Conjugated metallorganic macrocycles: Opportunities for coordination-driven planarization of bidentate, pyridine-based ligands

Article abstract of DOI:10.1039/c2dt31914d

Two conjugated systems that can be constrained to planarity via metal coordination have been generated and their metal complexes studied. The potential for these architectures to be incorporated into metal-sensing arylene ethynylene/vinylene oligomers and polymers was probed by verifying that these ligands (1) bind strongly to Ag(i) and Pd(ii) cations, and (2) that this event leads to complexes that are planar. Single crystal structures confirm that introduction of Ag(i) or Pd(ii) cations enforces planarity in the newly formed macrocycles. Likewise, ¹H-NMR titration studies reveal stoichiometric binding of Pd(ii) and strong binding of Ag(i) (K_{a (Ligand 1)} = 1.3 × 10² M^-1; K_{a (Ligand 2)} = 5.4 × 10² M^-1) for each conjugated ligand. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2dt31914d

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Conjugated metallorganic macrocycles: opportunities
for coordination-driven planarization of bidentate,
pyridine-based ligands†
Cite this: Dalton Trans., 2013, 42, 948
Danielle C. Hamm,a Lindsey A. Braun,a Alex N. Burazin,a Amanda M. Gauthier,a
Kendra O. Ness,a Casey E. Biebel,a Jon S. Sauer,a Robin Tanke,a Bruce C. Noll,b
Eric Boschc and Nathan P. Bowling*a
Two conjugated systems that can be constrained to planarity via metal coordination have been generated
and their metal complexes studied. The potential for these architectures to be incorporated into metal-
sensing arylene ethynylene/vinylene oligomers and polymers was probed by verifying that these ligands
(1) bind strongly to Ag(I) and Pd(II) cations, and (2) that this event leads to complexes that are planar.
Single crystal structures confirm that introduction of Ag(I) or Pd(II) cations enforces planarity in the newly
formed macrocycles. Likewise, 1H-NMR titration studies reveal stoichiometric binding of Pd(II) and strong
binding of Ag(I) (Ka (Ligand 1) = 1.3 × 102 M−1; Ka (Ligand 2) = 5.4 × 102 M−1) for each conjugated ligand.
Received 21st August 2012,
Accepted 16th October 2012
DOI: 10.1039/c2dt31914d
www.rsc.org/dalton
Introduction
Extensive electron delocalization in oligomeric and polymeric
conjugated organic molecules leads to electronic properties
that are desirable for a number of applications such as
photovoltaics,1–3 light-emitting diodes4–6 and organic tran-
sistors.7–9 One pitfall with conjugated structures is that elec-
tronic properties can differ considerably from what would be
predicted with a two-dimensional depiction. Conformational
effects in such oligomeric and polymeric structures can sub-
stantially perturb or enhance effective conjugation. The ability
to understand and perhaps control the conformational effects
of these structures is, therefore, paramount.
In appropriately designed molecules, metal cations can
alter the electronics of an unsaturated structure by both
restricting rotation and withdrawing electron density from the
π-system. Three of the most studied conjugated ligands used
for this purpose are the 2,2′-bipyridine,10–13 1,2-bis(2′-pyridinyl-
ethynyl)benzene,14–17 and quinoline/quinoxaline-ethynylene
based systems18,19 (Fig. 1). Metal complexation in each of
Fig. 1 Transition metal complexation to (a) 2,2’-bipyridines, (b) 1,2-bis(2’-pyri-
dinylethynyl)benzenes and (c) quinoline/quinoxaline-ethynylenes enforces pla-
narity in these conjugated systems.
these examples provides coordination-enforced planarity. This
behavior can be used to alter the properties of conjugated
oligomers/polymers in which these structures are incorpo-
rated20 and in the development of transition metal sensors.21
1,2-Bis(2′-pyridinylethynyl)benzenes are a particularly inter-
esting study in geometry. It is noted in a paper by Hu and co-
workers15 that a palladium complex of this ligand is nearly
“perfectly triangular with the C–CuC–C (4.05 Å) and N–Pd–N
(4.02 Å) distances being about equal.” With C–CuC–C and N–
Pd–N fragments acting as equivalently spaced building blocks,
one can think of these metal complexes as equilateral “pseudo-
dehydroannulene”22 triangles. With hopes of expanding the
possibilities for coordination-driven planarization of conju-
gated molecules, we envisioned other unsaturated structures
that might offer this same ability to be conformationally
restricted upon introduction of a transition metal.
aDepartment of Chemistry, University of Wisconsin-Stevens Point, 2001 Fourth
Avenue, Stevens Point, WI 54481, USA. E-mail: nbowling@uwsp.edu
bBruker AXS Inc., 5465 East Cheryl Parkway, Madison, WI 53711, USA
cDepartment of Chemistry, Missouri State University, 901 S. National Ave.,
Springfield, MO 65897, USA
†Electronic supplementary information (ESI) available: 1H NMR and 13C NMR
spectra for all compounds, Ag(I) and Pd(II) titration data for 1 and 2, and absor-
bance and fluorescence spectra for 1, 2 and complexes are provided. CCDC
896707–896711. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2dt31914d
Arylethynyl ligand 1 and arylethenyl ligand 2 (Fig. 2),
should they be incorporated into larger oligomeric/polymeric
948 | Dalton Trans., 2013, 42, 948–958
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absorbtivity upon complexation to Ag(I) or Pd(II), with no sig-
nificant spectral shifts in the absorption spectra. Absorbance
spectra of Pd(II) complexes show small “tails” to the red of the
major signals. These “tails” span into the visible region, giving
the complexes a colored appearance. As has been observed for
1,2-bis(2′-pyridinylethynyl)benzene-based
systems,20
fluo-
rescence in ligands 1 and 2 is quenched gradually with the
addition of AgOTf and abruptly upon introduction of
PdCl2(PhCN)2.
X-ray characterization of ligand
Ligands 1 and 2 are expected to have several low energy planar
conformations that maximize conjugation while also minimiz-
ing repulsion between pyridyl non-bonding electron pairs.
Single crystal X-ray diffraction studies, therefore, should
reinforce electronic studies that suggest little conformational
difference between ligand–metal complexes and the parent
ligands. Thus, a crystal of ligand 1 was selected and the X-ray
structure determined at −100 °C (crystals of ligand 2 suitable
for X-ray analysis could not be obtained). Indeed the structure
shown in Fig. 3A is essentially planar with the two pyridine
rings mutually anti with respect to the central tolan moiety.
