Zero-field Splitting Effects Research Articles

Field-Induced Single-Ion Magnet Behavior in Nickel(II) Complexes with Functionalized 2,2':6'-2″-Terpyridine Derivatives: Preparation and Magneto-Structural Study.

Two mononuclear nickel(II) complexes of the formula [Ni(terpyCOOH)2](ClO4)2∙4H2O (1) and [Ni(terpyepy)2](ClO4)2 MeOH (2) [terpyCOOH = 4'-carboxyl-2,2':6',2″-terpyridine and terpyepy = 4'-[(2-pyridin-4-yl)ethynyl]-2,2':6',2″-terpyridine] have been prepared and their structures determined by single-crystal X-ray diffraction. Complexes 1 and 2 are mononuclear compounds, where the nickel(II) ions are six-coordinate by the six nitrogen atoms from two tridentate terpy moieties. The mean values of the equatorial Ni-N bond distances [2.11(1) and 2.12(1) Å for Ni(1) at 1 and 2, respectively, are somewhat longer than the axial ones [2.008(6) and 2.003(6) Å (1)/2.000(1) and 1.999(1) Å (2)]. The values of the shortest intermolecular nickel-nickel separation are 9.422(1) (1) and 8.901(1) Å (2). Variable-temperature (1.9-200 K) direct current (dc) magnetic susceptibility measurements on polycrystalline samples of 1 and 2 reveal a Curie law behavior in the high-temperature range, which corresponds to magnetically isolated spin triplets, the downturn of the χMT product at lower temperatures being due to zero-field splitting effects (D). Values of D equal to -6.0 (1) and -4.7 cm-1 (2) were obtained through the joint analysis of the magnetic susceptibility data and the field dependence of the magnetization. These results from magnetometry were supported by theoretical calculations. Alternating current (ac) magnetic susceptibility measurements of 1 and 2 in the temperature range 2.0-5.5 K show the occurrence of incipient out-phase signals under applied dc fields, a phenomenon that is characteristic of field-induced Single-Molecule Magnet (SMM) behavior, which herein concerns the 2 mononuclear nickel(II) complexes. This slow relaxation of the magnetization in 1 and 2 has its origin in the axial compression of the octahedral surrounding at their nickel(II) ions that leads to negative values of D. A combination of an Orbach and a direct mechanism accounts for the field-dependent relation phenomena in 1 and 2.

Implementation of Quantum Level Addressability and Geometric Phase Manipulation in Aligned Endohedral Fullerene Qudits.

Endohedral nitrogen fullerenes have been proposed as building blocks for quantum information processing due to their long spin coherence time. However, addressability of the individual electron spin levels in such a multiplet system of 4 S3/2 has never been achieved because of the molecular isotropy and transition degeneracy among the Zeeman levels. Herein, by molecular engineering, we lifted the degeneracy by zero-field splitting effects and made the multiple transitions addressable by a liquid-crystal-assisted method. The endohedral nitrogen fullerene derivatives with rigid addends of spiro structure and large aspect ratios of regioselective bis-addition improve the ordering of the spin ensemble. These samples empower endohedral-fullerene-based qudits, in which the transitions between the 4 electron spin levels were respectively addressed and coherently manipulated. The quantum geometric phase manipulation, which has long been proposed for the advantages in error tolerance and gating speed, was implemented in a pure electron spin system using molecules for the first time.

Tris(oxalato)chromate(III) hybrid salts templated by pyridinium and mixed pyridinium-ammonium cations: synthesis, structures and magnetism

