Formal Gold-to-Gold Transmetalation of an Alkynyl Group Mediated by Palladium: A Bisalkynyl Gold Complex as a Ligand to Palladium

Article abstract of DOI:10.1002/chem.201501813

The reaction of [Au(C-C n-Bu)]_n with [Pd(η³-allyl)Cl(PPh₃)] results in a ligand and alkynyl rearrangement, and leads to the heterometallic complex [Pd(η³-allyl){Au(C-C n-Bu)₂}]₂ (3) with an unprecedented bridging bisalkynyl-gold ligand coordinated to palladium. This is a formal gold-to-gold transmetalation that occurs through reversible alkynyl transmetalations between gold and palladium. Back and forth! Direct gold-to-palladium and reverse palladium-to-gold alkynyl transmetalations effectively produce a deceptively simple gold-to-gold transfer. The resulting bisalkynyl-gold complexes act as bridging ligands to palladium atoms in an unprecedented structural motive (see scheme).

Full text of DOI:10.1002/chem.201501813

DOI: 10.1002/chem.201501813
Communication
&
Transmetalation
Formal Gold-to-Gold Transmetalation of an Alkynyl Group
Mediated by Palladium: A Bisalkynyl Gold Complex as a Ligand to
Palladium
Alberto Toledo, Isabel Meana, and Ana C. AlbØniz *[a]
turns out to be a complex process that involves multiple trans-
Abstract: The reaction of [Au(CꢀCÀn-Bu)]n with [Pd(h3-al-
metalations between Au and Pd, leading to a formal Au-to-Au
lyl)Cl(PPh3)] results in a ligand and alkynyl rearrangement,
and leads to the heterometallic complex [Pd(h3-allyl)-
{Au(CꢀCÀn-Bu)2}]2 (3) with an unprecedented bridging bi-
salkynyl–gold ligand coordinated to palladium. This is
a formal gold-to-gold transmetalation that occurs through
reversible alkynyl transmetalations between gold and pal-
ladium.
alkynyl transfer. The impact of gold catalysis makes this metal
an attractive candidate for the design of bimetallic cata-
lysts,[2,10] and this has been realized both in alkynyl coupling,
like in the gold cocatalyzed version of the Sonogashira reac-
tion,[11] or other Pd–Au catalyzed reactions.[12–17] In this context,
it is important to gather information about the transmetalation
processes between gold and palladium and the possible rear-
rangement events that can take place.
The reaction of [Pd(h3-allyl)Cl(PPh3)] (1) with a twofold molar
amount of [Au(CꢀCÀn-Bu)]n (2) leads to the heterometallic Pd–
The exchange of organic groups between metals is at the core
of many organometallic synthetic procedures and is a key step
in catalytic CÀC coupling reactions. Recently, the use of two-
transition-metal complexes in catalytic reactions, so-called bi-
metallic catalysis, is opening new avenues for the synthesis of
molecules.[1–4] These processes rely on the efficient exchange
of organic fragments between metal co-catalysts along with
a fast and efficient final coupling step that drives the reaction
to completion. The coupling of alkynyl groups can be per-
formed by utilizing the cooperative role of two metals, gener-
ally a Group 11 metal and palladium; an example of this is the
Sonogashira reaction.[5–7] The transmetalation of an alkynyl
group between palladium and copper, or palladium and silver
is a reversible reaction, which has been shown experimental-
ly.[8,9] After the transfer of the alkynyl group to the palladium
atom, the Group 11 metal may remain coordinated to the Pd–
alkynyl groups, forming stable bimetallic intermediates.[9] Both
features are important because they influence the formation of
the [Pd(alkynyl)RL2] intermediate that eventually give, by re-
ductive elimination, the coupling alkynyl–R derivative. We have
previously studied the transmetalation reaction between
copper or silver alkynyls and palladium allylic derivatives. The
reluctance of the allylic fragment to undergo reductive elimi-
nation in a [Pd(alkynyl)(allyl)L2] compound hampers the other-
wise preferred coupling process and allows the detection of bi-
metallic intermediate complexes.[9] We report herein the reac-
tion of gold alkynyls with palladium allylic derivatives; this
Au complex
3 and [AuClPPh3] as the only byproduct
(Scheme 1). Complex 3 is a zwitterionic species that shows cat-
ionic palladium allyl units coordinated to anionic bridging
Scheme 1. Reactions leading to the formation of complex 3.
