A new dimension in cyclic coinage metal pyrazolates: Decoration with a second ring of coinage metals supported by inter-ring metallophilic interactions

Article abstract of DOI:10.1021/ja211549e

When pyrazolate ligands with thioether chelate arms are used in cyclic coinage metal pyrazolates [Au(μ-pz)]_n, the inner gold ring can be framed with an outer silver ring to give novel heterometallic double-crowned complexes [AuAg(μ-L^x

Full text of DOI:10.1021/ja211549e

Communication
pubs.acs.org/JACS
A New Dimension in Cyclic Coinage Metal Pyrazolates: Decoration
with a Second Ring of Coinage Metals Supported by Inter-ring
Metallophilic Interactions
Ann Christin Jahnke, Kevin Propper, Catherine Bronner, Jorg Teichgraber, Sebastian Dechert,
̈
̈
̈
Michael John, Oliver S. Wenger, and Franc Meyer*
Institute of Inorganic Chemistry, Georg-August-University Gottingen, Tammannstr. 4, 37077 Gottingen, Germany
̈
̈
S
* Supporting Information
ABSTRACT: When pyrazolate ligands with thioether
chelate arms are used in cyclic coinage metal pyrazolates
[Au(μ-pz)]n, the inner gold ring can be framed with an
outer silver ring to give novel heterometallic double-
crowned complexes [AuAg(μ-Lx)(BF4)]4. They feature
short intramolecular in-plane Ag−Au interactions, are
stable as octanuclear species in solution, and show
promising luminescence properties.
Figure 1. Stacked pyrazolate trimers in staggered (A) and frontal (B)
modes based on linked bis(pyrazolato) ligands.
yrazoles are among the most popular building units in
ligand design, and cyclic complexes of monovalent coinage
P
While multidentate compartmental pyrazole ligands featuring
chelate arms are prominent scaffolds in biomimetic coordina-
tion chemistry and catalysis,8 they have rarely been used for
assembling cyclic Cu(I), Ag(I), and Au(I) complexes.9−11 We
reasoned that a single soft donor site such as a thioether
provided within the R substituents (ligands HLa−c in Figure 2)
metal ions represent benchmark systems in pyrazolate
coordination chemistry.1,2 These oligonuclear complexes may
assume trimeric, tetrameric, or hexameric structures depending
on the steric and electronic properties of the substituents on
the pyrazole ring, with nine-membered macrocycles [M(μ-
pz)]3 being the most prominent and most common ones.2,3
Such homoleptic pyrazolates of Cu(I), Ag(I), and Au(I) have
attracted significant interest in recent years, not only because of
their structural diversity but also because of their propensity to
form tunable supramolecular aggregates and their interesting
luminescence properties. Fluorinated pyrazolate ligands (e.g.,
with CF3 groups in the 3- and 5-positions) have proven
particularly valuable in this regard,4 as they may form layered π-
acid/π-base stacks with various arenes that give reversible
luminescence responses.5
The photophysical properties of [M(μ-pz)]n metallacycles
are strongly dependent on intramolecular and especially on
intermolecular metallophilic d10−d10 interactions.6 The latter
usually are encountered perpendicular to the metallacyclic
plane, leading to face-to-face aggregates of the trinuclear units.
This recently led to the targeted design of hexanuclear stacked
pairs of [M(μ-pz)]3 trimers based on linked bis(pyrazolato)
ligands in either the staggered (A) or frontal (B) mode (Figure
1).7 Here we introduce a new concept of extending the
dimension of the cyclic coinage metal pyrazolates by
surrounding the [M(μ-pz)]n core with a second peripheral
ring of monovalent coinage metals within the metallacyclic
plane. This is achieved through the synergetic effect of
appropriate donor substituents appended at the 3- and 5-
positions of the pyrazole heterocycle and in-plane metallophilic
interactions.
Figure 2. Pyrazolate ligands HLa−c with appended thioether
substituents and the new multinuclear coinage metal complexes 3a−c.
might bind additional monovalent coinage metal ions in the
periphery of the metallacycle without disrupting the central
[M(μ-Lx)]n core.
