Synthesis and equilibrium binding studies of cationic, two-coordinate gold(I) π-alkyne complexes

Article abstract of DOI:10.1016/j.jorganchem.2010.09.055

Reaction of a 1:1 mixture of (L)AuCl [L = P(t-Bu)₂o-biphenyl or IPr] and AgSbF₆ with internal alkynes led to isolation of the corresponding cationic, two-coordinate gold π-alkyne complexes in ≥ 90% yield. Equilibrium binding studies show that the binding affinities of alkynes to gold(I) are strongly affected by the electron density of the alkyne and to a lesser extent on the steric bulk of the alkyne. These substituent effects on alkyne binding affinity are greater than are the differences between the inherent binding affinities of alkynes and alkenes to gold(I).

Full text of DOI:10.1016/j.jorganchem.2010.09.055

Journal of Organometallic Chemistry 696 (2011) 1216e1220
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and equilibrium binding studies of cationic, two-coordinate gold(I)
p
-alkyne complexes
*
Timothy J. Brown, Ross A. Widenhoefer
Department of Chemistry, French Family Science Center, Duke University, Durham, NC 27708-0346, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Reaction of a 1:1 mixture of (L)AuCl [L ¼ P(t-Bu)2o-biphenyl or IPr] and AgSbF6 with internal alkynes led
Received 13 August 2010
Received in revised form
22 September 2010
to isolation of the corresponding cationic, two-coordinate gold
p
-alkyne complexes in ꢀ 90% yield.
Equilibrium binding studies show that the binding affinities of alkynes to gold(I) are strongly affected by
the electron density of the alkyne and to a lesser extent on the steric bulk of the alkyne. These
substituent effects on alkyne binding affinity are greater than are the differences between the inherent
binding affinities of alkynes and alkenes to gold(I).
Accepted 23 September 2010
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Gold
Alkyne
Pi-complex
Binding affinity
1. Introduction
We have recently reported the syntheses and X-ray crystal
structures of monomeric, cationic, two-coordinate gold(I) p-alkene
Over the past ten years, gold(I) complexes of the form (L)AuX
[L ¼ phosphine or N-heterocyclic carbene; X ¼ weakly coordinating
anionic ligand] have emerged as a soft, carbophilic Lewis acids for
the activation of CeC multiple bonds toward nucleophilic addition
[1]. These gold(I) complexes have shown particular utility as cata-
lysts for the functionalization of alkynes and typically activate
alkynes selectively in the presence of alkenes [1]. Although this
selectivity has been attributed to kinetic rather than thermody-
namic effects [2,3], little is know regarding the relative binding
affinities of alkynes to the cationic, twelve-electron gold(I) frag-
ment (L)Auþ or regarding the binding affinities of alkynes vis-a-vis
alkenes toward gold(I). Computational analyses suggest that
ethylene binds more strongly to cationic gold(I) than does acety-
lene [2,4] and Echavarren has shown that gold(I) binds preferen-
tially to the C]C moiety of a terminally unsubstituted 1,6-enyne
[2]. Although recent efforts have led to the synthesis of cationic,
complexes that contained the N-heterocyclic carbene ligand IPr
[IPr ¼ 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine] or the
sterically-hindered phosphine ligand P(t-Bu)2(o-biphenyl) [10e12].
An important component of these studies was the determination of
the relative binding affinities of alkenes to the twelve-electron
LAuþ [L ¼ IPr, P(t-Bu)2(o-biphenyl)] fragments. These experiments
revealed that the binding affinity of alkenes to gold(I) increases
dramatically with the increasing electron density of the alkene and
is attenuated by steric interactions between the alkene substituents
and the (L)Au moiety [10,11]. We therefore considered that a similar
approach might be suitable to generate data regarding the binding
affinities of alkynes to gold(I). Here we report the synthesis of
cationic, two-coordinate gold(I)
p-alkyne complexes and an anal-
ysis of the binding affinities of substituted alkynes to gold(I). These
studies reveal that the binding affinities of alkynes to gold(I) are
strongly affected by alkyne substitution and these effects are
greater than are the differences between the inherent binding
affinities of alkynes and alkenes to gold(I).
two-coordinate p-alkyne complexes that contain an N-heterocyclic
carbene [3,5,6], a cyclic alkyl amino carbene [7], or a phosphine
ligand [8], no information regarding the relative binding affinities
of alkynes to gold(I) have emerged [9].
