Single-crystal to single-crystal addition of H₂to [Ir(ⁱPr-PONOP)(propene)][BAr^F₄] and comparison between solid-state and solution reactivity

Abstract

The reactivity of the Ir(I) PONOP pincer complex [Ir(iPr-PONOP)(η2-propene)][BArF4], 6, [iPr-PONOP = 2,6-(iPr2PO)2C6H3N, ArF = 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 H2 and propene, and then the application of vacuum, to [Ir(iPr-PONOP)(η2-COD)][BArF4], 1, a reaction manifold that proceeds via the Ir(III) dihydrogen/dihydride complex [Ir(iPr-PONOP)(H2)H2][BArF4], 2, and the Ir(III) dihydride propene complex [Ir(iPr-PONOP)(η2-propene)H2][BArF4], 7, respectively. In solution (CD2Cl2) 6 undergoes rapid reaction with H2 to 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 H2 in 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 H2 to 6, the PONOP ligand moves by 90° within a cavity of [BArF4]- 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. H2 addition 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)][BArF4], 5, is also reported as part of these studies.

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