The central phenyl rings are essentially coplanar with a tor-
sional angle between them of approximately 3°. In contrast,
the pyridyls are twisted with respect to the central tolan moiety
in order to minimize the steric congestion between the hydro-
gens on the two pyridyl rings, specifically H4 and H26. The
pyridyl–phenyl torsional angle is about 15°. The major crystal
packing interaction in three dimensions is π-stacking with
closest interactions between adjacent rings around 3.4 Å. The
molecules are slip-stacked with a herringbone arrangement
between adjacent columns of π-stacked molecules (shown in
Fig. 3B and C) and one close C–H⋯π interaction of 2.743 Å
between H11 and phenyl ring (C16–C21) that is shown by the
dashed lines in Fig. 3B.
Fig. 2 Unsaturated ligands 1 and
2 have the appropriate alignment and
spacing for bidentate binding of the pyridinyl rings to select transition metals.
This binding event is predicted to enforce planarity in the conjugated backbone
of the structure.
structures, could serve the same purpose as 2,2′-bipyridines,
1,2-bis(2′-pyridinylethynyl)benzenes and quinoxaline-ethyny-
lenes. That is, introduction of transition metal cations should
enforce planarity, increasing the overall effective conjugation
in these structures. Because of the similar C–CuC–C and
N–Pd–N distances noted above, 1-M and 2-M can be thought
of as rhombus and parallelogram relatives, respectively, of the
triangular 1,2-bis(2′-pyridinylethynyl)benzene metal com-
plexes. Before incorporation of these structures into larger con-
jugated molecules, however, it must first be demonstrated (1)
that 1 and 2 act as bidentate ligands for transition metals, and
(2) that this binding event enforces coplanarity in the
backbone.
Results and discussion
Electronic properties of 1 and 2 and their complexes
X-ray analysis of coordination complexes
In a previous study, we have shown that introduction of tran-
sition metals to 1,2-bis(2′-pyridinylethynyl)benzene-based Single crystal X-ray structural analysis was used to confirm the
systems can lead to significant electronic changes in a conju- formation of the postulated planar rhombus and parallelo-
gated backbone.20 These types of changes likely require signifi- gram shaped coordination complexes (Table 1 and 2). Accord-
cant differences between the most favorable conformation of ingly, two coordination complexes of 1 were prepared and
the ligand and the planar complex. While this can be achieved analyzed by single crystal X-ray diffraction. Thus a solution of
in longer conjugated systems, small, conjugated molecules the ligand 1 in dichloromethane was mixed with an aceto-
such as ligand 1, ligand 2, and 1,2-bis(2′-pyridinylethynyl)- nitrile solution containing 1 equiv. of silver(I) trifluoroacetate.
benzene are unlikely to display dramatic, conformation-driven After crystal formation appeared complete, a single crystal was
electronic shifts upon metal coordination. For these small cut for analysis and the structure was determined at −100 °C.
molecules, the most favorable conformation likely involves The expected rhombus-shaped coordination complex was
coplanarity in the conjugated backbone. Any changes in conju- obtained as shown in Fig. 4A. The ligand is slightly bent and
gation upon metal complexation are therefore likely to be the tolan alkyne is not quite linear with bond angles C12–C14–
small.
C15 and C14–C15–C16 of 171.68 and 174.44° respectively. The
Consistent with this expectation, 1,2-bis(2′-pyridinylethynyl)- pyridyl–silver–pyridyl bond is similarly slightly bent with bond
benzene shows only a small bathochromic shift in its absorp- angle N1–Ag1–N2 of 165.07°. The silver nitrogen bond dis-
tion spectrum upon coordination to Pd(II).15 Similarly, dilute tances of 2.188 and 2.197 Å are within the normal range.23
samples (8.2 × 10−6 M–8.5 × 10−6 M in THF) of ligands 1 (λmax There is a relatively strong monodentate interaction between
= 275 nm, ε = 5.71 × 104 M−1 cm−1) and 2 (λmax = 277 nm, ε = the trifluoroacetate oxygen atom and the silver with a bond
5.89 × 104 M−1 cm−1) display only slight changes in molar distance O1–Ag1 of 2.514 Å and angles O1–Ag1–N1 and
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Fig. 3 (A) View of ligand 1 showing atom labeling. (B) Crystal packing of ligand 1 shown along the a-axis with the C–H-π interaction shown with dashed lines from
the H to the centroid of the benzene ring. (C) As for (B) viewed along the c-axis showing the herringbone arrangement of adjacent stacks of the ligand.
Table 1 Crystal data for ligand 1 and coordination complexes of ligand 1
Table 2 Crystal data for coordination complexes of ligand 2
2·AgCF3SO3·CH3OH
1
1·AgC2F3O2
1·PdCl2·CH2Cl2
2·PdCl2
Formula
C28H16N2
C30H16Ag
F5N2O2
601.32
Monoclinic
P21/n
16.8364(17)
7.9149(8)
19.851(2)
90
114.2540(10)
90
2411.8(4)
4
1.656
0.891
1200.0
C29H18 N2PdCl4
Formula
C30H24AgF3N2O4S
673.45
Monoclinic
P21/c
7.5430 (9)
18.135 (2)
20.575 (3)
90
95.252 (4)
90
2802.7 (6)
4
1.656
0.85
1360.0
0.60 × 0.10 × 0.06
200
2.9–17.6
43 347
4958
4958/0/372
1.046
C28H20N2PdCl2
561.76
Orthorhombic
PFddd
16.0921(13)
27.4790(13)
44.234(3)
90
90
90
19 560(2)
32
1.526
0.996
9024.0
0.29 × 0.20 × 0.05
173(2)
2.6–27.0
55 436
5472
5472/0/298
1.087
M/g mol−1
M/g mol−1
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
β/°
380.43
Monoclinic
P21/c
12.7773(14)
14.4079(15)
12.3216(13)
90
117.4760(10)
90
2012.5(4)
4
1.256
1.061
792
642.65
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
β/°
Triclinic
ˉ
P1
8.5163(5)
11.2205(6)
14.9946(12)
102.112(1)
95.543(1)
107.875(1)
1313.21(15)
2
1.625
1.136
640.0
0.40 × 0.20 ×
0.20
γ/°
γ/°
V/Å3
V/Å3
Z
Z
ρcalcd./g cm−3
ρcalcd./g cm−3
μ/mm−1
F(0,0,0)
μ/mm−1
F(0,0,0)
Crystal size, mm
Temp./K
Crystal size, mm
0.28 × 0.20 ×
0.13
173(2)
1.8–27.2
23 414
0.40 × 0.07 ×
0.04
173(2)
1.3–27.2
27 258
θ range/°
Temp./K
θ range/°
Reflections
collected
173(2)
1.4–27.2
15 308
Reflections collected
Independent reflections
Data/restraints/parameters
Goof
Independent
reflections
Data/restraints/
parameters
Goof
4482
5356
5797
R(int)
0.093
0.041/0.104
0.041
0.026/0.068
Final R indices[I > 2σ(I)],
R1/wR2
4482/0/271
5356/0/343
5797/0/325
Largest diff. peak/hole/e Å3
0.59/−0.55
0.45/−0.32
1.005
0.058
1.008
0.088
1.065
0.014
R(int)
Final R indices
[I > 2σ(I)], R1/wR2
Largest diff. peak/
hole/e Å3
0.045/0.115
0.047/0.104
0.020/0.053
solutions. After two weeks, large crystals had grown and a
single crystal was selected for analysis. The structure of the dis-
crete palladium complex, which includes a single dichloro-
methane solvent molecule, is shown in Fig. 5A. The palladium(II)
atom has square planar coordination geometry with angles
0.16/−0.18
0.83/−0.90
0.52/−0.68
O1–Ag1–N2 of 89.95 and 93.95° respectively. The slight butter- about palladium ranging from 87.66, 88.96, 90.22 and 93.12°.
