High-spin Ground State Research Articles

Magnetic interactions in the catalyst used by nature to split water: a DFT + U multiscale study on the Mn4CaO5 core in photosystem II

An important approach in the design of new environmentally friendly materials is represented by the study of analogous systems already existing in nature. In the search for new water splitting catalysts, the corresponding natural analogue is represented by the oxygen-evolving complex of photosystem II, which is a large membrane protein complex present in photosynthetic organisms. The understanding of the catalytic strategy of its active Mn4CaO5 core is important to unravel the mechanisms of water oxidation in photosynthesis and can serve as an inspiring model for the design of biomimetic catalysts based on largely non-toxic, earth abundant elements. The magnetic interactions between Mn ions are studied in the present work by means of DFT + U broken symmetry ab initio molecular dynamics within a quantum mechanics/molecular mechanics framework. The room temperature dynamics of two different structural models (i.e. with total high-spin and total low-spin ground states) was stable during the simulated time. We observed large fluctuations of the magnetic coupling constants calculated on both the structural models of the complex, causing occasionally instantaneous swapping of the ferromagnetic/antiferromagnetic coupling between the metal centers.

A semi-flexible aminotriazine-based bis-methylpyridine ligand for the design of nickel(ii) spin clusters

The self-assembly of a semi-flexible aminotriazine-based bis-methylpyridine ligand, N(2),N(2)-dibenzyl-N(4),N(6)-di(pyridylmethyl)-1,3,5-triazine-2,4,6-triamine (H2L), with NiCl2 and NiBr2 afforded two new nickel(II) clusters, (H2NMe2)2[Ni5(OH)2(H2L)2Cl10] (1) and [Ni6(OH)2(H2L)2Br10(THF)2] (2) showing a high spin ground state of S = 3.

Synthesis and Characterization of Divalent Manganese, Iron, and Cobalt Complexes in Tripodal Phenolate/N-Heterocyclic Carbene Ligand Environments

Two novel tripodal ligands, (BIMPN(Mes,Ad,Me))(-) and (MIMPN(Mes,Ad,Me))(2-), combining two types of donor atoms, namely, NHC and phenolate donors, were synthesized to complete the series of N-anchored ligands, ranging from chelating species with tris(carbene) to tris(phenolate) chelating arms. The complete ligand series offers a convenient way of tuning the electronic and steric environment around the metal center, thus, allowing for control of the complex's reactivity. This series of divalent complexes of Mn, Fe, and Co was synthesized and characterized by (1)H NMR, IR, and UV/vis spectroscopy as well as by single-crystal X-ray diffraction studies. Variable-temperature SQUID magnetization measurements in the range from 2 to 300 K confirmed high-spin ground states for all divalent complexes and revealed a trend of increasing zero-field splitting |D| from Mn(II), to Fe(II), to Co(II) complexes. Zero-field (57)Fe Mössbauer spectroscopy of the Fe(II) complexes 3, 4, 8, and 11 shows isomer shifts δ that increase gradually as carbenes are substituted for phenolates in the series of ligands. From the single-crystal structure determinations of the complexes, the different steric demand of the ligands is evident. Particularly, the molecular structure of 1-in which a pyridine molecule is situated next to the Mn-Cl bond-and those of azide complexes 2, 4, and 6 demonstrate the flexibility of these mixed-ligand derivatives, which, in contrast to the corresponding symmetrical TIMEN(R) ligands, allow for side access of, e.g., organic substrates, to the reactive metal center.

In Search of Organic Compounds Presenting a Double Exchange Phenomenon.

The objective of this paper is to design a consistent series of organic molecules that may present a double exchange mechanism and study their low energy spectrum using spin unrestricted Density Functional Theory. For this purpose, organic tetra-methylene methane units having an S = 1 spin ground state and diamagnetic organic bridges are taken as building blocks for constructing molecules having two or more magnetic units. When biunit systems are ionized, the ground state of the resulting molecular ions may be either a quartet, if the spectrum is ruled by a double exchange mechanism, or a doublet, if it obeys the logic of a monoelectronic picture. A strategy based on the physical analysis of the leading interactions is followed in order to energetically favor a high-spin ground state. It is shown that the most promising compounds involve bridges that have both a large gap between the highest occupied and the lowest unoccupied molecular orbitals and small coefficients on the atoms to which the magnetic units are connected. While the followed strategy enables one to conceive organic compounds exhibiting a double exchange phenomenon, it is shown that the electronic mechanism ruling the spectrum of such organic double exchange compounds is different from that of their inorganic homologues. A new method to reconstruct the spectrum of low energy from various spin unrestricted DFT solutions is proposed and applied. Finally monodimensional and bidimensional periodic lattices based on the most promising organic architecture are suggested.

