Synthesis of 4-[(2-imidazylethynyl)]-5-(2-pyridylethynyl)veratrole and characterization of the coordination complexes with silver(I) and palladium(II)

Article abstract of DOI:10.1016/j.ica.2019.119328

We describe the design, synthesis and characterization of the ligand, 4-[(2-imidazylethynyl)]-5-(2-pyridylethynyl)veratrole as well its coordination complexes with silver(I) and palladium(II). ¹H NMR titration clearly indicates that 1:1 coordination complexes are formed with both cations. The X-ray structure of the palladium dichloride complex confirms the trans coordination of palladium(II) and the silver trifluoromethanesulfonate complex confirms the linear coordination geometry about silver(I). [Formula presented]

Full text of DOI:10.1016/j.ica.2019.119328

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Research paper
Synthesis of 4-[(2-imidazylethynyl)]-5-(2-pyridylethynyl)veratrole and charac-
terization of the coordination complexes with silver(I) and palladium(II)
Chideraa I. Nwachukwu, Eric Bosch
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S0020-1693(19)31615-9
https://doi.org/10.1016/j.ica.2019.119328
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Inorganica Chimica Acta
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Please cite this article as: C.I. Nwachukwu, E. Bosch, Synthesis of 4-[(2-imidazylethynyl)]-5-(2-
pyridylethynyl)veratrole and characterization of the coordination complexes with silver(I) and palladium(II),
Inorganica Chimica Acta (2019), doi: https://doi.org/10.1016/j.ica.2019.119328
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Synthesis of 4-[(2-imidazylethynyl)]-5-(2-pyridylethynyl)veratrole and
characterization of the coordination complexes with silver(I) and palladium(II)
Chideraa I. Nwachukwu and Eric Bosch*
Chemistry Department, Missouri State University, 901 South National Avenue, Springfield,
Missouri, 65897, USA
*Dr. Eric Bosch
Chemistry Department
Missouri State University
901 South National Avenue
Springfield, MO USA 65897
Phone: 417-823-8735
FAX: 417-836-5507
email: ericbosch@missouristate.edu
web: https://chemistry.missouristate.edu/ericbosch.aspx

Abstract
We describe the design, synthesis and characterization of the ligand, 4-[(2-imidazylethynyl)]-5-
(2-pyridylethynyl)veratrole as well its coordination complexes with silver(I) and palladium(II).
1H NMR titration clearly indicates that 1:1 coordination complexes are formed with both cations.
The X-ray structure of the palladium dichloride complex confirms the trans coordination of
palladium(II) and the silver trifluoromethanesulfonate complex confirms the linear coordination
geometry about silver(I).
Keywords: Bidentate ligand, coordination complex, palladium(II), silver(I), trans-coordinating,
linear coordination

1. Introduction
Pyridines, and to a lesser extent imidazoles, have a rich history of forming coordination
complexes with silver(I) and palladium(II) that favor linear and square planar coordination
geometries, respectively. One of the enduring motivations for the preparation and study of silver
coordination complexes is their potential use in medicinal applications. Indeed, Medici and co-
workers recently outlined the ancient history and current potential of silver coordination
compounds in the medical field [1]. We have long been interested in the design of bipyridyls that
form discrete linear silver(I) coordination complexes and reported the synthesis of a trans
coordinating bidentate ligand, 1,2-bis(2-pyridylethynyl)benzene and its silver(I) and
palladium(II) complexes in their favored coordination geometries (linear and square planar
respectively) (A in Scheme 1) [2]. Thummel and coworkers simultaneously reported the
formation of both 1:1 and 2:1 complexes of the same ligand with palladium(II) [3], while Bunz
and coworkers reported the related ligand, 4,5-bis(2-pyridylethynyl)veratrole and its
coordination chemistry with cobalt(II) chloride, copper(II) acetate and rhodium(II) acetate [4].
More recently, we modified the ligand with heteroatom containing sidearms in the search for
enhanced binding to silver(I) cation [5]. It is noteworthy that palladium complexes based on the
trans-bipyridyl ligand framework and their analogues have been evaluated as catalysts in organic
transformations. For instance, 1,2-bis(2-pyridylethynyl)benzene palladium(II) complexes were
found to be highly efficient catalysts for the Heck reaction [6] and the Suzuki coupling reaction
[7], while the oxidative homocoupling of terminal acetylenes in air (Glaser coupling) and the
coupling of acid chlorides and alkynes were catalyzed by the palladium(II) complex of 1-(2-
pyridylethynyl)-2-(2-thienylethynyl)benzene [8,9]. The analogous bis thienyl ligand coupled
with PdCl2(PPh3)2 was found to catalyze the copper-free Sonogashira coupling reaction of

