Synthesis and study of cationic, two-coordinate triphenylphosphine- gold-π complexes

Article abstract of DOI:10.1002/chem.201204564

Cationic, two-coordinate triphenylphosphine-gold(I)-π complexes of the form [(PPh₃)Au(π ligand)]⁺ SbF₆^- (π ligand=4-methylstyrene, 1×SbF₆), 2-methyl-2-butene (3×SbF₆), 3-hexyne (6×SbF₆), 1,3-cyclohexadiene (7×SbF₆), 3-methyl-1,2-butadiene (8×SbF₆), and 1,7-diphenyl-3,4-heptadiene (10×SbF ₆) were generated in situ from reaction of [(PPh₃)AuCl], AgSbF₆, and π ligand at -78°C and were characterized by low-temperature, multinuclear NMR spectroscopy without isolation. The π ligands of these complexes were both weakly bound and kinetically labile and underwent facile intermolecular exchange with free ligand (ΔG ^≠≈9 kcal mol^-1 in the case of 6×SbF ₆) and competitive displacement by weak σ donors, such as trifluoromethane sulfonate. Triphenylphosphine-gold(I)-π complexes were thermally unstable and decomposed above -20°C to form the bis(triphenylphosphine) gold cation [(PPh₃)₂Au] ⁺SbF₆^- (2×SbF₆). Golden family: A family of cationic, two-coordinate gold complexes of the form [(PPh₃)Au(π ligand)]⁺SbF₆^- were generated in situ and characterized by low-temperature NMR spectroscopy (see scheme). The π ligands of these complexes underwent facile intermolecular exchange with free ligand (ΔG^≠≈9 kcal mol^-1) and competitive displacement by weak σ donors. Copyright

Full text of DOI:10.1002/chem.201204564

DOI: 10.1002/chem.201204564
Synthesis and Study of Cationic, Two-Coordinate Triphenylphosphine–
Gold–p Complexes
Rachel E. M. Brooner, Timothy J. Brown, and Ross A. Widenhoefer*[a]
Abstract: Cationic, two-coordinate tri-
phenylphosphine–gold(I)–p complexes
AgSbF6, and p ligand at À788C and
were characterized by low-temperature,
multinuclear NMR spectroscopy with-
out isolation. The p ligands of these
complexes were both weakly bound
and kinetically labile and underwent
facile intermolecular exchange with
free ligand (DG° ꢀ9 kcalmolÀ1 in the
case of 6·SbF6) and competitive dis-
placement by weak s donors, such as
trifluoromethane sulfonate. Triphenyl-
phosphine–gold(I)–p complexes were
thermally unstable and decomposed
above À208C to form the bis(triphenyl-
phosphine) gold cation [(PPh3)2Au] +
of the form [(PPh3)Au
ACHTUNGTRENNUNG
À
SbF6
(p ligand=4-methylstyrene,
1·SbF6), 2-methyl-2-butene (3·SbF6),
3-hexyne (6·SbF6), 1,3-cyclohexadiene
(7·SbF6),
3-methyl-1,2-butadiene
(8·SbF6), and 1,7-diphenyl-3,4-hepta-
diene (10·SbF6) were generated in situ
Keywords: alkene ligands · allenes ·
diene ligands · gold · p complexes
À
from
reaction
of
[(PPh3)AuCl],
SbF6 (2·SbF6).
Introduction
study of cationic, two-coordinate gold–p complexes.[7] From
these efforts, a number of mononuclear, cationic gold(I)
complexes containing a p alkene,[8,9] alkyne,[10,11] allene,[12]
conjugated diene,[13,14] or arene[15,16] p ligand and a sterically
hindered, electron-rich supporting ligand, such as 1,3-bis-
The utilization of soluble, gold(I) complexes as catalysts for
organic transformations has increased significantly over the
last decade.[1–4] In particular, cationic gold(I) complexes
have been applied extensively as catalysts for the electro-
AHCTUNGTRENNUNG
À
philic activation of C C p bonds. Included in this family of
transformations are the gold(I)-catalyzed hydrofunctionali-
U
ACHTUNGTRENNUNG
À
zation of C C multiple bonds with carbon and heteroatom
nucleophiles and the gold(I)-catalyzed cycloaddition of
enynes and related substrates.[3,4] Although a number of ster-
ically hindered, electron-rich supporting ligands, such as N-
heterocyclic carbenes (NHCs) and dialkyl o-biphenylphos-
phines, have been employed to good effect in gold(I) cataly-
sis, the simplest triarylphosphine, triphenylphosphine, re-
mains the workhorse ligand in gold(I) catalysis. In addition,
axially chiral bis(triarylphosphines) remain the most
common ligands utilized in enantioselective gold p-activa-
tion catalysis.[2]
Although direct experimental evidence is limited,[5–7] the
mechanisms most often invoked for gold(I) p-activation cat-
alysis involve activation of the C C multiple bond through
coordination to the carbophilic gold(I) center followed by
outer-sphere addition of the nucleophile to the p ligand.[6]
Due to the central role of gold–p complexes in these mecha-
nisms, there has been recent interest in the synthesis and
À
À [9,11–15]
Furthermore, the lower frequency value of d=28 ppm is
similar to that of weak s complexes, such as
[(PPh3)AuOTf][24] and [(PPh3)AuNTf2],[25] whereas the
higher frequency value of d=45 ppm is similar to the
bis(phosphine) cation [(PPh3)2Au]+ (2) (Table 1), complexes
that could be envisioned as byproducts or decomposition
products in the generation of triphenylphosphine–gold–
p complexes.[26,27] Perhaps best characterized of these tri-
[a] R. E. M. Brooner, Dr. T. J. Brown, Prof. R. A. Widenhoefer
Department of Chemistry, Duke University
French Family Science Center
Durham, NC 27708 (USA)
Fax : (+1)919-660-1605
E-mail: rwidenho@chem.duke.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201204564.
