Synthesis of trans-A2B2- and trans-A2BC-porphyrins with polar 4′-(dimethylamino)tolan-4-yl substituents, and a screening protocol for vapor-phase deposition on metal surfaces

FULL PAPER

DOI: 10.1002/ejoc.201402634

Synthesis of trans-A₂B₂- and trans-A₂BC-Porphyrins with Polar

4Ј-(Dimethylamino)tolan-4-yl Substituents, and a Screening Protocol for

Vapor-Phase Deposition on Metal Surfaces

Mariza N. Alberti,^[a]Sylwia Nowakowska,^[b]Manolis D. Tzirakis,^[a]Jan Nowakowski,^[c]

Petra Fesser,^[a]W. Bernd Schweizer,^[a]Aneliia Shchyrba,^[b]Carlo Thilgen,^[a]

Thomas A. Jung,*^[b,c]and François Diederich*^[a]

Keywords: Supramolecular chemistry / Self-assembly / Surface analysis / Dipole effects / Gold / Porphyrinoids

The role of polar 4-[p-(dimethylamino)phenylethynyl]phenyl

substituents, with a calculated dipole moment of 3.35 Debye,

in the self-assembly of trans-A₂B₂- and A₂BC-substituted

porphyrins was explored in the solid state by X-ray crystal-

lography, and on an Au(111) surface by scanning tunneling

microscopy (STM). Our results demonstrate that the dipolar

character of these substituents blocks the 2D self-assembly

of porphyrins into larger ordered domains on Au(111) at low

coverage, whereas antiparallel dipole–dipole interactions

govern the molecular ordering in the crystal. The STM analy-

sis revealed an adaptation of the conformation of the pro-

chiral building blocks and a site-selectivity of the adsorption.

We present a general protocol for testing the suitability of

higher-molecular-weight compounds, such as porphyrins, to

be deposited on surface by sublimation in ultra-high vacuum

(UHV). This protocol combines classical methods of chemical

analysis with typical surface science techniques.

Given the planarity and stability of many porphyrins, on-

surface assembly of porphyrin-based materials is of special

interest.^[5]A variety of meso substituents have been used

to induce the formation of specific arrangements through

noncovalent^[6]or covalent^[7]interactions. In some cases,

porphyrin-based systems showed complex multiphase be-

havior due to the presence of different competing interac-

tions.^[8]In this regard, the on-surface self-assembly of a

porphyrin bearing a polar 4Ј-(dimethylamino)tolan-4-yl

(DMAT) meso substituent (cf. 1, Figure 1; tolane = diphen-

ylacetylene) has recently been explored.^[9,10]Specifically, the

polarity of this group, with a computed^[11]dipole moment

along the molecular axis of 3.35 Debye (see Section 5 in the

Supporting Information), led to a complex interplay of

attractive and repulsive, short- and long-range forces that

allowed the dimensionality of on-surface assemblies to be

controlled.^[9]

Introduction

Supramolecular chemistry gives the opportunity to direct

the self-assembly of molecular architectures using a variety

of binding motifs or synthons that, depending on their rela-

tive association strengths, also give rise to subtle modifica-

tions of the resulting structures. In view of the rapidly grow-

ing number of well-defined supramolecular architectures^[1,2]

that can be constructed and investigated for potential uses,

for example, in electronics^[3]or medicine,^[4]there is great

demand for newly synthesized, shape-persistent π-conju-

gated molecules bearing various specific functional groups.

Importantly, there are fundamental differences between the

supramolecular architectures produced from solution by

crystallization and those created at solid/liquid or solid/vac-

uum interfaces. Structural analysis of these architectures

provides insight into the forces and interactions that lead

to self-assembly in the different environments.

In an extension of our studies, we were interested in fur-

ther examining the role of the DMAT substituent in the

solid state and in 2D supramolecular architectures. For this

purpose, a systematic series of trans-A₂B₂and trans-A₂BC

porphyrins 1–6 (Figure 1) with different meso substituents

was synthesized. The self-assembling behavior of these por-

phyrins in the solid state (porphyrins 1, 2, 4, and 6) and on

a metallic surface (porphyrins 2 and 6) was then investi-

gated. A key step in the preparation of porphyrins 1–6 was

the Suzuki–Miyaura cross-coupling of a porphyrinbis-

(boronic ester) with the appropriate aryl halides. When

[Pd(PPh₃)₄] was used as a catalyst in this reaction, two

[a] Laboratorium für Organische Chemie, ETH Zurich,

Vladimir-Prelog-Weg 3, 8093 Zurich, Switzerland

E-mail: diederich@org.chem.ethz.ch

http://ww w .diederich.ethz.ch/

[b] Institute of Physics, University of Basel,

Klingelbergstrasse 82, 4056 Basel, Switzerland

E-mail: thomas.jung@psi.ch

https://nanolab.unibas.ch/

[c] Laboratory for Micro- and Nanotechnology, Paul Scherrer

Institute,

5232 Villingen, Switzerland

E-mail: thomas.jung@psi.ch

Supporting information for this article is available on the

WWW under http://dx.doi.org/10.1002/ejoc.201402634.

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aldehydes (A-CHO and B-CHO), this method is practically

limited since it gives a statistical mixture of six different

porphyrins (types A₄, A₃B, cis-A₂B₂, trans-A₂B₂, AB₃, and

B₄). A selective way to obtain trans-A₂B₂systems involves

the MacDonald-type [2+2] condensation of a dipyrro-

methane with an aldehyde.^[13]Self-condensation of dipyrro-

methane-1-carbinols or 1-acyldipyrromethane has also been

used for the synthesis of porphyrins with this meso substitu-

tion pattern.^[14]Another common route to both trans-A₂B₂

and trans-A₂BC systems uses a stepwise approach where

meso-disubstituted trans-A₂-porphyrins are subjected to

further meso functionalization.^[15]In this case, trans-A₂-

porphyrins can simply be synthesized by MacDonald-type

[2+2] condensation of unsubstituted dipyrromethane with

an aldehyde A-CHO.

For the preparation of target porphyrins 1–6 (Figure 1),

we chose a stepwise approach to prevent scrambling of the

meso substituents. An initially synthesized trans-A₂-por-

phyrin was metalated and then further meso-functionalized

(see Section 2.1 in the Supporting Information). Using this

method, porphyrin-bis(boronic ester) 7^[16](Scheme 1), the

common precursor of all of the target porphyrins (i.e., 1–

6), was prepared in four steps and 10% overall yield (see

Scheme S2 in the Supporting Information).

In the key step of the synthesis, a Suzuki–Miyaura cross-

coupling of bis(boronic ester) 7 with aryl bromide 8 was

used to obtain porphyrin 1 (Scheme 1). Compound 8 was

prepared in 92% yield by Sonogashira cross-coupling of 1-

bromo-4-iodobenzene with 4-ethynyl-N,N-dimethylaniline

(see Section 2.2 in the Supporting Information). Unexpec-

tedly, the Suzuki–Miyaura cross-coupling of 7 with 8 in the

presence of [Pd(PPh₃)₄] as a catalyst (Route A, Scheme 1)

gave a mixture of two porphyrins with molecular masses of

1186.56 Da (major product) and 1043.49 Da (minor prod-

uct). Although attempts to fully separate these compounds

by flash column chromatography (FC) were unsuccessful,

we were able to isolate them by preparative recycling GPC

(2ϫ Jaigel-2H and Jaigel-2.5H, CHCl₃).^[17]Gratifyingly,

pure porphyrins 1 and 2 (Scheme 1) could be fully charac-

terized by NMR spectroscopy and MALDI-TOF mass

spectrometry. Importantly, an unambiguous assignment

was possible by ¹³C NMR spectroscopy. In the spectrum of

D_2h-symmetric trans-A₂B₂derivative 1, two resonances for

both the α- and β-carbon atoms of the pyrrole rings are

expected, and they were observed at δ = 150.04 and

150.64 ppm (C-α), and at δ = 133.00 and 134.45 ppm (C-β).

In contrast, C_2v-symmetric trans-A₂BC derivative 2 showed

four C-α resonances at δ = 150.04, 150.26, 150.61, and

150.65 ppm, and four C-β resonances at δ = 132.43, 132.56,

132.98, and 134.46 ppm.