By modifying the stoichiometric ratio of starting materials, two tris(oxalato)chromate(III) salts, (C7H11N2)3[Cr(C2O4)3] (1) and (C5H8N3)2(NH4)[Cr(C2O4)3]·2H2O (2) {(C7H11N2)+ = 2-amino-4,6-dimethylpyridinium, (C5H8N3)+ = 2,6-diaminopyridinium}, were synthesized and characterized by elemental and thermal analyses, single-crystal X-ray diffraction, IR and UV − Vis spectroscopies, EPR and SQUID measurements. Salt 1 exhibits a 3-D supramolecular framework based on [Cr(C2O4)3]3- and 2-amino-4,6-dimethylpyridinim cations, (C7H11N2)+, via N–H···O hydrogen bonds. Interestingly, π–π stacking interactions between pyridine rings contribute to the stabilization of the crystal packing. In contrast to salt 1, no π–π stacking interactions are observed in the mixed-cation salt 2 and its crystal packing is consolidated by N–H···O and O − H···O hydrogen bonds. EPR spectra of 1 and 2 are consistent with the oxidation state +3 of the chromium center in an octahedral environment. Temperature-dependence of the magnetic susceptibility data investigated from 2 to 300 K revealed the existence of zero-field splitting effects (ZFS) for Cr(III) ions in both compounds.

How the Ligand Field in Lanthanide Coordination Complexes Determines Magnetic Susceptibility Anisotropy, Paramagnetic NMR Shift, and Relaxation Behavior.

ConspectusComplexes of lanthanide(III) ions are being actively studied because of their unique ground and excited state properties and the associated optical and magnetic behavior. In particular, they are used as emissive probes in optical spectroscopy and microscopy and as contrast agents in magnetic resonance imaging (MRI). However, the design of new complexes with specific optical and magnetic properties requires a thorough understanding of the correlation between molecular structure and electric and magnetic susceptibilities, as well as their anisotropies. The traditional Judd–Ofelt–Mason theory has failed to offer useful guidelines for systematic design of emissive lanthanide optical probes. Similarly, Bleaney’s theory of magnetic anisotropy and its modifications fail to provide accurate detail that permits new paramagnetic shift reagents to be designed rather than discovered.A key determinant of optical and magnetic behavior in f-element compounds is the ligand field, often considered as an electrostatic field at the lanthanide created by the ligands. The resulting energy level splitting is a sensitive function of several factors: the nature and polarizability of the whole ligand and its donor atoms; the geometric details of the coordination polyhedron; the presence and extent of solvent interactions; specific hydrogen bonding effects on donor atoms and the degree of supramolecular order in the system. The relative importance of these factors can vary widely for different lanthanide ions and ligands. For nuclear magnetic properties, it is both the ligand field splitting and the magnetic susceptibility tensor, notably its anisotropy, that determine paramagnetic shifts and nuclear relaxation enhancement.We review the factors that control the ligand field in lanthanide complexes and link these to aspects of their utility in magnetic resonance and optical emission spectroscopy and imaging. We examine recent progress in this area particularly in the theory of paramagnetic chemical shift and relaxation enhancement, where some long-neglected effects of zero-field splitting, magnetic susceptibility anisotropy, and spatial distribution of lanthanide tags have been accommodated in an elegant way.

Synthsis, structures and magnetic properties of three new coordination polymers constructed by mixed-linker strategy

By the reaction of a rigid π-conjugated linker of H4L (5,5′-(1,3,6,8-tetraoxobenzo [lmn] [3,8] phenanthroline-2–7- diyl) bis-1,3-benzenedicarboxylic acid), nickel nitrate and N-donor auxiliary linkers under solvothermal conditions, three new Ni-based coordination polymers (CPs), namely {[Ni(L)0.5(1,4-bib)(H2O)2]·2·5H2O·DMA}n (1), {[Ni(H2L)(4,4′-bibp)(H2O)2]}n (2) and {[Ni(L)0.5(4,4′-bimb)(H2O)2]·3·5H2O·2DMA}n (3) {1,4-bib = 1,4-bis(1-imidazoly)benzene, 4,4′-bibp = 4,4′-bis(imidazolyl)biphenyl, 4,4′-bimb = 4,4′-bis(imidazol-1-ylmethyl) biphenyl} have been synthesized and characterized by EA, TG, IR, PXRD and magnetic properties. The structural analysis indicates both 1 and 2 are 2D layer structures, which are further extended to 3D supramolecular structures by H-bonding and intermolecular π…π interactions. While 3 exhibits a complicated 3D structure with the topology symbol of {64.82}. The difference of the structures in CPs 1–3 is mainly attributed to different auxiliary linkers used. Meanwhile, magnetic properties of CPs 1–3 show that the descent of χMT values is attributed to zero-field splitting (ZFS) effects of Ni2+ ions. In addition, on account of large Ni2+ Ni2+ separations within these frameworks, magnetic interactions between adjacent Ni2+ ions can be neglected.

