Heterotopic silver-NHC complexes: From coordination polymers to supramolecular assemblies

Article abstract of DOI:10.1039/c003352a

New coordination polymers based on different combinations of silver atoms and pyridyl-substituted N-heterocyclic carbene moieties are described. The addition of Zn(ii) templates leads to Ag-Zn supramolecular assemblies via selective Zn...N interactions; a process that can be reverted.

Full text of DOI:10.1039/c003352a

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Heterotopic silver–NHC complexes: from coordination polymers to
supramolecular assemblies†
Miguel Rubio,a Maxime A. Siegler,b Anthony L. Spekb and Joost N. H. Reek*a
Received 17th February 2010, Accepted 14th April 2010
First published as an Advance Article on the web 5th May 2010
DOI: 10.1039/c003352a
New coordination polymers based on different combinations
of silver atoms and pyridyl-substituted N-heterocyclic car-
bene moieties are described. The addition of Zn(II) tem-
plates leads to Ag–Zn supramolecular assemblies via selective
Zn ◊ ◊ ◊ N interactions; a process that can be reverted.
1
Since they were first reported by Olefe and Wanzlick,2 N-
¨
heterocyclic carbene (NHC) complexes have been extensively
studied,3 mainly after the isolation of the first stable NHC by
Arduengo.4 Due to their strong s donor ability, their high air and
moisture stability and their high affinity towards a wide range of
transition metals, NHCs have been successfully applied in several
homogeneous catalytic processes, such as olefin metathesis5 and
C–C cross-coupling reactions.6 These properties also make NHC
ligands particularly suitable for the incorporation into metal based
supramolecular architectures,7,8 but surprisingly NHCs have re-
ceived only little attention in this area: rigid bis-NHC-based main-
chain organometallic polymers have been described by Bielawski9
and a few examples of silver coordination polymers.10 These
examples involve exclusively metal–heteroatom or metal–carbon
interactions, but the combination of both has been less studied.11
In this contribution we report a ditopic pyridyl-substituted NHC
ligand that forms coordination polymers in the solid state and
supramolecular complexes in solution with zinc(II)porphyrins.
Whereas the NHC ligand remains coordinated to the silver atom,
the pyridine coordinates to silver in the coordination polymer
and to zinc in the assembly, leading to switchable supramolecular
structures.
Scheme 1 Synthesis of complexes 1–3.
2, used as a transfer agent15 in the synthesis of 3. Complex 2,
synthesized by the reaction of 1 with Ag2O (Scheme 1), was isolated
and crystallized‡ from a dichloromethane–diethyl ether mixture
and the solid state structure clearly showed the formation of a
coordination polymer (Fig. 1). The transoid ligand conformation
leads to an infinite one-dimensional double chain in which the
ligand acts as a spacer between two Ag(I) atoms via the carbene
and pyridyl donor groups in a “head-to-tail” orientation.16 The
parallel coordination polymers are connected via the silver atoms
through two bridging chlorine atoms per silver-pair. In addition,
there is a significant argentophilic attraction between Ag(I) atoms
We designed a heterotopic NHC ligand that can not act as a
bidentate ligand12 and therefore should be a proper building block
to produce larger well-defined supramolecular architectures. The
NHC building block 1 (Scheme 1) is a modification of previously
described13 pyridyl-substituted imidazolium salts. Complex 1
contains a meta-pyridine instead of an ortho-pyridine, which
prevents ligand coordination to metals in a chelating fashion. It
has been previously used in the preparation of rhodium complex
3a that has been applied, together with other related complexes
3 (Scheme 1), in “carbene polymerization” reactions.14 In the
current study, we focused our attention on the Ag compound
aVan’t Hoff Institute for Molecular Sciences, University of Amsterdam,
Nieuwe Achtergracht 166, 1018WV, Amsterdam, (The Netherlands).
E-mail: j.n.h.reek@uva.nl; Fax: +31 20 5255604; Tel: +31 20 5255265
bBijvoet Center for Biomolecular Research, Utrecht, (The Netherlands).
