Valérie Marvaud

Molecules to build solids: high TC molecule-based magnets by design and recent revival of cyano complexes chemistry

Abstract This paper was presented as a session lecture at the XXXIII ICCC in Florence (29 August–4 September 1998). It intends to point out some recent achievements in the chemistry and physics of transition metal polycyanides in the field of molecular magnetism. Prussian blue is sometimes considered as the first coordination compound and the paper shows how it is possible to obtain brand new results with Prussian blue analogues when looking at these antique systems with fresh eyes. Hexacyanometalates revealed in the last few years as very flexible molecular precursors to build three-dimensional molecule-based magnets with tunable and high Curie temperatures or to grow high nuclearity clusters with tunable high spins and anisotropy. The use of a localized electron orbital model allowed the authors’ team to push the Curie temperatures from 5.6 K in the Prussian blue itself to above room temperature in a vanadium–chromium Prussian blue analogue. Several groups confirmed the result and are improving it. In the same way, high spin molecules with ground spin states ranging from S=3/2 to 27/2 were obtained. The paper reviews some of the steps which lead to these spectacular findings and some of the prospects opened in molecular materials by this revival of polycyanide chemistry.

Photomagnetism in Clusters and Extended Molecule-Based Magnets

Photomagnetism in molecular systems is a new development in molecular magnetism. It traces back to 1982 and 1984 when a transient effect and then the light-induced excited-spin-state-trapping effect was discovered in spin-crossover complexes. The present contribution gives a definition of the phenomenon, a process that changes the magnetism of a (molecular) system after absorption of a photon. It is limited to the discussion of photomagnetism based on metal-metal electron transfer in clusters and extended molecule-based magnets. The paper is organized around the main pairs of spin bearers, which allowed us to evidence and to study the phenomenon: Cu-Mo, Co-Fe, and Co-W. For each metallic pair, we report and discuss the conditions of appearance of the effect and its characteristics, both in extended structures and in molecular units: structure, spectroscopy, magnetism, thermodynamics and kinetics, and applications. We conclude with some brief prospects. The field is in rapid expansion. We are convinced that the interaction of photons with magnetized matter, to provide original magnetic properties, will meet more and more interest in the future.

Metalloporphyrin Oligomers with Collapsible Cavities: Characterisation and Recognition Properties of Individual Atropisomers

A flexible porphyrin trimer is shown to exist in multiple exchanging forms, each of which displays unique molecular recognition properties (see the two atropisomers below). NMR spectroscopy has been used to characterise the equilibration and recognition properties of two of these individual trimer atropisomers.

[CrIII(L)(CN)4]−: a new building block in designing cyanide-bridged 4,2-ribbon-like chains {[CrIII(L)(CN)4]2Mn(H2O)2}·nH2O [L = 2-aminomethylpyridine (n = 6) and 1,10-phenanthroline (n = 4)]

The preparation, X-ray crystallography and magnetic study of compounds PPh4[Cr(ampy)(CN)4]·H2O (1), PPh4[Cr(phen)(CN)4]·H2O·CH3OH (2), {[Cr(ampy)(CN)4]2Mn(H2O)2}·6H2O (3) and {[Cr(phen)(CN)4]2Mn(H2O)2}·4H2O (4), with PPh4+ = tetraphenylphosphonium cation, ampy = 2-aminomethylpyridine and phen = 1,10-phenanthroline, are reported here. 1 and 2 are mononuclear complexes whereas 3 and 4 are 4,2-ribbon-like bimetallic chains. The magnetic properties of 1–4 were investigated in the temperature range 1.9–300 K. A quasi Curie law behaviour for a magnetically isolated spin quartet is observed for 1 and 2. Compounds 3 and 4 are ferrimagnetic CrIII2MnII chains, which exhibit a metamagnetic behaviour, the values of the critical field being Hc = 1 T (3) and 5000 G (4), which is due to the occurrence of weak interchain antiferromagnetic interactions. Below ca. 4.0 K, compound 4 shows a spin-canted structure with a narrow hysteresis loop, the value of the coercive field being 50 G.

Mixed valency and magnetism in cyanometallates and Prussian blue analogues

Prussian blue (PB) is a well-known archetype of mixed valency systems. In magnetic PB analogues {CxAy[B(CN)6]z}.nH2O (C alkali cation, A and B transition metal ions) and other metallic cyanometallates {Cx(AL)y[B(CN)8]z}.nH2O (L ligand), the presence of two valency states in the solid (either A–B, or A–A′ or B–B′) is crucial to get original magnetic properties: tunable high Curie temperature magnets; photomagnetic magnets; or photomagnetic high-spin molecules. We focus on a few mixed valency pairs: V(II)/V(III)/V(IV); Cr(II)/Cr(III); Fe(II)–Fe(III); Co(II)–Co(III); Cu(I)–Cu(II); and Mo(IV)/Mo(V), and discuss: (i) the control of the degree of mixed valency during the synthesis, (ii) the importance of mixed valency on the local and long-range structure and on the local and macroscopic magnetization, and (iii) the crucial role of the cyanide ligand to get these original systems and properties.

