Diastereoselective Ullmann Coupling to Bishelicenes by Surface

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Diastereoselective Ullmann Coupling to Bishelicenes by Surface Topochemistry Anaïs Mairena, Christian Wäckerlin, Martin Wienke, Konstantin Grenader, Andreas Terfort, and Karl-Heinz Ernst J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 01 Nov 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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Journal of the American Chemical Society

Diastereoselective Ullmann Coupling to Bishelicenes by Surface Topochemistry Anaïs Mairena,† Christian Wäckerlin,† Martin Wienke,‡ Konstantin Grenader,§ Andreas Terfort,§ and Karl-Heinz Ernst†,#, †

Empa, Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland

‡

Department of Chemistry, University of Hamburg, 20146 Hamburg, Germany

§ Institut

für Anorganische und Analytische Chemie, Goethe-Universität Frankfurt, Max-von-Laue-Straße 7, 60438

Frankfurt, Germany #

Department of Chemistry, University of Zurich, 8057 Zurich, Switzerland



Institute of Physics of the Czech Academy of Sciences, Cukrovarnická 10, 18221 Prague 6, Czech Republic

Supporting Information Placeholder ABSTRACT: The comparison of the self-assembly

better understand such complex intermolecular

9,9’-bisheptahelicene on the Au(111) surface,

interactions, self-assembly (SA) of chiral molecules

studied with scanning tunneling microscopy, with the

on

self-assembly of the same species obtained by on-

functionalization of surfaces with helical aromatic

surface synthesis via Ullmann coupling from 9-

hydrocarbons, so-called helicenes, is also of interest

bromoheptahelicene

diastereomeric

for new organic electronic devices, such as

excess for the (M,P)-meso-form of 50%. The

chiroptical sensors or electron spin filters.11-13

stereoselectivity is explained by a topochemical

Therefore, synthesis and SA of helicenes on

effect, in which the surface-alignment of the starting

surfaces has received significant attention.14-16

material

intermediate

Beyond plain SA, however, on-surface synthesis

sterically favor the (M,P)-transition state over the

towards larger and more robust compounds is

homochiral transition states.

increasingly applied as strategy for more stable

and

the

reveals

a

organometallic

surfaces

has

become

popular.8-10

The

films. For that purpose, the Ullmann reaction has been very successful for formation of covalent C-C Molecular chiral recognition on surfaces plays an important role in heterogeneous enantioselective catalysis,1 biomineralization,2 and for the resolution of chiral molecules into pure enantiomers via crystallization3,4

or

chromatography.5-7

In order to

bonds on surfaces.17-21 Topochemistry deals with stereoselectivity of reactions in confined environments such as a crystal or on a surface.22,23 Confinement on a surface may either favor a reaction product or intermediate,24 or

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it induces selectivity due to the constraint of dense lateral packing of the product.25,26 An example for such scenario is bistetrahelicene, but there the stereoselectivity is based on the low inversion barrier of enantiomers.26 It is therefore interesting to extend the problem to helicenes with much higher inversion barriers in order to see if there are still diasteromeric effects during surface C-C coupling. Here it is shown by means of scanning tunneling microscopy

(STM)

and

X-ray

photoelectron

spectroscopy (XPS) under ultrahigh vacuum (UHV) conditions that post-reaction-SA of on-surface synthesized 9,9’-bisheptahelicene (C60H34, bis[7]H) differs

substantially

from

SA

of

sublimation-

deposited bis[7]H on the Au(111) surface. While identical

racemic

phases,

composed

of

the

homochiral (M,M)- and (P,P)-combinations, are observed in both cases, the 2D lattices formed by the

meso-(M,P)-compounds

are

structurally

different. In particular the diastereomeric ratio

Figure 1. (a) Molecular structures of diastereomers of

(M,P)/[(M,M)+(P,P)]

9,9’-bis[7]H and Br[7]H. (b) STM image (65.5 nm × 65.5

of

on-surface

synthesized

bis[7]H deviates substantially from unity. Details of synthesis of 9-bromoheptahelicene (Br[7]H) and bis[7]H has been described recently.27 The Au surface was cleaned in UHV by Ar+ ion sputtering and annealing. Thermal sublimation of Br[7]H and bis[7]H onto the Au(111) surface held at room temperature was performed at 443 K and 573 K, respectively. The molecular layers were analyzed by STM at 60 K and by XPS at room temperature

nm, 30 pA, –3.5 V) of bis[7]H on Au(111). Two different phases are observed: a ‘diamond’ phase and a ‘zigzag’ phase. (c) STM image (13.7 nm × 13.7 nm, 63 pA, 3.2 V) of the zigzag phase. This phase is a racemate of

(M,M)- and (P,P)-enantiomers. The bright off-center protrusions,

highlighted

with

colored

circles,

correspond to the two uppermost parts of a bishelicene. The arrows follow the direction of the helices down. Molecular models are superimposed in the STM image accordingly. (d) STM image (27 nm ×

after annealing to different temperatures. Molecular

27 nm, 52 pA, 2.9 V) of the diamond phase. A

frontier orbitals (LUMO to LUMO+4) were simulated

‘diamond’ is built up by two (M,P)-bis[7]H, rotated by

using extended Hückel theory (EHT). AMBER force

180° with respect to each other. Molecular models are

field molecular mechanics was used to compare

superimposed accordingly. (e) Structure models for

different freely relaxed molecular structures of

zigzag (left) and diamond (right) phases. Matrix

intermediate atropisomers on a fixed metal slab (see

notations, choices of substrate and adlattice vectors

Supporting Information).

are indicated.