2. Results and discussion
2.1. Synthesis of gold p-alkyne complexes
Cationic, two-coordinate gold(I)
lated employing a procedure analogous to that used to synthesize
p-alkyne complexes were iso-
* Corresponding author. Tel.: þ1 919 660 1533; fax: þ1 919 660 1605.
E-mail address: rwidenho@chem.duke.edu (R.A. Widenhoefer).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.09.055

T.J. Brown, R.A. Widenhoefer / Journal of Organometallic Chemistry 696 (2011) 1216e1220
1217
cationic gold(I)
p-alkene complexes [10,11]. As an example, treat-
2.2. Determination of alkyne binding constants
ment of a methylene chloride suspension of [P(t-Bu)2o-biphenyl]
AuCl and AgSbF6 with 3-hexyne (1.5 equiv.) at room temperature
We have previously determined the relative binding affinities of
alkenes to the twelve-electron gold fragments [(L)Au]þ [L ¼ P
(t-Bu)2o-biphenyl, IPr] by measuring the equilibrium constants for
displacement of NCArF [NCArFeNbC-3,5-C6H3(CF3)2] from [LAu
(NCArF)]þ SbF6ꢁ [L ¼ P(t-Bu)2o-biphenyl (3a), IPr (3b)] with alkenes
in CD2Cl2 at ꢁ 60 ꢂC employing 1H NMR analysis [10,11]. Employing
a similar approach, we determined equilibrium constants for
displacement of NCArF from 3a with 2-hexyne (Keq ¼ 86 ꢃ 7), 4,4-
dimethyl-2-propyne (Keq ¼ 58 ꢃ 4), 2-butyne (Keq ¼ 25 ꢃ 2), and 1-
phenylpropyne (Keq ¼ 0.43 ꢃ 0.02) and for displacement of NCArF
from 3b with 2-butyne (Keq ¼ 24 ꢃ 1) and 1-phenylpropyne
(Keq ¼ 8.6 ꢃ 0.6) (Table 2).
for 10 min led to isolation of {[P(t-Bu)2o-bipheny]Au[h2
-
EtChCEt]}þ SbF6 (1a) in 98% yield as an air and thermally stable
white solid that was characterized by NMR spectroscopy and
combustion analysis (Table 1). Complexation of the alkyne to gold
in solution was established by spectroscopy. The 13C NMR spec-
ꢁ
trum of 1a displayed a resonance at
carbon atoms of the alkyne ligand, which was shifted downfield
relative to that of free 3-hexyne (
81.0). Similarly, 1H NMR spec-
trum of 1a displayed resonances at
d 91.4 corresponding to the sp
d
d
2.46 (q, J ¼ 7.5 Hz) and 1.21 (t,
J ¼ 7.5 Hz) corresponding to the ethyl groups of the 3-hexyne
ligand that were shifted downfield relative to those of free 3-
hexyne (
In addition to complex 1a, gold
(t-Bu)2(o-biphenyl)]Au[
2-alkyne]}þ SbF6 [alkyne
d
2.15, 1.11).
Attempts to determine the equilibrium constants for displace-
mentof NCArF fromeither 3a or 3bwith 3-hexynewereunsuccessful
due to the near quantitative displacement of NCArF in each case. To
circumvent this problem, we determined the equilibrium constants
p
ꢁ
-alkyne complexes {[P
h
¼
2-hexyne
(1b), 4,4-dimethyl-2-pentyne (1c), 2-butyne (1d)] and {(IPr)Au
2-alkyne]}þ SbF6 [alkyne ¼ 3-hexyne (2a), 1-phenylpropyne
for displacement of the more tightly binding 2-methyl-2-butene
ꢁ
[
h
2-Me(H)C¼CMe2]}þ SbF6 [L ¼ P(t-Bu)2o-biphenyl
ꢁ
(2b)] were isolated in ꢀ 90% yield and were fully characterized
from {(L)Au[
h
(Table 1). In addition, the
p
-alkyne complexes {[P(t-Bu)2(o-
(4a), IPr (4b)] with 3-hexyne (Keq ¼ 2.9 ꢃ 0.2 for 4a and 12.2 ꢃ 0.6 for
4b) (Scheme 1). From these values and from the equilibrium
constants previously determined for the displacement of NCArF
from 3a and 3b with 2-methyl-2-butene (Keq ¼ 156 ꢃ 8 for 3a and
64 ꢃ 4 for 3b), we estimate equilibrium constants for the displace-
ment of NCArF from 3a and 3b with 3-hexyne of Keq ¼ 450 ꢃ 39 and
780 ꢃ 62, respectively.
