Importance of the Anchor Group Position (Para versus Meta) in Tetraphenylmethane Tripods: Synthesis and Self-Assembly Features

DOI: 10.1002/chem.201602019

Full Paper

&

Molecular Tripods

Importance of the Anchor Group Position (Para versus Meta) in

Tetraphenylmethane Tripods: Synthesis and Self-Assembly

Features

[

a]

[a]

Marcin Lindner, Michal Valµ sˇ ek, Jan Homberg, Kevin Edelmann, Lukas Gerhard,

[

a]

[a]

[d]

[e]

Wulf Wulfhekel, Olaf Fuhr, Tobias Wächter, Michael Zharnikov, Viliam Kolivo sˇ ka,

[e]

[f]

[e]

[a, b, c]

Lubomír Pospí sˇ il, Gµbor MØszµros, MagdalØna Hromadovµ, and Marcel Mayor*

Abstract: The efficient synthesis of tripodal platforms based

on tetraphenylmethane with three acetyl-protected thiol

groups in either meta or para positions relative to the cen-

copy, near-edge X-ray absorption fine structure spectrosco-

py, and reductive voltammetric desorption studies. These ex-

periments indicated that the meta derivative forms a well-or-

dered monolayer, with most of the anchoring groups bound

to the surface, whereas the para derivative forms a multilayer

film with physically adsorbed adlayers on the chemisorbed

para monolayer. Single-molecule conductance values for

both tripodal platforms are obtained through an STM break

junction experiment.

3

tral sp carbon for deposition on Au (111) surfaces is report-

ed. These platforms are intended to provide a vertical ar-

rangement of the substituent in position 4 of the perpendic-

ular phenyl ring and an electronic coupling to the gold sub-

strate. The self-assembly features of both derivatives are an-

alyzed on Au (111) surfaces by low-temperature ultra-high-

vacuum STM, high-resolution X-ray photoelectron spectros-

[

1–3]

Introduction

organic frameworks (COFs),

porous organic polymers

[

4,5]

[6,7]

(

POPs),

hyperbranched polymers and dendrimers,

molec-

[

8,9]

Tetrahedral molecules based on tetraphenylmethane have

been studied extensively in recent decades. In particular, they

have been used as shape-providing core motives of covalent

ular gyroscopes,

and organic light-emitting diodes

[

10,11]

(OLEDs).

In addition, the potential of the tetrahedral syn-

[

12]

thon to act as a tripodal foot, organizing and controlling the

spatial arrangement of molecules in self-assembled monolayers

(SAMs) on flat substrates, has been investigated. For this pur-

pose, three of the four phenyl units are usually decorated with

anchor groups interacting with the substrate, and the fourth is

expected to protrude from the surface and might be substitut-

ed further. SAMs of tetraphenylmethane model compounds ex-

posing three functional groups comprising sulfur atoms (e.g.,

[

a] M. Lindner, Dr. M. Valµs

ˇek, J. Homberg, K. Edelmann, Dr. L. Gerhard,

Prof. Dr. W. Wulfhekel, Dr. O. Fuhr, Prof. Dr. M. Mayor

Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT)

P. O. Box 3640, 76021 Karlsruhe (Germany)

E-mail: marcel.mayor@unibas.ch

[

b] Prof. Dr. M. Mayor

Lehn Institute of Functional Materials (LIFM)

Sun Yat-Sen University (SYSU), XinGangXi Rd. 135

by ÀCH SH, ÀSAc, ÀSR, and ÀSH) to profit from their chemi-

2

5

10275 Guangzhou (P. R. China)

[13–19]

sorption on gold substrates have already been reported.

[

c] Prof. Dr. M. Mayor

Depending on the type of substrate (silver, titanium oxide,

indium-tin oxide), alternative anchor groups have been consid-

ered (selenium, pyridine, benzyl amine, alkyl ester, phospho-

Department of Chemistry, University of Basel

St. Johannsring 19, 4056 Basel (Switzerland)

[

d] T. Wächter, Prof. Dr. M. Zharnikov

[

20–26]

Applied Physical Chemistry, Heidelberg University

Im Neuenheimer Feld 253, 69120 Heidelberg (Germany)

nate, tertiary ammonium salts).

For binding provision on

graphene surfaces by p–p stacking, tetraphenylmethane-based

[

e] Dr. V. Kolivo ˇs ka, Dr. L. Pospí sˇ il, Dr. M. Hromadovµ

J. Heyrovsk y´ Institute of Physical Chemistry of ASCR v.v.i.

Dolej sˇ kova 3, 182 23 Prague 8 (Czech Republic)

model compounds functionalized with three “pyrene feet”

[27,28]

were investigated.

Structure stabilization by Van der Waals

forces in monolayers, and efficient packing in SAMs, have been

reported for backbones consisting of C tetrahedral symmetric

[

f] Dr. G. MØszµros

Research Centre for Natural Sciences, HAS

Magyar tudósok krt. 2, 1117 Budapest (Hungary)

3

[

13,15,29]

units equipped with Si or C cores.

Supporting information for this article is available on the WWW under

Our initial attempts toward tripodal model compounds were

1

13

http://dx.doi.org/10.1002/chem.201602019. H and C NMR spectra of com-

pounds 2–24 containing chemical structures and numbering used for the

complete peak assignment. CCDC-1475999 (22) contains the supplementary

crystallographic data for this paper. These data are provided free of charge

by The Cambridge Crystallographic Data Centre.

III

based on coordination complexes comprising both neutral M

[30,31]

II

[32]

complexes

and charged tris(bipyridine)-Ru derivatives,

but our recent molecular designs comprise purely organic scaf-

[

33]

folds such as 9,9’-spirobifluorenes.

13218

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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 1. Structure of molecular tripods (7, 21), and proposed arrangement of 21 on a gold substrate.

Active-tail groups have been introduced to demonstrate the

presence of tripodal molecules on electrodes. The concept was

pioneered by Tour and co-workers with their carbazole and oli-

of the foot structure and consequently good control over its

spatial arrangement on the substrate, we decided to use ar-

ylthiol anchor groups. The remaining question from a molecular

design viewpoint is whether the para or the meta position is

better suited to provide efficient footing architectures. To ad-

dress this question, we synthesized both structures (7 and 21),

and studied their self-assembling behavior on gold substrates.

The spacing of the anchoring group as well as the size of

these specific nanostructures is a compromise between tightly

packed mono- and dipodal systems and large multipodal plat-

forms with low surface coverage. Even though functional

groups allowing modular decoration of the structures (e. g.,

boronic acid derivatives or halides) would be very appealing

from a molecular design viewpoint, our initial focus was set on

tripods decorated with nitrile groups in para positions as spec-

troscopic markers facilitating the study of their behavior in

SAMs.

[34–37]

gophenylenethylene-fullerene hybrids.

In this paper, we compare two regioisomers of tetraphenyl-

methane footing architectures, namely the derivative with the

three acetyl-protected thiol anchor groups in para positions

(

7), and the analogue with the anchor groups in meta posi-

tions (21) (Figure 1). As the active tail group facilitating their

spectroscopic investigations, both model compounds expose

a nitrile group in the para position, decorating the fourth re-

maining phenyl subunit. Gold substrates were coated with the

model compounds 7 and 21, and the resulting self-assembled

molecular coatings were analyzed by ultra-high-vacuum STM

(

UHV-STM), laboratory X-ray photoelectron spectroscopy (XPS),

high-resolution XPS (HRXPS), near-edge X-ray absorption fine

structure (NEXAFS) spectroscopy, and electrochemical desorp-

tion experiments. In addition, the syntheses of both foot-struc-

tures as modular building blocks, enabling their further func-

tionalization by cross-coupling chemistry, are reported. These

modular building blocks are the boronic ester derivatives 8

and 22, as well as the iodine derivatives 10 and 24.

Results and Discussion

Synthesis

Our retrosynthetic approach is displayed in Figure 2, and con-

sists of the assembly of the tetraphenylmethane structure in

two steps. First, the three identical phenyl subunits decorated

with the masked anchor group (or a substituent enabling the

later introduction of the anchor group) are introduced by nu-

cleophilic addition of lithiated species to diethyl carbonate,

providing the suitably substituted trityl alcohol. The second

step is a Friedel–Crafts alkylation with phenol, providing the

tetraphenylmethane derivatives with one phenyl substituent

exposing a phenolic hydroxyl group in the para position. Func-

tional group transformation chemistry subsequently allows

both the establishment of the acetyl-protected thiol anchor

groups and the transformation of the phenolic hydroxy group

Molecular design

The rigid and well-defined tetrahedral geometry of tetraphe-

nylmethane is ideal as a tripodal foot architecture. Three

phenyl rings can be decorated with the anchor group of inter-

est (sulfur in this case), and the fourth one is arranged perpen-

dicular to the surface if all three anchor-group-decorated subu-

nits interact equally with the surface. Owing to the nonconju-

3

gated character of the central sp hybridized carbon, the mo-

lecular structures mounted on such foot architectures are ex-

pected to be arranged in a spatially well-controlled, but

electronically decoupled, manner. To guarantee a tight contact

Figure 2. Retrosynthetic analysis of tetraphenylmethane platforms.

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into the protruding functional group of choice. The same ret-

rosynthetic strategy can be applied for both model com-

pounds as the position of the anchor groups is selected by the

choice of the starting materials. Interestingly, in spite of the

self-evident molecular design and the straightforward synthe-

sis, none of the subunits assembled here have been reported

previously.

The synthetic sequence yielding the para-substituted tetra-

phenylmethane derivative 7 in six steps is outlined in

Scheme 1. The synthesis started with the thiol protection of

commercially available 4-bromothiophenol with trimethyl(vi-

nyl)silane through a radical reaction in the presence of 2,2’-

azobis(2-methylpropionitrile) (AIBN) as a radical initiator to pro-

vide 2-(trimethylsilyl)ethyl (TMSE) derivative

1

in 93%

[

38,39]

yield.

Subsequent addition of lithiated aryl bromide 1 to

diethyl carbonate afforded a mixture of benzophenone deriva-

tive 2 (24% yield) and the desired trityl alcohol 3 in 60% yield.

The benzophenone derivative 2 was again reacted with an

excess of lithiated aryl bromide 1 to obtain a second crop of

trityl alcohol 3 in 83% yield. Then, trityl alcohol 3 was subject-

ed to the acid-catalyzed Friedel–Crafts alkylation with phenol

to provide tetraphenylmethane derivative 4 in 84% yield. The

resulting phenol derivative 4 was esterified with triflic anhy-

dride to afford triflate 5 in almost quantitative yield. Subse-

quently, the palladium-catalyzed cyanation of the triflate 5

with zinc and copper cyanide at 1408C provided the corre-

sponding nitrile 6 in 93% yield. Final transprotection of the

thiols was performed successfully using AgBF and acetyl chlo-

4

ride (AcCl) in dichloromethane to afford the desired thioace-

tate 7 in 54% yield. The acetyl protection group provides im-

proved storing and handling features of the compound, as it

prohibits polymerization through intermolecular disulfide for-

mation. Its lability allows mild and efficient deprotection prior

to or during the self-assembly process on gold substrates.

Triflate 5 was converted to the more reactive iodo derivative

Scheme 1. Preparation of para-substituted tetraphenylmethane derivative 7.

Reagents and conditions: (i) trimethyl(vinyl)silane, AIBN; (ii) tert-BuLi,

(EtO)

2

CO, THF; (iii) tert-BuLi, 1, THF; (iv) PhOH, HCl, toluene; (v) Tf

2 3

O, Et N,

CH Cl

2

; (vi) Zn(CN) , CuCN, Pd(PPh , DMF; (vii) AgBF , AcCl, CH Cl .

2

3

)

4

2

was treated with copper iodide and N-iodosuccinimide in an-

1

0 (as outlined in Scheme 2) to have in hand the modular tri-

hydrous DMF and toluene (2:1, v/v) using the recently report-

[

40]

podal platform bearing labile thioacetates for further function-

alization through the Sonogashira cross-coupling reaction. The

Miyaura borylation of aryl triflate 5 with bis(pinacolato)diboron

ed Osuka protocol to afford the aryl iodide 9 in 91% yield.

The transprotection of the thiols using AgBF and acetyl chlo-

4

ride (AcCl) in dichloromethane provided the desired thioace-

tate 10 in 87% yield.

in the presence of Pd(dppf)Cl provided pinacol boronate 8 in

2

7

1% yield. This pinacol boron ester itself is an interesting

The same synthetic strategy was applied for the preparation

of the regioisomeric meta-substituted tetraphenylmethane de-

rivative 14, which is shown in Scheme 3. The thiol of 3-bromo-

thiophenol was protected as 2-(trimethylsilyl)ethyl derivative

building block, as it is suitable for the further functionalization

of the tetraphenylmethane platform through Suzuki cross-cou-

pling reactions. Then, the corresponding pinacol boronate 8

2 4

Scheme 2. Preparation of aryl iodide 10. Reagents and conditions: (i) bis(pinacolato)diboron, Pd(dppf)Cl , KOAc, dioxane; (ii) CuI, NIS, toluene, DMF; (iii) AgBF ,

AcCl, CH Cl

2

.

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Scheme 4. Synthesis of the meta-brominated tetraphenylmethane 18. Re-

agents and conditions: (i) nBuLi, (EtO)

2

CO, THF; (ii) 1,3-dibromobenzene,

O.

nBuLi, THF; (iii) PhOH, HCl; (iv) Et N, TMSCl, Et

3

2

15 afforded tris(3-bromophenyl)methanol 16 in 82% yield.