Figure 1. trans-A₂B₂and trans-A₂BC porphyrins investigated in

this study.

meso-phenylated porphyrins were formed as by-products. It

is worth mentioning that, in comparison to our previous

studies,^[9,10]the modification of the molecular architectures

described in this paper led, in some cases, to larger por-

phyrins with higher molecular weights. The larger the com-

pounds are, the higher the temperature required for their

sublimation, and, therefore, the more likely their fragmenta-

tion during the deposition process. In order to ensure that

our investigations were performed on supramolecular

assemblies formed from intact molecular modules, we de-

veloped a systematic screening protocol for the candidate

molecules consisting of (i) sublimation in a test chamber

under high vacuum (HV)^[12]followed by high-performance

liquid

chromatography

(HPLC),

gel-permeation

chromatography (GPC), NMR spectroscopy, high-resolu-

tion mass spectrometry (HRMS), and, optionally, thermo-

gravimetric analysis (TGA) of the sublimed compound;

(ii) electron spectroscopy for chemical analysis (ESCA)/X-

ray photoelectron spectroscopy (XPS); and (iii) scanning

probe microscopy (SPM) imaging.

To explain the origin of by-product 2, we considered two

routes through which 2 could be formed. These involve

either a further transformation of 1 or the transfer of a

phenyl group from the catalyst [Pd(PPh₃)₄] to porphyrin

7.^[18,19]To test the first possibility, we studied the stability

of porphyrin 1 under the conditions of synthetic Route A

Results and Discussion

Synthesis of Porphyrins 1–6

Although trans-A₂B₂porphyrins can, in principle, be ob- (Scheme 1). The lack of any transformation clearly indicates

tained by mixed condensation of pyrrole and two different that 2 is not generated via 1. With the hope of proving the

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Synthesis of trans-A₂B₂- and trans-A₂BC-Porphyrins

Scheme 1. Synthesis of porphyrins 1 and 2.

validity of our second hypothesis, we carried out the Su- (CA–RE) reaction^[21]with different electron-deficient ole-

zuki–Miyaura cross-coupling of 7 with 8 in the presence of fins. Specifically, the reaction of porphyrin 1 with 7,7,8,8-

[Pd(OAc)₂] and the bulky phosphine ligand dicyclohexyl-2- tetracyano-p-quinodimethane (TCNQ; 2.1 equiv.) was

(2,6-dimethoxyphenyl)phenylphosphine (S-Phos; Route B, quantitative and regioselective, giving product 14^[22]

Scheme 1).^[20]In this case, porphyrin 1 was exclusively (Scheme 3, a). To further expand the scope of the CA–RE

formed in 57% yield. This observation undoubtedly sup- on-surface reaction,^[10]we initially examined the possible

ports the proposal that by-product 2 is formed by phenyl– regular [AB]-polymerization of porphyrin 1 (A) with doubly

aryl exchange during the cross-coupling reaction (aryl = reactive bis(dicyanovinyl) arene 15 (B; 1.2–5 equiv.) in

DMAT). A plausible mechanism for this exchange is pre- 1,1,2,2-tetrachloroethane (Scheme 3, b). This reaction was

sented in Scheme 2.^[18]Specifically, the first step of the cata- monitored by MALDI-TOF mass spectrometry. The reac-

lytic cycle consists of the oxidative addition of bromide 8 tion of 15 with a similar substrate incorporating two

to Pd⁰to form Pd^IIcomplex 9. As an “irregular” step of reactive 4-ethynylanilino sites (i.e., 1,4-bis{[4-(N,N-dihex-

this mechanism, an exchange between a phosphorus-bound ylamino)phenyl]ethynyl}) had previously been demon-

phenyl ligand and the palladium-bound aryl group (R¹) strated to give oligomeric and macrocyclic [AB]_n, A[AB]_n

originating from 8 occurs to give complex 10. Reaction of as well as B[AB]_nstructures.^[23]In our reaction, the con-

the latter with Cs₂CO₃leads to an exchange of bromide sumption of porphyrin 1 and the formation of a mixture of

with carbonate to form 11, which, by transmetalation with oligomeric products were observed. The signals seen in the

boronate 12, gives complex 13. In the final reductive elimi- mass spectrum of the crude product correspond to m/z val-

nation step, the phenyl ligand of Pd^IIcomplex 13 is now ues that are consistent with products of three general for-

coupled (instead of the desired aryl residue R¹) to the por- mulae, i.e., [AB]_n(n = 1 or 2), A[AB], and B[AB]_n(n = 1 or

phyrin core to give the scrambled porphyrin (i.e., 2) and 2; A and B denote the two monomers 1 and 15, respec-

regenerate the palladium catalyst.

tively). B[AB]-type push–pull chromophore 16 was the

With diacetylene-appended porphyrin 1 in hand, we set major product, and was isolated in 64% overall yield (con-

out to study its [2+2] cycloaddition–retroelectrocyclization sistently with previous findings, this product was formed in

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Scheme 2. Proposed catalytic cycle accounting for the unexpected formation of porphyrin 2.^[18]

a regioselective manner). The other products were formed porphyrin 18 was formed exclusively in 82% yield. It can

in minor amounts (5–10%), and were only detected by be concluded that scrambled porphyrin 19 is formed analo-

MALDI-TOF mass spectrometry. At this point, we were gously to 2 (cf. Scheme 2). In the next step, the iPr₃Si pro-

interested in examining the reactivity of the two more ac- tecting groups were removed from 18 and 19 with nBu₄NF

cessible dicyanovinyl groups of 16. Hence, the reaction of in THF to give porphyrins 3 and 4 in 68 and 79% yields,

16 with a 30-fold excess of porphyrin 1 was investigated in respectively. Similarly, porphyrin 6 was prepared in 88%

1,1,2,2-tetrachloroethane at 120 °C. Under these condi- yield by Suzuki–Miyaura cross-coupling of porphyrin 7

tions, the starting material was consumed, but only un- with iodobenzene (Scheme 4).

identified products were formed. Similar studies were also

performed for porphyrin 2 (see Section 2.3 in the Support- coupling of 3 with 4-bromo-N,N-dimethylaniline (1 equiv.;

ing Information). see Scheme S2 in the Supporting Information). The reac-

Porphyrin 5 was synthesized by Sonogashira cross-

The synthesis of porphyrins 3, 4, and 6 was accomplished tion conditions^[25–27]screened for this transformation are

in a manner analogous to that described for 1 and 2 summarized in Table S2. In particular, porphyrin 5 (11%

(Scheme 4). Suzuki–Miyaura cross-coupling of porphyrin 7 yield) was formed only when [Pd₂(dba)₃] (dba = dibenz-

with bromide 17, which was obtained in 79% yield accord- ylideneacetone) and P(o-Tol)₃were used as the catalytic

ing to a known literature procedure,^[24]in the presence of system (Table S2, entry 4). It is worth mentioning that

[Pd(PPh₃)₄] as a catalyst, gave porphyrins 18 and 19 in 69 GPC facilitated purification of the desired porphyrin; using

and 21% yields, respectively. When the coupling reaction this method, aromatic impurities could be removed

was carried out in the presence of [Pd(OAc)₂] and S-Phos, smoothly.

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Synthesis of trans-A₂B₂- and trans-A₂BC-Porphyrins

Scheme 3. CA–RE reaction of porphyrin 1 with a) TCNQ and b) bis(dicyanovinyl) arene 15.

Scheme 4. Synthesis of porphyrins 3, 4, and 6.

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Figure 2. ORTEP plots of a) 1·2THF and b) 2·MeOH, with vibrational ellipsoids shown at the 50% probability level (T = 100 K).

Arbitrary numbering. Noncoordinating solvent molecules, hydrogen atoms, and disordered positions are omitted for clarity. c) View of

the unit cell of 2·MeOH. 3,5-Di(tert-butyl)phenyl rings, noncoordinating solvent molecules, hydrogen atoms, and disordered positions

are omitted for clarity. Selected bond lengths [Å] and angles [°]: 1·2THF: Zn-1–N-1: 2.059(2), Zn-1–N-8: 2.046(2), Zn-1–O-44: 2.336(2),

C-16–C-19: 1.443(3), C-19–C-20: 1.200(3), C-20–C-21: 1.437(3), N-1–Zn-1–N-1: 180.000(1), N-8–Zn-1–N-8: 180.00(6), N-1–Zn-1–O-44:

90.41(7), N-8–Zn-1–O-44: 89.93(7), O-44–Zn-1–O-44: 179.999(1), C-19–C-20–C-21: 178.9(3), C-20–C-19–C-16: 175.5(3); 2·MeOH: Zn-

1–N-1: 2.069(3), Zn-1–N-2: 2.064(3), Zn-1–N-3: 2.063(3), Zn-1–N-4: 2.063(3), Zn-1–O-1: 2.131(3), C-58–C-61: 1.423(5), C-61–C-62:

1.206(5), C-62–C-63: 1.444(5), N-3–Zn-1–N-1: 162.7(1), N-4–Zn-1–N-2: 164.5(1), N-1–Zn-1–O-1: 95.3(1), N-3–Zn-1–O-1: 102.0(1), C-

62–C-61–C-58: 178.6(4), C-61–C-62–C-63: 177.8(4). Further details are found in Sections 4.2 and 4.3 of the Supporting Information.

