Single-Crystal to Single-Crystal Addition of H₂to [Ir(ⁱPr-PONOP)(propene)][BAr^F₄] and Comparison between Solid-State and Solution Reactivity

The reactivity of the Ir(I) PONOP pincer complex [Ir(ⁱPr-PONOP)(η²-propene)][BAr^F₄], 6, [ⁱPr-PONOP = 2,6-(ⁱPr₂PO)₂C₆H₃N, Ar^F= 3,5-(CF₃)₂C₆H₃] was studied in solution and the solid state, both experimentally, using molecular density functional theory (DFT) and periodic-DFT computational methods, as well as in situ single-crystal to single-crystal (SC-SC) techniques. Complex 6 is synthesized in solution from sequential addition of H₂and propene, and then the application of vacuum, to [Ir(ⁱPr-PONOP)(η²-COD)][BAr^F₄], 1, a reaction manifold that proceeds via the Ir(III) dihydrogen/dihydride complex [Ir(ⁱPr-PONOP)(H₂)H₂][BAr^F₄], 2, and the Ir(III) dihydride propene complex [Ir(ⁱPr-PONOP)(η²-propene)H₂][BAr^F₄], 7, respectively. In solution (CD₂Cl₂) 6 undergoes rapid reaction with H₂to form dihydride 7 and then a slow (3 d) onward reaction to give dihydrogen/dihydride 2 and propane. DFT calculations on the molecular cation in solution support this slow, but productive, reaction, with a calculated barrier to rate-limiting propene migratory insertion of 24.8 kcal/mol. In the solid state single-crystals of 6 also form complex 7 on addition of H₂in an SC-SC reaction, but unlike in solution the onward reaction (i.e., insertion) does not occur, as confirmed by labeling studies using D₂. The solid-state structure of 7 reveals that, on addition of H₂to 6, the PONOP ligand moves by 90° within a cavity of [BAr^F₄]^-anions rather than the alkene moving. Periodic DFT calculations support the higher barrier to insertion in the solid state (ΔG^‡= 26.0 kcal/mol), demonstrating that the single-crystal environment gates onward reactivity compared to solution. H₂addition to 6 to form 7 is reversible in both solution and the solid state, but in the latter crystallinity is lost. A rare example of a sigma amine-borane pincer complex, [Ir(ⁱPr-PONOP)H₂(η¹-H₃B·NMe₃)][BAr^F₄], 5, is also reported as part of these studies.