Trigonal Ligand Field Research Articles

Theoretical Study of Electron Paramagnetic Resonance Spectra and Local Lattice Distortion for Mn2+ in Zn(ClO4)2·6(H2O)Mg(ClO4)2·6(H2O)

The electron paramagnetic resonance (EPR) spectra of trigonal Mn2+ centers in Zn(ClO4)2·6(H2O) and Mg(ClO4)2·6(H2O) crystals were studied on the basis of the complete energy matrices for a d5 configuration ion in a trigonal ligand field. It was demonstrated that the local lattice structure around a trigonal Mn2+ center has an compressed distortion along the crystalline c3 axis, and when Mn2+ is doped in the Zn(ClO4)2·6(H2O) and Mg(ClO4)2·6(H2O) crystals, there is a similar local distortion. From the EPR calculation, the local lattice structure parameters R=2.183 2 A, for Zn(ClO4)2·6(H2O), R=2.130 2 A, for Mg(ClO4)2·6(H2O) have been determined.

EPR investigation of local structure for the [Mn(H2O)6]2+cluster in M(ClO4)2· 6H2O:Mn2+(M = Cd, Hg) systems at different temperatures

Local lattice structure distortion studies of the octahedral Mn 2+ center in M(ClO 4 ) 2 · 6H 2 O:Mn 2+ (M = Cd, Hg) systems have been performed systematically based on the diagonalization of the complete energy matrices of electron-electron repulsion, spin-orbit coupling and trigonal ligand-field interaction for a d 5 configuration ion in a trigonal ligand field. From the EPR calculation, the local structure distortion parameters R and θ are determined, respectively. Results show that the local lattice structure around a trigonal Mn 2+ center has a compressed distortion along the crystalline C 3 axis. The compression distortion may be ascribed to the fact that the radius of the Mn 2+ ion is smaller than that of the Hg 2+ ion and the Cd 2+ ion. The local lattice structure parameters for Mn 2+ in M(ClO 4 ) 2 · 6H 2 O:Mn 2+ (M = Cd, Hg) systems are determined at liquid-nitrogen temperature and room temperature.

A Terminal N2 Complex of High-Spin Iron(I) in a Weak, Trigonal Ligand Field.

The role of Fe in biological and industrial N2 fixation has inspired the intense study of small molecule analogues of Fe-(NxHy) intermediates of potential relevance to these processes. Although a number of low-coordinate Fe-(N2) featuring varying degrees of fidelity to the nitrogenase active site are now known, these complexes frequently feature strongly donating ligands that either enforce low- or intermediate-spin states or result in linear Fe-(N2)-Fe bridging motifs. Given that the nitrogenase active site uses weak-field sulfide ligands to stabilize its reactive Fe center(s), N2 binding to high-spin Fe is of great interest. Herein, we report the synthesis and characterization of the first terminal N2 complex of high-spin (S = 3/2) Fe(I) as well as a bridging Fe-(N2)-Fe analogue. Electron paramagnetic resonance and solution magnetic moment determination confirm the high-spin state, and vibrational experiments indicate a substantial degree of activation of the N≡N bond in these complexes. Density functional theory calculations reveal an electronic structure for the terminal adduct featuring substantial delocalization of unpaired spin onto the N2 ligand.

Ambient Stable Trigonal Bipyramidal Copper(III) Complexes Equipped with an Exchangeable Axial Ligand.

A stable trigonal bipyramidal copper(III) complex, [PPN][Cu((TMS)PS3)Cl] (1, wherein PPN represents bis(triphenylphosphine)iminium), was synthesized from CuCl2/PPNCl via intramolecular copper(II) disproportionation. Under ambient conditions, the axial chloride of 1 is exchangeable in solution thus making 1 serve as an intermediate to prepare trigonal bipyramidal copper(III) derivatives, e.g., [PPN][Cu((TMS)PS3)(N3)] (2) and [Cu((TMS)PS3)(DABCO)] (3). Diamagnetic complexes 1-3 were fully characterized by X-ray crystallography, NMR, UV-vis, and Cu K-edge absorption spectroscopy. A series of UV-vis titrations were performed to investigate the relative ligand affinity toward the [Cu((TMS)PS3)] moiety, verifying the 1:1 binding equilibrium between various ligands. Compared to known copper(III) compounds, Cu K-edge absorptions of 1-3 possess lower pre-edge energy and higher shakedown transition energy, which, respectively, attribute to the electron donation from (TMS)PS3(3-) ligand and their trigonal ligand field.

