Isolation of neutral mono- and dinuclear gold complexes of cyclic (alkyl)(amino)carbenes

Article abstract of DOI:10.1002/anie.201304820

The smallest pieces of gold! Thanks to the presence of two π-accepting cyclic (alkyl)(amino)carbenes (CAACs),complexes featuring one and two atoms of gold in the formal oxidation state of zero can be isolated. Copyright

Full text of DOI:10.1002/anie.201304820

Angewandte
Chemie
Communications
Gold Complexes
D. S. Weinberger, M. Melaimi,
C. E. Moore, A. L. Rheingold, G. Frenking,
P. Jerabek, G. Bertrand*
&&&& — &&&&
Isolation of Neutral Mono- and Dinuclear
Gold Complexes of Cyclic
(Alkyl)(amino)carbenes
The smallest pieces of gold! Thanks to the
presence of two p-accepting cyclic
(alkyl)(amino)carbenes (CAACs),com-
plexes featuring one and two atoms of
gold in the formal oxidation state of zero
can be isolated.
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
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Communications
DOI: 10.1002/anie.201304820
Gold Complexes
Isolation of Neutral Mono- and Dinuclear Gold Complexes of Cyclic
(Alkyl)(amino)carbenes**
David S. Weinberger, Mohand Melaimi, Curtis E. Moore, Arnold L. Rheingold,
Gernot Frenking, Paul Jerabek, and Guy Bertrand*
The common oxidation states of gold are -I, + I, and + III,
with rare complexes featuring a gold + II, + IV, and + V.[1] To
date, apart in elemental gold,[2] the zero oxidation state has
been mentioned in mixed gold(0)/gold(I) complexes of the
general formula [(LAu)n]m+, mXꢀ as exemplified by the
gold. Cyclic (alkyl)(amino)carbenes (CAACs)[13,14] feature
the desired properties,[15] and furthermore they strongly
stabilize paramagnetic centers.[16] Herein we report the syn-
thesis and single-crystal X-ray diffraction study of neutral
mononuclear and dinuclear gold complexes, which are stable
for days at room temperature, both in solution and in the solid
state.
classical [(Ph3PAu)6]2+ 2(BF4 )[3a] and [(tBu3PAu)4]2+ 2-
ꢀ
ꢀ
(BF4 ),[3b] and the recently reported {[(NHC)Au]3}+ (TfO)ꢀ
complex.[4] There are also a few examples of multinuclear
To check our hypothesis, we first carried out the cyclic
voltammetry of a tetrahydrofuran solution of the known
2
mixed-metal clusters,[5] such as [cis-h -{(Ph3P)Au Au-
ꢀ
(PPh3)}Ph3PCr(CO)4],[6] in which the Au atom can be
considered to have an oxidation state of zero. However,
(CAAC)2AuI complex (Scheme 1),[17] containing 0.1m
1
ꢀ
mononuclear (LnAu), binuclear (LAu AuL), and polynuclear
[(LAu)n] neutral complexes in which atoms of gold are
coordinated end-on by L ligands have never been isolated.[7,8]
[9]
ꢀ
Earlier claims of the preparation of [(Ph3P)Au Au(PPh3)]
have not been confirmed, and calculations by Schwerdtfeger
and Boyd[10] on the model compound (H3P)Au Au(PH3)
ꢀ
leave serious doubts about the structural assignment. In fact,
this type of dinuclear gold(0) species supported by phos-
phines[11] has been postulated as intermediates to rationalize
the formation of gold nanoparticles. In their recent review,
Raubenheimer and Schmidbaur[7] wrote that “it is very likely
that N-heterocyclic carbenes (NHCs) may be the right choice to
prepare the much sought-after complexes of the Au2 molecule”.
Unfortunately, Sadighi et al.[12] have shown that the
Scheme 1. Synthesis of [(CAAC)2Au] complex 2. Dipp=2,6-diisopropyl-
phenyl.
nBu4NPF6 as electrolyte (Figure 1a). A reversible one-
electron reduction was observed at E1/2 = ꢀ2.24 V versus
Fc+/Fc, which prompted us to perform the chemical synthesis
of (CAAC)2Au complex 2. A benzene solution of 1 was stirred
at room temperature for 4 h in a Schlenk flask coated with
a potassium mirror. After filtration and evaporation of the
solvent under vacuum, a green solid was obtained. Recrystal-
lization from a THF solution stored at ꢀ788C under argon
afforded 2 (Scheme 1) as thermally stable (m.p. 1108C),
highly air- and water-sensitive, light green single crystals
(yield: 44%).
ꢀ
(NHC)Au Au(NHC) complex, generated by the deprotona-
tion of the corresponding cationic hydride-bridged species,
was also unstable and led to colloidal gold.