fly effect is likely a manifestation of the crystal packing where The N1–Pd1–N2 bond angle is 176.49° with palladium–nitro-
the silver interacts weakly with the alkyne of the adjacent gen bond distances of 2.022 and 2.024 Å that are within the
complex as shown in Fig. 4B to form discrete π-stacked pairs of expected range.14,15 The overall structure is mostly planar, with
the complex.24 The pairs of complexes are then π-stacked in torsional angles between pyridine and benzene rings of no
columns which have a herringbone-like arrangement between more than 2° and a phenyl–phenyl angle of about 2.5°.
adjacent stacks as shown in Fig. 4C.
The chlorides of the palladium dichloride moiety are
A palladium coordination complex was prepared in a almost orthogonal to the essentially planar organic ligand and
similar way – a solution of the ligand 1 in dichloromethane thereby preclude face-to-face π-stacking so the complexes are
was layered with a solution containing one equivalent of bis- offset-stacked as shown in Fig. 5B and C.
(acetonitrile) palladium(II) dichloride. Within several hours
Single crystals of ligand 2 with AgOTf were obtained by slow
orange crystals started to form at the interface between the two evaporation of a 2 : 1 methanol–dichloromethane mixture in
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Fig. 4 (A) View of complex 1·AgCF3CO2 showing atom labeling. (B) Side view of a pair of the coordination complexes with the weak silver–alkyne interaction
shown with a dashed line from the silver to the centroid of the alkyne. (C) Crystal packing of the coordination complex viewed along the a-axis showing the herring-
bone arrangement of adjacent stacks of the complex.
Fig. 5 (A) View of complex 1·PdCl2·CH2Cl2 showing the atom labeling. (B) Oblique view of the crystal packing showing the offset π-stacking of complexes. (C) View
of the crystal packing along the a-axis.
Fig. 6 (A) View of complex 2·AgCF3SO3 showing the atom labeling. (B) Crystal packing of the coordination complex viewed along the b-axis showing a staggered
arrangement of adjacent complexes.
the dark. A rod-shaped crystal was cut and placed into a between each pyridyl ring and the phenyl ring it is connected
sample holder, which was then cooled to −73 °C. As seen in to. The palladium has square planar geometry with palla-
Fig. 6A, the dipyridyl ligand is essentially flat with all torsional dium–nitrogen bond distances of 2.028 Å and 2.015 Å and
angles deviating from planarity by less than 4.0°. The N1–Ag1– bond angles about the palladium ranging from 88.38 to
N2 bond angle (174.66°) and the Ag–N bond lengths (Ag1–N1: 91.54°. The N1–Pd1–N2 bond is 179.81°. The crystal packing
2.127 Å, Ag1–N2: 2.141 Å) are close to what is expected for a involves π-stacking interactions with the orthogonal palladium
two coordinate dipyridyl silver complex.
dichloride moiety controlling the extent of overlap as shown in
The staggered orientation of the complex (Fig. 6B) is similar Fig. 7B and C.
to what has been reported for the triangular complex.14 Tri-
NMR titration of ligands 1 and 2 with AgOTf and
PdCl2(PhCN)2
fluoromethanesulfonate anions and methanol molecules are
located along the stacks of silver complexes with methanol
forming a hydrogen bond with an oxygen atom of the triflate.
It is clear from crystal structure analyses that ligands 1 and 2
The complex of ligand 2 with palladium dichloride was form bidentate complexes with Ag(I) and Pd(II) in the solid
formed and analyzed in a manner similar to complex 1·PdCl2. state. To confirm this behavior in solution, NMR titration
The coordination complex is planar with a torsional angle of studies were performed. Introduction of small amounts of
approximately 3° between the benzene rings and about 1.4° PdCl2(PhCN)2 led to a mixture of uncomplexed ligands 1 and 2
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1
and metal complexes 1-Pd and 2-Pd, respectively (Fig. 8). This dissociation of the ligands and Ag(I) cations, separate H NMR
complexation event was evidenced by significant downfield signals for complex and unbound ligand were not observed in
shifting of proton signals. The three signals for pyridine hydro- DMSO-d6. Instead, increased levels of complexation were
gens at the 2 and 6 positions, for instance, are considerably accompanied by gradual increases in chemical shifts (Fig. 9).
downfield in complexes 1-Pd and 2-Pd from those of uncom- Therefore, binding stoichiometry could not be ascertained via
plexed ligands 1 and 2 (Fig. 8). An additional two downfield integration, but rather continuous variation Job plot ana-
signals found for 2-Pd in this downfield region are tentatively lyses.25,26 To this end, a variety of solutions with different
assigned to intra-annular phenyl and vinyl hydrogens in the ligand : metal ratios, but a constant combined ligand + metal
metallorganic macrocycle. Both ligands were found to bind concentration (10.3 mM in DMSO-d6), were analyzed via 1H
stoichiometrically to Pd(II) in DMSO-d6 with mixtures of NMR spectroscopy. Plots of Δδ × [ligand] vs. mole fraction Ag(I)
unbound ligand and complex when less than one equivalent reveal maxima at 0.5 mole fraction AgOTf, which is consistent
of Pd(II) is introduced, but no signal from unbound ligands at with 1 : 1 binding ratios of ligand 1 and ligand 2 with the Ag(I)
one equivalent Pd(II) or above. As expected for a system with cation (Fig. 10).
1 : 1 binding, no further changes are observed with additional
PdCl2(PhCN)2.
