A gold(I) complex based on 1,8-naphthyridine functionalized N-heterocyclic carbene (NHC) and its catalytic activity

Article abstract of DOI:10.1002/jccs.201100618

A gold complex containing 2,7-bis(mesitylimidazolylidenyl)naphthyridine (NHC-NP) has been synthesized. Thus, reaction of [{Ag₃(NHC-NP) ₂}(PF₆)₃] with [Au(Me2S)Cl] provided an unusual digold complex bridged by two NHC-NP, forming a 20-membered dinuclear metallacycle [{Au₂ (NHC-NP) ₂Cl₂}-(PF ₆)₂] (2) in high yield. This complex was characterized by spectroscopic and elemental analysis. This gold complex is active for the hydrolysis of 2,2,5-trimethyl-1,3-dioxane-5-methanol and can be recycled without losing the activity.

Full text of DOI:10.1002/jccs.201100618

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
A Gold(I) Complex Based on 1,8-Naphthyridine Functionalized N-Heterocyclic Carbene
(NHC) and Its Catalytic Activity
Zu-Yin Liu and Shiuh-Tzung Liu*
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, R.O.C.
(Received: Oct. 21, 2011; Accepted: Dec. 12, 2011; Published Online: Dec. 21, 2011; DOI: 10.1002/jccs.201100618)
A gold complex containing 2,7-bis(mesitylimidazolylidenyl)naphthyridine (NHC-NP) has been synthe-
sized. Thus, reaction of [{Ag3(NHC-NP)2}(PF6)3] with [Au(Me2S)Cl] provided an unusual digold com-
plex bridged by two NHC-NP, forming a 20-membered dinuclear metallacycle [{Au2(NHC-NP)2Cl2}-
(PF6)2] (2) in high yield. This complex was characterized by spectroscopic and elemental analysis. This
gold complex is active for the hydrolysis of 2,2,5-trimethyl-1,3-dioxane-5-methanol and can be recycled
without losing the activity.
Keywords: Gold; Heterocyclic carbene; Naphthyridine; Hydrolysis; Acetal.
INTRODUCTION
Nowadays N-Heterocyclic carbenes (NHCs) are rela-
microcrystalline solid in 85% yield and fully characterized
by NMR spectroscopies and elemental analysis. The ESI-
HRMS exhibits a molecular ion signal with m/z = 695.2194
that is consistent with the formula of [C64H60N12Au2]2+
(Calcd. 695.2197). This information clearly demonstrates
the structure 2 which is in a coordination mode with two
metal ions with two NHC-NP ligands. In addition, The mo-
lar conductivity of the complex in methanol (LM ~ 170 cm2
tively common donors for various transition metal ions ow-
ing to its strong s-donating ability.1 Analogous to phos-
phines, design and investigation of NHC chelating molds
have received much attention because they allow to prepare
a variety of geometrical complexes.1-3 As part of our ongo-
ing research project on the coordinating capability of
polydentates containing NHC donors toward transition
metal ions, we have described the preparation of the palla-
dium complex 1 containing a bis(N-heterocyclic carbene)-
naphthyridine (NHC-NP) ligand and its catalytic activity
on Kumada coupling reactions.3 In this work, we would
like to illustrate the coordination chemistry of NHC-NPto-
ward Au(I) ions and the catalytic activity of the resulting
complex.
-1
W-1 mol ) is consistent with formation of a 2:1 electrolyte,
suggesting that the complex might be dissociated into
[Au2(bpnp)2]2+ and 2(PF6)-.
Mes
Mes
Mes
N
Mes
N
N
N
3+
[(PF6)- ]3
N
N
N
N
Ag
N
2+
[(PF6)-]2
N
Au
Au
N
N
N
L
Ag
N
N
N
N
N
(1)
(Me2S)AuCl
Ag
N
N
N
N
N
N
N
N
N
N
N
Mes
Mes
Mes
Mes
N
N
N
N
N
Mes
N
N
N
N
Mes
N
Mes
I
Pd I
Mes
I
Pd
I
Mes
Mes
N
N
N
2
3, L= CH3CN
NHC-NP
The 1H NMR spectrum of 3 shows five sets of signals
in the aromatic region (two from naphthyridine, two from
imidazole and one from mesityl group), which permits us to
establish the symmetrical structure in the molecule. The
13C NMR spectrum of 2 also shows only one resonance for
the carbene carbons at d 186.3, which is in the normal range
for Au-NHC complexes. These observations evidence the
symmetrical nature of the naphthyridine-NHC liagnd in
1
RESULTS AND DISCUSSION
Preparation of the Gold Complex 2
Silver NHC complex 3 was prepared according to the
reported procedure.4 Reaction of 3 with [(Me2S)AuCl] in
anhydrous dichloromethane generated the air-stable digold
complex (Eq. 1). Complex 2 was isolated as a light yellow
Dedicated to the memory of Professor Yung-Son Hon (1955–2011).
* Corresponding author. E-mail: stliu@ntu.edu.tw
J. Chin. Chem. Soc. 2012, 59, 321-323
© 2012 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
321