iodobenzene and alkynes [10]. Interestingly, the synthesis and evaluation of a rotaxane based
palladium catalyst in which the palladium was bound to a 1,2-bis(2-pyridylethynyl)benzene
moiety has been reported [11]. Surprisingly, no related imidazolyl ligands have been reported.
We reasoned that a variation of the original ligand including one pyridine and one imidazole
moiety would potentially provide access to a neutral silver(I) complex after coordination
followed by deprotonation. Given the increasing application of silver, we envisage the need for
silver selective ligands that can be modified, and perhaps incorporated on a solid substrate, for
use in recovery and extraction and reasoned that the neutral complex would be more stable and
have higher potential than a cationic analogue. Herein we describe the synthesis and
characterization of the pyridyl-imidazolyl ligand 4-[(2-imidazylethynyl)]-5-(2-
pyridylethynyl)veratrole (1 in Scheme 1) as well its coordination complexes with silver(I) and
palladium(II).
Scheme 1
2. Experimental
2.1 Materials

The bulk solvents, hexanes, ethylacetate and dichloromethane used were purchased from Fisher
Scientific and used as receivedDichlorobis(triphenylphosphine) palladium (II) was purchased
from Pressure Chemical co. Imidazole, 1,2-dimethoxybenzene (veratrole), silver (I)
trifluoromethanesulfonate, copper iodide, trifluoroacetic acid, n-butyllithium sodium hydride ,
triethylamine and tetrahydrofuran were purchased from Sigma Aldrich. 2-Ethynylpyridine was
purchased from GFS chemicals. 1-(Trimethylsilyl ethoxy methyl) chloride (SEMCl) was
purchased from TCI America. Silver trifluoroacetate and dichlorobis(acetonitrile)palladium II
were purchased from Strem chemicals. All chemicals were used as received. Column
chromatography was carried out using 230-400 mesh silica gel, purchased from Dynamic
Adsorbents, Inc.
1H and 13C NMR spectra were recorded on Varian 400 Unity INOVA spectrometer. All 1H NMR
spectra are reported in parts per million (ppm) relative to residual CHCl3 (7.23 ppm) or DMF-d7
(8.03 ppm). Coupling constants, J, are reported in Hertz (Hz). All 13C NMR spectra are reported
in ppm relative to residual CHCl3 (77.23 ppm). Elemental analyses were performed by Atlantic
Microlabs.
4,5-Diiodoveratrole, [12] 1-[[2-(trimethylsilyl)ethoxy]methyl]-imidazole [13] and 1-[[2-
(trimethylsilyl)ethoxy]methyl]-2-iodoimidazole [14] were synthesized as reported.
2.2 Synthesis
The ligand, 4-[(2-imidazoylethynyl)]-5-(2-pyridylethynyl)veratrole, was synthesized by
successive Sonogashira coupling reactions followed by deprotection as outlined in Scheme 2.

Scheme 2
2.2.1 Synthesis of 4-iodo-5-(2-ethynylpyridine) veratrole, 2.
1,2-Diiodo veratrole (5.00 g, 12.8 mmol), PdCl2(PPh3)2 (0.100 g, 0.142 mmol), CuI (92 mg, 0.49
mmol), 2-ethynylpyridine (0.7 ml, 0.660 g, 6.40 mmol), THF (30 mL) and anhydrous
triethylamine (10 mL) were reacted under argon atmosphere at room temperature for 4 days. The
mixture was diluted with ethyl acetate (200 mL) and washed with water (100 mL x 3) followed
by brine (100 mL x 3). The organic layer was separated, dried over anhydrous Na2SO4, filtered
and solvent removed in vacuo. The crude solid was purified by flash column chromatography
with a 5:1 mixture of hexane and ethyl acetate as the eluent. The product was obtained as a
brown oil which solidified after 2 days (1.44 g, 3.94 mmol, 62%). Rf = 0.27 [EtOAc/hexane
(2:1)]. Mp: 89.5–90.1 ˚C. 1H NMR (400 MHz, CDCl3): δ 8.64 (d, 3J = 4.8 Hz, 1 H), 7.74 (td, 3J =
7.8 Hz, 4J = 2.0 Hz, 1 H), 7.64 (d, 3J = 7.8 Hz, 1 H), 7.28 (m, 2 H), 7.26 (s, 1 H), 3.90 (s, 3 H),
3.88 ppm (s, 3 H). 13C NMR (100 MHz, CDCl3): δ = 150.5, 149.8, 149.2, 143.2, 136.8, 127.6,
123.1, 121.1, 121.0, 115.6, 92.5, 90.6, 90.2, 56.4, 56.3 ppm. 1,2-Bis(2-pyridylethynyl) veratrole
was also isolated as a brown solid (0.612 g, 1.80 mmol, 26%) along with a small amount of
unreacted 4,5-diiodoveratrole that was not quantified.
2.2.2 Synthesis of 1-[[2-(trimethylsilyl)ethoxy]methyl]-2-[2 (trimethylsilyl)ethynyl]imidazole

