Silver(I) complexation of linked 2,2′-dipyridylamine derivatives. Synthetic, solvent extraction, membrane transport and X-ray structural studies

Article abstract of DOI:10.1039/b609738c

Synthesis of the 2,2′-dipyridylamine derivatives di-2- pyridylaminomethylbenzene 1, 1,2-bis(di-2-pyridylaminomethyl)benzene 2, 1,3-bis(di-2-pyridylaminomethyl)benzene 3, 2,6-bis(di-2-pyridylaminomethyl) pyridine 4, 1,4-bis(di-2-pyridylaminomethyl)benzene

Full text of DOI:10.1039/b609738c

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PAPER
www.rsc.org/dalton | Dalton Transactions
Silver(I) complexation of linked 2,2ꢀ-dipyridylamine derivatives. Synthetic,
solvent extraction, membrane transport and X-ray structural studies†‡
Bianca Antonioli,a,b David J. Bray,b Jack K. Clegg,b Kerstin Gloe,a Karsten Gloe,*a Olga Kataeva,a,c
Leonard F. Lindoy,*b John C. McMurtrie,b Peter J. Steel,*d Christopher J. Sumbyd and Marco Wenzela
Received 10th July 2006, Accepted 31st August 2006
First published as an Advance Article on the web 12th September 2006
DOI: 10.1039/b609738c
Synthesis of the 2,2ꢀ-dipyridylamine derivatives di-2-pyridylaminomethylbenzene 1,
1,2-bis(di-2-pyridylaminomethyl)benzene 2, 1,3-bis(di-2-pyridylaminomethyl)benzene 3,
2,6-bis(di-2-pyridylaminomethyl)pyridine 4, 1,4-bis(di-2-pyridylaminomethyl)benzene 5, and
1,3,5-tris(di-2-pyridylaminomethyl)benzene 6 are reported together with the single-crystal X-ray
structures of 2, 3, and 5. Reaction of individual salts of the type AgX (where X = NO3−, PF6−, ClO4
−
,
or BF4−) with the above ligands has led to the isolation of thirteen Ag(I) complexes, nine of which have
also been characterised by X-ray diffraction. In part, the inherent flexibility of the respective ligands
has resulted in the adoption of a range of coordination arrangements. A series of liquid–liquid
(H2O/CHCl3) extraction experiments of Ag(I) with varying concentrations of 1–6 in the organic phase
have been undertaken, with the counter ion in the aqueous phase being respectively picrate, perchlorate
and nitrate. In general, extraction efficiencies for a given ionophore followed the Hofmeister order of
picrate > perchlorate > nitrate; in each case the tris-dpa derivative 6 acting as the most efficient
extractant of the six systems investigated. Competitive seven-metal bulk membrane transport
experiments (H2O/CHCl3/H2O) employing the above ligands as the ionophore in the organic phase
and equimolar concentrations of Co(II), Ni(II), Zn(II), Cu(II), Cd(II), Pb(II) and Ag(I) in the aqueous
source phase were also undertaken, with transport occurring against a pH gradient. Under the
conditions employed 1 and 5 yielded negligible transport of any of the metals present in the source
phase while sole transport selectivity for Ag(I) was observed for 2–4 and 6.
centres in such materials have frequently been associated with
Introduction
‘supramolecular’ interactions (including silver–p interactions)10,11
and in some instances give rise to unusual electronic and photo-
physical properties.12
The coordination chemistry of 2,2ꢀ-dipyridylamine (dpa) and
its derivatives has been the focus of a considerable number of
investigations. For example, mono-, di- and tri-dpa derivatives
have been reported in which the secondary nitrogen of each
dpa is directly bound to an aryl core with such species being
employed in studies that range from metal coordination and
supramolecular chemistry1,2 to the synthesis of new luminescent
materials.3–5 Pd(II) and Pt(II) complexes of such dpa derivatives
have also been investigated as potential anticancer agents due
to their structural similarity to cisplatin.6–8 The use of Ag(I)
in a range of metal complex and supramolecular materials has
received increasing attention, in part due to the coordination
flexibility of this d10 ion and its well documented tendency to form
strong complexes with nitrogen donor ligands.9 Further, the silver
In the above context, the dpa moiety has been demon-
strated to coordinate to Ag(I) and the solid-state structure of
[Ag(deprotonated dpa)]n has been reported.13 Under the synthetic
conditions employed, deprotonation of the central amine nitrogen
occurs resulting in a charge-neutral, one-dimensional polymeric
chain, with the backbone of the chain defined by weak Ag–Ag
interactions; each silver ion is coordinated to an amido nitrogen
as well as two pyridyl nitrogens from three different dpa ligands.
Under other reaction conditions, neutral dpa has also been shown
to form discrete complexes with silver.14 In each of these latter
systems the secondary amine nitrogen does not coordinate to
silver and each dpa unit acts as a bidentate ligand, coordinating
through both pyridyl nitrogens to form a six-membered chelate
ring. Coordination to silver solely through pyridyl nitrogen donors
is also typical of other substituted dpa ligands.2,4,15
In contrast to the above systems, examples of linked di- and
tri-dpa derivatives in which the linking groups between the dpa
fragments are flexible have not been extensively studied.7,8,16 Nev-
ertheless, complexes of such flexible poly-dpa derivatives bound
to Cu(II),16 as well as Pd(II) and Pt(II),7,8 have been investigated
for use in biological applications. However, no silver complexes
of dpa derivatives of this type have been described previously and
this, in part, provided one motivation for the present study.
aDepartment of Chemistry, Technical University Dresden, 01062, Dresden,
Germany
bCentre for Heavy Metals Research, School of Chemistry, University of
Sydney, NSW, 2006, Australia
cA. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan, 420088,
Russia
dDepartment of Chemistry, University of Canterbury, Christchurch, New
Zealand
† The HTML version of this article has been enhanced with colour images.
‡ Electronic supplementary information (ESI) available: Additional
crystallographic parameters and ORTEP diagrams. See DOI:
10.1039/b609738c
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We now report a comparative solid state and solution investi-
gation of the interaction of the linked 2,2ꢀ-dipyridylamine ligand
systems 1–6 with Ag(I).
0.25 g (16%); mp 76.5 ◦C. Found: C, 78.01; H, 5.72; N, 16.12. Calc.
for C17H15N3: C, 78.13; H, 5.79; N, 16.08%.
1H NMR (500 MHz, CDCl3, 298 K) d 8.40 (2H, d, py), 7.60
(2H, t, py), 7.37 (2H, d, py), 7.27 (2H, d, ph), 7.20 (1H, t, ph), 7.16
(2H, d, ph), 6.94 (2H, t, py), 5.55 (2H, s, CH2). MS (ESI): m/z
262.1 [M + H]+.
1,2-Bis(di-2-pyridylaminomethyl)benzene (2). Di-2-pyridyl-
amine (1.00 g, 5.84 mmol) and potassium hydroxide (1.33 g,
23.7 mmol) were stirred in DMSO (5 mL) for 1 h; 1,2-
bis(bromomethyl)benzene (0.70 g, 2.65 mmol) was added and
the reaction mixture was stirred for a further 40 h. Water
was then added until the solution turned cloudy. The yellow
precipitate that formed was isolated and recrystallised from an
ethyl acetate–petroleum ether (bp 30–60 ◦C) solution to yield
yellow crystals. Yield, 0.29 g (25%); mp 144–146 ◦C. Found: C,
75.37; H, 5.38; N, 19.04. Calc. for C28H24N6: C, 75.65; H, 5.44; N,
1
18.91%. H NMR (500 MHz, CDCl3, 296 K) d 8.32 (4H, d, py),
7.53 (4H, t, py), 7.27 (2H, d, ph), 7.22 (4H, d, py), 7.04 (2H, t,
ph), 6.86 (4H, dd, py), 5.64 (4H, s, CH2). 13C NMR (125.76 MHz,
CDCl3, 296 K) d 157.1 (C5), 148.2 (C1), 137.2 (C7), 136.1 (C3),
126.5, 126.4 (C8, C9), 117.2 (C2), 114.6 (C4), 48.6 (C6).
