Sulfate selective anion recognition by a novel tetra-imidazolium zinc metalloporphyrin receptor

Article abstract of DOI:10.1039/b609817g

Imidazolium groups have been successfully incorporated into the structure of a "picket fence" porphyrin molecule to produce a novel tetra-imidazolium zinc metalloporphyrin anion receptor. UV/visible spectroscopic studies reveal that this receptor is selec

Full text of DOI:10.1039/b609817g

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www.rsc.org/dalton | Dalton Transactions
Sulfate selective anion recognition by a novel tetra-imidazolium zinc
metalloporphyrin receptor
David P. Cormode, Sean S. Murray, Andrew R. Cowley and Paul D. Beer*
Received 10th July 2006, Accepted 2nd October 2006
First published as an Advance Article on the web 11th October 2006
DOI: 10.1039/b609817g
Imidazolium groups have been successfully incorporated into the structure of a “picket fence”
porphyrin molecule to produce a novel tetra-imidazolium zinc metalloporphyrin anion receptor.
UV/visible spectroscopic studies reveal that this receptor is selective for sulfate anions, capable of
strongly complexing sulfate in competitive water–DMSO (5 : 95) solvent mixtures. Cyclic and square
wave voltammetric studies demonstrate the receptor’s ability to sense a variety of anions
electrochemically.
Introduction
The vital role of anions in many chemical, biological and envi-
ronmental processes is well documented.1 As such, the design of
increasingly sophisticated molecular receptors for the strong and
selective recognition of anionic guests is a topic of great current
interest. The integration of photo- and redox-active reporter
groups into these anion hosts has allowed the binding of anions
to be reported by a measurable macroscopic physical response.2
As a consequence of its inherent optical and redox properties3
the porphyrin macrocycle is an attractive structural framework
on which to append anion recognition motifs to produce new
potential anion sensory reagents.4 Indeed, we4g,i and others4j,k have
Scheme 1
previously described amide and urea functionalised ‘picket fence’
porphyrin receptors which strongly bind anions in a cooperative
(Scheme 2) in 62% yield. Treatment of 4 with excess methyl iodide
1 : 1 stoichiometric fashion.
and, subsequently, excess NH4PF6 gave 1 in an overall yield of
44%.
Both new zinc porphyrin receptors 1 and 3 were characterised
by 1H and 13C NMR spectroscopy, mass spectrometry, elemental
analysis and UV/visible spectroscopy (see Experimental section).
The imidazolium group has recently been shown to be an
effective motif for complexing anions via favourable electrostatic
and hydrogen bonding interactions.5 We report here the synthesis
of a novel tetra-imidazolium zinc metalloporphyrin receptor which
selectively binds sulfate anions in competitive aqueous–DMSO
solvent mixtures.
Results and discussion
X-Ray crystal structure
Syntheses
Crystals of the zinc metalloporphyrin receptor 3, with pyridine as
the coordinating ligand, suitable for single crystal X-ray structure
determination, were grown by the slow diffusion of diethyl ether
into THF solutions of 3 with an excess of pyridine. The structure
(Fig. 1) shows the zinc is five coordinate, with pyridine as the fifth
coordinating ligand.
As is usual in zinc porphyrin complexes containing a single
additional ligand bonded to the metal, the zinc atom is displaced
from the plane of the macrocycle towards the pyridine. The zinc
5,10,15,20-Tetrakis(2-aminophenyl)porphyrin (H2TAPP) was
synthesized via the method of Collman,6 and the a,a,a,a-isomer
was isolated via Lindsey’s atropisomerization procedure.7 The
synthesis of a,a,a,a-tetrakis(2-chloroacetamidophenyl)porphyrin,
2, was achieved using a modified literature procedure.8 Conden-
sation of a,a,a,a-H2TAPP with excess chloroacetyl chloride in dry
dichloromethane in the presence of triethylamine afforded 2 in a
71% yield (Scheme 1).
˚
atom lies 0.35 A from the best plane of the porphyrin nitrogen
Insertion of zinc was achieved by reaction of 2 with Zn(OAc)2
in DMSO to afford 3 in a 90% yield. A dry THF solution of
3 was added to an excess of sodium imidazolate to produce 4
atoms.
