High-spin Ground State Research Articles

X-ray structure and spectroscopic characterization of divalent dinuclear cobalt complexes containing carboxylate- and phosphodiester- auxiliary bridges

Two dinuclear spin-coupled divalent cobalt complexes, [Co 2(P1-O)(μ 2-OAc)](ClO 4) 2, ( 1) and [Co 2(P1-O)(μ 2-BNPP)](ClO 4) 2, ( 2) containing μ-1,3 acetate (OAc) and bis(4-nitrophenyl)phosphate (BNPP) auxiliary bridges, respectively, were synthesized by the reaction of a classic dinucleating ligand, P1–OH with cobalt(II) perchlorate in presence of acetic acid/bis(4-nitrophenyl)phosphate. They were characterized by single crystal X-ray diffraction, to show a trigonal bipyramidal geometry around each cobalt center and the intervening bridging atoms that are responsible for spin-transfer between the two divalent cobalt centers; the alkoxo oxygen donor occupies an equatorial position, and the auxiliary ligand oxygens (OAc/BNPP) occupy the axial positions. Solution state magnetic moment measurement together with UV–Vis/NIR spectra revealed a high-spin ground state ( S = 3/2) for Co(II) in these compounds. Complexes 1 and 2 show interesting 1H NMR spectral features of resonances with relatively narrower linewidths in conjunction with a sizable chemical shift dispersion of −5 and 265 ppm. Complex 2 containing the bis(4-nitrophenyl)phosphate auxiliary bridge showed narrower spectral window than complex 1 that has the acetate auxiliary bridge.

Supramolecular interactions of fullerenes with (Cl)Fe- and Mn porphyrins. A theoretical study

The electronic structure and bonding in the noncovalent, supramolecular complexes of fullerenes (C(60), C(70)) with (Cl)Fe- and Mn porphyrins [(Cl)FeP, MnP] were investigated in detail with DFT methods. A dispersion correction was made for the fullerene-porphyrin binding energy through an empirical approach. Several density functionals were employed in the calculations in order to obtain reliable results. Our calculated results differ from those obtained in a previous paper (J. Phys. Chem. A, 2005, 109, 3704). The ground state of (Cl)FeP*C(60) is predicted to be high spin (S = 5/2), in agreement with the experimental results. MnP*C(70) is calculated to have a high-spin (S = 5/2) ground state as well; this is similar to (Cl)FeP*C(60), but at variance with the assignment of a low-spin (S = 1/2) state for this complex. According to the calculations, C(70) in MnP*C(70) does not have sufficient ligand-field strength to cause a high- to low-spin state change in MnP. An additional calculation on a comparable, high-spin (Py)MnP complex gives support for the calculated results on MnP*C(70). More detailed experimental investigations are desirable, which might help to resolve the question of the MnP*C(70) electronic structure. The estimated dispersion energies (E(disp)) in the fullerene-porphyrin systems are rather large, ranging from 0.6 to 1.0 eV. Including E(disp) improves the calculated binding energy considerably.

Two edge-sharing MnII4MnIII6 supertetrahedra give an anisotropic S = 28 ± 1 MnII6MnIII11 complex

A complex with a [Mn(II)(6)Mn(III)(11)] core results via formal fusion of two isotropic Mn(II)(4)Mn(III)(6) supertetrahedra along a common edge leading to a high spin ground state anisotropic system showing Mvs. H hysteresis loops below 1 K.

