Activation of a Hydroamination Gold Catalyst by Oxidation of a Redox-Noninnocent Chlorostibine Z-Ligand

Article abstract of DOI:10.1021/jacs.5b07998

In search of new platforms that support redox-controlled catalysis, we have investigated the noninnocent behavior of chlorostibine ligands coordinated to gold. The gold chlorostibine complex ((o-(Ph₂P)C₆H₄)₂SbCl

Full text of DOI:10.1021/jacs.5b07998

Article
pubs.acs.org/JACS
Activation of a Hydroamination Gold Catalyst by Oxidation of a
Redox-Noninnocent Chlorostibine Z‑Ligand
Haifeng Yang and Franc o̧ is P. Gabbaı*
̈
Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States
*
S Supporting Information
ABSTRACT: In search of new platforms that support redox-
controlled catalysis, we have investigated the noninnocent behavior
of chlorostibine ligands coordinated to gold. The gold chlorostibine
complex ((o-(Ph P)C H ) SbCl)AuCl (1-Cl) undergoes a clean
2
6
4 2
oxidation reaction on treatment with PhICl . This oxidation reaction
2
affords the corresponding trichlorostiborane complex ((o-(Ph P)-
2
C H ) SbCl )AuCl (2-Cl), which can be converted into the more
6
4 2
3
tractable trifluoride analogue ((o-(Ph P)C H ) SbF )AuCl (3-Cl) by
2
6
4 2
3
treatment with a fluoride source. As supported by experimental and
computational results, these complexes possess a Au→Sb donor−acceptor interaction which is distinctly stronger in the oxidized
complexes 2-Cl and 3-Cl. Both 1-Cl and 3-Cl undergo a clean chloride abstraction reaction to afford the corresponding cationic
+
+
+
+
−
gold species [((o-(Ph P)C H ) SbCl)Au] ([1] ) and [((o-(Ph P)C H ) SbF )Au] ([3] ), which have been isolated as SbF
6
2
6
4 2
2
6
4 2
3
+
salts. As a result of a stronger Au→Sb interaction, cation [3] features a more Lewis acidic gold center. It forms an isolable water
adduct and also activates terminal alkynes toward hydroamination with arylamines. These results demonstrate that the redox
state of noninnocent Z-ligands can be used to control the catalytic activity of the adjoining metal center.
INTRODUCTION
While often regarded as heavy phosphine analogues, stibine
ligands possess a number of unusual characteristics, including
The hydroamination of alkynes is a powerful synthetic
method which provides access to a diverse array of nitrogen-
containing organic compounds. These hydroamination reac-
tions are efficiently catalyzed by late-transition-metal com-
■
1
softer donor properties combined with increased accepting
7
8
2
plexes, including gold complexes. To date, most of the gold
properties. We have also demonstrated that triarylstibine
catalysts used are monoligated gold(I) complexes of the type
ligands are redox-noninnocent and can be converted from L-
type (A) to Z-type ligands (B) via oxidation (Chart 1).
+
3
4
[
LAu] in which the exposed and thus Lewis acidic metal ion is
readily available for substrate activation. Two-coordinate
+
gold(I) complexes of the type [L Au] are not active,
2
Chart 1. Oxidation of Coordinated Stibine Ligands
presumably because of the lack of Lewis acidity of the gold
center. Inspired by a recent report describing the use of a gold−
+
boron complex of the type [L Au→BAr ] as a catalyst for
2
3
9
enyne cyclization, we questioned whether the use of a redox-
active Z-ligand such as a halostibine could afford a complex
whose catalytic properties are controlled by the redox state of
the Z-ligand (Chart 2). In this paper, we describe a
chlorostibine gold complex whose catalytic activity for the
hydroamination of alkynes is turned on by oxidation of the
antimony center. These results add to the growing repertoire of
Oxidation proceeds without dissociation of the metal, provided
that ancillary ligands are employed. This redox process has
been shown to induce an umpolung of the M−Sb bond which
switches from Sb→M in the reduced state to M→Sb in the
10
redox-controlled catalytic processes and illustrate the role that
Chart 2. Representation of Possible Catalyst Structures
4
a
oxidized state. Interestingly, σ-accepting properties are also
5
observed with halostibine ligands (C). In this case, oxidation
of the antimony simply increases the Lewis acidity of the
antimony center, making it a stronger σ-acceptor and thus a
harder Z-ligand. While we have previously focused on the
impact of oxidation on the nature of the metal−antimony bond,
it occurred to us that such ligand-centered processes may
provide control over the Lewis acidity and reactivity of the
Received: July 30, 2015
6
adjoining metal center.
