Six-coordinated nickel(II) complexes with benzothiadiazole Schiff-base ligands: synthesis, crystal structure, magnetic and HFEPR study.

Two new nickel(II) complexes, namely Ni(L1)2 (1) and Ni(L2)2·CH2Cl2(2) were obtained by reacting nickel(II) acetate tetrahydrate with the benzothiadiazole Schiff base ligands HL1 = 2-[4-(2,1,3-benzothiadiazole)imino]methyl-phenol or HL2 = 2-[(2,1,3-benzothiadiazol-4-ylimino)methyl]-6-methoxyphenol in the presence of Et3N. The tridentate NNO chelate ligands induce a distorted octahedral environment around the nickel(II) ions. Single crystal X-ray diffraction analysis reveals elongated Ni-N bonds with the nitrogen atom of the benzothiadiazole ring in both complexes. Intermolecular hydrogen bonds and π-π stacking interactions create two-dimensional and three-dimensional supramolecular arrays, respectively, for complexes 1 and 2. Magnetic susceptibility and high-field electron paramagnetic resonance measurements show the presence of significant magnetic anisotropy, with an axial distortion parameter D of -8--10 cm-1.

Graphite Anodes for Li-Ion Batteries: An Electron Paramagnetic Resonance Investigation.

Graphite is the most commercially successful anode material for lithium (Li)-ion batteries: its low cost, low toxicity, and high abundance make it ideally suited for use in batteries for electronic devices, electrified transportation, and grid-based storage. The physical and electrochemical properties of graphite anodes have been thoroughly characterized. However, questions remain regarding their electronic structures and whether the electrons occupy localized states on Li, delocalized states on C, or an admixture of both. In this regard, electron paramagnetic resonance (EPR) spectroscopy is an invaluable tool for characterizing the electronic states generated during electrochemical cycling as it measures the properties of the unpaired electrons in lithiated graphites. In this work, ex situ variable-temperature (10-300 K), variable-frequency (9-441 GHz) EPR was carried out to extract the g tensors and line widths and understand the effect of metallicity on the observed EPR spectra of electrochemically lithiated graphites at four different states of lithiation. We show that the increased resolution offered by EPR at high frequencies (>300 GHz) enables up to three different electron environments of axial symmetry to be observed, revealing heterogeneity within the graphite particles and the presence of hyperfine coupling to Li nuclei. Importantly, our work demonstrates the power of EPR spectroscopy to investigate the local electronic structure of graphite at different lithiation stages, paving the way for this technique as a tool for screening and investigating novel materials for use in Li-ion batteries.

IL-17 and IL-22 are pivotal cytokines to delay wound healing of S. aureus and P. aeruginosa infected skin.

Introduction: Although the presence of pathogens in skin wounds is known to delay the wound healing process, the mechanisms underlying this delay remain poorly understood. In the present study, we have investigated the regulatory role of proinflammatory cytokines on the healing kinetics of infected wounds. Methods: We have developed a mouse model of cutaneous wound healing, with or without wound inoculation with Staphylococcus aureus and Pseudomonas aeruginosa, two major pathogens involved in cutaneous wound bacterial infections. Results: Aseptic excision in C57BL/6 mouse skin induced early expression of IL-1ß, TNFα and Oncostatin M (OSM), without detectable expression of IL-22 and IL-17A/F. S. aureus and P. aeruginosa wound inoculation not only increased the expression of IL-1ß and OSM, but also induced a strong cutaneous expression of IL-22, IL-17A and IL-17F, along with an increased number of infiltrating IL-17A and/or IL-22-producing γδ T cells. The same cytokine expression pattern was observed in infected human skin wounds. When compared to uninfected wounds, mouse skin infection delayed the wound healing process. Injection of IL-1α, TNFα, OSM, IL-22 and IL-17 together in the wound edges induced delayed wound healing similar to that induced by the bacterial infection. Wound healing experiments in infected Rag2KO mice (deficient in lymphocytes) showed a wound healing kinetic similar to uninfected Rag2KO mice or WT mice. Rag2KO infected-skin lesions expressed lower levels of IL-17 and IL-22 than WT, suggesting that the expression of these cytokines is mainly dependent on γδ T cells in this model. Wound healing was not delayed in infected IL-17R/IL-22KO, comparable to uninfected control mice. Injection of recombinant IL-22 and IL-17 in infected wound edges of Rag2KO mice re-establish the delayed kinetic of wound healing, as in infected WT mice. Conclusion: These results demonstrate the synergistic and specific effects of IL-22 and IL-17 induced by bacterial infection delay the wound healing process, regardless of the presence of bacteria per se. Therefore, these cytokines play an unexpected role in delayed skin wound healing.

