Unprecedented Formation of a Binuclear Au(II)-Au(II) Complex through Redox State Cycling: Electrochemical Interconversion of Au(I)-Au(I), Au(II)-Au(II), and Au(I)-Au(III) in Binuclear Complexes Containing the Carbanionic Ligand C6F4PPh2

Article abstract of DOI:10.1021/acs.inorgchem.9b01983

The rational design of binuclear Au(I)-Au(I), Au(II)-Au(II), and Au(I)-Au(III) complexes requires an understanding of how the redox states interconvert. Herein, the electrochemical interconversion of the three oxidation states I, II, and III is reported o

Full text of DOI:10.1021/acs.inorgchem.9b01983

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Unprecedented Formation of a Binuclear Au(II)−Au(II) Complex
through Redox State Cycling: Electrochemical Interconversion of
Au(I)−Au(I), Au(II)−Au(II), and Au(I)−Au(III) in Binuclear Complexes
Containing the Carbanionic Ligand C6F4PPh2
Chencheng Sun,†,§,∥ Nedaossadat Mirzadeh,*,‡,∥ Si-Xuan Guo,† Jiezhen Li,† Zhengkui Li,§
Alan M. Bond,*,† Jie Zhang,*,† and Suresh K. Bhargava*,‡
†School of Chemistry, Monash University, Clayton, Victoria 3800, Australia
‡School of Science, RMIT University, GPO Box 2476, Melbourne, Victoria 3001, Australia
§School of the Environment, Nanjing University, Nanjing 210046, P. R. China
ABSTRACT: The rational design of binuclear Au(I)−Au(I),
Au(II)−Au(II), and Au(I)−Au(III) complexes requires an
understanding of how the redox states interconvert. Herein, the
electrochemical interconversion of the three oxidation states I, II,
and III is reported on the voltammetric (cyclic and rotating disk
electrode) time scales for binuclear gold complexes containing
C6F4PPh2 as a ligand, to demonstrate for the first time formation
of a binuclear Au(II)−Au(II) from a Au(I)−Au(III) complex.
Results are supported by bulk electrolysis and coulometry with
reaction products being identified by 31P NMR and UV−vis
spectroscopy. All electrochemical processes involve an overall two-
electron charge-transfer process with no one-electron intermediate
being detected. Importantly, the kinetically rather than thermo-
II
dynamically favored isomer [Au2 X2(μ-2-C6F4PPh2)2] is formed
on redox cycling of [XAuI(μ-2-C6F4PPh2)(κ2-2-C6F4PPh2)AuIIIX] (X = Cl, ONO2). Finally, a mechanism is proposed to explain
the simultaneous change of coordination of the chelating carbanionic ligand to bridging mode and interconversion of oxidation
states in binuclear gold complexes.
Changes in the ligand framework can cause subtle or
extreme changes in the chemistry of metal complexes
containing carbanion ligands of the type 2-C6R4ER′2 (R =
hydrogen, halogen, alkyl; E = phosphorus, arsenic; R′ = phenyl,
alkyl).3,9 For example, the 6-methyl substituted complex
[Au2(μ-C6H3-6-Me-2-PPh2)2] behaves similarly to its tetra-
fluoro analogue. However, isomerization from Au(II)−Au(II)
to Au(I)−Au(III) is much faster in the 6-MeC6H3-2-PPh2
system than in the 2-C6F4PPh2 one; the former occurs below
room temperature within minutes, whereas the latter requires
hours in hot toluene.10 These data imply that Au(I)−Au(III) is
the thermodynamically favored isomer.