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After deposition of bis[7]H on Au(111) two distinct

unmodified by the molecule-substrate interactions,

structures were observed at full monolayer coverage

implying that the large distance observed between

(Figure 1): one appearing in a ‘zigzag’ pattern and

two consecutive zigzag rows is probably not due to

one as a ‘diamond’ pattern. Such structures have

another surface reconstruction.33 As previously

previously been observed for bis[7]H on Cu(111)

discussed,27 other reasons could be charge density

and were analyzed in great detail, including STM

waves or a substantial interface dipole moment due

manipulation of single molecules.27 Unlike on

to Pauli repulsion. That any of these effects would

Cu(111), the mobility of the molecules on Au(111) at

act

room temperature was large enough for SA into

(M,M)/(P,P)-racemate and not the (M,P)-diamond

these ordered structures without the need of any

structure is quite surprising. Despite the different

extra annealing. The occupied surface areas per

substrate lattice dimensions of Au and Cu, bis[7]H is

molecules are comparable to the ones on Cu(111)

engaged in identical long-range structures on both

(Table S1). Due to the larger substrate lattice

surfaces. Hence, the SA is essentially controlled by

vectors of Au, the matrix notations differ from those

intermolecular

determined for Cu(111). They are (6 –2, 3 8) and (6

structures are commensurate, i.e., also controlled to

–1, 5 14) for diamond and zigzag structure,

some extent by favored substrate sites.

diastereoselectively,

i.e., affect only the

interactions.

Nevertheless,

the

respectively (Figure 1e).28 The zigzag structure is a racemate, appearing as succession of rows of homochiral (M,M)- and (P,P)enantiomers, rotated by 90° with respect to each other (Figure 1c). Racemic lattice structures built-up by homochiral dimers are so far only known for the non-covalently bound pentahelicene dimers on Cu(111) and aminohexahelicene on Au(111).29,30 The diamond structure is assembled by (M,P)-

Figure 2. XP spectra of Br 3p and C 1s after deposition

diastereomers, in which two molecules in the unit

of 2 ML Br[7]H and annealing at different temperatures.

cell are related by a 2D center of inversion. That [7]H

Desorption of the second layer below 373 K is

forms surface dimers or quadruplets via van der

recognized by the decrease of signal area for C and Br.

Waals forces has been reported previously.31,32

Debromination occurs below 413 K, as indicated by a

As observed for bis[7]H SA on Cu(111), the zigzag

redshift in binding energy of the Br 3p signals.

phase does not show a dense close-packing and

The course of the Ullmann reaction on the surface

has voids between the molecules. A special surface

can be followed by XPS (Figure 2). After deposition

reconstruction as reason is excluded, because the

of two layers of Br[7]H at room temperature (RT) the

typical gold herringbone reconstruction of the

following changes are observed with increasing

topmost

observed

temperature: At first desorption of the 2nd layer

underneath both structures (Figure S1). It means

occurs, indicated by decrease of the signal areas by

that the standard reconstruction of the surface is

50%. A shift to lower binding energies at 413 K of

Au

surface

layer

is

still



Page 4 of 7

the Br 3p signal then reveals the debromination of

available via indirect pyrometric measurements. In

Br[7]H. Previous reports list the Ullmann reaction on

order

Au(111) to occur between 380 K and 473 K.19,25,34,35

temperature, the surface was annealed step-wise

Note that organometallic intermediates of the

until no Br[7]H features were observed anymore in

Ullmann coupling have been observed on Cu and

STM, which also led to well-ordered closed-packed

Ag surfaces, but rarely for Au surfaces.35 That is, this

layers of the product. Two different structures were

intermediate on Au is short-lived and C-C coupling

observed after post-reaction SA: one appearing in

occurs instantaneously after debromination.

the same zigzag pattern as observed after

to

find

the

lowest

sufficient

reaction

deposition of bis[7]H; and a second new structure, where the molecules are aligned in lines (Figure 3). The latter is completely different to the diamond structure of the self-assembled (M,P)-bis[7] isomers. No patches of the diamond structure were observed at all after on-surface coupling to bishelicene. As the zigzag structure is built up by both homochiral

