Olivier Guillou

In situ 2,5-pyrazinedicarboxylate and oxalate ligands synthesis leading to a microporous europium–organic framework capable of selective sensing of small molecules

A new microporous europium-containing metal–organic framework [Eu2(μ2-pzdc)(μ4-pzdc)(μ2-ox)(H2O)4]·8H2O (1) (H2pzdc = 2,5-pyrazinedicarboxylic acid, H2ox = oxalic acid) has been hydrothermally synthesized via an unprecedented in situ formation of 2,5-pyrazinedicarboxylate (pzdc) and oxalate (ox) ligands. Structure 1 was characterized by both single-crystal and powder X-ray diffraction analysis. It can be described as a 4-connected binodal Moganite network with the Schlafli symbol of (4·64·8)2(42·62·82), which exhibits a reversible and highly selective sensing function with respect to acetone.

One- and two-dimensional rare earth-copper molecular materials

Abstract This short review paper deals with the crystal structures and the magnetic properties of molecular materials arising from the reaction of the [Cu(pba)]2− recursor with trivalent rare earth ions M(III); pba stands for 1,3- propylenebis(oxamato). Three families of compounds have been obtained so far, abbreviated as M2Cu3Cu·1, M2Cu3·2 and M2Cu3·3, depending on the synthesis method, and/or the nature of the rare earth. The structure 3 is the simplest one; it consists of discrete and infinite ladder-motifs, the edges of which are made of MCu(pba) units (M going from Tb to Yb, and Y), and the rungs of Cu(pba) units linking two rare earth ions. In structure 1 (M going from La to Gd), the ladders are linked together by oxalato groups, promoting a double-sheet two- dimensional pattern. In addition, this structure contains discrete [Cu(H2O)5]2+ cations interspersed in the gap between the layers. Finally, structure 2 (M going again from La to Gd) consists of discrete and infinite tube- like motifs with a pseudo four-fold symmetry, resulting from the condensation of two ladder-motifs associated with a complete rearrangement of the rungs. The temperature dependences of the magnetic susceptibility and the field dependences of the magnetization for all compounds have been investigated. Only some of the findings are discussed. For the compounds with Mue5fbGd, Tb, Dy and Er, χMT (χM being the molar magnetic susceptibility, and T the temperature) increases as T is lowered. This result together with the magnetization data indicate that the M(III)-Cu(II) interaction is ferromagnetic. When Mue5fbLa, the copper(II) spin carriers are weakly antiferromagnetically coupled through the diamagnetic rare earth ion. The same situation holds for Mue5fbEu which has also a diamagnetic ground state. On the other hand, there is no propagation of the interaction through the Y(III) ion. The perspectives in this new field of the molecular materials incorporating rare earth and 3d metal ions are briefly discussed.

Magnetic Slow Relaxation in a Metal-Organic Framework Made of Chains of Ferromagnetically Coupled Single-Molecule Magnets

We report the study of a Dy-based metal-organic framework (MOF) with unprecedented magnetic properties. The compound is made of nine-coordinated DyIII magnetic building blocks (MBBs) with poor intrinsic single-molecule magnet behavior. However, the MOF architecture constrains the MBBs in a one-dimensional structure that induces a ferromagnetic coupling between them. Overall, the material shows a magnetic slow relaxation in absence of external static field and a hysteretic behavior at 0.5u2005K. Low-temperature magnetic studies, diamagnetic doping, and ab initio calculations highlight the crucial role played by the Dy-Dy ferromagnetic interaction. Overall, we report an original magnetic object at the frontier between single-chain magnets and single-molecule magnets that host intrachain couplings that cancel quantum tunneling between the MBBs. This compound is evidence that a bottom-up approach through MOF design can induce spontaneous organization of MBBs able to produce remarkable molecular magnetic materials.

Lanthanide-Based Coordination Polymers with a 4,5-Dichlorophthalate Ligand Exhibiting Highly Tunable Luminescence: Toward Luminescent Bar Codes

Reactions in water of 4,5-dichlorophthalate (dcpa2-) with the heaviest lanthanide ions lead to a family of compounds with the general chemical formula [Ln2(dcpa)3(H2O)5·3H2O]∞, where Ln = Tb-Lu, Y. The synthesis, crystal structure, thermal behavior, and luminescent properties of this series of homonuclear compounds are described. Additionally, this family can be extended to isostructural heteronuclear compounds that can contain some light lanthanide ions and therefore present some original photophysical properties. These compounds show potential interest as multiemissive materials (visible and infrared light between 450 and 1600 nm) and could find application as luminescent bar codes.

Strong Magnetic Coupling and Single-Molecule-Magnet Behavior in Lanthanide-TEMPO Radical Chains

The rational design of molecular chains made of 4f ions and substituted 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical is presented. The reaction of Ln(hfac)3·2H2O (hfac- = hexafluoroacetylacetonate) and the 4-cyano-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO-CN) radical affords air- and moisture-stable isostructural molecular chains of the formula [Ln(hfac)3TEMPO-CN] n for Ln = GdIII and TbIII, whereas zero-dimensional complexes of the formula [Dy(hfac)3(TEMPO-CN)2][Dy(hfac)3(H2O)2]2 are obtained for Ln = DyIII. To the best of our knowledge, the Gd derivative, Gd-TEMPO-CN, shows one of the strongest antiferromagnetic (AF) couplings for Gd-radical pairs ever reported with JGd-rad/ kB = -21.18 K, 14.72 cm-1 ( H = - JSrad SGd spin Hamiltonian convention). The TbIII derivative, Tb-TEMPO-CN, also shows strong Tb-radical AF coupling, which has been rationalized using the ab initio CASSCF approach ( JTb-rad = -23.02 K, -16.7 cm-1) and confirmed by luminescence measurements. Tb-TEMPO-CN shows remarkable properties for a Tb-radical-based single-molecule magnet ( Ueff = 69.3 ± 1 K; τ0 = 1.3 × 10-7 s) and two different relaxation processes triggered by interchain magnetic coupling.