Silver and gold complexes with a new 1,10-phenanthroline analogue n-heterocyclic carbene: A combined structural, theoretical, and photophysical study

Article abstract of DOI:10.1002/chem.201200465

Unusual coordination for gold: An imidazolium salt was synthesized and used as a precursor for an N-heterocyclic carbene, which can be considered as the carbene analogue of 1,10-phenanthroline. Like the diimine congener, this ligand gives luminescent metal complexes. Remarkably, the Au^III complex features a gold atom in an unusual environment: it is surrounded by six donor atoms, two of which interact electrostatically with the Au atom (see figure).

Full text of DOI:10.1002/chem.201200465

DOI: 10.1002/chem.201200465
Silver and Gold Complexes with a New 1,10-Phenanthroline Analogue N-
Heterocyclic Carbene: A Combined Structural, Theoretical, and
Photophysical Study
Margit Kriechbaum,[a] Manuela List,[b] Raphael J. F. Berger,[c] Michael Patzschke,[d] and
Uwe Monkowius*[a]
Transition-metal complexes with 1,10-phenanthroline
(phen) ligands frequently reveal interesting photo- and elec-
troluminescence properties, and thus are well suited for op-
toelectronic applications (organic light-emitting diodes
(OLEDs) and sensors).[1,2] On the other hand, N-heterocy-
clic carbenes (NHCs) are strongly s-donating ligands that
form very stable complexes with favorable photophysical
properties.[3] Furthermore, they show advantages as ligands
in various catalytically active transition-metal complexes.[4]
Therefore, the combination of both, the unique coordination
geometry of phen-type ligands and the electronic properties
of NHCs is a worthwhile synthetic goal.
The substitution of one or two pyridine moieties by azoli-
um moieties gave the ligand precursors A or B, respectively
(Scheme 1). We chose structure A because of its smaller
bite angle compared with B and the presence of two differ-
ent donor atoms combining the properties of both an imine
and a carbene function. The ligand precursor B and the co-
ordination chemistry of the respective biscarbene has been
reported just recently.[5]
Because the synthesis of gold complexes is straightfor-
ward, we chosen them as model substrates for probing the
ligand properties of the carbene ligand derived from struc-
ture A.[6,7,8] The most common oxidation states of gold are
+1 and +3 with linear or square-planar coordination geo-
metries, respectively. AuI complexes can in some cases
extend their coordination number to four or might be fur-
ther aggregated by aurophilic interactions,[9] whereas AuIII is
known to form square-pyramidal complexes as has been
demonstrated recently in complexes with dialkylamino-sub-
stituted NHC ligands.[10,11] The coordination number six is
rarely found (e.g., in a thiocrown ether AuII complex,[12] or
[13]
in AuACHTNUGRTNEUNG
(SO3F)3 and AuF3[14]), but to the best of our knowl-
edge, has never been reported for an organometallic AuIII
complex.
The NHC–AuI compounds were conveniently synthesized
by the reaction of [(Me2S)AuCl] with the NHC–Ag complex
3 or the free carbene. Subsequent oxidation with Br2 gives
the respective NHC–AuIII complexes (Scheme 2).[15] In the
absence of light, all complexes are perfectly stable both in
solution and as solids, whereas the AuIII complexes are sensi-
Scheme 1. 1,10-Phenanthroline and the precursors of its NHC analogues.
[a] M. Kriechbaum, Dr. U. Monkowius
Institut fꢀr Anorganische Chemie
Johannes-Kepler-Universitꢁt
Altenbergerstrasse 69, 4040 Linz (Austria)
Fax : (+43)732-2468-9681
E-mail: uwe.monkowius@jku.at
[b] Dr. M. List
Institut fꢀr Chemische Technologie Organischer Stoffe
Johannes-Kepler-Universitꢁt
Altenbergerstrasse 69, 4040 Linz (Austria)
[c] Dr. R. J. F. Berger
Fachbereich Materialwissenschaften und Physik
Paris-Lodron-Universitꢁt Salzburg
Hellbrunnerstrasse 34, 5020 Salzburg (Austria)
[d] Dr. M. Patzschke
Department of Chemistry
University of Helsinki
POB 55 AI Virtanens Plats 1, Helsinki 00014 (Finland)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200465.
Scheme 2. Synthesis of silver and gold complexes with a phen-analogue
NHC ligand (dms=dimethyl sulfide).
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Chem. Eur. J. 2012, 18, 5506 – 5509