Intriguingly, replacement of fluorine by hydrogen atoms in
the carbanion leads to the formation of the C−C coupled
Au(I)−Au(I) complex [Au2X2(2,2′-Ph2PC6H4C6H4PPh2)] as
the result of reductive elimination reaction at a Au(II)−Au(II)
center,11,12 a completely different reduction pathway to that
observed in the C6F4PPh2 system.4
INTRODUCTION
■
In addition to a variety of ligands which have played significant
roles in the development of gold chemistry,1,2 the use of
−
fluorine-substituted carbanions 2-C6F4PPh2 has enabled the
preparation of a diverse range of mono- and binuclear gold
complexes in which the formal oxidation state of the metal is I,
II, III, or mixed I/III.3−8 The ligand has established itself
recently as a desirable motif for preparation of gold-based
therapeutic agents.8
Digold(I) complexes of the type [Au2(μ-2-C6F4PPh2)2] (1,
in Scheme 1)3,4 are prone to oxidation and can readily undergo
two-center, two-electron oxidative addition reactions with
halogens to form Au(II)−Au(II) complexes [Au2X2(μ-2-
C6F4PPh2)2] (X = halide) [X = Cl (2), Scheme 1].4
Compound 2 is thermodynamically unstable and rearranges
in solution to form the Au(I)−Au(III) complex [ClAu(μ-2-
C6F4PPh2)(κ2-2-C6F4PPh2)AuCl] 3 (Scheme 1), in which
Au(I) is linearly coordinated by a halide atom and the donor
atom of the bridging group and the Au(III) atom is
coordinated by a halide atom and the carbanion in a four-
membered chelate ring.4
Received: July 4, 2019
© XXXX American Chemical Society
A
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Inorganic Chemistry
Article
I
2-C6F4PPh2)2] (2) to [Au2 (μ-2-C6F4PPh2)2] (1) along with
Scheme 1. Interconversion of Oxidation States I, II, and III
in Binuclear Gold Complexes Containing the C6F4PPh2
elimination of Cl−, as outlined in eq 1.
−
Ligand
II
[Cl2Au2 (μ‐2‐C6F4PPh2)2] (2) + 2e−
→ [Au2 (μ‐2‐C6F4PPh2)2] (1) + 2Cl−
I
(1)
II
The digold(II)−dinitrate complex [(ONO2)2Au2 (μ-2-
C6F4PPh2)2] (4) in CH2Cl2 displays an analogous irreversible
reduction process (labeled a) under conditions of cyclic
voltammetry but now at −0.43 V, which is 0.46 V more
positive than 2, reflecting the easier reducibility of the Au(II)−
Au(II) center with the axial ligand of ONO2. However,
whether the origin of this difference is thermodynamic or
kinetic is unknown. For 4, only a single new oxidation process
(labeled b) is observed in the second positive potential scan at
a potential of 1.15 V, which is attributed to the oxidation of
digold(I) back to digold(II). Process a is proposed to be the
overall two-electron reduction of [(ONO2)2Au2II(2-μ-
I
C6F4PPh2)2] (4) to [Au2 (2-μ-C6F4PPh2)2] (1) with elimi-
nation of NO3 , as outlined in eq 2, with process b being the
oxidation of the digold(I) species to reform 4.
−
Our interest in exploring the extensive redox behavior of
binuclear gold complexes containing 2-C6F4PPh2 has now
resulted in an electrochemical study which has provided the
first observation of interconversion of oxidation states I, II, and
III in digold complexes containing the carbanionic ligand
C6F4PPh2; a detailed account is presented here.
II
[(ONO2)2Au2 (2‐μ‐C6F4PPh2)2] (4) + 2e−
→ [Au2 (2‐μ‐C6F4PPh2)2] (1) + 2NO3−
I
(2)
Electrochemical oxidation of either 2 or 4 was not observed
within the anodic limit of the solvent potential window. This is
not surprising since oxidation of Au(I)−Au(I) to Au(II)−
Au(II) already requires very positive potentials as shown in the
second cycle of Figure 1.
RESULTS AND DISCUSSION
■
Cyclic Voltammetry of Dinuclear Gold Complexes
II
II
[Cl2Au2 (μ-2-C6F4PPh2)2] (2) and [(ONO2)2Au2 (μ-2-
C6F4PPh2)2] (4): Influence of the Axial Ligand. Cyclic
voltammograms at a scan rate of 2 V s−1 were obtained with
1.0 mM 2 (Figure 1A) or 1.0 mM 4 (Figure 1B) at a glassy
Cyclic Voltammetry of [ClAuI(μ-2-C6F4PPh2)(κ2-2-
C6F4PPh2)AuIIICl] (3) and [(O2NO)AuI(μ-2-C6F4PPh2)(κ2-
2-C6F4PPh2)AuIII(ONO2)] (5). The cyclic voltammograms
obtained at a scan rate of 2 V s−1 with 1.0 mM 3 (Figure
2A) or 1.0 mM 5 (Figure 2B) at a glassy carbon electrode in
Figure 1. Cyclic voltammograms obtained at a scan rate of 2 V s−1 for
(A) 1.0 mM [Cl2Au2II(μ-2-C6F4PPh2)2] (2) or (B) 1.0 mM
II
[(ONO2)2Au2 (μ-2-C6F4PPh2)2] (4) at a glassy carbon electrode
(diameter of 1 mm) in CH2Cl2 (0.1 M Bu4NPF6).