Enhancing the Quantum Properties of Manganese–Lanthanide Single‐Molecule Magnets: Observation of Quantum Tunneling Steps in the Hysteresis Loops of a {Mn12Gd} Cluster

Single-molecule magnets (SMMs) are individual molecules that function as single-domain nanoscale magnetic particles. A SMM derives its properties from a combination of a high-spin ground state (S) and an easy axis type of magnetoanisotropy (negative zero-field splitting parameter,D), which results in a significant energy barrier to the reversal of the magnetization vector. Such species display both classical magnetization hysteresis, quantum tunneling of magnetization (QTM), and quantum phase interference. Thus, SMMs represent a molecular (“bottom-up”) route to nanoscale magnetism, with potential technological applications in information storage and spintronics at the molecular level, and use as quantum bits (qubits) in quantum computation by exploiting the QTM through the anisotropy barrier. The upper limit to the barrier (U) is given by S jD j or (S-1/4) jD j for integer and half-integer S, respectively. In practice, QTM through upper regions of the barrier makes the true or the effective barrier (Ueff) lower than that of U. Ideally, the QTM can be observed and studied in magnetization vs. DC (direct current) field hysteresis loops, appearing as distinct step-like features at periodic field values, at which levels on either side of the anisotropy barrier to relaxation are in resonance. The steps are thus field positions at which the magnetization relaxation rate increases owing to the onset of QTM. Such steps are a diagnostic signature of resonant QTM, and have been clearly seen only for a few classes of compounds, such as manganese, iron, and nickel SMMs. 7, 8] The most fruitful source of SMMs is the manganese carboxylate chemistry. The prototype was the [Mn12O12(O2CR)16(H2O)4] family, [2,4, 9] and a number of others have since been discovered; almost all have been transition metal clusters, and the vast majority of them have been manganese clusters containing at least some manganese(III) ions. As the search for new SMMs expanded, several groups explored mixed transition metal/lanthanide (Ln) compounds, and particularly Mn–Ln ones, as an attractive area; these efforts were greatly stimulated by the Cu2Tb2 SMM reported by Matsumoto and co-workers. The strategy is obviously to take advantage of the lanthanide ion s significant spin, and/or its large anisotropy, as reflected in a largeD value, to generate SMMs distinctly different from the homometallic ones. Indeed, there are now several Mn–Ln SMMs, including Mn11Ln4, [11] Mn11Gd2, [12] Mn5Ln4, [13a] and Mn6Dy6 [13b] . Many of them have exhibited magnetization hysteresis loops, but unfortunately none of them have displayed resolved QTM steps in these loops. Thus, the incorporation of lanthanide ions has led to a degradation of the quantum properties, as reflected in the QTM steps. The likeliest reason for the degradation of the quantum properties is the step broadening owing to the low-lying excited states resulting from very weak exchange interactions involving the 4f metal ion(s). Herein we report a new structural type in mixed Mn–Ln SMMs having a {Mn12Gd} 38+ core, in which clear QTM steps have been observed in the hysteresis loops of a mixed 3d–4f SMM for the first time. As a result, the D value of a 3d–4f SMM can be measured directly for the first time from the hysteresis data, that is, from magnetic field separation between the steps. The reaction of Mn(O2CPh)2, nBu4NMnO4, Gd(NO3)3, and PhCO2H in a 4:1:4:32 molar ratio in nitromethane produced a dark brown solution, which upon filtration and slow evaporation of the solvent resulted in crystals of [Mn12GdO9(O2CPh)18(O2CH)(NO3)(HO2CPh)] (1) in 40% yield. The structure of 1 consists of a {MnMn11} 35+ cluster with a central Gd ion (Figure 1). The {Mn12Gd} 38+ core is held together by seven m4-O 2 and two m3-O 2 ions. Peripheral ligation is provided by a m4-, three m3-, fourteen m-benzoate groups, a m3-formate group, a chelating NO3 on Mn12, and a terminal benzoic acid on Mn5. The formate probably comes from oxidation of nitromethane by the highly oxidizing MnO4 reagent. The metal oxidation states and the protonation levels of O ions were established by bond valence sum (BVS) calculations and the observation of manganese(III) Jahn–Teller (JT) elongation axes (Figure S1). All manganese atoms are six-coordinate, whereas the gadolinium [*] Dr. T. C. Stamatatos, Prof. Dr. G. Christou Department of Chemistry, University of Florida Gainesville, FL 32611-7200 (USA) Fax: (+1)352-392-8757 E-mail: christou@chem.ufl.edu

