Theoretical Study On Dihydrogen Activation Of "Frustrated Lewis Pairs"

Enhanced effort to study and development new and efficient hydrogen storage materials has been very active field of hydrogen energy. As a class of potential chemical hydrogen storage materials,"frustrated Lewis pairs"(FLPs), which were first reported in pioneering studies by Stephan and coworkers in 2006 year, have received increasingly attention for experimental and theoretical chemists. Hydrogen activation is the basis of chemical hydrogen storage and synthetic organic chemistry. Thus, in order to gain valuable guidance for further experiment studies of"FLPs", it is important to clarify the mechanistic of hydrogen activation and explore the relationship between the structure and properties of"FLP". On the basis of previously experimental and theoretical work, the mechanism of heterolytic dihydrogen (H₂) splitting for 2,6-lutidine (Lu)/B(C₆F₅)₃ and 2,2,6,6-tetramethylpiperidine (Pi)/B(C₆F₅)₃ Lewis pairs, and the substituent effects of Lu/BR₃, pyridine (Py)/BR₃ (R=F, CH₃, C6F5) and PR₃/B(C₆F₅)₃ (R=CH₃, C(CH₃)₃, C₆H₅, C6F5) pairs on the"FLP"reactivity, as well as the nature of dihydrogen bonds in the products of the reactions of H₂ activation have been studied from theoretical point of view. The purpose is gaining a deeper insight into the structures and the chemical reactivities of"FLPs", and explaining the experimental findings, as well as establishing the relationship between the geometrical/electronic structures and the H₂ activation properties of the Lewis pairs.Part 1 provides theoretically studies on the mechanism of heterolytic H₂ splitting reaction for Lu/B(C₆F₅)₃ and Pi/B(C₆F₅)₃ Lewis pairs. The structures, the reactivities of Lewis acid and base centers, and the reaction paths of H-H bond splitting have been calculated and discussed. It has been found the existence of an equilibrium involving the classical Lewis acid-base adduct and"frustrated complex"in the Lu/B(C₆F₅)₃ pair, and the inability of the Lewis acid-base adduct formation as a result of the steric congestion in the Pi/B(C₆F₅)₃ pair. There is much larger reactivity of Lewis acid and base centers in the"frustrated complexes"of the two FLPs. The atom B serves as nucleophilic attack site and the atom N serves as electrophilic attack site, respectively. Pi/B(C₆F₅)₃ pair shows strongly exothermic and exergonic nature of H₂ cleavage reaction than that of Lu/B(C₆F₅)₃ pair. These calculated results offer satisfactory basis for explaining their reactivity differences: Lu/B(C₆F₅)₃ pair acts as both classical adduct and reversible H₂ activation, while Pi/B(C₆F₅)₃ pair shows no evidence of adduct formation and react with H₂ irreversibly.Part 2 is devoted to the substitute effects of Lu/BR₃, Py/BR₃ (R=F, CH₃, C₆F₅) and PR₃/B(C₆F₅)₃ (R=CH₃, C(CH₃)₃, C₆H₅, C₆F₅) Lewis pairs on the"FLP"reactivity. It is found that for Py/BR₃ (R=F, CH₃), Lu/BF₃ and PR₃/B(C₆F₅)₃ (R=CH₃, C₆H₅) pairs, only a minimum as covalently bound adduct can be identified on the potential energy curve (PEC) of Lewis acid and base interaction. Although the adduct and"frustrated complex"have also been found on PEC for Py/B(C₆F₅)₃ and Lu/B(CH₃)₃ pairs, contrary to Lu/B(C₆F₅)₃ pair, for"frustrated complex"of Py/B(C₆F₅)₃ pair, the N-B bond length is much longer than that of Lu/B(C₆F₅)₃ pair and there is much lower reactivity of Lewis base center. For Lu/B(CH₃)₃ pair, the addition reaction is thermodynamically unfeasible, Lewis acid and base centers are much lower reactive, and the H₂ cleavage reaction represents a larger barrier and thermodynamic unfeasibility. In addition, as for P(C(CH₃)₃)₃/B(C₆F₅)₃ and P(C₆F₅)3/B(C₆F₅)₃ pairs, only a weakly bound"frustrated complex"has been identified. The process of H₂ activation for P(C(CH₃)₃)₃/B(C₆F₅)₃ pair is thermodynamically feasible, however, the reactions for other PR₃/B(C₆F₅)₃ (R=CH₃, C₆H₅, C₆F₅) pairs are thermodynamically unfeasible process.The calculated results explained reasonably the differences of chemical reactivity on the addition and hydrogen activation reactions for these pairs, and showed that the existence of"frustrated complex"on the potential energy profile, much larger reactivity of Lewis acid and base centers in the"frustrated complex", and proper thermodynamic/kinetic properties are necessary to ensure H-H bond cleavage.Part 3 mainly focuses on the nature of dihydrogen bonds X-H···H-B (X = N, C:, P, C) in the four products of the H₂ cleavage reactions, as the dihydrogen bond not only stabilize the supramolecular structures of the products, but also is the active site for the reversible hydrogen activation of"FLPs". In analogy with normal hydrogen bond, the dihydrogen bond length, the electron density and its Laplacian at dihydrogen bond critical point, and the stabilization energy associated with the interaction of donor-acceptor orbitals are good correlated and can be used to describe the dihydrogen bond strength. The lengthening of proton donor bond X-H, the red shifting of its stretch frequency and the decreasing in electron density upon hydrogen bonded complexation are also reflected in the dihydrogen bond. In addition, the unusual P-H···H-B and C-H···H-B dihydrogen bonds have been found. Similar properties have been observed in that X-H contractions and blue shift of their stretching, and the increase of its electron density upon hydrogen bonded complexation, which provides the basis for the concept of blue-shifted or blue-shifting dihydrogen bonds.