Inverse Isotope Effects in Single-Crystal to Single-Crystal Reactivity and the Isolation of a Rhodium Cyclooctane σ-Alkane Complex.

The sequential solid/gas single-crystal to single-crystal reaction of [Rh(Cy 2 P(CH 2 ) 3 PCy 2 )(COD)][BAr F 4 ] (COD = cyclooctadiene) with H 2 or D 2 was followed in situ by solid-state 31 P{ 1 H} NMR spectroscopy (SSNMR) and ex situ by solution quenching and GC-MS. This was quantified using a two-step Johnson-Mehl-Avrami-Kologoromov (JMAK) model that revealed an inverse isotope effect for the second addition of H 2 , that forms a σ-alkane complex [Rh(Cy 2 P(CH 2 ) 3 PCy 2 )(COA)][BAr F 4 ]. Using D 2 , a temporal window is determined in which a structural solution for this σ-alkane complex is possible, which reveals an η 2 ,η 2 -binding mode to the Rh(I) center, as supported by periodic density functional theory (DFT) calculations. Extensive H/D exchange occurs during the addition of D 2 , as promoted by the solid-state microenvironment.