Benzobisimidazole Cruciform Fluorophores

Article abstract of DOI:10.1021/acs.joc.5b00616

A series of 11 cross-conjugated cruciform fluorophores based on a benzobisimidazole nucleus has been synthesized and characterized. Like in their previously reported benzobisoxazole counterparts, the HOMOs of these new fluorophores are localized along the vertical bisethynylbenzene axes, while their LUMOs remain relatively delocalized across the molecule, except in cruciforms substituted with electron-withdrawing groups along the vertical axis. Benzobisimidazole cruciforms exhibit a pronounced response to deprotonation in their UV/vis absorption and emission spectra, but their response to protonation is significantly attenuated. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.joc.5b00616

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Article
Benzobisimidazole Cruciform Fluorophores
Ha T. M. Le, Nadia S. El-Hamdi, and Ognjen Š. Miljani#
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The Journal of Organic Chemistry
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Benzobisimidazole Cruciform Fluorophores
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Ha T. M. Le, Nadia S. ElꢀHamdi, and Ognjen Š. Miljanić*
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Department of Chemistry, University of Houston, 112 Fleming Building, Houston, TX 77204ꢀ5003
miljanic@uh.edu
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RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
Table of Contents Graphic
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Abstract
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A series of eleven crossꢀconjugated cruciform fluorophores based on a benzobisimidazole nucleus has
been synthesized and characterized. Like in their previously reported benzobisoxazole counterparts, the
HOMOs of these new fluorophores are localized along the vertical bisethynylbenzene axes, while their
LUMOs remain relatively delocalized across the molecule—except in cruciforms substituted with
electronꢀwithdrawing groups along the vertical axis. Benzobisimidazole cruciforms exhibit pronounced
response to deprotonation in their UV/Vis absorption and emission spectra, but their reꢀsponse to
protonation is significantly attenuated.
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INTRODUCTION
Crossꢀconjugated molecular cruciforms1 have been a subject of much scrutiny in recent years as their
modular optoelectronic properties make them versatile elements in sensing and molecular electronics
applications. Nuckolls,2 JeffriesꢀEL,3 and we4 have extensively studied molecular cruciforms based on
the central benzobisoxazole motif, represented by structure 1 (Figure 1). Our group has demonstrated
that benzobisoxazole cruciforms represent a viable fluorescent sensor for a broad variety of analytes,
and that they are capable of detecting minute structural differences between those analytes.3d,3f In a
related effort, we have shown that cruciforms can be "cut in half" and that the oxazole nucleus can be
replaced by an imidazole, to give Lꢀshaped halfꢀcruciforms of the general structure 2.5 Bielawski et al.
have studied fluorophores based on benzobisimidazolium salts represented by general structure 3.6
In this contribution, we present "the missing link" in this series of fluorophores: the Xꢀshaped
benzobisimidazoleꢀbased cruciforms 4a–l (Scheme 1). These new fluorophores are direct analogs of 1 in
which the two oxazole nuclei have been replaced with imidazoles.
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Figure 1. Previous examples of fluorophores based on crossꢀconjugated benzobisoxazole (1) and
benzimidazole (2) geometries, and benzobisimidazole motif (3).
RESULTS AND DISCUSSION
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Synthesis. Despite structural similarity between 1 and 4, their syntheses had to be approached quite
differently. Cruciforms 1 could be easily elaborated from the readily available 2,5ꢀdiaminoꢀ3,6ꢀ
dibromobenzeneꢀ1,4ꢀdiol,7 whose oꢀaminophenol functionalities engaged in acyl condensation to
generate the horizontal axis, while the two bromine substituents reacted in a Sonogashira coupling to
establish the vertical axis. As the corresponding 1,2,4,5ꢀtetraaminoꢀ3,6ꢀdibromobenzene8 proved
difficult to prepare and handle, we chose to begin the synthesis of benzobisimidazole cruciforms with a
hydrochloride salt of 1,2,4,5ꢀtetraaminobenzene (5, Scheme 1).9 Its condensation with benzoic acid in
the presence of polyphosphoric acid (PPA) yielded intermediate 6,3g which was then doubly brominated
on the central benzene ring with Nꢀbromosuccinimide (NBS) to produce compound 7. The vertical axis
was established by the subsequent Sonogashira coupling with a terminal alkyne bearing functionalities
of interest, yielding cruciforms 4a and 4b (route A, highlighted in dark red in Scheme 1). In an
alternative approach (route B, highlighted in blue), the roles of coupling partners were switched:
compound 7 was first reacted with tris(isopropylsilyl)acetylene (TIPSA) and then desilylated to reveal
two terminal alkyne functionalities in precursor 8. Compound 8 was subsequently functionalized
through a second Sonogashira coupling to produce cruciforms 4c–k. Cruciform 4l was obtained after
acidic hydrolysis of the tetrahydropyranyl (THP) protecting group in 4k.
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It should be noted that this synthetic route was plagued with difficulties. First, compound 5 is very
difficult to handle on account of its sensitivity to oxidation and high polarity. Second, analogs of the
highly insoluble intermediate 6 bearing other functional groups along the horizontal axis proved
difficult to prepare and impossible to purify, and their subsequent bromination yielded mixtures of
products. Thus, we were not able to effectively vary the substitution along the horizontal axis of these
cruciforms. Finally, the closing Sonogashira couplings proceeded in moderate yields (27–60%), which
was likely a consequence of the low solubility of the precursors.
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Scheme 1. Synthesis of Benzobisimidazole Cruciforms 4a–l.
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Computational Studies. Frontier molecular orbitals (FMOs) of cruciform 4a–4j and 4l have been
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calculated using Gaussian 09W10 software using the B3LYP hybrid density functional and 3ꢀ21G as the
basis set. FMOs of two exemplary cruciforms 4a and 4f are shown in Figure 2. In both systems, the
highest occupied molecular orbital (HOMO) resides dominantly along the vertical axis. As observed
previously for the related benzobisoxazoleꢀbased cruciforms,3g this vertical localization of the HOMOs
is a consequence of orbital properties of the benzobisimidazole core and is largely independent of the
substitution. The lowest unoccupied molecular orbital (LUMO) of 4a is in contrast delocalized across
the molecule. In the case of a cruciform substituted with electronꢀwithdrawing groups along the vertical
axis—such as 4f (Figure 2, bottom)—the LUMO also resides along the electronꢀpoor vertical axis of the
molecule. As synthetic access to cruciform variants with a different substitution on the horizontal axis
was restricted, we were unable to probe the effects of placing strongly electronꢀwithdrawing or electronꢀ
donating groups along the horizontal axis.