IR and Raman spectroscopic analysis, DFT modeling, and magnetic properties of a nickel(II) complex, [Ni(succ)(H2O)4

A nickel(II) coordination polymer of succinic acid (1,4-butanedioic acid, H2succ), [Ni(succ)(H2O)4]n (1), was prepared and characterized in depth using IR and Raman spectroscopic methods and theoretical (DFT) calculations. The magnetic properties of 1 were investigated to explain the magneto-structural correlation. The experimental vibrational spectroscopic studies combined with the DFT calculations are indispensable to elucidate the molecular structures of such complexes. A complete assignment of the IR and Raman spectra was made on the basis of the calculated potential energy distribution (PED). The structures of two analogues, monomer [Ni(succ)(H2O)4] (2) and dimer [Ni(succ)(H2O)4]2 (3), were modeled by DFT calculations in good agreement with the experiment. The atomic charges and spin densities for the model complexes were calculated with natural bond orbital analysis. Magnetic susceptibility measurements for 1 indicated that Ni(II) paramagnetic centers bridged by succinic ligand in the 1 D chain are almost insolated, and also a great impact of zero-field splitting effect of the Ni(II) ions is visible.

Synthesis, characterization and X-ray crystal structure of an iron(III) complex of a tripodal pyridoxal Schiff base ligand: effects of positional disorder on its magnetic properties

The synthesis and characterization of fac-{tris[(E)-5-(hydroxymethyl)-2-methyl-4-((methylimino-κN)methyl)pyridin-3-olato-κO]amine}iron(III) 7.5-hydrate is described, together with its crystal structure at 95 K. The FeIII ion is six-coordinated in a trigonally distorted octahedral configuration with the hexadentate tripodal pyridoxal Schiff base ligand. A positional disorder of the pyridoxal hydroxymethyl group is identified, which partly undergoes a cyclization forming a five-membered ring with the imine moiety. The two configurations occur in the ratio 79.5:20.5. In the crystal, the hydrogen bonds and π–π stacking interactions expand the mononuclear units into 2D sheets which are further stabilized by a system of hydrogen bonds to the 3D supramolecular network. The magnetic susceptibility data and EPR measurements indicate that the octahedral Fe(III) centers exist as a mixture of low- and high-spin forms within the temperature range 2–300 K. The presence of a mixture of the majority high-spin and the minority low-spin components showing a spin transition is proposed. The effect of zero-field splitting on magnetic properties is discussed. EPR spectra at X-band display signals of a high-spin Fe(III) complex of the rhombohedral symmetry with the zero-field splitting parameters E/D ratio approximately 1/3, along with the signals of a low-spin Fe(III) complex. Temperature dependence of the EPR spectra revealed a bend temperature T ≈ 88 K attributed to spin transition of the minority Fe(III) centers. The thermal stability of the compound was investigated by (TG, DTG, DTA) techniques. New iron(III) complex with tripodal pyridoxal Schiff base ligand was prepared. Single-crystal XRD data revealed a positional disorder of a side hydroxymethyl group. The magnetic data were analysed using the spin Hamiltonian including the ZFS term. EPR measurements at X-band confirmed magnetic anisotropy of the complex. Thermal stability of the complex was investigated by (TG, DTG, DTA) techniques.