As the primary motivation of this study is to generate conju-
gated frameworks that can be incorporated into functional
In relation to Pd(II) studies, titrations with AgOTf were organic materials and/or metal sensors, the binding strength
much less straightforward. Due to rapid complexation/ of the ligands to the studied metals is of interest. As men-
tioned above, ligands 1 and 2, like 1,2-bis(2′-pyridinylethynyl)-
benzene, bind very efficiently to Pd(II) cations, even at high
dilution. Because complexation to Ag(I) is dynamic on the
NMR timescale, we anticipated this cation would provide a
better platform for ligand comparison. At high concentration,
complexation of these ligands to Ag(I) begins to approach
stoichiometric binding. Consequently, titration experiments
with our ligands had to be performed under very dilute con-
ditions in order to achieve optimal equilibrium conditions.
This high dilution was necessary to achieve an appropriate “p”
value (actual concentration of complex/maximum possible
concentration of complex) between 0.2 and 0.8, as defined by
Fig. 7 (A) View of complex 2·PdCl2 showing atom labeling. (B) View of offset
Weber.27 Even at high dilution, samples that contained a large
π-stacking of two adjacent coordination complexes. (C) Side view of the two
complexes in (B) with the atoms shown using the space filling option.
ratio of ligand to metal provided unreliable association
Fig. 8 Dilute samples in DMSO-d6 (0.3–0.9 mM) with different mole fractions of PdCl2(PhCN)2 and (a) ligand 1 and (b) ligand 2 were studied via 1H-NMR spec-
troscopy, revealing growth of signals corresponding to complexes 1-Pd and 2-Pd and diminishing signals for 1 and 2 upon addition of Pd(II).
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Fig. 9 Introduction of AgOTf to dilute solutions (0.28–0.88 mM) of ligands 1 and 2 in DMSO-d6 leads to gradual downfield shifting of 2/6 pyridine hydrogen
resonances in (a) ligand 1 and (b) ligand 2.
Fig. 10 Continuous variation Job plot analyses of 1H NMR titration data (initial ligand concentrations of 1.7–9.5 mM in DMSO-d6) confirm that (a) ligand 1 and
(b) ligand 2 bind to Ag(I) cations with 1 : 1 stoichiometry.
constants. Once these limitations were identified, we were able more tightly to the silver cation than ligands 1 and 2, as com-
to identify association constants with reproducibility.
plexation to the former requires restriction of fewer degrees of
A variety of NMR samples containing different ratios of freedom. Differences between 1 and 2 are a little more surpris-
ligand to metal were made. Final concentrations of ligand 1 ing, however, as they possess very similar structures. We tenta-
and ligand 2 ranged from 0.28 to 0.90 mM (in DMSO-d6) and tively ascribe the stronger binding of
2 to electronic
final Ag(I) concentrations ranged from 0.30 to 0.90 mM. The substituent effects. Based upon reported Hammet constants,
chemical shifts observed for these samples were compared to styryl substituents can be slightly electron-donating to margin-
the chemical shifts of the free ligands (0.6 mM) and the totally ally electron-withdrawing.29 Phenylethynyl substituents, on the
saturated ligands (solid AgOTf added until no further changes other hand, are significantly more electron-withdrawing, likely
in chemical shift were observed). The concentration of decreasing the basicity of the pyridine lone pair, and therefore
complex in each sample was calculated using the equation: decreasing the binding constant. Steric hindrance between
((δobs − δfree)/(δsat − δfree)) × [Ligand], where δobs is the chemical intra-annular hydrogen atoms could also destabilize a 1-Ag+
shift observed for the equilibrating complex, δfree is the chemi- complex. This interaction would be less problematic in the
cal shift of the free ligand, and δsat is the chemical shift of the 2-Ag+ complex, as the intra-annular hydrogens in this species
saturated system. With complex concentrations in hand, are offset.
association constants could be calculated. Averaging all asso-
Ligand preparation
ciation constants obtained from different hydrogens and
different trials, K values of 1.3 × 102 M−1 for ligand 1 and 5.4 × Ligand 1 was generated primarily via Sonogashira coupling
102 M−1 for ligand 2 were obtained.
reactions of commercially available haloarenes with commer-
Comparing these ligands with each other and 1,2-bis(2′- cially available ethynylpyridines (Scheme 1). 3-Ethynylpyridine
pyridinylethynyl)benzene (K = 1.3 × 103 M−1 28
is instructive. It is selectively coupled to the iodo position of 2-bromoiodo-
is not surprising that 1,2-bis(2′-pyridinylethynyl)benzene binds benzene upon treatment with appropriate catalysts to yield
)
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added and argon was bubbled through the mixture for
15 minutes. Terminal alkyne 4 (1.39 g, 6.85 mmol), dissolved
in a minimal amount of diisopropylamine, was then added.
After argon was bubbled through this mixture for 15 minutes,
the vessel was sealed and heated at 80 °C for 24 hours. The
resulting mixture was diluted with diethyl ether and the inso-
luble salts were removed via gravity filtration. The filtrate was
washed with water, dried with anhydrous Na2SO4, filtered and
concentrated under reduced pressure. The product was puri-
fied with consecutive applications of flash chromatography
(1st: 1% EtOAC/99% ether, 2nd: 100% ether; both on silica
gel). Appropriate fractions were concentrated to reveal product
1 as a white solid (1.19 g, 3.14 mmol, 49% yield, MP =
106–107 °C). 1H-NMR (400 MHz, DMSO-d6): δ = 8.82 (s, 1H),
8.68 (d, J = 4.2 Hz, 1H), 8.63 (d, J = 4.0 Hz, 1H), 8.04 (dt, J = 8.0,
1.9 Hz, 1H), 7.92 (td, J = 7.7, 1.8 Hz, 1H), 7.79 (m, 1H),
7.77–7.64 (m, 5H), 7.61–7.44 (m, 5H) ppm. 13C-NMR
(100.6 MHz, DMSO-d6): δ = 152.4, 151.1, 150.2, 142.9, 139.4,
137.7, 135.1, 133.0, 132.88, 132.86, 130.6, 130.3, 130.1, 128.5,
125.5, 125.2, 124.7, 123.7, 123.1, 120.1, 93.4, 91.7, 91.2, 90.6,
89.7, 87.9 ppm. ESI-MS (m/z) calcd for C28H16N2H+ 381.1386;
found 381.1401. λmax nm (ε, M−1 cm−1) (THF) = 275 (57 100).
