Metal-Ligand Proton Tautomerism, Electron Transfer, and C(sp(3))-H Activation by a 4-Pyridinyl-Pincer Iridium Hydride Complex

The para-N-pyridyl-based PCP pincerproligand3,5-bis(di-tert-butylphosphinomethyl)-2,6-dimethylpyridine(pN-(PCP)-P-tBu-H) was synthesized and metalated to give theiridium complex (pN-(PCP)-P-tBu)IrHCl (2-H). Inmarked contrast with its phenyl-based congeners, e.g., ((PCP)-P-tBu)IrHCl and derivatives, 2-H is highly air-sensitiveand reacts with oxidants such as ferrocenium, trityl cation, and benzoquinone.These oxidations ultimately lead to intramolecular activation of aphosphino-t-butyl C(sp(3))-H bondand cyclometalation. Considering the greater electronegativity ofN than C, 2-H is expected to be less easily oxidizedthan simple PCP derivatives; cyclic voltammetry and DFT calculationssupport this expectation. However, 2-H is calculatedto undergo metal-ligand-proton tautomerism (MLPT) to give anN-protonated complex that can be described with resonance forms representinga 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 tobe dramatically more favorable than direct oxidation of 2-H (& UDelta;& UDelta;G & DEG; = -31.3 kcal/mol).The resulting Ir(II) oxidation product is easily deprotonated to givemetalloradical 2 (& BULL;) which is observedby NMR spectroscopy. 2 (& BULL;) can be furtheroxidized to give cationic Ir(III) complex, 2(+) , which can oxidatively add a phosphino-t-butylC-H bond and undergo deprotonation to give the observed cyclometalatedproduct. DFT calculations indicate that less sterically hindered analoguesof 2(+) would preferentially undergo intermolecularaddition of C(sp(3))-H bonds, for example, of n-alkanes. The resulting iridium alkyl complexes could undergofacile & beta;-H elimination to afford olefin, thereby completinga catalytic cycle for alkane dehydrogenation driven by one-electronoxidation and deprotonation, enabled by MLPT.

Automated summary

In this paper, a new pN-pyridyl-based PCP pincer ligand was synthesized and metalated to give an iridium complex. In contrast to its phenyl-based congeners, the iridium complex is highly air-sensitive and reacts with oxidants, leading to the activation of a phosphino-tert-butyl C(sp(3))-H bond and cyclometalation.