Synthesis, structure, and properties of a T-shaped 14-electron stiboranyl-gold complex

Article abstract of DOI:10.1021/ja201092g

A cyclic stiboranyl-gold complex (1) supported by two 1,8-naphthalenediyl linkers has been synthesized and structurally characterized. The gold atom of this complex adopts a T-shaped geometry and is separated from the antimony center by only 2.76 A. Surprisingly, the trivalent gold atom of this complex is involved in an aurophilic interaction, a phenomenon typically only observed for monovalent gold complexes. This phenomenon indicates that the stiboranyl ligand possesses strong σ-donating properties making the trivalent gold atom of 1 electron rich. This view is supported by DFT calculations as well as Au L₃- and Sb K-edge XANES spectra which reveal that 1 may also be described as an aurate-stibonium derivative. In agreement with this view, complex 1 shows no reactivity toward the halides Cl^-, Br^-, and I^-. It does, however, rapidly react with F^- to form an unprecedented anionic aurate fluorostiborane complex ([2]^-) which has been isolated as the tetra-n-butylammonium salt. The increased coordination number of the antimony center in this anionic complex ([2]^-) does not notably affect the Au-Sb separation (2.77 A) or the geometry at the gold atom which remains T-shaped.

Full text of DOI:10.1021/ja201092g

ARTICLE
pubs.acs.org/JACS
Synthesis, Structure, and Properties of a T-Shaped 14-Electron
Stiboranyl-Gold Complex
Casey R. Wade,† Tzu-Pin Lin,† Ryan C. Nelson,‡ Elizabeth A. Mader,‡ Jeffrey T. Miller,*,‡ and
Franc-ois P. Gabbaï*,†
†Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States
‡Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439-4837, United States
S Supporting Information
b
ABSTRACT: A cyclic stiboranyl-gold complex (1) supported by two
1,8-naphthalenediyl linkers has been synthesized and structurally
characterized. The gold atom of this complex adopts a T-shaped
geometry and is separated from the antimony center by only 2.76 Å.
Surprisingly, the trivalent gold atom of this complex is involved in an
aurophilic interaction, a phenomenon typically only observed for
monovalent gold complexes. This phenomenon indicates that the
stiboranyl ligand possesses strong σ-donating properties making the
trivalent gold atom of 1 electron rich. This view is supported by DFT
calculations as well as Au L3- and Sb K-edge XANES spectra which reveal that 1 may also be described as an aurate-stibonium
derivative. In agreement with this view, complex 1 shows no reactivity toward the halides Clꢀ, Brꢀ, and Iꢀ. It does, however, rapidly
react with Fꢀ to form an unprecedented anionic aurate fluorostiborane complex ([2]ꢀ) which has been isolated as the tetra-n-
butylammonium salt. The increased coordination number of the antimony center in this anionic complex ([2]ꢀ) does not notably
affect the AuꢀSb separation (2.77 Å) or the geometry at the gold atom which remains T-shaped.
’ INTRODUCTION
T-shaped 14-electron transition metal complexes constitute
an interesting class of compounds because of their inherent
unsaturation.1 Such complexes can be stabilized through the use
of noncoordinating anions and/or bulky ligands, which sterically
hinder the coordination of a fourth ligand to the unsaturated
transition metal center.2 Another useful strategy relies on the use
of silyl3 or boryl ligands4 whose strong σ-donating properties can
be used to increase the lability of the trans-ligand. The properties
of these ligands can be illustrated by a series of cationic T-shaped
which feature R2X2Sb (type I)7a,b or R2RfO2Sb (type II)7cꢀf
bis(phosphine)platinum boryl derivatives, which can be formed,
albeit when a noncoordinating anion is employed.5 The pre-
ponderance of silyl and boryl ligands for the stabilization of
T-shaped 14-electron complexes has led us to question whether
other main group ligands could also behave as strong σ-donors.