Synthesis, characterization and magnetic properties of mixed-valence iron complexes with 2-pyridyl oximes

Two new mixed-valence iron complexes with 2-pyridyl oximes, [Fe(mpko)3Fe(H2O)2(NO3)](NO3)·2H2O (1) (mpko− = methyl(2-pyridyl)ketone oximate) and [{Fe(dpko)3}2Fe](ClO4)·4H2O (2) (dpko− = bis(2-pyridyl)ketone oximate), have been prepared by reaction of FeIII with mpkoH in methanol (1) and FeII with dpkoH in methanol/water (2). Dinuclear FeII(low-spin)FeIII(high-spin) and trinuclear FeII(low-spin)FeIII(high-spin)FeII(low-spin) cations are present in the crystal structure of 1 and 2, respectively. Intermolecular hydrogen bonds in 1 lead to weak antiferromagnetic interactions between pairs of neighboring FeIII centers, which allows observation of single-ion zero-field splitting effects.

Insights into Magnetic Interactions in a Monodisperse Gd12 Fe14 Metal Cluster.

The largest Ln-Fe metal cluster [Gd12 Fe14 (μ3 -OH)12 (μ4 -OH)6 (μ4 -O)12 (TEOA)6 (CH3 COO)16 (H2 O)8 ]⋅(CH3 COO)2 (CH3 CN)2 ⋅(H2 O)20 (1) and the core-shell monodisperse metal cluster of 1 a@SiO2 (1 a=[Gd12 Fe14 (μ3 -OH)12 (μ4 -OH)6 (μ4 -O)12 (TEOA)6 (CH3 COO)16 (H2 O)8 ]2+ ) were prepared. Experimental and theoretical studies on the magnetic properties of 1 and 1 a@SiO2 reveal that encapsulation of one cluster into one silica nanosphere not only effectively decreases intermolecular magnetic interactions but also significantly increases the zero-field splitting effect of the outer layer Fe3+ ions.

Spin Crossover in Double Salts Containing Six- and Four-Coordinate Cobalt(II) Ions.

The preparation and spectroscopic and structural characterization of three cobalt(II) complexes of formulas [Co(tppz)2](dca)2 (1), [Co(tppz)2][Co(NCS)4]·MeOH (2), and [Co(tppz)2][Co(NCO)4]·2H2O (3) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine and dca = dicyanamide] are reported here. Compounds 1-3 have in common the presence of the cationic [Co(tppz)2]2+ entity where each mer-tridentate tppz ligand coordinates to the cobalt(II) ion equatorially through two pyridyl donors and axially via the pyrazine, completing the six-coordination. The electroneutrality is achieved by the organic dca group (1) and the anionic tetrakis(thiocyanato-κN)cobaltate(II) (2) and tetrakis(cyanato-κN)cobaltate(II) (3) complexes. Direct current (dc) magnetic susceptibility measurements of 1 in the temperature range 1.9-400 K show the occurrence of a thermally induced spin crossover behavior of the [Co(tppz)2]2+ unit from a high spin (S = 3/2) at higher temperatures to a low-spin (S = 1/2) at lower temperatures, with the low spin phase being reached at T ≤ 200 K. X-band electron paramagnetic resonance (EPR) measurements in solution at low temperatures were used to characterize the low spin state. An analytical expression based on the combination of the spin-orbit coupling and both first- and second-order Zeeman effects for a d7 electronic configuration was used to fit the magnetic data of 1, the values of the best-fit parameters being Cvib = 0.1367(9), λ = -168(2) cm-1, α = 1.12(1), Δ = 1626(15) cm-1, and gLS = 2.12(1). The magnetic behavior of the four-coordinate cobalt(II) ions [Co(NCS)4]2- (2) and [Co(NCO)4]2- (3) with a 4A2 ground state overlaps with the spin crossover of the [Co(tppz)2]2+ entity, the abrupt decrease of the χMT product below 15.0 K being due to zero-field splitting effects between the spin components |±1/2> and |±3/2>. The combined analysis of the dc magnetic data and the Q-band EPR spectra in the solid state of 2 and 3 led to the following sets of best-fit parameters: Cvib = 0.105(5), λ = -170(4) cm-1, α = 1.10(2), Δ = 1700(25) cm-1, gLS = 2.10(1), gHS = 2.27(1), and |D| = 3.80(2) cm-1 (2) and Cvib = 0.100(1), λ = -169(5) cm-1, α = 1.10(3), Δ = 1500(30) cm-1, gLS = 2.10(1), gHS = 2.28(1), and |D| = 4.30(2) cm-1 (3). Some evidence of slowing of the relaxation of the magnetization has been found in the out-of-phase ac signal at very low temperatures under applied dc fields of 0.1-0.4 T for 3, suggesting the occurrence of single-ion magnet behavior of its [Co(NCO)4]2- anionic entity.