COMMUNICATION
tive towards light and are easily reduced to the AuI conge-
ner (vide infra). Just recently, we have proved the capability
of a picolyl-substituted NHC to act as a chelating ligand by
removing one bromide ligand in the [(NHC)AuBr3] complex
as AgBr.[16] Analogously, we added one equivalent of AgBF4
to a solution of 5 to facilitate the coordination of the nitro-
gen atom of the imine moiety to the gold atom, but exten-
sive ligand-scrambling reactions and a reduction to AuI and
elemental gold occurred, and no k-N derivative could be iso-
lated. Instead, upon recrystallization of the residue from
CH2Cl2/Et2O, different crystals could be isolated, two of
them were identified by X-ray diffraction analyses as
[(NHC)AuBr] (4-Br) and the trans–anti isomer of 7 as its
tetrafluoroborate salt [(NHC)2AuBr2]BF4, anti-7-BF4.
X-ray structures of all complexes and of the imidazolium
salt were determined. All metal–carbon bond lengths are
within the expected ranges with values in the narrow range
of 1.94–2.09 ꢃ. Due to the geometry of the ligand, all metal
atoms feature weak M···N contacts, which are significantly
lower than the sum of the van der Waals radii (r(M)+
r(N)=3.27 (Ag), 3.21 ꢃ (Au)).[17] The homoleptic silver and
ꢀ
gold complexes 3 and 6 crystallize isostructurally with Au N
distances of 2.72–3.05 ꢃ (3) and 2.87–3.11 ꢃ (6). Very simi-
ꢀ
lar are the Au N distances (ꢁ3.05 ꢃ) of the heteroleptic
[(NHC)AuX] (X=Cl (4), Br (4-Br), Figures S1–S5 in the
ꢀ
Supporting Information). For comparison, the Au N dis-
tance in the linear NHC-AuI-N fragment with pyridine-type
N-ligands account for about 2.1 ꢃ.[16] Particularly interesting
are the molecular structures of the AuIII complexes: com-
pound 5 shows a square-pyramidal coordination geometry
with the AuIII atom coordinated by the carbon atom of the
carbene and the halide ligands in the equatorial position
Figure 1. Molecular structures of [(NHC)AuBr3], 5 (left), the cation in
[(NHC)2AuBr2]BF4,
anti-7-BF4
(right),
and
the
cation
in
ꢀ
[(NHC)2AuBr2]PF6, syn-7. Selected bond lengths [ꢃ]: for 5: Au1 C1
ꢀ
ꢀ
ꢀ
ꢀ
2.02(1), Au1 Br1 2.416(1), Au1 Br2 2.428(1), Au1 Br3 2.435(1), Au1
ꢀ
ꢀ
ꢀ
N3 2.91(1); for anti-7-BF4: Au1 C1 2.06(1), Au1 Br1 2.427(1), Au1 N3
ꢀ
ꢀ
ꢀ
2.93(1); for syn-7: Au1 C1 2.07(1), Au1 C23 2.08(1), Au1 N3 2.96(1),
ꢀ
ꢀ
ꢀ
Au1 N6 2.932(7), Au1 Br1 2.604(1), Au1 Br2 2.629(1).
ꢀ
and the nitrogen atom N3 in the axial position [Au1 N3
2.910(6) ꢃ, Figure 1]. A square-pyramidal coordination en-
vironment of a AuIII atom is well documented for diimine-
type ligands like phen and 2,2’-bipyridine in their
[(N\N)AuCl3] complexes[18] and also for amine functional-
ized [(NHC)AuX3] complexes with significantly shorter
tural parameters can be found in the Supporting Informa-
tion.
Because there are only a few observations supporting the
possibility of a hexa-coordinated AuIII cation,[13,14,19,20] we in-
vestigated the nature of the Au···N interactions in anti-7 and
syn-7 more detailed by using DFT calculations. The solid-
state structures can be reproduced well within the expected
limits. Deviations between solid-state and (computed) gas-
phase conditions are expected in the distances of Lewis-
donor to Lewis-acceptor sites. Such structural parameters
tend to be shorter in a polar environment (solid state) than
in an unpolar environment (dilute gas or vacuum).[21–23] In
agreement with this known trend, we found longer Au···N
bond lengths of 3.028 ꢃ for anti-7 and 3.077 ꢃ for syn-7 in
the calculation, compared with shorter bond lengths of
2.93(1) ꢃ for anti-7-BF4 and 2.96(1) or 2.93(1) ꢃ for syn-7 in
the solid state. Thus, the Au···N contact resembles classic
donor–acceptor interactions. Our next question is whether
Au and N share any electrons. The calculated shared elec-
[10]
ꢀ
axial Au N bond lengths (2.58–2.84 ꢃ).
In the complex
anti-7-BF4, the AuIII exhibits a highly distorted octahedral
environment defined by two carbene carbon atoms, two bro-
mides, and two nitrogen atoms of the NHC ligand with an
ꢀ
Au N bond length of 2.933 ꢃ (Figure S6 in the Supporting
Information). The complex cation is Ci-symmetric with the
gold atom lying on the inversion center. The orientation of
the chelating carbene ligands of the cations in crystals of
syn-7 resemble the geometry of the NHC ligands in complex
6 and arise from the addition of two bromide ligands to the
gold atom still maintaining the conformation of the com-
plex. The geometry of the coordination sphere of the gold
atom is a trigonal prism with the triangular consisting of the
carbene carbon and nitrogen atoms and the bromide. The
gold atom is lying in the center of one of the side planes