All the arms of the molecule are on the same side. Three of the
amide bonds in this structure point into the cavity, while the other
amide (bond furthest right in Fig. 1) points out of the cavity. It can
be seen that a pocket is formed in which anions can potentially be
bound.
Department of Chemistry, Inorganic Chemistry Laboratory, University of
Oxford, South Parks Road, UK, Oxford. E-mail: Paul.Beer@chem.ox.ac.uk;
Fax: 44 (0)1865 275900; Tel: 44 (0)1865 275914
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Scheme 2
Fig. 1 Capped stick plot of the X-ray determined crystal structure of 3.
Hydrogen atoms (except for amide hydrogen atoms) and solvent molecules
omitted for clarity. Key: grey = C, white = H, blue = N, green = Cl, red =
O and mauve = Zn.
Fig. 2 Top: the assignment of the phosphorous atoms of ATP2−. Bottom:
change in the chemical shift of the c phosphorous atom of the ATP2−
anion (the Na2ATP salt was used) with increasing amounts of receptor 1
at 293 K. Solvent: a D2O–d6-DMSO (5 : 95) solvent mixture.
NMR Anion binding studies
was observed upon addition of one equivalent of TBA(H2PO4).
Unfortunately, precipitation problems prevented quantitative H
1
1
Qualitative H NMR binding investigations of 1 and a variety
of anions were initially undertaken in DMSO-d6 solutions. The
addition of 5 equivalents of the anions Cl− and H2PO4− produced
significant downfield shifts in the amide and methine imidazolium
peaks. Therefore it can be inferred that both the imidazolium and
the amide groups participate in anion binding. Unfortunately,
quantitative H NMR titration experiments proved impossible
to undertake as the changes in the chemical shift could not be
accurately monitored due to overlapping signals.
The changes in the 31P NMR chemical shift of the phosphorous
atoms of ATP2− were studied upon addition of receptor 1 in a D2O–
DMSO-d6 (5 : 95) solvent mixture (Fig. 2). Large perturbations
were seen in the chemical shift of all three phosphorous atoms. It
is noteworthy that the maximum shift values of 2.44 ppm for the
a phosphorous atom, 3.99 ppm for the b and 3.67 ppm for the
c were achieved after one equivalent, indicating a 1 : 1 binding
stoichiometry.
NMR titration data being obtained.
UV/Visible anion binding studies
The electronic spectral characteristics of receptors 1 and 3 are
summarised in Table 1. UV/visible spectroscopic anion binding
studies with 1 were undertaken in DMSO and in water–DMSO
(5 : 95) solvent mixtures with the anions ATP2− and SO42−. No
electronic spectral evidence of anion binding was observed with
3 in these solvents. However studies performed in acetonitrile
revealed 3 was capable of anion recognition.
1
Table 1 Wavelength k/nm (molar extinction coefficient e/103 M−1cm−1
of the absorbances of receptors 1 and 3 in DMSO
)
Band
1
3
Qualitative anion binding studies were also carried out with
receptor 3 using a MeCN-d3–THF-d8 (1 : 1) solvent mixture.
These studies revealed significant downfield shifts in the amide
protons upon addition of anions. For example, a 0.4 ppm shift
Soret
Q (b)
Q (a)
434 (187.0)
563 (12.6)
602 (3.6)
432 (243.7)
563 (18.1)
602 (3.4)
5136 | Dalton Trans., 2006, 5135–5140
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Table 2 Association constant (log K) data derived from UV/visible
spectroscopy for 1 and 3 determined at 293 K, errors 0.1
Anion
1a
1b
3c
d
Cl−
4.2
4.6
3.6
3.1
d
d
d
d
Br−
I−
H2PO4
−
−
4.8
4.1
5.4
d
HSO4
>6
>6
5.0
>6
ATP2−
e
f
f
f
f
2−
e
SO4
a Solvent: DMSO. b Solvent: a water–DMSO (5 : 95) solvent mixture,
c Solvent: acetonitrile, d Spectral changes deemed to be too small for
accurate association constants to be derived, e Na+ salts, f Anion was not
soluble in these solvents.