Scheme 3. Synthetic attempts toward the meta-substituted tetraphenylme-

thane 14. Reagents and conditions: (i) trimethyl(vinyl)silane, AIBN; (ii) tert-

BuLi, (EtO) CO, THF; (iii) tert-BuLi, 11, THF; (iv) PhOH, HCl, toluene.

2

After Friedel–Crafts alkylation of phenol with 16, meta-bromi-

nated tetraphenylmethane derivative 17 was obtained in 88%

yield. Subsequent protection of phenol as trimethylsilyl ether

provided 18 in 89% yield, and allowed the further introduction

of protected thiol moieties through the palladium-catalyzed re-

action.

11 in 86% yield. Subsequent lithiation of aryl bromide 11 with

tert-BuLi and stepwise addition to diethyl carbonate provided

the mixture of benzophenone 12 in 39% yield and trityl alco-

hol 13 in 44% yield. Isolated benzophenone 12 was again

treated with lithiated species to obtain a second crop of trityl

alcohol 13 in 76% yield. However, subsequent acid-catalyzed

Friedel–Crafts alkylation of phenol with trityl alcohol 13 did

not afford the pure single tetraphenylmethane product 14. We

used several Brønsted (HCl, CH SO H) and Lewis (BF ·Et O)

The thiol anchor groups were introduced in their TMS-ethyl-

protected form. Therefore, the palladium-catalyzed reaction of

[

41]

aryl bromide with alkyl thiol was employed, through which

the meta-brominated tetraphenylmethane derivative 18 was

treated with 2-(trimethylsilyl)ethanethiol in the presence of

Pd (dba) , Xantphos, and Hünig’s base to afford the deprotect-

2

3

ed phenolic species 14 in 92% yield upon acidic workup, as

shown in Scheme 5. After triflatation of 14 with triflic anhy-

dride, the corresponding triflate 19 was isolated in 83% yield.

The palladium-catalyzed cyanation of 19 yielded nitrile 20 in

74% yield and subsequent transprotection of the thiols provid-

ed the target thioacetate 21 in 77% yield.

Analogously to the isomeric para-substituted platform, tri-

flate 19 was converted to the iodo derivative 24 (Scheme 6),

which is more reactive in palladium-catalyzed cross-coupling

reactions than the triflate 19 itself. The Miyaura borylation of

aryl triflate 19 provided the corresponding pinacol boronate

22 in 82% yield. Then, the aryl boronate 22 was substituted to

the aryl iodide 23 in 83% yield, and finally, the previously men-

tioned transprotection of the thiols provided the desired thioa-

cetate 24 in 55% yield.

3

2

acids in combination with cosolvents (toluene, xylenes) at dif-

ferent temperatures (80–1408C), but all these attempts gave

a complex mixture comprising regioisomers and side products,

instead of the desired molecule. Even though NMR analysis in-

dicated the presence of the desired tetraphenylmethane deriv-

ative 14, its similarity with the side products did not allow its

separation.

We assumed that the increased proximity of the TMS-ethyl-

protected sulfanyl group in the meta position of the trityl alco-

hol handicaps its reactivity sterically, and therefore, we decided

to employ less sterically bulky substituents. The new synthetic

strategy leading to the meta-substituted tetraphenylmethane

derivative 18 is outlined in Scheme 4. The synthesis started

with halogen–lithium exchange of five equivalents of 1,3-di-

bromobenzene and its subsequent addition to diethyl carbon-

ate to provide the desired benzophenone 15 in 93% yield. It

should be noted that in the case of 1,3-dibromobenzene, the

formation of the trityl alcohol derivative 16 was not observed

and the reaction mixture contained almost pure benzophe-

none derivative 15. Additional lithiation of 1,3-dibromoben-

zene and subsequent addition to the benzophenone derivative

All newly synthesized compounds were fully characterized

by conventional analytical and spectroscopic methods such as

1

13

H and C NMR spectroscopy, mass spectrometry, IR and UV/

vis spectroscopy, and elemental analysis. In addition, the iden-

tity of the tetraphenylmethane boronic acid derivative 22 was

further corroborated by single-crystal XRD analysis. Single crys-

tals of the meta derivative 22 were obtained by slow evapora-

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C1,C5 or C1,B1 is 808, providing a first approximation of the ar-

rangement of the molecular rod with respect to the surface

plane.

All attempts to grow a single crystal from a tetraphenylme-

thane derivative with thiol groups in the para position failed

hitherto. Details of the crystallographic data, as well as bond

lengths and angles, can be found in the Supporting Informa-

tion.

Characterization of adlayers of molecular tripods 7 and 21

on the gold substrate

A detailed characterization of the adlayers of meta (21) and

para (7) tripodal molecules on various gold substrates (gold

single crystal, gold-coated silicon wafers, gold-coated mica,

and polycrystalline gold electrode) was performed using sever-

al surface-analytical techniques including UHV-STM, laboratory

XPS, synchrotron-based HRXPS and NEXAFS spectroscopy, and

cyclic voltammetry (CV). Depending on the analytical tech-

nique and the nature of the corresponding sample, different

sample preparation methods were applied, and during the in-

vestigation, a broad range of physical conditions was present-

ed. The variation in physical conditions ranged from

low to room temperature and from UHV, through

inert atmosphere, to the solid/liquid interface. The

particular sample preparation details will be provid-

ed briefly at the beginning of each corresponding

analytical section of the manuscript, and further

method-specific details are provided in the Experi-

mental Section.

Scheme 5. Synthesis of the meta-substituted tetraphenylmethane 21. Re-

agents and conditions: (i) Pd (dba) , Xantphos, diisopropylethylamine,

O, Et N, CH Cl ; (iii) Zn(CN) , CuCN,

, AcCl, CH Cl

2

3

+

TMSCH

Pd(PPh

2

CH

2

SH, dioxane, H ; (ii) Tf

2

3

2

3

)

4

, DMF; (iv) AgBF

4

2

.

Although the exact molecular arrangements in

Scheme 6. Preparation of the aryl iodide 24. Reagents and conditions: (i) bis(pinacolato)-

diboron, Pd(dppf)Cl , KOAc, dioxane; (ii) CuI, NIS, toluene, DMF; (iii) AgBF , AcCl, CH Cl

the different samples are expected to vary consider-

ably owing to the differences in both sample prepa-

ration and physical conditions during characteriza-

2

4

2

.

tion of a dichloromethane/hexane solution. The solid-state

structure obtained by single-crystal XRD is shown in Figure 3.

Compound 22 crystallizes in the triclinic space group P 1¯ with

two molecules per unit cell. The angle between the plane

made by three carbons C14, C25, C36 and the line through

tion, the comparison of regioisomers 7 and 21 was of interest.

The main goal of this comparison was to analyze the role of

the position of the anchoring groups in controlling the spatial

arrangement of tripodal platforms on gold surfaces, with the

aim of forming stable contacts with the surface.

STM analysis of the tripodal films performed at 77 K

Films of the tripodal platforms 7 and 21 for the STM investiga-

tion were prepared by immersing a sample of freshly prepared

gold on mica in a solution of either meta (21) or para (7) tetra-

phenylmethane derivatives in a mixture of methanol/THF (3:1,

v/v) containing 30% of 1m solution of ammonia in methanol

as a cleaving agent, at room temperature for 48 h (for more

details see Experimental Section). After rinsing, the roughness

of the prepared films was measured by STM at 77 K in UHV. In

the case of the para derivative 7, no signal of the tunneling

current could be detected, pointing to an insulating multilayer

of molecules. In contrast, in the case of the meta derivative 21,

STM imaging could be performed, albeit without molecular

resolution. Figure 4a shows a large-scale STM image of the

meta derivative 21 obtained at 77 K in UHV. Here, atomic steps

of the Au(111) surface can be seen, separating flat terraces of

Figure 3. Solid state structure of 22 obtained by single-crystal X-ray diffrac-

tion (50% probability of thermal ellipsoids). (C black, H white, O red, S

yellow, Si violet, B green).

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about 150 nm in width. The inset shows a cross section of

a monoatomic step with a measured height of 229 pm, close

to the expected step value of 235 pm. This clearly indicates 2D

growth of the molecular layer. This is in perfect agreement

with the HRXPS and CV data, which indicated multilayer ad-

sorption in the case of the para derivative 7 and a well-ordered

monolayer in the case of the meta derivative 21 (see below).

However, molecular resolution could not be achieved, presum-

ably owing to contamination on top of the relatively open sur-

face of the tripod layer.

7, para) solutions in dichloromethane. After deposition, the

surface was transferred into the UHV chamber (p<5

À10

10 mbar) and heated mildly to about 400 K for 60 min to

remove remnants of the solvent and promote the deprotection

of the thiol anchor groups, mediating the immobilization of

the tripodal molecules on the surface. Subsequently, low-tem-

perature STM investigations were performed at about 5.3 K to

characterize the meta and para molecules on the Au(111) sur-

face deposited by this method.

As seen in Figure 4b, deposition of a sub-monolayer

amount of para molecules 7 leads to islands with long-range

order and an apparent height of about 500 pm (see Figure 4c).

The hexagonal order agrees with both the threefold symmetry

of the molecule and the underlying lattice. Figure 4d shows an

enlarged view of a unit cell (marked in black) with a lattice pa-

rameter of 3.25 nm (for details, see Supporting Information)

STM analysis of spray-deposited tripods performed at 5.3 K

Low-coverage samples of the meta and para molecules on

a clean Au(111) single crystal were prepared to gain insight

into the adsorption geometry of the tripods on a well-defined

metal substrate. The tripods were deposited by spraying the

dissolved molecules through a pulsed valve onto the Au(111)

surface. Then, after an annealing procedure, the samples were

cooled to 5.3 K for the UHV-STM investigations (preparation

details are provided in the Experimental Section). The advant-

age of this method, described in detail in our recent publica-

3

0.5

2

and an area of ( = ) (3.25 nm) =9.15 nm . The precise ad-

4

sorption configuration of the molecules within the unit cell,

however, cannot be inferred from the STM measurements.

After further annealing at 450 K for 60 min, the molecular is-

lands rearrange and form a slightly more densely packed film

(see Figure 4e,f). Here, the lattice parameter of the unit cell is

[

33]

2

tion, over the deposition of a droplet lies in the reduced

total amount of solution that is deposited. The use of higher

concentrations of molecules in the deposition solution allows

further reduction of the influence of possible contamination of

the solvent. We used 0.16 mm (for 21, meta) and 0.20 mm (for

3.12 nm, corresponding to an area of 8.43 nm . The nature of

the six identical blobs that form the unit cell cannot be identi-

fied unambiguously. Intuition suggests a number of six mole-

cules, so that each bright spot would correspond to a single

molecule. Our latest measurements on similar derivatives with

Figure 4. a) Large-scale overview of the SAM of meta-molecule 21 (77 K). The inset shows the cross section along the white dashed line with a step height of

30 pm. b) Ordered island of para-molecules 7 deposited on clean Au(111) by spray deposition. c) Cross section along the white line in (b). d) Enlarged view

2

of the area marked in (b) with the unit cell marked in black. e) Well-ordered arrangement of para-molecules 7 after prolonged heating. f) Enlarged view of

the area marked in (e) with the unit cell marked in black. g) meta-Molecules 21 lined up at the step edges of the Au(111) substrate. h/i) Enlarged view of the

area marked in (g)/(h) with the proposed adsorption model of the meta-molecule superimposed to scale. Tunneling parameters: (b)/(d): 2 V, 5 pA, (e)/(f): 2 V,

10 pA, (g)/(h): 2 V, 20 pA, (i): 0.05 V, feed back off.

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the same anchoring platforms, however, indicate the formation

of molecular dimers, so interpretations of disulfide-bridged

dimer species forming one bright blob in the STM image (see

Figure 4e,f) should be considered, and this interpretation

serves as our current working hypothesis. Assuming that each

bright spot corresponds to a single molecule or a dimer, the

ing 30% of ammonia solution (1m) in methanol, at room tem-

perature for 48 h. Afterwards, the substrate was rinsed thor-

oughly with methanol, deionized water, and THF, and finally

dried under a stream of argon. In the following, results exclu-

sively from samples obtained by this procedure will be pre-

sented and discussed, as the alternative procedures provided

films of poorer quality (see Experimental Section).

2

area occupied per molecule is 1.41 or 0.70 nm respectively.

The precise adsorption configuration, however, cannot be in-

ferred from the STM measurements.

The Au4f , C1s, S2p, and N1s HRXP spectra of the meta

7/2

(21) and para (7) films are presented in Figure 5, along with fit-

ting and decomposition of these spectra by characteristic

emissions and doublets. The Au4f₇_/₂spectrum in Figure 5a

shows noticeably lower intensity for 7/Au than for 21/Au, sug-

gesting a higher film thickness in the former case. Indeed, ac-

cording to the evaluation of the HRXPS data (see Experimental

Section), the effective thicknesses of the para (7) and meta

(21) films were estimated to be 2.68 and 1.83 nm, respectively.

