Experimental and Theoretical Study of Molecular Response of Amine Bases in Organic Solvents

Abstract

Reorientational correlation times of various amine bases (namely, pyridine, 2,6-lutidine, 2,2,6,6-tetramethylpiperidine) and organic solvents (dichloromethane, toluene) were determined by solution-state NMR relaxation time measurements and compared with predictions from molecular dynamics (MD) simulations. The amine bases are reagents in complex reactions catalyzed by frustrated Lewis pairs (FLP), which display remarkable activity in metal-free H2 scission. The comparison of measured and simulated correlation times is a key test of the ability of recent MD and quantum electronic structure calculations to elucidate the mechanism of FLP activity. Correlation times were found to be in the range of 1.4-3.4 (NMR) and 1.23-5.28 ps (MD) for the amines and 0.9-2.3 (NMR) and 0.2-1.7 ps (MD) for the solvent molecules.

Ab Initio Molecular Dynamics Study of Hydrogen Cleavage by a Lewis Base [tBu3P] and a Lewis Acid [B(C6F5)3] at the Mesoscopic Level—Dynamics in the Solute–Solvent Molecular Clusters

“…It is shown that: i) the onset of H 2 activation is at much larger boron···phosphorus distances than previously thought; ii) the system evolves to the product in a roaming-like fashion because of quasi-periodic nuclear motion along the asymmetric normal mode of P···HÀH···B fragment; iii) transient configurations of a certain type are present despite structural interference from the solvent; iv) transient-state configurations with sub-picosecond lifetime have potentially interesting infrared activity in the organic solvent (toluene) as well as in the gas phase. [9][10][11][12][13] Recently, we found the first-but already convincing-indications that the FLP activity in [1] has fundamental characteristics that are not captured in the transition-state/minimum-energy-pathway (TS/MEP) description. [1, 2] The two central aspects of [1] are that: i) the dative adduct between bulky LB and LA species is not formed for steric reasons, hence the term (intermolecular) "frustrated" Lewis pairs (FLPs); ii) the encounter van der Waals (vdW) complexes between LB and LA species have not been unambiguously confirmed neither by experiment nor by theory.…”

“…[2] Despite considerable efforts, theoretical analyses of FLP activity carried out without applying a molecular dynamics methodology have not led to consensus with regard to the mechanism of [1]. [9][10][11][12][13] Recently, we found the first-but already convincing-indications that the FLP activity in [1] has fundamental characteristics that are not captured in the transition-state/minimum-energy-pathway (TS/MEP) description. [9][10][11][12][13] Recently, we found the first-but already convincing-indications that the FLP activity in [1] has fundamental characteristics that are not captured in the transition-state/minimum-energy-pathway (TS/MEP) description.…”

Experimental Insights into the Structure and Reactivity of Frustrated Lewis Pairs

“…[36] Further insights into the behaviour of some amine/BCF solutions were obtained by Autrey and Kathmann, who assessed the role of the structure of the amine in the strength of LA/LB interactions by means of NMR spectroscopy and calorimetric experiments. [37,38] In particular, 11 B NMR spectra of 5/BCF mixtures were reported to be identical to that of pure borane, in analogy to what is observed with 3 and 4 phosphines. This would mean that NH···F hydrogen bonding interactions do not favour the formation of dative adducts, at least in appreciable amounts.…”

“…This has been also confirmed by a 14 N NMR study, which indicated that chemical shift and relaxation constants of 5 are not affected by the presence of an equimolar amount of BCF. [38] Accordingly, no heat release was observed when the solution was formed, besides independent heats of dilution. On the other hand, for less encumbered bases, such as pyridine, forming only classical Lewis adducts with BCF, calorimetric experiments evidenced a heat release related to the formation of dative NÀB bonds, having a DG8 = À17 kcal mol…”

“…In fact, the reorientational dynamics of an LB is unaffected by the nature of the medium. [38] With regards to carbene/borane FLPs, the physical behaviour of their solutions was less explored, compared with that of phosphines and amines. Less encumbered carbenes react readily with BCF affording classical Lewis adducts that are inert towards H 2 , as observed for PR 3 and NR 3 bases.…”