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

The known complex [Ir­(tBu-PONOP)­MeH]­[BArF4], 1­[BArF4] [tBu-PONOP = κ3-2,6-(tBu2PO)2C5H3N); ArF = 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[BArF4] decomposes by methane loss. Crystalline 1­[BArF4] 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[BArF4], and its isotopologue [Ir­(tBu-PONOP)­(CD3)­D]­[BArF4], d4-1­[BArF4], suggest a rapid and reversible endergonic reductive bond formation is occurring in crystallo to access an Ir­(I) σ-methane complex. Heating 1­[BArF4] to 80 °C under high vacuum results in loss of methane and intramolecular C–H activation to form cyclometalated [Ir­(cyclo-tBu-PONOP′)­H]­[BArF4], 2­[BArF4], in a SC-SC reaction. This is reversible, and the addition of CH4 or CD4 to 2­[BArF4] at 80 °C results in an equilibrium with 1­[BArF4] or d4-1­[BArF4], respectively. Complex 2­[BArF4] is thus an operationally unsaturated source of 14-electron [Ir­(tBu-PONOP)]­[BArF4], III, that undergoes C–H activation with methane. Periodic DFT studies, alongside isotope labeling experiments, link 1­[BArF4] and 2­[BArF4]/CH4 via a reductive elimination/oxidative addition pathway. Heating 2­[BArF4] to 80 °C under N2 forms [Ir­(tBu-PONOP)­(κ1-N2)]­[BArF4], in a SC-SC transformation. Reaction with CO forms [Ir­(tBu-PONOP)­(CO)]­[BArF4] 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[BArF4] to 80 °C under an ethane atmosphere results in alkane dehydrogenation, via a SC–SC reaction, forming a ∼1:1 mixture of [Ir­(tBu-PONOP)­(η2-H2CCH2)]­[BArF4], and [Ir­(tBu-PONOP)­H2]­[BArF4].