Dinuclear Au(I), Au(II) and Au(III) Complexes with (CF2)n Chains: Insights into The Role of Aurophilic Interactions in the Au(I) Oxidation

Article abstract of DOI:10.1002/chem.202103153

New dinuclear Au(I), Au(II) and Au(III) complexes containing (CF₂)_n bridging chains were obtained. Metallomacrocycles [Au₂{μ-(CF₂)₄}{μ-diphosphine}] show an uncommon figure-eight structure, the helicity inversion barrier of which is influenced by aurophilic interactions and steric constraints imposed by the diphosphine. Halogenation of LAu(CF₂)₄AuL (L=PPh₃, PMe₃, (dppf)_1/2, (binap)_1/2) gave [Au(II)]₂ species, some of which display unprecedented folded structures with Au?Au bonds. Aurophilic interactions facilitate this oxidation process by preorganizing the starting [Au(I)]₂ complexes and lowering its redox potential. The obtained [Au(II)]₂ complexes undergo thermal or photochemical elimination of R₃PAuX to give Au(III) perfluorinated auracycles. Evidence of a radical mechanism for these decomposition reactions was obtained.

Full text of DOI:10.1002/chem.202103153

Accepted Article
Accepted Article
Title: Dinuclear Au(I), Au(II) and Au(III) Complexes with (CF2)n Chains:
Insights into the Role of Aurophilic Interactions in the Au(I)
Oxidation
Authors: Juan Gil-Rubio, Alejandro Portugués, and Delia Bautista
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To be cited as: Chem. Eur. J. 10.1002/chem.202103153
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01/2020

10.1002/chem.202103153
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Dinuclear Au(I), Au(II) and Au(III) Complexes with (CF2)n Chains:
Insights into the Role of Aurophilic Interactions in the Au(I)
Oxidation
Alejandro Portugués,[a] Delia Bautista[b] and Juan Gil-Rubio*[a]
[a]
A. Portugués, Dr. J. Gil-Rubio
Departamento de Química Inorgánica, Facultad de Química. Universidad de Murcia. Campus de Espinardo. 30100 Murcia (Spain)
E-mail: jgr@um.es
[b]
Dr. D. Bautista
ACTI. Universidad de Murcia. Campus de Espinardo. 30100 Murcia (Spain)
Supporting information for this article is given via a link at the end of the document.
Abstract: New dinuclear Au(I), Au(II) and Au(III) complexes
containing (CF2)n bridging chains were obtained. Metallomacrocycles
[Au2{µ-(CF2)4}{µ-diphosphine}] show an uncommon figure-eight
structure, whose helicity inversion barrier is influenced by aurophilic
interactions and steric constraints imposed by the diphosphine.
A
A
R2P
AuI
PR2
AuI
R2P
PR2
YZ
AuII AuII
Y
Z
Oxidative
addition
X
X
X
X
Halogenation of LAu(CF2)4AuL (L = PPh3, PMe3, (dppf)1/2, (binap)1/2
)
Rearrangements
gave [Au(II)]2 species, some of which display unprecedented folded
structures with Au–Au bonds. Aurophilic interactions facilitate this
oxidation process by preorganizing the starting [Au(I)]2 complexes
and lowering its redox potential. The obtained [Au(II)]2 complexes
undergo thermal or photochemical elimination of R3PAuX to give
Au(III) perfluorinated auracycles. Evidence of a radical mechanism
for these decomposition reactions was obtained.
