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, 8 (1), 405-410

Sizing the Role of London Dispersion in the Dissociation of All- meta tert-butyl Hexaphenylethane

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Sizing the Role of London Dispersion in the Dissociation of All- meta tert-butyl Hexaphenylethane

Sören Rösel et al. Chem Sci.

Abstract

The structure and dynamics of enigmatic hexa(3,5-di-tert-butylphenyl)ethane was characterized via NMR spectroscopy for the first time. Our variable temperature NMR analysis demonstrates an enthalpy-entropy compensation that results in a vanishingly low dissociation energy (ΔG298d = -1.60(6) kcal mol-1). An in silico study of increasingly larger all-meta alkyl substituted hexaphenylethane derivatives (Me, iPr, t Bu, Cy, 1-Ad) reveals a non-intuitive correlation between increased dimer stability with increasing steric crowding. This stabilization originates from London dispersion as expressed through the increasing polarizability of the alkyl substituents. Substitution with conformationally flexible hydrocarbon moieties, e.g., cyclohexyl, introduces large unfavourable entropy contributions. Therefore, using rigid alkyl groups like tert-butyl or adamantyl as dispersion energy donors (DED) is essential to help stabilize extraordinary bonding situations.

Figures

Scheme 1
Scheme 1. The Jacobsen–Nauta structure H-2 is exclusively observed for the parent trityl radical H-. All-meta tert-butyl substitution leads to equilibration between the tBu- and dimeric tBu-1 2 in solution.
Scheme 2
Scheme 2. Synthesis of tBu-1 2: (a) tBuCl, AlCl3, (–40) → (–15) °C, 81% (b) Br2, Fe, CCl4, rt, 86% (c) 1. tBuLi, Et2O, –78 °C → rt 2. (EtO)2CO, 78% (d) AcCl, n-hexane, 95% (e) Zn(Cu), C6D6 or C6D12, rt (f) only in C6D12.
Fig. 1
Fig. 1. (a) 1H-NMR of tBu- in benzene-d 6 (top) in equilibrium with tBu-1 2 in cyclohexane-d 12 (middle) and the computed spectrum(ii) of tBu-1 2 (bottom); see ESI for full spectra. (b) The structure of tBu-1 2 for NMR computations. R cc: exp. 1.67(3) Å, comp. 1.662 Å. (ii)(B3LYP-D3(BJ)/6-31G(d,p)/C-PCM:cyclohexane).
Fig. 2
Fig. 2. (a) Variable temperature 1H-NMR spectra of the equilibrium between tBu- () and tBu-1 2 () and (b) the corresponding van't Hoff plot (see ESI for complete spectra).
Fig. 3
Fig. 3. The experimental bond lengths R CC do not correlate well with the bond dissociation enthalpy, entropy, and free energy. Bond lengths given in Å, enthalpies and energies in kcal mol–1, entropies in cal K–1 mol–1. Bold numbers are from this work. aNo error given. tBu-1 2; H-2;, 12.
Fig. 4
Fig. 4. The computed free dissociation energies ΔG298d including dispersion corrections () correlate very well with the polarizability α and therefore the LD contributions of the R groups (R = H, Me, iPr, tBu, Cy, Ad) and gain stability with size. Neglecting LD leads to negative ΔG298d (○), bearing the opposite, negative trend. Basis set: cc-pVDZ.
Fig. 5
Fig. 5. Non-covalent interaction (NCI) plots (s = 0.5 au/–0.01 < ρ < +0.01 au) are depicted separately on the left from the molecular structure on the right for clarity. Repulsion is colour-coded red, “strong” attraction blue and weak interactions in green.

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