Heterolytic bond activation at gold: Evidence for gold(iii) H-B, H-Si complexes, H-H and H-C cleavage

Article abstract of DOI:10.1039/c8sc05229h

The coordinatively unsaturated gold(iii) chelate complex [(C^N-CH)Au(C₆F₅)]⁺ (1⁺) reacts with main group hydrides H-BPin and H-SiEt₃ in dichloromethane solution at -70 °C to form the corresponding σ-complexes, which were spectroscopically characterized (C^N-CH = 2-(C₆H₃Bu^t)-6-(C₆H₄Bu^t)pyridine anion; Pin = OCMe₂CMe₂O). In the presence of an external base such as diethyl ether, heterolytic cleavage of the silane H-Si bond leads to the gold hydrides [{(C^N-CH)AuC₆F₅}₂(μ-H)]⁺ (2⁺) and (C^N-CH)AuH(C₆F₅) (5), together with spectroscopically detected [Et₃Si-OEt₂]⁺. The activation of dihydrogen also involves heterolytic H-H bond cleavage but requires a higher temperature (-20 °C). H₂ activation proceeds in two mechanistically distinct steps: the first leading to 2 plus [H(OEt₂)₂]⁺, the second to protonation of one of the C^N pyridine ligands and reductive elimination of C₆F₅H. By comparison, formation of gold hydrides by cleavage of suitably activated C-H bonds is very much more facile; e.g. the reaction of 1·OEt₂ with Hantzsch ester is essentially instantaneous and quantitative at -30 °C. This is the first experimental observation of species involved in the initial steps of gold catalyzed hydroboration, hydrosilylation and hydrogenation and the first demonstration of the ability of organic C-H bonds to act as hydride donors towards gold.

Full text of DOI:10.1039/c8sc05229h

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Heterolytic Bond Activation at Gold: Evidence for Gold(III) H-B, H-Si
DOI: 10.1039/C8SC05229H
Complexes, H-H and H-C Cleavage†
Luca Rocchigiani,a* Peter H. M. Budzelaar,b* Manfred Bochmanna*
a School of Chemistry, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, United
Kingdom. E-mail: l.rocchigiani@uea.ac.uk; m.bochmann@uea.ac.uk
b Department of Chemistry, University of Naples Federico II, Via Cintia, 80126 Naples, Italy.
E-mail: p.budzelaar@unina.it
ORCID IDs:
L Rocchigiani https://orcid.org/0000-0002-2679-8407
P. H. M. Budzelaar https://orcid.org/0000-0003-0039-4479
M. Bochmann https://orcid.org/0000-0001-7736-5428
Abstract.
The coordinatively unsaturated gold(III) chelate complex [(C^N-CH)Au(C6F5)]+ (1+) reacts with main group
hydrides H-BPin and H-SiEt3 in dichloromethane solution at -70 °C to form the corresponding σ-complexes,
which were spectroscopically characterized (C^N-CH = 2-(C6H3But)-6-(C6H4But)pyridine anion; Pin =
OCMe2CMe2O). In the presence of an external base such as diethyl ether, heterolytic cleavage of the silane
H-Si bond leads to the gold hydrides [{(C^N-CH)AuC6F5}2(μ-H)]+ (2+) and (C^N-CH)AuH(C6F5) (5),
together with spectroscopically detected [Et3Si-OEt2]+. The activation of dihydrogen also involves
heterolytic H-H bond cleavage but requires a higher temperature (-20 °C). H2 activation proceeds in two
mechanistically distinct steps: the first leading to 2 plus [H(OEt2)2]+, the second to protonation of one of the
C^N pyridine ligands and reductive elimination of C6F5H. By comparison, formation of gold hydrides by
cleavage of suitably activated C-H bonds is very much more facile; e.g. the reaction of 1·OEt2 with
Hantzsch ester is essentially instantaneous and quantitative at -30 °C. This is the first experimental
observation of species involved in the initial steps of gold catalyzed hydroboration, hydrosilylation and
hydrogenation and the first demonstration of the ability of organic C-H bonds to act as hydride donors
towards gold.
Introduction
Gold catalysts, both heterogeneous and homogeneous, have experienced an explosive growth in interest in
recent years.1-3 In the absence of direct evidence, their mode of action tends to be explained in analogy to the
well-known chemistry of other noble metals, notably palladium and platinum, and indeed gold and platinum
1

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systems are often compared side-by-side.4,5 There are significant differences, however, not least in the
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DOI: 10.1039/C8SC05229H
distinct reluctance of gold to undergo oxidative addition reactions,6,7 and an experiment-based outline of the
reactivity of gold, and in particular of the much less well explored chemistry of gold(III), is only now
beginning to emerge.8-10 Heterogeneous gold catalysts show high activity in a multitude of reactions, for
example in hydrogenations,11-16 including the hydrogenation of nitro compounds17-19 and hydrogen transfer,20
in acetylene hydrochlorination,21 in the water-gas shift reaction,22 in hydrosilylations and in catalytic
dehydrogenative Si-O coupling reactions.23-28 Similarly, homogeneously gold-catalyzed hydrogenations and
hydrosilylations29-31 as well as alkyne hydroborations32 have been reported.
