A thermally stable gold(III) hydride: Synthesis, reactivity, and reductive condensation as a route to gold(II) complexes

Full text of DOI:10.1002/anie.201206468

Angewandte
Chemie
DOI: 10.1002/anie.201206468
Gold(III) Hydrides
A Thermally Stable Gold(III) Hydride: Synthesis, Reactivity, and
Reductive Condensation as a Route to Gold(II) Complexes**
Dragos¸-Adrian Ros¸ca, Dan A. Smith, David L. Hughes, and Manfred Bochmann*
Transition-metal hydride complexes are well-established as
key intermediates in numerous homogeneously and hetero-
geneously catalyzed reactions.[1] Hydride complexes of plat-
inum(II), for example, have been known for over 50 years.[2]
Analogous complexes of the isoelectronic gold(III), on the
other hand, appear to be unknown.[3] This is all the more
surprising as gold complexes have attracted much attention as
catalysts and catalyst precursors in recent years.[4] For
example, heterogeneous gold catalysts are used for selective
hydrogenations;[5] for zirconia-supported gold hydrogenation
catalysts, surface Au3+ ions have been proposed as active
sites.[6] Both gold(I) and gold(III) hydrides have been
postulated as intermediates in numerous homogeneously
catalyzed hydrogenations,[7,8] hydrosilylations,[9] dehydrogen-
ative alcohol silylations,[10] hydroborations,[11] and other
organic transformations.[12] Corma and co-workers used
immobilized gold(III) chelate complexes as hydrogenation
catalysts and explored the possible involvement of AuIII
hydride intermediates in detail by kinetic and computational
methods.[13] On the other hand, the highly positive standard
redox potentials for gold (Au3+/Au+ = 1.36 V; Au3+/Au0 =
1.52 V)[14] might suggest facile reduction of AuIII in the
presence of hydride. Binary hydrides, including AuH,
(H2)AuH, (H2)AuH3, and [AuH4]À, have been detected in
frozen gas matrices below 5 K.[15] We report herein the
synthesis and reactions of a thermally stable gold(III) hydride,
[(C^N^C)*AuH], wherein (C^N^C)* is a doubly cyclometa-
lated 2,6-bis(4’-tert-butylphenyl)pyridine ligand.
sponding gold deuteride complex 2-D by treating 1 with
LiDBEt3. The H NMR spectrum of 2-D shows the resonan-
1
ces for the C^N^C* ligand that are identical to those of 2-H,
without the hydride resonance, and the phenyl b proton shows
the expected doublet multiplicity (see the Supporting Infor-
mation). The 2H NMR spectrum of 2-D in CH2Cl2 shows
a singlet at d = À6.58.
The IR spectrum of 2-H shows a strong, sharp band at
À1
À
2188 cm , which is higher than the Au H stretch in
[(IPr)AuH] (1976 cmÀ1)[3a] but comparable to that found for
AuH and (H2)AuH in a frozen argon matrix (2226.6 and
2173.6 cmÀ1, respectively).[15] The Au D stretching frequency
À
of 2-D (expected at ca. 1550 cmÀ1) was not found and is likely
to be obscured by other ligand bands in this region.
As a solid, 2-H is stable to air and moisture at room
temperature. Although the hydride is cis to two phenyl
À
ligands, and although we do observe Au C cleavage reactions
of this pincer system under acidic conditions,[17] the rigidity of
the C^N^C pincer implies that reductive elimination is not
favorable. On the other hand, toluene and CH2Cl2 solutions of
2-H darkened rapidly when exposed to light, even though
little change was noticeable in the 1H NMR spectrum.
However, prolonged exposure to sunlight of CH2Cl2 solutions
of 2-H gave [(C^N^C)*AuCl] (t1/2 ꢀ 4 h), while heating 2-H in
C6D6 for 12 h generated a mixture of products consisting of
about 30% free ligand, with the loss of the hydride resonance.
The structure of 2-H was confirmed by X-ray diffraction
(Figure 1).[18]
The reaction of [(C^N^C)*AuOH][16] (1) with LiHBEt3 in
toluene at À788C followed by stirring at room temperature
for 15 min rapidly yielded [(C^N^C)*AuH] (2-H), which was
isolated as a yellow crystalline complex [Eq. (1)]. The
1H NMR spectrum showed a broad singlet resonance at d =
À6.51 in CD2Cl2 and at d = À5.73 in C6D6, while the protons
attached to the carbon atom in the b position with respect to
the gold center give rise to a pseudo triplet multiplicity,
4
suggesting a J coupling (1 Hz) to the hydride ligand. This
assignment was confirmed by the preparation of the corre-
Complex 2-H proved unreactive towards ethylene, 3-
hexyne, phenylacetylene, and even dimethyl acetylenedicar-
boxylate. There was also no reaction with CO2 or with
benzaldehyde in the dark or under photolysis. Also no
reaction was observed between 2-H and weak acids such as
acetic acid; however, 2-H reacts instantaneously with the
stronger trifluoroacetic acid (HOAcF) to give a mixture of
products, with the expected complex [(C^N^C)*AuOAcF]
not being observed among the reaction products. The results
suggest that the hydridic character of 2-H is less pronounced
than in the case of AuI hydrides stabilized by strong NHC
donor ligands.[3a] The hydride does however react with 1,1-
[*] D.-A. Ro¸sca, Dr. D. A. Smith, Dr. D. L. Hughes, Prof. 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. We thank
Johnson Matthey plc for a loan of gold salts and Dr. Yimin Chao
(UEA) for access to spectrophotometric facilities. D.A.R. thanks the
University of East Anglia for a studentship.