In search of such ligands, we were drawn by the fact that
antimony species such as Ph4SbX (X = halogen) become
increasingly ionic as the size of the halogen increases.6 This
behavior, which is promoted by the electropositive character of
antimony, suggested to us that a tetraorganostiboranyl (R4Sb)
ligand7 may possess strong σ-donating properties and may be
well-adapted for the formation of T-shaped 14-electron transi-
tion metal complexes.
stiboranyl ligands (R = alkyl or aryl, X = halogen, RfO =
fluorinated alkoxy group). A noteworthy feature of complexes
of type I and II pertains to the presence of electronegative
halogen or alkoxy groups whose electron-withdrawing character
may limit the σ-donating properties of the stiboranyl group.
While several derivatives of type I and II have been previously
described,7 transition metal complexes featuring the more σ-
donating tetraorganostiboranyl (R4Sb) ligands (type II) have to
our knowledge never been reported, possibly because of a lack of
adequate synthetic approaches.
Seeking inspiration from a series of reports dealing with the
coordination of σ-acceptor ligands to late transition metal
complexes,9 it occurred to us that a tetraorganostiboranyl
In contrast to triaryl and trialkyl stibines which have enjoyed a
wide precedent in coordination chemistry,8 few transition metal
complexes possessing stiboranyl ligands have been isolated. Most
examples of such complexes consist of derivatives of type I and II
Received: February 3, 2011
Published: May 13, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of the StiboranylꢀGold Complex 1a
a tmeda = tetramethylethylenediamine, THF = tetrahydrofuran, tht =
tetrahydrothiophene.
complex could be approached, at least conceptually, by coordi-
nation of a tetraorganostibonium ion to an electron-rich transi-
tion metal complex. Inspired by this idea and having recently
succeeded in the isolation of A, a compound best described as a
mercury stibonium derivative,10 we have now chosen to inves-
tigate the synthesis of late transition analogs of this mercury
derivative.
Figure 1. Crystal structure of (a) one of the independent molecules of 1
and (b) both independent molecules showing the presence of the
aurophilic contact. Thermal ellipsoids are drawn at the 50% probability
level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and angles (deg) (the metrical parameters of the second independent
molecule are given in brackets) for 1: AuꢀSb 2.7486(7) [2.7746(8)],
AuꢀAu0 3.2551(7); CNap1ꢀAuꢀCNap2 177.0(3) [172.4(3)], CNap1
ꢀ
AuꢀSb 88.8(3) [88.6(2)], CNap2ꢀAuꢀSb 89.2(3) [88.5(3)], Auꢀ
SbꢀCax 172.0(2) [171.4(3)].
In this paper, we report the first successful implementation of
this approach with the synthesis and characterization of a
T-shaped, 14-electron, trivalent stiboranyl-gold complex.11 Through
a combination of experimental, spectroscopic, and computational
methods, we show that the stiboranyl ligand present in this complex
possesses strong σ-donating properties and renders the gold atom of
this complex unusually electron rich.
geometry as confirmed by the average CNapꢀAuꢀSb and
CNap1ꢀAuꢀCNap2 angles of 88.8° and 174.7°, respectively.
Accordingly, the antimony atom displays a distorted trigonal
bipyramidal geometry with the gold atom in an axial position.
The AuꢀSbꢀCax angle of 171.8° (av.) as well as the sum of the
CeqꢀSbꢀCeq angles of 356.3° (av.) supports this view
(Figure 1). A last point of interest in this structure relates to
the presence of an aurophilic interaction of 3.2551(7) Å which
connects the two independent molecules (Figure 1). The
formation of such a contact as well as the perpendicular orienta-
tion of the two monomers is reminiscent of monovalent gold
compounds such as Me3PAuI15 or (2,6-Me2C6H3NC)AuCl16
which also form perpendicular aurophilic dimers with AuꢀAu
distances of 3.168(1) and 3.3555(5) Å, respectively.17 Aurophilic
contacts involving trivalent species are limited to two examples
which each display stacked square planar units and AuꢀAu
distances longer than 3.5 Å.18 This structural analysis indicates
that the gold atom of 1 may be as electron rich as that in gold
monohalide complexes. Thus, despite the short AuꢀSb bond
which supports the presence of a covalent linkage between the
two atoms as represented in resonance form a, the gold atom
behaves as that of simple monovalent complexes, pointing to the
relevance of resonance form b.
