Anion binding in (arene)ruthenium(II)-based hosts

Article abstract of DOI:10.1039/b712699a

Asymmetric ruthenium(II) complexes of a flexible aminomethylpyridine derivative exhibit diastereotopic ligand methylene protons, as measured by NMR spectroscopy; binding of external anions renders these protons equivalent possibly by increasing dynamicall

Full text of DOI:10.1039/b712699a

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Anion binding in (arene)ruthenium(II)-based hosts{
Sara Jane Dickson,a Stefano C. G. Biaginib and Jonathan W. Steed*a
Received (in Austin, TX, USA) 17th August 2007, Accepted 20th September 2007
First published as an Advance Article on the web 8th October 2007
DOI: 10.1039/b712699a
occurs as a singlet at 4.36 and 4.58 ppm, respectively. The
Asymmetric ruthenium(II) complexes of a flexible amino-
methylpyridine derivative exhibit diastereotopic ligand methy-
lene protons, as measured by NMR spectroscopy; binding of
external anions renders these protons equivalent possibly by
increasing dynamically averaged symmetry; the amount of
anion needed to raise average symmetry correlates to the anion
binding constant.
inequivalence of protons Ha and Hb must arise from the fact that
the molecule is point group Cs and Ha and Hb (Scheme 1) are not
related by the mirror symmetry of the molecule. Instead, the
mirror plane relates Ha to Ha9 and Hb to Hb9. These protons
remain diastereotopic even without invoking any restricted
rotation or effects such as intramolecular hydrogen bonding of
the amine protons to the coordinated chloride ligand.8,17,18 The
resonance assigned to the NH protons in 2a occurs at 5.37 ppm,
the downfield chemical shift suggests enhanced hydrogen bonding
compared to the free ligand and 1a (d 4.19 and 4.40 ppm,
respectively) and presumably arises from hydrogen bonding to the
There is currently a great deal of interest in the use of transition
metal ions as sensing and structural elements in the design of
supramolecular hosts for anions.1–4 An early report by Hamilton
et al. concerning Ru(II) terpyridyl-thiourea derivatives5 has been
followed by recent results on monodentate pyridylurea com-
plexes6–11 and anion binding by coordinated, chelating biimidazole
complexes.12,13 Anion templation has also been used to guide self-
assembly of coordination complexes, as in the elegant helicates
reported by Rice and co-workers.14 We have previously reported
anion binding and sensing by 3-aminopyridine ruthenium(II)
derivatives, in which the semi-labile Ru(II) centre acts as a
structural ‘core’, organising the two anion-binding aminopyridine
ligands.15 We now report the extension of this chemistry to
extended 3-aminomethylpyridine derivatives and the consequences
on the compounds’ anion-binding behaviour and symmetry.
Ligands L1 and L2 are readily prepared from the reaction of
pyridine 3-carboxaldehyde with aniline and p-nitroaniline, respec-
tively, followed by reduction with sodium borohydride. Reaction
of these ligands with the chloro-bridged dimer [{Ru(g6-
C6H4MeCHMe2)Cl(m-Cl)}2]16 in toluene solution results initially
in the adducts [Ru(g6-C6H4MeCHMe2)Cl2(L1)] (1a) and [Ru(g6-
C6H4MeCHMe2)Cl2(L2)] (1b). Treatment of complexes of type 1
with one equivalent of AgBF4 and additional L1 or L2 in
methanol–acetone (1 : 1 v/v) solution gives the monocationic
Ru(II) complexes [Ru(g6-C6H4MeCHMe2)Cl(L1)2]BF4 (2a) and
[Ru(g6-C6H4MeCHMe2)Cl(L2)2]BF4 (2b). The new complexes
were fully characterised by elemental analysis, ESI-MS, IR
spectroscopy (nujol) and 1H and {1H}-13C NMR spectroscopy
(see ESI{). The 1H NMR spectrum of complex 2a shows a broad
AB quartet resonance assigned to the methylene protons, Ha,a9 and
Hb,b9 (Scheme 1) at 4.40 and 4.32 ppm, with 2J 5 17.2 Hz,
2
BF4 anion. Examination of X-ray crystal structures of related
2-aminopyridine derivatives19 including the 2-amino-4-picoline
(2-apic) complex [Ru(g6-C6H4MeCHMe2)Cl2(2-apic)] (3) (see
ESI{) shows that intramolecular hydrogen bonding to halides in
these types of complexes is possible for ortho aminopyridines.