[1-Ag]OTf. 1H-NMR (400 MHz, DMSO-d6): δ = 9.10 (m, 1H),
8.79 (m, 1H), 8.74 (dd, J = 5.2, 1.4 Hz, 1H), 8.26 (dt, J = 8.0,
1.9 Hz, 1H), 8.14 (td, J = 8.0, 1.7 Hz), 7.96 (m, 2H), 7.75 (m,
6H), 7.57 (m, 3H) ppm. λmax nm (ε, M−1 cm−1) (THF) = 273
(50 900), 300 (35 000).
Scheme 1 Generation of Ligand 1.
Scheme 2 Generation of Ligand 2.
1-PdCl2. 1H-NMR (400 MHz, DMSO-d6): δ = 9.02 (d, J =
1.9 Hz, 1H), 8.96 (d, J = 5.7 Hz, 1H), 8.84 (d, J = 5.0 Hz, 1H),
8.30 (dt, J = 5.7, 1.6 Hz, 1H), 8.20 (t, J = 1.4 Hz, 1H), 8.07 (td,
7.8, 1.4 Hz, 1H), 7.89 (d, J = 7.2 Hz, 1H), 7.85 (m, 2H), 7.76 (m,
2H), 7.70 (m, 2H), 7.63 (m, 1H), 7.56 (m, 2H) ppm. λmax nm
(ε, M−1 cm−1) (THF) = 274 (67 600).
compound 3. Under similar conditions, trimethylsilylacetylene
is coupled to bromoarene 3. Removal of the trimethylsilane
protecting group can be achieved via treatment with a basic
methanol solution, yielding 4. Sonogashira coupling of 2-ethy-
nylpyridine with 1-bromo-3-iodobenzene at room temperature
provides compound 5. Terminal alkyne 4 and bromoarene 5
can then be linked using coupling catalysts with electron-rich/
sterically encumbered phosphine ligands at 80 °C, affording 1
in reasonable yields.
2-((E)-3-((2-((E)-2-(Pyridin-3-yl)vinyl)phenyl)ethynyl)styryl)-
pyridine (2)
Compound 8 (0.205 g, 0.778 mmol) was mixed with diisopro-
pylamine (10 mL) under an argon atmosphere. CuI (0.0105 g,
0.055 mmol), X-Phos (0.0255 g, 0.053 mmol) and
PdCl2(PhCN)2 (0.0208 g, 0.054 mmol) were added and argon
was bubbled through the mixture for 15 minutes. Terminal
alkyne 7 (0.245 g, 1.19 mmol) was added and the mixture was
heated at 40 °C for two days. The mixture was rinsed into a
flask with chloroform then concentrated under reduced
pressure. The resulting residue was purified via flash chro-
matography (99% CH2Cl2/1% CH3OH on silica gel). Appropri-
ate fractions were concentrated to reveal the solid product
The condensation of 2-picoline with 3-bromobenzaldehyde
in refluxing acetic anhydride offers the trans isomer of vinyl-
pyridine 6 in high yields after basic workup. Alkene 8 is
obtained in similarly high yields via Horner–Wadsworth–
Emmons reaction of a phosphoester (obtained from 2-bromo-
benzylbromide) with 3-pyridinecarboxaldehyde. Linking these
two isomers with an ethynyl group requires a familiar coup-
ling–deprotection–coupling sequence, which gives ligand 2 in
reasonable yields (Scheme 2).
1
(0.191 g, 0.497 mmol, 64% yield, MP = 136–137 °C). H-NMR
(400 MHz, CDCl3): δ = 8.79 (s, 1H), 8.61 (d, J = 3.6 Hz, 1H), 8.51
(s, 1H), 7.86 (d, J = 7.9 Hz, 1H), 7.78 (d, J = 16.3 Hz, 1H), 7.77
(s, 1H), 7.70 (d, J = 7.9 Hz, 1H), 7.64 (td, J = 5.8, 1.7 Hz, 1H),
7.63 (d, J = 16.0 Hz, 1H), 7.58 (dd, J = 7.7, 1.2 Hz, 1H), 7.55 (d,
Experimental
2-((3-((2-(Pyridin-3-ylethynyl)phenyl)ethynyl)phenyl)ethynyl)-
pyridine (1)
Bromoarene 5 (1.64 g, 6.37 mmol) was dissolved in a minimal J = 7.8 Hz, 1H), 7.48 (dt, J = 7.7, 1.3 Hz, 1H), 7.40–7.23 (m, 5H),
amount of diisopropylamine and was transferred to a reaction 7.18 (d, J = 16.0 Hz, 1H), 7.16 (d, J = 16.6 Hz, 1H), 7.14 (m, 1H)
vessel. PdCl2(PhCN)2 (0.128 g, 0.33 mmol), CuI (0.063 g, ppm. 13C-NMR (100.6 MHz, CDCl3): δ = 155.2, 149.7, 148.8,
0.33 mmol) and [(t-Bu)3PH]BF4 (0.096 g, 0.33 mmol) were 148.7, 138.0, 137.0, 136.5, 133.1, 132.83, 132.76, 131.6, 131.1,
954 | Dalton Trans., 2013, 42, 948–958
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129.9, 128.91, 128.87, 128.75, 128.7, 127.8, 127.3, 126.6, 124.9, 138.4, 132.4, 131.8, 128.4, 128.3, 125.8, 125.3, 123.0, 120.5,
123.7, 123.6, 122.4, 122.32, 122.27, 94.5, 87.9 ppm. ESI-MS 103.2, 99.0, 91.4, 89.8, 0.0 ppm. ESI-MS (m/z) calcd for
(m/z) calcd for C28H20N2H+ 385.1705; found 385.1708. λmax nm C18H17NSiH+ 276.1203; found 276.1213. This oil was dissolved
(ε, M−1 cm−1) (THF) = 277 (58 900), 314 (54 800).
in a mixture of methanol (10 mL) and THF (10 mL). Enough
[2-Ag]OTf. 1H-NMR (400 MHz, DMSO-d6): δ = 9.57 (m, 1H), K2CO3 was added to create a saturated solution and the reac-
8.78 (d, J = 5.6 Hz, 1H), 8.64 (m, 1H), 8.43 (s, 1H), 8.25 (d, J = tion was stirred at room temperature for three days. Undis-
8.4 Hz, 1H), 8.11 (m, 2H), 8.03 (d, J = 16.6 Hz, 1H), 7.99 (d, J = solved solid was removed via gravity filtration and the
7.8 Hz, 1H), 7.78 (m, 2H), 7.72 (m, 1H), 7.65 (d, J = 6.8 Hz, 1H), resulting mixture was flushed through a silica gel column with
7.63–7.47 (m, 6H), 7.42 (m, 1H) ppm. λmax nm (ε, M−1 cm−1
(THF) = 287 (63 900), 309 (53 500).