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Figure 2. HOMOs (left) and LUMOs (right) of cruciforms 4a and 4f (calculated in Gaussian, using
B3LYP hybrid density functional and 3ꢀ21G basis set).
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Optical Properties. Compounds 4a–l are yellow to red powders with moderate solubility in most
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common organic solvents. Their H NMR spectra are complicated by facile tautomerization of both
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imidazoles, but are otherwise consistent with their structures. They are highly fluorescent11 in solution
and mildly in the solid state when irradiated with a handheld UV lamp (λexc=365 nm). UV/Vis
absorption and fluorescence emission spectra of 4a–j and 4l are shown in Figure 3 and summarized in
Table 1. UV/Vis absorption spectra (Figure 3, top) of all cruciforms are essentially superimposable with
a single broad absorption band centered between 380 and 395 nm. The only exception is 4b, whose
absorption spectrum shows an additional band at 425 nm. In their fluorescence emission spectra (Figure
3, bottom), a somewhat greater level of distinction can be achieved. Compound 4b is a clear outlier,
with a relatively featureless emission band at 486 nm. All other cruciforms' emission spectra show two
distinct maxima: a more intense one, centered between 426 (for 4e) and 456 (for 4i) nm, and a lowerꢀ
intensity band, centered between 452 (for 4e) and 483 (for 4i) nm. Most of these trends are visible by
the naked eye (see Figure 5 below), in that colors of all cruciforms except 4b look similar under both
UV and visible light.
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Figure 3. Normalized UV/Vis absorption (top) and emission (bottom) spectra of cruciforms 4a–4j and
4l in THF. Excitation wavelengths for emission spectra: 335 (4a), 348 (4b), 336 (4c), 337 (4d), 333
(4e), 339 (4f–4h), 330 (4i–4j), 337 (4l) nm.
Table 1. Optical Properties and Calculated HOMO–LUMO Gaps for Cruciforms 4a‒4l.
Absorption λmax
Emission λmax
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Stokes shift
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Calculated HOMO–LUMO gap
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Compound
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2.93, 423
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Optical Response to Acids and Bases. In contrast to benzobisoxazoleꢀbased cruciforms, these new
benzobisimidazoleꢀbased systems are amphoteric and were expected to show significant changes in their
UV/Vis absorptions and emissions upon both protonation and deprotonation. To experimentally confirm
this hypothesis, we performed titrations of 4a–l with both an acid (trifluoroacetic acid, TFA) and a base
(tetrabutylammonium hydroxide, TBAOH) in THF. Full experimental details of these titrations are
given in the Supporting Information. In general, benzobisimidazole cruciforms show a rather moderate
response to acids: with excess acid (−log[TFA]<1.0), a blue shift in absorption of up to ꢁλ=−18 nm was
observed. This set of observation could be rationalized by protonation of benzobisimidazole nitrogen
atoms (pKa = 5.55 for benzimidazolium cation)12 at high acid concentration. The only cruciform with a
different response was 4b (Figure 4, top), whose significantly more basic –NMe2 groups became
protonated at lower concentrations of TFA. In the emission spectra of 4a–4j and 4l, small but less
consistent changes were observed: for some cruciforms, emission shifted minimally toward the red
region, while for others it shifted toward the blue or was quenched.
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Figure 4. Exemplary titrations of benzobisimidazole cruciforms with acid (top) and base (bottom). On
the top, changes in UV/Vis absorption spectra of cruciform 4b as a function of added TFA. On the
bottom, fluorescence response of cruciform 4a to the addition of TBAOH (λexc=335 nm). Both titrations
were performed in THF.
Response of benzobisimidazole cruciforms to bases is much more dramatic. Upon exposure to high
concentrations of base (−log[TBAOH]<2.5), red shifts in absorption of approx. ꢁλ=+100 nm occurred
for all studied cruciforms, and were accompanied by equally significant shifts in the emission (as
illustrated for 4a in Figure 4, bottom); for more acidic 4f, even lower concentrations of base induced
similar shifts. As noticed previously in Lꢀshaped halfꢀcruciforms,5 deprotonation of benzobisimidazole’s
N–H moieties (pKa = 12.78)12 causes these dramatic changes in fluorescence.
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Most of these changes were detectable by the naked eye as well. Digital photographs of vials
containing 10−5 M solutions of 4a–4j and 4l under visible and UV light are shown in Figure 5, and the
dramatic response of these cruciforms to a base is apparent.
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Figure 5. Acidꢀ and baseꢀinduced changes in the colors of 10−5 M solutions of cruciforms 4a–j and 4l in
THF, as observed under visible (top) and UV (bottom) light. For emission color photographs, λexc=365
nm, shutter speed 0.05 s.
CONCLUSION
In summary, we have synthesized a series of eleven crossꢀconjugated cruciform fluorophores based on
the benzobisimidazole nucleus, using a combination of acidꢀcatalyzed condensation and Sonogashira
coupling. These materials are fluorescent in the solid state and strongly so in dilute solutions. All of
them also show a pronounced red shift upon deprotonation of their imidazole N–H functionalities,
while, in contrast, their response to acids is much more attenuated. This lack of response is partially a
consequence of our inability to synthetically access diverse substitution patterns on the horizontal axis,
which in turn, meant that we could modulate the LUMO orbitals of 4a–l only to a limited extent. Thus,
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unlike in the benzobisoxazole series,4g systems with completely spatially separated FMOs could not be
produced and studied. At the same time, it is highly encouraging that almost all of the
benzobisimidazole cruciforms maintain a certain level of fluorescence regardless of their protonation
state. This feature could be explored in the preparation of e.g. fluorescent and guestꢀresponsive porous
materials, such as zeolitic imidazolate frameworks (ZIFs),13 utilizing these crossꢀconjugated
benzobisimidazoles.
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EXPERIMENTAL SECTION
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General Experimental Methods. All reactions were performed under nitrogen atmosphere in ovenꢀ
dried glassware. Reagents were purchased from commercial suppliers and used without further
purification. Solvents were used as received, except THF and N,Nꢀdimethylformamide (DMF), which
were degassed by a 30 minute nitrogen purge prior to use in Sonogashira couplings. Compounds 1,5ꢀ
dichloroꢀ2,4ꢀdinitrobenzene14 and 1,3ꢀdiaminoꢀ4,6ꢀdinitrobenzene15 were synthesized according to
literature procedures. Triethylamine (Et3N) was degassed by a 30 minute nitrogen purge prior to use.