Ti3+-, V2+/3+-, Cr2+/3+-, Mn2+-, and Fe2+-Substituted MOF-5 and Redox Reactivity in Cr- and Fe-MOF-5

The metal nodes in metal-organic frameworks (MOFs) are known to act as Lewis acid catalysts, but few reports have explored their ability to mediate reactions that require electron transfer. The unique chemical environments at the nodes should facilitate unusual redox chemistry, but the difficulty in synthesizing MOFs with metal ions in reduced oxidation states has precluded such studies. Herein, we demonstrate that MZn3O(O2C-)6 clusters from Zn4O(1,4-benzenedicarboxylate)3 (MOF-5) serve as hosts for V(2+) and Ti(3+) ions and enable the synthesis of the first MOFs containing these reduced early metal ions, which can be accessed from MOF-5 by postsynthetic ion metathesis (PSIM). Additional MOF-5 analogues featuring Cr(2+), Cr(3+), Mn(2+), and Fe(2+) at the metal nodes can be obtained by similar postsynthetic methods and are reported here for the first time. The inserted metal ions are coordinated within an unusual all-oxygen trigonal ligand field and are accessible to both inner- and outer-sphere oxidants: Cr(2+)- converts into Cr(3+)-substituted MOF-5, while Fe(2+)-MOF-5 activates NO to produce an unusual Fe-nitrosyl complex.

Studies of EPR spectra and defect structure for Er3+ ions in BaF2 and SrF2 crystals

The local lattice structure and electron paramagnetic resonance (EPR) spectra have been studied systematically by diagonalizing 364×364 complete energy matrices for a f11 ion in a trigonal ligand-field. By simulating the calculated Stark levels and EPR parameters to the experimental results, the shift parameters are determined for Er3+ ions in BaF2 and SrF2. The results show that the trigonal center is attributed to an interstitial F− ion located at the [111] axis of the cube, and the nearest ligand close to the charge compensator has a displacement towards central ion by 0.042Å for L center in BaF2:Er3+ and 0.026Å for J center in SrF2:Er3+, respectively. Moreover, the relationships between g-factors and shift parameter ΔZ as well as orbit reduction factor k′ are discussed.

Modelling the circularly and linearly polarised spectrum of [Ni(en)3]2+

The relative intensity and band shapes of the low energy spin-allowed transitions in the linearly polarised and circular dichroism spectrum of [Ni(en)(3)](2+) have been calculated using a time-dependent density functional theory approach. The effect of the trigonal ligand-field is minimal and no splitting of the bands is predicted by the simulations or observed experimentally. The 'd-d' transitions of the [Ni(en)(3)](2+) ion are electric dipole allowed but gain much of their intensity through Herzberg-Teller vibronic coupling. Its CD spectrum is dominated by the low energy band, which gains its rotatory strength through the magnetic dipole-allowed character of the parent octahedral transition and the electric dipole character due to the trigonal field. The simulation of the spectrum incorporates the contribution from all inducing vibrational modes with significant involvement of the {NiN(6)} unit. Vibrations which are centred on the chelate rings are not important in generating intensity, reflecting the localised d-d' character of the transitions. Simulated linearly polarised and circular dichroism spectra of such an open-shell system are presented for the first time and predict the essential elements of the experimental spectra.

Detailed Ab Initio First-Principles Study of the Magnetic Anisotropy in a Family of Trigonal Pyramidal Iron(II) Pyrrolide Complexes