Ferromagnetic Disklike MnIVMnII3NaI3 Heptanuclear Complex with a S = 9 Ground State

The wheel-shaped heptanuclear cluster complex {[Mn(4)Na(3)(thmp)(2)(acac)(6)(H(2)O)(6)]CH(3)COO} x 4 H(2)O is the first heterometallic mixed-valent disklike compound with apical alkali-metal ions. The weak ferromagnetic interaction between the central Mn(4+) ion and the peripheral Mn(2+) ones leads to a high-spin ground state (S = 9).

Development of electron holes across the temperature-induced semiconductor–metal transition inBa1−xSrxCo1−yFeyO3−δ(x, y = 0.2–0.8): a soft x-ray absorption spectroscopy study

The x-ray absorption spectra ofBa1−xSrxCo1−yFeyO3−δ (BSCF) powders quenched in air from 623 and 1173 K were measured at the oxygenK and transitionmetal LII,III edges. All the samples show a predominantly Fe high spin ground state of3d5L character,while the 3d6L Co ions are intermediate spin at 623 K and high spin at 1173 K. Further changes in the metalLII,III peaks caused by higher temperature quenching are attributed to changesin symmetry around the cations associated with oxygen loss. The oxygenK spectra show the development of unoccupied states just abovethe Fermi level for samples quenched from 1173 K. At 1173 K,Ba1−xSrxCo1−yFeyO3−δ shows metallic conductivity, while at 623 K it is a semiconductor; the states developed at hightemperature with strong oxygen character are pathways for hole conductivity. Splitting ofthe transition metal 3d energy levels by the ligand field was observed in the oxygenK spectra, andthe range for 10Dq is 1.6–1.8 eV, while the 3d bandwidth is 1.1–1.4 eV in samples quenched from623 K. On the basis of the soft x-ray absorption results, the classification ofBa1−xSrxCo1−yFeyO3−δ as a material with a negative charge-transfer energy is proposed.

Spin transition in Co3+ by optical absorption and time-resolved spectroscopy under pressure: an appraisal of the different spin states

This work investigates the electronic structure of CoF3 by optical absorption and its relationship with the ground-state spin and the Jahn–Teller (JT) effect exhibited by Co3+(d6) in octahedral coordination (CoF6)3−. The results are compared with other Co3+ fluorides and oxides, where Co3+ is high spin (HS) and low spin (LS), respectively. In CoF3 we detect an absorption band with a doublet structure at 1.89 eV (657 nm) and 1.45 eV (855 nm) at 2.5 GPa, which is associated with the T ⊗ e JT effect on the 5T2 HS ground state. Absorption measurements under pressure have been carried out around the HS–LS (5T2 ↔1A1) spin crossover transition in the 0–20 GPa range. We show that the JT effect is stable in a wide pressure range. No sign of intermediate spin (3T1, 2 states) is observed in the explored pressure range.

High-spin magnetic states in the two-orbital Hubbard model on a tetrahedron

We present a study of the two-orbital degenerate Hubbard model in which the exactnumerical solution on a regular tetrahedron is obtained via suitable implementation of thesymmetries generated by the spin, the pairing and the orbital pseudospin operators. Inparticular, we show that a large variety of high-spin magnetic ground states can developaway from half filling, depending on the values of the electron density and the parametersof the model. As the tetrahedron is the simplest finite-size cluster where hopping processesconnect all pairs of sites with constant probability, the study is extended by providing theexact analytical solution of the model on an infinite lattice in the unconstrained hoppinglimit.

