Rhodium pincer complexes: Coordination, reactivity and catalysis

The pioneering reports in the 1970s by the Nelson and the Shaw groups on the preparation of pyridine- and benzene-based tridentate ligands, that enforce the meridional coordination mode upon complexation to transition metals, have initiated a still fast-growing research field of “Pincer complexes” in coordination chemistry and catalysis due to the increased stability of corresponding metal complexes.^1,2 While “pincer” was first coined by van Koten to describe a class of tridentate ligands with a central anionic carbon and two donor arms for the meridional coordination in 1989,³ it is now a widely accepted term referring to a platform of meridional ligands (Fig. 1).⁴ The platform allows diversified designs adopting different donor atoms (X), ligand backbones, spacers (Y), donor groups (L), etc. to control the ligation environment, to modify the steric and electronic properties, and to manipulate the reactivity of the metal center, particularly attractive for applications in organometallic catalysis.^4–9 The pincer chemistry of rhodium (Rh) has attracted significant attention since the early works from Shaw^2,10,11 and Kaska.^12,13 These complexes are dominated by Rh(I) and Rh(III) low-spin states, and thus they are more predictable than other paramagnetic species. Rh complexes often have rich reactivities toward bond activations and catalysis.¹⁴ In this chapter, the development of Rh pincer complexes will be reviewed in fundamental aspects of the coordination chemistry, reactivity in related organometallic reactions, and selected catalytic applications. Complexes of three ligand classes will be addressed according to the different central donor atom: (1) C-centered pincer ligands (C_sp2 or C_sp3), (2)....