Structure and bonding in neutral and cationic 14-electron gold alkyne π complexes

Article abstract of DOI:10.1002/chem.200901062

Cyclododecyne (5) as a prototype unstrained alkyne was coordinated to either the neutral [AuCl] fragment or to two different cationic [Au(NHQ] ⁺ entities (NHC = N-heterocyclic carbene), and the resulting complexes 6, 8, and 10 were characterize

Full text of DOI:10.1002/chem.200901062

DOI: 10.1002/chem.200901062
Structure and Bonding in Neutral and Cationic 14-Electron Gold Alkyne
p Complexes
Susanne Flꢀgge, Anakuthil Anoop, Richard Goddard, Walter Thiel, and
Alois Fꢀrstner*[a]
Dedicated to Professor Yitzhak Apeloig on the occasion of his 65th birthday
Abstract: Cyclododecyne (5) as a pro-
totype unstrained alkyne was coordi-
nated to either the neutral [AuCl] frag-
ment or to two different cationic [Au-
clododecyne in the solid state could
also be obtained after in situ crystalli-
zation, a comparison was possible that
provides insights into structural
changes imposed on the alkyne by the
different gold fragments. These data
are interpreted on the basis of a DFT
analysis of the bonding situation in the
individual compounds, which provides
insights into the very first elementary
step common to many gold-catalyzed
transformations.
ACHTUNGTRENNUNG
(NHC)]+ entities (NHC=N-heterocy-
clic carbene), and the resulting com-
plexes 6, 8, and 10 were characterized
by X-ray crystallography and NMR
spectroscopy. Since the structure of cy-
Keywords: alkyne ligands · alkynes ·
carbene ligands · density functional
calculations · gold
Introduction
structures of an appreciable number of gold complexes of
metal acetylides are known in the literature,[6] only very few
gold complexes of regular alkynes have been isolated in
pure form. In addition to the unstrained and neutral 3-
The use of gold and platinum compounds for the selective
activation of p bonds has been a major thrust of catalysis re-
search during the last decade.[1] Gold, in particular, in its
function as a soft, carbophilic, Lewis acidic transition metal,
allows remarkably high degrees of molecular complexity to
be built up in reactions that are operationally simple, safe,
and convenient to perform. Therefore, it is finding increas-
ing application in target-oriented synthesis.[1–3]
Despite this success story, the underlying mechanisms are
not yet well understood. This is particularly true for trans-
formations of polyunsaturated substrates such as enynes,
which are believed to commence with coordination of only
one of the p bonds to the catalytically active metal center.[4]
Even though binding of AuI to the alkene might be thermo-
dynamically favored,[5] indirect evidence suggests that it is
coordination to the alkyne unit that triggers the subsequent
chemical transformation.[1] Pertinent experimental data to
support this view, however, are scarce; even though the
hexyneACHTNUTRGNE(NUG AuCl) complex 2 recently described by Dias and co-
workers,[7] Shapiro and Toste characterized the dimeric spe-
cies 1, which was formed on coordination of cationic AuI to
a substrate carrying a phosphine donor tethered to an
alkyne unit protected by a bulky triisopropylsilyl (TIPS)
group.[8,9] The only other structurally characterized gold
alkyne complexes are 3 and 4, each incorporating a highly
strained cycloalkyne as ligand.[10] Whereas 4 is a coordina-
tion polymer, 3 features a dimeric array with a trigonal gold
coordination mode and is devoid of any aurophilic interac-
tions. In this complex, pronounced backbonding helps to
reduce the angular strain of the thiacycloheptyne moiety.
ꢀ
This effect is clearly expressed in the C C bond length
ꢀ ꢁ
(1.259(12) ꢀ) and the C C C angles F (147.3(8) and
146.4(9)8), and may also explain the trigonal coordination
geometry, which is unusual for AuI but rather reminiscent of
the coordination modes of AuIII.[10]
As part of our investigations into the elementary steps of
p-acid catalysis,[11–15] we decided to study alkyne activation
in more detail. Here we report the preparation and charac-
terization of a series of 14-electron gold alkyne coordination
compounds that are more similar to the actual catalytic sys-
tems because they do not invoke any tailor-made substrates
and deliberately employ gold templates commonly used for
[a] Dr. S. Flꢁgge, Dr. A. Anoop, Dr. R. Goddard, Prof. W. Thiel,
Prof. A. Fꢁrstner
Max-Planck-Institut fꢁr Kohlenforschung
45470 Mꢁlheim an der Ruhr (Germany)
Fax : (+49)208-306-2994
E-mail: fuerstner@mpi-muelheim.mpg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901062.