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FULL PAPER
Table 1. 31P NMR chemical shifts of relevant triphenylphosphine–gold–
p complexes in CD2Cl2.
s Complex
[(PPh3)AuCl]
[(PPh3)AuOTf]
ACHTUNGTRENNUNG[(PPh3)AuNTf2]
d at 258C [ppm]
d at À908C [ppm]
30.6
28.0
27.8
G
32.7
30.6
29.9
ACHTUNGTRENNUNG
45.5
30.7
41.8
28.5
phenylphosphine–gold–p complexes is the 4-methylstyrene
À
complex [(PPh3)Au(h2-H2C=CH-4-C6H4CH3)]+BF4 (1·BF4),
which was interrogated by variable-temperature, multinu-
clear, one- and two-dimensional NMR analysis.[23] However,
significant broadening of the reported 31P NMR resonance
of 1·BF4 (d=37.6 ppm, n1/2 ꢀ250 Hz at À208C) suggested
the possibility of a rapidly equilibrating mixture of com-
plexes.
Figure 1. Synthesis and 31P NMR chemical shifts of triphenylphosphine–
gold–p-complexes in CD2Cl2.
Due to the apparent inconsistencies and discrepancies in
the extant reports of two-coordinate triphenylphosphine–
gold–p complexes, we sought to develop a more detailed un-
derstanding of the spectroscopy and solution behavior of
these complexes. To this end, a family of gold complexes of
10.5 Hz) relative to free 2-methyl-2-butene (C2, d=132.0;
C3, d=118.0 ppm) in the 13C NMR spectrum of 3·SbF6 at
À858C and by the large downfield shift of the vinylic proton
resonance at d=5.96 ppm relative to free 2-methyl-2-butene
1
the form [(PPh3)Au
G
(d=5.12 ppm) in the H NMR spectrum of 3·SbF6 at À858C.
arene, alkyne, conjugated diene, allene) was generated and
characterized without isolation, employing low-temperature,
multinuclear NMR spectroscopy. Key observations made in
the course of our investigation include the following: 1) the
p ligands of cationic triphenylphosphine–gold–p complexes
are considerably more labile than the p ligands of related
gold NHC and o-biphenylphosphine p complexes; 2) careful
control of reaction stoichiometry and employment of a
Although the 13C NMR chemical shifts of the alkenyl
carbon atoms of 3·SbF6 are quite similar to those observed
À
for
[h2-Me(H)C=CMe2]}+SbF6
[PACTHNGURTE(NNUG tBu2)-o-biphenyl]AuACTHUNGTRENNUNG
(4·SbF6; d=146.1, 111.4 ppm), the vinylic proton of 4·SbF6
(d=4.20 ppm) was shifted upfield relative to free 2-methyl-
2-butene, as opposed to the strong downfield shift observed
for 3·SbF6.[9] A similar relationship was observed in the spec-
tra of triphenylphosphine and di(tert-butyl)-o-biphenylphos-
phine–p-allene complexes (see below). Warming a CD2Cl2
solution of 3·SbF6 at 08C led to decomposition (t1/2 ꢀ90 min)
À
weakly coordinating counterion, such as SbF6 , was required
to generate solutions of cationic triphenylphosphine–gold–
p complexes free from byproducts; 3) all of the gold–p com-
plexes generated in this study are thermally unstable and
decompose above À208C to form the bis(triphenylphos-
phine) gold cation [(PPh3)2Au]+ (2); and 4) the 31P NMR
resonance of these complexes falls within the range of d=
37.1Æ1.7 ppm.
À
with concomitant formation of [(PPh3)2Au]+SbF6 (2·SbF6),
as was evidenced by the appearance of the sharp, diagnostic
resonance at d=43.8 ppm in the 31P NMR spectrum and by
comparison to an authentic sample.
Effect of reaction stoichiometry: To explore the effect of re-
action stoichiometry on the composition and solution behav-
ior of 3, complex 3 was generated: in situ 1) with a deficien-
cy of 2-methyl-2-butene; 2) with a deficiency of AgSbF6À;
and 3) with the more strongly coordinating anions, NTf2
and OTfÀ. When an equimolar mixture of [(PPh3)AuCl],
and AgSbF6 was treated with a deficiency of 2-methyl-2-
butene (0.5 equiv) at À788C, the 31P NMR spectrum of the
resulting solution at À948C displayed an approximately 2:1
ratio of sharp resonances at d=36.8 and 28.2 ppm
(Figure 2). Although the d=36.8 ppm resonance can be con-
fidently assigned to 3·SbF6, the composition of the complex
(A·SbF6) that gives rise to the d=28.2 ppm resonance re-
Results and Discussion
Triphenylphosphine–gold–p-alkene complexes: Addition of
2-methyl-2-butene (1 equiv) to
[(PPh3)AuCl] and AgSbF6 in CD2Cl2 at À788C for 5 min led
to formation of thermally unstable {(PPh3)Au
[h2-Me(H)C=
a
1:1 mixture of
AHCTUNGTRENNUNG
À
CMe2]}+SbF6 (3·SbF6; Figure 1), which was characterized
without isolation by NMR spectroscopy. The 31P NMR spec-
trum of 3·SbF6 at À858C displayed a sharp resonance at d=
36.8 ppm consistent with the presence of a single organome-
tallic complex (see below). Complexation of the alkene to
gold was established by the downfield shift of the C2 alkene
carbon resonance (d=145.4 ppm) and the upfield shift of
À
mains unclear; the free gold cation [(PPh3)Au]+SbF6 , the
À
solvated cation [(PPh3)Au
silver heterobimetallic complex [(PPh3)Au
G
À
(m-Cl)Ag]+SbF6
represent possible structures. In any case, reaction of a 1:1
the C3 alkene carbon resonance (d=111.8 ppm, d, JCP
=
mixture of [(PPh3)AuCl] and AgSbF6 at À788C in the ab-
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8277

R. A. Widenhoefer et al.
converting mixture of p-alkene complex 3·SbF6 (d=
36.8 ppm), [(PPh3)AuCl] (d=30.6 ppm), the chloride-bridg-
À
ed bis
G
E
28.5 ppm)[28]
and free 2-methyl-2-butene, which favors
5·SbF6 and free alkene (Scheme 1). The thermal stability of
the mixture presumably reflects the low equilibrium concen-
tration of 3·SbF6 present under these conditions.