Chemical tuning of spin clock transitions in molecular monomers based on nuclear spin-free Ni(ii).

We report the existence of a sizeable quantum tunnelling splitting between the two lowest electronic spin levels of mononuclear Ni complexes. The level anti-crossing, or magnetic "clock transition", associated with this gap has been directly monitored by heat capacity experiments. The comparison of these results with those obtained for a Co derivative, for which tunnelling is forbidden by symmetry, shows that the clock transition leads to an effective suppression of intermolecular spin-spin interactions. In addition, we show that the quantum tunnelling splitting admits a chemical tuning via the modification of the ligand shell that determines the crystal field and the magnetic anisotropy. These properties are crucial to realize model spin qubits that combine the necessary resilience against decoherence, a proper interfacing with other qubits and with the control circuitry and the ability to initialize them by cooling.

Correction: De novo prediction of cross-effect efficiency for magic angle spinning dynamic nuclear polarization.

Correction for 'De novo prediction of cross-effect efficiency for magic angle spinning dynamic nuclear polarization' by Frédéric Mentink-Vigier et al., Phys. Chem. Chem. Phys., 2019, 21, 2166-2176, DOI: 10.1039/C8CP06819D.

Quantum dynamics of a single molecule magnet on superconducting Pb(111).

Magnetic materials interfaced with superconductors may reveal new physical phenomena with potential for quantum technologies. The use of molecules as magnetic components has already shown great promise, but the diversity of properties offered by the molecular realm remains largely unexplored. Here we investigate a submonolayer of tetrairon(III) propeller-shaped single molecule magnets deposited on a superconducting lead surface. This material combination reveals a strong influence of the superconductor on the spin dynamics of the single molecule magnet. It is shown that the superconducting transition to the condensate state switches the single molecule magnet from a blocked magnetization state to a resonant quantum tunnelling regime. Our results open perspectives to control single molecule magnetism via superconductors and to use single molecule magnets as local probes of the superconducting state.

The Origin of Magnetic Anisotropy and Single-Molecule Magnet Behavior in Chromium(II)-Based Extended Metal Atom Chains.

Chromium(II)-based extended metal atom chains have been the focus of considerable discussion regarding their symmetric versus unsymmetric structure and magnetism. We have now investigated four complexes of this class, namely, [Cr3(dpa)4X2] and [Cr5(tpda)4X2] with X = Cl- and SCN- [Hdpa = dipyridin-2-yl-amine; H2tpda = N2,N6-di(pyridin-2-yl)pyridine-2,6-diamine]. By dc/ac magnetic techniques and EPR spectroscopy, we found that all these complexes have easy-axis anisotropies of comparable magnitude in their S = 2 ground state (|D| = 1.5-1.8 cm-1) and behave as single-molecule magnets at low T. Ligand-field and DFT/CASSCF calculations were used to explain the similar magnetic properties of tri- versus pentachromium(II) strings, in spite of their different geometrical preferences and electronic structure. For both X ligands, the ground structure is unsymmetric in the pentachromium(II) species (i.e., with an alternation of long and short Cr-Cr distances) but is symmetric in their shorter congeners. Analysis of the electronic structure using quasi-restricted molecular orbitals (QROs) showed that the four unpaired electrons in Cr5 species are largely localized in four 3d-like QROs centered on the terminal, "isolated" Cr2+ ion. In Cr3 complexes, they occupy four nonbonding combinations of 3d-like orbitals centered only on the two terminal metals. In both cases, then, QRO eigenvalues closely mirror the 3d-level pattern of the terminal ions, whose coordination environment remains quite similar irrespective of chain length. We conclude that the extent of unpaired-electron delocalization has little impact on the magnetic anisotropy of these wire-like molecular species.

De novo prediction of cross-effect efficiency for magic angle spinning dynamic nuclear polarization.

