Exchange bias in Ni4 single-molecule magnets

The syntheses and physical properties are reported for three single-molecule magnets (SMMs) with the composition [Ni(hmp)(ROH)Cl]₄, where R is CH₃ (complex 1), CH₂CH₃ (complex 2) or CH₂CH₂C(CH₃)₃ (complex 3) and hmp⁻ is the monoanion of 2-hydroxymethylpyridine. The core of each complex is a distorted cube formed by four Ni^II ions and four alkoxide hmp⁻ oxygen atoms at alternating corners. Ferromagnetic exchange interactions give a S=4 ground state. Single crystal high-frequency EPR spectra clearly indicate that each of the complexes has a S=4 ground state and that there is negative magnetoanisotropy, where D is negative for the axial zero-field splitting DŜ_z². Magnetization versus magnetic field measurements made on single crystals with a micro-SQUID magnetometer indicate these Ni₄ complexes are SMMs. Exchange bias is seen in the magnetization hysteresis loops for complexes 1 and 2.     

Single-molecule magnetism behavior of [Mn12O12(O2CR)16(H2O)4]2− salts

(PPh₄)₂[Mn₁₂O₁₂(O₂CCHCl₂)₁₆(H₂O)₄] (3) has been prepared by the two-electron reduction of [Mn₁₂O₁₂(O₂CCHCl₂)₁₆(H₂O)₄] (2) using iodide. Crystallization from CH₂Cl₂/hexanes yields a mixture of two crystal forms, 3·4CH₂Cl₂·H₂O (3a) and 36CH₂Cl₂ (3b), which are triclinic and monoclinic, respectively. They are both trapped valence 2Mn(II), 6Mn(III), 4Mn(IV). DC magnetization data for dried, unsolvated 3 in 1.80–4.00 K and 10–70 kG ranges were fit to give S=10, D=−0.28 cm⁻¹, g=2.00. Frequency-dependent out-of-phase (χ″_M) signals in AC susceptibility studies on crystalline sample of 3a and 3b combined with DC relaxation decay data were fit to the Arrhenius equation to give an effective energy barrier of U_eff=18.5 and 30.3 K, respectively. Magnetization vs. DC field sweeps on single crystals of 3a and 3b gave hysteresis loops containing steps due to quantum tunneling of magnetization (QTM). The step separations yielded ∣D∣/g values of 0.087 and 0.14 cm⁻¹, and consequently U=20 and 39 K (for g=2) for 3a and 3b, respectively, suggesting that the differences in U_eff are primarily caused by changes to D. This work demonstrates the sensitivity of the magnetic properties of [Mn₁₂]²⁻ single-molecule magnets to subtle differences in their environment.     

Anisotropic exchange in a tetranuclear CoII complex

Very high-frequency (50–715 GHz) electron paramagnetic resonance (EPR) studies of the tetranuclear Co^II complex [Co(hmp)(dmb)Cl]₄ (1), where dmb is 3,3-dimethyl-1-butanol and hmp^? is the monoanion of 2-hydroxy-methylpyridine, reveal the presence of significant zero-field-splitting (ZFS) within the ground state spin multiplet. Meanwhile, low-temperature hysteresis measurements of 1 (and related Co^II₄ complexes) provide evidence for slow magnetization relaxation, suggesting that it could be a single-molecule magnet (SMM). However, EPR studies of a Zn analog of 1, doped with a small quantity of Co^II, show the ground state of the Co^II ions to be an effective spin S′ = 1/2 Kramers doublet with a highly anisotropic g-tensor. The question then arises as to the origin of the ZFS within the ground state spin multiplet of 1, as well as the slow magnetization relaxation. Here, we consider the effect of anisotropic exchange interactions between the effective spin S′ = 1/2 Kramers ions within the tetranuclear complex. Such exchange anisotropy arises naturally when one treats the ground state of high-spin Co^II as a Kramers doublet. Our model provides an explanation for the ZFS in the ground state observed via EPR, and can also account for qualitative features observed through magnetic measurements.     