Scheme 1. Low-temperature reaction of a 2:1:2 mixture of [(PPh3)AuCl],
AgSbF6, and 2-methyl-2-butene in CD2Cl2; chemical-shift values refer to
the diagnostic 31P NMR resonances at À908C in CD2Cl2.
Effect of counterion: The effect of the more strongly coordi-
nating counterions, bistriflimide and triflate, on the forma-
tion and solution behavior of alkene complex 3 was also in-
vestigated. For example, when an equimolar mixture of
[(PPh3)AuNTf2] and 2-methyl-2-butene (73 mm) was com-
bined in CD2Cl2 at À788C [Eq. (1)], the 31P NMR spectrum
of the resulting mixture at À908C displayed an approxi-
mately 4:1 ratio of broad resonances at d=37 (n1/2 =105 Hz)
and d=28 ppm (n1/2 =38 Hz), consistent with an equilibrat-
ing mixture of p-alkene complex 3·NTf2 and bistriflimide
Figure 2. Variable-temperature 31P NMR spectra of the solutions generat-
ed from reaction of 1) a 2:2:1 mixture of [(PPh3)AuCl], AgSbF6, and
2-methyl-2-butene in CD2Cl2 at À788C (left); and 2) a 2:1:2 mixture of
[(PPh3)AuCl], AgSbF6, and 2-methyl-2-butene in CD2Cl2 at À788C
(right). Vertical amplitude is not consistent between spectra.
sence of alkene likewise displays a broad resonance at d
ꢀ28 ppm. Warming the aforementioned solution above
À508C resulted in broadening of the resonance of A·SbF6 at
dꢀ28 ppm without detectable broadening of the phospho-
rous resonance of 3·SbF6 below the onset of decomposition
at À208C (Figure 2), which argues against intermolecular
alkene exchange between 3·SbF6 and A·SbF6. Supporting
1
complex [(PPh3)AuNTf2]. Both the H and 13C NMR spectra
of the solution at À908C displayed broad, time-averaged
resonances for bound and free alkene, consistent with rapid
exchange of alkene and bistriflimide on the NMR time
scale. Similarly, when an equimolar mixture of
[(PPh3)AuCl], AgOTf, and 2-methyl-2-butene was combined
in CD2Cl2 at À788C [Eq. (2)], the 31P NMR spectrum at
À908C displayed an approximately 6:1 ratio of broad reso-
nances at d=38 (n1/2 ꢀ100 Hz) and 30 ppm (n1/2 ꢀ400 Hz),
consistent with an equilibrating mixture of 3·OTf and
[(PPh3)AuOTf].
1
this contention, the H NMR spectrum of this 2:1 mixture of
3·SbF6 and A·SbF6 displayed no broadening or shifting of
the alkene resonances of 3·SbF6 over the temperature range
À94 to 08C.
To evaluate the effect of a deficiency of silver salt on the
formation of 3·SbF6, a 2:1:2 mixture of [(PPh3)AuCl],
AgSbF6, and 2-methyl-2-butene was combined in CD2Cl2 at
À788C. The 31P NMR spectrum of this mixture at À908C
displayed a large broad resonance at d=29 and a small
broad peak at dꢀ35 ppm (Figure 2). When the temperature
was raised, these resonances coalesced to form a single peak
at d=30.6 ppm (n1/2 =84 Hz) at À508C, which sharpened
further upon warming the solution to 258C (d=30.0 ppm
(n1/2 =11 Hz); Figure 2); at this temperature, no formation
of 2·SbF6 was detected after 5 h. Likewise, the 1H NMR
spectrum of the same mixture at À908C revealed extremely
broad resonances in the vinylic (d=5.7–4.7 ppm) and ali-
phatic (d=2.4–0.9 ppm) regions, which sharpened upon
warming to À508C, forming one vinylic resonance at d=
5.27 and three methyl resonances at d=1.75, 1.68, and
1.60 ppm. These spectra are consistent with a rapidly inter-
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Synthesis and Study of Triphenylphosphine–Gold–p Complexes
FULL PAPER
À
[(PPh3)Au(h2-H2C=CH-4-C6H4Me)]+XÀ
SbF6
(1·SbF6)): The reported spectroscopy for 1·BF4,[23] in
(X=BF4
E
periment employing the less coordinating SbF6 counterion
R
was performed. To this end, an equimolar mixture of
[(PPh3)AuCl], AgSbF6, and 4-methylstyrene in CD2Cl2 was
mixed at À788C for 5 min. The H NMR spectrum of the re-
particular the significant perturbation of the vinylic proton
resonances of bound 4-methylstyrene relative to free 4-
methylstyrene, leave little doubt that complex 1·BF4 is gen-
erated in solution from mixtures of [(PPh3)AuCl], AgBF4,
and 4-methystyrene. However, as was noted in the introduc-
tion, broadening in the 31P NMR spectrum (À208C) suggest-
ed the presence of a mixture of complexes, presumably in-
cluding the free or solvated cationic species A·BF4, which
may be more prevalent than was A·SbF6 in the formation of
3·SbF6 owing both to the weaker ligating ability of 4-methyl-
styrene to gold(I) relative to 2-methyl-2-butene and the
1
sultant solution at À708C displayed a 1:1 ratio of doublets
at d=5.84 and 5.51 ppm, corresponding to the terminal vi-
nylic protons of bound 4-methylstyrene, whereas the inter-
nal vinylic proton was shifted downfield to the extent that it
was obscured by the aromatic resonances. The 31P NMR
spectrum of this solution at À308C displayed a broad reso-
nance at d=38.0 ppm (n1/2 =260 Hz), which broadened fur-
ther upon cooling to À508C (n1/2 =310 Hz). However, as the
temperature was lowered to À908C, the resonance sharp-
À
À [29]
stronger coordination of BF4 relative to SbF6 . Indeed,
ened and shifted downfield slightly to d=38.8 ppm (n1/2 =
the 31P NMR spectrum of
a
1:1:1.15 mixture of
120 Hz) with the appearance of a second broad resonance at
d=33–25 ppm, just visible above the baseline, consistent
with a mixture of 1·SbF6 and A·SbF6 (Figure 3).