MS (HR-ESI): m/z 444.2079; C28H24N6 requires 444.2063.
1,3-Bis(di-2-pyridylaminomethyl)benzene (3). Di-2-pyridyl-
amine (1.00 g, 5.84 mmol) and potassium hydroxide (1.33 g,
23.7 mmol) were stirred in DMF (30 mL) at 40 ◦C for 20 min.
1,3-Bis(bromomethyl)benzene (0.699 g, 2.65 mmol) was added
and stirring was continued at 40 ◦C for a further 40 h. The solvent
was removed under reduced pressure and the residue partitioned
between dichloromethane (200 mL) and water (150 mL). The
organic layer was washed twice with water (2 × 150 mL) and
then the combined aqueous phases washed with dichloromethane
(2 × 100 mL). The combined dichloromethane layers were dried
(Na2SO4) and then evaporated, resulting in a pale yellow solid.
This was recrystallised from acetone–water yielding 980 mg (83%)
of a yellow crystalline solid; mp 133–134 ◦C. Found: C, 75.46; H,
5.49; N, 19.03. Calc. for C28H24N6: C, 75.65; H, 5.44; N, 18.91%.
1H NMR (500 MHz, CDCl3, 296 K) d 8.25 (4H, d, py), 7.53 (4H,
t, py), 7.27 (2H, d, ph), 7.22 (4H, d, py), 7.04 (2H, t, py), 6.86
(4H, t, py), 5.64 (4H, s, CH2). 13C NMR (500 MHz, CDCl3, 296
K) d 157.0 (C5), 148.1 (C1), 139.3 (C7), 137.1 (C3), 128.4 (C9),
125.3 (C10), 125.1 (C8), 117.2 (C2), 114.5 (C4), 51.20 (C6).
Experimental
Physical methods
HRMS were determined on samples dissolved in dichloromethane
and run in an acetonitrile matrix. Column chromatography was
performed on silica gel 60 (0.040–0.063 mm Merck) or neutral
Al2O3 (FH 300 mm, HNS 29 Por.0). Melting points are uncor-
rected. NMR spectra were determined on a DRX-500 Bruker
spectrometer or on Varian 300 and 500 MHz spectrometers.
Ligand synthesis
Di-2-pyridylaminomethylbenzene (1). This synthesis was based
on literature procedures.17 Benzyl bromide (1.0 g, 5.88 mmol) in
DMF (3 mL) was added dropwise to 2,2ꢀ-dipyridylamine (1.0 g,
5.85 mmol) and KOH (1.33 g, 23.7 mmol) in DMF (5 mL).
The resulting solution was stirred at room temperature for 15 h
and then taken to dryness under reduced pressure. The residue
was washed with excess water and extracted with chloroform
(3 × 50 mL). The chloroform extracts were dried over anhydrous
sodium sulfate, filtered, and the filtrate taken to dryness under
reduced pressure. The residue was chromatographed on silica gel
by elution with CHCl3–MeOH (5 : 1). The crude yellow product
obtained was recrystallised from an acetone–water mixture. Yield,
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MS (HR-ESI): m/z 444.2064; C28H24N6 requires 444.2063.
with dichloromethane and methanol, and dried in vacuo. Yield,
25.2 mg (89%); mp 233–235 ◦C (decomp.). Found: C, 53.32; H,
3.71; N, 15.87. Calc. for C56H48Ag2N14O6·2H2O: C, 53.18; H, 4.14;
N, 15.50%.
2,6-Bis(di-2-pyridylaminomethyl)pyridine (4). This was syn-
thesised and characterised as described elsewhere.18
1,4-Bis(di-2-pyridylaminomethyl)benzene (5). Di-2-pyridyl-
amine (1.00 g, 5.84 mmol) and potassium hydroxide (1.33 g,
23.7 mmol) were stirred in DMF (30 mL) at 40 ◦C for 20 min.
1,3-Bis(bromomethyl)benzene (0.699 g, 2.65 mmol) was added
and stirring was continued at 40 ◦C for a further 40 h. The solvent
was removed under reduced pressure and the residue partitioned
between dichloromethane (200 mL) and water (150 mL). The
organic layer was washed twice with water (2 × 150 mL) and
then the combined aqueous phases washed with dichloromethane
(2 × 100 mL). The combined dichloromethane layers were dried
(Na2SO4) and then evaporated, resulting in a pale yellow solid.
This was recrystallised from acetone–water yielding 780 mg (66%)
as a yellow powder; mp 181–183 ◦C. Found: C, 75.47; H, 5.44; N,
19.01. Calc. for C28H24N6: C, 75.65; H, 5.44; N, 18.91%. 1H NMR
(500 MHz, CDCl3, 296 K) d 8.28 (4H, d, py), 7.47 (4H, t, py),
7.22 (4H, s, ph), 7.13 (4H, d, py), 6.82 (4H, dd, py), 5.43 (4H, s,
CH2). 13C NMR (500 MHz, CDCl3, 296 K) d 157.1, 148.1, 137.6,
137.1, 126.9, 117.1, 114.5, 51.02. MS (HR-ESI): m/z 444.2078;
C28H24N6 requires 444.2063.
[Ag2(2)2(ClO4)2]·CH3CN. Silver perchlorate (13.0 mg,
0.063 mmol) in acetonitrile (5 mL) was added to 2 (14.1 mg,
0.031 mmol) in dichloromethane (5 mL). Colourless crystals of
the title compound were obtained by slow diffusion of diethyl
ether vapour into the resulting solution. The crystals were
collected, washed with ether, and dried in vacuo. Yield, 10.3 mg
(79%); mp 230–232 ◦C. Found: C, 51.75; H, 3.68; N, 13.79. Calc.
for C56H48Ag2Cl2N12O8·CH3CN: C, 51.51; H, 3.82; N, 13.54%.
[Ag2(2)2](BF4)2. Silver
tetrafluoroborate
(18.7
mg,
0.096 mmol) in methanol was added to 2 (20.2 mg, 0.045 mmol) in
dichloromethane–methanol (1 : 1; 8 mL). The colourless crystals
that formed were collected, washed with dichloromethane and
methanol. These crystals were suitable for X-ray crystallography.
The remaining crystals were dried in vacuo before microanalysis.
Yield, 17.4 mg (60%); mp 245 ◦C (decomp.). Found: C, 52.84; H,
3.97; N, 13.27. Calc. for C56H48Ag2B2F8N12: C, 52.61; H, 3.78; N,
13.15%.
[Ag2(2)2](PF6)2·CH2Cl2. Silver hexafluorophosphate (23.9 mg,
0.095 mmol) in methanol (5 mL) was added to 2 (20.3 mg,
0.046 mmol) in dichloromethane–methanol (1 : 1; 8 mL). The
colourless crystals which formed were collected, washed with
dichloromethane and methanol. These crystals were suitable
for X-ray crystallography. The remaining crystals were dried
in vacuo before microanalysis. Yield, 27.5 mg (68%); mp 239–
1,3,5-Tris(di-2-pyridylaminomethyl)benzene (6). This was pre-
pared by an adaptation of a published procedure.7 Di-2-
pyridylamine (1.00 g, 5.84 mmol) and potassium hydroxide (1.31 g,
23.4 mmol) were stirred in DMSO (5 mL) for 1 h. 1,3,5-
Tris(bromomethyl)benzene19 (0.675 g, 1.89 mmol) was then added
and the solution stirred for an additional 48 h. Water was added
dropwise until the solution turned cloudy. On standing, a yellow
precipitate formed which was filtered off, washed with water and
recrystallised from an ethyl acetate–petroleum ether (bp 30–60 ◦C)
mixture. Yield, 0.583 g (49%); mp 158–160 ◦C. Found: C, 74.33; H,
5.07; N, 19.80. Calc. for C39H33N9: C, 74.62; H, 5.30; N, 20.08%.
