Single-ion Zero-field Splitting Research Articles

Antisymmetric Exchange in a Real Copper Triangular Complex

The antisymmetric exchange, also known as the Dzyaloshinskii-Moriya interaction (DMI), is an effective interaction that may be at play in isolated complexes (with transition metals or lanthanides, for instance), nanoparticles, and highly correlated materials with adequate symmetry properties. While many theoretical works have been devoted to the analysis of single-ion zero-field splitting and to a lesser extent to symmetric exchange, only a few ab initio studies deal with the DMI. Actually, it originates from a subtle interplay between weak electronic interactions and spin-orbit couplings. This article aims to highlight the origin of this interaction from theoretical grounds in a real tri-copper(II) complex, capitalizing on previous methodological studies on bi-copper(II) model complexes. By tackling this three-magnetic-center system, we will first show that the multispin model Hamiltonian is appropriate for trinuclear (and likely for higher nuclearity) complexes, then that the correct application of the permutation relationship is necessary to explain the outcomes of the ab initio calculations, and finally, that the model parameters extracted from a binuclear model transfer well to the trinuclear complex. For a more theory-oriented purpose, we will show that the use of a simplified structural model allows one to perform more demanding electronic structure calculations. On this simpler system, we will first check that the previous transferability is still valid, prior to performing more advanced calculations on the derived two-magnetic-center model system. To this end, we will explain in detail the physics of the DMI in the copper triangle of interest, before advocating further theory/experiment efforts.

Theoretical Studies on Hexanuclear [M3(μ3-O/OH)]2 (M = Fe(III), Mn(III), and Ni(II)) Clusters: Magnetic Exchange, Magnetic Anisotropy, and Magneto-Structural Correlations.

Controlling spin Hamiltonian parameters such as magnetic exchange and magnetic anisotropy of polynuclear clusters is of great interest in the area of single molecule magnets (SMMs). Among large polynuclear clusters, hexanuclear clusters offer the best compromise in terms of size as they are often rigid, solution stable, and chemically amenable. The {M6O2} core is one of the common architectures known for many hexanuclear clusters and generally reported to possess a diamagnetic ST = 0 spin ground state, barring a few exceptions. In these clusters, there are several open questions that are poorly understood: (a) What controls the nature of magnetic exchange, which in turn dictates the ground state spin values? (b) For clusters possessing a nonzero spin ground state, what dictates the magnetic anisotropy? Here, using density functional methods, we have attempted to shed light on these two question by evaluating the exchange coupling constants in [Fe6IIIO2(OH)2{(C4N2H2SMe)2C(OH)O}2( tBuCO2)10] (1), [Fe6III(O)2(O2CH2)(O2CCH2 tBu)12(py)2] (2), [Fe6III(O2)(O)2(O2CCMe3)12(py)2] (3), [FeIII6O3(O2CMe)9(OEt)2(bpy)2]ClO4 (4), [MnIII6O2(O2CH2)(O2CPe t)11(HO2CPe t)2(O2CMe)] (5), and [NiII6(OH)4(O2C tBu)8( tBuCO2H)4] (6) complexes. We have estimated all the eight near-neighbor exchange coupling constants in these clusters. Our calculations not only agree with the experimental results but also offer insight on the origin of the spin ground state. Extensive magneto-structural correlations developed by varying M-O-M angles and M-O distances reveal that J values are extremely sensitive to small structural distortions. Correlations developed indicate that both the parameters are important for Fe(III), but for Mn(III) and Ni(II), the angles were found to play a dominant role. Quite interestingly, the computed zero-field splitting parameter D S=5 of complex 1 reveals that the exchange contribution to the anisotropy controls the sign of the ground state D value-an observation which differs from the general perception that the ground state D is controlled by the single-ion zero-field splitting parameter.

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.

Magnetic Properties of a Dinuclear Nickel(II) Complex with 2,6-Bis[(2-hydroxyethyl)methylaminomethyl]-4-methylphenolate.

