Interaction of tripodal Schiff-base ligands with silver(i): Structural and solution studies

Article abstract of DOI:10.1039/c0ce00255k

The binding and extraction of Ag(i) by tripodal Schiff-base ligands incorporating different aromatic podand arms are reported. These ligands have been synthesized by condensation of tris(2-aminoethyl)amine with benzaldehyde (1), 4-phenylbenzaldehyde (2),

Full text of DOI:10.1039/c0ce00255k

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Interaction of tripodal Schiff-base ligands with silver(I): structural and
solution studies†
ab
b
b
b
c
*
*
€
Kathrin Wichmann, Kerstin Gloe, Karsten Gloe, Hans-Jurgen Buschmann,
Marco Wenzel,
d
e
e
e
b
f
€
Keisuke Otho, Martin Schroder, Alexander J. Blake, Claire Wilson, Allison M. Mills, Leonard F. Lindoy
and Paul G. Pliegera
Received 1st June 2010, Accepted 7th June 2010
DOI: 10.1039/c0ce00255k
The binding and extraction of Ag(I) by tripodal Schiff-base ligands incorporating different aromatic
podand arms are reported. These ligands have been synthesized by condensation of tris-
(2-aminoethyl)amine with benzaldehyde (1), 4-phenylbenzaldehyde (2), 2- (3) and
4-pyridinecarbaldehyde (4). The structures of 1 and of four Ag(I) complexes [Ag(1)]ClO4, [Ag(2)]ClO4,
[Ag3(3)2](ClO4)3 and {[Ag3(4)2](ClO4)3}n have been determined by single crystal X-ray diffraction. The
structure of 1 shows intramolecular C–H/p (aromatic) interactions between phenyl rings, while
bonding of Ag(I) via Ag–N interactions within the ionophore pocket is confirmed. In [Ag(1)]ClO4, and
[Ag(2)]ClO4 additional long-range Ag–H interactions are observed, while p–p stacking occurs in the
polynuclear species [Ag3(3)2](ClO4)3 and {[Ag3(4)2](ClO4)3}n. Potentiometric titration, liquid–liquid
1
extraction and H NMR spectroscopic studies were performed to probe the nature of the silver
complexes in solution. Potentiometric studies confirm increasing complex stability with Ag(I) in the
order 2 < 4 < 1 < 3, and enhanced Ag(I) extraction efficiency was observed with both increasing
1
lipophilicity of the ionophore and the presence of additional donor groups. H NMR spectroscopic
studies were employed to probe the solution complexation behaviour of 1–4 towards Ag(I) and these
confirm the formation of primarily 1 : 1 Ag : L complexes in solution.
Introduction
The complexation of Ag(I) in a range of supramolecular mate-
rials has received increasing attention, due to the coordinative
flexibility of the d10 ion as well as to its well documented tendency
to form strong complexes with nitrogen donor ligands.1 Tripodal
Scheme 1 Ligand structures for 1–4 with the labelling scheme used in the
amine ligands based on tris(2-aminoethyl)amine (tren) are well
known to interact with a range of transition and post transition
metal ions2 and typically complex metal ions within a well
defined ‘pocket’. For example, extending the structure of tren via
Schiff-base condensation at each of the terminal amine groups by
reaction with a range of aromatic aldehyde derivatives poten-
tially offers a wide array of derivatives that can be matched to
specific applications.3,4 A structural study of the Schiff-base
derivative 1 (Scheme 1), derived from tren and benzaldehyde
confirmed the presence of a pocket in the solid state,5 and
complexation of Cu(I) with this ligand6–9 confirms coordination
NMR investigations.
of the metal ion by the four N-donor atoms with the phenyl
substituents encapsulating the metal ion. This arrangement
prevents further coordination to the metal centre on the exposed
side10 resulting in a tendency to form long-range intramolecular
metal–hydrogen attractions.6
In contrast, the 2-pyridylmethyl substituted ligand 3 incor-
porates seven potential donors, and as described by Kirchner
et al.11 the presence of additional donors results in a variation of
the coordination number from six for Fe(II), Ni(II), Cu(II) and
Zn(II) (bridgehead tertiary nitrogen not coordinated) to pseudo-
seven coordination for Co(II) and Mn(II) by ‘‘weak bonding’’ of
the bridgehead amine. A subsequent study indicated the presence
of similar pseudo-seven coordination in the corresponding Fe(II)
complex.12 Cr(III)13 and Tc(II)14 complexes of 3 have also been
reported, with the smaller Cr(III) ion yielding a six-coordinate
species,13 while the larger Tc(II) ion shows a tendency for coor-
dination of the apical tertiary amine.14 More recent studies have
expanded the range of complexes of 3 to include those of Cd(II)15
and Ca(II).16 The larger Ca(II) ion is coordinated to all seven N
donor atoms,16 while Cd(II) results in a six-coordinate species.15
To the best of our knowledge only one example of a Ag(I)
tripodal Schiff-base complex of the above general type has been
aInstitute of Fundamental Sciences, Massey University, Private Bag 11
222, Palmerston North, New Zealand
bDepartment of Chemistry and Food Chemistry, TU Dresden, 01062
Dresden, Germany. E-mail: Karsten.Gloe@chemie.tu-dresden.de
cDeutsches Textilforschungszentrum Nord-West e.V., Institut an der
Universita€t Duisburg - Essen, Adlerstraße 1, 47798 Krefeld, Germany
dDepartment of Applied Chemistry, Saga University, Saga, 840-8502,
Japan
eSchool of Chemistry, The University of Nottingham, University Park,
Nottingham, UK NG7 2RD
fCentre for Heavy Metals Research, School of Chemistry, University of
Sydney, NSW, 2006, Australia
† Electronic supplementary information (ESI) available: Additional
crystallographic parameters and drawings. See DOI: 10.1039/c0ce00255k
4176 | CrystEngComm, 2010, 12, 4176–4183
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reported.17 The Ag(I) ion in this 4-phenylnitrile substituted
tripodal Schiff-base complex has a distorted coordination sphere
in which Ag(I) is 5-coordinated to four nitrogens from the
tripodal ligand and an acetonitrile molecule. The observed Ag–N
bond lengths differ significantly, with distances ranging between
(CDCl3) d (ppm) 160.19 (CHN), 150.83, 143.20, 122.12,
(C5H5N), 60.57 (CH2N), 55.59 (CH2N).