[Au(CꢀCÀn-Bu)2]À fragments. The formation of 3 results from
the reorganization of both the neutral ligands and the alkynyl
fragments between metals. When the reaction is carried out
with an equimolar amount of 1 and 2, it proceeds in the same
fashion, but only half of the starting amount of 1 is trans-
formed. A different combination of reagents with the same
total number of atoms can also be used, that is, a mixture of
the dimeric allylic palladium complex 4 and the gold deriva-
tives 2 and 5 (Scheme 1).
The synthetic routes depicted in Scheme 1 are not conven-
ient for the isolation of 3 because the separation of the gold
byproduct [AuCl(PPh3)] from 3 is difficult. Besides, 3 decompos-
es in solution, at room temperature, with a half-life of about
1 h. For this reason, a different reaction route, which involves
the use of [Au(CꢀCR)2]À as reagent (either generated in situ for
[a] A. Toledo, Dr. I. Meana, Prof. Dr. A. C. AlbØniz
IU CINQUIMA/Química Inorgµnica
Universidad de Valladolid
47071-Valladolid (Spain)
E-mail: albeniz@qi.uva.es
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501813.
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Scheme 2. Alternative synthesis of complexes 3 and 6.
R=n-Bu or previously prepared for R=C6H4OMe-p) and its co-
ordination to a preformed cationic palladium allyl, was devised
(Scheme 2). In this way 3 (R=n-Bu) was isolated and 6 (R=
C6H4OMe-p) was detected. They were characterized by NMR
spectroscopy; both the 1H and 13C NMR spectra are quite
simple, reflecting the high symmetry of the complexes. A char-
acteristic and revealing ABX2 system is observed for the diaste-
reotopic methylene hydrogens of the AuÀCꢀCCH2-CH2-Et
groups of complex 3 (see the Supporting Information).
The molecular structure of 3, determined by X-ray crystal dif-
fraction at 120 K, shows a centrosymmetric derivative with two
allyl palladium fragments, in a mutual trans arrangement,
bridged by two [Au(CꢀCÀn-Bu)2]À complexes (Figure 1).[18] The
AuÀC(alkynyl) bond lengths are in accordance with the litera-
ture data for other gold alkynyls.[19,20] The CꢀC bond also con-
forms to the values reported in the literature and it is not very
sensitive to coordination to another metal.[19] The bond distan-
ces and angles of the allylic fragments, as well as the PdÀ
C(allyl) distances, are within the range found for other palladi-
um allyl complexes with ligands of moderate trans influ-
ence.[21,22] The value of the Au–Au distance (3.255(10) Š) falls in
the range found for aurophilic contacts.[23] The Au–Pd distance
(3.231(10) Š) is shorter than the sum of the van der Waals radii
(3.29 Š) and could indicate weak metalophilic interactions. The
solid-state structure is kept in solution. In addition to the
Figure 1. Molecular structure of 3. Selected interatomic distances [Š] and
angles [8]: Pd1ÀC1, 2.141(14); Pd1ÀC2, 2.115(13); Pd1ÀC3, 2.114(11); Pd1ÀC4,
2.270(13); Pd1ÀC5, 2.390(12); Au1ÀAu2, 3.255(10); Au1ÀC4, 2.005(12); Au1À
C10, 1.980(12); C4ÀC5, 1.202(16); Au1-C4-C5, 177.7(11); C4-Au1-C10,
177.2(5).
tion of the stable [AuClPPh3] complex. A soluble gold alkynyl is
also required, because, the insoluble [Au(CꢀCR)]n (R=aryl) de-
rivatives react too slowly to reach a sufficiently high concentra-
tion of the unstable bimetallic complex. We carried out several
experiments to find out how this rearrangement occurs.
First of all, we analyzed the reactions of [Au(CꢀCÀn-Bu)]n in
the presence of the ligands provided by palladium in
Scheme 1, that is, PPh3 and chloride. The results are collected
in Scheme 3. The alkynyl gold complex 2 reacts with PPh3 to
give 5 and this occurs efficiently regardless of the amount of
chloride present in the solution. The anionic complex 7 is the
product of the reaction of 2 with chloride with an equilibrium
constant, K(298K) =[7]/([2][ClÀ])=32.6Æ1.2, that is much lower
than the equilibrium constant for the coordination of PPh3,
which should be Kꢁ104, because both 2 and PPh3 react com-
1
1H NMR pattern mentioned above, H DOSY experiments give
a hydrodynamic radius (RH =5.11 Š at 293 K) that is consistent
with the radius determined from the X-ray molecular structure
(RX-ray =4.86 Š).
