Variable temperature inelastic neutron scattering study of chromium(II) Tutton salt: manifestation of the 5E⊗e Jahn–Teller effect

Electronic Structure and Magnetic Properties of a Low-Spin CrII Complex: trans-[CrCl2(dmpe)2] (dmpe = 1,2-Bis(dimethylphosphino)ethane)

Octahedral coordination complexes of the general formula trans-[MX₂(R₂ECH₂CH₂ER₂)₂] (M^II = Ti, V, Cr, Mn; E = N, P; R = alkyl, aryl) are a cornerstone of both coordination and organometallic chemistry, and many of these complexes are known to have unique electronic structures that have been incompletely examined. The trans-[CrCl₂(dmpe)₂] complex (dmpe = Me₂PCH₂CH₂PMe₂), originally reported by Girolami and co-workers in 1985, is a rare example of a six-coordinate d⁴ system with an S = 1 (spin triplet) ground state, as opposed to the high-spin (S = 2, spin quintet) state. The ground-state properties of S = 1 systems are challenging to study using conventional spectroscopic methods, and consequently, the electronic structure of trans-[CrCl₂(dmpe)₂] has remained largely unexplored. In this present work, we have employed high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy to characterize the ground-state electronic structure of trans-[CrCl₂(dmpe)₂]. This analysis yielded a complete set of spin Hamiltonian parameters for this S = 1 complex: D = +7.39(1) cm^-1, E = +0.093(1) (E/D = 0.012), and g = [1.999(5), 2.00(1), 2.00(1)]. To develop a detailed electronic structure description for trans-[CrCl₂(dmpe)₂], we employed both classical ligand-field theory and quantum chemical theory (QCT) calculations, which considered all quintet, triplet, and singlet ligand-field states. While the high density of states suggests an unexpectedly complex electronic structure for this "simple" coordination complex, both the ligand-field and QCT methods were able to reproduce the experimental spin Hamiltonian parameters quite nicely. The QCT computations were also used as a basis for assigning the electronic absorption spectrum of trans-[CrCl₂(dmpe)₂] in toluene.

Spin-phonon couplings in transition metal complexes with slow magnetic relaxation

Spin–phonon coupling plays an important role in single-molecule magnets and molecular qubits. However, there have been few detailed studies of its nature. Here, we show for the first time distinct couplings of g phonons of Co^II(acac)₂(H₂O)₂ (acac = acetylacetonate) and its deuterated analogs with zero-field-split, excited magnetic/spin levels (Kramers doublet (KD)) of the S = 3/2 electronic ground state. The couplings are observed as avoided crossings in magnetic-field-dependent Raman spectra with coupling constants of 1–2 cm⁻¹. Far-IR spectra reveal the magnetic-dipole-allowed, inter-KD transition, shifting to higher energy with increasing field. Density functional theory calculations are used to rationalize energies and symmetries of the phonons. A vibronic coupling model, supported by electronic structure calculations, is proposed to rationalize the behavior of the coupled Raman peaks. This work spectroscopically reveals and quantitates the spin–phonon couplings in typical transition metal complexes and sheds light on the origin of the spin–phonon entanglement.

Transition metal complexes that display slow magnetic relaxation show promise for information storage, but our mechanistic understanding of the magnetic relaxation of such compounds remains limited. Here, the authors spectroscopically and computationally characterize the strength of spin–phonon couplings, which play an important role in the relaxation process.

Enhancing the Magnetic Anisotropy of Linear Cr(II) Chain Compounds Using Heavy Metal Substitutions

Magnetic properties of the series of three linear, trimetallic chain compounds Cr2Cr(dpa)4Cl2, 1, Mo2Cr(dpa)4Cl2, 2, and W2Cr(dpa)4Cl2, 3 (dpa = 2,2'-dipyridylamido), have been studied using variable-temperature dc and ac magnetometry and high-frequency EPR spectroscopy. All three compounds possess an S = 2 electronic ground state arising from the terminal Cr(2+) ion, which exhibits slow magnetic relaxation under an applied magnetic field, as evidenced by ac magnetic susceptibility and magnetization measurements. The slow relaxation stems from the existence of an easy-axis magnetic anisotropy, which is bolstered by the axial symmetry of the compounds and has been quantified through rigorous high-frequency EPR measurements. The magnitude of D in these compounds increases when heavier ions are substituted into the trimetallic chain; thus D = -1.640, -2.187, and -3.617 cm(-1) for Cr2Cr(dpa)4Cl2, Mo2Cr(dpa)4Cl2, and W2Cr(dpa)4Cl2, respectively. Additionally, the D value measured for W2Cr(dpa)4Cl2 is the largest yet reported for a high-spin Cr(2+) system. While earlier studies have demonstrated that ligands containing heavy atoms can enhance magnetic anisotropy, this is the first report of this phenomenon using heavy metal atoms as "ligands".

