Low-spin Isomers Research Articles

High-spin isomers production in (α,n)-reactions

Results of the studies of the relative yield of high-spin and low-spin isomers in reactions 112Sn(α,n)115Te and 134Ba(α,n)137Ce in the energy range of the α-particles 15 – 31 MeV based on off-beam measurements of induced activity of members of the isomeric pair are presented. Extremely large relative yield for the first reaction is obtained. The features of the second one, which are promising for its use in fundamental studies, are revealed.

Magnetic Properties of the Dicationic Iron o-Quinone Complexes with the Pyridinophane Ligands: A Quantum Chemical Study

Computer modeling of the dicationic iron o-benzoquinone complexes with the 2,11-diaza[3.3]-(2,6)pyridinophane ligands is performed. The ground states of the studied compounds are the low-spin isomers. Strong antiferromagnetic exchange interactions are predicted to take place between lone electrons of the trivalent iron ion and the radical-anionic form of the redox-active ligand. The compound capable of undergoing crossover is revealed by the variation of substituents in the tetraazamacrocyclic base.

Computational modeling of mixed-ligand cobalt complexes with o-quinone derivative of corannulene

The structure and energy characteristics of isomers of electrically neutral adducts of tetracoordinate cobalt complexes with o-quinone derivative of corannulene were studied quantum chemically within the density functional theory (DFT) in the UB3LYP*/6-311++G(d,p) approximation. It was shown that the framework rigidity of the redox-active ligand favors convergence of the energy levels of the high-spin and low-spin isomers of the complexes under study as compared to the o-benzoquinone-containing analogues. At the same time, steric strain in the corannulene moiety causes the stability of the adducts to reduce. A compound characterized by high stabilization energy and the ability to switch its spin states via valence tautomeric rearrangements was found by systematic variation of structural parameters of cobalt bis(chelate).

GGA/PBE study of the spin isomers of Fe34,40, Co23,34, and Co12Cu Clusters with Selected Geometries

In a recent beam deflecting experiment was found that high and low spin states of pure Fen and Con clusters with n ≤ 300 atoms coexist at cryogenic temperatures. In this work we have studied the high spin (HS) and low spin (LS) states of several structural isomers of Co23, Co34, Fe34, and Fe40 using the generalized gradient approximation (GGA) to density functional theory as implemented in the first-principles pseudo-potential code SIESTA. The calculated energy difference between these HS and LS isomers is not consistent with the observed coexistence, which can be due to an insufficient account of many body correlation effects in the GGA description, or to unknown isomer structures of these clusters. We have calculated within the same tools the magnetic isomers of Co12Cu cluster aimed to re-visit a former DFT prediction of an anti-ferromagnetic ground state. We find, however, a ferromagnetic ground state as expected on physical grounds. Our results exemplify the difficulties of the current DFT approaches to describe the magnetic properties of transition metal systems.

Nuclear inelastic scattering and density functional theory studies of a one-dimensional spin crossover [Fe(1,2,4-triazole)2(1,2,4-triazolato)](BF4) molecular chain