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b07998
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Z-ligands can play in the activation of otherwise inactive
catalytic centers.
induces a shortening of the Sb−Cl bond from 2.4829(13) Å in
6
,9,11
1-Cl to 2.4106(11) Å in [1][SbF ]. This contraction reflects
6
the increased Lewis acidity of the cationic antimony−gold
RESULTS AND DISCUSSIONS
Gold-chlorostibine complexes. The chlorostibine−gold
complex 1-Cl was synthesized from the known ligand L
complex. It also results in a lengthening of the Au−Sb bond
■
from 2.8527(6) Å in 1-Cl to 2.9318(5) Å in [1][SbF ], which
6
Cl 12
suggests a weakening of the Au−Sb interaction.
and
To better understand the nature of these changes, both 1-Cl
Au(tht)Cl (Scheme 1). This complex, which is an analogue of
and [1]+ have been studied computationally using DFT
1
4
15
13
methods (Gaussian program, functional BP86; mixed
Scheme 1. Synthesis of 1-Cl and [1][SbF6]
1
6
basis set Sb/Au cc-pVTZ-pp; P/Cl 6-31g(d′), C/H 6-31g).
The structure optimizations reproduce the trend observed
experimentally, with 1-Cl displaying a computed Au−Sb bond
+
shorter (2.911 Å) than that of [1] (2.955 Å). Analyses of these
structures using the natural bond orbital method (NBO)
suggest that the Au−Sb interactions are weak and are best
described as donor−acceptor interactions (Figure 2). As
5
the known complex [(o-(iPr P)C H ) SbCl]AuCl, reacts with
2
6
4 2
AgSbF6 in dichloromethane to afford the air-stable salt
3
1
[
1][SbF ]. The P NMR spectrum of [1][SbF ] features a
6
6
signal at 50.3 ppm, significantly downfield from that of 1-Cl at
8.4 ppm. Complexes 1-Cl and [1][SbF ] have been fully
3
6
characterized, and their structures have been determined using
single-crystal X-ray diffraction (Figure 1). In both complexes,
Figure 2. NBO plots of the major Sb−Au bonding interactions in 1-Cl
(
left) and [1][SbF ] (right) (isodensity value 0.05, the lp* orbital is a
6
vacant Sb orbital of p character). Hydrogen atoms are omitted.
observed for other chlorostibine−gold complexes investigated
5
by us, the NBO analysis shows that 1-Cl possesses a Au→Sb
rather than a Sb→Au interaction. In turn, we conclude that the
accepting properties of the chlorostibine moiety dominate its
ligative characteristics, as observed for related chlorobismuthine
17
complexes. This interaction, which has lp(Au)→lp*(Sb)
character, is associated with a deletion energy (or stabilization
energy) Edel of 70.79 kcal/mol. The donicity of the gold center
+
toward antimony is notably decreased in [1] , which possesses
a lp(Au)→σ*(Sb−Cl) interaction associated with a deletion
energy E of only 28.29 kcal/mol. This decrease directly
del
Figure 1. Solid-state structures of 1-Cl (top) and [1][SbF ] (bottom).
results from the removal of the gold-bound chloride anion
positioned trans from the antimony atom. The removal of this
ligand leads to a decreased electron density on the gold atom,
which becomes a weaker donor toward antimony. Such effects
have been previously discussed by Bourissou, who showed that
conversion of the gold boratrane [(o-(iPr P)C H ) B]AuCl
6
Thermal ellipsoids are drawn at the 50% probability level. Phenyl
groups are drawn in wireframe. Hydrogen atoms and solvent
molecules are omitted for clarity. Relevant metrical parameters can
be found in the text or the Supporting Information.
2
6
4 3
+
the antimony atom adopts a distorted seesaw geometry with a
into its cationic counterpart [((o-(iPr P)C H ) B)Au] is
2 6 4 3
Cl(1)−Sb−Au angle close to linearity (177.00(3)° for 1,
accompanied by a weakening of the Au→B interaction present
18
1
77.16(3)° for [1]SbF ) and a compressed C(1)−Sb−C(19)
in these complexes.
The electronic structure of [1] is reminiscent of that
6
+
angle (99.89(15)° for 1-Cl, 98.08(13)° for [1][SbF ]). The
6
+
9
gold center of 1-Cl displays a distorted square-planar geometry,
with the chloride bending away from the Sb−Au vector by
displayed by cations of type [D] (Chart 3). These cations,
which are derived from the corresponding gold chloride
11d
2
6.10(3)°. The primary ligands of [1][SbF ] are arranged
complex D-Cl,
have been reported as competent catalysts
6
about the gold atom in a distorted T-shaped geometry with
P(1)−Au−Sb(1), P(2)−Au−Sb(1), and P(1)−Au−P(2) angles
of 83.00(2), 81.39(3), and 159.46(4)°, respectively. The
coordination sphere of the gold atom is completed by the
for reactions that necessitate alkyne activation. Because the
+
chlorostibine moiety of [1] mimics the σ-acceptor properties
5
+
of boron, we decided to determine whether [1] could also be
used for the activation of alkynes. To test this idea, we chose to
study the hydroamination of alkynes, not only because of the
synthetic importance of this reaction but also because of its
−
SbF6 anion, which is engaged in a long secondary Au−F
contact of 2.931(3) Å. Conversion of 1-Cl into [1][SbF ]
6
B
DOI: 10.1021/jacs.5b07998
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Journal of the American Chemical Society
Chart 3. Side by Side Comparison Showing [1] and a
Article
+
+
9,13
Recently Reported Boron Complex of Type [D]
simplicity. A test reaction involving p-toluidine and phenyl-
Figure 3. Solid-state structure of 2-Cl. Thermal ellipsoids are drawn at
the 50% probability level. Phenyl groups are drawn in wireframe.