Magnetic properties of dinuclear nickel(II) complex [Ni2(sym-hmp)2](BPh4)2·3.5DMF·0.5(2-PrOH) (1), where (sym-hmp)- is 2,6-bis[(2-hydroxyethyl)methylaminomethyl]-4-methylphenolate anion and DMF indicates dimethylformamide, were investigated using high-frequency and -field electron paramagnetic resonance (HFEPR). To magnetically characterize the mononuclear nickel(II) species forming the dimer, its two dinuclear zinc(II) analogues, [Zn2(sym-hmp)2](BPh4)2·3.5DMF·0.5(2-PrOH) (2) and [Zn2(sym-hmp)2](BPh4)2·2acetone·2H2O (2'), were prepared. One of them (2') was structurally characterized by X-ray diffractometry and doped with 5% mol nickel(II) ions to prepare a mixed crystal 3. From the HFEPR results on complex 1 obtained at 40 K, the spin Hamiltonian parameters of the first excited spin state (S = 1) of the dimer were accurately determined as |D1| = 9.99(2) cm-1, |E1| = 1.62(1) cm-1, and g1 = [2.25(1), 2.19(2), 2.27(2)], and for the second excited spin state (S = 2) at 150 K estimated as |D2| ≈ 3.5 cm-1. From these numbers, the single-ion zero-field splitting (ZFS) parameter of the Ni(II) ions forming the dimer was estimated as |DNi| ≈ 10-10.5 cm-1. The HFEPR spectra of 3 yielded directly the single-ion parameters for DNi = +10.1 cm-1, |ENi| = 3.1 cm-1, and giso = 2.2. On the basis of the HFEPR results, the previously obtained magnetic data (Sakiyama, H.; Tone, K.; Yamasaki, M.; Mikuriya, M. Inorg. Chim. Acta 2011, 365, 183) were reanalyzed, and the isotropic interaction parameter between the Ni(II) ions was determined as J = -70 cm-1 (Hex = -J SA·SB). Finally, density functional theory calculations yielded the J value of -90 cm-1 in a qualitative agreement with the experiment.

Structure and magnetism of novel dinuclear cobalt(II) complexes

Two dinuclear cobalt(II) complexes of the formula [Co2X4(isonia)4] have been synthesized and structurally characterized; isonia is unidentate neutral iso-nicotinamide and X are furan-3- or furan-2-carboxylate ligands. The chromophore {CoO4N2} is formed of two iso-nicotinamide ligands and three furancarboxylate ligands where two of them bear a bridging function and one is a terminal-bidentate ligand. The magnetic data confirm a weak exchange interaction (J/hc=−2cm−1) and a high single-ion zero-field splitting (D/hc=63 and 33cm−1, respectively).

Strongly correlated one-dimensional magnetic behavior ofNiTa2O6

The magnetic properties of NiTa$_2$O$_6$ were investigated by magnetic susceptibility, specific heat, electron paramagnetic resonance, neutron powder diffraction and pulse field magnetization measurements. Accompanying \textit{ab initio} DFT calculations of the spin-exchange constants complemented and supported our experimental findings that NiTa$_2$O$_6$ must be described as a quasi-1D Heisenberg $S$ = 1 spin chain system with a nearest-neighbor only anti-ferromagnetic spin-exchange interaction of 18.92(2) K. Inter-chain coupling is by about two orders of magnitude smaller. Electron paramagnetic resonance measurements on Mg$_{1-x}$Ni$_x$Ta$_2$O$_6$ ($x \approx$ $1\%$) polycrystalline samples enabled us to estimate the single-ion zero-field splitting of the $S$ = 1 states which amounts to less than $4\%$ of the nearest-neighbor spin-exchange interaction. At 0 T NiTa$_2$O$_6$ undergoes long-range anti-ferromagnetic ordering at 10.3(1) K evidenced by a $\lambda$-type anomaly in the specific heat capacity. On application of a magnetic field the specific heat anomaly is smeared out. We confirmed the magnetic structure by neutron powder diffraction measurements and at 2.00(1) K refined a magnetic moment of 1.93(5) $\mu_{\rm{B}}$ per Ni$^{2+}$ ion. Additionally, we followed the magnetic order parameter as a function of temperature. Lastly we found saturation of the magnetic moment at 55.5(5) T with a $g$-factor of 2.14(1), with an additional high field phase above 12.8(1) T. The onset of the new high field phase is not greatly effected by temperature, but rather smears out as one approaches the long-range ordering temperature.

