Tuning of tetrathiafulvalene properties: Versatile synthesis of N-arylated monopyrrolotetrathiafulvalenes via Ullmann-type coupling reactions

Article abstract of DOI:10.3762/bjoc.11.96

An Ullmann-type coupling reaction was employed for the preparation of several N-arylated monopyrrolotetrathiafulvalenes with variable substitution patterns. Spectroscopic and electrochemical properties of the coupling products strongly depend on the electronic nature of the aromatic substituents due to their direct conjugation with the tetrathiafulvalene chromophore. The crystal packing of the arylated monopyrrolotetrathiafulvalenes is primarily defined by networks of C-H...X weak hydrogen bonds and short S...S contacts involving the tetrathiafulvalene moieties.

Full text of DOI:10.3762/bjoc.11.96

Tuning of tetrathiafulvalene properties: versatile synthesis of
N-arylated monopyrrolotetrathiafulvalenes via Ullmann-type
coupling reactions
Vladimir A. Azov*1, Diana Janott1, Dirk Schlüter1 and Matthias Zeller2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 860–868.
1Department of Chemistry, University of Bremen, Leobener Str. NW
2C, D-28359 Bremen, Germany and 2One University Plaza,
Department of Chemistry, Youngstown State University, Youngstown,
OH 44555-3663, USA
doi:10.3762/bjoc.11.96
Received: 17 February 2015
Accepted: 22 April 2015
Published: 21 May 2015
Email:
Vladimir A. Azov* - vazov@uni-bremen.de
This article is part of the Thematic Series "Tetrathiafulvalene chemistry".
Guest Editor: P. J. Skabara
* Corresponding author
Keywords:
© 2015 Azov et al; licensee Beilstein-Institut.
License and terms: see end of document.
cyclic voltammetry; N-arylation; pyrrolotetrathiafulvalene;
Ullmann-type coupling; X-ray crystallography
Abstract
An Ullmann-type coupling reaction was employed for the preparation of several N-arylated monopyrrolotetrathiafulvalenes with
variable substitution patterns. Spectroscopic and electrochemical properties of the coupling products strongly depend on the elec-
tronic nature of the aromatic substituents due to their direct conjugation with the tetrathiafulvalene chromophore. The crystal
packing of the arylated monopyrrolotetrathiafulvalenes is primarily defined by networks of C–H···X weak hydrogen bonds and
short S···S contacts involving the tetrathiafulvalene moieties.
Introduction
For the last four decades tetrathiafulvalenes [1,2] (Figure 1) 1 moiety in macrocycles usually leads to poorly separable
have been the subject of extensive studies due to their mixtures of cis/trans isomers [7-9]. Even if separation is
outstanding electron-donating properties and ability to induce possible, TTFs are prone to cis/trans isomerization, which can
reversible electrochemically-induced switching processes in be induced by light [10] or traces of acid [11]. These problems
molecular and supramolecular systems [3,4]. Availability of are aggravated by the fact that each reversible oxidation–reduc-
selective synthetic methods [5,6] gave access to differently tion cycle of the TTF moiety always leads to formation of
substituted tetrathiafulvalene (TTF) moieties which allowed cis/trans isomer mixtures.
tuning of oxidation potential, donating ability, as well as other
physical and chemical properties. The regioselective functional- Bis-pyrrolotetrathiafulvalenes 2 and monopyrrolotetrathia-
ization of TTF, however, remains problematic due to the pres- fulvalenes (MPTTFs) 3 represent a significant modification of
ence of four identical attachment sites. Incorporation of the TTF the TTF backbone featuring a more extended electron-rich
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Being interested in the preparation of calix[4]arene receptors
with MPTTF moieties directly attached to an aromatic
calixarene backbone, we have chosen the copper-catalysed
N-arylation reaction as a method for coupling of aromatic and
MPTTF moieties with each other and successfully employed it
for the preparation of two bis-MPTTF-calixarene conjugates, as
well as two model low molecular weight aromatic derivatives
4a and 4c [22]. To explore the scope of the reaction, we decided
to conduct a deeper investigation varying reaction conditions
and testing different substituents on the aromatic as well as the
MPTTF components of the reaction. It led to the preparation of
a family of MPTTF aromatic derivatives, whose properties and
crystal structures are discussed below.
Figure 1: Molecular structures of tetrathiafulvalenes 1, bis-pyrrolotetra-
thiafulvalenes 2 and monopyrrolotetrathiafulvalenes 3.
