Complexation of silver(I) cation by a series of heterosubstituted dipyridyl ligands

Article abstract of DOI:10.1016/j.ica.2010.07.078

The synthesis of a series of dipyridyl ligands based on 1,2-bis(2′-pyridylethynyl)benzene and their complexation of silver cation is described. NMR binding studies confirm that the incorporation of thioether appendages results in an increased binding cons

Full text of DOI:10.1016/j.ica.2010.07.078

Inorganica Chimica Acta 363 (2010) 3987–3992
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Complexation of silver(I) cation by a series of heterosubstituted dipyridyl ligands
Nathan L. Brennan a, Charles L. Barnes b, Eric Bosch a,
⇑
a Chemistry Department, Missouri State University, Springfield, MO 65897, USA
b Chemistry Department, University of Missouri, Columbia, MO 65211, USA
a r t i c l e i n f o
a b s t r a c t
The synthesis of a series of dipyridyl ligands based on 1,2-bis(20-pyridylethynyl)benzene and their com-
plexation of silver cation is described. NMR binding studies confirm that the incorporation of thioether
appendages results in an increased binding constant while ether appendages result in similar or lower
binding constants as compared to the unsubstituted ligand. X-ray crystallographic analysis confirms that
steric hinderance is critical.
Article history:
Received 3 June 2010
Received in revised form 22 July 2010
Accepted 27 July 2010
Available online 3 August 2010
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Silver complexes
Bipyridyl ligands
X-ray crystal structures
1. Introduction
(15 mg, 0.057 mmol), and diethylamine (15 mL) were added to a
50 mL round bottom flask under an argon atmosphere. Argon
We earlier reported the design and synthesis of a simple trans
coordinating ligand, 1,2-bis(20-pyridylethynyl) benzene (1 in
Scheme 1A), that was particularly well suited for the coordination
of silver(I) and palladium(II) cation [1,2]. We modeled the planar
conformation of the ligand as shown in Scheme 1 and noted that
this conformation would position the pyridyl N-atoms in the ideal
position at 4.3 Å apart to coordinate transition metal cations like
silver(I) that favor linear coordination geometry [3]. Note that
the pyridyl–pyridyl distance in simple linear 2:1 pyridine–silver
complexes is approximately 4.26 Å for the perchlorate, tetrafluoro-
borate and hexafluorophosphate salts [4]. Indeed, we reported the
formation and the X-ray structural characterization of the essen-
tially planar coordination complexes of the ligand with silver(I)
and palladium(II) cations (Scheme 1B). In this paper we report
our results with a series of modified ligands with heteroatom-con-
taining side-arms that we envisioned would wrap around the
bound metal cation and possible even assist in the initial binding
process. These ligands are shown in Scheme 1C.
was bubbled through the mixture for 10 min. Copper iodide
(4 mg, 0.02 mmol), and dichloro bis(triphenylphosphine)palla-
dium(II) (15 mg, 0.02 mmol) were added and the solution warmed
to 80 °C under an argon atmosphere. After 96 h TLC indicated that
the reaction was complete. The crude reaction mixture was puri-
fied using flash chromatography with ethyl acetate as eluant. Com-
plex 2 was recrystallized from dichloromethane as light yellow
crystals (62% yield). M.p.: 149–154 °C. 1H NMR (400 MHz, DMSO
d-6) d 7.89 (t, J = 8 Hz, 2H), 7.74 (dd, J = 3.2, 6.0 Hz, 2H), 7.71 (d,
J = 8.4 Hz, 2H), 7.54 (dd, J = 3.2, 6.0 Hz, 2H), 7.53 (d, J = 7.2 Hz,
2H), 5.53 (t, J = 5.6 Hz, 2H), 4.61 (d, J = 6.0 Hz, 4H); 13C NMR
(100 MHz, DMSO d-6) d 163.5, 141.7, 137.9, 132.8, 130.3, 126.5,
125.2, 121.0, 94.0, 87.2, 64.72.