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FULL PAPER
preparative purposes. Moreover, we give first insights into
the effects induced by the neutral [AuCl] unit as compared
to those of two different cationic [AuACTHNUTRGNEUNG
(NHC)]+ fragments.[16]
Results and Discussion
Scheme 1. Reagents and conditions: a) AuCl, CH2Cl2, 08C, 90%; b) 7,
AgSbF6, CH2Cl2, RT, 80%; c) 9, AgSbF6, CH2Cl2, RT, 92%; bond angles
Starting point for our study was to choose cyclododecyne
(5) as a supposedly unhindered and relatively unstrained
substrate.[17] Even though this compound is liquid at ambient
temperature, we were able to determine its structure in the
solid state by in situ crystallization (m.p. ꢁ548C by differen-
tial scanning calorimetry). Figure 1[18] shows one of the two
ꢀ
F are mean values for each compound, d is the C C bond length.
tane, and crystallization of the crude product from Et2O at
ꢁ808C furnished complex 6 (Scheme 1) in the form of color-
less crystals, which can be stored for several weeks at
ꢁ508C but rapidly decompose at or above 08C even under
inert atmosphere. Comparison of the structure of
6
(Figure 2) with that of the free ligand reveals that the triple
Figure 1. Structure of cyclododecyne (5) in the solid state at 100 K, show-
ꢁ ꢀ ꢁ
ing the relatively linear, almost unstrained C C C C unit; note that the
compound adopts a planar-chiral conformation and the crystal is a race-
Figure 2. Structure of complex 6 in the solid state with almost linearly co-
mate.[18]
ordinated gold center.[18]
independent molecules in the asymmetric unit, and
Scheme 1 gives pertinent distances and angles. As expected,
the alkyne bond is almost straight and thus relatively un-
strained.
Addition of 5 to a greenish suspension of AuCl in careful-
ly dried CH2Cl2 at 08C results in rapid (<1 min) formation
of a clear and colorless solution.[19] Careful evaporation of
the solvent at 08C, trituration of the residue with cold pen-
bond is noticeably extended from 1.196(4) ꢀ in 5 to
ꢀ ꢁ
1.224(5) ꢀ in 6. Moreover, the two C C C bond angles F
are reduced from 175.9(9)8 (av) in free cyclododecyne to
165(1)8 (av) in 6. The structure of 6 also features short
Au···Au contacts (av 3.33(2) ꢀ) due to the aurophilicity of
AuI (Figure 3).[20] The Cl-Au-alkyne
ACTHNUTRGNE(UNG midpoint) angle is nev-
ertheless almost linear (av 176(1)8), that is, the Au···Au in-
teraction is very weak. This contrasts with the structure of 1,
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A. Fꢁrstner et al.
Figure 3. The four independent molecules in the crystal structure of 6, showing the Au···Au interactions (Au1···Au2 3.318(1), Au3···Au4 3.349(1) ꢀ).
in which the two independent P-Au-alkyne
(midpoint) units
Addition of AgSbF6 to a solution of 9 and cyclododecyne
in CH2Cl2 caused instantaneous precipitation of AgCl; care-
ful evaporation of the resulting filtrate followed by crystalli-
zation of the crude material furnished single crystals of the
desired alkyne complex 10 suitable for X-ray structure anal-
ysis. The corresponding complex 8, bearing the imidazol-2-
ylidene moiety as ancillary two-electron donor, was pre-
pared analogously (Scheme 1). Both products are highly
sensitive and slowly decompose even below ꢁ208C under
inert conditions. Nevertheless, it was possible to obtain good
X-ray diffraction data that allowed the molecular structures
of 8 and 10 shown in Figure 5 and Figure 6 to be deter-
mined.
are noticeably bent (1738), and the gold atomsꢃ pointing
toward dichloromethane of solvation (mean Au···Cl distance
3.66 ꢀ) indicates possible interaction with the donating sol-
vent, though the effect of ring strain cannot be ruled out
(see Scheme S-8 in the Supporting Information).[8,18]
Since activation of the alkyne is thought to occur by with-
drawal of electron density by the electrophilic gold center, it
seemed appropriate to extend the study to the effects of cat-
ionic gold templates, as such species dominate contemporary
gold catalysis.[1] To this end, complexes 7 and 9 carrying N-
heterocyclic carbene (NHC) ligands of different ring size
were prepared in good yields by established routes;[21] the
structure of the previously unknown compound 9 in the
solid state is shown in Figure 4.
ꢀ
As expected, the C C triple bond is again extended in
both 8 and 10 relative to parent alkyne 5, but the effect is
less pronounced than in the neutral AuCl complex 6
(Scheme 1). One may be tempted to ascribe the slightly
Figure 4. Structure of 9 in the solid state.[18,22]
Figure 5. Structure of 8 in the solid state.[18]
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Structure and Bonding in Gold Alkyne Complexes
FULL PAPER
aryl group of the NHC in a h
fashion. Overall, this results in
the intricate bonding pattern
shown in Figure 6.[18]
For further analysis, we cal-
culated the bonding situation in
the different complexes using
density functional theory at the
BP86/def2-TZVP level. The
computed distances and angles
(Table 2) reproduce the experi-
mental trends very well, except
for the observed high degree of
bending of the alkyne in 10,
which, however, is likely steric
in origin; the crystal structure
of 10 contains two equivalents
of AgSbF6 (see above), which
are not included in the calcula-
tions. Second-order perturba-
tion analysis in the natural
bond order (NBO) framework
shows that the orbital interac-
ꢀ
tion energies for p
AHCTUNGERTG(NNUN AuL) donation are on the
G
order of 70–100 kcalmolꢁ1 and
thus 3–4 times larger than those
Figure 6. Solid-state structure of 10, which cocrystallized with two equivalents of AgSbF6; the silver cations
engage in h coordination with the aromatic rings of the N-aryl groups of the NHC ligand.[18] Color code:
white: C; blue: N; gray: Ag; yellow: Au; light green: F; green-brown: Sb; red: O of cocrystallized H2O.
