Oxidative rearrangement in gold organometallics

Article abstract of DOI:10.1021/om3001422

The one-electron oxidizing agent [NO]PF₆ was reacted with Bu₄N[Au(C₆X₅)₂] (X = F, Cl) complexes in CH₃CN. The gold(III) complexes [Au(C₆F ₅)₃(CH₃

Full text of DOI:10.1021/om3001422

Communication
pubs.acs.org/Organometallics
Oxidative Rearrangement in Gold Organometallics
Atiya T. Overton,† Jose M. Lopez-de-Luzuriaga,‡ M. Elena Olmos,‡ and Ahmed A. Mohamed*,†
́
́
†Department of Chemistry, Delaware State University, 1200 N DuPont Highway, Dover, Delaware 19901, United States
‡
́
́
Departamento de Quımica, Universidad de la Rioja, Grupo de Síntesis Quımica de La Rioja, UA-CSIC Complejo Científico
Tecnolog
́
ico, E-26001 Logrono, Spain
̃
S
* Supporting Information
ABSTRACT: The one-electron oxidizing agent [NO]PF6 was
reacted with Bu4N[Au(C6X5)2] (X = F, Cl) complexes in
CH3CN. The gold(III) complexes [Au(C6F5)3(CH3CN)] and
cis-[Au(C6Cl5)2(CH3CN)2]PF6 were synthesized by the oxidation
of gold(I) to gold(III) with the concomitant ligand rearrangement
“oxidative rearrangement”. The supramolecular crystal packing in
the perfluorinated aryl gold(III) complex is dictated by the
notably short C(sp2)−F···H(CH3CN) bond distance. The
[Au(C6Cl5)2(CH3CN)2]PF6 complex exhibits antisymbiosis,
displaying equal ligands in the cis position. The electrochemical oxidation of Bu4N[Au(C6X5)2] (X = F, Cl) complexes in
CH3CN showed two irreversible peaks at 0.71 and 1.21 V (X = F) and at 0.65 and 0.91 V (X = Cl) vs Ag/AgCl.
erhalogenated arylgold complexes and in particular the
perfluorinated derivatives have been widely utilized as
transfer of two C6F5 groups via an inner-sphere mechanism.9
Also, nonoxidative routes have been used to modify the gold−
carbon bonding such as the synthesis of [Au(C6F5)3(tht)] by
the addition of the arylating agents Ag(C6F5) and LiC6F5 to
trans-[AuCl2(C6F5)(tht)].9
The easy removal of the gaseous byproduct nitric oxide from
the nitrosonium cation [NO]+ oxidation reactions provides an
advantage over other oxidizing and oxidative addition reagents
utilized in gold chemistry. The oxidation potential of [NO]+ is
comparable with that of gold (approximately 0.87 V vs Fc/Fc+
in CH3CN).10 The one-electron-oxidizing agent [NO]PF6 was
reacted with Bu4N[Au(C6X5)2] (X = F, Cl) complexes in
CH3CN (2/1) followed by crystallization from CH3CN/ether
(Scheme 1). Unlike the abundant oxidative addition reactions,
the oxidative rearrangement reaction is described for the first
time.11,12
The gold(III) complex [Au(C6F5)3(CH3CN)] was synthe-
sized in a 64% yield by the addition of 2 equiv of [NO]PF6 in
CH3CN with respect to the Bu4N[Au(C6F5)2] complex
(Supporting Information). An unidentifiable product in a
small yield was formed according to the reaction equation, but
no attempts have been made to characterize it. The oxidation
reaction of Bu4N[Au(C6Cl5)2] with 2 equiv of [NO]PF6 in
CH3CN formed colorless crystals of the cationic complex
[Au(C6Cl5)2(CH3CN)2]PF6 in a 68% yield (Supporting
Information). The off-white powder turns gray, provided the
acetonitrile is removed in vacuo. The two gold(III) complexes
displayed 13C NMR peaks in CDCl3 at δ 2.25 and 118.7 ppm.
The ν(CN) stretching vibrational modes appeared at 2346
and 2311 cm−1 for [Au(C6F5)3(CH3CN)] and at 2360 and
2337 cm−1 for [Au(C6Cl5)2(CH3CN)2]PF6.
P
volatile organic compound (VOC) sensors,1 liquid crystal
materials,2 and strongly luminescent materials with tunable
emissions.3 Arylgold(III) complexes are good catalysts for the
addition of nucleophiles to alkynes.4 Gold(III) complexes with
nitrogen and carbon donor ligands are widely utilized in
catalytic cycle design and occupy a leading place in the
cycloaurated derivatives for C(sp3)−H bond activation.5
Knowledge gained from gold(III) oxidative chemistry should
significantly aid in the advance of catalytic gold(III) systems for
the selective functionalization of sp3-hybridized C−H bonds.
Although the terminology “oxidative rearrangement” is not
used uniformly in the organic literature, it refers in this study to
the alteration in one or more gold−carbon connectivities, in
which the complex undergoes a net oxidation.6 Generally these
rearrangements occur simultaneously and are driven by the
metal oxidation. The oxidative rearrangement reactions are
predicted to occur in gold organometallics in a mannaer
comparable to that for organic compounds, as gold and carbon
possess similar Pauling electronegativities (gold 2.54, carbon
2.55).7 The rearrangement in organic reactions has been
catalyzed by some metals, including gold. Toste reported the
gold(I)-catalyzed oxidative rearrangement reactions of alkynes
using sulfoxides as stoichiometric oxidants.8a Zhang developed
a convenient and reliable method to access reactive α-oxo gold