2.1.2. Synthesis of dimesylate 3
Diol 2 (1 mmol, 0.340 g), triethylamine (3.0 mmol, 0.42 mL) and
THF (15 mL) were added to a dry 25 mL round bottom flask under
an argon atmosphere. The mixture was cooled in an ice bath and
methylsulfonyl chloride (3.0 mmol, 0.232 mL) added. After the
reaction was complete saturated aqueous sodium bicarbonate
was added to the reaction mixture which was then extracted using
dichloromethane. The dichloromethane solution was washed with
a saturated brine solution and then dried over magnesium sulfate
and the solvent removed yielding 3 as off-white fluffy crystals in
almost quantitative yield. M.p.: 149–150 °C. 1H NMR (400 MHz,
CDCl3) d 7.80 (t, J = 7.6 Hz, 2H), 7.69 (d, J = 8.4 Hz, 2H), 7.66 (dd,
J = 3.2, 6.0 Hz, 2H), 7.47 (d, J = 8.0 Hz, 2H), 7.40 (dd, J = 3.2 Hz,
6.0 Hz, 2H), 5.36 (s, 4H), 3.10 (s, 6H); 13C NMR (100 MHz, CDCl3)
d 154.3, 143.2, 137.5, 132.3, 129.1, 127.6, 125.2, 121.6, 92.3, 88.4,
71.1, 38.2.
2. Experimental
2.1. Synthesis
2.1.1. Synthesis of 2
Diethynyl benzene (0.126 g, 1 mmol), 2-bromo-5-hydroxy-
methyl pyridine [5] (0.419 g, 2.25 mmol), triphenylphosphine
⇑
Corresponding author. Address: 901 South National Avenue, Springfield, MO
65897, USA. Tel.: +1 417 836 4277; fax: +1 417 836 5507.
E-mail address: ericbosch@missouristate.edu (E. Bosch).
URL: http://chemistry.missouristate.edu/31072.htm (E. Bosch).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.07.078

3988
N.L. Brennan et al. / Inorganica Chimica Acta 363 (2010) 3987–3992
Scheme 1.
2.1.3. Synthesis of 4
2.1.6. Synthesis of 7
Sodium hydride (60% in mineral oil, 0.2 mmol, 0.008 mg) and
THF (15 mL) were placed in a dry 25 mL round bottom flask under
an argon atmosphere. Butanol (0.25 mmol, 0.023 mL) was added
and the resultant mixture stirred for 30 min under argon. The solu-
tion was then cooled in an ice bath and the dimesylate 3
(0.1 mmol, 50 mg) added as a solid. After 30 min the reaction mix-
ture was allowed to warm to room temperature and stirred for 7 h
when TLC indicated that the reaction was complete. Saturated
aqueous ammonium chloride and ethyl acetate were added to
the solution. The organic layer was separated, washed with water
and brine and dried over anhydrous sodium sulfate. The solvent
was removed under vacuum and the product isolated by flash
chromatography using a 1:4 mixture of ethyl acetate and hexane
as eluant yielding the product as a yellow oil in 60% yield. 1H
NMR (400 MHz, DMSO d-6) d 7.90 (t, J = 3.6 Hz, 2H), 7.735 (m,
4H), 7.55(dd, J = 3.2 Hz, 6.0 Hz, 2H), 7.48 (d, J = 7.6 Hz, 2H), 4.57
(s, 4H), 3.52 (t, J = 6.4 Hz, 4H), 1.56 (quintet, J = 1.6 Hz, 4H), 1.35
(sextet, J = 7.2 Hz, 4H), 0.89 (t, J = 7.2 Hz, 6H); 13C NMR (100 MHz,
CDCl3) d 159.7, 142.5, 136.6, 132.1, 128.6, 126.2, 125.5, 120.3,
92.9, 87.7, 73.5, 71.0, 31.8, 19.3, 13.9.