Results and Discussion
Synthesis
π-system with only two or three easily accessible attachment Our initial synthetic efforts were focused on optimizing reac-
points for external substituents, respectively [12,13]. The asym- tion conditions using the PrS-MPTTF derivative 7a [23] and
metric nature of MPTTFs 3 opens the possibility for the intro- bromoanisole 8a as starting materials (Scheme 2), as well as
duction of different R- and R1-groups on the two sides of the comparing two possible copper(I) ligands (Figure 2): trans-
TTF moiety. If R1 = alkyl (or a similar group with an diaminocyclohexane (DACH) 9a, which was employed before
sp3-hybridized carbon atom), such substituents can be readily in N-arylations with pyrrolo-TTFs [17-19], and its Schiff base
attached to the pre-formed MPTTF moiety using a variety derivative 9b, which was reported to be one of the most
of common nucleophilic substitution reactions. In the case effective ligands in similar N-arylation reactions with other
of R1 = aryl, two possible approaches for the preparation substrates [20,21].
of N-arylated MPTTF derivatives 4 have been reported
(Scheme 1). In the first procedure [14-16], the aryl substituent is
incorporated during the initial synthetic steps to form a N-aryl-
1,3-dithiolo[4,5-c]pyrrole-2-thione 5, which is then coupled to
1,3-dithiole-2-thione 6 in hot triethyl or trimethyl phosphite.
Using the second approach [17-19], the aryl group is attached to
the MPTTF moiety using a direct copper-mediated Ullmann-
type N-arylation reaction [20,21]; this method was also used for
Figure 2: Copper(I) ligands 9a and 9b.
the preparation of arylated bis-pyrrolotetrathiafulvalenes 2.
Although being reported in the literature, it has so far not found
wide spread use and was employed only with a narrow scope of Initial experiments gave evidence of much lower efficiency of
aromatic derivatives with electron-donating substituents and ligand 9b in comparison to 9a in terms of conversion and pres-
alkylthio-substituted (R = SAlkyl) MPTTFS.
ence of undesired side-products. Reaction utilizing trans-
Scheme 1: The two synthetic approaches used for the preparation of arylated monopyrrolotetrathiafulvalenes 1.
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Beilstein J. Org. Chem. 2015, 11, 860–868.
Scheme 2: Synthesis of arylated monopyrrolotetrathiafulvalenes 4a–f.
diaminocyclohexane 9aalso allowed for fast optimization and sloping shoulder at ca. 390 nm, which is missing in the non-
reproducibility, which prompted us to focus our attention on the substituted 4c. In contrast, the spectrum of nitro-derivative 4d
CuI/9a catalysis system. Although copper(I) is supposed to play displays an additional strong absorption band centred at
a catalytic role [20,21], smaller CuI loadings led to diminished ca. 425 nm, arising most likely due to charge transfer from the
reaction yields, supposedly due to lower solubility/activity of electron-rich MPTTF moiety to the electron-deficient aromatic
the CuBr formed in the coupling reaction. Taking into account substituent. This absorption manifests itself in the dark red
the affordability of CuI, its excess cannot be considered a disad- colour of 4d, both in solid state as well as in solution.
vantage of the method. Additionally, use of the inexpensive
Cu(I) catalyst allows to avoid Buchwald–Hartwig amination
[24,25], which employs more expensive Pd-based catalysts for a
similar type of C–N coupling reactions.
In a typical procedure, 1 equiv of MPTTF 7a, 7b [12], or 7c
[26], 1.5–1.6 equiv of a brominated aromatic derivative,
0.5 equiv of CuI/9a and 3–4 equiv of K3PO4 were heated at
110–115 °C overnight in absolute dioxane in a Schlenk tube.
The reaction yields amounted to 70–80% for stable MPTTF
derivatives 4a,b,d,e, but were lower for 4c due to sensitivity of
the starting material 7b, as well as for 4f due to its tendency
towards oxidation.
Figure 3: UV–vis spectra of compounds 4a,c–f (CH2Cl2,
c = 4 × 10−5 M).
Thus, the N-arylation reaction can be readily employed with
electron-rich as well as electron-deficient aromatic derivatives,
as well as with thioalkyl-substituted and non-substituted
MPTTFs. The successful reaction with bromophenol 8d to form MPTTF derivatives are readily soluble in non-polar organic
the adduct 4f also confirmed the possibility of the reaction with solvents, such as CH2Cl2, toluene or acetone (with the excep-
hydroxy-substituted aryl derivatives, paving the way for tion of poorly soluble 4c), giving solutions that are stable at
application of this method with non-protected calix[4]arene room temperature in air.
derivatives [27].