Details of the reactions of H2, silanes and boranes with gold compounds, and particularly of their
modes of activation and reaction pathways, are however rather scarce. The common feature of all these
reactions is that they involve, or are postulated to involve, Au-H species. While in a number of
heterogeneous systems surface gold hydride species could indeed be detected spectroscopically,33-35 the
mechanisms of gold-catalysed reactions have been explored by computational modelling. Naturally, most
studies are based on mechanistic analogies to better known noble metal catalysts; for example, homolytic H-
H and H-Si bond scission and oxidative addition of dihydrogen or of silanes to surface gold atoms are
assumed, to give Au-H and Au-SiMe3 species which initiate the catalytic cycles.24-28,35-37 There are however
also cases where the possible involvement of polar solvents in the catalytic process has been recognized,
leading to models of heterolytic, solvent-facilitated H-H bond cleavage.38-41
We present here the first experimental evidence for detectable gold(III) adducts with borane H-B and
silane H-Si bonds, elucidate the important role of the solvent and of basic ligand sites in the formation of Au-
H species via heterolytic H-Si and H-H bond scission, and demonstrate the ability of gold(III) to generate
gold hydrides by H-C(sp3) bond cleavage.
Results and discussion.
As we showed recently,42 the reaction of the pincer complex (C^N^C)AuX with the strong Brønsted
acid [H(OEt2)2]+[H2N{B(C6F5)3}2]- (HAB2)43 creates the ether-stabilized C^N chelate complex [(C^N-
-
CH)AuX(OEt2)]+AB2 (1·OEt2) (C^N^C = 2,6-(C6H3But)2pyridine dianion). In the present work the
pentafluorophenyl derivative (X = C6F5) was chosen, firstly because this ligand provides additional stability,
and secondly because the fluorine atoms can act as additional reporter nuclei and aid spectroscopic
characterization. The ether ligand is labile and can be removed to afford ether-free 1 with a “dangling”
-C6H4But substituent, of a “Pacman”-type structure and which is capable of supporting ligand binding to the
coordination pocket, such that adducts of weak ligands with 1 are accessible which would not be feasible in
C^N complexes without this dangling aryl moiety. We have also shown44 that treatment of 1·OEt2 with
HSi(OMe)3 leads quantitatively to hydride transfer and the formation of the hydrido-bridged binuclear
gold(III) complex 2 (Scheme 1).
2

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–
-
AB2
DOI: 10.1039/C8SC05229H
AB2
PhCl/
CD2Cl2
+
N
N
H(OEt2)2
N
But
But
But
But
But
But
Au
Au
Au
–
OEt2
AB2
- OEt2
C6F5
C6F5
C6F5
HSi(OMe)3
–50 oC
.
1 OEt2
1
–
+
CH
H
AB2
CD2Cl2
N
C6F5
C
Au
Au
N
C
C6F5
2
Previous work
This work
HC
1
2'
2
5
-
AB2
6'
-
5'
adduct formation
HBPin
CD2Cl2
AB2
6
N
N
HSiEt3
CD2Cl2
But
But
1
But
But
Au
Au
C6F5
H
H
8
BPin
SiEt3
C6F5
heterolytic
cleavage
OEt2
H H
R
R
H-H
X
H-SiEt3
R
R
+ [H(OEt2)2]+
+
2
2
X
+ [Et3Si-OEt2]+
2
Scheme 1. Reaction outlines including the atom labelling scheme used in NMR assignments.
By contrast, we now find that gold hydride formation is preceded by formation of observable adducts
between Au(III) and H-B and H-Si bonds.
Borane σ-complexes. In contrast to the reaction of 1·OEt2 with HSi(OMe)3, no reaction took place when
this gold complex was mixed with 2 molar equivalents of HBPin at –70 °C, suggesting that the borane is
unable to displace the coordinated ether ligand. However, when the solution was warmed to –20 °C, the
starting material reacted slowly over a period of 30 minutes, affording the bridging hydride 2 quantitatively.