’ RESULTS AND DISCUSSION
Synthesis Structure and Bonding. Reaction of Ph3SbBr2
with 1,8-dilithionaphthalene and, subsequently, AuCl(tht) and
Ph3P in THF afforded, after recrystallization from CH2Cl2, the
stiboranyl-gold complex 1 in 37% yield (Scheme 1). This
compound, which can be represented on the basis of two
resonance structures (form a and b), is air stable. It has been
1
characterized by conventional means including H NMR spec-
troscopy which confirms the unsymmetrical substitution of the
naphthalenediyl backbones. Compound 1 crystallizes in the
space group P1 with two independent molecules in the asym-
metric unit (Figure 1). The two independent molecules possess
almost identical structures and feature a short AuꢀSb bond
distance of 2.76 Å (av.). This bond distance, which is essentially
equal to the sum of the covalent radii of the two elements
(2.75 Å),12 is only marginally longer than the AuꢀSb bonds
found in stibineꢀgold complexes such as [Au(SbMes3)2][ClO4]
(2.58 Å)13 or [Au(SbPh3)4][ClO4] (2.656 ꢀ 2.658 Å).14 The
short AuꢀSb bond indicates a strong interaction. This view is
reinforced by a comparison of 1 with its isoelectronic mercury
analogue A.10 Indeed, despite the similarity of the covalent radii
of mercury (1.32 Å) and gold (1.36 Å),12 the average AuꢀSb
bond distance in 1 (2.76 Å, respectively) is markedly shorter than
the HgfSb bond observed in A (3.0601(7) Å). Owing to the
presence of this AuꢀSb bond, the gold atom adopts a T-shaped
X-ray Absorption Spectroscopy. To gather additional in-
formation about the electronic structure of 1, we decided to study
the atomic properties of the gold nucleus in 1 by measuring its
room temperature, solid state Au L3-X-ray Absorption Near Edge
Structure (XANES) spectrum along with that of two reference
compounds, namely AuCl(PPh3) and CsAuCl4 (Figure 2). In
agreement with the trend established for gold compounds at the
L3-edge,19 we observed that the edge position of the trivalent
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recordings of each spectra showed very little variance (less than
0.1 eV) confirming that the difference is not the result of random
measurement error. Collectively, these results indicate that an
aurate-stibonium resonance structure (form b, Scheme 1) for 1
must be considered, possibly as the most important contributor.
It follows that 1 is closely related to a series of complexes such
as B21 and C,22 which have been described according to a
donorꢀacceptor formalism with the late transition metal center
acting as a σ-donor and the main group element as a σ-acceptor
ligand.9,23
Computational Studies. To better understand the electronic
structure of 1, we carried out DFT calculations using the ADF
program. All calculations were carried out at the BP86/TZP level
of theory using the zero-order regular approximation (ZORA).
Geometry optimization starting from the solid state structure of
one of the independent molecules of 1 afforded a structure that is
in excellent agreement with that observed experimentally (see
Supporting Information). A first assessment of the nature of the
AuꢀSb bond was derived from the Electron Localization Func-
tion (ELF), a function which can be used to map the electron pair
localization in a molecule.24 The ELF map of 1 is characterized by
a continuum of relatively elevated ELF values along the AuꢀSb
vector, indicating some degree of electron sharing between the
two heavy atoms (Figure 3). Additional insight into the nature of
the AuꢀSb bond was provided by a molecular orbital analysis.