Synthesis, Structures, and Dearomatization by Deprotonation of Iron Complexes Featuring Bipyridine-based PNN Pincer Ligands

The synthesis and characterization of new iron pincer complexes bearing bipyridine-based PNN ligands is reported. Three phosphine-substituted pincer ligands, namely, the known (t)Bu-PNN (6-((di-tert-butylphosphino)methyl)-2,2'-bipyridine) and the two new (i)Pr-PNN (6-((di-iso-propylphosphino)methyl)-2,2'-bipyridine) and Ph-PNN (6-((diphenylphosphino)methyl)-2,2'-bipyridine) ligands were synthesized and studied in ligation reactions with iron(II) chloride and bromide. These reactions lead to the formation of two types of complexes: mono-chelated neutral complexes of the type [(R-PNN)Fe(X)2] and bis-chelated dicationic complexes of the type [(R-PNN)2Fe](2+). The complexes [(R-PNN)Fe(X)2] (1: R = (t)Bu, X = Cl, 2: R = (t)Bu, X = Br, 3: R = (i)Pr, X = Cl, and 4: R = (i)Pr, X = Br) are readily prepared from reactions of FeX2 with the free R-PNN ligand in a 1:1 ratio. Magnetic susceptibility measurements show that these complexes have a high-spin ground state (S = 2) at room temperature. Employing a 2-fold or higher excess of (i)Pr-PNN, diamagnetic hexacoordinated dicationic complexes of the type [((i)Pr-PNN)2Fe](X)2 (5: X = Cl, and 6: X = Br) are formed. The reactions of Ph-PNN with FeX2 in a 1:1 ratio lead to similar complexes of the type [(Ph-PNN)2Fe](FeX4) (7: X = Cl, and 8: X = Br). Single crystal X-ray studies of 1, 2, 4, 6, and 8 do not indicate electron transfer from the Fe(II) centers to the neutral bipyridine unit based on the determined bond lengths. Density functional theory (DFT) calculations were performed to compare the relative energies of the mono- and bis-chelated complexes. The doubly deprotonated complexes [(R-PNN*)2Fe] (9: R = (i)Pr, and 10: R = Ph) were synthesized by reactions of the dicationic complexes 6 and 8 with KO(t)Bu. The dearomatized nature of the central pyridine of the pincer ligand was established by X-ray diffraction analysis of single crystals of 10. Reactivity studies show that 9 and 10 have a slightly different behavior in protonation reactions.

Prospects for ultracold polar and magnetic chromium–closed-shell-atom molecules

The properties of the electronic ground state of the polar and paramagnetic chromium--closed-shell-atom molecules have been investigated. State-of-the-art \textit{ab initio} techniques have been applied to compute the potential energy curves for the chromium--alkaline-earth-metal-atom, CrX (X = Be, Mg, Ca, Sr, Ba), and chromium--ytterbium, CrYb, molecules in the Born-Oppenheimer approximation for the $X^7\Sigma^+$ high-spin electronic ground state. The spin restricted open-shell coupled cluster method restricted to single, double, and noniterative triple excitations, RCCSD(T), was employed and the scalar relativistic effects within Douglas-Kroll-Hess Hamiltonian or energy-consistent pseudopotentials were included. The permanent electric dipole moments and static electric dipole polarizabilities were computed. The leading long-range coefficients describing the dispersion interaction between atoms at large interatomic distances, $C_6$, are also reported. Molecules under investigation are an example of species possessing both large magnetic and electric dipole moments making them potentially interesting candidates for ultracold many-body physics studies.