Synthesis, Structure and Magnetic Behavior of Five-Coordinate Bis(iminopyrrolyl) Complexes of Cobalt(II) containing PMe3 and THF Ligands

The new bis-iminopyrrolyl five-coordinate Co(II) complexes [Co(kappa (2) N, N'-NC 4H 3C(R)N-2,6- (i)Pr 2C 6H 3) 2(PMe 3)] (R = H 3a; Me 3b) were synthesized in high yields (ca. 80-90%), using THF and diethyl ether as solvents, respectively, by (a) treatment of CoCl 2(PMe 3) 2 with the corresponding iminopyrrolyl Na salts ( Ie or If) or (b) reaction of anhydrous CoCl 2 and PMe 3 with Ie or If. A third route was tested, involving the addition of excesses of PMe 3 to the complexes [Co(kappa (2) N, N'-NC 4H 3C(R)N-2,6- (i)Pr 2C 6H 3) 2] (R = H 1e; Me 1f), which was only successful for the synthesis of 3a, in lower yields (ca. 30%). The synthesis of 3b in THF was unfruitful because of the kinetic competition of the solvent, giving rise to mixtures of 1f and its coordinated THF adduct 4b. The synthesis of the new bis-iminopyrrolyl five-coordinate Co(II) complexes [Co(kappa (2) N, N'-NC 4H 3C(R)N-2,6- (i)Pr 2C 6H 3) 2(THF)] (R = H 4a; Me 4b) were carried out in high yields (ca. 80-90%) by the reaction of CoCl 2(THF) 1.5 with the corresponding iminopyrrolyl Na salt. All the compounds have been characterized by X-ray diffraction, with 3a and 3b showing axially compressed trigonal bipyramidal geometry (with the PMe 3 ligand lying on the equatorial plane), whereas complexes 4a and 4b exhibit distorted square pyramidal geometries with the THF molecule occupying the axial position. Complex 4a shows clearly a compressed geometry, but for complex 4b, two polymorphs were characterized, exhibiting molecules with different Co-O (THF) bond lengths, one of them being compatible with an elongated form. Magnetic measurements either in the solid or in the liquid phases indicate that complexes 3a and 3b have low-spin ground states ( S = 1/2). In toluene solution, the geometry is fully confirmed by EPR data, which further indicates a d x (2) - y (2) /d xy ground state. However, compounds 4a and 4b behave unusually because they show magnetic moments that are compatible with high-spin ground states ( S = 3/2) in the solid state, but conform to low-spin ground states ( S = 1/2) when both complexes are dissolved in toluene solutions. The low-spin ground states in toluene solution are confirmed by EPR spectroscopy, which further supports, for complexes 4a and 4b, an axially elongated square pyramidal geometry and a d z (2) ground state. Thus the change in the ground-state and, consequently, in the geometry of complexes 4a and 4b from solid state to toluene solution might be a consequence of the elongation of the Co-O(THF) bond length. DFT studies performed on complexes 3 and 4 corroborate their different structure and magnetic behaviors and verify, for the latter complexes, the structural differences observed in the solid state and in toluene solution.

The Key Role of the Intermolecular π−π Interactions in the Presence of Spin Crossover in Neutral [Fe(abpt)2A2] Complexes (A = Terminal Monoanion N Ligand)