Compound 1 was subjected to a Boys localization analysis
because of extensive delocalization of the occupied KohnꢀSham
orbitals. This analysis identifies two CꢀAu σ-bonding orbitals
connecting the Au atom to the two naphthalene ligands. These
σ-orbitals show little polarization, in agreement with the low
electronegativity difference between carbon (χ = 2.54) and gold
(χ = 2.55). By contrast, the only orbital of σ-symmetry oriented
along the AuꢀSb axis is very strongly polarized toward the gold
atom with very little contribution from the antimony atom. This
observation supports the notion that the stiboranyl ligand acts as
a strong σ-donor. It also lends further support to the relevance
of the aurateꢀstibonium contribution (resonance form b in
Scheme 1) in the description of 1. This view is further supported
by the electrostatic potential map of 1 which shows a clear
accumulation of negative charge on the gold atom (see Support-
ing Information). Collectively these computational results show
that the AuꢀSb bond is highly polarized, making the trivalent
gold center electron rich.
Figure 2. Solid-State XANES spectrum: (Top) Au L3-edge of 1 (—)
compared to reference complexes of Au3þ (CsAuCl4, ---) and Au1þ
Photophysical Properties. The electronic absorption spec-
trum of 1 in CH2Cl2 displays a broad band spanning the
350ꢀ440 nm range (Figure 4). This low energy feature is
attributed to a transition from the naphthalene-based HOMO
to the LUMO which bears a significant contribution from a gold
6p and SbꢀCPh σ* orbitals (Figure 4). The solid state emission
spectrum of 1 measured at 77 K shows a broad emission centered
at around 660 nm (figure 4). This band is not observed at room
temperature suggesting that it corresponds to a triplet excited
state arising from a gold heavy atom effect. This view is
confirmed by the excited state lifetime of 6.8 μs which is
comparable to that observed for other aurated aromatic
derivatives.25 Oxygen-free solutions of 1 in CH2Cl2 are not
visibly luminescent at room temperature. Upon freezing, how-
ever, these solutions give rise to a bright orange emission
centered at 560 nm. In turn, the emission of 1 in frozen solution
is distinctly blue-shifted when compared to that of the solid state,
a phenomenon that we assign to the absence of aurophilic
interactions under these conditions.
(AuCl(PPh3),
). (Bottom) Sb K-edge of 1 (—) compared to
3 3 3
reference complexes of Sb5þ ([SbPh4]OTf, ---) and Sb3þ (SbPh3,
).
3 3 3
gold reference CsAuCl4 (11918.9 eV) is at lower energy than that
measured for the monovalent gold reference compounds AuCl-
(PPh3) (11921.2 eV). Notably, the Au L3 XANES spectrum of 1
shows an edge position of 11921.4 eV, a value very close to that
measured for the monovalent gold reference AuCl(PPh3). Next,
we decided to study compound 1 by Sb K-edge-XANES spec-
troscopy. The Sb K-edge of XANES spectra is typically domi-
nated by 1sfp transitions whose energy increases with the
valence of the antimony atom.20 Our measurements shown in
Figure 2 indicate that the Sb K-edge of 1 (30493.3 eV) is
essentially identical to that measured for the stibonium salt
[SbPh4][OTf] (30493.3 eV) and at higher energy than that of
the trivalent reference compound SbPh3 (30492.0 eV). Although
only a small energy difference (1.3 eV) was observed between the
Sb K-edge of SbPh3 and those of 1 and [SbPh4][OTf], multiple
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Figure 3. (aꢀc) Gold-centered Boys orbitals for 1 (drawn with a 0.02 isodensity value). (d) Plot of the electron localization function for 1.
Scheme 2. Synthesis of [n-Bu4N][2]a
a TBAF = tetra-n-butylammonium fluoride.
salt (Scheme 2). Evidence for the formation of an SbꢀF bond
rather than a AuꢀF bond was provided by the detection of a
19F NMR signal (in acetone-d6) at ꢀ41 ppm, a chemical shift
close to that observed for Me4SbF.26 The selectivity of the anti-
mony center for fluoride over the larger halides is a well docu-
mented phenomenon which can be assigned to the increased
Figure 4. (a) UVꢀvis absorption spectra of 1 (___) in CH2Cl2. (b) Low
contribution of the stibonium halide form as the size of the
temperature (77 K) emission spectra of 1 in the solid state (___) and
halogen increases.27 Formation of [2]ꢀ can also be monitored by
UVꢀvis spectroscopy which indicates that the fluoride binding to
the antimony quenches the low energy band present in the spectrum
of 1(Figure 5). This observation can be rationalized by an increase in
the energy of the LUMO of [2]ꢀ caused by anion coordination.