Synthesis, magnetic and spectroscopic characterization of a new Fe7 cluster with a six-pointed star topology

A heptanuclear FeIII7 cluster is reported, prepared from the reaction of [Fe3O(O2CPh)6(H2O)3]+ and a combination of aliphatic aminoalcohol ligands, namely apH (3-amino-1-propanol) and hmpipH (2-hydroxymethyl piperidine). The (apH2)[Fe7O3(OH)3(hmpip)6(O2CPh)7] complex possesses a metal core resembling a metal-centered non-planar six-pointed star. Thermogravimetric analysis was used to monitor the thermal stability of 1, and shows stepwise loss of mass for this complex upon heating, while Mössbauer spectroscopy was used to evaluate the chemical environment of the Fe ions. The latter confirmed the existence of only high-spin FeIII ions in the cluster, as also suggested by X-ray crystallography. Dc and ac magnetic susceptibilities were measured on microcrystalline samples of 1, and the ground state for this cluster was found to be S=21/2, with low lying excited states. Attempts to rationalize the observed spin using two different magnetostructural correlation relationships, previously derived for oxide-bridged FeIII dimers, suggested the presence of spin frustrated pathways along the peripheral hexagon, which in turn produce a high-spin ground state. The observed S=21/2 was finally rationalized using the results of this magnetostructural analysis, and assuming a combination of Si=5/2 and intermediate Si=3/2 spin projections. This Fe7 cluster is the third reported example of a heptanuclear Fe cluster with this topology, featuring a tetrahedral Fe ion at the center, and joins a small family of Fe clusters based on the aliphatic ligands apH and hmpipH.

Chemisorption of Exchange-Coupled [Ni2L(dppba)]+Complexes on Gold by Using Ambidentate 4-(Diphenylphosphino)benzoate Co-Ligands

A new strategy for the fixation of redox-active dinickel(II) complexes with high-spin ground states to gold surfaces was developed. The dinickel(II) complex [Ni2L(Cl)]ClO4 (1ClO4), in which L(2-) represents a 24-membered macrocyclic hexaaza-dithiophenolate ligand, reacts with ambidentate 4-(diphenylphosphino)benzoate (dppba) to form the carboxylato-bridged complex [Ni2L(dppba)](+), which can be isolated as an air-stable perchlorate [Ni2L(dppba)]ClO4 (2ClO4) or tetraphenylborate [Ni2L(dppba)]BPh4 (2BPh4) salt. The auration of 2ClO4 was probed on a molecular level, by reaction with AuCl, which leads to the monoaurated Ni(II)2Au(I) complex [Ni(II)2L(dppba)Au(I)Cl]ClO4 (3ClO4). Metathesis of 3ClO4 with NaBPh4 produces [Ni(II)2L(dppba)Au(I)Ph]BPh4 (4BPh4), in which the Cl(-) is replaced by a Ph(-) group. The complexes were fully characterized by ESI mass spectrometry, IR and UV/Vis spectroscopy, X-ray crystallography (2BPh4 and 4BPh4), cyclic voltammetry, SQUID magnetometry and HF-ESR spectroscopy. Temperature-dependent magnetic susceptibility measurements reveal a ferromagnetic coupling J = +15.9 and +17.9 cm(-1) between the two Ni(II) ions in 2ClO4 and 4BPh4 (H = -2 JS1S2). HF-ESR measurements yield a negative axial magnetic anisotropy (D<0), which implies a bistable (easy axis) magnetic ground state. The binding of the [Ni2L(dppba)]ClO4 complex to gold was ascertained by four complementary surface analytical methods: contact angle measurements, atomic-force microscopy, X-ray photoelectron spectroscopy, and spectroscopic ellipsometry. The results indicate that the complexes are attached to the Au surface through coordinative Au-P bonds in a monolayer.

DFT study of gadolinium aluminohydrides and aluminofluorides

First-principles calculations using density functional theory were performed to study the structure, stability and electronic properties of clusters formed by attaching AlH4 and AlF4 complexes to the Gd atom. The calculated electron affinity of AlH4 is 3.58eV, close to that of chlorine, and for the Gd (AlH4)4 cluster it is 4.95eV, indicative of superhalogen behavior. For AlF4 the calculated electron affinity is 7.36eV, indicating superhalogen behavior. When this complex is used to decorate Gd in number that exceeds its formal valence (+3) the electron affinity increases to 8.42eV in the Gd (AlF4) 4 cluster, which is an indication of hyperhalogen behavior. High-spin ground states are predicted for these moieties, which have same spin multiplicities in both aluminohydrides and aluminofluorides.