Fig. 3 UV/Visible titration of 1 with TBA(HSO4) in a water–DMSO (5 :
95) solvent mixture at 293 K. Addition of 15 aliquots up to 1 equivalent
of anion are shown.
that for Cl−. 1 showed no affinity for the halide anions Br− or I−,
however in less competitive solvents such as acetone, these anions
−
The anion titration experiments focused on perturbations of
the Q bands of the respective porphyrin receptors. In a typical
titration experiment (Fig. 3) a bathochromic shift can be seen in
the maxima of the peaks (5 nm for this example) and the intensity
of the longer wavelength a band increases relative to that of the
b band. These changes correspond to those found by Valentine
for anion addition to Zn tetraphenylporphyrin and are caused by
axial ligation of the anion to the Lewis acidic zinc centre.3a Three
isosbestic points were observed as the titration progressed. It is
noteworthy that anion addition caused a significant colorimetric
response visible by the naked eye. Acetonitrile solutions of 3 are
pink in colour and addition of any strongly binding anion, such as
H2PO4−, resulted in the solution changing colour to green (Fig. 4).
Similar colorimetric responses were seen for 1.
and NO3 were shown to be bound. In the water–DMSO (5 : 95)
−
solvent mixture, as expected, the association constant for H2PO4
is reduced in magnitude and the changes in the electronic spectrum
upon addition of Cl− were deemed too small for a association
−
constant to be derived. However, it is noteworthy that for HSO4
2−
and SO4 extremely strong complexes are observed as there are
few previous examples of receptors that are selective for these
anions.10 Attempts were made to gain association constants for
these anions using a water–DMSO (1 : 1) solvent mixture, however,
unfortunately, 1 was not sufficiently soluble in this solvent mixture.
It is difficult to rationalise the preference of this receptor for HSO4
over H2PO4 as these anions are of similar size and shape, and
H2PO4 is the more basic anion. ATP2− is also bound strongly by
1, with an association constant an order of magnitude higher than
that for H2PO4−, which is a consequence of the former anion’s
higher charge.
Receptor 3, in contrast to 1, has no affinity for HSO4 in
acetonitrile. In the less competitive solvent DCM, HSO4 and
−
−
−
−
−
−
NO3 were shown to be bound by 3. The differing anion binding
motif of 3 compared to 1 must be much less favourable for HSO4
−
.
−
The anions are bound by 3 in the order of preference H2PO4
>
Cl− > Br− > I−, following the trend of anion basicities.
It is clear that the four positively charged imidazolium groups of
1 amplify the anion binding strength of this receptor as compared
to neutral receptor 3. The combination of attractive electrostatic
charges and additional potential imidazolium methine hydrogen
bond donating groups of 1 has produced a superior anion receptor
which binds anions in polar organic and organic/aqueous solvent
media.
Fig. 4 Colour of 3 before (left) and after (right) addition of 2 equiv. of
TBA(H2PO4). Solvent: acetonitrile.
Job plot analyses showed the anion binding to be in a 1 : 1 ratio
for both receptors. Association constant values for 1 : 1 complexes
were calculated using the SpecfitꢀC program9 and are summarised
in Table 2. No evidence of ClO4 and NO3 complexation was
observed with either 1 or 3 in these solvent systems.
Electrochemical anion binding studies
The electrochemistry of 1 and 3 was investigated by both square-
wave and cyclic voltammetry in acetonitrile solutions with TBA
BF4 as the supporting electrolyte. Both these receptors displayed
two quasi-reversible oxidation potentials at ca. 0.6 and 1.0 V
relative to a Ag/Ag+ reference electrode (Table 3).11 Interestingly,
the respective oxidation wave values of both porphyrin receptors
are similar, which is surprising given that 1 is a tetracation whereas
3 is neutral. This suggests that the cationic imidazolium groups
are electronically insulated from the porphyrin macrocyclic ring.
−
−
As was hoped, Table 2 shows that 1 binds specific anions very
strongly, with large association constant values for the anions Cl−,
−
−
H2PO4 and especially HSO4 in DMSO, a highly competitive
solvent. The association constant data suggests this receptor has a
preference for anions of tetrahedral geometry, with the association
constants for H2PO4− and HSO4− being of greater magnitude than
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Table 3 Electrochemical data.