Microwaveꢀassisted reactions were performed in a Biotage Initiator 2.0 microwave reactor, producing
monochomatic microwave radiation with the frequency of 2.45 GHz. NMR spectra were obtained on
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spectrometers with working frequencies (for H nuclei) of 400, 500, and 600 MHz. All 13C NMR
spectra were recorded with simultaneous decoupling of 1H nuclei. 1H NMR chemical shifts are reported
in ppm units relative to the residual signal of the solvent (CDCl3: 7.25 ppm, (CD3)2SO: 2.48 ppm, D2O:
4.79 ppm, (CD3)2CO: 2. 05 ppm). All NMR spectra were recorded at 25 oC for samples in CDCl3, D2O,
and (CD3)2CO, while samples in DMSOꢀd6 were recorded at temperatures ranging from 25 to 90 °C.
Mass spectra of compound 7 and 2,6ꢀdiphenylꢀ4,8ꢀbis((triisopropylsilyl)ethynylꢀ1,5ꢀdihydrobenzo[1,2ꢀ
d:4,5ꢀd’]diimidazole were obtained using magnetic sector CI mass analyzer, while cruciforms 4a–4l
were analyzed using QTof mass analyzer. Melting point measurements were performed in open
capillary tubes and the reported values are uncorrected. Column chromatography was carried out on
silica gel 60, 32–63 mesh and basic aluminum oxide Act. 1, 50–200µm. Analytical TLC was performed
on plasticꢀbacked silica gel IBꢀF plates and aluminum oxide IBꢀF plates.
Synthesis of 1,2,4,5-benzenetetramine tetrahydrochloride (5).9 A 1 L Schlenk flask was flushed
with nitrogen and charged with 1,3ꢀdiaminoꢀ4,6ꢀdinitrobenzene15 (6.00 g, 30.3 mmol), concentrated
HCl (311 mL), and EtOH (155 mL). The reaction flask was attached to the condenser and heated at 50
°C with stirring for 10 min. After 10 min, a solution of anhydrous SnCl2 (52.0 g, 274 mmol) in EtOH
(80 mL) was added to the reaction mixture. The beaker containing the SnCl2 solution was washed with
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an additional portion of EtOH (40 mL) and the washings were added to the reaction flask. The reaction
was heated at 86 °C for 2 d, to ensure that the reaction went to completion. The reaction resulted in a
white precipitate forming in the flask. After cooling, the solution was filtered and the residue was
washed with EtOH to give the product as a light pink solid (8.1 g). The crude product was used without
further purification. 5. IR (neat): 2923 (s, ꢀȬN+–H), 2495 (s), 1601 (m, ꢀȬC–C), 1556 (s), 1511(s, ꢀȬN+–H),
1221 (s, ꢀȬC–N), 1127 (m), 1054 (w), 860 (m, ꢀȬC–H) cm−1. 1H NMR (D2O, 400 MHz): δ 6.77 (s) ppm. 13C
NMR (D2O, 100 MHz): δ 125.1, 113.5 ppm.
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Synthesis of 2,6-diphenyl-1,5-dihydrobenzo[1,2-d:4,5-d’]diimidazole (6).4g In a 500 mL round
bottom flask equipped with a condenser, a mixture of 5 (10.3 g, 36.3 mmol), benzoic acid (17.7 g, 145
mmol), and polyphosphoric acid (PPA, 70 g) was heated up to 140 °C. The reaction temperature was
slowly increased to 145 °C and maintained at that temperature for 40 h. After the reaction was finished,
the mixture was allowed to cool down, and was then poured into ice water. Stirring was continued while
the mixture was made very basic by the addition of NaOH pellets (pH=14). The precipitate was filtered,
washed with H2O, and dried in air overnight. The solid was further dried in a rotary evaporator whose
bath was heated for 2 h at 80 °C, finally give crude 6 (11.25 g). Because of its low solubility, this
material was used for the next step without further purification.
Synthesis of 4,8-dibromo-2,6-diphenyl-1,5-dihydrobenzo[1,2-d:4,5-d’]diimidazole (7). A 250 mL
round bottom flask was charged with DMF (165 mL) and the solvent was stirred vigorously while the
ground powder of compound 6 (10.8 g, 34.7 mmol) was added to it. NꢀBromosuccinimide (NBS, 12.3 g,
69.3 mmol) was added next, and the reaction mixture was stirred at 20 °C for 1 h. The formed
precipitate was filtered under reduced pressure, washed with EtOH, and dried in air to obtain 13 g of a
o
white powder, which was the identified as the target compound 7 (mp >350 C) .The overall yield for
the synthesis of compound 7 from compound 5 in 2 steps is 80%. 7. IR (neat): 3160 (w, ꢀȬN–H), 1560 (m,
ꢀȬC=N), 1480 (s), 1460 (s), 1330 (m, ꢀȬC–N), 1280 (s), 1230 (s), 867 (s), 772 (m), 725 (s), 685 (s, ꢀȬC–Br
)
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cm−1. H NMR (DMSOꢀd6, 400 MHz): δ 13.01 (s, 2H), 8.32 (m, 4H), 7.53 (m, 6H) ppm. 13C NMR
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(DMSOꢀd6, 100 MHz): δ 153.6, 153.3, 140.4, 139.9, 133.1, 132.6, 130.9, 130.0, 129.3, 127.9, 90.9 ppm.
1
2
3
+
HRMS (CI/[M]+): calcd for C20H12Br2N4 : 465.9429, found: 465.9435.