Spin-Crossing in an Organometallic Pt-Benzene Complex.

The interaction between the Pt atom and the benzene (Bz) molecule was investigated theoretically, using a symmetric Pt-Bz half-sandwich complex. Various levels of wave function theory (HF, MP2, CCSD(T), CASPT2, multistate (MS) MS-CASPT2) together with several functionals (PBE, PBE+D3, PBE+vdW, PBE/EE+vdW) of the density functional theory (DFT) were complemented by quantum Monte Carlo (QMC) calculations. Spin-orbit coupling (SOC) effects were also taken into account at the CASPT2 and DFT levels of theory. The inclusion of dynamic correlation energy was found to be essential to maintaining the stability of the complex. The dative type of bonding was identified to be responsible for the Pt-Bz binding in the ground state. Single-reference CCSD(T) and MP2 as well as multireference CASPT2 and MS-CASPT2 methods reveal that the interaction curve has a single energy minimum (corresponding to the (1)S0 state) and a shoulder (arising from the crossing between the (1)S0 and (3)D3 states) at longer distances. The inclusion of SOC at the CASPT2 level leads to the appearance of another well-separated minimum, which corresponds to the triplet state (3)F4. The PBE/EE+vdW functional, which includes a fraction of exact-exchange (EE) and nonlocal electron correlation, shows the best qualitative agreement with respect to the CCSD(T) data among all DFT functionals used. Large-scale QMC calculations, based on DFT wave functions constructed using TPSSh and M11 functionals, were confronted with the CCSD(T) results. The QMC-TPSSh protocol favorably agrees with the CCSD(T) data, suggesting its possible use in other Pt-containing organometallic systems. All of the methods used (except for HF) show that the Pt-benzene binding leads to the quenching of the Pt high-spin triplet ground state, and the low-spin closed-shell singlet state is found to be preferred in the ground-state of the complex.

The Inherent Single‐Molecule Magnet Character of Trivalent Uranium

SMMs are often based on transition-metal clusters, but significant attention has recently focused on complexes of single and multiple lanthanoid ions, because the crystal-field splitting of the lowest Russell–Saunders multiplet engenders large magnetic anisotropies. [5–11] These anisotropies are responsible for high relaxation barriers and therefore slow magnetic relaxation. However, well isolated high-spin ground states are difficult to achieve within polynuclear lanthanoid SMMs because the valence 4f orbitals have limited radial extension and are usually energetically incompatible with ligand orbitals. These inherent 4f orbital properties give predominantly ionic interactions and results in weak magnetic exchange coupling with neighboring spin centers, with very few exceptions. [7, 12] In principle, actinoids, and in particular uranium, possess properties that render these ions ideal candidates from which to construct SMMs. This is because, compared to the lanthanoids, uranium exhibits enhanced crystal field splitting, [13, 14] as well as increased covalency, the latter enabling significant spin couplings in polynuclear systems, [15] and therefore both stronger magnetic exchange and anisotropies can be envisaged. This premise was realized recently with reports of several closely related single-ion pyrazolylborate uranium(III) complexes which were shown to display SMM behavior. [14, 16–20] In addition, two neptunium complexes were shown to display slow relaxation of the magnetization. [15, 21] The fact that all uranium(III) SMMs reported to date are closely related to each other raises the question as to how sensitive SMM behavior in trivalent uranium is to the composition of the coordination sphere and its symmetry. SMM behavior in all published examples is much more pronounced in an external field than in zero field, which suggests that quantum tunneling of the magnetization plays a significant role in shortening the relaxation times. However, in principle, low-symmetry crystal field components cannot induce tunneling of the magnetization, because uranium(III) is a Kramers half-integer angular momentum ion. Also the nuclear spin I = 0o f 238 U cannot induce tunneling of the