ꢀ
for d(Au)!p
tion, which lie in the range of
20–30 kcalmolꢁ1
(Table 2).
ACHTUNGTREN(NGNU C C)* backdona-
Table 1. 13C NMR shifts of the alkyne carbon atoms of the new gold-
alkyne complexes and those of gold alkyne NHC complexes known in
the literature.[7,25,26]
ꢁ
longer C1 C2 bond in 10, as compared with 8, to the some-
what stronger donor capacity of the six-membered NHC.[22]
Although this explanation is consistent with the notion that
N-heterocyclic carbenes with enlarged rings are stronger
donors than the parent imidazol-2-ylidenes,[23,24] care must
be taken not to overinterpret the data in view of the experi-
mental error bars. Interestingly, however, the degree of de-
shielding of the alkyne sp C atom on complexation to the
different gold fragments, as reflected in the 13C chemical
shifts (Table 1), seems to corroborate the view that 10 car-
ries the slightly more electron donating ancillary ligand.[22]
Significant differences between the two NHC complexes,
however, are observed in the degree of bending of the
alkyne moiety. Whereas a rather weak flow of electron den-
sity between the ligand and the cationic gold fragment in 8
entails little change, the corresponding bond angles F in 10
deviate considerably from linearity (160(4)8). This effect,
however, is unlikely to be electronic in nature but rather
caused by the shape of the six-membered NHC, which di-
rects the bulky N-aryl substituents more toward the alkyne
and hence forces the ligand to adopt a rather distorted con-
formation (Scheme 1).
Compound
dC [ppm] in CD2Cl2
cyclododecyne (5)
2
6
8
81.7
86.4
85.9
88.0
86.6
10
87.7[a]
71/95[a,b]
[a] Ar=2,6-diisopropylphenyl. [b] The supporting information to ref. [26]
reports shifts of 71.7 and 93.8 ppm.
Hence, consistent with previous theoretical results,[7,8] p!s*
donation clearly dominates over d!p* backdonation in all
cases investigated, and thus explains the highly electrophilic
character of the alkyne within the coordination sphere of
any chosen AuI fragment. Interestingly, [AuCl] is not only
the strongest donor, but also the strongest acceptor. The
fact that both bonding interactions are more pronounced
The structure of 10 in the solid state also deserves com-
ment, since the complex cocrystallized with two molar
equivalents of AgSbF6. These escorting silver cations are by
no means innocent, but coordinate to the p system of the N-
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A. Fꢁrstner et al.
AuCl, we find B3LYP binding energies of 146.5 kJmolꢁ1
(35.0 kcalmolꢁ1) for acetylene and 157.0 kJmolꢁ1 (37.5 kcal
molꢁ1) for cyclododecyne, which are consistent with previ-
Table 2. Computed structural parameters, natural bond order orbital
(NBO) interaction energies [kcalmolꢁ1], and partial charges from NBO
analysis; experimental data are given in brackets for comparison.
Cyclododecyne (5)
(C1 C2) [ꢀ] 1.216
[1.196(4)]
174.9
[175.9(9)]
174.8
[175.9(9)]
6
8
10
ously
published
B3LYP
values
for
acetylene
(142.8 kJmolꢁ1), propyne (151.4 kJmolꢁ1),[27] and 3-hexyne
(157 kJmolꢁ1).[7] Clearly, the computed binding energies in-
crease with increasing donor strength of the alkyne. For the
cationic complexes, we obtain values of 165.7 kJmolꢁ1
(39.6 kcalmolꢁ1) for 8 and 138.4 kJmolꢁ1 (33.1 kcalmolꢁ1)
for 10. The weaker donor–acceptor interaction and the
larger steric strain (see above) both contribute to the lower
binding energy of 10 compared with 8.
ꢀ
d
N
1.256
T
1.246
T
1.247
ACHTUNGTRENNUNG
G
F
[a] [8]
R
N
N
ACHTUNGTRENNUNG
F’[a] [8]
A
R
R
ACHTUNGTRENNUNG
p!s*
100.1
27.2
72.9
0.00
77.4
19.8
57.6
0.88
69.7
20.3
49.4
0.90
d!p*
difference[b]
qACHTUNGTRENNUNG
(AuL)[c]
[a] The experimental values are mean bond angles. [b] Reflects the differ-
ꢀ
ent strengths of donation from the p
and backdonation from the d(Au) NBO to the p*
G
R
Conclusions
charges on AuL, where L is Cl in 6 and the NHC ligand in 8 and 10; note
that 8 and 10 are cations; the group charges on the alkyne are thus 0.00
in 6, 0.12 in 8, and 0.10 in 10.