While DFT calculations (BP86/TZP level of theory with ZORA)
indicate that the makeup of this orbital remains similar to that of 1
(Figure 5), this perturbation causes the HOMOꢀLUMO transition
to shift to higher energies. In line with this argument, the UVꢀvis
spectrum of [n-Bu4N][2] shows the emergence of a new absorption
band centered at around 310 nm.
frozen CH2Cl2 solution (
a 0.02 isodensity value).
). (c,d) Frontier orbitals of 1 (drawn with
3 3 3 3
Reactions with Halide Ions. In line with an accumulation of
electron density on the gold atom, and hence the strong σ-
donating properties of the stiboranyl ligand, the gold atom of 1
shows no affinity for chloride, bromide, or iodide anions in
CDCl3. These NMR experiments also showed no measurable
interaction between the halide anions and the antimony center of
1. Interestingly, however, 1 quickly reacts with tetra-n-butylam-
monium fluoride (TBAF) to form an aurated fluoroantimonate
anion ([2]ꢀ) which has been isolated as the tetra-n-butylammonium
Recrystallization of [n-Bu4N][2] from THF/toluene (1:1)
afforded colorless, plate-like crystals that are not luminescent.
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Figure 5. (a) Spectral changes in the UVꢀvis absorption spectra of 1 (5.0 ꢁ 10ꢀ5 M in CH2Cl2) upon incremental addition of a TBAF solution (4.0 ꢁ
10ꢀ3 M in CH2Cl2). (b) Plot of the absorbance monitored at 314 nm versus [Fꢀ]. The curve was fit to a 1:1 binding model to yield a calculated fluoride
binding constant K = 1.0 ꢁ 107 Mꢀ1. (c,d) Frontier orbitals of [2]ꢀ (drawn with a 0.02 isodensity value).
Figure 6. (a) Crystal structure of [2]ꢀ in the [n-Bu4N][2] salt. Only one of the independent molecules is shown. Thermal ellipsoids are drawn at the
50% probability. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg) (the metrical parameters of the second
independent molecule are given in brackets). [2]ꢀ: AuꢀSb 2.7694(8) [2.7728(9)], SbꢀF 2.091(5) [2.060(5)]; CNap1ꢀAuꢀCNap2 179.2(4)
[177.3(3)], CNap1ꢀAuꢀSb 89.9(3) [91.4(3)], CNap2ꢀAuꢀSb 90.4(2) [89.4(3)], AuꢀSbꢀCPh2 170.4(3) [174.9(3)], FꢀSbꢀCPh1 174.5(3)
[174.9(3)], CNap3ꢀSbꢀCNap4 164.0(4) [162.9(4)]. (b) Plot of the Electron Localization Function for [2]ꢀ. (c) Gold-centered Boys orbitals for
[2]ꢀ (drawn with a 0.02 isodensity value).
The salt crystallizes in the space group P1 with two independent
molecules in the asymmetric unit. The structure of [2]ꢀ shows
fluoride coordination to the antimony center with an average
SbꢀF bond distance of 2.08(2) Å (Figure 6). This SbꢀF
distance is similar to that observed in Ph4SbF (2.0530(8)
Å)27 or Ph3MeSbF (2.069(3) Å).28 Fluoride coordination
results in a nearly octahedral geometry around antimony with
average angles of 174.7° for CPh1ꢀSbꢀF, 172.6° for
CPh2ꢀSbꢀAu, and 163.4° for CNap1ꢀSbꢀCNap2. The coordi-
nation environment of the Au atom remains T-shaped with
average CNaphꢀAuꢀSb and CNap1ꢀAuꢀCNap2 angles of 90.3°
and 178.2°, respectively. The AuꢀSb distance of 2.771(2) Å
(av.) is virtually unchanged when compared to that in 1.