Sodium hydride (0.008 mg, 60% in mineral oil, 0.2 mmol) and
THF (15 mL) were placed in a dry 25 mL round bottom flask under
an argon atmosphere. The sodium hydride solution was cooled to
ꢀ15 °C before butanethiol (0.2 mmol, 0.021 mL) was added. The
resultant solution was stirred 15 min before dimesylate
3
(0.1 mmol, 50 mg) was then added as a solid. TLC analysis indi-
cated that the reaction was complete after 40 min at ꢀ15 °C. The
usual workup yielded 7 in low yield as an orange oil. 1H NMR
(400 MHz, DMSO d-6) d 7.848 (t, J = 7.6 Hz, 2H), 7.744 (dd, J = 3.2,
6.0 Hz, 2H), 7.695 (d, J = 6.8, 2H), 7.545 (dd, J = 3.2, 6.0 Hz, 2H),
7.481 (d, J = 7.6 Hz, 2H), 3.848 (s, 4H), 2.472 (t, J = 7.2 Hz, 4H),
1.464 (quintet, J = 7.6 Hz, 4H), 1.291 (sextet, J = 7.2 Hz, 4H), 0.812
(t, J = 7.2 Hz, 6H). 13C NMR (400 MHz, DMSO d-6) d 160.2, 141.8,
137.8, 132.6, 130.1, 126.3, 124.8, 123.6, 93.7, 87.0, 37.4, 31.3,
31.0, 21.7, 13.9.
2.1.7. Synthesis of 8
A solution of the dimesylate 3 (0.2 g, 0.4 mmol) in acetonitrile
(40 mL) was prepared in a dry round bottom flask under an argon
atmosphere. Diisopropylethylamine (0.4 mL, 2.5 mmol) and dieth-
ylamine (0.17 mL, 1.6 mmol) were added and the resultant mixture
was heated to 45 °C. After the reaction was complete it was diluted
with dichloromethane and washed with water and dried over so-
dium sulfate. The solvent was evaporated and the product purified
by flash chromatography using methanol as eluant to yield 8 as a
dark brown gel. 1H NMR (400 MHz, DMSO d-6) d 7.882 (t,
J = 7.6 Hz, 2H), 7.738 (dd, J = 3.6, 6.0 Hz, 2H), 7.373 (d, J = 7.6 Hz,
2H), 7.530 (m, 4H), 3.681 (s, 4H), 2.510 (q, J = 7.2 Hz, 8H), 0.985
(t, J = 6.8 Hz, 12H). 13C NMR (400 MHz, CDCl3) d 161.6, 142.3,
136.3, 132.1, 128.5, 125.8, 125.6, 122.0, 93.2, 87.4, 59.7, 47.5, 12.0.
2.1.4. Synthesis of 5
Sodium hydride (60% in mineral oil, 0.2 mmol, 0.008 mg) and
THF (5 mL) were placed in a dry 25 mL round bottom flask under
an argon atmosphere. The mixture was cooled to 0 °C and
methoxyethanol (0.25 mmol, 0.02 mL) added. The solution was
stirred for 30 min under argon and then a separate solution of
the dimesylate 3 (0.1 mmol, 50 mg) in THF (10 mL) was then added
dropwise. The reaction mixture was allowed to warm to room tem-
perature and stirred until TLC indicated that the reaction was com-
plete. Following the usual workup 5 was obtained as a clear yellow
oil in near quantitative yield. 1H NMR (400 MHz, CDCl3) d 7.697 (t,
J = 7.6 Hz, 2H), 7.649 (dd, J = 3.2, 6.0 Hz, 2H), 7.609 (d, J = 7.6 Hz,
2H), 7.490 (d, J = 8.0 Hz, 2H), 7.359 (dd, J = 3.2, 6.0 Hz, 2H), 4.742
(s, 4H), 3.740 (m, 4H), 3.624 (m, 4H), 3.419 (s, 6H). 13C NMR
(400 MHz, CDCl3) d 159.5, 142.8, 137.0, 132.4, 129.0, 126.6,
125.7, 120.7, 93.1, 87.9, 74.2, 72.1, 70.4, 59.3.