)
chloroform. Concentration of appropriate fractions revealed
the product as a yellow solid (0.785 g, 3.86 mmol, 78% yield).
2-PdCl2. 1H-NMR (400 MHz, DMSO-d6): δ = 9.47 (d, J = 1H-NMR (400 MHz, CDCl3): δ = 8.80 (dd, J = 2.1, 0.8 Hz, 1H),
17.0 Hz, 1H), 9.45 (s, 1H), 8.94 (d, J = 5.3 Hz, 1H), 8.64 (m, 8.56 (dd, J = 4.9, 1.7 Hz, 1H), 7.84 (dt, J = 8.0, 1.8 Hz, 1H), 7.56
2H), 8.16 (d, J = 7.4 Hz, 1H), 8.13 (d, J = 16.5 Hz, 1H), 8.02 (m, (d, J = 2.0, 1H), 7.55 (d, J = 2.2 Hz, 1H), 7.39–7.27 (m, 3H), 3.38
3H), 7.75 (d, J = 16.4 Hz, 1H), 7.66 (m, 2H), 7.60 (m, 3H), (s, 1H) ppm. 13C-NMR (100.6 MHz, CDCl3): δ = 152.4, 148.8,
7.55–7.41 (m, 4H) ppm. λmax nm (ε, M−1 cm−1) (THF) = 285 138.5, 132.7, 131.9, 128.6, 128.5, 125.6, 124.8, 123.0, 120.4,
(61 300), 311 (46 400).
91.0, 90.0, 82.0, 81.4 ppm. ESI-MS (m/z) calcd for C15H9NH+
204.0808; found 204.0808.
3-((2-Bromophenyl)ethynyl)pyridine (3)
2-((3-Bromophenyl)ethynyl)pyridine (5)
PdCl2(PPh3)2 (0.284 g, 0.40 mmol) and CuI (0.209 g,
1.10 mmol) were added to a reaction vessel. The tube was eva- PdCl2(PPh3)2 (0.249 g, 0.35 mmol) and CuI (0.082 g,
cuated and purged with Ar (×3). Freshly distilled triethylamine 0.43 mmol) were added to a reaction vessel. The tube was evac-
(20 mL) was added, followed by 2-bromoiodobenzene uated and purged with Ar (×3). Freshly distilled triethylamine
(0.91 mL, 7.07 mmol) and 3-ethynylpyridine (0.893 g, (20 mL) was added, followed by 3-bromoiodobenzene
8.66 mmol). The tube was sealed and the contents stirred for (0.90 mL, 7.07 mmol) and 2-ethynylpyridine (0.79 mL,
4 days at room temperature. The resulting mixture was diluted 7.78 mmol). The tube was sealed and the contents stirred for
with diethyl ether and gravity filtered. The filtrate was washed 24 hours. The resulting mixture was diluted with diethyl ether
with water, dried with anhydrous Na2SO4, filtered and concen- then gravity filtered to remove insoluble salts. The filtrate was
trated. This residue was purified via flash chromatography washed with water, dried with anhydrous Na2SO4, filtered and
(25% EtOAc/75% hexanes on silica gel). The product was concentrated under reduced pressure. The product was puri-
revealed as a yellowish oil (1.54 g, 5.98 mmol, 85% yield). fied using flash chromatography (10% EtOAc/90% hexanes
1H-NMR (400 MHz, CDCl3): δ = 8.81 (dd, J = 2.8, 0.7 Hz, 1H), with slow increases in polarity, on silica gel). Appropriate frac-
8.56 (dd, J = 4.9, 1.7 Hz, 1H), 7.83 (dt, J = 7.9, 1.9 Hz, 1H), 7.61 tions were concentrated to reveal the product as an off-white
(dd, J = 8.0, 1.1 Hz, 1H), 7.56 (dd, J = 7.7, 1.6 Hz, 1H), 7.29 (td, solid (1.64 g, 6.35 mmol, 90% yield). 1H-NMR (400 MHz,
J = 7.6, 1.2 Hz, 1H), 7.27 (m, 1H), 7.19 (td, J = 7.8, 1.7 Hz, 1H) CDCl3): δ = 8.62 (d, J = 4.8 Hz, 1H), 7.74 (t, J = 1.7 Hz, 1H), 7.67
ppm. 13C-NMR (100.6 MHz, CDCl3): δ = 152.2, 148.9, 138.5, (td, J = 7.7, 1.6 Hz, 1H), 7.50 (m, 3H), 7.22 (m, 2H) ppm.
133.3, 132.5, 129.9, 127.1, 125.6, 124.7, 123.0, 120.1, 91.1, 13C-NMR (100.6 MHz, CDCl3): δ = 150.13, 143.0, 136.2, 134.7,
90.4 ppm. ESI-MS (m/z) calcd for C13H879BrNH+ 257.9913; 132.1, 130.5, 129.8, 127.2, 124.2, 123.0, 122.2, 89.8, 87.4 ppm.
found 257.9913.
ESI-MS (m/z) calcd for C
13H879BrNH+ 257.9913; found
257.9937.
3-((2-Ethynylphenyl)ethynyl)pyridine (4)
(E)-2-(3-Bromostyryl)pyridine (6)
PdCl2(PPh3)2 (0.218 g, 0.31 mmol) and CuI (0.0.057 g,
0.30 mmol) were added to a reaction vessel. The tube was eva- 3-Bromobenzaldehyde (3.15 mL, 27.0 mmol), acetic anhydride
cuated and purged with Ar (×3). Bromoarene 3 (1.59 g, (5.10 mL, 54.0 mmol) and 2-picoline (2.67 mL, 27.0 mmol)
6.18 mmol) was dissolved in freshly distilled triethylamine were refluxed in a 140 °C oil bath for one day. Another equi-
(20 mL) and transferred to the reaction vessel. Trimethylsilyl- valent of 2-picoline (2.67 mL, 27.0 mmol) was added and the
acetylene (0.95 mL, 6.75 mmol) was added and the tube sealed mixture was refluxed for another three days. Acetic anhydride
and heated at 100 °C for three days. The resulting mixture was was removed via distillation. While cooling with an ice bath,
diluted with diethyl ether and gravity filtered. The filtrate was the reaction was quenched with water. 5% NaOH was added
washed with water, dried with anhydrous Na2SO4, filtered and until the mixture was basic. The product was extracted with
concentrated. The residue was purified via flash chromato- diethyl ether. This organic phase was washed with water, dried
graphy (5% EtOAc/95% hexanes on silica gel) revealing the with anhydrous Na2SO4, filtered and concentrated under
TMS-protected product as a nearly colorless oil (1.37 g, reduced pressure. The resulting residue was purified via flash
4.97 mmol, 81% yield). 1H-NMR (400 MHz, CDCl3): δ = 8.80 chromatography (80% hexane/20% EtOAc, on silica gel).