Inspection of the ELF map and Boys orbitals calculated for
[2]ꢀ shows that they are strikingly similar to those of 1 thus
indicating that the nature of the AuꢀSb bond is not significantly
altered by coordination of a fluoride anion to the antimony
center (Figure 6). In turn, compound [2]ꢀ can also be
described according to two resonance structures, the first one
corresponding to an aurated antimonate anion (resonance form a),
the second one to an aurate fluorostiborane derivative
(resonance form b).
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’ CONCLUSIONS
Upon standing, [n-Bu4N][2] precipitated as a colorless microcrystalline
solid which was washed with Et2O (2 mL) and dried in vacuum to yield
9.1 mg (70%). Crystals of [n-Bu4N][2] suitable for X-ray diffraction
were obtained by slow evaporation of a solution of the compound in
We have prepared and structurally characterized the first
example of a stiboranyl-gold complex (1). The structural and
computational results that we have obtained for this compound
indicate that tetraorganostiboranyl ligands have strong σ-donat-
ing properties. These strong σ-donating properties lead to an
accumulation of electron density on the gold center, giving rise in
the case of 1 to the emergence of aurophilicity, a phenomenon
typically preponderant in monovalent gold species.17 Another
manifestation of these strong σ-donating properties is the lack of
reactivity of the gold center toward halide anions and its
stabilization in a T-shaped geometry. These observations, com-
plemented by the results of the XANES measurements and DFT
calculations, underscore the importance of a diarylaurate-stibo-
nium resonance structure in the description of complex 1.
Adopting the donorꢀacceptor formalism that has been recently
popularized for late transition metals coordinated to Lewis acidic
ligands, we can describe complex 1 as a diarylaurate anion
stabilized by partial donation to a Lewis acidic stibonium ligand.
The same reasoning can be applied to [2]ꢀ, which can be
described as a diarylaurate anion intramolecularly stabilized by
a Lewis acidic fluorostiborane.9,21ꢀ23
1
THF/toluene (1/1 vol). H NMR (399.59 MHz; acetone-d6): δ 7.95
(br, 2H, Ph-CH), 7.75 (d, 2H, Naph-CH, 3JHꢀH = 7.88 Hz), 7.63 (d, 2H,
Naph-CH, 3JHꢀH = 6.59 Hz), 7.58 (d, 2H, Naph-CH, 3JHꢀH = 8.06 Hz),
7.42ꢀ7.52 (m, 3H, Ph-CH), 7.37 (dd, 2H, Naph-CH, 3JHꢀH = 8.06, 6.59
3
Hz), 7.33 (d, 2H, Ph-CH, JHꢀH = 6.96 Hz), 7.07 (t, 2H, Naph-CH,
3JHꢀH = 6.96 Hz), 6.97 (t, 1H, Ph-CH, 3JHꢀH = 6.96 Hz), 6.78ꢀ6.88
(m, 4H, Naph-CH, Ph-CH), 3.24 (m, 8H, n-Bu4Nþ), 1.63 (m, 8H,
3
n-Bu4Nþ), 1.26 (tq, 8H, n-Bu4Nþ, JHꢀH = 7.51 Hz), 0.88 (t, 8H,
n-Bu4Nþ, 3JHꢀH = 7.33 Hz). 13C{1H} NMR (100.45 MHz; acetone-d6):
δ 187.97, 145.86, 137.87, 136.86, 136.37, 135.74, 135.64, 135.08, 134.70,
131.45, 131.32, 130.06, 129.47, 129.03, 127.96, 125.71, 124.07, 123.95,
59.10, 24.33, 20.25, 13.86. 19F NMR (375.99 MHz; acetone-d6):
δ ꢀ43.26. HRMS: m/z calculated for C32H22FAuSbꢀ, 743.0409; found,
743.0425.
