Odd-electron, redox-active ligands are discussed in the context of catalysis. We focus on ligand-based, non-singlet state intermediates and their participation in catalytic processes and related stoichiometric transformations.

Pincers, tridentate ligands that prefer a meridional geometry, are a rising class because of their distinctive combination of properties. They permit a good level of control on the nature of the coordination sphere by holding the donor groups in a predictable arrangement. Some groups, such as an aryl or a pyridine, that would normally be easily lost as monodentate ligands, become reliably coordinated, especially if they form the central donor unit of the three. Many pincer complexes show exceptional thermal stability, a property that is particularly prized in homogeneous catalysis where they can permit high temperature operation. The connectors between the three donor groups are often rigid, enforcing a strict mer geometry but flexible linkers permit fac binding and even fluxionality between the two forms. Rigid pincers can make good ligands for asymmetric catalysis—if the wingtip groups cannot easily rotate they may instead maintain a geometry in which suitable substituents project into the active site area of the catalyst where they help enantio-differentiation of the relevant transition states. Examples have been selected to illustrate these and other properties of this promising ligand class.

Although enzymes often incorporate molecular recognition elements to orient substrates selectively, such strategies are rarely achieved by synthetic catalysts. We combined molecular recognition through hydrogen bonding with C-H activation to obtain high-turnover catalytic regioselective functionalization of sp ³ C-H bonds remote from the –COOH recognition group. The catalyst contains a Mn(μ-O) ₂ Mn reactive center and a ligand based on Kemp's triacid that directs a –COOH group to anchor the carboxylic acid group of the substrate and thus modify the usual selectivity for oxidation. Control experiments supported the role of hydrogen bonding in orienting the substrate to achieve high selectivity.