Hydride Rebound: A Frustrated Lewis Pair (FLP)‐Type Cooperative Mechanism for H₂ Activation by a Potassium Aluminyl Compound

Combining experiment and theory, the mechanisms of H₂ activation by the potassium-bridged aluminyl dimer K₂[Al(NON)]₂ (NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tertbutyl-9,9-dimethylxanthene) and its monomeric K⁺-sequestered counterpart have been investigated. These systems show diverging reactivity towards the activation of dihydrogen, with the dimeric species undergoing formal oxidative addition of H₂ at each Al centre under ambient conditions, and the monomer proving to be inert to dihydrogen addition. Noting that this K⁺ dependence is inconsistent with classical models of single-centre reactivity for carbene-like Al(I) species, we rationalize these observations instead by a cooperative frustrated Lewis pair (FLP)-type mechanism (for the dimer) in which the aluminium centre acts as the Lewis base and the K⁺ centres as Lewis acids. In contrast to previous theoretical work on this precise system by Schaefer and co-workers, the potassium ions are shown to play explicit roles in stabilizing a nascent ₂-bridging hydride, formed by heterolytic H-H bond cleavage (with accompanying protonation of the aluminium-centred lone pair). K-to-Al hydride "rebound" into the vacant aluminium-centred p-orbital then completes the net addition of H₂ via sequential H⁺/H^- transfer. The experimentally determined kinetic isotope effect (k_H/k_D=2.6) reflects a high degree of bond activation in the transition state (as predicted quantum chemically).