New iron(II) complexes of formulas [Fe(abpt) 2(tcm) 2] ( 1), [Fe(abpt) 2(tcnome) 2] ( 2), and [Fe(abpt) 2(tcnoet) 2] ( 3) (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, tcm (-) = [C(CN) 3] (-) = tricyanomethanide anion; tcnome (-) = [(NC) 2CC(OCH 3)C(CN) 2] (-) = 1,1,3,3-tetracyano-2-methoxypropenide anion; tcnoet (-) = [(NC) 2CC(OC 2H 5)C(CN) 2] (-) = 1,1,3,3-tetracyano-2-ethoxypropenide anion) have been synthesized and characterized by infrared spectroscopy, magnetic properties and by variable-temperature single-crystal X-ray diffraction. The crystal structure determinations of 1 and 2 reveal in both cases centrosymmetric discrete iron(II) monomeric structures in which two abpt chelating ligands stand in the equatorial plane and two terminal polynitrile ligands complete the distorted octahedral environment in trans positions. For 3, the crystallographic studies revealed two polymorphs, 3- A and 3- B, exhibiting similar discrete molecular structures to those found for 1 and 2 but with different molecular arrangements. In agreement with the variable-temperature single-crystal X-ray diffraction, the magnetic susceptibility measurements, performed in the temperature range 2-400 K, showed a spin-crossover phenomenon above room temperature for complexes 1, 3- A, and 3- B with a T 1/2 of 336, 377, and 383 K, respectively, while complex 2 remains in the high-spin ground state ( S = 2) in the whole temperature range. To understand further the magnetic behaviors of 1, 3-A, and 3-B, single-crystal X-ray diffraction measurements were performed at high temperatures. The crystal structures of both polymorphs could not be obtained above 400 K because the crystals decomposed. However, single-crystal X-ray data have been collected for compound 1, which reaches the full high-spin state at lower temperatures. Its crystal structure, solved at 400 K, showed a strong modification of the iron coordination sphere (average Fe-N = 2.157(3) A vs 1.986(3) A at 293 K). In agreement with the magnetic properties. Such structural behavior is a signature of the spin-state transition from low-spin (LS) to high-spin (HS). On the basis of the intermolecular pi stacking observed for the series described in this paper and for related complexes involving similar discrete structures, we have shown that complexes displaying frontal pi stacking present spin transition such as 1, 3-A, and 3-B and those involving sideways pi stacking such as complex 2 remain in the HS state.

On the Mechanisms of Degenerate Ligand Exchange in [M(CH3)]+/CH4 Couples (M=Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt) as Explored by Mass Spectrometric and Computational Studies: Oxidative Addition/Reductive Elimination versus σ‐Complex‐Assisted Metathesis

The degenerate ligand exchange in [M(CH(3))](+)/CH(4) couples occurs in the gas phase at room temperature for M=Ni, Ru, Rh, Pd, and Pt, whereas the complexes containing Fe and Co are unreactive. Details of hydrogen-atom scrambling versus direct ligand switch have been uncovered by labeling experiments with CD(4) and (13)CH(4), respectively. The reactivity scale ranges from unreactive (M=Fe, Co) or inefficient (M=Ni, Pd) to moderately (M=Ru) and rather reactive (M=Rh, Pt). Quite extensive, but not complete, H/D exchange between the hydrogen atoms of the incoming and outgoing methyl groups is observed for M=Pt, whereas for M=Ni and Pd a predominantly direct ligand switch prevails. DFT calculations performed at the B3LYP level of theory account well for the thermal nonreactivity of the Fe and Co couples. For [Ni[CH(3))](+)/CH(4), a sigma-complex-assisted metathesis (sigma-CAM) is operative such that, in a two-state reactivity (TSR) scenario, two spin flips between the (3)A ground and (1)A excited states take place at the entrance and exit channels of the encounter complexes. For M=Ru and Rh, only oxidative addition/reductive elimination (OA/RE) is favored energetically, and the reaction is confined to the electronic ground states (3)A and (2)A. In contrast, for the [Pd(CH(3))](+)/CH(4) system, on the (1)A ground-state potential-energy surface both the OA/RE and sigma-CAM variants are energetically comparable, and the small reaction efficiency for the ligand switch is reflected in transition states located energetically close to the reactants. For the [M(CH(3))](+)/CH(4) complexes of the 5d elements, the sigma-CAM mechanism does not play a role. For M=Pt, the energetically most favored path proceeds in a spin-conserving manner on the (1)A potential-energy surface, which accounts for the extensive single and double hydrogen-atom exchange preceding ligand exchange. Although for M=Os and Ir the [M(CH(3))](+) complexes could not be generated experimentally, computational studies predict that both systems may undergo thermal reaction with CH(4), and an OA/RE mechanism will commence on the respective high-spin ground states; however, the bond-activation and ligand-exchange steps will occur on the excited low-spin surfaces in a TSR scenario.