The three ligands HLa−c were chosen to evaluate the effect of
different backbone substituents on either the pyrazole C4 or the
side-arm sulfur donor; they were synthesized in close analogy
Received: December 10, 2011
Published: January 23, 2012
© 2012 American Chemical Society
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Communication
to previously reported procedures.9,12 Treatment with an excess
of Ag2O cleanly gave the silver(I) complexes [Ag(μ-Lx)]3 (1a−
c; Figure 3), and subsequent transmetalation of the isolated
Figure 4. (top) Top view and (bottom) side view of the molecular
structure of 3a. H atoms have been omitted for clarity. For atom
distances and bond angles, see the SI.
Figure 3. Molecular structures of (top) 1a and (bottom) 2a. H atoms
have been omitted for clarity. For atom distances and bond angles, see
the SI.
The four gold(I) ions in the inner circle are coordinated by
two pyrazolate N atoms in the typical linear fashion; they are
located in a square with Au···Au distances of 3.03−3.21 Å,
which are slightly shorter than those in 2a−c and in the range
of weak intramolecular interactions.13 In the saddlelike
tetrameric structure, the pyrazolate groups are disposed
alternately above and below the {Au4} square (see the bottom
panel of Figure 4). Each silver(I) ion in the outer circle is held
between the thioether functions of two adjacent pyrazole
ligands, and all of the silver ions come to lie roughly within the
{Au4} plane with short Ag···Au distances of 2.84−2.90 Å. These
latter values are clearly shorter than the sum of the van der
Waals radii13 and indicate significant metallophilic d10−d10
interactions.14 The S−Ag−S angles in 3a−c deviate from
linearity and were found to be in the range 143.3−165.7°, since
each silver atom is displaced toward one of the BF4− anions and
also slightly displaced toward the gold atom. The anions are
hosted between the flaps of the peripheral thioether
substituents, alternately above and below the plane formed by
the eight metal ions. The Ag−F distances of 2.48−2.67 Å
suggest weak but non-negligible coordination in the solid state.
It is interesting to note that the present homometallic
complexes [Au(μ-Lx)]3 all adopt trinuclear structures but
transform to octametallic complexes with an inner tetrameric
[Au(μ-Lx)]4 core upon addition of AgBF4. Saddlelike tetrameric
structures are less abundant than planar trimeric coinage metal
pyrazolates but can be induced by large pyrazole substituents
(e.g., tert-butyl).2,3 The approximate (noncrystallographic) D2d
symmetry in 3a−c is obviously imposed by symmetry
constraints required for accommodation of the outer circle of
metal ions: the alternate disposition of the thioether donors
above and below that plane requires an even number of
silver(I) complexes with AuCl(SMe2) led to the corresponding
gold(I) species [Au(μ-Lx)]3 (2a−c; Figure 3). Single-crystal X-
ray crystallography of 1a, 1c, 2a, and 2c revealed that all of
these complexes adopt the common trinuclear pyrazolate-
bridged motif [M(μ-Lx)]3 with the thioether side arms dangling
[Figure 3 and Figures S10−S13 in the Supporting Information
(SI)]. The central [M(μ-pz)]3 cores are almost planar, with
Ag···Ag distances of 3.47−3.63 Å in 1a and 1c and Au···Au
distances of 3.36−3.38 Å in 2a and 2c. Each of these values is
longer than the sum of the van der Waals radii, and the
intradimer d10−d10 interactions are probably weak if present at
all.13,14 The NMR spectra of 2a−c each showed a single set of
signals typical for a planar [Au(μ-pz)]3 motif; in particular, only
one narrow singlet at 4.01 ppm was observed for the hinge CH2
groups. Furthermore, the Hpz singlet in 2a and 2b was shifted
slightly downfield relative to the free ligand, while virtually no
signal shifts were observed for the thioether side arms,
confirming that they are not involved in metal coordination.