Magnetic and HFEPR Studies of Exchange Coupling in a Series of μ-Cl Dicobalt Complexes.

We report three dinuclear cobalt(II) complexes, [Co(L)Cl2]2 (L = bpy, mbpy, and dmpbt), that are bridged solely by chloride ions. High-field electron paramagnetic resonance and magnetometric measurements were applied to investigate the magnetic intramolecular Co-Co interactions. Simulation results based on the multispin model reveal that the complexes are weakly ferromagnetically coupled and that the isotropic exchange coupling constants differ slightly for the three complexes. Moreover, the competing effects of zero-field splitting and magnetic coupling on the temperature-dependent magnetic susceptibility were analyzed.

Self assembly of dialkoxo bridged dinuclear Fe(III) complex of pyridoxal Schiff base with C[sbnd]C bond formation – Structure, spectral and magnetic properties

The new dinuclear Fe(III) complex, [Fe2(µ-L)2]·3(H2O)·0.38(O), [L=(E)-2-(((5-(hydroxymethyl)-2-methyl-3-oxido-1κO-pyridin-4-yl)methylene)amino-1κN)-3-(3-(hydroxymethyl)-2-methyl-5-oxido-1κO-pyridin-1-ium-4-yl)-3-oxido-1:2κ2O-propanoato-2κO]∗ (∗The second ligand has swapped indices 1 and 2, denoting the individual Fe centers.), was synthesized by reaction of FeCl3·6H2O with potassium pyridoxylidenglycinate in ethanol-water solution. Pentadentate pyridoxal Schiff base ligand was generated in situ by aldol condensation of potassium pyridoxylidenglycinate and pyridoxal. A CC coupling was achieved at room temperature under metal template reaction. The complex was characterized by X-ray structural analysis at 100K, elemental analysis, spectral and magnetic measurements. The compound crystallizes in the monoclinic space group P 21/n, with unit cell parameters: a=9.7445(15)Å, b=19.489(3)Å, c=20.862(6)Å, β=92.595(17)°. Structural analysis of the complex showed that each Fe(III) center has a distorted octahedral FeNO5 core, which is sixfold coordinated through iminic nitrogen atom, two phenoxido oxygen atoms, alkoxo oxygen atom and carboxylato oxygen atom from two Schiff-base ligands forming a rhombic arrangement of metal ions and oxygen atoms. In the structure, two Fe(III) atoms are connected with two alkoxo-bridging ligands providing covalent superexchange pathway by linkage Fe1–O–Fe2 with the Fe1⋯Fe2 distance of 3.195Å. In the crystal lattice, the noncovalent interactions, hydrogen bonds, and π-π stacking interactions expand the mononuclear units to 3D supramolecular network. The temperature dependent magnetic susceptibility data (6.5–300K)indicate that the octahedral Fe(III) centers in this complex retain high-spin S=5/2 (6A1g) in the whole temperature range. The effects of zero-field splitting (D=7.67cm−1) and an antiferromagnetic coupling (J=−14.66cm−1) between the two alkoxo-oxygen bridged Fe(III) ions are discussed.

Uranyl and uranyl-3d block cation complexes with 1,3-adamantanedicarboxylate: crystal structures, luminescence, and magnetic properties.