1-[[2-(Trimethylsilyl)ethoxy]methyl]-2-iodoimidazole (2.00 g, 6.17 mmol) and
anhydrous triethylamine (15 mL) were added to a pressure flask under argon. PdCl2(PPh3)2 (100
mg, 0.142 mmol), CuI (45 mg, 0.24 mmol) and trimethylsilyl acetylene (1.77 g, 2.50 mL, 18.2
mmol) were added under continuous argon flow. After 5 minutes the argon flow was stopped and
the pressure flask sealed. The reaction was heated to 60 °C for 40 h and then cooled to room
temperature. The solvent was removed in vacuo and the residue dissolved in CH2Cl2 (100 mL).
The organic layers was washed with brine (80 mL x 2) and subsequently dried over anhydrous
Na2SO4. The solvent was removed under reduced pressure, and the crude product purified by
flash column chromatography with a mixture of 20:1 hexane and ethyl acetate as the eluent.
After purification, the product was obtained as a yellow oil (0.812 g, 44%). Rf = 0.3
[EtOAc/hexane (2:1)]. 1H NMR (400 MHz, CDCl3): δ 7.04 (d, 3J = 3.2 Hz, 2 H), 5.34 (s, 2 H),
3.43 (t, 3J = 8.4 Hz, 2 H), 0.87 (t, 3J = 8.4 Hz, 2 H), - 0.12 (s, 9 H), -0.06 ppm (s, 9 H).
2.2.3 Synthesis of 1-{[2-(trimethylsilyl)ethoxy]methyl}-2-ethynylimidazole, 3.
A solution of potassium hydroxide (300 mg, 5.35 mmol) in water (0.5 mL) was added to
a solution of 1-[[2-(trimethylsilyl)ethoxy]methyl]-2-[2 (trimethylsilyl)ethynyl]imidazole (0.810
g, 2.72 mmol) in ethanol (10 mL). The mixture was stirred at room temperature and the reaction
monitored by TLC. After 2 h, the solvent was evaporated, and the residue dissolved in CH2Cl2
(60 mL), washed with brine (50 mL x 2) and dried over anhydrous Na2SO4. The solvent was
removed in vacuo and the crude product purified by flash column chromatography with a
mixture of 10:1 hexane and ethyl acetate as the eluent. Upon purification, the product was
obtained as a yellow oil (0.390 g, 65%). Rf = 0.3 [hexane/EtOAc (2:1)]. 1H NMR (400 MHz,
CDCl3): δ 7.05 (d, 3J = 3.2 Hz, 2 H), 5.37 (s, 2 H), 3.50 (t, 3J = 8.4 Hz, 2 H), 3.32 (s, 1 H), 0.87

(t, 3J = 8.4 Hz, 2 H), -0.06 ppm (s, 9 H). 13C NMR (100 MHz, CDCl3): δ 131.1, 130.1, 120.5,
81.5, 75.2, 72.9, 66.5, 17.6, -1.5 ppm.
2.2.4 Synthesis of 4-[1-(2-trimethylsilylethoxymethyl)]-1H-imidazylethynyl [5-(2pyridylethynyl)]
veratrole, 4
Compound 3 (410 mg, 1.85 mmol) and compound 2, (1.24 g, 3.38 mmol) were reacted in
anhydrous triethylamine (3 mL) and anhydrous THF (5 mL) with PdCl2(PPh3)2 (100 mg, 0.142
mmol) and CuI (45 mg, 0.24 mmol) as the catalyst under argon at 60 °C for 24 h. The mixture
was cooled to room temperature, diluted with CH2Cl2 (100 mL) and washed with brine (50 mL x
2). The organic layer was separated, dried over anhydrous Na2SO4, and solvent removed in
vacuo. The crude product was purified by flash column chromatography with a 1:1 mixture of
hexane and ethyl acetate as the eluent. The product, 4, was obtained as a red oil, which solidified
after 5 days (390 mg, 46%). Rf = 0.30 [EtOAc/hexane (2:1)]. Mp: 110.8–111.9 ˚C. 1H NMR
(400 MHz, CDCl3): δ 8.65 (d, 3J = 4.8 Hz, 1 H), 7.66 (td, 3J = 7.8 Hz, 4J = 2.0, Hz, 1 H), 7. 55
(d, 3J = 7.8 Hz, 1 H), 7.23 (ddd, 3J = 7.8 Hz, 4J = 5.0 Hz , 5J = 1.2 Hz, 1 H), 7.18 (s, 1 H), 7.08
(d, 3J = 1.2 Hz, 3 H), 5.52 (s, 2 H), 3.89 (s, 6 H, OCH3), 3.49 (t, 3J = 8.4Hz, 2 H), 0.76 (t, 3J =
8.4 Hz, 2 H), -0.15 ppm (s, 9 H). 13C NMR (100 MHz, CDCl3): δ 150.2, 149.8, 149.7, 143.3,
136.3, 132.5, 130.7, 127.3, 123.0, 120.3, 117.7, 117.6, 114.7, 114.3, 91.7, 91.6, 88.1, 81.5, 75.6,
66.6, 56.2, 17.7, -1.4 ppm. Anal. Calcd. for C26H29N3O3Si: C, 67.94; H, 6.36; N, 9.14. Found: C,
65.85; H, 6.13; N, 8.82.
2.2.5 Synthesis of 4-[(2-imidazylethynyl)]-5-(2-pyridylethynyl) veratrole, 1
Trifluoroacetic acid (4.3 mL) was added to a solution of 12 (100 mg, 0.27 mmol) in THF (0.6
mL). The mixture was stirred at room temperature and monitored by TLC. After 7 h the

