This facile interconversion of dinuclear double helicates and side-by-side species: A reprogrammable ligand with potential sensor applications

Article abstract of DOI:10.1039/b600597g

The ligand L¹ forms a dinuclear double helicate with Cu⁺ but upon addition of Ba²⁺ to the system a side-by-side species is formed both in solution and in the solid state; in the presence of Na⁺ both the helicate and the side-by-side species are formed in roughly equal amounts in solution. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b600597g

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COMMUNICATION
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This facile interconversion of dinuclear double helicates and side-by-side
species: a reprogrammable ligand with potential sensor applications{
Georgios Bokolinis,a T. Riis-Johannessen,b Lindsay P. Harding,a John. C. Jeffery,b Neil McLaya and
Craig R. Rice*a
Received (in Cambridge, UK) 16th January 2006, Accepted 20th March 2006
First published as an Advance Article on the web 3rd April 2006
DOI: 10.1039/b600597g
The ligand L1 forms a dinuclear double helicate with Cu+ but
upon addition of Ba2+ to the system a side-by-side species is
formed both in solution and in the solid state; in the presence of
Na+ both the helicate and the side-by-side species are formed in
roughly equal amounts in solution.
The self-assembly of metallosupramolecular helicates from transi-
tion metal ions and multidentate N-donor ligands has gained
considerable attention in recent years.1 Recently we have
Scheme 1 Reagents: yields: a) penta(ethylene glycol) di-p-toluenesulfo-
demonstrated that the formation of helicate species can be
nate, DMF, NaH: 69%; b) mCPBA (0.9 equiv.), CH2Cl2: 56%; c)
controlled by a mixture of allosteric and electrostatic factors.
TMSCN, PhCOCl, CH2Cl2: 80%; d) H2S, Et3N, EtOH: 87%; e)
For example, we have shown that the ditopic ligand L which
a-bromoacetylpyridine, EtOH: 81%.
contains a potentially tetradentate pyridyl-thiazole ligand chain
crystals for which single crystal diffraction studies established the
formation of a dinuclear double helicate structure [Cu2(L1)2]2+
and an additional ‘‘external’’ crown ether binding site connecting
the two ligand halves can be reprogrammed by addition of s-block
metal ions. Thus upon reaction with Hg2+ the helicate species
[Hg2(L)2]4+ is formed. However, upon addition of either Sr2+ or
Ba2+ metal ions a mononuclear species results.2 This change in the
binding sites also changes the specificity towards different metal
ions.3 Furthermore, formation of a helicate from Cu2+ with a
similar type of ligand but which contains a potentially hexadentate
pyridyl-thiazole ligand chain results in a system where the pitch
length can be tuned dependent upon the size and charge of the
s-block metal ions present within the crown ether host.4 In this
paper we describe the first example of a ligand chain which forms a
dinuclear double helicate with Cu+ but upon addition of Ba2+ to
the crown ether moiety forms a side-by-side species both in
solution and in the solid state. In the presence of Na+ both species
are formed in roughly equal amounts in solution. The accom-
panying colour changes associated with the different species are
striking, leading to possible colorimetric sensor applications.
The ditopic ligand L1 was prepared from 2,29-bipyridine-
3,39-diol as outlined in Scheme 1. This ligand has a potentially
tetradentate pyridyl-thiazole/bipyridyl ligand chain with an
‘‘external’’ crown ether binding site bridging the bipyridyl moiety.
Reaction of L1 with [Cu(NCMe)4]PF6 in acetone afforded a dark
red solution and ESI-MS suggested the formation of a dinuclear
1
(Fig. 1).{ In the crystal, the ligand partitions into two bis-bidentate
binding domains, comprising pyridyl-thiazole and bipyridyl units,
with both Cu+ ions coordinated by two bridging ligands in a
‘‘head-to-tail’’ double stranded helicate arrangement. Both of the
copper centres have a distorted tetrahedral geometry that arises
from coordination of the pyridyl-thiazole ‘‘head’’ of the first ligand
and the bipyridyl N-donor ‘‘tail’’ of the second ligand. A
consequence of this binding mode is that it places the two crown
ether moieties on the same side of the molecule, leading to the
formation of a hydrophilic cavity or ‘‘molecular cup’’ which it was
anticipated might act as an efficient cation receptor site.