A series of neutral and cationic 14-electron AuI complexes
has been obtained in which the gold atoms are free from ex-
traneous donors in the crystal. Among the two NHC com-
plexes investigated, we observe that the five-membered imi-
dazol-2-ylidene ligand in 8 allows for more electron with-
drawal from the alkyne than its six-membered homologue in
than in the cationic counterparts is reflected in the tighter
contact between the gold atom and the triple bond (the dis-
tance between the midpoint of the alkyne and the gold
center is only 2.064(3) ꢀ in 6 but 2.142(5) ꢀ in 8; for a com-
parison of relevant structural data of all known AuI alkyne
complexes characterized by X-ray crystallography, see
Table 3). Somewhat surprisingly, the computed NBO charg-
ꢀ
10. Moreover, the net energetic effect of p
ACHTUNGTRENNUNG
ꢀ
tion and Au!p
ACHTUNGTRENNUNG
larger in 8 than in 10. This suggests that the use of a classical
Arduengo carbene[23,28] as ancillary ligand for cationic AuI
combines strong p!Au acceptor properties with weak
Au!p* donation, which may be the reason for the rapidly
growing number of applications of such catalysts in the
preparative context.[1,21,29] This conclusion must also be seen
in the light of the NBO analysis of the dimeric phosphine
complex 1,[8] which indicates that donation of p-electron
density to the gold atom as well as backdonation from the
metal center are both less pronounced than in the NHC
complexes presented herein.
Table 3. Structural data of all currently known AuI alkyne complexes
characterized by X-ray crystallography, excluding acetylides.[6]
ꢀ
Complex
d
(C C) [ꢀ]
d
(Au···AM[a]) [ꢀ]
Ref.
1
2
3
4
6
8
10
1.221(8)
1.224(6)
1.259(12)
1.244(11)
1.224(5)
1.206(11)
1.218(16)
2.159(13)
2.074(9)
1.967(15)
1.980(15)
2.064(3)
2.142(5)
2.14(1)
[8]
[7]
[10]
[10]
[b]
–
–
–
[b]
[b]
[a] AM=alkyne midpoint. [b] This work.
Experimental Section
es indicate that there is essentially no net flow of electrons
between the two fragments in 6, since AuCl and cyclodode-
cyne remain almost neutral. In cationic complexes 8 and 10,
net electron donation from the alkyne ligand occurs which is
slightly larger in the case of the five-membered NHC ligand
(0.12 vs. 0.10).
We also computed the binding energies of the complexes
from the difference between the total energies of the con-
stituent molecules (alkyne and AuL) and those of the corre-
sponding complexes (Table 4). For the complexes with
General: All reactions were carried out under Ar in flame-dried glass-
ware. Solvents were purified by distillation over the indicated drying
agents prior to use and were transferred under Ar: Et2O, 1,2-dimethoxy-
ethane (Mg/anthracene), CH2Cl2, CHCl3, (CaH2), pentane (Na/K),
MeOH (Mg). Flash chromatography: Merck silica gel 60 (230–400 mesh).
NMR: Spectra were recorded on Bruker DPX 300, AMX 300, or AV 400
spectrometers in the solvents indicated. The solvent signals were used as
references, and the chemical shifts converted to the TMS scale (CDCl3:
dC =77.23 ppm; residual CHCl3 in CDCl3: dH =7.27 ppm; CD2Cl2: dC =
53.8 ppm; residual CHDCl2 in CD2Cl2: dH =5.32 ppm). IR: Magna IR750
(Nicolet) or Spectrum One (Perkin–Elmer) spectrometer. MS (EI): Fin-
nigan MAT 8200 (70 eV), ESI-MS: ESQ3000 (Bruker), accurate mass de-
terminations: Bruker APEX III FT-MS (7 T magnet) or Mat 95 (Finni-
gan). Melting points: Bꢁchi melting point apparatus B-540 (corrected).
Elemental analyses: H. Kolbe, Mꢁlheim/Ruhr. All commercially avail-
able compounds (Fluka, Lancaster, Aldrich) were used as received. Cy-
clododecyne was purified by distillation over LiAlH4 prior to use.
Table 4. Bond dissociation energies [kcalmolꢁ1].