Magic angle spinning dynamic nuclear polarization (MAS-DNP) has become a key approach to boost the intrinsic low sensitivity of NMR in solids. This method relies on the use of both stable radicals as polarizing agents (PAs) and suitable high frequency microwave irradiation to hyperpolarize nuclei of interest. Relating PA chemical structure to DNP efficiency has been, and is still, a long-standing problem. The complexity of the polarization transfer mechanism has so far limited the impact of analytical derivation. However, recent numerical approaches have profoundly improved the basic understanding of the phenomenon and have now evolved to a point where they can be used to help design new PAs. In this work, the potential of advanced MAS-DNP simulations combined with DFT calculations and high-field EPR to qualitatively and quantitatively predict hyperpolarization efficiency of particular PAs is analyzed. This approach is demonstrated on AMUPol and TEKPol, two widely-used bis-nitroxide PAs. The results notably highlight how the PA structure and EPR characteristics affect the detailed shape of the DNP field profile. We also show that refined simulations of this profile using the orientation dependency of the electron spin-lattice relaxation times can be used to estimate the microwave B1 field experienced by the sample. Finally, we show how modelling the nuclear spin-lattice relaxation times of close and bulk nuclei while accounting for PA concentration allows for a prediction of DNP enhancement factors and hyperpolarization build-up times.

Crowding out: ligand modifications and their structure directing effects on brucite-like {Mx(µ3-OH)y} (M = Co(ii), Ni(ii)) core growth within polymetallic cages.

Previous employment of the ligands 2-methoxy-6-[(methylimino)methyl]phenol (L1H) and 2-methoxy-6-[(phenylimino)methyl]phenol (L2H) has resulted in the self-assembly of pseudo metallocalix[6]arene complexes of general formulae: [M7(µ3-OH)6(Lx)6](NO3)y (M = Ni(ii), x = 1, y = 2 (1) and Co(ii/iii), x = 2, y = 3 (2)). Extrapolating upon this work, we report the coordination chemistry of ligands 2-methoxy-6-{[(2-methoxyphenyl)imino]methyl}phenol (L3H), 2-[(benzylimino)methyl]-6-methoxyphenol (L4H), 2-[(benzylamino)methyl]-6-methoxyphenol (L5H) and 2-[(benzylamino)methyl]-4-bromo-6-methoxyphenol (L6H), whose structures are modifications of ligands L1-2H. These ligands are employed in the synthesis and characterisation of the dimetallic complex [Ni(ii)2(L3)3(H2O)](NO3)·2H2O·3MeOH (3); the monometallic complexes [Ni(ii)(L4)2] (4) and [Co(iii)(L4)3]·H2O·MeOH (5a); and the tetranuclear pseudo metallocalix[4]arene complexes: [(NO3)⊂Co(ii)4(µ3-OH)2(L5)4(H2O)2](NO3)·H2O (6), [(NO3)⊂Ni(ii)4(µ3-OH)2(L5)4(H2O)2](NO3)·H2O (7) and [Ni(ii)4(µ3-OH)2(L6)4(NO3)2]·MeCN (8). The tetrametallic 'butterfly' core topologies in 6-8 are discussed with respect to their structural and topological relationship with their heptanuclear [M7] (M = Co(ii), Ni(ii)) pseudo metallocalix[6]arene ancestors (1 and 2).

Oncostatin M is overexpressed in skin squamous-cell carcinoma and promotes tumor progression.

Cutaneous squamous cell carcinoma (cSCC) is the second most common keratinocyte malignancy and accounts for 20% of skin cancer deaths. Cancer is closely related to inflammation, but the contribution of the tumor microenvironment to cSCC development is poorly understood. We previously showed that oncostatin M (OSM), a cytokine belonging to the IL-6 family, promotes normal keratinocyte proliferation and migration, skin inflammation, and epidermal hyperplasia, both in vitro and in vivo. Here, we show that OSM is overexpressed in human cSCC and is associated with type 1 immune polarization. In vitro, OSM induced STAT-3 and ERK signaling, modified the expression of genes involved in cytokine signaling, proliferation, inhibition of apoptosis, and immune responses, and promoted proliferation and migration of malignant keratinocyte PDVC57 cells. PDVC57 cells grafted in the skin of mice led to rapid cSCC development, associated with OSM expression by tumor-infiltrating neutrophils. Finally, the absence of OSM (OSM-KO mice) led to a 30% reduction of tumor size and reduced M2 polarization in the tumor microenvironment. Globally, these results support a pro-tumoral role of OSM in cSCC development and suggest that a new therapeutic approach targeting this cytokine could be considered.

Filling the Gap in Extended Metal Atom Chains: Ferromagnetic Interactions in a Tetrairon(II) String Supported by Oligo-α-pyridylamido Ligands.

