Cationic ligands: from monodentate to pincer systems: synthesis, electronic properties and applications in catalysis

For a long time, the small group of cationic ligands stood out as an obscure phenomenon within the general landscape of coordinative chemistry. However, this situation has started to change rapidly during the last decade, with more and more examples of metal-coordinated cationic species being reported. The growing interest in these systems is not only of a purely academic nature, but also driven by the accumulating evidence of their high catalytic utility. Overcoming the inherently poor coordinating ability of cationic species often required an additional structural stabilization. In numerous cases this was realized by functionalizing them with a pair of chelating side-arms, effectively constructing a pincer-type scaffold. This comprehensive review aims to encompass all cationic ligands possessing such pincer architecture reported to date. Herein each cationic species ever embedded in a pincer framework is described in terms of its electronic structure, followed by an in-depth discussion of its donor/acceptor properties, based on computational studies (DFT), and available experimental data (IR, NMR or CV). We then elaborate on how the positive charge of these ligands affects the spectroscopic and redox properties, as well as the reactivity, of their complexes, compared to those of the structurally related neutral ligands. Among other systems discussed, this review also surveys our own contribution to the field, namely, the introduction of sulfonium-based pincer ligands and their complexes, recently reported by our group.

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