NH vs. CH hydrogen bond formation in metal-organic anion receptors containing pyrrolylpyridine ligands

Article abstract of DOI:10.1039/b510506d

Anion complexation studies on a series of platinum(II) tetrakis(pyrrolylpyridine) salts demonstrate the importance of CH-anion hydrogen bonds in coordinating anionic guests in solution and the solid-state. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b510506d

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COMMUNICATION
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NH vs. CH hydrogen bond formation in metal–organic anion receptors
containing pyrrolylpyridine ligands{
Ismael El Drubi Vega,a Philip A. Gale,*a Mark E. Lighta and Stephen J. Loebb
Received (in Cambridge, UK) 22nd July 2005, Accepted 16th August 2005
First published as an Advance Article on the web 12th September 2005
DOI: 10.1039/b510506d
complexes of these pyridines, 2(BF4)2 and 3(BF4)2, were prepared
in an analogous fashion to 1(BF4)2 (see ESI for details).{
Crystals of 2(BF4)2 were obtained by slow evaporation of a
nitromethane solution of the complex.{ The structure (see Fig. 1)
Anion complexation studies on a series of platinum(II)
tetrakis(pyrrolylpyridine) salts demonstrate the importance
of CH–anion hydrogen bonds in coordinating anionic guests
in solution and the solid-state.
shows that the complex adopts the 1,2-alternate conformation in
2
the solid state binding the BF4 anions via NH F and CH
…
…
The coordination of anionic guest species by hydrogen bond
donating receptors is an area of supramolecular chemistry that
continues to attract attention.1 A high proportion of the anion
receptors reported thus far are ‘built’ upon organic scaffolds
such as the calix[4]arenes.2 However, the preparation of this type
of receptor can often be synthetically challenging, a fact that
prompted us to look for alternative means of arranging hydrogen
bond donating groups in space. It occurred to us that some easy to
prepare metal ligand complexes could be exploited as simple pieces
of inorganic molecular scaffolding.3 We initially chose to study
square planar platinum(II) complexes due to their relative inertness
towards ligand substitution and pyridine or iso-quinoline ligands
due to their ease of synthesis and possibility of the hydrogen in the
F
…
hydrogen bonds (including N2 F1 2.882(3); N4 F2 2.884(3);
…
…
…
…
˚
C1 F3 3.322(3); C5 F1 3.447(3); C10 F3 3.442(4) A).
…
2-position of the pyridine involving itself in potential CH anion
hydrogen bonding.4 Brammer and co-workers have studied the
2+
application of related PtL4 (L 5 nicotinamide) complexes in
The anion binding properties of complexes 1(BF4)2, 2(BF4)2 and
3(BF4)2 were explored using 1H NMR titration techniques in
DMSO-d6. Complex 1(BF4)2, despite containing no NH hydrogen
bond donor groups, was observed to interact with anions with
downfield shifts of up to 0.50 ppm for the hydrogen in the
2-position of the pyridine ring in the presence of three equivalents
of chloride ion (Table 1). In most cases, this data could be fitted to
a 1 : 1 receptor : anion binding model with the results shown in
Table 2. However, the possibility exists that a second anion may
associate with the 1 : 1 complex with a low K2 that has not been
detected in this experiment. This is true of all the 1H NMR
titration experiments reported in this communication.
the construction of crystal-engineered networks5 whilst Steed and
co-workers have investigated the role anions can play in stabilising
discrete complexes using similar organic ligands and kinetically
labile templating metal centres.6
7
…
We decided to investigate the role CH anion hydrogen bonds
play in stabilising the anion complexes in solution by synthesising a
series of receptors containing either no NH hydrogen bond donors
or a combination of NH and CH donors and to compare the
affinity of these complexes for anions. Compound 2 offers the
putative anionic guest a choice of hydrogen bond donor (NH or
CH) as the pyrrole is free to rotate around the pyrrole–pyridine
bond without significantly changing the shape of the binding site. On
the other hand, complex 3 offers the anionic guest an electrostatic
interaction and CH hydrogen bond donors at the centre of the
complex or an NH hydrogen bond donor at the periphery.