Synthesis of the Ag(I) complexes
17
ꢀ
2.21 and 2.76 A. We report herein a comparative synthetic,
solid-state and solution study of the interaction of Ag(I) with the
tripodal Schiff-base ligands 1–4 (Scheme 1).
All reaction flasks were covered with aluminium foil to protect
the Ag(I) species from light. For ligands 1, 3 and 4 an equimolar
amount of AgClO4 in anhydrous methanol (5 ml) was added to
the required ligand (0.5 mmol) in anhydrous methanol (10 ml)
and the solution was stirred for 30 min at room temperature.
Diethyl ether was added and the resulting precipitate collected by
filtration was washed with methanol. The synthesis of the Ag(I)
complex of the 4-biphenyl substituted ligand 2 was carried out in
a similar manner except that a chloroform/methanol mixture was
employed.
Experimental
Physical methods
All reagents and solvents were purchased from commercial
sources and were used without further purification. NMR
spectra were recorded on Bruker DPX300, DPX 500 or Jeol 300
MHz spectrometers; values are referenced to solvent or Me4Si at
0 ppm in the case of the titrations. Electron Impact (EI) mass
spectra were run on a Micromass Autospec mass spectrometer.
[Ag(1)]ClO4. White solid, 87% yield, MS (EI) m/z 517;
1
C27H30AgN4 requires 517, H NMR (CDCl3) d (ppm) 8.54 (s,
CH, 1 H), 7.86 (d, ortho phenyl, 2 H), 7.46 (m, para phenyl, 2 H),
7.32 (m, meta phenyl, H), 3.77 (t, CH2N, 2 H), 3.02 (t, NCH2, 2
H). Anal. Calc. for C27H30AgClN4O4: C 52.49, H 4.89, N 9.07.
Found: C 52.40, H 4.93, N 9.19%. Single crystals suitable for
X-ray determination were grown by slow diffusion of diethyl
ether into a dimethylformamide solution of the complex over one
week in the dark.
Ligand synthesis
The ligands were synthesized by slight modifications of literature
methods.7,17,18 Reaction of tris(2-aminoethyl)amine (tren) (0.94
g; 1 ml) with three equivalents of the corresponding aldehyde at
60 ꢀC for 4 h was carried out under anhydrous conditions in dry
methanol or ethanol (30 ml). The solvent was removed under
reduce pressure to give the crude ligands. Ligand 1 was recrys-
tallised from n-hexane/diethyl ether while 2 was recrystallised
from chloroform/diethyl ether. Ligands 3 and 4 were obtained
after any unreacted starting material was removed by vacuum
distillation.
[Ag(2)]ClO4. White solid, 59% yield, MS (EI) m/z 745,
C45H42AgN4 requires 745, 1H NMR (DMSO-d6) d (ppm) 8.68 (s,
CH, 1 H), 8.00 (m, ArH, 2 H), 7.65 (m, ArH, 2 H), 7.58 (m, ArH, 2
H), 7.37 (m, ArH, 3 H), 3.86 (t, CH2, 2 H), 3.03 (t, CH2, 2 H).
Anal. Calc. for C45H42AgClN4O4: C 63.80, H 5.00, N 6.62.
Found: C 63.50, H 4.92, N 6.50%. Single crystals suitable for
X-ray determination were grown by slow diffusion of diethyl ether
into a dimethylformamide solution over one week in the dark.
N,N0,N00-Tris(2-benzyliminoethyl)amine (1). Yellowish solid;
yield: 80%; C27H30H4 (410.6), MS (EI) m/z 411 [M + 1]+; H
1
NMR (CDCl3) d (ppm) 8.28 (s, 1H, CH), 7.49 (d, 2H, ortho
C6H5), 7.31–7.39 (m, 3H, meta and para C6H5), 3.68 (t, 2H,
CH2), 2.93 (t, 2H, CH2); 13C NMR (CDCl3) d (ppm) 161.23,
(CHN), 135.54, 129.74, 127.36 (C6H5), 59.43 (CH2N), 55.00
(CH2N).
[Ag3(3)2](ClO4)3. Yellow solid, 33% yield, MS (EI) m/z 520,
1
C24H27AgN7 requires 520, H NMR (CD3CN) d (ppm) 8.49 (s,
CH, 1 H), 8.44 (s, py, 1 H), 7.84 (t, py, 1 H), 7.72 (d, py, 1 H), 7.46
(t, py, 1 H), 3.79 (t, CH2, 2 H), 2.95 (t, CH2, 2 H). Anal. Calc. for
C48H54Ag3Cl3N14O12: C 39.79, H 3.76, N 13.53. Found: C 40.13,
H 3.78, N 13.60%. Single crystals suitable for X-ray determina-
tion were grown by slow diffusion of diethyl ether into a dime-
thylformamide solution of the complex over 3 weeks in the dark.
Tris[2-(40-phenylbenzyl)iminoethyl]amine (2). White solid, 98%,
C45H42N4 (638.9) MS (EI) m/z 639 [M + 1]+; 1H NMR (CDCl3)
d (ppm) 8.14 (s, 1H, CH), 7.36–7.57 (m, 9H, ArH), 3.73 (t, 2H,
CH2), 2.94 (t, 2H, CH2); 13C NMR (CDCl3) d (ppm) 161.53
(CHN), 143.06, 140.34, 135.19, 128.84, 128.49, 127.68, 127.19,
127.07 (C12H9), 60.16 (CH2N), 55.62 (CH2N).
{[Ag3(4)2](ClO4)3}n. White solid, 80% yield, MS (EI) m/z 520,
1
C24H27AgN7 requires 520, H NMR (CD3CN) d (ppm) 8.53 (d,
py and CH, 3 H), 7.56 (d, py, 2 H), 3.81 (t, CH2, 2 H), 2.99 (t,
CH2, 2 H). Anal. Calc. for C48H54Ag3Cl3N14O12$2H2O: C 38.82,
H 3.94, N 13.20, found: C 38.72, H 3.75, N 12.91%. Single
crystals suitable for X-ray determination were grown by slow
diffusion of diethyl ether into a solution of the complex in MeCN
over 3 weeks in the dark.
Tris[2-(20-pyridylmethyl)iminoethyl]amine (3). Brown/yellow
oil, 87%, C24H27N7 (413.5) MS (EI) m/z 436 [M + Na]+; 1H NMR
(CDCl3) d (ppm) 8.60 (m, 1H, C5H4N), 8.34 (s, 1H, CH), 7.90 (m,
1H, C5H4N), 7.70 (m, 1H, C5H4N), 7.28 (m, 1H, C5H4N), 3.77 (t,
2H, CH2), 2.98 (t, 2H, CH2); 13C NMR (CDCl3) d (ppm) 162.72,
(CHN), 154.46, 149.39, 136.50, 124.59, 122.25 (C5H4N), 59.87
(CH2N), 55.27 (CH2N).
