Reactions of a stable monomeric gold(I) hydride complex

Article abstract of DOI:10.1002/anie.200803842

(Figure Presented) Golden opportunity: A terminal mononuclear gold(I) hydride complex has been stabilized by an N-heterocyclic carbene ligand (see picture: Au orange, N blue, C gray). The complex is stable to a wider range of conditions than other transition metal hydride compounds and reacts to form a number of different gold(I) species.

Full text of DOI:10.1002/anie.200803842

Angewandte
Chemie
DOI: 10.1002/anie.200803842
Gold Hydride Complexes
Reactions of a Stable Monomeric Gold(I) Hydride Complex**
Emily Y. Tsui,* Peter Müller, and Joseph P. Sadighi
Gold hydride complexes have been postulated as
intermediates in a number of homogeneous gold-
catalyzed reactions,[1] but relatively little is known
about these compounds.[2] The simplest gold hy-
dride, AuH, has been observed in the gas phase[3,4]
and trapped in an inert matrix,[5] but has been
extensively studied through theoretical calcula-
tions.[4b] Complexes with hydride bridges between
gold and other metals are well known,[6] but gold-
only hydride-bridged complexes have been
observed solely in the gas phase.[7]
The N-heterocyclic carbene (NHC) ligand IPr
(IPr= 1,3-bis(2,6-diisopropylphenyl)imidazol-2-yli-
dene) stabilizes a copper(I) hydride of low nucle-
arity, dimeric [{(IPr)CuH}2], synthesized from
[(IPr)CuCl] in two steps.[8] Since NHC ligands
have been shown to effectively support many
unusual gold species,[9] we believed the same
methodology could yield an analogous gold com-
plex. Herein, we present the structure of a stable
Scheme 1. Preparation and reactions of 2. OTf=CF3SO3, DMAD=dimethyl acetyle-
nedicarboxylate, EDA=ethyl diazoacetate.
gold(I) hydride complex and some of its reactions.
The reaction of the known compound
[(IPr)AuCl][10] with sodium tert-butoxide generated
[(IPr)AuOtBu] (1), the unsaturated analogue of
[(SIPr)AuOtBu] (SIPr= 1,3-bis(2,6-diisopropylphenyl)imida-
zolin-2-ylidene).[9b] Treatment of 1 in benzene or dichloro-
methane with trimethoxysilane yielded [(IPr)AuH] (2) after
1.5 h at room temperature (Scheme 1). A new singlet
resonance in the 1H NMR spectrum, appearing at d =
5.11 ppm (C6D6) or d = 3.38 ppm (CD2Cl2), is assigned as
the hydride resonance. This assignment was confirmed
through the preparation of the corresponding deuteride
complex by reaction of [(IPr)AuCl] with LiDBEt3 [Eq. (1)].
gives rise to a singlet resonance at 3.34 ppm. A minor
byproduct (ca. 10%) of this reaction is assigned as
[(IPr)AuEt]. The IR spectrum of 2 displays a sharp, intense
band at 1976 cmꢀ1,
a plausible value in light of the
2305.01 cmꢀ1 band measured for AuH in the gas phase[11]
and the strong s-donor characteristics of NHC ligands.[12] In
the IR spectrum of 2D, no band is observed at 1976 cmꢀ1, and
ꢀ1
ꢀ
the expected Au D stretch at 1407 cm appears to be
obscured by bands resulting from ligand-derived stretching
modes.
The X-ray crystal structure[13] confirms that 2 is mono-
meric in the solid state (Figure 1), with the shortest gold–gold
separations in the crystal measured at 8.9174(7) Š. When
LiDBEt3
½ðIPrÞAuClŠ
½ðIPrÞAuDŠ ð2 Þ þ ½ðIPrÞAuEtŠ
ð1Þ
ƒƒƒƒƒƒ!