The magnetic circular dichroism spectra of the linear trinuclear clusters [Fe3S4(SR)4]3− in purple aconitase and in a synthetic model

The low-temperature magnetic circular dichroism (MCD) spectra and MCD magnetization properties of the linear three-iron cluster [Fe(μ₂-S)₂Fe(μ₂-S)₂Fe]⁺ have been compared in a solution of the model compound (Et₄N)₃[Fe₃S₄(SEt)₄] and in the “purple” form of beef heart aconitase. The similarity between the spectra confirms the close equivalence of the structures in the model and the protein. Analysis of the MCD magnetization curves has been carried out in terms of a ground state spin S = 5/2 and E / D values of 0.33 (model) and 0.31 (aconitase) established from the electron paramagnetic resonance (EPR) spectra. This gives estimates of the axial zero-field splitting parameters D = +0.7 ± 0.2 cm⁻¹ and +1.5 ± 0.2 cm⁻¹ for the model and protein, respectively.     

A high-frequency EPR characterization of the S = 2 linear tri-atomic chain in Cr3(dpa)4Cl2·CH2Cl2

We report the high frequency electron paramagnetic resonance (HF-EPR) study of Cr₃(dpa)₄Cl₂, a linear tri-atomic (Cr^II)₃ chain, that was reported to be EPR silent at the X-band (9.5 GHz). Higher frequencies yield well resolved spectra for this S = 2 system even at room temperature. At 30 K, our variable frequency (34-400 GHz) EPR spectra yield a large axial zero-field splitting (D) of −1.643(1) cm⁻¹ as well as a small rhombic ZFS parameter E = 0.0339(4) cm⁻¹; with g_x = 1.9978(4), g_y = 1.9972(4), and g_z = 1.9808(4). The magnetic susceptibility measurements fully support the earlier magnetic susceptibility studies and the current EPR results. The observation of the EPR spectrum of only the S = 2 state at room temperature suggests that the ground state is well isolated from the excited states.     

EPR Studies on Branched High‐Spin Arylnitrenes

The UV (λ>305 nm) photolysis of triazide 3 in 2‐methyl‐tetrahydrofuran glass at 7 K selectively produces triplet mononitrene 4 (g=2.003, D_T=0.92 cm^?1, E_T=0 cm^?1), quintet dinitrene 6 (g=2.003, D_Q=0.204 cm^?1, E_Q=0.035 cm^?1), and septet trinitrene 8 (g=2.003, D_S=?0.0904 cm^?1, E_S=?0.0102 cm^?1). After 45 min of irradiation, the major products are dinitrene 6 and trinitrene 8 in a ratio of ～1:2, respectively. These nitrenes are formed as mixtures of rotational isomers each of which has slightly different magnetic parameters D and E. The best agreement between the line‐shape spectral simulations and the experimental electron paramagnetic resonance (EPR) spectrum is obtained with the line‐broadening parameters Γ(E_Q)=180 MHz for dinitrene 6 and Γ(E_S)=330 MHz for trinitrene 8 . According to these line‐broadening parameters, the variations of the angles Θ in rotational isomers of 6 and 8 are expected to be about ±1 and ±3°, respectively. Theoretical estimations of the magnetic parameters obtained from PBE/DZ(COSMO)//UB3LYP/6‐311+G(d,p) calculations overestimate the E and D values by 1 and 8 %, respectively. Despite the large distances between the nitrene units and the extended π systems, the zero field splitting (zfs) parameters D are found to be close to those in quintet dinitrenes and septet trinitrenes, where the nitrene centers are attached to the same aryl ring. The large D values of branched septet nitrenes are due to strong negative one‐center spin–spin interactions in combination with weak positive two‐center spin–spin interactions, as predicted by theoretical considerations.     

