Gold(III) olefin complexes

Article abstract of DOI:10.1002/anie.201208356

Zeise's salt gets company: 185 years after the report of the well-known platinum(II) ethylene compound, examples of isolable olefin complexes of its isoelectronic neighbor in the periodic table, gold(III), have been prepared (see picture). The complexes a

Full text of DOI:10.1002/anie.201208356

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Angewandte
Communications
DOI: 10.1002/anie.201208356
Gold(III) Alkene Complexes
Gold(III) Olefin Complexes**
Nicky Savjani, Dragos¸-Adrian Ros¸ca, Mark Schormann, and Manfred Bochmann*
Zeiseꢀs salt, K[PtCl3(C2H4)], was first reported in 1827 and is
generally regarded as the prototypical transition-metal olefin
complex.[1] It displays the square-planar structure[2] typical of
a heavy transition metal with the d8 electron configuration,
a structural feature shared with innumerable d8 noble-metal
olefin complexes of the cobalt and nickel triads. It is
surprising then that analogous olefin complexes of its
isoelectronic and isostructural neighbor in the periodic
table, gold(III), are not known. By contrast, alkene and
alkyne complexes of gold(I) were first isolated in the 1960s
and 70s, mainly by the pioneering work of Hꢁttel.[3] This area
has experienced a dramatic resurgence in recent years, an
interest fuelled by the growing importance of gold complexes
as catalysts where formation of gold olefin or acetylene
complexes is frequently postulated as the first step of
a catalytic cycle.[4] Gold(III) catalysts or catalyst precursors
are employed in a multitude of reactions including inter- and
alongside perfluorobiphenyl as the product of reductive
elimination.
However, employing gold(III) complexes based on the
reduction-resistant (C^N^C)* pincer ligand, where
(C^N^C)* = 2,6-bis(4-tBuC6H3)2pyridine dianion, were
more successful. Thus adding cyclopentene to a solution of
the gold trifluoroacetato complex 1[12] in dichloromethane at
À408C in the presence of B(C6F5)3 gave the cyclopentene
complex 2a (Scheme 1). The norbornene and ethylene
À
intramolecular additions of nucleophiles to C C double and
À
À
triple bonds, C H bond activations, C C bond formations,
hydrogenations and the like, most commonly in the form of
gold(III) halide compounds.[5–9] These reactions can only be
rationalized by assuming the coordination of an alkene or
alkyne to the metal center as the first step of the catalytic
cycle. On the other hand, earlier efforts to generate gold(III)
olefin complexes were unsuccessful, and reactions of typical
gold(III) starting materials, such as AuCl3, AuBr3, HAuCl4 or
NaAuCl4, with olefins or dienes invariably led to reduction to
gold(I) or gold metal,[3] which is not surprising perhaps in view
of the strongly positive standard redox potentials for Au3+
ions (Au3+/Au+ = 1.36 V; Au3+/Au0 = 1.52 V).[10] We decided
to reinvestigate the possible synthesis of gold congeners of PtII
p-complexes and report herein the synthesis and some
reactions of gold(III) alkene complexes.
Scheme 1. Synthetic routes to gold(III) alkene complexes.
complexes 3a and 4a, respectively, were obtained in analo-
gous manner. An alternative route to these complexes starts
with the gold(III) hydroxide (C^N^C)*AuOH, which reacts
with [H(OEt2)2]+XÀ (X = H2N[B(C6F5)3]2 or B[3,5-C6H3-
(CF3)2]4) in the presence of olefins under the same conditions.
In this case, a few granules of 4 ꢂ molecular sieves were
added to remove the produced water. The yields were
essentially quantitative in all cases.
Initial experiments seemed to confirm the earlier reports.
For example, attempts to displace the ether ligands in the
known[11] gold(III) cation [(C6F5)2Au(OEt2)2]+ with norbor-
nene or 1,5-cyclooctadiene showed no reaction, while chlo-
ride abstraction from [(C6F5)2AuCl2]À with AgSbF6 in the
presence of norbornene gave mainly [Au(norbornene)3]+
In solution, the cyclopentene and ethylene complexes are
stable up to about 0 and À208C, respectively, when changes in
the pincer ligand pattern are becoming apparent. The
norbornene complex is stable at 208C. Removal of volatiles
below À308C (2a, 4a) or below 08C (3a) affords the
complexes as yellow powders which are stable under nitro-
gen; 3c was isolated as yellow needles. Once isolated, these
complexes are stable in the solid state for hours at room
temperature and in air with minimal degradation. Up to now,
crystals suitable for X-ray diffraction could unfortunately not
be obtained, even though various different counteranions and
crystallization methods were tried.
[*] Dr. N. Savjani, D.-A. Ro¸sca, Dr. M. Schormann,
Prof. Dr. M. Bochmann
Wolfson Materials and Catalysis Centre, School of Chemistry,
University of East Anglia
Norwich, NR4 7TJ (UK)
E-mail: m.bochmann@uea.ac.uk
[**] This work was supported by the Leverhulme Trust and Johnson
Matthey plc. N.S. and D.A.R. thank the University of East Anglia for
studentships.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208356.
