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

M-Ge-Si thermolytic molecular precursors and models for germanium-doped transition metal sites on silica

The synthesis, thermolysis, and surface organometallic chemistry of thermolytic molecular precursors based on a new germanosilicate ligand platform, -OGe[OSi(OtBu)3]3, is described. Use of this ligand is demonstrated with preparation of complexes containing the first-row transition metals Cr, Mn, and Fe. The thermolysis and grafting behavior of the synthesized complexes, Fe{OGe[OSi(OtBu)3]3}2 (FeGe), Mn{OGe[OSi(OtBu)3]3}2(THF)2 (MnGe) and Cr{OGe[OSi(OtBu)3]3}2(THF)2 (CrGe), was evaluated using a combination of thermogravimetric analysis; nuclear magnetic resonance (NMR), ultraviolet-visible (UV-Vis), and electron paramagnetic resonance (EPR) spectroscopies; and single-crystal X-ray diffraction (XRD). Grafting of the precursors onto SBA-15 mesoporous silica and subsequent calcination in air led to substantial changes in transition metal coordination environments and oxidation states, the implications of which are discussed in the context of low-coordinate and low oxidation state thermolytic molecular precursors.

Synergy of Magnetic Anisotropy and Ferromagnetic Interaction Triggering a Dimeric Cr(II) Zero-Field Single-Molecule Magnet

A novel Cr^II-dimeric complex, [Cr^IIN(SiⁱPr₃)₂(μ-Cl)(THF)]₂ (1), has been successfully constructed using a bulky silyl-amide ligand. Single-crystal structure analysis reveals that complex 1 exhibits a binuclear motif, with a Cr₂Cl₂ rhombus core, where two equivalent tetra-coordinate Cr^II centers in the centrosymmetric unit display quasi-square planar geometry. The crystal structure has been well simulated and explored by density functional theory calculations. The axial zero-field splitting parameter (D < 0) with a small rhombic (E) value is unambiguously determined by systematic investigations of magnetic measurements, high-frequency electron paramagnetic resonance spectroscopy, and ab initio calculations. Remarkably, ac magnetic susceptibility data unveil that 1 features slow dynamic magnetic relaxation typical of single-molecule magnet behavior with U_eff = 22 K in the absence of a dc field. This increases up to 35 K under a corresponding static field. Moreover, magnetic studies and theoretical calculations point out that a non-negligible ferromagnetic coupling (FMC) exists in the dimeric Cr-Cr units of 1. The coexistence of magnetic anisotropy and FMC contributes to the first case of Cr^II-based single-molecule magnets (SMMs) under zero dc field.

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.

1,2-Diaza-4-phospholide complexes of chromium(ii): dipotassium organochromates behaving as single-molecule magnets

The 1,2-diaza-4-phospholide (dp^-) dipotassium ate complexes of chromium(ii) {[(η¹-N-3,5-tBu₂dp)₄Cr][(η⁵-(N,N,C,C,P))₂-K(η¹-O-THF)₂]₂} (5) and {[(η¹-N-3,5-Ph₂dp)₄Cr][(η⁵-(N,N,C,C,P))₂-K(η¹-O-THF)₂]₂}_∞ (6) were synthesized and characterized by X-ray single crystal structure analysis. Complex 5 with a near-square planar geometry at the chromium(ii) ion was unambiguously characterized by the high field electron paramagnetic resonance (HF-EPR) technique and magnetic measurements, revealing that it is a field-induced single-molecule magnet (SMM).

Electronic properties and vibrational spectra of (NH4)2M″(SO4)2·6H2O (M = Ni, Cu) Tutton's salt: DFT and experimental study

The complex crystals of the family of the Tutton's salt have been investigated through the numerous experimental and theoretical studies to understand their physical properties and their potential technological applications. In spite of the more than 60 years of research, there are very few studies about the electronic properties of Tutton's salt. In our present work, we have calculated the stability, electronic properties and the first theoretical study of band structure of the three different crystals of the Tutton's salt, ammonium nickel sulfate hexahydrate ((NH₄)₂Ni(SO₄)₂·6H₂O), ammonium nickel-copper sulfate hexahydrate ((NH₄)₂Ni_0.5Cu_0.5(SO₄)₂·6H₂O) and ammonium copper sulfate hexahydrate ((NH₄)₂Ni(SO₄)₂·6H₂O) with the help of periodic ab-initio calculations based on density functional theory (DFT). In addition to this, the internal Raman and FTIR modes of the ionic fragments [Ni(H₂O)₆]²⁺, [Cu(H₂O)₆]²⁺ NH₄⁺ and SO₄^2- of the sample crystals were obtained by employing the ab initio and the orientation of the molecular vibrations of the ionic fragments have been presented in picturized form. Furthermore, the Raman and FTIR spectroscopy of the sample crystals were measured in the range 100-4000 cm^-1 and 400-4000 cm^-1 respectively, and the internal vibrational modes obtained from experimental measurement have been compared with those obtained from DFT calculations.

