Mechanism of Hydrogen Activation by Frustrated Lewis Pairs: A Molecular Orbital Approach

A detailed molecular orbital treatment of the heterolytic hydrogen splitting by bulky Lewis acid-base pairs is presented. The frontier molecular orbitals of the proposed reactive intermediate are shown to be preorganized but otherwise practically identical to those of the free acid and base molecules. The concerted interaction of the Lewis centers with hydrogen leading to the polarization and, ultimately, to the cleavage of the H-H bond is examined, and the bridge role of hydrogen molecule in the electron transfer is pointed out. The formation of the new covalent bonds is monitored by bond order and natural localized molecular orbital calculations, and found to be synchronous. The stability of the product is interpreted on the basis of favorable orbital interactions. A comparison of various hydrogen activation mechanisms emphasizes the common donation/back-donation motifs and the different ways of making them feasible. (C) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 109: 2416-2425, 2009