Gold(III) diazonium complexes for electrochemical reductive grafting

Article abstract of DOI:10.1021/ic300307z

Gold(III) diazonium complexes were synthesized for the first time and studied for electrochemical reductive grafting. The diazonium complex [CN-4-C₆H₄N≡N]AuCl₄ was synthesized by protonating CN-4-C₆H₄

Full text of DOI:10.1021/ic300307z

Communication
pubs.acs.org/IC
Gold(III) Diazonium Complexes for Electrochemical Reductive
Grafting
Atiya T. Overton and Ahmed A. Mohamed*
Department of Chemistry, Delaware State University, 1200 N. DuPont Highway, Dover, Delaware 19901, United States
S
* Supporting Information
Au covalent bond.1 Density functional theory structure and
ABSTRACT: Gold(III) diazonium complexes were syn-
thesized for the first time and studied for electrochemical
reductive grafting. The diazonium complex [CN-4-
C6H4NN]AuCl4 was synthesized by protonating CN-
4-C6H4NH2 with chloroauric acid H[AuCl4]·3H2O to
form the ammonium salt [CN-4-C6H4NH3]AuCl4, which
was then oxidized by the one-electron oxidizing agent
[NO]PF6 in CH3CN. The highly irreversible reduction
potential of 0.1 mM [CN-4-C6H4NN]AuCl4 observed
at −0.06 V versus Ag/AgCl in CH3CN/0.1 M [Bu4N]PF6
encompasses both gold(0) deposition and diazonium
reduction. Repeated scans showed the absence of the
reduction peak on the second run, which indicates that
surface modification with a blocking gold aryl film has
occurred and is largely complete.
bonding studies between an aryl group and the gold surface
shows that the phenyl favors a C−Au σ bond in a vertical
pattern and facilitates the perpendicular growth of the aryl
layers compared with grafting to other metals such as iron.16
Conversely, the drawback of a modification approach based
on the aryldiazonium salts is their synthesis and isolation, which
is not always simple. More broadly, many diazonium complexes
are incompatible with the extremely harsh conditions for
diazotization, which involve either strong acids, oxidizing
agents, or both. In comparison with the extensive literature
on the synthesis and application of gold−nitrogen complexes,
gold diazonium complexes have not been synthesized.17 We
describe a facile procedure for the synthesis of stable gold(III)
diazonium complexes and the success in the in situ “synthesis”
of an aryl-modified gold surface. Thus, the synthesis of easily
reduced diazonium salts containing metal complex anion-based
anions is very important from the preparative and application
standpoint.
odification of surfaces by the electrochemical reduction
Diazotization necessitates an acidic medium, which can be
provided by the chloroauric acid H[AuCl4]·3H2O (Scheme 1).
M
of diazonium salts is a progressing area of materials
chemistry.1 Organic films generated by diazonium reduction
and grafting demonstrated a distinctive performance in the
formation of a superior corrosion inhibitor film on iron
surfaces,2 graphene surface modification,3 the preparation of
diazonium-modified enzyme electrodes,4 immobilization of
proteins,5 the grafting of polymers to surfaces,6 the attachment
of oligonucleotides to surfaces,7 and the printing of gold
surfaces via soft lithography.8 Heck reactions involving
diazonium salts have been extensively developed and widely
used in the synthesis of natural products and other organic
compounds.9
Scheme 1. Synthesis of Gold(III) Diazonium Salt 3
In the electrochemical reductive grafting process, one-
electron reduction of an aryldiazonium ion results in the
attachment of the aryl group to the electrode. Electrochemical
reduction of a cobaltocenium diazonium complex resulted in
the covalent attachment of the cobaltocenium ion to a glassy
carbon surface and the formation of an “organometallic
electrode”.10 Some of the limitations that arise from the low
stability of the aryldiazonium salts have been overcome by in
situ diazotization, followed by the cathodic deposition
process.10 Electrochemical reduction of the aryldiazonium
salts to grafted films has been studied at carbon,3,12 semi-
conductors,13 and metals.14
Despite the advantages of self-assembled monolayer grafting
to gold, there are some serious limitations, and an alternative
modification strategy has shown great success: diazonium
grafting.15 Strong and stable adhered films derived from
diazonium salt reduction on gold surfaces are due to the C−
The gold precursor plays several important roles in this
chemistry; it provides the acidic medium necessary in the
diazotization reactions, a stabilizing counteranion, and a gold
surface for aryl grafting. One more advantage of the chloroauric
acid is the ability to dissolve the amine completely as an
ammonium salt. The trial to protonate CN-4-C6H4NH2 with
H[AuCl4]·3H2O by suspending in water resulted in the loss of
one HCl molecule and the formation of the gold(III)
trichloride amine complex CN-4-C6H4NH2AuCl3 (1). Because
of the absence of the acidic medium necessary for diazotization,
Received: February 13, 2012
Published: April 26, 2012
© 2012 American Chemical Society
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Inorganic Chemistry
Communication
1 was not susceptible to oxidation by [NO]PF6 in CH3CN
(Supporting Information). However, the protonation of CN-4-
C6H4NH2 with H[AuCl4]·3H2O in CH3CN formed the
tetrachloroaurate ammonium salt [CN-4-C6H4NH3]AuCl4
(2), which was oxidized by [NO]PF6 to form the yellow
gold(III) diazonium salt [CN-4-C6H4NN]AuCl4 (3) in 92%
yield.
The gold(III) diazonium complex 3 was synthesized for the
first time in the present work and without the need for mineral
or organic acids. Although caution should be exercised when
handling all diazonium salts, there were no indications that this