Nuclear inelastic scattering (NIS) experiments have been performed in order to study the vibrational dynamics of the low- and high-spin states of the polynuclear 1D spin crossover compound [Fe(1,2,4-triazole)2(1,2,4-triazolato)](BF4) (1). Density functional theory (DFT) calculations using the functional B3LYP* and the basis set CEP-31G for heptameric and nonameric models of the compound yielded the normal vibrations and electronic energies for high-spin and low-spin isomers of three models differing in the distribution of anionic trz- ligands and BF4- anions. On the basis of the obtained energies a structural model with a centrosymmetric Fe(trzH)4(trz-)2 coordination core of the mononuclear unit of the chain is proposed. The obtained distribution of the BF4- counteranions in the proposed structure is similar to that obtained on the basis of X-ray powder diffraction studies by Grossjean et al. (Eur. J. Inorg. Chem., 2013, 796). The NIS data of the system diluted to 10% Fe(ii) content in a 90% Zn(ii) matrix (compound (2)) show a characteristic change of the spectral pattern of the low-spin centres, compared to the low-spin phase of the parent Fe(ii) complex (1). DFT calculations reveal that this is caused by a change of the structure of the neighbours of the low-spin centres. The spectral pattern of the high-spin centres in (2) is within a good approximation identical to that of the high-spin Fe(ii) isomer of (1). The inspection of the molecular orbitals of the monomeric model systems of [Fe(trzH)4(trz-)2] and [Fe(trzH)6], together with calculations of spin transition energies, point towards the importance of an electrostatic effect caused by the negatively charged ligands. This results in the stabilisation of the low-spin state of the complex containing the anionic ligand and shortening of the Fe-N(trz-) compared to the Fe-N(trzH) bond in high-spin, but not in low-spin [Fe(trzH)4(trz-)2].

Computational design of mixed-ligand adducts of Co aminovinyl ketonates with redox-active o-quinones and their derivatives

The adducts of cobalt aminovinyl ketonates with o-benzoquinones, o-benzoquinone imines, and o-benzoquinone diimines were studied by the density functional theory method (B3LYP*/6-311++G(d,p)). For all of the compounds, the ground states are low-spin structures containing a trivalent cobalt atom and a redox-active ligand in the semiquinonate form. The bulky substituents at the bis-chelate and ligand nitrogen atoms favor a decrease in the energy difference between the low-spin and high-spin isomers of the adducts containing LSCoIII and HSCoII atoms, respectively. These compounds can exhibit valence tautomerism in the crystalline state, but the relatively low stabilization energies of the high-spin isomers in solutions may induce a competing process: dissociation into the original bis-chelate and redox-active ligand.

Magnetic structure variation in manganese-oxide clusters

Negative-ion photoelectron spectroscopy and ab initio simulations are used to study the variation in magnetic structure in Mn(x)O(y) (x = 3, 4[semicolon] y = 1, 2) clusters. The ferrimagnetic and antiferromagnetic ground-state structures of Mn(x)O(y) are 0.16-1.20 eV lower in energy than their ferromagnetic isomers. The presence of oxygen thus stabilizes low-spin isomers relative to the preferred high-spin ordering of bare Mn(3) and Mn(4). Each cluster has a preferred overall magnetic moment, and no evidence is seen of competing states with different spin multiplicities. However, non-degenerate isomags, which possess the same spin multiplicity but different arrangements of local moments, do contribute additional features and peak broadening in the photoelectron spectra. Proper accounting for all possible isomags is shown to be critical for accurate computational prediction of the spectra.

Structure of 191Pb from α- and β-decay spectroscopy

Complementary studies of 191Pb have been made in the β decay of 191Bi at LISOL (CRC) and in the α decay of 195Po at ISOLDE (CERN). Fine structures in the α decay of the low-spin and high-spin isomers of 195Po have been fully resolved. Identification of the parent state is made possible via isomer selection based on narrow-band laser frequency scanning. The α-particle and γ-ray energies have been determined with greater precision. New α-particle and γ-ray energies are identified. Branching ratios in the decay of 195Po and 191Pb have been examined.

A DFT Study on the Magnetostructural Property of Ferromagnetic Heteroverdazyl Diradicals with Phenylene Coupler