Synthesis, structure and magnetic properties of Ni(II)–Co(II) heterodinuclear complexes with ONNO type Schiff bases as ligands

Four new heterodinuclear Ni(II)–Co(II) complexes, NiL·CoCl2·(DMF)2 (I), NiL·CoBr2·(DMF)2 (II), NiL′·CoCl2·(DMF)2 (III) and NiLDM·CoCl2(DMF)2 (IV), have been prepared in non-aqueous DMF using the ONNO type Schiff bases N,N′-bis(2-hydroxyphenylidene)-1,3-propanediamine (LH2), N,N′-bis(2-hydroxyphenylidene)-1,4-butanediamine (L′H2) and N,N′-bis(2-hydroxyphenylidene)-2,2′-dimethyl-1,3-propanediamine (LDMH2), and the inorganic salts NiCl2 and CoCl2. The complexes were characterized by elemental analysis, IR spectroscopy, thermogravimetry and X-ray diffraction. An XRD study revealed that the Ni(II) ion is situated at the center of a distorted octahedral coordination sphere formed by two iminic nitrogen and two phenolic oxygen atoms of the ONNO type Schiff base and two other oxygen atoms belonging to coordinated DMF molecules. On the other hand, the Co(II) ion is situated inside a distorted tetrahedral coordination sphere, members of which being the two halogen atoms and two oxygen atoms, bonded to form a μ-bridge.The magnetic susceptibility of the complexes was studied using a SQUID magnetometer between 2.0 and 300K at B=0.1T, and the magnetization data were taken up to B=7T at T=2.0 and 4.6K. The exchange interaction between the metal centers is of a ferromagnetic nature (typically JCo–Ni/hc=+2cm−1) and at low temperature single-ion zero-field splitting applies (DNi/hc=+8cm−1, DCo/hc=+14cm−1).

Magnetism of dinuclear benzoato cobalt(II) complexes modeled by a general bilinear exchange

Combination of the cobalt(II) benzoate with N-heterocyclic ligands L-pyridine, 4-methylpyridine, iso-quinoline, furo[3,2-c]pyridine, 2-methylfuro[3,2-c]pyridine and 2,3-dimethylfuro[3,2-c]pyridine resulted in dinuclear complexes containing two benzoato bridging ligands (bz) and one aqua ligand of the composition [(μ2-H2O)(μ2-bz)2{Co(bz)(L)2}2]·Sol. The cobalt(II) centers show a weak exchange interaction of an antiferromagnetic nature. The problem of non-collinear orientation of the local D-tensors is solved by a numerical procedure considering a general bilinear exchange. The susceptibility and magnetization data were fitted by a joint functional allowing to determine the single-ion zero-field splitting that adopts large values: D/hc of the order of 50–100cm−1.

