Gold(III) Alkyne Complexes: Bonding and Reaction Pathways

Article abstract of DOI:10.1002/anie.201708640

The synthesis and characterization of hitherto hypothetical Au^III π-alkyne complexes is reported. Bonding and stability depend strongly on the trans effect and steric factors. Bonding characteristics shed light on the reasons for the very different stabilities between the classical alkyne complexes of Pt^II and their drastically more reactive Au^III congeners. Lack of back-bonding facilitates alkyne slippage, which is energetically less costly for gold than for platinum and explains the propensity of gold to facilitate C?C bond formation. Cycloaddition followed by aryl migration and reductive deprotonation is presented as a new reaction sequence in gold chemistry.

Full text of DOI:10.1002/anie.201708640

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Accepted Article
Title: Gold(III) alkyne complexes: bonding and reaction pathways
Authors: Manfred Bochmann, Luca Rocchigiani, Julio Fernandez-
Cestau, Gabriele Agonigi, Isabelle Chambrier, and Peter
Budzelaar
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10.1002/anie.201708640
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Gold(III) alkyne complexes: bonding and reaction pathways
Luca Rocchigiani,*[a] Julio Fernandez-Cestau,[a] Gabriele Agonigi,[a] Isabelle Chambrier,[a] Peter H. M.
Budzelaar,*[b] and Manfred Bochmann*[a]
Abstract: The synthesis and characterization of hitherto hypothetical
Au(III) π-alkyne complexes is reported. Bonding and stability depend
strongly on the trans effect and steric factors. The bonding
characteristics shed light on the reasons for the very different
stabilities between the classical alkyne complexes of Pt(II) and their
drastically more reactive Au(III) congeners. Lack of back-bonding
facilitates alkyne slippage which is energetically less costly for gold
than for platinum and explains the propensity of gold to facilitate C-C
bond formation. Cycloaddition, followed by aryl migration and
reductive deprotonation, are new reaction sequences in gold
chemistry.
sterically less hindered alkynes [2+2] cyclodimerization occurs.
In the case of 3-hexyne this leads to the gold-bound
cyclobutenyl cation 3, whose structure and intermolecular
contacts were elucidated by 2D NMR methods (see Supporting
Information). By contrast, the interaction of 1-phenyl-1-propyne
with 2 leads to a complex NMR spectrum consistent with the
formation of the cyclopentenyl complex 4, apparently produced
by a combination of cyclodimerization and 1,3-H shifts.[10,11] The
observed 13C NMR data are in excellent agreement with the
calculated NMR chemical shifts for complexes 3 and 4 (see
Supporting Information). These cyclizations are an expression of
the vinyl cation character of Au(III) alkynes and reflect the
tendency for C-C bond formation that typifies gold catalyzed
alkyne reactions.
The reactions of gold complexes with alkynes form the basis
of a multitude of organic catalytic transformation involving
starting materials with CC triple bonds.[1] Invariably these
reactions are initiated by coordination of the alkyne to the metal,
which induces polarization and susceptibility towards
nucleophilic attack. Remarkably, unlike alkyne adducts of gold(I),
which have been known since the 1970s,[2-4] alkyne π-
complexes of gold(III) have remained unknown.[4] This is in
sharp contrast to alkyne compounds of isoelectronic platinum(II),
which were first reported over 50 years ago and provide
textbook examples of the Dewar-Chatt-Duncanson π-bonding
model.[5-6] Elucidating the reaction chemistry of gold(III) has
provided major challenges and has highlighted that drawing
mechanistic analogies between gold(III) with the chemistry of
isolectronic Pd and Pt systems is often not valid.[7-9] We show
here that the hitherto elusive Au(III) alkyne complexes can in
fact readily be generated and have Au-alkyne bond energies
comparable to those of platinum, but are drastically more
reactive. The results have enabled the first experiment-based
comparison of alkyne bonding in Au(III) and Pt(II) systems and
shed light on their reactivity differences, which helps explain the
superior performance of gold in alkyne catalysis.