Computational Details. DFT structural optimizations for 1 and
[2]ꢀ were carried out using the ADF program (2008.01).32 All
calculations were carried out using the BP86 functional33 with the all
electron TZP basis sets for all atoms.34 These calculations were
performed using ZORA.35 Electron localization function (ELF)24
and Boys36 localization were carried out in the ADF program. ELF
plots as well as Boys localized orbitals were visualized in the ADFview
program. The structure of C has been previously optimized at the same
level of theory.10
’ EXPERIMENTAL SECTION
General Considerations. Antimony compounds are highly toxic
and should be handled cautiously. Triphenylphosphine was purchased
from Aldrich. Tetra-n-butylammonium fluoride was purchased from Alfa
Crystallographic Measurements. All crystallographic measure-
ments were performed at 110(2) K using a Bruker SMART APEX II
diffractometer with a CCD area detector (graphite monochromated Mo
KR radiation, λ = 0.71073 Å, ω-scans with a 0.5° step in ω) at 110 K. In
each case, a specimen of suitable size and quality was selected and
mounted onto a nylon loop. The semiempirical method SADABS was
applied for absorption correction. The structures were solved by direct
methods and refined by the full-matrix least-squares technique against F2
with the anisotropic temperature parameters for all non-hydrogen
atoms. All H-atoms were geometrically placed and refined in riding
model approximation. Data reduction and further calculations were
Aesar. 1,8-Dilithionaphthalene TMEDA,29 triphenyl-dibromoantimony,30
3
and AuCl(tht)31 (tht = tetrahydrothiophene) were prepared according
to the reported procedures. All preparations were carried out under an
atmosphere of dry N2 employing either a glovebox or standard Schlenk
techniques. Solvents were dried by passing through an alumina column
(CH2Cl2) or refluxing under N2 over Na/K (Et2O, n-hexane, and THF).
NMR spectra were recorded on a Varian Unity Inova 400 FT NMR
(399.59 MHz for 1H, 375.99 MHz for 19F, 100.45 MHz for 13C)
spectrometer at ambient temperature. Chemical shifts are given in
37
ppm and are referenced to residual H and 13C solvent signals and
1
performed using the Bruker SAINTþ and SHELXTL NT program
packages.38
external BF3 Et2O for 19F. Elemental analyses were performed by
3
After numerous modeling attempts, heavily disordered solvent mol-
ecules in the structure of [n-Bu4N][2] were handled using the Squeeze
program implemented in PLATON.39 The program calculated a sol-
vent-accessible volume of 726.9 Å3 (15.68% of the total unit cell
volume), which was then removed from subsequent structure factor
calculations.
Atlantic Microlab (Norcross, GA).
Synthesis of 1. A solution of Ph3SbBr2 (500 mg, 0.976 mmol) in
THF (3 mL) was added dropwise to a solution of 1,8-dilithionaphtha-
lene (250 mg, 0.976 mmol) in THF (2 mL) at ambient temperature. The
mixture was allowed to stir for 10 min before adding a solution of
AuCl(tht) (313 mg, 0.976 mmol) and Ph3P (256 mg, 0.976 mmol) in
THF (5 mL). The resulting clear yellow solution was layered with Et2O
(5 mL) and hexane (5 mL) and allowed to stand at room temperature.
After 2 days, a mixture of yellow and colorless crystals was obtained.
Both types of crystals were suitable for X-ray diffraction, which identified
the colorless crystals as AuBr(Ph3P)2 and the yellow crystals as complex 1.
Multiple recrystallizations of the crude mixture from hot CH2Cl2
X-ray Absorption Spectroscopy. X-ray absorption measure-
ments were acquired on Materials Research Collaborative Access Team
(MR-CAT) beamlines at the Advanced Photon Source, Argonne
National Laboratory. The Au L3-edge (11,919 eV) spectra were acquired
on an insertion-device beamline, and measurements of the Sb K-edge
(30,491 eV) were made on a bending magnet beamline. In both cases,
spectra of elemental foils (Au and Sb, respectively) were collected
simultaneously with the sample measurements for energy calibration
and multiple scans were taken to ensure spectrum reproducibility.