Polydentate-ligand-supported self-assembly of heterometallic T-shaped Co4RE (RE = Gd, Tb, Y) clusters: synthesis, structure and magnetism

A series of mixed-valent heterometallic pentanuclear Co(III)(3)Co(II)RE(III) (RE = Gd (1), Tb (2), Y (3)) clusters have been rationally assembled by taking advantage of a bifunctional ligand with o-vanillin and tripodal tris(hydroxymethyl)aminomethane units. Structural determinations reveal that all compounds are isomorphous and possess a T-shaped Co(4)RE core, which comprises two nearly linear Co(2)RE subunits sharing a common RE ion. Their magnetic properties were thoroughly studied. The static magnetic susceptibility studies of 1 demonstrate the presence of weak ferromagnetic interactions between the magnetic centres and magnetic anisotropy reflected by the single ion zero-field splitting (ZFS) D term. Both 1 and 2 behave magnetically as heterodinuclear metal systems, while the magnetic behaviour of 3 is identical to an isolated Co(II) ion. Moreover, alternating-current susceptibility measurements did not exhibit any out-of-phase signal, suggesting that slow magnetic relaxation is absent above 2 K within them. These isomorphous Co(4)RE clusters offer an opportunity to systematically probe the contribution of different metal ions to the overall magnetic behaviour in Co(II)-RE(III) systems.

Structure and magnetism of mono-, di-, and trinuclear benzoato cobalt(II) complexes

Three cobalt(II) – benzoato ( bz) complexes have been prepared and structurally characterized. In the mononuclear complex trans-[Co( bz) 2(H 2O) 2( nca) 2] the benzoato ligand is unidentate ( nca = nicotinamide). The dinuclear complex [(μ 2- bz) 4{Co( qu)} 2] is a structural analog of the copper acetate ( qu = quinoline) where four bidentate benzoato ligands link two cobalt(II) pentacoordinate centers. The trinuclear complex of the composition [Co 3( bz) 6( inca) 6] contains a central hexacoordinate {( bz) 2Co( inca) 2( bz) 2} unit in which the bidentate benzoato ligands held the central and peripheral cobalt(II) centers ( inca = iso-nicotinamide); the peripheral hexacoordinate {( bz)Co( inca) 2<} units contain the terminal benzoato ligand in its bidentate function. The magnetic susceptibility data down to T = 2 K and the magnetization data up to B = 7 T reveal a considerable magnetic anisotropy due to the single-ion zero-field splitting.

Antiferromagnetic interactions through phenoxo bridges and lattice water: Synthesis, structure, and magnetic properties of new Mn(III) Schiff base complexes in combination with thiocyanate ligand

The Schiff base ligands, N,N′-bis(2-hydroxyacetophenone)-1,2-diaminoethane (acphenH 2) and N,N′-bis(2-hydroxyacetophenone)-1,3-diaminopropane (acphpnH 2), prepared in situ were used to synthesise two new Mn(III) complexes which were characterised by crystallography and variable temperature magnetic measurements. [Mn(acphen)NCS] 2 is a phenoxo-bridged dimeric compound with the thiocyanate coordinating in the usual bent mode (Mn–N–C angle, 152°) and is weakly antiferromagnetic. Since there are no significant inter-dimer contacts in the crystal, the low temperature magnetic behaviour is influenced by single ion zero-field splitting. Exact diagonalisation of the spin Hamiltonian was performed to derive the following parameters: J = −0.7 cm −1, D = −0.6 cm −1. Mn(acphpn)(H 2O)NCS is monomeric with an unusual linearly coordinated thiocyanate (Mn–N–C angle, 178°). Two lattice water molecules link the Mn(III) complex molecules through hydrogen bonds to form one-dimensional chains in the crystal. Magnetic exchange along the chain makes this compound also weakly antiferromagnetic with J ∼ –2 cm −1.

A Planar Carboxylate-Rich Tetrairon(II) Complex and Its Conversion to Linear Triiron(II) and Paddlewheel Diiron(II) Complexes