Structural and electronic data of three first-row transition octahedral hexaaquametal(II) ions, metal=Cr, Ni or Cu

Structural and density functional theory calculated data of three octahedral hexaaquametal(II) ions containing different metals (Cr, Ni or Cu) are presented to illustrate different geometries these octahedral hexaaquametal(II) ions can have. The density functional theory optimized geometries exhibit either a regular octahedral geometry, an octahedral elongated or an octahedral compressed geometry. Experimental structures exhibit octahedral or distorted octahedral geometries, the latter includes octahedral elongated, octahedral compressed and orthorhombic distorted geometries.

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⁺ (1), MePh₃P⁺ (2), and Ph₄As⁺ (3), 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. 1 and 2 exhibit highly irregular seven-coordinate geometries, while the central Co(II) ion of 3 is in a distorted-dodecahedral configuration. The sole magnetic transition observed in the INS spectroscopy of 1-3 corresponds to the zero-field splitting (2(D² + 3E²)^1/2) from 22.5(2) cm^-1 in 1 to 26.6(3) cm^-1 in 2 and 11.1(5) cm^-1 in 3. The positive sign of the D value, and hence the easy-plane magnetic anisotropy, was demonstrated for 1 by INS studies under magnetic fields and HF-EPR spectroscopy. The combined analyses of INS and HF-EPR data yield the D values as +10.90(3), +12.74(3), and +4.50(3) cm^-1 for 1-3, respectively. Frequency- and temperature-dependent alternating-current magnetic susceptibility measurements reveal the slow magnetization relaxation in 1 and 2 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 1-3 were revealed by calculations at the CASPT2/NEVPT2 level.

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".

Uniaxial magnetic anisotropy of square-planar chromium(ii) complexes revealed by magnetic and HF-EPR studies

Two chromium(ii) complexes exhibit single-molecule-magnet behavior due to uniaxial magnetic anisotropy revealed by magnetic and HF-EPR studies.

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.

High-frequency/high-field EPR spectroscopy of the high-spin ferrous ion in hexaaqua complexes

Electron paramagnetic resonance (EPR) at conventional magnetic fields and microwave frequencies, respectively, B0 < or = 1.5 T, nu < or = 35 GHz, has been widely applied to odd electron-number (S = 1/2) transition metal complexes. This technique is less successfully applied to high-spin systems that have even electron configurations, e.g. Fe2+ (S = 2). The recently developed technique of high-frequency and high-field EPR (HFEPR), employing swept fields up to 25 T combined with multiple, sub-THz frequencies readily allows observation of EPR transitions in such high-spin systems. A parallel spectroscopic technique is frequency-domain magnetic resonance spectroscopy (FDMRS), in which the frequency is swept while at zero, or at discrete applied magnetic fields. We describe here the application of HFEPR and FDMRS to two simple high-spin (HS) ferrous (Fe2+) salts: ferrous perchlorate hydrate, [Fe(H2O)6](ClO4)2 and (NH4)2[Fe(H2O)6](SO4)2, historically known as ferrous ammonium sulfate. Both compounds contain hexaaquairon(II). The resulting spectra were analyzed using a spin Hamiltonian for S = 2 to yield highly accurate spin-Hamiltonian parameters. The complexes were also studied by powder DC magnetic susceptibility and zero-field Mössbauer effect spectroscopy for corroboration of magnetic resonance results. In the case of [Fe(H2O)6](ClO4)2, all the magnetic techniques were in excellent agreement and gave as consensus values: D = 11.2(2) cm(-1), E = 0.70(1) cm(-1). For (NH4)2[Fe(H2O)6](SO4)2, FDMRS and HFEPR gave D = 14.94(2) cm(-1), E = 3.778(2) cm(-1). We conclude that the spin-Hamiltonian parameters for the perchlorate best represent those for the isolated hexaaquairon(II) complex. To have established electronic parameters for the fundamentally important [Fe(H2O)6]2+ ion will be of use for future studies on biologically relevant systems containing high-spin Fe2+.

Electronic Raman spectroscopy of the vanadium(III) hexaaqua cation in guanidinium vanadium sulphate: Quintessential manifestation of the dynamical Jahn–Teller effect

Single-crystal Raman spectra are presented for the salt [C(NH2)3][V(OH2)6](SO4)2, displaying electronic transitions between the trigonal components of the vanadium(III) 3T1g(Oh) ground term. The 3A-->3E(C3) electronic Raman band is centered at approximately 2720 cm-1, and exhibits extensive structure, revealing the energies of the spinor components of the 3E(C3) term for the two crystallographically distinct [V(OH2)6]3+ cations. The data are interpreted in conjunction with parameters previously reported from an electron paramagnetic resonance study of the salt. A satisfactory reproduction of the electronic Raman profile and ground-state spin-Hamiltonian parameters is achieved by employing a (3A plus sign in circle3E)multiply sign in circle e vibronic coupling model, in which the spin-orbit splitting of the 3E(C3) is quenched significantly by the Ham effect, and the intensity of harmonics of the Jahn-Teller active vibration enhanced by their proximity to the electronic Raman bands. The model gives an excellent account of the intensities of the electronic Raman bands, which are shown to depend profoundly on both temperature and the selected component of the polarizability tensor. The electronic Raman profile changes notably upon deuteriation, a result that exposes deficiencies in the single-mode coupling model.