Insertion-device experiments utilized a cryogenically cooled double-
crystal Si (111) monochromator in conjunction with an uncoated glass
mirror to minimize the presence of harmonics. The monochromator was
scanned continuously during the measurements with data points
integrated over 0.6 eV for 0.07 s per data point. Measurements were
made in transmission mode with the ionization chambers optimized for
the maximum current with linear response (∼1010 photons detected/s)
using a mixture of nitrogen and helium in the incident X-ray detector and
a mixture of ca. 20% argon in nitrogen in the transmission X-ray detector.
1
(10 mL) provided a pure sample of 1 (130 mg, 37% yield). H NMR
3
(399.59 MHz; CDCl3): δ 8.02 (d, 2H, Naph-CH, JHꢀH = 7.81 Hz),
7.89 (dd, 2H, Naph-CH, 3JHꢀH = 6.6 Hz, 4JHꢀH = 1.2 Hz), 7.77 (dd, 2H,
Naph-CH, 3JHꢀH = 8.1 Hz, 4JHꢀH = 1.2 Hz), 7.64ꢀ7.58 (m, 4H, Naph-
CH), 7.44 (t, 2H, p-PhꢀCH, 3JHꢀH = 7.3 Hz), 7.34 (t, 4H, m-Ph-CH,
3JHꢀH = 7.3 Hz), 7.27ꢀ7.20 (m, 6H, o-Ph-CH and Naph-CH); 13C{1H}
NMR (100.45 MHz; CDCl3): δ 183.71, 145.82, 139.66, 136.68, 135.69,
134.80, 134.62, 134.20, 133.13, 130.49, 129.66, 126.78, 124.33, 124.18.
Elemental analysis calculated (%) for C32H22AuSb: C, 52.99; H, 3.06.
Found: C, 52.38; H, 3.12.
Synthesis of [n-Bu4N][2]. Solid TBAF 3H2O (4.4 mg, 0.013 mmol)
was added to a solution of 1 (10 mg, 0.013 mmol) in CHCl3 (1 mL).
3
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Journal of the American Chemical Society
ARTICLE
Photon energies at the bending magnet were selected using a water-
cooled, double-crystal Si(111) monochromator, which was detuned by
approximately 50% to reduce harmonic reflections. Measurements
were made in transmission mode with argon filled ionization chambers.
Data points were acquired in four separate regions (energies relative
to the elemental Sb K-edge): a pre-edge region ꢀ250 to ꢀ30 eV (step
size = 10 eV, dwell time = 0.3 s), the XANES region from ꢀ30 to þ40
eV (step size = 0.5 eV, dwell time = 0.2 s), an initial EXAFS region
from þ40 eV to 6 k (step size = 0.07 k, dwell time = 0.5 s), and a
final EXAFS region from 6 to 14 k (step size = 0.07 k, dwell
time = 0.5 s).
Samples were prepared by pressing ∼20 mg of finely ground powders
of the compounds mixed with silica gel into a 4 mm diameter cylindrical
sample holder. Mixtures were made to be approximately 7.5 wt % Au for
Au L3-edge and 10 wt % Sb for Sb K-edge, and enough sample was used
to obtain a step height of about 1.5. Grinding and sample preparation
were done in air, and the sample spectra were acquired in air at room
temperature.
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Corresponding Author
francois@tamu.edu
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Support by the National Science Foundation (CHE-0646916
and CHE-0952912), the Welch Foundation (A-1423), and Texas
A&M University (Davidson Professorship) is gratefully acknowl-
edged. The use of the Advanced Photon Source was supported
by the U.S. Department of Energy, Office of Science, Office of
Basic Energy Sciences, under Contract DE-AC02-06CH11357.
MRCAT operations are supported by the Department of Energy
and the MRCAT member institutions. We thank the referees for
their help in improving this manuscript.
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