1H NMR (500 MHz, CDCl3, 296 K) d 8.20 (6H, d, py), 7.37 (6H,
t, py), 7.07 (3H, s, ph), 6.91 (6H, d, py), 6.78 (6H, dd, py), 5.34
(6H, s, CH2). 13C NMR (125.76 MHz, CDCl3, 296 K) d 157.0,
148.0, 139.4, 137.0, 123.6, 116.9, 114.5, 51.18. MS (HR-ESI): m/z
◦
240 C (decomp.). Found: C, 41.76; H, 3.09; N, 10.27. Calc. for
C28H24AgF6N6P·CH2Cl2: C, 41.55; H, 3.25; N, 9.69%.
[Ag(3)(NO3)]. Silver nitrate (15.8 mg, 0.093 mmol) in
methanol (5 mL) was added to 3 (19.9 mg, 0.045 mmol) in
dichloromethane–methanol (1 : 1; 8 mL). The colourless crystals
which formed on slow evaporation of the above solution were col-
lected, and washed with dichloromethane and methanol. A crystal
from this crop was used for the X-ray crystallographic study. The
remaining crystals were dried in vacuo before microanalysis. Yield,
22.6 mg (82%); mp 232–235 ◦C (decomp.). Found: C, 54.78; H,
3.84; N, 16.21. Calc. for C28H24Ag N7O3: C, 54.74; H, 3.94; N,
15.96%.
+
628.2937. C39H34N9 requires 628.2953.
Complex synthesis
CAUTION. Perchlorate complexes are potentially explosive and
appropriate caution should be exercised in their synthesis and
handling.
{[Ag(3)(CH3CN)](ClO4)}n. Silver perchlorate (16.8 mg,
0.099 mmol) in acetonitrile (5 mL) was added to 3 (28.1 mg,
0.063 mmol) in dichloromethane solution (5 mL). Colourless
crystals of the title compound were obtained by slow diffusion of
ether vapour into the resulting solution. A crystal from this crop
was used for the X-ray structure determination. The remaining
crystals were collected, washed with ether, and dried in vacuo.
Yield, 14.5 mg (86%); mp 233–235 ◦C. Found: C, 51.95; H, 3.78;
N, 13.99. Calc. for C28H24AgClN6O4·CH3CN: C, 51.99; H, 3.94;
N, 14.15%.
[Ag(1)(NO3)]·CH3CN. Silver nitrate (13.3 mg, 0.078 mmol)
in acetonitrile (5 mL) was added to 1 (20.5 mg, 0.078 mmol) in
dichloromethane (5 mL). Colourless crystals of the title compound
were obtained by slow diffusion of diethyl ether vapour into the
resulting solution. The crystals were collected and washed with
ether. Yield, 10.0 mg (76%); mp 160–162 ◦C (decomp.). Found:
C, 48.17; H, 3.65; N, 14.79. Calc. for C17H15N4AgO3·CH3CN: C,
48.32; H, 3.84; N, 14.68%.
{[Ag(3)(CH3OH)](PF6)·H2O}n. Silver hexafluorophosphate
(24.4 mg, 0.097 mmol) in methanol (5 mL) was added to a
dichloromethane–methanol solution (1:1; 8 mL) of 3 (20.7 mg,
0.047 mmol). On standing, colourless crystals grew in the reaction
mixture. These were collected and washed with dichloromethane
and methanol. A crystal from this crop was used for the X-ray
[Ag2(2)2(NO3)2]·2H2O. Silver nitrate (0.168 g, 0.099 mmol)
in methanol (5 mL) was added to 2 (0.201 g, 0.045 mmol)
in dichloromethane–methanol (1:1; 8 mL). Colourless crystals
that were suitable for X-ray crystallography formed on slow
evaporation of the resulting solution. These were collected, washed
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crystallography. The remaining crystals were dried in in vacuo
before microanalysis. Yield, 11.0 mg (31%); mp 179–181 ◦C
(decomp.). Found: C, 46.93; H, 3.87; N, 11.61. Calc. for
C28H24AgF6N6P·CH3OH·H2O: C, 46.60; H, 4.05; N, 11.24%.
duplicate 100 lL samples of the aqueous and organic phases were
removed for analysis.
Membrane transport
The transport experiments employing 1–6 as ionophores were
carried out using a ‘concentric cell’ in which the aqueous source
phase (10 mL) and receiving phase (30 mL) were separated by a
chloroform phase (50 mL). Details of the cell design have been
published previously.20 For each experiment both the aqueous
phases and the organic phase were stirred separately at 10 rpm; the
cell was enclosed by a water jacket and thermostatted at 25 ◦C. The
aqueous source phase was buffered (CH3CO2H–CH3CO2Na) at
pH 4.9 ( 0.1) (6.95 mL of 2 M sodium acetate solution and
3.05 mL of 2 M acetic acid made up to 100 mL) and contained
an equimolar solution of the nitrate salts of Co(II), Ni(II), Zn(II),
Cu(II), Cd(II), Pb(II) and Ag(I), each at a concentration of 1 ×
10−2 M. The chloroform phase contained the ligand at 1 ×
10−3 M. The aqueous receiving phase was buffered (HCO2H–
HCO2Na) at pH 3.0 0.1 (56.6 mL of 1 M formic acid and 10.0 mL
of 1 M sodium hydroxide made up to 100 mL). All transport runs
were terminated after 24 h and atomic absorption spectroscopy
was used to determine the amount of metal ion transported over
this period. Transport rates (J values) are in mol (24 h−1). The
transport results are quoted as the average values obtained from
duplicate runs (error ca. 10% of reported value).
[Ag(5)(NO3)]. Silver nitrate (16.7 mg, 0.098 mmol) in
methanol (5 mL) was added to 5 (20.5 mg, 0.046 mmol) in
dichloromethane–methanol (1 : 1; 8 mL). A pale yellow crystalline
solid formed following slow evaporation of the reaction mixture.
This was collected, washed with dichloromethane and methanol,
and dried in vacuo. Yield 27.5 mg (97%). Found C, 54.85; H, 3.87;
N, 16.02. Calc. for C28H24N7O3Ag: C, 54.74; H, 3.94; N, 15.96%.
[Ag2(5)](BF4)2. Silver tetrafluoroborate (19.3 mg, 0.099 mmol)
in methanol (5 mL) was added to 5 (19.8 mg, 0.045 mmol) in
dichloromethane–methanol (1 : 1; 8 mL). Pale yellow crystals
formed. These were collected, washed with dichloromethane and
methanol, and then dried in vacuo. Yield 14.5 mg (39%). Found:
C, 41.03; H, 2.94; N, 10.13. Calc. for C28H24N6B2F8Ag2: C, 40.33;
H, 2.90; N, 10.08%.