Electrochemistry
Compounds 4a–c,e,f (Figure 3) display UV–vis spectra typical Solution oxidation potentials of aromatic MPTTF conjugates
for TTF derivatives with absorption maxima at λmax of were determined using cyclic voltammetry (CV) in CH2Cl2/
ca. 310–330 nm as well as a long tail with very low absorption Bu4NClO4 solution and are summarized in Table 1. The CVs of
spanning to ca. 500 nm and rendering the yellow colour to the all compounds displayed two reversible oxidation waves on the
compounds. Thioalkyl TTF derivatives 4a,b,e,f also show a cathodic scan (Figure 4) characteristic to TTFs [1], the first one
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Beilstein J. Org. Chem. 2015, 11, 860–868.
ously confirm the identity of the compounds and analyse their
structural properties and arrangement in the solid state. Bond
lengths and angles in all structures may be considered normal.
With the exception of alkylS-substituents, the molecular frame-
works of 4a,b,d,e display relatively low deviations from
planarity. Angles between the least-square planes, defined by
the heavy atoms of the aromatic ring and neighbouring pyrrole
ring do not exceed 17.3° (in 4e, see Table 2), ensuring good
conjugation between the MPTTF and aromatic moieties. Boat-
type deviations of the TTF groups (folding along the S–S
vectors in the five-membered rings) are minor for electron-defi-
cient 4d lying below 5°, whereas they are much larger in elec-
tron-rich derivatives, where they reach 20.45° in 4a, 13.58° in
4b and 11.23° in 4e. This observation corresponds well
with previously reported data: the electron-deficient N-Ts
derivative has an almost planar arrangement of the TTF moiety,
whereas N-alkyl derivatives show significant deviation from
planarity [12].
Table 1: Electrochemical data.a
Compound
E1/2ox1 (V)
E1/2ox2 (V)
E1/2ox3 (V)
4a [22]
4b
0.47
0.48
0.40
0.55
0.46
0.47
0.84
0.83
0.84
0.92
0.84
0.86
–
–
4c [22]
4d
–
–
4e
1.06
4f
1.76 (irrev.)
aData were obtained using a one-compartment cell in CH2Cl2/0.1 M
Bu4NClO4, Pt as the working and counter electrodes and a non-
aqueous Ag/Ag+ reference electrode; scan rate 100 mV/s. Values
given at room temperature vs SCE; the Fc/Fc+ couple (0.480 V vs
SCE) was used as an internal reference [30].
Table 2: Angles (°) between the planes of the aromatic ring and neigh-
bouring pyrrole ring in 4a,b,d,e.
Compound
Molecule1
Molecule 2
4a
4b
4d
4e
3.41
3.69
1.36
13.72
3.42
12.20
10.68
17.28
Interestingly, the crystal structures of all four compounds
feature similar packing arrangements with two crystallographi-
cally distinct molecules (Z’ = 2) with quasi-parallel tilted edge-
to face arrangements. In the crystal packing, molecules are
Figure 4: Cyclic voltammograms of compounds 4a,c [22] and 4d–f
(plotted vs SCE; CH2Cl2/0.1 M Bu4NClO4).
leading to the radical cation and the second to the dication. interconnected by multiple non-classical weak intermolecular
Non-substituted derivative 4c shows a lower first oxidation hydrogen bonds [31] and C–H···π and S···S interactions, the
potential than its alkylS-substituted counterpart 4a,b, as latter being common in the crystals of sulfur-rich compounds
expected due to the electron-withdrawing effect of the two such as TTFs [32]. Most of the close contacts involve the
thioalkyl groups [28,29]. The strong electron-withdrawing central parts of neighbouring molecules, thus connecting them
effect of the 4-nitrophenyl group in 4d manifests itself in an with each other and leading to formation of supramolecular
increased oxidation potential with a shift of ca. 0.08 V for both layers. Parallel layers are only loosely bound to each other [33].
oxidation waves. Aromatic electron-donating groups as in 4e This layered arrangement manifests itself in the crystal
and 4f do barely influence the potential of the two oxidation morphology: all Ar-MPTTF derivatives crystallize in the form
waves. Instead, they induce an additional oxidation wave at of thin platelets, with the plane of the parallel molecular layers
higher potentials of 1.06 V and 1.77 V, in 4e and 4f (see Figure coinciding with the direction normal to the largest crystal face
S7, Supporting Information File 1), respectively. For the phenol of the platelets. This observation can be rationalized by
derivative 4f, this oxidation is irreversible.
