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

Abstract

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 (kH/kD=2.6) reflects a high degree of bond activation in the transition state (as predicted quantum chemically). Show more