Article
Liu and Liu
this gold complex. In addition, comparison of the NMR
shifts between 2 and the palladium analog 1 can tell the
similar structural features between two compounds.3
Stability of the Gold Complex 2
OH
OH
O
2
O
(2)
+
O
H2O, rt
OH
OH
4
Complex 2 is stable toward air and water. In order to
investigate further about the stability of Au-C(carbene) bonds,
this digold complex was subjected to react with trifluoro-
acetic acid. The reaction of 2 with excess of CF3COOH in
CDCl3 in a sealed NMR tube was monitored by 1H NMR
spectroscopy. Complex 2 remained retained in the solution
of CF3COOH/CDCl3 over a temperature range (r.t to 50 oC)
even for a longer period, indicating that Au-Ccarbene bond is
stable toward acid.
In a typical experiment for the reaction, acetal 4 (0.1
mmol) and complex 2 (0.005 mmol) in water (1 mL) were
placed in the flask with stirring at room temperature for 6 h.
The complex 2 was separated by extraction with dichloro-
methane and then analyzed by 1H NMR spectroscopy. The
spectral data of the extracted product are essentially identi-
cal to those of complex 2, showing that this complex is sta-
ble under the catalytic reaction. Furthermore, it can be re-
used as the catalyst for the hydrolysis of 4. We have learned
that complex 2 remains the same activity after five cycles.
To the best of our knowledge, there is no precedent
report concerning [(NHC)2Au]+ species as catalysts on the
hydrolysis of acetals. NHCs are known to be strong donat-
ing ligands and do not dissociate from the metal centers.
Thus the species of [(NHC)2Au]+ species acts as a Lewis
acid for catalyzing the hydrolysis of acetals. Certainly, the
hydrophobic effect of the catalyst in aqueous solution
might also play a role in this hydrolysis particularly for the
homogeneity of this reaction medium.7 Due to the strong
bonding strength in Au-C bonds, complex 2 is stable under
catalytic conditions.
It is documented that reaction of diamino-substituted
carbenes with sulfur yields the corresponding thiourea.5
We have found that some NHC copper complexes react
with sulfur to yield the corresponding thiourea. Reactivity
of 2 with sulfur was investigated to probe the stability of
Au-Ccarbene bond. Treatment of 2 with excess of sulfur in
refluxing acetone did not cause any decomposition of 2
with recovery of 2.
The coordination ability of phosphine toward Au(I)
ions is fairly strong. However, the NHC moiety on the gold
center cannot be replaced by phosphines. Under refluxing
conditions, complex 2 did not undergo ligand substitution
with excess of triphenylphosphine even for a prolong heat-
ing. From these observations, the NHC moiety is stable to-
ward oxidation with sulfur or aerial oxygen, acidic cleav-
age, and ligand substitution with phosphines. In all in-
stances, the NHC moiety remains intact, again demonstrat-
ing the good strength of Au-CNHC bond.
In summary, we have successfully prepared and char-
acterized a new dinuclear gold(I) complex containing a
bis(N-heterocyclic carbene)naphthyridine ligand. This
dinuclear bis-NHC complex 2 appears fairly stable toward