Hydrogen atoms are omitted for clarity. Relevant metrical parameters
can be found in the text or in the Supporting Information.
acetylene was carried out in CDCl under ambient conditions,
3
with a catalyst loading of 3.5 mol % (Scheme 2). Catalysts
Scheme 2. Model Hydroamination Reaction Used To
Evaluate [1][SbF6]
support for the increased Au→Sb dative interaction upon
oxidation of the antimony center. Finally, the antimony center
adopts an octahedral geometry which shows little distortion, as
indicated by the values of the Cl1−Sb−Au (167.95(11)°),
Cl2−Sb−Cl3 (175.75(10)°), and C1−Sb−C19 angles
(
169.6(4)°). Complex 2-Cl is poorly soluble in most organic
[
1][SbF ] displayed a non-negligible but very low activity, with
solvents, including CH Cl and CHCl , which complicated an
2
2
3
6
only 2.7% conversion into the Markovnikov imine product after
h. When the same reaction was repeated with [(Ph P) Au]-
exploration of its chemical reactivity. Confronted with this
difficulty, we decided to explore the generation of a more
soluble analogue. Gratifyingly, we observed that 2-Cl could be
easily converted into the more soluble trifluoride complex 3-Cl
3
3
2
SbF as a catalyst, no measurable conversion was observed.
6
+
These results suggest that the chlorostibine moiety of [1]
plays an activating role, albeit a very weak one. This prompted
us to question whether oxidation of the antimony atom could
be used to strengthen the Au→Sb interaction, leading to a
more acidic and thus more catalytically active gold center.
Gold−Trihalostiborane Complexes. With oxidation of
the antimony center as a goal, complex 1 was allowed to react
with PhICl in CH Cl (Scheme 3). This reaction afforded the
by reaction with TBAF or TASF in CH
P NMR resonance of 3-Cl at 83.5 ppm is split into a doublet,
2
Cl
2
(Scheme 3). The
31
indicating coupling to the axial antimony-bound fluorine atom
(JP−Fax = 16 Hz). The 19F NMR spectrum shows two signals
(
Figure 4). The two equatorial fluorine atom give rise to a
2
2
2
13
Scheme 3. Synthesis of 2-Cl, 3-Cl, and [3][SbF6]
Figure 4. Resonances observed in the 19F NMR spectrum of 3-Cl
measured in CDCl3.
doublet at −72 ppm with JFax−Feq = 17 Hz. The axial fluorine
resonance appears as a pseudoquintet at −133 ppm because of
the near-degeneracy of J
(16 Hz) and J
(17 Hz). The
ax−Feq
P−Fax
F
absence of coupling between the phosphorus and equatorial
fluorine nuclei is certainly noteworthy and may be rationalized
by invoking the Karplus rule and the orthogonality of the F−
Sb−F and P−Au−P vector. Crystals of 3-Cl were readily
obtained through Et O diffusion into a CH Cl solution. The
trichlorostiborane complex 2-Cl as a pale yellow precipitate.
The 31P NMR spectrum of 2-Cl shows a signal at 72.0 ppm,
which is significantly downfield from that of complex 1-Cl.
Crystals of 2-Cl were grown from CH Cl . Inspection of the
2
2
2
structure of 3-Cl is very similar to that of 2-Cl. The Sb−Au
bond of 2.7069(7) Å in 3-Cl is almost identical with that in 2-
Cl (2.6985(14) Å), suggesting little change in the Au→Sb
interaction. As in 2-Cl, (i) the gold atom of 3-Cl adopts a
square-planar geometry with a Cl−Au−Sb angle close to
linearity (176.37(7)°) and (ii) the antimony atom, now at the
center of a trifluorostiborane unit, retains an octahedral
geometry (F1−Sb1−Au1 = 178.3(2)°, F2−Sb1−F3 =
179.2(3)°, and C1−Sb1−C19 = 173.7(3)°).
2
2
structure confirms oxidation of the antimony center and
formation of a trichlorostiborane unit that directly engages the
gold atom in a Au→Sb interaction of 2.6985(14) Å (Figure 3).