Theory of High-Spin d(4) Complexes: An Angular-Overlap Model Parametrization of the Ligand Field in Vibronic-Coupling Calculations

A new theoretical approach for the calculation of the electronic and molecular structures of octahedrally-coordinated high-spin d(4) complexes is described. A prescription for the construction of an effective (3)T1 + (5)E (O) Hamiltonian from the ligand-field matrices of a complex with general trigonal symmetry is given, where the ligand field is parametrized in terms of the angular-overlap model (AOM). The Jahn-Teller matrices for the (3)T1 + ((5)E⊗e) vibronic Hamiltonian are constructed and the lowest eigenvalues are calculated by a numerical method. The model obviates the need to assume a temperature dependence of bonding parameters, inherent to the conventional ligand-field-theory approach and is applicable over the whole range of vibronic-coupling strengths, as demonstrated by example calculations on the [Mn(OD2)6](3+) cation and MgO:Cr(2+).

Influence of lattice interactions on the Jahn-Teller distortion of the [Cu(H2O)6]2+ ion: dependence of the crystal structure of K2(x)Rb(2-2x)[Cu(H2O)6](SeO4)2 upon the K/Rb ratio

The temperature dependence of the structures of a wide range of mixed-cation Tutton's salts of general formula K2(x)Rb(2-2x)[Cu(H2O)6](SeO4)2 has been determined over the temperature range 90 to 320 K. Crystals with a high proportion of potassium adopt a different structure (form B) from those with a low ratio (form A). In both forms, the [Cu(H2O)6](2+) ion has an orthorhombically distorted tetragonally elongated coordination geometry, but the long and intermediate bonds occur with a different pair of water molecules in form A compared with form B. The alkali metal is surrounded by seven close oxygen atoms in form B but eight oxygen atoms in form A, and this difference in coordination number is associated with the change in the Cu-O bond distances via the hydrogen-bonding network. For crystals with between 32 and ∼41% potassium, a relatively sharp change from form B to A occurs on cooling, and the temperature at which this occurs increases approximately linearly as the proportion of potassium falls. For the whole range of mixed crystals, the bond lengths have been determined as a function of temperature. The data have been interpreted as a thermal equilibrium of the two structural forms of the [Cu(H2O)6](2+) ion that develops gradually as the temperature increases, with this becoming more pronounced as the proportions of the two cations become more similar. The temperature dependence of the bond lengths in this thermal equilibrium has been analyzed using a model in which the Jahn-Teller potential surface of the [Cu(H2O)6](2+) ion is perturbed by lattice strain interactions. The magnitude and sign of the orthorhombic component of this strain interaction depends upon the proportion of potassium to rubidium ions in the structure.

Pressure-Induced Switch of the Direction of the Unique Jahn−Teller Axis of the Chromium(II) Hexaqua Cation in the Deuterated Ammonium Chromium Tutton Salt

Inelastic neutron scattering (INS) spectra are presented for chromium(II) Tutton salts, as a function of the temperature and pressure. Transitions are observed between the levels of the 5Ag (Ci) ground term and the data modeled with a conventional S = 2 spin Hamiltonian. At 10 K and ambient pressure, the zero-field-splitting parameters of the ammonium salt, (ND4)2Cr(D2O)6(SO4)2, are determined as D = -2.431(4) cm(-1) and E = 0.091(4) cm(-1), evolving to D = -2.517(4) cm(-1) and E = 0.127(5) cm(-1) upon application of 7.5(1.0) kbar of quasi-hydrostatic pressure. By contrast, the change in the INS spectrum of the rubidium salt in this pressure range is comparitively minor. The results are interpreted using a 5Ee vibronic-coupling Hamiltonian, in which low-symmetry strain, perturbing the adiabatic potential-energy surface, is pressure-dependent. It is argued that, for the ammonium salt, the change with pressure of the anisotropic strain impinging upon the [Cr(D2O)6]2+ cation is sufficient to cause a switch of the long and intermediate Cr-OD2 bonds, with respect to the crystallographic axes.