[(PPh3)AuCl], AgBF4, and 4-methylstyrene in CD2Cl2 at
À208C displayed a single broad resonance at d=33.9 ppm
(n1/2 =112 Hz).[30] However, when the temperature was low-
ered, this resonance broadened significantly (n1/2 >1000 Hz;
Figure 3), indicating the presence of a mixture of complexes
in solution.
2
+
À
ꢁ
N
ACHTUNGTERNUN(NG 6·SbF6): Re-
action of an equimolar mixture of [(PPh3)AuCl], AgSbF6,
and 3-hexyne at À788C led formation of the thermally sensi-
2
Unable to resolve the 31P NMR spectrum of the solution
generated from reaction of [(PPh3)AuCl], AgBF4, and 4-
methylstyrene into its constituent resonances, a similar ex-
tive
3-hexyne
complex
[(PPh3)Au(h -CH3CH2C
ꢁ
CCH2CH3)]+SbF6 (6·SbF6). The 31P NMR spectum of
6·SbF6 at À708C displayed a single sharp resonance at d=
36.1 ppm (n1/2 =5.7 Hz), indicating the presence of a single
organometallic complex. Coordination of 3-hexyne to gold
was established by the downfield shift and carbon–phospho-
rous coupling of the sp-carbon resonance of 6·SbF6 at d=
91.9 ppm (d, JCP =8.6 Hz) relative to free 1-hexyne (d=
81.7 ppm) in the 13C NMR spectrum at À908C. Similarly, the
1H NMR spectrum of 6·SbF6 at À858C displayed a quartet
at d=2.79 and a triplet at 1.26 ppm corresponding to equiv-
alent ethyl groups of the 3-hexyne ligand, both of which
were shifted to higher frequency relative to free 3-hexyne
(d=2.29 and 1.12 ppm, respectively, at À908C). Warming a
CD2Cl2 solution of 6·SbF6 at À208C for 20 min led to ap-
proximately 15% decomposition to form 2·SbF6.
À
Mixtures of 6·SbF6 and free 3-hexyne displayed well-re-
solved resonances for free (d=2.79 ppm) and bound (d=
2.24 ppm) 3-hexyne in the 1H NMR spectrum at À858C.
This peak separation was exploited to analyze the kinetics
of intermolecular ligand exchange employing spin-saturation
techniques. To this end, the rate of intermolecular 3-hexyne
exchange with 6·SbF6 (80 mm) was recorded as a function of
[3-hexyne] from 18 to 60 mm in CD2Cl2 at À858C. A plot of
kobs versus [3-hexyne] was linear and established the second-
order rate law for intermolecular 3-hexyne exchange with
6·SbF6 of rate=k
G
E
14mÀ1 sÀ1 (DG°188K =9.10Æ0.05 kcalmolÀ1; Figure 4). These
data support an associative pathway for 3-hexyne exchange
via the cationic bisACTHNUTRGNEUNG
(alkyne) intermediate [(PPh3)Au(h2-
ꢁ
CH3CH2C CCH2CH3)2] SbF6 . It is worth noting that the
energy barrier for intermolecular 3-hexyne exchange with
6·SbF6 is ꢂ6 kcalmolÀ1 lower than the energy barriers deter-
mined for the intermomolecular p-ligand exchange with [P-
Figure 3. Variable-temperature 31P NMR spectra of the solutions generat-
ed from reaction of 1) a 1:1:1.15 mixture of [(PPh3)AuCl], AgBF4, and
4-methylstyrene in CD2Cl2 at À788C (left); and 2) a 1:1:1 mixture of
[(PPh3)AuCl], AgSbF6, and 4-methylstyrene in CD2Cl2 at À788C (right).
Vertical amplitude is not consistent among the spectra.
N
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R. A. Widenhoefer et al.
corresponding to the allylic protons (Figure 5). When the
temperature was raised, the pairs of resonances at d=6.50
and 6.41, and at 6.89 and 6.31 ppm broadened and coalesced
Figure 4. Plot of kobs versus [3-hexyne] for the intermolecular exchange of
the 3-hexyne ligand of 6·SbF6 ([6·SbF6]=80 mm) from 18 to 60 mm at
À858C in CD2Cl2.
ment about gold and the weaker gold–p ligand bond of
6·SbF6 relative to that of the P
p complexes.
ACHTUNGRTEN(NUNG tBu)2-o-biphenyl–gold–
The effect of a deficiency of alkyne on the formation and
solution behavior of 6·SbF6 was analogous to the effect of a
deficiency of 2-methyl-2-butene on the formation of 3·SbF6.
When a 2:2:1 ratio of [(PPh3)AuCl], AgSbF6, and 3-hexyne
was mixed thoroughly in CD2Cl2 at À788C, subsequent
31P NMR analysis at À908C revealed a 1.5:1 ratio of sharp
resonances at d=35.1 and 28.2 ppm corresponding to 6·SbF6
and A·SbF6, respectively. When the solution was warmed
above À508C, the 31P NMR resonance at d=28 ppm, corre-
sponding to A·SbF6, broadened without detectable broaden-
ing of the d=35 ppm resonance, corresponding to 6·SbF6.
Warming the solution above À208C led to rapid decomposi-
tion of 6·SbF6 (ca. 30% decomposition after 25 min) with
concomitant formation of bis(phosphine) complex 2·SbF6.
Figure 5. Temperature dependence of the alkenyl and aliphatic 1H NMR
resonances of 7·SbF6 from À94 to À258C in CD2Cl2.
