Reactivity Models of Hydrogen Activation by Frustrated Lewis Pairs: Synergistic Electron Transfers or Polarization by Electric Field?

Rokob, Tibor András and Bakó, Imre and Stirling, András and Hamza, Andrea and Pápai, Imre (2013) Reactivity Models of Hydrogen Activation by Frustrated Lewis Pairs: Synergistic Electron Transfers or Polarization by Electric Field? Journal of the American Chemical Society, 135 (11). pp. 4425-4437. ISSN 0002-7863 (print), 1520-5126 (online)

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Abstract

Two alternative qualitative reactivity models have recently been proposed to interpret the facile heterolytic cleavage of H(2) by frustrated Lewis pairs. Both models assume that the reaction takes place via reactive intermediates with preorganized acid/base partners; however, they differ in the mode of action of the active centers. In the electron transfer (ET) model, the hydrogen activation is associated with synergistic electron donation processes with the simultaneous involvement of active centers and the bridging hydrogen, showing similarity to transition-metal-based and other H(2)-activating systems. In contrast, the electric field (EF) model suggests that the heterolytic bond cleavage occurs as a result of polarization by the strong electric field present in the cavity of the reactive intermediates. To assess the applicability of the two conceptually different mechanistic views, we examined the structural and electronic rearrangements as well as the electric fields along the H2 splitting pathways for a representative set of reactions. The analysis reveals that electron donations developing already in the initial phase are general characteristics of all studied reactions, and the related ET model provides qualitative interpretation for the main features of the reaction pathways. On the other hand, several arguments have emerged that cast doubt on the relevance of electric field effects as a conceptual basis in FLP-mediated hydrogen activation.

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