A N-Heterocyclic Carbene-Stabilized Coinage Metal-Chalcogenide Framework with Tunable Optical Properties

Article abstract of DOI:10.1021/jacs.7b09025

A new class of coinage-metal chalcogenide compounds [Au₄M₄(μ₃-E)₄(IPr)₄] (M = Ag, Au; E = S, Se, Te) has been synthesized from the combination of N-heterocyclic carbene-ligated gold(I) trimethylsilylc

Full text of DOI:10.1021/jacs.7b09025

Communication
pubs.acs.org/JACS
A N‑Heterocyclic Carbene-Stabilized Coinage Metal-Chalcogenide
Framework with Tunable Optical Properties
Alexander M. Polgar,† Florian Weigend,‡,§ Angel Zhang,† Martin J. Stillman,† and John F. Corrigan*,†,⊥
†Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada
‡Institut fur Nanotechnologie, Karlsruher Institut fur Technologie, Hermann-von Helmholtz-Platz 1, 76344
̈
̈
Eggenstein-Leopoldshafen, Germany
§Institut fur Physikalische Chemie, Karlsruher Institut fur Technologie, Kaiserstr. 12, 76131 Karlsruhe, Germany
̈
̈
⊥Centre for Advanced Materials and Biomaterials Research, The University of Western Ontario, London, Ontario N6A 3K7, Canada
S
* Supporting Information
phosphines.8 This is in stark contrast to other areas of gold
ABSTRACT: A new class of coinage-metal chalcogenide
compounds [Au4M4(μ3-E)4(IPr)4] (M = Ag, Au; E = S, Se,
Te) has been synthesized from the combination of N-
heterocyclic carbene-ligated gold(I) trimethylsilyl-
chalcogenolates [(IPr)AuESiMe3] and ligand-supported
metal acetates. Phosphorescence is observed from these
clusters in glassy 2-methyltetrahydrofuran and in the solid
state at 77 K, with emission energies that depend on the
selection of metal/chalcogen ion composition. The ability
to tune the emission is attributed to electronic transitions
of mixed ligand-to-metal-metal-charge-transfer (IPr →
AuM2) and interligand (IPr → E) phosphorescence
character, as revealed by time-dependent density func-
tional theory computations.N-heterocyclic carbenes
(NHCs) have been applied as ancillary ligands in the
synthesis of luminescent gold(I) chalcogenide clusters and
this approach allows for unprecedented selectivity over the
metal and chalcogen ions present within a stable
octanuclear framework.
chemistry, where N-heterocyclic carbene (NHC) ligands rival
phosphines as the ligand of choice for applications in catalysis,9
medicine,10 supramolecular assembly,11 surface chemistry,12
and recently nanocluster chemistry.13 NHC−Au(I) complexes
demonstrate fascinating and tunable optical effects such as
mechano-, iono-, and vapochromism.14 Furthermore, reports
have emerged recently where NHCs stabilize chalcogenide
clusters of other d-block metals.15
Herein, we promote a fusion of the fields of luminescent
gold(I) chalcogenide clusters and NHC−Au(I) complexes, by
using gold(I) trimethylsilylchalcogenolate reagents [(IPr)-
AuESiMe3] (E = S (1a), Se (1b), Te (1c); IPr = 1,3-bis(2,6-
diisopropylphenyl)imidazole-2-ylidene) as precursors to homo-
and heterometallic clusters with NHC ancillary ligands.
Treatment of these synthons with [LM(OAc)] (L = Ph3P,
(ArO)3P; M = Ag, Au) led to the formation of the isostructural
octanuclear clusters [Au4Ag4(μ3-E)4(IPr)4] (E = S (2a), Se
(2b)) and [Au8(μ3-E)4(IPr)4] (E = S (3a), Se (3b), Te (3c)).
The enhanced σ-donating ability of NHCs compared to
phosphines conferred exceptional stability to the silylchalcoge-
nolate precursors and resulting cluster framework. This allowed
for extensive tuning of the metal/chalcogen ion composition,
with a marked shift in the luminescence upon controlled
variation of E and M. The present work demonstrates that
NHCs have a special role in the continued development of
gold(I) chalcogenide chemistry and its application toward new
light emitting metal complexes.