We reasoned that for stabilizing electron-rich gold(0)
centers, the most suitable ligands should be p-electron
accepting while also capable of forming strong bonds with
[*] D. S. Weinberger, Dr. M. Melaimi, Dr. C. E. Moore,
Prof. A. L. Rheingold, Prof. G. Bertrand
An X-ray diffraction study[18] (Figure 1c) first confirmed
that complex 2 is neutral, and features a linear geometry (C1-
Au-C1’ 180.08), with the five-membered ring ligands being co-
UCSD-CNRS Joint Research Laboratory (UMI 3555)
Department of Chemistry and Biochemistry, University of California
San Diego, La Jolla, CA 92093-0343 (USA)
and
UCSD Crystallography Facility, Department of Chemistry and
Biochemistry, University of California
San Diego, La Jolla, CA 92093-0343 (USA)
E-mail: guybertrand@ucsd.edu
Homepage: http://bertrandgroup.ucsd.edu
ꢀ
planar (N1-C1-C1’-N1’ dihedral angle 1808). The Au C1
ꢀ
(1.991(2) ꢀ) and C1 N1 (1.344(3) ꢀ) bonds of 2 are slightly
shorter and longer, respectively, than in the gold(I) precursor
1 (2.0321(11) and 1.304(2) ꢀ), indicating a significant p-back-
donation of the unpaired electron from gold to the ligands. As
expected, solutions of 2 are NMR-silent and EPR-active. The
room-temperature EPR spectrum of 2 in benzene shows
a broad signal at g = 1.9607, while in the solid state (Fig-
ure 1b) the anisotropic tensors are gxx = 1.9410, gyy = 1.9543,
and gzz = 1.9864.[19] Calculations at the (U)M05-2X/def2-SVP
level using the NBO method show that the spin density in 2 is
mainly localized on the carbene carbons (60%) and the
nitrogen atoms (20%), while only 17% stays at gold. The
Prof. G. Frenking, P. Jerabek
Fachbereich Chemie, Philipps-Universitꢀt Marburg
Hans-Meerwein-Strasse, 35032 Marburg (Germany)
[**] The authors gratefully acknowledge financial support from the DOE
(DE-SC0009376) and the NSF (CHE-0924410 and CHE-1316956).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304820.
2
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Angewandte
Chemie
the carbene gold(I) precursor 3; a similar trend has been
observed with a variety of metal–NHC complexes when
comparing oxidation states zero and one.[20] A single-crystal
ꢀ
X-ray diffraction study confirmed the desired (CAAC)Au
Au(CAAC) structure. There are six molecules in the asym-
metric unit, three of them are nearly identical (Figure 2), the
others differ merely by the respective orientation of the two
Figure 2. Solid-state structure of 4 with only one rotamer shown
(hydrogen atoms are omitted for clarity). Selected bond lengths (ꢁ)
and angles (8), with calculated values at M05-2X/def2-SVP in square
brackets: Au1–Au2 2.5520(6) [2.579], Au1–C1 2.082(10) [2.123], Au2–
C2 2.088(9) [2.133], C1–N1 1.261(12) [1.307], C2–N2 1.337(10)
[1.308]; C1-Au1-Au2 173.8(2) [171.3], C2-Au2-Au1 171.3(3) [169.3]; N1-
C1-C2-N2 177.65(66) [146.0].
Figure 1. a) Cyclic voltammogram of 1 (nBu4NPF6 as electrolyte, poten-
tial versus Fc+/Fc, scan rate 100 mVsꢀ1). b) Experimental (c) and
simulated (a) EPR spectrum of 2 in a frozen benzene solution at
235 K. B=magnetic field. c) Solid-state structure of 2 (hydrogen atoms
are omitted for clarity). Selected bond lengths (ꢁ) and angles (8), with
calculated values at (U)M05-2X/def2-SVP in square brackets: Au1–C1
1.991(2) [2.006], C1–N1 1.344(3) [1.348]; C1-Au1-C1’ 180 [180]; N1-C1-
C1’-N1’ 180 [180]; d) Graphical representation of the SOMO of 2.
shape of the SOMO (Figure 1d) shows that the unpaired
electron is delocalized over the p(p) AOs of the carbene
ligand and the p(p) AO of Au. This means that the electronic
reference configuration for the bonding in (CAAC)2Au is
d10s0p1 but not the ground state configuration d10s1p0.
carbene ligands; in other words these molecules are rotamers.
As predicted by Schwerdtfeger and Boyd[10] for (H3P)Au
ꢀ
Au(PH3), the C1-Au1-Au2-C2 skeleton is almost linear (C1-
Au1-Au2 173.8(2), Au1-Au2-C2: 171.3(3)8), and the gold–
gold bond is short (2.5520(6) ꢀ).
From these results, it can be concluded that CAACs
behave as redox active ligands, and it could be argued that 2
has only a weak gold(0) character. Thus, the next question was
to investigate if a neutral mono-CAAC gold complex would
The EDA-NOCV results (see the Supporting Informa-
tion) show that the total interaction energy and the orbital
interactions between Au2 and the CAAC ligands in 4 are
much weaker than between Au and the CAACs in 2. This is
because Au2 binds in 4 to the ligands through its electronic 2S
(d10s1p0, J = 1/2) ground state, while Au binds in 2 through the
2P (d10s0p1, J = 1/2) excited state, which is 106.8 kcalmolꢀ1
ꢀ
undergo a dimerization into a (CAAC)Au Au(CAAC) com-
plex, and if the latter would be stable enough to be isolated.