2.1.8. Formation of complex [2, Ag]+ CF3SO3
ꢀ
A solution of silver(I) trifluoromethane sulfonate (10.3 mg,
0.04 mmol) and 2 (11.2 mg, 0.04 mmol) was prepared in warm
nitromethane (2 mL). This was cooled to room temperature and
carefully layered over dichloromethane (3 mL) in a screw-cap vial.
The vial was sealed and allowed to stand in the dark. Clear block-
shaped crystals formed after 24 h. The solution was removed and
the crystals dried in the atmosphere.
2.1.5. Synthesis of 6
Sodium hydride (60% in mineral oil, 0.2 mmol, 0.008 mg) and
THF (5 mL) were placed in a dry 25 mL round bottom flask under
an argon atmosphere. The mixture was cooled to 0 °C and phenol
(0.2 mmol, 0.019 mg) was then added and the solution stirred for
30 min. A solution of the dimesylate 3 (0.1 mmol, 50 mg) in THF
(10 mL) was then added dropwise. The resultant mixture was
brought to 40 °C and stirred under argon until TLC indicated that
the reaction was complete. The usual workup yielded 6 as white
fluffy crystals in near quantitative yield. M.p.: 123–126 °C. 1H
NMR (400 MHz, DMSO d-6) d 7.888 (t, J = 7.6 Hz, 2H), 7.782 (d,
J = 8.0 Hz, 2H), 7.769 (dd, J = 3.2 Hz, 6.0 Hz, 2H), 7.545 (d,
J = 8.0 Hz, 2H), 7.559 (dd, J = 3.2, 6.0 Hz, 2H), 7.303 (t, J = 7.6 Hz,
4H), 7.027 (d, J = 8.8 Hz, 4H), 6.962 (t, J = 6.8 Hz, 2H), 5.217 (s,
4H). 13C NMR (400 MHz, DMSO d-6) d 157.9, 157.6, 141.7, 137.6,
132.1, 129.7, 129.5, 126.6, 124.2, 121.5, 120.9, 114.6, 92.9, 86.9,
69.7.
2.1.9. Formation of complex [6, Ag]+ CF3SO3
ꢀ
A solution of silver(I) trifluoromethane sulfonate (10.3 mg,
0.04 mmol) and 6 (15.8 mg, 0.04 mmol) was prepared in warm
dichloromethane (3 mL). The vial was sealed and allowed to stand
in the dark. Clear plate-like crystals formed.
2.2. Binding constants
We titrated each of the ligands with silver(I) triflate and used 1H
NMR to monitor changes in the chemical shifts to determine bind-
ing constants. All titrations were performed by stepwise addition
of silver(I) triflate into a solution of the ligand. The titrations were
continued until there was no further change in chemical shift indi-
cating that there was no ‘‘free”, or uncomplexed, ligand. Plots of
the change in chemical shift against the ratio of added silver(I) con-

N.L. Brennan et al. / Inorganica Chimica Acta 363 (2010) 3987–3992
3989
firmed the formation of 1:1 complexes. The initial and final chem-
ical shifts were then used to determine the relative concentrations
of free ligand, complexed ligand and free and complexed silver(I) in
solution. An initial titration experiment was usually run to obtain
approximate values of the formation constant along with estimates
of the reliability of the results using the ‘‘p” value as defined by
Weber [6]. The concentrations of ligand and silver(I) to be used
in the experiment were then modified to ensure that valid reliable
results were obtained with the next titration experiment.