2-PhChCMe]}þ SbF6 (1e) and {(IPr)Au[
h
2-MeCh
ꢁ
biphenyl)]Au[
h
CMe]}þ SbF6 (2c) were generated cleanly in solution and were
characterized spectroscopically without isolation (Table 1). Worth
noting, the 13C NMR chemical shifts of the alkyne sp carbon atoms
of the phosphine complexes 1a, 1d, and 1e were, in each case,
shifted downfield relative to the alkyne sp carbon resonances of
the corresponding IPr complexes 2ae2c. This observation points
to greater positive charge on the alkyne carbon atoms of the
phosphine complexes relative to the IPr complexes owing to the
greater donor properties of the IPr ligand relative to the phos-
phine, as has been previously noted [13].
ꢁ
Taken together, the equilibrium binding affinities of alkynes to
the gold fragment [P(t-Bu)2o-biphenyl]Auþ decrease by a factor of
w1000 in the order 3-hexyne > 2-hexyne > 4,4-dimethyl-2-
pentyne > 2-butyne > 1-phenylpropyne. Likewise, the equilibrium
binding affinities of alkynes to the gold fragment (IPr)Auþ decrease
Table 1
Synthesis and alkyne C(sp) 13C NMR chemical shifts of gold
p-alkyne complexes 1ae1e and 2ae2c.
entry
1
L
alkyne
complex
yield (%)a
98
d ChC
P(t-Bu)2(o-biphenyl)
1a
91.4
2
3
4
5
6
7
8
P(t-Bu)2(o-biphenyl)
1b
1c
1d
1e
2a
2b
2c
96
88.7, 75.4
98.3, 87.0
84.9
P(t-Bu)2(o-biphenyl)
90
P(t-Bu)2(o-biphenyl)
91
P(t-Bu)2(o-biphenyl)
(92)
94
90.6, 86.0
87.2
IPr
IPr
IPr
99
86.9, 83.5
81.5
(97)
a
Isolated yield. Values in parentheses refers to 1H NMR yield for in situ generated complexes.

1218
T.J. Brown, R.A. Widenhoefer / Journal of Organometallic Chemistry 696 (2011) 1216e1220
Table 2
(Keq ¼ 12.5 ꢃ 0.5). These data suggest that there is little inherent
difference in the binding affinities of alkenes and alkynes to gold(I).
Equilibrium constants for displacement of NCArF from 3a [L ¼ P(t-Bu)2o-biphenyl] or
3b (L ¼ IPr) with alkynes in CD2Cl2 at e 60 ꢂC.
3. Conclusions
We have synthesized and fully characterized a family of cationic,
linear gold
p-alkyne complexes that contain either the sterically-
hindered phosphine P(t-Bu)2(o-biphenyl) ligand or the N-hetero-
cyclic carbene ligand IPr. Equilibrium binding studies reveal that
the binding affinities of alkynes to gold(I) are strongly affected by
the electron density of the alkyne and to a lesser extent on the
steric bulk of the alkyne. The effect of the electron density of the
alkyne on binding affinity was more pronounced for the phosphine
series than for the NHC series. The safest conclusion we can draw
from the data we have collected regarding the binding affinities of
alkynes and alkenes [10,11] to gold(I) is that the variability in
binding affinity as a function of substitution of the alkene or alkyne
is greater than is the inherent difference between the binding
affinities of alkenes and alkynes to gold(I).