(dd, J = 2.1, 0.8 Hz, 1H), 8.56 (dd, J = 4.9, 1.7 Hz, 1H), 7.82 (dt, Appropriate fractions were concentrated to reveal the product
J = 7.9, 1.9 Hz, 1H), 7.56–7.49 (m, 2H), 7.34–7.25 (m, 3H), 0.27 as a yellow-brown solid (5.34 g, 20.5 mmol, 76% yield).
(s, 9H) ppm. 13C-NMR (100.6 MHz, CDCl3): δ = 152.3, 148.7, 1H-NMR (400 MHz, CDCl3): δ = 8.61 (d, J = 4.0 Hz, 1H), 7.72
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(t, J = 1.5 Hz, 1H), 7.65 (td, J = 7.7, 1.8 Hz, 1H), 7.56 (d, J = 16.0 diluted in anhydrous THF (7.5 mL). This solution was slowly
Hz, 1H), 7.46 (d, J = 7.7 Hz, 1H), 7.40 (d, J = 7.9 Hz, 1H), 7.35 added to a cold mixture (5 °C) of KOtBu in THF (35 mL). At
(d, J = 7.8 Hz, 1H), 7.25 (d, J = 8.2 Hz, 1H), 7.21 (d, J = 7.8 Hz, this temperature, a solution of pyridine-3-carboxaldehyde
1H), 7.15 (m, 1H), 7.13 (d, J = 16.0 Hz, 1H) ppm. 13C-NMR (2.07 mL, 22.1 mmol) in THF (5 mL) was added. After addition
(100.6 MHz, CDCl3): δ = 155.0, 149.7, 138.9, 136.6, 131.09, was complete, the resulting mixture was stirred for one hour at
131.06, 130.2, 129.7, 129.3, 125.8, 122.9, 122.44, 122.40 ppm. 0 °C. The reaction was quenched with 1 N HCl (40 mL). After
ESI-MS (m/z) calcd for C13H1079BrNH+ 260.0075; found separation of layers, the organic phase was washed with brine,
260.0070.
dried with anhydrous Na2SO4, filtered and concentrated under
reduced pressure. The resulting residue was purified via flash
chromatography (85% ether/15% hexane on silica gel). Appro-
(E)-2-(3-Ethynylstyryl)pyridine (7)
Compound 6 (1.13 g, 4.34 mmol) was mixed with diisopropyl- priate fractions were concentrated to reveal the product
amine (10 mL) under an atmosphere of argon. To this was (3.55 g, 13.6 mmol, 68% yield). 1H-NMR (400 MHz, CDCl3): δ =
added PdCl2(PPh3)2 (0.152 g, 0.22 mmol), CuI (0.058 g, 8.71 (d, J = 1.6 Hz, 1H), 8.50 (dd, J = 4.7, 0.7 Hz, 1H), 7.85 (dt,
0.30 mmol), and another 10 mL diisopropylamine followed by J = 8.0, 1.7 Hz, 1H), 7.64 (dd, J = 3.8, 1.4 Hz, 1H), 7.57 (dd, J =
trimethylsilylacetylene (0.61 mL, 4.3 mmol). This reaction 8.0, 0.8 Hz, 1H), 7.50 (d, J = 16.3 Hz, 1H), 7.25–7.32 (m, 2H),
mixture was heated at 80 °C for four days. The volatile contents 7.12 (td, J = 7.7, 1.6 Hz, 1H), 6.97 (d, J = 16.3 Hz, 1H) ppm.
were removed under reduced pressure. The remaining residue 13C-NMR (100.6 MHz, CDCl3): δ = 148.9, 148.8, 136.4, 133.1,
was dissolved in diethyl ether, washed with water (×2) and 132.8, 132.6, 129.5, 129.3, 127.62, 127.56, 126.75, 124.2,
saturated NaCl solution, dried with anhydrous Na2SO4, filtered 123.6 ppm. ESI-MS (m/z) calcd for C13H10BrNH+ 260.0075;
and concentrated under reduced pressure. Purification was found 260.0069.
achieved via flash chromatography (70% hexane/30% EtOAc
on silica gel). Appropriate fractions were concentrated to reveal While ligands 1 and 2 are quite stable under standard atmos-
the TMS-protected product as beige solid (0.617 g, pheric conditions, Ag(I) and Pd(II) complexes show slow dis-
2.22 mmol, 51% yield). 1H-NMR (400 MHz, CDCl3): δ = 8.59 (d, coloration upon extended exposure to air/light.
Preparation of coordination complexes for X-ray analysis.
a
J = 4.7 Hz, 1H), 7.70 (t, J = 1.5 Hz, 1H), 7.61 (td, J = 7.7, 1.8 Hz,
1·AgCF3CO2. A solution of 1 (21.0 mg, 0.055 mmol) in
1H), 7.58 (d, J = 16.0 Hz, 1H), 7.47 (dt, J = 7.7, 1.3 Hz, 1H), dichloromethane (2 mL) was mixed with a solution of silver(I)
7.38 (dt, J = 7.7, 1.3 Hz, 1H), 7.32 (d, J = 7.8 Hz, 1H), 7.28 (t, J = trifluoroacetate (12.1 mg, 0.055 mmol) in acetonitrile (2 mL).
7.7 Hz, 1H), 7.14 (d, J = 16.0 Hz, 1H), 7.11 (ddd, J = 7.4, 4.7, After two days, crystals started to form and the solvent was
1.0 Hz, 1H), 0.27 (s, 9H) ppm. 13C-NMR (100.6 MHz, CDCl3): allowed to slowly evaporate until 1 mL of solvent remained.
δ = 155.6, 150.0, 137.1, 136.8, 132.0, 131.9, 130.6, 129.0, 128.9, Clear rod-shaped crystals (30.7 mg, 92%) were isolated.