Pairs of dangling thioether substituents from adjacent
pyrazolate ligands in 2a−c appear to be well-positioned to
bind additional coinage metal ions in close proximity to the
inner gold(I) ions. Indeed, addition of stoichiometric amounts
of AgBF4 (3 equiv per [Au(μ-Lx)]3) in CH2Cl2 gave the sought-
after heteromultimetallic complexes [AuAg(μ-Lx)(BF4)]n (3a−
c). Single crystals could be obtained for all three derivatives,
and X-ray crystallography in all cases revealed the formation of
an oblate octanuclear species with a central [Au(μ-Lx)]4 core
decorated by an outer ring of four silver(I) ions (Figure 4 and
Figures S14−S16).
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Communication
pyrazole ligands in the ring, since the S−Ag−S axes are roughly
With complexes 3a−c in hand, preliminary investigations of
the luminescence properties of these novel double-crowned
coinage metal complexes were performed. First measurements
were carried out in a frozen 2-methyl-THF glass matrix at 77 K.
Under these experimental conditions, all of these complexes
exhibited luminescence upon exposure to UV radiation (Figure
perpendicular to the metal plane.
(CF3)2
A recent osmometry study of [Ag(μ-pz(CF ) )]3 [pz
=
3
2
3,5-bis(trifluoromethyl)pyrazolido] in toluene revealed the
presence of monomeric, dimeric, trimeric, and hexameric
species in a concentration-dependent dynamic equilibrium.15
To characterize the identity of the new complexes 3a−c in
solution, some crystals were dissolved in dichloromethane-d2
1
and investigated by NMR spectroscopy. Again, the H NMR
spectra of complexes 3a−c at room temperature contained
single sets of resonances, but here the side-arm signals were
substantially shifted (Figure 5). As expected for a saddle-shaped
Figure 6. Photoluminescence spectra of 3b. The emission spectrum
(blue) was detected after excitation at 280 nm, while the excitation
spectrum (dashed black) monitored the emission at 415 nm. The red
line is the absorption spectrum of 3b in 2-methyl-THF at room
temperature.
6 and Figures S5 and S6). The solid blue trace in Figure 6 is a
representative (normalized) luminescence spectrum detected
after excitation of compound 3b at 280 nm. Vibrational fine
structure with at least four members of a progression in an
∼1600 cm−1 mode is easily discernible, and this observation
appears to be consistent with coupling of the luminescence
transition to breathing modes of the aromatic moieties present
in the emissive complex.18 Analogous vibrational progressions
were observed for the other complexes investigated in this
work, albeit in attenuated (i.e., spectrally less well resolved)
form (Figures S5 and S6). Even though the exact nature of the
electronic transition responsible for the luminescence is unclear
at this point, the occurrence of the above-mentioned vibrational
progression points toward a non-negligible contribution from
ligand-localized excited states. Prior investigations of emissive
cyclic coinage metal complexes are in line with this
interpretation.3,11c The superposition of the excitation spec-
trum (dashed black line, detected at 415 nm) and the
absorption spectrum (solid red line) of 3b in the left part of
Figure 6 strongly supports our notion that complex 3b is
indeed the emissive species in the glassy matrix.
In conclusion, the present work adds a new turn to coinage
metal pyrazolate chemistry, since it demonstrates how the
popular and common [M(μ-pz)]n core can be framed by a
further ring of coinage metal ions supported by in-plane closed-
shell d10−d10 interactions. It suggests further investigations to
explore all of the homo- and heterometallic permutations of
coinage metal ions (Cu+, Ag+, Au+) in the inner and outer rings
as well as the use of pyrazolate ligands that have multiple donor
sites in their chelate arms for the targeted synthesis of high-
nuclearity complexes. More detailed studies of the lumines-
cence properties of the new complexes are on the way.
Figure 5. 1H NMR spectra of HLa, complexes 1a and 2a in
chloroform-d and complex 3a in dichloromethane-d2.
tetrameric [Au(μ-Lx)]4 structure, the CH2 groups yielded AB-
type patterns.11 One of the two diastereotopic protons
appeared to be broad and at 0 °C split into an 8 Hz doublet,
3
which could be assigned to JH,Ag coupling [see the SI for
1
variable-temperature H NMR and 109Ag heteronuclear multi-
ple bond correlation (HMBC) spectra]. The extraordinary
large coupling constant may be explained by a H−C−S−Ag
torsion angle that deviates on average only 10° from linearity.