trifluoroacetic acid was removed in vacuo and the crude product dissolved in CH2Cl2 (50 mL).
The solution was stirred with a saturated solution of NaHCO3 (10 mL) and the organic layer was
separated, washed with brine (40 mL), and dried over anhydrous Na2SO4. The solvent was again
removed in vacuo and the crude product was purified by preparative-TLC with a 5:1 mixture of
ethyl acetate/ hexane (Rf = 0.45). After purification, the product was isolated as a brown solid
(36 mg, 0.11 mmol, 50%) and recrystallized from a 1:1 mixture of acetonitrile and
dichloromethane to afford brown crystals suitable for x-ray determination. The reaction was
repeated multiple times in an attempt to improve the overall yield. Mp: 185.1–186.9 ˚C. 1H
NMR (400 MHz, CDCl3): δ 8.58 (d, 3J = 4.4 Hz, 1 H), 7.74 (td, 3J = 7.8 Hz, 4J = 1.6 Hz, 1 H), 7.
55 (d, 3J = 7.8 Hz, 1 H), 7.30 (ddd, 3J = 7.6 Hz, 4J = 4.8 Hz, 5J = 1.2 Hz, 1 H), 7.15 (s, 2 H), 7.02
(d, 3J = 2.0 Hz, 2 H), 3.90 (s, 3 H), 3.89 ppm (s, 3 H). 13C NMR (100 MHz, CDCl3): δ 150.2,
149.4, 149.2, 143.0, 137.3, 131.0, 127.2, 123.2, 120.1, 118.0, 113.5, 112.6, 91.8, 90.0, 89.0, 85.4,
56.3 ppm. Anal. Calcd. for C20H15N3O2: C, 72.94; H, 4.59; N, 12.76. Found: C, 72.74; H, 4.52;
N, 12.63.
2.2.7 Synthesis of {4-[(2-imidazylethynyl)]-5-(2-pyridylethynyl) veratrole} palladium (II)
dichloride
The ligand, 1 (3 mg, 0.0091 mmol) was dissolved in anhydrous CH3CN (2 mL), and the solution
was added to a separate solution of PdCl2(CH3CN)2 (0.0024 g, 0.0091 mmol) in CH3CN (0.3
mL) in a vial. The vial was capped and placed in the hood. Orange crystals were harvested after
5 days (2.2 mg, 54 %). Analysis was performed on the accumulated product obtained from
several similar experiments. 1H NMR (400 MHz, DMF-d7): δ 15.86 (br, 1H), 8.87 (d, 3J = 7.8
Hz, 1 H), 8.06 (td, 3J = 7.8 Hz, 4J = 1.6 Hz, 1 H), 7. 87 (d, 3J = 7.8 Hz, 1 H), 7.62–7.60 (m, 1 H),
7.56–7.52(m, 2H) 7.27 (t, 3J = 1.56 Hz, 1 H), 7.24 (s, 1 H), 4.03 (s, 3H, OCH3), 4.02 ppm (s, 3H,

