Self-assembly of molecular cylinders from polycarbene ligands and Ag I or AuI

Article abstract of DOI:10.1021/ja101490d

Tris- and tetrakis(imidazolium) salts react with Ag₂O to give cylindrical polynuclear silver(I) complexes such as the tetrasilver derivative [Ag₄(1)₂](PF₆)₄. The silver(I) complexes undergo transmetal

Full text of DOI:10.1021/ja101490d

Published on Web 03/11/2010
Self-Assembly of Molecular Cylinders from Polycarbene Ligands and AgI or AuI
Arnab Rit, Tania Pape, and F. Ekkehardt Hahn*
Institut fu¨r Anorganische und Analytische Chemie, Westfa¨lische Wilhelms-UniVersita¨t Mu¨nster,
Corrensstrasse 30, D-48149 Mu¨nster, Germany
Received February 20, 2010; E-mail: fehahn@uni-muenster.de
1
The formation of supramolecular structures via metal-directed
self-assembly has become a field of intensive research initiated with
the first report on dinuclear helicates obtained from CuI and 2,2′-
bipyridine.1 Subsequently, a large number of metallosupramolecular
structures have been described.2 Some of these can act as molecular
hosts by encapsulating small molecular guests, which might undergo
selective chemical transformations within the host molecule.3 Most
of the known “molecular containers” have been built from Werner-
type metal centers (i.e., metal centers coordinated by nitrogen and/
or oxygen donor atoms). Metallosupramolecular structures con-
taining ligands with carbon donor atoms are rare,2e although some
examples of such structures with bridging diisocyanides,4 acyclic
diaminocarbenes,5a or remote N-heterocyclic carbene (NHC)
ligands5b are known. Metallosupramolecular structures derived from
the ubiquitous NHC ligands6 have been unknown until recently.
In our search for macrocyclic ligands with carbene donor groups,7
we serendipitously prepared the first three-dimensional metallosu-
pramolecular assembly featuring exclusively M-C(NHC) bonds7b
from two hexa-NHC ligands and six silver(I) ions. This was
followed by the planned preparation of a molecular rectangle8 from
two 4,4′-bipyridine and two rigid ditopic benzobis-NHC ligands.9
In this contribution, we describe the use of in situ-generated poly-
NHC ligands for the generation of cylindrical structures containing
three or four silver atoms sandwiched between two tri- or tetra-
NHC ligands.
The tetraimidazolium salts H4-1(X)4 (X ) Br-, PF6-; Scheme
1) were prepared by the reaction of imidazole with 1,2,4,5-
tetrabromobenzene followed by alkylation of the remaining free
imine with n-butyl bromide and, for H4-1(PF6)4, anion exchange
with NH4PF6 (see the Supporting Information). Salt H4-1(Br)4 is
soluble only in methanol and dimethyl sulfoxide (DMSO), while
H4-1(PF6)4 is freely soluble in acetonitrile and acetone. These
solubility differences determine the choice of solvent for the
subsequent preparation of the silver complexes. The reactions of 4
equiv of Ag2O with 2 equiv of H4-1(Br)4 (in methanol) and 2 equiv
of H4-1(PF6)4 (in acetonitrile) yielded the cyclindrical tetrasilver
octacarbene complexes [Ag4(1)2](Y)4 (Y- ) [AgBr2]- and/or Br-)
and [Ag4(1)2](PF6)4, respectively, in ∼70% yield as white solids
(Scheme 1). While salt [Ag4(1)2](Y)4 is extremely sensitive toward
light and soluble only in methanol and DMSO, the hexafluoro-
phosphate salt [Ag4(1)2](PF6)4 is stable toward light and soluble in
acetonitrile and acetone.
coupling to both silver isotopes [dd, J(C-Ag107) ) 184.3 Hz,
1J(C-Ag109) ) 212.9 Hz].7b The positive-ion electrospray ionization
(ESI) mass spectra of both [Ag4(1)2](Y)4 and [Ag4(1)2](PF6)4
showed the mass of the cationic complex ion [Ag4(1)2]4+ as the
peak of highest intensity.
Scheme 1. Preparation of Compounds [Ag4(1)2](Y)4 (Y ) [AgBr2]-,
Br-, PF6-) and Synthesis of [Au4(1)2](PF6)4
Crystallization of [Ag4(1)2](Y)4 proved to be impossible because
of its sensitivity to light and the different anions present. Crystals
of [Ag4(1)2](PF6)4 ·2CH3CN were grown by slow diffusion of
diethyl ether into a saturated acetonitrile solution of the salt at room
temperature. The structure analysis (Figure 1) confirmed the
formation of the cylindrical tetrasilver octacarbene complex cation,
which exhibits some structural relationship to the assemblies of
metal ions between disk-shaped template ligands reported by
Shionoya.10
Formation of the carbene complexes [Ag4(1)2](Y)4 and
[Ag4(1)2](PF6)4 was confirmed by 1H and 13C NMR spectroscopy.
In the 1H NMR spectra, for example, the resonance for the C2-H
imidazolium proton in H4-1(PF6)4 (9.55 ppm) disappeared upon
complex formation. The imidazolium C2 resonance for H4-1(PF6)4
(137.68 ppm) was absent in the 13C NMR spectrum of
[Ag4(1)2](PF6)4, and a new carbene resonance was observed at
181.30 ppm. This resonance was found at 182.50 ppm in the 13C
NMR spectrum of [Ag4(1)2](Y)4, showing the rarely observed
Figure 1. Molecular structure of the tetracation [Ag4(1)2]4+ in
[Ag4(1)2](PF6)4 ·2CH3CN (hydrogen atoms have been omitted for clarity,
and only the first atom of each N-nBu substituent is depicted).
The metric parameters found in the [Ag4(1)2]4+ cation
[Ag-Ccarbene, 2.073(4)-2.089(4) Å; Ccarbene-Ag-Ccarbene, 175.2(2)-
178.5(2)°] fall in the ranges previously described for linear
dicarbene silver complexes.11 The four silver atoms form an
essentially planar rectangle featuring two short [Ag1-Ag2, 3.4032(1)
9
4572 J. AM. CHEM. SOC. 2010, 132, 4572–4573
10.1021/ja101490d  2010 American Chemical Society