We report a series of oligonuclear carboxylate-rich high-spin ironII complexes with three different [FeIIn(mu-O2Cbiph)2n(L)m] (n = 2-4; m = 2 or 4) structural motifs, where -O2Cbiph is 2-biphenylcarboxylate and L is an exogenous ligand bound to terminal iron atoms. Solid compounds were isolated and their structural, spectroscopic, and magnetic properties thoroughly investigated. The discrete tetranuclear complexes [Fe4(mu-O2Cbiph)8(L)2] crystallize in a planar tetraironII motif in which two diiron paddlewheel units are linked in an unprecedented manner involving a mu3-1,1,3-bridging mode. X-ray crystallography reveals average Fe-Oanti bond lengths of 2.081[2] A at the dimer-dimer interface. Terminal axial positions are capped by ligands L, where L is tetrahydrofuran (THF) (1), indazole (2), pyrazole (3), 3,5-dimethylpyrazole (4), or acetamide (5). Reaction of 1 with an excess of acetonitrile affords the linear compound [Fe3(mu-O2Cbiph)6(MeCN)4] (6). The acetonitrile ligands in 6 can be replaced by THF or dimethoxyethane at elevated temperatures with retention of the structure to afford 7 and 8, respectively. Reaction of 1 or 6 with pyridine or 1-methylimidazole results in the isolation of paddlewheel dimers 9 and 10, respectively, with [Fe2(mu-O2Cbiph)4(L)2] composition. Mössbauer spectroscopy confirms the presence of high-spin ferrous ions and indicates that the two iron sites of the dimer are geometrically indistinguishable. For the tri- and tetrairon compounds, two quadrupole doublets are observed, suggesting that the iron centers do not have identical geometries. Plots of magnetic susceptibility versus temperature reveal intramolecular antiferromagnetic exchange coupling for all complexes under study. The magnetic data were fit to a theoretical model incorporating exchange coupling, single-ion zero-field splitting, and g-tensor anisotropy. The resulting magnetic parameters reveal in most cases weak antiferromagnetic exchange coupling (J typically <3 cm(-1)) and dominant zero-field-splitting parameters.

Structural, Spectroscopic, and Magnetic Study of Bis(9,10-dihydro-9-oxo-10-acridineacetate)bis(imidazole)bis(methanol) Nickel(II)

The mixed ligand complex [Ni(CMA)2(im)2(MeOH)2] (where CMA = 9,10-dihydro-9-oxo-10-acridineacetate ion, im = imidazole) was prepared, and its crystal and molecular structure were determined. The nickel ions are hexa-coordinated by four oxygen atoms of the carboxylate and hydroxyl groups and by two imidazole nitrogen atoms, to form a distorted octahedral arrangement. The structure consists of a one-dimensional network of the complex molecules connected by strong intermolecular hydrogen bonds. The weak intermolecular C-H...X hydrogen bonds and stacking interactions make up the 2-D structure. Very strong intramolecular hydrogen bonds significantly affect the geometry and vibrational characteristics of the carboxylate group. The UV-vis-NIR electronic spectrum was deconvoluted into Gaussian components. Electronic bands of the Ni(II) ion were assigned to suitable spin-allowed transitions in the D4h symmetry environment. The single ion zero-field splitting (ZFS) parameters for the S = 1 state of Ni(II), as well as the g components, have been determined by high-field and high-frequency EPR (HF-HFEPR) spectroscopy over the frequency range of 52-432 GHz and with the magnetic fields up to 14.5 T: D = 5.77(1) cm-1, E = 1.636(2) cm-1, gx = 2.29(1), gy = 2.18(1), and gz = 2.13(1). These values allowed us to simulate the powder magnetic susceptibility and field-dependent magnetization of the complex.