The structure of complexes 3 and 6 is unprecedented. Heter-
ometallic platinum–gold alkynyl complexes are known, but
most examples show h2 coordination of the triple bond of
a platinum alkynyl fragment to a gold atom. In some cases,
these complexes result from the reaction of a gold alkynyl
complex with a platinum precursor, so that Au-to-Pt alkynyl
transmetalation precedes the formation of the complex.[24] Very
few examples of [Au(CꢀCR)2]À fragments that act as ligands
have been reported. Most of them are heteronuclear alkynyl
complexes that combine several Group 11 metals.[25,26] As for
palladium allyl fragments bound to two metal alkynyls, only
a few examples of platinum alkynyls that act as ligands have
been reported.[22]
The formation of 3 in Scheme 1 involves an effective rear-
rangement of ligands and organometallic fragments. Complex
3 seems to act as a thermodynamic sink aided by the forma-
Scheme 3. Reactions of 2 with PPh3 and chloride.
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pletely, as shown by NMR. When both ligands are present in
substoichiometric amounts, both 5 and 7 are formed along
with small amounts of the dialkyne and [AuCl(PPh3)] (5% and
10% mol, respectively). No experiment showed the formation
of [Au(CꢀCR)2]À species.
According to these results, palladium must play a direct role
in the rearrangement, more important than a mere ligand
source, and we believe that the formal gold-to-gold alkynyl
transfer occurs through palladium. This is supported by several
independent experiments that show that gold-to-palladium al-
kynyl transmetalation is reversible and can lead to complex 3.
First, we synthesized the new allyl alkynyl palladium complex 8
by the reaction of the allylic dimeric complex 4 and a slight
excess of Bu3Sn(CꢀCÀn-Bu) at 243 K. Consistent with its dimer-
ic nature, 8 is a mixture of cis and trans isomers, in a 1.2:1
ratio, as a result of the two different arrangements of both al-
lylic moieties. The addition of an excess of [Au(CꢀCÀn-Bu)]n to
a solution of 8 at 243 K, produces the clean formation of the
bimetallic complex 3 (Scheme 4). Thus, the formation of the
Scheme 5. Plausible route for the formation of 3.
transmetalation of the alkynyl group. In any case, available co-
ordination sites are needed either on palladium or in the gold
center, so the later can trap PPh3. The presence of PPh3 on
both metal centers is detrimental for the alkynyl transfer as
shown by the lack of reaction between complexes 1 and 5 in
Equation (2). On the other hand, the chloro-bridged palladium
complex 4 does react with 5 to give 3 albeit in low yield be-
cause the presence of PPh3 inhibits further reaction [Eq. (3)].
No shift of the allyl ligand from a h3 to a s coordination seems
to occur. The transmetalation transition states depicted in
Scheme 5 are analogous to those proposed by Espinet et al.[27b]
and Hashmi et al.[28] for the Au–Pd transmetalation of aryl and
vinyl groups.
Scheme 4. Transmetalation from Pd (8) to Au to form complex 3.
bisalkynyl gold species [Au(CꢀCR)2]À occurs by transmetalation
of the alkynyl moiety from palladium to gold. The reverse
transmetalation, from gold to palladium, occurs by the addi-
tion of an excess of PPh3 on complex 3, which leads to the for-
mation of the monoalkynyl allyl palladium complex 9,[9] and
gold derivatives 5 and [AuClPPh3] [Eq. (1)].
The decomposition of complex 3 leads to a mixture of the
allyl–alkynyl coupling product 10 and the dialkyne in a 2:1
molar ratio [Eq. (4)]. The formation of 10 is a result of reductive
elimination on a [Pd(alkynyl)(allyl)L2] intermediate, so the pres-
ence of this compound shows again that the transmetalation
from the bisalkynyl gold moiety to palladium takes place.
Thus, the transmetalation of the alkynyl group from gold to
palladium is a reversible reaction that occurs in either direc-
tion, depending on the presence of other ligands that influ-
ence the thermodynamics of the process. This has been ob-
served before for the transmetalation of methyl and perfluoro-
phenyl groups.[27] In this case the solubility of [Au(CꢀCÀn-Bu)]n
and the formation of the stable [AuClPPh3] complex, which
takes up all the ligands coordinated to palladium, drives the
equilibrium to the formation and coordination of [Au(CꢀCR)2]À.
The above mentioned experiments support the route repre-
sented in Scheme 5 for the formation of 3 by reaction of
1 and 2 in Scheme 1.