1H NMR titration studies
Tris[2-(4-pyridylmethyl)iminoethyl]amine (4). Brown/yellow
solid, 86%, C24H27N7 (413.5) MS (EI) m/z 414 [M + 1]+; 1H NMR
(DMSO) d (ppm) 8.60 (q, 2H, C5H4N), 8.23 (s, 1H, CH), 7.49 (q,
2H, C5H4N), 3.66 (t, 2H, CH2), 2.85 (t, 2H, CH2); 13C NMR
1H NMR titration experiments were carried out to investigate the
formation of the Ag(I) complexes of 1–4. The concentration of
the ligand was 2 mM and defined aliquots of a 0.1 M AgClO4
1
stock solution were added in each titration step. The H NMR
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titrations of all ligands were carried out in a d6-DMSO/d3-MeCN
2 : 1 mixture.
electrolyte. A solution of a ligand (1 ꢂ 10ꢃ2 to 1 ꢂ 10ꢃ3 M) was
added to a solution of AgNO3 (1 ꢂ 10ꢃ3 to 1 ꢂ 10ꢃ4 M). Under
these experimental conditions the activity coefficients are
constant. From the measured potential E during the titrations
the activity of the uncomplexed Ag(I) ion was calculated using
the Nernst equation:
Liquid–liquid extraction experiments
The liquid–liquid extraction experiments were performed at 23 ꢁ
1
ꢀC in microcentrifuge tubes (2 cm3) undergoing mechanical
EꢃE
1Þ
D
Â
Ã
ð
shaking. The phase ratio V(w) : V(org) (500 ml each) was 1 : 1, and
an initial concentration for AgClO4 of 1 ꢂ 10ꢃ4 M was used.
Perchloric acid was added as counter ion at a concentration of
5 ꢂ 10ꢃ3 M. The ligand concentration in the organic phase was
1 ꢂ 10ꢃ3 M (except where variable concentration experiments
were employed), and a mixing time of 30 min was used. All
experiments were performed at a pH of 5.5 employing citric acid/
NaOH buffer. The equilibrium pH was measured using an InLab
micro pH electrode. After extraction, all samples were centri-
fuged and the phases separated. The distribution of metal ion
concentrations was measured in both phases radiometrically by
g-radiation of 110mAg in a NaI (TI) scintillation counter (Cobra
II/Canberra-Packard). In the solvent-extraction studies the
distribution ratio, DM, the quotient of the metal concentrations
in the organic and aqueous phase, or the percentage extraction
efficiency (E%) were obtained using: DM ¼ cM(org)/cM(w) and
E[%] ¼ 100 ꢂ DM/(DM + 1), respectively.
AgðIÞ ¼ cAg ꢂ 10
cAg is the total concentration of the silver ions and E1 is the
potential between the silver electrode and the reference electrode
ꢀ
before the titration. The constant D at 25 C is defined by D ¼
ꢃ2.303RT/nF ¼ ꢃ59.16/n [mV] with the gas constant R, the
temperature T, the Faraday constant F and the charge of the
cation n. As reference, an identical solution of AgNO3 together
with a second silver electrode was used.
Crystallography†
The data for the crystal structures of 1, [Ag(1)]ClO4 and
[Ag(2)]ClO4 were collected on a STOE-IPDSII-diffractometer at
150 K (Mo-Ka) with STOE X-AREA.19 SHELXS-9720 was used
for the structure solution and SHELXL-9721 for the refinement.
Absorption corrections were carried out empirically using
X-RED,22 X-SHAPE21 and MULABS in PLATON.23 Data for
[Ag3(3)2](ClO4)3 and {[Ag3(4)2](ClO4)3}n were collected on
a Bruker SMART1000 CCD area detector diffractometer at 150
K (Mo-Ka). Bruker SMART version 5.62424 was used for the
data collection with SIR9225 and SHELXS-9720 for the structure
solution, and SHELXL-9721 for refinement. Absorption correc-
tions were carried out empirically using SADABS.20
Potentiometric titrations
The potentiometric titrations experiments were carried out in
duplicate in a 716 DMS Titrino (Methrom) titrator. Anhydrous
silver nitrate (Merck) and tetraethylammonium perchlorate
(Fluka) of the highest purity were used for all experiments. As
solvent, anhydrous methanol (Merck; H2O content less than
0.01%) was used. All stability constants were measured by means
of an ion selective electrode for silver(I) (Methrom EA 282).
During the titration the ionic strength is kept constant at I ¼ 0.05
Crystal and structure refinement data for all structures are
summarised in Table 1. Where applicable, further details relating
to the X-ray structure refinements are given in the ESI† along
with tables of bond lengths and angles and selected H-bond and
p-interaction geometries.