The NHC-stabilized gold trimethylsilylchalcogenolates 1a−c
were prepared by salt metathesis of [(IPr)AuCl] with
Li[ESiMe3] in tetrahydrofuran (THF; Scheme 1). Repeated
crystallization attempts produced fragile and rapidly desolvating
crystals unsuitable for diffraction analysis. The molecular
structures were thus confirmed by 1D and 2D multinuclear
NMR spectroscopy and elemental analysis (Supporting
Information). Given the propensity for gold(I) to adopt linear,
terminally coordinated species, we propose mononuclear
complexes with gold(I) ligated to one IPr ligand and one
E(SiMe3). Compared to previous metal trimethylsilyl-
chalcogenolate reagents,15b,c,16 complexes 1a−c exhibit excep-
ver the past 50 years, complexes of gold in the +1
Ooxidation state have been studied for their rich
photophysical properties.1 Early crystallographic studies of
linear Au(I) complexes revealed intermolecular aggregation of
Au+ centers into supramolecular chains and sheets through
what is now understood as a dispersive d10−d10 attraction
strengthened by relativistic effects, which has been termed
“aurophilicity”.2 It was soon recognized that this interaction is
intimately connected to photoluminescence in Au(I) com-
plexes.3 It was also shown that aurophilicity could exist
intramolecularly in the clustering of Au+ centers about main
group donor atoms.4 Of these, gold(I) chalcogenide clusters
have demonstrated particular promise for their stability,
synthetic accessibility, and intense photoluminescence.5
When combined with phosphine ancillary ligands, gold(I)
chalcogenides form clusters typically based on [(μ3-E)Au3]+
units aggregated through aurophilic interactions.6 By varying
the steric and electronic parameters of the stabilizing
phosphine, clusters of varying size and geometry have been
prepared.7 Curiously, there have been very few reports on
gold(I) chalcogenide clusters with ancillary ligands other than
Received: August 23, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.7b09025
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Journal of the American Chemical Society
Communication
a
and residing about a crystallographic 4 center (Figures 1 and
S4). The molecules consist of a central {M4E4} cycle (M = Ag
Scheme 1. Synthesis of 1a−c
̅
a
See the Supporting Information for detailed experimental conditions.
THF = tetrahydrofuran. Dipp = 2,6-diisopropylphenyl.
tional stability in solution, as shown by monitoring in CDCl3 at
1
room temperature with H NMR spectroscopy (Supporting
Information). Both 1a and 1b show negligible degradation after
2 weeks in solution. 1c is, expectedly, less stable but
decomposes only after several days to produce Te(SiMe3)2
and insoluble polynuclear gold-tellurides.
Figure 1. Molecular structures of [Au4Ag4S4(IPr)4] 2a (left) and
[Au8Te4(IPr)4] (3c) (right) in the crystal (50% thermal ellipsoids for
core atoms; all protons have been omitted for clarity). Molecules of 2a
Stoichiometric reactions of 1a with a suspension of AgOAc at
−78 °C resulted in instant darkening of the reaction solutions,
and ultimately production of intractable mixtures. The reaction
was subsequently performed via ligation of AgOAc with a
tertiary phosphine to retard uncontrolled cluster growth. The
weak σ donor PPh3 was chosen so that the phosphine would
not be present in the final product bound to Ag and may be
removed by recrystallization. Thus, reaction of 1a and 1b with
[(Ph3P)AgOAc] yielded the ternary Au-E-Ag clusters
[Au4Ag4(μ3-S)4(IPr)4] (2a) and [Au4Ag4(μ3-Se)4(IPr)4] (2b),
respectively, as the only crystalline materials (Scheme 2). In a
reside about a crystallographic 4 center.
̅
(2a,b), Au (3a,b)) with the four “M” atoms close to planarity
(torsion angles between 0.63° and 1.47°), and the chalcogen
atoms lying alternately above and below this plane. Each
chalcogen atom is ligated to two M and one Au. In 2a and 2b,
these arrangements result in unprecedented [Ag2Au(μ3-S)]+
and [Ag2Au(μ3-Se)]+ tetrahedra as structural units, respectively.
