El-Eulmi Bendeif

Giant Ising-Type Magnetic Anisotropy in Trigonal Bipyramidal Ni(II) Complexes: Experiment and Theory

This paper reports the experimental and theoretical investigations of two trigonal bipyramidal Ni(II) complexes, [Ni(Me(6)tren)Cl](ClO(4)) (1) and [Ni(Me(6)tren)Br](Br) (2). High-field, high-frequency electron paramagnetic resonance spectroscopy performed on a single crystal of 1 shows a giant uniaxial magnetic anisotropy with an experimental D(expt) value (energy difference between the M(s) = ± 1 and M(s) = 0 components of the ground spin state S = 1) estimated to be between -120 and -180 cm(-1). The theoretical study shows that, for an ideally trigonal Ni(II) complex, the orbital degeneracy leads to a first-order spin-orbit coupling that results in a splitting of the M(s) = ± 1 and M(s) = 0 components of approximately -600 cm(-1). Despite the Jahn-Teller distortion that removes the ground term degeneracy and reduces the effects of the first-order spin-orbit interaction, the D value remains very large. A good agreement between theoretical and experimental results (theoretical D(theor) between -100 and -200 cm(-1)) is obtained.

Charge Density and Electrostatic Interactions of Fidarestat, an Inhibitor of Human Aldose Reductase

The charge density and the topological features of fidarestat, an inhibitor of human aldose reductase, have been determined from ultra high-resolution X-ray diffraction data at 100 K. The modeled electron density was used to calculate the electrostatic interaction energy of fidarestat and its (2R,4S) stereoisomer with the human aldose reductase by using the ELMAM database as coded in the MoPro program. Such calculation may be extended to other protein complexes for which accurate high resolution X-ray data are available. The paper also discusses the hydrogen bonds in the fidarestat crystal. There are notably two hydrogen bonds with a pi system as an acceptor. All the chemical bonds and the intermolecular interactions, especially these two pi...H bonds, have been quantitatively studied by topological analysis. The three-dimensional electrostatic potential calculated on the molecular surface emphasizes the preferential polar binding sites of fidarestat. Theses interacting features in the molecule are crucial for drug-receptor recognition. The interactions between chemical groups in the crystal are also analyzed by computing the electrostatic energy using the latest advancements of the MoPro crystallographic software. The complexes of fidarestat and its (2R,4S) stereoisomer with human aldose reductase were modeled with a multipolar atom model transferred from our experimental electron density database. Accurate estimation of electrostatic interaction energy between inhibitors and the main residues of the protein active site is derived from this high detail level of the electron density.

Cyanocarbanion-based spin-crossover materials: photocrystallographic and photomagnetic studies of a new iron(II) neutral chain.

A new iron(II) chain of formula [Fe(abpt)(2)(tcpd)] [1; (tcpd)(2-) = [C(10)N(6)](2-) = (C[C(CN)(2)](3))(2-) = 2-dicyanomethylene-1,1,3,3-tetracyanopropanediide anion, abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole] has been synthesized and characterized by IR spectroscopy, detailed variable-temperature single-crystal X-ray diffraction, magnetic and photomagnetic measurements. The crystal structure determination of 1 reveals a one-dimensional structural architecture in which the (tcpd)(2-) cyanocarbanion acts as a μ(2)-bridging ligand and the two abpt molecules act as chelating ligands. Detailed X-ray diffraction studies as a function of the temperature (293-10 K) showed a strong modification of the iron coordination sphere, whose characteristics are in agreement with the presence of a spin-crossover transition from high spin (HS) to low spin (LS) in 1. The average Fe-N distances at room temperature, at 10 K following a flash cooling, and at 10 K after subsequent HS-to-LS relaxation are in the range expected for 100%, 50%, and 25% fractions of HS Fe(II), respectively. These observations are consistent with the presence of ca. 25% residual HS species at low temperatures, as derived from the magnetic data. The signature of a photoinduced metastable HS state in 1 has been detected by performing coupled photomagnetic and photocrystallographic analyses. The limiting T(LIESST) value associated with the light-induced excited-spin-state trapping effect was derived as 35 K, in good agreement with the thermal dependence of the unit cell volume upon irradiation. Kinetic studies governing the photoinduced HS/LS process have been recorded at different temperatures; a reverse-LIESST effect has been evidenced at 10 K as a reduction of the residual HS fraction by irradiating the sample at 830 nm.

The experimental library multipolar atom model refinement of l-aspartic acid

The crystal structure of L-aspartic acid, C(4)H(7)NO(4), has been determined using two types of refinement, viz. the standard independent atom model (IAM) and the experimental library multipolar atom model (ELMAM). The ELMAM refinement shows a good improvement of the statistical indices compared with the IAM model, notably in terms of thermal displacement parameters and bond distances involving H atoms.

Light‐Induced Bistability in the 2 D Coordination Network {[Fe(bbtr)3][BF4]2}∞: Wavelength‐Selective Addressing of Molecular Spin States

Whereas the neat polymeric Fe(II) compound {[Fe(bbtr)3 ][ClO4 ]2 }∞ (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) shows an abrupt spin transition centered at 107 K facilitated by a crystallographic symmetry breaking, in the covalently linked 2D coordination network of {[Fe(bbtr)3 ][BF4 ]2 }∞ , Fe(II) stays in the high-spin state down to 10 K. However, strong cooperative effects of elastic origin result in reversible, persistent, and wavelength-selective photoswitching between the low-spin and high-spin manifolds. This compound thus shows true light-induced bistability below 100 K. The persistent bidirectional optical switching behavior is discussed as a function of temperature, irradiation time, and intensity. Crystallographic studies reveal a photoinduced symmetry breaking and serve to establish the correlation between structure and cooperative effects. The static and kinetic behavior is explicated within the framework of the mean-field approximation.