The stringlike complex [Fe4(tpda)3Cl2] (2; H2tpda = N2, N6-bis(pyridin-2-yl)pyridine-2,6-diamine) was obtained as the first homometallic extended metal atom chain based on iron(II) and oligo-α-pyridylamido ligands. The synthesis was performed under strictly anaerobic and anhydrous conditions using dimesityliron, [Fe2(Mes)4] (1; HMes = mesitylene), as both an iron source and a deprotonating agent for H2tpda. The four lined-up iron(II) ions in the structure of 2 (Fe···Fe = 2.94-2.99 Å, Fe···Fe···Fe = 171.7-168.8°) are wrapped by three doubly deprotonated twisted ligands, and the chain is capped at its termini by two chloride ions. The spectroscopic and electronic properties of 2 were investigated in dichloromethane by UV-vis-NIR absorption spectroscopy, 1H NMR spectroscopy, and cyclic voltammetry. The electrochemical measurements showed four fully resolved, quasi-reversible one-electron-redox processes, implying that 2 can adopt five oxidation states in a potential window of only 0.8 V. Direct current (dc) magnetic measurements indicate dominant ferromagnetic coupling at room temperature, although the ground state is only weakly magnetic. On the basis of density functional theory and angular overlap model calculations, this magnetic behavior was explained as being due to two pairs of ferromagnetically coupled iron(II) ions ( J = -21 cm-1 using JS i·S j convention) weakly antiferromagnetically coupled with each other. Alternating-current susceptibility data in the presence of a 2 kOe dc field and at frequencies up to 1.5 kHz revealed the onset of slow magnetic relaxation below 2.8 K, with the estimated energy barrier Ueff/ kB = 10.1(1.3) K.

From Positive to Negative Zero-Field Splitting in a Series of Strongly Magnetically Anisotropic Mononuclear Metal Complexes.

A series of mononuclear [M(hfa)2(pic)2] (Hhfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; pic = 4-methylpyridine; M = FeII, CoII, NiII, ZnII) compounds were obtained and characterized. The structures of the complexes have been resolved by single-crystal X-ray diffraction, indicating that, apart from the zinc derivative, the complexes are in a trans configuration. Moreover, a dramatic lenghthening of the Fe-N distances was observed, whereas the nickel(II) complex is almost perfectly octahedral. The magnetic anisotropy of these complexes was thoroughly studied by direct-current (dc) magnetic measurements, high-field electron paramagnetic resonance, and infrared (IR) magnetospectroscopy: the iron(II) derivative exhibits an out-of-plane anisotropy (DFe = -7.28 cm-1) with a high rhombicity, whereas the cobalt(II) and nickel(II) complexes show in-plane anisotropy (DCo ∼ 92-95 cm-1; DNi = 4.920 cm-1). Ab initio calculations were performed to rationalize the evolution of the structure and identify the excited states governing the magnetic anisotropy along the series. For the iron(II) complex, an out-of-phase alternating-current (ac) magnetic susceptibility signal was observed using a 0.1 T dc field. For the cobalt(II) derivative, the ac magnetic susceptibility shows the presence of two field-dependent relaxation phenomena: at low field (500 Oe), the relaxation process is beyond single-ion behavior, whereas at high field (2000 Oe), the relaxation of magnetization implies several mechanisms including an Orbach process with Ueff = 25 K and quantum tunneling of magnetization. The observation by µ-SQUID magnetization measurements of hysteresis loops of up to 1 K confirmed the single-ion-magnet behavior of the cobalt(II) derivative.

Evidence of Organic Luminescent Centers in Sol-Gel-Synthesized Yttrium Aluminum Borate Matrix Leading to Bright Visible Emission.

Yttrium aluminum borate (YAB) powders prepared by sol-gel process have been investigated to understand their photoluminescence (PL) mechanism. The amorphous YAB powders exhibit bright visible PL from blue emission for powders calcined at 450 °C to broad white PL for higher calcination temperature. Thanks to 13 C labelling, NMR and EPR studies show that propionic acid initially used to solubilize the yttrium nitrate is decomposed into aromatic molecules confined within the inorganic matrix. DTA-TG-MS analyses show around 2 wt % of carbogenic species. The PL broadening corresponds to the apparition of a new band at 550 nm, associated with the formation of aromatic species. Furthermore, pulsed ENDOR spectroscopy combined with DFT calculations enables us to ascribe EPR spectra to free radicals derived from small (2 to 3 rings) polycyclic aromatic hydrocarbons (PAH). PAH molecules are thus at the origin of the PL as corroborated by slow afterglow decay and thermoluminescence experiments.

Electronic Structure and Magnetic Anisotropy in Lanthanoid Single-Ion Magnets with C3 Symmetry: The Ln(trenovan) Series.