Furthermore, the 19F diffusion-ordered spectroscopy
(DOSY) NMR spectrum of 3a showed a decrease in the
diffusion coefficient (5.5 × 10−10 m2 s−1) relative to 2a (7.0 ×
10−10 m2 s−1) (Figure S3), which is in agreement with the
increase in molecular size. Both the Ar−F and BF4− signals
displayed identical diffusion coefficients, indicating that the
BF4− counterions are tightly bound in the complex. This was
also confirmed by 1H,19F heteronuclear Overhauser effect
spectroscopy (HOESY) (Figure S4), which yielded several
short H···F distances that are in agreement with the crystal
structure. The combined NMR data confirmed that the
complexes [AuAg(μ-Lx)(BF4)]4 (3a−c) remain intact in
CD2Cl2 solution and retain the BF4− tightly associated with
the octametallic core, explaining the good solubility in relatively
nonpolar solvents.
The electrospray ionization (ESI) mass spectra of freshly
prepared solutions of complexes 3a−c in a 0.01:1 CH2Cl2/
MeCN mixture showed peaks at m/z values corresponding to
[L4Au4AgxCl(x−1)]+ (x = 2, 3);16 Field-desorption (FD) mass
spectrometry of 3a showed a peak for [L4Au4Ag2(BF4)]+. After
some time, only signals for [L3Au3] were discernible, indicating
that the peripheral silver(I) ions are rather labile and tend to
dissociate in coordinating solvents such as MeCN.17
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic procedures and complete experimental details;
selected NMR spectra; luminescence spectra of 3a, 3c, and
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(12) Teichgraber, J.; Leibeling, G.; Dechert, S.; Meyer, F. Z. Anorg.
Allg. Chem. 2005, 631, 2613.
(13) Bondi, A. J. Phys. Chem. 1964, 68, 441.
(14) (a) Schmidbaur, H.; Schier, A. Chem. Soc. Rev. 2008, 37, 1931.
(b) Katz, M. J.; Ramnial, T.; Yu, H.-Z.; Leznoff, D. B. J. Am. Chem. Soc.
̈
the free ligands; and crystallographic details (CIF) and ORTEP
plots for 1a, 1c, 2a, 2c, and 3a−c. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
́ ́
2008, 130, 10662. (c) Fernandez, E. J.; Lopez-de-Luzuriaga, J. M.;
■
Monge, M.; Olmos, M. E.; Puelles, R. C.; Laguna, A.; Mohamed, A. A.;
Fackler, J. P. Jr. Inorg. Chem. 2008, 47, 8069. (d) Yam, V. W.-W.; Lo,
K. K. W. Chem. Soc. Rev. 1999, 28, 323.
Corresponding Author
franc.meyer@chemie.uni-goettingen.de
Notes
(15) Krishantha, D. M. M.; Palepepitiya Gamage, C. S.; Schelly, Z. A.;
Rasika Dias, H. V. Inorg. Chem. 2008, 47, 7065.
(16) The chloride likely comes from adventitious trace amounts of
chloride present under the ESI-MS conditions.
(17) In some experiments, crystalline [Ag(MeCN)4]BF4 was
obtained from solutions of 3a−c in MeCN/CH2Cl2 mixtures.
(18) Bronner, C.; Wenger, O. S. Dalton Trans. 2011, 40, 12409.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by Georg-August Universitat and the Fonds
̈
der Chemischen Industrie is gratefully acknowledged. We thank
Dr. Regine Herbst-Irmer (Institute of Inorganic Chemistry at
the Georg-August-University) for help with some of the
crystallographic analyses and Dr. Holm Frauendorf (Institute
of Organic and Biomolecular Chemistry at the Georg-August-
University) for collecting the FD-MS data.
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