D
CH2Cl2, RT
The 1H NMR spectrum of the reaction product mixture shows
ligand-derived resonances identical to those of 2 for the major
(ca. 90%) component, assigned as 2D, but no hydride
2
resonance. The H NMR spectrum of this product in CH2Cl2
[*] E. Y. Tsui, Dr. P. Müller, Dr. J. P. Sadighi
Department of Chemistry, Massachusetts Institute of Technology
Cambridge, MA02139-4307 (USA)
Fax: (+1)617-268-5700
E-mail: etsui@alum.mit.edu
[**] This work was supported by the NSF (CAREER award, CHE-
0349204) and the MIT Department of Chemistry. E.Y.T. thanks the
MIT UROP office and the Paul E. Gray UROP fund. We are grateful
to Prof. Timothy Swager (MIT) and to Dr. David Laitar for help in the
preparation of this paper.
Figure 1. Solid-state structure of 2·(C5H12); thermal ellipsoids are set
at the 50% probability level. For clarity, hydrogen atoms (except for
that bound to the gold atom) and solvent molecule are omitted.
Selected bond lengths [Š] and angles [8]: Au(1)–C(1) 2.045(3), C(1)–
N(1) 1.362(3); N(1)-C(1)-Au(1) 127.98(14), N(1)-C(1)-(N1A) 104.0(3).
N(1A) is a crystallographic symmetry equivalent of N(1), generated by
the operator ꢀx+2,y, ꢀz+1/2.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803842.
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exposed to air and moisture, 2 decomposed slowly in solution
toward alkynes such as 3-hexyne and diphenylacetylene at
room temperature, it reacted cleanly with 1.05 equivalents
dimethyl acetylenedicarboxylate (DMAD) in benzene to
form the vinylgold complex 5. The crystal structure of 5
(Figure 3)[13] reveals a trans orientation of the gold and
at room temperature (t1/2 = 6 h). In the solid state, 2 was stable
for weeks under ambient conditions. In contrast,
[{(IPr)CuH}2] decomposes within seconds after exposure to
air in the solid state.[8]
We first explored the hydridic character of 2 toward a
strong Lewis acid. The trityl salt [Ph3C]+[BF4]ꢀ, a known
hydride acceptor, reacted readily with 2 in CD2Cl2. The
1H NMR spectrum of the reaction mixture indicates the
formation of a new species (4) associated with a hydride
resonance at d = 0.42ppm. Integration of this resonance
relative to ligand resonances suggests a ratio of two IPr
ligands per hydride ligand. This result suggested a cationic
hydride-bridged species related to known heterometallic
examples,[6] which led us to believe that an alternative
method for preparing 4 could be found through the reaction
of complex 2 with a gold(I) cation. Indeed, treatment of
[(IPr)AuCl] with AgOTf in CH2Cl2 afforded [(IPr)AuOTf]
(3), which reacted with 2 in benzene to form a white
precipitate. The 1H NMR spectrum of the precipitate is
identical to that of the gold complex formed by reaction of
[Ph3C]+ with 2, and the resonance at d = 0.42ppm is assigned
to the bridging hydride ligand.
Figure 3. Solid-state structure of 5; thermal ellipsoids are set at the
50% probability level. For clarity, hydrogen atoms are omitted.
Selected bond lengths [Š] and angles [8]: Au(1)–C(1) 2.0178(18),
Au(1)–C(7) 2.037(2), C(6)–C(7) 1.340(3), C(5)–C(6) 1.482(3), C(7)–
C(8) 1.494(3); C(1)-Au(1)-C(7) 173.59(7), C(6)-C(7)-Au(1) 131.02(15),
C(8)-C(7)-Au(1) 113.55(13), C(7)-C(6)-C(5) 123.23(18), C(6)-C(7)-C(8)
115.42(17).