Point Defects in SiC as a Promising Basis for Single-Defect, Single-Photon Spectroscopy with Room Temperature Controllable Quantum States

The unique quantum properties of the nitrogen–vacancy (NV) center in diamond have motivated efforts to find defects with similar properties in silicon carbide (SiC), which can extend the functionality of such systems not available to the diamond. As an example, results of experiments on electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR) are presented suggests that silicon vacancy (VSi) related point defects in SiC possess properties the similar to those of the NV center in diamond, which in turn make them a promising quantum system for single-defect and single-photon spectroscopy in the infrared region. Depending on the defect type, temperature, SiC polytype, and crystalline position, two opposite schemes have been observed for the optical alignment of the high-spin ground state spin sublevels population of the VSi-related defects upon irradiation with unpolorized light. Spin ensemble of VSi-related defects are shown to be prepared in a coherent superposition of the spin states even at room temperature. Zero-field (ZF) ODMR shows the possibility to manipulate of the ground state spin population by applying radiofrequency field. These altogether make VSi-related defects in SiC very favorable candidate for spintronics, quantum information processing, and magnetometry.

Computational study of the spin-state energies and UV-Visspectra of bis(1,4,7-triazacyclononane) complexes of some first-row transition metal cations

We report here computed spin-state energies and UV-Vis spectra for several transition metal complexes with a triazacyclononane ligand. Our results show that the spin ground-state is correctly obtained with either OPBE or SSB-D, except for the high-spin ground-state of the Co(ii) complex that was properly described only by SSB-D. The UV-Vis spectra from TD-DFT reproduce in general rather well the experimental spectra, but in cases of the Cr(III) and Co(II) complexes it clearly failed. Better results for the UV-Vis spectra have been obtained by using Ligand Field DFT.

Low-Spin Hexacoordinate Mn(III): Synthesis and Spectroscopic Investigation of Homoleptic Tris(pyrazolyl)borate and Tris(carbene)borate Complexes

Three complexes of Mn(III) with "scorpionate" type ligands have been investigated by a variety of physical techniques. The complexes are [Tp(2)Mn]SbF(6) (1), [Tp(2)*Mn]SbF(6) (2), and [{PhB(MeIm)(3)}(2)Mn](CF(3)SO(3)) (3a), where Tp(-) = hydrotris(pyrazolyl)borate anion, Tp*(-) = hydrotris(3,5-dimethylpyrazolyl)borate anion, and PhB(MeIm)(3)(-) = phenyltris(3-methylimidazol-2-yl)borate anion. The crystal structure of 3a is reported; the structures of 1 and 2 have been previously reported, but were reconfirmed in this work. The synthesis and characterization of [{PhB(MeIm)(3)}(2)Mn]Cl (3b) are also described. These complexes are of interest in that, in contrast to many hexacoordinate (pseudo-octahedral) complexes of Mn(III), they exhibit a low-spin (triplet) ground state, rather than the high-spin (quintet) ground state. Solid-state electronic absorption spectroscopy, SQUID magnetometry, and high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy were applied. HFEPR, in particular, was useful in characterizing the S = 1 spin Hamiltonian parameters for complex 1, D = +19.97(1), E = 0.42(2) cm(-1), and for 2, D = +15.89(2), E = 0.04(1) cm(-1). In addition, frequency domain Fourier-transform THz-EPR spectroscopy, using coherent synchrotron radiation, was applied to 1 only and gave results in good agreement with HFEPR. Variable-temperature dc magnetic susceptibility measurements of 1 and 2 were also in good agreement with the HFEPR results. This magnitude of zero-field splitting (zfs) is over 4 times larger than that in comparable hexacoordinate Mn(III) systems with S = 2 ground states. Complexes 3a and 3b (i.e., regardless of counteranion) have a yet much larger magnitude zfs, which may be the result of unquenched orbital angular momentum so that the spin Hamiltonian model is not appropriate. The triplet ground state is rationalized in each complex by ligand-field theory (LFT) and by quantum chemistry theory, both density functional theory and unrestricted Hartree-Fock methods. This analysis also shows that spin-crossover behavior is not thermally accessible for these complexes as solids. The donor properties of the three different scorpionate ligands were further characterized using the LFT model that suggests that the tris(carbene)borate is a strong σ-donor with little or no π-bonding.