[Ag(5)](PF6)·CH3OH. Silver hexafluorophosphate (25.8 mg,
0.102 mmol) in methanol (5 mL) was added to 5 (20.5 mg,
0.046 mmol) in dichloromethane–methanol (1 : 1; 8 mL). A
yellow crystalline solid formed. This was collected, washed
with dichloromethane and methanol, and dried in vacuo. Yield,
32.2 mg (95%). Found: C, 48.08; H, 4.35; N, 11.55. Calc. for
C28H24N6PF6Ag·CH3OH: C, 47.75; H, 3.87; N 11.52%. Subse-
quently, further pale yellow crystals, suitable for X-ray crystallog-
raphy, were isolated from the filtrate; the structure determination
revealed that these have a different composition to that of the bulk
sample, namely: [Ag3(5)2(H2O)0.5](PF6)3·1.5H2O.
X-Ray structure determinations
Crystals employed for the X-ray determinations were obtained
directly from the respective reaction solutions and were used
without further drying. Structures of 2, {[Ag(3)(CH3CN)](ClO4)·
CH3CN}n and [Ag2(3)2(ClO4)2]·2H2O were collected on a Nonius
Kappa CCD with x and w scans to approximately 56◦ 2h at
198(2) K. Data collections were undertaken with COLLECT,21
cell refinement with Dirax/lsq,22 data reduction with EvalCCD23
and structure solution with SHELXS-97.24 Structures of
5, [Ag2(2)2(NO3)2], [Ag2(2)2](BF4)2, {[Ag2(2)2](PF6)2}3·2CH2Cl2·
2CH3OH, [Ag(3)(NO3)], {[Ag(3)(CH3OH)](PF6)}n and [Ag3(5)2-
(H2O)0.5](PF6)3·1.5H2O, were collected at 168(2) K using a Bruker
SMART 1000 diffractometer with a CCD area detector with x and
w scans to approximately 53◦ 2h. Data integration and reduction
were undertaken with SAINT and XPREP.25 The structures were
solved by direct methods using SHELXS-97.24 Structures of 3
and [Ag(6)NO3] were collected at 150(2) K with x scans to
approximately 56◦ 2h using a Bruker SMART 1000 diffractometer.
Data integration and reduction were undertaken with SAINT and
XPREP25 and subsequent computations were carried out using
the WinGX-32 graphical user interface.26 These structures were
solved by direct methods using SIR97.27 All three diffractometers
employed graphite-monochromated Mo-Ka radiation generated
[Ag(6)(NO3)]. Silver nitrate (21.1 mg, 0.125 mmol) in ace-
tonitrile (5 mL) was added to 6 (26.1 mg, 0.042 mmol) in
dichloromethane (5 mL). Yellow crystals of the title compound
were obtained by slow diffusion of diethyl ether vapour into the
resulting solution. The crystals were collected and washed with
ether. Yield, 7.01 mg (33%); mp 210–212 ◦C (decomp.). Found: C,
58.53; H, 4.13; N, 17.66. Calc. for C39H33N10AgO3: C, 58.73; H,
4.17; N, 17.56%. MS (ESI): m/z 735.3 [L + Ag]+.
Liquid–liquid extraction experiments
Liquid–liquid extraction experiments were performed at 23 1 ◦C
in microcentrifuge tubes (2 cm3) with a phase ratio V(org): V(aq)
of 1 : 1 (500 lL each) with the silver in each phase measured
radiometrically by c-emission of 110mAg by means of a NaI (TI)
scintillation counter (Cobra/Canberra-Packard). The aqueous
phase contained Ag(I) perchlorate (1 × 10−4 M), a supporting
anion (either sodium nitrate (5 × 10−3 M), sodium perchlorate
(5 × 10−3 M) or picric acid (5 × 10−3 M)) and a selected buffer. The
zwitterionic buffer system, MES/NaOH, was used to maintain a
pH of 6.2 in the aqueous phase; as a precaution, the pH of this
phase was measured before and after each experiment using a
InLab423 pH electrode. The organic phase contained a known
˚
from a sealed tube (0.71073 A). Multi-scan empirical absorption
corrections were applied to all data sets, where appropriate,
using the program SADABS.28 All structures were refined and
extended with SHELXL-97.29 In general, ordered non-hydrogen
atoms with occupancies greater than or equal to 0.5 were refined
anisotropically. Partial occupancy carbon, nitrogen and oxygen
atoms were refined isotropically. Carbon-bound hydrogen atoms
were included in idealised positions and refined using a riding
model. Oxygen-bound hydrogen atoms that were structurally
concentration of ligand in chloroform (normally 1 × 10−3
M
except where variable concentration experiments were employed).
All experiments involved the mechanical shaking of the two-phase
system for 30 min by which time equilibrium was reached. At the
end of this time, the phases were separated, centrifuged and then
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evident in the difference Fourier map were included and refined
with bond length and angle restraints.
and S5 in the ESI‡ while schematic depictions of structures are
shown in Fig. 4–12, S1 and S4.‡ Where applicable, additional
details relating to the X-ray crystal structure refinement are given
in the ESI‡ (along with tables of selected bond lengths and
angles).
Crystal data
Crystal and structure refinement data for all structures are sum-
marised in Table 1. ORTEP30 depictions of the crystal structures
are given in Fig. 1–3 and are also provided in Fig. S2, S3
CCDC reference numbers 614167–614178.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b609738c
Table 1 Crystal data for the ligands 2, 3 and 5 and some of their complexes with Ag(I)
{[Ag(3)(CH3CN)]-
(ClO4)·CH3CN}n
Compound
2
3
5
[Ag2(2)2(NO3)2]
[Ag(3)(NO3)]
Formula
Mr
C28H24N6
444.53
C28H24N6
444.53
C28H24N6
444.53
C56H48Ag2N14O6
1228.82
C28H24AgN7O3
614.41
C32H30AgClN8O4
733.96
Crystal system
Triclinic
P1
9.218(1)
11.011(1)
12.294(1)
104.89(1)
95.32(1)
108.07(1)
1126.08(18)
1.311
Monoclinic
C2
14.635(3)
9.2483(19)
16.571(3)
Monoclinic
P21/n
8.483(2)
16.999(4)
8.759(2)
Monoclinic
P21/n
12.659(5)
12.417(5)
17.195(5)
Triclinic
P1
Monoclinic
P21/c
8.522(1)
22.118(1)
17.633(1)