The initial ligand concentration was in the range of 0.5–8.8 mM
(2, 8.0; 4, 8.8; 5, 8.5; 6, 8.2; 7, 0.5 and 8, 8.0 mM). For each exper-
iment a stock solution of the silver(I) triflate was prepared in
DMSO d-6 (140–212 mM) and titrated into the solution of the li-
out using the Apex II softwaresuite [7]. Structures were solved by
direct methods using SHELXS-97 and refined using standard alternat-
ing least-squares cycles against F2 using SHELXL-97 [8]. The program
X-Seed was used as a graphical interface [9]. Methine and methy-
lene hydrogen atoms attached to carbon were placed in idealised
positions and refined with a riding model. Methyl hydrogen atoms
attached to carbon were positioned according to maximum elec-
tron density. Hydrogen atoms attached to oxygen were placed in
idealised positions based on maximum electron density. For com-
plex [2, Ag]+ OTfꢀ the highest residual electron density peak ap-
peared close to C22 suggesting disorder between the two
hydroxymethyl sidearms. For complex [6, Ag]+ OTfꢀ the triflate
was disordered over two positions and it was modeled using a free
variable, which converged to 0.89, as well as bond distance and
bond angle restraints. The oxygen and fluorine were allowed to re-
main isotropic during refinement since they tended to become
non-positive definite when refined anisotropically. The highest
two residual electron density peaks were centered around the dis-
ordered triflate suggesting that there may be other less populated
positions for the triflate. The crystallographic data for these struc-
tures is collected in Table 1.
gand in DMSO d-6 in aliquots of between 5 and 50 lL. In a few in-
stances where the ‘‘endpoint” of the titration was unclear the final
concentration of silver salt was increased by addition of a small
portion of the solid salt. In all cases the solutions and NMR samples
were mixed using a vortex genie to ensure homogeneous samples
before measurement of the NMR spectra. All 1H NMR spectra were
recorded using 32 scans. The resulting concentration and chemical
shift data was entered into an Excel worksheet and calculations
were performed in Excel. A similar titration was performed with
the unsubstituted ligand, 1,2-bis(20-pyridylethynyl)benzene, com-
pound 1 (see Scheme 1) as reference.
3. Results and discussion
3.1. Synthesis
2.3. X-ray structure determinations
The synthetic strategy that we used to synthesize compounds
2–8 is shown specifically for compound 4 in Scheme 2.
Crystals were mounted on small cryoloops using viscous hydro-
carbon oil. Data were collected using a Bruker Apex2 CCD diffrac-
The synthetic route began with the preparation of 2-bromo-6-
hydroxymethylpyridine from 2,6-dibromopyridine using the pro-
cedure developed by Verhoeven et al. [5]. This bromopyridine
was then coupled with 1,2-diethynylbenzene to yield the dihy-
0
tometer equipped with Mo K
a radiation with k = 0.71073 ÅA. Data
collection at low temperatures was facilitated by use of a Kryoflex
system with an accuracy of 1 K. Initial data processing was carried
Table 1
Crystal data and structure refinement for compound 2, [2, Ag]+ OTfꢀ and [6, Ag]+ OTfꢀ
.
Compound
2
[2, Ag]+ OTfꢀ
[6, Ag]+ OTfꢀ
Empirical formula
Formula weight
Color, habit
Temperature (K)
Wavelength (Å), source
Crystal system, space group
Unit cell dimensions
a (Å)
C22H16N2O2
340.37
colorless, block
173(2)
0.71073 Mo K
monoclinic, P21/c
C23H16AgF3N2O5S
597.31
colorless, block