entry
1
L
alkyne
Keq
P(t-Bu)2(o-biphenyl)
86 ꢃ7
2
3
4
5
6
P(t-Bu)2(o-biphenyl)
P(t-Bu)2(o-biphenyl)
P(t-Bu)2(o-biphenyl)
IPr
58 ꢃ4
25 ꢃ2
0.43 ꢃ 0.02
24 ꢃ1
4. Experimental
IPr
8.6 ꢃ0.6
4.1. General methods
Reactions were performed under
a nitrogen atmosphere
employing standard Schlenk and glovebox techniques unless
specified otherwise. NMR spectra were obtained on a Varian
spectrometer operating at 500 MHz for 1H NMR, 101 MHz for 13C
NMR, and 202 MHz for 31P NMR in CD2Cl2 at 25 ꢂC unless noted
otherwise. IR spectra were obtained on a Nicolet Avatar 360-FT IR
spectrometer. Elemental analyses were performed by Complete
Analysis Laboratories (Parsippany, NJ). Mass spectra were obtained
on an Applied Biosystems Voyager-DE Pro MALDI mass spectrom-
eter operating at a mass range of 500e4000u with a dihydrox-
yacetophenone matrix (10 mg/1 mL DCM) and was calibrated with
PEG1000.
by a factor of w90 in the order 3-hexyne > 2-butyne > 1-phenyl-
propyne. Several points are worth noting. The strong sensitivity of
alkyne binding affinity to the electron density of the alkyne is
revealed both by the precipitous drop in the binding affinity of
2-hexyne relative to 3-hexyne and in the binding affinity of 1-
phenylpropyne relative to 4,4-dimethyl-2-pentyne in the phos-
phine series. In comparison, the modest decrease in the binding
affinity of 4,4-dimethyl-2-pentyne relative to 2-hexyne suggests
that steric factors play only a minor role in the determination of
alkyne binding affinity. The greater sensitivity of alkyne binding
affinity to the electron density of the alkyne in the phosphine series
Methylene chloride was purified by passage through columns of
activated alumina under nitrogen. CDCl3 and CD2Cl2 were dried
over CaH2 and distilled under nitrogen prior to use. (IPr)AuCl,
hexanes, AgSbF6, 3-hexyne, 2-butyne, 1-phenylpropyne, 2-hexyne,
and 4,4-dimethyl-2-pentyne were purchased from major chemical
suppliers and used as received. [P(t-Bu)2o-bipheny]AuCl [14], {[P(t-
relative to the carbene series is consistent with the greater L/M s-
component of the goldealkyne binding interaction in the former
series relative to the latter [13], which was also suggested by the 13C
NMR data (see above). In a similar manner, the binding affinities of
alkenes to [P(t-Bu)2o-biphenyl]Auþ were more strongly dependent
on alkene electron density than were binding affinities of alkenes to
(IPr)Auþ [10,11].
Interestingly, 3-hexyne binds more tightly to the gold(I) frag-
ments (L)Auþ [L ¼ P(t-Bu)2o-biphenyl, IPr] than do anyof the alkenes
we have investigated. Perhaps the most direct comparison between
the inherent binding affinities of alkynes and alkenes to cationic
gold(I) is between 2-butyne and cis- and trans-2-butene. The equi-
librium constant for displacement of NCArF from 3a with 2-butyne
(Keq ¼ 25 ꢃ 2) falls between the values determined for displacement
of NCArF from 3a with cis-2-butene (Keq ¼ 126 ꢃ 9) and trans-2-
butene (Keq ¼ 14.1 ꢃ 0.1). Similarly, the equilibrium constant for
displacement of NCArF from 3b with 2-butyne (Keq ¼ 24 ꢃ 1) falls
between the values determined for displacement of NCArF from 3b
Bu)2o-bipheny]Au(NCArF)}þ SbF6 (3a) [11], {(IPr)Au(NCArF)}þ
ꢁ
SbF6ꢁ (3b) [10], {[P(t-Bu)2o-bipheny]Au[
h
2-Me(H)C¼CMe2]}þ SbF6
ꢁ
ꢁ
(4a) [11], and {(IPr)Au[
h
2-Me(H)C¼CMe2]}þ SbF6 (4b) [10] were
prepared employing published procedures.