New compounds in the cesium sulfobromide rhenium octahedral cluster chemistry: syntheses and crystal structures of Cs4Re6S8Br6 and Cs2Re6S8Br4

The exploration of the CsReSBr system, in order to identify new phases based on octahedral cluster anions, has produced single crystals of Cs₄Re₆S₈Br₆ (1) (trigonal, space group P-6c2, a=9.7825 (3) Å, c=18.7843 (5) Å, V=1556.77 (1) ų, Z=2, density=5.09 g cm⁻³, μ=36.07 mm⁻¹) and Cs₂Re₆S₈Br₄ (2) (monoclinic, space group P2₁/n, a=6.3664 (1) Å, b=18.4483 (4) Å, c=9.3094 (2) Å, β=104.2618 (8)°, V=1059.69 (4) ų, Z=2, density=6.14 g cm⁻³, μ=45.83 mm⁻¹). These two compounds have been obtained by high-temperature solid state route. Their structures have been solved and refined from single crystal X-ray diffraction data. The structure of Cs₄Re₆S₈Br₆ presents isolated anionic cluster units inscribed in a (Cs⁺)₁₂ cuboctahedron and the one of Cs₂Re₆S₈Br₄ exhibits ReS^i-a,a-iRe inter-unit bridges. The framework of the latter presents then a strongly 1-D character.     

Breaking the Semi‐Quinoid Structure: Spin‐Switching from Strongly Coupled Singlet to Polarized Triplet State

2,7‐TMPNO (4,5,9,10‐tetramethoxypyrene‐2,7‐bis(tert‐butylnitroxide)) was found to exist in semi‐quinoid form with unprecedented strong intramolecular magnetic exchange interaction of 2 J/k_B=1185 K operating over a distance of 10 Å. Structural transformations with the activation energy of ΔE_eq=949 K were observed by varying the temperature, from more quinoid structure at low temperature to more biradicaloid structure at higher temperature. Moreover, this molecule undergoes a transient spin transition from singlet to polarized triplet state upon photoexcitation revealed by TREPR spectroscopy. The spin Hamiltonian parameters were determined to be S=1, g=2.0065, D=?0.0112 cm^?1, and E=?0.0014 cm^?1 by spectral simulation with the hybrid Eigenfield/exact diagonalization method.     

Rational design of large-spin clusters based on the hexacopper(II) siloxanolate core

The hexacopper(II) siloxanolate cage [Cu₆{(PhSiO₂)₆}₂(BuOH)₅]·3 ⁿBuOH has been synthesized by reaction between CuCl₂ and potassium phenylsiloxanolate in n-butanol, and characterized by single-crystal X-ray diffraction, magnetic measurements and high-frequency EPR (HF-EPR). The complex exhibits a torus-like structure featuring a layer of six copper(II) ions sandwiched between two cyclic (PhSiO₂)₆^6– ligands and surrounded by ⁿBuOH molecules. The magnetic properties are characterized by moderate ferromagnetic exchange interactions between the S = 1/2 copper(II) centres to give an S = 3 ground spin state. Variable temperature HF–EPR spectra evidence a hard-axis magnetic anisotropy with g_|| = 2.063, g_⊥ = 2.225 and D = 0.31 cm^–1. The cage is very soluble in organic solvents and, upon exchange of the labile ⁿBuOH ligands, it functions as a high-spin hexatopic receptor for monodentate units. Reaction with the trigonal bipyramidal copper(II) mononuclear [Cu(tmpa)CN]⁺ in CHCl₃/MeOH solution (tmpa = tris(2-pyridylmethyl)amine) affords the decacopper(II) complex [Cu₆{(PhSiO₂)₆}₂{NCCu(tmpa)}₄](PF₆)₄, whose X-ray structure and magnetic behaviour are presented. To cite this article: G.L. Abbati et al., C. R. Chimie 6 (2003).     