Effect of Fe(II) spin crossover on charge distribution in and lattice properties of thiospinels

The spinels AgFe0.5Sn1.5S4 and CuFe0.5Sn1.5S4 belong to the rare examples of purely inorganic compounds in which Fe(II) exhibits a thermally induced transition from a low spin 3d(t2g)6(eg)0 to a high spin 3d(t2g)4(eg)2 electronic ground state. The extremely rare situation of having 119Sn as a second Mössbauer isotope in the lattice besides 57Fe is used to obtain deeper insight in the consequences the spin transition has on the lattice properties. To this end, 119Sn Mössbauer spectra were recorded between 5 and 500K. The temperature dependence of the 119Sn hyperfine parameters is analysed. The data are compared to those obtained for the spinels AgMn0.5Sn1.5S4 and CuMn0.5Sn1.5S4 which are characterised by a temperature-independent high spin ground state. The results are discussed in terms of Fe–Sn charge transfer, local distortions on the tin site and changes of the vibrational lattice properties induced by the Fe spin transition.

Employment of a New Tripodal Ligand for the Synthesis of Cobalt(II/III), Nickel(II), and Copper(II) Clusters: Magnetic, Optical, and Thermal Properties

The employment of 2-(β-naphthalideneamino)-2-(hydroxymethyl)-1-propanol (LH(3)) in cobalt, nickel, and copper chemistry has led to the isolation of five new metallic complexes with interesting magnetic properties. More specifically, the reaction of Co(OAc)(2)·4H(2)O with LH(3) in MeOH in the presence of NEt(3) under solvothermal conditions forms the complex [Co(III)(2)Co(II)(3)(L)(2)(LH)(2)(L')(OAc)]·8.5MeOH (1·8.5MeOH; L' = monoanion of 2-hydroxy-1-naphthaldehyde), while in nickel chemistry, a similar reaction of Ni(OAc)(2)·6H(2)O with LH(3) in MeCN in the presence of NEt(3) under high pressure/temperature forms the complex [Ni(II)(LH(2))(2)]·2MeCN (2·2MeCN). Repeating the same reaction in MeOH and switching from Ni(OAc)(2)·4H(2)O to NiSO(4)·4H(2)O produces the complex [Ni(II)(4)(HL)(3)(OMe)(MeOH)(3)](SO(4))(0.5)·2MeOH (3·2MeOH) under solvothermal conditions. Furthermore, in copper chemistry, the reaction of Cu(2)(OAc)(4)·2H(2)O with LH(3) in the presence of NEt(3) in MeOH under solvothermal conditions affords the complex [Cu(II)(4)(LH)(4)] (4), while the same reaction under ambient temperature and pressure conditions forms [Cu(II)(4)(LH)(4)] ·3.5MeOH·2.25H(2)O (5·3.5MeOH·2.25H(2)O). Complex 1 is a mixed-valent [Co(III)(2)Co(II)(3)] complex, consisting of three edge-sharing [Co(3)] triangles. Complex 2 is a nickel(II) monomer in which the central metal is found in an octahedral geometry, while complex 3 describes a [Ni(II)(4)] cubane. Complexes 4 and 5 may be considered as structural isomers because they possess the same formulas but different topologies: 4 describes a highly distorted [Cu(II)(4)(OR)(4)](4+) eight-membered ring, while 5 consists of a distorted [Cu(II)(4)(μ(3)-OR)(4)](4+) cubane. In addition, 5 can be converted to 4 in excellent yield under solvothermal conditions. Direct-current magnetic susceptibility studies have been carried out in the 5-300 K range for complexes 1 and 3-5, revealing the possibility of a high-spin ground state for 1, an S = 4 ground state for 2, and diamagnetic ground states for 4 and 5.