Table 5 Wavelength k/nm (intensity/AU) of the emissions of receptors
1 and 3 in DMSO. kex = 424 nm. [receptor] = 10−5
M
P/P+
1
3
Band
1
3
E
1/2/V
DEp/V
pa/Ipc
0.555
0.125
2.4
0.560
0.130
1.6
Qꢁ (a)
Qꢁ (b)
615 (1682)
666 (1890)
610 (1674)
662 (1795)
I
P+/P2+
E
1/2/V
DEp/V
pa/Ipc
1.005
0.065
1.1
0.980
0.140
1.4
were carried out with receptors 1 and 3, using DMSO for 1 and
acetonitrile for 3. Addition of dihydrogen phosphate produced
dramatic changes in the spectrum of both 1 and 3. The changes
seen for 1 are displayed in Fig. 5. There is a distinct red shift of
the peaks of the Qꢁ bands upon addition of H2PO4−, the intensity
of the b peak decreases relative to the intensity of the a peak
and there are three isosbestic points to be seen: at 616, 633 and
675 nm. These changes are similar to those found by UV/visible
spectroscopy (Fig. 3). Large increases in intensity of emission
were seen for chloride upon addition to receptor 3. Only small
changes were observed upon addition of NO3−, ClO4−, Br− and
I−. Significant perturbations were also observed upon addition of
HSO4− to solutions of 1, however itproved impossible to determine
an association constant value for the spectral titration data.
I
P−/P
E
1/2/V
−1.185
0.225
0.8
−1.140
0.130
0.9
DEp/V
I
pa/Ipc
The reference electrode used was Ag/Ag+ at 293 K. Scan rate = 100 mV s−1
.
Solvent: acetonitrile. TBA(BF4) was used as the supporting electrolyte.
Receptor concentration was 0.001 M.
Table 4 Cathodic shifts (mV) of the first oxidation potential of the
porphyrin receptors 1 and 3 produced on addition of 5 equiv. of the anion.
Solvent: acetonitrile. TBA(BF4) was used as the supporting electrolyte
Anion
1
3
Cl−
H2PO4
HSO4
NO3
175
200
260
105
105
<5
−
a
−
140
95
<5
−
−
ClO4
a Addition of TBA(H2PO4) caused a precipitate to form, so no accurate
value can be given.
For the purposes of electrochemical anion recognition studies,
attention focused on the first oxidation wave as this was easiest to
monitor. The voltammograms were recorded after addition of 5
equivalents of the TBA salt of the anion. In general, significant
cathodic shifts of the oxidation potential were observed upon
addition of anions
Fig. 5 Luminescent titration of 1 with TBA(H2PO4) in DMSO. Addition
of 29 aliquots up to 10 equivalents of anion are shown.
Table 4 shows that receptor 1 exhibits large cathodic shifts upon
addition of Cl− and HSO4−, a result that corresponds with the
association constant values for these anions found via UV/visible
−
spectroscopy. Although the addition of ClO4 was expected to
Job plot analyses showed the anion binding to be in a 1 : 1 ratio
for both receptors with each anion. The association constants for
1 : 1 complexes were calculated using the SpecfitꢀC program and are
summarised in Table 6. These values do not correspond exactly,
but are of similar magnitude, to those determined by UV/visible
spectroscopy (Table 2).
cause little perturbation, the 95 mV shift observed upon addition
of NO3 was surprising, as 1 showed no affinity for this anion
−
as evidenced by UV/visible studies. Table 4 also shows that 3
displays large cathodic perturbations upon addition of anions Cl−
−
and H2PO4 which again indicate a strong association, as noted
in the UV/visible binding studies. The significant cathodic shifts
seen upon addition of NO3− and HSO4− are difficult to rationalise
as UV/visible titration studies indicated no binding.
Table 6 Association constant (log K) data derived from emission spectra
for 1 and 3 determined at 293 K, errors 0.1
Luminescence
Anion
1a
3b
Zinc porphyrins are known to luminesce strongly.12 The wave-
lengths and intensities of the emission bands of 1 and 3 in DMSO
are shown in Table 5.