We have theoretically designed five different m-phenylene coupled high-spin bis-heteroverdazyl diradicals and their analogous p-phenylene coupled low-spin positional isomers. The geometry-based aromaticity index, harmonic oscillator model of aromaticity (HOMA) values for both the couplers (local HOMA), and the whole diradicals (global HOMA) have been calculated for all the diradicals. We also qualitatively relate these HOMA values with the intramolecular magnetic exchange coupling constants (J), calculated using a broken symmetry approach within unrestricted density functional theory framework. Structural aromaticity index HOMA of linkage specific benzene rings in our designed diradical systems shows that the aromatic character depends on the planarity of the molecule and it controls the sign and magnitude of J. The predicted J values are explained on the basis of spin polarization maps, average dihedral angles, and magnetic orbitals. The effect of the spin leakage phenomenon on magnetic exchange coupling constant and that on HOMA values of certain phosphaverdazyl systems has been explicitly discussed. In addition, a similar comparison is made between the calculated exchange coupling constants and corresponding HOMA values. The main novelty of this work stands on the consideration of the aromatic behavior by means of the geometrical index HOMA. We also estimate another aromaticity index, nucleus independent chemical shift (NICS) values for the phenylene coupler in each diradical to measure aromaticity and compare its value with that of HOMA. The ground state stabilities of these diradicals have also been compared.

Ising-like hamiltonian and free energy functional: Two fundamentally different models of spin crossover

The theoretical models of spin crossover (equilibrium between the high- and low-spin isomers of transition metal complexes) are compared. The models yield similar descriptions of the one-step spin crossover, but different descriptions of the two-step transition curves because of different approaches to the calculation of molecular interactions. The parameters of the Ising-like Hamiltonian reflect the lattice response to the distribution of molecules, and the parameters of the free energy functional reflect the molecular interactions, due to which the sharpness of transition can be correlated with the molecular properties.

Relevance of supramolecular interactions, texture and lattice occupancy in the designer iron(II) spin crossover complexes

New Fe II complexes of formula [Fe(3-Br-phen) 2(NCS) 2]·Solvent (Solvent=0.5 CH 3OH ( 1), 2 CH 2Cl 2 ( 2), desolvation of 2 ( 3), 0.5 CH 3COCH 3 ( 4) and 0 ( 5)) have been synthesized. 57Fe Mössbauer and magnetic investigation reveal unique features atypical of classic [Fe(phen) 2(NCS) 2] polymorphs. Complex 1, prepared by precipitation in MeOH, undergoes upon cooling below room temperature an incomplete and gradual thermally induced spin conversion, while 4 prepared by an extraction method remains mostly in the low-spin state. The non solvated compounds 3 and 5, display a more abrupt spin crossover on cooling around T 1/2=175 K and T 1/2=198 K, respectively. Defects/soft lattice inclusion due to different methods of material synthesis, extent of aging, reaction medium and associated solvent molecules have enormous influence on the particle size and magnetic properties of these complexes. Scanning electron micrographs helps to establish a logical relationship among methods employed for synthesis, texture of materials and their effect on magnetic properties. The crystal structure of 2 determined in the monoclinic space group P2 /c (100 K) reveals a mononuclear complex consisting of a distorted FeN 6 octahedron in the low-spin state, constructed from two 3-bromo-1, 10-phenanthroline and two isothiocyanato anions in cis position. Intermolecular interactions between mononuclear units of the S⋯Br, S⋯C(H) and π–π type afford a 2D supramolecular network. DFT calculations for the single molecule 2 reveals an energy difference between high-spin and low-spin isomers of 7 kJ/mol suggesting a slight destabilization of the low-spin state compared to [Fe(phen) 2(NCS) 2]. Normal co-ordinate analysis was also carried out for 3 and compared with experimental temperature dependent Raman spectra for 5.

Fe(ii) complex with the octadentate btpa ligand: a DFT study on a spin-crossover system that reveals two distinct high-spin states