M
using tetraethylammonium perchlorate as supporting
Table 1 Crystal data for ligand 1 and the Ag(I) complexes of 1–4
Compound
1
[Ag(1)]ClO4
[Ag(2)]ClO4
[Ag3(3)2](ClO4)3
{[Ag3(4)2](ClO4)3}n
Formula of refinement model
Crystal system
Space group
C27H30N4
Monoclinic
C2/c
29.238(3)
9.5541(8)
17.1115(15)
90.00
99.104(7)
90.00
4719.8(8)
1.156
C27H30AgClN4O4
Triclinic
C45H42AgClN4O4
Orthorhombic
Pccn
18.0927(4)
29.2012(6)
14.3480(4)
90.00
90.00
90.00
7580.5(3)
1.483
C48H54Ag3Cl3N14O12.5
Monoclinic
I2
21.150(2)
12.4365(12)
22.095(2)
90.00
105.399(2)
90.00
5603.1(15)
1.727
C48H55Ag3Cl3N14O12.5
Triclinic
ꢁ
P1
ꢁ
P1
ꢀ
a/A
10.6570(7)
11.2647(7)
12.1892(7)
108.952(5)
106.558(5)
90.962(5)
1316.89(14)
1.558
14.4780(9)
14.9423(9)
15.0134(10)
110.035(1)
98.355(1)
92.026(1)
3006.3(3)
1.611
ꢀ
b/A
ꢀ
c/A
a/ꢀ
b/ꢀ
g/ꢀ
V/A
Dc/g cmꢃ3
3
ꢀ
Z
8
2
8
4
2
Crystal size/mm
Crystal colour
Crystal habit
Temperature/K
Reflections measured
0.34 ꢂ 0.28 ꢂ 0.26
Colourless
Block
150(2)
21019
0.45 ꢂ 0.40 ꢂ 0.25
Colourless
Block
150(2)
25 341
0.45 ꢂ 0.25 ꢂ 0.10
Colourless
Block
150(2)
90 447
0.35 ꢂ 0.20 ꢂ 0.08
0.57 ꢂ 0.24 ꢂ 0.21
Colourless
Block
150(2)
25 737
Yellow
Tablet
150(2)
17 497
Unique reflections (Rint
R1(F), wR2(F2)
GoF
)
4628 (0.067)
0.0492, 0.1108
1.02
ꢃ0.17, 0.17
5582 (0.035)
0.0187, 0.0487
1.01
ꢃ0.35, 0.52
8285 (0.064)
0.0460, 0.1078
1.14
ꢃ0.64, 2.49
10 062 (0.047)
0.0405, 0.1100
1.03
ꢃ0.68, 1.18
13 305 (0.016)
0.0451, 0.1345
1.03
ꢃ0.94, 1.69
ꢃ3
Residual density/eꢃ
A
ꢀ
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Results and discussion
X-Ray structure of ligand 1
Crystals of the benzyl substituted ligand 1 suitable for X-ray
structure analysis were grown from an n-hexane/diethyl ether
mixture. Although the room temperature structure was reported
in 2002,5 it was re-determined as part of the present study at 150 K
(Fig. 1) since it was of interest to re-examine the structure for the
presence of potential intra- and inter-molecular interactions; this
had not been carried out in the previous study. No significant
intermolecular interactions were apparent within the structure.
However, intramolecular CH–p (aromatic) interactions between
the C–H of a phenyl group contacting the centre of a neighbouring
aromatic ring (Fig. 1) are observed. The CH–p (centroid)
Fig. 2 ORTEP plot of [Ag(1)]ClO4 with selected labeled atoms, 50%
probability ellipsoids, non-interacting H-atoms omitted for clarity; CH/
Ag interactions are shown as dashed lines.
ꢀ
distances lie between 2.65 and 2.75 A with the C–H–centroid
angles ranging between 136 and 138ꢀ (Table 2).26 The presence of
three CH–p (aromatic) interactions within 1 results in a cage-like
arrangement, where the three podand arms are involved in
CH–centroid interactions.
Table 3 CH/Ag1 interactions in [Ag(1)]ClO4
ꢀ
C–H/A
ꢀ
C–H
H/Ag/A
C9–H9A
C18–H18A
C27–H27A
0.95
0.95
0.95
2.54
2.55
2.51
X-Ray structures of the Ag(I) complexes
Crystals of the Ag(I) complexes of 1–4 suitable for X-ray struc-
ture analysis were obtained by slow diffusion of diethyl ether into
a solution of the respective complexes in dimethylformamide or
acetonitrile.
ꢀ
A). The rigidity of Schiff-base linkages and the positioning of the
aromatic end groups appear to inhibit further coordination of
solvent or anion groups to the Ag(I) centre. Ag–Caryl separations
The structure of [Ag(1)]ClO4 (Fig. 2) confirms the metal centre
to be four-coordinate with a distorted trigonal pyramidal
geometry. The metal is bound to the tertiary bridgehead nitrogen
donor N1 as well as to the three imine nitrogen donor atoms. The
ꢀ
of 3.2760(17) to 3.3435(18) A are observed between the Ag(I)
centre and the centre of the aromatic rings, but these are outside
27
ꢀ
the normal range (2.47–2.76 A) of significant Ag/p interac-
tions. Interestingly however, three hydrogen atoms, one from
each phenyl substituent, point towards the Ag(I) centre, corre-
ꢀ
Ag(I) is located 0.53 A out of trigonal plane defined by the three
imine nitrogen atoms, away from the bridgehead N atom.
ꢀ
sponding to CH/Ag distances of 2.51–2.55 A (Table 3); these
are significantly shorter than the sum of the corresponding van
The Ag–N bond lengths to the imine nitrogen atoms (Ag1/
N2 2.2948(13) A, Ag1/N3 2.3138(12) A, Ag1/N4 2.2946(13)
ꢀ
A) are significantly shorter than the Ag1–N1 distance (2.4554(12)
ꢀ
ꢀ
28
ꢀ
ꢀ
der Waals radii (H 1.20 A; Ag 1.72 A).
It should be noted, that a similar placement of Cu(I) centres in
two Cu(I) complexes of this ligand showing slightly longer metal–
hydrogen distances have been described by Alyea et al.6 and
Brown et al.9 Such C–H/metal contacts can perhaps be best
described as weak hydrogen bonds.29 Electron-rich transition
metal ions have previously been proposed to act as such hydrogen
bond acceptors.30,31 The CH/Ag distances of 2.51–2.55 A, the C–
ꢀ
H–Ag angles of 135–141ꢀ30 and the significant low-field shift (0.35
1
ppm) of the corresponding protons in the H NMR spectrum
studies (see below) are consistent with the presence Ag/H
hydrogen-bonding interactions.32
In [Ag(1)]ClO4 the cation and the counter anion are arranged in
ꢃ
layers (Fig. S1, ESI†). A weak C25–H/O1 contact with the ClO4
ꢀ
counter ion is observed with a separatꢀion of 3.192(7) A (H/O
ꢀ
2.55 A) and a C–H/O angle of 125 (2 ꢃ x, 1 ꢃ y, 1 ꢃ z
Fig. 1 X-Ray structure of 1, selected atom labels and CH/p (centroid)
interactions (dashed lines).
symmetry). The cationic [Ag(1)]+ units are connected in a 2D plane
by two weak intermolecular CH–p-aryl interactions with
ꢀ
distances of 3.80 A, from C11 to centroid Cg2 of the ring C22–C27
Table 2 CH/p (aromatic) interactions in 1 (Cg: ring centroid)
and from C23 to centroid Cg1 of the ring C4–C9 (Table S2, ESI†).
In order to increase the possibility of forming a pseudo-cage
complex via intramolecular p–p or CH–p interactions involving
the aromatic end groups, the Ag(I) complex of the analogous 4-
biphenyl substituted ligand 2 was synthesised. The X-ray struc-
ture of [Ag(2)]ClO4 is shown in Fig. 3.
