Anion complexation via C-H...X interactions using a palladacyclic receptor

Article abstract of DOI:10.1039/b801823e

A novel organometallic receptor binds anions in solution and in the solid state, with complexes stabilised through a series of C-H...X interactions, as evidenced by ¹H NMR spectroscopy, X-ray crystallography and computational models. The Royal Society of Chemistry.

Full text of DOI:10.1039/b801823e

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Anion complexation via C–Hꢀ ꢀ ꢀX interactions using a palladacyclic
receptorw
Robin B. Bedford,*a Michael Betham,a Craig P. Butts,a Simon J. Coles,b
Michael B. Hursthouse,b P. Noelle Scully,c James H. R. Tucker,*d John Wilkied
and Yasmine Willenerd
Received (in Cambridge, UK) 31st January 2008, Accepted 20th March 2008
First published as an Advance Article on the web 23rd April 2008
DOI: 10.1039/b801823e
A novel organometallic receptor binds anions in solution and in
the solid state, with complexes stabilised through a series of
three cations (d(31P) 132.1, 131.8 and 130.8 for 2a–c, respec-
tively) and the corresponding 1H NMR spectra show two
signals for the methylene protons associated with the coordi-
nated [9]aneS3 ligand (e.g. 2b: 2.60 ppm and 2.98 ppm), each
integrating to six protons. The upfield and downfield environ-
ments observed in each case correspond to the exo and endo
protons of the [9]aneS3 ring, respectively (confirmed by NOE
experiments) with the apparent equivalence of all the protons
in each environment due to rapid interchange of the S-donor
sites.
1
C–Hꢀ ꢀ ꢀX interactions, as evidenced by H NMR spectroscopy,
X-ray crystallography and computational models.
Although the C–Hꢀ ꢀ ꢀX hydrogen bond (where X is any
acceptor atom) is generally accepted as an important weak
interaction in the solid state,1 there are relatively few examples
of synthetic supramolecular host–guest complexes that are
stabilized by such interactions in solution.2–4 While some of
these complexes involve a combination of various H-bonding
interactions,3 others have the C–Hꢀ ꢀ ꢀX interaction as the sole
H-bonding motif,4 with the majority of these multiply charged
imidazolium systems.4c,d Here we report a novel anion recep-
tor incorporating a cationic organometallic palladacycle and
present evidence for it binding various anions in solution via
concerted C–Hꢀ ꢀ ꢀX hydrogen bonding interactions.
One important difference observed between the spectra of
the complexes in CDCl3 is that whereas the signals for both the
exo and endo [9]aneS3 protons of 2b match those of 2c (i.e. 2c:
2.59 ppm and 2.97 ppm, respectively), in the case of 2a, the
endo proton signal is shifted significantly downfield (2.51 ppm
and 3.62 ppm, respectively). This suggests that only in the
chloride complex is there a pronounced interaction in solution
between the anion and cation, with the chloride small enough
to be located in the cavity formed by the metal-bound [9]aneS3
ring. Further support for such an interaction was garnered by
examining the effect of adding aliquots of Bu4N+Clꢁ to a
solution of 2b in CDCl3 (ca. 20 mM), which induced a
significant downfield shift in the endo proton signal, until with
a slight excess of guest, the NMR spectrum resembled that of
2a. A similar effect was found upon the addition of Bu4NBr or
Bu4NI to 2b. Job plots confirmed the binding stoichiometry as
1 : 1 (Fig. 1).
As part of our interest in five-coordinate palladium(II)
complexes with orthometallated triaryl phosphite ligands,
and the effect that this coordination mode has on the bonding
of the cyclometallated ligand,5 the palladacycle 1 was treated
with two equivalents of the macrocycle 1,4,7-trithiacyclono-
nane ([9]aneS3) in dichloromethane. This led to the formation
of the chloride salt of five-coordinate palladacycle 2, complex
2a, in good yield.ꢁThe chloride counter-anion can be readily
ꢁ
replaced by SbF6 or PF6 by salt metathesis to give com-
plexes 2b and 2c (Scheme 1).