Anisotropic double exchange in valence-delocalized [Fe 2.5+ Fe 2.5+] cluster

Anisotropic double exchange (DE) interaction is considered for the valence-delocalized [Fe 2.5+ Fe 2.5+] clusters with strong isotropic Anderson-Hasegawa DE coupling. The combined effect of the spin–orbit interaction and electron transfer between the excited states of the mixed-valence Fe ions results in the inter-ion anisotropic DE coupling. The coefficients of anisotropic DE coupling are linearly proportional to the parameters t 0 and t μ of the electron transfer (ET) in the ground and excited cluster states. The anisotropic DE coupling results in the zero-field splitting (ZFS) specific for the valence-delocalized clusters. The ZFS of the delocalized states of the anisotropic DE origin is described by the new effective ZFS Hamiltonian H ZFS t , which is active between the cluster states of different localizations, whereas the standard ZFS Hamiltonian, H ZFS 0 , acts in the localized states. The contributions of the anisotropic DE origin to the cluster ZFS parameters are linearly proportional to the ET (DE) parameters t 0 and t μ . The anisotropic DE contribution to the cluster ZFS parameters of the ground delocalized state Φ - 0 ( S max = 9 / 2 ) is of the same order of magnitude as the single-ion ZFS contributions.

Anisotropic double exchange in a valence-delocalized [Fe 2.5+Fe 2.5+] cluster. Spin–orbit coupling in the Anderson–Hasegawa double exchange model

Anisotropic double exchange interaction is considered for the valence-delocalized [Fe 2.5+Fe 2.5+] cluster with strong Anderson–Hasegawa double exchange (DE) coupling. The combined effect of spin–orbit interaction and DE between the excited states of the mixed-valence Fe ions results in the inter-ion anisotropic DE coupling. The coefficients of anisotropic DE coupling are linearly proportional to the parameters B 0 and B μ of the DE interaction in the ground and excited cluster states. The anisotropic DE interaction operates between the cluster states of different localizations of the extra electron. The anisotropic DE coupling results in the zero-field splitting (ZFS) of the valence-delocalized ground state of the [Fe 2.5+Fe 2.5+] cluster. The ZFS of the anisotropic DE origin is specific for the delocalized mixed-valence systems and is described by the new effective ZFS Hamiltonian H ZFS t . The ZFS Hamiltonian H ZFS t includes the operator of the Anderson–Hasegawa DE interaction and is active between the cluster states of different localizations, whereas the standard ZFS Hamiltonian, H ZFS 0 , acts in the localized states. The contributions of the anisotropic DE origin to the cluster ZFS parameters are linearly proportional to the DE parameters B 0, B μ and depend on the total spin S. The anisotropic DE contribution to the cluster ZFS parameter of the ground valence-delocalized state is of the same order of magnitude as the single-ion ZFS contributions.

Single-ion and molecular contributions to the zero-field splitting in an iron(III)-oxo dimer studied by single crystal W-band EPR

Detailed knowledge of the type and strength of pair interactions between high-spin metal ions is paramount to the understanding and design of molecular magnetic materials. In this work, the anisotropic magnetic interactions in a β-diketonate-alkoxide iron(III) dimer compound, [Fe 2(OCH 3) 2(dbm) 4, Hdbm = dibenzoylmethane] (Fe2) have been investigated by single crystal electron paramagnetic resonance (EPR) in the W-band (at 95 GHz). The diamagnetic substitution method was employed using the isomorphous gallium(III)-based compound doped with iron(III) to produce Ga–Fe dimers (GaFe). The single-ion zero-field splitting (ZFS) tensor could be separately determined in GaFe with the iron ion in a local environment quasi-identical to the one in Fe2. Its principal directions are found to point in arbitrary directions, uncorrelated with the Fe–O bonds. The Fe2 EPR spectra consist of transitions within the lowest multiplet states S = 1, 2, 3, which were analyzed using the full spin Hamiltonian description of an exchange coupled pair of s = 5/2 spins. The anisotropic spin–spin interaction tensor of Fe2 possesses a principal axis close to the Fe–Fe direction and was shown to arise both from through-space (dipolar) and through-bond (anisotropic exchange) contributions. The latter involves an rhombic component J E = ( J X − J Y )/2 ≈ 0.093 cm −1 of magnitude comparable to the dipolar interaction, and even to the rhombic part of the single-ion ZFS ( E = 0.097 cm −1). Our results show that the anisotropic exchange, usually neglected for S-type ions, is significant for the anisotropic interactions in exchange-coupled iron(III) clusters, including the Fe 4 and Fe 8 families of single-molecule magnets and the antiferromagnetic iron wheels.