Note that the above thickness difference is much less pro-

nounced in the C1s spectra of the tripods’ films in Figure 5b,

owing to the strong self-attenuation of the photoemission

In contrast to the para derivative 7, spray deposition of the

meta derivative 21 never led to ordered molecular films or is-

lands. Instead, these molecules adsorb preferentially along the

step edges of the gold substrate, as seen in Figure 4g. We pro-

pose an array of identically adsorbed meta derivatives equally

spaced with a periodicity of 1.15 nm (see enlarged view, Fig-

ure 4h). From the periodicity of 1.15 nm, we can estimate the

molecular footprint as the area of an equilateral triangle to

1

2

0.5

2

=

(1.15 nm) (3) =0.57 nm . This model is supported by

4

the high-resolution image of a single molecule shown in Fig-

ure 4i, and agrees intuitively with the assumption of dimer for-

[

42]

signal at the given kinetic energy. These spectra are domi-

nated by an intense emission at a binding energy of approxi-

mately 284.7 eV accompanied by a weak shoulder at around

286.5 eV. The dominant emission stems from the aromatic

backbone, and the shoulder can be assigned to the nitrile

2

mation and a slightly larger footprint of 0.70 nm in the case

of the para derivative 7. Here, the molecule can be identified

by two brighter lobes directed toward the gold step edge and

a third lobe that is slightly darker. The head group appears as

a brighter protrusion in the center, which jumps from the left

to the right during the scan with the tip close to the sample.

In summary, the low-temperature UHV studies demonstrate

that both tetraphenylmethane derivatives 7 and 21 can be de-

posited in sub-monolayer concentration by “spraying” tech-

niques, and after an annealing procedure, both derivatives

form considerably different ordered molecular assemblies, with

large islands for the para derivative 7, and rows at the step

edges for the meta molecule 21. Although the dimensions of

the structures’ subunits match with the molecules’ sizes and

corroborate the molecular origin of the structures, the investi-

gations do not allow conclusions to be drawn about the pre-

cise arrangement of the individual molecules on the substrate.

The observed patterns match with tripodal structures immobi-

lized with all three anchor groups, but the experiments cannot

prove this hypothesis as the tetrahedral shape of the model

compounds allows various arrangements; such proof is

beyond the resolution of the experiment.

[

43]

carbon. It cannot be excluded, however, that the shoulder

contains a small contribution from CO, at least in the case of

7/Au, for which a small signal at approximately 288.9 eV (COO)

is observed as well.

The S2p XP spectra of the tripod films in Figure 5c exhibit

two doublets at around 162.0 eV and 163.6–163.9 eV (S2p ),

3/2

assigned to the thiolate species bound to noble metal surfaces

[

44,45]

and disulfide or unbound sulfur, respectively.

The relative

weights of these components are distinctly different for 21/Au

and 7/Au. Whereas the spectrum of 21/Au is dominated by

High-resolution X-ray photoelectron spectroscopy

For these experiments, films of the tripodal molecules were

prepared by immersing a freshly prepared sample of gold on

a silicon substrate into the solution of either the meta (21) or

the para (7) tripodal molecule. Three different preparation pro-

cedures were tested for these films, as described in detail in

the Experimental Section. On the basis of the results of the

preliminary characterization by laboratory XPS as well as pre-

liminary HRXPS and NEXAFS experiments at the synchrotron,

the following optimal procedure was selected. Freshly an-

nealed gold-coated silicon wafers (typically 105 mm) were

immersed in a 0.25 mm solution of meta (21) or para (7) tripo-

dal molecules in a mixture of methanol/THF (3:1, v/v) contain-

Figure 5. a) Au4f7/2, b) C1s, c) S2p, and d) N1s HRXP spectra of the meta

(

21) and para (7) films acquired at photon energies of either 350 or 580 eV

as marked in the panels (open circles). The fitting and decomposition of the

spectra are shown (thin red and blue solid lines and thick black solid line),

including the respective background (thin black solid lines).

Chem. Eur. J. 2016, 22, 13218 – 13235

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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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the thiolate-related doublet (75%), the opposite is observed

for 7/Au, for which the disulfide/unbound sulfur doublet is the

dominant feature (85%). This suggests that the meta (21) film

represents a monolayer, with most of the terminal ÀSAc moiet-

ies being deprotected and bound to the substrate as the thio-

lates. The respective signal is then strongly attenuated by the

hydrocarbon overlayer, resulting in its comparably low intensi-

ty. In contrast, the para (7) film presumably represents a multi-

layer, with some unbound sulfur moieties buried not deep in

the matrix or even close to the film-ambience interface. Ac-

cordingly, the total intensity of the S2p signal is significantly

higher than in the 21/Au case. These findings support the hy-

pothesis of dimer formation in the case of the para (7) sub-

monolayer film prepared on a Au(111) single crystal studied by

low-temperature UHV-STM (see Figure 4e,f).

fects, and are therefore exclusively representative of the elec-

[

50]

tronic structure of unoccupied molecular orbitals.

The C K-edge spectra of the tripod films in Figure 6 exhibit

the characteristic absorption structure of the phenyl rings.

They are dominated by a pronounced absorption resonance at

approximately 285.1 eV (p *), accompanied by several weaker

1

and broader resonances at ꢀ287.6 (mixture of R* and CÀH*),

ꢀ288.4 (p *), ꢀ292.5 (s *), ꢀ297.0 (s *), and ꢀ305.0 eV

2

CÀC

[

50–52]

(s *).

The characteristic p* resonance of the nitrile

CÀC

[

43,53]

moiety at around 286.7 eV

can only be traced in the spec-

trum of 7/Au, as a shoulder at the absorption edge. The reso-

nance at approximately 288.4 eV can also contain some contri-

[

50]

butions from the p* feature of COOH.

The N1s XP spectra of the tripod films in Figure 5d exhibit

a single emission at approximately 399.1 eV, assigned to the ni-

trile nitrogen. The intensity of this emission is much lower in

the case of 21/Au as compared with 7/Au, suggesting much

higher molecular coverage in the latter case, in agreement

with the analysis of the S2p spectra. Significantly, in the case

of SAM, the nitrile group is located at the film-ambience inter-

face in the upright adsorption geometry, so the respective

signal is not affected by attenuation and can be taken as

a measure of the surface coverage. With respect to 4’-(pyridin-

4

-yl)biphenyl-4-yl)methanethiol (PyPP1)/Au, which also has ni-

trogen in the terminal position (see Experimental Section), the

intensity of the N1s signal in 21/Au and 7/Au was estimated

at approximately 14% and 59%, respectively. Accordingly, in

view of the known parameters of PyPP1/Au, namely a packing

1

4

2

density of 4.6310 molecules/cm corresponding to a molec-

Figure 6. C K-edge NEXAFS spectra of the meta (21) and para (7) films ac-

quired at an X-ray incident angle of 558. The characteristic absorption reso-

nances are marked.

2

[46,47]

ular footprint of 0.216 nm ,

the effective packing densities

14

of 21/Au and 7/Au could be estimated at around 0.6510

and 2.7510 molecules/cm , corresponding to a molecular

14

2

footprint of approximately 1.5 nm for 21/Au (because of the

multilayer character of the sample, an estimation of the molec-

ular footprint for 7/Au is not possible). An alternative evalua-

tion of the effective packing density of 21/Au, based on the

S2p/Au4f intensity ratio and using the well-defined hexadeca-

[

48,49]

nethiolate (HDT) SAM on Au(111) as a reference,

resulted

1

4

in an effective packing density of around 1.110 molecules/

2

cm , corresponding to a molecular footprint of 0.9 nm . These

values are somewhat different from those derived from N1s,

presumably because of the limitations of both evaluation pro-

cedures. On the other hand, the values are not too far from

each other, giving a reasonable estimate of the packing densi-

ty in the SAM-like 21/Au.

NEXAFS spectroscopy

NEXAFS spectroscopy provides complementary information

about the tripod films, in addition to HRXPS. C and N K-edge

NEXAFS spectra of the meta (21) and para (7) films acquired at

an X-ray incident angle of 558 are presented in Figures 6 and

Figure 7. N K-edge NEXAFS spectra of the meta (21) and para (7) films ac-

quired at an X-ray incident angle of 558 (black solid curves), along with the

respective difference between the spectra collected under the normal (908)

and grazing (208) incidence geometry (gray solid curves). The spectra of the

meta (21) films are magnified by a factor of five. The characteristic absorp-

tion resonances are marked. The horizontal dashed lines correspond to zero.

7

, respectively. At this particular orientation, denoted as the

“

magic angle”, the spectra are unaffected by orientational ef-

Chem. Eur. J. 2016, 22, 13218 – 13235

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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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The C K-edge spectra of the meta (21) and para (7) films

differ to some extent regarding the intensity and exact shape

of the absorption resonances. This difference is not surprising

in view of the different coverages and distinctly different char-

acters of these films. In particular, a spectrum similar to that of

Au, corresponding to the higher intensity at 208 (E almost ?

to the surface). This suggests a strong molecular inclination in

the multilayer para (7) film (on average), probably with a cer-

tain supramolecular arrangement involving such an inclination.

[

50,51]

[52]

benzene

or long-chain thio-oligophenyl SAMs can be ex-

Surface electrochemistry

pected for the multilayer para (7) film, as is indeed observed in

Figure 6. In contrast, the p* resonances can be quenched to

some extent in the monolayer-like meta (21) films, as observed

in the respective spectrum, similar to the SAMs of phenylthiol

Chemisorbed meta (21) and para (7) molecules were desorbed

from the gold substrate at negative applied potential during

the cathodic voltammetric scan, giving a corresponding de-

sorption peak (see Figure 8). Integration of the electric current

[

52]

on Au(111). In addition, there can be a featureless absorption

edge contribution from contamination, which presumably is

present to some extent in the meta (21) and para (7) films, but

is more pronounced in the former case because of the lower

coverage of the target molecules. Note that, under ambient

conditions, contamination is always present on the surface of

the Au substrate but is mostly removed upon efficient self-as-

sembly. The respective “self-cleaning” is, however, presumably

less efficient in the case of tripod precursors owing to the

comparably weak interaction between the molecular back-

bones, so that contamination cannot be avoided completely.

Whereas the features associated with the nitrile group are

hardly visible in the C K-edge NEXAFS spectra of the tripod

films, they are clearly perceptible in the N K-edge spectra of

these films in Figure 7, evidencing the intact character of the

adsorbed molecules. Indeed, these spectra are dominated by

the characteristic double resonance of benzonitrile at approxi-

mately 398.8 and 399.75 eV. The same feature is observed in

Figure 8. Cyclic voltammograms of meta (21) (left) and para (7) (right) self-

assembled monolayers on gold electrode in 0.5m NaOH aqueous electrolyte,

À1

scan rate 0.10 Vs (black curves). Gray curves show the response of the

bare gold electrode (i.e., with no adlayer) under otherwise the same condi-

tions.

gives the electric charge Q, which, according to Equation (1),

provides information on the surface concentration of the ad-

[

54]

[58]

the spectra of molecular benzonitrile in the gas phase, ad-

sorbed molecules, G.

[

55]

sorbed benzonitrile, and well-defined benzonitrile-terminat-

[

43,53,56,57]

ed SAMs.

The appearance of the double resonance

G ¼ ðQÀQdlÞ=nFA

ð1Þ

stems from the conjugation between the p* orbitals of the ni-

trile group and the adjacent phenyl ring. The degeneracy of

the former orbital is consequently lifted, and it splits into two

states with different energies, oriented either perpendicular

For the meta (21) derivative, assuming negligible double-

layer charging current (Q =0) and the number of transferred

dl

electrons n equal to three (one for each thiolate anchor),

[

54,55]

À10

À2

(

p *) or parallel (p *) to the plane of the adjacent ring.

The

a value of (4.9Æ0.2)10 molcm

is obtained for the

1

3

delocalization of the p * orbital over the entire benzonitrile

surface concentration G. This represents (3.0Æ0.2)

14 À2

1

moiety leads to the lower intensity of the respective resonance

10 moleculescm and leads to an area occupied by one mol-

2

À1

as compared to p *, corresponding to the orbital localized at

ecule of 0.34 nm molecule . This value is in good agreement

with that obtained from an STM image of individual molecules

3

the nitrile group.