The reaction of (C^N^C)AuOAcF (OAcF = trifluoroacetate) 1-
OAcF with B(C6F5)3 in dry dichloromethane generates the ion
pair [(C^N^C)Au···Y] [Y = AcFOB(C6F5)3][9a] 2 (Scheme 1).
Internal alkynes were added to these solutions at -78 °C and the
mixture was monitored by H NMR spectroscopy at -20 °C. The
outcome of the reactions proved to be dramatically affected by
nature of the substituents at the triple bond (Scheme 1). With
Scheme 1. Formation and reactivity of Au(III) alkyne complexes, including 13C
NMR chemical shifts of the product complexes (CD2Cl2, -20 °C). The structure
of 2 indicates the numbering system used for NMR assignments. Crystal
structure of 6[SbF6]·CH2Cl2, selected bond distances [Å] and angles [°]: N1-Au
2.13(1), C7-Au 2.06(2), Au-O1 2.09(2), Au-C27 2.07(2), O1-C26 1.23(2), N1-
Au-C7 81.8(7), C7-Au-C27 100.6(9), C7-Au-O1 166.7(7), N1-Au-C27 174.9(8),
C27-Au-O1 66.2(8), O1-Au-N1 111.2(6), O1-C26-C27 110(2). The angle
through best planes of pyridine and the C6H4tBu ring is 46°.
1
[a]
[b]
Dr. L. Rocchigiani, Dr. J. Fernandez-Cestau, G. Agonigi, Dr. I.
Chambrier, Prof. Dr. M. Bochmann
School of Chemistry, University of East Anglia,
Norwich Research Park, Norwich NR4 7TJ, UK
E-mail: m.bochmann@uea.ac.uk, L.Rocchigiani@uea.ac.uk
Prof. Dr. P. H. M. Budzelaar
Department of Chemistry,
University of Naples Federico II, Via Cintia,
80126 Naples, Italy.
Cycloaddition is suppressed by steric hindrance. For
example, mixing 2 with ca. 2.5 molar equiv of BuCCMe or
t
AdCCAd showed that only 1 equiv of alkyne reacted to give the
π-complexes 5a and 5b, respectively, without cyclodimeriza-
tion.[11,12] Complexation was confirmed by the 13C NMR and the
selective dipolar interactions of the Me and tBu substituents with
the pincer ligand in the 1H NOESY NMR spectrum. Alkyne
coordination induces only modest 13C NMR shifts: 5a: C(Me) δC
= 85.7 (ΔδC = 11.9 ppm); C(tBu) δC = 97.1 (ΔδC = 9.4); 5b: δC =
98.3 (ΔδC = 10.6), very similar to the values observed for
[(R3P)Au(alkyne)]BF4.[13] Solutions of 5a show no exchange
between free and coordinated alkyne and are stable for hours
at -30 C but decompose at 25 C within minutes. By contrast,
e-mail: p.budzelaar@unina.it
Supporting information for this article is given via a link at the
end of the document. CCDC 1559726 (6) and 1569528 (11)
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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10.1002/anie.201708640
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the even more bulky bis(1-adamantyl)acetylene complex 5b is
stable for days in solution at room temperature and can be
isolated as a yellow microcrystalline solid.[11]
very different AuIII-C distances (2.29(2) and 2.52(2) Å) and which
lacks the CC bond elongation (Scheme 3).
The bonding of alkynes to Au(III) was explored by Density
Functional Theory (DFT) calculations (Gaussian 09, TPSSh/cc-
pVTZ// B3LYP/SVP), and compared to bonding in structurally
related Pt(II) fragments.[16] Surprisingly, the dissociation energies
of Au(III) and Pt(II) with pyridine ligands in trans position were
remarkably similar, suggesting that the observed differences in
reactivity are not due to thermodynamic effects. Alkyne bonding
to [(C^N—CH)Au(C6F5)]+ was weaker by 30-40 kcal/mol
compared to [(C^N^C)Au]+, for both electronic and steric
reasons: the empty coordination site in [(C^N—CH)Au(C6F5)]+ is
trans to a strong Au-C bond, and it is shielded by the dangling
aryl arm. General bonding trends are very similar between the
two types of complexes, with the more π-donating alkynes
binding most strongly.