Density functional theory calculations (DFT) were performed for the spin-crossover system [Fe(btpa)]2+ (btpa = N,N,N',N'-tetrakis(2-pyridylmethyl)-6,6'-bis(aminomethyl)-2,2'-bipyridine), and for the predominantly low-spin [Fe(b(bdpa))]2+ complex (in the solid state) (b(bdpa) = N,N'-bis(benzyl)-N,N'-bis(2-pyridylmethyl)-6,6'-bis(aminomethyl)-2,2'-bipyridine). The calculations confirmed that the former complex exhibits two high-spin isomers of the complexes, i.e. with C1 quasi hepta-coordinated (long-lived isomer) and C2 hexa-coordinated (short-lived isomer) structures that have been suggested previously based on time-resolved Raman and flash photolysis experiments. Application of B3LYP and B3LYP* functionals together with the CEP-31G basis yielded reasonable estimates of electronic energies (E(el) = E(el)(HS) - E(el)(LS)) for both isomers (calculated E(el) of ca. 24 and 31 kJ mol(-1) for long- and short-lived HS isomers, respectively, vs. the experimentally determined value of 27.5 kJ mol(-1)). Further calculations yielded the electronic structure of the low-spin isomer together with lowest lying singlet and triplet excited states of the [Fe(btpa)]2+ as well as the energy profile of the C2 <--> C1 isomerisation pathway for the high-spin [Fe(btpa)]2+ within the framework of the QST (quadratic synchronous transit) approach. The data obtained are discussed in relation to the observed ultrafast intersystem crossing in Fe(II) polypyridine complexes. The importance of ligand strain in relation to the destabilisation of the low-spin isomers is also discussed. In that context, calculations for a further 15 Fe(II) spin-crossover complexes of hexa-coordinating nitrogen-donor ligands have shown that the LS-HS conversion is associated with a release of ligand stress of 95 +/-16 kJ mol(-1), on average. On the basis of the calculations presented in this paper we propose that octahedral high-spin d-6 isomers are far more elastic regarding the angular distortions (equatorial and meridional strain, i.e. the declination of cis- and trans-L-M-L angle from the regular values of 90 and 180 degrees, respectively) than their low-spin counterparts.

Molecular statistical model of spin crossover equilibrium in the crystal state taking into account the phenomenon of ordering

Formalism is developed in which contributions of binary and ternary interactions towards free energy of a mixture of low-spin (A) and high-spin (B) isomers of spin crossover compounds as well as effects of ordering are taken into account. Parameters characterising non-ideality in this formalism are the excess free energy (ΔFex) and absolute asymmetries (ΔA, ΔB) of splittings of free energy levels. The excess free energy characterises the effects of binary interactions whereas asymmetries arise from ternary interactions. According to this model, the plateau in the spin crossover transition curve originates from the phenomenon of ordering taken into account in the Gorsky-Bragg-Williams approximation.

A DFT study of transition metal complexes with 1,10-phenanthroline, C-C-dimeric 2,2′-bi-1,10-phenanthroline, and its tetraaza chromophore anion

Quantum-chemical calculations of the 1,10-phenanthroline complexes [M(en)(1,10-phen)]2+ (M = Pt, Pd, Ni; en = NH2C2H4NH2) were performed by the DFT B3LYP method in the 6-31G** basis set using the GAMESS-2006 program package. The calculations were also performed for the nickel complexes with 2,2′-bi-1,10-phenanthroline, [Ni(2,2′-bi-1,10-phen)]2+, and with its electron-excessive analog, [Ni(2,2′-bi-1,10-phen)]0, and also for the octahedral complex cation [Ni(2,2′-bi-1,10-phen)Cl(H2O)]+ characterized by single crystal X-ray diffraction. For the Ni(II) complexes, the stabilities of their high-and low-spin isomers were evaluated, and the structural features were revealed. The barriers to mutual transformations of the low-and high-spin Ni(II) complexes are low.