173(2)
C35H24AgF3N2O5S
749.49
colorless, block
173(2)
0.71073 Mo Ka
monoclinic, P121/c1
a
0.71073 Mo K
a
ꢀ
triclinic, P1
13.258(3)
7.6640(15)
17.501(4)
90
108.49(3)
90
1686.5(6)
4, 1.341
7.4221(7)
12.4394(12)
13.7826(13)
114.6540(10)
96.553(2)
97.641(2)
1125.73(19)
2, 1.762
18.2234(13)
9.6662(7)
19.1686(14)
90
114.5620(10)
90
3071.0(4)
4, 1.621
0.79
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
Volume (Å3)
Z, Dcalc (mg/m3)
Absorption coefficient (mmꢀ1
F(0 0 0)
)
0.087
712
1.052
596
1512
Crystal size (mm)
h Range for data collection (°)
0.15 ꢁ 0.25 ꢁ 0.30
0.30 ꢁ 0.15 ꢁ 0.05
0.19 ꢁ 0.44 ꢁ 0.54
2.45–25.00
1.66–27.15
1.23–27.14
Limiting indices
ꢀ15 6 h 6 15, ꢀ9 6 k 6 9,
ꢀ20 6 l 6 20
17297/2966 [Rint = 0.0265]
100, 25.0
ꢀ9 6 h 6 9, ꢀ15 6 k 6 15,
ꢀ17 6 l 6 15
ꢀ23 6 h 6 23, ꢀ12 6 k 6 12,
ꢀ24 6 l 6 24
37674/6794 [Rint = 0.0245]
99.8, 27.14
Reflections collected/unique
Completeness (h) (%, °)
8020/4852 [Rint = 0.0245]
97.0, 27.15
Absorption correction
multi-scan
semi-empirical
numerical
Maximum and minimum transmission
Refinement method
0.98 and 0.89
Full-matrix least-squares on F2
2966/0/237
0.95 and 0.74
0.87 and 0.69
Full-matrix least-squares on F2
6794/12/467
1.035
Full-matrix least-squares on F2
4852/0/320
Data/restraints/parameters
Goodness-of-fit on F2
1.039
1.053
Final R indices [I > 2h(I)]
R indices (all data)
R1 = 0.0306, wR2 = 0.0775
R1 = 0.0370, wR2 = 0.0839
0.17 and ꢀ0.18
R1 = 0.0433, wR2 = 0.1125
R1 = 0.0591, wR2 = 0.1227
1.86 and ꢀ0.73
R1 = 0.0242, wR2 = 0.0618
R1 = 0.0289, wR2 = 0.0653
0.55 and ꢀ0.40
Largest difference in peak and hole
(e Aꢀ3
)

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N.L. Brennan et al. / Inorganica Chimica Acta 363 (2010) 3987–3992
Scheme 2.
Fig. 1. Series of 1H NMR spectra obtained on titration of ligand 2 with silver(I) triflate in dimethylsulfoxide-d6. The top spectrum, (5), corresponds to 2 alone (no silver ion
present). The ratio of silver(I):2 for each of the other spectra is: 0.5 in (4); 1.0 in (3); 2.0 in (2); and 3.0 in (1).
droxy ligand 2 in moderate yield. Compound 2 was then trans-
formed into the corresponding dimesylate 3 using methanesulfo-
nylchloride and triethylamine in tetrahydrofuran solvent. Further
transformation of the dimesylate by reaction with a series of nucle-
ophiles, prepared by reaction of the corresponding alcohol or thiol
with sodium hydride, yielded the desired ligands 4–8. Thus, for
example, reaction of 3 with sodium butoxide yielded the diether
4 in moderate yield. Reaction of the dimesylate 3 with diethyl-
amine in the presence of diisopropylethylamine and acetonitrile
yielded the diamine 8.
For clarity and consistency in the subsequent discussion the
hydrogen atoms were assigned and numbered according to the
diagram in Scheme 3.
In order to confirm that a 1:1 complex was formed the change
in chemical shift of each of the protons was plotted against the mo-
lar ratio of ligand:silver. A typical plot is shown in Fig. 2 for proton
H-4. The fitted lines confirm that a 1:1 complex is indeed formed. It
is noteworthy that a similar plot was obtained for unsubstituted li-
gand 1 as well as the other substituted ligands.
The chemical shift changes were collected for each of the li-
gands and are collated in Table 2. It is important to note that they
are similar in magnitude and direction to those obtained in a titra-
tion using the unsubstituted ligand 1,2-bis(20-pyridylethynyl)ben-
zene 1.
