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

The known complex [Ir( t Bu-PONOP)MeH][BAr F 4], 1[BAr F 4 ] [ t Bu-PONOP = κ 3-2,6-( t Bu 2PO) 2C 5H 3N); Ar F = 3,5-(CF 3) 2(C 6H 3); 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 (CD 2Cl 2) behavior, where 1[BAr F 4 ] decomposes by methane loss. Crystalline 1[BAr F 4 ] 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 F 4 ], and its isotopologue [Ir( t Bu-PONOP)(CD 3)D][BAr F 4], d 4 -1[BAr F 4 ], suggest a rapid and reversible endergonic reductive bond formation is occurring in crystallo to access an Ir(I) σ-methane complex. Heating 1[BAr F 4 ] to 80 °C under high vacuum results in loss of methane and intramolecular C-H activation to form cyclometalated [Ir(cyclo- t Bu-PONOP')H][BAr F 4], 2[BAr F 4 ], in a SC-SC reaction. This is reversible, and the addition of CH 4 or CD 4 to 2[BAr F 4 ] at 80 °C results in an equilibrium with 1[BAr F 4 ] or d 4 -1[BAr F 4 ], respectively. Complex 2[BAr F 4 ] is thus an operationally unsaturated source of 14-electron [Ir( tBu-PONOP)][BAr F 4], III, that undergoes C-H activation with methane. Periodic DFT studies, alongside isotope labeling experiments, link 1[BAr F 4 ] and 2[BAr F 4 ]/CH 4 via a reductive elimination/oxidative addition pathway. Heating 2[BAr F 4 ] to 80 °C under N 2 forms [Ir( t Bu-PONOP)(κ 1-N 2)][BAr F 4], in a SC-SC transformation. Reaction with CO forms [Ir( t Bu-PONOP)(CO)][BAr F 4] at room temperature. Calculations suggest reaction with N 2 occurs via an associative process or competitively through III, while with CO only an associative process operates. Heating 2[BAr F 4 ] to 80 °C under an ethane atmosphere results in alkane dehydrogenation, via a SC-SC reaction, forming a ∼1:1 mixture of [Ir( t Bu-PONOP)(η 2-H 2C═CH 2)][BAr F 4], and [Ir( t Bu-PONOP)H 2][BAr F 4].