Treatment of a dark red MeCN solution of [Cu2(L1)2]2+ with
excess Ba(ClO4)2 afforded a pale yellow solution. The ESI-MS
+
species with an ion at m/z 1372 corresponding to [Cu2(L1)2]PF6 .
Slow diffusion of CH3CO2Et into an acetone solution of the
complex, in the presence of excess NH4BPh4, produced deep red
aDepartment of Chemical and Biological Sciences, University of
Huddersfield, Huddersfield, UK HD1 3DH. E-mail: c.r.rice@hud.ac.uk;
Fax: (+44)148-447-2182
bSchool of Chemistry, University of Bristol, Cantocks Close, Bristol,
UK BS8 1TS
{ Electronic supplementary information (ESI) available: experimental
details and H NMR spectra. See DOI: 10.1039/b600597g
Fig. 1 Crystal structure of the helicate cation 1 [Cu2(L1)2]2+
.
1
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Ba2+ complex retains a side-by-side structure. In contrast, addition
of excess Na+ ions to a MeCN solution of 1 only partially
quenches the deep red colour ascribed to the helicate, which
indicates that in this case there may be an equilibrium in solution
between red helicate and pale yellow side-by-side species.
Consistent with these observations, a 1H NMR spectrum
(MeCN, 295 K) of the Ba2+ complex 2 shows 10 aromatic
resonances as expected for a Ci symmetric side-by-side complex.
The latter resonances are reasonably sharp at room temperature
and there is no evidence for other species or exchange processes
occurring in solution.
Interestingly, the 1H NMR spectra in MeCN of the Na+
complex 3 and of its precursor 1, are both very broad at room
temperature, which suggests that exchange between helicate and
side-by-side species is taking place on the NMR timescale. At low
temperature (MeCN, 243 K) the exchange is to some extent frozen
Fig. 2 Crystal
structure
of
the
side-by-side
cation
2
[Cu2(L1)2Ba2](ClO4)4(MeCN)4
.
2+
showed an ion at m/z 949 corresponding to [Cu2(L1)2Ba2]
(ClO4)42+, thus confirming that a dinuclear metal structure is
retained whilst providing evidence for the incorporation of Ba2+
ions in the crown ether cavities. Crystallisation of the solution by
slow diffusion of CH3CO2Et gave pale yellow crystals, which
X-ray analysis revealed had the dinuclear structure [Cu2(L1)2Ba2]6+
2 (Fig. 2).{ The solid-state structure of 2 retains the head-to-tail
arrangement of ligands found in the precursor 1 and the pyridyl-
thiazole ends of the ligand chains still act as bidentate donors to
the Cu centres. However, in marked contrast to 1, the bipyridyl
units in 2 are twisted about their backbones (C–N–N–C dihedral
angle ca. 81u) such that only the terminal pyridyl units coordinate
to the Cu centres. The Cu atoms in 2 retain a pseudo tetrahedral
geometry by coordination of a molecule of MeCN and the
resultant ‘‘opened out’’ geometry of 2 may be best described as a
side-by-side complex. Both crown ether moieties incorporate Ba2+
ions that are bound by five of the six oxygen atoms of the crown
and further stabilised by interaction with two perchlorate
counteranions and a molecule of MeCN. The related reaction of
[Cu2(L1)2]2+ with excess Na(ClO4), following crystallisation,
afforded pale yellow crystals of [Cu2(L1)2Na2]4+ 3. The crystal
structure of 3 (not shown) is effectively isostructural with the Ba2+
complex 2, with Na+ ions replacing Ba2+ ions in the crown ether
cavities. Conversion of the helicate 1 to the side-by-side complexes
2 and 3 on addition of Ba2+ or Na+, respectively, may be primarily
ascribed to an allosteric effect. Coordination of the s-block cations
in the crown ether moieties tends to impose a non-coplanar
geometry on the bipyridyl ligands (C–N–N–C dihedral angle ca.