In conclusion, a formal gold to gold transfer of an alkynyl
fragment occurs by sequential gold-to-palladium and palladi-
um-to-gold transmetalations. Thus, palladium acts as a media-
tor in the rearrangement of the alkynyl gold complex. Gold
shows a distinct behavior compared with the other Group 11
members. In the latter cases, alkynyl transmetalation to palladi-
um takes place when [M(alkynyl)]n (M=Cu, Ag) are reacted
with 1, and M remains h2-coordinated to the Pd–alkynyl frag-
ment; this leads to the heterometallic complexes [{Pd(h3-allyl)-
(alkynyl)L}CuCl]2 and [{Pd(h3-allyl)(alkynyl)L}2AgCl] where s-al-
The substitution of the PPh3 by a gold alkynyl must be the
first step of the reaction. This may be an actual decoordination,
as depicted in Scheme 5, or a concerted process that leads to
PPh3 coordination to gold, and takes place along with the
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kynyl–Pd and h2-alkynyl–M coordination modes are found. In
the analogous reaction, gold forms no complex of that type,
but acts as a ligand scavenger, forming [AuCl(PPh3)], and un-
dergo more facile transmetalation processes that show a prefer-
ence for the s-alkynyl–Au coordination mode, present in com-
plexes 3 and 6, leaving palladium coordinated in a h2-alkynyl–
Pd fashion.
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Experimental Section
Synthesis of 3
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Method A:
A
5 mm NMR tube was charged with [Pd(h3-
C3H5)Cl(PPh3)] (0.024 g, 0.054 mmol) and [Au(CꢀCÀn-Bu)]n (0.030 g,
0.108 mmol). Then, CDCl3 (0.6 mL) was added under nitrogen at
room temperature. A yellow solution was obtained and it slowly
turned dark brown. After 15 min, the solution was cooled at 243 K
and then characterized by NMR spectroscopy. Equimolar amounts
of 3 and [AuCl(PPh3)][29] were formed in solution.
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Method B: A solution of n-butyl lithium (0.49 mL, 1.6m in hexane,
0.79 mmol) in dry THF (5 mL) was added dropwise under nitrogen
to a stirred solution of 1-hexyne (0.091 mL, 0.79 mmol) in dry THF
(5 mL) at À408C. After 90 min at this temperature, [Au(CꢀCÀn-Bu)]n
(0.22 g, 0.79 mmol) was added under nitrogen and the solution
stirred at À408C for 3 h. In another Schlenk tube, [Pd(m-Cl)(h3-
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(153.8 mg, 0.79 mmol) in dry THF (5 mL). The mixture was stirred
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À208C. The brownish residue was extracted with CH2Cl2 (3”5 mL),
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The light-brown residue was triturated with dry cold hexane (5 mL)
in a bath at À808C. The resulting solid was filtered, and dried in
vacuo (227 mg; 56.7% yield). This complex was stored at À308C.
1H NMR (500 MHz, CDCl3 243 K): d=5.62 (m, 2H, H2 allyl), 4.12 (d,
J=7.1 Hz, 4H; H1syn, H3 allyl), 3.41 (d, J=12.5 Hz, 4H; H1anti, H3
syn
anti
[18] CCDC 1063517 contains the supplementary crystallographic data for
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Crystallographic Data Centre.
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2.051 Š; CꢀC: 1.167–1.275 Š). The Cambridge Structural Database,
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allyl), 2.26, 2.15 (ABX2 system, JAB =16.4, JAX =JBX =7.7 Hz, 8H, CꢀC-
CH2), 1.48 (m, 8H; CH2-CH2-CH2), 1.33 (m, 8H; CH2-CH3), 0.83 ppm
(t, J=7.3 Hz, 12H; CH3); 13C{1H} NMR (100.61 MHz, CDCl3): d=
112.01 (s, C2 allyl), 100.8 (s, Au-CꢀC), 90.8 (s, Au-CꢀC), 67.1 (s, C1, C3
allyl), 31.85 (s, CH2CH2CH3), 22.15 (s, CH2CH3), 21.84 (s, CꢀCCH2),
13.97 ppm (s, CH3); MS (ESI-TOF, CH2Cl2): m/z: 1037.09 [M+Na]+
(Na+ comes from HCOONa used for calibration). The hydrodynamic
radius determined by 1H DOSY is RH =5.11 Š (293 K).
Acknowledgements
Financial support from the Spanish MINECO (DGI, grant
CTQ2013-48406-P; fellowships to IM (FPU) and AT (FD)) and
the Junta de Castilla y León (grant VA302U13) is gratefully ac-
knowledged.
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Keywords: bimetallic · gold · ligand exchange · palladium ·
transmetalation
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Received: May 8, 2015
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