On the other hand, adding 1 equivalent of HBPin to the ether-free complex 1 in dichloromethane at –70
°C immediately afforded a mixture of the hydride 2 (15%), together with a new species 3 (85%). The relative
positions of the borane and the C^N ligand framework are most clearly indicated by nuclear Overhauser
effect (NOE) experiments, which show selective interactions between the “dangling” –C6H4But substituent
of the C^N ligand and the methyl groups of the boron-pinacolato moiety (Figure 1). These data suggest that
3 is a gold(III) -borane adduct, to our knowledge the first such observation. By contrast, as a reviewer
pointed out, M-HB interactions in copper(I) and silver(I) complexes of hydroborates and anionic borane
clusters are well known,45-49 while intramolecular Au(I)-HB interactions were observed in heterometallic Au-
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50
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DOI: 10.1039/C8SC05229H
Rh carborane clusters. Further characterization of 3 is hampered by its poor thermal stability, even at very
low temperatures. On warming the sample to -20 °C 3 is quickly and quantitatively converted into the
hydride 2. This suggests that heterolytic B–H splitting occurs readily without the involvement of solvent (in
contrast to silanes, see below), and it seems reasonable to assume that the B-H cleavage process is assisted
by the O-donor of a second molecule of HBPin. However, the 11B NMR spectrum is uninformative and no
clear indication about the nature of the boron side-product could be obtained. The formation of a gold-HB σ-
complex, as well as its lability, are in agreement with computational results (vide infra).
Figure 1. Reactivity of 1 and 1·OEt2 with HBPin.
Gold-silane σ-complexes. As stated above, the reaction of 1·OEt2 with HSi(OMe)3 provided a
convenient route to the μ-H gold hydride 2. However, a very different course of reaction was observed when
the ether-free cation 1 was treated with HSiEt3 at -70 °C in CD2Cl2. Under these conditions, there is no
HSiEt3 cleavage and no hydride transfer. Instead, the Au(III) σ-silane complex 4 is formed in essentially
quantitative yield (Figure 2). The identity of 4 was confirmed by multinuclear and multidimensional NMR
spectroscopy. The H–Si moiety shifted by 2.19 ppm with respect to free silane, to δH = 1.27 ppm, while the
ethyl substituents showed only moderate shifts of Δδ = 0.25 and 0.28 ppm for the CH2 and CH3 groups,
respectively. The coordination of HSiEt3 in 4 is readily reversible; resonances are broad even at –70 °C, and
the 1H NOESY NMR spectrum shows extensive chemical exchange between free and coordinated silane. On
the basis of these data it is therefore not possible on to discriminate between side-on and end-on coordination
modes, although DFT calculations indicate that end-on Au-H-Si bonding is preferred (vide infra).
4

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The coordination of HSiEt3 to the Au(III) centre in 4 is weak enough to be displaced by other weakly
DOI: 10.1039/C8SC05229H
coordinated ligands such as 2-butyne. Under these conditions, no hydrosilylation of the alkyne was observed.
Instead, the alkyne complex evolved by carbocationic alkyne cyclodimerization, as we observed previously
in reactions of Au(III) centres with sterically undemanding alkynes,53 while HSiEt3 remains unreacted.
In the presence of diethyl ether hydride transfer to gold involving heterolytic splitting of the H-SiEt3
bond takes place immediately, as shown by the quantitative formation of the Et2O-stabilized triethylsililium
cation [Et3SiOEt2]+ which is stable at –60 °C and was characterized spectroscopically. Relative to free Et2O
the ethyl groups of Et2O in [Et3SiOEt2]+ are high-frequency shifted, to δH = 4.55 ppm (CH2, Δδ = 1.14) and
δH = 1.57 ppm (Δδ = 0.45), respectively.
Contrary to what we observed for HSi(OMe3), when 1·OEt2 was treated with 8 equivalents of HSiEt3 at
–30 °C, a new major product 5 was formed, accompanied by only small traces of 2 (<5%). Compound 5 is
characterized by an Au–H signal at δH = 0.20 ppm; this chemical shift is in good agreement with the value
expected for a neutral Au(III) hydride, with H in trans-position to an anionic aryl-C.44 The 1H NMR signal
of the hydride moiety in 5 appears as a pseudo-quartet, due to the simultaneous coupling of the hydride with
one o-fluorine atom of the pentafluorophenyl ligand and the  proton H8 of the cyclometalated C^N chelate
(for the numbering system used for NMR assignments see Scheme 1). Probing the hydrogen-fluorine
heterocoupling by means of 1H{19F} and 19F{1H} experiments revealed a coupling constant 4JHF = 5.7 Hz,
which might arise from a through-space correlation mechanism.54 This pattern is not observed in 2, where
the steric constraints imposed by μ-H dimer formation hold the pentafluorophenyl rings tightly in positions
perpendicular to the square-planar coordination plane. On the basis of these data 5 is identified as a neutral
mononuclear gold(III) hydride complex.