The reaction of 1,3-adamantanedicarboxylic acid (LH2) with uranyl nitrate under solvo-hydrothermal conditions, either alone or in the presence of additional metal cations (Co(2+), Ni(2+), or Cu(2+)) gives a series of nine complexes displaying a wide range of architectures. While [UO2(L)(H2O)]·1.25CH3CN (1) and [UO2(L)(DMF)] (2) are one-dimensional (1D) species analogous to that previously known, [H2NMe2]2[(UO2)2(L)3]·1.5H2O (3), which includes dimethylammonium counterions generated in situ, is a three-dimensional (3D) framework, and [UO2(L)(NMP)] (4) (NMP = N-methyl-2-pyrrolidone) is a braid-shaped 1D polymer. When 3d block metal ions are present and bound to 2,2'-bipyridine (bipy) coligands, their role is reduced to that of decorating species attached to uranyl-containing 1D polymers, as in [UO2M(L)2(bipy)2]·0.5H2O with M = Co (5) or Ni (6), and [(UO2)2Cu2(L)3(NO3)2(bipy)2]·0.5H2O (9), or of counterions, as in [Ni(bipy)3][(UO2)4(O)2(L)3]·3H2O (7), in which a two-dimensional (2D) assembly is built from tetranuclear uranyl-containing building units. In contrast, the heterometallic 3D framework [UO2Cu(L)2] (8) can be isolated in the absence of bipy. The emission spectra measured in the solid state display the usual uranyl vibronic fine structure, with various degrees of resolution and quenching, except for that of complex 7, which shows emission from the nickel(II) centers. The magnetic properties of complexes 5, 6, 8, and 9 were investigated, showing, in particular, the presence of zero-field splitting effects in 6 and weak antiferromagnetic interactions in 9.

Relationships between electron density and magnetic properties in water-bridged dimetal complexes.

The electron densities in two analogous dimetallic transition metal compounds, namely, [M2(μ-OH2)((t)BuCOO)4((t)BuCOOH)2(C5H5N)2] (M = Co(1), Ni(2)), were determined from combined X-ray and neutron single-crystal diffraction at 100 K. Excellent correspondence between the thermal parameters from X- and N-derived atomic displacement parameters is found, indicating high-quality X-ray data and a successful separation of thermal and electronic effects. Topological analysis of electron densities derived from high-resolution X-ray diffraction, as well as density functional theory calculations, shows no direct metal-metal bonding in either compound, while the total energy density at the bond critical points suggests stronger metal-oxygen interactions for the Ni system, in correspondence with its shorter bond distances. The analysis also allows for estimation of the relative strength of binding of terminal and bridging ligands to the metals, showing that the bridging water molecule is more strongly bound than terminal carboxylic acid, but less so than bridging carboxylates. Recently, modeling of magnetic and spectroscopic data in both of these systems has shown weak ferromagnetic interactions between the metal atoms. Factors related to large zero-field splitting effects complicate the magnetic analysis in both compounds, albeit to a much greater degree in 1. The current results support the conclusion drawn from previous magnetic and spectroscopic measurements that there is no appreciable direct communication between metal centers.

Synthesis and crystal structure determination of Mn(II) Schiff base complexes and their performance in ethene polymerization

Mn(II) complexes 1–10, of which eight are novel, were prepared in high yields by reacting MnCl2 with bidentate N,N′-imine-pyridine and N,N′-imine-quinoline-type donor ligands having different kinds of chemical modifications. The complexes were fully characterized by elemental analysis, mass spectrometry and IR spectroscopy. Crystal structures of 6, 7 and 9 were determined by X-ray crystallography and the results showed that 6 has a dimeric square-pyramidal geometry, whereas 7 and 9 have distorted octahedral structures. Magnetic studies on the high spin complexes 6 and 9 showed that the former exhibits a weak intramolecular antiferromagnetic interaction through the chloride-bridging ligands with a magnetic exchange coupling J=−0.46(6) cm−1, whereas the latter, as expected for its mononuclear structure, only shows very weak intermolecular interactions and/or zero-field splitting effects. All the complexes were activated with methylaluminoxane and investigated under low pressure (5bar) for ethene polymerization at 60°C. Complexes 1 and 6–10 showed catalytic activity. The prepared polyethenes exhibited high molar masses (296000–399000g/mol) and high melting temperatures (133–140°C). The low molar mass distribution values of 2.15–2.55 indicated single-site polymerization behavior.