Trinuclear Heterobimetallic Ni2Ln complexes [L2Ni2Ln][ClO4] (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er; LH3= (S)P[N(Me)N═CH−C6H3-2-OH-3-OMe]3): From Simple Paramagnetic Complexes to Single-Molecule Magnet Behavior

The reaction of LH3 with Ni(ClO4)(2).6H 2O and lanthanide salts in a 2:2:1 ratio in the presence of triethylamine leads to the formation of the trinuclear complexes [L2Ni2Ln][ClO4] (Ln=La (2), Ce (3), Pr (4), Nd (5), Sm (6), Eu (7), Gd (8), Tb (9), Dy (10), Ho (11) and Er (12) and L: (S)P[N(Me)NCH-C6H3-2-O-3-OMe]3). The cationic portion of these complexes consists of three metal ions that are arranged in a linear manner. The two terminal nickel(II) ions are coordinated by imino and phenolate oxygen atoms (3N, 3O), whereas the central lanthanide ion is bound to the phenolate and methoxy oxygen atoms (12O). The Ni-Ni separations in these complexes range from 6.84 to 6.48 A. The Ni-Ni, Ni-Ln and Ln-O phenolate bond distances in 2-12 show a gradual reduction proceeding from 2 to 12 in accordance with lanthanide contraction. Whereas all of the compounds (2-12) are paramagnetic systems, 8 displays a remarkable ST=(11)/2 ground state induced by an intramolecular Ni. . .Gd ferromagnetic interaction, and 10 is a new mixed metal 3d/4f single-molecule magnet generated by the high-spin ground state of the complex and the magnetic anisotropy brought by the dysprosium(III) metal ion.

Why Does Cyanide Pretend to be a Weak Field Ligand in [Cr(CN)5]3−?

Chemical reasoning based on ligand-field theory suggests that homoleptic cyano complexes should exhibit low-spin configurations, particularly when the coordination sphere is nearly saturated. Recently, the well-known chromium hexacyano complex anion [Cr(CN)6](4-) was shown to lose cyanide to afford [Cr(CN)5](3-) in the absence of coordinating cations. Furthermore, (NEt 4)3[Cr(CN)5] was found to be in a high-spin (S=2) ground state, which challenges the common notion that cyanide is a strong field ligand and should always enforce low-spin configurations. Using density functional theory coupled to a continuum solvation model, we examined both the instability of the hexacyanochromate(II) anion and the relative energies of the different spin states of the pentacyanochromate(II) anion. By making direct comparisons to the analogous Fe (II) complex, we found that cyanide electronically behaves as a strong-field ligand for both metals because the orbital interaction is energetically more favorable in the low-spin configuration than in the corresponding high-spin configuration. The Coulombic repulsion between the anionic cyanide ligands, however, dominates the overall energetics and ultimately gives preference to the high-spin complex, where the ligand-ligand separation is larger. Our calculations highlight that for a quantitative understanding of spin-state energetic ordering in a transition metal complex, ligand-ligand electrostatic interactions must be taken into account in addition to classical ligand-field arguments based on M-L orbital interaction energies.

