An Operationally Unsaturated Iridium-Pincer Complex That C-H Activates Methane and Ethane in the Crystalline...

The known complex [Ir(Bu-PONOP)MeH][BAr^F₄], 1[BAr] [Bu-PONOP = κ³-2,6-(Bu₂PO)₂C₅H₃N); Ar^F = 3,5-(CF₃)₂(C₆H₃); J. Am. Chem. Soc. 2009, 131, 8603], is a robust precursor for in crystallo single-crystal to single-crystal (SC-SC) C-H activation of methane and ethane at 80 °C. This contrasts with the reported solution (CD₂Cl₂) behavior, where 1[BAr] decomposes by methane loss. Crystalline 1[BAr] is accessed as a single polymorph on a gram scale. A single-crystal neutron diffraction study locates the hydride. ¹³C{¹H} SSNMR experiments on 1[BAr], and its isotopologue [Ir(Bu-PONOP)(CD₃)D][BAr^F₄], d-1[BAr], suggest a rapid and reversible endergonic reductive bond formation is occurring in crystallo to access an Ir(I) σ-methane complex. Heating 1[BAr] to 80 °C under high vacuum results in loss of methane and intramolecular C-H activation to form cyclometalated [Ir(cyclo-Bu-PONOP')H][BAr^F₄], 2[BAr], in a SC-SC reaction. This is reversible, and the addition of CH₄ or CD₄ to 2[BAr] at 80 °C results in an equilibrium with 1[BAr] or d-1[BAr], respectively. Complex 2[BAr] is thus an operationally unsaturated source of 14-electron [Ir(^tBu-PONOP)][BAr^F₄], III, that undergoes C-H activation with methane. Periodic DFT studies, alongside isotope labeling experiments, link 1[BAr] and 2[BAr]/CH₄ via a reductive elimination/oxidative addition pathway. Heating 2[BAr] to 80 °C under N₂ forms [Ir(Bu-PONOP)(κ¹-N₂)][BAr^F₄], in a SC-SC transformation. Reaction with CO forms [Ir(Bu-PONOP)(CO)][BAr^F₄] at room temperature. Calculations suggest reaction with N₂ occurs via an associative process or competitively through III, while with CO only an associative process operates. Heating 2[BAr] to 80 °C under an ethane atmosphere results in alkane dehydrogenation, via a SC-SC reaction, forming a ∼1:1 mixture of [Ir(Bu-PONOP)(η²-H₂C═CH₂)][BAr^F₄], and [Ir(Bu-PONOP)H₂][BAr^F₄].