Figure 2. Cyclic voltammograms obtained at a scan rate of 2 V s−1
with a glassy carbon electrode (diameter of 1 mm) in CH2Cl2 (0.1 M
Bu4NPF6) for (A) 1.0 mM [ClAuI(μ-2-C6F4PPh2)(κ2-2-C6F4PPh2)-
AuIIICl] (3) and (B) 1.0 mM [(O2NO)AuI(μ-2-C6F4PPh2)(κ2-2-
C6F4PPh2)AuIII(ONO2)] (5).
carbon electrode in CH2Cl2 (0.1 M Bu4NPF6). For 2, when the
potential was initially scanned positively from the open circuit
value of −0.30 to 1.35 V vs Fc0/+, no oxidation process was
observed. However, on reversing the scan direction, a well-
defined irreversible reduction process (labeled 1) was detected
at −0.89 V. In the second scan, in the positive potential
direction, two new oxidation processes (labeled 2 and 3) were
detected at 0.91 and 1.20 V and are associated with oxidation
of the products of reduction process 1. Oxidation process 3 is
coincident with oxidation of Cl− from Et4NCl or PPh4Cl,
implying that Cl− is released upon reduction of 2.
Consequently, process 2 corresponds to the oxidation of
digold(I) back to digold(II). Reduction process 1 therefore is
CH2Cl2 (0.1 M Bu4NPF6) differ significantly from 2 and 4. 3
exhibited an irreversible reduction process (labeled 4′) at
−1.68 V, which is 0.79 V more negative than that for
II
[Cl2Au2 (2-μ-C6F4PPh2)2] (2), reflecting the greater difficulty
of reducing the Au(III)−carbon δ-bond. However, in the
reverse positive direction scan, the two oxidation processes
(labeled 2′ and 3′) occur at very similar potentials to those for
2 and 3 in Figure 1A, implying that reduction process 4′
generates the same products as reduction process 1. However,
following the oxidation processes 2′ and 3′, in the second
II
attributed to an overall two-electron reduction of [Cl2Au2 (μ-
B
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Article
negative potential direction scan, a second reduction process
working electrode (0.0707 cm2); D and c* are the diffusion
coefficient and concentration of the gold compound,
respectively; ω (=2πf) is the angular frequency of rotation;
and ν is the kinematic viscosity of CH2Cl2 (at 20 °C, ν =
viscosity/density = 0.43/1.3266 cm2 s−1 = 0.32 cm2 s−1).
RDE voltammograms for compounds 2 and 4 are shown in
Figure 3 as a function of rotation rate. The D values obtained
from the Levich equation using n = 2.0 were (1.2 0.1) ×
10−5 (2) and (1.3 0.1) × 10−5 cm2 s−1 (3, 4, 5).
(labeled 1′) emerged at a potential of −0.82 V which is
II
consistent with reduction of [Cl2Au2 (2-C6F4PPh2)2] (2).
When the potential was switched before oxidation process
3′, reduction process 1′ remained. However, if the potential
was switched prior to oxidation process 2′, reduction process
1′ was absent. The results imply that reduction process 1′ is
derived from the product of oxidation process 2′. Thus,
oxidation processes 2 and 2′ are both due to the oxidation of
I
II
[Au2 (2-C6F4PPh2)2] (1) to [Au2 Cl2(2-C6F4PPh2)2] (2), so
that on redox cycling, the kinetically stable isomer 2 is favored
over the thermodynamically favored one, as in eqs 3 and 4.
