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

Abstract

The sequential solid/gas single-crystal to single-crystal reaction of [Rh(Cy₂P(CH₂)₃PCy₂)(COD)][BAr^F ₄] (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)][BAr^F ₄]. 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.