A theoretical, computational, and conceptual framework for the interpretation and prediction of the magnetic anisotropy of transition metal complexes with orbitally degenerate or orbitally nearly degenerate ground states is explored. The treatment is based on complete active space self-consistent field (CASSCF) wave functions in conjunction with N-electron valence perturbation theory (NEVPT2) and quasidegenerate perturbation theory (QDPT) for treatment of magnetic field- and spin-dependent relativistic effects. The methodology is applied to a series of Fe(II) complexes in ligand fields of almost trigonal pyramidal symmetry as provided by several variants of the tris-pyrrolylmethyl amine ligand (tpa). These systems have recently attracted much attention as mononuclear single-molecule magnet (SMM) complexes. This study aims to establish how the ligand field can be fine tuned in order to maximize the magnetic anisotropy barrier. In trigonal ligand fields high-spin Fe(II) complexes adopt an orbitally degenerate (5)E ground state with strong in-state spin-orbit coupling (SOC). We study the competing effects of SOC and the (5)E⊗ε multimode Jahn-Teller effect as a function of the peripheral substituents on the tpa ligand. These subtle distortions were found to have a significant effect on the magnetic anisotropy. Using a rigorous treatment of all spin multiplets arising from the triplet and quintet states in the d(6) configuration the parameters of the effective spin-Hamiltonian (SH) approach were predicted from first principles. Being based on a nonperturbative approach we investigate under which conditions the SH approach is valid and what terms need to be retained. It is demonstrated that already tiny geometric distortions observed in the crystal structures of four structurally and magnetically well-documented systems, reported recently, i.e., [Fe(tpa(R))](-) (R = tert-butyl, Tbu (1), mesityl, Mes (2), phenyl, Ph (3), and 2,6-difluorophenyl, Dfp (4), are enough to lead to five lowest and thermally accessible spin sublevels described sufficiently well by S = 2 SH provided that it is extended with one fourth order anisotropy term. Using this most elementary parametrization that is consistent with the actual physics, the reported magnetization data for the target systems were reinterpreted and found to be in good agreement with the ab initio results. The multiplet energies from the ab initio calculations have been fitted with remarkable consistency using a ligand field (angular overlap) model (ab initio ligand field, AILFT). This allows for determination of bonding parameters and quantitatively demonstrates the correlation between increasingly negative D values and changes in the σ-bond strength induced by the peripheral ligands. In fact, the sigma-bonding capacity (and hence the Lewis basicity) of the ligand decreases along the series 1 > 2 > 3 > 4.

Analysis of Ground-State Zero-Field Splitting for Mn2+ in ZnNbOF5·6(H2O) and CoNbOF5·6(H2O)

The electron paramagnetic resonance spectra of trigonal Mn2+ centers in ZnNbOF5·6(H2O) and CoNbOF5·6(H2O) crystals are studied on the basis of the complete energy matrices for a d5 configuration ion in a trigonal ligand field. It is demonstrated that the local lattice structure around a trigonal Mn2+ center has an elongation distortion along the crystalline c3 axis, and when Mn2+ is doped in the ZnNbOF5·6(H2O) and CoNbOF5·6(H2O) crystals, there is a similar local distortion. From the EPR calculation, the local lattice structure parameters for trigonal Mn2+ centers in ZnNbOF5·6(H2O) and CoNbOF5·6(H2O) are determined.

EPR parameters and local lattice structure study of V 2+ ions in CdCl 2 and CsMgCl 3 crystals

The local lattice structure and EPR parameters ( D, g ‖, g ⊥ ) have been studied systematically on the basis of the complete energy matrix for a d 3 configuration ion in a trigonal ligand field. By simulating the calculated optical and EPR spectra data to the experimental results, the local distortion parameters (Δ R, Δ θ) are determined for V 2+ ions in CdCl 2 and CsMgCl 3 crystals, respectively. The results show that the local lattice structure of CdCl 2:V 2+ system exhibits a compression distortion (Δ R=−0.0868 Å) while that of CsMgCl 3:V 2+ system exists an elongation distortion (Δ R=0.0165 Å). The different distortion may be ascribed to the fact that the radius of V 2+ ion is smaller than that of Cd 2+ ion or larger than that of Mg 2+ ion. Moreover, the relationships between EPR parameter D and local structure parameters ( R, θ) as well as the orbital reduction factor k and g ‐factors (g ‖, g ⊥ ) are discussed.

Role of Orbital Degeneracy in the Single Molecule Magnet Behavior of a Mononuclear High-Spin Fe(II) Complex

To explain the single-molecule magnet behavior of the mononuclear complex [(tpaMes)Fe](-) we have developed a model that takes into account the trigonal ligand field splitting of the atomic (5)D term of the Fe(II) ion, and the spin-orbital splitting and mixing of the ligand field terms. The ground ligand field term is shown to be the orbital doublet (5)E possessing an unquenched orbital angular momentum. We demonstrate that the splitting of this term cannot be described by the conventional zero-field splitting Hamiltonian proving thus the irrelevance of the spin-Hamiltonian formalism in the present case. The first-order orbital angular momentum is shown to lead to the strong magnetic anisotropy with the trigonal axis being the easy axis of the magnetization.