The Au−Sb bond is notably shorter than that in 1-Cl
(
2.8527(6) Å), indicating that oxidation of the antimony
center increases its Lewis acidity, leading to a stronger Au→Sb
dative interaction. The gold atom displays a square-planar
geometry which, as indicated by the Cl−Au−Sb angle
(
177.88(5)° in 2-Cl vs 153.90(3)° in 1-Cl), is much less
With the intent of accessing a well-defined alkyne activation
catalyst, complex 3-Cl was treated with AgSbF in CH Cl ,
distorted than in 1-Cl. This square-planar geometry points to
6
2
2
the trivalent character of the gold atom, providing further
which resulted in the clean formation of the corresponding
C
DOI: 10.1021/jacs.5b07998
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Journal of the American Chemical Society
Article
+
cationic complex [3] as a hexafluoroantimonate salt (Scheme
of 2.728(4) Å. The presence of this contact, which is shorter
3
). A similar reaction could not be cleanly carried out with 2-Cl,
than that observed in [1][SbF ] (2.931(3) Å), attests to the
greater Lewis acidity of the gold atom. In turn, these
comparisons show that oxidation of the antimony center in
6
which gave an intractable product mixture when treated with
AgSbF . The salt [3][SbF ] has been characterized by
31
P
6
6
NMR spectroscopy, which shows a resonance at 66.3 ppm. The
F NMR spectrum displays two new resonances corresponding
[3][SbF ] translates into a stronger Au→Sb dative interaction
and a harder gold atom.
6
1
9
to the axial (−147.1 ppm) and equatorial fluorine ligands
Lewis Acidic Behavior and Electronic Structures of
(
−55.1 ppm). In addition to chemical shift changes, these
the Gold−Trihalostiborane Complexes. The increased
+
resonances are all singlets, indicating that the F−F and P−F
scalar coupling has become too small to observe. The
weakening of the Sb−Au bond upon conversion of 3-Cl into
hardness of the gold atom in [3] is reminiscent of the
+
mercury stibonium complex [E] , in which the Lewis acidity of
the d10 ion is enhanced by juxtaposition with a strongly Lewis
19
+
[
3][SbF ] (vide infra) may be responsible for the disappear-
acidic pentavalent antimony moiety. Complex [E] forms
adducts with both neutral and anionic donors (D), which
coordinate to the mercury center in a direction perpendicular
to the two primary ligands. The Lewis acidity enhancement
6
ance of the P−F scalar coupling. The disappearance of the F−F
coupling is more difficult to rationalize but is nevertheless
firmly established on the basis of the spectra. Crystals of
+
[
3][SbF ] could be obtained through pentane diffusion into a
observed for [3] has the same origin: namely, the presence of
6
CH Cl2 solution of the complex under moisture-free
conditions. The main distinguishing feature in the structure
an adjoining and highly acidic pentavalent antimony center. In
support of this analogy, we observe formation of the water
2
of [3][SbF ] is a Au−Sb bond length of 2.8196(4) Å, which is
adduct [3-OH
][SbF ], which was isolated as a byproduct in
2 6
6
significantly elongated in comparison to that in 3-Cl (2.7069(7)
small quantities due to the presence of adventitious water
Å) (Figure 5). As argued in the case of [1][SbF ], the removal
6
13
Scheme 4. Formation of [3-OH ][SbF ]
2
6
Figure 5. Solid-state structures of 3-Cl (top) and [3][SbF ] (bottom).
6
Thermal ellipsoids are drawn at the 50% probability level. Phenyl
groups are drawn in wireframe. Hydrogen atoms and solvent
molecules are omitted for clarity. Relevant metrical parameters can
be found in the text or in the Supporting Information.
Figure 6. Solid-state structure of [3-OH ][SbF ]. Thermal ellipsoids
2
6
are drawn at the 50% probability level. Phenyl groups are drawn in
wireframe. Hydrogen atoms and solvent molecules are omitted for
clarity. Relevant metrical parameters can be found in the text or in the
Supporting Information.
of the chloride anion from the gold center decreases the
metallobasicity of the latter, leading to a weaker Au→Sb
1
8
interaction. This weaker Au→Sb dative bond also affects the
length of the Sb−F bonds, which are notably shorter in
[
[
[
3][SbF ] (Sb−F = 1.942(7) Å in 3-Cl vs 1.897(4) Å in
6
ax
3][SbF ] and Sb−F = 1.961(6) Å in 3-Cl vs 1.942(2) Å in
(Scheme 4). X-ray analysis of this adduct (Figure 6) shows that
the water ligand is coordinated to the gold center via a bond of
length 2.383(3) Å. The Au−Sb bond of 2.7775(6) Å is
intermediate between that of 3-Cl (2.7069(7) Å) and
6
eq
3][SbF ]). The P(1)−Au−Sb, P(2)−Au−Sb, and P1−Au−P2
6
angles of 86.689(18), 86.689(18), and 173.38(4)°, respectively,
suggest that the gold coordination geometry is best described as
T-shaped. The structure of [3][SbF ] can also be compared
[3][SbF ] (2.837(4) Å). Accordingly, the Sb−F bonds in the
6
6
with that of [1][SbF ]. A comparison of the Au−Sb distance
water adduct [3-OH ][SbF ] (Sb−F = 1.914(3) Å and Sb−
6
2
6
ax
(
2.9318(5) Å in [1][SbF ] vs 2.8196(4) Å in [3][SbF ])
F = 1.947(3) Å) also fall between those in 3-Cl and [3][SbF ]
eq 6
6
6
illustrates the higher acidity of the oxidized antimony center
and the comparatively stronger Au→Sb dative interaction.