New pyridine-2,3,5,6-tetracarboxylato (H4pdtc) complexes: Synthesis, crystal structures and magnetic properties of K2[Mn(H2O)(pdtc)]·3H2O 1, Na2[M3(H2O)6(pdtc)2]·6H2O (M=Mn 2, Co 3)

Three new pyridine-2,3,5,6-tetracarboxylato (H₄pdtc) complexes K₂[Mn(H₂O)(pdtc)]·3H₂O 1, Na₂[M₃(H₂O)₆(pdtc)₂]·6H₂O (M=Mn 2, Co 3) were obtained and characterized by single-crystal X-ray diffraction methods and magnetic measurements. The characteristic building blocks of 1–3 are the pdtc bridged stair-like chains ∞¹([M(H₂O)](pdtc){sub 3/3})²⁻, which results from the six-coordinated transition metal atoms bridged by pdtc⁴⁻ ligands. The infinite chains in 1 are assembled by hydrogen bonds into 2D supramolecular networks, which are held together by (4·8²) topological K⁺–H₂O networks to complete 3D architecture. While the stair-like chains in 2 and 3 are interconnected by trans-[M(H₂O)₄]²⁺ moieties to 2D polymeric layers, which are bridged by dimeric [Na₂(μ-H₂O)₂(H₂O)₂]²⁺ moieties to build up 3D framework. The magnetic properties of 1–3 were analyzed on the basis of (i) linear trinuclear M₃ models and (ii) the free Mn²⁺ and Co²⁺ ions with the zero-field splitting effect and spin–orbit coupling effect, respectively. - Graphic abstract: Synopsis. The characteristic building blocks and magnetic model of K₂[Mn(H₂O)(pdtc)]·3H₂O 1 and Na₂[M₃(H₂O)₆(pdtc)₂]·6H₂O (M=Mn 2, Co 3). Highlights: • The characteristic building blocks of 1–3 are the pdtc bridged stair-like chains. • The compound 2 and 3 are interlinked by trans-[M(H₂O)₄]²⁺ moieties to 2D layers. • The magnetic behavior of 1 was analyzed with zero-field splitting effects.more » • The magnetic behaviors were modulated with linear trinuclear model for 2 and 3.« less

Synthesis, crystal structures and magnetic behavior of NiII–4f–NiII compounds

We report here synthesis, crystal and molecular structures as well as magnetic measurement results for new heterotrinuclear Schiff base complexes having a general formula [Ni2Ln(L)2(CH3COO)2(MeOH)2]NO3·4H2O (where Ln=LaIII (1) and NdIII (2) and L=C19H18N2O4Br2=N,N′-bis(5-bromo-3-methoxysalicylidene)-1,3-diaminopropanato) and the mononuclear [NiL(H2O)2] (3) complex. The NiII/4f compounds are isostructural and crystallize in the monoclinic space group P21/n. The lanthanide(III) ion lies on the center of symmetry and is ten-coordinated forming pentagonal antiprism. Compound 3 crystallizes in orthorhombic space group Pnma with the molecule lying on the mirror plane. The complex 1 indicated a weak antiferromagnetic interaction between the terminal NiII ions. The magnetic properties of 2 revealed that the NiII–NdIII interaction is antiferromagnetic. The magnetic data for 3 show dominant zero-field splitting effect of NiII ions.