4.2. Synthesis of gold(I)
p-alkyne complexes
ꢁ
4.2.1. {[P(t-Bu)2o-bipheny]Au[
h
2-EtChCEt]}þ SbF6 (1a)
A suspension of [P(t-Bu)2o-bipheny]AuCl (35 mg, 0.066 mmol),
AgSbF6 (22.7 mg, 0.066 mmol), and 3-hexyne (8.2 mg, 0.10 mmol)
in CH2Cl2 (1.5 mL) was stirred in a sealed flask at room temperature
for 10 min. The resulting suspension was filtered through a pad of
Celite, which was flushed with additional CH2Cl2. The filtrate was
concentrated to w2 mL, diluted with two volumes of hexanes, and
cooled at 4 ꢂC for 24 h to give 1a (52.5 mg, 98%) as a white solid. 1H
with cis-2-butene (Keq
¼
38
ꢃ
2) and trans-2-butene
NMR:
d 7.95e7.90 (m, 1 H), 7.70e7.60 (m, 2 H), 7.57e7.52 (m, 1 H),
7.48 (t, J ¼ 7.5 Hz, 2 H), 7.35e7.31 (m, 1 H), 7.24e7.20 (m, 2 H), 2.46
(q, J ¼ 7.5 Hz, 4 H),1.45 (d, J ¼ 16 Hz,18 H),1.21 (t, J ¼ 7.5 Hz, 6 H). 13C
{1H} NMR:
d
149.1 (d, J ¼ 12.3 Hz), 143.4 (d, J ¼ 6.9 Hz), 134.1, 133.7
(d, J ¼ 7.8 Hz), 132.1 (d, J ¼ 3.1 Hz), 130.0, 129.1, 128.6, 128.3 (d,
J ¼ 7.0 Hz), 123.7 (d, J ¼ 49.3 Hz), 91.4, 39.0 (d, J ¼ 24.6 Hz), 31.2
(d, J ¼ 6.2 Hz),16.4,14.1. 31P{1H} NMR:
d 65.5. Anal. calcd (found) for
Scheme 1.
C26H37PF6AuSb: H, 4.59 (4.51); C, 38.40 (38.49).

T.J. Brown, R.A. Widenhoefer / Journal of Organometallic Chemistry 696 (2011) 1216e1220
1219
All remaining gold(I)
employing similar procedures unless noted otherwise.
p
-alkyne complexes were synthesized
86.9, 83.5, 28.7, 24.5, 23.9, 6.6. MALDI-MS calcd (found) for
[C36H44N2Au]þ (Mþ): 701.3 (700.9).
2-MeChCCH2CH2CH3]}þ SbF6 (1b)
7.95e7.90 (m, 1 H), 7.70e7.60 (m,
4.2.8. {(IPr)Au(
h
2-MeChCMe)}þ SbF6 (2c)
ꢁ
ꢁ
4.2.2. {[P(t-Bu)2o-bipheny]Au[
h
White solid, 96%. 1H NMR:
d
An NMR tube containing a suspension of (IPr)AuCl (35 mg,
0.066 mmol), AgSbF6 (22.7 mg, 0.066 mmol), 2-butyne (3.6 mg,
0.066 mmol), and toluene (1.2 mg, internal standard) in CD2Cl2
(0.5 mL) was shaken briefly and allowed to stand at room
temperature. Following precipitation of AgCl, the sample was
analyzed by 1H and 13C NMR spectroscopy at 25 ꢂC, which revealed
2 H), 7.58e7.52 (m, 1 H), 7.49 (t, J ¼ 7.5 Hz, 2 H), 7.36e7.30 (m, 1 H),
7.23 (d, J ¼ 7.0 Hz, 2 H), 2.37 (qt, J ¼ 2.5, 7.0 Hz, 2 H), 2.16 (t,
J ¼ 2.0 Hz, 3 H), 1.61 (sextet, J ¼ 7.5 Hz, 2 H), 1.45 (d, J ¼ 16.5 Hz,
18 H), 1.00 (t, J ¼ 7.5 Hz, 3 H). 13C{1H} NMR:
149.8 (d, J ¼ 12.8 Hz),
d
143.1 (d, J ¼ 6.7 Hz), 133.8, 133.3 (d, J ¼ 7.7 Hz), 131.8, 129.6, 128.7,
128.3, 128.0 (d, J ¼ 7.7 Hz), 123.4 (d, J ¼ 48.0 Hz), 88.7 (d, J ¼ 7.7 Hz),
86.3 (d, J ¼ 6.7 Hz), 38.5 (d, J ¼ 25.1 Hz), 30.8 (d, J ¼ 6.8 Hz), 24.0,
formation of 2c in 97 ꢃ 5% yield. 1H NMR (300 MHz):
d 7.60
(t, J ¼ 8.1 Hz, 2H), 7.48 (s, 2H), 7.39 (d, J ¼ 7.8 Hz, 4H), 2.51 (septet,
22.4, 13.2, 7.2. 31P{1H} NMR:
C26H37PF6AuSb: H, 4.59 (4.36); C, 38.40 (38.35).
