Ruthenium Carbon-Rich Group as a Redox-Switchable Metal Coupling Unit in Linear Trinuclear Complexes.

The preparation and properties of novel ruthenium carbon-rich complexes [(Ph-C≡C-)2-nRu(dppe)2(-C≡C-bipyM(hfac)2)n] (n = 1, 2; M = CuII, MnII; bipy = 2,2'-bipyridin-5-yl) characterized by single-crystal X-ray diffraction and designed for molecular magnetism are reported. With the help of EPR spectroscopy, we show that the neutral ruthenium system sets up a magnetic coupling between two remote paramagnetic CuII units. More specifically, these copper compounds are unique examples of bimetallic and linear heterotrimetallic compounds for which a complete rationalization of the magnetic interactions could be made for exceptionally long distances between the spin carriers (8.3 Å between adjacent Cu and Ru centers, 16.6 Å between external Cu centers) and compared at two different redox states. Surprisingly, oxidation of the ruthenium redox-active metal coupling unit (MCU), which introduces an additional spin unit on the carbon-rich part, leads to weaker magnetic interactions. In contrast, in the simpler parent complexes bearing only one paramagnetic metal unit [Ph-C≡C-Ru(dppe)2-C≡C-bipyCu(hfac)2], one-electron oxidation of the ruthenium bis(acetylide) unit generates an interaction between the Cu and Ru spin carriers of magnitude comparable to that observed between the two far apart Cu ions in the above corresponding neutral trimetallic system. Evaluation and rationalization of this coupling with theoretical tools are in rational agreement with experiments for such complex systems.

Polarized Neutron Diffraction to Probe Local Magnetic Anisotropy of a Low-Spin Fe(III) Complex.

We have determined by polarized neutron diffraction (PND) the low-temperature molecular magnetic susceptibility tensor of the anisotropic low-spin complex PPh4 [Fe(III) (Tp)(CN)3]⋅H2O. We found the existence of a pronounced molecular easy magnetization axis, almost parallel to the C3 pseudo-axis of the molecule, which also corresponds to a trigonal elongation direction of the octahedral coordination sphere of the Fe(III) ion. The PND results are coherent with electron paramagnetic resonance (EPR) spectroscopy, magnetometry, and ab initio investigations. Through this particular example, we demonstrate the capabilities of PND to provide a unique, direct, and straightforward picture of the magnetic anisotropy and susceptibility tensors, offering a clear-cut way to establish magneto-structural correlations in paramagnetic molecular complexes.

Conformational Effects on the Magnetic Properties of an Organic Diradical: A Computational Study.

A theoretical study on the singlet triplet energy splitting in a m-phenylene bridged organic diradical has been performed using an original computational protocol developed in our group. The method is based on post Hatree-Fock calculations and has proven to provide accurate results with reasonable computational effort. By virtue of such efficiency, the full PES of both the singlet and triplet states as a function of the two "soft" torsional degrees of freedom at the meta position of the ring has been explored. In agreement with literature findings, we found a pronounced dependence of the sign of the energy gap from the torsional angles. Finally, exploiting the two-dimensional surface, a statistical analysis is carried out at low temperatures and a comparison with available experimental data addressed.

Structure-Properties Relationships in Triplet Ground State Organic Diradicals: A Computational Study.

A fast and efficient computational protocol, devised for the accurate calculation of singlet-triplet magnetic splittings in organic diradicals, is here applied to several promising organic magnets, recently considered in the literature. The very good agreement with the measured values, obtained for all investigated compounds, suggests that the present approach could successfully flank the experiment in the design of novel magnetic materials. Indeed, some structure-magnetic properties relationships were rationalized thanks to the theoretical soundness of the adopted multireference approach. In particular the different effects of N· and NO· magnetic moieties, as well as the role of lateral aliphatic chains and phenyl pendant substituents, are discussed in detail.

Ab initio study of the influence of structural parameters on the potential energy surfaces of spin-crossover Fe(II) model compounds.