This confirms that similar linear 1:1 complexes as shown in
Scheme 1B were obtained with the sidearm substituted ligands
and the unsubstituted ligand. There are slight differences for thio-
ether 7 which will be discussed later. The titrations for ligands 2
and 4–7 were performed at concentrations that were chosen to
give ratios of the concentration of complex to the maximum possi-
ble concentration of complex between 0.2 and 0.8 – this is called
3.2. Binding constants
There were several things to verify with the binding experi-
ments. Most notably, we needed to establish that a 1:1 complex
was formed and that all the ligands indeed bound silver as a linear
1:1 complex as shown in Scheme 1B. Ideally we were hoping to
clearly evaluate the involvement of the side arm in the binding
process. A representative titration experiment, the titration of
dihydroxy ligand 2 with silver(I) triflate, is shown in Fig. 1. In this
plot the aromatic, methylene and hydroxyl protons shift downfield
however it is noteworthy that there is no change in chemical shift
between spectra 2 and 1 indicating that the ligand was totally
bound to silver.
Fig. 2. Plot of change in chemical shift of H4 against the ratio of silver(I) cation to
ligand 2.
Scheme 3.

N.L. Brennan et al. / Inorganica Chimica Acta 363 (2010) 3987–3992
3991
Table 2
Total chemical shift change (ppm) of each proton in ligands 1, 2 and 4–7.
Table 4
Selected bond lengths (Å) and angles (°) for complexes [2, Ag]+ OTfꢀ and [6, Ag]+ OTfꢀ
.
Ligand
H1
H2
H3
H4
H5
H6
Complex [2, Ag]+ OTfꢀ
Complex [6, Ag]+ OTfꢀ
1
2
4
5
6
7
0.105
0.120
0.128
0.128
0.123
0.086
0.123
0.143
0.147
0.146
0.144
0.084
0.207
0.207
0.236
0.236
0.221
0.300
0.216
0.29
0.309
0.262
0.332
0.247
0.305
0.227
0.293
0.279
0.299
0.243
–
Bond lengths
Ag(1)–N(1)
Ag(1)–N(2)
Ag(1)–O(1)
0.232
0.281
0.295
0.292
0.387
2.203(3)
2.181(3)
2.570(4)
2.163 (1)
2.169 (1)
2.404(1)a
Bond angles
N(2)–Ag(1)–N(1)
N(1)–Ag(1)–O(1)
177.19(11)
68.80(11)
165.34(5)
–
a
Interaction with the triflate anion.
the ‘‘p-value” [6]. In our studies most p-values were in the range of
0.4–0.76. For each of the ligands the formation constant was indi-
vidually calculated for each of the protons listed in Table 1 and
then averaged. It is noteworthy that the values were consistent
throughout. The formation constants are presented in Table 3.
Surprisingly the hydroxymethyl substituent had no noticeable
effect on the binding of silver cation while the addition of the ether
substituents generally lowered the binding constant with the low-
est binding constant at 1.8 ꢁ 102 Mꢀ1 for the diphenylether 6. Im-
proved binding and the highest binding constant was observed for
the dithioether 7 with K = 7.8 ꢁ 103 Mꢀ1. The diamine ligand 8
gave anomalous results and will not be further discussed here.
These results can be put into context by comparing these bind-
ing constants to binding constants reported for other 1:1 polypyr-
idyl:silver(I) complexes in DMSO solution. Thus, the ligands 2 and
7 have higher binding constants than the bidentate ligand 2,20-
bipyridine and the tridentate ligand 2,2060,200-terpyridine which
have log K values of 2.08 and 3.03, respectively. The binding is,
however, significantly weaker than that of the flexible tridentate li-
gand bis((2-pyridyl)methyl)amine with log K = 4.37.[10]
in hydrogen bonding, O(2)–Hꢂ ꢂ ꢂN(1), which establishes one-
dimensional hydrogen-bonded chains of the ligand within the
crystals.