[(PPh3)Au(h2-1,3-cyclohexadiene)]+SbF6
ACHTUNGTERN(NUNG 7·SbF6): Reac-
À
tion of an equimolar mixture of [(PPh3)AuCl], AgSbF6, and
1,3-cyclohexadiene in CD2Cl2 at À788C led to formation of
the thermally unstable h2-diene complex 7·SbF6, which dis-
played a single sharp resonance at d=36.3 ppm in the
31P NMR spectrum at À908C. Binding of gold to one of the
two C=C bonds of the cyclohexadiene ligand was establish-
independently forming a 1:1 ratio of resonances at d=6.48
and 6.62 ppm at À258C (Figure 5). Over the same tempera-
ture range, the broad allylic multiplet collapsed to form a
single sharp peak at d=2.65 ppm at À258C (Figure 5). Line-
shape analysis of the vinyl resonances at d=6.50 and
6.41 ppm near the coalescence temperature provided an
energy barrier for exchange of the complexed and uncom-
plexed C=C bonds of the 1,3-cyclohexadiene ligand of
7·SbF6 of DG°185K ꢃ9.0 kcalmolÀ1.[31]
1
ed by low-temperature 13C and H NMR analysis, notably by
the presence of four alkenyl resonances (d=137.9, 121.7,
119.8, 117.9 ppm) and two aliphatic resonances (25.0,
22.2 ppm) corresponding to the six chemically inequivalent
carbon atoms of the cyclohexadiene ligand in the 13C NMR
spectrum of 7·SbF6 at À908C.
The presence of a single aliphatic resonance in the fast-ex-
change spectrum of 7·SbF6 points to an intermolecular path-
way for interconversion of the complexed and uncomplexed
C=C bonds of the cyclohexadiene ligand. Because an intra-
molecular pathway for exchange would occur without ex-
change of the two faces of the 1,3-cyclohexadiene ligand,
the fast-exchange spectrum should display two allylic reso-
nances for the time-averaged allylic protons positioned cis
Complex 7·SbF6 displayed fluxional behavior consistent
with facile interconversion of the complexed and uncom-
plexed C=C bonds of the cyclohexadiene ligand. For exam-
1
ple, the H NMR spectrum of 7·SbF6 at À948C displayed a
1:1:1:1 ratio of four broad vinylic resonances at d=6.89,
6.50, 6.41, and 6.31 and a broad multiplet at 2.8–2.3 ppm,
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Synthesis and Study of Triphenylphosphine–Gold–p Complexes
FULL PAPER
and trans to the gold–phosphine moiety, barring accidental
equivalence. Indeed, we have previously established an in-
tramolecular mechanism for the interconversion of the com-
plexed and uncomplexed C=C bonds of {[P
G
N
À
A
U
u(h2-1,3-cyclohexadiene)}+SbF6 with an energy barri-
er of DG°233K =10.4 kcalmolÀ1.[14] Conversely, an intermolec-
ular pathway for exchange would necessarily occur with
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
resonance in the fast-exchange spectrum, as was observed
for 7·SbF6. Supporting this contention, the 1H NMR spec-
trum of a solution of 7·SbF6 (42 mm) that contained a trace
of free 1,3-cyclohexadiene (ca. 4 mm) at À908C displayed
only two alkenyl (d=6.56 and 6.42 ppm) and one allylic-
proton resonance (d=2.61 ppm), which established rapid,
intermolecular exchange of bound and free 1,3-cyclohexa-
diene.
Triphenylphosphine–gold–p-allene complexes: We have pre-
viously reported the synthesis of gold–p allene complexes of
À [12]
(p allene)]+SbF6 . Solu-
the form [{PACHTUNGTRENNUNG(tBu)2-o-biphenyl}AuCAHTUNGTRENNUGN
tion-phase analysis of these complexes established h2-allene
ground state species that underwent facile (DG° =8.9–
11.4 kcalmolÀ1) p-face exchange through staggered h2-allene
transition states. Solution-state analysis of gold complexes
containing a chiral allene ligand, such as the gold 4,5-nona-
diene complex {[P
ACHTUNGTRENNUNG
(tBu)2-o-biphenyl}Au[h2-CH3CH
ACHUTGTNRENNUG 2CAHTUNGTERNCNUGN H2-
Figure 6. Temperature dependence of the allenyl methyl 1H NMR reso-
nances of 8·SbF6 in CD2Cl2. Amplitude is not consistent between spectra.
À
ed by the presence of both cis- and trans-diastereomers (DG
ꢀ0.4 kcalmolÀ1), which underwent p-face exchange and cis/
trans isomerization through multiple, diastereomeric h1-
allene transition states. Building from these studies, we ex-
tended our analysis of triphenylphosphine-gold–p complexes
to include p-allene complexes. In one experiment, reaction
of an equimolar mixture of [(PPh3)AuCl], AgSbF6, and 3-
methyl-1,2-butadiene in CD2Cl2 at À788C for 5 min generat-
broadened and coalesced, forming a single resonance at d=
1.93 ppm at À308C, with an energy barrier of DG°
=
215K
10.9 kcalmolÀ1, as was determined from line-shape analysis.
Although this value is similar to the energy barrier deter-
mined for intramolecular p-face exchange of the 3-methyl-
1,2-butadiene ligand of gold–allene complex {[P
(tBu)2-o-bi-
À
À
ed the [(PPh3)Au(h2-H2C=C=CMe2)]+SbF6 (8·SbF6), which
phenyl]Au(h2-H2C=C=CMe2)}+SbF6
(DG°220K =11.0 kcal
displayed a single sharp resonance at d=36.3 ppm in the
molÀ1),[12] we could not distinguish between intra- and inter-
molecular pathways for interconversion of the diastereotop-
ic methyl groups of 8·SbF6 (see below). Warming a solution
of 8·SbF6 at 08C for 1.5 h led to 90% decomposition to form
the bis(triphenylphosphine)–gold cation 2·SbF6.