carbenes via gold-catalyzed pyridine N-oxide intermolecular
oxidation of terminal alkynes.8b
Oxidative addition, the use of the oxidizing and arylating
agent [Tl(C6F5)2X]2 (X = Cl, Br), is a common reaction used
in the oxidation and modification of the gold−carbon bonding
in perhalogenated arylgold organometallics.9 [Au(C6F5)3(tht)]
(tht = tetrahydrothiophene) was prepared by the oxidative
addition of [Tl(C6F5)2Cl]2 to [Au(C6F5)(tht)] with the
Received: February 24, 2012
Published: April 13, 2012
© 2012 American Chemical Society
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Communication
Scheme 1. Synthesis of [Au(C6F5)3(CH3CN)] and
[Au(C6Cl5)2(CH3CN)2]PF6 Complexes
Colorless crystals of [Au(C6F5)3(CH3CN)] were obtained as
one polymorph, unlike the two polymorphs of [Au-
(C6F5)3(tht)] due to the orientation of the bound tht ligand
(Figure 1). The gold−carbon distance trans to CH3CN,
Figure 2. Structure of [Au(C6Cl5)2(CH3CN)2]+ with ellipsoids given
at the 50% probability level. Bond distances (Å): Au(1)−N(1) =
2.080(15), Au(1)−N(2) = 2.045(17), Au(1)−C(1) = 1.980(17),
Au(1)−C(7) = 1.995(15). Bond angles (deg): C(1)−Au(1)−N(2) =
173.6(6), C(7)−Au(1)−N(1) = 175.8(6).
competition theory, which states that in four-coordinate
complexes, two soft ligands in mutually trans positions will
have a destabilizing effect on each other.14 There are a few
examples of perfluorinated aryl gold(III) complexes in which
the cis arrangement has been isolated. For example, the
reaction of cis-Bu4N[Au(C6F5)2Cl2] with AgClO4 in ether
formed the cis-[Au(C6F5)2(Et2O)2]ClO4 complex.9
The irreversible electrochemical oxidation of [Au(C6F5)2]−
in CH3CN/0.1 M [Bu4N]PF6 displayed two peaks at 0.71 and
1.21 V vs Ag/AgCl at a platinum working electrode (Figure 3).
Figure 1. Structure of [Au(C6F5)3(CH3CN)] complex with ellipsoids
given at the 50% probability level. Bond distances (Å): Au(1)−N(1) =
2.062(2), Au(1)−C(7) = 1.987(2), Au(1)−C(13) = 2.063(2),
Au(1)−C(1) = 2.063(2). Bond angles (deg): C(7)−Au(1)−N(1) =
178.51(9), C(1)−Au(1)−C(13) = 177.58(9).
1.987(2) Å, is slightly shorter than the other two identical Au−
C distances, 2.063(2) Å. The fluorine-directed crystal packing is
believed to be the reason for the complex stability. The notable
C(sp2)−F···H(CH3CN) bond distance of 2.33 Å leads to dimer
formation. Additional weaker F···H bonding of 2.85 Å added
more interactions and leads to a supramolecular arrangement.
The crystal structure of [Au(C6Cl5)2(CH3CN)2]PF6 shows
the ligands bonded to the gold center in a cis disposition
(Figure 2). The repulsive Cl···Cl interactions and the larger size
of the chlorine compared with the fluorine substituents are
believed to contribute to the structural differences in the
perhalogenated aryls. The modification in the aryls about the
gold(III) center to cis positions cannot rule out the ligand
scrambling. Kochi reported the trans-CH3CH2(CH3)2AuPPh3
to cis-CH3CH2(CH3)2AuPPh3 isomerization which is hypothe-
sized to occur via an intermolecular rearrangement involving a
tetrahedral intermediate.13
Figure 3. Cyclic voltammograms of the oxidation of 0.1 mM
[Au(C6X5)2]− (X = F, Cl) complexes in CH3CN/0.1 M [Bu4N]PF6
at a 100 mV/s scan rate vs Ag/AgCl.
Similarly, the complex [Au(C6Cl5)2]− showed irreversible
oxidation potentials at lower values of 0.65 and 0.91 V. It is
predictable that the strongly electronegative fluorine will show
higher oxidation potential compared with the chlorine
substituents. The electron-withdrawing effect of the C6F5
group is comparable with that of the haloborons BF3 and
BCl3.15 The [NO]+ chemical oxidation complex [Au-
(C6Cl5)2(CH3CN)2]PF6 is sparingly soluble in most organic
solvents, and it is likely that the electrochemical oxidation
complex blocks the electrode surface from further oxidation at
the second potential. This observation is in support of the
Square-planar gold(III) complexes of the general formula
[AuR2L2] exhibit antisymbiosis, displaying equal ligands in the
cis positions. This is in agreement with the Pearson π-
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* Supporting Information
CIF files and text giving details of the synthesis and
characterization data for the complexes [Au(C6F5)3(CH3CN)]
and [Au(C6Cl5)2(CH3CN)2]PF6. This material is available free
of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: amohamed@desu.edu. Tel: 302-857-6531.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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The NSF-SMILE (0928404), NSF-AMP (HRD-0903924), the
Center for Teaching and Learning of Delaware State
University, and the DGI (MEC)/FEDER (CTQ2010-20500-
C02-02) are acknowledged for the financial support of this
work.
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