2

À1

Along with the identification of the adsorbed species,

NEXAFS data provide information on their orientation, based

on the linear dichroism in X-ray absorption (see Experimental

Section). The intensity of an absorption resonance is maximal if

the orientation of the electric field vector of the linearly polar-

ized light coincides with the transition dipole moment (TDM)

of a molecular orbital of interest, and is zero if the electric field

(0.57 nm molecule ) as well as with the area of a triangle

made by three outer para hydrogen atoms H17, H28, H39

2

À1

(0.32 nm molecule ) of the similar meta derivative 22 ob-

tained from single-crystal X-ray diffraction (Figure 3 and corre-

sponding X-ray data in Supporting Information), and would

provide even better agreement once the value of the double

layer charge Q is known and taken into account. In any case,

dl

[50]

vector is perpendicular to the TDM. The TDMs of the p₁*

the above result confirms unequivocally that meta (21) is anch-

ored to the gold substrate through three thiolate bonds form-

ing a chemisorbed compact monolayer (consistent with STM

images in Figure 4a). This finding is crucial if the well-ordered

SAMs of meta (21) tripods are to be used in further applica-

tions.

and p * orbitals are perpendicular to the axis of the benzoni-

3

[

53]

trile moiety and should be directed parallel to the substrate

at the upright orientation of the tripod molecules. This is

indeed the case for 21/Au, for which the difference between

the NEXAFS spectra collected under the normal (908) and graz-

ing (208) incidence geometries exhibits positive peaks at the

positions of the p * and p * resonances (Figure 7), correspond-

Integration of the total voltammetric response obtained

upon desorption of the para (7) derivative led to a surface con-

1

3

À10

À2

ing to the higher intensity at 908 (E j j to the surface). In con-

trast, these peaks are negative in the difference spectrum of 7/

centration value of (5.9Æ0.4)10 molcm or (3.6Æ0.3)

10 moleculescm , which is slightly higher than the value ob-

14

À2

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tained for the meta (21) derivative [(3.0Æ0.2)

moleculescm ] using the same assumptions for the G cal-

terminated by nitrile anchoring groups on each end form

a symmetric junction that breaks in such a way that the cur-

rent–distance curves exhibit two distinct slopes in the current

plateau region (just before junction breaking). These two

slopes lead to a broad peak in the 1D conductance histogram

that can be fitted by two conductance values, indicating the

conductance of a single molecule in distinctly different stages

14

À2

1

0

culation. A small voltammetric prewave at À0.85 V was ob-

served reproducibly during desorption of the para (7) deriva-

tive (Figure 8, right). This could indicate the presence of an ad-

ditional, physically adsorbed adlayer on the chemisorbed para

[

59]

(

7) monolayer or desorption of the disulfide dimers of the

[

63]

para (7) bound to the gold surface by free thiol(s). The main

desorption peak for the para (7) derivative was observed at

a more negative potential than for the meta (21) derivative

of the junction formation. Similar behavior was observed in

Figure 9, confirming that the junction breaks on the side of

the nitrile contacting group whereas the tripod anchor repre-

sents a very robust contact.

(

Figure 8), indicating stronger interaction of (7) with the under-

[

60]

[64]

lying gold substrate. At this point we cannot tell how many

electrons are being transferred (as we cannot exclude desorp-

tion of the para (7) dimer) and thus provide additional infor-

mation on the composition of the film from the desorption

data.

It is known from the literature that single-molecule con-

ductance depends strongly on the type of anchoring group,

and that the probability of junction formation and its stability

decrease on going from the SH to CN anchoring group, with

the latter having lower binding energy. For example, in a series

of tolanes, symmetrically terminated on both sides by either

SH or CN groups, the reported conductance and junction for-

mation probability for the SH-terminated molecule were G =

H

Single-molecule conductance measurements

À2.7

1

0

G and approximately 90%, respectively, whereas for the

0

À4.6

The scanning tunneling microscopy break junction (STM-BJ)

CN-terminated one the values were G =10 G and approxi-

H 0

[

61]

[64]

technique was applied to investigate the charge transport

through the tetraphenylmethane-based molecular tripods. The

STM-BJ was formed repeatedly by perpendicular movement of

the electrochemically etched gold tip (0.25 mm wire, 99.99%

Goodfellow, UK) toward the gold substrate immersed in

a 1 mm solution of 7 or 21 in mesitylene, which was spiked

with triethylamine to achieve in situ deprotection of the an-

choring groups. All conductance–distance traces obtained

during molecular junction elongation were converted cumula-

tively to a 1D conductance histogram. Very similar 1D conduc-

mately 70%, respectively. Previously reported junction evolu-

[

63,64]

tion studies

lead us to propose that the conductance cor-

responding to G values is given predominantly by the differ-

H

ences on the side of thiolate–gold anchor, whereas G is relat-

L

ed to the CN–gold anchor. Note that the slightly higher G_H

values obtained for the para (7) compared to the meta (21)

molecules are in agreement with theoretical calculations,

which suggest higher conductance for para substitution on

[

65]

the aromatic ring than for substitution in the meta position.

tance histograms of log(G/G ) were obtained for both mole-

0

Conclusion

cules, showing a broad conductance feature, which can be

fitted by two peaks (see Figure 9). For the para derivative (7),

these two peaks give high and low conductance values equal

In summary, two regioisomeric tripodal model compounds

consisting of a tetraphenylmethane backbone and three

acetyl-protected thiol substituents directly mounted on the

phenyl ring in either para (7) or meta position (21) have been

synthesized and fully characterized, and their self-assembly be-

havior has been investigated in different coverage and prepa-

ration regimes through a variety of experimental techniques,

including low-temperature UHV-STM, HRXPS, NEXAFS spectros-

copy, and reductive voltammetric desorption experiments. The

modular syntheses, introducing the protruding functionality in

the para position of the fourth phenyl ring late in the se-

quence, enabled the provision not only of the model com-

pounds 7 and 21 exposing a nitrile group, but also tripodal de-

rivatives exposing other functional groups, for example, for

transition-metal-catalyzed coupling chemistry such as the

iodine derivatives (10 and 24) or the boronic acid esters (8 and

À3.16Æ0.46

À4.07Æ0.59

to G =10

G and G =10

G , and for the meta

0

H

0

L

À3.32Æ0.54

derivative (21) values of G_H= 10

G₀and G_L=

À4.05Æ0.40

1

0

G , respectively. The low conductance values are

0

almost identical for both molecules, whereas the high conduc-

tance value is slightly higher for the para derivative (7). It was

proposed recently that a low conductance value G may be

L

caused by a weakly conducting cofacial p–p stacked dimer for-

mation, whereas G represents true single-molecule conduc-

H

[

62]

tance. On the other hand, it was also shown that molecules

22). The electronic conductance of the model compounds 7

and 21 was investigated through single-molecule transport

studies.

The experimental data obtained from the different tech-

niques, and above all, the predominant thiolate signal in the

S2p XP spectra, suggest consistently that the adsorption of

the meta-substituted model compound 21 on Au(111) by the

standard immersion procedure results in the formation of

Figure 9. Logarithmically binned 1D histogram of log(G/G

0

) for meta (21)

(

left) and para (7) (right) molecules in mesitylene constructed from all data

À1

points; Vbias =0.13 V, retraction rate=36 nms , bin size 0.005. Conductance

is presented in units of quantum conductance, G =77.5 mS.

0

Chem. Eur. J. 2016, 22, 13218 – 13235

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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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[

33]

a well-defined SAM-like film with the majority of its constitu-

ents attached with all three legs to the gold surface. Other

findings supporting a monolayer character for this compound

are the intensity of the Au4f₇_/₂HRXPS signal, the UHV-STM

data, the N1s XP data, and the results of the electrochemical

experiments. This interpretation was also supported by the N

K-edge NEXAFS data, suggesting an upright orientation for the

nitrile groups in 21/Au. Sub-monolayer samples prepared by

spraying solutions of 21 into the UHV showed parallel rows of

paired molecules at the gold step edges in the STM analysis,

which allowed analysis of the dimensions of the deposited

molecules.

lyl)ethylsulfanyl]benzene 11

published procedures.

were prepared according to the

Equipment and measurements

All NMR spectra were recorded at 258C in CDCl or CD Cl . H NMR

500.16 MHz) spectra were referenced to TMS as internal standard

(d =0 ppm) or to the solvent residual proton signal (CDCl , d =

Cl , d =5.32 ppm). C NMR (125.78 MHz) spectra

2 H

with total decoupling of protons were referenced to the solvent

CDCl₃, d_H=77.23 ppm; CD Cl , d_H=54.00 ppm).

470.57 MHz) spectra were referenced to CFCl as an external stan-

dard in a coaxial capillary (d =0.00 ppm). UV/Vis spectra were re-

corded with a UV/Vis spectrometer in a 1 cm quartz cell at ambient

temperature (excitation coefficients are given below in units of

Lmol cm ). HRMS spectra were obtained with an ESI-TOF mass

spectrometer. IR spectra were measured in KBr pellets. Analytical

samples were dried at 40–1008C under reduced pressure

10 mbar). Melting points were measured with a melting point

apparatus. Elemental analyses were obtained using an elemental

analyzer.

1

3

2

(

H

3

H

13

7.26 ppm; CD

2

19

1

(

F{ H} NMR

2

3

F

The observations for the regioisomer 7 with the thiol anchor

groups is the para position suggest the formation of molecular

multilayers, with a lower degree of orientational order com-

pared with 21. The multilayer nature of these films is reflected

in their thickness, as follows from the analysis of the intensity

of the Au4f₇_/₂HRXPS signal as well as from the insulated char-

acter of these films as observed in the UHV-STM experiments.

A large fraction of the unbound docking groups was observed

in the S2p XP spectrum, and no preferred orientation of the ni-

trile group was found in the NEXAFS data. However, deposited

as a sub-monolayer in the UHV-STM experiment, large ordered

domains consisting of molecules ordered in hexagonal tiles

were observed. Although the experiment does not allow con-

clusions concerning the orientation of the molecule at the sur-

face, the dimensions of the molecular footprint were obtained.

Single-molecule conductance was measured for both tripo-

dal molecules 7 and 21 through STM break junction experi-

ments. Comparable values were obtained, which might point

À1

À2

(

Film characterization

The SAMs for the STM characterization at 77 K were prepared on

freshly annealed gold-coated (300 nm) mica substrates (typically

5 mm), which were purchased from Georg-Albert-PVD, Germa-

ny. The substrates were immersed in a solution (0.25 mm) of meta

21) or para (7) tripodal molecules in a mixture of methanol/THF

3:1, v/v) containing 30% of a 1m solution of ammonia in metha-

nol as cleaving agent, at room temperature for 48 h. The samples

were rinsed thoroughly with methanol, DI water, and THF, dried

under a stream of argon, and placed into the UHV-STM chamber.

5

(

The SAMs for the STM characterization at 5.3 K were prepared on

a Au(111) single crystal, which was cleaned by repeated cycles of

argon ion sputtering and annealing to 720 K. The tripodal mole-

cules 7 and 21 were deposited onto the substrate surface by spray

deposition. For this purpose, the substrate was transferred to a sep-

arate vacuum chamber with a pressure of approximately 5 mbar. A

droplet of 7 or 21 dissolved in CH Cl (0.16 mm for meta derivative

3

to the central sp hybridized C atom as the structural feature

controlling the electronic coupling irrespective of the connec-

tion of the structure to the substrate.

Currently, we are exploring further to what extent the mo-

lecular arrangement on the substrate surface can be controlled

by the deposition method, and the size of the molecular struc-

tures that can be mounted on these platforms.

2

1 and 0.20 mm for para derivative 7) was sprayed into the cham-

ber by opening the pulse valve for 10 ms. Thus, a mist containing

the dissolved molecules was deposited onto the substrate. The

sample was then transferred to UHV and annealed mildly at T=

400 K for 60 min to remove remaining solvent molecules. Subse-

quently, the sample was transferred to a custom-built STM setup.

STM images were measured in constant current mode at 5.3 K.

Experimental Section

Materials

All starting materials and reagents were obtained from commercial

suppliers and used without further purification. TLC was performed

on silica gel 60 F₂₅₄plates; spots were detected by fluorescence

quenching under UV light at 254 nm. Column chromatography

was performed on silica gel 60 (0.040–0.063 mm). All experimental

manipulations with anhydrous solvents were performed in flame-

dried glassware under an inert argon atmosphere. Degassed sol-

vents were obtained through three freeze-pump-thaw cycles. Tetra-

hydrofuran, toluene, and diethyl ether were dried and distilled

from sodium/benzophenone under an argon atmosphere. Di-

chloromethane and triethylamine were dried and distilled from

The samples for the spectroscopic characterization were prepared

on freshly annealed gold-coated Si(100) wafers (typically 10

5 mm) with 5 nm Ti and 100 nm Au. These substrates were pur-

chased from Georg-Albert-PVD, Germany. Three different prepara-

tion procedures for the formation of the tripod films were tried. In

the first procedure, the substrates were immersed in a 0.25 mm so-

lution of meta (21) or para (7) tripodal molecules in a mixture of

methanol/THF (3:1, v/v) containing 30% of a 1m solution of am-

monia in methanol as cleaving agent, at room temperature for

48 h. In the second procedure, the samples were immersed in

a 0.5 mm solution of tripodal molecules in the mixture of metha-

nol/THF (3:1, v/v) at 508C for 48 h. Finally, in the third procedure,

the samples were immersed in a 0.5 mm solution of tripodal mole-

cules in THF containing 15% triethylamine as cleaving agent, at

room temperature for 48 h. In all three cases, the substrates were

rinsed thoroughly with methanol, DI water, and THF, and then

dried under a stream of argon. The first procedure was found to

CaH under an argon atmosphere. Ultrapure deionized (DI) water

2

with a minimum resistivity of 18.2 MWcm was obtained by means

of a Milli-Q Integral 5 water purification system. All glassware for

the electrochemical experiments was cleaned in boiling 20% nitric

acid and washed copiously in ultrapure water. 4-Bromo-1-[2-(trime-

[38,39]

thylsilyl)ethylsulfanyl]benzene 1

and 3-bromo-1-[2-(trimethylsi-

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be the most suitable and reliable. Consequently, only samples pre-

pared by this procedure were used for the subsequent HRXPS and

NEXAFS experiments at the synchrotron.

ray absorption, that is, the strong dependence of the cross section

of the resonant photoexcitation process on the orientation of the

electric field vector of the linearly polarized light with respect to

[50]

the molecular orbital of interest.