31P NMR at À908C. p Complexation of 3-methyl-1,2-buta-
1
diene to gold was established by H and 13C NMR spectro-
scopy at À908C. Notably, the 13C NMR chemical shifts of
the allenyl carbon atoms of 8·SbF6 [d=105.3 (CMe2), 198.2
(=C=), 62.6 ppm (CH2)] were shifted significantly from
those of free 3-methyl-1,2-butadiene [d=94.4, 205,
72.6 ppm] and were similar to those observed for the related
Both the 31P NMR chemical shift and thermal instability
of 8·SbF6 appeared incongruent with the data reported for
the
related
gold–p allene
complex
[(PPh3)Au(h2-
PhCH2CH2(H)C=C=C(H)CH2CH2Ph)]+XÀ
[X=NTf2
À
p-allene complex {[P
G
G
À
À
(CH3)2]}+SbF6 (9·SbF6) [d=105.6 (CMe2), 197.0 (=C=),
(10·NTf2), BF4 (10·BF4)].[21] Complex 10 was reportedly
63.1 ppm (CH2)]. In comparison, the allenyl proton of
8·SbF6 (d=5.23 ppm) was shifted downfield relative to the
allenyl resonance of both free 3-methyl-1,2-butadiene (d=
4.46 ppm) and (9·SbF6) (d=4.07 ppm).[12] Selective binding
of the 3-methyl-1,2-butadiene ligand of 8·SbF6 through the
less substituted allenyl C=C bond was established by the
presence of a pair of diastereotopic allenyl methyl resonan-
ces in both the 13C (d=21.4, 20.2 ppm) and the 1H NMR
spectrum (d=1.93, 1.87 ppm) of 8·SbF6 at À908C (Figure 6).
When the temperature was raised, the proton resonances
generated in situ from reaction of 1,7-diphenylhepta-3,4-
diene with either [(PPh3)AuNTf2] or
a
mixture of
[(PPh3)AuCl] and AgBF4 in CD3NO2 and/or CD2Cl2 and
was characterized solely on the basis of sharp resonance at
d=45.2 ppm in CD3NO2 (44.8 ppm in CD2Cl2) in the
31P NMR spectrum at 25–458C. Although the chemical shift
reported for 10 is not within the range defined by triphenyl-
phosphine-gold–p complexes 1, 3, and 6–8 (d=36.1–
38.1 ppm, Figure 1), it is near the value expected for the
bis(triphenylphosphine)–gold cation 2, and the rapid decom-
Chem. Eur. J. 2013, 19, 8276 – 8284
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8281

R. A. Widenhoefer et al.
position of 10 to form 2 at or above room temperature is in
accord with the observed thermal stability of 8.
To gain insight into the spectroscopy and solution behav-
ior of gold–p allene complex 10, the reaction of an equimo-
lar mixture of [(PPh3)AuCl], AgSbF6, and 1,7-diphenyl-3,4-
heptadiene was investigated in CD2Cl2 at À788C for 5 min.
Low-temperature one- and two-dimensional NMR analysis
of the resulting solution was most consistent with an equili-
brating approximately 1:2.5 mixture of diastereomeric h2-
allene complexes, presumably cis-10·SbF6 and trans-10·SbF6,
although we have no information that would allow assign-
ment of the major and minor isomers and little information
regarding the structure of the minor isomer. For example,
the 31P NMR spectrum of 10·SbF6 at À898C displayed an ap-
proximately 1:2.5 ratio of resonances at d=36.5 and
35.4 ppm, whereas the 1H NMR spectrum of 10·SbF6 at
À898C displayed an approximately 1:2.5 ratio of broad al-
lenyl resonances at d=6.14 and 6.03 ppm; assignment of the
6.03 ppm resonance to both allenyl protons (time-averaged
or coincidental) of the major diastereomer was further sup-
ported by 1H–1H COSY and heteronuclear multiple-quan-
tum-coherence (HMQC) analysis at À908C (see below, Fig-
ures S1 and S2 in the Supporting Information). When the
1
temperature was raised, the approximately 1:2.5 ratio of H
and 31P NMR resonances broadened and coalesced, forming
single resonances in both the 31P (d=36.6 ppm at À508C)
and 1H (d=6.06 ppm at À608C) NMR spectra, with an
energy barrier for interconversion of the major and minor
diastereomers of 10·SbF6 of DG°215K =9.5 kcalmolÀ1, as was
determined from 31P NMR line-shape analysis (Figure 7).
Assignment of the major diastereomer of 10·SbF6 as an
h2-allene complex was further supported by low-temperature
13C NMR analysis. The 13C NMR spectrum of 10·SbF6 at
Figure 7. Temperature dependence of the 31P resonances (left) and allenyl
1H resonances (right) of 10·SbF6 in CD2Cl2. Amplitude is not consistent
between spectra.