[ClAuI(μ‐2‐C6F4PPh2)(κ2‐2‐C6F4PPh2)AuIIICl] (3) + 2e−
I
→ [Au2 (μ‐2‐C6F4PPh2)2] (1) + 2Cl−
(3)
I
[Au2 (μ‐2‐C6F4PPh2)2] (1) + 2Cl−
→ [Cl2Au2 (μ‐2‐C6F4PPh2)2] (2) + 2e−
II
(4)
[(O2NO)AuI(μ-2-C6F4PPh2)(κ2-2-C6F4PPh2)AuIII(ONO2)]
(5) in CH2Cl2 displays (Figure 2B) an irreversible reduction
process (labeled c′) at −1.41 V, which is 0.98 V more negative
(Figure 1B) than for digold(II)-(ONO2)2, establishing the
greater difficulty in reducing the Au(III)−carbon δ-bond.
However, this peak potential is 0.27 V less negative than the
reduction peak for digold(I,III)-Cl2, reflecting the easier
reducibility when the axial ligand is −ONO2. For 5, on the
second scan in the positive potential direction, an oxidation
process (labeled b′) is observed at a similar potential to that
for process b in Figure 1B. Additionally, after oxidation process
b′ and reversing the scan direction, a new reduction process
(labeled a′) emerges at a potential −0.42 V, consistent with
Figure 3. RDE voltammograms obtained as a function of rotation rate
for reduction of 1.0 mM [Cl2Au2 (μ-2-C6F4PPh2)2] (2) in CH2Cl2
II
(0.1 M Bu4NPF6) with a 3 mm diameter GC rotating disk electrode.
Scan rate = 20 mV s−1.
Coulometric analysis of the current−time derived from
controlled potential bulk electrolysis of the gold compounds
gives the charge (Q, coulomb) calculated by eq 8,11 where V is
the volume of the solution (10 mL). n values of 1.8 0.2 (2),
1.9 0.2 (3), 1.9 0.2 (4), and 2.1 0.2 (5) were obtained
via coulometric analysis.
II
reduction of [(ONO2)2Au2 (μ-2-C6F4PPh2)2] (4). When the
potential was switched prior to oxidation process b′, reduction
process a′ was absent confirming that reduction process a′ is
associated with the product of oxidation process b′. This again
confirms that the oxidation processes b and b′ are both due to
I
the oxidation of [Au2 (2-μ-C6F4PPh2)2] (1). Again, on redox
Q = nFVc*
(8)
cycling, the thermodynamically favored Au(I)−Au(III) isomer
is converted to the kinetically favored isomer, as in eqs 5 and 6.
It is likely that the overall electroreduction of digold(II)−
dichloride proceeds by a one-electron reduced intermediate.
However, cyclic voltammetry of digold(II)−dichloride with a
50 μm diameter Pt microelectrode at scan rates up to 100 V
s−1 and lower temperature (−15 °C) did not allow detection of
intermediates, probably due to their very high level of
reactivity.
[(O2NO)AuI(μ‐2‐C6F4PPh2)(κ2‐2‐C6F4PPh2)AuIII
(ONO2)] (5) + 2e− → [Au2 (2‐μ‐C6F4PPh2)2] (1)
I
+ 2NO3−
(5)
Characterization of Products after Bulk Electrolysis
by RDE Voltammetry, UV−Vis, and 31P NMR Spectros-
copies. In order to unequivocally identify the product(s)
derived from the irreversible reduction of 2, 3, 4, or 5, bulk
electrolysis experiments were undertaken with 1.0 mM of gold
compound using a glassy carbon tube working electrode. RDE
voltammetry along with UV−vis and 31P NMR spectroscopy
on solutions obtained after bulk electrolysis were used to
identify the products.