E-mail: a.l.spek@uu.nl; Fax: +31 30 2533940; Tel: +31 30 2532538
† Electronic supplementary information (ESI) available: Characterization
of compounds 2, 3, 4, 5A and 5B, GPC and UV-vis spectra of 2, and
UV-vis titration experiments for assemblies 5A and 5B. CCDC reference
numbers 773279–773281. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c003352a
Fig. 1 ORTEP diagram (top) of 2 with 50% probability ellipsoids and
different perspectives of the polymeric chains (bottom). Hydrogen atoms
are omitted for clarity.
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◦
˚
cations which are balanced with Ag(OTf)2 anions. The heterotopic
NHC ligand acts as a linker between inequivalent and formally
charged Ag(I) centres in a “head-to-head” orientation, that is,
[Ag(I)]+ selectively bonded to C and a formal [Ag(I)]- to N. It is
noteworthy that the donor groups have relative syn orientation
upon coordination to each Ag(I) ions, as can be observed in
Table 1 Selected bond lengths (A) and angles ( )
2
4
5A
Ag(1)–C(1)
Ag(1)–Cl(1)
Ag(2)–O(1)
Ag–N(3)
Zn(1)–N(3)
C(1)–N(1)
C(1)–N(2)
C(2)–C(3)
2.1322(13)
2.6845(4)
—
2.3409(13)
—
1.3572(19)
1.3544(16)
1.345(2)
111.80(3)
—
2.081(3)
2.071(3)
2.3483(8)
—
2.639(2)
2.200(3)
—
1.348(3)
1.352(3)
1.351(4)
—
+
—
the disposition of Ag(carbene)2 moieties with mesityl groups
2.182(2)
1.348(4)
1.359(4)
1.326(5)
166.46(9)
—
in close proximity (despite steric interactions). As a result, the
+
Ag(NHC)2 moieties adopt “loop” arrangements interconnected
˚
by Ag(OTf)2 anions. The distances Ag1–C1 (2.081(3) A) and
C(1)–Ag(1)–Cl(1)
C(1)–Ag(1)–C(1¢)
N(3)–Ag(2)–N(3¢)
N(3)–Zn(1)–N(7)
˚
Ag2–N3 (2.200(2) A) are considerably shorter compared to those
177.15(10)
151.28(8)
˚
˚
distances in 2 (Ag1–C1 2.1322(13) A, Ag1–N3 2.3408(11) A).
The most interesting aspect of this structure is the presence of
2D undulating channels determined by the packing mode of the
polymeric chain.18 In a perpendicular view (y-axis), the Ag ions are
aligned (Ag+ ◊ ◊ ◊ Ag- ◊ ◊ ◊ Ag+), matching with a C2-symmetry axis,
—
—
—
93.21(8)
17
˚
as indicated by the distance Ag1–Ag1 of 3.0965(2) A. The
Ag(I) atoms are in a distorted tetrahedral environment where
the pyridine coordinates nearly perpendicularly to the plane
formed by the Ag(I) and chlorine atoms (N–Ag1–Ag1: 100.74(3)◦).
The Ag1–C1 bond (2.1322(13) A, Table 1) bisects the rest of
the coordination sphere. Additional p–p stacking arrangement
˚
˚
but the distances are too large (Ag1–Ag2 5.209 A and 8.734 A)
to consider any argentophilic interaction. However, electrostatic
interactions between ion pairs at this distance are still sufficiently
large to significantly contribute to the solid state structure. In addi-
tion, some C ◊ ◊ ◊ F, H ◊ ◊ ◊ F and H ◊ ◊ ◊ O intermolecular interactions
˚
˚
between pyridine rings (distance 3.532 A) of adjacent polymers are
˚
˚
˚
(C2–F1 3.151 A, H18A–F2 2.454 A, O1–H4A 2.705 A) involving
also present. Although 2 is sparingly soluble in dichloromethane,
the macrostructure is not retained in diluted solutions. According
to 1H-NMR, GPC and UV/vis studies,19 2 dissociates into silver–
NHC monomeric species.10 Most strikingly, a band at 260 nm in
the UV/vis absorption spectrum19 is observed, which indicates
that in diluted solutions of 2 the pyridine lone pair is not involved
in coordinative bonds.20
Since the chloride plays an important role in the overall solid
structure of 2, we anticipated that the cationic derivatives would
lead to different coordination polymers. The cationic form of 2 was
simply accessible by a reaction of 2 with silver triflate (Scheme 2).