Figure 2. Reactivity of 1·OEt2 and 1 with HSiEt3.
5

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DOI: 10.1039/C8SC05229H
The 19F NMR spectrum of the mixture shows only one signal for both the ortho fluorine atoms of 5,
suggesting that the monomeric environment makes aryl tilting and C6F5 rotation possible, thus enabling close
H–F proximity. Homocoupling between the hydride signal and H8 has previously been observed in
biphenyl-based Au(III) dihydrides Li[(C^C)AuH2];44 however, the coupling in 5 is larger (4JHH = 5.0 versus
2.3 Hz). This suggests that minor alteration of the ligand structure has a dramatic impact on ligand flexibility
and hence the spectroscopic features of this class of compounds.
Species 2 and 5 do not show chemical exchange in the 1H NOESY NMR spectrum recorded at –60 °C,
suggesting that they form as kinetic products upon the addition of the silane. Warming the samples at
temperatures above –30 °C results in the decomposition of 5 by reductive elimination, to give C6F5H. On the
other hand, 2 remains unaltered up to –10°C, as observed previously. Evidently, terminal neutral gold(III)
hydrides are less stable and more reactive than cationic H-bridged analogues. On warming the sample at
room temperature C6F5H was formed quantitatively, indicating that ArF–H reductive elimination is the only
decomposition pathway accessible to both 2 and 5.
The observation of Au(III) -silane complexes as detectable intermediates is unprecedented in gold
chemistry. There are apparently no previous examples of silane-gold complexes,51 and Bourissou and co-
workers pointed out recently that whereas copper(I) forms intramolecular σ-Si-H adducts, gold(I) does not.52
Mechanistically, their formation under base-free conditions suggests that the heterolytic Si–H bond cleavage
requires the presence of a Lewis base, which is capable of reacting with and stabilizing silylium cations
generated after the hydride abstraction (see DFT section for details). This process contrasts with the reactions
of heterogeneous gold nanoparticle hydrosilylation catalysts, for which the evidence points to the formation
of H• radicals and Au-silyl surface species.26
Hydrogen activation. The ability of gold(III) cations to promote the heterolytic cleavage of Si–H and B–
H bonds via a base-assisted non-redox mechanism is reminiscent of the behaviour of frustrated Lewis pairs.55
We therefore envisioned that same reaction principle might enable the heterolytic splitting of molecular
hydrogen. When a solution of 1·OEt2 in CD2Cl2 was saturated with H2 (1 bar) at –50 °C, no reaction was
observed over a period of 1 hour. However, upon warming the sample to –20 °C, a slow reaction took place
and gave a mixture containing 2 (41%), C6F5H (43%) and a minor component of a second hydride complex 6
+
with a hydride chemical shift at H = –4.47 ppm (16%). At the same time the evolution of H(OEt2)2 (H =
16.6 ppm) was observed. These results show that heterolytic hydrogen splitting had occurred, and that some
of the gold(III) hydride formed had undergone reductive elimination of C6F5H.
The 1H NMR spectrum of the mixture shows the presence of two AA′XX′ systems for 6 at H =
7.52/7.35 and H = 7.18/6.66 ppm, integrating as 8 and 4 protons, respectively, which give dipole
interactions with the Au-H signal in the 1H NOESY NMR spectrum (see ESI). The chemical shift of the Au–
H moiety in 6 matches that of Au(III) bridging hydrides, where the hydrogen is simultaneously trans to an
Au–C(aryl) and an Au–N(pyridine) bond.44 Consistent with this, the 19F NMR spectrum shows only one
signal for the two ortho fluorine atoms, suggesting that the Au–C6F5 ring in 6 can rotate freely. These
6

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DOI: 10.1039/C8SC05229H
observations suggest that 6 is an H-bridged mixed valence Au(III)/Au(I) complex, formed from 2 by a
second heterolytic H-H cleavage step followed by reductive elimination (Scheme 2).
–
+
CH
AB2
–
AB2
N
C6F5
Au
N
H2
R
Au
+
R
H
+ [ (OEt2)2]
OEt2
H
C
Au
C
–20 oC
C6F5
C6F5
N
.
HC
1 OEt2
2
R = But
–20 oC
H2
–
AB2
R
CH
N
C6F5
Au
N
–
R
AB2
N
Au
R
H
C
Au
C
H
Au
N
C6F5
H
H
H
C6F5
H
C6F5
HC
6
R
Scheme 2. Formation of gold hydrides 2 and 6 by two successive heterolytic H-H cleavage steps.