Gold(III) is well known for its ability to catalyze alkyne
hydration.[2a] This process is modelled by 5, which reacts with
water at -20 °C to give the ketonyl complex 6. Evidently the
proton released after the nucleophilic attack of H2O at the alkyne
cleaves the pincer. The same complex was obtained directly
t
from 1-Cl, AgSbF6, BuCCMe and H2O (D2O), in >90% yield.
The structure was confirmed by X-ray diffraction (Scheme 1).
A second type of alkyne complexes could be generated after
Au-C bond cleavage of 1-C6F5 with [H(OEt2)2][H2N{B(C6F5)3}2] in
C6D5Cl, followed by removal of associated Et2O.[14] Ether-free 7
reacts with 3-hexyne at -78 °C in CD2Cl2 to generate 8a
(Scheme 2). Alkyne coordination was indicated by a significant
shift in the ligand signals. In this type of compounds the π-ligand
is trans to an anionic C-atom, which, unlike the pyridine donors
in 3 - 5, exerts a strong trans influence, resulting in substantial
weakening of the Au-alkyne bond.
The Au(III)-C(alkyne) bond distances calculated for the
model [(C^N^C)Au(2-butyne)]+ correspond remarkably closely to
those found for the π-bonded alkynyl ligand in 11 (Figure 1)
Scheme 2. Generation of C^N chelated alkyne complexes.
At -50 C, 3-hexyne coordination to 8a is strong enough to
allow separate observation of the ethyl signals of bound and free
alkyne. The 13C NMR signal of the coordinated triple bond was
located at δC = 91.7 (ΔδC = 10.7 ppm). 1H NOESY NMR
experiments revealed the presence of chemical exchange,
indicating that the complex is still fluxional at -50 °C. Raising the
temperature broadens the resonances, and coalescence
between free and bound 3-hexyne occurs at about -30 °C. The
simulation of the VT 1H NMR spectra of 8a allowed the activation
parameters of alkyne exchange to be estimated, ΔHǂ = 19.9
kcal/mol and a positive ΔSǂ = 34 cal mol–1 K–1, suggesting that
the exchange has a dissociative character. For the tBuCCMe
complex 8b, well-resolved spectra were obtained at -60 °C, allowing
complete NMR characterization and a direct comparison with
complex 5, where the alkyne is trans to a pyridine donor. The alkyne
13C signals of 8b are very similar to those of 5; there is no detectable
trans influence on the 13C NMR chemical shifts.
Scheme 3. Formation of the p-bonded Au(III) alkynyl complex 11. Hydrogen
bonds are omitted for clarity; ellipsoids are drawn at 50%. Selected bond
distances [Å] and angles []: Au1-C59 2.33(2), Au1-C60 2.40(2),Au1-C21
1.98(3), Au2-C53 2.29(2), Au2-C54 2.52(2), Au2-C47 2.07(2), C53-C54
1.17(3), C59-C60 1.23(3); C53-Au3-C59 175.6(9), Au1-C60-C59 72(1), Au1-
C59-C60 78(1), Au2-C53-C54 87(2), Au2-C54-C53 65(2).