(Figure S7 in the Supporting Information). All attempts to
isolate the trans-isomer by recrystallization of compound 7
failed and repeatedly gave the syn-7 isomer. Further struc-
tron numbers (SEN)[24] for the Au N atom pairs of 0.01 for
ꢀ
both the anti and the syn conformer can be interpreted in
such a way that the interaction between Au and N in both
cases is not influenced by a stabilizing fragment orbital in-
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U. Monkowius et al.
teraction. This is in agreement with the ligand-field-theory-
based view of AuIII as a fourfold, square-planar acceptor
site. A population analysis based on occupation numbers
(PABOON)[24] resulted in a positive partial charge for Au of
0.83 and in negative partial charges for N3/N3ꢄ of the syn-
and N3/N6 of the anti-conformer of ꢀ0.16, respectively. All
remaining nitrogen atoms in both conformers have positive
partial charges. Thus, there is an intramolecular electrostatic
attractive component (possibly weak) between these nega-
tively charged nitrogen atoms and the positively charged
gold atom. In such cases, weak nonorbital-based attractive
intramolecular interactions are found to be enhanced by dis-
persion-type interactions.[23] DFT-calculations cannot repro-
duce dispersion-type (“van der Waals”) interactions, but em-
pirical corrections (e.g., D3) are available.[25,26] However, we
found no significant structural change upon application of
the dispersion correction, nor any significant changes in the
isomerization energy [syn!anti: ꢀ0.9 kcalmolꢀ1 (DFT),
ꢀ1.3 (DFT–D3)]. We conclude that the experimentally ob-
served sub van der Waals contacts between Au and N in
anti-7-BF4 and syn-7 are not significantly orbital based, but
may be stabilized by negative partial charges found exclu-
sively on the two N atoms with the closest contacts to the
positively charged Au atom, and we note that this intramo-
lecular electrostatic stabilization contribution is not en-
hanced significantly by van der Waals interactions.
S13–15 in the Supporting Information), we assign the HE
and LE bands to an intraACTHNUGRTENNUGliACHUTGTNRENNUG
gand fluorescence (1IL) and phos-
phorescence (3IL), respectively. The S1–T1 energy gap is
large (e.g., 7300 cmꢀ1 for 6) and is in agreement with pub-
lished values of other gold complexes containing ligands
with an extended p system.[28] The AuIII complexes under-
went photoreductive elimination of Br2 upon irradiation
with polychromatic light giving the AuI congener (l
>335 nm, Figure S17–S20 in the Supporting Information).
As a further deactivation path of the excited state exists, the
photoreactivity is also accompanied by a significant drop of
the emission-quantum yields. All photophysical data are
summarized in Table S1 in the Supporting Information.
In this work, we presented a phenanthroline analogue
NHC ligand and studied its coordination properties by the
synthesis of neutral and ionic AuI and AuIII complexes that
exhibit unusual coordination environments with six donor
atoms around the gold atom in two of the AuIII compounds.
All complexes revealed luminescence that originated from
intraACHTNUTRGENNUGliCAHUTGTNRENNUGgand excited states. Application of this new NHC as
bidentate ligand for further transition-metal complexes (e.g.,
as catalysts or triplet emitters for OLEDs) may be envis-
aged. The synthesis and photophysical characterization of
Pd, Pt, and Ir complexes are the subject of current investiga-
tions.
The electronic absorption and emission spectra of the imi-
dazolium salt 2 featured typical signals for rigid annulated
aromatic systems (Figure S8 in the Supporting Informa-
tion).[27] For all gold compounds, comparable luminescence
behavior with both a structured high-energy (HE) and
a low-energy (LE) band (the latter dominating at 77 K)
were observed (Figure 2). The silver complex is emissive
only at 77 K exhibiting the LE band (Figures S10–12, S16 in
the Supporting Information). Due to the similarity to the
emission of 2 and their emission decay times (Figure S9,
Acknowledgements
We thank Prof. G. Knçr for his support and Prof. Dr. H. Yersin for the
opportunity to measure the emission life times in his laboratory. B.R.
thanks Prof. Dr. Hꢀsing for support.
Keywords: density functional calculations
luminescence · silver · X-ray diffraction
·
gold
·
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Silver and Gold Complexes with 1,10-Phenanthroline
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Received: February 13, 2012
Published online: March 27, 2012
Chem. Eur. J. 2012, 18, 5506 – 5509
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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