Spin and charge fluctuation occurring in valence tautomerism and cooperative effects in the solid-state

The character of low-spin and high-spin redox isomer conversion of [Co(3,6-di- t-butyl-1,2-benzoquinone) 2(2,2′-bipyridine)] in the solid-tate was investigated by magnetic susceptibility, solid-state high-resolution 13C NMR, and dielectric measurements. We found anomalously large entropy change for relatively sharp transition between the low-spin and high-spin state, when we assumed a random equilibrium between the low-spin and high-spin isomers. A model of domain formation by a cooperative effect for the isomer conversion accounted for the sharp transition with a typical entropy change. It was suggested that three or four molecules form a domain in average. Solid-state high-resolution 13C NMR spectrum revealed the conversion process in a microscopic point of view. Two differently reduced states of 3,6-di- t-butyl-1,2-benzoquinone, i.e. semiquinoate (SQ) and catecholate (Cat), are averaged by rapid electron exchange even in the low-spin state. The exchange rate is much larger than the NMR time scale. Since fluctuation of molecular dipole moment is expected for an electron hopping between SQ and Cat, temperature dependence of dielectric constant was measured at different frequencies between 1 and 100 kHz. The result showed no evidence of freezing of the fluctuation above 10 K. Almost the same result was obtained for the stable crystalline phase of [Co(3,5-di- t-butyl-1,2-benzoquinone) 2(2,2′-bipyridine)], which showed a gradual isomer conversion around 300 K as a random equilibrium process.

A Range of Spin‐Crossover Temperature T1/2&gt;300 K Results from Out‐of‐Sphere Anion Exchange in a Series of Ferrous Materials Based on the 4‐(4‐Imidazolylmethyl)‐2‐(2‐imidazolylmethyl)imidazole (trim) Ligand, [Fe(trim)2]X2 (X=F, Cl, Br, I): Comparison of Experimental Results with Those Derived from Density Functional Theory Calculations

The synthesis and characterization of [FeII(trim)2]Cl2 (2), [FeII(trim)2]Br2MeOH (3), and [FeII(trim)2]I2MeOH (4), including the X-ray crystal structure determinations of 2 (50 and 293 K) and 4 (293 K), have been performed and their properties have been examined. In agreement with the magnetic susceptibility results, the Mössbauer data show the presence of high-spin (HS) to low-spin (LS) crossover with a range of T1/2 larger than 300 K (from approximately 20 K for [FeII(trim)2]F2 (1) to approximately 380 K for 4). All complexes in this series include the same [Fe(trim)2]2+ complex cation: the ligand field comprises a constant contribution from the trim ligands and a variable one originating from the out-of-sphere anions, which is transmitted to the metal center by the connecting imidazole rings and hydrogen bonds. The impressive variation in the intrinsic characteristics of the spin-crossover (SCO) phenomenon in this series is then interpreted as an inductive effect of the anions transmitted to the nitrogen donors through the hydrogen bonds. Based on this qualitative analysis, an increased inductive effect of the out-of-sphere anion corresponds to a decreased SCO temperature T1/2, in agreement with the experimental results. Electronic structure calculations with periodic boundary conditions have been performed that show the importance of intermolecular effects in tuning the ligand field, and thus in determining the transition temperature. Starting with the geometries obtained from the X-ray studies, the [FeII(trim)2]X2 complex molecules 1-4 have been investigated both for the single molecules and the crystal lattices with the local density approximation of density functional theory. The bulk geometries of the complex cations deduced from the X-ray studies and those calculated are in fair agreement for both approaches. However, the trend observed for the transition temperatures of 1-4 disagrees with the trend for the spin-state splittings ES (difference EHS-ELS between the energy of the HS and LS isomers) calculated for the isolated molecules, whereas it agrees with the trend for ES calculated with periodic boundary conditions. The latter calculations predict the strongest stabilization of the HS state for the fluoride complex, which actually is essentially HS above T=50 K, while the most pronounced stabilization of the LS state is predicted for 4, in line with the experimental results.