Complex
BP86
B3LYP
DE
DG
DE
6
8
10
44.58
43.82
37.30
42.65
33.26
27.59
24.66
33.81
37.52
39.60
33.08
35.01
Complex 6: Cyclododecyne (318 mg, 1.936 mmol) was added to a cold
(08C) suspension of AuCl (225 mg, 0.968 mmol) in CH2Cl2 (1 mL) and
the resulting clear solution was stirred for 5 min before all volatile mate-
rials were evaporated at this temperature with the aid of a gentle stream
(acetylene)AuCl
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Structure and Bonding in Gold Alkyne Complexes
FULL PAPER
of argon. The residue was triturated three times with cold (ꢁ788C) pen-
tane and dried under Ar to give complex 6 as a colorless powder
(345 mg, 90%). Single crystals were grown by cooling a saturated solu-
tion in Et2O from ꢁ108C to ꢁ808C over 24 h. The resulting crystals can
be stored at ꢁ508C under Ar for weeks but decompose rapidly at above
08C. 1H NMR (400 MHz, CD2Cl2): d=2.72 (t, J=5.5 Hz, 4H), 1.72–1.65
(m, 4H), 1.56–1.49 (m, 4H), 1.45–1.39 ppm (m, 8H); 13C NMR
(100 MHz, CD2Cl2): d=85.9, 26.8, 25.9, 25.8, 24.8, 21.5 ppm.
The corresponding chloride salt was obtained as a yellow solid by stirring
a solution of the bromide (2.71 g, 5.59 mmol) in MeOH with DOWEX
(22 Cl anion-exchange resin) for 12 h at ambient temperature and flash
chromatography of the crude material (MeOH/CH2Cl2 1/10) (2.36 g,
96%). M.p. 326–3278C (decomp); 1H NMR (400 MHz, CDCl3): d=7.54
(s, 1H), 7.41 (t, J=7.8 Hz, 2H), 7.22 (d, J=7.8 Hz, 4H), 4.20 (t, J=
5.6 Hz, 4H), 3.01 (sept, J=6.8 Hz, 4H), 2.82–2.75 (m, 2H), 1.35 (d, J=
6.8 Hz, 12H), 1.20 ppm (d, J=6.8 Hz, 12H); 13C NMR (100 MHz,
CDCl3): d=152.8, 145.5, 135.7, 131.1, 125.0, 49.0, 28.8, 24.7, 24.7,
Complex 8: AgSbF6 (55 mg, 0.161 mmol) was added to a solution of cy-
~
19.3 ppm; IR (film): v=3306, 3210, 2964, 2867, 2745, 1652, 1466, 1447,
clododecyne (79 mg, 0.483 mmol) and
7
(100 mg, 0.161 mmol)[22] in
1383, 1316, 1206, 1182, 1059, 1042, 807, 754 cmꢁ1; MS (EI): m/z (%): 406
(23), 405 [M+ꢁCl] (84), 404 (80), 403 (60), 390 (30), 389 (100), 361 (13),
202 (32), 201 (20), 186 (33), 172 (20), 146 (21); HRMS (ESI): m/z calcd
for C28H41N2 [M+ꢁCl]: 405.3264, found: 405.3260; elemental analysis
calcd (%) for C28H41ClN2: C 76.24, H 9.37, N 6.35; found: C 76.25, H
9.40, N 6.44.
CH2Cl2 (1 mL) and the resulting mixture stirred at ambient temperature
for 15 min. Evaporation of the solvent with the aid of a gentle stream of
Ar and trituration of the residue with cold (08C) pentane afforded the
title complex as a colorless powder (126 mg, 80%). Single crystals were
grown by carefully layering a saturated solution of the complex in
CH2Cl2 with pentane and storage of the resulting mixture overnight. The
crystals can be stored under Ar at ꢁ208C but decompose within a few
days at ambient temperature. 1H NMR (300 MHz, CD2Cl2): d=7.65–7.59
(m, 2H), 7.51 (s, 2H), 7.42–7.38 (m, 4H), 2.52 (sept, J=6.8 Hz, 4H),
2.27–2.21 (m, 4H), 1.83–1.17 (m, 16H), 1.29 (d, J=6.8 Hz, 12H),
1.27 ppm (d, J=6.8 Hz, 12H); 13C NMR (75 MHz, CD2Cl2): d=177.6,
146.3, 133.5, 131.0, 125.1, 124.6, 88.0, 29.2, 25.8, 25.6, 24.6, 24.0, 20.9 ppm.
Complex 9: Silver oxide (315 mg, 1.36 mmol) was added to a solution of
1,3-bis(2,6-diisopropylphenyl)-3,4,5,6-tetrahydro-pyrimidin-1-ium chloride
(1.00 g, 2.27 mmol) in CH2Cl2 (14 mL) and the resulting mixture stirred
in the dark for 4 h. The precipitate was filtered off with an HPLC filter
and the filtrate treated with AuCl·SMe2 (667 mg, 2.27 mmol). After stir-
ring for 1 h in the dark, the mixture was again filtered through an HPLC
filter, the filtrate evaporated, and the residue triturated with pentane and
dried in vacuo to give complex 9 as a colorless powder. Single crystals
were grown from a solution in CHCl3 layered with pentane. M.p.>1208C
(decomp); 1H NMR (400 MHz, CD2Cl2): d=7.41 (t, J=7.7 Hz, 2H), 7.26
(d, J=7.7 Hz, 4H), 3.46 (t, J=5.8 Hz, 4H), 3.07 (sept, J=6.9 Hz, 4H),
2.41–2.35 (m, 2H), 1.32 (d, J=6.9 Hz, 12H), 1.32 ppm (d, J=6.9 Hz,
12H); 13C NMR (100 MHz, CD2Cl2): d=205.1, 146.0, 142.9, 129.6, 125.0,
46.4, 46.4, 28.9, 25.0, 24.7, 20.6 ppm; IR (film): v~=2962, 2921, 2867, 1514,
1462, 1338, 1312, 1209, 1059, 1034, 995, 804, 760, 735 cmꢁ1; MS (EI): m/z
(%): 636 (18) [M+], 600 (11), 405 (27), 404 (100), 403 (89), 401 (23), 390
(17), 389 (58), 361 (19), 202 (15), 186 (19); HRMS (ESI): m/z calcd. for
C28H40AuClN2 [M+]: 636.2545; found: 636.2539.