assuming fast crystal growth within each supramolecular layer,
assisted by the presence of directed weak hydrogen bonds as
well as C–H···π and S···S interactions. Addition of new parallel
Crystal structures
Compounds 4a,b,d,e afforded high quality crystals that could layers, connected to the previous layers via dispersive interac-
be analysed using X-ray crystallography, allowing to unambigu- tions of the van der Waals type, can be assumed to be a much
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Beilstein J. Org. Chem. 2015, 11, 860–868.
slower process, leading to plate like crystals that mimic the lay- Figures 5–8 display some aspects of molecular packing of
ered makeup at the molecular level. Crystals of 4d, for example, MPTTF derivatives 4a,b,d,e.
form plates with an aspect ratio of 10 and above. Such layered
arrangements make these derivatives possible candidates for Conclusion
organic electronics [34] and may serve as a motivation for eval- In summary, several arylated monopyrrolotetrathiafulvalene
uation of their electronic properties in the solid state.
derivatives have been conveniently prepared using a copper-
Figure 5: Crystal packing of 4a viewed along the a axis and showing one layer of molecules. Short S···S contacts are shown as black and S···C
contacts as green dashed lines. Only the major orientation of the disordered propyl chain is shown.
Figure 6: Crystal packing of 4b showing a group of four molecules interconnected by multiple weak hydrogen bonds, C–H···π, and S···S close
contacts.
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Figure 7: Crystal packing of 4d viewed along the b axis. Molecules of 4d form layers parallel to the (001) plane being interconnected with each other
by means of short S···S contacts (green) and C–H···O2N weak hydrogen bonds (blue).
Figure 8: Crystal packing of 4e showing a group of four molecules interconnected by multiple weak hydrogen bonds, C–H···π, and S···S close
contacts.
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Beilstein J. Org. Chem. 2015, 11, 860–868.
mediated Ullmann-type N-arylation reaction. The reaction was λmax (ε): 309 nm (25600 L∙mol−1∙cm−1), 329 (24500); MS (EI)
shown to tolerate substituents of different nature on the m/z (%): 441 (100) [M]+•, 426 (10) [M − Me]+; HRMS (EI)
aromatic group, as well as can be employed with substituted m/z: [M]+• calcd for C17H15NOS6+•, 440.94780; found,
and non-substituted MPTTFs, opening the way for its applica- 440.94675; CV (vs SCE, CH2Cl2): E1/2ox1 = 0.48 V,
tion for the synthesis of more complex molecular systems, such E1/2ox2 = 0.83 V.
as conjugates with non-protected calixarenes. New aromatic-
MPTTF conjugates were characterized using different analyt- 2-[4,5-Bis(propylthio)-1,3-dithiol-2-ylidene]-5-(4-nitro-
ical methods and X-ray crystallography.
phenyl)-5H-1,3-dithiolo[4,5-c]pyrrole (4d). Prepared from 7a
(0.055 g, 0.140 mmol), CuI (0.014 g, 0.070 mmol), K3PO4
(0.089 g, 0.42 mmol), trans-diaminocyclohexane (7.5 µL,
Experimental
Copper-catalyzed N-arylation reaction of monopyrrolo- 0.062 mmol) and 4-bromonitrobenzene (8b, 0.045 g,
tetrathiafulvalenes, general procedure. The reaction was 0.224 mmol) in 2 mL of dry dioxane. The product was purified
performed in a similar manner as described before [22]. A by flash chromatography (CH2Cl2) to afford deep red crystals.
heavy walled Schlenk tube with a wide bore Teflon screw stop- X-ray quality crystals were grown by slow evaporation of
cock was charged with MPTTF derivatives 7a, 7b or 7c, CuI, CDCl3/heptane solution. Yield: 59.7 mg (0.116 mmol, 83%).