We performed some additional experiments that evaluat-
ed the spectroscopy and solution behavior of 10 a function
of counterion and solvent. In one experiment, an equimolar
mixture of [(PPh3)AuNTf2] and 1,7-diphenyl-3,4-heptadiene
was mixed at À788C in CD2Cl2 for 5 min. The 31P NMR
spectrum of the resulting solution at À908C displayed an ap-
proximately 2.5:1 ratio resonances at d=35.2 and 27.8 ppm,
assigned to the time-averaged resonance of cis- and trans-
10·NTf2 and [(PPh3)AuNTf2], respectively. When the tem-
perature was raised, these resonances collapsed to form a
single resonance d=28.4 ppm at À308C. In contrast to
10·SbF6, warming this solution to 258C led to only about
9% conversion to the bis(triphenylphosphine) cation 2·NTf2
after 12 h, which is perhaps due to low concentration of
10·NTf2 in solution. In comparison, when an equimolar mix-
ture of [(PPh3)AuNTf2] and 1,7-diphenyl-3,4-heptadiene was
combined in CD3NO2 at À258C, the 31P NMR spectrum ob-
tained within 10 min of mixing displayed an approximately
1:5 ratio of a sharp resonance at d=42.4 ppm, correspond-
À908C displayed a resonance at d=190.2 ppm (d, JCP
=
6.9 Hz), an approximately 1:1 ratio of resonances at d=
100.0 and 86.0 ppm (d, JCP =6.7 Hz), and an approximately
1:1:1:1 ratio of resonances at d=34.8, 33.7, 32.5, and
31.8 ppm, assigned to the allenyl sp-carbon atom, the allenyl
sp2-carbon atoms, and the methylene-carbon atoms, respec-
tively, of the allene ligand of the major diastereomer of 10
under conditions of slow exchange.[32] Corroborating this as-
signment, HMQC analysis of 10·SbF6 at À908C revealed a
strong correlation between the allenyl sp2-carbon resonances
at d=100.0 and 86.0 with the 1H NMR resonance at d=
6.03 ppm assigned to the allenyl protons of the major dia-
stereomer (Figure S2 in the Supporting Information). In ad-
dition to these resonances, the 13C NMR spectrum of
10·SbF6 at À908C displayed a resonance at d=188.6 ppm,
which was approximately half the intensity of the 190.2 ppm
resonance and was assigned to the allenyl sp-carbon atom of
the minor diastereomer. The associated allenyl sp2-carbon
resonances and methylene-carbon resonances of the minor
diastereomer were not observed, presumably due to more
facile p-face exchange of this diastereomer and failure to re-
alize the slow-exchange regime in the 13C NMR spectrum at
À908C. Warming a solution of 10·SbF6 at 08C for 75 min led
to 37% decomposition to form 2·SbF6.
ing to 2·NTf2, and a broad resonance at d=36.1 ppm (n1/2
=
150 Hz), corresponding either to 10·NTf2 or to an equilibrat-
ing mixture of 10·NTf2 and [(PPh3)AuNTf2]. Warming this
mixture above À208C resulted in rapid disappearance of the
d=36.1 ppm resonance with concomitant increase in the
42.4 ppm resonance of 2·NTf2 (Table 1). From these experi-
ments, we conclude that the spectroscopy and solution be-
havior previously attributed to 10 correspond instead to the
bis(triphenylphosphine)–gold complex 2 generated through
8282
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 8276 – 8284

Synthesis and Study of Triphenylphosphine–Gold–p Complexes
FULL PAPER
6H), 5.23 (br s, 2H), 1.93 (s, 3H), 1.87 ppm (s, 3H); 13C{1H} NMR
(125 MHz, À908C): d=198.2, 133.5 (d, J=13.3 Hz), 132.4, 129.3 (d, J=
11.9 Hz), 125.0 (d, J=62.6 Hz), 105.3, 62.6 (d, J=10.4 Hz), 21.4,
decomposition of 10 under the conditions employed for its
synthesis and spectroscopic analysis.[21]
20.2 ppm; 31P{1H} NMR (202 MHz, À908C): d=36.3 ppm.
À
G
ACHTUNGTRENNUNG(10·SbF6):
Approximately 2.5:1 mixture of diastereomers; 1H NMR (500 MHz,
À898C): d=7.75–7.36 (m, 29H), 7.36–6.86 (m, 45H), 6.78 (br s, 9H), 6.42
(br d, J=7.4 Hz, 4H), [6.14 (br s), 6.03 (br s), ca. 1:2.5, 2 H] 3.15–
2.16 ppm (m, 26H); 13C{1H} NMR (125 MHz, À858C): d=[190.2 (d, J=
6.9 Hz), 187.8 (d, J=7.2 Hz), ca. 2.5:1], 139.4, 139.0, 133.5 (d, J=
13.5 Hz), 132.5 (m), 129.1 (d, J=12.0 Hz), 128.5, 128.1, 127.9, 126.0,
125.7, [124.8 (d, J=62.5 Hz), 124.5 (d, J=62.5 Hz), 1:2.5], 100.0, 86.0 (d,
J=6.7 Hz), 34.0, 32.9, 31.6, 31.4 ppm; 31P{1H} NMR (202 MHz, À898C):
d=36.5, 35.4 ppm (ca. 1:2.5).
Conclusion
Monomeric, cationic triphenylphosphine–gold(I) complexes
that contain a p-alkene, vinyl arene, internal alkyne, conju-
gated diene, or allene ligand were synthesized and charac-
terized without isolation by low-temperature NMR spec
ACHTUNGTNERtNUNG ros-
ACHTUNGTRENNUNG
es is thermally unstable and decomposes above À208C in
CD2Cl2 to form the bis(triphenylphosphine)–gold cation
[(Ph3P)2Au]+ (2). The p ligands of these complexes undergo
rapid intermolecular exchange with free ligand and are com-
petitively displaced by weak s donors, such as OTfÀ. One of
the key ramifications of the weak gold–p-ligand interaction
of these complexes is that careful control of reaction stoichi-
ometry and employment of a weakly coordinating counter-
Acknowledgements
Acknowledgements is made to the NSF (CHE-0555425) for support of
this research. T.J.B. thanks Duke University for Zielik and Hauser fellow-
ships and R.E.M.B. thanks Duke University for Charles Bradsher and
Joe Taylor Adams fellowships.
À
ion, such as SbF6 , is required to generate pure triphenyl-
phosphine–gold–p complexes. In many cases, the presence
of rapidly equilibrating impurities was revealed only by low-
temperature 31P NMR spectroscopy (ꢃÀ808C). The
31P NMR resonance of these triphenylphosphine–gold–
p complexes was diagnostic and fell within the range d=
37.1Æ1.7 ppm.