4
5
Synthesis
of
2,6-diphenyl-4,8-bis((triisopropylsilyl)
ethynyl-1,5-dihydrobenzo[1,2-d:4,5-
6
7
8
9
d’]diimidazole. A 500 mL pearꢀshaped Schlenk flask was charged with compound 7 (13.0 g, 27.8
mmol), PdCl2(PPh3)2 (1.95 g, 2.77 mmol), and CuI (1.06 g, 5.56 mmol). The flask was sealed, then
evacuated and backfilled with N2 three times. In a separate flask, a mixture of (iꢀPr)2NH (200 mL),
DMF (100 mL), and tris(isopropyl)silylacetylene (TIPSA, 25 mL, 111 mmol) was degassed for 20 min
and then slowly transferred, under positive N2 pressure, via cannula to the reaction flask. The reaction
mixture was stirred and heated at 90 °C for 2 d and then cooled to 20 °C. Afterwards, silica gel (150
mL) was added to the reaction mixture, and the solvent was removed under reduced pressure until a
brown powder was formed. This powder was subjected to column chromatography on silica gel, using a
mixture of CH2Cl2 and hexane as an eluent (starting with a 1:1 volume ratio, and proceeding until 7:3
ratio). The solvent was removed in vacuo to give a yellow crude product which was further purified by
recrystallization from EtOH (150 mL), finally giving the desired product as a bright yellow powder
(16.4 g, mp 310 oC). IR (neat): 3460 (s, ꢀȬN–H), 2940 (s, ꢀȬC–H), 2860 (s), 2140 (m, ꢀȬC≡C), 1480 (m, ꢀȬSi–C),
1450 (s), 1330 (s, ꢀȬSi–C), 997 (w), 883 (w), 775 (w), 740 (s), 668 (s) cm−1. 1H NMR (CDCl3, 500 MHz):
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60
3
13
δ 9.43 (s, 2H), 8.08 (d, JH–H = 6.9 Hz, 4H), 7.52 (m, 6H), 1.27 (m, 42H) ppm. C NMR (CDCl3, 125
MHz): δ 152.3, 151.8, 142.7, 141.9, 134.4, 133.7, 130.5, 130.3, 129.6, 129.2, 129.1, 126.7, 102.2, 99.8,
+
97.1, 19.0, 18.9, 11.7, 11.5, 11.3 ppm. HRMS (CI/[M]+): calcd for C42H54N4Si2 : 670.3887, found:
670.3871.
Synthesis of 4,8-diethynyl-2,6-diphenyl-1,5-dihydrobenzo[1,2-d:4,5-d’]diimidazole (8). In a
nitrogenꢀflushed 100 mL flask, tris(isopropyl)silylꢀsubstituted alkyne produced in the previous step
(2.00 g, 2.98 mmol) was dissolved in THF (20 mL), and treated with 1M solution of TBAF in THF
(6.56 mL, 6.56 mmol). The resulting solution was stirred at 20 °C for 1 h, and a yellow precipitate was
formed. The precipitate was collected by filtration and washed with THF. The residue was mixed with
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The Journal of Organic Chemistry
H2O (150 mL) and sonicated for 10 min to give fraction A. Meanwhile, the solvent in the filtrate was
removed under reduced pressure. The remaining solid was mixed with EtOH (10 mL), followed by H2O
(40 mL), to give fraction B. Subsequently, fraction A was combined with fraction B. The mixture was
sonicated for 10 min, filtered, washed with H2O, and dried in vacuo to yield 1.06 g of crude 8 as a dark
yellow powder, which was used in the next step without purification. 1H NMR (DMSOꢀd6, 500 MHz): δ
13.03 (s, 2H), 8.32 (d, J=6.9 Hz, 4H), 7.52 (m, 6H), 4.88 (m, 2H) ppm. 13C NMR (DMSOꢀd6, 125
MHz): δ 153.5, 153.2, 143.0, 142.7, 134.8, 134.5, 130.6, 130.3, 129.3, 127.8, 95.8, 91.4, 90.6, 90.0, 78.3
ppm.
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Synthesis of Cruciform 4a. Phenylacetylene (781 mg, 7.65 mmol) was added to a thickꢀwalled
microwave pressure vial that contained a mixture of compound 7 (300 mg, 0.64 mmol), PdCl2(PPh3)2
(90 mg, 0.13 mmol), CuI (45 mg, 0.24 mmol), Et3N (5 mL), and DMF (5 mL). The vial was sealed
under nitrogen and exposed to microwave irradiation for 12 h at 110 °C. After cooling, the reaction
mixture was extracted with EtOAc, washed with brine, and dried over anhydrous MgSO4. Afterward,
Al2O3 (80 g) was added to the combined organic layer and the solvent was removed under reduced
pressure until a brown powder formed, which was purified by gradient elution column chromatography
on alumina (300 g) using EtOAc and hexane as eluents (starting volume ratio 3:17, ending volume ratio
9:11). The solvent was removed in vacuo to yield the crude product which was further purified by
recrystallization from THF to give 4a as a yellow powder (153 mg, 47%, mp 309 °C with
decomposition). UV/Vis (THF): λmax (log ε) 386 (4.90) nm. IR (neat): 3056 (w, ꢀȬ N−H), 2160 (w, ꢀȬC≡C),
1958 (w), 1596 (m, ꢀȬC=N), 1557 (w), 1526 (w), 1476 (s), 1454 (s), 1441 (w), 1402 (m), 1342 (s), 1280
(m), 1215 (w), 1177 (w), 1156 (w), 1115 (w), 1069 (m), 1028 (m), 999 (w), 943 (m), 913 (w), 775 (m),
752 (s), 728 (m), 686 (s), 624 (s), 565 (w) cm−1. 1H NMR (DMSOꢀd6, 600 MHz): δ 12.96 (s, 2H), 8.34
3
3
(d, JH–H = 7.6 Hz, 4H), 7.78 (m, 4H), 7.56 (t, JH–H = 7.6 Hz, 4H), 7.51 (m, 8H) ppm. 13C NMR
(DMSOꢀd6, 150 MHz, 40 °C): δ 153.5, 142.6, 134.0, 130.5, 129.3, 127.9, 123.7, 103.7, 98.7, 96.6, 89.9,
+
86.4, 84.6, 82.9 ppm. HRMS (ESI/[M]+): calcd for C36H22N4 : 510.1844, found: 510.1849.
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Synthesis of Cruciform 4b. Anhydrous K2CO3 (1.11 g, 8.00 mmol) was added to a solution of 2ꢀ(4ꢀ
(N, Nꢀdimethylamino)phenyl) trimethylsilylethyne (875 mg, 4.00 mmol) in a mixture of MeOH (5 mL)
and THF (5 mL). After stirring for 30 min under nitrogen, the reaction mixture was filtered through
Celite. The solvent was removed under reduced pressure to yield crude 4ꢀethynylꢀN,Nꢀdimethylaniline,
which was used without purification in the next step. To minimize manipulations of this compound, we
assumed a 95% yield for this reaction.4g
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The entire amount of 4ꢀethynylꢀN,Nꢀdimethylaniline (prepared as described above) was added to a
thickꢀwalled microwave pressure vial that contained a mixture of compound 7 (300 mg, 0.64 mmol),
PdCl2 (PPh3)2 (90 mg, 0.13 mmol), CuI (45 mg, 0.24 mmol), Et3N (5 mL), and DMF (5 mL). The vial
was sealed under nitrogen and exposed to microwave irradiation for 12 h at 100 °C. After cooling, the
reaction mixture was extracted with EtOAc, washed with brine, and dried over anhydrous MgSO4.