However, examination of molecular models and the structure of
the related nicotinamide (nic) complex [Tl2Cl4(m-Cl)2(nic)4]?2nic20
shows that it is impossible for complexes of type 2. The
intrinsic asymmetry of 2a is supported by a dilution study. The
chemical shift of the NH resonance remains essentially unchanged
across the concentration range 1.9–31.0 mol dm23
. Any
intermolecular association would be expected to exhibit concen-
tration dependence.
1
consistent with geminal coupling. In contrast, in the H NMR
spectrum of the free ligand and in complex 1a this resonance
aDepartment of Chemistry, University of Durham, South Road, Durham,
UK DH1 3LE. E-mail: jon.steed@durham.ac.uk;
Fax: +44 (0)191 384 4737; Tel: +44 (0)191 334 2085
bSchool of Physical Sciences, University of Kent Canterbury, Kent, UK
CT2 7NH
{ Electronic supplementary information (ESI) available: Experimental
details and crystal structure information for compound 3 (CCDC 657924)
in CIF or other electronic format. See DOI: 10.1039/b712699a
Compound 2b behaves similarly to 2a, with the presence of the
nitro group designed to enhance the hydrogen bond acidity of the
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 4955–4957 | 4955

View Article Online
Fig. 1 Job plot for 2a binding Br2.
second binding constant is presumably much smaller than the first.
The single formal charge on the host presumably allows two hosts
to fit around a single anion without severe electrostatic repulsion.
Interestingly, virtually no displacement of the unidentate pyridyl
ligands was observed during the titrations even in the presence of
nucleophilic anions such as Cl2, in contrast to previous work on
3-aminopyridine derivatives.15 However, leaving a sample of 2a
overnight in the presence of one equivalent of Cl2 led to the
formation of ca. 20% 1a.
Scheme 1 Anion binding by complex 2 forms both 1 : 1 and 2 : 1 host :
guest complexes.
secondary amine. The NH resonance occurs at 6.67, 5.94 and
4.92 ppm for 2b, 1b and L2, respectively. The strongly hydrogen
bonded nature of the NH proton is evident from the observation
of coupling to the methylene resonances Ha and Hb, while this
coupling is barely resolved for 2a. Similarly, in the IR spectrum of
2b the n(NH) stretch occurs at lower wavenumber than in 2a.