Theoretical investigations of the local structure distortion and relationship between the EPR parameter and spin-orbit coupling coefficients for CsCdX3:Ni2+ (X = Cl, Br) systems

The EPR zero-field splitting parameters and structural distortion of Ni2+-doped CsCdX3 (X = Cl, Br) systems have been studied on the basic of 45f × 45 complete energy matrices for a configuration ion in trigonal ligand field, in which the contributions from the spin-orbit coupling coefficients of the central ions and ligands are taken into account simultaneously. It is shown that the local structure exhibits compression distortions for CsCdX3 (X = Cl, Br):Ni2+ systems, and the contribution from the s.o. coupling coefficient of ligands to the covalent effect should not be neglected. Simultaneously, the relationship between the EPR parameter D and the spin-orbit coupling coefficients (ζ, ) as well as the average parameter ζ1() and the divergent parameter ζ2() have been discussed.

EPR studies for [Mn(H 2O) 6] 2+ complex in MSnF 6·6H 2O:Mn 2+ (M = Zn, Co) and Cd(BF 4) 2·6H 2O:Mn 2+ systems at different temperature

The relationship between the local structure and EPR spectrum is studied by diagonalizing the complete energy matrices for Mn 2+ in a trigonal ligand-field. The results show that the degree of distortion increases with temperature increasing whether it is compressed or elongate distortion, and the distorted tendency of the local structure is different according to the sign of D. Meanwhile, the distortion of Cd(BF 4) 2·6H 2O:Mn 2+ system is more sensitive to temperature than that of ZnSnF 6·6H 2O:Mn 2+ system. Moreover, the EPR parameter D is closely associated with the local structure parameters R and θ while ( a − F) is mainly concerned with R.

Theoretical study of local structure and ground-state splitting of Cs2NaMF6(M=Al, Ga):Cr3+ complex molecule systems

Based on the two spin orbit coupling parameter model and the complete energy matrices for a d3 configuration ion in a trigonal ligand field, the local structure and the ground-state splitting of Cr3+ ion in Cs2NaMF6(M=Al, Ga):Cr3+ systems have been studied. By simulating the EPR parameter D and the optical spectra, the distorted angles of two inequivalent sites formed by Cr3+ replacing M3+ are determined and the obvious difference of the distorted angles Δθ calculated by using two spin orbit coupling parameter model and single spin orbit coupling parameter model is found. Our results indicate that the influence of ligand F- can not be ignored with respect to spin orbit coupling mechanism when the energy level fine structure and local structure distortion of fluoride complex molecules are studied. Meanwhile, the dependence of EPR parameter D on angle θ, average parameter ζ1 and divergent parameter ζ2 is discussed, respectively.

Theoretical studies of the optical and EPR spectra for Fe3+ ion in the MF3:Fe3+ (M=Al, Ga) systems

By analyzing the EPR spectra of Fe 3+ ion in the fluorinde glasses, the local lattice structures around impurity Fe 3+ ion in MF 3 :Fe 3+ (M=Al, Ga) systems have been studied by means of diagonalizing the complete energy matrices of the electron–electron repulsion, the ligand-field and the spin–orbit coupling for a d 5 configuration ion in a trigonal ligand-field. Both the second-order and fourth-order EPR parameters D and ( a − F ) are taken simultaneously in the structural investigation. The results indicate that the local lattice structure around octahedral Fe 3+ center has an expansion distortion for Fe 3+ in MF 3 :Fe 3+ (M=Al, Ga). The expansion distortion may be ascribed to the fact that the radius of Fe 3+ ion is larger than that of Al 3+ ion and Ga 3+ ion, and the Fe 3+ ion will push the fluoride ligands upwards and downwards, respectively. The local lattice structure parameters R= 1.927 A, θ =55.538° for Fe 3+ in AlF 3 :Fe 3+ and R =1.931 A, θ =56.09° for Fe 3+ in GaF 3 :Fe 3+ are determined, respectively, and the EPR spectra of the MF 3 :Fe 3+ (M=Al, Ga) systems are satisfactorily explained.