A
A
A
R2P
AuI
PR2
R2P
AuI
PR2
R2P
PR2
Y
Z
Z
AuIII
X
AuIII
AuIII AuI
X
Y
X
X
Y
X
X
Z
Reductive elimination
PR2
A
R2P
Coupling
products
Introduction
AuI
AuI
+
X-X, X-Y
or XZ
= X, Y or Z
The applications of gold compounds in Catalysis,[1–5] Medicine,[6–
9]
and Materials Science[10–12] have triggered an spectacular
development of the Organometallic and Coordination Chemistry
of gold during the last two decades.[13–16] One of the most
captivating features of gold is aurophilicity, defined as the
tendency of closed-shell Au centers to approach at short
distances. Abundant manifestations of this phenomenon have
been found in the conformations and supramolecular arrays of
gold compounds, mainly in crystal structures, but also in
solution.[17–21] Besides, these interactions influence the
catalytic[22–26] properties of gold complexes. Thus, complexes
containing two AuI centers connected by a three-atom bridging
ligand show enhanced activity in gold-catalyzed coupling
reactions, compared to their mononuclear congeners.[23,25,26] In
these systems, the aurophilic interaction enables oxidative
addition of YZ reagents to the AuI···AuI unit, to give [AuII–AuII]
intermediates,[23,26] which can rearrange to [AuI, AuIII] species
(Scheme 1). Fast reductive elimination on the AuIII centers
produces coupling products.[27,28]
AuII complexes are much less abundant than AuI or AuIII
ones. Most reported examples present a Au–Au covalent bond,
and are obtained by oxidative addition of halogens or similar
oxidants to a AuI···AuI unit containing one or two short bridging
ligands.[29–31] Although Au–Au bond energies around 200 KJ/mol
have been theoretically[32] and experimentally[33] determined, a
very small number of stable [AuII–AuII] complexes without
supporting bridging ligands are known.[34–40]
Scheme 1. Sequence of oxidative addition and reductive elimination reactions
leading to coupling products in dinuclear Au complexes.
Despite the catalytic relevance of AuII complexes,[22] its reactivity
has been much less explored compared to that of AuI or AuIII
derivatives.[29,31,37,39] Thermal rearrangement to mixed-valent
[AuIII, AuI] species has been commonly observed,[41–44] while
photochemical disproportionation[40] or reductive elimination[27,45–
47] reactions have been rarely reported.
We have recently shown that complexes LAu(CF2)4AuL (L =
PPh3 or PMe3) adopt a folded structure stabilized by an
aurophilic interaction (Figure 1, A).[21] The study of their
conformational equilibria by NMR techniques provided clear
evidence of the existence of aurophilic interactions in solution.
Herein we report macrocyclic complexes containing two AuI
centres bridged by (CF2)4 and diphosphine ligands (Figure 1, B),
which show an interesting dynamic behaviour. In addition, we
have studied the oxidation reactions of complexes
LAu(CF2)4AuL (L = PPh3, PMe3, PCy3, IPr, (dppf)1/2, (binap)1/2).
Unprecedented [AuII–AuII] complexes containing a (CF2)4 chain
as the only supporting bridging ligand (Figure 1, C), or with both
(CF2)4 and diphosphine ligands (Figure 1, D) have been
obtained. Reactivity and electrochemical studies suggest that
aurophilic interactions are key for the formation of these
1
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dinuclear AuII species, which undergo unprecedented thermal or
photochemical rearrangements involving the cleavage and
formation of Au–C bonds.
F2C CF2
F2 F2
F2C
CF2
LAu
C
C
Aurophilic interaction
depending on L
C
C
AuL
Au
L
Au
F2
F2
L
A
This work
F2C CF2
F2C CF2
F2C CF2
F2C
CF2
F2C CF2
CF2
F2C
Cl Au Au Cl
X
Au Au
X
Au
Au
PPh2
Ph2P
PPh2
PR3
Ph2P
R3P
X = F, Cl, Br
B
C
D
Figure 2. ORTEP diagram (thermal ellipsoids set at 50% probability) of the
molecular structure of 2. H atoms were omitted except those involved in weak
interactions (Relevant distances (Å): Au1···Au2 2.9799(2), H36-F7 2.49, H42-
F2 2.38).[48]
Figure 1. Previously reported dinuclear AuI complexes (A). AuI (B) and AuII (C,
D) complexes reported in this work.
