Structural chemistry of metal coordination complexes at high pressure

The application of pressures of up to about 10 GPa may induce significant geometric, configurational, conformational and packing changes in molecular solids. This review highlights and describes recent advances in high pressure studies of coordination complexes, many of which have been conducted at synchrotrons or other central facilities. The main focus is on the wide range of geometric changes which occur with pressure. In some cases these changes have associated physical effects, and the review describes materials exhibiting negative linear compressibility, spin cross-over phenomena, magnetism and molecular conduction, as well as detailing the exciting possibilities for future developments in this area of research

Spin dynamics of the one-dimensional magnet Na2MnF5

The spin dynamics of the one-dimensional (1D) magnet Na2MnF5 have been investigated by inelastic neutron scattering. Measurements of the spin-wave dispersion resulted in values of the exchange constant J/k(B)=-8.5(1) K and the anisotropy constant D/k(B)=-2.70(2) K, respectively. The temperature dependence of the magnetic correlation length showed significant deviations from the Heisenberg/ Fischer behavior at low temperatures. The observed exponential increase of the inverse correlation length upon temperature is a typical feature of strongly anisotropic systems. Soliton excitations have been measured directly by high-resolution neutron spectroscopy. From these measurements, a soliton activation energy of E-s/k(B)=65(5) K has been determined. The results are discussed with respect to other 1D magnetic systems among the fluoromanganates

A 57Fe MÖSSBAUER STUDY OF THE HIGH PRESSURE PHASE TRANSITION IN THE SYSTEM V1-xFexO2 (x = 0.024)

La transition M2, M3 de V0.976Fe0.024O2 a été étudiée jusqu'à 60 kbar en utilsant l'effet Mössbauer dans 57Fe. On observe une variation brutale de l'effet quadrupolaire et une variation continue du déplacement isomérique avec inversion des valeurs relatives au moment de la transition M2, M3.The M2, M3 transition of V0.976Fe0.024O2 was studied up to 60 kbar using the Mössbauer effect of 57Fe. A discontinuous change of the quadrupole splittings and a continuous change of the isomer shift with a crossing over at the M2, M3 phase transition is found

[5-5]Bitrovacene: Synthesis, Structure, and Intermetallic communication

Lithiation of trovacene, (eta(7)-C7H7)V(eta(5)-C5H5), Li/I exchange, and Pd(dppf)Cl-2-catalyzed homocoupling affords [5-5]bitrovacene 5(2.). Intermetallic communication manifests itself in typical biradical features of the EPR spectrum and in the resolution of redox splittings of 147 mV for the two subsequent oxidations and 224 mV for the reductions, respectively. 5(2.) adopts a trans conformation in the single crystal (X-ray diffraction) as well as in rigid solution (EPR spectroscopy). A magnetic susceptibility study of 5(2.) reveals weak antiferromagnetic coupling (J = -2.78 cm(-1)) of the two V(d(5)) centers.</p

Synthesis and characterization of novel charge transfer complexes formed by N,N%-bis(ferrocenylmethylidene)-p-phenylenediamine and N-(ferrocenylmethylidene)aniline

N,N'-Bis(ferrocenylmethylidene)-p-phenylenediamine 1 and N-(ferrocenylmethylidene) aniline 2 are readily synthesized by Schiff base condensation of appropriate units. Iodine (I-2), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetrachloro-1,4-benzoquinone (CA), tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) form charge transfer complexes with 1 and 2. IR spectroscopy suggests an increase in the amount of charge transferred from the ferrocenyl ring to the oxidant in the order, I-2 < CA < TCNQ < TCNE approximate to DDQ. EPR spectra of the oxidized binuclear complexes are indicative of localized species containing iron- and carbon-centered radicals. The Mossbauer spectrum of the iodine oxidized complex of 1 reveals the presence of both Fe(III) and Fe(II) centers. Variable temperature magnetic and Mossbauer studies show that the ratio of Fe(III)/Fe(II) centers varies as a function of temperature. The larger Fe(II)/Fe(III) ratio at lower temperatures is best explained by a retro charge transfer from the iodide to the iron(III) metal center. There is negligible solvent effect on the formation of the iodine oxidized charge transfer complex of 1. (C) 1999 Elsevier Science S.A. All rights reserved

Head-on versus Side-on [5-5] Bitrovacenes Featuring Benzene and Naphthalene Units as Spacers: How p-Stacking Affects Exchange Coupling and Redox Splitting

The biradicals 1,8-di([5]trovacenyl)naphthalene (6), 1,5-di((5]trovacenyl)naphthalene (7), and 1,3-di([5]trovacenyl)benzene (8) and the monoradical 1-[51trovacenylnaphthalene (9) have been prepared and studied by means of single-crystal X-ray diffraction 6, 8, 9), cyclic voltammetry, EPR spectroscopy, and magnetic susceptometry. Comparison of the, magnetic properties of 6-9 reveals that pi-stacking, as encountered in 6, largely enhances exchange interaction between the two singly occupied V(3d(z)(2)) orbitals. The effect of pi-stacking on the electrochemical properties, as manifested in the redox splitting between consecutive electron transfer steps, is less pronounced. Redox splittings deltaE(1/2) for consecutive reductions exceed those for oxidations of binuclear trovacenes, deltaE(1/2)(2+/+, +/0) being apparent for pi-stacked 6 only, Because of the orthogonality of the metal-centered redox orbitals V1(d(z)(2)) and V2(d(z)(2)) and the eta(5)-cyclopentadienyl pi-orbitals, electro- and magnetocommunication are indirect processes. Electrocommunication rests on changes of the donor/acceptor properties of V1,V2 caused by oxidation or reduction which govern metal-ligand charge distribution; the latter changes are transmitted via the spacer. Magnetocommunication takes the form of antiferromagnetic coupling, which can be traced to spin polarization of filled pi-orbitals of the bridge by orthogonal singly occupied vanadium 3d(z)(2)orbitals.</p