A control experiment in which pre-formed 2 was mixed with [H(OEt2)2][AB2] at -50 °C revealed no
formation of 6, suggesting that the latter is not the product of a side reaction of 2 with protons. We suggest
that a dissociated pyridine moiety of one C^N chelate ligand in 2 acts as base and enables a second, slower
heterolytic H-H cleavage step. This leads to an unstable transient Au(III)/Au(III) dihydride, which undergoes
fast reductive elimination of C6F5H coupled with internal protodeauration of the remaining Au-aryl bond of
that phenylpyridine ligand, to give 6. Finally, upon warming the sample to room temperature a clean and
quantitative conversion to C6F5H and protonated bis(4-t-butylphenyl)pyridine was observed, without any
trace of C–C reductive elimination products (Figure 3).56
7

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Figure 3. Left: Reactions of 1·OEt2 and of 1 with H2. Right: kinetic profile for the reaction with 1·OEt2 at
DOI: 10.1039/C8SC05229H
-20 °C.
Ether-free 1 is unreactive towards H2 at –10 °C over a period of several hours. No gold dihydrogen
complex was observed, suggesting that H2 binding to gold(III) is not strong enough to displace the -
interaction with the dangling aryl ring. However, 1 and H2 do react slowly over a period of 1 week at room
temperature to generate quantitatively C6F5H and protonated bis(4-t-butylphenyl)pyridine, as was the case in
the reaction with 1·OEt2. It seems reasonable to assume that H2 is activated by the cooperation between gold
and base (following pyridine dissociation) to give pyridinium salts and Au(III)–H, which then decomposes
upon further reaction with H2 as suggested in Scheme 2.
We note in this context that Corma and co-workers prepared heterogenised Schiff-base complexes of
gold(III) which proved to be highly active in olefin hydrogenation.38 The possible active site was probed by
DFT calculations which assumed formation of Au(III) hydrides and persistence of the Schiff-base structure.
In light of the mechanistic results described above, it seems probable however that ligands with N and O-
donor sites are far from innocent and may facilitate both H2 activation and reduction of the gold catalyst.
Heterolytic C-H bond cleavage. Organic hydride donors can exceed dihydrogen in their ability to
generate gold hydrides. For example, 1·OEt2 reacts with 1 equivalent of Hantzsch ester at –30 °C essentially
instantaneously and much faster than with H2, to give 2 in quantitative yield (Scheme 3). Since the by-
product is the corresponding pyridinium salt, the heterolytic C-H bond cleavage does not require the action
of an external base. Indeed, ether-free 1 reacts with Hantzsch ester readily to give 2 quantitatively even at
-70 °C.
N-Benzyl nicotinamide reacts rather more slowly under the same conditions, to give a mixture
containing 2 and 5 in a 30/70 molar ratio. Interestingly, the 1H NMR spectrum recorded soon after mixing at
–30 °C reveals the formation of the Au–nicotinamide adduct 7. The coordination of the amide group to the
metal was revealed by the chemical shift changes of the NH2 signal ( = 0.4 ppm) and those of the benzyl
group and C–H moiety  to the nitrogen. The formation of 7 affects the kinetics of C–H activation and
product distribution: as most of 1·OEt2 is captured by the nicotinamide, the concentration of gold cations
with sufficient electrophilicity for hydride abstraction is reduced, leading to a slower C-H activation rate and
formation of the mononuclear terminal hydride 5 as the major product.
Less activated C-H compounds react only at higher temperature and significantly more slowly. For
example, 1,4-cyclohexadiene (8.5 equivalents) undergoes hydride transfer to 1·OEt2 over the period of 2
weeks. Since the gold hydrides formed in these reactions are not thermally stable under these conditions,
their intermediacy is deduced from the appearance of the reductive elimination product, C6F5H. Under
similar conditions the analogous reaction with cycloheptatriene proceeds significantly faster and is complete
+
in <5 min, to give C6F5H and the tropylium cation C7H7 in about 40% yield, accompanied by a number of
side products including evidence for reductive C6F5-phenylpyridine C-C coupling (Scheme 3). NMR
monitoring at -15 C confirmed the formation of the gold hydride 2 as reaction intermediate.
8

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DOI: 10.1039/C8SC05229H
The facile formation of gold hydrides from substrates such as benzyl-nicotinamide is remarkable and
may have wider implications. Compounds like the Hantzsch ester and nicotinamide are well-known
analogues of NADH, which is involved in electron transport in the mitochondrial respiratory chain.57 Many
gold(III) chelate complexes exhibit pronounced cytotoxicity, although their modes of action remain to be
elucidated.58,59 The hydride abstraction ability of Au(III) demonstrated here raises the possibility that
interference in the NAD/NADH intracellular redox pathways may have to be added to the pathways
responsible for the anti-tumour activity of gold(III) compounds.