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Synthesis of trans-A₂B₂- and trans-A₂BC-Porphyrins

X-ray Crystallography

shown in Figure S2 in the Supporting Information). Of

course, we were interested in exploring whether this distinct

X-ray crystal structures of porphyrins 1, 2, 4, 6, and 19 antiparallel dipolar alignment of porphyrin 2 would also be

were solved. The structures of 1 and 2, which have DMAT found in 2D self-assembly on metal surfaces.

substituents, are discussed here, whereas all other structural

information can be found in the Supporting Information.

A Three-Step Protocol for Determining the Suitability of

Molecular Modules for Deposition on Surfaces by

Sublimation in Ultra-High Vacuum

Single crystals of porphyrin 1 were obtained from a mixture

of THF and CH₃CN (2:1) by slow evaporation at 3–5 °C.

The compound (Figure 2, a) cocrystallizes with two THF

molecules axially coordinated to the zinc(II) ion, and fur-

ther gap-filling, disordered THF/CH₃CN molecules, in the

A comparison between the solid-state ordering induced

by the DMAT moiety in the synthesized porphyrins and 2D

self-assemblies formed on the chemically inert and atomi-

cally clean Au(111) surface under UHV (ultra-high vac-

uum) conditions is possible only if the molecular modules

can be assembled at the solid/vacuum interface in the ab-

sence of impurities and decomposition products. Although

deposition by sublimation is well established, it has been

reported that for some molecules, thermal decomposition

takes place before or during sublimation.^[32]In addition, de-

composition may occur rapidly after deposition on some

metallic surfaces.^[33]The fact that molecules can decompose

during UHV deposition asks for a careful investigation of

this process. As degradation may occur at different stages

of the process (i.e., in the bulk of the heated material or at

crystal surfaces), and considering the fact that SPM meth-

ods are quite time-consuming, a three-tiered approach (Fig-

ure 3) was developed to detect possible decomposition prior

to microscopic investigations.

¯

centrosymmetric, triclinic space group P1 with the octahe-

drally coordinated zinc(II) ion at the inversion point. The

macrocycle is essentially planar with an average deviation

from the 24-atom porphyrin plane of 0.03 Å. The O-atoms

of the THF ligands lie on an axis, which is nearly perpen-

dicular to the porphyrin plane. Both Zn–O bond lengths

are 2.336(2) Å, which is in the expected range for por-

phyrins with axial oxygen-containing ligands.^[28,29]The di-

hedral angles between the porphyrin plane and the meso

aryl rings are 72° and 62°, respectively. The phenyl rings

interconnected by an acetylene unit are twisted by 74° rela-

tive to each other.

Single crystals of porphyrin 2 were obtained from a mix-

ture of CH₂Cl₂and MeOH (2:1) by slow evaporation at 3–

¯

5 °C. The structure of 2·MeOH (triclinic space group P1,

Figure 2, b) shows an almost planar porphyrin core with

an average deviation from the 24-atom porphyrin plane of

0.04 Å. The zinc(II) ion prefers a square-pyramidal coordi-

nation geometry, and is axially coordinated by a MeOH

ligand. Similar to other pentacoordinate metalloporphyrin

structures, the zinc(II) ion deviates from the mean plane of

the four pyrrolic N-atoms towards the axial MeOH ligand

by 0.295 Å. The dihedral angles between the porphyrin

plane and the meso aryl rings vary between 63° and 86°.

The perpendicular arrangement of the 4-(dimethylamino)-

phenyl substituent between two porphyrin rings related by

a translation along the b-axis causes big gaps between the

molecules, which are filled with a mixture of heavily disor-

dered methanol and dichloromethane molecules. Each 4-

(dimethylamino)phenyl ring is in close proximity to the por-

phyrin ring of an adjacent molecule (Figure 2, c), partici-

pating in one C–H···N^[30]and two C–H···π^[31]interactions

(for selected interactions and geometrical parameters, see

also Figure S6 and Table S5 in the Supporting Infor-

mation).

Porphyrin 2 has a substantial permanent electric dipole

moment of 3.72 Debye (calculated value,^[11]see Section 5 in

the Supporting Information), which is induced across the

molecule by its N,N-dimethylanilino (DMA) moiety. This

dipole moment gives rise to electrostatic interactions, which

lead to an antiparallel dipolar alignment of the two por-

phyrins in the unit cell. Consistently with this observation,

porphyrin 2 undergoes analogous self-assembly in solution,

as suggested by the characteristic upfield shifts of the

Figure 3. A general protocol for conveniently checking the in-

tactness of molecules deposited on a surface by sublimation in

UHV. Molecules passing the entire test are suitable for on-surface

UHV studies by, among other methods, scanning tunneling micro-

scopy (STM), atomic force microscopy (AFM), angle-resolved pho-

toemission spectroscopy (ARPES), near edge X-ray absorption fine

1

N(CH₃)₂, 9-H, and 10-H protons that are seen in the H

NMR spectrum (CDCl₃) when the concentration is in-

1

creased (carbon atom numbering and H NMR spectra are structure (NEXAFS), X-ray magnetic circular dichroism (XMCD).

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In the first screening step, a specially built apparatus for mally activated chemical reactions. For this reason, the de-

sublimation under HV (high vacuum) conditions is used.^[12]posited sample is checked by ESCA (XPS) with a particular

The compound to be tested (1–2 mg) is placed in a quartz focus on its elemental composition. If the results deviate

flask, heated with a tin bath, and sublimed onto a cold fin- from the theoretical values, the experiment is repeated for

ger. The sublimed material is washed from the cold finger, a different molecular coverage to find out whether contami-

and analyzed by NMR spectroscopy, HRMS, HPLC, and/ nations are present in the starting sample or whether a

or GPC to assure that it is chemically intact. Ideally, the chemical reaction occurs during degassing, sublimation, or

analysis of the sublimate would be complemented by an deposition.

examination of the unsublimable residue. If these leftovers

However, it may also happen that the molecules decom-

correspond to a considerable amount of decomposed/poly- pose on the surface and the resulting fragments remain ad-

merized material, then such a thermal conversion may pre- sorbed. Such decomposition cannot be detected by ESCA,

vent successful deposition in UHV,^[34]which is usually pre- and neither can it be detected in the first screening step,

ceded by extended degassing procedures. In this case, ther- since the cold finger is made of a different material (glass,

mogravimetric analysis (TGA) under the conditions of the quartz) from the substrate (metal), and relatively few of the

degassing procedure is advisable (see Section 6 in the Sup- sublimed molecules are in direct contact with it. Therefore,

porting Information). The sublimation of impure com- in the third and final screening step, the integrity of the

pounds may also release a significant amount of contami- material deposited at sub-ML^[35]coverage on the desired

nants. This is due to the temperature dependence of subli- surface is checked with an SPM technique that is able to

mation and the possibility of impurities being more volatile resolve single molecules or even submolecular units.

than the target compound. Therefore, all compounds are

carefully purified by column chromatography, GPC, and/or

HPLC before entering the first screening stage.

In the second screening step, the molecular material is

introduced into a UHV chamber, properly degassed, and

Examining the Suitability of Porphyrins 1–6 for Deposition

by Sublimation in UHV

sublimed onto a sample surface with the rate being con-

Despite their high molecular weight and labile functional

trolled by a quartz crystal microbalance (QCMB). Fluctua- groups (absent in 6), porphyrins 1–4 and 6 could be sub-

tions in the deposition rate at a constant crucible tempera- limed without fragmentation or decomposition at 10^–6–

ture may be attributed to pressure bursts originating from 10^–7mbar and bath temperatures of 400–440, 320–340,

undesired thermal gradients in the crucible and/or ther- 300–330, 290–320, and 270–310 °C, respectively, as con-

Figure 4. C1s and N1s XPS spectra of a multilayer of a) porphyrin 6 and b) porphyrin 2 on Au(111). c) Summary of the quantitative

analysis comparing the experimental C/N and N_pyrrole/N_DMAratios with the theoretical values; n/a: not applicable.