EPR characterization of half-integer-spin iron molecule-based magnets

We report single crystal high-frequency electron paramagnetic resonance studies of two recently discovered half-integer Fe₇ complexes: [Fe₇O₃(O₂CBu^t)₉(mda)₃(H₂O)₃] (1) and [Fe₇O₄(O₂CPh)₁₁(dmem)₂] (2). The obtained spectra confirm spin S = 5/2 ground states for both complexes. On the basis of detailed frequency and field orientation dependent studies, we find that complex 1 is a single-molecule magnet (uniaxial zero-field-splitting parameter, D < 0) while complex 2 is not (D > 0). The EPR linewidth for complex 1 is considerably narrower in comparison to spectra obtained for other single-molecule magnets, suggesting the possibility of reduced decoherence from nuclear spins (⁵⁶Fe has no nuclear moment).     

[Mn18]2+ and [Mn21]4+ single-molecule magnets

The synthesis and structural characterization of the two new Mn complexes [Mn₁₈O₁₄(O₂CMe)₁₈(hep)₄(hepH)₂(H₂O)₂](ClO₄)₂ (1) and [Mn₂₁O₁₆(O₂CMe)₁₆(hmp)₆(hmpH)₂(pic)₂(py)(H₂O)](ClO₄)₄ (3) are presented, together with a detailed study of their magnetic properties. Complex 1 possesses a ground-state spin of S=13, and the ground-state spin for 3 is estimated to be S=17/2 or 19/2. Both complexes 1 and 3 are new examples of single-molecule magnets (SMMs), displaying frequency-dependent out-of-phase AC signals, as well as magnetization vs. DC field hysteresis at temperatures below 1 K. Complex 1 straddles the classical/quantum interface by also displaying quantum tunneling of the magnetization (QTM).     

Magnetic properties of nitronylnitroxide and iminonitroxide triradicals as model compounds for generalized ferrimagnets

A novel iminonitroxide triradical, p-triIN (1), has been synthesized, in which a π-conjugated biradical with a singlet ground state and a doublet monoradical are united by σ-bonds. The intramolecular exchange interaction within the para-phenylene-based biradical moiety has been found to be antiferromagnetic (2J(π)/k_B = −30.8 K) from magnetic susceptibility measurements on a parent biradical, p-bisIN (2). The magnetic properties of 1 have been examined by magnetic susceptibility χ_m. Upon cooling, the χ_mT value for 1 has decreased and passed across 0.38 emu mol⁻¹ K, which is expected for one mol of S = 1/2 spin with g = 2.0. The magnetic behavior of 1 indicates that the ground state of the molecular assemblage of 1 in the crystal is diamagnetic, which is attributable to one of the exotic spin states as predicted in a theoretical model of generalized ferrimagnetism. The possible occurrence of the generalized ferrimagnetic spin alignment is concluded in view of the magnetic properties of 1.     

Synthesis and properties of a monomeric and a μ-oxo-bridged dimeric iron(III) complex with a tetradentate pyridine amide in-plane ligand. X-ray structure of [Fe(bpc)Cl(DMF)] [H2bpc=4,5-dichloro-1,2-bis(pyridine-2-carboxamido)benzene]