ꢀ
ꢀ
Cg/A
C–H
Cg
CH/Cg/A
C–H/Cg/ꢀ
C11–H11
C21–H21
C27–H27
Cg2
Cg3
Cg1
2.75
2.73
2.65
3.494(2)
3.487(2)
3.4226(18)
136
138
138
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disordered over two positions with 50% occupancy in each and
ꢃ
occupies voids between columns of cations. One ClO4 counter
ion is disordered over two orientations of three of the O atoms.
The 2-pyridylmethyl substituents in 3 provide additional donor
atoms resulting in the prospect of higher coordination numbers
being generated if a 1 : 1 complex is formed. However, reaction of
Ag(I) with 3 results in a 3 : 2 (M : L) complex [Ag3(3)2](ClO4)3 in
the solid state (Fig. 4) in which the Ag(I) ion adopts trigonal
pyramidal coordination via coordination to two imine and two
pyridine nitrogen donor atoms. Two metal centres (Ag1 and Ag3)
are bound to the imine nitrogen atoms and the pyridine nitrogen
atoms of two podand arms from different ligand molecules. The
third Ag(I) centre (Ag2) acts as a bridge between the above two
complex units, being bound to the nitrogen donors of the
remaining podand arms from each of the above ligand molecules.
There is a slight difference in the coordination geometry of this Ag
ꢀ
site; Ag2 is located 0.409 A out of the plane formed by N7, N9 and
N10 towards the fourth donor atom N6, whereas Ag1 and Ag3 are
ꢀ
located 0.145 and 0.122 A out of the planes formed by N2, N3 and
Fig. 3 ORTEP plot of [Ag(2)]ClO4 with 50% probability ellipsoids;
ꢃ
ClO4 counter ion disordered over two positions with 50% occupancy,
N5 and N12, N13 and N14, respectively. There are no significant
differences in the Ag–N bond lengths for (bridging) Ag2 relative to
Ag1 and Ag3. In general, shorter bond lengths are observed for
non-interacting H-atoms omitted for clarity; CH/Ag interactions are
shown as dashed lines.
ꢀ
the Ag–Nimine interactions (Ag–Nimine: 2.270(6) to 2.355(6) A)
ꢀ
than for the Ag–Npy interactions (Ag–Npy: 2.345(6) to 2.481(6) A),
The coordination geometry of the Ag(I) centre in [Ag(2)]ClO4
is similar to that in [Ag(1)]ClO4 and is pseudo-trigonal pyramidal
involving coordination of the tertiary bridgehead nitrogen atom
N1 and the three imine nitrogen atoms N4, N20 and N36. The
(Table S5 ESI†).
There is no evidence for intramolecular or intermolecular
interactions between Ag(I) centres, with the internuclear sepa-
ꢀ
Ag1–N1 bond (2.444(9) A) is again significantly longer than the
ꢀ
rations Ag1 to Ag2 (3.47 A) and Ag2 to Ag3 (3.54 A) being
ꢀ
ꢀ
ꢀ
28
ꢀ
Ag–Nimine bonds (Ag1/N2 2.283(2) A, Ag1/N3 2.309(4) A,
greater than the van der Waals diameter for Ag (3.44 A).
The intra-centroid distances between pyridyl substituents lie
ꢀ
ꢀ
Ag1/N4 2.314(4) A), with the Ag(I) ion located 0.53 A out of
the trigonal plane defined by the three imine N atoms, away from
the bridgehead N atom. As in [Ag(1)]ClO4 CH/Ag interactions
ꢀ
between 3.577(4) and 3.851(5) A with b-angles (angle between
the ring normal of the aromatic ring plane and the centroid
vector)33 from 20 to 34ꢀ indicating the presence of long-range
ꢀ
ꢀ
ꢀ
(C11H/Ag1 2.51 A, C23H/Ag1 2.60 A, C43H/Ag1 2.44 A)
are observed in [Ag(2)]ClO4. However, there is an increase in
twist of the aromatic substituents away from the N1–Ag1 axis
compared to [Ag(1)]ClO4.
ꢃ
p–p stacking between the pyridine units. The ClO4 counter
ions and the solvate water molecules are located in layers beside
the columns of face-to-face trinuclear Ag(I) complexes (see
ꢃ
No intramolecular p–p or CH–p interactions between the
biphenyl end groups are present in this complex. However, there
is evidence for weak intermolecular CH–p interactions between
ligand molecules (Fig. S2, ESI†) and CH/O interactions
involving the ClO4ꢃ anion to form a 3D network throughout the
Fig. S4 ESI†). Two of the three ClO4 ions are disordered.
ꢃ
Numerous weak CH/O interactions involving the ClO4
counter ions are also present; these connect the trinuclear cation
complexes along the crystallographic a- and b-axes (but not
along the c-axis). The CH/O distances lie between 2.38 and
ꢃ
ꢀ
lattice (Fig. S3 and Table S4 ESI†). The ClO4 counter ion is
2.51 A (Table S6 ESI†).
Fig. 4 The X-ray structure of [Ag3(3)2]ClO4)3$0.5H2O, H atoms, water
and counter ions are omitted for clarity, selected intramolecular p–p
interactions are shown (dashed lines).
Fig. 5 The X-ray crystal structure of {[Ag3(4)2]ClO4)3}n, dashed lines
ꢃ
represent p–p interactions, H atoms and ClO4 anions are omitted for
clarity.
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In 4 the pyridine nitrogen atoms are present in the 4-position
of the aromatic substituents. Simultaneous coordination of the
same Ag(I) ion to the pyridine donors as well as to the imine and
amine nitrogen atoms positioned around the ligand bridgehead is
thus sterically not possible. Reaction of this ligand with AgClO4
affords the polymeric complex {[Ag3(4)2](ClO4)3}n, the structure
of which is shown in Fig. 5.
The observed stabilities for the Ag(I) complexes of these
tripodal ligands follow the order 2 < 4 < 1 < 3. This order
appears to reflect both the degree of steric hindrance of the N-
donor sites within the tripodal pocket and the availability of the
additional N-donor atoms for coordination to the Ag(I) centre.
For example, the benzyl substituted ligand 1 gave a value of
logK[AgL] ¼ 9.10 while a significantly lower value of logK[AgL]
¼
The Ag1 and Ag2 centres are not equivalent in
{[Ag3(4)2](ClO4)3}n; Ag1 has disordered tetrahedral coordina-
tion involving the bridgehead nitrogen atom N1 (Ag–N1 2.496(3)
7.26 was obtained for the 40-phenylbenzyl substituted ligand 2,
possibly reflecting the presence of enhanced steric hindrance to
coordination by the bulky biphenyl substituents in the latter case.