¯
¯
Space group
˚
a/A
8.8253(18)
10.437(2)
14.732(3)
85.759(2)
84.787(2)
70.861(2)
1275.2(4)
1.600
˚
b/A
˚
c/A
◦
a/
b/
◦
90.094(3)
115.643(3)
106.291(5)
103.41(1)
◦
c /
3
˚
V/A
2243.0(8)
1.316
4
1138.6(5)
1.297
2
2594.3(16)
1.573
2
3233.0(4)
1.508
4
Dc/g cm−3
Z
2
2
Crystal size/mm
Crystal colour
Crystal habit
T/K
0.83 × 0.26 × 0.24
Light yellow
Block
198(2)
0.71073
0.39 × 0.31 × 0.21
Colourless
Multi-face
150(2)
0.71073
0.081
0.61 × 0.53 × 0.35
Pale yellow
Block
168(2)
0.71073
0.70 × 0.55 × 0.40
Colourless
Block
168(2)
0.71073
0.60 × 0.58 × 0.21
Colourless
Block
168(2)
0.71073
0.33 × 0.16 × 0.10
Colourless
Block
198(2)
0.71073
k(Mo-Ka)
l(Mo-Ka)/mm−1
0.081
0.080
0.822
0.836
0.756
T(Empirical)min,max
0.9360, 0.9809
54.80
N/A
56.58
0.7672, 1.0000
52.80
0.8558, 1.0000
52.72
0.6367, 1.0000
52.76
0.9282, 0.7885
56.00
◦
2hmax
/
hkl Range
−11 to 11, −14 to
14, −15 to 15
35395
19 to 18, −11 to
12, −21 to 21
10605
−10 to 10, −21 to
21, −5 to 10
14413
−15 to 15, −15 to
15, 21 to 21
31748
−10 to 10, −13 to
6, −18 to 18
16279
−11 to 11, −29 to
29, −23 to 23
73465
N
Nind (Rmerge
)
5013 (0.0376)
3900
0.0395,
0.1037
0.0534, 0.2205
1.024
2821 (0.0268)
2684
0.0313,
0.0800
0.0446, 0.4092
0.987
2292 (0.0160)
2102
0.0403,
0.1024
0.0481, 0.3332
1.063
5275 (0.0191)
4891
0.0235,
0.0597
0.0256, 1.8526
1.092
5067 (0.0211)
4815
0.0234,
0.0626
0.0398, 0.6586
1.026
7770 ( 0.0380)
6170
0.0266,
0.0626
0.0279, 1.7272
1.009
Nobs (I > 2r(I))
R1a (I > 2r(I)),
wR2a (all)
Aa, Ba
GoF
−3
Dq/e−
0.218, −0.244
0.147, −0.177
0.147, −0.161
0.574, −0.391
0.284, −0.688
0.363, −0.377
˚
A
[Ag3(5)2(H2O)0.5]-
(PF6)3·1.5H2O
{[Ag2(2)2](PF6)2}3·
{[Ag(3)(CH3OH)-
(PF6)]}n
Compound
[Ag2(2)2](BF4)2
2CH2Cl2·2CH3OH
[Ag2(2)2(ClO4)2]·2H2O
[Ag(6)NO3]
Formula
Mr
Crystal system
C56H52Ag3F18N12O2P3
1683.62
Monoclinic
P21/c
14.7918(9)
14.7643(9)
29.0604(17)
C56H48Ag2B2F8N12
1278.42
Monoclinic
P21/n
12.618(4)
12.798(4)
17.110(6)
C172H156Ag6Cl4F36N36O2P6
C56H52Ag2Cl2N12O10
1339.74
Orthorhombic
Pccn
20.973(1)
29.544(1)
17.798(1)
C29H28AgF6N6OP
729.41
Monoclinic
C2/c
8.623(3)
25.164(9)
14.262(5)
C39H33AgN10O3
797.62
Triclinic
¯
P1
9.1320(15)
12.801(2)
15.264(3)
81.910(3)
81.998(3)
71.440(3)
1666.2(5)
1.590
4418.16
Triclinic
¯
P1
14.361(5)
17.001(6)
20.092(7)
69.969(4)
73.690(4)
86.817(5)
4419(2)
Space group
˚
a/A
˚
b/A
˚
c/A
◦
a/
b/
◦
94.881(1)
106.394(4)
94.626(4)
◦
c /
3
˚
V/A
6323.5(7)
1.768
4
2650.8(15)
1.602
2
11028.1(9)
1.614
8
3084.7(18)
1.571
4
Dc/g cm−3
Z
1.660
1
2
Crystal size/mm
Crystal colour
Crystal habit
T/K
0.45 × 0.45 × 0.30
0.45 × 0.36 × 0.20
Colourless
Block
168(2)
0.71073
0.46 × 0.44 × 0.08
Colourless
Plate
168(2)
0.71073
0.30 × 0.20 × 0.05
Colourless
Plate
198(2)
0.71073
0.70 × 0.29 × 0.28
Colourless
Rod
168(2)
0.71073
0.138 × 0.123 × 0.096
Brown
Yellow
Block
168(2)
0.71073
1.102
Block
150(2)
0.71073
0.663
k(Mo-Ka)
l(Mo-Ka)/mm−1
0.819
0.870
0.879
0.775
T(Empirical)min,max
0.7908, 1.0000
52.78
0.7998, 1.0000
52.70
0.7133, 1.0000
50.00
0.7785, 0.9574
50.80
0.8307, 1.0000
52.76
0.745, 0.938
56.80
◦
2hmax
/
hkl Range
−12 to 18, −18 to 17,
−27 to 36
47897
−15 to 14, −15 to
15, −21 to 21
32363
−12 to 17, −18 to 20, −23 to
−23 to 25, −35 to 35,
−16 to 21
44563
−10 to 10, −30 to
31, −10 to 17
17476
−12 to 12, −16 to 17,
−20 to 20
16508
22
N
51734
Nind (Rmerge
)
12819 (0.0328)
9727
0.0488,
0.1418
0.0702,19.7685
1.052
5348 (0.0374)
4246
0.0278,
0.0663
0.0332, 1.5060
0.994
15404 (0.0952)
9767
0.0496,
0.1377
0.0640, 0.0000
0.941
9832 (0.0380)
6090
0.0628,
0.1747
0.0597, 87.2719
1.053
3121 (0.0289)
2833
0.0572,
0.1457
0.0449, 17.7216
1.228
7771 (0.0352)
5892
0.0424,
0.0921
0.0433, 0.0000
1.032
Nobs (I > 2r(I))
R1a (I > 2r(I)),
wR2a (all)
Aa, Ba
GoF
−3
Dq/e−
2.604, −1.389
0.466, −0.377
1.045, −1.063
1.375, −1.500
0.720, −1.888
0.628, −0.365
˚
A
ꢀ
ꢀ
ꢀ
ꢀ
a R1 = ꢁFo| − |Fcꢁ/ |Fo| for Fo > 2r(Fo) and wR2 = { [w(Fo − Fc2)2]/ [w(Fc2)2]} where w = 1/[r2(Fo2) + (AP)2 + BP], P = (Fo + 2Fc2)/3
2
1/2
2
and A and B are listed in the crystal data information supplied.
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Fig. 1 ORTEP plot of 2 shown with 50% probability ellipsoids.
Fig. 3 ORTEP plot of 5 shown with 50% probability ellipsoids. Symmetry
operator used to generate equivalent atoms: −x + 1, −y, −z + 1.
Results and discussion
X-Ray structures of ligands 2, 3 and 5
The 2,2ꢀ-dipyridylamine ligands 1–3 and 5–6 were prepared by the
reaction of 2,2ꢀ-dipyridylamine with the appropriate alkyl halide
derivative in the presence of base; while the synthesis of 4 has
been reported elsewhere by our group.18 In order to investigate
the silver ion chemistry of 1–6 and make structural comparisons
between these derivatives and their resulting silver complexes,
an attempt was made to obtain crystalline products suitable
for X-ray diffraction of both the ligands and their correspond-
ing complexes. Solid complexes of type [Ag(1)(NO3)]·CH3CN,
[Ag2(2)2(NO3)2]·2H2O, [Ag2(2)2(ClO4)2]·CH3CN, [Ag2(2)2](BF4)2,
[Ag2(2)2](PF6)2·CH2Cl2, [Ag(3)(NO3)], {[Ag(3)(CH3CN)]ClO4)n,
{[Ag(3)(CH3OH)]PF6·H2O}n, [Ag(5)NO3], [Ag3(5)2(H2O)0.5](PF6)3·
1.5H2O, [Ag2(5)](BF4)2, [Ag(5)]PF6·CH3OH and [Ag(6)NO3] were
isolated following the reaction of the appropriate silver salt in
methanol or acetonitrile with the required ligand in dichloro-
methane and/or methanol. We have also recently reported the
synthesis of the related complex, [Ag2(4)CH3CN]2[Ag2(NO3)6]·
0.75CH3CN·0.25CH2Cl2.18
Suitable crystals for X-ray structure analysis of the isomeric
bis(di-2-pyridylaminomethyl)benzenes 2, 3 and 5 were obtained.
