A di-palladium urea complex as a molecular receptor for anions

Article abstract of DOI:10.1039/b509565d

A new di-nuclear palladium complex containing thiol-urea ligands has been prepared, structurally characterized and its interaction with anionic species studied in solution. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b509565d

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A di-palladium urea complex as a molecular receptor for anions{
Jorge A. Tovilla,ab Ramo´n Vilar*ac and Andrew J. P. Whiteb
Received (in Cambridge, UK) 6th July 2005, Accepted 5th August 2005
First published as an Advance Article on the web 5th September 2005
DOI: 10.1039/b509565d
hydrochloride—in which the thiol is protected as disulfide—and
phenylisocyanate (PhNCO) was first carried out to yield the urea
disulfide [–SCH2CH2NHC(LO)NHPh]2 (1). This species was then
reduced with PPh3 and protonated with HCl to yield the
corresponding thiol. The thiol was purified by forming the
insoluble lead thiolate Pb[SCH2CH2NHC(LO)NHPh]2 (2) which
was easily isolated by precipitation. Upon treating 2 with HCl the
thiol-urea ligand HSCH2CH2NHC(LO)NHPh (3) was isolated as
an analytically pure compound.
A new di-nuclear palladium complex containing thiol-urea
ligands has been prepared, structurally characterized and its
interaction with anionic species studied in solution.
The synthesis of molecular receptors that can selectively recognize
specific guests is a topic of current interest due to the important
applications that these species can have in the development of
sensors, probes and selective catalysts.1 Although initially more
attention was paid to receptors that could bind cations and neutral
species, over the past few years an increasing number of molecular
hosts for the selective recognition of anionic guests have been
developed.2,3 A particularly appealing approach for the design of
receptors for anions is to combine the structural and functional
properties of metal centres with the recognition capabilities of
hydrogen bonding groups.4–8 Not only do metal centres often
The
metalla-receptor
[Pd(dppp){m-SCH2CH2NHC(LO)-
NHPh}]2[CF3SO3]2 (4) was obtained from the reaction between
Pd(dppp)(CF3SO3)2 and one equivalent of the thiol in the presence
of NaOtBu. Complex 4 was crystallized out of this solution and
characterized by spectroscopic, analytical and structural methods
(see the ESI for experimental details of all the compounds
prepared in this work{). The 31P{1H} NMR spectra of 4 indicate
that there is only one type of phosphorus in the complex since
only a singlet was observed (at 11.0 ppm). The FAB(+) mass
spectrum of this compound showed strong peaks corresponding to
[4–CF3SO3]+. In order to establish the exact conformation of the
di-palladium complex and gain some insight into the host–anion
interactions, the single crystal X-ray structure of 4 was determined
(see Fig. 1).{
provide
a positive charge to the receptor (increasing the
electrostatic attraction between the host and the anionic guest)
but they also act as structural motifs to organize the hydrogen
bonding moieties for optimal binding. Furthermore, the metal can
confer to the receptor useful optical, electrochemical or catalytic
properties.
Herein we report a new di-palladium complex with urea-
containing ligands that binds several anions in dmso solutions. The
design of the metalla-receptor is based on two square-planar
palladium centres linked by bridging thiolate ligands that contain
urea groups as hydrogen bonding units (see Scheme 1).
The urea-containing ligand HSCH2CH2NHC(LO)NHPh was
prepared in three steps. The reaction between di-cystamine
Scheme 1
aInstitute of Chemical Research of Catalonia (ICIQ), Avgda. Pa¨ısos
Catalans 16, 43007 Tarragona, Spain. E-mail: rvilar@iciq.es;
Fax: +34 977 920 228; Tel: +34 977 920 212
bDepartment of Chemistry, Imperial College London, London, UK
SW7 2AZ
Fig. 1 The molecular structure of the complex cation in the structure of
4 showing the encapsulation of one of the disordered triflate anions (see
…
cInstitution of Advanced Studies and Research of Catalonia (ICREA),
Spain
text) by N–H O hydrogen bonding interactions. The hydrogen bonding
{ Electronic supplementary information (ESI) available: Synthetic proce-
dures for 1–5 and for the titration experiments. X-Ray structure figures for
4 and 5. See http://dx.doi.org/10.1039/b509565d
…
…
…
˚
geometries, [N O], [H O] (A), [N–H O] (u) are (a) 3.38(3), 2.53, 159;
(b) 3.16(2), 2.27, 173; (c) 3.30(3), 2.59, 136; (d) 3.23(3), 2.35, 165.