Along with the SAMs of tripodal molecules 7 and 21, monolayers

of hexadecanethiol (HDT) and 4’-(pyridin-4-yl)biphenyl-4-yl)metha-

nethiol (PyPP1) were prepared on a Au(111) substrate following the

The raw NEXAFS spectra were normalized to the incident photon

flux by division through a spectrum of a clean, freshly sputtered

gold sample. The energy scale was referenced to the most intense

p* resonance of highly oriented pyrolytic graphite (HOPG) at

[47,66]

literature procedures.

These monolayers were used as referen-

ces to calculate the effective thicknesses (HDT) and packing densi-

ties (HDT and PyPP1) of the triad films. Significantly, the PyPP1

monolayer has a nitrogen atom in the terminal position, similarly

to the triad films in the proper adsorption geometry. The structure

[71]

285.38 eV.

Single-molecule conductance was measured with an original STM

tubular scanner Agilent 5500 Scanning Probe Microscope with an

in-house-implemented current-to-voltage converter circuit with

p

of this monolayer is similar to the basic 3 3 arrangement in

[66]

[72]

the alkanethiolate SAMs, with a packing density close to 4.63

a wide dynamic range. Current–time curves were converted to

1

4

À2

2

À1 [46,47]

1

0

moleculescm , corresponding to 0.216 nm molecule .

conductance–distance curves, and corresponding histograms were

constructed using a combination of software developed in-

The SAMs of the meta (21) and para (7) tripodal molecules were

characterized by laboratory XPS, HRXPS, and NEXAFS spectroscopy.

The laboratory XPS measurements were used for a preliminary

screening only, to optimize the preparation procedure. All experi-

ments were performed at room temperature. The spectroscopy

measurements were conducted under UHV conditions at a base

[73]

house and OriginPro. A logarithmic bin size of 0.005 was used

for the construction of 1D histograms of log(G/G ) values from the

0

original data without any selection, in which G is the conductance,

[

74]

presented in units of quantum conductance G =77.5 mS.

0

For electrochemical desorption studies, monolayers of meta deriva-

tive 21 and para derivative 7 were prepared on gold bead electro-

À9

pressure <1.510 mbar. Special care was taken to avoid

[44,67]

2

damage induced by X-rays.

des (area 0.17–0.21 cm ) by deposition from a 0.2 mm solution in

ethanol containing triethylamine (10% v/v ratio) at 608C for 16 h.

Subsequently, the electrodes were rinsed copiously with pure etha-

nol. Cyclic voltammograms were measured using a Potentiostat/

Laboratory XPS measurements were performed using a MgK X-ray

a

source (1253.6 eV) and a dedicated spectrometer (MAX200, Ley-

bold–Heraeus). The spectra were acquired in normal emission ge-

ometry with an energy resolution of around 0.9 eV. The X-ray

source was operated at a power of 200 W and positioned approxi-

mately 1.5 cm away from the samples. The recorded spectra were

normalized to the transmissions function of the spectrometer, and

the binding energy scale was referenced to the Au4f peak of

À1

Galvanostat PGSTAT12 at a scan rate of 0.10 Vs . Desorption was

performed in 0.5m NaOH in ultrapure water in a three-electrode

system containing a gold bead working electrode in a hanging me-

niscus arrangement and pseudo-reference and auxiliary (both

gold) electrodes.

7

/2

[68]

clean gold at 84.0 eV.

The HRXPS experiments were performed at the D1011 beamline

bending magnet) at the MAX II storage ring of the MAX-IV syn-

(

Tris{4-[2-(trimethylsilyl)ethylsulfanyl]phenyl}methanol (3)

chrotron radiation facility in Lund, Sweden. The spectra were ac-

quired in normal emission geometry at photon energies of 350

and 580 eV, depending on the acquisition range. The energy reso-

lution was better than 100 meV, allowing clear separation of indi-

vidual spectral components. The binding energy scale was refer-

Method A: A dry, argon-flushed 100 mL Schlenk flask was charged

with 1 (3.76 g, 13 mmol), and anhydrous THF (35 mL) was added.

The solution was cooled to À788C and tert-BuLi (17.8 mL,

2

6 mmol, 15% in pentane) was added dropwise over 20 min. The

[68]

reaction mixture was stirred at À788C for 2 h under argon. In

enced to the Au4f₇_/₂peak of clean gold at 84.0 eV.

XP and HRXP spectra were fitted by symmetric Voigt functions and

a Shirley-type background. To fit the S2p₃_/₂₁_/₂doublet we used

two peaks with the same full-width at half-maximum (fwhm), the

a

3

second 100 mL Schlenk flask, diethyl carbonate (0.44 mL,

.71 mmol) was diluted with anhydrous THF (5 mL) under an inert

,

atmosphere and cooled to À788C. The solution of lithiated species

was added slowly through a cannula into the flask containing di-

ethyl carbonate solution; after 1 h, the reaction mixture was al-

lowed to warm to room temperature and stirred for an additional

[68]

standard spin-orbit splitting of approximately 1.18 eV (verified

by fit), and a branching ratio of 2 (S2p₃_/₂/S2p₁_/₂). The fits were per-

formed self-consistently, that is, the same fit parameters were used

for identical spectral regions.

1

6 h. The reaction mixture was quenched with saturated NH Cl so-

4

lution (100 mL). The aqueous layer was washed with CH Cl (2

2

HRXPS data were used to calculate the effective thicknesses of the

100 mL). The combined organic layer was washed with brine

[69]

triad films. A standard procedure was used, based on the C1s/

Au4f intensity ratio and an SAM (HDT/Au) of well-defined thick-

ness (1.84 nm) as a reference. Attenuation length values character-

istic of densely packed alkanethiolate SAMs on Au(111) were

(100 mL), dried over MgSO , and filtered. All volatiles were re-

4

moved under reduced pressure and the residue was purified by

column chromatography on silica gel (300 g) in hexane/EtOAc (9:1)

to afford pure 3 (1.46 g) as a yellowish solid in 60% yield (R =0.45,

f

[70]

used.

hexane/EtOAc=9:1) and pure 2 (0.4 g) as a white solid in 24%

yield (R =0.54, hexane/EtOAc=9:1).

The NEXAFS measurements were performed at same beamline as

the HRXPS experiments. The spectra were acquired at the carbon

and nitrogen K-edges in the partial electron yield mode with re-

tarding voltages of À150 and À300 V, respectively. Linear polarized

synchrotron light with a polarization factor of about 95% was

used. The energy resolution was better than 100 meV. The inci-

dence angle of the primary X-rays was varied from 908 (E-vector in

the surface plane) to 208 (E-vector nearly normal to the surface) in

steps of 108–208 to monitor the orientational order within the

tripod films. This approach is based on the linear dichroism in X-

f

1

For 3: m.p. 121–1318C; H NMR (500 MHz, CDCl ): d=0.04 (s, 27H),

3

0.92–0.96 (m, 6H), 2.71 (s, OH), 2.94–2.96 (m, 6H), 7.16–7.18 (m,

13

6H), 7.22–7.23 ppm (m, 6H); C NMR (125.88 MHz, CDCl ): d=

3

À1.5, 17.0, 29.4, 81.6, 128.0, 128.5, 136.9, 144.1 ppm; IR (KBr) v˜ =

3446 (s), 3025 (w), 2952 (s), 1594 (m), 1490 (s), 1417 (m), 1249 (s),

À1

1161 (m), 838 (s), 693 cm (w); UV/Vis (CH Cl ): l

(e)=269 nm

2

max,

À1

3

À1

(43186 mol dm cm ); ESI(+) MS: m/z calcd for C H OS Si Na:

3

4

52

3

+

679.2289 [M+Na] ; found: 679.2380; elemental analysis calcd (%)

for C H OS Si (656.25): C 62.14, H 7.98; found: C 62.23, H 8.05.

34

52

3

Chem. Eur. J. 2016, 22, 13218 – 13235

www.chemeurj.org

13229

ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper

1

For 2: m.p. 71–748C; H NMR (500 MHz, CDCl ): d=0.02 (s, 18H),

layer was washed with brine (100 mL), dried over MgSO , and fil-

3

4

0

7

1

.97–1.00 (m, 4H), 3.02–3.06 (m, 4H), 7.29–7.31 (m, 4H), 7.70–

tered. The volatiles were removed under reduced pressure, and

the residue was purified by column chromatography on silica gel

(130 g) with hexane/EtOAc (9:1) as an eluent to yield 0.64 g (97%)

1

3

.71 ppm (m, 4H); C NMR (125.8 MHz, CDCl ): d=À1.5, 16.7, 28.4,

3

26.5. 130.7, 134.4, 144.5, 195.1 ppm; IR (KBr) v˜ =3037 (w), 2952 (s),

À1

1

657 (s), 1553 (s), 1401 (m), 1288 (s), 1088 (s), 830 (s), 692 cm (w);

of 5. R_f=0.88 (hexane/EtOAc=9:1); m.p. 98–1018C; H NMR

À1

3

À1

UV/Vis (CH Cl ): l

MS: m/z calcd for C H OS Si Na: 469.1378 [M+Na] ; found:

4

6

(e)=329 nm (29245 mol dm cm ); ESI(+)

(500 MHz, CDCl ): d=0.05 (s, 27H), 0.93–0.96 (m, 6H), 2.94–2.98

2

max

3

+

(m, 6H), 7.04–7.05 (m, 6H), 7.14–7.17 (m, 8H), 7.27–7.28 ppm (d,

2

3

34

2

1

3

69.1482; elemental analysis calcd (%) for C H OS Si (446.16): C

J=8.85 Hz, 2H), C NMR (125.8 MHz, CDCl ): d=À1.6, 16.9, 29.1,

2

3

34

2

3

1.83, H 7.67; found: C 61.59, H 7.88.

63.7, 120.5, 127.4, 131.4, 132.9, 136.1, 143.1, 147.2, 147.8 ppm;

19

F NMR (470.57 MHz, CDCl ): d=À72.92 ppm; IR (KBr) v˜ =3025

Method B: In a 100 mL Schlenk flask, 1 (1.16 g, 4.01 mmol) was dis-

solved in anhydrous THF (12 mL) under argon, cooled to À788C,

and degassed. Then, tert-BuLi (5.6 mL, 8.24 mmol, 15% in pentane)

was added dropwise over 25 min and the resulting mixture was

stirred at À788C for 2 h. In a second 100 mL Schlenk flask, 2 (0.9 g,

3

(

w), 2953 (s), 1557 (m), 1490 (s), 1429 (s), 1250 (s), 1214 (s), 1141 (s),

À1

8

(

12 (s), 694 cm

(w); UV/Vis (CH Cl ): ^lmax (e)=273 nm

2 2

À1

3

À1

33589 mol dm cm ); ESI(+) MS: m/z calcd for C H O S Si FNa:

41 55 3 4 3

+

8

87.2186 [M+Na] ; found: 887.2121; elemental analysis calcd (%)

for C H O S Si F (864.23): C 56.91, H 6.41; found: C 56.97, H 6.55.

2

.02 mmol) was dissolved in THF (8 mL) under argon, and cooled

41 55

3

4

3

to À788C. The solution of lithiated species was added slowly

through a cannula into the flask containing the solution of 2. The

yellow solution was stirred at À788C for 2 h, then allowed to warm

to room temperature and stirred for an additional 12 h. The reac-

4-(Tris{4-[2-(trimethylsilyl)ethylsulfanyl]phenyl}methyl)ben-

zonitrile (6)

A 50 mL pressure tube was charged with triflate 5 (0.25 g,

tion mixture was quenched with saturated NH Cl solution

4

0

(

.29 mmol), zinc cyanide (0.14 g, 1.19 mmol), copper(I) cyanide

⁰^.⁰²⁶^g^,⁰^.²⁹^m^m^o^l⁾^,^P^d⁽^P^P^h3⁾4 (0.034 g, 0.029 mmol) and anhy-

drous DMF (8 mL) under argon. The tube was sealed and stirred at

408C for 16 h. The solution was cooled to room temperature, and

^t^h^e^r^e^s^u^l^tⁱⁿ^g^s^o^l^u^tⁱ^oⁿ^w^a^s^q^u^eⁿ^c^h^e^d^wⁱ^t^h^N^a^C^O3 (60 mL). The

(

100 mL). The aqueous layer was washed with CH Cl (2150 mL).

2 2

The combined organic layer was washed with brine (100 mL), dried

over MgSO , and filtered. The volatiles were removed under re-

duced pressure and the residue was purified by column chroma-

tography on silica gel (200 g) in hexane/EtOAc (9:1) to yield 3

4

1

2

aqueous layer was washed with CH Cl (2150 mL). The combined

(

1.1 g, 83%) as a yellowish solid.

2

organic layer was washed with brine (100 mL), dried over MgSO₄,

and filtered. All volatiles were removed under reduced pressure

and the residue was purified by column chromatography on silica

gel (70 g) in a mixture of hexane/EtOAc (9:1), yielding 0.2 g of 6 as

4

-(Tris{4-[2-(trimethylsilyl)ethylsulfanyl]phenyl}methyl)phe-

nol (4)

a white powder in 93% yield. R =0.44 (hexane/EtOAc=9:1); m.p.

f

A 100 mL two-necked round-bottomed flask was charged with 3

1

56–1588C; H NMR (500 MHz, CDCl ): d=0.04 (s, 27H), 0.92–0.96

3

(

0.69 g, 1.05 mmol), phenol (1.97 g, 21 mmol), and toluene (10 mL)

(

m, 6H), 2.93–2.97 (m, 6H), 7.03–7.05 (m, 6H), 7.15–7.17 (m, 6H),

.32–7.34 (d, J=8.85 Hz, 2H), 7.53–7.55 ppm (d, J=8.5 Hz, 2H),

under argon, and a few drops of concentrated HCl were added.