Table 1. Calculated alkyne dissociation free energies from gold(III) and
platinum(II) fragments (kcal/mol).[a][11]
Ligand
[(C^N^C)Au]+
46.5
[(C^N^N)Pt]+
55.7
[(C^N^CH)Au(C6F5)]+
CO
8.5
4.3
HC≡CH
MeC≡CMe
EtC≡CEt
MeC≡CtBu
MeC≡CPh
38.9
41.7
48.7
47.6
12.3
15.0
13.4
13.8
53.0
50.8
While these alkyne complexes have so far resisted all
attempts at crystallization, structural confirmation of alkyne
bonding was unexpectedly obtained from the reaction of 9[15]
with AgCCtBu, which gives two thermally stable crystalline
products, 10 and 11. The latter, a product of partial reduction,
contains a π-bonded Au(CCtBu)2 anion which, probably due to
packing effects, displays two types of Au(III)-alkynyl interactions:
one symmetrical Au(III)-alkynyl bond with an elongated CC
distance of 1.23(3) Å, and an asymmetric alkynyl bridge with two
53.1
51.0
50.6
49.7
[a] C^N^N = 2,2’-bipyridyl-6-C6H4.
In order to assess the trans-influence on back-donation,
charge decomposition analysis was carried out.[17] Back-
donation for all gold(III) compounds is very low and is further
reduced by strong trans ligands; e.g. 5 shows a back-
donation/donation (B/D) ratio of 0.43, which drops to B/D = 0.17
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for 8a, much less than in structurally analogous [(C^N^N)PtII(3-
hexyne)]+ (B/D = 0.77). The stability of alkyne complexes
depends almost entirely on π-donation from the triple bond. We
ascribe the relative stability of 8 and the reduced tendency of
this complex to undergo [2+2] cycloaddition to the steric
shielding provided by the “Pacman”-type C^N-aryl ligand
structure, which provides a kinetically protected binding pocket
and thus enables detection of these thermodynamically labile
species.
contributes significantly to the higher reactivity of gold(III)
acetylene complexes.
Warming a mixture of 8a and 3-hexyne to room temperature
gives the thermally stable product 12 (Scheme 4). Its formation
involves [2+2] cycloaddition, followed by C6F5 migration to the
resulting cyclobutenyl cation. The C=C bond is significantly
polarized due to a weak interaction with the gold centre (δC
=
138.0 and 185.1; cf. calculated values (Me model): δC =131.4
and 189.3). This reaction sequence – alkyne cycloaddition
followed by migratory insertion into an Au-C(aryl) bond – is to
the best of our knowledge unprecedented in gold chemistry.
Alkenes are typically released from metal alkyls by β-H
elimination, as recently shown for gold(III) n-alkyls.[8c] This
process requires a vacant site in cis position and is suppressed
by donor ligands. By contrast, alkene release from 12 is actually
induced by donors (SMe2 or xylylNC). Monitoring 12/SMe2
mixtures at -30 °C, where the reaction is quite slow, showed no
evidence for the formation of a transient Au(III)–H species,[9b-d]
nor was there evidence for a C(aryl)-H coupling product resulting
from such an intermediate. On the other hand, a broad signal at
δH = 13.4 appeared, typical of a protonated pyridine. These
findings suggest that 12 may follow an alternative path,
reductive deprotonation, involving dissociation into a gold(I) aryl
and a cyclobutenyl cation (Scheme 4), likely assisted by
stabilizing SMe2 coordination to Au(I), followed by deprotonation
of the cyclobutenyl cation by pyridine to give 13 as an E/Z
mixture.
Figure 1. Comparison of Au-C and C-C distances of the calculated model
[(C^N^C)Au(2-butyne)]+ (left) and the π-alkynyl fragment of the crystal
structure of 11.
The lability of Au(III) alkyne complexes and their high
reactivity contrasts sharply with the thermal and chemical
stability of their Pt(II) analogues. Surprisingly, the alkyne binding
energies to Au(III) are comparable to or slightly higher than
those to Pt(II): -50.3 kcal/mol for [(C^N^C)Au(3-hexyne)]+,
versus -42.6 kcal/mol for [(C^N^N)Pt(3-hexyne)]+. However, the
reduced back-bonding permits more facile alkyne slippage, with
consequent accumulation of positive charge on the β-carbon
atom and an increase in its susceptibility to nucleophilic attack.