Finally, the vacant site trans from the antimony is flanked by
the SbF6− counteranion with which it forms an Au- - -F contact
(vide supra). These structural features can be assigned to the
donor strength of the water ligand, which is weaker than that of
the chloride anion in 3-Cl and yet stronger than that of the
weakly coordinating SbF6− anion in [3][SbF ]. The ability of
6
D
DOI: 10.1021/jacs.5b07998
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Journal of the American Chemical Society
Article
+
the gold center of [3] to coordinate water bears a parallel with
some recently described triorganogold(III) complexes, which
20
engage water among other hard donors. A further measure of
+
the Lewis acidity of [3] is provided by a simple Gutmann−
Beckett measurement using Ph PO (δ 27.3, CH Cl ) as a
3
2
2
+
Lewis base. When the base is mixed 1/1 with a [Au(PPh ) ]
3
2
3
1
salt in CH Cl , no shift of the Ph PO P NMR resonance is
2
2
3
observed, indicating that coordination does not occur. When
the same experiment is repeated with [1][SbF ] and [3][SbF ],
6
6
the resonance shifts downfield to 30.6 and 32.9 ppm,
respectively. These changes speak to the increased acidity of
the gold atom imparted by the presence of a chlorostibine unit
+
+
in [1] and an oxidized trifluorostiborane unit in [3] .
The ligand push/antimony pull effects observed in these
complexes can also be analyzed computationally. The structures
of 3-Cl, [3-OH ][SbF ], and [3][SbF ] have been optimized
2
6
6
computationally at the same level of theory as that used for 1-
+
Cl and [1] (basis set for F 6-31+g(d′)) (Figure 7). The
+
+
13
Figure 8. Resonance structures for [3] to [3-OH ] and 3-Cl.
2
resonance structures contribute to the electronic structures of
all three complexes, the structural and computational results
described above show that the importance of structure b
+
+
increases on going from [3] to [3-OH ] and 3-Cl. This
2
smooth transition illustrates the dynamic and adaptive nature of
the Au−Sb interaction, which increases in strength when the
gold atom accepts a donor ligand.
+
Figure 7. NBO plots of the major Sb−Au bonding interactions [3]
(
isodensity value 0.05). Hydrogen atoms are omitted. The same
Catalytic Properties of the Cationic Gold−Trifluor-
+
donor−acceptor interactions are also observed for 3-Cl and [3-OH ] .
2
ostiborane Salt [3][SbF ]. With [3][SbF ] in hand, we
6
6
became eager to test whether the stronger Au→Sb interaction
would indeed correlate with a higher catalytic activity in
reactions involving alkynes. While the reaction of p-toluidine
and phenylacetylene was very poorly catalyzed by [1][SbF6],
lengthening of the Sb−Au bond observed experimentally on
+
+
going from 3-Cl to [3-OH ] and [3] is nicely reproduced
2
computationally with distances of 2.76(6), 2.82(4), and 2.87(1)
Å for these three compounds, respectively. The NBO analyses
support the dative nature of the Au−Sb interaction (Figure 7).
In addition to lp(Au)→σ*(Sb−F) interactions, each complex
also features lp(Au)→σ*(Sb−C) donor−acceptor interactions
that make an important contribution to the stability of the
complexes. This is confirmed by deletion calculations, which
we found [3][SbF ] to be a very competent catalyst for this
6
reaction, with an essentially complete conversion after 40 min
at room temperature in CDCl , with a catalyst loading of 3.5
3
mol % (Table 1, entry 1). Carrying out the reaction under
strictly anhydrous conditions or in air gives identical results,
suggesting that adventitious water does not inhibit the activity
afford E = 148.52, 91.18, and 59.18 kcal/mol for 3-Cl, [3-
of the catalysts. In fact, when [3][SbF ] is treated with p-
del
6
+
+
31
OH ] , and [3] , respectively. These energies correlate with the
toluidine (4 equiv) in CH Cl , the P NMR resonance of the
2
2
2
1
9
donor strength of the ligand bound to gold and provide a
further illustration of the ligand push/antimony pull effect at
play in these complexes. Accordingly, the natural population
analysis (NPA) charges calculated for the antimony atom of
complex shifts from 63.6 to 76.5 ppm while the F NMR
resonances shift from −52.3 and −148.8 ppm to −63.0 and
−140.1 ppm. These changes suggest coordination of p-
+
toluidine to the gold center of [3] in solution. No notable
+
+
[
3] (2.312), [3-OH ] (2.274), and 3-Cl (2.139) decrease as
changes are observed when [3][SbF ] is treated with
2
6
the trans influence of the gold-bound ligand increases. The gold
phenylacetylene in CH Cl . The coordination of amines to
2
2
+
+
NPA charge also increases ([3] (0.276), [3-OH ] (0.379),
gold hydroamination catalysts is not unprecedented. It is
usually assumed that these adducts dissociate prior to activation
2
and 3-Cl (0.484)), suggesting a greater extent of Au→Sb
electron transfer. Finally, a comparison of the Au→Sb
stabilization energies in [1]+ (28.29 kcal/mol) and [3]+
2
2
of the alkyne. Since many late-transition-metal hydro-
7,23
8
amination catalysts,
including those with gold, often
(
59.18 kcal/mol) illustrates the increased σ-accepting proper-
necessitate an inert atmosphere, high temperature, and
ties of the trifluorostiborane, which behaves as a Z-type ligand.
relatively long reaction time, the fact that [3][SbF ] catalyzes
6
The increasing transfer of electron density from gold to
the reactions under mild conditions and in air is noteworthy.
