Asymmetric Molecular Adsorption and Regioselective Bond Cleavage on Chiral PdGa Crystals

Abstract Homogenous enantioselective catalysis is nowadays the cornerstone in the manufacturing of enantiopure substances, but its technological implementation suffers from well‐known impediments like the lack of endurable catalysts exhibiting long‐term stability. The catalytically active intermetallic compound Palladium‐Gallium (PdGa), conserving innate bulk chirality on its surfaces, represent a promising system to study asymmetric chemical reactions by heterogeneous catalysis, with prospective relevance for industrial processes. Here, this work investigates the adsorption of 10,10′‐dibromo‐9,9′‐bianthracene (DBBA) on the PdGa:A(1¯1¯1¯) Pd3‐terminated surface by means of scanning tunneling microscopy (STM) and spectroscopy (STS). A highly enantioselective adsorption of the molecule evolving into a near 100% enantiomeric excess below room temperature is observed. This exceptionally high enantiomeric excess is attributed to temperature activated conversion of the S to the R chiral conformer. Tip‐induced bond cleavage of the R conformer shows a very high regioselectivity of the DBBA debromination. The experimental results are interpreted by density functional theory atomistic simulations. This work extends the knowledge of chirality transfer onto the enantioselective adsorption of non‐planar molecules and manifests the ensemble effect of PdGa surfaces resulting in robust regioselective debromination.


SUPPORTING INFORMATION: S1. RT-deposition of DBBA molecules on Au(111): self-assembled islands and electronic structure
We deposit DBBA at RT on Au(111), as an achiral surface reference.Once that step edges are saturated, DBBA molecules self-assemble into islands along the fcc regions, either as linear rows along the herringbone ditches (Supp.Figure 1c) or as irregular islands at domain boundaries (Supp.Figure 1b).Among these latter irregular aggregates, some DBBA molecules self-assemble in circular aggregates of pentamers or hexamers (blue arrows .inpanel b), like previously reported on Ag(111) for high molecular coverages 1 .A minor portion of molecules pins to surface 'elbow' dislocations, presumably fixing the nucleation point of the self-assembled islands.In contrast to the well-defined adsorption of DBBA molecules on the A:Pd3 surface, the assembly of adsorbates on Au(111) is dominated by intermolecular interactions overruling the relatively weak adsorbate-substrate interaction in this case.DBBA molecules are known to form armchair graphene nanoribbons on Au(111) upon thermal annealing 2 .Contrary to A:Pd3 samples at RT where all molecules are mono-debrominated, DBBA molecules are intact on Au(111) after RT-deposition.According to the d-band model introduced by Nörskov and coworkers 3 , this stronger catalytic effect of Pd3 surface for the mono-debromination is expected because of similar reactivity of PdGa with respect to Ag(111)/Cu(111) 3 , where DBBA molecules are known to undergo partial/complete debromination at RT 4 .Additionally supporting this stronger adsorbate-substrate interaction on A:Pd3 with respect to Au(111), we observe a smaller band gap on the former (3.17 V, Figure 4a) than on the latter (4.02 V).

S2. Simulated STM images of intact and debrominated DBBA molecules on the A:Pd3 surface:
We perform simulated STM images of the intact and debrominated DBBA molecules on the A:Pd3 surface by DFT, easing their visual identification in experimental STM images.The detailed chiral motifs of the simulated species fit precisely with those observed experimentally, and remark the apparent (counter-) clockwise helicity of the anthracene subunits of intact DBBA molecules imaged by STM.

S4. DBBA in gas phase: forbidden anti-parallel rotation of anthracene units
Simulations using constrained geometry optimizations illustrate the DBBA internal energy as a function of the dihedral angle between anthracenes.The parabolic growth of DBBA energy when anthracene units are brought to the same plane supports molecular fragmentation happening before any successful anti-parallel rotation of anthracene units, as reported previously for similar molecules 5 .The difference in energy between the angle 0° and 180° is due to the initial conditions of the optimization with the central dihedral fixed at a certain value.The energy profile is however largely symmetric around the minimum.Constrained optimizations were performed with the software ORCA 6 using the hybrid functional B3LYP and the split valence polarization Karlsruhe basis set (def2-SVP) (A triple-zeta basis set was also tested but the differences were found to be negligible) 7 .

S5. Tip-induced on-surface enantiomer flipping on A:Pd3
On-surface flipping of single molecules can be attained by tip-manipulation methods, enabling the in situ interconversion of enantiomers.This tip-induced single-molecule process supports the collective flipping of all enantiomer S into R by thermal means (see Figure 1).

Figure 4 :
DBBA in gas phase.(a) Energy as a function of dihedral angle between anthracene units.(b-d) DBBA models for dihedral angles of (b) 180°, (c) 140° and (d) 100°.The hydrogens causing the most steric repulsion for high torsional angles are highlighted in green, and the bonds defining the torsional angle in yellow.Black arrow in panel b indicates a high steric hindrance situation between (overlapping) H atoms located in-between anthracenes.