I
[Au2 (μ‐2‐C6F4PPh2)2] (1) + 2NO3−
→ [(ONO2)2Au2 (μ‐2‐C6F4PPh2)2] (4) + 2e−
II
(6)
Determination of the Diffusion Coefficients and
Number of Electrons Transferred. In order to quantita-
tively determine the diffusion coefficients and the number of
electrons transferred in the reduction of the four dinuclear gold
compounds, steady-state voltammetric data were obtained and
analyzed in combination with the chronoamperometric
(coulometric) data derived from bulk electrolysis. The
steady-state limiting current, Iss, obtained at a rotating disk
electrode is given by the Levich equation13 (eq 7)
II
Bulk electrolysis of [Cl2Au2 (μ-2-C6F4PPh2)2] (2) in
CH2Cl2 with an applied potential of −0.9 V (vs Fc0/+) led to
a color change from yellow to colorless. As shown in Figure
4A, RDE voltammetry before and after the reductive bulk
electrolysis of 2 leads to transition from a fully reductive to a
fully oxidative process. Bulk electrolysis of digold(I,III)−
dichloride (3) with an applied potential of −1.8 V (vs Fc0/+)
also exhibits an almost complete transition from a reductive to
Iss = 0.62nFAD2/3ω1/2ν−1/6c*
(7)
where n is the total number of electrons transferred; F is
Faraday’s constant (96485 C mol−1); A is the area of the
C
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Inorganic Chemistry
Article
−
condition can lead to PF6 hydrolysis. It is now spectroscopi-
cally confirmed that reduction of [ClAuI(μ-2-C6F4PPh2)(κ2-2-
C6F4PPh2)AuIIICl] (3) in CH2Cl2 results in the cleavage of the
I
Au(III)−carbon δ-bond and the generation of [Au2 (2-
C6F4PPh2)2] (1), as shown in eq 3.
Bulk electrolysis of digold(I,III)−dinitrate (5) at −1.5 V (vs
Fc0/+) in CH2Cl2 (0.1 M Bu4NPF6) gave a 31P NMR spectrum
which also contains a singlet at δ 41.4, along with a resonance
I
at δ 44.8, assigned to tetragold(I) [Au4 (μ-2-C6F4PPh2)4] (6)
4
−
(Figure 5), also obtained a very small amount of PO2F2 , a
Figure 4. RDE voltammograms obtained before and after reductive
II
bulk electrolysis of (A) 1.0 mM [Cl2Au2 (μ-2-C6F4PPh2)2] (2) and
I
(B) 1.0 mM [ClAuI(μ-2-C6F4PPh2)(κ2-2-C6F4PPh2)AuIIICl] (3) in
CH2Cl2 (0.05 M Bu4NPF6) using a scan rate of 20 mV s−1 and
rotation rate of 1000 rpm. UV−visible spectra of (C) 0.05 mM
Figure 5. Molecular structure of tetragold(I) [Au4 (μ-2-C6F4PPh2)4]
(6). Ellipsoids show 30% probability levels, and hydrogen atoms have
been omitted for clarity. Only the ipso carbons of the phenyl groups
attached to the phosphorus atoms are shown. Only the two carbon
atoms of the C6F4 ring that form the macrocycle are shown.
II
[Cl2Au2 (μ-2-C6F4PPh2)2] (2) and (D) 0.05 mM [ClAuI(μ-2-
C6F4PPh2)(κ2-2-C6F4PPh2)AuIIICl] (3) before (black line) and after
(red line) exhaustive electrolysis. The spectrum of 0.05 mM digold(I)
(blue line) is also shown for comparison. For the electrolysis of
II
−
[Cl2Au2 (μ-2-C6F4PPh2)2] (2), the potential was held at −0.9 V,
hydrolysis product of PF6 (see above). Thus, during the
while for [ClAuI(μ-2-C6F4PPh2)(κ2-2-C6F4PPh2)AuIIICl] (3), the
exhaustive reduction of [(O2NO)AuI(μ-2-C6F4PPh2)(κ2-2-
potential was held at −1.8 V vs Fc0/+
.
C6F4PPh2)AuIII(ONO2)] (5), cleavage of the Au(III)−carbon
I
δ-bond to generate [Au2 (μ-2-C6F4PPh2)2] (1) occurs along
with the formation of [Au4 (μ-2-C6F4PPh2)4] (6), the 31P
I
an oxidative process (Figure 4B) when monitored by RDE
voltammetry. Figures 4C and D show the UV−visible spectra
NMR of which appears at 44.8, ca. 4 ppm higher frequency
from that for 1.4 Compounds 1 and 6 are formed in a 2:1 ratio
under electrochemical reduction conditions; repeated chemical
preparations of 6 give mixtures of 1 and 6 in varying
proportions.4
of [Cl2Au2 (μ-2-C6F4PPh2)2] (2) and [ClAuI(μ-2-C6F4PPh2)-
II
(κ2-2-C6F4PPh2)AuIIICl] (3), respectively, before and after
bulk electrolysis of 1.0 mM solutions followed by a 20-fold
I
dilution. The spectrum of 0.05 mM [Au2 (μ-2-C6F4PPh2)2]
(1) is also shown for comparison.