Receptor 1(BF4)2 was synthesised via literature procedures.4,8
3-(Pyrrol-2-yl)pyridine and 4-(pyrro-2-yl)pyridine were synthesised
via a Paal Knorr9 reaction with 4-oxo-4-(pyridin-3-yl)butanal10
and 4-oxo-4-(pyridin-4-yl)butanal,10 respectively, to afford sub-
stituted pyridines in up to 95 and 60% yields. The platinum
aSchool of Chemistry, University of Southampton, Southampton, UK
SO17 1BJ. E-mail: philip.gale@soton.ac.uk; Fax: +44 23 80596805
bDepartment of Chemistry Biochemistry, University of Windsor,
Windsor, ON, Canada N9B 3P4. E-mail: loeb@uwindsor.ca
{ Electronic supplementary information (ESI) available: Spectroscopic
data for 1, 2 and 3. See http://dx.doi.org/10.1039/b510506d
Fig. 1 A ball-and-stick representation of the X-ray crystal structure of 2
crystallized from nitromethane. The hydrogen bonding network is shown
with BF4 anions bound above and below the plane of the complex by
… …
CH F and NH F hydrogen bonds.
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Table 1 Proton NMR chemical shifts upon addition of three
equivalents of tetrabutylammonium anion salt to solutions of
1(BF4)2, 2(BF4)2 and 3(BF4)2 in DMSO-d6
Table 2 Stability constants of the tetrafluoroborate salts of 12+, 22+
and 32+ with anionic guests in DMSO-d6 at 298 K calculated from the
shift of the resonance of the pyridine hydrogen in the 2-position except
where noted assuming 1 : 1 stoichiometry in all cases except where
noted
Anion
12+
22+
32+
Cl2
195
121
50
84
—
132
45
—
,10
960
796
216
211
113
117a
—
Br2
I2
CH3CO2
462
K1 5 216a
2
K2 5 2400a
111a
,10
,10
,10
2
PhCO22
MeSO3
NO3
HSO4
b
1115
29
837
2
2
a
Calculated following the shift in the pyrrole NH proton.
Precipitation during the titration, however shifts of protons suggest
a strong interaction of 2 with benzoate.
b
Complex 3(BF4)2 does not contain a potential convergent
anion-binding site that could involve all four of the pyrrole NH
groups. Indeed similar, although not identical, shifts of the pyri-
dine CH resonance in the 2-position were observed with complexes
1(BF4)2 and 3(BF4)2. The exception to this are the interactions
observed with the carboxylate anions. Here, as with complex
2(BF4)2, the pyrrole NH group was observed to shift downfield by
between 0.61 and 0.85 ppm whilst only small shifts of the pyridine
a-CH resonance were observed. These results are evidence that a
different mode of binding occurs between complexes 2(BF4)2 and
3(BF4)2 and carboxylate anions than that which occurs between
the complexes and the other less basic anions.
Crystals of 2(MeSO3)2.H2O were obtained by slow evaporation
of a nitromethane solution of the salt (prepared by an analogous
method using AgMeSO3).§ Once again the complex adopts the 1,2-
alternate conformation (see Fig. 2), binding the methanesulfonate
…
…
anions via both pyrrole NH O and pyridine CH O hydrogen
…
bonds in the solid state (including N2 O3A 2.926(8) A; N4 O2A
…
˚
…
…
˚
˚
˚
2.850(8) A; C1 O1A 3.542(7) A; C10 O1A 3.100(6) A).
The solution binding experiments and the crystal structure of
the methanesulfonate complex of 22+ serve to reiterate the
important point that crystal structure data cannot be relied upon
to predict the structure of complexes in solution. There is no
evidence that the pyrrole NH groups interact with the methane-
sulfonate anion in DMSO-d6 solution, but rather that the pyrrole
Complex 2(BF4)2 shows the highest affinity for anions of the
three receptors reported. Shifts of up to 1.15 ppm for the pyridine
CH proton were observed with three equivalents of chloride. Most
surprisingly however, the pyrrole NH proton does not shift signi-
ficantly upon addition of Cl2, Br2, I2, HSO42, MeSO32 or NO3
2
.
…
CH group is involved in a CH O hydrogen bond with the anion
In the cases of the most basic anions studied (acetate and benzoate)
the NH resonance does shift downfield by 1.36 and 1.03 ppm,
respectively, in the presence of three equivalents of the anions
(although precipitation occurs in the case of benzoate and the ratio
of the receptor to anion in solution is uncertain). Acetate binds in a
1 : 2 receptor : anion stoichiometry (planar anions were found to
bind in a 1 : 2 fashion to the previous generation of nicotinamide
based platinum complexes)4 with K2 . K1. This suggests that an
allosteric effect is operating which preorganises the second binding
site upon complexation of the first equivalent of acetate. The same
effect was also observed in the first generation nicotinamide sys-
tems.4 In the case of the other anions, downfield shifts of a pyrrole
CH proton were observed (Table 1). Amongst the non-carboxylate
anions, the highest affinity was found with methanesulfonate
(1115 M21) whilst HSO42 was bound considerably more strongly
(837 M21) than was the case with complex 12+ (,10 M21).