The effect of anion ligation on the emission spectra of por-
phyrins has not been previously investigated. Anion titrations
c
Cl−
H2PO4
4.3
5.0
−
5.5
a Solvent: DMSO. b solvent: acetonitrile. c Spectral changes deemed to be
too small for accurate association constants to be derived.
5138 | Dalton Trans., 2006, 5135–5140
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sulfoxide, giving a cherry-red solution. To this stirred solution
was added a ten-fold excess (2.91 g, 13.30 mmol), of Zn(OAc)2.
After stirring for 16 h, 50 ml of brine and 70 ml of CHCl3 was
added. The organics were separated and washed with 50 ml of
brine and 2 × 50 ml of H2O. The solvent was removed in vacuo
and the residue was taken up in 5 ml dimethyl sulfoxide. 50 ml of
Conclusions
New zinc porphyrin receptors appended with amide and amide-
imidazolium groups have been successfully prepared. UV/visible
spectroscopic anion titration studies reveal the positively charged
tetra-imidazolium zinc metalloporphyrin receptor 1 strongly binds
anions in a cooperative 1 : 1 stoichiometric manner. Receptor 1 is a
superior anion binding reagent to the neutral amide zinc porphyrin
3, being capable of strong and selective complexation of sulfate
in aqueous–DMSO solvent mixtures. Cyclic and square wave
voltammetric investigations demonstrate both receptors to sense
anions electrochemically via significant cathodic perturbations of
the respective porphyrin’s first oxidation wave.
◦
methanol was added and, after 4 h at −20 C, the solution was
filtered to yield a light purple powder (1.25 g, 90.2%). Found: C,
59.7; H, 3.5; N, 10.6. Calc. for C52H36Cl4N8O4Zn: C, 59.8; H, 3.5;
N, 10.7%; kmax(DMSO)/nm 432 (e/dm3 mol−1 cm−1 243 700), 563
(18 100), 603 (3 400); dH (300 MHz; d6-DMSO;) 3.58 (8 H, s, CH2),
7.57 (4 H, t, J 9 Hz, aryl H), 7.84 (4 H, t, J 9 Hz, aryl H), 7.92 (4
H, d, J 9 Hz, aryl H), 8.34 (4 H, d, J 9 Hz, aryl H), 8.67 (8 H, s,
pyrrole H), 8.95 (4 H, s, C(O)NH); dC (75.43 MHz; d6-DMSO)
43.25, 115.81, 123.36, 124.43, 129.60, 132.29, 135.30, 136.58,
137.89, 150.25, 165.08; MS (ESI positive ion, MeOH–MeCN):
m/z 1041.15 [M + H+].
Experimental
Materials and general procedures
Solvents (HPLC grade) were purchased from Rathburn Chemi-
cals, Alfa Aesar and Fisher Scientific. Solvent drying was carried
out by degassing via sparging with dinitrogen and then passing
through a column of activated alumina using Grubbs apparatus.13
Solvent thus dried was used immediately. The starting materials
were used as received (Acros, Aldrich) without any further
purification except for pyrrole which was distilled under nitrogen
just before use.
1H NMR spectra were recorded on a Varian Mercury spec-
trometer operating at 300 MHz, 13C NMR at 75.48 MHz, and
31P NMR at 282.46 MHz. Absorption spectra were obtained on
a Perkin Elmer Lambda 6 UV/visible spectrophotometer. Cyclic
voltammetry was performed on an Eco Chemie lAutolab Type II
combined with General Purpose Electrochemical Software version
4.9. Glassy carbon working electrode, Ag/Ag+ reference electrode
and platinum counter electrode were used in the instrumental
setup. TBA BF4 was used as the supporting electrolyte. Mass Spec-
trometry was performed in the Inorganic Chemistry Department
at the University of Oxford. Elemental analyses were performed by
Stephen Boyer of the SACS at London Metropolitan University.