OCH3). Anal. Calcd. for C20H15Cl2N3O2Pd: C, 47.41; H, 2.98; N, 8.29. Found: C, 47.26; H, 3.14;
N, 8.49.
2.2.8 Synthesis of {4-[(2-imidazylethynyl)]-5-(2-pyridylethynyl) veratrole} silver (I)
trifluoroacetate
The ligand, 1 (3 mg, 0.0091 mmol) was dissolved in anhydrous CH3CN (2 mL), and the solution
was added to a separate solution of 2CF3CO2Ag (0.0024 g, 0.0091 mmol) in CH3CN (0.3 mL) in
a vial. 1 mL each of ethanol and dichloromethane were added. The vial was capped and placed in
the freezer. Colourless crystals were harvested after 7 days (1.8 mg, 35 %). Analysis was
performed on the accumulated product obtained from several similar experiments. 1H NMR (400
MHz, DMF-d7): δ 8.87 (d, 3J = 7.8 Hz, 1 H), 8.26 (td, 3J = 7.8 Hz, 4J = 1.6 Hz, 1 H), 8.05 (m, 1
H), 7. 79 (ddd, 3J = 7.8 Hz, 4J = 5.1 Hz, 5J = 1.6 Hz, 1 H), 7.71 (s, 1 H), 7.54 (s, 1H), 7.29 (s, 1
H), 7.19 (s, 1 H), 3.94 (s, 3H, OCH3), 3.93 ppm (s, 3H, OCH3). Anal. Calcd. for
C22H15AgF3N3O4.H2O: C, 46.50; H, 3.02; N, 7.39. Found: C, 46.59; H, 2.87; N, 7.13.
2.3 Titrations.
2.3.1 Titration of ligand 1 with silver(I) triflate
A solution of ligand 1 (9 mg, 0.027 mmol) in DMF-d7 (0.9 mL, 30.36 mM) was carefully
transferred into a clean and dry NMR tube and the 1H NMR spectrum recorded. A stock solution
of CF3SO3Ag was then prepared in a vial by dissolving CF3SO3Ag (78 mg, 0.30 mmol) in DMF-
d7 (0.9 mL, 337.29 mM). The titration begun with addition of a 10 µL aliquot of the CF3SO3Ag
solution to the solution of the ligand. The mixture was vortexed to ensure homogeneity and the
1H NMR spectrum recorded. The procedure was repeated with subsequent aliquots of 10 µL of

CF3SO3Ag and collecting 1H NMR spectra until a total of 150 µL of the silver triflate was added.
At this stage larger aliquots were added until 5 eq. of CF3SO3Ag was added.
2.3.2 Titration of ligand 13 with bis(acetonitrile)palladium(II) dichloride
A solution of ligand 1 (23 mg, 0.068 mmol) in DMF-d7 (0.9 mL, 75.71 mM) was carefully
transferred into a clean and dry NMR tube and the 1H NMR spectrum of the ligand recorded. A
stock solution of PdCl2(CH3CN)2 was prepared in a vial by dissolving PdCl2(CH3CN)2 (78 mg,
0.30 mmol) in DMF-d7 (0.9 mL, 759.04 mM). The titration began on addition of a 20 µL aliquot
of the PdCl2(CH3CN)2 stock solution to the solution of 1. The mixture was vortexed to ensure
homogeneity and the 1H NMR spectrum recorded. Injection of 20 µL into the tube and collection
of 1H NMR spectrum continued until proton peaks associated with the ligand protons completely
disappeared (100 µL added in total).
2.4 Crystallography
For each sample, a single crystal was mounted on a Kryoloop using viscous hydrocarbon
oil. Data were collected using a Bruker Apex 2 CCD diffractometer equipped with Mo
Kradiation with  = 0.71073 Å. Data collection at low temperature was facilitated by the use
of a Kryoflex system with an accuracy of ±2 K. Initial data processing was carried out using the
Apex 3 software suite [15]. Structures were solved by direct methods using SHELXT-2018 [16]
and refined against F2 using SHELXL-2018 [17]. The program X-Seed was used as a graphical
interface [18]. All H atoms were located in the difference maps. The hydrogen atom on the
imidazole N in both structures was restrained in the refinement with N—H = 0.88 (2) Å with
Uiso(H) = 1.2Ueq(N). All other hydrogen atoms were treated as riding atoms in geometrically
idealized positions with C—H = 0.95 Å and Uiso(H) = 1.2Ueq(C) for aryl protons and C—H =