Origin of the fast magnetization tunneling in tetranuclear nickel single-molecule magnets

Abstract Several tetranuclear nickel(II) single-molecule magnets (SMMs) have been prepared with the general composition of [Ni(hmp)(ROH)X]4 · S, where hmp− is the monoanion of 2-hydroxymethylpyridine, X− is either Cl− or Br− and S is the solvate molecule. Magnetization versus magnetic field hysteresis loops for these Ni4 SMMs show that there is a relatively fast rate of magnetization tunneling (small coercive field) and, in certain cases, an exchange bias present. Detailed measurements have been carried out in order to determine the origin of the fast magnetization tunneling. High-field electron paramagnetic resonance (HFEPR) data were collected on a single crystal of [Zn(hmp)(dmb)Cl]4 doped with a small amount of Ni(II), where, dmb is 3,3-dimethyl-1-butanol. These variable-frequency/temperature data give values of the single-ion zero-field splitting parameters Di and Ei, and the orientations of these interactions, for the single NiII ions in a Zn3Ni complex doped into a Zn4 crystal. HFEPR data were also obtained at many frequencies and temperatures for a single crystal of isostructural [Ni(hmp)(dmb)Cl]4. Rotation of the single crystal such that the external field is positioned in the hard plane clearly establishes that the transverse zero-field interaction B 4 4 is the cause of the fast magnetization tunneling in the S = 4 ground state of this SMM. The magnitude of B 4 4 and the Ni4 D value can be related to the directionality and magnitude of the Di and Ei interactions at the individual NiII ions, determined for the doped crystal. The microenvironments and ligand dynamics were probed by means of a single-crystal X-ray structure at 12 K and by heat capacity data.

Synthesis, structures, and magnetic properties of novel mononuclear, tetranuclear, and 1D chain Mn(III) complexes involving three related asymmetrical trianionic ligands.

The manganese(III) complexes studied in this report derive from asymmetrical trianionic ligands abbreviated H(3)L(i) (i = 4-6). These ligands are obtained through reaction of salicylaldehyde with "half-units", the latter resulting from monocondensation of different diamines with phenylsalicylate,. Upon deprotonation, L(i) (i = 4-6) possess an inner N(2)O(2) coordination site with one amido, one imine, and two phenoxo functions, and an outer amido oxygen donor. The trianionic character of such ligands yields original neutral complexes with the L/Mn stoichiometry. The crystal and molecular structures of three complexes have been determined at 190 K (1) or 180 K (2 and 3). Complex 1 crystallizes in the triclinic space group P (No. 2): a = 7.8582(14) A, b = 10.9225(16) A, c = 12.4882(18) A, alpha = 67.231(14) degrees, beta = 72.134(14) degrees, gamma = 82.589(13) degrees, V = 940.6(3) A(3), Z = 2. Complex 2 crystallizes in the orthorhombic space group Pbcn (Nuomicron. 60): a = 23.8283(15) A, b = 11.1605(7) A, c = 26.152(2) A, V = 6954.8(8) A(3), Z = 8, while complex 3 crystallizes in the monoclinic space group P2(1)/c (No. 14) with a = 11.7443(14) A, b = 7.5996(10) A, c = 18.029(2) A, beta = 100.604(10) degrees, V = 1581.6(3) A(3), Z = 4. Owing to hydrogen bonds and pi-pi stackings, the mononuclear neutral molecules of 1 are arranged in a 2D network while complexes 2 and 3 are tetranuclear and polymeric (1D chain) species, respectively, owing to the bridging ability of the oxygen atom of the amido function. The experimental magnetic susceptibilities of complexes 2 and 3 indicate the occurrence of similarly weak Mn(III)-Mn(III) antiferromagnetic interactions (J = -1.1 cm(-1)). Single ion zero-field splitting of manganese(III) must be taken into account for satisfactorily fitting the data by exact calculation of the energy levels associated to the spin Hamiltonian through diagonalization of the full matrix for axial symmetry in 2 (J = - 1.1 cm(-1), D(1) = 2.2 cm(-1), D(2) = -2.8 cm(-1)), D(1) and D(2) being associated to the six- and five-coordinate Mn ions, respectively. A weaker antiferromagnetic interaction (J = - 0.2 cm(-1)) operates through pi-pi stacking in complex 1. Complex 3 is a weak ferromagnet (ordering temperature approximately 7 K) as a result of the spin canting originating from the crystal packing.