O
O
H
H
–
AB2
OEt
O
O
.
EtO
OEt
t < 10s
1 OEt2
+
EtO
2 +
+
–30 oC
N
H
N
H
CD2Cl2
–
AB2
R
O
H
O
H
N
H2N
t = 2h
.
H2N
2 + 5 +
+
N
1 OEt2
+
Au
R
–30 oC
NH2
–
O
H
AB2
N
CD2Cl2
C6F5
Ph
Ph
H
7
N
Ph
H
F
H
H
–
25 oC
AB2
F
F
F
.
+
1 OEt2
+
+
+
N
CD2Cl2
slow
F
R
R
H
H
H
H
H
H
H
–
25 oC
AB2
F
F
F
.
+
N
+ side products
1 OEt2
+
+
+
CD2Cl2
<5 min
F
R
R
H
H
F
Scheme 3. Reactions of 1·OEt2 with activated organic hydride donors.
DFT studies. The reactions of several substrates HE (E = BPin, SiMe3 or H) with [(C^N-CH)Au(C6F5)]+
(here abbreviated to LAu+) were studied by Density Functional Theory (DFT) calculations. Geometry
optimization and thermal corrections were carried out at B3LYP/def2-SVP/PCM(CH2Cl2) level, final
electronic energies from M06/cc-pVTZ/PCM(CH2Cl2), with free energies calculated at the compromise
temperature of 250 K. The model ligand L has the tBu substituents of the C^N-CH ligand replaced by
hydrogens, and substrates were replaced by minimal models. For further details see the SI.
The basic reaction studied is given in equ. (1):
LAu+ + HE
[LAu··H··E]+
LAuH + E+
(1)
Table 1 lists the most relevant free energies for different choices of E; complete listings can be found in
Table S1.
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Before going into specific reactions, a few general points should be noted: (i) Several reactions start
with ether complex LAu(OR2)+, see Scheme 4 below. The initial dissociation of coordinated ether costs 8.9
kcal/mol. (ii) The initially produced LAuH would have the hydride trans to the aryl-C atom of the C^N-CH
ligand. The isomer with H trans to N(pyridine) is actually preferred by 6.5 kcal/mol, although it is not clear
how such an isomerization would happen. However, any LAuH formed can be quickly trapped by unreacted
LAu+ to form the dimer [(LAu)2(μ-H)]+, for which a structure with H trans to both aryl-C atoms is preferred.
(iii) In many of the cases studied the basic reaction (1) is endergonic. However, the trapping of the initial
product LAuH by LAu+ adds -18.7 kcal/mol to the reaction free energy, pulling the equilibrium to the right
(Scheme 4).
LAu+ + HE
[LAu··H··E]+
LAuH + E+
LAu+
- OR2 + OR2
LAu(OR2)+
[LAu
H
AuL]+
Scheme 4. H-E activation equilibria.
(iv) The formation of free E+ is energetically unfavourable. However, the presence of a Lewis base B (ether
in this case) may help trap and stabilize the H abstraction product E+ as an adduct BE+. Stabilization by B
may start at various stages of reaction (1): by initial coordination of B to HE, or by having B as a nucleophile
displacing LAuH from E, or even after hydride displacement has already completed. This results in a set of
similar but subtly different "mechanisms" as detailed below.
It is convenient to start with a silane (HSiMe3) substrate. The relevant energy profile is shown in Figure
4. Reaction of HSiMe3 with LAu+ produces a weakly bound silane complex (-5.3 kcal/mol relative to
“naked” LAu+), but this low binding energy is misleading: both Si-H and Au-H bonds are clearly elongated
relative to the parent molecules HSiMe3 and LAuH, indicating extensive electronic reorganization to the
separated hydrides. Relevant bond lengths and Wiberg bond indexes (WBIs) are shown in Figure 5; the WBI
+
suggests that at this stage H transfer has progressed to about 30%. Simple dissociation of SiMe3 is not
feasible, and even trapping of LAuH by LAu+ is not enough to make this reaction exergonic. However, if the
same reaction is carried out in the presence of ether, an SN2-like attack of OMe2 on the Si atom of the
coordinated silane leading to loss of [Me2OSiMe3]+ is remarkably easy, requiring only 4.4 kcal/mol. The
rate-limiting step for this process is actually the initial ether dissociation, which costs 8.9 kcal/mol,
corresponding to a reaction that is extremely fast even at -60 °C.