Afterward, 80 g of aluminum oxide was added to the combined organic layer and the solvent was
removed under reduced pressure until a brown powder was formed, which was purified by gradient
elution column chromatography on alumina, using EtOAc/hexane eluent system (solvent ratios ranging
from 4:16 to 9:11). The solvent was removed in vacuo to give the crude product, which was further
purified by recrystallization from THF to afford 214 mg (56%, 0.36 mmol) of a tangeloꢀcolored powder
(4b, mp 220 oC, with decomposition). UV/Vis (THF): λmax (log ε) 386 (4.81), 425 (4.70) nm. IR (neat):
3086 (w, ꢀȬN−H), 2190 (s, ꢀȬC≡C), 1604 (s, ꢀȬC=N), 1530 (m), 1514 (m), 1479 (m), 1457 (m), 1402 (w), 1345
(s), 1290 (m), 1232 (m), 1183 (s), 1065 (m), 1029 (m), 999 (w), 945 (m), 920 (w), 843 (w), 810 (s), 777
(m),725 (m), 690 (s), 634 (w), 580 (w) cm−1. 1H NMR (DMSOꢀd6, 500 MHz): δ 12.90 (d, 3JH–H=5.2 Hz,
2H), 8.34 (m, 4H), 8.57 (m, 10H), 6.79 (d, 3JH–H=8.6 Hz, 4H), 2.98 (s, 12H) ppm. 13C NMR (DMSOꢀd6,
with 20 mg of TBAF trihydrate added, 150 MHz): δ 159.5, 149.3, 146.5, 139.9, 132.1, 127.9, 126.6,
+
125.3, 114.0, 112.8, 96.1, 94.0, 91.8, 51.9 ppm. HRMS (ESI/[M]+): calcd for C40H32N6 596.2688,
found 596.2692.
Synthesis of Cruciform 4c. A mixture of compound 8 (300 mg, 0.84 mmol), 4ꢀiodotoluene (913 mg,
4.19 mmol), PdCl2(PPh3)2 (50 mg, 0.07 mmol) and CuI (27 mg, 0.14 mmol) was added to a 100 mL
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The Journal of Organic Chemistry
pearꢀshaped Schlenk flask. The flask was sealed, and then evacuated and backfilled with nitrogen three
times. In a separate flask, a mixture of Et3N (25 mL) and THF (17 mL) was degassed for 30 min and
transferred slowly under positive N2 pressure via cannula to the reaction flask. The reaction mixture was
stirred and heated at 65 °C for 24 h and then cooled to 20 °C. Afterwards, aluminum oxide was added to
the reaction mixture and the solvent was removed under reduced pressure until a brown powder was
formed, which was subsequently purified by gradient elution column chromatography on alumina, using
EtOAc/hexane eluent system (volume ratios ranging from 3:17 to 9:11). The solvent was removed in
vacuo to give the crude product, which was further purified by recrystallization from THF to afford
cruciform 4c as a yellow powder (174 mg, 38%, mp 293 °C). UV/Vis (THF): λmax (log ε) 386 (4.91) nm.
IR (neat): 3000 (w, ꢀȬN−H), 2188 (w, ꢀȬC≡C), 1603 (m, ꢀȬC=N), 1560 (w), 1528 (w), 1505 (m), 1480 (s), 1454
(s), 1401 (m), 1336 (s), 1286 (m), 1254 (m), 1236 (m), 1176 (m), 1109 (w), 1070 (w), 1030 (m), 987
(w), 935 (m), 809 (s), 774 (s), 738 (w), 723 (m), 692 (s), 632 (w) cm−1. 1H NMR (DMSOꢀd6, 500 MHz):
1
2
3
4
5
6
7
8
9
10
11
12
13
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3
δ 12.97 (s, 2H), 8.34 (d, JH–H = 7.5 Hz, 4H), 8.57 (m, 10H), 7.32 (d, JH–H = 5.7 Hz, 4H), 2.38 (s, 6H)
ppm. 13C NMR (DMSOꢀd6, 125 MHz, 90 °C): δ 153.3, 138.9, 132.1, 130.7, 130.5, 129.8, 129.2, 128.0,
+
120.9, 21.6 ppm. HRMS (ESI/[M]+): calcd for C38H26N4 : 538.2157, found: 538.2138.
Synthesis of Cruciform 4d. A mixture of compound 8 (300 mg, 0.84 mmol), 4ꢀiodoanisole (979 mg,
4.19 mmol), PdCl2(PPh3)2 (50 mg, 0.07 mmol), and CuI (27 mg, 0.14 mmol) was added to a 100 mL
pearꢀshaped Schlenk flask. The flask was sealed, then evacuated and backfilled with nitrogen three
times. In a separate flask, a mixture of Et3N (25 mL) and THF (17 mL) was degassed for 30 min and
transferred slowly under positive N2 pressure via cannula to the reaction flask. The reaction mixture was
stirred and heated at 65 °C for 24 h and then cooled to 20 °C. Afterwards, 80 g of aluminum oxide was
added to the reaction mixture and the solvent was removed under reduced pressure until a brown
powder was formed. This powder was purified by gradient elution column chromatography on alumina,
using EtOAc/hexane eluent system (volume ratio ranging from 7:13 to 4:1). The solvent was removed in
vacuo to give the crude product, which was further purified by recrystallization from THF to afford
cruciform 4d as a yellow powder (196 mg, 41%, mp 280 °C). UV/Vis (THF): λmax (log ε) 385 (4.93)
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nm. IR (neat): 3062 (w, ꢀȬN−H), 2834 (w, ꢀȬC−H), 2195 (m, ꢀȬC≡C), 1603 (s, ꢀȬC=N), 1567 (w), 1524 (w), 1504
(s), 1478 (s), 1453 (s), 1399 (m), 1340 (s), 1288 (s), 1244 (s), 1169 (s), 1106 (m), 1071 (w), 1027 (s),
942 (m), 827 (s), 774 (s), 727 (m), 689 (s), 644 (w), 633 (w), 589 (s), 564 (m), 546 (m), 529 (s) cm−1. 1H
1
2
3
4
5
6
7
8
o
3
NMR (DMSOꢀd6, 500 MHz at 90 C): δ 12.69 (s, 2H), 8.33 (d, JH–H = 6.9 Hz, 4H), 7.71 (s, 4H), 7.52
9
3
(m, 6H), 7.05 (d, JH–H = 8.6 Hz, 4H), 3.84 (s, 6H) ppm. 13C NMR (DMSOꢀd6, 125 MHz, 90 °C): δ
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160.4, 153.2, 142.0, 133.7, 130.7, 130.4, 129.1, 128.0, 116.0, 115.0, 98.9, 96.9, 83.4, 56.0 ppm. HRMS
+
(ESI/[M]+): calcd for C38H26N4O2 : 570.2056, found: 570.2056.