The reaction mixture was heated at 1208C for 14 h under argon.

After cooling to room temperature, the navy blue reaction mixture

was diluted with toluene (120 mL) and quenched with NaOH solu-

tion (2m, 160 mL). The combined organic layer was washed with

7

13

C NMR (125.8 MHz, CDCl ): d=À1.5, 16.9, 29.1, 64.3, 110.2, 119.0,

3

1

27.4, 131.4, 131.6, 131.8, 136.3, 142.7, 152.2 ppm; IR (KBr) v˜ =3024

(

2

w), 2953 (s), 2231 (m), 1605 (m), 1489 (s), 1401 (w), 1248 (s), 1094

À1

s), 859 (s), 694 cm (w); UV/Vis (CH Cl ): l

(e)=224 (13100),

2

max

water (150 mL), dried over MgSO , and filtered. All volatiles were

À1

3

À1

4

70 nm (23341 mol dm cm ); ESI(+) MS: m/z calcd for

removed under reduced pressure and the residue was purified by

column chromatography on silica gel (150 g) in hexane/EtOAc (5:1)

+

C H OS Si NNa: 764.2697 [M+Na] ; found: 764.2548; elemental

41

55

3

analysis calcd (%) for C H OS Si N (741.28): C 66.34, H 7.47, N

41

55

3

to afford 4 (0.65 g) as an orange solid in 84% yield. R =0.26

f

1

.89; found: C 66.56, H 7.61, N 1.92.

1

(

hexane/EtOAc=9:1); m.p. 98–1018C; H NMR (500 MHz, CDCl₃):

d=0.03 (s, 27H), 0.92–0.95 (m, 6H), 2.93–2.96 (m, 6H), 4.79 (s, OH),

S,S’,S’’-{4,4’,4’’-[(4-Cyanophenyl)methanetriyl)]tris(benzene-

,1-diyl)} tris(thioacetate) (7)

6

7

.69–6.71 (d, J=8.6 Hz, 2H), 7.00–7.02 (d, J=8.65 Hz, 2H), 7.06–

.08 (m, 6H), 7.14–7.15 ppm (m, 6H); C NMR (125.8 MHz, CDCl₃):

1

3

4

d=À1.6, 17.0, 29.3, 63.4, 114.5, 127.4, 131.6, 132.4, 135.3, 139.0,

In 50 mL Schlenk flask, tetraphenylmethane

a

6

(150 mg,

1

44.3, 153.8 ppm; IR (KBr) v˜ =3403 (s), 3022 (w), 2951 (s), 1591 (m),

0

.20 mmol) was dissolved in a mixture of dry CH Cl (16 mL) and

À1

2

488 (s), 1428 (m), 1248 (s), 837 (s), 692 cm (w); UV/Vis (CH Cl ):

2

acetyl chloride (1.6 mL) under argon. The solution was treated with

À1

3

À1

l_m_a_x(e)=271 nm (74485 mol dm cm ); ESI(+) MS: m/z calcd for

C H OS Si Na: 771.2698 [M+Na] ; found: 771.2609; elemental

AgBF (0.2 g, 1.02 mmol), and the green suspension was stirred at

+

4

40

56

3

room temperature for 12 h. The completion of the reaction was

checked by TLC (hexane/EtOAc=3:1). The reaction mixture was di-

luted with dichloromethane (100 mL) and poured slowly into a sa-

turated solution of NaHCO₃(60 mL). The precipitate was filtered

through a pad of Celite, washed with CH Cl , and concentrated in

analysis calcd (%) for C H OS Si (732.28): C 65.52, H 7.70; found:

4

0

56

3

C 65.30, H 7.76.

4-(Tris{4-[2-(trimethylsilyl)ethylsulfanyl]phenyl}methyl)phen-

2

vacuo. The crude product was purified by column chromatography

on silica gel (150 g) with hexane/EtOAc (3:1) to yield 62 mg (54%)

yl trifluoromethanesulfonate (5)

Trifluoromethanesulfonic anhydride (0.22 mL, 1.3 mmol) was added

dropwise to a solution of 4 (0.56 g, 0.76 mmol) in dry triethylamine

of 7 as a white powder. R =0.11 (hexane/EtOAc=3:1); m.p. 202–

f

1

2048C; H NMR (500 MHz, CDCl ): d=2.42 (s, 9H), 7.21–7.22 (m,

3

(

0.2 mL, 1.4 mmol) and CH Cl (20 mL) at À788C under an argon at-

6H), 7.32–7.34 (m, 6H), 7.36–7.37 (d, J=8.5 Hz, 2H), 7.56–7.58 ppm

2

13

mosphere. The solution was stirred for 6 h at À788C, and then al-

(d, J=8.5 Hz, 2H); C NMR (125.8 MHz, CDCl ): d=30.47, 65.05,

3

lowed to warm to room temperature and stirred for an additional

110.7, 118.7, 126.9, 128.63, 128.69, 131.6, 131.7, 131.9, 134.05,

146.1, 150.9, 193.7 ppm; IR (KBr) v˜ =3029 (w), 2920 (s), 2227 (m),

1704 (s), 1603 (w), 1486 (m), 1396 (m), 1194 (w), 1117 (m), 820 (s),

1

0 h, before being quenched with water (100 mL). The aqueous

layer was washed with CH Cl (2150 mL). The combined organic

2

Chem. Eur. J. 2016, 22, 13218 – 13235

www.chemeurj.org

13230

ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper

À1

6

(

18 cm

(s); UV/Vis (CH CN): l

(e)=223 (31640), 250 nm

S,S’,S’’-{4,4’,4’’-[(4-Iodophenyl)methanetriyl)]tris(benzene-4,1-

diyl)} tris(thioacetate) (10)

3

max

À1

3

À1

27521 mol dm cm ); ESI (+) HRMS: m/z calcd for

+

C H NO S Na: 590.0889 [M+Na] ; found: 590.0883; elemental

32

25

3 3

The desired product was prepared according to the method de-

scribed for the preparation of 7, starting from 9 (0.11 g, 0.13 mmol)

analysis calcd (%) for C H NO S (567.10): C 67.70, H 4.44, N, 2.47;

found: C 67.89, H 4.53, N 2.58.

3

2

25

3 3

and AgBF (0.2 g, 1.05 mmol) in a mixture of dry dichloromethane

4

(

12 mL) and acetyl chloride (1.2 mL) under argon. The suspension

was stirred at room temperature for 12 h. The crude product was

purified by column chromatography on silica gel (150 g) in

4

-(Tris{4-[2-(trimethylsilyl)ethylsulfanyl]phenyl}methyl)phe-

hexane/EtOAc (5:1) to provide 0.076 g of 10 as a white powder in

1

8

7% yield. R =0.41 (hexane/EtOAc=3:1); m.p. 272–2738C; H NMR

nylboronic acid pinacol ester (8)

f

(

500 MHz, CDCl ): d=2.42 (s, 9H), 6.95–6.97 (d, J=9.5 Hz 2H),

3

A 50 mL pressure tube was charged with triflate 5 (0.65 g,

7

2

.22–7.24 (m, 6H), 7.30–7.32 (m, 6H), 7.58–7.60 ppm (d, J=8 Hz,

0

.75 mmol), anhydrous potassium acetate (0.22 g, 2.24 mmol), bis(-

pinacolato)diboron (0.29 g, 1.14 mmol),^P^d⁽^d^p^p^f⁾^C^l2 (0.061 g,

.075 mmol), and dry dioxane (15 mL) under argon. The tube was

13

H); C NMR (125.8 MHz, CDCl ): d=30.5, 64.6, 92.7, 126.4, 131.8,

3

131.9, 133.1, 133.9, 137.2, 145.5, 146.9, 194.0 ppm; IR (KBr) v˜ =2924

0

(

s), 2854 (w), 1706 (s), 1485 (m), 1467 (w), 1390 (w), 1261 (m), 1092

sealed and stirred under at 1208C for 16 h. The reaction mixture

was cooled, then diluted with dichloromethane (200 mL) and

washed with water (100 mL). The combined organic layer was

À1

m), 817 (m), 616 cm

(s); UV/Vis (CH Cl ) l

(e)=251 nm

max

2

À1

3

À1

(

18696 mol dm cm ); ESI(+) MS: m/z calcd for C H IO S Na:

31 25 3 3

+

6

90.9903 [M+Na] ; found: 690.9890; elemental analysis calcd (%)

washed with brine (100 mL), dried over MgSO , and filtered. The

4

for C H IS Si (668.00): C 55.69, H 3.77; found: C 55.52, H 3.68.

40 55 3 3

volatiles were removed under reduced pressure and the residue

was purified by column chromatography on silica gel (130 g) with

hexane/EtOAc (9:1) to provide 0.45 g of 8 as a yellow powder in

1

7

1% yield. R =0.6 (hexane/EtOAc=9:1); m.p. 174–1758C; H NMR

f

(

500 MHz, CDCl ): d=0.03 (s, 27H), 0.91–0.95 (m, 6H), 1.32 (s, 12H),

3

2

.92–2.96 (m, 6H), 7.08–7.09 (m, 6H), 7.13–7.15 (m, 6H), 7.19–7.20

Tris{3-[2-(trimethylsilyl)ethylsulfanyl]phenyl}methanol (13)

13

(

1

d, J=7.8 Hz, 2H), 7.67–7.69 ppm (d, J=8 Hz 2H); C NMR

125.8 MHz, CDCl ): d=À1.5, 17.0, 25.1., 29.3, 64.3, 84.0, 127.5,

Method A: The desired product was prepared according to the

method described for the preparation of 3, starting from 11 (3 g,

10.4 mol) in anhydrous THF (30 mL), tert-BuLi (14 mL, 21 mmol,

15% in pentane), and diethyl carbonate (0.36 mL, 2.97 mmol) in

anhydrous THF (6 mL) under argon. After 16 h, the reaction mix-

ture was quenched, and the crude product was purified by column

chromatography on silica gel (350 g) with hexane/EtOAc (9:1) to

obtain 0.85 g of 13 as yellow oil in 44% yield and 0.52 g of 12 as

3

30.5, 131.5, 131.6, 134.2, 135.3, 143.9, 149.7 ppm; IR (KBr) v˜ =3437

(

m), 2952 (s), 2924 (m), 1608 (m), 1486 (m), 1398 (m), 1361 (s), 1247

À1

(

s), 1248 (s), 1145 (m), 1091 (s), 860 (s), 837 (s), 751 cm (w); UV/

À1

3

À1

Vis (CH Cl ): l (e)=271 nm (36961 mol dm cm ); ESI(+) MS:

max

m/z calcd for C H BO S Si Na: 865.3365 [M+Na] ; found:

8

6

2

+

4

6

67

2

3

65.3076; elemental analysis calcd (%) for C H BO S Si (842.37): C

46 67 2 3 3

5.52, H 8.01; found: C 65.74, H 8.06.

a yellowish solid in 39% yield.

1

For 13: R =0.43 (hexane/EtOAc=9:1); m.p. 92–948C; H NMR

f

(

500 MHz, CDCl ): d=0.01 (s, 27H), 0.85–0.89 (m, 6H), 2.75 (s, OH),

3

2

7

8

.86–2.89 (m, 6H), 7.01–7.03 (m, 3H), 7.19–7.20 (m, 5H); 7.20–

1

-Iodo-4-(tris{4-[2-(trimethylsilyl)ethylsulfanyl]phenyl}me-

13

.23 ppm (m, 5H); C NMR (125.8 MHz, CDCl ): d=À1.6, 16.9, 29.4,

3

thyl)benzene (9)

1.9, 125.4, 127.6, 127.9, 128.6, 137.5, 147.2 ppm; IR (KBr) v˜ =3460

(

6

m), 2950 (s), 1584 (m), 1467 (m), 1411 (m), 1247 (s), 846 (s),

A 50 mL, two-necked, round-bottomed flask was charged with pi-

À1

95 cm

(w);

UV/Vis

(CH Cl )

l_m_a_x

(e)=257 nm

2

nacol ester

8 (0.15 g, 0.18 mmol), copper iodide (0.051 g,

À1

3

À1

(

25970 mol dm cm ); ESI(+) MS: m/z calcd for C H OS Si Na:

34 52 3 3

0

.27 mmol), N-iodosuccinimide (0.06 g, 0.27 mmol), anhydrous

+

6

79.2380 [M+Na] ; found: 697.2307; elemental analysis calcd (%)

DMF (8 mL), and toluene (4 mL) under argon. The reaction mixture

was stirred and heated at 110 8C for 24 h under an argon atmos-

phere. After cooling, the reaction mixture was diluted with di-

chloromethane (200 mL) and washed with Na SO solution (10%,

for C H OS Si (656.25): C 62.14, H 7.98; found: C 62.29, H 8.08.