1,3-Bis(2,6-diisopropylphenyl)hexahydropyrimidine: An aqueous solution
of formaldehyde (37% w/w, 0.82 g, 9.94 mmol) was added to a solution
of N,N’-bis(2,6-diisopropylphenyl)pro-
pan-1,3-diamine (3.57 g, 9.04 mmol)[30]
in MeOH (20 mL) and the resulting
mixture stirred at 458C for 16 h. All
volatile materials were evaporated and
the residue was dried in vacuo to give
the title compound as a colorless solid
(3.65 g, 99%). M.p. 154–1558C;
1H NMR (400 MHz, CDCl3): d=7.20–
7.10 (m, 6H), 4.31 (s, 2H), 3.73 (sept,
J=6.9 Hz, 4H), 3.28 (t, J=5.3 Hz,
Complex 10: AgSbF6 (53.9 mg, 0.156 mmol) was added to a solution of
complex
9
(100 mg, 0.156 mmol) and cyclododecyne (25.7 mg,
4H), 1.98–1.91 (m, 2H), 1.28 (d, J=6.9 Hz, 12H), 1.23 ppm (d, J=
6.8 Hz, 12H); 13C NMR (100 MHz, CDCl3): d=149.4, 144.8, 126.4, 123.8,
70.7, 50.6, 28.4, 27.4, 24.8, 24.2 ppm; IR (film): v~=2957, 2916, 2865, 1464,
1442, 1398, 1379, 1360, 1323, 1249, 1232, 1125, 1106, 1047, 957, 806,
760 cmꢁ1; MS (EI): m/z (%): 406 (32) [M+], 405 (100); HRMS (ESI):
m/z calcd. for C28H42N2Na [M++Na]: 429.3240; found: 429.3238; elemen-
tal analysis calcd (%) for C28H42N2: C 82.70, H 10.41, N 6.89; found: C
82.81, H 10.50, N 6.79.
0.156 mmol) in CH2Cl2 (2 mL) and the resulting mixture was stirred at
ambient temperature for 5 min. All solid materials were filtered off and
the filtrate was diluted with pentane (15 mL) to precipitate the product
(144 mg, 92%). Single crystals were grown at ꢁ788C from a solution in
CH2Cl2 layered with cold pentane. They can be stored at ꢁ208C under
Ar but decompose at ambient temperature within a few days. 1H NMR
(300 MHz, CD2Cl2): d=7.48–7.40 (m, 2H), 7.32–7.27 (m, 4H), 3.64 (t, J=
5.8 Hz, 2H), 3.58 (t, J=5.8 Hz, 2H), 3.07 (sept, J=6.8 Hz, 2H), 3.02
(sept, J=6.8 Hz, 2H), 2.78 (brs, 2H), 2.48 (dq, J=5.8, 11.7 Hz, 2H),
1.75–1.66 (m, 2H), 1.40–1.10 ppm (m, 40H); 13C NMR (75 MHz,
CD2Cl2): d=194.5, 194.2, 146.7, 146.5, 146.3, 142.6, 142.6, 141.0, 140.7,
130.6, 130.4, 130.4, 125.6, 125.5, 125.3, 86.6, 48.2, 48.0, 46.5, 46.4, 29.1,
29.0, 26.2, 26.0, 25.6, 25.3, 25.3, 25.2, 24.9, 24.8, 24.8, 24.7, 24.7, 24.6, 24.5,
22.8, 20.5, 20.3, 20.0, 20.0, 19.6 ppm.