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Coordination of cyclopentene in 2a was indicated by
a downfield shift of the olefinic proton signal from d = 5.74 to
6.3 ppm, as well as by the loss of equivalence of the methylene
protons which in the complex appear as four multiplets.[13] By
contrast, the norbornene complexation in 3a is associated
with a small upfield shift of the olefinic resonances, from d =
6.02 to 5.8.
plexes for comparison. The data suggest comparable p-
donation and back-bonding in gold and platinum complexes.
A different course of reaction was observed when 1 was
allowed to react with ethylene at room temperature in the
absence of B(C6F5)3 over the course of 14–72 h. Quantitative
F
À
insertion of ethylene into the Au OAc bond was observed to
give 5 [Eq. (1)]. Attempts at crystallization resulted in a small
It is well-established that norbornene has a greater ring-
strain than cyclopentene, a factor that strengthens alkene
coordination and back-bonding.[14] The small chemical shift
change of the alkene signals in 3a can be understood as the
consequence of two opposing trends: p-donation to a cationic
metal center and back-donation, which almost cancel one
another out. Clear and unequivocal indication for norbornene
coordination is however provided by the bridgehead protons,
which display a strong shift to lower frequency, from d = 2.84
to 3.49, as well as by the protons in the 5- and 6-positions of
the norbornene framework, which are also downfield shifted
from d = 1.62 to 1.97. These signals remain invariant from À40
to + 208C.[15] Neither 2a nor 3a show evidence for alkene
rotation or ligand exchange processes between free and
coordinated alkenes. For all of the alkene complexes, the
addition of SMe2 to the NMR samples produced
[(C^N^C)*Au(SMe2)]+ together with the signals of the free
olefins.
crop of crystals of 5·3AgOAcF that proved suitable for X-ray
diffraction studies (Figure 1). Closer inspection showed that
this particular sample of 1 contained about 6 mol% AgOAcF
1
The coordination of ethylene in 4a is indicated by a H
chemical shift change from d = 5.38 in free C2H4 to d = 6.29 at
À408C. In the presence of excess ethylene, the 1H NMR
signals broaden on warming from À70 to À108C, which is
indicative of ligand exchange, while in the absence of excess
ethylene a sharp signal for coordinated C2H4 is observed over
this temperature range. The formation of an ethylene com-
plex was confirmed using 13C2H4; the 13C{1H} NMR resonance
of coordinated ethylene was observed at d = 108.9, up from
Figure 1. Partial view of 5·3AgOAcF showing the AuIII–Ag interactions.
Ellipsoids are set at 50% probability; hydrogen atoms are omitted.
Selected bond lengths [ꢀ] and angles [8]: Au1–N1 2.048(10), Au1–C31
2.055(11), Au1–C6 2.063(10), Au1–C17 2.080(10), Au1–Ag2
2.9367(10), Au1–Ag1 3.0789(10); N1-Au1-C31 178.4(4), C6-Au1-C17
162.0(4), N1-Au1-C6 80.9(4), C31-Au1-C6 97.8(5), N1-Au1-C17 81.1(4),
C31-Au1-C17 100.2(4), C6-Au1-Ag2 122.6(3), C17-Au1-Ag2 58.0(3), C6-
Au1-Ag1 51.3(3), C17-Au1-Ag1 128.7(3), Ag2-Au1-Ag1 72.07(3).
À
d = 122.8 for the free alkene, while the C H coupling
constant increased on coordination from 156 to 166 Hz.
Similar JCH values have been found for gold(I) ethylene
complexes, although in those cases the changes in the
13C NMR chemical shifts are larger, of the order of more
than 60 ppm.[16,17] Table 1 summarizes pertinent NMR spec-
troscopic parameters of AuI, AuIII, and PtII ethylene com-
remaining from its preparation (by 19F NMR), which
explained the low yield of crystalline material. The geometric
parameters of 5 are as expected, but the gold complex is
associated with a polymeric ribbon of silver trifluoroacetate
(Figure 2). This ribbon is similar to
the structure[23] of [AgOAcF]n, with
Table 1: Comparison of NMR data of selected gold(I), platinum(II), and gold(III) ethylene and
subtle deviations (see the Support-
ing Information).
cyclopentene complexes.[a]
d1H
Dd (H)
d13C
Dd (C)
JCH
Ref.