127.7, 123.9, 122.6, 122.5, 105.2, 94.7, 0.3 ppm. ESI-MS (m/z)
1·PdCl2·CH2Cl2. A solution of bis(acetonitrile)palladium(II)
calcd for C18H19NSiH+ 278.1365; found 278.1364. Part of this dichloride (18.5 mg, 0.071 mmol) in acetonitrile (2 mL) was
solid (0.437 g, 1.58 mmol) was dissolved in equal parts metha- carefully layered over a solution of 1 (27.3 mg, 0.072 mmol) in
nol and THF. Enough K2CO3 was added to create a saturated dichloromethane (2 mL) and the solution allowed to stand.
solution and the reaction was monitored via TLC. After Block-shaped orange crystals started to form after several
40 minutes, the solvents were removed under reduced hours and, after standing for seven days, 39 mg (86%) of the
pressure. The residue was dissolved in diethyl ether, washed product was isolated.
with brine and H2O (×2), then was dried with anhydrous
2·PdCl2. A solution of bis(acetonitrile)palladium(II) dichlor-
Na2SO4, filtered and concentrated under reduced pressure. ide (10.6 mg, 0.041 mmol) in acetonitrile (2 mL) was carefully
NMR spectroscopy revealed this product to be of sufficient layered over a solution of 2 (17.6 mg, 0.045 mmol) in dichloro-
purity (0.257 g, 1.25 mmol, 79% yield). 1H-NMR (400 MHz, methane (2 mL) and the solution allowed to stand. Trapezoidal
CDCl3): δ = 8.59 (d, J = 4.0 Hz, 1H), 7.69 (s, 1H), 7.61 (td, J = orange crystals started to form after 16 hours and, after stand-
7.7, 1.8 Hz, 1H), 7.58 (d, J = 16.1 Hz, 1H), 7.52 (d, J = 7.8 Hz, ing for seven days, 17 mg (74%) of the product was isolated.
1H), 7.40 (dt, J = 7.7, 1.3 Hz, 1H), 7.32 (d, J = 7.8 Hz, 1H), 7.30
2·AgOTf. To a solution of 2 (5.0 mg, 0.013 mmol) in methyl-
(t, J = 7.7 Hz, 1H), 7.14 (d, J = 16.1 Hz, 1H), 7.11 (m, 1H), 3.11 ene chloride was carefully added methanol (1 mL) followed by
(s, 1H) ppm. 13C-NMR (100.6 MHz, CDCl3): δ = 155.2, 149.7, a solution of AgOTf (4.3 mg, 0.017 mmol) in methanol (1 mL).
136.9, 136.5, 131.7, 131.5, 130.5, 128.8, 128.7, 127.5, 122.6, After approximately 10 days in the dark, single crystals were
122.3, 122.3, 83.4, 77.4 ppm. ESI-MS (m/z) calcd for C15H11NH+ obtained by slow evaporation of the methanol–dichloro-
206.0970; found 206.0962.
methane mixture. The crystals were rod shaped growing
several hundred micrometers in length.
X-ray structure determination (1, 1-Ag, 1-Pd, 2-Pd). Crystals
2(E)-3-(2-Bromostyryl)pyridine (8)
2-Bromobenzyl bromide (5.00 g, 20.0 mmol) was converted to were mounted on a Kryoloop using viscous hydrocarbon oil.
its corresponding phosphoester via dropwise addition of Data were collected using a Bruker Apex2 CCD diffractometer
triethylphosphite (3.7 mL, 21.6 mmol) to a hot solution equipped with MoKα radiation with λ = 0.71073 Å. Data collec-
(100 °C) of the aryl halide solution in toluene (5 mL). After tion at low temperature, −100 °C, was facilitated by use of a
toluene was distilled off, the phosphoester was cooled and Kryoflex system with an accuracy of
1 K. Initial data
956 | Dalton Trans., 2013, 42, 948–958
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processing was carried out using the Apex II software suite.30 these ligands has been confirmed via X-ray crystallography
Structures were solved by direct methods using SHELXS-97 and 1H-NMR spectroscopy. This binding event leads to
and refined using standard alternating least-squares cycles metallorganic macrocycles that are constrained to planarity.
against F2 using SHELXL-97.31 The program X-Seed was used While this coplanarity does not lead to major electronic
as a graphical interface.32 Hydrogen atoms were placed in changes in the parent systems studied, we anticipate that
idealized positions and refined with a riding model. Crystallo- incorporation of these conjugated units into more elaborate
graphic details are summarized in ESI.† For complex structures will lead to unsaturated molecules with enhanced
1·AgCF3CO2 the largest Q-peaks corresponded to a second electronic properties and metal-ion sensing capabilities.
minor conformation of the trifluoromethyl group and this dis-
order was not incorporated into the final solution given its low
contribution. For complex 1·PdCl2·CH2Cl2 the largest Q-peaks
Acknowledgements
corresponded to a second minor position of the chlorides of
the dichloromethane and this disorder was not incorporated
into the final solution given its low contribution.
Acknowledgement is made to the donors of The American
Chemical Society Petroleum Research Fund for support of this
research. Additional support was provided by the Philip &
Helen Marshall Chemistry Research Fund, the UWSP College
of Letters and Sciences Undergraduate Education Initiative
and the UWSP University Personnel Development Committee.
NMR characterization was based upon work supported by the
National Science Foundation under CHE-0957080. HR-MS
characterization was based upon work supported by the
National Science Foundation under CBET-0958711. The Mis-
souri State University Provost Incentive Fund funded the pur-
chase of the X-ray diffractometer.
X-ray structure determination (2-Ag). A crystal was cut (0.60
× 0.10 × 0.06 mm) and placed quickly into the sample holder
of a Bruker Benchtop SMART X2S (Bruker AXS, 2011). The
sample in the instrument was cooled to −73 °C. Cell refine-
ment and data reduction was completed using SAINT v7.68a
(Bruker AXS, 2010). The structure was solved using
SHELXS-97.31 Software used to prepare the structure for publi-
cation included the Apex II software suite30 and PublCIF.33
Full details are provided in the ESI.†
NMR titration studies/Job’s plot analysis. Stock solutions of
1, 2, AgOTf and PdCl2(PhCN)2 were prepared by adding these
solids to 5.00 mL volumetric flasks and diluting to mark with
DMSO-d6: ligand 1 (29.9 mg, 0.0786 mmol, 0.0157 M), ligand 2
(29.0 mg, 0.0754 mmol, 0.0151 M), AgOTf (20.9 mg,
0.0813 mmol, 0.0163 M), and PdCl2(PhCN)2 (27.5 mg,
0.0717 mmol, 0.0143 M). Into an NMR tube was added
10.0–30.0 μL of stock ligand solution followed by 10.0–30.0 μL
of stock metal solution. These samples were diluted to a total
volume of 540 μL (final concentration: 0.3–0.9 mM), mixed
thoroughly (manual shaking of NMR tube), then analyzed via
NMR spectroscopy. Metal solids were added to one sample
until an endpoint was reached (i.e. no further changes in
chemical shift were observed). K values were calculated by
comparing chemical shifts of mixed samples with those of free
ligands and the saturated sample.
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