We have also carried out this reaction on a larger scale (6.5
mmol), with a lower catalyst loading (0.5 mol %). Full
conversion was also observed but necessitated a longer reaction
time of 6 h. We have studied the scope of this reaction with
bulky, electron-rich, and/or electron-poor amines. As shown in
Table 1, hydroamination of phenylacetylene with the bulky 2,6-
+
+
antimony observed on going from [3] to [3-OH ] and 3-Cl
2
can also be depicted on the basis of the resonance structures
shown in Figure 8. Structures of type a correspond to Au(I)−
Sb(V) species with the gold atom in the monovalent state.
Resonance structures of type b, which account for the donation
of an electron pair from gold to antimony, correspond to
Au(II)−Sb(IV) species. Remembering that valence and formal
diisopropylaniline gave 93% of the desired products in 70 min
21
8h,23b
oxidation states are different concepts, the gold atom in
structures of type b is in the trivalent state. While both
under ambient conditions (entry 2).
The reaction
proceeded more slowly with the electron-poor pentafluoroani-
E
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Journal of the American Chemical Society
Article
Synthesis of 1-Cl. A CH Cl solution (8 mL) of (tht)AuCl (250
Table 1. Results Obtained in the Hydroamination of
Terminal Alkynes with Aromatic Amines with [3]SbF as a
Catalyst
2
2
mg, 0.78 mmol) was slowly added to a CH Cl solution (10 mL) of
2
2
6
Cl
L
(530 mg, 0.78 mmol) at ambient temperature. After the mixture
was stirred for 12 h, the solvent was removed under vacuum, affording
an oily residue. This residue was treated with Et O (10 mL), leading to
2
the precipitation of 1-Cl as a yellow solid. Compound 1-Cl was dried
under vacuum and obtained in a 93% yield (660 mg). Crystals of 1-Cl·
2
CH Cl suitable for X-ray diffraction were obtained by slow diffusion
2 2
of Et O into a concentrated CH Cl solution of 1-Cl at low
temperature (2−8 °C). H NMR (499.42 MHz; CDCl ): δ 7.01 (m,
2
2
2
1
3
3
2
7
7
H, o-P(Sb)C H ), 7.35 (t, 2 H, o-P(Sb)C H , J
= 7.49 Hz),
= 7.49 Hz),
6
4
6
4
H−H
3
.45−7.54 (m, 16 H), 7.64 (t, 2 H, o-P(Sb)C H , J
6
H−H
C−P
4
H−H
3
13
1
.86 (m, 4 H), 8.59 (d, 2 H, o-P(Sb)C H , J
= 7.49 Hz). C{ H}
= 28.9 Hz), 129.3 (t,
6
4
NMR (125.58 MHz; CDCl ): δ 128.5 (q, J
3
JC−P = 4.0 Hz), 129.4 (t of d, −C H , CH, J = 42.7 Hz, J = 5.9 Hz),
6
5
C−P
1
31.9 (d, −C H , CH, J
= 4.9 Hz), 132.7 (s), 133.5 (t, JC−P = 2.8
6
5
C−P
Hz), 134.2 (t of d, −C H , CH, J = 24.6 Hz, J = 7.2 Hz), 135.3 (t,
C−P
6
5
J
= 8.5 Hz), 137.7 (t, J
P{ H} NMR (202.16 MHz; CDCl ): δ 38.4 (s). Mp: 186 °C dec.
= 29.1 Hz), 149.3 (t, JC−P = 17.7 Hz).
C−P
C−P
3
1
1
3
Anal. Calcd for 1-Cl·2CH Cl (C H Cl P AuSb): C, 42.18; H, 2.98.
2
2
38 32
6 2
Found: C, 43.25; H, 3.15. These elemental analysis results may suggest
partial loss of the interstitial CH Cl . Anal. Calcd for 1-Cl·1.5CH Cl
2
2
2
2
(
C37.5H Cl P AuSb): C, 43.33; H, 3.01. For additional information on
31 5 2
the purity of this compound, please see the NMR spectra provided in
the Supporting Information.
