Iridium pincer complexes: Coordination, reactivity and catalysis

The chemistry of pincer complexes started from the pioneering studies in the 1970s by the Nelson and the Shaw groups on the metal complexes arising from tridentate ligands in a meridional coordination mode.^1–3 As we discussed earlier in the Rh chapter (6.03), “pincer” was first referred to tridentate ligands with a central anionic carbon and two binding units that enforce the meridional coordination by van Keton.⁴ “Pincer” has later been increasingly used to describe meridional tridentate ligands and their metal complexes (Fig. 1). The steric and electronic properties of these complexes can be fine-tuned by modifying the donor atoms (X), ligand backbones, spacers (Y), and donor groups (L), etc. Extraordinary reactivities and applications in catalysis have been developed.^5–18 The pincer chemistry of iridium (Ir) started to gain attention since the pioneer works by Shaw,^2,3,19,20 and later by Jensen and Kaska, particularly in the catalytic alkane dehydrogenation.²¹ These discoveries prompted more research efforts in expanding the applications and related catalytic reactions with Ir pincer complexes.^22–28 In a similar setting to the Rh chapter, the coordination chemistry, organometallic reactivities and catalytic activities will be reviewed in this chapter, and again, these complexes will be discussed in three classes according to the different central donor atom: (1) C-centered pincer ligands (C_sp2 or C_sp3), (2) N-centered pincer ligands (N_sp2 or N_sp3), and (3) X-centered ligands (X ¼ P, O, Si, B, Ge etc.). Classes (1) and (2) are further divided based on the donor groups L/L⁰. Selected key literature will be....