Fig. 2 A ball-and-stick representation of the X-ray crystal structure of
the methanesulfonate salt of cation 22+. The hydrogen bonding network is
2
shown with MeSO3 anions bound above and below the plane of the
…
…
complex by CH O and NH O hydrogen bonds.
4914 | Chem. Commun., 2005, 4913–4915
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phenomenon and should probably be looked at more closely as a
design feature for future anion receptors containing NH hydrogen
bond donor groups.12
We would like to thank the EPSRC for a project studentship (to
IEDV) and the EPSRC together with Prof. Mike Hursthouse
for access to the crystallographic facilities at the University of
Southampton. PAG thanks the Royal Society for a University
Research Fellowship and the Royal Society of Chemistry for an
International Journals Grant. SJL thanks the NSERC of Canada
for award of a Discovery Grant for financial support of this work.
Notes and references
{ All data were collected on a Bruker Nonius Kappa CCD with a Mo
rotating anode following standard procedures. Crystal data for
2(BF4)2(CH3NO2)2 C38H38N10B2F8O4Pt, Mr 5 1067.49, T 5 120(2) K,
Fig. 3 Proton NMR titration of compound 2 in DMSO-d6 with
tetrabutylammonium methanesulfonate (left) and acetate (right) at 298 K
showing a downfield shift of the pyridine CH proton in the 2-position
(red) in both cases, the pyrrole proton in the 3-position is shown in blue
whilst the pyrrole NH proton (green) does not shift upon addition of
methanesulfonate but does shift downfield upon addition of acetate.
¯
˚
triclinic, space group P, a 5 9.4410(3), b 5 10.4860(3), c 5 11.2470(4) A,
3
˚
a 5 77.568(2), b 5 89.272(2), c 5 68.613(2)u, V 5 1009.80(6) A ,
rcalc 5 1.755 g cm23, m 5 3.566 mm21, Z 5 1, reflections collected: 15299,
independent reflections: 4626 (Rint 5 0.0362), final R indices [I . 2sI]:
R1 5 0.0211, wR2 5 0.0466, R indices (all data): R1 5 0.0212. wR2 5 0.0467.
§ Crystal data for 2(MeSO3)2H2O C38H40N8O7PtS2, Mr 5 979.99,
¯
T 5 120(2) K, triclinic, space group P, a 5 9.059(2), b 5 10.489(3),
˚
c 5 11.080(2) A, a 5 81.790(17), b 5 73.740(16), c 5 69.240(14)u,
3
V 5 944.0(3) A , rcalc 5 1.724 g cm23, m 5 3.889 mm21, Z 5 1, reflections
˚
collected: 16456, independent reflections: 4313 (Rint 5 0.0423), final R
indices [I . 2sI]: R1 5 0.0343, wR2 5 0.0777, R indices (all data):
R1 5 0.0357. wR2 5 0.0785. CCDC 281270–281271. See http://dx.doi.org/
10.1039/b510506d for crystallographic data in CIF or other electronic
format.
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Fig. 4 Binding modes of one equivalent of methanesulfonate (A2) to 22+
in (a) nitromethane-d3 and (b) DMSO-d6 solution.
(Fig. 3). A comparison of the shifts observed in compound 32+
with acetate and chloride shows the halide binding presumably to
the centre of the complex so taking advantage of the electrostatic
interaction, whilst the more basic carboxylate induces a larger shift
in the pyrrole NH group.
In nitromethane, more typical behaviour is observed with
compound 22+ upon addition of anions. For example, addition of
aliquots of tetramethylammonium methanesulfonate to a nitro-
methane-d3 solution of 2(BF4)2 causes the pyrrole NH resonance
to shift downfield from 9.79 ppm to 11.56 ppm (Dd 5 1.77 ppm at
2.0 equiv MeSO32). Unfortunately, upon further additions of this
anion, the complex precipitates. These results led us to conclude
that a solvent competition mechanism, such as that shown in
Fig. 4, may be operating in DMSO solution. Nitromethane is a
notoriously poor hydrogen-bond acceptor11 and hence when an
anion is added to the receptor in this solvent it will bind to the
most acidic hydrogen bond donor (the NH). However in DMSO
(a good hydrogen bond acceptor), the anionic guest will either bind
to the pyrrole NH if it is a basic anion, or if it is less basic, the
pyrrole NH groups may orientate themselves into the solvent
allowing the formation of hydrogen bonds to the DMSO whilst
the anion accepts CH hydrogen bonds from the platinum complex.
This presumably leads to a greater overall stability of the complex
than if the NH groups were oriented inwards and the solvent
presented with the CH groups of the pyrrole. The profound effect
of solvent on the binding mode of this receptor is an important
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This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 4913–4915 | 4915