oxidants and acids. Moreover, complex 2 shows an excel-
lent reactivity on the hydrolysis of 2,2,5-trimethyl-1,3-di-
oxane-5-methanol.
Catalysis
According to the coordination behavior presented
above, it would be interesting to test the catalytic activity of
2. Quite a number of reports have demonstrated that NHC-
Au are good catalysts for C-H activation, alkyne hydration
and polymerization.6 Among this context, the catalytic ac-
tive NHC-Au species responsible for these catalysis are
limited to those with the formula of [(NHC)AuL], i.e. the
mono NHC substituted complexes. We have screened the
catalytic activity of 2 toward alkyne hydration and poly-
merization, but complex 2 shows no activity at all. This is
presumably due to the shortage of the coordination site for
catalysis. However, we found that the complex 2 is active
for the hydrolysis of 2,2,5-trimethyl-1,3-dioxane-5-metha-
nol (4) in aqueous medium (Eq. 2).
EXPERIMENTAL
General information
Nuclear magnetic resonance spectra were recorded in
CDCl3 on either a Bruker AM-300 or a Bruker AVANCE
400 spectrometer. Chemical shifts are given in parts per
million relative to Me4Si for H and 13C NMR. All reac-
1
tions, manipulations and purifications steps were per-
formed under a dry nitrogen atmosphere. Tetrahydrofuran
was distilled under nitrogen from sodium/benzophenone.
Dichloromethane and acetonitrile were dried over CaH2
and distilled under nitrogen. Other chemicals and solvents
were of analytical grade and were used after degassed pro-
cess. Silver carbene complex 3 were prepared accordingly
322
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CHEMICAL SOCIETY
N-Heterocyclic Carbene
to the method reported previously.4
support (NSC100-2113-M-002-001-MY3).
Complex 2
Amixture of 3 (196 mg, 0.13 mmol) and [(Me2S)AuCl]
(80 mg, 0.27 mmol) in a flask capped with a septum was
evacuated and flashed with nitrogen. Anhydrous CH2Cl2
(5 mL) was syringed into the above flask and the resulting
mixture was stirred at room temperature for 6 h. The reac-
tion mixture was then filtered through Celite to remove
silver salts. The filtrate was concentrated and the residue
was chromatographed on silica gel with elution of
CH2Cl2. Upon concentration, the desried complex was ob-
tained as light-yellow solids (187 mg, 85 %): 1H NMR (400
MHz, d6-acetone) d 2.00 (s, C6H2(CH3)3, 24H), 2.41 (s,
C6H2(CH3)3, 12H), 7.10 (s, C6H2(CH3)3, 8H), 7.70 (d, J =
2.0 Hz, NCHCHN, 4H), 8.24 (d, J = 8.7 Hz, naphthyridine,
4H), 8.30 (d, J = 2.0 Hz, NCHCHN, 4H), 8.39 (d, J = 8.7
Hz, naphthyridine, 4H); 13C NMR (100 MHz, d6-acetone) d
17.9, 21.3, 117.9, 121.8, 122.9, 125.2, 130.3, 135.7, 136.2,
140.5, 141.4, 152.6, 153.7, 186.3 (Ccarbene); 31P{1H} (161.9
MHz, d6-acetone) d -144.2 (sept, JP-F = 705.8 Hz); ESI-
HRMS calcd. for [C64H60N12Au2]2+: m/z = 695.2197; found
m/z = 695.2194; Anal. Calcd for C64H60Au2F12N12P2: C,
45.73; H, 3.60; N, 10.00. Found: C, 45.33; H, 3.36; N, 9.83.
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ACKNOWLEDGEMENTS
We thank the National Science Council for financial
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