High Spin and Anisotropic Molecules Based on Polycyanometalate Chemistry

This paper points out some recent achievements in the chemistry and physics of high spin and anisotropic molecules based on polycyanometalate complexes. Following a step by step synthetic strategy and using a localized electron orbital model, isotropic high spin molecules were obtained with ground spin states ranging from S = 9/2 to 27/2. In the same way, anisotropic molecules with various nuclearities (bi, tri, tetra, hexa, and hepta-nuclear complexes) have been synthesized. Mixing these two approaches, it has been possible to obtained anisotropic high spin molecules that behave as single molecule magnets. The paper reviews some of the steps that lead to these findings and some of the prospects opened in the field of single molecule magnets.

Electron transfer through 2,7,9,10-tetraazaphenanthrene : a quantum interference effect ?

Abstract Intramolecular electron transfer has been detected in a binuclear complex of ruthenium using 2,7,9,10-tetraazaphenanthrene as bridging ligand. From the characteristics of the intervalence transition, the effective coupling V ab between ruthenium sites has been determined. It is found to be lower than in the corresponding complex with 4,4′-bipyridine, despite the fact that 2,7,9,10-tetraazaphenanthrene presents addition connections. Molecular orbital calculations confirm this effect. The analysis of these differences is performed in terms of “interference” effects between the different branches connecting the metal atoms.

Photophysical properties of monomeric and oligomeric ruthenium (II) porphyrins

Abstract The present Letter deals with three ruthenium(II) porphyrins: RuTBP(CO) (EtOH), RuTBP(pyz) 2 and [RuTBP(pyz)] n where TBP = tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl) porphyrin, EtOH = ethanol and pyz = pyrazine. Their photophysical properties are studied by steady-state and time-resolved absorption and emission spectroscopy. Each one of the examined compounds shows weak luminescence originating from a different electronic state: porphyrin triplet 3 (π,π ∗ ) for RuTBP (CO) (EtOH), equatorial 3 LCT for RuTBP(pyz) 2 and axial 1 MLCT for [RuTBP(pyz)] n .

Molecular Dyads of Ruthenium(II)– or Osmium(II)–Bis(terpyridine) Chromophores and Expanded Pyridinium Acceptors: Equilibration between MLCT and Charge-Separated Excited States

The synthesis, characterization, redox behavior, and photophysical properties (both at room temperature in fluid solution and at 77 K in rigid matrix) of a series of four new molecular dyads (2-5) containing Ru(II)- or Os(II)-bis(terpyridine) subunits as chromophores and various expanded pyridinium subunits as electron acceptors are reported, along with the reference properties of a formerly reported dyad, 1. The molecular dyads 2-4 have been designed to have their (potentially emissive) triplet metal-to-ligand charge-transfer (MLCT) and charge-separated (CS) states close in energy, so that excited-state equilibration between these levels can take place. Such a situation is not shared by limit cases 1 and 5. For dyad 1, forward photoinduced electron transfer (time constant, 7 ps) and subsequent charge recombination (time constant, 45 ps) are evidenced, while for dyad 5, photoinduced electron transfer is thermodynamically forbidden so that MLCT decays are the only active deactivation processes. As regards 2-4, CS states are formed from MLCT states with time constants of a few dozens of picoseconds. However, for these latter species, such experimental time constants are not due to photoinduced charge separation but are related to the excited-state equilibration times. Comparative analysis of time constants for charge recombination from the CS states based on proper thermodynamic and kinetic models highlighted that, in spite of their apparently affiliated structures, dyads 1-4 do not constitute a homologous series of compounds as far as intercomponent electron transfer processes are concerned.

Understanding the Photomagnetic Behavior in Copper Octacyanomolybdates

The mechanism of photomagnetism in copper octacyanomolybdate molecules is currently under debate. Contrary to the general belief that the photomagnetic transition occurs only due to a photoinduced electron transfer from the molybdenum to the copper atom, recent X-ray magnetic dichroic (XMCD) data clearly indicate that this phenomenon is associated at low temperature to a local low-spin-high-spin transition on the molybdenum atom. In this article we provide theoretical justification for these experimental facts. We show the first simulation of X-ray absorption (XAS) and magnetic circular dichroism (XMCD) spectra at the L(2,3) edges of molybdenum from the joint perspective of density functional theory (DFT) calculations and ligand field multiplet (LFM) theory. The description of electronic interactions seems mandatory for reproducing the photomagnetic state.