Single-Crystal to Single-Crystal Addition of H2 to [Ir(iPr-PONOP)(propene)][BArF4] and Comparison Between Solid-State and Solution Reactivity

The reactivity of the Ir(I) PONOP pincer complex [Ir( i Pr-PONOP)(η 2 -propene)][BAr F 4 ], 6 , [ i Pr-PONOP = 2,6-( i Pr 2 PO) 2 C 6 H 3 N, Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ] 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 H 2 and propene, and then the application of vacuum, to [Ir( i Pr-PONOP)(η 2 -COD)][BAr F 4 ], 1 , a reaction manifold that proceeds via the Ir(III) dihydrogen/dihydride complex [Ir( i Pr-PONOP)(H 2 )H 2 ][BAr F 4 ], 2 , and the Ir(III) dihydride propene complex [Ir( i Pr-PONOP)(η 2 -propene)H 2 ][BAr F 4 ], 7 , respectively. In solution (CD 2 Cl 2 ) 6 undergoes rapid reaction with H 2 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 H 2 in an SC-SC reaction, but unlike in solution the onward reaction (i.e., insertion) does not occur, as confirmed by labeling studies using D 2 . The solid-state structure of 7 reveals that, on addition of H 2 to 6 , the PONOP ligand moves by 90° within a cavity of [BAr F 4 ] - anions rather than the alkene moving. Periodic DFT calculations support the higher barrier to insertion in the solid state (Δ G ‡ = 26.0 kcal/mol), demonstrating that the single-crystal environment gates onward reactivity compared to solution. H 2 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( i Pr-PONOP)H 2 (η 1 -H 3 B·NMe 3 )][BAr F 4 ], 5 , is also reported as part of these studies.