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Synthesis of trans-A₂B₂- and trans-A₂BC-Porphyrins

Figure 5. Role of the polar DMAT residue in on-surface self-assembly. a and b) STM images of β-enantiomers^[36a]of polar porphyrin 2

deposited on Au(111): a) two different chiral conformers denoted β₁and β₂; b) trimer consisting of identical chiral conformers denoted

β₃. c and d) STM images of apolar porphyrin 6 deposited on Au(111): c) single α-enantiomer [α and β differ in the signs of the dihedral

angles between the porphyrin core and each of the 3,5-di(tert-butyl)phenyl groups]; d) close-packed domain consisting of two enantiomeric

conformers (α and β), arranged in alternating order. Tentative models are superimposed on the STM images. Removal of the polar

substituent leads to a uniform condensation, which shows that the polarity of porphyrin 2 is blocking the self-assembly into ordered

domains. All STM measurements were carried out at 5 K; tunneling parameters: a) 1.00 V/10 pA, b) 1.20 V/5 pA, c) 0.05 V/5 pA, and

d) 0.20 V/10 pA.

firmed by the analytical techniques used in the first screen- ment with the data published for a similar (N,N-di-

ing step (Figure 3). In contrast, porphyrin 5 decomposed methylanilino)ethynyl-substituted

tetraarylporphyrin.^[10]

during sublimation at 10^–6–10^–7mbar and bath tempera- Furthermore, the quantitative analysis presented in Fig-

tures of 370–420 °C, as shown by NMR spectroscopy and ure 4 (c) reveals a good agreement between the experimen-

HRMS. Therefore, it was excluded from further studies.

tal (16.9) and the theoretical (16.0) C/N ratios for 6. In the

For the second screening stage, porphyrins 1–4 and 6 case of porphyrin 2, the agreement between the C/N ratios

were placed in a home-made stainless steel crucible and in- is acceptable (experimental 17.0 vs. theoretical 14.8); the ex-

troduced into a UHV system with a base pressure of cess of carbon possibly results from incomplete degassing

5ϫ10^–11mbar. Stable deposition rates were achieved only of the material in the crucible. Taking into account the per-

for porphyrins 2 and 6, of which several monolayers were fect N_pyrrole/N_DMAratio, a repetition of the deposition for

sublimed onto Au(111). The recorded XPS spectra are XPS measurements was not necessary. Although porphyrin

shown in parts a and b of Figure 4 (for general XPS condi- 1 passed the first screening stage, its deposition onto

tions, see Section 1.4 in the Supporting Information). Por- Au(111) by sublimation under UHV conditions was prob-

phyrin 6 bears four pyrrolic nitrogen atoms, which are char- lematic. A factor that needs to be taken into account is

acterized by a single peak at the binding energy (BE) of the higher molecular mass of 1 (1186.56 Da) compared to

398.1 eV. The N1s spectrum of porphyrin 2 has two compo- porphyrins 2–6 (900.41–1067.48 Da). Remarkably, success-

nents, one originating from the four pyrrolic nitrogen atoms ful STM studies on a trans-A₂B₂porphyrin lighter by only

(398.2 eV), and the other from the nitrogen atom of the 84 Da,^[26,36]show that, in addition to the molecular mass,

DMA moiety (399.6 eV). All N1s BEs are in good agree- also the chemical nature of the residues {i.e., two meso-[4-

Eur. J. Org. Chem. 2014, 5705–5719

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5713

T. A. Jung, F. Diederich et al.

FULL PAPER

(4-pyridyl)ethynyl]phenyl^[26,36]vs. two meso-DMAT (in 1) the surface, only β is shown in Figure 5 (a), as chirality is

substituents} strongly influences the stability of the por- not in the focus of the current study. The β₁and β₂con-

phyrins during sublimation.

formers (Figure 5, a) differ in the absolute value of the dihe-

Porphyrins 3–5, which did not pass the second screening dral angles between the porphyrin core and each of the 3,5-

step, bear either one or two 4-ethynylphenyl meso substitu- di(tert-butyl)phenyl groups. The DMA group of both con-

ents. Previous studies have shown that a diethynyl- formers appears with a dimmer contrast than the 3,5-di-

substituted π-system polymerizes on Ag(111), Au(111), (tert-butyl)phenyl groups.^[9,10]The supramolecular trimers,

and Cu(111) after annealing at ca. 125 °C,^[37]a temperature shown in Figure 5 (b) (tunneling parameters 1.20 V/5 pA),

which is much lower than the sublimation temperature of consist of identical conformational enantiomers (β₃) which,

3–5 (290–420 °C). This brings us to the conclusion that the similar to β₁and β₂, show specific torsional angles between

sublimation rate of ethynylated porphyrins steadily de- the porphyrin core and each of the 3,5-di(tert-butyl)phenyl

creases over time because of competitive polymerization groups. In any case, these dihedral angles are smaller than

processes such as Glaser coupling^[38]in the crucible. those observed in the X-ray crystal structure of 2. The por-

This is consistent with the observed degradation of por- phyrins in these trimers are arranged “back-to-back”, prob-

phyrins 3–5 upon prolonged heating at high temperatures ably due to van der Waals interactions between the tert-

(200–250 °C).

butyl groups and C–H···π interactions between the tert-

Last but not least, the STM images of single molecules butyl and the phenyl groups, while the polar DMA groups

of porphyrins 2 and 6 (vide infra; Figure 5, a and c) are in stick out, which possibly stabilizes the porphyrin trimer

good agreement with their molecular structures and, conse- against condensation to form higher oligomers. Similar self-

quently, these molecules successfully passed the whole assembly behavior was observed with a related A₃B-por-

screening test.

phyrin on Au(111).^[10]That A₃B-porphyrin was found to

self-assemble in “open” (head-to-head) dimers, whereas no

dimers were observed with 2. This clearly shows that even

a small modification in the structure of a porphyrin can

lead to noticeable changes in its self-assembly behavior.

In contrast to 2, porphyrin 6 does not include a polar

On-Surface (2D) Self-Assembly: STM Investigations

Among the six porphyrins 1–6 synthesized in this study,

only 2 and 6 successfully passed the whole screening test group, which significantly changes the way it organizes on

(Figure 3), and could thus be further used for on-surface the surface. As seen in Figure 5 (d) (tunneling parameters

studies. At this point, it is important to point out that por- 0.20 V/10 pA), it forms close-packed assemblies with alter-

phyrin 6 has no net dipole moment due to its trans-A₂B₂nating conformational enantiomers (α and β) held together

substitution pattern with two 3,5-di(tert-butyl)phenyl and by van der Waals interactions between the tert-butyl groups,

two phenyl substituents. In contrast, the DMA moiety of and C–H···π interactions between the tert-butyl and the

trans-A₂BC-porphyrin 2 induces a permanent electric di- phenyl groups, similarly to the trimers of porphyrin 2. This

pole moment across the molecule (calculated to be 3.72 De- behavior further confirms that the DMA tail of 2 is block-

bye; see Section 5 in the Supporting Information).

ing its self-assembly into larger oligomers or extended is-

Detailed STM studies were performed at 5 K on Au(111) lands. In neither case (2 or 6) is the herringbone reconstruc-

with a coverage of ca. 0.2 ML of 2. Individual molecules tion, which is characteristic for Au(111),^[40]influenced. As

can be identified (Figure 5, a; tunneling parameters 1.00 V/ is often the case for heteroepitaxy on Au(111), the single

10 pA) by their characteristic signature, which manifests molecules of both porphyrins, the trimers and the higher

itself in the conformation-dependent STM contrast of the order islands of porphyrin 2 are adsorbed preferentially in

3,5-di(tert-butyl)phenyl and DMA groups in conjunction the elbows of the reconstruction on the electronically poor

with the contrast of the porphyrin macrocycle. The charac- fcc region. Furthermore, the edges of the self-assembled

teristic contrast of different subunits appears even more close-packed domain created from porphyrin 6 are straight

clearly in the STM image of porphyrin 6 (Figure 5, c; tun- (see Figure S1 in the Supporting Information), and the do-

neling parameters 0.05 V/5 pA). The different levels of main boundary is always located at the end of the fcc (face-

STM contrast observed for the 3,5-di(tert-butyl)phenyl centred cubic) region. This means that both porphyrins ad-

groups depend on the dihedral angle between this group sorb in a site-selective manner. Moreover, the facts that

and the porphyrin core, and they can be used to analyze the (i) the trimer is composed of identical conformers of por-

conformation of the molecule, including its conformational phyrin 2, and (ii) the close-packed islands are made up of

chirality, as established in earlier studies.^[9,10,36,39]If the tor- conformers of opposite chirality sense, arranged in a chess-

sion angles around the bonds interconnecting the porphyrin board pattern, demonstrate the conformational adaptability

and the 3,5-di(tert-butyl)phenyl rings are negative (a viewer of both porphyrins.