The structure of 2 is presented in Fig. 1. The aliphatic nitrogen
atoms associated with each of the dpa moieties in this structure
are considerably distorted from the usually observed trigonal
pyramidal arrangement for simple tertiary amines towards a
more trigonal planar geometry, undoubtedly reflecting electron-
pair delocalisation that also involves the attached aromatic rings,
resulting in the nitrogens showing some sp2 character. Thus, the
angles between the bonds to the pyridyl rings from the aliphatic
nitrogen atoms (N(11) and N(12)), are 124 and 122◦, respectively.
The two pyridyl rings of each dpa moiety are not co-planar.
In the N(11)-containing dpa moiety the two pyridyl rings are
‘reversed’ (that is approaching a ‘trans’ orientation) with respect
to each other so that the ring nitrogens are not adjacent, while in
the N(12)-containing fragment the two rings are oriented so that
the nitrogen atoms are somewhat closer. Adjacent molecules pack
Fig. 2 ORTEP plots of 3 shown with 50% probability ellipsoids. The asymmetric unit contains two crystallographically independent but chemically
identical molecules, each with crystallographic C2 symmetry. Symmetry operators used for generating equivalent atoms: left: −x + 1, y, −z + 1; right:
−x + 1, y, −z.
4788 | Dalton Trans., 2006, 4783–4794
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in two-dimensional sheets which propagate via a combination of
phenylene to pyridyl nitrogen interactions and edge to face p–p
interactions (Fig. S1, ESI‡).
The structure of 3 is shown in Fig. 2. Again the tertiary amines
are considerably closer to a trigonal planar than a tetrahedral
geometry with, for example, the angle between the bonds from
N(1) to the N(2)- and N(3)-containing rings in one dpa moiety
being 121◦ while in the other (the N(4)-containing dpa fragment)
the corresponding angle is 123◦. Like the structure of 2 there are
two arrangements of the dpa pyridyl rings present in this structure.
The N(1)-containing moiety is arranged in a generally similar
manner to that in the N(11)-containing moiety in the structure
of 2. The conformation of the N(4)-containing moiety is similar
to the N(12)-containing fragment in the above structure, once
again stabilised by the presence of weak nitrogen to hydrogen
interactions in the structure. However, in contrast to 2 there is
also evidence for p–p stacking in this structure. An intermolecular
edge-to-face p-interaction occurs between the N(3)- and N(5)-
Fig. 4 A schematic representation of the X-ray crystal structure of
˚
[Ag2(2)2(NO3)2]. Ag(I) ions are shown as spheres; Ag(1)–Cg(1) 3.1 A and
˚
Ag(1)–Cg(2) 3.3 A (Cg = ring centroid).
˚
˚
containing rings, reflected by a H(25) to C(15) distance of 2.7 A;
a similar interaction, given by a H(28) to C(24) distance of 2.8 A,
also indicates the presence of an edge-to-face interaction between
the N(5)- and N(6)-containing rings.
As shown in Fig. 3, the structure of 5 also contains tertiary
amines that are almost planar, with the pyridyl moieties again
oriented in a similar manner to those in the structures of 2
and 3. Some edge-to-face p–p stacking is present between the
N(21)- and N(31)-containing rings as well as between the N(21)-
containing ring and the central arene ring. These are reflected
uncoordinated. This results in the formation of a discrete [2 + 2]
metallacyclic structure. Other examples of dpa moieties failing to
chelate to Ag(I) have also been documented.4,5,33 Two weak silver–
arene p-interactions involving the non-coordinated pyridine rings
are present in the structure, these presumably aid stabilisation of
the complex and contribute to the overall rigidity of the system.
As observed for free 2, the tertiary nitrogen groups of each
dpa moiety are again distorted from ideal tetrahedral geometry
towards trigonal planar and are similar to those observed in the
crystal structure of free ligand 2. There are also short contacts
between phenylene hydrogen atoms and both tertiary amines,
again stiffening the structure.
˚
˚
by distances of 2.7 A (H(36A)–C(26)) and 2.9 A (H(26A)–C(6)),
respectively. Again there are weak nitrogen–hydrogen interactions
present which presumably aid in reducing the flexibility of the
structure (see below).
An investigation of the effect of variation of the anion on the
resulting structure adopted in the solid state was carried out. Thus
an analogous synthesis to that used to obtain [Ag2(2)2(NO3)2]
was repeated in which silver perchlorate was substituted for the
nitrate salt. Reaction of two equivalents of AgClO4 with 2 again
resulted in a discrete [2 + 2] macrocyclic product of stoichiometry
[Ag2(2)2(ClO4)2]·2H2O (Fig. 5). Despite the addition of Ag(I) to
2 in a 2 : 1 ratio, only the above 2 : 2 complex was isolated. The
X-ray structure of this product shows that each silver is bound to
two pyridine N donors from different ligands and to one oxygen
atom of the perchlorate in a trigonal planar geometry. The latter
A common feature of each of the above ligand structures is
that the tertiary nitrogens almost lie in the plane of the central
substituted phenyl ring. This leads to close contacts between the
aliphatic nitrogen atoms and adjacent aryl hydrogen atoms present
on the phenylene cores. Namely, in 2 inter-atomic distances of
˚
˚
2.5 A (H(6)–N(11)) and 2.5 A (H(3)–N(12)) occur, in 3 distances
˚
˚
of 2.5 A (H(4)–N(1)) and 2.5 A (H(19)–N(4)) are present while
˚
in 5 a distance of 2.6 A (H(6A)–N(11)) occurs. Such distances
are typical of interactions between phenyl hydrogens and amine
nitrogens. Interactions of this type are well documented to play
a role in structure stabilisation in other systems31,32 and this also
appears to be the case in the present solid state arrangements
of 2, 3 and 5. Taken together with the tendency towards sp2
hybridisation of the aliphatic nitrogens, such interactions will
act to restrict the flexibility in these ligand systems. Any such
decreased flexibility serves to add a degree of preorganisation to
the respective structures which is also evident in the structures
of the corresponding Ag(I) complexes in particular instances (see
below).
X-Ray crystal structures of Ag(I) complexes
Each Ag(I) ion in [Ag2(2)2(NO3)2] (Fig. 4), has a four-coordinate,
pseudo-tetrahedral geometry with a coordination sphere con-
sisting of two pyridine N-donors from different ligands and a
chelating nitrate anion. Thus only one of the pyridyl groups from
each dpa unit is bound to a Ag(I) ion, while the second remains
Fig. 5 A schematic representation of the X-ray crystal structure of
[Ag2(2)2(ClO4)2]·2H2O. Ag(I) ions are shown as spheres. Water molecules
˚
˚
are not shown; Ag(1)–Cg(1) 3.2 A, Ag(1)–Cg(2) 3.2 A (Cg = ring centroid).
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contrasts with the distorted tetrahedral geometry obtained with
the nitrate anion. Nevertheless, the structure shows remarkable
similarity to that of [Ag2(2)2(NO3)2].
two-coordinate, again forming a [2 + 2] metallacycle with each
silver bound to a pyridine donor from a different ligand. In the
tetrafluoroborate case, the coordination to each silver involves a
‘bent’ two-coordinate geometry with, for example, the N(21A)–
Ag(1)–N(41) bond angle being 152.84(7)◦. Interestingly, there are
some short-range interactions present between both Ag(I) ions
and a disordered tetrafluoroborate anion, with the presence of
In the presence of the non-coordinating anions, tetrafluorobo-
rate or hexafluorophosphate, the complexes [Ag2(2)2](BF4)2 and
[Ag2(2)2]3(PF6)6·2CH2Cl2·2CH3OH were obtained. These were
synthesised directly by the reaction of two equivalents of AgBF4 or
AgPF6 with 2 and, as was observed for the nitrate and perchlorate
salts, gave 2:2 complexes. As expected, the X-ray analysis of each
species revealed that neither anion coordinates to a Ag(I) centre
(Figs. 6 and 7). In each case the silver ions are each formally
˚
silver–fluorine distances of 2.681(2) and 2.686(2) A.