We report the syntheses and the magnetic characterization of a new series of lanthanide complexes, in which the Ce, Nd, Gd, Dy, Er, and Yb derivatives show single-molecule magnet behavior. These complexes, named Ln(trenovan), where H3trenovan is tris(((3-methoxysalicylidene)amino)ethyl)amine, exhibit trigonal symmetry and the Ln(III) ion is heptacoordinated. Their molecular structure is then very similar to that of the previously reported Ln(trensal) series, where H3trensal is 2,2',2″-tris(salicylideneimino)triethylamine. This prompted us to use the spectroscopic and magnetic properties of the Ln(trensal) family (Ln = Nd, Tb, Dy, Ho, Er, and Tm) to obtain a set of crystal-field parameters to be used as starting point to determine the electronic structures and magnetic anisotropy of the analogous Ln(trenovan) complexes using the CONDON computational package. The obtained results were then used to discuss the electron paramagnetic resonance (EPR) and ac susceptibility results. As a whole, the obtained results indicate for this type of complexes single-molecule magnet behavior is not related to the presence of an anisotropy barrier, due to a charge distribution of the ligand around the lanthanoid, which results in highly mixed ground states in terms of MJ composition of the states. The crucial parameter in determining the slow relaxation of the magnetization is then rather the number of unpaired electrons (only Kramers ions showing in-field slow relaxation) than the shape of the charge distribution for different Ln(III).

Pentagonal Bipyramid FeII Complexes: Robust Ising-Spin Units towards Heteropolynuclear Nanomagnets.

Pentagonal bipyramid FeII complexes have been investigated to evaluate their potential as Ising-spin building units for the preparation of heteropolynuclear complexes that are likely to behave as single-molecule magnets (SMMs). The considered monometallic complexes were prepared from the association of a divalent metal ion with pentadentate ligands that have a 2,6-diacetylpyridine bis(hydrazone) core (H2 LN3O2R ). Their magnetic anisotropy was established by magnetometry to reveal their zero-field splitting (ZFS) parameter D, which ranged between -4 and -13 cm-1 and was found to be modulated by the apical ligands (ROH versus Cl). The alteration of the D value by N-bound axial CN ligands, upon association with cyanometallates, was also assessed for heptacoordinated FeII as well as for related NiII and CoII derivatives. In all cases, N-coordinated cyanide ligands led to large magnetic anisotropy (i.e., -8 to -18 cm-1 for Fe and Ni, +33 cm-1 for Co). Ab initio calculations were performed on three FeII complexes, which enabled one to rationalize the role of the ligand on the nature and magnitude of the magnetic anisotropy. Starting from the pre-existing heptacoordinated complexes, a series of pentanuclear compounds were obtained by reactions with paramagnetic [W(CN)8 ]3- . Magnetic studies revealed the occurrence of ferromagnetic interactions between the spin carriers in all the heterometallic systems. Field-induced slow magnetic relaxation was observed for mononuclear FeII complexes (Ueff /kB up to 53 K (37 cm-1 ), τ0 =5×10-9  s), and SMM behavior was evidenced for a heteronuclear [Fe3 W2 ] derivative (Ueff /kB =35 K and τ0 =4.6 10-10  s), which confirmed that the parent complexes were robust Ising-type building units. High-field EPR spectroscopic investigation of the ZFS parameters for a Ni derivative is also reported.

Efficient cross-effect dynamic nuclear polarization without depolarization in high-resolution MAS NMR.

Dynamic nuclear polarization (DNP) has the potential to enhance the sensitivity of magic-angle spinning (MAS) NMR by many orders of magnitude and therefore to revolutionize atomic resolution structural analysis. Currently, the most widely used approach to DNP for studies of chemical, material, and biological systems involves the cross-effect (CE) mechanism, which relies on biradicals as polarizing agents. However, at high magnetic fields (≥5 T), the best biradicals used for CE MAS-DNP are still far from optimal, primarily because of the nuclear depolarization effects they induce. In the presence of bisnitroxide biradicals, magic-angle rotation results in a reverse CE that can deplete the initial proton Boltzmann polarization by more than a factor of 2. In this paper we show that these depolarization losses can be avoided by using a polarizing agent composed of a narrow-line trityl radical tethered to a broad-line TEMPO. Consequently, we show that a biocompatible trityl-nitroxide biradical, TEMTriPol-1, provides the highest MAS NMR sensitivity at ≥10 T, and its relative efficiency increases with the magnetic field strength. We use numerical simulations to explain the absence of depolarization for TEMTriPol-1 and its high efficiency, paving the way for the next generation of polarizing agents for DNP. We demonstrate the superior sensitivity enhancement using TEMTriPol-1 by recording the first solid-state 2D 13C-13C correlation spectrum at natural isotopic abundance at a magnetic field of 18.8 T.