looking along an interannular bond can make the ring pla-

Although the polar DMAT tails led to isolated trimers

nes eclipse by anticlockwise rotation of the proximal ring on the Au(111) surface, their favorable antiparallel dipolar

through less than 90°), the conformational enantiomer is alignment enforced a close-packed arrangement of the mol-

termed “α”; if the torsion angles are positive, the enantio- ecules in the crystal (Figure 2, c). This shows that the as-

mer is termed “β”.^[36a]Although conformers of both chiral- sembly in 3D, as seen in the crystal structure, does not nec-

ity senses, “α” and “β”, can be observed for porphyrin 2 on essarily predict the 2D assembly on metal surfaces, which

5714

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Eur. J. Org. Chem. 2014, 5705–5719

Synthesis of trans-A₂B₂- and trans-A₂BC-Porphyrins

erature procedures.^[24]The carbon-atom numbering is defined in

the structures of compounds 1–6, 18, and 19 shown in the NMR

section of the Supporting Information (Figures S28, S33, S38, S40,

S42, S44, S59, and S61, respectively). The matrix used for MALDI

MS was trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]-

malononitrile (DCTB).

is also influenced by the intrinsic properties of the surface

and its interaction with the molecular modules.

Conclusions

A series of meso-substituted trans-A₂B₂and trans-A₂BC

poprhyrins 1–6 was synthesized in order to systematically

study the role of the polar DMAT substituent in the 3D

self-assembly of crystals and in the 2D supramolecular

architectures formed on metal surfaces. The key step in

their preparation was the Suzuki–Miyaura cross-coupling

of a porphyrin-derived diboronate with the appropriate aryl

halides in the presence of [Pd(PPh₃)₄]. Interestingly, por-

phyrins 2 and 4 were unexpectedly formed by transfer of a

phenyl group from the PPh₃ligand of the palladium cata-

lyst to the metal and subsequent coupling to the porphyrin

core.

The molecular structures and crystal packings of por-

phyrins 1, 2, 4, 6, and 19 were determined by X-ray crystal-

lography. In all cases, the porphyrin core was found to be

almost planar, and the zinc(II) ion preferred to be tetra-,

penta-, or hexacoordinate with cocrystallized solvent mol-

ecules acting as axial ligands. The intermolecular interac-

tions in crystalline porphyrin 2 – which shows a permanent

dipole moment of 3.72 Debye, mainly originating from the

DMAT substituent – are dominated by antiparallel dipole–

dipole interactions.

A general protocol for testing the suitability of new com-

pounds for deposition by sublimation under UHV condi-

tions was developed and applied to porphyrins 1–6. In par-

ticular, it became apparent that porphyrins having 4-ethyn-

ylphenyl groups with terminal acetylenes at the meso posi-

tions decompose upon prolonged heating. Also, the deposi-

tion by UHV sublimation of porphyrin 1, which has the

highest molecular mass (1186.56 Da) in the series, was

problematic. These results underline the importance of hav-

ing a convenient preselection procedure to identify worth-

while candidate molecules, thus avoiding time-consuming

acquisition of inconclusive SPM data by imaging structures

that have been modified during the deposition. We are con-

vinced that this efficient screening protocol with its decent

throughput will stimulate the development of molecular

architectures suitable for SPM investigations.

STM studies of porphyrins 2 and 6 – which successfully

passed the screening protocol – on Au(111) showed that the

polar DMAT tail blocks the 2D self-organization into

higher order oligomers or extended islands at low coverage.

This contrasts with the 3D self-organization in a single

crystal, where the antiparallel arrangement of the polar tails

is the governing force of ordering. Moreover, the investiga-

tion of both porphyrins provided a deeper insight into the

on-surface conformational adaptation of molecules and

their site-selective adsorption.

[5,15-Bis[3,5-di(tert-butyl)phenyl]-10,20-bis(4-{2-[4-(dimethyl-

amino)phenyl]ethynyl}phenyl)porphyrinato(2–)-κN²¹,κN²²,κN²³,κN²⁴]-

zinc(II) (1) and [5,15-Bis[3,5-di(tert-butyl)phenyl]-10-(4-{2-[4-(di-

methylamino)phenyl]ethynyl}phenyl)-20-phenylporphyrinato(2–)-

κN²¹,κN²²,κN²³,κN²⁴]zinc(II) (2): A round-bottomed flask

(100 mL) was evacuated and purged with N₂(3ϫ). Porphyrin 7

(500 mg, 0.50 mmol), [Pd(PPh₃)₄] (58 mg, 0.05 mmol), and Cs₂CO₃

(2.12 g, 6.51 mmol) were added. The flask was carefully evacuated

and purged with N₂(3ϫ). Bromide 8 (374 mg, 1.25 mmol), toluene

(50 mL), and H₂O (200 μL) were added. The mixture was degassed

by three freeze–pump–thaw cycles and put under N₂. It was heated

to 100 °C for 16 h, and cooled to 25 °C. SiO₂(ca. 15 g) was added,

and the solvent was evaporated. Flash chromatography in the dark

(3ϫ SiO₂; n-pentaneǞCH₂Cl₂, 1% v/v Et₃N), followed by GPC

(2ϫ Jaigel-2H and Jaigel-2.5 H, CHCl₃), gave 1 (208 mg, 35%) and

2 (99 mg, 19%) as purple solids.

Data for porphyrin 1: R_f= 0.06 (SiO₂; n-pentane/CH₂Cl₂, 2:1),

m.p. Ͼ 300 °C. ¹H NMR (600 MHz, CDCl₃): δ = 1.53 (s, 36 H,

tBu), 3.02 (s, 12 H, Me), 6.73 (d, J = 8.4 Hz, 4 H, 8-H), 7.56 (d, J

= 9.0 Hz, 4 H, 7-H), 7.80 (t, J = 1.8 Hz, 2 H, 4-H), 7.89 (d, J =

7.8 Hz, 4 H, 6-H), 8.09 (d, J = 1.8 Hz, 4 H, 3-H), 8.19 (d, J =

7.8 Hz, 4 H, 5-H), 8.97 (d, J = 4.8 Hz, 4 H, 1-H or 2-H), 9.01 (d,

J = 4.8 Hz, 4 H, 1-H or 2-H) ppm. ¹³C NMR (150 MHz, CDCl₃):

δ = 31.89, 35.19, 40.43, 87.65, 91.77, 110.29, 112.10, 120.53, 120.99,

122.80, 123.44, 129.61, 129.83, 131.82, 132.57, 133.00, 134.45,

141.83, 142.36, 148.70, 150.04, 150.32, 150.64 ppm. IR (neat): ν =

˜

2959 (m), 2901 (m), 2866 (m), 2796 (m), 2210 (m), 1807 (w), 1727

(w), 1697 (w), 1609 (m), 1592 (m), 1519 (m), 1491 (m), 1475 (m),

1442 (m), 1423 (w), 1391 (w), 1361 (m), 1337 (m), 1286 (m), 1246

(m), 1219 (m), 1196 (m), 1133 (m), 1101 (w), 1069 (m), 996 (s), 928

(m), 899 (w), 886 (w), 875 (w), 860 (m), 814 (s), 795 (s), 752 (w),

735 (w), 716 (s), 665 (w), 645(w) cm^–1. UV/Vis (CHCl₃): λ_max

(ε, m^–1cm^–1) = 425 (680300), 550 (34535), 589(11500) nm. HRMS

(MALDI, DCTB): m/z (%) = 1197.5604 (66), 1192.5596 (13),

1191.5571 (30), 1190.5555 (44), 1189.5577 (47), 1188.5574 (57),

1186.5570 (100); calcd. for C₈₀H₇₈N₆⁶⁴Zn [M]⁺1186.5574.

