Abstract
The known complex [Ir((t)Bu-PONOP)MeH][BAr(F)(4)], 1[BAr(F)(4)] [(t)Bu-PONOP = κ(3)-2,6-((t)Bu(2)PO)(2)C(5)H(3)N); Ar(F) = 3,5-(CF(3))(2)(C(6)H(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(2)Cl(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. (13)C{(1)H} 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((t)Bu-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(2)C═CH(2))][BAr(F)(4)], and [Ir((t)Bu-PONOP)H(2)][BAr(F)(4)].