From these results it appears that the rearrangement of
Au(I)−Au(III) to Au(II)−Au(II) proceeds via σ-aryl migra-
tion. One-ended dissociation of the chelating group allows the
carbanion to transfer from one gold atom and reassociate with
the neighboring gold atom via a two-electron, three-center
transition (stage A, shown in Scheme 1). This transition state
is supported by an ab initio calculation of the isomerization of
Au(II)−Au(II) complexes [Au2X2(μ-2-carbanion)2] (X = Cl,
The 31P NMR spectrum after bulk electrolysis of digold-
(II)−dichloride (2) contains a resonance at δ 41.4, together
with the expected septet from the supporting electrolyte anion
[PF6]−. The resonance at δ 41.5 closely matches the 31P NMR
spectrum of digold(I). Bulk electrolysis of digold(II)−dinitrate
4 in CH2Cl2 with an applied potential of −0.5 V (vs Fc0/+) also
led to a color change from yellow to colorless. The 31P NMR
spectrum after bulk electrolysis contained a resonance at δ
41.4, which again corresponds to formation of the digold(I)
compound (1). NMR spectroscopy identification of the
products therefore confirms that the reduction of
Br, I; carbanion = C6H4PPh2, C6H3-n-Me-2-PPh2 n = 5, 6).14
A
related mechanism is assumed to apply when 3 (or 5) is
reduced to 2 (or 4) and 1.
Comparison with the Chemical Reduction of the
Dinuclear Gold Complexes. Based on the electrochemical
observations, digold(II)−dichloride (2) is much easier to
reduce than digold(I,III)−dichloride (3). Consistent with
these observations, 2 but not 3 is reduced to the parent
digold(I) 1 (31P NMR evidence) by zinc powder, which is a
moderately strong reductant. In contrast, digold(I,III)−
dinitrate (5) undergoes rapid and clean reduction to 1, in
the presence of sodium ethanethiolate. No intermediates were
identified during the course of reaction by 31P NMR
spectroscopy. Interestingly, compound 3 is reduced to 1 in
II
II
[Cl2Au2 (μ-2-C6F4PPh2)2] (2) and [(ONO2)2Au2 (μ-2-
C6F4PPh2)2] (4) occur be an overall two-electron reductive
I
elimination reactions to give [Au2 (μ-2-C6F4PPh2)2] (1) with
cleavage of the Au(II)−Au(II) bond and the Au(II)−Cl or
Au(II)−ONO2 bond, respectively.
The 31P NMR spectrum after bulk electrolysis of digold-
(I,III)−dichloride (3) contains a singlet at δ 41.4 (correspond-
ing to the digold(I) compound) and a triplet at δ −9.0, −14.9,
−
−
and −20.8, due to PO2F2 , a hydrolysis product of PF6 .
Reduction at very negative potentials under bulk electrolysis
D
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The reference potentials were calibrated to that of the ferrocene/
ferricenium (Fc0/+) couple.
the presence of sodium ethanethiolate, although it is noted that
this reaction involves the formation and subsequent reaction of
intermediate species evident from 31P NMR spectra.
AUTHOR INFORMATION
■
CONCLUSION
Corresponding Authors
■
*E-mail: nedaossadat.mirzadeh@rmit.edu.au.
*E-mail: alan.bond@monash.edu.
*E-mail: jie.zhang@monash.edu.
*E-mail: suresh.bhargava@rmit.edu.au.