Unlike the above structure, there are no silver–anion inter-
actions present in [Ag2(2)2]3(PF6)6·2CH2Cl2·2CH3OH. However,
pyridine coordination of the silver atom (and hence the stability
of the system) is complemented by intermolecular silver–p inter-
actions between each silver and the uncoordinated pyridine rings
of adjacent [2 + 2] metallacycles. These Ag–p interactions appear
significant11,34 and have Ag–ring centroid distances of 3.3 A. Their
˚
presence results in the formation of an infinite sheet-like two-
dimensional network (Fig. 7). Once again the arrangement of the
ligands in this complex shows similarities to that adopted by the
free ligand.
Overall, the X-ray investigations show that all four of the
complexes of 2 form 2 : 2 metallacyclic structures in which
each dpa moiety bridges two silver ions irrespective of the anion
present. Minor differences in the ring structure arise due to the
−
−
coordination of NO3 or ClO4 anions to the silver centres while
−
−
in the case of the BF4 and PF6 derivatives, (as expected) these
anions do not coordinate, while essentially the same metallacyclic
structural motif is maintained.
The reaction of 3 with AgX salts (X = NO3−, ClO4 and
−
PF6−) resulted in crystals suitable for X-ray analysis in each case.
Variation of the reaction conditions (solvent type, anion present
and/or ratio of metal ion to ligand employed) resulted in two very
different structures forming; namely, a discrete [1 + 1] metallacycle
and an infinite one-dimensional, polymeric chain species.
Fig. 6 A schematic representation of the X-ray crystal structure of
[Ag2(2)2](BF4)2. Ag(I) ions are shown as spheres. Tetrafluoroborate anions
are not shown; Ag(1)–Cg(1) 3.1 A, Ag(1)–Cg(2) 3.2 A (Cg = ring centroid).
Discrete [Ag(3)NO3] was obtained from the reaction of
Ag(NO3) with 3 in a 2 : 1 ratio in methanol–dichloromethane.
˚
˚
Fig. 7 A schematic representation of the Ag–p interactions in [Ag2(2)2]3(PF6)6·2CH2Cl2·2CH3OH, giving rise to a two-dimensional network.
4790 | Dalton Trans., 2006, 4783–4794 This journal is The Royal Society of Chemistry 2006
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As shown in Fig. 8, the resulting 1 : 1 complex has the Ag(I) in a
distorted tetrahedral coordination environment composed of two
−
pyridine N atoms from different dpa units and a chelating NO3
ion. As occurs for the other silver complex structures discussed so
far, one 2-pyridyl ring from each dpa moiety does not participate
in coordination; however, weak silver–p interactions with the non-
coordinated pyridine rings and a silver—arene C–H interaction of
˚
2.7 A (involving H(2) of the central benzene core) help to stabilise
the structure. Once again, short central phenylene hydrogen to
tertiary nitrogen distances are present in this structure.
Fig. 10 The molecular structure of {[Ag(3)(CH3CN)]ClO4·CH3CN}n.
The perchlorate anion and the non-coordinated acetonitrile are not shown;
˚
˚
Ag(1)–Cg(1) 3.2 A. Ag(1)–Cg(2) 3.4 A (Cg = ring centroid).
ion and the structures are stabilised via weak offset face to face
p–p and silver–p interactions.
The reaction of the 2,6-lutidinyl bridged, bis-dipyridylamine
ligand 4 with silver nitrate in acetonitrile–dichloromethane re-
sults in polymeric species of type {[Ag2(4)CH3CN]2(Ag2(NO3)6)·
0.75CH3CN·0.25CH2Cl2}n. The structure of this product in-
corporates a molecular ladder motif that consists of pairs of
cationic [Ag2(4)CH3CN]2+ units bridged by an unprecedented
[Ag2(NO3)6]4− anion.18 The silver ions in the cationic dinuclear
silver species are not equivalent. Each is bound to an oxygen
from a ‘terminal’ nitrate group in an [Ag2(NO3)6]4− unit. The
coordination sphere of Ag(1) is completed by two pyridyl nitrogens
from 4 and the lutidinyl nitrogen donor in a distorted tetrahedral
arrangement. Ag(2) also has a distorted tetrahedral geometry with
the coordination sphere composed of the remaining (two) pyridyl
nitrogens from 4 and an acetonitrile molecule.
Fig. 8 A schematic representation of the X-ray crystal structure of
˚
[Ag(3)(NO3)]. The silver ion is shown as a sphere; Ag(1)–Cg(1) 3.4 A,
˚
Ag(1)–Cg(2) 3.4 A (Cg = ring centroid).
The structure of [Ag3(5)2(H2O)0.5]PF6)3·1.5H2O, generated from
the reaction of AgPF6 with 5 in a 2 : 1 ratio, is shown in
Fig. 11. An M3L2 complex is formed, where at one end of the
complex the di-2-pyridylamine units coordinate in a monodentate
The reaction of 3 with AgPF6 and with AgClO4 in a
1 : 2 ratio in methanol–dichloromethane and acetonitrile/
dichloromethane, respectively, resulted in the isolation of corres-
ponding one-dimensional coordination polymers of stoichio-
metries {[Ag(3)(CH3OH)](PF6)}n (Fig. 9) and {[Ag(3)(CH3CN)]-
(ClO4)·CH3CN}n (Fig. 10). The Ag(I) in each case exhibits a
distorted trigonal planar arrangement, with two sites coordi-
nated to pyridyl moieties and the third to a methanol molecule
(which is disordered over two sites) in the case of {[Ag(3)-
(CH3OH)](PF6)}n, or an acetonitrile molecule in the case of
{[Ag(3)(CH3CN)](ClO4)·CH3CN}n. Again, in both products only
one pyridyl group from each dpa unit coordinates to each silver
Fig. 11 The molecular structure of [Ag3(5)2(H2O)0.5](PF6)3·1.5H2O.
Fig. 9 The molecular structure of {[Ag(3)(CH3OH)](PF6)}n. Hexafluoro-
˚
Hexafluorophosphate anions and water molecules are not shown.
phosphate anions are not shown; Ag(1)–Cg(1) 3.3 A (Cg = ring centroid).
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fashion towards one silver ion Ag(2), while at the opposite end,
a twelve-membered dimetallacycle is formed involving two silver
ions (Ag(1)/Ag(3). The ligand strands have a helical twist between
the Ag(2) and the Ag(1)/Ag(3)) ends of the complex. The Ag(3)
centre is also bound to a half-occupancy water molecule. The
water molecule incorporating O(60) is hydrogen bonded to a
nearby hexafluorophosphate anion and to a non-coordinated
water molecule. Ag(3) is also involved in a silver–p interaction
technique.35 A summary of the extraction results for each of 1–
6 under comparable experimental conditions in the presence of
different counterions in the aqueous phase is given in Fig. 13.
˚
with an aryl ring of the ligand (Ag–C distances of 2.8 A). Ag(1)
and Ag(2) are only bound to pyridine nitrogens and show close
to linear geometries; however, there are also weak contacts with
the fluorine atoms of nearby hexafluorophosphate anions present.
Silver-arene p-interactions involving Ag(1) and Ag(2) also occur.
