The discovery and development of organometallic catalysts is of paramount importance in modern organic synthesis, among which the ligand scaffolds play a crucial role in controlling the activity and selectivity. Over the past several decades, d 8 transitionmetal complexes of pincer ligands have been developed extensively thanks to their easy structural modification, versatile reactivities, and high stability. One paradigm is the bis(phosphine)-based pincer iridium complexes PCP-Ir, which are highly active for alkane dehydrogenation, partly due to their high thermostability. However, except for alkane dehydrogenation and related transformations, few applications of pincer iridium catalysis have been seen in organic synthesis. This mainly arises from the low functional-group compatibility and poor substrate scope and the limited catalytic chemistry that invariably involves Ir(I/III) redox processes initiated by oxidative addition of substrates to 14-electron (PCP)Ir fragments (the proposed catalytically active intermediates). In this Account, we describe our endeavor on the development of a new family of PCN-Ir complexes with initial intention on creating more efficient alkane dehydrogenation catalysts. The replacement of a soft, σdonor phosphine arm in the PCP ligands by a harder, π-acceptor N-heteroarene (pyridine or oxazoline) not only provides an additional platform to modify the structural properties but also offers new modes of bond activation and novel reactivities and catalysis. One uniqueness of the PCN-Ir system lies in the formation, via ortho-C(sp 2 )−H cyclometalation of the pyridine unit in the PCN Py ligand, of the neutral monohydride (PCC)Ir III HL (L = neutral ligand), which catalyzes positional and stereoselective 1alkene-to-(E)-2-alkene isomerization. Moreover, the PCN-Ir catalysts effect ethanol dehydrogenation without decarbonylation, allowing for transfer hydrogenation of unactivated alkenes and trans-selective semihydrogenation of internal alkynes with userfriendly ethanol as the H-donor. Another feature originates from the ability of the pentacoordinate hydrido chloride complex (PCN)Ir III HCl to undergo reversible solvent-coordination-induced-ionization (SCII), furnishing a cationic monohydride [(PCN)Ir III H(Sol)] + Cl − bearing an uncoordinated Cl anion that effects selective hydrometalation of internal alkynes over the corresponding (Z)-alkenes; the resulting (PCN)Ir III (vinyl)Cl complex undergoes amine-assisted formal alcoholysis involving the protonation of the Cl anion by the activated Ir III -bound EtOH, again via the SCII pathway. Together these elementary reactions lay the foundation for cis-selective semihydrogenation of alkynes with EtOH. Further, the design of the oxazoline-containing chiral complexes (PCN Oxa )Ir III HCl enables asymmetric transfer hydrogenation of alkenes/ketones with ethanol. The efficient catalytic αalkylation of unactivated esters/amides with alcohols is another case showing the benefit that the PCN-Ir catalyst can offer. These examples illustrate the prof...

A deeply ingrained assumption in the conventional understanding and practice of organometallic chemistry is that an unactivated aliphatic C­(sp3)–H bond is less reactive than an aromatic C­(sp2)–H bond within the same molecule given that they are at positions unbiasedly accessible for activation. Herein, we demonstrate that a pincer-ligated iridium complex catalyzes intramolecular dehydrogenative silylation of the unactivated δ-C­(sp3)–H (δ to the Si atom) with exclusive site selectivity over typically more reactive ortho δ-C­(sp2)–H bonds. A variety of tertiary hydrosilanes undergo δ-C­(sp3)–H silylation to form 5-membered silolanes, including chiral silolanes, which can undergo further oxidation to produce enantiopure β-aryl-substituted 1,4-diols. Combined computational and experimental studies reveal that the silylation occurs via the Si–H addition to a 14-electron Ir­(I) fragment to give an Ir­(III) silyl hydride complex, which then activates the C­(sp3)–H bond to form a 7-coordinate, 18-electron Ir­(V) dihydride silyl intermediate, followed by sequential reductive elimination of H2 and silolane. The unprecedented site selectivity is governed by the distortion energy difference between the rate-determining δ-C­(sp3)–H and δ-C­(sp2)–H activation, although the activation at sp2 sites is much more favorable than sp3 sites by the interaction energy.

Chiral iridium complexes ligated by anionic oxazoline-bearing NCP-type pincer ligands were developed and applied to the asymmetric transfer hydrogenation (ATH) of diarylethenes using environmentally benign ethanol as the hydrogen donor. High enantioselectivities could be achieved for substrates bearing ortho-Me, ortho-Cl, or ortho-Br substituents on one of the aryl groups. The ATH of ortho-Br-substituted diarylethenes is particularly attractive due to the propensity of the C(aryl)–Br bond to undergo various new bond-forming events.