Optical Determination of the Single-Ion Zero-Field Splitting in Large Spin Clusters

The dodecametallic Cr(III) cluster has an S = 6 ground state with an axial zero-field splitting (ZFS) of DS=6 = +0.088 cm-1. Analysis of high-resolution optical data (MCD) allows us to determine the single-ion ZFS of the constituent Cr(III) ions directly (D = -1.035 cm-1). A vector coupling analysis demonstrates that the cluster ZFS is almost entirely single-ion in origin. Thus, the relative orientations of the local and cluster magnetic axes can lead to cluster ZFS of opposite sign to the single-ion even when this is the only significant contribution.

Dinuclear (Fe(II), Gd(III)) complexes deriving from hexadentate Schiff bases: synthesis, structure, and Mössbauer and magnetic properties.

The dinuclear (Fe(II), Gd(III)) complexes studied in this report derive from hexadentate Schiff base ligands abbreviated H(2)L(i)() (i = 1, 2, 3). H(2)L(1) = N,N'-bis(3-methoxysalicylidene)-1,3-diamino-2,2'-dimethyl-propane, H(2)L(2) = N,N'-bis(3-methoxysalicylidene)-1,2-diamino-2-methylpropane, and H(2)L(3) = N,N'-bis(3-methoxysalicylidene)-1,2-diaminoethane. The crystal and molecular structures of three complexes have been determined at 160 K. Depending on the solvent used in the preparation, L(1)Fe(CH(3)OH)Gd(NO(3))(3)(CH(3)OH)(2), 1, or L(1)Fe((CH(3))(2)CO)Gd(NO(3))(3), 1', is obtained from H(2)L(1). A similar complex, L(2)Fe((CH(3))(2)CO)Gd(NO(3))(3), 2, is obtained from H(2)L(2). Complex 1 crystallizes in the orthorhombic space group Pca2(1) (No. 29): a = 22.141(3) A, b = 9.4159(16) A, c = 15.2075(17) A, V = 3170.4(7) A(3), Z = 4. Complexes 1' and 2 crystallize in the monoclinic space group P2(1)/c (No. 14): 1', a = 9.6264(17) A, b = 19.662(3) A, c = 16.039(3) A, beta = 95.15(2) degrees, V = 3023.6(9) A(3), Z = 4; 2, a = 9.7821(13) A, b = 18.7725(17) A, c = 16.100(2) A, beta = 96.497(16) degrees, V = 2937.5(6) A(3), Z = 4. Complexes 1, 1', and 2 possess an Fe(O(phenoxo))(2-)Gd core. The mononuclear L(3)Fe complex could be prepared from H(2)L(3) but not the related dinuclear (Fe, Gd) species. Mössbauer spectroscopy evidences that the iron center is in the +2 oxidation state for the six complexes. The experimental magnetic susceptibility and magnetization data of complexes 1, 1', and 2 indicate the occurrence of weak Fe(II)-Gd(III) ferromagnetic interactions. Single ion zero-field splitting of the iron(II) must be taken into account for satisfactorily fitting the data by exact calculation of the energy levels associated to the spin Hamiltonian through diagonalization of the full matrix for axial symmetry (1, J = 0.50 cm(-1), D = 2.06 cm(-1); 1', J = 0.41 cm(-1), D = 3.22 cm(-1); 2, J = 0.08 cm(-1), D = 4.43 cm(-1)).