For 3 the availability of additional pyridine N-donor atoms in
the 2-position of the aromatic moiety results in a marked increase
in the complex stability of approximately four orders of magni-
tude to logK[AgL] ¼ 12.95. In 4 the pyridyl donor atoms are not
correctly positioned for simultaneous binding to the same Ag(I)
centre as the remaining ligand donors; consistent with this, the
stability of the corresponding 1 : 1 Ag(I) complex was found to
be significantly lower (Table 4).
ꢀ
ꢀ
A), the two imine nitrogen atoms N4 (Ag–N4 2.319(4) A) and N6
ꢀ
(Ag–N6 2.477(4) A) and the pyridyl nitrogen donor N12 (Ag–
ꢀ
N12 2.242(3) A). Ag2 is coordinated by the bridgehead nitrogen
ꢀ
N7 (Ag–N7 2.540(5) A), the imine nitrogen N8 (Ag–N8 2.281(4)
ꢀ
ꢀ
A) and the pyridyl nitrogen atom N14 (Ag–N14 2.238(4) A).
Further interactions with the imine nitrogen atoms N10 and N11
ꢀ
(Ag–N 2.675(4) and 2.607(4) A, respectively) give Ag2 an overall
five-coordinate distorted square-pyramidal geometry (s
¼
0.43).34 This difference in the coordination geometry between
Ag1 and Ag2 is possibly in part a reflection of weak p–p stacking
of the ‘bridging’ pyridine substituents. The centroid–centroid
distance between the ring Cg1 (N12–C46) and the ring Cg2
1H NMR titration
In order to investigate the complex stoichiometries in d6-DMSO/
1
d3-MeCN (2 : 1 v/v) solution, H NMR titrations of 1–4 with
ꢀ
33
ꢀ
(N14–C24) is 3.76 A (angle b is 22 ).
Ag3 is coordinated to the 4-pyridine nitrogen donor atoms of
four different ligand molecules to give a distorted tetrahedral
AgClO4 were carried out. Fast NMR exchange was observed for
all systems. As an example, representative spectra for 1 in the
presence of increasing concentrations of AgClO4 are shown in
Fig. 6. The shifts of the proton signals on addition of Ag(I) are
in agreement with interactions occurring between this metal ion
and all of the N-donor atoms.
geometry with bond lengths lying between 2.288(5) and 2.366(4)
i
A (Ag3–N3 2.288(5) A, Ag3–N5 2.291(4) A, Ag3–N9 2.366(4) A,
ꢀ
ꢀ
ꢀ
ꢀ
ii
ꢀ
Ag3–N13 2.323(4) A; symmetry codes: (i) ꢃx, 1 ꢃ y, 2 ꢃ z; (ii)
ꢃx, ꢃy, 1 ꢃ z). The Npy–Ag3–Npy angles lie between 98 and 121ꢀ
(Table S7 ESI†).
For each of 1–4, the respective proton shifts were plotted
against the molar ratio of Ag(I) (Fig. 7). In the case of 1 and 2, the
expected 1 : 1 complex formation was observed. Shifts in the
signals for protons 1, 2 and 3 are in accord with the involvement
of all four nitrogen donor atoms in complexation.
The three ClO4ꢃ counter ions associated with each [Ag3(4)2]3+
unit are disordered over four positions with occupancy of 1, 1,
0.5 and 0.5, respectively. Two of the anions, namely Cl2 and Cl4,
are further disordered with 50% occupancy in each position. All
anions are situated in the voids of the 1-D polymer formed by the
complex cation and are involved in numerous short CH/O
In the case of 3 significant shifts of the proton signals were
observed up to a metal to ligand ratio of 3 : 2, in agreement with
the stoichiometry observed in the X-ray structure of this
ꢀ
interactions. The CH/O distances lie between 2.23 and 2.52 A
ꢀ
(C/O distances between 3.108(8) and 3.470(5) A). In addition,
an intermolecular edge-to-face p–p interaction connects the
polymeric chains. The separation between C9–H9A and the
ꢀ
centroid of the ring N13–C48 (Cg3) is 3.836(5) A (CH/Cg: 2.84
ꢀ
ꢀ
A) and the X–H/p (centroid) angle is 173 (x, y, 1 + z
symmetry). A schematic drawing of the weak interactions and
a summary of the bond distances are given in Fig. S5 and Table
S8 in the ESI.†
Stability constants for the Ag(I) complexes
Thermodynamic stability constants for the Ag(I) complexes of 1–
4 in methanol were determined by the Ag(I) potentiometric
titration technique (Table 4).
Table 4 Stability constants for the 1 : 1 (M : L) complexes of Ag(I) with
1–4 in MeOH. [Ag]+ ¼ 1 ꢂ 10ꢃ3 M, [L] ¼ 1 ꢂ 10ꢃ3 M, [Et4NClO4] ¼ 5 ꢂ
10ꢃ2 M; T ¼ 25 ꢀ
C
Ligand
1
2
3
4
Fig. 6 1H NMR spectra of free ligand 1 and in the presence of molar
logK[AgL]
9.10 ꢁ 0.02
7.26 ꢁ 0.03
12.95 ꢁ 0.01
7.96 ꢁ 0.04
increments of added AgClO4 in d6-DMSO/d3-MeCN (2 : 1 v/v).
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Fig. 8 Variable ligand concentration experiments for 1–4 with Ag(I) and
perchlorate as the anion. [AgClO4] ¼ 1 ꢂ 10ꢃ4 M, [HClO4] ¼ 5 ꢂ 10ꢃ3 M;
pH 5.5 (citric acid/NaOH buffer); [ligand] ¼ 5 ꢂ 10ꢃ4 M – 1.5 ꢂ 10ꢃ3 M in
CHCl3; shaking time 30 min; T ¼ 23 ꢁ 1 ꢀC. Slopes: 1 s ¼ 1.14; 2 s ¼ 1.14;
3 s ¼ 0.9; 4 s ¼ 1.14.