0.98 Å and Uiso(H) = 1.5Ueq(C) for methyl protons. For complex 1.PdCl2, the program PLATON
SQUEEZE was used to ameliorate the negative effect on highly disordered solvent on the
refinement [19]. Several crystals of the complex 1.AgCF3CO2 were also selected for X-ray
analysis, however, these were very weak diffractors with multiple silver centers and high
disorder and did not lead to reasonably refined data. Subsequently, data was collected from
multiple crystals of the complex 1.AgCF3SO3 prepared in a similar way and also culled from
titration experiments. These crystals were also weakly diffracting even with extended time of
data collection. The result presented here corresponds to the best data obtained. The structure of
the complex 1.AgCF3SO3 included four unique coordination complexes in the unit cell. Two of
the four coordination complexes were readily refined. On the contrary, the third and fourth
complexes required repeated refinement with the aid of multiple restraints (DFIX, DANG,
EADP, FLAT and SIMU). While it was apparent that the ligands are disordered with respect to
the interchange of the imidazole and pyridine moieties, this was not modeled given the
complexity of the refinement. Two of the triflate anions were also refined with the aid of
multiple restraints. We were unable to adequately model and refine two triflate anions and the
presence of disordered solvent was also suspected. Consequently, the program SQUEEZE run
through the Platon interface was used to facilitate refinement.
3. Results and discussion
3.1 Ligand Synthesis and Characterization
The ligand was synthesized in moderate overall yield starting from 4,5-diiodoveratrole.
The synthesis began with the Sonogashira coupling of 2-ethynylpyridine with 4,5-
diiodoveratrole in triethylamine, catalyzed by bis(triphenylphosphine)palladium(II) dichloride
and copper(I) iodide to form 4-iodo-5-(2-ethynylpyridine) veratrole, 2, in 62% yield along with

the disubstituted pyridyl product, 1,2-Bis(2-pyridylethynyl) veratrole in 26% and a small
amount of 4,5-diiodoveratrole. The synthesis of the second moiety began with protection of
imidazole with the trimethylsilylethyoxymethyl group (SEM) [13], followed by iodination
according to the procedure described by Knapp [14]. Subsequent Sonogashira coupling with
trimethylsilylacetylene followed by base promoted deprotection of the alkyne provided alkyne 3
in relatively low yield, 29 % overall for the two steps, possibly due to the solubility of the
product in water and difficulty monitoring the reactions. The alkyne 3 was then coupled with 4-
iodo-5-(2-ethynylpyridine) veratrole, 2 using standard Sonogashira coupling reaction conditions.
Several attempts were made to improve the yield that resulted in the modest yield of 4 (46 %)
after reaction at 60 oC for 24 h. The coupled SEM-protected imidazole product was deprotected
with trifluoroacetic acid to yield the desired ligand. The 1H NMR of the ligand clearly
established the presence of protons unique to each of the three aromatic rings and further
characterization by 13C NMR (see Figure S1-S3), elemental analysis and single crystal X-ray
crystallography confirmed the structure.
The single crystal X-ray structure of 1 shown in Figure 1 contains a single molecule in its
asymmetric unit with an intramolecular hydrogen bond with distances H2N---N1 and N2---N1
1.97(2) and 2.872(4) Å respectively with angle N1---H2N-N2 of 167(3)°. The imidazole ring is
essentially coplanar with the benzene ring with interplanar dihedral angle of 2.11(5)° although
the pyridyl ring is slightly twisted with respect to the benzene ring with a dihedral angle of
15.00(8)°. The ethynyl bond to the imidazole is slightly distorted from linearity to accommodate
the intramolecular hydrogen bond with C(13)-C(14)-C(15) and C(14)-C(15)-C(16) angles of
172.8(3) and 172.8(3)° respectively (Table 2). The molecules of 1 form of planar sheets where
adjacent molecules interact through C-H---N and C-H---O interactions (see Table 3).

Figure 1. The labelled asymmetric unit of ligand, 1 with displacement ellipsoids drawn at the
50% level and the intramolecular hydrogen bond shown as a dashed line.
1
3.2 H NMR studies of metal complexation
Binding of silver(I) salts by 1,2-bis(2′-pyridinylethynyl)benzenes [5] and related bipyridyl
ligands [20] is characterized by rapid exchange between complexed and un-complexed ligands
that is significantly faster than the time scale of the NMR measurement. Consequently, the
observed NMR spectrum is a weighted average spectrum of bound and unbound ligand rather
than distinct peaks corresponding to bound and unbound ligand. Therefore the proton chemical
shift of the pyridine protons gradually shift downfield as increments of Ag(I) are added until no
further chemical shift change is observed [5,20]. The incremental addition of silver(I) triflate to a
0.03 M solution of ligand 1 in dimethylformamide-d7 demonstrates the change in chemical shift
of the pyridyl protons as shown in Figure 2. It is clear that after addition of 1 equivalent of
silver(I), at this concentration, there was no further change in the chemical shift of the ligand
protons thereby indicating the formation of a 1:1 ligand:silver(I) complex in solution at this

