The ABC in pincer chemistry – From amine- to borylene- and carbon-based pincer-ligands

Abstract

The platform of pincer ligands offers the opportunity to stabilize elusive groups in the coordination sphere of a metal. These groups often exhibit unusual bonding properties and can show unprecedented reactivity patterns, which are potentially useful for (cooperative) catalysis. In the current review, we compare pincer ligands whose central donor group exhibits a sp3-hybridized atom. The primary focus lies on the comparison of reactivity patterns of these formally isoelectronic donor groups. In particular, secondary amine groups, well known to act cooperatively as a proton relay, are compared with recently reported ligands that are based on boron and carbon. In the ABC in pincer chemistry, we will demonstrate that in contrast to Amine donor-groups, ligand-stabilized Borylenes are not acting as a proton source and rather undergo a reversible redox-reaction with the coordinated metal centre. The different types of Carbon-based ligands rather show characteristics of amine reactivity, but there is evidence for some complexes that a reversible redox reaction might be a possible exchange mechanism. Moreover, this review tries to analyze the bonding situation of the aforementioned species and to distinguish them from other related ligand types and their reactivity patterns.