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DOI: 10.1039/C8SC05229H
+
LAuH + SiMe3
+22.1
+
[LAu]2H+ + SiMe3
+3.4
LAu+
(0)
[LAu··H··SiMe3··OMe2]+
-0.9
-5.3
LAu(HSiMe3)+
+
LAuH + Me2OSiMe3
--7.2
-8.9
LAu(OMe2)+
-25.8
+
[LAu]2H+ + Me2OSiMe3
Figure 4. Free energy profile of the reaction on LAu+ with HSiMe3 in the presence (black and blue parts)
and absence (black and red parts) of OMe2.
Figure 5. Structure of the silane complex [LAu(C6F5)(HSiMe3)]+ and activation of HSiMe3 by LAu+ / OMe2.
Bond lengths in Å, Wiberg bond indexes shown in purple and italics.
Coordination of borane HBPin to LAu+ (see profile in Figure 6) is even weaker than coordination of
silane (less than 1 kcal/mol). The resulting species is best regarded as a borane σ-complex, with a short B-H
bond and a large Au-H distance, as is clear from the B-H/Au-H bond distances and the corresponding WBIs
(Figure 7). Again, direct dissociation of BPin+ is not feasible. Approach of OMe2 does not result in
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displacement of the hydride from B, the ether simply coordinates weakly (~1 kcal/mol) to the B atom. This
DOI: 10.1039/C8SC05229H
results in considerable reorganization and has the effect of making the borane a better hydride donor, as is
clear from the elongation of the B-H bond and considerable shortening of the Au-H bond: the ether complex
can now be seen as a gold hydride donating density to the B atom of (Me2O)BPin+. Finally, dissociation of
(Me2O)BPin+ produces the free gold hydride, to be captured by LAu+. The subtle difference with the silane
case is that here the LAu(HBPin)(OMe2)+ represents a local minimum (intermediate) instead of an SN2-like
transition state. Similar to the silane case, this is predicted to be a very fast reaction.
LAuH + BPin+
+64.1
+45.4
[LAu]2H+ + BPin+
LAu+
-0.7
[LAu-HBPin-OMe2]+
(0)
LAu(HBPin)+
LAuH + Me2OBPin+
-4.7
-1.7
-8.9
LAu(OMe2)+
-23.4
[LAu]2H+ + Me2OBPin+
Figure 6. Free energy profile (kcal/mol) for the reaction of HBpin with LAu+ in the presence (black and blue
parts) and absence (black and red parts) of ether OMe2.
Figure 7. Activation of HBPin by LAu+ / OMe2. Bond lengths in Å, Wiberg bond indexes shown in purple
and italics.
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The reaction with dihydrogen obviously requires a base (here OMe2) to bind the proton released in the
DOI: 10.1039/C8SC05229H
heterolytic cleavage of H2. However, one molecule of ether is apparently not enough. The free energy profile
(Figure 8) shows that H2 binding is endergonic by 6.1 kcal/mol. The coordinated dihydrogen molecule is
hardly activated, as is clear from bond lengths and WBIs in Figure 9. From there, attack by OMe2 at one of
the hydrogen atoms of H2 leads to heterolytic cleavage, forming the tightly bound pair [LAuH·HOMe2]+ at
+10.5 kcal/mol relative to ether-free LAu+. Dissociation into separate LAuH and HOMe2 would cost
+
another 5.3 kcal/mol, leading to an effective barrier of 24.7 kcal/mol which would correspond to a reaction
that is slow at room temperature. More likely, the tightly bound pair [LAuH·HOMe2]+ binds another
molecule of ether, forming a less tightly bound combination of LAuH and [H(OMe2)2]+,42 after which
separation and trapping of LAuH would lead to the final products. This variation would have an effective
barrier of 19.4 kcal/mol which would be more compatible with a reaction that is slow at -20 °C. The main
difference with the silane and borane variations is that for H2 coordination of the substrate is endergonic and
hence contributes to the effective reaction barrier.
+
LAuH + HOMe2
+15.8
+
LAuH··HOMe2
[LAu··H··H··OMe2]+
+10.5
+9.7
+6.2
LAu(H2)+
+2.6
+
LAu+
(0)
LAuH + H(OMe2)2
+
[LAu]2H+ + HOMe2
-0.9
-8.9
LAu(OMe2)+
-21.3
[LAu]2H+ + H(OMe2)2
+
Figure 8. Free energy profile (kcal/mol) for the reaction of H2 with LAu+ assisted by one (black and red
parts) and two (black and blue parts) molecules of ether OMe2.
Figure 9. Activation of H2 by LAu+ / OMe2. Bond lengths in Å, Wiberg bond indexes shown in purple and
italics.
