Metal–Ligand Proton Tautomerism, Electron Transfer, and C(sp3)–H Activation by a 4‑Pyridinyl-Pincer Iridium Hydride Complex

The para-N-pyridyl-based PCP pincer proligand 3,5-bis­(di-tert-butylphosphinomethyl)-2,6-dimethylpyridine (pN-tBuPCP-H) was synthesized and metalated to give the iridium complex (pN-tBuPCP)­IrHCl (2-H). In marked contrast with its phenyl-based congeners, e.g., (tBuPCP)­IrHCl and derivatives, 2-H is highly air-sensitive and reacts with oxidants such as ferrocenium, trityl cation, and benzoquinone. These oxidations ultimately lead to intramolecular activation of a phosphino-t-butyl C­(sp3)–H bond and cyclometalation. Considering the greater electronegativity of N than C, 2-H is expected to be less easily oxidized than simple PCP derivatives; cyclic voltammetry and DFT calculations support this expectation. However, 2-H is calculated to undergo metal–ligand-proton tautomerism (MLPT) to give an N-protonated complex that can be described with resonance forms representing a zwitterionic complex (with a negative charge on Ir) and a p-N-pyridylidene (a remote N-heterocyclic carbene) Ir­(I) complex. One-electron oxidation of this tautomer is calculated to be dramatically more favorable than direct oxidation of 2-H (ΔΔG° = −31.3 kcal/mol). The resulting Ir­(II) oxidation product is easily deprotonated to give metalloradical 2 • which is observed by NMR spectroscopy. 2 • can be further oxidized to give cationic Ir­(III) complex, 2+ , which can oxidatively add a phosphino-t-butyl C–H bond and undergo deprotonation to give the observed cyclometalated product. DFT calculations indicate that less sterically hindered analogues of 2+ would preferentially undergo intermolecular addition of C­(sp3)–H bonds, for example, of n-alkanes. The resulting iridium alkyl complexes could undergo facile β-H elimination to afford olefin, thereby completing a catalytic cycle for alkane dehydrogenation driven by one-electron oxidation and deprotonation, enabled by MLPT.