1,3-Bis(2,6-diisopropylphenyl)-3,4,5,6-tetrahydropyrimidin-1-ium halide
(X=Cl, Br): N-Bromosuccinimide (1.15 g, 6.47 mmol) was added to a
suspension of 1,3-bis(2,6-diisopropyl-
phenyl)hexahydropyrimidine (2.63 g,
6.47 mmol) in 1,2-dimethoxyethane
(15 mL) and the resulting mixture
X-ray crystal structure analysis of 5: C12H20, Mr =164.28 gmolꢁ1, colorless
cylinder, crystal size 0.50ꢄ0.40ꢄ0.40 mm, monoclinic, space group P21/c,
a=9.1005(7), b=13.5069(11), c=16.9276(14) ꢀ, b=90.073(4)8, V=
stirred for 12 h. After evaporation of
the solvent, the residue was dissolved
in CH2Cl2 (30 mL), the organic layer
was washed with water, dried over
2080.7(3) ꢀ3, T=100 K, Z=8, 1calcd =1.049 gcmꢁ3
,
l=1.54178 ꢀ, m-
MgSO4 and evaporated, and the resi-
A
=
due purified by flash chromatography
(MeOH/CH2Cl2 1/20) to give 1,3-
bis(2,6-diisopropylphenyl)-3,4,5,6-tetrahydropyrimidin-1-ium bromide as
yellow solid (3.11 g, 99%). M.p. 300–3018C (decomp). 1H NMR
(400 MHz, CDCl3): d=7.55 (s, 1H), 7.41 (t, J=7.8 Hz, 2H), 7.22 (d, J=
7.8 Hz, 4H), 4.21 (t, J=5.6 Hz, 4H), 3.00 (sept, J=6.8 Hz, 4H), 2.76–2.70
(m, 2H), 1.34 (d, J=6.7 Hz, 12H), 1.20 ppm (d, J=6.8 Hz, 12H);
13C NMR (100 MHz, CDCl3): d=152.8, 145.5, 135.7, 131.1, 125.0, 48.9,
28.7, 24.7, 24.6, 19.3 ppm; IR (film): v~=3322, 2962, 2926, 2866, 1650,
0.86), Proteum X8 diffractometer, 4.19<q<68.118, 36475 measured re-
flections, 3664 independent reflections, 2927 reflections with I>2s(I).
Structure solved by direct methods and refined by full-matrix least-
squares techniques against F2 to R1 =0.076 [I>2s(I)], wR2 =0.175; 214
parameters, H atoms riding, S=1.158, residual electron density +0.3/
a
ꢁ0.3 eꢀꢁ3
.
X-ray crystal structure analysis of 6: C12H20AuCl, Mr =396.70 gmolꢁ1, col-
orless prism, crystal size 0.13ꢄ0.04ꢄ0.02 mm, monoclinic, space group
C2/c, a=54.7070(11) ꢀ, b=11.7361(2), c=15.6081(3), b=97.166(1)8, V=
1461, 1445, 1313, 1223, 1203, 1100, 1058, 984, 937, 809, 761, 729, 696 cmꢁ1
;
9942.9(3) ꢀ3, T=100 K, Z=32, 1calcd =2.120 gcmꢁ3
(MoKa)=12.014 mmꢁ1
max =0.74), Nonius KappaCCD diffractometer, 5.10<q<28.288, 128853
measured reflections, 12273 independent reflections, 9302 reflections
,
l=0.71073 ꢀ, m-
MS (EI): m/z (%): 485 (<1) [M+], 406 (30), 405 (100), 404 (11), 389
(12); HRMS (ESI): m/z calcd for C28H41N2 [M+ꢁBr]: 405.3264, found:
405.3261.
A
,
empirical absorption correction (Tmin =0.22,
T
Chem. Eur. J. 2009, 15, 8558 – 8565
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8563

A. Fꢁrstner et al.
with I>2s(I). Structure solved by direct methods and refined by full-
matrix least-squares techniques against F2 to R1 =0.026 [I>2s(I)], wR2 =
0.065; 522 parameters, H atoms riding, S=1.030, residual electron density
Acknowledgements
Generous financial support by the MPG and the Fonds der Chemischen
Industrie (Kekulꢅ stipend to S.F.) is gratefully acknowledged. We thank
Umicore AG & Co KG, Hanau, for a gift of noble-metal salts.
+1.7/ꢁ1.4 eꢀꢁ3
.
X-ray crystal structure analysis of 8: [C39H56AuN2] CHTNUGTRNENUG
+A[SbF6]ꢁ, Mr =
985.57 gmolꢁ1, colorless plate, crystal size 0.14ꢄ0.09ꢄ0.05 mm, ortho-
rhombic, space group Pna21, a=16.1859(2), b=14.7429(2), c=
34.0181(4) ꢀ, V=8117.64(18) ꢀ3, T=100 K, Z=8, 1calcd =1.613 gcmꢁ3
l=0.71073 ꢀ,
,
mACHTUNGTRENNUNG , empirical absorption correction
(MoKa)=4.331 mmꢁ1
(Tmin =0.51, Tmax =0.77), Nonius KappaCCD diffractometer, 5.86<q<
33.228, 164022 measured reflections, 30681 independent reflections,
24434 reflections with I>2s(I). Structure solved by direct methods and
refined by full-matrix least-squares techniques against F2 to R1 =0.035
[I>2s(I)], wR2 =0.057; 880 parameters, absolute structure parameter=
0.4144(19), H atoms riding, S=1.020, residual electron density +2.5/
[1] Selected reviews: a) A. Fꢁrstner, P. W. Davies, Angew. Chem. 2007,
119, 3478–3519; Angew. Chem. Int. Ed. 2007, 46, 3410–3449;
