Review
Tetrathiafulvalene-based group XV ligands: Synthesis, coordination chemistry and radical cation salts

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Abstract

This review deals with the synthesis, properties and applications of metal complexes of tetrathiafulvalene-based group XV (N, P, As, Sb) ligands. The different synthetic methods for the preparations of tetrathiafulvalenes (TTFs) incorporating coordination functions such as N atoms (essentially sp2 N atoms in pyridines, pyrazines, bipyridines, imines, etc.) and P atoms (essentially in tertiary phosphines) are reviewed, together with the redox properties of these free ligands, showing in most cases that the electron donating ability of the TTF core is only scarcely affected upon association with the coordinating group. Metal complexes of such ligands have been reported with a large variety of metal centers, from group 6 (Cr, Mo, W) to group 12 (Zn, Cd, Hg). Their structural and electronic and magnetic properties are described, with an emphasis on the evolution of the properties of the ligand (redox potentials, 31P NMR chemical shifts) upon coordination. Two applications of these complexes are further discussed, elaboration of multi-functional conducting materials upon TTF oxidation, redox modulation of the reactivity of the metal complex used as catalyst.

Introduction

The synthesis and coordination chemistry of electro-active molecules functionalized by various mono or polydentate ligands have developed steadily during the past two decades. A prominent redox-active class of compounds is represented by tetrathiafulvalene (TTF) derivatives, which have been extensively studied in the search for molecular conductors and superconductors [1], [2]. In this respect, the association of the redox active TTF unit with coordinating heteroatom-based groups is particularly promising in the perspective of preparing electroactive transition metal complexes with original structural and electronic properties. First, the metal may serve as a template in order to assemble two or more TTF units, by means of metal-ligand interactions, in a rigid predefined manner ruled by the stereochemical preferences of the metal ion and the number of free coordination sites. Interplay between the electron donating properties of the electroactive ligand and the electron density on the metallic center could possibly occur, especially when the ligand is directly bound to TTF. In addition, the functional redox ligands may be further activated and assembled into electroactive supramolecular edifices by chemical or electrochemical oxidation. Such radical cation salts would provide unique models to probe the structural and electronic consequences of mixing the metal d and ligand π electrons and spins. In addition, when the coordinated metal is paramagnetic, the coexistence of magnetic and conducting properties hold much promise for the development of bifunctional molecular materials, a field of much current activity [3], [4], [5], [6]. Electroactive complexes could provide new interesting applications in the field of luminescent compounds and also in that of homogenous catalysis.

Among the numerous TTF ligand systems investigated in these last 20 years, those containing pyridine or bipyridine ligands in the one hand, those containing phosphines in the other hand are the most extensively studied so far, particularly for the elaboration of metal complexes. In this review article, we therefore concentrate on group XV ligands containing N, P or Sb atoms, readily available for further metal coordination. We will first describe in Section 2 the different synthetic methods developed for the syntheses of these original electroactive ligands together with a strong emphasis on the evolution of their electrochemical properties depending on the nature and location of the coordinating group. Section 3 is devoted to a thorough description of the different metal complexes reported so far with these ligands, from group 6 (Cr, Mo, W) to group 12 (Zn, Cd, Hg) metal triad, as no N or P complexes have been described so far, neither with the early transition metals, nor with lanthanide or actinides. In Section 4, we describe in detail the few reported examples of cation radical salts derived from these TTF-containing complexes, while in Section 5, we report on the catalytic applications of several Pd and Ir complexes.

Section snippets

Syntheses of tetrathiafulvalenyl ligands

For almost four decades, the imagination of the chemists involved in the field of molecular materials based on TTF gave rise to numerous TTF derivatives which have been thoroughly reviewed along the years by Schukat and Fanghänel [7]. The various strategies used to prepare these numerous functionalized TTF derivatives have also been recently reviewed [8]. However, until now, no compendium of TTFs as electroactive ligands has been reported, this is the object of the present part of this section.

The Cr, Mo, W triad (group 6)

While no chromium complexes containing TTF-pyridine or TTF-phosphine ligands have been reported so far, all the molybdenum-based complexes known to date contain TTF-phosphines and the fragment Mo(CO)4, with the metallic center in the 0 oxidation state. The common synthetic procedure involves the thermal displacement of both piperidine ligands from the precursor cis-Mo(CO)4(piperidine)2 upon moderate heating (50 or 80 °C) in toluene in the presence of the TTF-phosphine. The first complexes were

Cation radical salts of TTF metal complexes

A large number of metal complexes associating the redox active tetrathiafulvalenyl core have been described above in Section 3. In almost all of them, the first oxidation wave is fully reversible and associated with the TTF redox moiety. We have seen that complexes with pyridine ligands and analogs exhibit a limited anodic shift of the TTF first oxidation potential of the free ligand while complexes of TTF phosphines with the PPh2 groups directly linked to the TTF core exhibit a 200–250 mV

Catalytic applications of complexes based on TTF ligands

Besides the obvious interest in multifunctional molecular materials, another attractive field of investigation related to the TTF-containing ligands concerns homogenous catalysis based on transition metal complexes. The peculiarity of these catalysts with respect to the vast majority of transition metal complexes utilized in various catalytic processes relies on their electroactive behavior. This feature could possibly induce a modulation of the reactivity of the metal complex used as catalyst

Conclusions and perspectives

In this review paper, which is unprecedented with respect to the topic addressed, we presented in an exhaustive manner the electroactive ligands based on the TTF unit and phosphorous and nitrogen coordinating groups, together with their coordination metallic complexes. It turns out that all the phosphorous ligands described so far are phosphines. However, as superior pnictogen homologues, we reported also a couple of examples concerning TTF-stibines. While the nitrogen ligands containing TTF

Acknowledgement

This work was supported by the CNRS, University of Rennes I and University of Angers.

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