Synthesis of [1][SbF ]. A CH Cl (1 mL) solution of AgSbF
6
2
2
6
a
1
The conversion (conv.) was measured by H NMR spectroscopy
(
15.5 mg, 45 μmol) was slowly added to a CH Cl solution (2 mL) of
2 2
using an internal standard. The yield given in parentheses corresponds
to the isolated yield.
complex 1-Cl (41 mg, 45 μmol). The resulting cloudy solution was
31
stirred for 15 min, filtered, and analyzed by P NMR spectroscopy,
which confirmed full conversion of 1-Cl into [1][SbF ]. The salt
6
[
1][SbF ] was purified by evaporation of the solvent and washing with
line, affording a 68% yield after 48 h (entry 3). The low yield of
this reaction is caused by partial hydrolysis of the imine product
and formation of acetophenone. The reaction also proceeded
quickly with phenylhydrazine (entry 4) but failed with
alkylamines such as cyclohexylamine, which on the basis of
NMR spectroscopy lead to formation of multiple species,
suggesting catalyst decomposition. We also examined the
hydroamination reactions of the less activated terminal aliphatic
alkyne. 1-Hexyne reacted with p-toluidine to produce the
desired imine within 60 min (entry 5).
6
two portions of Et O (2 mL). The salt [1][SbF ] was obtained in 56%
2
6
yield (38 mg) as a pale yellow powder. Light yellow plates of
1][SbF ]·CH Cl were obtained by slow diffusion of Et O into a
[
6
2
2
2
1
concentrated CH Cl solution of [1][SbF ] at room temperature. H
NMR (499.42 MHz; CDCl ): δ 7.15 (q, 2 H, o-P(Sb)C H , J
6
Hz), 7.64−7.66 (m, 12 H), 7.72 (t, 2 H, J
H, o-P(Sb)C
CDCl ): δ 125.0 (t, J
(
Hz), 133.3 (s, −C H , CH), 133.6 (s), 133.9 (t, J = 4.2 Hz), 134.1
(
Hz), 148.8 (t, J
δ 50.3 (s). Mp: 255 °C dec. Anal. Calcd for [1][SbF ]·CH Cl
(C H Cl F P AuSb ): C, 37.11; H, 2.53. Found: C, 37.58; H, 2.53.
For additional information on the purity of this compound, please see
the NMR spectra provided in the Supporting Information.
2
2
6
3
=
3
6
4
H−H
= 7.49
3
3
.49 Hz), 7.51 (q, 6 H, J
= 7.49 Hz), 7.60 (t, 4 H, J
H−H
H−H
3
= 7.99 Hz), 8.48 (d, 2
H−H
3
13
1
H , JH−H = 7.49 Hz). C{ H} NMR (125.58 MHz;
6
4
= 30.9 Hz), 125.8 (t, JC−P = 30.1 Hz), 130.3
= 37.7 Hz, J = 6.0 Hz), 131.1 (t, JC−P = 4.5
3
C−P
t of d, −C H , CH, J
6
5
C−P
6
5
C−P
t of d, −C H , CH, J
= 27.9 Hz, J = 7.4 Hz), 135.3 (t, J
= 16.1 Hz). P{ H} NMR (202.16 MHz; CDCl ):
= 7.8
C−P
6
5
C−P
CONCLUSIONS
■
31
1
C−P
3
In this paper, we have shown that the σ-accepting properties of
chlorostibine ligands can be greatly enhanced by oxidative
conversion into the corresponding trihalostiboranes. This
increase leads to a strengthening of the Au→Sb interaction.
As a result of this strengthening, the gold center of [3]+
acquires a trivalent character, making it a harder Lewis acid, as
confirmed by the isolation of a water adduct. More importantly,
the changes induced by oxidation of the antimony center also
6
2
2
37
30
3
6
2
2
Synthesis of 2-Cl. A CH Cl solution (5 mL) of PhICl (151 mg,
2
2
2
0
(
.55 mmol) was slowly added to a CH Cl solution (8 mL) of 1-Cl
2 2
387 mg, 0.42 mmol) at ambient temperature, resulting in the slow
precipitation of the product. After 2 h, the resulting mixture was
evacuated to dryness and the resulting pale yellow residue was washed
with ether. This procedure afforded 2-Cl in 94% yield (390 mg).
Complex 2-Cl is only slightly soluble in CHCl and CH Cl . Single
+
affect the reactivity of the gold center, making [3] a competent
catalyst for the hydroamination of alkynes.
3
2
2
crystals of 2-Cl were obtained by slow evaporation of the CH Cl
2
2
1
EXPERIMENTAL SECTION
solution. H NMR (499.42 MHz; CDCl ): δ 7.41 (t of d, 2 H, o-
■
3
3
3
3
General Considerations. [(tht)AuCl] (tht = tetrahydrothio-
P(Sb)C
6 4
H , JH−H = 6.49 Hz, JH−P = 4.00 Hz), 7.47 (t 10 H, JH−H =
, JH−H = 7.49 Hz), 7.65 (q 8 H,
H−H = 6.99 Hz), 7.74 (t, 2 H, JH−H = 8.49 Hz), 9.44 (d, 2 H, o-
24
25
3
phene) and PhICl2 were prepared according to the reported
procedures. Solvents were dried by passing through an alumina
7.49 Hz), 7.54 (t, 4 H, o-P(Sb)C
6
H
4
3
3
J
3
31
1
column (n-pentane and CH Cl ) or by reflux under N over Na/K
P(Sb)C H , J
= 7.99 Hz). P{ H} NMR (202.16 MHz; CDCl ):
H−H 3
2
2
2
6
4
(
Et O and THF). All other solvents were used as received.