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Table 1 Association constants (log Ka)a in dmso solution
As expected, the dicationic complex has both of the thiolate
ligands on the same side of the central Pd2S2 ring (see Fig. 1 below
and Fig. S1 in the supplementary data{). This ring is folded about
Anion
4
2
PF62
SO4
…
y0
the S S vector, the two square-planar palladium coordination
2.4 (¡0.05)b
2.9 (¡0.05)
3.3 (¡0.09)
3.4 (¡0.03)
3.5 (¡0.08)
planes being inclined by ca. 35u. Of most interest is the
encapsulation of one of the triflate anions between two thiolate
…
Cl2
Br2
P(LO)(OH)(OPh)O2
H2PO4
ligand arms by virtue of N–H O hydrogen bonding from the
urea groups (see Fig. 1).9
2
a
Determination by 1H NMR spectroscopy monitoring the changes
in the chemical shift of the urea protons. Standard deviation
Interestingly, slight changes in the purification and crystal-
lization techniques employed to obtain crystalline samples of 4 led
to the formation of a different compound, namely the di-nuclear
complex Na{[(dppp)Pd(m-SCH2CH2NHC(LO)NHC6H5)]2[TfO]3}
(5) in which an Na+ cation (coming from the NaOtBu used to
deprotonate the thiol) and an extra triflate anion are included in
the structure. This was established by an X-ray crystallographic
study (see Fig. 2).§ As was seen in the structure of 4, in 5 the
thiolate ligands are again oriented on the same side of the Pd2S2
ring (Fig. S4), and the two palladium coordination planes are
inclined similarly (here by ca. 37u cf. 35u in 4). The presence of a
sodium cation, however, leads to a very different orientation of the
urea group within each thiolate ligand (Fig. 2). In the structure of 4
the N–H groups were oriented inwards to allow the formation of
b
corresponding to the fit of the experimental data to the calculated
curve.
interactions were investigated by 1H NMR spectroscopy. The
changes in chemical shift of the urea protons (both from the NH’s
and the phenyl group) of the complex were recorded upon adding
increasing amounts of the different anions. As can be seen in
Table 1, 4 indeed acts as a good molecular receptor for anionic
species. Although this complex is di-positively charged and hence
could bind two mono-charged anions independently, with the
model employed to fit the experimental titration data there is no
indication of the presence of two distinct binding constants.
As discussed before, the crystal structures of 4 and 5 show that
in the solid state the triflate anions hydrogen bond to the urea
groups of the metalla-receptor. To establish if these interactions
were retained in dmso solution, a dilution experiment of 4 and a
titration of the ‘‘free’’ thiol-urea 3 with triflate, were carried out.
Both of these experiments indicate that in dmso, there are no
significant hydrogen bonding interactions between triflate and the
urea groups of 3 and 4 (which is not surprising since dmso is
known to interact strongly with ureas while triflate binds to them
weakly).
N–H O hydrogen bonds to one of the triflate anions,10 whereas
…
here in 5 it is the carbonyl oxygens that point inwards to
encapsulate the sodium cation. The binding to the sodium cation is
…
supplemented by an Na p contact at ca. 3.26 A to the terminal
˚
…
phenyl ring of one of the thiolate arms (the Na centroid vector is
inclined by ca. 73u to the ring plane). It is thus clear that these
thiolate-urea ligands give the complex the versatility to act as a
host for both cationic and anionic guests.
To evaluate the anion binding abilities of the di-metallic
complex 4 in solution, titrations of the hosts with several anionic
species in deuterated dmso were carried out. The host–guest
The metalla-receptor shows a degree of selectivity towards
anions such as phosphate and phosphoester. Although the
discrimination between anions is not very high, there is a clear
22
trend in the binding abilities of 4: PF62 y CF3SO32 , SO4
,
2
Cl2 , Br2 y H2PO4 y P(LO)(OH)(OPh)O2.
In conclusion, a new type of metal-containing receptor for
anionic guests has been prepared and structurally characterized.
Due to the presence of phenylurea groups, under certain
conditions, the receptor is also able to bind simultaneously anions
and cations. We are currently carrying out further studies to
increase the selectivity of the receptor for phosphorylated species.
The authors wish to thank CONACYT (Me´xico) and ORS
(UK) for a studentship to J.A.T., the ICIQ Foundation and the
Ministerio de Educacio´n y Ciencia (Spain) for financial support,
and Johnson Matthey for a loan of palladium chloride. Prof. P.
Ballester is thanked for his help in calculating the association
constants and for useful discussions.
Notes and references
…
Fig. 2 The molecular structure of 5 showing the Na O interactions.