Data for porphyrin 2: R_f= 0.06 (SiO₂; n-pentane/CH₂Cl₂, 2:1),

m.p. Ͼ 300 °C. ¹H NMR (600 MHz, CDCl₃): δ = 1.53 (s, 36 H,

tBu), 2.99 (s, 6 H, Me), 6.71 (d, J = 8.4 Hz, 2 H, 10-H), 7.55 (d, J

= 8.4 Hz, 2 H, 9-H), 7.73–7.77 (m, 3 H, 12-H and 13-H), 7.80 (br.

s, 2 H, 6-H), 7.89 (d, J = 7.8 Hz, 2 H, 8-H), 8.10 (br. d, J = 1.2 Hz,

4 H, 5-H), 8.20 (d, J = 7.8 Hz, 2 H, 7-H), 8.23 (d, J = 6.6 Hz, 2

H, 11-H), 8.95 (d, J = 4.2 Hz, 2 H, 1-H, 2-H, 3-H or 4-H), 8.98

(d, J = 4.2 Hz, 2 H, 1-H, 2-H, 3-H or 4-H), 9.00 (d, J = 4.2 Hz, 2

H, 1-H, 2-H, 3-H or 4-H), 9.02 (d, J = 4.2 Hz, 2 H, 1-H, 2-H, 3-

H or 4-H) ppm. ¹³C NMR (150 MHz, CDCl₃): δ = 31.90, 35.19,

40.42, 87.65, 91.77, 110.32, 112.10, 120.45, 120.95, 121.16, 122.75,

123.44, 126.64, 127.58, 129.62, 129.88, 131.79, 131.98, 132.43,

132.56, 132.98, 134.46, 141.87, 142.38, 143.08, 148.70, 150.04,

150.26, 150.31, 150.61, 150.65 ppm. IR (neat): ν = 2958 (m), 2923

˜

(m), 2854 (m), 2209 (w), 2158 (w), 1809 (w), 1700 (w), 1611 (w),

1591 (m), 1521 (m), 1487 (w), 1474 (m), 1459 (m), 1452 (m), 1425

(w), 1392 (w), 1361 (m), 1337 (m), 1287 (m), 1247 (m), 1214 (m),

1202 (m), 1154 (w), 1133 (m), 1102 (w), 1068 (m), 999 (s), 929 (m),

900 (m), 886 (w), 875 (w), 860 (w), 821 (m), 812 (m), 796 (s), 752 (s),

715 (s), 701 (s), 665 (m) cm^–1. UV/Vis (CHCl₃): λ_max(ε, m^–1cm^–1) =

Experimental Section

General Remarks: The procedures for the synthesis of 1–6, 18, and

19 are described below. Bromide 17 was prepared according to lit-

Eur. J. Org. Chem. 2014, 5705–5719

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5715

T. A. Jung, F. Diederich et al.

FULL PAPER

423 (702400), 549 (31800), 589 (9200) nm. HRMS (MALDI,

(w), 660 (m), 635 (s) cm^–1. UV/Vis (CHCl₃): λ_max(ε, m^–1cm^–1) =

DCTB): m/z (%) = 1049.4852 (15), 1048.4827 (41), 1047.4808 (66),

421 (383400), 548 (26800), 586 (15500) nm. HRMS (MALDI,

1046.4832 (69), 1045.4818 (82), 1044.4866 (100), 1043.4833 (79); DCTB): m/z (%) = 930.4134 (9), 928.4051 (32), 927.4103 (43),

calcd. for C₇₀H₆₉N₅⁶⁴Zn [M]⁺1043.4844.

926.4064 (51), 924.4102 (100); calcd. for C₆₂H₆₀N₄⁶⁴Zn [M]⁺

924.4104.

{5,15-Bis[3,5-di(tert-butyl)phenyl]-10,20-bis(4-ethynylphenyl)por-

phyrinato(2–)-κN²¹,κN²²,κN²³,κN²⁴}zinc(II) (3):^[26,41]A round-bot-

tomed flask (50 mL) equipped with a magnetic stirrer and a sep-

tum, was charged with 18 (300 mg, 0.24 mmol) and THF (30 mL).

nBu₄NF (1 m in THF; 2.4 mL, 0.48 mmol) was added dropwise.

The mixture was stirred for 1 h at 25 °C (after which time TLC

showed complete conversion), and then it was poured into a mix-

ture of EtOAc and satd. aq. NH₄Cl (2:1; 80 mL). The phases were

separated, and the aqueous phase was extracted with EtOAc (3ϫ

80 mL). The combined organic phases were dried (Na₂SO₄) and

filtered, and the solvents were evaporated. Flash chromatography

[5,15-Bis[3,5-di(tert-butyl)phenyl]-10-(4-{2-[4-(dimethylamino)-

phenyl]ethynyl}phenyl)-20-(4-ethynylphenyl)porphyrinato(2–)-κN²¹

,

κN²²,κN²³,κN²⁴]zinc(II) (5): A round-bottomed flask (100 mL) was

evacuated and purged with N₂(3 ϫ). Porphyrin 3 (100 mg,

0.10 mmol), [Pd₂(dba)₂] (14 mg, 0.015 mmol), P(o-Tol)₃(37 mg,

0.12 mmol), and 4-bromo-N,N-dimethylaniline (21 mg, 0.10 mmol)

were added. The flask was carefully evacuated and purged with N₂

(3ϫ). Toluene (33 mL) and dry Et₃N (7 mL) were added. The mix-

ture was degassed by three freeze–pump–thaw cycles and put under

N₂. It was warmed to 25 °C, heated to 40 °C for 16 h, and cooled

in the dark (SiO₂; n-pentane/CH₂Cl₂, 2:1ǞCH₂Cl₂, 1% v/v Et₃N) to 25 °C. SiO₂(ca. 5 g) was added, and the solvents were evapo-

gave 3 (155 mg, 68%) as a purple solid. R_f= 0.50 (SiO₂; n-pentane/

rated. Flash chromatography in the dark (SiO₂; n-pent-

CH₂Cl₂, 2:1), m.p. Ͼ 300 °C. ¹H NMR (400 MHz, CDCl₃): δ = aneǞCH₂Cl₂, 1% v/v Et₃N), followed by GPC (2ϫ Jaigel-2H and

1.54 (s, 36 H, tBu), 3.31 (s, 2 H, 7-H), 7.82 (t, J = 2.0 Hz, 2 H, 4- Jaigel-2.5 H, CHCl₃), gave 5 (12 mg, 11%) as a purple solid. R_f=

1

H), 7.89 (d, J = 8.0 Hz, 4 H, 6-H), 8.10 (d, J = 2.0 Hz, 4 H, 3-H),

8.21 (d, J = 8.0 Hz, 4 H, 5-H), 8.94 (d, J = 4.8 Hz, 4 H, 1-H), 9.03

(d, J = 4.8 Hz, 4 H, 2-H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ =

31.91, 35.21, 78.22, 83.96, 120.14, 121.06, 121.48, 122.99, 129.91,

0.80 (SiO₂; n-pentane/CH₂Cl₂, 1:1), m.p. Ͼ 300 °C (decomp.). H

NMR (600 MHz, CDCl₃): δ = 1.54 (s, 36 H, tBu), 2.96 (s, 6 H,

Me), 3.31 (s, 1 H, 13-H), 6.68 (d, J = 9.0 Hz, 2 H, 10-H), 7.54 (d,

J = 9.0 Hz, 2 H, 9-H), 7.81 (t, J = 1.8 Hz, 2 H, 6-H), 7.89 (d, J =

130.51, 131.76, 132.73, 134.41, 141.75, 143.78, 148.79, 149.91, 8.4 Hz, 4 H, 8-H), 8.09 (d, J = 1.8 Hz, 4 H, 5-H), 8.20 (d, J =

150.74 ppm. IR (neat): ν = 3268 (m), 3050 (w), 2952 (m), 2924 (m),

2904 (m), 2860 (m), 2105 (w), 1800 (w), 1590 (m), 1523 (w), 1494

8.4 Hz, 2 H, 7-H or 11-H), 8.21 (d, J = 8.4 Hz, 2 H, 7-H or 11-H),

8.92 (d, J = 4.8 Hz, 2 H, 1-H), 8.98 (d, J = 4.8 Hz, 2 H, 2-H), 9.01

˜

(m), 1473 (m), 1463 (w), 1425 (w), 1391 (w), 1361 (m), 1336 (m), (d, J = 3.0 Hz, 2 H, 3-H or 4-H), 9.02 (d, J = 3.0 Hz, 2 H, 3-H or

1289 (w), 1261 (m), 1246 (m), 1219 (w), 1204 (m), 1178 (w), 1105

(w), 1070 (m), 1053 (w), 1022 (w), 998 (s), 958 (w), 931 (m), 899

4-H) ppm. ¹³C NMR (150 MHz, CDCl₃): δ = 31.91, 35.21, 40.40,

78.17, 83.99, 87.65, 91.83, 110.33, 112.11, 119.96, 120.71, 121.03,

(w), 888 (w), 875 (w), 857 (m), 816 (s), 793 (s), 711 (s), 699 (m), 121.42, 122.89, 123.50, 129.63, 129.88, 130.49, 131.65, 131.92,

678 (m), 656 (w), 638 (m), 626 (m) cm^–1. UV/Vis (CHCl₃): λ_max132.66, 132.98, 134.42, 134.48, 141.82, 142.32, 143.86, 148.75,

(ε, m^–1cm^–1) = 422 (510800), 549 (36400), 587 (21600) nm. HRMS

(MALDI, DCTB): m/z (%) = 953.4095 (27), 951.4102 (49),

950.4067 (52), 949.4139 (71), 948.4102 (100); calcd. for

C₆₄H₆₀N₄⁶⁴Zn [M]⁺948.4104.