δ 79.2 (s). The poor solubility of this product did not allow for a
satisfactory 13C NMR spectrum to be collected. Mp: 220 °C dec. Anal.
Calcd for 2-Cl (C H Cl P AuSb): C, 43.98; H, 2.87. Found: C,
2
Commercially available chemicals were purchased and used as
provided (commercial sources: Aldrich for SbCl and Bu NF; Strem
3
4
36 28
4 2
Chemicals for AgSbF ). Ambient-temperature NMR spectra were
43.30; H, 2.81. For additional information on the purity of this
compound, please see the NMR spectra provided in the Supporting
Information.
6
recorded on a Varian Unity Inova 500 FT NMR spectrometer (499.42
1
13
19
MHz for H, 125.58 MHz for C, 469.89 MHz for F, 202.16 MHz
for 31P). Chemical shifts δ are given in ppm and are referenced against
Synthesis of 3-Cl. TBAF·3H O (530 mg, 1.68 mmol) dissolved in
2
1
13
19
residual solvent signals ( H, C) or external BF ·Et O ( F) and
CH Cl (6 mL) was added was added within 1 min to a CH Cl (10
3
2
2
2
2
2
31
H PO ( P).
mL) suspension of 2-Cl (460 mg, 0.47 mmol). The yellowish
3
4
F
DOI: 10.1021/jacs.5b07998
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
suspension of 2-Cl became immediately clear and colorless. After the
mixture was stirred for 2 h, the solvent was removed under vacuum.
The withe residue was washed with three portions of methanol (4 mL)
and dried under vacuum, affording 3-Cl in 91% yield (397 mg).
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b07998.
■
*
S
Colorless crystals of 3-Cl·CH Cl2 were easily obtained by slow
2
1
Additional experimental and computational details
diffusion of ether into a CH Cl solution. H NMR (499.42 MHz;
2
2
3
(PDF)
CDCl ): δ 7.40−7.48 (m, 12H), 7.53 (t, 4 H, o-P(Sb)C H , J
=
3
6
3
4
H−H
3
7
.00 Hz), 7.60 (q 8 H, J
= 7.00 Hz), 7.70 (q, 2 H, J
= 8.00
Optimized stuructures (XYZ)
Crystallographic data (CIF)
H−H
H−H
3
13
1
Hz), 8.74 (d, 2 H, o-P(Sb)C H , J
= 8.00 Hz). C{ H} NMR
6
4
H−H
(
125.58 MHz; CDCl ): δ 121.8 (broad, weak, t, JC−P = 29.5 Hz), 127.0
3
(
t, JC−P = 31.0 Hz), 127.4 (t, JC−P = 31.3 Hz), 129.0 (t of d, −C H ,
6
5
AUTHOR INFORMATION
Corresponding Author
E-mail for F.P.G.: francois@tamu.edu.
■
*
CH, JC−P = 7.8 Hz, JC−F = 5.9 Hz) 130.9 (t of d, JC−P = 11.8 Hz, J
3
Hz), 133.8 (broad), 134.0 (t of d, −C H , CH, J = 9.4 Hz, JC−F
6
=
C−F
.1 Hz), 132.3 (d, −C H , CH, J
= 3.6 Hz), 133.3 (d, JC−P = 17.1
6
5
C−P
=
6
5
C−P
.4 Hz), 134.9 (m). 3 P{ H} NMR (202.16 MHz; CDCl ): δ = 83.5
1
1
Notes
3
d, JP−Fax = 16.2 Hz). 1 F{ H} NMR (469.89 MHz; CDCl ): δ −72.0
9
1
The authors declare no competing financial interest.
(
3
(
d, 2F , JF
= 17.0 Hz), −133.3 (pseudoquintet, 1F , JF
= 17.0
eq
eq−Fax
a
ax−Feq
ACKNOWLEDGMENTS
■
Hz, JFax−P = 16.2 Hz). Mp: 238 °C dec. Anal. Calcd for 3-Cl
This work was supported by the National Science Foundation
(CHE-1300371), the Welch Foundation (A-1423), and Texas
A&M University (Arthur E. Martell Chair of Chemistry).
(
C H ClF P AuSb): C, 46.31; H, 3.02. Found: C, 46.01; H, 3.22. For
36
28
3 2
additional information on the purity of this compound, please see the
NMR spectra provided in the Supporting Information.
Synthesis of [3][SbF ]. A CH Cl (2 mL) solution of AgSbF
6
2
2
6
REFERENCES
■
(
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(
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31
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(
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1
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depicted with a positive charge on the gold atom, in line with the
notion that these complexes contain a gold(I) cation ligated by two
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+
−
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3
2
formal charge on each phosphorus atom and a negative charge on the
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DOI: 10.1021/jacs.5b07998
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