34

52

3

1

For 12: R =0.5 (hexane/EtOAc=9:1); m.p. 123–1258C; H NMR

(

f

500 MHz, CDCl ): d=0.04 (s, 18H), 0.93–0.96 (m, 4H), 2.97–3.01

3

2

3

(

m, 4H), 7.36–7.39 (m, 2H), 7.49–7.50 (m, 2H), 7.52–7.54 (m, 2H),

1

00 mL). The combined organic layer was washed with brine

13

7

.69 (s, 2H), 7.70 ppm (s, 1H); C NMR (125.8 MHz, CDCl ): d=

3

(

100 mL), dried over MgSO , and filtered. All volatiles were re-

4

À1.6, 16.9, 29.4, 128.8, 129.5, 132.5, 138.2, 138.6, 196.1 ppm; IR

moved under reduced pressure, and the residue was purified by

column chromatography on silica gel (100 g) with hexane/EtOAc

(

KBr) v˜ =3055 (w), 2951 (s), 1658 (s), 1561 (s), 1431 (m), 1414 (m),

À1

1

260 (s), 847 (s), 695 cm (w); UV/Vis (CH Cl ) l

(e)=265 nm

(

0

10:1) to obtain 0.14 g of 9 as a yellow powder in 91% yield. R =

.92 (hexane/EtOAc=9:1); m.p. 70–718C; H NMR (500 MHz,

2

max

f

À1

3

À1

1

(23085 mol dm cm ); ESI(+) HRMS: m/z calcd for C H OS Si K:

23 34 2 2

85.1221 [M+K] ; found: 485.1247; elemental analysis calcd (%) for

+

4

CDCl ): d=0.04 (s, 27H), 0.92–0.95 (m, 6H), 2.93–2.96 (m, 6H),

3

C H OS Si (446.16): C 61.83, H 7.67; found: C 61.67, H 7.90.

6

7

.92–6.93 (d, J=9 Hz, 2H), 7.04–7.06 (m, 6H), 7.14–7.16 (m, 6H),

23 34

2

1

3

.55–7.57 ppm (d, J=8 Hz 2H); C NMR (125.8 MHz, CDCl ): d=

Method B: The desired product was prepared according to the

method described for the preparation of 3, starting from 11

(0.97 g, 3.35 mmol) in anhydrous THF (15 mL), tert-BuLi (4.55 mL,

6.71 mmol, 15% in pentane), and 12 (0.75 g, 1.6 mmol) in anhy-

drous THF (10 mL) under argon. After 12 h, the reaction mixture

was quenched, and the crude product was purified by column

chromatography on silica gel (200 g) in hexane/EtOAc (9:1) to yield

1 g (76%) of 13 as a yellowish oil.

3

À1.5, 16.9, 29.2, 63.8, 92.1, 127.4, 131.5, 131.6, 133.2, 135.7, 136.9,

1

(

43.4, 146.5 ppm; IR (KBr) v˜ =2952 (s), 2923 (s), 2853 (w), 1590 (w),

À1

484 (w), 1399 (w), 1248 (s), 1006 (m), 858 (s), 809 (s), 753 cm

À1

3

À1

w); UV/Vis (CH Cl ) l

(e)=271 nm (38610 mol dm cm ); ESI

2

max

+

(

+) HRMS: m/z calcd for C H IS Si Na: 865.1711 [M+Na] ; found:

40 55 3 3

8

5

65.1716; elemental analysis calcd (%) for C H IS Si (842.18): C

40 55 3 3

6.98, H 6.57; found: C 57.09, H 6.65.

Chem. Eur. J. 2016, 22, 13218 – 13235

www.chemeurj.org

13231

ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper

Bis(3-bromophenyl)methanone (15)

under argon. The residue was purified by column chromatography

on silica gel (300 g) in hexane/EtOAc (10:1) to afford 2.17 g of 17

A flame-dried, argon-flushed 100 mL Schlenk flask was charged

with 1,3-dibromobenzene (2 g, 8.47 mmol) and anhydrous THF

as a white solid in 88% yield. R =0.35 (hexane/EtOAc=10:1); m.p.

f

1

78–1808C; H NMR (500 MHz, CDCl ): d=5.17 (s, OH), 6.75–6.76

3

(

30 mL). The solution was cooled to À788C and nBuLi (1.6m in

(

d, J=8.8 Hz, 2H), 6.97–6.99 (d, J=8.8 Hz, 2H), 7.06–7.07 (d, J=

hexane, 5.3 mL, 8.48 mmol) was added dropwise over 20 min. The

8

9

1

.05 Hz, 3H), 7.13–7.16 (m, 3H), 7.32 (s, 3H), 7.35–7.37 ppm (d, J=

reaction mixture was stirred at À788C for 2 h under argon. In

13

.25 Hz, 3H); C NMR (125.8 MHz, CDCl ): d=64.1, 115.1, 122.4,

3

a

1

second 100 mL Schlenk flask, diethyl carbonate (0.21 mL,

.7 mmol) was diluted in anhydrous THF (10 mL) under an inert at-

29.5, 129.9, 130.0, 132.3, 133.5, 137.3, 148.4, 154.3 ppm; IR (KBr)

v˜ =3422 (s), 3058 (m), 2922 (m), 1590 (s), 1468 (m), 1404 (m), 1215

mosphere and cooled to À788C. The solution of lithiated species

was added slowly through a cannula into the flask containing the

carbonate solution. After 1 h, the reaction mixture was allowed to

warm to room temperature and stirred for an additional 15 h. The

À1

(

5

m), 832 (s), 782 cm

(s); UV/Vis (CH Cl ): l

(e)=280 nm

2

max

À1

3

À1

4420 mol dm cm ); ESI(+) HRMS: m/z calcd for C H Br ONa:

25 17 3

94.8682 [M+Na] ; found: 596.8680; elemental analysis calcd (%)

+

for C H Br O (569.88): C 52.39, H 2.99; found: C 52.42, H 2.88.

25

17

3

reaction mixture was quenched with saturated NH Cl solution

4

(

100 mL). The aqueous layer was washed with CH Cl (3100 mL).

2 2

Trimethyl{4-[tris(3-bromophenyl)methyl]phenoxy}silane (18)

The combined organic layer was washed with brine (100 mL), dried

over MgSO , and filtered. The volatiles were removed under re-

duced pressure and the residue was purified by column chroma-

tography on silica gel (200 g) in hexane/EtOAc (20:1) to afford

4

Trimethylsilyl chloride (0.50 g, 4.17 mmol) was added dropwise to

a solution of 17 (1.32 g, 2.32 mmol) in a mixture of anhydrous trie-

thylamine (0.43 g, 4.17 mmol) and diethyl ether (11 mL) at À788C

under an argon atmosphere. The reaction mixture was stirred for

0

2

.53 g of 15 as a white solid in 93% yield. R =0.4 (hexane/EtOAc=

f

1

0:1); m.p. 123–1258C; H NMR (500 MHz, CD Cl ): d=7.38–7.41

2

3

h, then allowed to warm to room temperature and stirred for an

additional 12 h. The resulting solution was quenched with water

100 mL). The aqueous layer was washed with CH Cl (2150 mL).

(

m, 2H), 7.68–7.69 (m, 2H), 7.75–7.77 (m, 2H), 7.91–7.92 ppm (m,

H); C NMR (125.8 MHz, CD Cl ): d=123.0, 128.9, 130.5, 133.0,

2 2

1

3

2

1

(

3

(

2

36.0, 139.3, 193.8 ppm; IR (KBr) v˜ =3061 (w), 2922 (m), 1651 (s),

The combined organic layer was washed with brine (100 mL), dried

À1

563 (s), 1470 (m), 1417 cm (m); UV/Vis (CH Cl ): l (e)=265 nm

2

max

over MgSO , and filtered. The volatiles were removed under re-

4

À1

3

À1

9490 mol dm cm ); ESI(+) HRMS: m/z calcd for C H Br ONa:

13 8 2

62.8814 [M+Na] ; found: 362.8802; elemental analysis calcd (%)

duced pressure and the residue was purified by column chroma-

tography on silica gel (130 g) with hexane/EtOAc (9:1) to obtain

+

for C H Br O (337.89): C 45.92, H 2.37; found: C 45.97, H 2.34.

1

3

8

2

1

.32 g of 18 as a white powder in 89% yield. R =0.84 (hexane/

f

1

EtOAc=9:1); m.p. 42–458C; H NMR (500 MHz, CDCl ): d=0.28 (s,

3

9

7

H), 6.75–6.77 (d, J=8.8 Hz, 2H), 6.96–6.98 (d, J=8.8 Hz, 2H),

.07–7.08 (d, J=8.05 Hz, 3H), 7.13–7.17 (m, 3H), 7.32 (s, 3H), 7.36–

Tris(3-bromophenyl)methanol (16)

13

In a 100 mL Schlenk flask, 1,3-dibromobenzene (1.9 g, 8.1 mmol)

was dissolved in anhydrous THF (30 mL) under argon, cooled to

À788C, and degassed. Then, nBuLi (1.6m in hexane, 5.3 mL,

7.37 ppm (d, J=7.8 Hz, 3H); C NMR (125.8 MHz, CDCl ): d=0.5,

64.1, 119.4, 122.3, 129.4, 129.8, 130.0, 132.1, 133.6, 137.7, 148.4,

153.9 ppm; IR (KBr) v˜ =3034 (m), 2956 (m), 1585 (s), 1470 (m), 1406

(m), 1215 (m), 832 (s), 782 cm (s); UV/Vis (CH Cl ): l

3

À1

8

.4 mmol) was added dropwise over 20 min, and the resulting mix-

(e)=

2

max

À1

3

À1

ture was stirred at À788C for 2 h. In a second 100 mL Schlenk

flask, 15 (1.8 g, 5.33 mmol) was dissolved in THF (10 mL) under

argon, and cooled to À788C. The solution of lithiated species was

added slowly through a cannula into the flask containing the solu-

tion of 15. The yellow solution was stirred at À788C for 2 h, then

allowed to warm to room temperature and stirred for an additional

274 nm (9059 mol dm cm ); ESI(+) HRMS: m/z calcd for

+

C H Br ONa: 596.8682 [M+Na, ÀTMS] ; found: 594.8690; elemen-

25

17

3

tal analysis calcd (%) for C H Br OSi (641.92): C 52.12, H 3.91;

28

25

3

found: C 52.34, H 3.97.

4

-(Tris{3-[2-(trimethylsilyl)ethylsulfanyl]phenyl}methyl)phe-

1

2 h. The reaction mixture was quenched with saturated NH Cl so-

4

nol (14)

lution (100 mL). The aqueous layer was washed with CH Cl (3

2

1

00 mL). The combined organic layer was washed with brine

A 25 mL pressure tube was charged with 18 (1.3 g, 2.03 mmol),

(

100 mL), dried over MgSO , and filtered. The volatiles were re-

4

Xantphos (0.094 g, 0.016 mmol), Pd (dba) (0.093 g, 0.0089 mmol),

2

3

moved under reduced pressure and the residue was purified by

column chromatography (300 g) in hexane/EtOAc (10:1) to provide

and anhydrous dioxane (14 mL). The tube was evacuated and re-

filled with argon three times. Then N,N-diisopropylethylamine

(2.3 g, 18 mmol) and 2-(trimethylsilyl)ethanethiol (2.5 g, 12 mmol)

were added under argon and the tube was sealed quickly. The re-

action mixture was heated at 1208C for 24 h, then cooled, diluted

with dichloromethane (200 mL), and washed with a solution of HCl

2

1

7

1

.15 g of 16 as a yellow oil in 82% yield. R =0.29 (hexane/EtOAc=

f

1

0:1); H NMR (500 MHz, CDCl ): d=3.07 (s, OH), 7.13–7.14 (d, J=

3

.9 Hz, 3H), 7.19–7.22 (m, 3H), 7.44–7.46 (d, J=7.85 Hz, 3H),

1

3

.48 ppm (s, 3H); C NMR (125.8 MHz, CDCl ): d=68.1, 122.9,

3

26.8, 130.0, 130.8, 131.2, 148.1 ppm; IR (KBr) v˜ =3442 (s), 3061

(

1m, 100 mL). The combined organic layer was washed with brine

(

7

(

5

m), 2926 (s), 1564 (s), 1470 (s), 1416 (m), 1416 (m), 1200 (m),

100 mL), dried over MgSO , and filtered. The volatiles were re-

4

À1

85 cm

(w);

UV/Vis

(CH Cl ):

^lmax

(e)=267 nm

2

moved under reduced pressure and the residue was purified by

column chromatography on silica gel (300 g) in hexane/EtOAc

À1

3

À1

5540 mol dm cm ); ESI(+) HRMS: m/z calcd for C H Br ONa:

19 13 3

+

18.8389 [M+Na] ; found: 518.8376; elemental analysis calcd (%)

(

10:1) to provide the title compound 14 (1.36 g) as a yellow oil in

for C H Br O (493.25): C 45.92, H 2.64; found: C 45.99, H 2.56.