The triflate salt (4) was crystallized‡ from a dichloromethane–
diethyl ether mixture to yield crystals suitable for X-ray diffraction
analysis. The solid state structure of 4 significantly differs from that
of 2. It also forms a 1D polymeric structure (Fig. 2) but it contains
two different silver atoms. The polymers consist of alternating
silver atoms, one surrounded by two NHC carbene ligands and the
other by two pyridyl ligands. The structure contains Ag biscarbene
triflate groups, that may be of importance, are observed.
The binding properties of this heterotopic NHC can also be ap-
plied in the design of heterometallic supramolecular architectures
as it was found previously in the Rh assemblies with different Zn(II)
templates.14 This self-assembly approach has been applied by our
group in homogeneous catalysis to make bidentate ligands21–23 and
to encapsulate metal complexes.24 The above mentioned pyridine
lability in 2 enables the construction of assemblies with Zn(II)–TPP
(A) and Zn(II)–salphen (B). Thus, upon addition of the template to
a solution of 2, the pyridine coordinates selectively to Zn forming
5A and 5B in quantitative yields (Scheme 3). Despite the use of
Ag–NHC as transfer agents and the existence of Zn–NHC
complexes,25 products derived from the coordination of carbene to
Zn were not observed in these reactions. Assemblies 5A and 5B are
stable both in solution and in the solid state and, interestingly, their
solubilities are increased as compared to the poor solubility of 2.26
An X-ray diffraction study of a suitable crystal of 5A‡ revealed the
structure of the Zn(II)–TPP assembly based on Ag–NHC (Fig. 3).
Scheme 2 Synthesis of compound 4.
Fig. 3 ORTEP view (a) of 5A with 50% probability ellipsoids and
perspective (b). Hydrogen atoms and solvent molecule are omitted for
clarity.
Fig. 2 ORTEP diagram with 50% probability ellipsoids of 4.18 Hydrogen
atoms are omitted for clarity.
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The NRSCC and the Spanish Ministerio de Ciencia e Inno-
vacio´n and FECYT are acknowledged for financial support, Dr H.
Peters for mass spectroscopy analyses, E. Jellema for GPC analysis
and J. Iglesias-Sigu¨enza and J. M. Lassaletta for preliminary
catalytic studies on 1,3-dipolar cycloaddition reactions.
Notes and references
‡ Crystal data for 2: C18H19AgClN3, M = 420.68, triclinic, space group
◦
¯
˚
˚
˚
P1, a = 8.7708(2) A, b = 9.4691(2) A, c = 11.1568(3) A, a = 86.174(2) ,
◦
◦
3
˚
b = 78.509(2) , g = 66.335(2) , V = 831.57(4) A , Z = 2, T = 150 K,
Dc = 1.680 g cm-3, m(Mo-Ka) = 13.75 cm-1, Mo-Ka radiation (l =
˚
0.71073 A), F(000) = 424.0, R1 = 0.0153, wR2 = 0.0393. Crystal data
for 4: C19H19AgF3N3O3S, M = 534.30, monoclinic, space group C2/c, a =
Scheme 3 Synthesis of assemblies 5A and 5B.
◦
˚
˚
˚
19.9067(13) A, b = 13.9335(7) A, c = 15.7007(5) A, b = 103.914(4) , V =
3
-3
-1
˚
4227.1(4) A , Z = 8, T = 110 K, Dc = 1.679 g cm , m(Mo-Ka) = 11.04 cm ,
˚
Mo-Ka radiation (l = 0.71073 A), F(000) = 2144, R1 = 0.0272, wR2 =
The molecule can be considered as a dimer connected by weak
0.0626. Crystal data for 5A: 2(C62H47AgClN7Zn) + C7H8, M = 2289.65,
˚
˚
˚
monoclinic, space group P21/c, a = 10.8301(4) A, b = 21.1321(7) A, c =
Ag ◊ ◊ ◊ Cl interactions (3.041 A) between heterometallic monomers
◦
3
˚
˚
24.3335(10) A, b = 101.511(2) , V = 5457.0(4) A , Z = 2, T = 110 K, Dc =
(Fig. 3(b)), where the C(1)–Ag–Cl(1) angle (166.46(9)◦) is slightly
-3
-1
˚
1.393 g cm , m(Mo-Ka) = 8.96 cm , Mo-Ka radiation (l = 0.71073 A),
deviated from linearity. The pyridine coordinates to the Zn atom
F(000) = 2348, R1 = 0.0403, wR2 = 0.0954.