The controlled nucleophilic halide displacement reaction of [NEt₄][Fe(bpc)Cl₂] [H₂bpc=4,5-dichloro-1,2-bis(pyridine-2-carboxamido) benzene] with AgClO₄ in MeCN afforded a crystalline iron(III) complex Fe(bpc)Cl·H₂O 1. The mixed chloro-dimethylformamide (DMF) axially ligated complex [Fe(bpc)Cl(DMF)] (obtained during recrystallization of 1 from DMF; however, it loses DMF quite readily to revert back to 1) has been structurally characterized. It belongs to only a handful of mononuclear high-spin iron(III) complexes having deprotonated picolinamide ligand. The iron(III) centre is co-ordinated in the equatorial plane by two pyridine nitrogens and two deprotonated amide nitrogens of the ligand, and two axial sites are co-ordinated by a chloride ion and a DMF molecule. The metal atom has a distorted octahedral geometry. Reaction of 1 with [ⁿBu₄N][OH] in MeOH afforded a μ-oxo-bridged diiron(III) complex, [Fe(bpc)]₂O·DMF·2H₂O, 2. The spin state and the co-ordination environment of the iron(III) centres in 1 and 2 have been determined by temperature-dependent (25–300 K) magnetic susceptibility measurements in the solid state (Faraday method) and Mössbauer spectral studies at 300 K. Complex 1 behaves as a perfect S=5/2 system, in the solid-state as well as in DMF solution. The two iron(III) centres in 2 are antiferromagnetically coupled (J=−117.8 cm⁻¹) and the bridged dimeric structure is retained in DMF solution. Bridge-cleavage reactions of 2 have been demonstrated by its ready reaction with mineral acids such as HCl and MeCO₂H to generate authentic S=5/2 complexes, [Fe(bpc)Cl₂]⁻ and [Fe(bpc)(O₂CMe)₂]⁻, respectively.     

Single crystal EPR study of Mn(II)-doped cis-catena-μ-sulphato-aquotris(imidazole)cadmium(II): Mn(II) in imidazole ligated low symmetry site

EPR of Mn(II)-doped single crystals of cis-catena-μ-sulphato-aquotris(imidazole)cadmium(II) has been studied in the temperature range 300-90 K. Mn(II) replaces Cd(II) substitutionally giving rise to two magnetically inequivalent sites. The large magnitude and high asymmetry component of the D-tensor is in accordance with the low symmetry of the substitutional site. Due to the non-first order nature of the spectrum all the work had to be carried out at Q-band frequency. Forbidden transitions corresponding to Δm_I = ± 1 and ±2 are observed for a number of orientations and their positions have been calculated by perturbation theory taking into account the second order admixtures due to cross terms in D and hyperfine coupling constant A. The spin—Hamiltonian parameters at 300 K are A = −83.5, D = −790.0, E = −80.4 (all in units of 10⁻⁴ cm⁻¹) and g = 2.0014.     

Antiferromagnetic spin chain behavior and a transition to 3D magnetic order in Cu(D,L-alanine)2: Roles of H-bonds

We study the spin chain behavior, a transition to 3D magnetic order and the magnitudes of the exchange interactions for the metal-amino acid complex Cu(D,L-alanine)₂∙H₂O, a model compound to investigate exchange couplings supported by chemical paths characteristic of biomolecules. Thermal and magnetic data were obtained as a function of temperature (T) and magnetic field (B₀). The magnetic contribution to the specific heat, measured between 0.48 and 30 K, displays above 1.8 K a 1D spin-chain behavior that can be fitted with an intrachain antiferromagnetic (AFM) exchange coupling constant 2J₀=(−2.12±0.08) cm⁻¹ (defined as ℋ_ex(i,i+1) = −2J₀S_i⋅S_i+1), between neighbor coppers at 4.49 Å along chains connected by non-covalent and H-bonds. We also observe a narrow specific heat peak at 0.89 K indicating a phase transition to a 3D magnetically ordered phase. Magnetization curves at fixed T = 2, 4 and 7 K with B₀ between 0 and 9 T, and at T between 2 and 300 K with several fixed values of B₀ were globally fitted by an intrachain AFM exchange coupling constant 2J₀=(−2.27±0.02) cm⁻¹ and g = 2.091±0.005. Interchain interactions J₁ between coppers in neighbor chains connected through long chemical paths with total length of 9.51 Å cannot be estimated from magnetization curves. However, observation of the phase transition in the specific heat data allows estimating the range 0.1≤|2J₁|≤0.4 cm⁻¹, covering the predictions of various approximations. We analyze the magnitudes of 2J₀ and 2J₁ in terms of the structure of the corresponding chemical paths. The main contribution in supporting the intrachain interaction is assigned to H-bonds while the interchain interactions are supported by paths containing H-bonds and carboxylate bridges, with the role of the H-bonds being predominant. We compare the obtained intrachain coupling with studies of compounds showing similar behavior and discuss the validity of the approximations allowing to calculate the interchain interactions.     