Bimetallic Fe−Ni Cluster Alloys: Stability of Core(Fe)−Shell(Ni) Arrays and Their Role Played in the Structure and Magnetic Behavior

We present density functional theory as well as tight-binding calculations to analyze the stability and magnetic properties of small fcc and polyicosahedral Fe−Ni cluster alloys having different compositions and chemical orderings. In agreement with the gas-phase experiments of Parks et al. (Chem. Phys. 2000, 262, 151), we obtain in all our considered alloys that iron atoms prefer to accumulate in the core region of the structures leading to the formation of highly stable core(Fe)−shell(Ni) arrays. We found that the low energy atomic configurations of our fcc Fe−Ni clusters strongly depend on the chemical order within the structures. Interestingly, fcc particles with Fe-rich core regions are characterized by unusual and considerably expanded Fe−Fe bond lengths (∼2.7 Å), a result that induces a highly nonuniform relaxation profile in which both sizable expansions and contractions of the interatomic distance between neighboring atoms can be obtained. In contrast, disordered alloys and core(Ni)−shell(Fe) arrays are characterized by Fe−Ni bond lengths of ∼2.5 Å, being of the order of the arithmetic weighted average of the Fe−Fe and Ni−Ni pairs. A common feature of our results is that all our considered Fe−Ni clusters exhibit high-spin ground states (larger than the corresponding bulk alloy values for the same composition); however, we have found that iron clustering within the particles, the presence of chemisorbed hydrogens on the surface, as well as the existence of antiferromagnetic order in the iron-rich regions could lead to a sizable quenching of the average magnetization in the nanoalloys, which is in good agreement with the experimental data of Li et al. (J. Magn. Magn. Mater. 1997, 170, 339). Finally, we show that the value of the orbital-to-spin ratio in both fcc and polyicosahedral Fe−Ni clusters is very sensitive to the internal location of the Fe impurities, a result that suggests that X-ray magnetic circular dichroism experiments can be very useful to reveal precise features of the chemical order in magnetic cluster alloys.

Assessment of the Anisotropy in the Molecule Mn19 with a High-Spin Ground State S = 83/2 by 35 GHz Electron Paramagnetic Resonance

35 GHz electron paramagnetic resonance experiments on a powder sample of the magnetic molecule Mn 19 with a high-spin ground state S = 83/2 are presented. At low temperatures, the data are well described by the simulated spectra for an isolated spin with a zero-field-splitting parameter D = 0.004 cm (-1), which is, in particular, positive. Hence, Mn 19 is not a single-molecule magnet; the previously observed magnetic hysteresis at ultralow temperatures is likely due to intermolecular dipolar interactions.

Light induced excited spin state trapping in the binuclear spin crossover compound [Fe(bpym)(NCS) 2] 2(bpym) exhibiting a high-spin ground state

A photo-magnetic effect is evidenced using near-infrared light in the binuclear complex [Fe(bpym)(NCS) 2] 2(bpym). This compound has a 5T 2g– 5T 2g ground state and exhibits no thermal spin crossover – in contrast to the analogous [Fe(bpym)(NCSe) 2] 2(bpym). The estimated photo-conversion ratio is ca. 30%. By means of magnetic susceptibility measurements as well as Raman and infrared absorption spectroscopies, the nature of the photo-induced phase was established as the 5T 2g– 1A 1g state, which means that only one iron center is converted to low-spin. The photo-induced state was completely converted back to the ground state either by visible light excitation or by heating.

Assembly of Iron(II)-Triazole Polymers from 1D Chains to 3D Interpenetrated Frameworks: Syntheses, Structures, and Magnetic Properties

By the reactions of FeX2 with α,ω-bis(1,2,4-triazol-1-yl)alkane ligands (X = ClO4−, BF4−, NCS−, and NCSe−), a series of novel iron(II) coordination polymers have been synthesized and characterized. Complexes [Fe(btp)3](ClO4)2 (1a), [Fe(btp)3](BF4)2 (1b), and [Fe(btp)2(NCS)2] (2) (btp = 1,3-bis(1,2,4-triazol-1-yl)propane) are infinite 1D-linear chains. [Fe(btb)3](ClO4)2 (3a) and [Fe(btb)3](BF4)2 (3b) (btb = 1,4-bis(1,2,4-triazol-1-yl)butane) are 3-fold interpenetrated α-polonium cubic frameworks. The two sorts of btb ligands with different conformations alternately link to the Fe(II) centers in [Fe(btb)2(NCS)2]·H2O (4) to form a 3D interpenetrating architecture of CdSO4-like topology. When NCSe− instead of NCS− was used as the framework-regulator, a 2D grid layer [Fe(btb)2(NCSe)2]·2H2O (5) was obtained. The counteranions, spacer length, and conformation of the α,ω-bis(1,2,4-triazol-1-yl)alkane ligands are responsible for the assembled topological architectures. Magnetic studies indicate that all of these iron(II) polymers retain a high-spin ground state upon cooling from 300 to 4 K.