(a,a,a,a-Tetrakis(2-imidazolylacetamidophenyl)porphyrinato)-
zinc(II), 4. 1.36 g (20.0 mmol) of imidazole was added to 50 ml
of dry THF cooled in an ice bath. To this stirred solution was
added 0.80 g of NaH (20.0 mmol, 60% dispersion in mineral
oil), portionwise, to create a white suspension. When the bubbling
ceased, 0.52 g (0.5 mmol) of 3, dissolved in 50 ml dry THF (a violet
solution), was added dropwise to this suspension. The addition of
the solution of 3 turned the suspension green. After 16 h stirring,
the solution was filtered, the THF was removed in vacuo and
resulting green oil was taken up in 50 ml dichloromethane with
5 ml of DMSO. This solution was washed with 2 × 50 ml H2O. The
solution turned purple after the first wash. The volume of the
solution was reduced to 3 ml and the product was precipitated by
addition of 50 ml diethyl ether, to give a very dark purple powder
(0.36 g, 61.5%). kmax(DMSO)/nm 432 (e/dm3 mol−1 cm−1 245 200),
563 (18 200), 603 (4600); dH (300 MHz; d6-DMSO) 3.93 (8 H, s,
CH2), 5.09 (4 H, s, imidazole), 5.29 (4 H, s, imidazole), 5.96 (4 H, s,
imidazole), 7.60 (4 H, t, J 8 Hz, aryl H), 7.84 (4 H, t, J 8 Hz, aryl
H), 8.02 (4 H, d, J 8 Hz, aryl H), 8.14 (4 H, d, J 8 Hz, aryl H),
8.50 (8 H, s, pyrrole H), 9.63 (4 H, s, C(O)NH); MS (ESI positive
ion, MeOH–MeCN): m/z 1169.36 [M + H+].
Syntheses
H2TAPP was synthesized by the Collman method.6
(a,a,a,a-Tetrakis(2-[3-methylimidazolium]acetamidophenyl)-
porphyrinato)zinc(II), 1. 0.10 g (0.09 mmol) of 4, was taken up
in 20 ml DMSO. To this stirred solution was added 3 ml (xs)
of methyl iodide. The mixture was heated for 16 h at 50 ◦C after
which a white precipitate appeared. The solution was filtered, then
poured into 30 ml of H2O. The resulting purple precipitate was
filtered off, washed copiously with H2O and Et2O, before being
taken up in 20 ml MeOH and 3 ml DMSO. To this stirred solution
was added 10 ml of sat. NH4PF6(aq). The solution was stirred for
16 h. The resulting purple precipitate was filtered off and washed
copiously with H2O and Et2O (0.08 g, 0.04 mmol, 44%). Found: C,
45.2; H, 3.2; N, 12.5. Calc. for C52H36Cl4N8O4Zn: C, 45.1; H, 3.3;
N, 12.4%; kmax(DMSO)/nm 434 (e/dm3 mol−1 cm−1 187 000), 563
(12 600), 602 (3600); dH (300 MHz; CDCl3) 3.55 (12 H, s, CH3),
4.12 (8 H, s, CH2), 7.09 (4 H, br s, aryl H), 7.39 (4 H, br s, aryl H),
7.61 (4 H, br s, aryl H), 7.86 (8 H, br s, aryl H), 8.15 (4 H, br s, aryl
H), 8.57 (4 H, br s, aryl H), 8.67 (8 H, s, pyrrole H), 9.61 (4 H, s,
C(O)NH); dC (75.43 MHz; CDCl3) 36.01, 50.73, 115.98, 123.20,
123.55, 124.80, 129.26, 136.51, 136.84 137.68, 149.90, 164.54;
a,a,a,a-Tetrakis(2-chloroacetamidophenyl)porphyrin, 2. 1.26 g
(1.87 mmol) of H2TAPP was dissolved in a stirred solution of 3 ml
(excess) NEt3 in 50 ml dry CH2Cl2, cooled in an ice bath. To this
solution was added 0.89 ml (1.26 g, 11.16 mmol) chloroacetyl
chloride in 50 ml dry CH2Cl2, dropwise. After 16 h a purple
suspension was produced. The volume of the solution was reduced
to 20 ml and filtered, leaving a light purple powder. This powder
was washed copiously with Et2O, EtOH and was dried in vacuo
(1.30 g, 70.9%). kmax(DMSO)/nm 423 (e/dm3 mol−1 cm−1 243 600),
519 (18 600), 552 (5600), 593 (6000), 652 (2700); dH (300 MHz; d6-
DMSO) −2.79 (2H, s, pyrrole NH), 3.55 (8 H, s, CH2), 7.59 (4 H,
t, J 9 Hz, aryl H), 7.86 (4 H, t, J 9 Hz, aryl H), 7.92 (4 H, d, J
9 Hz, aryl H), 8.22 (4 H, d, J 9 Hz, aryl H), 8.70 (8 H, s, pyrrole
H), 9.32 (4 H, s, C(O)NH); MS (ESI positive ion, MeOH–MeCN):
m/z 979.18 [M + H+].
