Gold(l) chloride coordinated 3-hexyne

Article abstract of DOI:10.1021/ic8020854

A linear gold(l) complex featuring a simple, unstrained alkyne hasbeen synthesized using AuCI and 3-hexyne and characterized using X-ray crystallography. Density functional theory calculations show that σ donation from alkyne → Au dominates over the Au →

Full text of DOI:10.1021/ic8020854

Inorg. Chem. 2009, 48, 423-425
Gold(I) Chloride Coordinated 3-Hexyne
Jiang Wu, Peter Kroll,* and H. V. Rasika Dias*
Department of Chemistry and Biochemistry, The UniVersity of Texas at Arlington,
Arlington, Texas 76019
Received October 30, 2008
A linear gold(I) complex featuring a simple, unstrained alkyne has
been synthesized using AuCl and 3-hexyne and characterized using
X-ray crystallography. Density functional theory calculations show
that σ donation from alkyne f Au dominates over the Au f
alkyne π back-donation.
Gold was long considered to be chemically inert and a
poor catalyst.1 However, during the past few years, novel
reactions mediated by gold have been reported in the
literature on a regular basis and in ever-increasing num-
bers.2-5 Alkynes are some of the most commonly used
substrates in gold-catalyzed organic synthetic processes.
Furthermore, alkyne complexes of gold are believed to be
key intermediates in numerous homogeneous and heteroge-
neous processes. However, isolable gold alkyne complexes
are rare. Structurally authenticated gold alkyne compounds
that have been described in the literature (Figure 1) have
strained alkynes like 3,3,6,6-tetramethyl-1-thiacyclohept-4-
yne 1,1-dioxide (1)6 and 3,3,6,6-tetramethyl-1-thiacyclohept-
4-yne (2)6 or an alkyne in a tethered framework (3).7 A
Figure 1. Structurally characterized gold(I) alkyne complexes.
limited number of gold(I)-coordinated metal acetylides and
spectroscopically detected gold alkynes are also known.8
In this paper, we report the isolation and structural
characterization of Au(EtCt CEt)Cl (4), a metal coordination
complex featuring a simple unstrained alkyne bonded to
AuCl (gold chlorides are used routinely in catalytic homo-
geneous processes mediated by gold). First attempts at the
isolation of simple gold(I) alkynes including Au(EtCt CEt)Cl
were reported by Hu¨ttel and Forkl in 1972.9 However, they
have not been able to characterize them in detail.
The treatment of AuCl with 3-hexyne (EtCt CEt) in
methylene chloride at -20 °C led to Au(EtCt CEt)Cl, which
could be crystallized to obtain colorless, needle-shaped
crystals (Scheme 1).10 Au(EtCt CEt)Cl is an air-sensitive
solid and decomposes easily and rapidly at room temperature,
forming black deposits. Au(EtCt CEt)Cl is stable in solution
for hours at low temperature but decomposes slowly at room
temperature, as is evident from the deposition of dark solids,
* To whom correspondence should be addressed. E-mail: dias@uta.edu.
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10.1021/ic8020854 CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 2, 2009 423
Published on Web 12/15/2008