Influence of Lattice Interactions on the Jahn−Teller Distortion of the [Cu(H2O)6]2+ Ion: Dependence of the Crystal Structure of K2[Cu(H2O)6](SO4)2x(SeO4)2-2x upon the Sulfate/Selenate Ratio

The temperature dependence of the structure of the mixed-anion Tutton salt K2[Cu(H2O)6](SO4)(2x)(SeO4)(2-2x) has been determined for crystals with 0, 17, 25, 68, 78, and 100% sulfate over the temperature range of 85-320 K. In every case, the [Cu(H2O)6]2+ ion adopts a tetragonally elongated coordination geometry with an orthorhombic distortion. However, for the compounds with 0, 17, and 25% sulfate, the long and intermediate bonds occur on a different pair of water molecules from those with 68, 78, and 100% sulfate. A thermal equilibrium between the two forms is observed for each crystal, with this developing more readily as the proportions of the two counterions become more similar. Attempts to prepare a crystal with approximately equal amounts of sulfate and selenate were unsuccessful. The temperature dependence of the bond lengths has been analyzed using a model in which the Jahn-Teller potential surface of the [Cu(H2O)6]2+ ion is perturbed by a lattice-strain interaction. The magnitude and sign of the orthorhombic component of this strain interaction depends on the proportion of sulfate to selenate. Significant deviations from Boltzmann statistics are observed for those crystals exhibiting a large temperature dependence of the average bond lengths, and this may be explained by cooperative interactions between neighboring complexes.

Electronic and Molecular Structure of High-Spin d4 Complexes: Experimental and Theoretical Study of the [Cr(D2O)6]2+ Cation in Tutton's Salts

Variable-temperature spectroscopic and crystallographic studies on the chromium(II) Tutton's salts, (MI)2Cr(X2O)6(SO4)2, where MI = ND4+, Rb+, or Cs+, and X = H or D, are reported. Inelastic neutron scattering (INS) and multifrequency EPR experiments facilitate a rigorous definition of the ground-state electronic structure from 1.5 up to 296 K, which is unprecedented for a high-spin d4 complex. Modeling of the INS data using a conventional S = 2 spin Hamiltonian reveals a dramatic variation in the axial and rhombic zero-field-splitting parameters. For the ammonium salt, D and E are -2.454(3) and 0.087(3) cm(-1) at 10 K and -2.29(2) and 0.16(3) cm(-1) at 250 K, respectively. A temperature variation in the stereochemistry of the [Cr(D2O)6]2+ complex is also identified, with an apparent coalescence of the long and medium Cr-O bond lengths at temperatures above 150 K. The corresponding changes for the rubidium and cesium salts are notable, though less pronounced. The experimental quantities are interpreted using a 5Ee Jahn-Teller Hamiltonian, perturbed by anisotropic strain. It is shown that good agreement can be obtained only by employing a model in which the anisotropic strain is itself temperature dependent. A new theoretical approach for calculating variable-temperature EPR spectra of high-spin d4 complexes, developed within the 5Ee coupling model, is described. Differences between spin-Hamiltonian parameters determined by INS and EPR are consistent with those of the different time scales of the two techniques.

Structure and Bonding of the Vanadium(III) Hexa-Aqua Cation. 2. Manifestation of Dynamical Jahn−Teller Coupling in Axially Distorted Vanadium(III) Complexes

Ground-state spin-Hamiltonian parameters, magnetic data, and electronic Raman spectra of hexacoordinate vanadium(III) complexes are calculated with consideration to the ((3)A (3)E) e vibronic interaction and compared to experimental data. It is shown that the zero-field-splitting of the (3)A(g) (S(6)) ground term may be reduced significantly by the dynamical Jahn-Teller effect, particularly when the pi-anisotropy of the metal-ligand bonding interaction is significant, and the energy of the Jahn-Teller active vibration is comparable to the diagonal axial field. The dynamical Jahn-Teller effect may also give rise to a significant enhancement in the Raman intensity of overtones and higher harmonics of Jahn-Teller active vibrations, when the energies of these transitions fall in the proximity of intra-(3)T(1g) (O(h)) electronic Raman transitions. A simple method of conducting vibronic coupling calculations is described, employing ligand field matrices generated by angular overlap model calculations, which may in principle be applied to any transition metal complex.