In conclusion, we have presented the first electrochemical
investigation of interconversion of various oxidation states (I,
II, and III) in binuclear gold complexes containing the 2-Br-
C6F4PPh2 ligand. This ligand is well-established in organogold
chemistry and displays two coordination modes, chelate and
bridging; the interconversion of the former to the latter has not
been reported until now.3,5 We also report the electrochemical
formation of the kinetically rather than thermodynamically
ORCID
Nedaossadat Mirzadeh: 0000-0002-3092-2634
Alan M. Bond: 0000-0002-1113-5205
Jie Zhang: 0000-0003-2493-5209
Suresh K. Bhargava: 0000-0002-3127-8166
Author Contributions
The manuscript was written through contributions of all
authors.
II
favored isomer gold(II)−gold(II) [Au2 X2(μ-2-C6F4PPh2)2]
from the redox cycling of gold(I)−gold(III) [XAuI(μ-2-
C6F4PPh2)(κ2-2-C6F4PPh2)AuIIIX] (X = Cl, NO3). A mecha-
nism to describe this rare behavior has been proposed.
An understanding of how gold I, II, and III oxidation states
interconvert provides a pathway for the rational design of gold
complexes. This report aims at realizing the potential of
manipulation of oxidation states in gold complexes, a critical
factor in fine-tuning their chemical properties.
Author Contributions
∥C.S. and N.M. contributed equally.
Notes
The authors declare no competing financial interest.
EXPERIMENTAL SECTION
■
ACKNOWLEDGMENTS
■
Chemicals and Reagents. Dichloromethane (CH2Cl2, Aldrich)
is of HPLC grade, and acetone (BDH) is of laboratory reagent grade.
Ferrocene (BDH), tetraethylammonium chloride (Et4NCl), tetraphe-
nylphosphonium chloride (PPh4Cl), zinc powder, and sodium
ethanethiolate (Sigma-Aldrich) were used as received. Tetrabutylam-
monium hexafluorophosphate (Bu4NPF6) (GFS) was purified
according to literature procedures15 and used as the supporting
electrolyte in dichloromethane.
Authors acknowledge Dr. Nedaossadat Mirzadeh for use of her
artwork ‘wheel of gold conversion’ as graphical abstract.
REFERENCES
■
(1) Schmidbauer, H. Gold: chemistry, biochemistry and technology, 1st
ed.; Wiley: England, 1999.
(2) Gold chemistry: applications and future directions in the life sciences;
Mohr, F., Ed.; Wiley: Germany, 2009.
Instrumentation. 31P NMR spectra, referenced to external 85%
H3PO4 (31P), were recorded on a Bruker DPX 400 spectrometer at a
frequency of 121 MHz. UV−visible spectra over the wavelength range
of 250 to 800 nm were obtained with a Cary 5 spectrophotometer
(Varian OS/2 software) using a 1 mm path length quartz cuvette.
Preparation of Dinuclear Gold Complexes. The digold
complexes 1−5 were prepared following established literature
procedures.4
(3) Mirzadeh, N.; Bennett, M. A.; Bhargava, S. K. Cycloaurated
complexes of aryl carbanions: digold(I), digold(II) and beyond.
Coord. Chem. Rev. 2013, 257, 2250−2273.
́
(4) Bennett, M. A.; Bhargava, S. K.; Mirzadeh, N.; Priver, S. H.;
Wagler, J.; Willis, A. C. Synthesis and interconversions of digold(I),
tetragold(I), digold(II), gold(I)-gold(III) and digold(III) complexes
of fluorine-substituted aryl carbanions. Dalton Trans 2009, 7537−
7551.