COMMUNICATION
Scheme 1. Synthesis of Au(EtCt CEt)Cl
presumably metallic gold, on the walls of the container.11
The 1H NMR spectrum of Au(EtCt CEt)Cl in CD2Cl2 at -20
°C displays a quartet and a triplet at δ 2.61 and 1.23,
respectively. These values did not change significantly upon
warming to room temperature. The corresponding signals
of the free 3-hexyne were observed at δ 2.15 (qt) and 1.11
(t), respectively (indicating that AuCl coordination leads to
a downfield shift of both signals). The 13C NMR signal of
the acetylenic carbons also shows a downfield shift (δ 81.1
peak of the free ligand shifts to δ 86.4, upon coordination).
The corresponding signals of [HC{(F3C)CO}2Ag]2(EtCt
CEt),12 Cu2(µ-O2CCF3)2(EtCt CEt)2,13 (Ph3P)2Ni(EtCt CEt),14
and [K(18-crown-6)[PtCl3(EtCt CEt)]15 were observed at δ
81.5, 88.3, 126.0, and 72.6, respectively. Note that, in contrast
to the observation with the alkyne adduct Au(EtCt CEt)Cl,
alkene carbons show large upfield 13C NMR shifts upon
coordination of AuI centers to olefins.16,17 For example, the
13CNMRsignalofthecoordinatedethylenein[N{(C3F7)C(2,6-
Cl2C6H3)N}2]Au(C2H4) appears at δ 59.1, while the corre-
sponding peak of the free ethylene appears at δ 123.5.18
Figure 2. Molecular structure of Au(EtCt CEt)Cl. Selected bond lengths
(Å) and angles (deg): Au-C4 2.152(4), Au-C3 2.172(5), Au-Cl 2.2703(11),
C1-C2 1.530(7), C2-C3 1.472(7), C3-C4 1.224(6), C4-C5 1.470(6),
C5-C61.526(6);C4-Au-C332.88(17),C4-Au-Cl167.13(13),C3-Au-Cl
159.99(12), C3-C2-C1 112.4(4), C4-C3-C2 166.9(5), C4-C3-Au
72.7(3), C2-C3-Au 120.3(3), C3-C4-C5 163.0(5), C3-C4-Au 74.4(3),
C5-C4-Au 122.6(3), C4-C5-C6 111.6(4).
Table 1. Selected Bond Lengths (Å) and Angles (deg) of
Au(EtCt CEt)Cla
Parameter
experimental
computed
C3t C4
1.224(6)
2.152(4)
2.172(5)
2.2703(11)
166.9(5)
163.0(5)
1.247
2.206
2.231
2.304
165.7
163.0
Au-C4
Au-C3
Au-Cl
C4-C3-C2
C3-C4-C5
a See Figure 2 for the atom numbering scheme. The calculated Ct
C
distance of free 3-hexyne is 1.215 Å.
The X-ray crystal structure of Au(EtCt CEt)Cl is il-
lustrated in Figure 2. 3-Hexyne coordinates to gold in a η2
fashion with Au-C bond lengths of 2.152(4) and 2.172(5)
Å (Table 1). The gold adopts a linear coordination environ-
ment (Cl-Au-centroidCt C angle of 176.3°). These distances
are shorter than the Au-C distances of cationic species 3
[2.197(5) and 2.270(5) Å] reported by Shapiro and Toste.7
Gold complexes of angle-strained alkynes 1 and 2 from
Behrens group feature shorter (and presumably stronger)
Au-C bonds [2.050(7)-2.100(8) Å].6 The Ct C bond length
of the bonded alkyne in Au(EtCt CEt)Cl [1.224(6) Å] is only
marginally longer than the typical alkyne Ct C bond distance
1
(10) Au(EtCt CEt)Cl: H NMR (CD2Cl2, 258 K) δ 1.23 (t, J ) 7.2 Hz,
6H, CH3), 2.61 (q, J ) 7.2 Hz, 4H, CH2); 13C{1H} NMR (CD2Cl2,
258 K) δ 14.7, 15.2, 86.4 (Ct C). Crystal data: empirical formula
C6H10AuCl, fw ) 314.56, T ) 100(2) K, crystal system ) tetragonal,
space group ) P41, a ) 8.3936(4) Å, b ) 8.3936(1) Å, c ) 11.2283(5)
Å, V ) 791.06(5) Å3, Z ) 4, final R indices [I > 2σ(I)] R1 ) 0.0145,
wR2 ) 0.0343; R indices (all data) R1 ) 0.0149, wR2 ) 0.0345.
(11) A mixture of 3-hexyne and gold(I) chloride (1:4 molar ratio) in CH2Cl2,
upon warming to room temperature, generates metallic gold and
various decomposition products after about 2 h. Filtering of this
mixture, upon cooling to -20 °C, produced few crystals. X-ray analysis
(see Figure S1 in the Supporting Information, CCDC 703381) revealed
this to be a dimer of a gold(III) complex, 1-chloroaura-2,5-trans-
dichloro-2,3,4,5-tetraethylcyclopent-3-ene (see Figure S3 in the Sup-
porting Information), presumably formed by a cycloaddition pathway.
This could be an intermediate during the gold chloride mediated
formation of trans-3,4-dichloro-tetraalkyl-1-cyclobutenes from alkynes
(because reductive elimination of the organic ligand fragment from
this gold(III) adduct would lead to such compounds and AuCl). See:
Hu˝ttel, R.; Forkl, H. Chem. Ber. 1972, 105, 2913–2921, and ref 9.
(12) Chi, K.-M.; Lin, C.-T.; Peng, S.-M.; Lee, G.-H. Organometallics 1996,
15, 2660–2663.
of 1.202(5) Å19 [e.g., BuCt CtBu, 1.2022(15) Å]. Note
that often changes in the C-C bond distance upon coordina-
tion of coinage metals to alkyne (or alkene) moieties are
harder to discern using routine X-ray crystallography because
relatively small lengthenings are often overshadowed by high
estimated standard deviations (esd’s).17 However, the average
Ct C-C angle of 165.0(5)° indicates a significant deviation
from linearity. The Au-Cl bond distance of 2.2703(11) Å
is similar to the corresponding distance in two-coordinate
compounds like Au(PPh3)Cl [2.279(3) Å],21 Au(cis-cy-
clooctene)Cl [2.266(4) Å],22 and Au(CO)Cl [2.261(6) Å].23
20
t
(13) Reger, D. L.; Huff, M. F. Organometallics 1990, 9, 2807–2810.
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V. B.; Vol’pin, M. E. J. Org. Chem. 1990, 391, 119–122.
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H. Inorg. Chim. Acta 1995, 234, 47–53.
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Manassero, M.; Arca, M. Dalton Trans. 2006, 5703–5716, and
references cited therein.
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York, 1972; p 108.
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C55, IUC9900016.
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1976, B32, 962–963.
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424 Inorganic Chemistry, Vol. 48, No. 2, 2009