These data are consistent with the formation of a 1 : 1
complex between the cationic complex 2y and halide guests,
Halꢁ, with each anion situated in the cavity formed by the
The crystal structure of complex 2bz (see ESIw) shows the
five-coordinate palladium cation interacting with the SbF6
counter-anion through a series of C–Hꢀ ꢀ ꢀF interactions from
the methylene protons on the [9]aneS3 ligand.
The 31P NMR spectra of the complexes in CDCl3 reveal that
the P-donors are in essentially identical environments in all
a School of Chemistry, University of Bristol, Cantock’s Close, Bristol,
UK BS8 1TS. E-mail: r.bedford@bristol.ac.uk
b EPSRC National Crystallography Service, Department of
Chemistry, University of Southampton, Highfield Road,
Southampton, UK SO17 1BJ
c Department of Chemistry, Trinity College Dublin, Dublin 2, Ireland
d School of Chemistry, University of Birmingham, Edgbaston,
Birmingham, UK B15 2TT. E-mail: j.tucker@bham.ac.uk
w Electronic supplementary information (ESI) available: Experimental
procedures, X-ray data for complexes 2b and 2d, NMR binding data
and vapour pressure osmometry experiments. CCDC 662914 and
662915. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/b801823e
Scheme 1 Reagents and conditions: (i) [9]aneS3, CH2Cl2, r.t., 1 h; (ii)
AgSbF6 or NH4PF6, CH2Cl2, r.t., 1 h; (iii) Bu4NCl, CH2Cl2 r.t., 1 h.
ꢂc
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Pd-bound macrocycle (Scheme 2) and acting as an H-bond
acceptor to the six endo protons of the [9]aneS3 ligand.z
The titration experiments indicated that the binding
strength followed the trend Clꢁ 4 Brꢁ 4 Iꢁ. A slight
dissociation of the macrocycle from the chloride complex in
CDCl3 (vide infra) precluded the accurate determination of its
binding constant in this solvent. However, this effect was much
less marked for bromide and was not observable with iodide,
allowing a binding constant with Iꢁ to be determined as
K = 2.2 ꢃ 103 Mꢁ1 (ꢄ10%) (see ESIw).
Scheme 2 Complexation of halide ions, Halꢁ by cation 2 (initial
counter-ion is SbF6, complex 2b).
The partial dissociation of the chloride complex 2a in
CDCl3 was revealed by the appearance of signals for free
[9]aneS3, with the ratio of free to coordinated ligand at 25 1C
approximately 15 : 85 ([2a] = 38 mM). Furthermore, the 31P
NMR spectrum of this solution showed an additional small
singlet at 124.0 ppm, corresponding to the complex 3, which
could also be made directly from 1 (Scheme 1). However in the
presence of a slight excess (1.25 equiv.) of [9]aneS3, 2a was
observed as the sole phosphorus containing species in solution.
The extent of dissociation was also increased slightly by
dilution and by changing the solvent to CD3CN. These find-
ings indicate that 2a is in equilibrium with species 2 and 3 in
solution, as shown below in Scheme 3.
with the observation from the NMR studies that only the
signal from these protons is affected by complexation. The
range of Clꢀ ꢀ ꢀH distances in 2d (2.55–2.95 A) fall within the
sum of the van der Waals radii and can be considered as weak
hydrogen bonds.1 The extent of aggregation of complex 2d in
solution was investigated by vapour pressure osmometry (see
ESIw) in benzene solution, which indicates that in solution, it is
the ion-pair dimer that predominates in solution, rather than
the tetramer.
In order to test whether the close Clꢀ ꢀ ꢀH contacts in the
X-ray structure of 2d were indeed genuine interactions rather
than a consequence of crystal packing, the core complex (i.e. a
dimer with the two uncoordinated aryl groups and the two tBu
groups on the coordinated aryl of each ion replaced with
hydrogens) was optimised using the Guassian03 program,6
LANL2DZ7 basis and MPW1PW918 functional. The resulting
complex retained the close contacts between chloride and the
endo C–H protons, with three such contacts being less than
2.5 A, which in fact are closer than those observed in the X-ray
structure.8 Alternative arrangements of the anion 3 with
respect to cation 2 that would rely on through space interac-
tions alone were then optimised. However even though shorter
Pd–Pd and Pd–Cl distances were observed in some cases, all
complexes optimised with a higher energy overall. Thus it
would appear that there is an attractive interaction local to the
close C–Hꢀ ꢀ ꢀCl contacts that helps to stabilise the complex 2d.