149.85, 150.11, 150.33, 150.66, 150.72 ppm. IR (neat): ν = 3297

˜

(w), 2951 (s), 2923 (s), 2854 (m), 2616 (w), 2494 (w), 2211 (w), 1809

(w), 1741 (w), 1696 (w), 1611 (w), 1591 (m), 1516 (m), 1491 (w),

1474 (m), 1461 (m), 1451 (m), 1425 (w), 1391 (w), 1361 (m), 1337

(m), 1287 (m), 1247 (m), 1216 (m), 1204 (m), 1178 (w), 1154 (w),

1133 (m), 1101 (w), 1069 (m), 1051 (w), 1036 (w), 1022 (w), 997

(s), 928 (m), 901 (m), 885 (w), 875 (w), 860 (m), 822 (s), 809 (s),

796 (s), 752 (s), 733 (s), 715 (s), 666 (m), 650 (m), 621 (m) cm^–1.

UV/Vis (CHCl₃): λ_max(ε, m^–1cm^–1) = 423 (175100), 549 (13000),

589 (7500) nm. HRMS (MALDI, DCTB): m/z (%) = 1073.4860

(11), 1071.4790 (37), 1070.4828 (50), 1069.4785 (50), 1067.4836

(100); calcd. for C₇₂H₆₉N₅⁶⁴Zn [M]⁺1067.4839.

{5,15-Bis[3,5-di(tert-butyl)phenyl]-10-(4-ethynylphenyl)-20-phenyl-

porphyrinato(2–)-κN²¹,κN²²,κN²³,κN²⁴}zinc(II) (4): A round-bot-

tomed flask (50 mL) equipped with a magnetic stirrer and a sep-

tum, was charged with 19 (124 mg, 0.12 mmol) and THF (15 mL).

nBu₄NF (1 m in THF; 580 μL, 0.12 mmol) was added dropwise.

The mixture was stirred for 1 h at 25 °C (after which time TLC

showed complete conversion), and then it was poured into a mix-

ture of EtOAc and satd. aq. NH₄Cl (2:1; 50 mL). The phases were

separated, and the aqueous phase was extracted with EtOAc (3ϫ

50 mL). The combined organic phases were dried (Na₂SO₄) and

filtered, and the solvents were evaporated. Flash chromatography

in the dark (SiO₂; n-pentane/CH₂Cl₂, 2:1, 1% v/v Et₃N) gave 4

(87 mg, 79%) as a purple solid. R_f= 0.59 (SiO₂; n-pentane/CH₂Cl₂,

{5,15-Bis[3,5-di(tert-butyl)phenyl]-10,20-diphenylporphyrinato(2–)-

κN²¹,κN²²,κN²³,κN²⁴}zinc(II) (6): A round-bottomed flask (50 mL)

was evacuated and purged with N₂(3ϫ). Porphyrin 7 (195 mg,

0.2 mmol), [Pd(PPh₃)₄] (23 mg, 0.02 mmol), and Cs₂CO₃(828 mg,

2.54 mmol) were added. The flask was carefully evacuated and

purged with N₂(3ϫ). Iodobenzene (245 μL, 2.19 mmol), toluene

(20 mL), and H₂O (200 μL) were added. The mixture was degassed

by three freeze–pump–thaw cycles and put under N₂. It was

warmed up to 25 °C, heated to 100 °C for 20 h, and cooled to

25 °C. SiO₂(ca. 10 g) was added, and the solvents were evaporated.

Flash chromatography in the dark (SiO₂; n-hexane Ǟn-hexane/

CH₂Cl₂, 2:1, 1% v/v Et₃N), followed by GPC (2ϫ Jaigel-2H and

1

2:1), m.p. Ͼ 300 °C. H NMR (400 MHz, CDCl₃): δ = 1.55 (s, 36

H, tBu), 3.31 (s, 1 H, 9-H), 7.76 (m, 3 H, 11-H and 12-H), 7.82

(br. s, 2 H, 6-H), 7.90 (d, J = 8.0 Hz, 2 H, 8-H), 8.11 (br. s, 4 H,

5-H), 8.22 (d, J = 8.8 Hz, 2 H, 7-H), 8.24 (br. d, J = 8.0 Hz, 2 H,

10-H), 8.95 (d, J = 4.8 Hz, 2 H, 1-H), 8.97 (d, J = 4.8 Hz, 2 H, 4-

H), 9.02 (d, J = 4.8 Hz, 2 H, 3-H), 9.04 (d, J = 4.8 Hz, 2 H, 2-H)

ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 31.92, 35.22, 78.18, 83.99,

119.89, 121.01, 121.35, 121.44, 122.86, 126.68, 127.64, 129.94, Jaigel-2.5 H, CHCl₃), gave 6 (154 mg, 88%) as a purple solid. R_f

130.50, 131.64, 132.10, 132.51, 132.66, 134.43, 134.48, 141.85, = 0.68 (SiO₂; n-pentane/CH₂Cl₂, 2:1), m.p. Ͼ 300 °C. ¹H NMR

143.05, 143.88, 148.76, 149.87, 150.34, 150.65, 150.75 ppm. IR

(400 MHz, CDCl₃): δ = 1.53 (s, 36 H, tBu), 7.71–7.77 (m, 6 H, 6-

(neat): ν = 3319 (w), 3058 (w), 2962 (s), 2864 (m), 2108 (w), 1801 H and 7-H), 7.80 (t, J = 1.8 Hz, 2 H, 4-H), 8.10 (d, J = 2.0 Hz, 4

˜

(w), 1590 (s), 1523 (w), 1490 (w), 1476 (m), 1463 (w), 1443 (w),

1391 (w), 1361 (m), 1337 (m), 1288 (w), 1261 (w), 1246 (m), 1204

(m), 1178 (w), 1051 (w), 1000 (s), 932 (m), 899 (m), 887 (m), 875

H, 3-H), 8.23 [dd, J = 2.0, 7.6 Hz, 4 H, 5-H), 8.94 (d, J = 4.8 Hz,

4 H, 1-H or 2-H), 8.99 (d, J = 4.8 Hz, 4 H, 1-H or 2-H) ppm. ¹³C

NMR (100 MHz, CDCl₃): δ = 31.90, 35.20, 120.87, 120.98, 122.61,

(m), 860 (m), 817 (s), 793 (s), 757 (s), 745 (m), 736 (m), 712 (s), 677 126.61, 127.53, 129.96, 131.90, 132.37, 134.49, 142.00, 143.22,

5716

www.eurjoc.org

Eur. J. Org. Chem. 2014, 5705–5719

Synthesis of trans-A₂B₂- and trans-A₂BC-Porphyrins

148.65, 150.23, 150.60 ppm. IR (neat): ν = 2958 (m), 2924 (m), 668 (s) cm^–1. UV/Vis (CHCl₃): λ_max(ε, m^–1cm^–1) = 422 (1389000),

˜

2859 (m), 1811 (w), 1590 (m), 1524 (w), 1488 (m), 1474 (m), 1461

549 (52000), 587 (52100) nm. HRMS (MALDI, DCTB): m/z (%)

(m), 1441 (m), 1425 (m), 1392 (m), 1361 (m), 1338 (m), 1287 (m), = 1084.5422 (44), 1083.5444 (56), 1082.5404 (67), 1080.5446 (100);

1259 (m), 1246 (m), 1217 (m), 1206 (m), 1174 (m), 1158 (m), 1069 calcd. for C₇₁H₈₀N₄Si⁶⁴Zn [M]⁺1080.5438.