(a,a,a,a-Tetrakis(2-chloroacetamidophenyl)porphyrinato)zinc(II),
3. 1.30 g (1.33 mmol) of 2 was taken up in 50 ml dimethyl
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dP (282.46 MHz; CDCl3) −144.28 (q, J 713) MS (MALDI-TOF,
Acknowledgements
+
MeOH–MeCN): m/z 1663.13 [M–PF6 ].
We thank the EPSRC for a studentship (D. P. C.). Oxford Diffrac-
tion Ltd. are thanked for the loan of the Gemini diffractometer.
UV/Visible anion titration protocol
In a typical experiment, aliquots of guest (2.5 × 10−5 mol in
5 ml) were added to a 3 ml solution of the host (5 × 10−5 M)
at 293 K. Twenty eight aliquots were added (13 × 2 ll, 1 × 4 ll,
6 × 5 ll, 4 × 15 ll, 4 × 60 ll). Spectra were recorded and the data
was analysed by the computer program SpecfitꢀC . The spectra
together with the host and guest concentrations were read into the
program for every titration point and the complex stoichiometry
and whether the components species were coloured was entered.
The parameters were refined by global analysis that uses singular
value decomposition and non-linear modelling by the Levenberg–
Marquardt method. Using the calculated stability constants, the
program plots the predicted spectra of the component species
together with the observed and calculated absorption vs. guest
concentration at a given wavelength, both of which reveal the
accuracy of the experimental data and the suitability of the model.
The program also gives the best-fit values of the stability constants
together with their errors. The parameters were varied until the
values for the stability constants converged.
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Luminescence anion titration protocol
In a typical experiment, aliquots of guest (1.5 × 10−5 mol in
5 ml) were added to a 3 ml solution of the host (1 × 10−5 M)
at 293 K. Twenty nine aliquots were added (15 × 2 ll and 14 ×
5 ll). Spectra were recorded and the data was analysed by the
computer program SpecfitꢀC , in a method as described above.
X-Ray crystallography
Single crystals of the 3:pyridine complex were grown by slow
diffusion of diethyl ether into a THF solution of 3 and an excess of
pyridine. Crystals were mounted on a glass fibre and cooled rapidly
to 150 K in a stream of cold nitrogen using an Oxford Cryosystems
CRYOSTREAM unit. Diffraction data were measured using
an Oxford Diffraction Gemini CCD diffractometer (graphite-
˚
monochromated Cu-Ka radiation, k = 1.54248 A). Intensity
data were processed using the Crysalis package.14 Structures were
solved by direct methods using the SIR92 program.15 Full-matrix
least-squares refinement was carried out using the CRYSTALS
program suite.16 Hydrogen atoms were positioned geometrically
after each cycle of refinement. A three-term Chebychev polyno-
mial weighting scheme was applied.
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435.
Crystal data for 3. Chemical formula: C57H41Cl4N9O4-
Zn·xC4H8O (x ∼ 1.63), M = 1240.96, monoclinic, space group =
˚
P21/n, a = 13.3651(2), b = 27.1137(3), c = 16.4073(2) A, T =
150 K, Z = 4, l = 2.711 mm−1, reflections measured = 27764,
Rint = 0.035, R = 0.0574.
CCDC reference number 618393.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b609817g
16 CRYSTALS Issue 12,P. W. Betteridge, J. R. Cooper, R. I. Cooper, K.
Prout and D. J. Watkin, J. Appl. Crystallogr., 2003, 36, 1487.
5140 | Dalton Trans., 2006, 5135–5140
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