81u) making it energetically less favourable for them to act as
bidentate donors. As a result the crown ether domain is
allosterically reprogrammed by addition of s-block cations and a
side-by-side geometry is observed for the complexes in the solid
state.
1
1
out and the H NMR spectra of 1 and 3 are sharper. The H
NMR spectrum of 3 reveals two sets of ten, partially overlapping,
resonances in the aromatic region which are attributed to the
presence of both helicate and side-by-side species in equal
amounts. The sharper signals can be fully assigned by a
combination of chemical shift and 2D NMR techniques. These
signals correspond to the helicate species on the basis of their
relatively high field shift, common to helicate moieties due to
shielding by aromatic ring currents. The remaining set of 10
broader signals are attributed to the side-by-side structure. In
1
contrast the H NMR spectrum of 1 is still quite broad even at
243 K and the signals are correspondingly difficult to assign.
However, careful comparison with the spectrum of 3 does indicate
that two sets of 10 signals are present with one set of signals
substantially less intense. Thus it is clear that the two species are in
equilibrium. In the case of 1 the equilibrium favours the helicate
(helicate : side-by-side 3 : 1) whilst excess Na+ ions shift the
equilibrium in favour of the side-by-side structure (helicate : side-
by-side 1 : 1).
In summary, these results demonstrate that even in the absence
of s-block cations a MeCN solution of 1, which has a helicate
structure in the solid state, contains both helicate and side-by-side
species, which interconvert on the NMR time scale at room
temperature. In the presence of Na+ ions this equilibrium is
allosterically driven towards the side-by-side species, whilst
addition of highly charged s-block cations such as Ba2+, which
bind very strongly to the crown ether ligands, further shifts the
equilibrium so that only the side-by-side structure 2 is present in
both the solid state and solution. The marked colour changes
which occur on addition of s-block cations demonstrate the proof
of principle that these and related systems have potential for cation
sensing applications. Further investigations into their ability to act
as sensors are ongoing.
The interconversion of helicate and side-by-side structures does
not require ligand dissociation and could occur via an intra-
molecular rearrangement involving an untwisting of the helicate
structure with concomitant dissociation of two pyridyl donor
ligands. As such, this is likely to be a low energy process and both
1H NMR and UV/VIS spectroscopy provide evidence for facile
interconversion of helicate and the side-by-side species in solution.
Thus, addition of an excess of Ba2+ cations (3 equivalents) to a
MeCN solution of 1 completely quenches the deep red colour
associated with the helicate affording a pale yellow solution.5 This
suggests that both in solution and in the solid state, the pale yellow
We thank the EPSRC and the University of Huddersfield for
financial support
Notes and references
{ X-Ray single-crystal diffraction data were collected on a Bruker
PROTEUM-CCD area detector diffractometer under a stream of cold
nitrogen. Crystal data for 1(BPh4)2; M = 1866.74, Monoclinic, P21/c,
˚
a = 14.3802(3), b = 20.4058(4), c = 37.9889(7) A, b = 94.8080(10)u V =
11108.2(4) A , Z = 4, rc = 1.116 Mg m23, F(000) = 3904, m(CuKa) =
3
˚
1.275 mm21, T = 100 K. A total of 52309 reflections were measured in the
range 2.33 ¡ h ¡ 70.40u (hkl range indices 215 ¡ h ¡ 15, 223 ¡ k ¡24,
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20.849 eA23. CCDC reference number 285238. For crystallographic data
˚
244 ¡ l ¡ 38), 18266 independent reflections (Rint = 0.0599). The
structure was refined on F2 to Rw = 0.3838 R = 0.1439 (9000 reflections
with I > 2s(I)) and GOF = 1.346 on F2 for 741 refined parameters, largest
in CIF or other electronic format see DOI: 10.1039/b600597g
difference peak and hole 1.630 and 21.519 eA23.The crystal suffered from
˚
1 J.-M. Lehn, Supramolecular Chemistry, VCH, Weinheim, 1995;
J. W. Steed and J. L. Atwood, Supramolecular Chemistry, John Wiley
and Sons, Chichester, 2000; M. J. Hannon and L. J. Childs, Supramol.