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Experimental Section
À
{(PPh3)Au
[h2-Me(H)C=CMe2]}+SbF6
N
A
solution
of
[(PPh3)AuCl] (22 mg, 4.4ꢁ10À2 mmol) in CD2Cl2 (0.50 mL) was added to
an NMR tube containing AgSbF6 (15 mg, 4.4ꢁ10À2 mmol); the mixture
was shaken vigorously, and the resultant suspension was cooled to
À788C. To this suspension was added 2-methyl-2-butene (3.1 mg, 4.4ꢁ
10À2 mmol) at À788C. The contents of the tube were thoroughly mixed
by inverting the tube repeatedly over about 2 min period while maintain-
ing temperature at À788C, causing a fine white powder to precipitate
from solution. Low-temperature 1H and 31P NMR analyses of the result-
ing solution revealed complete conversion of the starting materials to
3·SbF6. 1H NMR (500 MHz, À858C): d=7.68–7.24 (m, 15H), 5.98–5.90
(m, 1H), 2.32 (s, 3H), 2.27 (s, 3H), 2.03 ppm (d, J=1.5 Hz, 3H);
13C{1H} NMR (125 MHz, À858C): d=145.4, 133.5 (d, J=13.4 Hz), 132.4,
129.2 (d. J=11.7 Hz), 125.1 (d, J=62.7 Hz), 111.8 (d, J=10.5 Hz), 28.5,
21.5, 16.0 ppm; 31P{1H} NMR (202 MHz, À708C): d=36.8 ppm.
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Complexes 1·SbF6, 6·SbF6–8·SbF6, and 10·SbF6 were generated in solution
by employing a procedure similar to that used to generate 3·SbF6.
2
+
À
[(PPh3)Au(h -CH3CH2CC CCCH2CH3)] SbF6
G
1H NMR
ꢁ
(500 MHz, À708C): d=7.65 (t, J=7.4 Hz, 3H), 7.57 (dt, J=1.5, 7.1 Hz,
6H), 7.48–7.40 (m, 6H), 2.80 (q, J=7.0 Hz, 4H), 1.27 ppm (t, J=7.2 Hz,
6H); 13C{1H} NMR (125 MHz, À908C): d=133.4 (d, J=13.6 Hz), 132.4
(d, J=2 Hz), 129.2 (d, J=12.2 Hz), 125.0 (d, J=63.7 Hz), 91.9 (d, J=
8.6), 15.0, 14.0 ppm; 31P{1H} NMR (202 MHz, À708C): d=36.1 ppm.
À
[(PPh3)Au(h2-1,3-cyclohexadiene)]+SbF6
(7·SbF6): 1H NMR (500 MHz,
À948C): d=7.62 (t, J=7.5 Hz, 3H), 7.54 (dt, J=1.4, 7.2 Hz, 6H), 7.43–
7.36 (m, 6H), 6.89 (br s, 1H), 6.50 (br s, 1H), 6.41 (br s, 1H), 6.31 (br s,
1H), 2.82–2.32 ppm (m, 4H); 13C{1H} NMR (125 MHz, À908C): d=
137.9 (br), 133.5 (br), 132.4, 129.2 (d, J=12.1 Hz), 125.2 (d, J=62.0 Hz),
121.7 (br), 119.8 (br), 117.9 (br), 25.0 (br), 22.2 ppm (br); 31P{1H} NMR
(202 MHz, À908C): d=36.3 ppm.
À
[(PPh3)Au(h2-H2C=C=CMe2)]+SbF6
(8·SbF6): 1H NMR (500 MHz,
ACHTUNGTRENNUNG
À908C): d=7.64 (t, J=7.1 Hz, 3H), 7.59–7.52 (m, 6H), 7.49–7.40 (m,
Chem. Eur. J. 2013, 19, 8276 – 8284
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. K. Patra, M. Green, C. A. Russell, Chem. Commun. 2012, 48,
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[29] We have previously shown that the equilibrium-binding affinity of 2-
methyl-2-butene to the twelve-electron gold fragment [PACHTNUTRGNEUNG(tBu)2-o-bi-
phenyl]Au+ fragment is about 17 fold greater than is that of 4-meth-
ylstyrene.[9]
[14] R. E. M. Brooner, R. A. Widenhoefer, Organometallics 2011, 30,
3182–3193.
[30] The discrepancy between the chemical shift reported by Macchoni
(d=37.8 ppm) and in this work (d=34.3 ppm) appears to be due to
the different referencing systems employed in the two studies (85%
H3PO4 versus (PPh3)O in CD2Cl2). Supporting this contention, Mac-
choni reported the 31P NMR chemical shift of the decomposition
[15] a) E. Herrero-Gómez, C. Nieto-Oberhuber, S. López, J. Benet-
Buchholz, A. M. Echavarren, Angew. Chem. 2006, 118, 5581–5585;
Angew. Chem. Int. Ed. 2006, 45, 5455–5459; b) P. Pꢂrez-Galµn, N.
Delpont, E. Herrero-Gómez, F. Maseras, A. M. Echavarren, Chem.
Eur. J. 2010, 16, 5324–5332.
product 2·ACHTNUTRGNENG[U BF4] at d=45.4 ppm (253 K), whereas we observed this
resonance at 43.3 ppm.[23]
[16] V. Lavallo, G. D. Frey, S. Kousar, B. Donnadieu, G. Bertrand, Proc.
Natl. Acad. Sci. USA 2007, 104, 13569–13573.
[31] Due to the width of the peaks at low temperature, line-shape simu-
lation of the proton NMR spectra provided an upper limit of the
energy barrier for interconversion of vinyl protons of DG°185K ꢃ
[17] Toste has described the isolation and structural characterization of
dimeric gold–p alkyne and polymeric gold–p-alkene complexes, in
which the p-hydrocarbon moiety is tethered to a triarylphosphine
ligand. However, detailed solution-state characterization of these
complexes was precluded by facile disproportionation, see: N. D.
Shapiro, F. D. Toste, Proc. Natl. Acad. Sci. USA 2008, 105, 2779–
2782.
9.0 kcalmolÀ1
.
[32] Pertinant 13C NMR data for free 1,7-diphenyl-3,4-heptadiene
(À908C, CD2Cl2): d=202.6 (sp), 90.6 (allenyl sp2), 34.3, and
30.6 ppm (sp3).
Received: December 22, 2012
Published online: April 16, 2013
8284
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