Metal–Ligand Proton Tautomerism, Electron Transfer, and C(sp <sup>3</sup> )–H Activation by a 4-Pyridinyl-Pincer Iridium Hydride Complex
Tariq M. Bhatti, Akshai Kumar, Ashish Parihar, Hellan K. Moncy, Thomas J. Emge, Kate M. Waldie, Faraj Hasanayn, Alan S. Goldman
Abstract
High Resolution Image Download MS PowerPoint Slide The para -N-pyridyl-based PCP pincer proligand 3,5-bis(di- tert -butylphosphinomethyl)-2,6-dimethylpyridine (pN- tBu PCP-H) was synthesized and metalated to give the iridium complex (pN- tBu PCP)IrHCl ( 2-H ). In marked contrast with its phenyl-based congeners, e.g., ( tBu PCP)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(sp 3 )–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 (ΔΔ G ° = −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(sp 3 )–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.