Fig. 7 Observed shifts in the proton signals of (a) 1, (b) 2, (c) 3 and (d) 4
in d6-DMSO/d3-MeCN (2 : 1 v/v) in the presence of increasing amounts
of Ag(I) (for proton labels see Scheme 1).
extraction was only 9%. As anticipated, the Ag(I) extraction
obtained for 3 (66%) is also higher than obtained for 1 (50%). As
mentioned earlier, in 4 the pyridine nitrogen atoms can not be
aligned in a similar manner to those in 3 and hence are only
available for intermolecular coordination. Further, the likely
presence of ’free’ polar pyridine groups in the case of 4 would be
expected to induce enhanced water solubility, especially under
acidic conditions, and result in increased bleeding of this ligand
into the water phase and, if present, such behaviour may
contribute to the dramatic decrease in Ag(I) extraction observed
for this system.
complex. The shift of proton 1 is very small, (Ddꢀ ¼ 0.06 ppm),
suggesting no significant interaction of the tertiary bridgehead
nitrogen with Ag(I) (as was also observed in the corresponding
X-ray structure). Shifts of 0.2 to 0.4 ppm in all other proton
signals are in accord with the expected interactions between the
imine and pyridine nitrogens and Ag(I) being present.
The 1H NMR titration of 4 with Ag(I) is in agreement with the
formation of a 1 : 1 complex. All proton signals shift up to a 1 : 1
concentration ratio, with the shifts for protons 2 and 3 (Fig. 6)
being the largest at Ddꢀ ¼ 0.25 and 0.45 ppm respectively, as
expected for the presence of strong interactions between the
imine nitrogens and the Ag(I) ion. In this case the shift of
the signal for proton 1 (Ddꢀ ¼ 0.17 ppm) is in accord with the
involvement of the bridgehead nitrogen in coordination to Ag(I).
In order to probe the stoichiometries of the extracted species,
extraction experiments were performed over a range of ligand
concentration while the metal ion concentration was maintained
at 1 ꢂ 10ꢃ4 M and the anion concentration was effectively
constant. The value of logDAg (DAg ¼ cAg(org)/cAg(w)) was plotted
against log(clig) (clig ¼ ligand concentration) (Fig. 8). Provided
a relatively simple equilibrium is involved, the slope of this plot
gives the stoichiometry of the extracted species directly.4,35
The results show linear relationships between the logarithm of
the distribution ratio (DAg) and the log(clig) at pH 5.5 (Fig. 8)
with slopes between 0.9 and 1.1 obtained for all four ligands
consistent with the formation of 1 : 1 (Ag : L) species in chlo-
roform under the conditions employed. Thus, the formation of
3 : 2 M : L complexes [Ag3(3)2](ClO4)3 and {[Ag3(4)2](ClO4)3}n
reflects the isolated solid-state structures rather than the 1 : 1
complexes that, as for 1 and 2, exist in solution.
Liquid–liquid extraction
The extraction behaviour of 1–4 toward Ag(I) was studied using
the radiotracer technique.35 An overview of the extraction results
under comparable experimental conditions is given in Table 5.
Under the conditions employed, the phenyl substituted deriva-
tive 1 showed an extraction efficiency of 50%, while for 40-phe-
nylbenzyl substituted ligand 2 the value was 76%. The presence
of larger aromatic end-groups in 2 will result in an enhancement
of the lipophilic properties of this ligand which, as observed, is
expected to be reflected by higher extraction efficiency than for 1
(even through the thermodynamic stability of the 1 : 1 Ag(I)
complex of 2 is in fact lower – see above).
Conclusions
The binding of Ag(I) to a series of tripodal Schiff-base ligands
based on tren scaffolds has been investigated in both the solid-
state and solution. These ligands show different Ag(I) coordi-
nation behaviour that in part reflects the number of available
donor atoms as well as steric effects arising from the appended
terminal aromatic groups. The incorporation of additional
nitrogen donors in ligands 3 and 4 leads to the formation of both
discrete trinuclear and polymeric solid-state complexes, respec-
tively. In solution, structure/function relationships underlying
the interaction of these species with Ag(I) have been probed
Use of the 2-pyridyl substituted derivative 3 resulted in 66%
extraction of Ag(I), whereas for the 4-pyridine derivative 4 the
Table 5 Percent Ag(I) extraction by 1–4. [AgClO4] ¼ 1 ꢂ 10ꢃ4 M,
[HClO4] ¼ 5 ꢂ 10ꢃ3 M; pH 5.5 (citric acid/NaOH buffer); [ligand] ¼ 1 ꢂ
10ꢃ3 M in CHCl3; shaking time 30 min; T ¼ 23 ꢁ 1 ꢀ
C
Ligand
1
2
3
4
% Ag(I) extraction
50
76
66
9
1
employing potentiometric stability constant determinations, H
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7 D. A. Tocher, M. G. B. Drew, S. Chowdhury and D. Datta, Indian J.
Chem., 2003, 42A, 983.
8 D. Walther, K. Hamza, H. Gorls and W. Imhof, Z. Anorg. Allg.
Chem., 1997, 623, 1135.
9 E. C. Brown, B. Johnson, S. Palavicini, B. E. Kucera, L. Casella and
W. B. Tolman, Dalton Trans., 2007, 3035.
NMR titration studies and liquid–liquid extraction experiments.
The results indicate that the solution behaviour, while resembling
the solid-state behaviour to some degree, tends to be more
diverse, being clearly solvent dependent in particular instances.
10 M. Becker, F. W. Heinemann and S. Schindler, Chem.–Eur. J., 1999,
5, 3124.
11 R. M. Kirchner, C. Mealli, M. Bailey, N. Howe, L. P. Torre,
L. J. Wilson, L. C. Andrews, N. J. Rose and E. C. Lingafelter,
Coord. Chem. Rev., 1987, 77, 89.
12 I. Morgenstern-Badarau, F. Lambert, J. Philippe Renault,
M. Cesario, J. D. Marechal and F. Maseras, Inorg. Chim. Acta,
2000, 297, 338.
13 G. A. McLachlan, G. D. Fallon and L. Spiccia, Acta Crystallogr.,
Sect. C: Cryst. Struct. Commun., 1996, 52, 309.
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft, the Deutscher
Akademischer Austauschdienst, the Marie Curie program of
European Union, the Australian Research Council and Massey
University for support. MS gratefully acknowledges receipt of
a Royal Society Wolfson Merit Award and of an ERC Advanced
Grant.
14 J. A. Thomas, A. Davison and A. G. Jones, Inorg. Chim. Acta, 1991,
184, 99.
15 H. S. He, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2007, 63,
m346.
16 H. S. He, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2007, 63,
m521.
17 G. K. Patra and I. Goldberg, Eur. J. Inorg. Chem., 2003, 969.
18 M. M. Ibrahim, N. Shimomura, K. Ichikawa and M. Shiro, Inorg.
Chim. Acta, 2001, 313, 125.