compound is particularly hazardous. Complex 3 is stable to air
and light in the solid state and in a CH3CN solution. The IR
spectrum of 3 showed the νNN stretching frequency at 2277
cm−1, and the 1H NMR spectrum in CDCl3/dimethyl sulfoxide
displayed two doublets assigned to the phenyl protons at 7.71
and 8.33 ppm, which are noticeably shifted from 6.65 and 7.45
ppm in the 4-aminobenzonitrile ligand (Supporting Informa-
tion).
Figure 2. Structure of 2 at 50% probability. Bond distances (Å): C1−
N1 1.479(8) and C7−N2 1.142(10).
To the best of our knowledge, this work reports the first X-
ray structure of an amine-AuCl3 complex, 1 (Figure 1).16
Figure 3. Structure of 3 at 50% probability. Bond distances (Å): N2−
N3 1.087(5), N2−C1 1.414(6), and C7−N1 1.133(6). Bond angles
(deg): N3−N2−C1 177.2(5) and N1−C7−C4 177.4(5).
Figure 1. Structure of 1 at 50% probability. Bond distances (Å): Au1−
N2 2.073(9), C4−N2 1.443(13), and C7−N1 1.139(14). Bond angles
(deg): Au1−N2−C4 109.0(6), Cl3−Au1−Cl1 177.33(11), and Cl2−
Au1−N2 173.3(3).
The highly irreversible reduction potential of 0.1 mM 3
occurs at −0.06 V versus Ag/AgCl in CH3CN/0.1 M
[Bu4N]PF6 (Figure 4). The reduction potential of the
Coordination of gold(III) trichloride to the CN-4-C6H4NH2
ligand caused a difference in the bond angles about the gold
atom in the square-planar arrangement. The Cl−Au−N and
Cl−Au−Cl bond angles measure 173.3(3)° and 177.33(11)°,
respectively. Protonation of the amine by tetrachloroauric acid
resulted in a slight increase in the C−N bond distance,
1.479(8) Å in 2 versus 1.443(13) Å in 1 (Figure 2). The close
proximity to the NN group, the Cl···NN bond distance
3.23 Å, and the fact that the halide is less nucleophilic in the
tetrachloroaurate anion than the free chloride are presumably
the contributing factors for the increased stability of the
diazonium salt (Figure 3).
The irreversible electrochemical reduction of 0.1 mM
complex 2 in CH3CN/0.1 M [Bu4N]PF6 at a glassy carbon
electrode at a scan rate of 500 mV/s was observed at −0.56 V
versus Ag/AgCl. The prepeak at −0.1 V probably indicates the
two-step reduction of gold(III). The AuIII → Au0 reduction in 2
was confirmed by the presence of a black gold deposit on the
glassy carbon (GC) electrode surface after several runs. The
peak assigned to the gold(III) reduction is known to be
sensitive to the coordinating ligand, among many other
factors,18 and can proceed directly to gold(0) or in two
distinctive steps.
Figure 4. Cyclic voltammogram of 0.1 mM 3 at a GC electrode in
CH3CN/0.1 M [Bu4N]PF6 at 100 mV/s versus Ag/AgCl. Top:
Reduction at the first cycle. Bottom: Reduction at the second cycle.
diazonium tetrafluoroborate salt is observed at 0.16 V versus
SCE.12b The first scan of 3 showed the reduction of gold(III)
and diazonium and the formation of black deposits on the
glassycarbon working electrode surface. The absence of the
irreversible reduction peak after repeated scans indicates that
surface modification with a blocking gold−aryl film has
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Inorganic Chemistry
Communication
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oxidation in the anodic backward scan from 0 to +1.8 V
presumably because of its protection by the grafted aryl layer.
The oxidation of gold(0) films at ∼1.0 V has been reported in
gold halide complexes.18
The electrochemical reduction of the diazonium tetrafluor-
oborate salt [CN-4-C6H4NN]BF4 at a glassy carbon
electrode showed the one-electron peak at 0.16 V to the fifth
run, in contrast to the gold(III) diazonium salt, which is
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the gold and diazonium.
It cannot be concluded from this study whether gold was
completely deposited before the diazonium reduction. More
detailed studies are underway to support the sequence of the
deposition as concerted or stepwise. Chronoamperometry
measurements for 3 showed a sharp decrease in the current
within a short time (∼5 s), which indicates that the first
monolayer of grafted aryl is completed rather quickly. The
current change after the turning point is much smaller because
electron transfer through the first grafted aryl monolayer is
strongly suppressed. The modified electrode does not appear to
undergo a significant loss of the gold−aryl layer if it is washed
with organic solvents, and it retains its coverage even when
subjected to sonication in water for 1 h.
In summary, we synthesized a gold diazonium complex that
showed electrochemical reduction behavior typical of grafting
onto the in situ deposited gold film on a glassy carbon
electrode. Synthesis of the gold diazonium complex following
our procedure is simple and can be carried out without the
need for mineral or organic acids.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis, characterization, and CIF files for 1−3. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: amohamed@desu.edu. Phone 302-857-6531.
■
Notes
The authors declare no competing financial interest.
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6030−6031.
ACKNOWLEDGMENTS
■
(17) (a) Mohamed, A. A. Coord. Chem. Rev. 2010, 254, 1918−1947.
(b) Hashmi, S. K.; Weyrauch, J. P.; Rudolph, M.; Kurpejov, E. Angew.
Chem., Int. Ed. 2004, 43, 6545−6547.
The NSF-SMILE (0928404), NSF-AMP (HRD-0903924), and
CTL (Center for Teaching and Learning) of Delaware State
University are acknowledged for financial support of this work.
(18) Mohamed, A. A.; Bruce, A. E.; Bruce, M. R. Electrochemistry of
Gold and Silver Complexes. In Organic Derivatives of Gold and Silver;
Patai, S., Ed.; John Wiley & Sons: New York, 1999; pp 313−352.
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