Aurophilic and/or metallophilic interactions are present in all
clusters,17 with the interaction becoming weaker (as judged by
longer metal−metal contacts) when the size of the bridging
chalcogen increases. Compound 3c crystallizes as a THF
solvate in the orthorhombic space group P212121 and its
geometry is distorted compared to the lighter congeners of this
series, owing to both the longer Au−Te bonds and aurophilic
contacts present (Figure 1). Selected structural parameters are
presented in Table S3.
a
Scheme 2. Synthesis of 2a,b and 3a−c
In contrast to many coinage metal-chalcogenide clusters
stabilized by phosphine ligands,18 crystals of NHC-stabilized
clusters 2a,b and 3a−c readily redissolve in solution without
evidence of ligand dissociation, allowing for detailed solution-
state spectroscopic characterization. Multinuclear NMR spectra
have been recorded for all clusters and are consistent with the
molecules remaining intact in solution (Supporting Informa-
tion). The luminescence of clusters 2a,b and 3a,b in glassy 2-
methyl tetrahydrofuran (2-Me-THF) and the solid state spectra
for 2a,b and 3c have been measured at 77 K (Figure 2). The
spectroscopic results are summarized in Table S5. Except for
a
See the Supporting Information for detailed experimental conditions.
similar reaction of 1a with [(Ph3P)AuOAc], the crystallization
of the octanuclear gold(I)-sulfido cluster [Au8(μ3-S)4(IPr)4]
(3a) was observed. However, a major byproduct was
[(IPr)Au(PPh3)](OAc) (ca. 33%), resulting in low yields of
the cluster (Figures S1 and S2). A weaker Lewis base (Ar*O)3P
(Ar* = 2,4-tBu(C6H3)) was employed instead to promote P−
Au bond scission. This modified procedure selectively yielded
[Au8(μ3-S)4(IPr)4] and [Au8(μ3-Se)4(IPr)4] (3b). Surprisingly,
reaction of 1c with [(Ph3P)AuOAc] allowed for the clean
isolation of [Au8(μ3-Te)4(IPr)4] (3c) without observation of
any cationic gold(I) species. The binary gold(I) chalcogenide
clusters 3a−c represent a rare series for which all the heavier
chalcogen congeners have been structurally characterized,
another being [{(Ph3P)Au}3(μ3-E)]+.7l,n,o The benefits of
using metal trimethylsilylchalcogenolate reagents 1a−c are
apparent in the retention of reaction stoichiometry in the
resultant cluster, and the ease of purification by simply layering
the reaction solutions with hydrocarbons.
max
3c, all clusters exhibit a single excitation band with λex
between 310 and 327 nm, increasing in wavelength in the order
2a < 2b < 3a < 3b. The same ordering applies to the low-
energy λemmax of the emission spectra in glassy 2-Me-THF. This
gives rise to emission colors ranging from yellow-green (2a) to
red (3b). Each of the clusters exhibited large Stokes’ shifts
(>14 000 cm−1) and microsecond lifetimes of ca. 10 to 40 μs,
indicating a triplet parentage for the emissions. Bathochromic
shifting of the emission maxima are observed in 2a, 2b, 3b, and
3c (Figures 2 and S11) from glassy 2-Me-THF to the solid state
and is attributed to strengthened Au···Ag/Au···Au interactions
in the absence of solvent.19 The cluster 2b displays dual
luminescence in glassy 2-Me-THF and as a powder with an
additional weak band at ∼500 nm; such features have been
observed in complementary gold(I)−chalcogenide systems and
were attributed to metal-perturbed ligand-centered phosphor-
escence.7e The relative intensity of this band decreases
markedly at temperatures above 77 K (Figure S6). The
Compounds 2a, 2b, 3a, and 3b are isostructural and
isomorphous, crystallizing in the tetragonal space group I 4
̅
B
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Journal of the American Chemical Society
Communication
Figure 3. Difference density plot for the singlet excitations of 2a using
the method of ref 25. Red regions indicate a surplus of electron density
in the ground electronic state and blue indicates a surplus in the
excited singlet state.