For this purpose, the bulkier menthyl-substituted CAAC was
chosen.[13] Reduction at room temperature of the (CAA-
C)AuCl complex 3 with a suspension of lithium sand in THF
afforded after work up the dinuclear complex 4, which was
isolated in 20% yield as highly air- and water-sensitive light
brown crystals (m.p. 988C; Scheme 2). Its non-ionic character
was indicated by its solubility in non-polar solvent, such as n-
hexane, and its diamagnetic nature apparent from NMR
spectroscopy. Interestingly, a 13C NMR signal was observed at
d = 286 ppm, downfield-shifted by d = 50 ppm compared to
higher in energy.[21] This explains why the Au C bond in 2 is
ꢀ
significantly shorter than in 4. The calculated bond dissoci-
ꢀ
ation energies De for breaking both Au C bonds in the
reactions 2!Au + 2CAAC (91.2 kcalmolꢀ1) and 4!Au2 +
2CAAC (90.6 kcalmolꢀ1), which yield the fragments in the
electronic ground state, are nearly the same. Naked Au2 has
ꢀ
a calculated Au Au bond length of 2.546 ꢀ, which is very
close to the observed (2.5520(6) ꢀ) and calculated (2.579 ꢀ)
ꢀ
gold–gold distance in 4. The Au Au bond comes from the
coupling of the unpaired electrons in the valence s AO of
gold, that is, from the d10s1p0 ground configuration, which is
sd-hybridized. This leaves one vacant sd-hybridized s* orbital
of Au2 as acceptor MO for donation from the minus
combination of the carbon lone-pair donor orbitals of the
!
CAAC ligands. The CAAC!Au2 CAAC donation from the
plus combination of the ligand donor orbitals occurs into
a vacant s MO of Au2, which has mainly p(s) character. A
Scheme 2. Synthesis of [(CAAC)2Au2] complex 4.
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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detailed analysis of the bonding situation in 2 and 4 will be
presented elsewhere.
When phosphines are used as ligands, a variety of small
and large gold clusters, including nanoparticles, are available,
and have found numerous applications;[22] all of these
particles possess positive charges. We are currently inves-
tigating the synthesis of similar clusters using cyclic (alkyl)-
(amino)carbenes as ligands. Importantly, as these results
demonstrate that CAACs can stabilize gold in the formal zero
oxidation state, we believe that neutral clusters of different
sizes, mimicking gold surfaces, will be accessible.
Experimental Section
Synthesis of mononuclear gold complex 2: Gold complex 1[17]
(200 mg) was added to a Schlenk flask with a potassium mirror (ca.
50 mg, excess). Benzene (30 mL) was added, and the solution was
stirred for 4 h at room temperature. Subsequently, the solution was
filtered and the solvent was removed under vacuum. The residue was
extracted with 3 ꢁ 20 mL of benzene and the solvent was removed to
yield a green solid. Crystals suitable for X-ray diffraction were grown
at ꢀ788C from a concentrated THF solution. Yield: 85 mg (44%).
Mp: 1108C.
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von Hopffgarten, S. Klein, R. Tonner, G. Frenking, G. Bertrand,
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Synthesis of the dinuclear gold complex 4: In a glove box, gold
complex 3[17] (150 mg) and lithium sand (4.5 mg, 2 equiv) were stirred
in THF (20 mL) for 1.2 hours. The solution was subsequently filtered
and dried in vacuo. The product was extracted with 3 ꢁ 20 mL of
hexane, and the solvent evaporated. The solid residue was washed
with 3 ꢁ 0.5 mL of hexane to yield a pale brown solid. Crystals suitable
for X-ray diffraction were grown at ꢀ308C from a concentrated
hexanes solution. Yield: 56 mg (20%). Mp: 988C (dec.);
13C{1H} NMR (C6D6, 500 MHz) = 287.2 (Ccarbene), 145.8 (Cortho), 145.5
(Cortho), 136.0 (Cipso), 128.6 (Cpara), 124.2 (Cmeta), 76.5 (Cquat), 66.7
(Cquat), 53.6 (CH2), 52.5 (CH), 50.3 (CH2), 36.9 (CH2), 29.6, 29.3, 29.2,
29.1, 29.0, 28.6, 28.3, 27.3, 25.6, 25.0 (CH2), 23.5, 23.3, 23.2, 20.0.
Computational details: The geometry optimization of the mole-
cules have been carried out using the DFT functional M05-2X[23] with
def2-SVP basis sets[24] and quasi-relativistic effective core potentials
for Au[25] using the program package Gaussian09.[26] The nature of the
stationary points was verified by calculation of the vibrational
frequencies. The nature of the gold–ligand bonding was analyzed
with the EDA-NOCV method[27] at BP86/TZ2P[28,29] where relativistic
effects are considered with the ZORA approximation[30] using the
program ADF.[31]
Received: June 4, 2013
Published online: && &&, &&&&
Keywords: carbenes · complexes · gold · zero oxidation state
.
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