High-Field, Multifrequency EPR Study of the [Mn(OH2)6]3+ Cation: Influence of π-Bonding on the Ground State Zero-Field-Splitting Parameters

High-field, multifrequency EPR data are presented for the alum CsMn(SO4)2.12D2O, containing the [Mn(OD2)6](3+) cation. The data are interpreted using the conventional S=2 spin Hamiltonian, and the following parameters determined for the data obtained below 30 K: D=-4.491(7) cm(-1), E=0.248(5) cm(-1), gx=1.981(5), gy=1.993(5), gz=1.988(5). Although the deviation of the MnO6 framework from idealized D(4h) symmetry is small, the magnitude of E/D is significant. The E parameter is related to ligand field parameters derived from the optical absorption spectrum. The rhombic anisotropy is shown to arise as a consequence of the pi-anisotropic nature of the manganese(III)-water interaction.

Low-Temperature Single-Crystal Raman and Neutron-Diffraction Study of the Hydrogenous Ammonium Copper(II) Tutton Salt and the Deuterated Analogue in the Metastable State

Low-temperature (15 K) single-crystal neutron-diffraction structures and Raman spectra of the salts (NX4)2[Cu(OX2)6](SO4)2, where X=H or D, are reported. This study is concerned with the origin of the structural phase change that is known to occur upon deuteration. Data for the deuterated salt were measured in the metastable state, achieved by application of 500 bar of hydrostatic pressure at approximately 303 K followed by cooling to 281 K and the subsequent release of pressure. This allows for the direct comparison between the hydrogenous and deuterated salts, in the same modification, at ambient pressure and low temperature. The Raman spectra provide no intimation of any significant change in the intermolecular bonding. Furthermore, structural differences are few, the largest being for the long Cu-O bond, which is 2.2834(5) and 2.2802(4) A for the hydrogenous and the deuterated salts, respectively. Calorimetric data for the deuterated salt are also presented, providing an estimate of 0.17(2) kJ/mol for the enthalpy difference between the two structural forms at 295.8(5) K. The structural data suggest that substitution of hydrogen for deuterium gives rise to changes in the hydrogen-bonding interactions that result in a slightly reduced force field about the copper(II) center. The small structural differences suggest different relative stabilities for the hydrogenous and deuterated salts, which may be sufficient to stabilize the hydrogenous salt in the anomalous structural form.

Spectroscopic and structural characterization of the [Fe(imidazole)(6)](2+) cation

Spectroscopic, structural, and magnetic data are presented for Fe(C(3)H(4)N(2))(6)(NO(3))(2), which facilitate a precise definition of the electronic and molecular structure of the [Fe(Im)(6)](2+) cation. The structure was determined at 120(1) K by X-ray diffraction methods. The salt crystallizes in the trigonal space group R3 with unit-cell parameters a = 12.4380(14) A, c = 14.5511(18) A, and Z = 3. All the imidazole ligands are equivalent with an Fe-N bond distance of 2.204(1) A. Variable-temperature inelastic neutron scattering (INS) measurements identify a cold magnetic transition at 19.4(2) cm(-1) and a hot transition at 75.7(6) cm(-1). The data are interpreted using a ligand field Hamiltonian acting in the weak-field (5)D basis, from which the diagonal trigonal field splitting of the (5)T(2g) (O(h)) term is estimated as approximately 485 cm(-1), with the (5)A(g) (S(6)) component lower lying. High-field multifrequency (HFMF) EPR data and measurements of the magnetic susceptibility are also reported and can be satisfactorily modeled using the energies and wave functions derived from analysis of the INS data. The electronic and molecular structures are related through angular overlap model calculations, treating the imidazole ligand as a weak pi-donor.