Figure 2. (Top) Photoluminescence excitation and emission spectra of
compounds 2a (red), 2b (blue), 3a (green), and 3b (purple) in glassy
2-Me-THF at 77 K. Samples were excited at 315 nm (2a and 2b) and
320 nm (3a and 3b). Photoluminescence excitation spectra were
recorded by monitoring emissions at 600 nm (2a and 2b) and 650 nm
(3a and 3b). (Bottom) Solid state photoluminescence excitation and
emission spectra of compounds 2a (red), 2b (blue), and 3c (dark red)
at 77 K. Samples were excited at 300 nm. Photoluminescence
excitation spectra were recorded by monitoring emissions at 650 nm
(2a) and 700 nm (2b and 3c).
gold(I) chalcogenide clusters. NHCs are found to be valuable
for stabilizing a particular metal cluster framework, allowing the
photophysical properties to be fine-tuned by selective doping of
metal ions and chalcogen centers. Furthermore, the nature of
the emission is found to influenced by the electron accepting
ability of the NHC ligands, so further tuning of the optical
properties through ancillary ligand modifications should be
possible.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b09025.
■
S
emission maxima of 3c in the solid state occur at low energies,
which are similar to those reported for the octanuclear gold(I)
tellurolates [Au8(μ-TePh)8(PR3)4] and [Au8(μ-TeTol)8-
(PR3)4].20
Experimental details, NMR and combustion analysis of
the reported complexes, stability studies of 1a−c in
solution, crystal structure data for 2a,b and 3a−c and
electronic spectroscopy data (PDF)
For cluster 2a, the character of the excitations causing the
excitation band starting at a wavelength of ca. 350 nm was
determined via time-dependent density functional theory
(TDDFT). Calculations were carried out with TURBO-
MOLE21 at the B3LYP/def2-SV(P) level.22,23 These transitions
dominantly involve the highest 8 occupied and the lowest 16
unoccupied orbitals. The latter are part of the π-system of the
NHCs, with the former mainly showing contributions of the 3p
orbitals of sulfur (55%), the 5s/4d orbitals of silver (29%), and
the 6s/5d orbitals of the gold centers (13%), as determined
from Mulliken population analysis.24 The character of this
transition is also evident from the difference density (Figure 3)
between the excited states and the ground state.25 The
electronic excitations in 2a may thus be formulated as
metal−metal to ligand charge transfer (1MMLCT; AuAg2 →
IPr) with an interligand (1IL; S → IPr) admixture. Based on the
photophysical data, it is expected that the reverse process
(3LMMCT; IPr → AuAg2 and 3IL; IPr → S) is responsible for
emission from the first excited triplet state. As more electron
density is injected into the {M4E4} core (via replacement of S
with Se or Ag with Au), a higher-lying S0 state should induce a
bathochromic shift of the electron transition energy, as is
indeed observed in the ordering of λemmax. A similar
involvement of the ancillary ligands in the triplet excited state
has been documented recently in copper(I) chalcogenide
clusters.26
Crystallographic data for 2a,b and 3a−c (CIF)
AUTHOR INFORMATION
Corresponding Author
*corrigan@uwo.ca
ORCID
Florian Weigend: 0000-0001-5060-1689
John F. Corrigan: 0000-0003-2530-5890
■
Notes
The authors declare no competing financial interest.
CCDC 1526645−1526646 for 2a and 2b, respectively, and
CCDC 1559057−1559059 for 3a−c, respectively, contain the
supplementary crystallographic data for this paper. These data
are provided free of charge by The Cambridge Crystallographic
Data Center.
ACKNOWLEDGMENTS
■
Dedicated with best wishes to Prof. Dr. Dieter Fenske on the
occasion of his 75th birthday. We are grateful to the Natural
Sciences and Engineering Research Council (NSERC) of
Canada for supporting this research in the form of Discovery
and Research Tools and Instruments to J.F.C. and M.J.S.
A.M.P. thanks the NSERC for scholarship support and the
Chemistry Department at the University of Western Ontario
for an ASPIRE travel grant. A.Z. acknowledges scholarship
support from a Queen Elizabeth II Graduate Scholarship. The
In summary, NHC ligands provide a remarkable increase in
thermal stability to [LAuESiMe3] reagents, while these
complexes retain their high cluster-forming reactivity, readily
yielding highly crystalline, solution processable, and functional
C
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Journal of the American Chemical Society
Communication
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data.
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