Complexes 2 were designed as hosts for binding anions external
to the complex. Their anion complexation ability was determined
from 1H NMR spectroscopic titrations with a variety of anions as
their tetrabutylammonium salts in CDCl3 solution. Anion binding
constants for the formation of 1 : 1 and 2 : 1 host : guest complexes
are given in Table 1. The use of both 1 : 1 and 2 : 1 stoichiometry
models for complexes 2 is consistent with previous work on
3-aminopyridine derivatives that suggests the formation of
compounds in which a single anion is sandwiched between two
hosts15 and resulted in a much improved fit to the titration data
than a 1 : 1 model alone. The stoichiometry was confirmed by Job
plot analysis for 2a binding Br2 which gave a maximum of 0.66
(Fig. 1). Furthermore, an ESI-mass spectrum of 2a in the presence
of 0.5 equivalents of NBu4Br gave a peak at m/z 1357,
corresponding to {(2a)2Br}+. Job plot analysis of the correspond-
ing chloride complex suggests only 1 : 1 binding, however, and the
The titration results reveal that control compound 1a does not
bind anions, while a very small affinity is noted for the more acidic
hydrogen bond donor 1b. Compounds 2a and 2b are both effective
anion hosts, however, with the nitro-substituted 2b consistently
binding more strongly than 2a. The hosts are moderately selective
for chloride and acetate, consistent with anion basicity. Titrations
with HSO42 and H2PO42 resulted in precipitation in the case of 2b
and small chemical shift changes for 2a. Surprisingly, monitoring
the appearance of the 1H NMR resonance assigned to the
methylene protons Ha and Hb as a function of added anion
showed that addition of strongly bound anions that are good
hydrogen bond acceptors rapidly results in the collapse of this
resonance to a singlet. In contrast, weakly interacting anions such
2
as CF3SO3 do not affect the appearance of the resonance even
after the addition of a fivefold excess, Fig. 2. In all cases, more
equivalents of anion are required to collapse the AB quartet to a
singlet for 2a than for 2b and the number of equivalents required
approximately inversely correlates with the anion binding affinity,
thus a singlet methylene resonance is observed for 2a upon
addition of only 0.4 equivalents of Cl2, while 0.8 equivalents of
acetate and two equivalents of nitrate are required. The anion
binding equilibria observed for complexes 2 are summarised in
Scheme 1. In order to remove the inequivalence of protons Ha and
Hb in complexes 2, anion binding must induce a time-averaged
plane of symmetry running along the N–Ru–N axis or involve
temporary dissociation of the pyridyl ligand. The latter explana-
tion seems relatively unlikely because we have shown that the
displacement of ligand to form 1a in the presence of chloride takes
hours to days (vide supra). The magnetic equivalence is also
unlikely to occur by anion association to form a transient 20-
electron complex since this would not result in increased symmetry
in any case other than Cl2 binding. Chloride loss to form a
transient 16-electron species is, however, possible and would result
in the required symmetry. Thus we tentatively suggest that the
evidence is consistent with strong anion association in hosts 2
Table 1 Anion binding constants (M21) for complexes +1 and 2 in
CDCl3 at 20 uC. Errors are ,10%, anions added as NBu4 salts, host
concentration 0.006 mol dm23 (dash indicates not measured)
K11 and K21/M21
Anion
Cl2
1a
1b
102
—
—
—
—
2a
2b
y0
—
K11 4325
K21 209a
K11 1870
K21 102
K11 115
K21 72
K11 11 020
K21 357
K11 4630
K21 432
K11 2770
K21 468
K11 198
K21 175
K11 13 240
K21 962
Br2
2
NO3
y0
y0
—
2
CF3SO3
K11 87a
K21 69
K11 3390
K21 174a
2
MeCO2
a
Large error.
4956 | Chem. Commun., 2007, 4955–4957
This journal is ß The Royal Society of Chemistry 2007

View Article Online
In conclusion, we have prepared two robust coordination
compound hosts able to bind a range of anions forming both 1 : 1
and 2 : 1 host : guest complexes. These compounds exhibit a novel
equilibrium in which anion binding promotes time-averaged
equivalence of Ha and Hb. This synergic situation offers a new
and, to our knowledge, hitherto unrecognised and unutilised way
to fine-tune and monitor anion binding and selectivity. Work is
currently in progress on fluorescent sensing analogues of these
compounds.
We thank the EPSRC for a studentship (SJD) and one of the
referees for very useful comments.
Notes and references
1 P. D. Beer and E. J. Hayes, Coord. Chem. Rev., 2003, 240, 167.
2 P. D. Beer and S. R. Bayly, Top. Curr. Chem., 2005, 255, 125.
3 J. W. Steed, Chem. Commun., 2006, 2637.
4 C. R. Rice, Coord. Chem. Rev., 2006, 250, 3190.
5 M. S. Goodman, V. Jubian and A. D. Hamilton, Tetrahedron Lett.,
1995, 36, 2551.