Synthesis of Cruciform 4e. A mixture of compound 8 (300 mg, 0.84 mmol), 4ꢀfluoroiodobenzene
(0.56 mL, 4.85 mmol), PdCl2(PPh3)2 (50 mg, 0.07 mmol), and CuI (27 mg, 0.14 mmol) was added to a
100 mL pearꢀshaped Schlenk flask. The flask was sealed, then evacuated and backfilled with nitrogen
three times. In a separate flask, a mixture of Et3N (25 mL) and THF (17 mL) was degassed for 30 min
and then transferred slowly under positive N2 pressure via cannula to the reaction flask. The reaction
mixture was stirred and heated at 65 °C for 24 h and then cooled to 20 °C. Afterwards, alumina was
added to reaction mixture and the solvent was removed under reduced pressure until a brown powder
was formed. This powder was then purified by gradient elution column chromatography on alumina,
using EtOAc/hexane eluent system (volume ratio ranging from 1:3 to 1:1). The solvent was removed by
vacuum to give the orange crude product which was further purified by recrystallization from EtOAc, to
afford cruciform 4e (187 mg, 41%, mp 287 °C). UV/Vis (THF): λmax (log ε) 383 (4.89) nm. IR (neat):
3066 (w, ꢀȬN−H), 2158 (w, ꢀȬC≡C), 1602 (m, ꢀȬC=N), 1558 (w), 1525 (w), 1503 (s), 1477 (s), 1455 (s), 1402
(m), 1342 (s), 1317 (w), 1288 (s), 1235 (s), 1169 (s), 1105 (m), 1064 (w), 1028 (s), 943 (m), 832 (s),
775 (s), 747 (w), 728 (m), 689 (s), 634 (w), 582 (s), 566 (w), 556 (w), 547 (w), 530 (s) cm−1. 1H NMR
(DMSOꢀd6, 500 MHz): δ 13.05 (d, 3JH–H = 8.0 Hz, 2H), 8.33 (m, 4H), 7.84 (m, 4H), 7.55 (m, 6H), 7.36
(m, 4H) ppm. 13C NMR (DMSOꢀd6, 125 MHz): δ 163.7, 161.6, 153.4, 142.3, 134.5, 130.7, 129.3, 127.7,
+
120.0, 116.6, 97.6, 96.4, 84.3 ppm. HRMS (ESI/[M]+): calcd for C36H20F2N4 : 546.1656, found:
546.1656.
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The Journal of Organic Chemistry
Synthesis of Cruciform 4f. A mixture of compound 8 (300 mg, 0.84 mmol), 1ꢀiodoꢀ4ꢀnitrobenzene
(834 mg, 3.35 mmol), PdCl2(PPh3)2 (50 mg, 0.07 mmol), and CuI (27 mg, 0.14 mmol) was placed in a
100 mL pearꢀshaped Schlenk flask. The flask was sealed, then evacuated and backfilled with nitrogen
three times. In a separate flask, a mixture of Et3N (30 mL) and THF (20 mL) was degassed for 30 min
and then transferred slowly under positive nitrogen pressure via cannula into the reaction flask. The
reaction mixture was stirred and heated at 50 °C for 24 h, then cooled to 20 °C, filtered, and washed
with THF (5 mL). The residue collected was recrystallized from THF to give the crude product, which
was then mixed with H2O (150 mL), sonicated for 30 min, filtered, and dried in air to give cruciform 4f
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o
(136 mg, 27%) as a red powder (mp 327 C, with decomposition). UV/Vis (THF): λmax (log ε) 388
(4.80) nm. IR (neat): 3060 (w, ꢀȬN−H), 2199 (w, ꢀȬC≡C), 1591 (m, ꢀȬC=N), 1514 (s, ꢀȬNO2 ), 1476 (m), 1454
(m), 1402 (w), 1337 (s, ꢀȬNO2), 1175 (w), 1106 (m), 1028 (w), 994 (w), 943 (w), 854 (s), 775 (m), 748
(m), 728 (w), 687 (s), 568 (w) cm−1. 1H NMR (DMSOꢀd6, 500 MHz): δ 13.29 (s, 2H), 8.34 (apparent d,
3JH–H = 6.9 Hz, 8H), 8.01 (br s, 4H), 7.57 (m, 6H) ppm. 13C NMR (DMSOꢀd6, 125 MHz): δ 133.2,
131.1, 130.0, 129.5, 128.1, 124.4 ppm (because of the low solubility of cruciform 4f, a satisfactory 13C
NMR spectrum could not be obtained; listed are all the observed peaks). HRMS (ESI/[M]+): calcd for
+
C36H20N6O4 : 600.1546, found: 600.1526.