Electrochemical Instrumentation and Procedures. Voltam-
metric measurements were conducted in nitrogen degassed CH2Cl2
using a CHI 760E electrochemical workstation (CH Instruments,
Austin, TX). A silver wire immersed in a glass tube containing 0.1 M
Bu4NPF6 in CH2Cl2 with a frit was used as a quasi-reference
electrode. For cyclic voltammetric studies, a glassy carbon disk
electrode with a diameter of 1 mm was used as the working electrode,
and a platinum wire was used as the auxiliary electrode. For rotating
disk electrode (RDE) studies, an ALS rotating disk electrode rotator
(RRDE-3A) was connected to the electrochemical workstation. In
this case, a 3 mm diameter GC disk was used as the working
electrode, along with the same platinum wire auxiliary and reference
electrodes used for cyclic voltammetry. Bulk electrolysis was
performed inside a nitrogen filled glovebox (oxygen- and moisture-
free system). The glassy carbon tube working electrode and the
reference electrode were separated from the platinum gauze auxiliary
electrode by a fine porosity frit. Ten mL of 1.0 mM of dinuclear gold
complex dissolved in CH2Cl2 (0.05 M Bu4NPF6) was placed in the
inner compartment which contained the working and reference
electrodes, while 15 mL of CH2Cl2 with 0.05 M Bu4NPF6 was placed
in the outer compartment which contained the platinum gauze
auxiliary electrode. During the course of bulk electrolysis, the
solutions in both compartments were magnetically stirred. Prior to
use, the GC electrodes were polished using an aqueous 0.3 μm Al2O3
slurry (Buehler), rinsed thoroughly with distilled water, sonicated,
rinsed with water again, finally rinsed with acetone, and dried under
nitrogen. All electrochemical studies were undertaken at 20 1 °C.
́
(5) Bennett, M. A.; Bhargava, S. K.; Mirzadeh, N.; Priver, S. H. The
use of [2-C6R4PPh2]−(R = H, F) and related carbanions as building
blocks in coordination chemistry. Coord. Chem. Rev. 2018, 370, 69−
128.
(6) Bennett, M. A.; Bhargava, S. K.; Takahashi, M.; Mirzadeh, N.;
197
́
̈
Au Mossbauer Spectroscopic Studies of Cyclo-
Priver, S. H.
metalated Gold Dimers Containing 2-C6F4PPh2 Ligands. Bull. Chem.
Soc. Jpn. 2009, 82 (12), 1506−1509.
(7) Bhargava, S. K.; Bennett, M. A.; Mirzadeh, N.; Priver, S. H.;
́
Wagler, J. Trinuclear Mixed-valent Gold Complexes Derived from 2-
C6F4PPh2: Phosphine Oxide Complexes of Gold(III) and an Ortho-
Metallated Complex of Gold(I). Z. Naturforsch., B: J. Chem. Sci. 2009,
64b, 1463−1468.
́
(8) Reddy, T. S.; Priver, S. H.; Mirzadeh, N.; Bhargava, S. K.
Synthesis of gold(I) phosphine complexes containing the 2-
BrC6F4PPh2 ligand: Evaluation of anticancer activity in 2D and 3D
spheroidal models of HeLa cancer cells. Eur. J. Med. Chem. 2018, 145,
291−301.
́
(9) Mohr, F.; Priver, S. H.; Bhargava, S. K.; Bennett, M. A. Ortho-
metallated transition metal complexes derived from tertiary phosphine
and arsine ligands. Coord. Chem. Rev. 2006, 250, 1851−1888.
(10) Bhargava, S. K.; Mohr, F.; Bennett, M. A.; Welling, L. L.; Willis,
A. C. Ortho-metallated transition metal complexes derived from
tertiary phosphine and arsine ligands. Organometallics 2000, 19,
5628−5635.
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B. Coupling of cyclometalatedphenylphosphanes in dinuclear gold(II)
complexes. Angew. Chem., Int. Ed. Engl. 1987, 26, 260−261.
(12) Bennett, M. A.; Bhargava, S. K.; Hockless, D. S. R.; Welling, L.
L.; Willis, A. C. Dinuclearcycloauratedcomplexes containing bridging
(2-diphenylphosphino)phenylphosphine and (2-diethylphosphino)-
phenylphosphine, C6H4PR2 (R = Ph, Et). carbon-carbon bond
formation by reductive elimination at a gold(II)-gold(II) center. J.
Am. Chem. Soc. 1996, 118, 10469−10478.
(13) Bard, A. J.; Faulkner, L. R. Electrochemical Methods:
Fundamentals and Applications, 2nd ed.; John Wiley & Sons, Inc.:
New York, 2001.
(14) Hermann, H. L.; Schwerdtfeger, P.; Mohr, F.; Bhargava, S. K.
Unusual strong ortho effects in the rearrangement of binuclear
gold(I) complexes. Organometallics 2003, 22, 2373−2377.
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electroanalytical chemistry; Marcel Dekker: New York, 1984;
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