Mutual induced fit transition structure stabilization of corannulene's bowl-to-bowl inversion in a perylene bisimide cyclophane

Corannulene is known to undergo a fast bowl-to-bowl inversion at r.t. via a planar transition structure (TS). Herein we present the catalysis of this process within a perylene bisimide (PBI) cyclophane composed of chirally twisted, non-planar chromophores, linked by para-xylylene spacers. Variable temperature NMR studies reveal that the bowl-to-bowl inversion is significantly accelerated within the cyclophane template despite the structural non-complementarity between the binding site of the host and the TS of the guest. The observed acceleration corresponds to a decrease in the bowl-to-bowl inversion barrier of 11.6 kJ mol−1 compared to the uncatalyzed process. Comparative binding studies for corannulene (20 π-electrons) and other planar polycyclic aromatic hydrocarbons (PAHs) with 14 to 24 π-electrons were applied to rationalize this barrier reduction. They revealed high binding constants that reach, in tetrachloromethane as a solvent, the picomolar range for the largest guest coronene. Computational models corroborate these experimental results and suggest that both TS stabilization and ground state destabilization contribute to the observed catalytic effect. Hereby, we find a “mutual induced fit” between host and guest in the TS complex, such that mutual geometric adaptation of the energetically favored planar TS and curved π-systems of the host results in an unprecedented non-planar TS of corannulene. Concomitant partial planarization of the PBI units optimizes noncovalent TS stabilization by π–π stacking interactions. This observation of a “mutual induced fit” in the TS of a host–guest complex was further validated experimentally by single crystal X-ray analysis of a host–guest complex with coronene as a qualitative transition state analogue.


General Methods
Chemicals: All chemicals and solvents were purchased from commercial suppliers and used without further purification.[3] NMR spectroscopy: 1 H NMR spectra were recorded on a Bruker Avance III HD 600 MHz spectrometer.Chemical shift data are reported in parts per million (ppm, δ scale) downfield from tetramethylsilane and referenced internally to the residual proton (for proton NMR) in the solvent (CD2Cl2: δ = 5.32).
UV/vis absorption spectroscopy: All spectroscopic measurements were carried out under ambient conditions using solvents of spectroscopic grade.The absorption spectra were recorded on a JASCO V-770 or V-670 spectrometer equipped with a PAC-743R Peltier for temperature control.
Steady-State Fluorescence Spectroscopy: Fluorescence spectra were recorded on an Edinburgh Instruments FLS981 fluorescence spectrometer.

Single crystal X-ray analysis:
The diffraction images for X-ray crystallographic analysis were collected on a Bruker D8 Quest Kappa diffractometer with a Photon II CMOS detector and multi-layered mirror monochromated Cu Kα radiation.The solvent molecules had to be partially removed by a SQUEEZE routine. 4][7][8][9][10][11] Subsequent frequency calculations at the same level of theory were employed to confirm the resulting structures as equilibrium structures (all real frequencies) or transition structures (one imaginary frequency along the reaction coordinate of the bowlinversion of interest).The connectivities of the optimized transition structures to the respective reactants and products of interest were further confirmed via intrinsic reaction coordinate (IRC) calculations 12,13 at the B3LYP-D3(BJ)/def2-SVP level of theory.14 Corrections for bulk solvent effects of tetrachloromethane were included using the SMD continuum solvation model 15 at the recommended 15 M05-2X/6-31G(d) level of theory.More S4 detailed insights into noncovalent interaction energies were obtained using the second generation of the Absolutely Localised Molecular Orbital (ALMO) energy decomposition analysis (EDA) scheme 16 at the recommended ωB97M-V/Def2-SVP level.All geometry optimizations, frequency, IRC and single point energy calculations were performed using the Gaussian16 software package. 17was titrated to a solution of the pure cyclophane in the same solvent (mixture) of the same concentration keeping the host concentration constant during the experiment.The UV/vis or fluorescence titration data were fitted to a 1:1 binding model. 18Data evaluation was furthermore performed by using bindfit. 19 [G] / M K a = (2.0 ± 0.6) x 10     [G] / M K a = (2.9 ± 0.4) x 10    l / nm calculated spectrum of complex corannulene@1-PP in DCM complex spectrum corannulene@1-PP in CCl

Variable Temperature NMR Studies
From the coalescence temperature and the signal splitting of the methylene protons at the lowest measured temperature, the corresponding barrier for the bowl-to-bowl inversion can be determined according to equation S1 with the universal gas constant R, the coalescence temperature Tc, the Avogadro constant NA, Planck's constant h and the signal splitting at the lowest measured temperature .S19

Single Crystal X-Ray Analysis
The crystals of coronene⸦1-PP were grown from a host-guest mixture in chlorobenzene by slow evaporation of n-hexane into the solution and were obtained as red blocks.Table S3.Breakdowns of the intermolecular interaction energies (ΔEINT) in kJ mol -1 between host and guest in reactant complex corannulene⸦1-PP and transition structure complex [corannulene⸦1-PP] ‡ from second generation ALMO-EDA at the ωB97MV/Def2-SVP level of theory.

Titration Studies:
For the titration experiments, a solution of PBI cyclophane 1-PP and the respective guest in excess (see corresponding graphs for exact amount of the individual guest)

1 ) 1 )
Fig. S1.a) UV/vis spectra of cyclophane 1-PP in CHCl3 at 22 °C (c = 10 x 10 - M) upon the addition of anthracene as a guest and b) the resulting plot of the absorption at l = 490 nm with nonlinear curve fit (1:1 binding model, red curve).

Fig. S6 .
Fig. S6.a) Fluorescence spectra (lexc = 507 nm) of cyclophane 1-PP in CHCl3 at 22 °C (c = 5 x 10 - M) upon the addition of coronene as a guest and b) the resulting plot of the fluorescence at l = 560 nm with nonlinear curve fit (1:1 binding model, red curve).c) UV/vis spectrum of free 1-PP and coronene⸦1-PP in chloroform at 22 °C.

Fig. S7 .
Fig. S7.a) UV/vis spectra of cyclophane 1-PP in CHCl3 at 22 °C (c = 10 x 10 - M) upon the addition of corannulene as a guest and b) the resulting plot of the absorption at l = 490 nm with nonlinear curve fit (1:1 binding model, red curve).

Fig. S8 .
Fig. S8.a) Fluorescence spectra (lexc = 507 nm) of cyclophane 1-PP in CCl4 at 22 °C (c = 1.2 x 10 - M) upon the addition of anthracene as a guest and b) the resulting plot of the fluorescence at l = 600 nm with nonlinear curve fit (1:1 binding model, red curve).c) UV/vis spectrum of free 1-PP and anthracene⸦1-PP in tetrachloromethane at 22 °C.

Fig. S14 .
Fig. S14.UV/vis spectrum of corannulene⸦1-PP in CCl4 and the corresponding calculated spectrum (the spectrum of the 1:1 complex was calculated from the titration data) in DCM.

Fig. S20 .
Fig. S20.Packing of coronene⸦1-PP in the crystalline state, obtained by single crystal X-ray analysis (thermal ellipsoids set at 50% probability).Hydrogen atoms are omitted for clarity.

Fig. S21 .
Fig. S21.Comparison between the bowl-depths of the freely optimized corannulene and the compressed corannulene from the optimized complex characterized by the distance between the centroids of the central 5membered ring and the 10 carbon atoms making up the corannulene rim.

Table S1 .
Crystal data and structure refinement for coronene⸦1-PP.

Table S2 .
Core twist of the calculated complex structures with corannulene as a guest.