Georges Bravic

Co(II) molecular complexes as a reference for the spin crossover in Fe(II) analogues

The crystal structures of a series of cobalt(II) molecular complexes, [Co(PM-L)2(NCS)2] [PM = N-2-pyridylmethylene, L = 4-(aminobiphenyl) or 4-(phenylethynyl)aniline], are investigated and compared to the analogous iron(II) complexes, [Fe(PM-L)2(NCS)2], already known in the literature. At room temperature, the Co(II) complexes prove to be isostructural with the iron(II) complexes. An interesting point is that the iron complexes, unlike the cobalt complexes, undergo a spin crossover at low temperature. Hence, a comparison of the temperature dependence of the structural properties of the Co(II) and the Fe(II) complexes underlines some structural features of the spin crossover. Comparative deformation of the lattices and thermal expansion tensors are first discussed. Then, new parameters to estimate the distortion and the contraction at the spin crossover of the FeN6 coordination sphere are presented, thereby allowing the estimation of the reduction of the volume of the octahedron to around 3 A3 (25%). As well, comparative discussions on the intermolecular contact modifications with temperature are proposed. In the above considerations the cobalt series is therefore used as a reference to distinguish between the effects of the spin crossover and the purely thermal effects.

Structural approach of the features of the spin crossover transition in iron(II) compounds

We have determined the crystal structures, both in high and low spin state, of four Fe(PM-L) 2 (NCS) 2 complexes, where PM is N-2′-pyridylmethylene and the aromatic subunit L is 4-aminoterphenyl (TeA), 4-(phenylazo)aniline (AzA), 4-aminobiphenyl (BiA) or 4-(phenylethynyl)aniline (PEA). As previously reported, these compounds undergo a spin crossover at low temperature with different features of transition: very smooth and incomplete for Fe(PM-TeA) 2 (NCS) 2 , smooth with almost no hysteresis for Fe(PM-AzA) 2 (NCS) 2 , unusually abrupt for Fe(PM-BiA) 2 (NCS) 2 and abrupt with a very large hysteresis (37 K) for Fe(PM-PEA) 2 (NCS) 2 . In Fe(PM-BiA) 2 (NCS) 2 , Fe(PM-TeA) 2 (NCS) 2 and Fe(PM-AzA) 2 (NCS) 2 the spin conversion is not associated with a large structural phase transition and the space group is the same above and below the temperature of transition: orthorhombic Pccn for the two first and monoclinic P2 1 /c for the third. On the other hand, Fe(PM-PEA) 2 (NCS) 2 undergoes a change in the crystal symmetry from P2 1 /c (high spin) to Pccn (low spin) which corresponds to a strong re-organisation of the iron atom network. The evolution as a function of temperature of the FeN 6 core as well as of the intramolecular characteristics are almost identical in all four compounds. To a first approximation, the crystal packing is similar in all of the structures except that the P2 1 /c structures develop an asymmetrical molecular environment. Nevertheless, a close examination of the intermolecular interactions, classified as intra- and inter-sheet, show some differences. The intrasheet and the intersheet interactions are stronger in Fe(PM-BiA) 2 (NCS) 2 and Fe(PM-PEA) 2 (NCS) 2 than either in Fe(PM-TeA) 2 (NCS) 2 where no ‘second’ neighbour intrasheet contacts are created, or in Fe(PM-AzA) 2 (NCS) 2 where the intersheet interactions are weak. Thus, the abruptness of the transition is attributed to the combination of close intrasheet and intersheet contacts. The hysteresis effect in Fe(PM-PEA) 2 (NCS) 2 is connected to the phase transition which could occur due to an irregular iron atom network associated with very short carbon-carbon intermolecular contacts at high temperature, not found in Fe(PM-AzA) 2 (NCS) 2 which shows the same irregular iron atom network.

A spin transition molecular material with a wide bistability domain

[Fe(hyptrz)3](4-chloro-3-nitrophenylsulfonate)22 H2O (1; hyptrz=4-(3-hydroxypropyl)-1,2,4-triazole) has been synthesized and its physical properties have been investigated by several physical techniques including magnetic susceptibility measurements, calorimetry, and Mössbauer, optical, and EXAFS spectroscopy. Compound 1 exhibits a spin transition below room temperature, together with a very wide thermal hysteresis of about 50 K. This represents the widest hysteresis loop ever observed for an FeII-1,2,4-triazole spin transition material. The cooperativity is discussed on the basis of temperature-dependent EXAFS studies and of the structural features of a CuII analogue. The EXAFS structural model of (1) in both spin states is compared to that obtained for a related material whose spin transition occurs above room temperature. EXAFS spectroscopy suggests that 1,2,4-triazole chain compounds retain a linear character whatever the spin state of the iron(II).