Although these contacts fall comfortably within the sum of the
van der Waals radii, the calculated change in C–H bond length
in 2d compared to 2 alone is very small (increasing by an
average of 0.002 A for each of the three sub 2.5 A contacts),
which is consistent with C–Hꢀ ꢀ ꢀX H-bonding interactions
being relatively weak.
Complex 2d, which consists of cation 2 and its counter-
anion 3, could be made in excellent yield by the addition of one
equivalent of [9]aneS3 to the dimer 1 in benzene. Whereas in
CDCl3, the complex is dissociated to some extent, its 1H NMR
spectrum in C6D6 ([2d] = 28 mM) gives rise to one set of
signals, with the endo proton signal shifted downfield com-
pared to 2b in the same solvent (3.51 ppm vs. 2.66 ppm,
respectively). This data is consistent with a strong pairing
interaction between 2 and 3 in this solvent. As anticipated, its
crystal structurez consists of an H-bonded complex between 2
and 3, but with two identical units of each species in the unit
cell, assembled in a tetrameric array (Fig. 2).
The two metal-bound chlorides on each molecule of 3 are
oriented towards a proximate [9]aneS3 ligand of one of the
five-coordinate cations. Each chloride forms three hydrogen
bonds with separate endo hydrogens on this ligand, while both
chlorides in the same complex form close contacts with one
endo hydrogen of the more remote [9]aneS3. The fact that the
close contacts occur with only the endo protons is consistent
In summary, these studies indicate that the new cationic
palladium-containing thiacrown species 2 is able to complex
various anions in solution, which appear to be mediated
by C–Hꢀ ꢀ ꢀX H-bonds. Complexation of either free halides
Fig. 1 1H NMR Job plots in CDCl3 for 2b and Bu4NBr (total conc.
= 5.17 mM, blue squares), Bu4NI (total conc. = 5.07 mM, pink
circles) and Bu4NCl (total conc. = 4.75 mM, brown triangles).
Scheme 3 Equilibrium between complexes 2a and 2d in solution.
ꢂc
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ment on 2b (20 mM to 1 mM) in CDCl3 gave no evidence for such an
interaction.
z Signals for the NCH2 protons of the Bu4N+ counter-cation also
shifted slightly upfield upon complexation, presumably due to appre-
ciable ion-pairing within the guest salt in this solvent. Studies with
tetraphenylphosphonium halide salts, which are expected to be dis-
sociated in solution to a greater extent, revealed very similar (but
stronger) halide binding trends, but were hampered by solubility
problems. For an in-depth discussion on the effect of counter-cations
on anion binding, see ref. 9.
8 The orientation of complex 3 with respect to the thiamacrocycle in 2
in the optimised structure of the dimer was found to be closer to
perpendicular than that observed in the X-ray structure of the tetra-
mer, reflecting the absence of packing effects present in the solid state,
which would be similarly absent in solution.
1 For recent review articles, see: (a) T. Steiner, Angew. Chem., Int.
Ed., 2006, 45, 6920; (b) G. R. Desiraju, Chem. Commun., 2005,
2995.
2 For a recent theoretical study of C–Hꢀ ꢀ ꢀAnion binding, see: V. S.
Bryantsev and B. P. Hay, Org. Lett., 2005, 7, 5031.
3 For examples of supramolecular complexes stabilized by C–Hꢀ ꢀ ꢀX
bonds amongst other H-bonding interactions, see: (a) C. Fujimoto,
Y. Kusunose and H. Maeda, J. Org. Chem., 2006, 71, 2389; (b) L.
Ion, D. Morales, S. Nieto, J. Perez, L. Riera, V. Riera, D. Miguel,
R. A. Kowenicki and M. McPartlin, Inorg. Chem., 2007, 46, 2846;
(c) M. R. Sambrook, P. D. Beer, J. A. Wisner, R. L. Paul, A. R.
Cowley, F. Szemes and M. G. B. Beer, J. Am. Chem. Soc., 2005,
127, 2292; (d) J. W. Steed, Chem. Commun., 2006, 2637; (e) P. S.
Lakshminarayanan, D. K. Kumar and P. Ghosh, Inorg. Chem.,
2005, 44, 7540.