Properties of metastable linkage NO isomers in Na2[Fe(CN)5NO]·2H2O incorporated in mesopores of silica xerogels

We study the properties of photoinduced metal-nitrosyl linkage isomers in sodium nitroprusside (SNP) as a function of particle size. By embedding the molecular complex at various concentrations into mesopores of silica xerogels the size of the particles can be adjusted. The ground state is characterized by X-ray diffraction, absorption and infrared spectroscopy. The physical properties of the photoswitched molecules were analysed by steady-state low-temperature absorption, infrared spectroscopy and by nanosecond transient absorption spectroscopy. The electronic structure as well as the activation energies of the metastable linkage isomers are independent of the particle size down to single isolated molecules, indicating that the SNP complexes are quasi-free inside the pores of the gel.

Isostructural phase transition in m-carboxyphenyl­ammonium monohydrogenphosphite

Crystals of m-carboxyphenylammonium monohydrogenphosphite, C7H8NO2+.H2PO3- (m-CPAMP), space group P2(1)/c, grown from aqueous solution undergo a reversible first-order single-crystal phase transition at Tc = 246 (2) K with a hysteresis of 3.6 K. The thermal behaviour of the sample was characterized by differential scanning calorimetry (DSC) experiments. Variations of the unit-cell parameters versus temperature between 100 and 320 K are reported. The transition from the higher-temperature phase (HTP) to the lower-temperature phase (LTP) is characterized by a unit-cell volume contraction of 1.77%. The average structure and unit-cell packing of m-CPAMP at lower temperature (100 K) are reported from accurate X-ray data sets and compared with those of the higher-temperature phase (293 K) in order to investigate the mechanism of the phase transition. The reciprocal lattice reconstruction showed a few very weak satellite reflections which will be discussed in a forthcoming paper.

Tautomerism and hydrogen bonding in guaninium phosphite and guaninium phosphate salts.

The crystal structures of three similar guaninium salts, guaninium monohydrogenphosphite monohydrate, C(5)H(6)N(5)O(+).H(2)O(3)P(-).H(2)O, guaninium monohydrogenphosphite dihydrate, C(5)H(6)N(5)O(+).H(2)O(3)P(-).2H(2)O, and guaninium dihydrogenmonophosphate monohydrate, C(5)H(6)N(5)O(+).H(2)O(4)P(-).H(2)O, are described and compared. The crystal structures have been determined from accurate single-crystal X-ray data sets collected at 100 (2) K. The two phosphite salts are monoclinic, space group P2(1)/c, with different packing and the monophosphate salt is also monoclinic, space group P2(1)/n. An investigation of the hydrogen-bond network in these guaninium salts reveals the existence of two ketoamine tautomers, the N9H form and an N7H form.

Can a Formally Zwitterionic Rhodium(I) Complex Emulate the Charge Density of a Cationic Rhodium(I) Complex? A Combined Synchrotron X-ray and Theoretical Charge-Density Study

The molecular electron densities of structurally related cationic ([(κ(2)-3-P(i)Pr(2)-2-NMe(2)-indene)Rh(COD)](CF(3)SO(3)), [1c](CF(3)SO(3))) and formally zwitterionic ([(κ(2)-3-P(i)Pr(2)-2-NMe(2)-indenide)Rh(COD)], 1z) complexes were accurately determined using synchrotron bright-source X-ray radiation at 30 K followed by multipolar refinement (COD = η(4)-1,5-cyclooctadiene). The densities were also obtained from density functional theory calculations with a large, locally dense basis set. A 28-electron ([Ar]3d(10)) core of the Rh atom was modeled by an effective core potential to obtain a density that was then augmented with relativistic cores according to the Keith-Frisch approximation. Calculations were performed at the experimental geometry and after vacuum-phase geometry optimization starting from the experimental geometry. Experimental and calculated geometries and electron-density distributions show that the electron density and electronic structure in the region of the Rh center are not significantly altered by protonation of the aromatic ring and that formal removal of CF(3)SO(3)H from [1c](CF(3)SO(3)) affords a complex 1z possessing substantial zwitterionic character (with a charge separation of ca. 0.9 electronic charge) featuring a negatively charged aromatic indenide framework. Further, the molecular electrostatic potentials of 1c and 1z exhibit similar topography around the metal, despite being drastically different in the vicinity of the indene or indenide portion of the cation (1c) and zwitterion (1z), respectively. Collectively, these observations obtained from high-level experimental and theoretical electron-density analysis confirm, for the first time, that appropriately designed zwitterionic complexes can effectively emulate the charge distribution found within ubiquitous cationic platinum-group metal catalyst complexes, in keeping with recent catalytic investigations.

The structural properties of a noncentrosymmetric polymorph of 4-aminobenzoic acid.

The crystal structure of a polymorph of 4-aminobenzoic acid (PABA), C7H7NO2, at 100 K is noncentrosymmetric, as opposed to centrosymmetric in the structures of the other known polymorphs. The two crystallographically independent PABA molecules form pseudocentrosymmetric O-H...O hydrogen-bonded dimers that are further linked by N-H...O hydrogen bonds into a three-dimensional network. The benzene rings stack in the b direction. The CO2 moieties are bent out slightly from the benzene ring plane.