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

The -N-pyridyl-based PCP pincer proligand 3,5-bis(di--butylphosphinomethyl)-2,6-dimethylpyridine (pN-PCP-H) was synthesized and metalated to give the iridium complex (pN-PCP)IrHCl (). In marked contrast with its phenyl-based congeners, e.g., (PCP)IrHCl and derivatives, 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--butyl C(sp)-H bond and cyclometalation. Considering the greater electronegativity of N than C, is expected to be less easily oxidized than simple PCP derivatives; cyclic voltammetry and DFT calculations support this expectation. However, 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 -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 (ΔΔ° = -31.3 kcal/mol). The resulting Ir(II) oxidation product is easily deprotonated to give metalloradical which is observed by NMR spectroscopy. can be further oxidized to give cationic Ir(III) complex, which can oxidatively add a phosphino--butyl C-H bond and undergo deprotonation to give the observed cyclometalated product. DFT calculations indicate that less sterically hindered analogues of would preferentially undergo intermolecular addition of C(sp)-H bonds, for example, of -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.