The molecule of 2 lacks any symmetry in its crystal structure,
although the differences between both halves of the molecule
are small. In agreement with this, at sufficiently low
temperatures the 19F NMR spectrum shows seven doublets
corresponding to eight inequivalent 19F nuclei, two of which are
coincidentally isochronous. On increasing temperature these
signals coalesce into four doublets, as expected for a folded ring
with average C2 symmetry (Figure 3). Further temperature
increase produces the coalescence of the four doublets into two
broad signals, due to fast inversion of the ring helicity in the
NMR time scale. The rate constants for this inversion process
were determined by line shape analysis of the 19F NMR
spectrum at different temperatures. An activation enthalpy of
49.1 KJ/mol was calculated by Eyring analysis,[48] which is
significantly larger than the activation enthalpy determined for
the helicity inversion of 1 (35.8 KJ/mol).[21] This difference is
rationalized by considering that inversion of 1 can easily take
place after breaking the Au···Au and C–H···F interactions and
formation of an extended species, while in 2 the distance
between both Au centers is constrained by the limited
conformational mobility of the macrocycle, raising the activation
energy.
Results and Discussion
Macrocyclic AuI complexes
The reaction of Ph3PAu(CF2)4AuPPh3 (1) with dppf or rac-binap
gave PPh3 and macrocyclic complexes 2 or 3, which were
isolated as soluble orange or white solids, respectively (Scheme
2). In contrast, the analogous reactions with (CH2)n(PPh2)2 (n = 2,
4
or
5)
gave
slightly
soluble
solids
with
a
[Au2(CF2)4(PPh2(CH2)nPPh2)]x composition, as determined by
elemental analysis. The solid with n = 5 was partially soluble and
showed characteristic 19F NMR signals associated to the
Au(CF2)4Au chain, which were similar to those of 2, suggesting
the presence of [Au2{µ-(CF2)4}{µ-PPh2(CH2)5PPh2}] (4) in the
soluble fraction. Oligomeric species with larger x values, could
be present in the insoluble fractions.
The crystal structure of 2 shows a macrocycle formed by two
AuI centres bridged by a (CF2)4 chain and a dppf ligand (Figure
2). The ring is folded, adopting a figure-eight shape, where the
Au atoms approach at short distance. Two C-H···F interactions
cooperate to stabilize this conformation. Similar Au···Au and C-
H···F interactions have been observed in the crystal structure of
1.[21]
Similarly, the variable-temperature NMR spectra of
3
suggest that at low temperatures the macrocycle adopts an
unsymmetrical conformation where all 19F and 31P nuclei are
inequivalent (Figures S54–S56).[48] As the temperature
increases (T > 211 K), fast conformational motion leads to an
average C2 symmetry. However, further temperature raise to
349 K did not produce coalescence of the observed signals,
which is attributable to the intrinsic chirality of the binap ligand.
Remarkably, only one diastereomer was observed despite
racemic binap was used, which suggests that the configuration
of the binaphthyl unit determines the helicity sense of the
Au(CF2)4Au chain. The 19F NMR spectrum of 4 agrees with a
cyclic structure of C2 symmetry analogous to that of 2 (Figure
S57).[48]
F2C CF2
F2C
CF2
F2C CF2
F2C CF2
PPh2
Ph2P
Au
Au
Au Au
- 2 PPh3
PPh3
Ph3P
PPh2
Ph2P
1
2, 3
Ph2P PPh2
Ph2P
PPh2
Ph2P
PPh2
=
Fe
dppf (2)
rac-binap (3)
Scheme 2. Synthesis of macrocyclic dinuclear AuI complexes.
2
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Although the molecule does not present any crystallographic
symmetry element, its geometry is very close to that expected
for an ideal C2 symmetry.
R3PAuCl
F2C CF2
F2C
CF2
F2C CF2
CF2
hν (fast)
or
(slow)
F2C
+
F2
C
PhICl2
-PhI
R3P
CF2
CF2
Au Au
Cl Au Au Cl
PR3
Au
+
Cl
C
R3P
R3P
PR3
F2
R
R
R
F2C CF2
6
7
8
Ph
Ph
Me
1
Ph
5 Me
(traces)
9
Me
F2C CF2
Scheme 3. Oxidation of dinuclear AuI complexes and decomposition of the
resulting AuII complexes.
