Ambiphilic ligands coordination

By design, ambiphilic ligands combine donor and acceptor sites on the same skeleton (Figure 1). The basic idea is to use donor sites as anchors to introduce Lewis acids in the coordination sphere of transition metals. Fine tuning of the structure of the ambiphilic ligands (coordination sites and linker) gives the possibility to control the position of the Lewis acid moiety and the way it participates in bonding and/or reactivity. 

In the present volume, our intention was to cover several modern approaches to phosphorus chemistry which were not, or at least not completely, covered in the previous volumes. The selected topics are expected to have broader relevance and to be interesting to a more general readership, since key aspects of phosphorus chemistry are pointed out. Indeed, several fields are investigated coordination chemistry, catalysis, supramolecular chemistry, biochemistry, hybrid organic-inorganic materials, new ambiphilic ligands, and biology. 

The four coordination modes E-H (Figure 18) have been substantiated spectroscopically and structurally for PBs and related ambiphilic ligands. They differ in the participation of the Lewis acid moiety that may remain... 

The simplest bonding situation conceivable upon coordination of an ambiphilic ligand to a transition metal is that involving the sole coordination of the Lewis base moiety, the Lewis acid fragment remaining pendant. Several examples of such complexes have been described in the recent years, most frequently upon coordination of preformed ambiphilic ligands. 

The ability of ambiphilic ligands to bridge M-X or M-L fragments (coordination mode F resulting from the concomitant coordination of the donor site to the metal and interaction of the Lewis acid with a coligand) has also been demonstrated experimentally. Such bridging coordination remains rare, and most of the known examples involve PB ligands.74... 

The bridging coordination of M-X and M-L fragments by ambiphilic ligands, as described in this section, open interesting perspectives in catalysis. Complex 94 nicely illustrates the possibility to cooperatively activate functionalized 7i-systems by concomitant coordination to a metal center and a Lewis acid. In addition, and in line with what is typically encountered upon activation of metal complexes with Lewis... 

Over the past few years, the coordination of preformed ambiphilic ligands, especially PBs, has been shown to afford straightforward access to complexes featuring metal-Lewis acid interactions. As discussed hereafter, this approach has allowed the significant extension of the scope of M-Z interactions, and to gain better understanding of their bonding situation. 

As discussed in the previous section, the coordination properties of PBs and related ambiphilic ligands have been quite extensively studied over the last decade. Comparatively, the reactivity and catalytic application of the ensuing complexes have only been scarcely explored. The few investigations reported in this area are summarized in this section. They nicely illustrate the various possibilities offered by the presence of the Lewis acid in the coordination sphere of a metal or at proximity. 

Ambiphilic Ligands Unusual Coordination and Reactivity Arising from Lewis Acid Moieties... 

Ligand Design in Metal Chemistry 9.3 Coordination of ambiphilic ligands... 

The coordination properties of ambiphilic ligands have been thoroughly investigated. Depending on the role of the Lewis acid, four different situations can be distinguished and they have all been authenticated experimentally (Figure 4). Representative examples will be discussed in the following sections. Special attention will be devoted to the three coordination modes in which the Lewis acid site actively participates. 

Such bridging coordination of M—X bonds with ambiphilic ligands is expected to be associated with some degree of bond activation. M—X bond lengths proved hardly informative in that respect, but computational studies, including comparison with related Lewis acid-free complexes, have clearly evidenced substantial weakening of M—Cl bonds upon interaction with boron. 

Brigding coordination of M—X bonds by ambiphilic ligands opens interesting perspectives. It can contribute to facilitate the activation of o-bonds and can participate in reversible abstraction/transfer of atoms. 

So far, the potential of ambiphilic ligands in reactivity and catalysis has only been scarcely investigated. The kinetic, thermodynamic and even the outcome of metal-mediated transformations can be influenced by the presence of a Lewis acid in the coordination sphere. As discussed hereafter, the first studies performed in this area are very promising and further developments are certainly to be expected. 

Guegan F, Mignon P, Tognetti V, Joubert L, Morell C (2014) Dual descriptor and molecular electrostatic potential complementary tools for the study of the coordination chemistry of ambiphilic ligands. Phys Chem Chem Phys 16 15558-15569... 

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