C O M M U N I C A T I O N S
Å; Ag3-Ag4, 3.5207(2) Å] and two long [Ag2-Ag3, 5.8373(2)
Å; Ag1-Ag4, 5.9592(2) Å] Ag · · · Ag separations.
Silver NHC complexes are excellent carbene transfer agents.12
All four silver atoms in [Ag4(1)2](PF6)4 can be substituted by AuI
to give [Au4(1)2](PF6)4 in 65% yield without destruction of the
supramolecular structure. Complete conversion was achieved by
stirring a mixture of [Ag4(1)2](PF6)4 and 4 equiv of [AuCl(SMe2)]
in acetonitrile for 12 h (Scheme 1). Complex [Au4(1)2](PF6)4 was
characterized by NMR spectroscopy and mass spectrometry. The
13C NMR spectrum showed the resonance for the carbene carbon
atoms shifted only slightly to 183.91 ppm as a singlet. These
spectroscopic data agree well with data reported for simple gold
dicarbene complexes.11b
Figure 2. Molecular structure of the trication [Au3(2)2]3+ in
[Au3(2)2](PF6)3 ·0.5Et2O·1.5CH3CN·0.5(CH3)2CO (hydrogen atoms have
been omitted for clarity, and only the first atom of each N-Et substituent is
depicted).
Cl(SMe2)] to give the homonuclear AuI complexes with retention
of the three-dimensional structure. Current studies are focused on
the incorporation of additional metal ions (Au+) or small substrates
(acetylenes) into the metallosupramolecular assemblies.
Supramolecular structures similar to [M4(1)2]4+ (M ) AgI, AuI)
can also be prepared from tricarbene ligands. The trisimidazolium
salt H3-2(PF6)3 reacts with Ag2O in acetonitrile to give the trisilver
salt [Ag3(2)2](PF6)3 in an excellent yield of 86% (Scheme 2; also
see the Supporting Information). The NMR spectroscopic data for
[Ag3(2)2](PF6)3 [13C NMR: δ(Ccarbene) ) 181.19 ppm] resembled
those of the tetrasilver salt [Ag4(1)2](PF6)4. The ESI mass spectrum
showed the mass of cation [Ag3(2)2]3+ as the most intense peak.
Acknowledgment. The authors thank the Deutsche Forschungs-
gemeinschaft and the Fonds der Chemischen Industrie for financial
support. A.R. thanks the NRW Graduate School of Chemistry
Mu¨nster for a predoctoral grant.
Scheme 2. Preparation of Compound [Ag3(2)2](PF6)3 and
Transmetalation to [Au3(2)2](PF6)3
Supporting Information Available: Experimental details for the
synthesis of all compounds and X-ray crystallographic data (CIF) for
compounds [Ag4(1)2](PF6)4 · 2CH3CN and [Au3(2)2](PF6)3 · 0.5Et2-
O·1.5CH3CN·0.5(CH3)2CO. This material is available free of charge
via the Internet at http://pubs.acs.org.
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