Figure 4. ORTEP diagram (thermal ellipsoids set at 50% probability, H atoms
omitted) of the molecular structure of 6.[48]
Figure 3. 19F NMR spectra of 2 at selected temperatures (CDCl3, 564.6 MHz).
Different 19F chemical environments are labelled as Fa, Fb, Fc, Fd.
Interconversion between both conformers originates a/b and c/d exchange.
Between 210 and 227 K the molecule adopts an unsymmetrical conformation
(not shown).
The 19F and 31P{1H} NMR spectra of 6 and 7 are very similar and
suggest that the main features of the solid-state structure of 6
are essentially maintained in solution for both complexes. Thus,
the room temperature 19F NMR spectra show four sharp signals
corresponding to a C2-symmetric species, indicating that the
covalent Au–Au bond hampers the inversion of the loop. By
contrast, the weak non-covalent interactions that stabilize the
folded conformation in 1 and 5 can be broken at relatively low
temperatures.
The reactions of 1 or 5 with PhICl2 in a 1:2 molar ratio gave
mainly complexes 6 or 7 instead of the expected dinuclear AuIII
species resulting from dichlorination of each gold centre. The
formation of the AuII complexes was fast, but subsequent
reaction with PhICl2 was sluggish. Indeed, at long reaction times
(24 h), unreacted PhICl2, 6 or 7, and their decomposition
products (8 or 9 and LAuCl) were observed.[48]
AuII and AuIII complexes
Oxidation of 1 or Me3PAu(CF2)4AuPMe3 (5) with an equimolar
amount of PhICl2 gave AuII complexes 6 or 7, respectively
(Scheme 3), which were isolated as yellow light-sensitive solids.
Photodecomposition of 6 and 7 in solution was fast under
irradiation at 402 nm, giving an almost equimolar mixture of
R3PAuCl and [Au{k2-(CF2)4}Cl(PR3)] (R = Ph (8), Me (9)) in 2–5
minutes. Traces of octafluorocyclobutane were detected by
NMR spectroscopy and MS. In the absence of light, conversion
of 6 into Ph3PAuCl and 8 took more than 8 days at room
temperature, or 1 h at 80 ºC. Photodecomposition was also
observed when solid 6 was exposed to ambient light.[49]
To get a closer insight into the influence of the auxiliary
ligands in the stability of the AuII dinuclear complexes, we
carried out the reactions of LAu(CF2)4AuL (L = PCy3 (10) or IPr
(11)) with PhICl2. We have previously observed that bulky
ligands disfavour the folded conformation of these dinuclear AuI
complexes.[21] Thus, considering the shorter Au-Au distance in
the AuII complexes, we expected that an increase of the
bulkiness of L should have a substantial effect in the outcome of
these reactions. In line with this, no AuII complexes were
detected in the reactions of 10 or 11 with an equimolar amount
The crystal structure of 6 shows an almost linear Cl–Au–Au–
Cl unit (Figure 4). The Au–Au distance shortens from 3.0394(3)
Å in 1 to 2.5454(3) Å in 3, a value that is comparable to those
found in the two reported perfluoroalkyl AuII complexes,
[35]
[Au(CF3)2(py)]2
and [Au2(CF3)2{µ-(CH2)2PPh2}][50] (Au–Au
distances: 2.5062(9) and 2.679(1) Å, respectively). Owing to the
reduction of the Au-Au distance, the P-Au-Au-P dihedral angle is
larger for 6 (96.177º) than for 1 (72.299º). The looped structure
is chiral and both enantiomers are present in the unit cell.