6 C. R. Bondy, P. A. Gale and S. J. Loeb, Chem. Commun., 2001, 729.
7 C. R. Bondy, P. A. Gale and S. J. Loeb, J. Am. Chem. Soc., 2004, 126,
5030.
8 D. R. Turner, B. Smith, A. E. Goeta, I. R. Evans, D. A. Tocher,
J. A. K. Howard and J. W. Steed, CrystEngComm, 2004, 6, 633.
9 D. R. Turner, E. C. Spencer, J. A. K. Howard, D. A. Tocher and
J. W. Steed, Chem. Commun., 2004, 1352.
10 D. R. Turner, B. Smith, E. C. Spencer, A. E. Goeta, I. R. Evans,
D. A. Tocher, J. A. K. Howard and J. W. Steed, New J. Chem., 2005,
29, 90.
11 D. R. Turner, M. Henry, C. Wilkinson, G. J. McIntyre, S. A.
Mason, A. E. Goeta and J. W. Steed, J. Am. Chem. Soc., 2005, 127,
11063.
12 L. Ion, D. Morales, J. Pe´rez, L. Riera, V. Riera, R. A. Kowenicki and
M. McPartlin, Chem. Commun., 2006, 91.
13 S. Nieto, J. Pe´rez, L. Riera, V. Riera and D. Miguel, New J. Chem.,
2006, 30, 838.
14 L. P. Harding, J. C. Jeffery, T. Riis-Johannessen, C. R. Rice and
Z. T. Zeng, Dalton Trans., 2004, 2396.
15 K. J. Wallace, R. Daari, W. J. Belcher, L. O. Abouderbala,
M. G. Boutelle and J. W. Steed, J. Organomet. Chem., 2003, 666, 63.
16 M. A. Bennett and A. K. Smith, J. Chem. Soc., Dalton Trans., 1974,
233.
Fig. 2 1H NMR spectra of the methylene region of host 2a as a function
of (a) added NBu4+Br2 and (b) added NBu4+CF3SO32 (bottom to top 0,
0.2, 0.5, 1.0 and 5.0 equivalents of anion). In the absence of added anion,
the methylenic protons Ha and Hb are inequivalent and appear as a
geminal AB quartet with added coupling to the NH proton just visible.
Addition of strongly bound anions such as Br2 results in the collapse of
the resonance to a singlet after ca. one equivalent. Analogous titration
2
with weakly bound anions such as CF3SO3 results in the persistence of
the AB quartet to at least 5 equivalents of added anion.
17 G. Aullo´n, D. Bellamy, L. Brammer, E. A. Bruton and A. G. Orpen,
Chem. Commun., 1998, 653.
18 L. Brammer, E. A. Bruton and P. Sherwood, Cryst. Growth Des., 2001,
1, 277.
19 R. Aronson, M. R. J. Elsegood, J. W. Steed and D. A. Tocher,
Polyhedron, 1991, 10, 1727.
20 M. Toma, A. Sanchez, E. Castellano, I. Berdan and M. S. Garcia-
Tasende, Rev. Chim. (Bucharest, Rom.), 2003, 54, 476.
21 B. N. Chaudret, D. J. Cole-Hamilton, R. S. Nohr and G. Wilkinson,
J. Chem. Soc., Dalton Trans., 1977, 1546.
leading to increased lability of the coordinated chloride ligand as a
result of electrostatic repulsion, such that in the presence of tightly
bound anions X2 the complex is rapidly interconverting between
the 18-electron [Ru(g6-C6H4MeCHMe2)Cl(L1/2)2]+?X2 and the
transient 16-electron [Ru(g6-C6H4MeCHMe2)(L1/2)2]2+?X2?Cl2.
Sixteen-electron Ru(II) compounds such as [RuCl2(PPh3)3] are
well known in the literature.21 At present this mechanism remains
unproven and is the subject of further study.
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 4955–4957 | 4957