1

9

13

3

92% yield. R_f=0.2 (hexane/EtOAc=10:1); H NMR (500 MHz,

CDCl ): d=À0.02 (s, 27H), 0.80–0.84 (m, 6H), 2.77–2.81 (m,6H),

3

5

3

.04 (s, OH), 6.69–6.71 (d, J=8.75 Hz, 2H), 6.95–6.99 (d, J=8.54 Hz,

H), 7.02–7.03 (d, J=8.75 Hz, 2H), 7.11–7.12 (m, 6H), 7.13–

4

-[Tris(3-bromophenyl)methyl]phenol (17)

13

The desired product was prepared according to the method de-

scribed for the preparation of 4, starting from 16 (2.13 g,

7.17 ppm (m, 3H); C NMR (125.8 MHz, CDCl ): d=À1.6, 16.9, 29.5,

3

68.2, 114.6, 126.6, 128.1, 128.8, 131.2, 132.5, 136.7, 138.4, 147.4,

4

.32 mmol), phenol (4.81 g, 51 mmol), and a few drops of concen-

154.0 ppm; IR (KBr): v˜ =3435 (s), 3055 (m), 2951 (s), 1582 (m), 1509

(m), 1471 (m), 1430 (m), 1248 (s), 857 (s), 840 (s), 687 cm (w); UV/

À1

trated HCl. The reaction mixture was heated at 1208C for 14 h

Chem. Eur. J. 2016, 22, 13218 – 13235

www.chemeurj.org

13232

ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper

À1

3

À1

1

Vis (CH Cl ): l

(e)=275 nm (51292 mol dm cm ); ESI(+) MS:

3:1); m.p. 151–1548C; H NMR (500 MHz, CDCl ): d=2.35 (s. 9H),

2

max

3

+

m/z calcd for C H OS Si Na: 755.2693 [M+Na] ; found: 755.2515;

7.26 (s, 3H), 7.28–7.30 (d-d, J=4.25 Hz, 3H), 7.35–7.38 (m, 6H),

7.43–7.45 (d, J=8.35 Hz 2H), 7.59–7.61 ppm (d, J=8.4 Hz, 2H);

C NMR (125.8 MHz, CDCl ): d=30.4, 65.1, 110.6, 118.8, 128.2, 129.1,

4

0

56

3

1

3

elemental analysis calcd (%) for C H OS Si (732.28): C 65.52, H

4

0

56

3

7

.70; found: C 66.15, H 7.81.

3

1

31.6, 131.8, 132.0, 132.7, 137.2, 146.1, 151.0, 193.7 ppm; IR (KBr):

v˜ =3060 (s), 2923 (s), 2228 (s), 1705 (s), 1584 (m), 1470 (s), 1408

4

-(Tris{3-[2-(trimethylsilyl)ethylsulfanyl]phenyl}methyl)phen-

À1

(

m), 1416 (m), 1243 (m), 1118 (s), 613 cm (s); UV/Vis (CH CN): l

3

max

yl trifluoromethanesulfonate (19)

À1

3

À1

e)=223 nm (48219 mol dm cm ); ESI(+) HRMS: m/z calcd for

+

The desired product was prepared according to the method de-

C H NO S Na: 590.0889 [M+Na] ; found: 590.0872; elemental

32 25 3 3

scribed for the preparation of 5, starting from 14 (0.3 g,

analysis calcd (%) for C H NO S (567.10): C 67.70, H 4.44, N 2.47;

32 25 3 3

0

.41 mmol), triflic anhydride (0.2 g, 0.71 mmol) in a mixture of an-

found: C 67.82, H 4.49, N 2.52.

hydrous triethylamine (0.12 mL, 0.82 mmol) and dichloromethane

(

25 mL) at À788C under argon. The reaction mixture was stirred

4

-(Tris{3-[2-(trimethylsilyl)ethylsulfanyl]phenyl}methyl)phe-

for 6 h, then allowed to warm to room temperature and stirred for

an additional 10 h. The residue was purified by column chromatog-

raphy on silica gel (150 g) in hexane/EtOAc (9:1) to provide to the

title compound 19 (0.29 g) as a yellowish oil in 83% yield. R =0.81

(

nylboronic acid pinacol ester (22)

The desired product was prepared according to the method de-

scribed for 8, starting from triflate 19 (0.29 g, 0.34 mmol), anhy-

drous potassium acetate (0.14 g, 1.36 mmol), bis(pinacolato)dibor-

on (0.13 g, 0.51 mmol), and Pd(dppf)Cl₂(0.028 g, 0.034 mmol) in

anhydrous dioxane (15 mL) under argon. After 16 h at 1208C, the

residue was purified by column chromatography on silica gel

(130 g) in hexane/EtOAc (10:1) to provide 0.23 g of 22 as a yellow

f

1

hexane/EtOAc=9:1); H NMR (500 MHz, CDCl ): d=À0.01 (s, 27H),

3

0

(

.81–0.84 (m, 6H), 2.78–2.82 (m, 6H), 6.92–6.93 (m, 3H), 7.07–7.08

m, 3H), 7.13–7.17 (m, 6H), 7.17–7.20 (m, 2H), 7.28–7.30 ppm (d,

1

3

J=3 Hz, 2H); C NMR (125.8 MHz, CDCl ): d=À1.6, 17.2, 29.7.,

3

6

1

5.2, 120.9, 127.0, 128.7, 128.8, 131.2, 133.4, 137.9, 146.9, 147.3,

1

9

48.3 ppm; F NMR (470.57 MHz, CDCl ): d=À73.30 ppm; IR (KBr):

powder in 82% yield. R =0.48 (hexane/EtOAc=10:1); m.p. 129–

3

f

1

v˜ =3060 (w), 2953 (m), 1582 (m), 1497 (w), 1427 (m), 1250 (s), 1213

1308C; H NMR (500 MHz, CDCl ): d=À0.02 (s, 27H), 0.79–0.83 (m,

3

À1

(

s), 1142 (s), 840 (s), 785 (w), 697 cm (w); UV/Vis (CH Cl ): l

6H), 1.32 (s, 12H), 2.76–2.80 (m, 6H), 6.97–6.99 (m, 3H), 7.10–7.11

2

max

À1

3

À1

e)=268 nm (25170 mol dm cm ); ESI(+) MS: m/z calcd for

(m, 3H), 7.13–7.16 (m, 6H), 7.21–7.22 (d, J=8.25 Hz, 2H), 7.67–

+

13

C H O S FSi Na: 887.2186 [M+Na] ; found 887.2126; elemental

analysis calcd (%) for C H O S Si F (864.23): C 56.91, H 6.41;

7.69 ppm (d, J=8.4 Hz 2H); C NMR (125.8 MHz, CDCl ): d=À1.6;

41

55

3

4

3

16.9, 25.1, 29.5, 65.3, 83.9, 126.6, 127.8, 128.1, 128.7, 130.6, 131.2,

4

1

55

3

4

3

found: C 57.03, H 6.59.

134.3, 136.8, 147.0, 149.3 ppm; IR (KBr): v˜ =3059 (s), 2950 (s), 2921

(

s), 2852 (w), 1733 (m), 1609 (m), 1580 (s), 1470 (w), 1427 (m), 1364

À1

s), 1247 (s), 1213 (s), 1143 (s), 1091 (m), 860 (s), 838 (s), 704 cm

4

-(Tris{3-[2-(trimethylsilyl)ethylsulfanyl]phenyl}methyl)ben-

À1

3

À1

w); UV/Vis (CH Cl ): l (e)=265 nm (27425 mol dm cm );

max

2

zonitrile (20)

+

ESI(+) MS: m/z calcd for C H BO S Si Na: 865.3605 [M+Na] ;

46

67

2

3

The desired product was prepared according to the method de-

found: 865.3226; elemental analysis calcd (%) for C46

H67BO S Si

2 3 3

scribed for the preparation of 6, starting from triflate 19 (0.15 g,

(842.37): C 65.52, H 8.01; found: C 65.73, H 8.12.

0

.17 mmol), zinc cyanide (0.049 g, 0.42 mmol), copper(I) cyanide

(

0.015 g, 0.17 mmol), and Pd(PPh ) (0.029 g, 0.025 mmol) in anhy-

3

4

1-Iodo-4-(tris{3-[2-(trimethylsilyl)ethylsulfanyl]phenyl}me-

thyl)benzene (23)

drous DMF (10 mL) under argon. After 16 h at 1408C, the reaction

mixture was purified by column chromatography on silica gel

(

100 g) in a mixture of hexane/EtOAc (9:1) to provide 0.13 g of 20

The desired product was prepared according to the method de-

scribed for 9, starting from pinacol ester 22 (0.36 g, 0.43 mmol),

copper iodide (0.12 g, 0.64 mmol), and N-Iodosuccinimide (0.14 g,

0.64 mmol) in anhydrous DMF (13 mL) and toluene (6.5 mL) under

argon. After 24 h at 1108C, the residue was purified by column

chromatography on silica gel (120 g) in hexane/EtOAc (10:1) to

1

as a yellow oil in 74% yield. R =0.67 (hexane/EtOAc=9:1); H NMR

(

7

f

500 MHz, CDCl ): d=À0.01 (s, 27H), 0.80–0.84 (m, 6H), 2.78–2.81

3

m, 6H), 6.91–6.93 (d, J=10.5 Hz, 3H), 7.07 (s, 3H), 7.13–7.15 (d, J=

.8 Hz,2H), 7.17–7.20 (m, 3H), 7.34–7.36 (d, J=8.55 Hz, 2H), 7.54–

1

3

.56 ppm (d, J=8.6 Hz, 2H); C NMR (125.8 MHz, CDCl ): d=

3

À1.57, 16.84, 29.45, 65.29, 110.39, 118.87, 126.79, 128.43, 128.47,

give 0.3 g of 23 as a yellow powder in 83% yield. R =0.75

f

1

30.83, 131.65, 131.87, 137.49, 154.96, 151.72 ppm; IR (KBr): v˜ =262

(hexane/EtOAc=10:1); m.p. 136–1378C; H NMR (500 MHz, CDCl ):

3

(

1

17540), 3059 (s), 2951 (s), 2228 (m), 1582 (s), 1471 (m), 1407 (m),

d=À0.0 (s, 27H), 0.81–0.85 (m, 6H), 2.80–2.84 (m, 6H), 6.98–7.00

À1

248 (m), 1161 (s), 875 (s), 860 (s), 696 cm (w); UV/Vis (CH CN):

(m, 5H), 7.11–7.14 (m, 6H), 7.17–7.20 (m, 3H), 7.59–7.61 ppm (d,

3

À1

3

À1

13

l_m_a_x(e)=224 nm (19670 mol dm cm ); ESI(+) MS: m/z calcd for

C H S Si K: 780.2436 [M+K] ; found: 780.2485; elemental analysis

J=7.8 Hz, 2H); C NMR (125.8 MHz, CD Cl ): d=À1.55, 17.27,

2

+

29.73, 65.27, 92.49, 126.86, 128.66, 128.72, 131.13, 133.55, 137.33,

41

55

3

calcd (%) for C H OS Si (741.28): C 66.34, H 7.47, N 1.89; found: C

137.77, 146.63; 147.1 ppm; IR (KBr): v˜ =3058 (s), 2952 (s), 2921 (s),

4

1

55

3

À1

6

6.49, H 7.65, N 1.91.

2853 (w), 1581 (s), 1473 (w), 1248 (s), 1006 (m), 840 (s), 728 cm

À1

3

À1

(

w); UV/Vis (CH Cl ): l

(e)=267 nm (24704 mol dm cm );

max

2

+

ESI(+) MS: m/z calcd for C H S ISi Na: 865.1711 [M+Na] ; found:

S,S’,S’’-{3,3’,3’’-[(4-Cyanophenyl)methanetriyl)]tris(benzene-

,1-diyl)} tris(thioacetate) (21)

40 55

3

8

5

87.1564; elemental analysis calcd (%) for C H IS Si (842.18): C

40 55 3 3

6.98, H 6.57; found: C 57.11, H 6.70.

4

The desired product was prepared according to the method de-

scribed for 7, starting from 20 (0.12 g, 0.16 mmol), AgBF (0.22 g,

4

S,S’,S’’-{3,3’,3’’-[(4-Iodophenyl)methanetriyl)]tris(benzene-4,1-

diyl)} tris(thioacetate) (24)

1

.12 mmol), and acetyl chloride (1.2 mL) in anhydrous dichlorome-

thane (12 mL) under argon. After 12 h at room temperature, the re-

action mixture was quenched and purified by column chromatog-

raphy on silica gel (150 g) in hexane/EtOAc (3:1) to give 0.07 g of

The desired product was prepared according to the method de-

scribed for 7, starting from 23 (0.29 g, 0.43 mmol), with AgBF₄

(0.4 g, 2.06 mmol) in a mixture of anhydrous dichloromethane

2

1 as an orange powder in 77% yield. R =0.11 (hexane/EtOAc=

f

Chem. Eur. J. 2016, 22, 13218 – 13235

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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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(

18 mL) and acetyl chloride (1.8 mL) under argon. The suspension

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purified by column chromatography on silica gel (150 g) in

hexane/EtOAc (5:1) to provide 0.2 g of 24 as a yellow powder in

5

(

7

6

1

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.26–7.27 (m, 6H), 7.27–7.29 (m, 3H), 7.32–7.35 (m,3H), 7.61–

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583 (m), 1467 (m), 1402 (m), 1119 (s), 947 (m), 613 cm (s); UV/Vis

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À1

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À1

(

CH CN): l

(e)=209 nm (39207 mol dm cm ); ESI (+) HRMS:

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m/z calcd for C H IO S Na: 690.9903 [M+Na] ; found; 690.9909;

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1

25

3 3

elemental analysis calcd (%) for C H IO S (668.00): C 55.69, H

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Received: April 29, 2016

Published online on August 9, 2016

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Article Doi

Importance of the Anchor Group Position (Para versus Meta) in Tetraphenylmethane Tripods: Synthesis and Self-Assembly Features

DOI: 10.1002/chem.201602019

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