d 65.3. Anal. calcd (found) for
J ¼ 6.9 Hz, 4H), 1.88 (s, 6H), 1.29 (d, J ¼ 6.9 Hz, 12H), 1.27 (d,
J ¼ 6.9 Hz, 12H). 13C{1H} NMR:
d 177.1, 146.1, 133.3, 131.8, 125.2,
124.9, 81.5, 29.2, 24.8, 24.1, 6.1.
4.2.3. {[P(t-Bu)2o-bipheny]Au[
h
2-MeChCC(CH3)3]}þ SbF6 (1c)
7.96e7.90 (m, 1 H), 7.70e7.60 (m,
ꢁ
White solid, 90%. 1H NMR:
d
4.3. Determination of alkyne binding constants
2 H), 7.59e7.52 (m, 1 H), 7.49 (t, J ¼ 7.5 Hz, 2 H), 7.35e7.28 (m, 1 H),
7.22 (d, J ¼ 7.5 Hz, 2 H), 2.19 (s, 3 H),1.47 (d, J ¼ 16.5 Hz,18 H), 1.27 (s,
4.3.1. Reaction of 2-butyne with {[P(t-Bu)2o-bipheny]Au(NCArF)}þ
SbF6 (3a)
9 H). 13C{1H} NMR:
d
149.2 (d, J ¼ 12.6 Hz), 143.4 (d, J ¼ 6.8 Hz),
ꢁ
134.2, 133.9 (d, J ¼ 7.7 Hz), 132.2, 130.0, 129.4, 128.5, 128.3 (d,
J ¼ 7.7 Hz), 123.5 (d, J ¼ 47.9 Hz), 98.3, 87.0, 39.2 (d, J ¼ 24.9 Hz),
2-Butyne (1.03 mg, 0.019 mmol) was added via gas tight syringe
to an NMR tube sealed with a rubber septum that contained a CD2Cl2
solution of 3a (20 mg, 0.021 mmol) at ꢁ 60 ꢂC. The tube was shaken,
placed in the probe of an NMR spectrometer cooled at ꢁ 60 ꢂC and
allowed to equilibrate for 10 min. The relative concentrations of 3a,
1d, NCArF, and 2-butyne were determined by integrating the reso-
31.9, 31.2 (d, J ¼ 5.8 Hz), 8.4. 31P{1H} NMR:
d 65.9. Anal. calcd
(found) for C27H39PF6AuSb: H, 4.75 (4.66); C, 39.20 (39.14).
ꢁ
4.2.4. {[P(t-Bu)2o-bipheny]Au[
h
2-MeChCMe]}þ SbF6 (1d)
White solid, 91%. 1H NMR:
d
8.00e7.84 (m, 1 H), 7.72e7.44 (m,
nances corresponding to the aromatic protons of bound [
(2:1)] and free [ 8.15, 8.14 (2:1)] NCArF and the resonances corre-
sponding to the methyl protons of bound ( 2.06) and free ( 1.70) 2-
d 8.41, 8.38
5 H), 7.38e7.30 (m, 1 H), 7.30e7.16 (m, 2 H), 2.11 (s, 6 H), 1.44 (d,
d
J ¼ 16.5 Hz, 18 H). 13C{1H} NMR:
d
149.1 (d, J ¼ 13.0 Hz), 143.5
d
d
(d, J ¼ 7.0 Hz), 134.0, 133.6 (d, J ¼ 6.9 Hz), 132.1, 129.9, 128.9, 128.7,
128.3 (d, J ¼ 7.8 Hz), 123.8 (d, J ¼ 48.6 Hz), 84.9, 39.7 (d, J ¼ 24.6 Hz),
butyne. An equilibrium constant of Keq ¼ [1d][NCArF]/[3a][2-
butyne] ¼ 25 ꢃ 2 was determined (Table 2).
