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].