Spin-Crossover in an Iron(III)−Bispidine−Alkylperoxide System

The iron(II) complex of a tetradentate bispidine ligand with two tertiary amines and two pyridine groups (L = dimethyl [3,7-dimethyl-9,9'-dihydroxy-2,4-di-(2-pyridyl)-3,7-diazabicyclo nonan-1,5-dicaboxylate]) is oxidized with tert-butyl hydroperoxide to the corresponding end-on tert-butylperoxo complex [Fe(III)(L)(OOtBu)(X)]n+ (X = solvent, anion). UV-vis, resonance Raman, and EPR spectroscopy, as a function of the solvent, show that this is a spin-crossover compound. The experimentally observed Raman vibrations for both low-spin and high-spin isomers are in good agreement with those computed by DFT.

Vibrational spectrum of the spin crossover complex [Fe(phen)2(NCS)2] studied by IR and Raman spectroscopy, nuclear inelastic scattering and DFT calculations

The vibrational modes of the low-spin and high-spin isomers of the spin crossover complex [Fe(phen)(2)(NCS)(2)] (phen = 1,10-phenanthroline) have been measured by IR and Raman spectroscopy and by nuclear inelastic scattering. The vibrational frequencies and normal modes and the IR and Raman intensities have been calculated by density functional methods. The vibrational entropy difference between the two isomers, DeltaS(vib), which is--together with the electronic entropy difference DeltaS(el)--the driving force for the spin-transition, has been determined from the measured and from the calculated frequencies. The calculated difference (DeltaS(vib) = 57-70 J mol(-1) K(-1), depending on the method) is in qualitative agreement with experimental values (20-36 J mol(-1) K(-1)). Only the low energy vibrational modes (20% of the 147 modes of the free molecule) contribute to the entropy difference and about three quarters of the vibrational entropy difference are due to the 15 modes of the central FeN(6) octahedron.

Chemical Bonds and Spin State Splittings in Spin Crossover Complexes. A DFT and QTAIM Analysis

Density functional theory (DFT) calculations have been performed for the high-spin (HS) and low-spin (LS) isomers of a series of iron(II) spin crossover complexes with nitrogen ligands. The calculated charge densities have been analyzed in the framework of the quantum theory of atoms in molecules (QTAIM). For a number of iron(II) complexes with substituted tris(pyrazolyl) ligands the energy difference between HS and LS isomers, the spin state splitting, has been decomposed into atomic contributions in order to rationalize changes of the spin state splitting due to substituent effects.

Sub-nanoscale, single-molecule, magnetic–electronic switching from externally perturbedspin states in iron (III)-based complexes

Both the temperature and pressure dependent spin state transition behaviour of two Fe(III)complexes will be exemplified. Such compounds are considered attractive as potential molecularswitch devices for low power, low weight, high density nanomemory arrays in computerapplications, or as sensors. Spin state configurations are readily investigated by 57Fe Mössbauer spectroscopy in the range 300–4 K, or at high pressures up to 20 GPa indiamond anvil cells.An iron coordination compound with the chemical formula {[FeIIIL2] [ClO4]}2·EtOH, where L− is a uni-negative ligand, HL is N-(pyridin-2-ylmethyl)salicylideneamine and EtOH is ethanol, is in the high spin (HS), 6A1g, state at room temperature. The onset of low spin (LS) population occurs at K and this increases monotonically as the temperature is lowered to 100 K.At 70 K the sample is predominantly in the LS state although a remnant HS population persists to the lowest recorded temperature of K. Thermodynamic parameters associated with the energy barrier between the twospin states are deduced from the temperature dependence of the equilibrium constantK.Pressure induced spin switching from the high moment HS state tothe low moment LS electronic state in a haem-porphyrin [FeIII(TPP)(NCS)] will also be shown. Spin pairing onset occurs at a moderate pressure of GPa. HS and LS isomers exist in equilibrium up to GPa at room temperature. The sample is fully LS populated at P>12 GPa. Temperature and pressure dependent spin–spin relaxation is used to account forthe pressure evolution of the (asymmetric) quadrupole split resonance profile. Densityfunctional theory calculations, in conjunction with crystallographic data, areused to consider the structural response to such a spin state variation, perhapssuggesting the possibility of a mechanically operated single-molecule magnetic switch.