Chem., 2004, 16, 7; M. Albrecht, Chem. Rev., 2001, 101, 3547;
M. Albrecht, Chem. Soc. Rev., 1998, 27, 281; C. Piguet,
G. Bernardinelli and G. Hopfgartner, Chem. Rev., 1997, 97, 2005;
E. C. Constable, in Comprehensive Supramolecular Chemistry, vol. 9;
Polynuclear Transition Metal Helicates, ed J.-P. Sauvage, Elsevier,
Oxford, 1996, pp. 213.
2 C. J. Baylies, L. P. Harding, J. C. Jeffery, T. Riis-Johannessen and
C. R. Rice, Angew. Chem., Int. Ed., 2004, 43, 4515.
3 C. J. Baylies, T. Riis-Johannessen, L. P. Harding, J. C. Jeffery, R. Moon,
C. R. Rice and M. Whitehead, Angew. Chem., Int. Ed., 2005, 44, 6909.
4 C. J. Baylies, J. C. Jeffery, T. A. Miller, T. Riis-Johannessen and
C. R. Rice, Chem. Commun., 2005, 4158.
rapid solvent loss on removal from the mother liquor, resulting in extensive
structural disorder and subsequent weak high angle data. Accordingly, only
the Cu, C, N and S atoms comprising the core of the complex moiety were
assigned anisotropic displacement parameters and refined without
positional restraints. The remaining fragments which were located in the
electron density map (crown ethers and tetraphenyl borate anions) were
very poorly resolved. Despite exhaustive attempts at modelling the disorder
in the crown ethers and tetraphenyl borate anions, the poor quality of the
data precluded complete parameterisation. Accordingly, the refinement
converged to a rather unsatisfactory final R-value of ca. 14% and the
resulting model does not merit detailed analysis beyond establishing the
basic atomic connectivity of the complex cation. Solutions obtained from
data collections on several different crystals were in agreement, but all
suffered from similar problems. CCDC reference number 285237. For
crystallographic data in CIF or other electronic format see DOI: 10.1039/
¯
5 The absorbance occurs ,350 nm, well within the ligand absorption
region. Addition of either Na+ or Ba2+ metal ions to 1 results in a decrease
in the absorption intensity in this region with the effect greatest for Ba2+.
The dark red colour observed is presumably due to MLCT to the p*
orbital of the planar bipyridine, which is usual for a Cu(I) system
coordinated by bipyridine ligand. Upon reprogramming of the ligand
strand the torsion angle of the bipyridine unit increases to 82u; with the
bipyridine unit no longer planar and acting as a mono-dentate ligand the
MLCT is quenched.
b600597g Crystal data for 2(ClO4)6?2MeCN; M = 2380.07, Triclinic, P1,
˚
a = 11.5874(7), b = 12.9425(8), c = 16.1980(10) A, a = 86.496(2), b =
3
23
˚
88.891(2), c = 66.515(2)u V = 2223.8(2) A , Z = 1, rc = 1.777 Mg m
,
F(000) = 1196, m(CuKa) = 10.266 mm21, T = 100 K. A total of 17363
reflections were measured in the range 2.73 ¡ h ¡ 70.15u (hkl range
indices 212 ¡ h ¡ 13, 215 ¡ k ¡ 15, 219 ¡ l ¡ 18), 7570 independent
reflections (Rint = 0.0440). The structure was refined on F2 to Rw
=
0.108222 R = 0.0414 (6354 reflections with I > 2s(I)) and GOF = 0.916 on
F2 for 588 refined parameters, largest difference peak and hole 1.342 and
1982 | Chem. Commun., 2006, 1980–1982
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