Structure and Magnetism in Fe–Gd Based Dinuclear and Chain Systems. The Interplay of Weak Exchange Coupling and Zero Field Splitting Effects

The synthesis and characterization of two Fe-Gd systems based on bpca(-) (Hbpca = bis(2-pyridilcarbonyl)amine) as bridging ligand is presented, taking the systems as a case study for structure-property correlations. Compound 1, [Fe(LS)(II)(μ-bpca)(2)Gd(NO(3))(2)(H(2)O)]NO(3)·2CH(3)NO(2), is a zigzag polymer, incorporating the diamagnetic low spin Fe(LS)(II) ion. The magnetism of 1 is entirely determined by the weak zero field splitting (ZFS) effect on the Gd(III) ion. Compound 2 is a Fe(III)-Gd(III) dinuclear compound, [Fe(LS)(III)(bpca)(μ-bpca)Gd(NO(3))(4)]·4CH(3)NO(2)·CH(3)OH, its magnetism being interpreted as due to the antiferromagnetic coupling between the S(Fe) = ½ and S(Gd) = 7/2 spins, interplayed with the local ZFS on the lanthanide center. In both systems, the d-f assembly is determined by the bridging capabilities of the ambidentate bpca(-) ligand, which binds the d ion by a tridentate moiety with nitrogen donors and the f center by the diketonate side. We propose a spin delocalization and polarization mechanism that rationalizes the factors leading to the antiferromagnetic d-f coupling. Although conceived for compound 2, the scheme can be proposed as a general mechanism. The rationalization of the weak ZFS effects on Gd(III) by multiconfiguration and spin-orbit ab initio calculations allowed us to determine the details of the small but still significant anisotropy of Gd(III) ion in the coordination sites of compounds 1 and 2. The outlined methodologies and generalized conclusions shed new light on the field of gadolinium coordination magnetochemistry.

Computation of Hyperfine Tensors for Dinuclear MnIIIMnIV Complexes by Broken‐Symmetry Approaches: Anisotropy Transfer Induced by Local Zero‐Field Splitting

Based on broken-symmetry density functional calculations, the (55)Mn hyperfine tensors of a series of exchange-coupled, mixed-valence, dinuclear Mn(III) Mn(IV) complexes have been computed. We go beyond previous quantum chemical work by fully including the effects of local zero-field splitting (ZFS) interactions in the spin projection, following the first-order perturbation formalism of Sage et al. [J. Am. Chem. Soc. 1989, 111, 7239]. This allows the ZFS-induced transfer of hyperfine anisotropy from the Mn(III) site to the Mn(IV) site to be described with full consideration of the orientations of local hyperfine and ZFS tensors. After scaling to correct for systematic deficiencies in the quantum chemically computed local ZFS tensors, good agreement with experimental (55)Mn anisotropies at the Mn(IV) site is obtained. The hyperfine coupling anisotropies on the Mn(III) site depend sensitively on structural distortions for a d(4) ion. The latter are neither fully reproduced by using a DFT-optimized coordination environment nor by using experimental structures. For very small exchange-coupling constants, the perturbation treatment breaks down and a dramatic sensitivity to the scaling of the local ZFS tensors is observed. These results are discussed with respect to ongoing work to elucidate the structure of the oxygen-evolving complex of photosystem II by analysis of the EPR spectra.

First Principle Simulation of the Temperature Dependent Magnetic Circular Dichroism of a Trinuclear Copper Complex in the Presence of Zero Field Splitting

We present a test of a recently developed density functional theory (DFT) based methodology for the calculation of magnetic circular dichroism (MCD) spectra in the presence of zero-field splitting (ZFS). The absorption and MCD spectra of the trinuclear copper complex μ(3)O ([Cu(3)(L)(μ(3)-O)](4+)), which models the native intermediate produced in the catalytic cycle of the multicopper oxidases, have been simulated from first principle within the framework of adiabatic time dependent density functional theory. The effects of the ZFS of the quartet (4)A(2) ground state on the theoretical MCD spectrum of μ(3)O have been analyzed. The simulated spectra are consistent with the experimental ones. The theoretical assignments of the MCD spectra are based on direct simulation as well as a detailed analysis of the molecular orbitals in μ(3)O. Some of the assignments differ from those given in previous studies. The ZFS effects in the presence of a strong external magnetic field (7 T) prove negligible. The change of the sign of the ZFS changes systematically the intensity of the MCD bands of the z-polarized excitations. The effect of the ZFS on the x,y-polarized excitations is not uniform.