Analysis of the complex by X-ray crystallography
(Figure 2)[13] revealed that 4, the isolobal analogue of H3 ,
+
has a triangular structure[7] with a notably short intramolec-
I
I
ꢀ
ular Au Au separation (2.7099(4) Š) compared to typical
hydrogen atoms about the double bond. The trans insertion of
alkynes into various transition metal hydride complexes has
been proposed to occur through a variety of mechanisms.[16]
For example, the analogous formation of a trans-vinylic
product from a platinum(II) hydride complex has been
ascribed to the participation of a radical intermediate.[17]
Such a mechanism, in this case, would suggest the intriguing
possibility of [(IPr)Au0] as an intermediate. However, we
cannot rule out other mechanisms, such as syn insertion
followed by isomerization of the resulting vinyl complex.[18,19]
In light of the demonstrated effectiveness of some
(NHC)AuI complexes for carbene transfer catalysis,[20] we
explored the reactivity of 2 with a carbene precursor,
expecting possibly to observe a net carbene insertion into
the gold–hydrogen bond. Complex 2 reacted slowly with ethyl
diazoacetate (EDA) in C6D6 to form a yellow product. The
1H NMR spectrum of the crude reaction mixture shows no
resonances suggesting the formation of a carboxy alkyl gold
complex, but a small singlet observed at d = 4.47 ppm is
consistent with the presence of dihydrogen. The yellow
product 6 accounts for 77% of the total IPr ligand resonances
by integration of the 1H NMR spectrum, and the IR spectrum
of 6 shows a sharp, intense band at 2040 cmꢀ1 characteristic of
a diazo moiety. These data are consistent with the deproto-
nation of EDA to form an a-metalated diazo complex and
dihydrogen.
Figure 2. Solid-state structure of 4·(C4H10O); thermal ellipsoids are set
at the 50% probability level. Hydrogen atoms, solvent, and OTfꢀ
counterion are omitted for clarity, and only one crystallographically
independent molecule is shown. Selected bond lengths [Š] and
angles[8]: Au(1)–C(1) 2.018(4), Au(1)–Au(2) 2.7099(4), Au(2)–C(4)
2.004(4); C(1)-Au(1)-Au(2) 158.25(12), C(4)-Au(2)-Au(1) 154.22(11).
ꢀ
Au Au distances in aurophilic interactions or in metallic
gold.[14] Complex 4 is stable to air and water in the solid state.
Preliminary attempts to deprotonate 4 and obtain a dimeric
gold(0) complex were unsuccessful. Complex 4 underwent
nucleophilic attack by sodium tert-butoxide to form 1 and 2,
and the reaction of 4 with n-butyllithium resulted in the
precipitation of colloidal gold.
Metalation of diazo compounds by metal amides,[21] by
nBuLi,[22] by silver(I) oxide,[23] and by mercuric oxide[24] are
well known. Although there have been no examples of related
Alkyne insertion into transition metal hydride complexes
is well established.[15] Although 2 displayed no reactivity
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Chemie
gold diazo compounds, to our knowledge, gold alkoxides have
been shown to deprotonate carbonyl compounds such as
methyl cyanoacetate to form the resulting C-bound gold(I)
enolates.[25] In contrast to these highly basic species, however,
complex 2 is fairly stable to acids. For example, no reaction
occurred between 2 and water or tert-butyl alcohol in C6D6
under inert atmosphere. The attempted reaction of 2 with the
more acidic hexafluoro-tert-butyl alcohol in C6D6 led to some
broadening of the hydride resonance, but no hydrogen
evolution was observed. Complex 2 reacted with benzoic
acid, but the reaction required several hours and formed a
number of unidentified decomposition products. Despite
these results, the observed deprotonation and auration of
EDA might be more favorable owing to the formation of a
stabilized carbanion, rather than a hard oxyanion, bound to
gold(I).
To determine whether the auration of EDA is hydride-
specific or occurs more generally with Brønsted-basic gold(I)
complexes, we examined the reaction of the tert-butoxide
complex 1 with EDA in benzene. Within 7 h at ambient
temperature this reaction formed complex 6 and tert-butyl
alcohol quantitatively as judged by 1H NMR spectroscopy. A
single-crystal X-ray diffraction study of 6, prepared in this
manner, confirmed its structure as an a-metalated diazo
compound (Figure 4).[13] The distance between the Au center
and the diazoalkyl carbon atom (2.0328(19) Š) is similar to
Experimental Section
See the supporting information for synthetic details.
Received: August 5, 2008
Published online: October 15, 2008
Keywords: carbene ligands · diazo compounds · gold · hydrides ·
.
insertion
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Figure 4. Solid-state structure of 6; thermal ellipsoids are set at the
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Selected bond lengths [Š] and angles [8]: Au(1)–C(1) 2.0069(16),
Au(1)–C(4) 2.0328(19), N(3)–N(4) 1.134(3), N(3)–C(4) 1.311(3), C(4)–
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N(3)-C(4)-C(5) 111.37(18), N(3)-C(4)-Au(1) 116.10(15), C(5)-C(4)-
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