concentration. It is noteworthy that the imidazole N-H proton is broadened and is shifted
downfield in the NMR spectrum. Thus, under these conditions, the ligand is not deprotonated
after forming the coordination complex with silver(I).
Figure 2. Spectral changes observed on incremental addition of silver(I) trifluoromethane
sulfonate into a DMF-d7 solution of ligand 13 (0.03 mM).
In contrast, the complexation of palladium(II) salts by 1,2-bis(2′
pyridinylethynyl)benzenes and related bipyridyl ligands revealed that binding of Pd(II) was slow
on the NMR time scale. Accordingly, proton signals for both the bound ligand and the unbound
ligand are observed in solutions where only a portion of the ligand is bound [20]. The results of
the titration of ligand, 1, with bis(acetonitrile)palladium(II) dichloride is shown in Figure 3. It is
noteworthy that the signal for un-complexed ligand completely disappeared after addition of 1.0
equiv. of Pd(II), thereby confirming the formation of a 1:1 complex in solution (Figure 3).

Figure 3. Stacked 1H NMR spectra obtained on sequential addition of PdCl2(CH3CN)2 into a
0.076M DMF-d7 solution of 1.
3.3 X-ray crystallographic studies of metal complexation
The palladium complex 1.PdCl2 crystallized from an equimolar solution of 1 and
PdCl2(CH3CN)2 in acetonitrile in the monoclinic space group P21/c as orange blocks. In the
complex, the ligand is essentially planar with dihedral angles of 2.35(2) and 1.69(18)° between
the benzene ring and the pyridine and imidazole rings respectively (Figure 4). The palladium has
square planar coordination geometry with N1-Pd1-N2 and Cl1-Pd1-Cl2 angles of 176.86(12) and
174.40(4)° respectively and N-Pd-Cl angles between 89.28(9) and 91.06(9)°. The imidazole N-
H has a close contact to a chloride from an adjacent complex with an H---Cl distance of 2.42(3)Å
that is 80% of the sum of the Vander Waals radii [21].
Figure 4. View of the palladium coordination complex with ligand (1) showing the atom
labeling with displacement ellipsoids drawn at the 50% level.

The 1:1 silver(I) triflate complex crystallized in the monoclinic space group Cc. However, the
unit cell contained 4 unique silver complexes with triflate counterions. Of these 4 complexes,
two were severely disordered, as shown in Figure 5, as were all four triflate anions. In addition,
the ligand complexes all exhibited some level of positional disorder between the imidazole and
pyridyl rings. Refinement of the structure was only successful when the two most highly
disordered triflate anions and the disordered incorporated solvent were removed using the
program SQUEEZE run through the Platon interface. Subsequent refinement was performed with
multiple restraints as described in the experimental section. Despite the fact that the final
refinement is below generally accepted expectations for publication, it does confirm the
coordination mode of the ligand. While the Ag-N bond distances and the N-Ag-N angles,
collected in Table 4, are typical for pyridyl and imidazolyl silver complexes, it should be noted
that positional disorder in all four complexes and the use of restraints is expected to have
affected these values. Nevertheless, it is clear that the ligand does indeed bind silver(I) cations in
an intramolecular linear mode similar to the related dipyridyl ligand.
Figure 5. View of the asymmetric unit of the silver(I) coordination complex with ligand, 1 with
the unique silver atoms labeled and displacement ellipsoids drawn at the 50% level.

The program SQUEEZE reported 8 distinct void spaces in the unit cell after removal of the two
highly disordered triflates and disordered solvent from the refinement. Given that the unit cell
contains a total of 16 silver complexes, 4 per asymmetric unit, and, with 2 triflates from the
asymmetric unit modelled in the refinement, each of these 8 void spaces reasonably contain a
triflate anion with an electron count of 73. Four of these voids have SQUEEZEd electron count
of 84 electrons and the other 4 have a SQUEEZEd electron count of 112 electrons. The smaller
void likely also includes a water molecule, for a total electron count of 83, while the larger voids
must contain a disordered triflate and a solvent molecule such as dichloromethane with a total
electron count of 115. A view of the complexes stacked along the b-axis, as shown in Figure S12
reveals the two distinct void spaces with different volumes.
4. Conclusions
In summary, we reveal the first synthesis of a trans coordinating ditopic ligand
containing both pyridyl and imidazolyl moieties and characterization of its coordination to both
silver(I) and palladium(II) cations. 1H NMR studies confirm the 1:1 stoichiometry between the
ligand and the silver and palladium cations respectively. X-ray studies confirms the trans
coordination of palladium(II) cation to form a square planar complex and the linear coordination
of silver(I) by the pyridyl and imidazolyl moieties. Our future studies will focus on improving
the synthesis of this, and related, imidazolyl ligands with a goal of forming neutral coordination
complexes and exploring their potential uses.
5. Acknowledgment
We thank the Missouri State University Provost Incentive Fund that funded the purchase
of the X-ray diffractometer and the Missouri State University Graduate College for funding
C.I.N.