K3PO4, (±)-trans-1,2-diaminocyclohexane and an aromatic bro- Mp 240–244 °C; Rf = 0.72 (CH2Cl2); 1H NMR (360 MHz,
mide, then absolute dioxane was added via syringe. The reac- CDCl3) δ 8.33–8.29 (m, 2H), 7.44–7.40 (m, 2H), 6.99 (s, 2H),
tion mixture was degassed by three freeze-pump-thaw cycles, 2.81 (t, 3J = 7.2 Hz, 4H), 1.68 (sext, 3J = 7.2 Hz, 4H), 1.02 (t,
the vessel was filled with nitrogen, tightly sealed and stirred at 3J = 7.2 Hz, 6H); 13C NMR (90 MHz, CDCl3) δ 144.7, 144.2,
110–115 °C. The reaction was complete in 18–24 h (TLC 127.5, 125.8, 125.3, 118.6, 117.3, 113.7, 110.1, 38.2,
control). The solvent was removed under reduced pressure 23.1, 13.2; UV–vis (CH2Cl2) λmax (ε): 325 nm
directly from the Schlenk tube, the residue was dissolved in (25600 L∙mol−1∙cm−1), 426 (11200); MS (EI) m/z (%): 512
CH2Cl2, filtered through a plug of celite, and evaporated to (100) [M]+•, 469 (5) [M − Pr]+∙, 436 (20) [M − HSPr]+•; HRMS
dryness. The crude products were triturated with n-hexane to (EI) m/z: [M]+• calcd for C20H20N2O2S6+•, 511.98435; found,
remove the unreacted aromatic starting material and then puri- 511.98296; CV (vs SCE, CH2Cl2): E1/2ox1 = 0.55 V,
fied by flash chromatography on silica gel to afford pure E1/2ox2 = 0.92 V.
N-arylated MPTTFs.
2-[4,5-Bis(propylthio)-1,3-dithiol-2-ylidene]-5-(4-dimethyl-
Preparation and characterization of 2-[4,5-bis(propylthio)-1,3- aminophenyl)-5H-1,3-dithiolo[4,5-c]pyrrole (4e). Prepared
dithiol-2-ylidene]-5-(4-methoxyphenyl)-5H-1,3-dithiolo[4,5- from 7a (0.050 g, 0.128 mmol), CuI (0.012 g, 0.063 mmol),
c]pyrrole (4a) and 2-[1,3-dithiol-2-ylidene]-5-(4- K3PO4 (0.082 g, 0.386 mmol), trans-diaminocyclohexane
methoxyphenyl)-5H-1,3-dithiolo[4,5-c]pyrrole (4c) was (7.5 µL, 0.062 mmol) and 4-iodo-N,N-dimethylaniline (8c,
described before [22], also including the X-ray data of 4a (see 0.050 g, 0.202 mmol) in 2 mL of dry dioxane. The product was
Supporting Information File 1 and Supporting Information purified by flash chromatography (CH2Cl2/cyclohexane, 1:1) to
File 2). For 4a, corrected oxidation potentials are reported in afford light orange crystals. X-ray quality crystals were grown
Table 1.
by slow evaporation of CDCl3/heptane solution. Yield: 53.8 mg
(0.105 mmol, 82%); Mp 196–199 °C; Rf = 0.5 (CH2Cl2/cyclo-
2-[4,5-Bis(methylthio)-1,3-dithiol-2-ylidene]-5-(4- hexane, 1:1); 1H NMR (360 MHz, CD2Cl2) δ 7.22–7.17 (m,
methoxyphenyl)-5H-1,3-dithiolo[4,5-c]pyrrole (4b). Prepared 2H), 6.81 (s, 2H), 6.76–6.71 (m, 2H), 2.96 (s, 6H), 2.81 (t,
from 7b (0.040 g, 0.119 mmol), CuI (0.023 g, 0.119 mmol), 3J = 7.2 Hz, 4H), 1.66 (sext, 3J = 7.2 Hz, 4H), 1.01 (t,
K3PO4 (0.020 g, 0.954 mmol), trans-diaminocyclohexane 3J = 7.2 Hz, 6H); 13C NMR (90 MHz, CD2Cl2) δ 149.6, 130.7,
(22 µL, 0.179 mmol) and 4-bromoanisole (8a, 0.033 g, 127.9, 122.0, 120.6, 119.8, 113.1, 111.9, 111.1, 40.8, 38.5,
0.179 mmol) in 3 mL of dry dioxane. The product was purified 23.5, 13.2; UV–vis (CH2Cl2) λmax (ε): 314 nm
by flash chromatography (CH2Cl2/cyclohexane, 1:2) to afford (34000 L∙mol−1∙cm−1), 330 (33600); MS (EI) m/z (%): 510
bright yellow crystals. X-ray quality crystals were grown by (100) [M]+•, 434 (20) [M − HSPr]+•; HRMS (EI) m/z: [M]+•
slow evaporation of CDCl3 solution. Yield: 43.2 mg calcd. for C22H26N2S6+•, 510.04147; found, 510.04104; CV (vs
(0.098 mmol, 82%). Mp 198–200 °C; Rf = 0.32 (CH2Cl2/cyclo- SCE, CH2Cl2): E1/2ox1 = 0.46 V, E1/2ox2 = 0.84 V,
hexane, 1:2); 1H NMR (360 MHz, CDCl3) δ 7.25–7.20 (m, 2H), E1/2ox3 = 1.06 V.