COMMUNICATION
agreement with calculated acetylene and propyne binding
energies to AuCl of -142.8 and -151.4 kJ/mol, respec-
tively).5
In order to understand the difference between AuCl
adducts of strained alkynes and simple unstrained EtCt CEt,
we examined the bonding and energetics of the AuCl
complex of 1-thiacyclohept-4-yne (C6H8S). The optimized
structure shows that C6H8S coordinates to AuCl in a
symmetrical fashion. The adduct Au(C6H8S)Cl has an
enthalpy of formation of -204.9 kJ/mol. The activation
barrier for adduct formation between AuCl and C6H8S or
EtCt CEt is essentially zero. Thus, the enthalpy of formation
data of AuCl adducts indicate that the dissociation of C6H8S
from Au(C6H8S)Cl is significantly more unfavorable relative
to EtCt CEt dissociation from Au(EtCt CEt)Cl. Second-
order perturbation analysis of bonding in Au(C6H8S)Cl shows
that the magnitude of σ donation from alkyne f Au (419
kJ/mol) dominates over the Au f alkyne π back-donation
(185 kJ/mol). Comparison to the corresponding values in
Au(EtCt CEt)Cl shows that Au(C6H8S)Cl features much
stronger gold alkyne σ and π components. The Au-C
distances are also relatively short in the Au(C6H8S)Cl adduct
(see the Supporting Information).
We also calculated a downfield shift of the 13C NMR signal
of the acetylenic carbons of Au(EtCt CEt)Cl upon coordina-
tion, which is consistent with the experimental observation.
However, the calculated downfield shift of δ 11.6 for the
symmetrical arrangement (shifts of δ 8.5 and 13.8 for the
slipped geometry) is somewhat larger than the δ 5.3 shift
observed in the experiment.
In summary, we describe the first X-ray structural data of
a simple, unconstrained alkyne complex of gold(I). Density
functional theory calculations show that σ donation from
alkyne f Au dominates over the Au f alkyne π back-
donation. Gold chlorides are widely used as effective
π-activation catalysts for alkynes, and Au(η2-EtCt CEt)Cl
may be viewed as a good model for the likely intermediate
in some of these processes.
Figure 3. Diagram showing the formation of chains via Au · · ·Au contacts.
There are weak aurophilic interactions24 [Au · · · Au separa-
tions of 3.26260(15) Å and Au · · · Au · · · Au angles of
137.753(4)°] between the neighboring Au(EtCt CEt)Cl
molecules, leading to the formation of helical chains (Figure
3). Helices lie on a 4-fold screw axis (see Figure S1 in the
Supporting Information).
Optimization of complex Au(EtCt CEt)Cl at the B3LYP/
LANL2DZ(Au), ccpVDZ(C,H), and AUG-ccpVDZ(Cl) lev-
els of theory reproduces the experimental geometry (Table
1). Interestingly, the computed structure predicts a partial
slippage of the gold(I) away from the symmetrical η2
coordination.4 However, the calculated energy difference
between the slipped and symmetric structures is very small,
about 1 kJ/mol. Moreover, we found no barrier between the
two geometries. The experimentally observed Au-C bond
distances show slight asymmetry, but the difference is not
significant at the 3σ level of esd. Nevertheless, the close
correspondence between the computed and experimental
structures suggests that crystal packing factors do not
significantly influence the geometry of the complex.
The optimized geometry was used to analyze the energetics
of the complex, bonding between the metal center and the
alkyne, and to compute the NMR chemical shift. Molecular
orbital and natural bond orbital analysis corroborate the
expected σ donation from the occupied in-plane alkyne π
orbital to the empty metal-chlorine σ* orbital and Au f
alkyne back-donation from the occupied d orbital of gold to
the π* orbital of the alkyne.
Acknowledgment. This work has been supported by the
Welch Foundation (Grant Y-1289) and employed NMR
instruments funded by the NSF (Grants CHE-9601771 and
CHE-0234811). We thank Charles Savage for performing
the low-temperature NMR measurements.
Second-order perturbation analysis indicates that the
magnitude of σ donation from alkyne f Au (319 kJ/mol)
dominates over the Au f alkyne π back-donation (111 kJ/
mol). For comparison, corresponding values reported for the
simplified version of cationic gold alkyne complex 3 are 237
and 55.6 kJ/mol.7 Lower values for the latter species are
not surprising because it has a relatively bulkier and
somewhat weakly donating alkyne,25 a competing phosphine
donor, and a cationic gold center. We also find a formation
enthalpy of Au(EtCt CEt)Cl with reference to molecular
AuCl and 3-hexyne at -157 kJ/mol (this is in good
Supporting Information Available: Further details of the
synthesis and characterization, additional data, and figures. This
material is available free of charge via the Internet at http://
pubs.acs.org. CCDC 703380 contains the supplementary crystal-
lographic data for Au(EtCt CEt)Cl. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html
(or from the Cambridge Crystallographic Data Centre (CCDC), 12
Union Road, Cambridge CB2 1EZ, U.K.).
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(25) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165–195.
Mares, F.; Streitwieser, A., Jr. J. Am. Chem. Soc. 1967, 89, 3770–
3771.
IC8020854
Inorganic Chemistry, Vol. 48, No. 2, 2009 425