(M,M)- and (P,P)-enantiomers the ‘line’-structure is likely to present the (M,P)-coupled species. The line structure building block (Figure 3d, black rectangle) appears with two bright protrusions on each half of the molecules. Such non-symmetric contrast indeed corresponds to the spatial density distribution in an (M,P)-bis[7]H (Figure 3d, inset). Figure 3. STM images taken after Ullmann coupling of Br[7]H on Au(111). (a) Long-range STM image (500 nm × 500 nm) showing that the surface is covered with two types of domains. (b) STM image (99 nm × 99 nm) revealing that one domain type appears in a zigzag pattern (Z), while the other consists of lines of molecules (L). (c) STM image (27.3 nm × 27.3 nm) of a domain of the zigzag structure. (d) STM image (9.7 nm × 9.7 nm) of the line structure formed by on-surface C-C coupling. The rectangle indicates a single bis[7]H

The

different

appearance

of

(M,P)-bis[7]H,

whether adsorbed as such or formed by on-surface chemistry, might be either due to the residual bromine present on the surface or due to stereochemistry. Bromine atoms are usually easily observed in STM, even between helicenes.25 Longrange STM images show dark areas without bis[7]H that probably contain the released bromine (Figure S2).

molecule. Inset: EHT electron density map of (M,P)-

Note that a substantial different shift of the protons

bis[7]H for comparison with the molecules observed in

in 8,8’ and 10,10’ positions is observed for bis[7]H in

this structure. (parameters: a,b: 30 pA, 1.4 V, c: 35 pA,

NMR, being subject to atropisomerism.27 Due to

–2.9 V; d: 30 pA, 1.5 V).

such steric constraint the two helicene subunits are

The Au surface temperature during Ullmann

expected to be mutually perpendicular to each other

coupling in the STM was not exactly known and only

in the free molecule. The tendency on the surface,


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however, to have the terminal rings of both helicene

complex under Ullmann coupling conditions, but the

subunits aligned as much as possible parallel to the

tendency

surface imposes substantial steric stress to the

homochiral combinations clearly confirms the

bishelicene.27

experimental

Due

to

the

high

sublimation

meso-complexes

favoring

observations.

Under

over reaction

temperature of 573 K of bis[7]H the barriers between

conditions the monolayer is disordered with highly

the atropisomers could be easily overcome, and

mobile single helicene units. A racemic mix situation

consequently only one type of surface conformation

is therefore favored over a ‘conglomerate’ mixing by

for each diastereomer is observed. The equal

RTln(2). Moreover, it also excludes an alignment-

surface-alignment of Br[7]H, however, will also

due-to-coverage

impose a substantial steric constrain during C-C

proposed for Ullmann coupling of bromobiphenyl.36

mechanism,

as

previously

coupling and may easily explain the formation of a

In conclusion, post-reaction self-assembly of on-

different atropisomer on the surface, leading to the

surface Ullmann-coupled bishelicenes reveal a

different appearance in STM.

profound diastereoselectivity, which is assigned to

Besides the different structure for the (M,P)-

the surface-induced alignment of the educts and the

isomers, the most striking difference for the on-

intermediates in combination with the different

surface-synthesized bis[7]H layer with respect to the

stereochemical constraint during reaction. It is

solution-synthesized

shown

surface-self-assembled

here

that

a

surface

may

induce

bis[7]H layer is the fact that significantly more

stereoselectivity by topochemistry. A clever design

surface area is covered by the line structure (Figure

of surface anchor groups in educts may therefore

3a,b). A statistical analysis of six large-scale STM

offer new routes in stereoselective heterogeneous

images (sizes from 150 × 150

nm2

to 250 × 250

nm2)

revealed that the line structure occupies 64% of the surface. Taking the higher density of the line structure

(Table

S1)

into

account,

the

diastereomeric ratio (M,P)/[(M,M)+(P,P)] is 3:1 (74.6%  5%). Hence, the on-surface Ullmann coupling of 9-Br[7]H to 9,9’-bis[7]H proceeds such that a diasteriomeric excess of 50% results. alignment, molecular mechanics simulations of organometallic

intermediates

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Details of experimental and theoretical methods, additional STM images, molecular modelling results and details of 2D-structures.

In order to identify steric constraint by surface potential

catalysis.

were

performed. Considering three different atropisomers of the two diastereomers (P,P)- and (M,P)-[7]H-Au[7]H intermediates, it is found that in all cases (M,P)[7]H-Au-[7]H configurations are sterically favored by roughly 15 kcal/mol (Figure S3). Such large values might be not entirely representative for the transition

AUTHOR INFORMATION Corresponding Author

* E-mail: [email protected] Funding Sources

University Research Priority Program LightChEC of the University of Zürich, Switzerland Swiss National Science Foundation (Grant 163296)

ACKNOWLEDGMENT



(15)

Financial support by the Swiss National Science

Part 3: applications and properties of carbohelicenes. Chem.

Foundation and the by the University of Zürich

Soc. Rev. 2013, 42, 1051–1095.

Research Priority Program LightChEC is gratefully

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Diastereoselective Ullmann Coupling to Bishelicenes by Surface

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