(m), 1001 (s), 972 (m), 933 (m), 917 (m), 898 (m), 885 (m), 875 (m),

CCDC-982227 (for 1·2THF), -982228 (for 2·MeOH), -982229 (for

824 (m), 798 (s), 754 (s), 729 (m), 715 (s), 701 (s), 665 (m), 622 (m)

4·MeOH), -982230 (for 6) and -982231 (for 19·MeOH) contain the

cm^–1. UV/Vis (CHCl₃): λ_max(ε, m^–1cm^–1) = 421 (686900), 548

crystallographic data for this paper and can be obtained free of

(25600), 587 (5200) nm. HRMS (MALDI, DCTB): m/z (%) =

charge from the Cambridge Crystallographic Data Centre via

906.4130 (6), 905.4091 (22), 904.4048 (32), 903.4102 (41), 902.4065

www.ccdc.cam.ac.uk/data_request/cif.

(54), 901.4138 (64), 900.4102 (100); calcd. for C₆₀H₆₀N₄⁶⁴Zn [M]⁺

900.4104.

Supporting Information (see footnote on the first page of this arti-

cle): General considerations, methods, and materials; synthesis and

(5,15-Bis[3,5-di(tert-butyl)phenyl]-10,20-bis{4-[(triisopropylsilyl)-

characterization of compounds 7, 8, 14, and 16; [2+2] cycload-

ethynyl]phenyl}porphyrinato(2–)-κN²¹,κN²²,κN²³,κN²⁴)zinc(II) (18)

dition–retroelectrocyclization (CA–RE) reaction of 2 with bis(di-

and (5,15-Bis[3,5-di(tert-butyl)phenyl]-10-{4-[(triisopropylsilyl)-

cyanovinyl) arene 15; concentration-dependent ¹H NMR spec-

ethynyl]phenyl}-20-phenylporphyrinato(2–)-κN²¹,κN²²,κN²³,κN²⁴)-

troscopy studies. DFT-optimized structures of 4-(dimethylamino)-

zinc(II) (19): A round-bottomed flask (100 mL) was evacuated and

tolane, 2, and 6; TGA analysis of 1–4 and 6; copies of UV/Vis

purged with N₂(3ϫ). Porphyrin 7 (500 mg, 0.50 mmol), [Pd(PPh₃)₄]

spectra, and 1D or 2D NMR spectra of all new compounds.

(58 mg, 0.05 mmol), and Cs₂CO₃(21 mg, 6.50 mmol) were added.

The flask was carefully evacuated and purged with N₂(3ϫ). Sub-

sequently, bromide 17 (0.42 g, 1.25 mmol), toluene (50 mL), and

H₂O (500 μL) were added. The mixture was degassed by three

Acknowledgments

freeze–pump–thaw cycles and put under N₂. It was warmed up to

25 °C, heated to 110 °C for 20 h, and cooled to 25 °C. SiO₂(ca.

tion (SNF) (grant number 206021-113149). The research of

10 g) was added, and the solvents were evaporated. Flash

chromatography in the dark (2ϫ SiO₂; n-pentaneǞn-pentane/

CH₂Cl₂, 2:1, 1% v/v Et₃N), followed by GPC (2ϫ Jaigel-2H and

This work was supported by the Swiss National Science Founda-

M. N. A. was carried out within the framework of the action “Sup-

porting Postdoctoral Researchers” of the operational program

“Education and Lifelong Learning” (action’s beneficiary: General

Jaigel-2.5 H, CHCl₃) gave 18 (435 mg, 69%) and 19 (113 mg, 21%)

Secretariat for Research and Technology), and was co-financed by

as purple solids.

the European Social Fund (ESF) and the Greek State. S. N. and

Data for porphyrin 18: R_f= 0.83 (SiO₂; n-pentane/CH₂Cl₂, 2:1),

m.p. Ͼ 300 °C. ¹H NMR (400 MHz, CDCl₃): δ = 1.27 (s, 42 H,

iPr), 1.55 (s, 36 H, tBu), 7.82 (br. s, 2 H, 4-H), 7.89 (d, J = 7.6 Hz,

4 H, 6-H), 8.10 (br. s, 4 H, 3-H), 8.19 (d, J = 7.6 Hz, 4 H, 5-H),

8.90 (d, J = 4.6 Hz, 4 H, 1-H), 9.01 (d, J = 4.6 Hz, 4 H, 2-H) ppm.

¹³C NMR (100 MHz, CDCl₃): δ = 11.63, 18.97, 31.92, 35.22, 91.72,

107.40, 120.30, 120.99, 122.81, 122.88, 129.97, 130.41, 131.76,

132.62, 134.38, 141.87, 143.36, 148.74, 149.95, 150.69 ppm. IR

T. A. J. acknowledge the SNF (grant numbers 200020-137917,

200020-149713, and 206021-121461). J. N. and T. A. J. gratefully

acknowledge the PhD School of the Swiss Nanoscience Institute

(SNI) and the SNF in addition to funding from the hosting insti-

tutions. The authors thank Dr. Christain Wäckerlin, Dr. Susanne

Martens, Dr. Toni Ivas, Mr. Haraldur Gardarsson, Dr. Pablo Ri-

vera Fuentes, Mr. Oliver Dumele, Dr. Nils Trapp, and Dr. Ori Gid-

ron for fruitful discussions. Mr. Marco Martina and Mr. Rolf

Schelldorfer are acknowledged for technical support during UHV

studies. The authors are also grateful to Mr. Boris Tchitchanov for

providing compound 15, and to Mr. Thomas Schweizer for per-

forming the TGA measurements.

(neat): ν = 2945 (m), 2863 (m), 2154 (m), 1811 (w), 1698 (w), 1591

˜

(m), 1525 (w), 1491 (m), 1461 (m), 1425 (w), 1392 (w), 1362 (m),

1338 (m), 1288 (w), 1246 (m), 1219 (m), 1205 (m), 1176 (m), 1101

(w), 1071 (m), 998 (s), 952 (w), 929 (m), 903 (w), 883 (m), 859 (m),

825 (s), 808 (s), 798 (s), 767 (w), 733 (m), 719 (m), 674 (s), 644 (m),

618 (m) cm^–1. UV/Vis (CHCl₃): λ_max(ε, m^–1cm^–1) = 423 (518700),

549 (20600), 588 (5600) nm. HRMS (MALDI, DCTB): m/z (%) =

1262.6778 (71), 1261.6821 (89), 1260.6796 (100); calcd. for

C₈₂H₁₀₀N₄Si₂⁶⁴Zn [M]⁺1260.6773.

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= 1.8 Hz, 2 H, 6-H), 7.88 (d, J = 8.4 Hz, 2 H, 8-H), 8.10 (d, J =

1.8 Hz, 4 H, 5-H), 8.19 (d, J = 7.8 Hz, 2 H, 7-H), 8.23 (br. d, J =

6.6 Hz, 2 H, 9-H), 8.94 (d, J = 4.8 Hz, 2 H, 1-H, 2-H, 3-H or 4-

H), 8.95 (d, J = 4.8 Hz, 2 H, 1-H, 2-H, 3-H or 4-H), 9.00 (d, J =

4.8 Hz, 2 H, 1-H, 2-H, 3-H or 4-H), 9.01 (d, J = 4.6 Hz, 2 H, 1-H,

2-H, 3-H or 4-H) ppm. ¹³C NMR (150 MHz, CDCl₃): δ = 11.62,

18.96, 31.90, 35.21, 91.70, 107.40, 120.13, 120.95, 121.24, 122.79,

126.65, 127.60, 129.95, 130.41, 131.70, 132.03, 132.46, 132.58,

134.36, 134.48, 141.88, 143.09, 143.36, 148.72, 149.92, 150.28,

150.61, 150.70 ppm. IR (neat): ν = 2957 (s), 2922 (s), 2853 (s), 2155

˜

(w), 1710 (w), 1592 (m), 1524 (w), 1484 (w), 1462 (m), 1393 (w),

1378 (w), 1363 (m), 1337 (w), 1287 (w), 1260 (m), 1247 (w), 1219

(w), 1198 (m), 1186 (w), 1203 (w), 1099 (w), 1071 (m), 996 (s), 928

(m), 899 (w), 883 (m), 856 (w), 822 (m), 795 (s), 733 (w), 717 (m),

Eur. J. Org. Chem. 2014, 5705–5719

www.eurjoc.org

5717

T. A. Jung, F. Diederich et al.

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Received: May 26, 2014

Published Online: July 30, 2014

Eur. J. Org. Chem. 2014, 5705–5719

www.eurjoc.org

5719

Article Doi

Synthesis of trans-A2B2- and trans-A2BC-porphyrins with polar 4′-(dimethylamino)tolan-4-yl substituents, and a screening protocol for vapor-phase deposition on metal surfaces

DOI: 10.1002/ejoc.201402634

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