Magnetic Anisotropy Transfer from Mono- to Polymetallic Complexes

The reaction of multi-bidentate oxamate-based copper(II) complexes with the [Ni(ⁱPrtacn)Cl₂] complex (ⁱPrtacn:1,4,7-triisopropyl-1,4,7-triazacyclononane) has been investigated. X-ray diffraction studies reveal that for all compounds the oxamato κO,κΟ’ bidentate coordination site replaces the two chloride ions in [Ni(iPrtacn)Cl₂] to form trimetallic {CuNi₂} ( 1 – 3) , hexametallic {Cu₂Ni₄} ( 4 ) and enneametallic {Cu₃Ni₆} ( 5 ) complexes. The investigation of the magnetic properties shows that Cu−Ni interactions through the oxamato bridge are in the expected range (−111 cm⁻¹, −68 cm⁻¹). For 1 – 3 , both the sign and strength of the magnetic couplings are computed independently from DFT calculations, and these estimates broadly agree with the experiments. The magnetization measurements and EPR studies reveal that 1 – 3 are anisotropic: a significant portion of the large anisotropy of the [Ni(ⁱPrtacn)Cl₂] complex is retained, resulting in a D value for the S=3/2 ground state of 5 cm⁻¹ on average. This is no longer the case for 4 and 5 where the anisotropy of the Ni(II) complexes is diluted due to the high nuclearity of the final edifices. These results show that it is possible to obtain trimetallic complexes with a high anisotropy and a high spin value for the ground state by a judicious choice of the interacting metal ions.     

Magnetic and theoretical characterization of a ferromagnetic Mn(III) dimer

Reaction of 1,1,1-tris(hydroxymethyl)ethane (H₃thme) with the complex [Mn₂O₂(bpy)₄](ClO₄)₃ produces the dimeric species [Mn₂(Hthme)₂(bpy)₂](ClO₄)₂ in high yield. Magnetic measurements in the temperature range 1.8–300 K and in fields up to 7 T reveal weak ferromagnetic exchange between the metal centres with J = +2.13 cm⁻¹. A fit of the magnetization data, assuming only the ground state is populated, gives S = 4, g = 1.71 and D = −0.65 cm⁻¹. Low temperature single crystal measurements suggest the co-existence of SMM behaviour and strong intermolecular interactions. Density functional calculations also support a weak exchange interaction between the Mn ions.     

Ruthenium(II/III) bipyridine complexes incorporating thiol-based imine functions: Synthesis,spectroscopic and redox properties

A group of five new ruthenium(II) bipyridine heterochelates of the type [Ru^II(bpy)₂L]⁺ 1a–1e have been synthesized (bpy=2,2′-bipyridine; L=anionic form of the thiol-based imine ligands, HS–C₆H₄NC(H)C₆H₄(R) (R=OMe, Me, H, Cl, NO₂). The complexes 1a−1e are 1:1 conducting and diamagnetic. The complexes 1a−1e exhibit strong MLCT transitions in the visible region and intra-ligand transitions in the UV region. In acetonitrile solvent complexes show a reversible ruthenium(III)–ruthenium(II) couple in the range 0.2–0.4 V and irreversible ruthenium(III)→ruthenium(IV) oxidation in the range 1.15–1.73 V vs. SCE. Two successive bipyridine reductions are observed in the ranges −1.43 to −1.57 and −1.67 to −1.78 V vs. SCE. The complexes are susceptible to undergo stereoretentive oxidations to the trivalent ruthenium(III) congeners. The isolated one-electron paramagnetic ruthenium(III) complex, 1c⁺ exhibits weak rhombic EPR spectrum at 77 K (g₁=2.106, g₂=2.093, g₃=1.966) in 1:1 chloroform–toluene. The EPR spectrum of 1c⁺ has been analyzed to furnish values of distortion parameters (Δ=8988 cm⁻¹; V=0.8833 cm⁻¹) and energy of the expected ligand field transitions (ν₁=1028 nm and ν₂=1186 nm) within the t₂ shell. One of the ligand field transitions has been experimentally observed at 1265 nm.     