19 Stoe and Cie GmbH, Darmstadt, Germany, 1999.
20 G. M. Sheldrick, Institut fu€r Anorganische Chemie der Universita€t
Go€ttingen, 1999–2003.
21 G. M. Sheldrick, Institut fu€r Anorganische Chemie der Universita€t
Go€ttingen, 1997.
22 Stoe and Cie GmbH, edn 1.22, Darmstadt, Germany, 2001.
23 A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7.
24 Bruker AXS Inc., Madison, Wisconsin, USA, 2001.
25 A. Altomare, G. Cascarano, C. Giacovazzo and A. Guagliardi,
J. Appl. Crystallogr., 1994, 27, 1045.
Notes and references
1 (a) B. Antonioli, D. J. Bray, J. K. Clegg, K. Gloe, K. Gloe,
O. Kataeva, L. F. Lindoy, J. C. McMurtrie, P. J. Steel, C. J. Sumby
and M. Wenzel, Dalton Trans., 2006, 4783; (b) H.-J. Kim, W.-
C. Zin and M. Lee, J. Am. Chem. Soc., 2004, 126, 7009; (c)
O. V. Dolomanov, D. B. Cordes, N. R. Champness, A. J. Blake,
€
L. R. Hanton, G. B. Jameson, M. Schroder and C. Wilson, Chem.
Commun., 2004, 642; (d) J. J. M. Amoore, C. A. Black,
L. R. Hanton and M. D. Spicer, Cryst. Growth Des., 2005, 5, 1255;
(e) P. Blondeau, A. van der Lee and M. Barboiu, Inorg. Chem.,
2005, 44, 5649; (f) C. Shimokawa and S. Itoh, Inorg. Chem., 2005,
44, 3010; (g) C. S. Purohit and S. Verma, J. Am. Chem. Soc., 2006,
128, 400; (h) A. N. Khlobystov, A. J. Blake, N. R. Champness,
€
D. A. Lemenovskii, A. G. Majouga, N. V. Zyk and M. Schroder,
Coord. Chem. Rev., 2001, 222, 155; (i) S.-L. Zheng, M.-L. Tong and
X.-M. Chen, Coord. Chem. Rev., 2003, 246, 185; (j) C.-L. Chen,
B.-S. Kang and C.-Y. Su, Aust. J. Chem., 2006, 59, 3; (k)
A. G. Young and L. R. Hanton, Coord. Chem. Rev., 2008, 252,
1346; (l) P. J. Steel and C. M. Fitchett, Coord. Chem. Rev., 2008,
€
252, 990; (m) A. J. Blake, G. Reid and M. Schroder, J. Chem. Soc.,
Dalton Trans., 1991, 615; (n) A. J. Blake, N. R. Champness,
€
A. N. Khlobystov, D. A. Lemenovski, W.-S. Li and M. Schroder,
Chem. Commun., 1997, 1339; (o) A. J. Blake, N. R. Champness,
€
S. S. M. Chung, W.-S. Li and M. Schroder, Chem. Commun., 1997,
1675; (p) J. Fan, Y. Wang, A. J. Blake, C. Wilson, E. S. Davies,
€
A. N. Khlobystov and M. Schroder, Angew. Chem., Int. Ed., 2007,
46, 8013.
2 A. G. Blackman, Polyhedron, 2005, 24, 1.
3 C. Chartroux, K. Wichmann, G. Goretzki, T. Rambusch, K. Gloe,
U. Mueller, W. Mueller and F. Voegtle, Ind. Eng. Chem. Res., 2000,
39, 3616.
4 (a) K. J. Bell, A. N. Westra, R. J. Warr, J. Chartres, R. Ellis,
€
C. C. Tong, A. J. Blake, P. A. Tasker and M. Schroder, Angew.
Chem., Int. Ed., 2008, 47, 1745; (b) R. J. Warr, A. N. Westra,
K. J. Bell, J. Chartres, R. Ellis, C. Tong, T. G. Simmance,
€
A. J. Blake, P. A. Tasker and M. Schroder, Chem.–Eur. J., 2009,
15, 4836.
5 N. Weibel, L. J. Charbonniere and R. F. Ziessel, J. Org. Chem., 2002,
67, 7876.
6 E. C. Alyea, G. Ferguson, M. C. Jennings and Z. Xu, Polyhedron,
1990, 9, 739.
26 M. Nishio, CrystEngComm, 2004, 6, 130.
27 M. Munakata, L. P. Wu, T. Kuroda-Sowa, M. Maekawa,
Y. Suenaga, G. L. Ning and T. Kojima, J. Am. Chem. Soc., 1998,
120, 8610.
28 A. Bondi, J. Phys. Chem., 1964, 68, 441.
29 (a) T. S. Thakur and G. R. Desiraju, Chem. Commun., 2006, 552; (b)
L. Brammer, J. M. Charnock, P. L. Goggin, R. J. Goodfellow,
T. F. Koetzle and A. G. Orpen, J. Chem. Soc., Chem. Commun.,
1987, 443; (c) L. Brammer, Dalton Trans., 2003, 3145.
30 D. Braga, F. Grepioni and G. R. Desiraju, J. Organomet. Chem.,
1997, 548, 33.
31 D. Braga, F. Grepioni and G. R. Desiraju, Chem. Rev., 1998, 98, 1375.
32 (a) W. Yao, O. Eisenstein and R. H. Crabtree, Inorg. Chim. Acta,
1997, 254, 105; (b) M. Bortolin, U. E. Bucher, H. Ruegger,
L. M. Venanzi, A. Albinati, F. Lianza and S. Trofimenko,
Organometallics, 1992, 11, 2514; (c) M. Brookhart, M. L. H. Green
and G. Parkin, Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 6908.
33 C. Janiak, J. Chem. Soc., Dalton Trans., 2000, 3885.
34 A. W. Addison, T. N. Rao, J. Reedijk, J. Van Rijn and
G. C. Verschoor, J. Chem. Soc., Dalton Trans., 1984, 1349.
35 (a) H. Stephan, S. Juran, B. Antonioli, K. Gloe and K. Gloe, in
Analytical Methods in Supramolecular Chemistry, ed. C. Schalley,
Wiley-VCH, Weinheim, 2006, pp. 79; (b) J. Rydberg,
G. R. Choppin, C. Musikas and T. Sekine, in Solvent Extraction
Principles and Practice, ed. M. C. J. Rydberg, C. Musikas, G. R.
Choppin, Marcel Dekker, New York, 2004, pp. 109.
This journal is ª The Royal Society of Chemistry 2010
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