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DOI: 10.1039/C8SC05229H
The reaction with dihydropyridine 3,5-dicarboxylate (PyCarH) does not require further assistance by a
base: after initial dissociation of ether from the starting complex LAu(OMe2)+, the actual H abstraction step
has a barrier of 3.4 kcal/mol. This leads to an effective activation free energy of 12.3 kcal/mol, indicating a
rather fast reaction. On the other hand, hydride abstraction from toluene or triphenylmethane is
thermodynamically not feasible, while abstraction from 1,4-cyclohexadiene (CHDH) in the absence of ether
is just on the edge; this reaction is exergonic by only 2.9 kcal/mol if LAuH trapping is included) with a
calculated barrier of 16.2 kcal/mol. This is in agreement with the observed slow reaction at room
temperature.
To summarize, LAu+ is a strong hydride acceptor, made even more potent by trapping of the initial
product LAuH to form the binuclear H-bridged complex [(LAu)2(μ-H)]+. It is comparable in strength to the
3,5-dicarboxylatopyridinium cation, and considerably weaker than the cyclohexadienyl and trityl cations. A
common feature of the reactions studied is the surprisingly low barrier for the H transfer step, typically less
than 5 kcal/mol relative to the substrate complex. These results illustrate how effective gold(III) is in
polarizing the H-E bond in preparation for heterolytic cleavage.
Table 1. Free energies of main stationary points for H-E activation by LAu+ (at -20 °C, in kcal/mol).a
HE
LAu(HE)+
[LAu··H··E··B]+ LAuH + BE+ [LAu]2H+ + BE+
HSiMe3
HBPin
H2
-5.28
-0.87b
-1.71c
-7.15
-4.72
-25.83
-23.40
-16.06f
-26.25
-2.85
-0.72
+6.21
+9.72b
+10.51e
-7.57d
PyCarH
CHDH
Ph3CH
PhCH3
-
-
-
-
+3.67b,d
+16.22b,d
+15.83d
+19.24d
+45.80d
-
+0.56
g
+27.12
a LAu+ + HE as reference (0 kcal/mol); B = OMe2; reference energy of LAu(B)+ is -8.86 kcal/mol if the
reaction is done in presence of ether. bTransition state. c Intermediate (local minimum). dNo assistance by
+
+
ether. eNot separated products but complex LAuH··BE+. fForming H[OMe2]2 instead of HOMe2 .
gBarrierless to reactants.
Conclusion
Gold(III) complexes have been shown to be able to form σ-complexes with boranes and silanes, species
that have long been postulated as the first steps in gold catalysed hydroboration and hydrosilylation
reactions. These complexes are sufficiently stable for spectroscopic characterisation. DFT calculations
suggest end-on Au-H-E bonding, with binding free energies of the order of 1 – 5 kcal/mol. The key to these
weak adducts is the use of the gold(III) chelate complex [(C^N-CH)Au(C6F5)]+, which contains a gold-arene
π-interaction to the “dangling” aryl substituent which is weak enough to be displaced by H-E, while at the
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same time the coordinated substrate is protected by this “Pacman”-type ligand structure. In the presence of
DOI: 10.1039/C8SC05229H
ether as base heterolytic H-E bond scission is facile. The same holds for the activation of dihydrogen. The
observations reported here on gold(III) mediated H2 activation follow the same principle as postulated in
earlier computational studies on hydrogen activation by gold(I) compounds,39 where a polar protic solvent
like ethanol was shown to be required in the activation process. These results suggest that for gold catalysts
in which the active centre is positively polarized to some extent, heterolytic rather than homolytic H-E and
H-H bond cleavage processes prevail, at least in the presence of weakly basic solvents or reagents. The range
of H-donors for gold could be extended to activated hydrocarbons, where in favourable cases C-H bond
activation was shown to be orders of magnitude faster than dihydrogen activation. Overall, these reactions
have provided routes to unprecedented types of gold complexes and serve to illustrate the remarkable
capacity of gold to polarize and activate substrate H-E bonds.
Acknowledgements.
This work was supported by the European Research Council. M.B. is an ERC Advanced Investigator Award
holder (grant no. 338944-GOCAT). We thank S. Woodhouse and Prof. G. G. Wildgoose (UEA) for access to
the hydrogen generator.
†Electronic Supplementary Information (ESI) available: Synthesis, NMR spectroscopic and computational
details See DOI: 10.1039/x0xx00000x
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Table of Contents Entry
Gold(III) forms spectroscopically detectable H-B and H-Si σ-complexes; experiments and DFT calculations
demonstrate heterolytic H-Si, H-H and H-C bond cleavage.
18