31.2 (d, J ¼ 6.2 Hz), 7.4. 31P{1H} NMR:
d
65.2. Anal. calcd (found) for
To ensure that equilibrium was achieved under these conditions,
the following control experiment was performed. NCArF (4.9 mg,
0.02 mmol) was added via syringe to an NMR tube that contained
a CD2Cl2 solution of 2-butyne complex 1d (16 mg, 0.02 mmol) at
ꢁ 60 ꢂC. The tube was shaken, placed in the probe of an NMR
spectrometer cooled at ꢁ 60 ꢂC and allowed to equilibrate for
10 min. The relative concentrations of 3a, 1d, NCArF, and 2-butyne
were determined as was described in the preceding paragraph. The
equilibrium constant determined from this experiment {Keq ¼ [1d]
[NCArF]/[3a][2-butyne] ¼ 24 ꢃ 1} was not significantly different
from that obtained from treatment of 3a with 2-butyne.
C24H33PF6AuSb: H, 4.24 (4.23); C, 36.71 (36.62).
4.2.5. {[P(t-Bu)2o-bipheny]Au[h
2-PhChCMe]}þ SbF6 (1e)
ꢁ
An NMR tube containing a suspension of [P(t-Bu)2o-bipheny]
AuCl (30 mg, 0.057 mmol), AgSbF6 (19.4 mg, 0.057 mmol), 2-
butyne (6.6 mg, 0.057 mmol), and 1,3-dimethoxybenzene
(1.4 mg, internal standard) in CD2Cl2 (0.5 mL) was shaken briefly
and allowed to stand at room temperature. Following precipita-
tion of AgCl, the sample was analyzed by 1H and 13C NMR
spectroscopy at 25 ꢂC, which revealed formation of 1e in 92 ꢃ 5%
yield. 1H NMR:
d
7.93e7.81 (m, 1H), 7.70e7.50 (m, 4H), 7.46 (t,
Similar procedures were employed to determine the equilib-
rium constants for the displacement of NCArF from 3a with
2-hexyne, 4,4-dimethyl-2-pentyne, and 1-phenylpropyne and for
displacement of NCArF from 3b with 2-butyne and 1-phenyl-
propyne. Error limits refer to the standard deviation in Keq deter-
mined from three independent experiments.
J ¼ 7.5 Hz, 2H), 7.43e7.32 (m, 4H), 7.32e7.24 (m, 1H), 7.16 (d,
J ¼ 7.0 Hz, 2H), 2.35 (s, 3H), 1.34 (d, J ¼ 16.5 Hz, 18H), 1.00. 13C{1H}
NMR:
d
147.9 (d, J ¼ 12.5 Hz), 142.9 (d, J ¼ 6.7 Hz), 133.4, 132.7 (d,
J ¼ 7.2 Hz), 132.1, 131.4, 131.1, 129.2, 129.0, 128.2, 127.8 (d,
J ¼ 7.7 Hz), 127.1, 122.6 (d, J ¼ 49.6 Hz), 117.9, 90.6 (d, J ¼ 8.6 Hz),
86.0 (d, J ¼ 7.2 Hz), 37.8 (d, J ¼ 24.9 Hz), 30.3 (d, J ¼ 5.8 Hz), 8.4.
31P{1H} NMR:
d
65.7.
4.3.2. Reaction of 3-hexyne with {[P(t-Bu)2o-biphenyl]Au[
h
2-Me(H)
C¼CMe2]}þ SbF6 (4a)
ꢁ
4.2.6. {(IPr)Au(
h
2-EtChCEt)}þ SbF6 (2a)
3-Hexyne (1.6 mg, 0.02 mmol) was added via gas tight syringe to
an NMR tube sealed with a rubber septum that contained a CD2Cl2
solution of 4a (16 mg, 0.02 mmol) at ꢁ 60 ꢂC. The tube was shaken,
placed in the probe of an NMR spectrometer cooled at ꢁ 60 ꢂC and
allowed to equilibrate for 10 min. The relative concentrations of 4a,
1a, 2-methyl-2-butene, and 3-hexyne were determined by inte-
grating the resonances corresponding to the olefinic proton of
ꢁ
White solid, 94%. 1H NMR (400 MHz, CDCl3):
d
7.52 (t, J ¼ 8 Hz,
2H), 7.49 (s, 2 H), 7.30 (d, J ¼ 7.6 Hz, 4H), 2.48 (sept, J ¼ 7.2 Hz, 4H),
2.20 (q, J ¼ 7.6 Hz, 4H), 1.23 (d, J ¼ 6.8 Hz, 24H), 0.57 (t, J ¼ 7.6 Hz,
6H). 13C{1H} NMR (CDCl3):
d 177.2, 145.7, 133.0, 131.4, 125.1, 124.5,
87.2, 28.8, 24.6, 23.9, 14.7, 13.1. Anal. calcd (found) for
C33H46AuF6N2Sb: H, 5.13 (5.05); C, 43.87 (43.76); N, 3.10 (3.37).
