A π-Basic Rhenium Center that Effects Cyclohexene Isomerization to a β-Agostic Carbene Ligand

The molecule (PNPR)Re(H)4 (PNPR = (R2PCH2SiMe2)2N, R = iPr or cyclohexyl) reacts at 20 °C with 2 mol of cyclohexene to form equimolar cyclohexane and (PNPR)Re(H)2[C(CH2)5]. This product is characterized by 1H, 13C, and 31P NMR and by X-ray diffraction as having one CH2 hydrogen (from a carbon located β to Re) donating to the metal (“agostic CH”). This interaction occurs in preference to PNPR amide nitrogen π-donation. DFT calculations confirm this agostic interaction, and show that the (PNPR)Re(H)2 fragment indeed reverses the greater stability of free olefin vs free carbene.