6.95–6.91 (m, 2H), 6.79 (s, 2H), 3.83 (s, 3H), 2.43 (s, 6H);
13C NMR (90 MHz, CDCl3) δ 158.0, 134.0, 127.1, 121.9, 2-[4,5-Bis(propylthio)-1,3-dithiol-2-ylidene]-5-(4-hydroxy-
121.2, 120.2, 114.7, 111.4, 111.1, 55.6, 19.2; UV–vis (CH2Cl2) phenyl)-5H-1,3-dithiolo[4,5-c]pyrrole (4f). Prepared from 7a
866

Beilstein J. Org. Chem. 2015, 11, 860–868.
3. Becher, J.; Jeppesen, J. O.; Nielsen, K. Synth. Met. 2003, 133–134,
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(0.082 g, 0.386 mmol), trans-diaminocyclohexane (7.5 µL,
0.062 mmol) and 4-bromophenol (8d, 0.035 g, 0.202 mmol) in
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(0.052 mmol, 42%); Mp 123–125 °C; Rf = 0.27 (CH2Cl2);
1H NMR (200 MHz, CDCl3) δ 7.21–7.14 (m, 2H), 6.91–6.83
(m, 2H), 6.77 (s, 2H), 4.91 (s, 1H), 2.81 (t, 3J = 7.2 Hz, 4H),
1.66 (sext, 3J = 7.2 Hz, 4H), 1.01 (t, 3J = 7.2 Hz, 6H);
13C NMR (50 MHz, CDCl3) δ 153.9, 134.1, 129.7, 127.5,
122.2, 121.4, 119.2, 116.2, 111.4, 38.2, 23.1, 13.2; UV–vis
(CH2Cl2) λmax (ε): 309 nm (26000 L∙mol−1∙cm−1), 326 (25900);
MS (EI) m/z (%): 483 (100) [M]+•, 440 (5) [M − Pr]+∙, 407 (25)
[M −HSPr]+•; HRMS (EI) m/z: [M]+• calcd. for
C20H20N2O2S6+•, 482.99419; found, 482.99474; CV (vs SCE,
CH2Cl2): E1/2ox1 = 0.47 V, E1/2ox2 = 0.86 V, E1/2ox3 = 1.77 V.
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Supporting Information File 2
Zip archive containing X-ray crystallographic data for 4a
(CCDC 987551), 4b (CCDC 1049639), 4d (CCDC
1049638) and 4e (CCDC 1049637).
16.Nygaard, S.; Leung, K. C.-F.; Aprahamian, I.; Ikeda, T.; Saha, S.;
Laursen, B. W.; Kim, S.-Y.; Hansen, S. W.; Stein, P. C.; Flood, A. H.;
Stoddart, J. F.; Jeppesen, J. O. J. Am. Chem. Soc. 2007, 129,
960–970. doi:10.1021/ja0663529
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-11-96-S2.zip]
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Acknowledgements
19.Solano, M. V.; DellaPia, E. A.; Jevric, M.; Schubert, C.; Wang, X.;
van der Pol, C.; Kadziola, A.; Nørgaard, K.; Guldi, D. M.;
Nielsen, M. B.; Jeppesen, J. O. Chem. – Eur. J. 2014, 20, 9918–9929.
doi:10.1002/chem.201402623
We are grateful to Dr. T. Dülcks, Ms. D. Kemken (MS) and Ms.
Ziyan Wang (NMR) for their help with the characterization of
new compounds, and Dr. Arunpatcha Nimthong-Roldán for
collection of X-ray data and structure refinement for compound
4b. The X-ray diffractometers were funded by NSF Grants
0087210 and 1337296, Ohio Board of Regents Grant CAP-491,
and by Youngstown State University.
20.Monnier, F.; Taillefer, M. Angew. Chem., Int. Ed. 2009, 48, 6954–6971.
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Chem. Soc. Rev. 2014, 43, 3525–3550. doi:10.1039/c3cs60289c
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