3
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of PhICl2 (Scheme 4). In the reaction with 10, the main products
were Cy3PAuCl and 12, likely formed by decomposition of the
unstable dinuclear AuII intermediate A. Small amounts of AuIII
complexes (13 and 14) were observed. By contrast, the reaction
of 11 with PhICl2 led to complexes resulting mainly from
dichlorination of one (15) or both (16) metal atoms, and complex
17. Remarkably, no [Au{k2-(CF2)4}Cl(IPr)] was detected in the
reaction mixture. Compound 16 was the main product in the
reaction of 11 with 2 equivalents of PhICl2. Small amounts of
unidentified products were also formed in these reactions,
accounting for less than 9 % of the total integral of the 19F NMR
spectra of the mixtures.
Reaction of 1 with XeF2 in CD2Cl2 gave the fluorocomplex 20
(Scheme 6). 20 is light sensitive and thermally unstable in
solution at room temperature. Its decomposition produced an
untractable mixture, where only [Au{k2-(CF2)4}F(PPh3)] (21)
could be identified by NMR spectroscopy. Similarly, the reaction
of 1 with Br2 gave the bromocomplex 22, which decomposed in
the dark at room temperature to give [Au{k2-(CF2)4}Br(PPh3)]
(23) and Ph3PAuBr. Owing to their instability, 20 and 22 could
not be isolated. The reaction of 1 with I2 gave mainly Ph3PAuI
and [Au{k2-(CF2)4}I(PPh3)].[21]
Remarkably, complex 20 and [Au2F2(µ-dfmt)2] (dfmt = N,N'-
bis(2,6-dimethylphenyl)formamidinate),[51] are the only AuII
fluorocomplexes reported.[52] The gold-bound fluorine atoms in
20 were evidenced in its 19F NMR spectrum by a broad singlet at
-186.1 ppm. The 19F NMR signals of the (CF2)4 chain and the
31P{1H} NMR signals of 20 and 22 were similar to those of 6 and
7, and agree with a similar structure of C2 symmetry. Auracyclic
complexes 21 and 23 were identified by means of their
characteristic 19F and 31P{1H} NMR signals, which were
compared with those of samples generated by halide
substitution in [Au{k2-(CF2)4}I(PPh3)].[48]
F2
C
F2C CF2
CF2
F2C
F2C CF2
F2C
CF2
Cy3P
Cl
CF2
CF2
Au
+
1 PhICl2
C
Au
Cl Au Au
Au
Cl
F2
12
PCy3
Cy3P
PCy3
Cy3P
A
fold-10
Cy3PAuCl
Main products
+
Not detected
Cl
Cy3PAu (CF2)4 AuPCy3
Cy3P Au (CF2)4 AuPCy3
13
ext-10
Minor
products
F2C CF2
CF2
+
F2C CF2
F2C
CF2
Cl
Cl
X
F
Cl
XeF2, -Xe
or Br2
F2C
20
Cy3P Au (CF2)4 Au PCy3 14
Cl Cl
22 Br
X
Au Au
X
Au Au
Unstable, not isolated
PPh3
Ph3P
Ph3P
PPh3
1
Cl
IPr Au (CF2)4 Au IPr
1 PhICl2
15
Scheme 6. In situ formation of AuII fluoro- or bromo-complexes.
IPr Au (CF2)4 Au IPr
Main
Cl
Cl
11
+
Cl
IPr Au (CF2)4 Au IPr 16
2 PhICl2
Oxidation of complexes 2 or 3 with an equimolar amount of
PhICl2 gave AuII macrocyclic complexes 24 or 25 (Scheme 7),
which were isolated as brown or yellow solids, respectively. Both
are light-sensitive in solution, but in the mixtures of
decomposition products only the anionic complex [Au{k2-
(CF2)4}Cl2]- could be identified by NMR spectroscopy. The
oxidation of AuI dinuclear macrocyclic complexes to [AuII–AuII]
complexes has been previously observed in macrocyclic bis-
carbene complexes.[53–55]
Cl
Cl
Cl
+
16 (Main) + 17
IPr Au (CF2)4 Cl
Cl
17
Scheme 4. Reactions of AuI complexes containing bulky ligands with PhICl2.