The X-ray structure of two silver complexes were obtained and
these help understand the results. In Fig. 3B, the structure of the sil-
ver(I)complexwithligand2 itis apparentthatin thecrystallinesolid
only one hydroxyl oxygen interacts with the silver cation. While it is
reasonable to conclude that this is also a function of crystal packing
which would favor a planar conformation and therefore restrict the
side arm interaction to one side it is also quite likely that in solution
there is a dynamic interaction between both hydroxyl groups and
themetalcenter. This latterproposalis supportedbytheobservation
that the hydroxyl protons shift downfield by 0.64 ppm on complex-
ation with silver(I). The structure of the complex between the phen-
ylether 6 and silver(I) triflate shown in Fig. 3C shows no interaction
between the ether oxygen atoms and the silver cation. In this case
the conformation is unlikely to be influenced significantly by crystal
packing since one phenyl ring is not coplanar with the rest of the
complex. Instead there is a weak interaction with the triflate anion,
shownasadashedlinein Fig. 3C, whichisunlikelyto persistinDMSO
solution. It is more reasonable to assume that the large phenyl rings
hinder the binding and do not allow for interaction with the ether
oxygen atoms. In both these complexes the N–Ag bond lengths are
3.3. X-ray analyses
The X-ray structure of ligand 2 was first obtained followed by
the structures of the silver complexes [2, Ag]+ OTfꢀ and [6, Ag]+
OTfꢀ. The structure of 2 in Fig. 3A shows the planar conformation
of the ligand with only one of the nitrogen atoms of the two pyri-
dine rings facing towards the metal binding site. This is reasonable
due to electronic repulsion of the pyridyl N non bonding electron
pairs. Interestingly only one of the two hydroxyl groups is involved
typical for pyridine–silver complexes ranging from 2.163 to
0
2.203 ÅA(see Table 4) [4]. The N–Ag–N bond angle is essentially linear
in unhindered complex [2, Ag]+ OTfꢀ at 177.19° while the bond angle
is significantly distorted in the more hindered complex [6, Ag]+ OTfꢀ
at 165.34°.
Table 3
Binding of silver cation by ligands 1, 2 and 4–7.
Ligand
1
2
4
5
6
7
Formation constant (Mꢀ1
Log K
)
1.3 ꢁ 103
1.3 ꢁ 103
7.0 ꢁ 102
6.6 ꢁ 102
1.8 ꢁ 102
7.8 ꢁ 103
3.11
3.11
2.85
2.82
2.26
3.89
Fig. 3. (A) View of the asymmetric unit of the X-ray crystal structure of compound 2. (B) View of complex [2, Ag]+ OTfꢀ crystalized from nitromethane solution. (C) Oblique
view of complex [6, Ag]+ OTfꢀ crystalized from dichloromethane solution. All displacement ellipsoids drawn at the 50% level.

3992
N.L. Brennan et al. / Inorganica Chimica Acta 363 (2010) 3987–3992
4. Conclusions
Appendix A. Supplementary material
It is reasonable to conclude that oxygen-containing sidearms
have minimal effect on silver(I) binding except for the very hin-
dered ethers which have a slightly negative effect on binding by
the ligands. Thus the binding constant for ligands 1 and 2 were
essentially the same – despite the involvement of the alcohol oxy-
gen as evidenced both by the large downfield shift of the OH and
methylene protons shown in Fig. 1 and the clear Ag–O interaction
shown in the X-ray crystal structure. The steric effect of the larger
ether sidearm apparently is more pronounced than any coopera-
tive oxygen–silver coordination leading to decreased binding con-
stants. This was most noticeable with the phenylether ligand 6.
This ligand has a binding constant almost one order of magnitude
lower than unsubstituted ligand 1. Indeed the crystal structure
shown in Fig. 3C shows no interaction between the ether O atoms
and the silver cation. In contrast the thioether derivative displayed
60% stronger binding so the sulfur atom is clearly involved. This is
presumably a result of the stronger affinity of silver for sulfur due
to the softer nature of the sulfur.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.07.078.
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Acknowledgment
[10] S. Del Piero, R. Fedele, A. Melchior, R. Portanova, M. Tolazzi, E. Zangrando,
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This research was supported by the National Science Founda-
tion, Grant 0415711. The Missouri State University Provost Incen-
tive Fund funded the purchase of the X-Ray diffractometer.