Anisotropic susceptibility of La2- xSrxCoO4 related to the spin states of cobalt

We present a study of the magnetic susceptibility of La2- xSrxCoO4 single crystals in a doping range 0.3 ⩽ x ⩽ 0.8. Our data show a pronounced magnetic anisotropy for all compounds. This anisotropy is in agreement with a low-spin ground state (S = 0) of Co3+ for x ⩾ 0.4 and a high-spin ground state (S = 3/2) of Co2+. We compare our data with a crystal-field model calculation assuming local moments and find a good description of the magnetic behavior for x ⩾ 0.5. This includes the pronounced kinks observed in the inverse magnetic susceptibility, which result from the anisotropy and low-energy excited states of Co2+ and are not related to magnetic ordering or temperature-dependent spin-state transitions.

Synthesis, crystal structure and magnetic properties of single-molecule magnet: [Mn4(CF3COO)4(hmp)6

The preparation, X-ray structure and magnetic properties are presented for a new mixed-valence tetranuclear manganese complex that functions as a single-molecule magnet (SMM): [Mn4(CF3COO)4(hmp)6], where hmp− is the anion of 2-(hydroxymethyl) pyridine and is a N,O bidentate chelate. The compound crystallizes in a monoclinic system, space group P 21/c (No. 14) with unit cell parameters a = 13.663(3) A, b = 14.705(3) A, c = 14.734(3) A, β = 98.51(3)°, V = 2927.6 A3 and Z = 2. The structure of the complex shows a novel coordination of the trifluoroacetate (TFA) anions, with one anion acting as a monodentate ligand while the second one coordinating through both oxygens to the same Mn center. Direct current magnetic susceptibility measurement in the 2–300 K temperature range supports a high-spin ground state. The presence of a frequency-dependent alternating current susceptibility signal indicates that the individual molecule is acting as magnet.

Chromium-Doped Germanium Clusters CrGen (n = 1−5): Geometry, Electronic Structure, and Topology of Chemical Bonding

The structure and properties of small neutral and cationic CrGen(0,+) clusters, with n from 1 to 5, were investigated using quantum chemical calculations at the CASSCF/CASPT2 and DFT/B3LYP levels. Smaller clusters prefer planar geometries, whereas the lowest-lying electronic states of the neutral CrGe4, CrGe5, and cationic CrGe5+ forms exhibit nonplanar geometries. Most of the clusters considered prefer structures with high-spin ground state and large magnetic moments. Relative to the values obtained for the pure Gen clusters, fragmentation energies of doped CrGen clusters are smaller when n is 3 and 4 and larger when n = 5. The averaged binding energy tends to increase with the increasing number of Ge atoms. For n = 5, the binding energies for Ge5, CrGe5, and CrGe5+ are similar to each other, amounting to approximately 2.5 eV. The Cr atom acts as a general electron donor in neutral CrGen clusters. Electron localization function (ELF) analyses suggest that the chemical bonding in chromium-doped germanium clusters differs from that of their pure or Li-doped counterparts and allow the origin of the inherent high-spin ground state to be understood. The differential DeltaELF picture, obtained in separating both alpha and beta electron components, is consistent with that derived from spin density calculations. For CrGen, n = 2 and 3, a small amount of d-pi back-donation is anticipated within the framework of the proposed bonding model.