˚
(Zn1–N3 2.182(2) A) almost perpendicularly to the porphyrin
¨
1 K. Olefe, J. Organomet. Chem., 1968, 12, P42.
plane (Npor–Zn1–N3: in the range 89–104◦). Notably, the NHC
transoid conformation maximizes the distance between Zn(II) and
Ag(I) ions. To the best of our knowledge, this is the first X-ray
structure of a supramolecular assembly of this type involving NHC
ligands reported to date.
2 H.-W. Wanzlick and H.-J. Scho¨nherr, Angew. Chem., Int. Ed. Engl.,
1968, 7, 141.
3 For reviews: (a) F. E. Hahn and M. C. Jahnke, Angew. Chem., Int. Ed.,
2008, 47, 3122; (b) N-Heterocyclic Carbenes in Synthesis. ed. S. P. Nolan,
Wiley-VCH, Weinheim, 2006; (c) W. A. Herrmann, Angew. Chem., Int.
Ed., 2002, 41, 1290.
4 A. J. Arduengo, III, R. L. Harlow and M. Kline, J. Am. Chem. Soc.,
1991, 113, 361.
5 (a) S. J. Connon and S. Blechert, Angew. Chem., Int. Ed., 2003, 42, 1900;
(b) H. Clavier, K. Grela, A. Kirschning, M. Mauduit and S. P. Nolan,
Angew. Chem., Int. Ed., 2007, 46, 6786.
The assembly formation was further investigated by NMR
and UV/vis.19 As previously described with analogous pyridine–
Zn(II) assemblies,27 the complex induced shift of the ortho-pyridyl
1
protons, observed in the H-NMR spectrum, is much higher in
5A (Dd = 5.0 ppm) than in 5B (Dd = 0.2 ppm). Apart from
this, UV/vis titration experiments showed binding constants of
Kass = 3.0 ¥ 103 M-1 for 5A; and Kass = 6.5 ¥ 105 M-1 for 5B.
The magnitudes of the binding are similar to that reported on
phosphine-assemblies23a,c and Rh–NHC complexes.14
Finally, it is worth mentioning that the dissolution of 5B in
a 5 : 1 mixture of dichloromethane–THF displayed a decrease
of about two orders of magnitude in the binding constant
(Kass = 7.4 ¥ 103 M-1). Furthermore, 2 completely precipitated
out when 5B was stirred in pure THF for few minutes. From
the above mentioned experiments, it appears that addition of
polar coordinating solvents weakens the Zn ◊ ◊ ◊ N interactions and,
finally, excess of coordinating solvents replaces Zn ◊ ◊ ◊ N bond,
reforming the Ag ◊ ◊ ◊ N interaction, which leads to the precipitation
of the insoluble polymer 2.
In summary, we report a novel interesting building block to
construct supramolecular architectures, based on non-chelating
heterotopic NHC-pyridyl ligand. This ligand forms unique coor-
dination polymers in combination with silver, providing materials
with alternating metals arrays; Ag+–Ag-. The addition of Zn(II)–
TPP to these complexes leads to pyridyl–zinc coordination, and as
a consequence to well-defined soluble supramolecular assemblies.
Changing the solvent makes this equilibrium reversible, facilitating
the controlled interconversion between the various structures.
Future studies include the application of these principles in
material science and catalysis. Preliminary results show that the
silver carbene complexes are active in several reactions, such as the
copper-catalyzed Michael addition of ZnEt2 to enones28 or the 1,3-
dipolar cycloaddition to imines for the synthesis of polysubstituted
prolines.29
6 E. A. B. Kantchev, C. J. O’Brien and M. G. Organ, Angew. Chem., Int.
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˚
17 The Ag1–Ag2 distance of 3.097 A is considerably lower than the sum
˚
of the van der Waals radii (3.44 A).
18 For other perspectives of 4: see ESI†.
19 See ESI†.
20 H. P. Stephenson, J. Chem. Phys., 1954, 22, 1077.
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5434 | Dalton Trans., 2010, 39, 5432–5435
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J. N. H. Reek, J. Am. Chem. Soc., 2004, 126, 4056; (d) J. N. H. Reek, M.
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