Unraveling σ and π Effects on Magnetic Anisotropy in cis‐NiA4B2 Complexes: Magnetization,HF‐HFEPR Studies,First‐Principles Calculations,and Orbital Modeling

By using complementary experimental techniques and first‐principles theoretical calculations, magnetic anisotropy in a series of five hexacoordinated nickel(II) complexes possessing a symmetry close to C_2v, has been investigated. Four complexes have the general formula [Ni(bpy)X₂]ⁿ⁺ (bpy=2,2′‐bipyridine; X₂=bpy ( 1 ), (NCS^?)₂ ( 2 ), C₂O₄^2? ( 3 ), NO₃^? ( 4 )). In the fifth complex, [Ni(HIM₂‐py)₂(NO₃)]⁺ ( 5 ; HIM₂‐py=2‐(2‐pyridyl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazolyl‐1‐hydroxy), which was reported previously, the two bpy bidentate ligands were replaced by HIM₂‐py. Analysis of the high‐field, high‐frequency electronic paramagnetic resonance (HF‐HFEPR) spectra and magnetization data leads to the determination of the spin Hamiltonian parameters. The D parameter, corresponding to the axial magnetic anisotropy, was negative (Ising type) for the five compounds and ranged from ?1 to ?10 cm^?1. First‐principles SO‐CASPT2 calculations have been performed to estimate these parameters and rationalize the experimental values. From calculations, the easy axis of magnetization is in two different directions for complexes 2 and 3 , on one hand, and 4 and 5 , on the other hand. A new method is proposed to calculate the g tensor for systems with S=1. The spin Hamiltonian parameters (D (axial), E (rhombic), and g_i) are rationalized in terms of ordering of the 3 d orbitals. According to this orbital model, it can be shown that 1) the large magnetic anisotropy of 4 and 5 arises from splitting of the e_g‐like orbitals and is due to the difference in the σ‐donor strength of NO₃^? and bpy or HIM₂‐py, whereas the difference in anisotropy between the two compounds is due to splitting of the t_2g‐like orbitals; and 2) the anisotropy of complexes 1 – 3 arises from the small splitting of the t_2g‐like orbitals. The direction of the anisotropy axis can be rationalized by the proposed orbital model.     

2D H-bonding molecular magnets based on [Ni(mnt)2]− monoanion: Syntheses,crystal structures and magnetic properties

Two new H-bonding molecular magnets based on [Ni(mnt)₂]⁻ monoanion have been synthesized and characterized structurally. In crystal of 1, [Ni(mnt)₂]⁻ monoanions form the π-stacked sheets and the neighboring anionic sheets are held together via H-bonding interactions between –NH groups of diprotonated benzene-1,4-diamine and CN groups of mnt²⁻ ligands. In crystal of 2, the neighboring anionic dimers form an anionic column via Ni…S contacting interactions, and these anionic stacks arrange into a sheet which is parallel to crystallographic ab-plane. The H-bonding interactions between diprotonated 1,4-diazabicyclo[2.2.2]octane cations and [Ni(mnt)₂]⁻ monoanions as well as between cations and solvent MeCN molecules stabilize the lattice. Magnetic susceptibility measurement for 1 indicates an activated magnetic behavior in the high-temperature range together with a Curie tail at the lower temperature range, but the magnetic feature deviates from the magnetic exchange model for a spin dimer with S = 1/2. The magnetic nature of 2 reveals the presence of strongly antiferromagnetic interactions between the nearest-neighboring spins, the larger energy gap between spin ground and excited states results in a weakly paramagnetic property.