Determination of the local molecular structure of metal perchlorate complex from the electron paramagnetic resonance spectra for a substitution of Mn2+ ion: a complete energy matrices study

AbstractA simple theoretical method is introduced for studying the local molecular structure of the (MnO6)10– coordination complex. By diagonalizing the complete energy matrices of the electron–electron repulsion, the ligand field and the spin–orbit coupling for a d5 configuration ion in a trigonal ligand field, the local distortion structure of the (MnO6)10– coordination complex for Mn2+ ions in A–PCC:Mn2+ (PCC = {[(CH3)2N]2OPOPO[N(CH3)2]2}(ClO4)2, A = Mg, Zn) complex systems is investigated. Both the second‐order zero‐field splitting parameter b20 and the fourth‐order zero‐field splitting parameter b40 are taken simultaneously in the structural investigation. From electron paramagnetic resonance (EPR) calculations, the local structure parameters R = 2.18 Å, θ = 56.38° for Mn2+ ions in Mg–PCC:Mn2+ and R = 2.20 Å, θ = 56.52° for Mn2+ ions in Zn–PCC:Mn2+ complex systems are determined, respectively. It is found that the theoretical calculation results of the EPR spectra for Mn2+ ions in A–PCC:Mn2+ complex systems are in good agreement with the experimental values. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural, spectral characterization and electron paramagnetic resonance studies of Ni2+ ions in various compounds: KZnF3, CdCl2, CdBr2 and CsMgI3

The electron paramagnetic resonance (EPR) parameters (zero-field splitting D, and g-factors g ∥, g ⊥) for d 8 Ni2+ ions in tetragonal ligand fields KZnF3 : Ni2+ or in trigonal ligand fields CdCl2(CdBr2 and CsMgI3) : Ni2+ complexes have been systematically studied on the basis of the 45 × 45 complete energy matrices. The local structural distortion parameters of the crystals obtained are ΔR 1 = 0.0355 Å, ΔR 2 = 0.0296 Å for KZnF3 : Ni2+, ΔR = −0.1730 Å, Δθ = −1.5695○ for CdCl2 : Ni2+, Å, Δθ = −2.6236○ for CdBr2 : Ni2+ and ΔR = −0.0267 Å, Δθ = −1.8967○ for CsMgI3 : Ni2+. The calculated results show that the local structures exhibit elongation distortion for KZnF3 : Ni2+ and compression distortions for Ni2+ : CdCl2 (CdBr2 and CsMgI3) systems. Further, the relationships between ZFS parameter D and structural parameters (θ, R) are studied and the influence of the orbit reduction factor k on g-factors (Δg, g) are discussed.

Theoretical study of spin-singlet contributions to zero-field splitting of a 3d6 ion in a trigonal ligand field and applications to Fe2+ in FeSiF6 · 6H2O and FeCO3

The complete energy matrix (210 × 210) for d6(d4) configuration ions in a trigonal ligand field is constructed. The energy levels and zero-field splitting parameters in ferrous fluosilicate and ferrous carbonate are studied. The contributions of spin singlets to zero-field splitting parameters in a trigonal ligand field are investigated for the first time. The calculation results indicate that the spin-singlet contribution to second-order zero-field splitting parameter D is negligible, but the contributions to fourth-order zero-field splitting parameter a − F cannot be neglected. Moreover, it is found that the stronger the trigonal ligand field, the larger are the spin-singlet contributions to the zero-field splitting parameters.

Local defect structures and EPR studies on (FeO6)9− and (FeO4)5− clusters in YGG: Fe3+ system

The octahedral (FeO6)9− and tetrahedral (FeO4)5− clusters in yttrium gallium garnet (YGG): Fe3+ system are investigated based on the 252 × 252 complete energy matrices for d5 configuration ions in trigonal and tetragonal ligand fields, moreover, the EPR and optical spectra are made unified calculation. The results indicate that the defect structures around Fe3+ centres display expansion effects at different temperatures 4.2 and 295 K, and which are close to those in YIG garnet, respectively. Simultaneously, the defect structure parameters for Fe3+ centres in YGG are determined, and the relationship between the defect structure and the temperature has been discussed.

Effect of temperature on the local angular thermal expansion and local structure for nickel-doped zinc fluosilicate

A theoretical method for studying the local properties about Ni 2+ ion in a trigonal ligand-field has been proposed by diagonalizing the complete energy matrix and considering the electron paramagnetic resonance (EPR) parameters D and g. For nickel-doped ZnSiF 6 · 6H 2O and ZnSiF 6 · 6D 2O systems, the local lattice distortion and local angular thermal expansion coefficient have been investigated at various temperatures. The calculated results indicate that the local structure around octahedral Ni 2+ centre has a compression distortion in the two systems. Moreover, the temperature dependence of the local angular thermal expansion coefficient has also been analysed.