Theoretical study of the influence of vacancies in the magnetic stability of V-, Cr-, and Mn-doped SnO2

In this work we study, theoretically, the magnetic properties of transition metals (TMs)-doped SnO2 (with TM=V, Cr, and Mn) in a diluted magnetic oxide configuration, focusing in particular in the role played by the presence of O vacancies, VO, nearby the TM. We present the results of first-principles electronic structure calculations of Sn0.96TM0.04O2 and Sn0.96TM0.04O1.98(VO)0.02 alloys. The calculated total energy as a function of the total magnetic moment per cell shows a magnetic metastability, corresponding to a high-spin (HS) ground state, respectively, with 2 and 3μB/cell, for Cr and Mn, and a metastable low-spin (LS) state, with 0 (Cr) and 1 (Mn)μB/cell. For vanadium, only a state with 1μB/cell was found. The spin-crossover energy (ESCO) from the LS to the HS is 114meV for Cr and 42meV for Mn. By creating O vacancies close to the TM site, we show that the metastability and ESCO change. For chromium, a new HS state appears (4μB/cell), with an energy barrier of 32meV relative to the 2μB/cell state. For manganese, the metastable LS state of 1μB/cell disappears, while for vanadium the HS state of 1μB/cell remains. In all cases, the ground state corresponds to the expected HS. These findings suggest that these materials may be used in applications that require different magnetization states.

Structural, magnetic and electronic characterization of an isostructural series of dinuclear complexes of 3d metal ions bridged by tpbd

Dinuclear complexes of the type [{M(H2O)(phen)}2(μ-tpbd)](ClO4)4 with N,N,N′,N′-Tetrakis-(2-pyridylmethyl)-benzene-1,4-diamine (tpbd) as bridging ligand and M=Mn(II) (1), Fe(II) (2), Co(II) (3), Ni(II) (4), Cu(II) (5), Zn(II) (6) were synthesized, structurally analyzed and their magnetic as well as their electrochemical properties are determined. A ligand-centered one-electron oxidation leads to radical complexes for which the lifetimes are strongly dependent on the coordinated metal ion as followed from time-resolved UV–Vis absorption spectroscopy. In addition to the six homometallic dinuclear complexes, one analog heterometallic Mn(II)/Ni(II) compound (7) of the same constitution was structurally and magnetically investigated, confirming an high spin ground state.

Tetragonally compressed high-spin Mn(III) Schiff base complex: Synthesis, crystal structure, magnetic properties and theoretical calculations

The synthesis, X-ray crystal structures, magnetic properties and Density Functional Theory (DFT) calculations of a new six-coordinated high-spin (HS) Mn(III) Schiff base complex, [MnIII(3-MeO-sal-N(1,5,9,13))]NO3 (1), are described. The crystal structure analysis show that the Mn site has a tetragonally compressed octahedral geometry. The magnetic susceptibilities measurements indicate the HS ground state in the temperature range 1.8–300K, in contrast with a previously reported analogue, [Mn(5-Br-sal-N-1,5,8,12)]ClO4, which showed spin-crossover (SCO) behavior. The magnetic data are well fitted by Zero-Field-Splitting (ZFS) Hamiltonian, showing a D>0 parameter, in line with the compressed octahedron pattern. Various DFT computations certified, in semi-quantitative respects, the fitted Spin Hamiltonian parameters. Computational experiments using fractional population schemes or Time-Dependent DFT approach yielded the specific ligand field parameters, contributing to the theoretical insight in the rare case of compressed Mn(III) complexes.

Theoretical Investigation of Magnetic and Conducting Properties of Substituted Silicon Chains. I. Hydrogen and Oxo-Verdazyl Ligands

In this work, we investigate the magnetic and conducting properties of polysilene chains attached to the stable free radical oxo-Verdazyl (o-VER) via C-linkage, by using quantum chemical and solid state methods. Calculations are first carried out on 39 possible monomers in their triplet and broken symmetry states following the density functional methodology UB3LYP and using Gaussian 03 and 09 codes. The magnetic exchange coupling constant (J) is calculated for each species. Here, the magnetic exchange coupling constant J equals the negative of the ratio of the energy difference and difference between the low-spin and the high-spin ground states, in accordance with the Heisenberg spin exchange Hamiltonian. Geometry is optimized using the 6-31G(d) basis set. Single-point calculations on the triplet and broken symmetry states are performed with the optimized geometries and 6-311G(d,p) basis set. Calculations on three series of molecules of interest are finally done by employing the 6-311++G(d,p) basis s...