Crystal structures and non-linear optical properties of borane derivatives

[4′-(Dimethylamino)biphenyl-4-yl]dimesitylborane (BNB) and 4-[4′-(Dimethylamino)pnenylazo]phenyldimesityl-borane (BNA) have been investigated by X-ray diffraction; crystallographic data were used to calculate dipole moments in the ground state and the hyperpolarizability coefficient βo was measured by the EFISH technique for each derivative. The results show that the trivalent boron group is an efficient electron acceptor compared to other known acceptors such as the nitro group. Organoboron derivatives are promising candidates for nonlinear optical materials.

X-ray and neutron studies of a displacive phase transition in N,N-dimethylnitramine (DMN)

N,N-Dimethylnitramine has a phase transition at 107 K for the hydrogenated compound and 111 K for the deuterated compound, associated with a soft mode previously observed by Raman spectroscopy at low temperature. The purpose of this work is to determine the structural modifications at the transition. Powder diffraction patterns (120, 95, 15 K) and rotating-crystal and Weissenberg photographs (155, 95 K) obtained with neutrons show the existence of a superlattice corresponding to a doubling of the lattice below the transition temperature. The structures at 300, 180, 125 K (X-rays) and 125, 91, 85 K (neutrons) have been determined. The structural modifications between the high-temperature form (space group P2~/m, Z = 2, planar molecule) and low-temperature form (P2~/c, Z = 4, deformed molecule) can essentially be described by a rotation of the whole molecule (2 4 o at T t - 20 K) plus an intramolecular deformation: the N--N bond and the C-N-C plane form an angle of about 10 ° at Tt-- 20 K.

Structure and non-linear optical properties of phosphine oxide derivatives

The crystal structures of three different phosphine oxides derivatives possessing high quadratic susceptibilities, β, in solution have been investigated by X-ray diffraction. Slight modification of the transmitter chain leads to an acentric molecule; however, it is shown that these polar molecules have specific arrangements in the crystalline state, leading to a weak non-linear optical (NLO) response. The crystallographic data were entered into the MOPAC program to calculate the quadratic hyperpolarizability of each structure. The results were compared to those obtained by the use of standard internal data of the MOPAC program.

Synthetic models related to deoxyribonucleic acid base–psoralen interactions. Syntheses and studies of bichromophore systems containing psoralen moieties

The syntheses of new bispsoralen derivatives (IIn) and (IIIm) are reported. The photophysical properties of these potential deoxyribonucleic acid (DNA) bisintercalating drugs are studied and compared with compounds (I) and (IV) where the psoralen moiety is linked with a DNA base. The singlet and triplet deactivation processes underline the role of intercalation in the photobiological activity of psoralen with DNA. Some attempts at complexation of compounds (IIn) and (IIIm) with DNA are also reported.

Room- and high-pressure neutron structure determination of tetrathiafulvalene-7,7,8,8-tetracyano-p-quinodimethane (TTF–TCNQ). Thermal expansion and isothermal compressibility; errata

The T T F T C N Q structures at 10 -I and 4.6 x 102 MPa have been refined to R = 0.046 from full sets of intensities (1084 and 1141 independent reflections respectively) up to sin 0/2 = 0.59 A -1. The space group is P2Jc and cell dimensions are at 10 -1 MPa: a = 12.291 (6), b = 3.825 (2), c = 18.425 (10)A, t = 104.49 (5)°; and at 4.6 x 102 MPa: a = 12.188 (6), b = 3.741 (2), c = 18.307 (10) A, t = 104.27 (5) °. An increase of the charge transfer by 10% for 4.6 x 102 MPa has been estimated from the change in intramolecular bond lengths. The major effect of pressure on the crystal packing is the change in the stacking distance in the columns of TTF molecules [3.476 (2) to 3.417 (1)A] and of TCNQ molecules [3.170 (3) to 3 .104(2)A] while reorientational movements are small. The isothermal compressibility and the thermal expansion of T T F T C N Q have been measured, leading to results in partial disagreement with some published earlier. The anisotropies and directions of principal eompressibilities and thermal expansion have been calculated and explained by means of a simple model based on structural considerations.