Slow magnetic relaxation in a dimeric Mn2Ca2 complex enabled by the large Mn(iii) rhombicity.

In this paper we present the characterization of a complex with the formula [Mn2Ca2(hmp)6(H2O)4(CH3CN)2](ClO4)4 (1), where hmp-H = 2-(hydroxymethyl)pyridine. Compound 1 crystallizes in the monoclinic space group C2/c with the cation lying on an inversion centre. Static magnetic susceptibility, magnetization and heat capacity measurements reflect a unique Mn(iii) valence state, and single-ion ligand field parameters with remarkable large rhombic distortion (D/kB = -6.4 K, E/kB = -2.1 K), in good agreement with the high-field electron paramagnetic resonance experiments. At low temperature Mn2Ca2 cluster behaves as a system of ferromagnetically coupled (J/kB = 1.1 K) Mn dimers with a ST = 4 and mT = ±4 ground state doublet. Frequency dependent ac susceptibility measurements reveal the slow magnetic relaxation characteristic of a single molecule magnet (SMM) below T = 4 K. At zero magnetic field, an Orbach-type spin relaxation process (τ ∼ 10-5 s) with an activation energy Ea = 5.6 K is observed, enabled by the large E/D rhombicity of the Mn(iii) ions. Upon the application of a magnetic field, a second, very slow process (τ ∼ 0.2 s) is observed, attributed to a direct relaxation mechanism with enhanced relaxation time owing to the phonon bottleneck effect.

Slow Magnetic Relaxation in Weak Easy-Plane Anisotropy: the Case of a Combined Magnetic and HFEPR Study.

Two pseudotetrahedral cobalt(II) complexes exhibiting slow magnetic relaxation under an applied direct-current field are investigated. The weak easy-plane anisotropy is accurately determined by high-field/high-frequency electron paramagnetic resonance spectroscopy as D = 2.57 cm-1 and E = 0.82 cm-1 for 1 and D = 5.56 cm-1 and E = 1.05 cm-1 for 2. In addition, hysteresis loops are observed for the two compounds at very low temperatures.

Magnetic Anisotropy in Pentacoordinate NiII and CoII Complexes: Unraveling Electronic and Geometrical Contributions.

The magnetic properties of the pentacoordinate [MII (Me4 cyclam)N3 ]+ (Me4 cyclam=tetramethylcyclam; N3 =azido; M=Ni, Co) complexes were investigated. Magnetization and EPR studies indicate that they have an easy plane of magnetization with axial anisotropy parameters D close to 22 and greater than 30 cm-1 for the Ni and Co complexes, respectively. Ab initio calculations reproduced the experimental values of the zero-field splitting parameters and allowed the orientation of the anisotropy tensor axes with respect to the molecular frame to be determined. For M=Ni, the principal anisotropy axis lies along the Ni-Nazido direction perpendicular to the Ni(Me4 cyclam) mean plane, whereas for M=Co it lies in the Co(Me4 cyclam) mean plane and thus perpendicular to the Co-Nazido direction. These orientations match one of the possible solutions experimentally provided by single-crystal cantilever torque magnetometry. To rationalize the geometry and its impact on the orientation of the anisotropy tensor axis, calculations were carried out on model complexes [NiII (NCH)5 ]2+ and [CoII (NCH)5 ]2+ by varying the geometry between square pyramidal and trigonal bipyramidal. The geometry of the complexes was found to be the result of a compromise between the electronic configuration of the metal ion and the structure-orienting effect of the Me4 cyclam macrocycle. Moreover, the orientation of the anisotropy axes is mainly dependent on the geometry of the complexes.

Unraveling σ and π Effects on Magnetic Anisotropy in cis-NiA4 B2 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 C2v , has been investigated. Four complexes have the general formula [Ni(bpy)X2 ]n+ (bpy=2,2'-bipyridine; X2 =bpy (1), (NCS- )2 (2), C2 O42- (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 gi ) 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 eg -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 t2g -like orbitals; and 2) the anisotropy of complexes 1-3 arises from the small splitting of the t2g -like orbitals. The direction of the anisotropy axis can be rationalized by the proposed orbital model.