bound (
nances corresponding to the methylene protons of bound (
and free (
methyl-2-butene]/[4a][3-hexyne] ¼ 2.9 ꢃ 0.2 was determined
(Scheme 1). Multiplication of this result by the equilibrium
constant for displxacement of NCArF from 3a with 2-methyl-2-
butene (Keq ¼ 156 ꢃ 8) [10] provided an equilibrium constant for
d
3.99) and free (
d
5.12) 2-methyl-2-butene and the reso-
2-PhChCMe)}þ SbF6 (2b)
d
2.40)
ꢁ
4.2.7. {(IPr)Au(
h
Pale green solid, 99%. 1H NMR (CDCl3):
d
7.58 (t, J ¼ 8 Hz, 2H),
d
2.10) 3-hexyne. An equilibrium constant of Keq ¼ [1a][2-
7.51 (s, 2 H), 7.36 (d, J ¼ 8.0 Hz, 1H), 7.31 (d, J ¼ 8 Hz, 4H), 7.15 (t,
J ¼ 7.5 Hz, 2H), 6.81 (d, J ¼ 7.5 Hz, 2H), 2.45 (sept, J ¼ 7 Hz, 4 H), 2.05
(s, 3 H), 1.22 (d, J ¼ 7 Hz, 12 H), 1.14 (d, J ¼ 7 Hz, 12 H). 13C{1H} NMR
(CDCl3):
d 175.7, 145.8, 132.9, 131.8, 131.4, 129.1, 125.1, 124.5, 117.1,

1220
T.J. Brown, R.A. Widenhoefer / Journal of Organometallic Chemistry 696 (2011) 1216e1220
(c) T. Ziegler, A. Rauk, Inorg. Chem. 18 (1979) 1558e1565;
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[6] (a) For reviews on gold N-heterocyclic carbene complexes see: N. Marion,
S.P. Nolan Chem. Soc. Rev. 37 (2008) 1776e1782;
(b) S. Díez-González, N. Marion, S.P. Nolan, Chem. Rev. 109 (2009) 3612e3676.
[7] (a) V. Lavallo, G.D. Frey, S. Kousar, B. Donnadieu, G. Bertrand, Proc. Natl. Acad.
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[8] (a) T.N. Hooper, M. Green, C.A. Russell, Chem. Commun. (2010) 2313e2315;
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the displacement of NCArF from 3a with 3-hexyne of Keq ¼ [1a]
[NCArF]/[3a][3-hexyne] ¼ 450 ꢃ 45.
A similar procedure was employed to determine the equilibrium
constant for the displacement of 2-methyl-2-butene from 4b with
3-hexyne (Keq ¼ 12.2 ꢃ 0.6) and to calculate the equilibrium
constant for the displacement of NCArF from 3b with 3-hexyne
(Keq ¼ 780 ꢃ 62). Error limits for the reactions of 3-hexyne with
complexes 4a and 4b refer to the standard deviation in Keq deter-
mined from three independent experiments. Error limits for equi-
librium binding affinities of 3-hexyne relative to NCArF were
estimated from propagation of the errors of the associated Keq
measurements.
(c) N.D. Shapiro, F.D. Toste, Proc. Natl. Acad. Sci. USA 105 (2008)
2779e2782.
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423e425;
Acknowledgements
(b) H.V.R. Dias, J.A. Flores, J. Wu, P. Kroll, P.J. Am, Chem. Soc. 131 (2009)
11249e11255;
The authors thank the NSF (CHE-0555425) for support of this
research and the NCBC (2008-IDG-1010) for support of the Duke
University NMR facility. TJB thanks Duke University for a Zeilik
Fellowship.
(c) P. Schulte, U. Behrens, Chem. Commun. (1998) 1633e1634;
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