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(Cy2P­(CH2)3PCy2)­(COD)]­[BArF4] (COD = cyclooctadiene) with H2 or D2 was followed in situ by solid-state 31P­{1H} 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 H2, that forms a σ-alkane complex [Rh­(Cy2P­(CH2)3PCy2)­(COA)]­[BArF4]. Using D2, a temporal window is determined in which a structural solution for this σ-alkane complex is possible, which reveals an η2,η2-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 D2, as promoted by the solid-state microenvironment.