Supporting information for this article (synthetic, spectroscopic,
and crystallographic details) is available on the WWW under http://
dx.doi.org/10.1002/anie.201206468.
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reduction of the two metal centers thus amounts to reductive
condensation. Dimeric metal–metal-bonded gold(II) com-
plexes are typically made by oxidation of AuI precursors[19–21]
or comproportionation;[22] we are not aware of a precedent for
the reductive route observed. We note however that the
inverse of reductive condensation, the oxidative addition of
I
I
À
H2O and NH3 across a Pd Pd bond, has recently been
reported.[23]
To confirm its identity, product 4 was also independently
synthesized by the one-electron reduction of [(C^N^C)*Au-
(SMe2)]+ or of [(C^N^C)*AuOAcF] with cobaltocene. In fact,
the formation of 4 is observed as a side reaction in several
transformations of 2-H and is possibly indicative of competing
one-electron reaction pathways. The crystal structure of 4
Figure 1. Molecular structure of [(C^N^C)*AuH] (2-H) in the solid
state. Ellipsoids are set at 50% probability. Hydrogen atoms (except
for that bound to the gold atom) are omitted. The hydrogen atom
attached to the gold center was located on the density map. Selected
bond distances [ꢀ] and angles [8]: Au–N1 2.035(3), Au–C17 2.073(4),
Au–C1 2.074(4); C17-Au-C1 161.63(16).
confirmed the complex as
a
dinuclear AuII complex
À
(Figure 2). The gold gold bond length of 2.4941(4) ꢀ is
II
II
À
among the shortest separations for an unsupported Au Au
bond.[24]
dimethylallene and cyclohexylallene regiospecifically to give
single insertion products, the vinyl complexes 3a and 3b,
respectively (Scheme 1).
Figure 2. Molecular structure of [(C^N^C)*2Au2] (4) in the solid state.
Hydrogen atoms and tert-butyl groups have been omitted for clarity.
Ellipsoids are set at 50% probability. Selected bond distances [ꢀ] and
angles [8]: Au1–Au2 2.4941(4); N1-Au1-Au2 179.4(2), C1-Au1-C17
162.3(5).
Scheme 1. Allene insertion reactions of [(C^N^C)*AuH].
More surprisingly perhaps, 2-H also reacts slowly with the
hydroxide 1 to give the gold(II) dimer 4. Compound 4 is
formed at room temperature in benzene or dichloromethane
solutions in the dark over a period of several days and was
Complex 4 emits green photoluminescence in the solid
state (lemiss = 444, 457, 479 nm). The compound is stable to air
and moisture in the solid state and in dichloromethane
solution in air at À258C for long periods of time. No reaction
was observed between a mixture of 4 and elemental sulfur in
toluene (608C, 12 h). The lack of reactivity is most probably
a reflection of the steric shielding provided by the four tert-
isolated
as
yellow
crystals.
The
reaction
of
[(C^N^C)*AuOAcF] with 2-H is faster, occurring within
a few minutes, and gives 4 in 75% yield, together with some
unidentified side products (Scheme 2).
The reaction of a metal hydride and a metal hydroxide (or
carboxylate) to eliminate water (or acid) with concomitant
À
butyl substituents that envelope the Au Au bond. On the
À
other hand, the Au Au bond is oxidatively cleaved by iodine
to give [(C^N^C)*AuI]. Photolytic bond homolysis is also
possible, and 4 decomposes slowly in CH2Cl2 at room
temperature under ambient light.
In conclusion, gold(III) hydrides, which have long been
postulated as catalytic intermediates, are indeed isolable and
thermally stable, given a suitable ligand environment. Reac-
À
tivity studies show that the Au H bond is highly covalent and
undergoes regioselective insertion reactions with allenes to
give gold(III) vinyl compounds, while there was no reaction
with simple noncyclic alkynes. A contributory factor to the
Scheme 2. Formation of the AuII–AuII dimer 4 by various routes.
stability of the Au H bond in the present case is probably the
À
2
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Received: August 10, 2012
Published online: && &&, &&&&
Keywords: gold · insertion reaction · metal hydrides · metal–
.
metal bonds · reduction
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Gold(III) Hydrides
Going for gold: The first thermally stable
gold(III) hydride [(C^N^C)*AuH] is pre-
sented. It undergoes regioselective
D.-A. Ros¸ca, D. A. Smith, D. L. Hughes,
M. Bochmann*
&&&& — &&&&
insertions with allenes to give gold(III)
vinyl complexes, and reductive conden-
sation with [(C^N^C)*AuOH] to the air-
stable AuII product, [(C^N^C)*2Au2], with
a short nonbridged gold–gold bond.
A Thermally Stable Gold(III) Hydride:
Synthesis, Reactivity, and Reductive
Condensation as a Route to Gold(II)
Complexes
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