectroscopy

Three mononuclear cobalt­(II) tetranitrate complexes (A)₂[Co­(NO₃)₄] with different countercations, Ph₄P⁺ (<b>1</b>), MePh₃P⁺ (<b>2</b>), and Ph₄As⁺ (<b>3</b>), have been synthesized and studied by X-ray single-crystal diffraction, magnetic measurements, inelastic neutron scattering (INS), high-frequency and high-field EPR (HF-EPR) spectroscopy, and theoretical calculations. The X-ray diffraction studies reveal that the structure of the tetranitrate cobalt anion varies with the countercation. <b>1</b> and <b>2</b> exhibit highly irregular seven-coordinate geometries, while the central Co­(II) ion of <b>3</b> is in a distorted-dodecahedral configuration. The sole magnetic transition observed in the INS spectroscopy of <b>1</b>–<b>3</b> corresponds to the zero-field splitting (2­(<i>D</i>² + 3<i>E</i>²)^1/2) from 22.5(2) cm^–1 in <b>1</b> to 26.6(3) cm^–1 in <b>2</b> and 11.1(5) cm^–1 in <b>3</b>. The positive sign of the <i>D</i> value, and hence the easy-plane magnetic anisotropy, was demonstrated for <b>1</b> by INS studies under magnetic fields and HF-EPR spectroscopy. The combined analyses of INS and HF-EPR data yield the <i>D</i> values as +10.90(3), +12.74(3), and +4.50(3) cm^–1 for <b>1</b>–<b>3</b>, respectively. Frequency- and temperature-dependent alternating-current magnetic susceptibility measurements reveal the slow magnetization relaxation in <b>1</b> and <b>2</b> at an applied dc field of 600 Oe, which is a characteristic of field-induced single-molecule magnets (SMMs). The electronic structures and the origin of magnetic anisotropy of <b>1</b>–<b>3</b> were revealed by calculations at the CASPT2/NEVPT2 level

Slow Magnetic Relaxations in Cobalt(II) Tetranitrate Complexes. Studies of Magnetic Anisotropy by Inelastic Neutron Scattering and High-Frequency and High-Field EPR Spectroscopy

Three mononuclear cobalt­(II) tetranitrate complexes (A)₂[Co­(NO₃)₄] with different countercations, Ph₄P⁺ (<b>1</b>), MePh₃P⁺ (<b>2</b>), and Ph₄As⁺ (<b>3</b>), have been synthesized and studied by X-ray single-crystal diffraction, magnetic measurements, inelastic neutron scattering (INS), high-frequency and high-field EPR (HF-EPR) spectroscopy, and theoretical calculations. The X-ray diffraction studies reveal that the structure of the tetranitrate cobalt anion varies with the countercation. <b>1</b> and <b>2</b> exhibit highly irregular seven-coordinate geometries, while the central Co­(II) ion of <b>3</b> is in a distorted-dodecahedral configuration. The sole magnetic transition observed in the INS spectroscopy of <b>1</b>–<b>3</b> corresponds to the zero-field splitting (2­(<i>D</i>² + 3<i>E</i>²)^1/2) from 22.5(2) cm^–1 in <b>1</b> to 26.6(3) cm^–1 in <b>2</b> and 11.1(5) cm^–1 in <b>3</b>. The positive sign of the <i>D</i> value, and hence the easy-plane magnetic anisotropy, was demonstrated for <b>1</b> by INS studies under magnetic fields and HF-EPR spectroscopy. The combined analyses of INS and HF-EPR data yield the <i>D</i> values as +10.90(3), +12.74(3), and +4.50(3) cm^–1 for <b>1</b>–<b>3</b>, respectively. Frequency- and temperature-dependent alternating-current magnetic susceptibility measurements reveal the slow magnetization relaxation in <b>1</b> and <b>2</b> at an applied dc field of 600 Oe, which is a characteristic of field-induced single-molecule magnets (SMMs). The electronic structures and the origin of magnetic anisotropy of <b>1</b>–<b>3</b> were revealed by calculations at the CASPT2/NEVPT2 level

Slow Magnetic Relaxations in Cobalt(II) Tetranitrate Complexes. Studies of Magnetic Anisotropy by Inelastic Neutron Scattering and High-Frequency and High-Field EPR Spectroscopy

Three mononuclear cobalt­(II) tetranitrate complexes (A)₂[Co­(NO₃)₄] with different countercations, Ph₄P⁺ (<b>1</b>), MePh₃P⁺ (<b>2</b>), and Ph₄As⁺ (<b>3</b>), have been synthesized and studied by X-ray single-crystal diffraction, magnetic measurements, inelastic neutron scattering (INS), high-frequency and high-field EPR (HF-EPR) spectroscopy, and theoretical calculations. The X-ray diffraction studies reveal that the structure of the tetranitrate cobalt anion varies with the countercation. <b>1</b> and <b>2</b> exhibit highly irregular seven-coordinate geometries, while the central Co­(II) ion of <b>3</b> is in a distorted-dodecahedral configuration. The sole magnetic transition observed in the INS spectroscopy of <b>1</b>–<b>3</b> corresponds to the zero-field splitting (2­(<i>D</i>² + 3<i>E</i>²)^1/2) from 22.5(2) cm^–1 in <b>1</b> to 26.6(3) cm^–1 in <b>2</b> and 11.1(5) cm^–1 in <b>3</b>. The positive sign of the <i>D</i> value, and hence the easy-plane magnetic anisotropy, was demonstrated for <b>1</b> by INS studies under magnetic fields and HF-EPR spectroscopy. The combined analyses of INS and HF-EPR data yield the <i>D</i> values as +10.90(3), +12.74(3), and +4.50(3) cm^–1 for <b>1</b>–<b>3</b>, respectively. Frequency- and temperature-dependent alternating-current magnetic susceptibility measurements reveal the slow magnetization relaxation in <b>1</b> and <b>2</b> at an applied dc field of 600 Oe, which is a characteristic of field-induced single-molecule magnets (SMMs). The electronic structures and the origin of magnetic anisotropy of <b>1</b>–<b>3</b> were revealed by calculations at the CASPT2/NEVPT2 level