4 For examples of supramolecular complexes for which C–Hꢀ ꢀ ꢀX
interactions represent the only H-bonding motif, see: (a) Z. R.
Laughrey, T. G. Upton and B. C. Gibb, Chem. Commun., 2005,
970; (b) C. L. D. Gibb, E. D. Stevens and B. C. Gibb, J. Am. Chem.
Soc., 2001, 123, 5849; (c) V. Amendola, M. Boiocchi, B. Colasson,
L. Fabbrizzi, M. J. R. Douton and F. Ugozzoli, Angew. Chem., Int.
Ed., 2006, 45, 6920; (d) For a review of imidazolium receptors, see:
J. Yoon, S. K. Kim, N. J. Singh and K. S. Kim, Chem. Soc. Rev.,
2006, 35, 355 and references therein; (e) S. J. Loeb, J. Tiburcio, S. J.
Vella and J. A. Wisner, Org. Biomol. Chem., 2006, 4, 667.
5 R. B. Bedford, M. Betham, C. P. Butts, S. J. Coles, M. Cutajar, T.
Gelbrich, M. B. Hursthouse, P. Scully and S. Wimperis, Dalton
Trans., 2007, 459.
Fig. 2 Crystal structure of complex 2d showing the tetranuclear motif
supported by hydrogen bonds between chlorides and [9]aneS3.
(i.e. Clꢁ in 2a), or metal-bound halides (in the form of the
palladium dichloride complex 3) can occur, and in the latter
case, such an interaction is supported by crystallographic and
modelling studies. It appears that the Pd(II) centre plays an
important role in the anion binding process; firstly to pre-
organize six of the methylene protons on the [9]aneS3 ring in a
convergent manner and secondly to render them sufficiently
acidic (partially through the effect of the metal charge) so that
they are effective H-bond donors in solution. We anticipate
that the coordinated thiaazacrown motif has considerable
promise in the development of novel H-bonding tectons and
receptors for crystal engineering and supramolecular chemis-
try, respectively.
6 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N.
Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V.
Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R.
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J.
B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
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Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J.
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Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P.
Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A.
Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M.
W. Gill, B. G. Johnson, W. Chen, M. W. Wong, C. Gonzalez and
J. A. Pople, GAUSSIAN 03 (Revision C.02), Gaussian, Inc.,
Wallingford, CT, 2004.
We thank the EPSRC for funding and for the provision of
an Advanced Research Fellowship (R. B. B.), the RSC for the
provision of the Sir Edward Frankland Fellowship (R. B. B.)
and Johnson Matthey for the loan of palladium salts.
Notes and references
z Crystal data: Complex 2b: [C42H68O3PPdS3]SbF6ꢀ4C6H6,
l =
ꢀ
0.71073 A, space group P1, a = 12.32(2), b = 12.719(13), c =
22.10(5) A, a = 83.91(11), b = 79.8(2), g = 78.56(8)1, V =
3332(10) A3, Z = 2, m = 0.853 mmꢁ1, 73574 data were collected of
which 15255 were independent. The structure was refined on F2 to give
R1 = 0.0757 (F2 4 2s(F2)) and wR2 (all data) = 0.1868. CCDC
662914.
Complex 2d: [C42H63Cl2O3PPd][C48H74O3PPdS3]ꢀCD3CN,
l =
ꢀ
0.71073 A, space group P1, a = 15.297(4), b = 18.455(4), c =
19.724(7) A, a = 108.554(19), b = 107.430(19), g = 98.853(15)1, V
= 4842(2) A3, Z = 2, m = 0.572 mmꢁ1, 36518 data were collected of
which 21041 were independent. The structure was refined on F2 to give
R1 = 0.0549 (F2 4 2s(F2)) and wR2 (all data) = 0.1259. CCDC
662915.
7 P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 270.
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P. Perdew, K. Burke and Y. Wang, Phys. Rev. B: Condens. Matter
Mater. Phys., 1996, 54, 16533.
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Schmidtchen, G. W. Bates, M. E. Light and P. A. Gale, J. Am.
Chem. Soc., 2006, 128, 12281.
y A weak H-bonding interaction between the SbF6 anion and receptor
2 in solution cannot be ruled out, although an NMR dilution experi-
ꢂc
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Chem. Commun., 2008, 2429–2431 | 2431