Synthesis of Cruciform 4g. A mixture of compound 8 (300 mg, 0.84 mmol), 4ꢀiodobenzotrifluoride
(0.50 mL, 3.40 mmol), PdCl2(PPh3)2 (50 mg, 0.07 mmol), and CuI (27 mg, 0.14 mmol) was placed in a
100 mL pearꢀshaped Schlenk flask. The flask was sealed, then evacuated and backfilled with nitrogen
three times. A mixture of Et3N (30 mL) and THF (20 mL) was degassed for 30 min and transferred
slowly under positive nitrogen pressure via cannula into the reaction flask. The reaction mixture was
stirred and heated at 65 °C for 24 h, then cooled to 20 °C. Afterwards, silica gel was added to the
reaction mixture, and the solvent was removed under reduced pressure until a brown powder was
formed. This powder was purified by gradient elution column chromatography on silica gel, using
EtOAc/hexane eluent system (volume ratio ranging from 1:3 to 1:1). The solvent was removed in vacuo
to give the yellow crude product, which was further purified by recrystallization from Et2O, to yield
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cruciform 4g (220 mg, 41%) as a yellow powder (mp 245 °C, with decomposition). UV/Vis (THF): λmax
(log ε) 392 (4.86) nm. IR (neat): 3062 (w, ꢀȬN−H), 2201 (w, ꢀȬC≡C), 1613 (m, ꢀȬC=N), 1563 (w), 1529 (w),
1505 (w), 1478 (s), 1455 (s), 1403 (m), 1341 (s), 1317 (s), 1290 (s), 1246 (m), 1167 (s), 1119 (s), 1103
(s), 1064 (s), 1028 (s), 1018 (s), 945 (m), 837 (s), 774 (s), 747 (w, ꢀȬCF3), 728 (m), 689 (s), 634 (w), 592
1
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8
9
1
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(s), 537 (m) cm−1. H NMR (DMSOꢀd6, 500 MHz): δ 13.16 (s, 2H), 8.34 (d, JH–H = 7.5 Hz, 4H), 8.00
10
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(m, 8H), 7.55 (m, 6H) ppm.13C NMR (DMSOꢀd6, 125 MHz): δ 153.5, 142.6, 134.2, 132.9, 130.9, 130.2,
+
129.4, 127.9, 126.2, 125.7, 123.5, 97.4, 96.3, 87.1 ppm. HRMS (ESI/[M]+): calcd for C38H20F6N4 :
646.1592, found: 646.1592.
Synthesis of Cruciform 4h. A mixture of compound 8 (300 mg, 0.84 mmol), 2ꢀiodothiophene (0.55
mL, 4.98 mmol), PdCl2(PPh3)2 (50 mg, 0.07 mmol), and CuI (27 mg, 0.14 mmol) was placed in a 100
mL pearꢀshaped Schlenk flask. The flask was sealed, then evacuated and backfilled with nitrogen three
times. A solution of Et3N (25 mL) and THF (17 mL) was degassed for 30 min and then transferred
slowly under positive nitrogen pressure via cannula into the reaction flask. The reaction mixture was
stirred and heated at 65 °C for 24 h and then cooled to 20 °C. Afterwards, alumina was added to the
reaction mixture and the solvent was removed under reduced pressure until a brown powder was
formed. This powder was purified by gradient elution column chromatography on alumina, using
EtOAc/hexane eluent system (volume ratio ranging from 1:4 to 3:2). The solvent was removed in vacuo
to give the crude product which was further purified by recrystallization from THF/hexane to yield
cruciform 4h (153 mg, 35%) as a dark yellow powder (mp 268 °C, with decomposition). 4h: UV/Vis
(THF): λmax (log ε) 391 (4.89) nm. IR (neat): 3111 (w, ꢀȬC−Hꢀthiophene), 3060 (w, ꢀȬN−H), 2190 (w, ꢀȬC≡C),
1613 (w, ꢀȬC=N), 1562 (w), 1531 (w), 1506 (w), 1479 (m), 1454 (s), 1396 (w), 1341 (s), 1325 (s), 1276
(m), 1226 (s), 1174 (w), 1071 (w), 1043 (s), 1029 (m), 965 (w), 942 (m), 918 (w), 851 (s), 831 (m), 774
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(s), 744 (w), 686 (s), 625 (m) cm−1. H NMR (DMSOꢀd6, 500 MHz): δ 13.12 (s, 2H), 8.32 (d, JH–H
=
7.5 Hz, 4H), 8.56 (m, 10H), 7.20 (m, 2H) ppm.13C NMR (Me2COꢀd6 and 200 mg of TBAF trihydrate,
150 MHz): δ 131.2, 128.0, 127.2, 127.1, 126.2, 125.8, 88.7 ppm. HRMS (ESI/[M]+): calcd for
+
C32H18N4S2 : 522.0973, found: 522.0959.
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Synthesis of Cruciform 4i. A mixture of compound 8 (300 mg, 0.84 mmol), 4ꢀiodoacetophenone
(618 mg, 2.51 mmol), PdCl2(PPh3)2 (50 mg, 0.07 mmol), and CuI (27 mg, 0.14 mmol) was placed in a
100 mL pearꢀshaped Schlenk flask. The flask was sealed, then evacuated and backfilled with nitrogen
three times. In a separate flask, a mixture of Et3N (30 mL) and THF (20 mL) was degassed for 30 min
and transferred slowly under positive nitrogen pressure via cannula into the reaction flask. The reaction
mixture was stirred and heated at 65 °C for 2 d, then cooled to 20 °C, filtered, and washed with THF (5
mL). The residue collected was recrystallized from THF to give the crude product, which was mixed
with H2O (150 mL), sonicated for 30 min, filtered, and dried in air to give cruciform 4i (300 mg, 60%)
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as a yellow powder (mp 352 C, with decomposition). UV/Vis (THF): λmax (log ε) 382 (4.60) nm. IR
(neat): 3199 (w, ꢀȬN−H), 2204 (w, ꢀȬC≡C), 1668 (s), 1602 (s, ꢀȬC=N), 1557 (w), 1505 (w), 1480 (m), 1455 (s),
1402 (m), 1336 (s), 1312 (w), 1273 (s), 1168 (m), 1105 (m), 1064 (m), 1029 (m), 1017 (w), 940 (w),
1
829 (s), 777 (s), 747 (w), 728 (m), 689 (s), 593 (m) cm−1. H NMR (DMSOꢀd6, 500 MHz, 40 °C): δ
13.10 (s, 2H), 8.35 (d, 3JH–H = 6.9 Hz, 4H), 8.06 (d, 3JH–H = 7.5 Hz, 4H), 7.90 (s, 4H), 7.55 (m, 6H), 2.62
(s, 6H) ppm.13C NMR (DMSOꢀd6, 125 MHz, 40 °C): δ 197.8, 154.1, 136.8, 132.3, 130.8, 130.3, 129.3,
13
129.0, 128.0, 98.3, 27.3 ppm (because of the low solubility of cruciform 4i, a satisfactory C NMR
spectrum could not be obtained; listed are all the observed peaks). HRMS (ESI/[M]+): calcd for
+
C40H26N4O2 : 594.2056, found: 594.2050.