The reaction of 6 with Ag(I) nitrate in a 1 : 3 ratio results in the
formation of a 1 : 1 (metal : ligand) complex [Ag(6)(NO3)]. Like
the structures discussed above, only one pyridyl from each pair of
pyridyl rings is coordinated to silver (Fig. 12). A bidentate nitrate
anion is also bound such that the Ag(I) ion adopts a distorted
tetrahedral geometry. No donors from the third dpa moiety are
coordinated. As with the above structures, the central nitrogen of
each dpa moiety exhibits a trigonal planar arrangement and lies
close to the plane of the central aryl ring; the phenylene hydrogen
to nitrogen interactions discussed previously are also present in
this structure. The presence of the free dpa moiety suggests that this
complex may be useful for the formation of extended heteronuclear
structures on reaction with other metal ions.
Fig. 13 Percentage Ag(I) extraction by 1–6 with nitrate, perchlorate and
picrate as the anion. [AgNO3] = 1 × 10−4 M, [NaNO3] = 5 × 10−3 M;
[AgClO4] = 1 × 10−4 M, [NaClO4] = 5 × 10−3 M; [AgNO3] = 1 × 10−4 M,
[Hpic] = 5 × 10−3 M; pH 6.2 (MES/NaOH buffer); [ligand] = 1 × 10−3
in CHCl3; shaking time 30 min; T = 23 1 ◦C.
M
As expected there are pronounced differences in the extraction
properties between 1–6. We consider first the results obtained in
the presence of perchlorate. In the case of the monosubstituted dpa
derivative 1, no Ag(I) extraction was detected while the (ortho- and
meta-linked) bis-dpa derivatives 2 and 3 gave extraction values
of 22 and 14%, respectively and the corresponding 2,6-lutidinyl
bridged, bis-dpa system 4 yielded a value of 44%. Increasing
the number of dpa residues to three in 6 leads to a further
enhancement of extraction efficiency, with the value now being
54%. Interestingly, the para-disubstituted isomer of 2 and 3,
namely 5, shows only trace extraction of Ag(I) into the organic
phase. Ligand 4, incorporating an additional pyridine donor
function, extracted silver (44%) more efficiently when compared
to its analogue ligand 3 (14%).
More lipophilic systems may aid the extraction process by
inhibiting the bleeding of the amine ligand or its metal complex
from the organic to the aqueous phase as well as enhancing
extraction through favorable solvation effects. As is clearly evident
from Fig. 13, the use of picrate improves the extraction efficiency
over perchlorate which in turn yields significantly better extraction
than nitrate. This is undoubtedly a reflection of the different
lipophilicities of these anions and thus follows the expected
anion influence on metal extraction predicted by the well-known
Hofmeister series.36
In order to investigate the stoichiometries of the extracted
species, extraction experiments were performed in which the
concentration of the ligand was varied while the metal ion concen-
tration was maintained at 1 × 10−4 M and the anion concentration
was effectively constant. The log DM = [Mn+](org)/[Mn+](aq) was then
plotted against the log of the ligand concentration. Provided a
‘simple’ equilibrium is involved, the slope of this plot gives the
stoichiometry of the extracted species directly.37
The results show linear relationships between the logarithms of
the distribution ratio (DAg) and the ligand concentration (pH 6.2,
ligand excess) (Fig. 14). Slopes of ca. 1.2 were obtained for 2,
3, and 4 and hence indicate approximate 1 : 1 Ag(I) : L stoi-
chiometries in each of these cases. This stoichiometry corresponds
with that observed in the X-ray crystal structures of the Ag(I)
Fig. 12 The molecular structure of [Ag(6)(NO3)].
Liquid–liquid extraction studies
The extraction behaviour of the di-2-pyridylaminomethylbenzene
derivatives 1–6 towards Ag(I) was studied using the radiotracer
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monosubstituted dpa ligand 1 fails to act as ionophore for all
seven metals under the conditions employed. The isomeric ligands
incorporating two 2,2ꢀ-dipyridylamine units 2 (J = 45 × 10−7
mol/24 h) (ortho substituted) and 3 (J = 277 × 10−7 mol/24
h) (meta substituted) both promote significant Ag(I) transport (in
parallel with their solvent extraction behaviour—although theory
dictates that this need not necessarily be the case) but nevertheless
they give quite different values indicating the influence of the
different substitution positions on the ‘linking’ aryl ring. Indeed,
for the corresponding para derivative 5, no metal-ion transport was
observed under the conditions employed—again this parallels the
extraction results where this ligand was shown to be an inferior
extractant of Ag(I) relative to the ortho and meta-linked isomers 2
and 3 (see Fig. 13). Ligands 4 and 6 also yield results that broadly
mimic those obtained in the corresponding solvent extraction
experiments. Ionophore 4, incorporating an additional pyridine
donor function relative to 3, acts as an efficient transporter of Ag(I)
(J = 367 × 10−7 mol/24 h) whereas the tri-substituted derivative
6 was found to show the highest transport efficiency towards this
ion with a J value of 397 × 10−7 mol/24 h being obtained.
Fig. 14 Variable ligand concentration experiments for 2–6 with Ag(I) and
perchlorate as the anion. [AgClO4] = 1 × 10−4 M, [NaClO4] = 5 × 10−3 M;
pH 6.2 (MES/NaOH buffer); [ligand] = (0.5–2.5) × 10−3 M in CHCl3;
shaking time 30 min; T = 23 1 ◦C. Slopes: 6, s = 1.4; 4, s = 1.2; 2, s =
1.2; 3, s = 1.2; 5, s = 1.3.
complexes of 2 and 3 in the solid state which have formulas
of [Ag2(2)2(ClO4)2]·2H2O and {[Ag(3)(CH3CN)](ClO4)·CH3CN}n
respectively (but not for 4 where the solid-state stoichiometry is
given by {[Ag2(4)CH3CN]2(Ag2(NO3)6)). In contrast, the slopes
obtained for ligands 5 and 6 fall a little higher at 1.3 and 1.4,
respectively, thus suggesting that species of higher metal : ligand
ratio(s) make a higher contribution to extraction in these latter
cases.
Concluding remarks
The Ag(I) chemistry of a series of linked dipyridylamine derivatives
in both the solid state and solution has been explored. These
ligands have been demonstrated to yield a range of new silver
complex derivatives showing both discrete and polymeric architec-
tures. In solution, structure/function relationships underlying the
interaction of these species with Ag(I) have been probed. In seven-
metal competitive transport experiments across bulk chloroform
membranes several of the ligands were demonstrated to show sole
selectivity for Ag(I) over the remaining six transition and post
transition metal ions present in the aqueous source phase.
Bulk membrane transport studies
Competitive mixed metal transport experiments across a bulk
chloroform membrane (water/chloroform/water) have been un-
dertaken. The chloroform membrane phase contained the
ionophore at 1 × 10−3 M chosen from 1–6, respectively. The
aqueous source phase contained equimolar concentrations of the
nitrate salts of Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Ag(I) and Pb(II),
with individual metal ion concentrations being 1 × 10−2 M. As
mentioned in the Experimental, transport was performed against
a back gradient of protons. Under the conditions employed
(and ignoring any apparent transport showing J values <15 ×
10−7 mol/24 h as being within experimental error of zero), sole
selectivity for Ag(I) was observed for each of the ionophores 2,
3, 4 and 6, with no transport of any ion occurring for 1 and 5
(Fig. 15).
Acknowledgements
We thank the Australian Research Council, the Royal Society of
New Zealand Marsden Fund and the Deutsche Forschungsge-
meinschaft for support. B. A. gratefully acknowledges the Max-
Buchner-Stiftung for a research grant. O. K. acknowledges the
DFG for assistance under grant 436RUS 17/20/06.
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