Scheme 4. Reactivity of alkyne complexes: C-C coupling and alkene release.
Figure 2. Fraction of binding energy lost vs 2-butyne slippage (Å) for AuCl3 (■)
and PtCl2(H2O) (♦).
Alkyne slippage was probed using 2-butyne adducts of AuCl3
and of PtCl2(H2O) as models, by varying the angle α in the range
35° - 120° with steps of 5°, resulting in slippage values of ~ -1 to
+2 Å (Figure 2).[18] In the (non-slipped) π-complexes this angle is
close to 75° and 73° for Cl3Au(butyne) and trans-
Cl2(H2O)Pt(butyne), respectively. Back-bonding can be expected
to increase the barrier to geometric deformation, and indeed, at
a slippage of +1 Å, the Au complex has lost only 13% of its
acetylene binding energy, while the Pt complex has lost 23%
(Figure 2). We believe this easier deformation of Au complexes
Scheme 5. Free energy profile (298 K, chlorobenzene) for cyclodimerization
and formation of 12.
The free energy pathway for alkyne dimerization and
cyclobutenyl-aryl coupling is shown in Scheme 5. Starting with
the mono-alkyne π-complex B1, the Au atom moves to one of
the two sp-C atoms, creating a gold-bound vinyl cation, which
attacks
a
π-bond of
a
second alkyne. The resulting
cyclopropenylmethylene cation B2 is a shallow local minimum.
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[2]
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Opening of one of the two ring single bonds leads to the aurated
cyclobutenyl cation B3. The rate-limiting step is the initial alkyne-
alkyne coupling. The transition state for coupling of the C6F5 and
cyclobutenyl fragments (B3 to B4/B5) looks somewhat like
insertion of cyclobutadiene in the Au-C6F5 bond. The calculated
barrier of ~20 kcal/mol indicates that this reaction should be
rather slow at room temperature, as was indeed observed. The
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With additional stabilisation by SMe2, intermediate B4 may also
provide a pathway for the release of the organic product by
reductive deprotonation (for the possibility of alkene release by
β-H elimination see Supporting Information).
In conclusion, several distinct types of well-defined gold(III)
alkyne complexes are now accessible and have been
characterized by spectroscopic, structural and computational
methods. Their synthesis has enabled an outline of their reaction
pathways. Alkyne bonding is subject to a strong trans influence:
ligands trans to pyridine-N are bound significantly more strongly
than trans to an anionic C-donor. Although Au(III) and Pt(II)
alkyne adducts show comparable binding energies, the much
lower back-bonding in Au(III) complexes reduces the energy
cost of alkyne slippage and facilitates CC bond polarisation.
This greatly enhances the susceptibility of gold-bound alkynes
towards nucleophilic attack and C-C bond formation, in line with
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complexes display reaction pathways which are unprecedented
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Acknowledgements
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This work was supported by the European Research Council. M.
B. is an ERC Advanced Investigator Award holder (grant no.
338944-GOCAT). We are grateful to the EPSRC National
Crystallographic Service for data acquisition[19] of 6[SbF6]·CH2Cl2.
[11] For full experimental details see the Supporting Information.
[12] Dimethyl acetylenedicarboxylate was unreactive; PhCCSiMe3 acts as
a transmetallation agent to give (C^N^C)Au-CCPh.
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Keywords: Gold • alkyne • bonding • reaction mechanisms •
density functional calculations
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10.1002/anie.201708640
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The synthesis of hitherto hypothetical
gold(III) π-alkyne complexes
highlights the differences between
classical platinum alkyne complexes
and their drastically more reactive
Au(III) congeners.
Luca Rocchigiani, Julio Fernandez-
Cestau, Gabriele Agonigi, Isabelle
Chambrier, Peter H. M. Budzelaar,
and Manfred Bochmann
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Gold(III) alkyne complexes: bonding
and reaction pathways
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