Synthesis of Cruciform 4j. A mixture of compound 8 (300 mg, 0.84 mmol), ethyl 4ꢀiodobenzoate
(0.56 mL, 3.33 mmol), PdCl2(PPh3)2 (50 mg, 0.07 mmol), and CuI (27 mg, 0.14 mmol) was added to a
100 mL pearꢀshaped Schlenk flask. The flask was sealed, then evacuated and backfilled with nitrogen
three times. In a separate flask, a solution of Et3N (30 mL) and THF (20 mL) was degassed for 30 min
and then transferred slowly under positive nitrogen pressure via cannula into the reaction flask. The
reaction mixture was stirred and heated at 65 °C for 24 h and then cooled to 20 °C, filtered and washed
with THF (5 mL). The residue collected was recrystallized from a THF/EtOAc mixture (volume ratio
2:3), to give the crude product which was then mixed with H2O (150 mL), sonicated for 30 min, filtered
and dried in air, to finally give cruciform 4j (310 mg, 57%) as a yellow powder (mp 268 °C, with
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decomposition). UV/Vis (THF): λmax (log ε) 394 (4.89) nm. IR (neat): 3253 (m), 2981 (w, ꢀȬN−H), 2205
(w, ꢀȬC≡C), 1694 (s, ꢀȬC=O), 1605 (m, ꢀȬC=N), 1561 (w), 1524 (w), 1504 (m), 1480 (s), 1454 (s), 1404 (m),
1365 (w), 1339 (s), 1308 (w), 1287 (s, ꢀȬCO−O), 1275 (s), 1172 (m), 1128 (m), 1106 (s), 1071 (w), 1021
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(m), 987 (w), 941 (m), 853 (m), 830 (w), 773 (s), 765 (m), 725 (m), 687 (s), 652 (w) cm−1. H NMR
9
(DMSOꢀd6, 400 MHz): δ 13.15 (d, 3JH–H = 5.0 Hz, 2H), 8.35 (m, 4H), 8.00 (m, 8H), 7.55 (m, 6H), 4.33
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(q, JH–H = 7.3 Hz, 4H), 1.33 (t, JH–H = 7.3 Hz, 6H) ppm.13C NMR (DMSOꢀd6, 100 MHz): δ 165.8,
153.6, 142.7, 134.1, 132.4, 130.9, 130.2, 129.9, 129.4, 128.0, 98.1, 96.3, 87.6, 61.6, 14.7 ppm. HRMS
(ESI/[M]+): calcd for C42H30N4O4 : 654.2267, found: 654.2270.
+
Synthesis of Cruciform 4l. A mixture of compound 8 (500 mg, 1.39 mmol), 4ꢀ
(iodophenyl)tetrahydropyranꢀ2ꢀyl ether16 (1.73 g, 5.58 mmol), PdCl2(PPh3)2 (83 mg, 0.12 mmol), and
CuI (45 mg, 0.24 mmol) was added to a 100 mL pearꢀshaped Schlenk flask. The flask was sealed, then
evacuated and backfilled with nitrogen three times. In a separate flask, a solution of Et3N (40 mL) and
THF (27 mL) was degassed for 30 min and then transferred slowly under positive N2 pressure via
cannula into the reaction flask. The reaction mixture was stirred and heated at 60 °C for 36 h, then
cooled to 20 °C. Afterwards, alumina was added to the reaction mixture, and the solvent was removed
under reduced pressure until a brown powder was formed. This powder was purified by gradient elution
column chromatography on alumina, using EtOAc/hexane eluent system (volume ratio ranging from
25:75 to 55:45). The solvent was removed in vacuo to give 590 mg of the crude cruciform 4k as the
intermediate.
In a 250 mL roundꢀbottom flask, trifluoroacetic acid (4.5 mL) was added to a solution of the
aforementioned cruciform 4k (590 mg, 0.83 mmol) in DCM (150 mL), which was kept in a dry
ice/Me2CO bath. The reaction was stirred for 2 h at –78 °C and then warmed up to 20 °C.17 The
precipitate formed in the reaction was collected by filtration and washed with DCM. Afterwards, it was
mixed with water (500 mL), sonicated for 30 min, filtered, and dried in air to give the crude product,
which was further purified by recrystallization from THF to afford cruciform 4l (310 mg, 41%) as a
yellow powder (mp 320 °C, with decomposition). UV/Vis (THF): λmax (log ε) 383 (4.95) nm. IR (neat):
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3410 (w, ꢀȬO−H), 3063 (w, ꢀȬN−H), 2204 (w, ꢀȬC≡C), 1604 (s, ꢀȬC=N), 1525 (w), 1505 (s), 1479 (s), 1454 (s),
1402 (w), 1380 (w), 1345 (m), 1278 (m), 1265 (w), 1243 (s), 1169 (s), 1105 (w), 1072 (w), 1028 (m),
946 (w), 828 (s), 776 (s), 727 (w), 692 (s), 684 (s), 634 (w), 616 (w), 591 (s), 567 (w), 534 (s) cm−1. 1H
NMR (DMSOꢀd6, 500 MHz): δ 12.95 (s, 2H), 9.97 (s, 2H), 8.33 (d, 3JH–H = 7.5 Hz, 4H), 8.52 (m, 10H),
6.87 (d, 3JH–H = 8.0 Hz, 4H) ppm.13C NMR (DMSOꢀd6, 125 MHz): δ 158.7, 153.1, 142.4, 133.9, 130.6,
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+
129.3, 127.8, 116.3, 113.9, 99.4, 96.6, 82.5 ppm. HRMS (ESI/[M]+): calcd for C36H22N4O2 : 542.1743,
found: 542.1742.
ACKNOWLEDGMENTS
This research was supported by the University of Houston (UH) and its Grant to Advance and
Enhance Research, the National Science Foundation (grant CHEꢀ1151292 to O. Š. M.), and the Welch
Foundation (grant Eꢀ1768 to O. Š. M.). O. Š. M. is a Cottrell Scholar of the Research Corporation for
Science Advancement. N. S. E. acknowledges the Texas Center for Superconductivity at UH (TcSUH)
for a Paul C. W. Chu scholarship. Parts of this manuscript were written at New York University Abu
Dhabi (NYUAD), where O. Š. M. was on a sabbatical stay.
Supporting Information. Additional experimental and computational details, and spectroscopic
characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.
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