References
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Table 1. Crystallographic data for compound 1 and complex 1.PdCl2
Compound 1
C20H15N3O2
329.35
Complex 1.PdCl2
C20H15Cl2N3O2Pd C20H15AgF3N3O2S
506.65 526.28
Complex 1.Ag(CF3SO3)
Formula
Formula weight
Crystal dim. (mm3)
Crystal system
Crystal color
Space group
a (Ǻ)
b (Ǻ)
c (Ǻ)
α (deg)
β (deg)
0.25 x 0.21 x 0.02 0.20 x 0.15 x 0.02 0.40 x 0.10 x 0.05
Triclinic
Colorless
P-1
6.698 (3)
9.659 (4)
13.379 (6)
110.387 (6)
95.185 (6)
92.835 (7)
2
Monoclinic
Gold
P 21/c
Monoclinic
Colorless
Cc
11.1420 (7)
27.3615 (17)
7.4271 (5)
90
53.94(2)
6.826(3)
24.457(9)
90
93.972
90
95.878 (1)
90
γ (deg)
Z
4
16
V (Ǻ3)
804.9 (6)
1.359
100 (2)
0.090
2252.3 (3)
1.494
8983 (6)
1.557
ρcalcd (mg m-3)
T (K)
100 (2)
1.080
10 (2)
1.035
µ (mm-1)
Measured, independent &
observed reflections
50289/19753/10759
10255/3575/1868 28387/5037/4568
Data/ parameters/restraints 3575/232/1
5037/259/1
0.041
19753/1061/817
0.114
Rint
0.059
Goodness of fit
R1/wR2
R1/wR2 (all data)
CCDC
0.992
1.147
1.044
0.065/0.165
0.128/0.207
1956600
0.045/0.102
0.050/ 0.102
1956601
0.116/0.335
0.181/0.335
1960422

Table 2. Selected bond lengths [Å] and angles [ °] for compound 1 and complex 1.PdCl2
Bond length
Compound 1
C6-C7
Bond angle
1.198(4)
1.200(4)
C5-C6-C7
178.7(3)
174.4(3)
172.8(3)
172.8(3)
C14-C15
C6-C7-C8
C14-C15-C16
C15-C16-C17
Complex 1.PdCl2
Pd1-N1
2.018(3)
2.006(3)
2.3335(10)
2.3185(9)
1.197(5)
1.201(5)
N1-Pd1-N2
Cl1-Pd1-Cl2
N1-Pd1-Cl1
N1-Pd1-Cl2
N2-Pd1-Cl1
N2-Pd1-Cl2
176.86(12)
174.40(4)
90.41(9)
89.28(9)
89.56(9)
91.06(9)
Pd1-N2
Pd1-Cl1
Pd1-Cl2
C6-C7
C14-C15
Table 3. Hydrogen bond geometry [Å, °] for compound 1
D-H---A
D-H
0.92(2)
0.95
H---A
1.97(2)
2.38
D---A
D-H---A
167(3)
164
N2-H2N---N1
C2-H2---O1i
C9-H9---N3ii
2.872(4)
3.303(4)
3.369(4)
0.95
2.43
168
Symmetry codes: (i) x, y, z+1; (ii) x, y+1, z.

Table 4. Selected bond lengths [Å] and angles [ ° ] for complex 1.Ag(CF3SO3)
Bond length
Ag1-N1
Bond angle
N1-Ag1-N2
N4-Ag2-N5
N7-Ag3-N8
2.059(17)
2.14(2)
2.09(3)
2.13(2)
Ag1-N2
Ag2-N5
Ag3-N8
Ag4-N11
2.098(18)
2.073(19)
2.19(3)
173.6(6)
174.9(6)
174(1)
Ag2-N4
Ag3-N7
Ag4-N10
2.15(4)
N10-Ag4-N11 174(1)

Highlights

Synthesis and characterization of a trans-coordinating ligand including one pyridine and one
imidazole moiety.


NMR studies of silver(I) and palladium(II coordination by the pyridyl-imidazolyl ligand.
X-ray structural characterization of linear coordination of Ag(I) and trans-coordination of Pd(II).

Author Contribution Statement
Eric Bosch:
Conceptualization, Data curation, Methodology, Validation, Formal analysis, Funding
acquisition, Investigation, Visualization, Writing- Original draft, Project administration,
Resources, Software, Supervision.
Chideraa I. Nwachukwu:
Methodology, Investigation, Validation, Data curation, Visualization, Writing –review & editing.