[b]
Compound
There is much interest in “met-
allophilic” interactions between
electron-rich heavy metals. These
form a common feature for closed-
shell d10 elements and are a prom-
inent feature of gold(I) chemistry,
but evidence is scant for the smaller
and less electron-rich gold(III),[24]
and indeed we have been unable
to find structurally characterized
precedents for AuIII–Ag interac-
tions in the CCDC crystallographic
database. In 5·3AgOAcF, each Au
C2H4
5.38
4.94
3.81
3.09
4.83
4.93
4.12
6.29
6.23
6.30
122.8
92.7
63.7
61.6
67.1
156
[Au(C2H4)3]SbF6
HB(pzCF3)3Au(C2H4)[c,d]
[(2-R-bipy)Au(C2H4)]PF6
K[PtCl3(C2H4)][f]
(py)PtCl2(C2H4)[g]
À0.44
À1.6
À2.3
À0.55
À0.45
À1.26
0.91
[17]
[16]
[18]
[19,22]
[20]
[21,22]
This work
[20]
À59.8
À55.2
À55.7
165
166
[e]
[g]
[MePt(PMe2Ph)2(C2H4)]PF6
84.8
108.9
À38.4
À13.9
4a
(py)PtCl2(cyclopentene)[g]
0.49
0.56
2a
This work
[a] In CD2Cl2 unless indicated otherwise. [b] In Hz. [c] pzCF3 =3,5-(CF3)2C3N2. [d] In CDCl3. [e] R=2,6-
Me2C6H3. [f] In 1m methanolic HCl. The chemical shift of free C2H4 in this solvent is d=5.37. [g] Trans
isomer, py=pyridine.
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Figure 2. Side view of 5·3AgOAcF, showing the interactions of 5 with
the polymeric AgOAcF ribbon through Ag–Au and Ag–arene p-interac-
tions.
Figure 3. Molecular structure of 6·SbF6. Ellipsoids are set at 50%
probability. Selected bond lengths [ꢀ] and angles [8]: Au–C1 1.995(3),
Au–C26 2.049(3), Au–O1 2.159(2), Au–N1 2.180(2); C1-Au-C26
98.23(11), C1-Au-O1 166.31(10), C1-Au-N1 81.20(10), N1-Au-C26
179.26(10), O1-Au-N1 112.42(8).
atom has two close Au···Ag contacts of 2.9367(10) and
3.0789(10) ꢂ, which could be interpreted as AuIII···Ag metal-
lophilic bonding. These Au···Ag distances are only slightly
longer than those found in a number of AuI···Ag clusters
containing silver trifluoroacetate.[25] The association of 5 with
the silver ions is further reinforced by h1- and h2-interactions
of Ag+ ions with the p-electron density of the aromatic
À
C^N^C* ligand system. The Ag C distances range from
2.405(9) to 2.679(11) ꢂ and are closely comparable to the
metal p-ligand bond lengths found in silver–benzene com-
plexes.[26] The structure of 5·3AgOAcF may be regarded as
a model for the adsorption of a planar aromatic system to
a polar solid support. The relative contributions of p-donation
to Ag+ and metallophilic AuIII···Ag+ interactions to this
molecular association have yet to be determined; at this stage
we feel that dipolar Ag+···p interactions are likely to be
important factors.
In an attempt to obtain crystallographic confirmation for
the norbornene complex, a different synthetic approach was
tried, with the reaction of (C^N^C)*AuCl with AgSbF6 in the
presence of norbornene. The resulting product was identified
by X-ray diffraction as the norbornolyl complex 6·SbF6
(Figure 3). Evidently this is the product of the reaction of
3a with adventitious water, which has led to nucleophilic
attack by OHÀ on coordinated norbornene, while the gold–
phenyl bond was cleaved by H+. The structure of 6·SbF6
suggests that in 3a the norbornene is bonded by its exo-
face, as is usually observed. Cleavage of the tridentate pincer
in (C^N^C)*Au complexes has been seen before, but only by
strong acids.[27] Interestingly, while the hydration of alkynes
by gold catalysts has been widely studied, alkene hydration
has received scant attention. Similarly, the reaction of
(C^N^C)*AuOH with [H(OEt2)2][B{3,5-C6H3(CF3)2}4] in
Scheme 2. Nucleophilic attack on gold(III) norbornene complexes.
À
to facilitate Au C bond cleavage. This was confirmed by the
failure of this bond to react with excess HOAcF or [H(OEt2)2]
[B{3,5-C6H3(CF3)2}4].
In summary, we have shown, some 185 years after the
well-known ethylene complex of platinum(II), that olefin
complexes of its isoelectronic neighbor in the periodic table,
gold(III), could be prepared and isolated. An important
stabilizing factor is a ligand framework that disfavors
reductive elimination. The complexes are thermally less
stable than platinum analogues and, being cationic, are
highly susceptible to nucleophilic attack, including by weak
nucleophiles such as trifluoroacetate. The resulting gold(III)
alkyls are stable towards protonolysis. Structural studies have
also provided a rare example of a structurally characterized
mixed-metal complex suggestive of metallophilic AuIII···Ag
interactions.
the presence of norbornene and water generated 6·BArF in
4
high yield. The addition of methanol to coordinated norbor-
nene proceeds in analogous fashion and affords the norbornyl
methyl ether complex 7 (Scheme 2).
In principle, this alkene hydration reaction should be
capable of proceeding under catalytic conditions, provided
the gold–alkyl bond is efficiently protolyzed. In practice this
does not prove to be the case. It was shown recently that the
I
À
rates of protolysis of Au C bonds strongly depend on the
Received: October 17, 2012
Revised: November 8, 2012
Published online: November 23, 2012
donor strength of the ligands in the trans position,[28] and we
suspect that in 6+, the pyridine-N donor is not strong enough
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