{ Crystal data for 4:[C72H74N4O2P4Pd2S2](CF3SO3)2?C4H8O, M51798.40,
˚
monoclinic, P21/c (no. 14), a 5 14.557(3), b 5 23.299(3), c 5 26.168(3) A,
Two of the three triflate anions are linked to the periphery of the thiolate
…
3
b 5 98.921(11) u, V 5 8768(2) A , Z 5 4, Dc 5 1.362 g cm23, m(Mo-
˚
arms by N–H O hydrogen bonds, whilst the third binds to the sodium
Ka) 5 0.644 mm21, T 5 293 K, pale brown/orange blocks; 12837
independent measured reflections, F2 refinement, R1 5 0.065, wR2 5 0.145,
7744 independent observed absorption-corrected reflections [|Fo| . 4s(|Fo|),
2hmax 5 47u], 1000 parameters. CCDC 276098. See http://dx.doi.org/
10.1039/b509565d for crystallographic data in CIF or other electronic
format.
cation (all three triflate anions are disordered, see text). The H-bonding
…
…
…
˚
geometries, [N O], [H O] (A), [N–H O] (u) are (a) 3.365(18), 2.50, 165;
(b) 2.785(14), 1.93, 158; (c) 3.086(12), 2.30, 146; (d) 2.901(12), 2.01, 172.
…
˚
The Na O contact separations (A) are (e) 2.511(8); (f) 2.412(8); (g)
2.509(11).
4840 | Chem. Commun., 2005, 4839–4841
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§ Crystal data for 5: [C72H74N4O2P4Pd2S2](Na)(CF3SO3)3?4C2H2Cl2?H2O,
¯
M 5 2256.07, triclinic, P1 (no. 2), a 5 13.819(4), b 5 15.492(3),
6 S.-T. Cheng, E. Doxiadi, R. Vilar, A. J. P. White and D. J. Williams,
J. Chem. Soc., Dalton Trans., 2001, 2239; P. D´ıaz, D. M. P. Mingos,
R. Vilar, A. J. P. White and D. J. Williams, Inorg. Chem., 2004, 43,
7597.
7 S. L. Tobey, B. D. Jones and E. V. Anslyn, J. Am. Chem. Soc., 2003,
125, 4026.
8 L. Fabbrizzi, A. Leone and A. Taglietti, Angew. Chem., Int. Ed., 2001,
40, 3066; B. Verdejo, J. Aguilar, A. Dome´nech, C. Miranda, P. Navarro,
H. R. Jime´nez, C. Soriano and E. Garc´ıa-Espan˜a, Chem. Commun.,
2005, 3086.
˚
c 5 23.828(4) A, a 5 103.025(9), b 5 96.742(18), c 5 99.811(18) u,
3
V 5 4832.6(19) A , Z 5 2, Dc 5 1.550 g cm23, m(Mo-Ka) 5 0.846 mm21
,
˚
T 5 183 K, pale yellow blocks; 12438 independent measured reflections, F2
refinement, R1 5 0.056, wR2 5 0.128, 8557 independent observed
absorption-corrected reflections [|Fo| . 4s(|Fo|), 2hmax 5 45u], 1130
parameters. CCDC 276099. See http://dx.doi.org/10.1039/b509565d for
crystallographic data in CIF or other electronic format.
9 This triflate anion is actually highly disordered, four separate
orientations having been identified in the crystal (Fig. S3). In each case
however, the three oxygen atoms are in approximately the same
position. Thus, the hydrogen bond geometries quoted in the figure
caption (which are for the major occupancy orientation) should be
regarded as indicative only.
10 Again the triflate anions are disordered (Fig. S6), two positions having
been located in each case. Only the major occupancy orientations are
drawn in Fig. 2 and described in the caption.
1 J. W. Steed and J. L. Atwood, Supramolecular Chemistry, John Wiley &
Sons, Chichester, 2000.
2 P. D. Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001, 40, 486.
3 Supramolecular Chemistry of Anions, ed. A. Bianchi, K. Bowman-James
and E. Garc´ıa-Espan˜a, Wiley-VCH, New York, 1997.
4 P. D. Beer and E. J. Hayes, Coord. Chem. Rev., 2003, 204, 167;
P. D. Beer, Acc. Chem. Res., 1998, 31, 71 and references therein.
5 C. R. Bondy, P. A. Gale and S. J. Loeb, J. Am. Chem. Soc., 2004, 126,
5030; C. R. Bondy, P. A. Gale and S. J. Loeb, Chem. Commun., 2001, 729.
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Chem. Commun., 2005, 4839–4841 | 4841