In spin-crossover (SCO) compounds exhibiting a light induced excited spin state trapping (LIESST) effect, the thermodynamic T(1∕2) and kinetic T(LIESST) temperature values depend on the features of the potential energy surfaces (PES) of the two lowest singlet and quintet states but also on vibrational contributions, collective effects, such as electrostatics, for instance, spin-orbit couplings to a lesser extent, etc. In this work, the question of the link between the shape of the PES of SCO compounds exhibiting a LIESST effect and their first coordination sphere structure is addressed from wave function theory based ab initio calculations. Fe(II) complexes based on model ligands suited to reproduce the main characteristics of the PES of such compounds are distorted to emphasize selectively the role played by the metal-ligand distances and the ligand-metal-ligand angles. The studied angular deformations are those usually observed in many Fe(L)(2)(NCS)(2) complexes. It is shown that the larger the deformation between the low spin and high spin equilibrium geometries, the higher the energy barrier from the high spin state and the weaker the energy difference between the bottom of the wells. These results corroborate observations made by experimentalists on a large number of complexes. While the PES features only constitutes one of the contributions to these temperatures, it is worth noticing that, relating T(1∕2) to the energy difference between the bottoms of the singlet and quintet wells and the T(LIESST) to the energy barrier from the quintet bottom well, the same slope of the empirical law T(LIESST) = -0.3T(1∕2)+T(0) is observed.

A bottom-up valence bond derivation of excitation energies in 1D-like delocalized systems.

Using the chemically relevant parameters hopping integral t(0) and on-site repulsion energy U, the charge gap (lowest dipolarly allowed transition energy) in 1D systems is examined through a bottom-up strategy. The method is based on the locally ionized states, the energies of which are corrected using short-range delocalization effects. In a valence bond framework, these states interact to produce an excitonic matrix which accounts for the delocalized character of excited states. The treatment, which gives access to the correlated spectrum of ionization potentials, is entirely analytical and valid whatever the U/|t(0)| ratio for such systems ruled by Peierls-Hubbard Hamiltonians. This second-order analytical derivation is finally confronted to numerical results of a renormalized excitonic treatment using larger blocks as functions of the U/|t(0)| ratio. The method is applied to dimerized chains and to fused polybenzenic 1D lattices. Such approaches complement the traditional Bloch-function based picture and deliver a conceptual understanding of the charge gap opening process based on a chemical intuitive picture.

Light-induced excited spin state trapping: ab initio study of the physics at the molecular level.

This paper provides a qualitative analysis of the physical content of the low-energy states of a spin-transition compound presenting a light-induced excited spin state trapping (LIESST) phenomenon, namely, [Fe(dipyrazolpyridine)2](BF4)2, which has been studied using the wave function-based CASPT2 method. Both the nature of the low-energy states and the relative position of their potential energy wells as a function of the geometry are rationalized from the analysis of the different wave functions. It is shown that the light-induced spin transition occurring in such systems could follow several pathways involving different excited spin states. In an ideal octahedral geometry, the interconversion from the excited singlet state to the triplet of lower energy, which is usually seen as an intermediate state in the LIESST mechanism, is quite unlikely since there is no crossing between the potential energy curves of these two states. On the contrary, in lower-symmetry complexes, the geometrical distortion of the coordination sphere due to ligand constraints is responsible for the occurrence of a crossing between these two states in the Franck-Condon region, leading to a possible participation of this triplet state in the LIESST mechanism. In the reverse LIESST process, a crossing between the potential energy curves of another triplet state and the excited quintet state occurs in the Franck-Condon region as well.

Is it possible to determine rigorous magnetic Hamiltonians in spin s = 1 systems from density functional theory calculations?

The variational energies of broken-symmetry single determinants are frequently used (especially in the Kohn-Sham density functional theory) to determine the magnetic coupling between open-shell metal ions in molecular complexes or periodic lattices. Most applications extract the information from the solutions of m(s)(max) and m(s)(min) eigenvalues of S(z) magnetic spin momentum, assuming that a mapping of these energies on the energies of an Ising Hamiltonian is grounded. This approach is unable to predict the possible importance of deviations from the simplest form of the Heisenberg Hamiltonians. For systems involving s=1 magnetic centers, it cannot provide an estimate of neither the biquadratic exchange integral nor the three-body operator interaction that has recently been proven to be of the same order of magnitude [Phys. Rev. B 70, 132412 (2007)]. The present work shows that one may use other broken-symmetry solutions of intermediate values of m(s) to evaluate the amplitude of these additional terms. The here-derived equations rely on the assumption that an extended Hubbard-type Hamiltonian rules the interactions between the magnetic electrons. Numerical illustrations on a model problem of two O(2) molecules and a fragment of the La(2)NiO(4) lattice are reported. The results obtained using a variable percentage of Fock exchange in the BLYP functional are compared to those provided by elaborate wave function calculations. The relevant percentage of Fock exchange is system dependent but a mean value of 30% leads to acceptable amplitudes of the effective exchange interaction.