b) D. J. Gorin, B. D. Sherry, F. D. Toste, Chem. Rev. 2008, 108, 3351–
3378; c) E. Jimꢅnez-NfflÇez, A. M. Echavarren, Chem. Rev. 2008,
108, 3326–3350; d) A. S. K. Hashmi, Chem. Rev. 2007, 107, 3180–
3211; e) N. Bongers, N. Krause, Angew. Chem. 2008, 120, 2208–
2211; Angew. Chem. Int. Ed. 2008, 47, 2178–2181; f) V. Michelet,
P. Y. Toullec, J.-P. GenÞt, Angew. Chem. 2008, 120, 4338–4386;
Angew. Chem. Int. Ed. 2008, 47, 4268–4315; g) Z. Li, C. Brouwer,
C. He, Chem. Rev. 2008, 108, 3239–3265; h) A. Arcadi, Chem. Rev.
2008, 108, 3266–3325.
ꢁ1.4 eꢀꢁ3
.
X-ray crystal structure analysis of 9: C28H40AuClN2·CHCl3, Mr =
756.40 gmolꢁ1, colorless plate, crystal size 0.33ꢄ0.24ꢄ0.18 mm, ortho-
rhombic, space group P212121, a=10.6707(2), b=16.5339(4), c=
17.7935(4) ꢀ, V=3139.28(12) ꢀ3, T=100 K, Z=4, 1calcd =1.600 gcmꢁ3
l=0.71073 ꢀ,
,
mACHTUNGTRENNUNG , empirical absorption correction
(MoKa)=5.047 mmꢁ1
[2] Review: A. S. K. Hashmi, M. Rudolph, Chem. Soc. Rev. 2008, 37,
1766–1775.
(Tmin =0.62, Tmax =1.00), Nonius KappaCCD diffractometer, 2.98<q<
31.568, 10398 measured reflections, 10398 independent reflections, 10014
reflections with I>2s(I). Structure solved by direct methods and refined
by full-matrix least-squares techniques against F2 to R1 =0.041 [I>2s(I)],
wR2 =0.127; 333 parameters, absolute structure parameter=0.178(7), H
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1998, 120, 8305–8314; b) A. Fꢁrstner, P. Hannen, Chem. Commun.
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[4] This feature distinguishes Au and Pt catalysis from many other tran-
sition-metal catalyzed reactions of enynes and related substrates,
which require coordination of both unsaturated sites to a single
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Trost, M. J. Krische, Synlett 1998, 1–16; b) see also: A. Fꢁrstner, K.
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atoms riding, S=1.027, residual electron density +2.2/ꢁ1.7 eꢀꢁ3
.
ACTHNUTRGNEUNG
X-ray crystal structure analysis of 10: [C40H60AuN2]+·2[Ag]+·3[SbF6]ꢁ·
[H2O], Mr =1706.87 gmolꢁ1
, colorless plate, crystal size 0.16ꢄ0.10ꢄ
0.04 mm, monoclinic, space group C2m, a=18.1953(3), b=15.7767(3),
c=19.0602(3) ꢀ, b=97.675(1)8, V=5422.44(16) ꢀ3, T=100 K, Z=4,
ACTHNUTRGNEUNG
1calcd =2.091 gcmꢁ3, l=0.71073 ꢀ, m(MoKa)=4.970 mmꢁ1, empirical ab-
sorption correction (Tmin =0.42, Tmax =0.88), Nonius KappaCCD diffrac-
tometer, 6.83<q<33.218, 68408 measured reflections, 10510 independent
reflections, 8640 reflections with I>2s(I). Structure solved by direct
methods and refined by full-matrix least-squares techniques against F2 to
R1 =0.044 [I>2s(I)], wR2 =0.130; 313 parameters, H atoms riding, S=
1.045, residual electron density +2.9/ꢁ3.4 eꢀꢁ3
.
CCDC-715728 (5), CCDC-715727 (6), CCDC-715729 (8), CCDC-715730
(9), and CCDC-715731 (10) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
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appropriate auxiliary basis sets to speed up the calculations.[35–37] All geo-
metries were fully optimized without symmetry constraints. The resulting
structures were confirmed to be minima by force constant analysis.
Single-point calculations with B3LYP[38,39]/def2-TZVP were carried out at
the geometries optimized with BP86. The rigid-rotor harmonic-oscillator
approximation was used to compute the zero-point vibrational energies
as well as the thermal and entropic corrections needed to determine the
bond-dissociation free enthalpies DG at 300 K. Bond dissociation ener-
gies DE were calculated as the energy difference between the constituent
molecules (alkyne and AuL) and the complex (Table 4). Natural bond
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8564
www.chemeurj.org
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 8558 – 8565

Structure and Bonding in Gold Alkyne Complexes
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Received: April 22, 2009
Published online: June 30, 2009
Chem. Eur. J. 2009, 15, 8558 – 8565
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8565