The reaction of Ph3PAu(CF2)6AuPPh3 (18) with an equimolar
amount of PhICl2 gave a mixture of AuI and AuIII species
resulting from partial chlorination of the metal atoms and Au–C
bonds (Scheme 5). No AuII complexes were detected. The 19F
and 31P{1H} NMR spectra showed characteristic NMR signals for
the Ph3PAuCF2- and Ph3PAu(Cl)2CF2- moieties, but an
unambiguous identification of the components of the mixture
was not possible because of extensive signal overlapping.[48]
The reaction of 18 with PhICl2 in a 1:2 molar ratio gave 19,
which was isolated.
F2C CF2
F2C CF2
F2C CF2
F2C CF2
PPh2
dppf
rac-binap
PhICl2
Ph2P
Au Au
Cl Au Au Cl
24
25
PPh2
Ph2P
PPh2
Ph2P
2 or 3
Scheme 7. Reactions of dinuclear macrocyclic complexes with PhICl2.
Au(PPh ),
AuCl2(PPh3),
Cl
X
(CF2)6
+
Y
X, Y =
3
The crystal structure of 24 shows an almost linear Cl–Au–Au–Cl
unit (Figure 5), with a Au–Au distance (2.5274(3) Å) similar to
that of 6 (2.5454(3) Å). The PAu(CF2)4AuP chain also adopts a
looped conformation, although the PAuAuP dihedral angle
(84.827º) is smaller than in 6 because of the conformational
constraints imposed by the diphosphine ligand. Both
enantiomers of 24 are present in the unit cell. The ferrocenediyl
unit is not symmetrically disposed, making both halves of the
molecule significantly different.
1 PhICl2
Ph3PAuCl
+
Ph3PAuCl3 (traces)
Cl
Ph3P Au (CF2)6 Au PPh3
18
Cl
Ph3P Au (CF2)6 Au PPh3
Cl Cl
19
2 PhICl2
Scheme 5. Reaction of a AuI complex containing a (CF2)6 chain with PhICl2.
4
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As observed for the other dinuclear AuII complexes 6, 7, 20
and 22, the 19F NMR spectra of the macrocyclic AuII complexes
24 and 25 show four sharp doublets of multiplets corresponding
to two diastereotopic CF2 units, in agreement with an average C2
symmetry. The d(19F), and 2JPF values are similar to those of the
other members of the series.
Complexes 1 and 10 share in common their ability to adopt a
folded structure stabilized by an aurophilic interaction. Thus, at
room temperature 1 is mainly in the folded conformation and 10
is mainly in the extended one, although a significant amount of
fold-10 (23%) in exchange with ext-10 was observed at low
temperature (Scheme 4).[21] In contrast, 11, 18 and
Cy3PAu(CF2)6AuPCy3 display extended conformations in their
crystal structures and their NMR spectra do not show any
evidence of stable folded conformers.[21] This suggests that short
Au···Au contacts could be responsible for the observed
decrease in the first-oxidation potentials of 1 and 10.
Figure 5. ORTEP diagram (thermal ellipsoids set at 50% probability, H atoms
omitted) of the molecular structure of 24.[48]
Cyclic Voltammetry
To gain information about the influence of the neutral ligands
and the length of the (CF2)n chain on the outcome of the
oxidation reactions of dinuclear AuI complexes, we carried out
cyclic voltammetry measurements on complexes LAu(CF2)4AuL
(L = PPh3 (1), PCy3 (10), IPr (11)) and LAu(CF2)6AuL (L = PPh3
(18), Cy3P). Mononuclear complexes LAu(n-C4F9) (L = PPh3,
IPr)[56] were also studied for the purpose of comparison. The
potential was scanned in the oxidative direction until solvent
oxidation became dominant. Then, the scan direction was
reversed toward negative values.
Figure 6. Cyclic voltammograms of: (top) (CF2)4-bridged; (bottom) (CF2)6-
bridged and mononuclear AuI complexes. Recorded in 0.001 M CH2Cl2
solutions, with 0.1 M NBu4PF6 as electrolyte and a 0.1 V/s scan rate.
Table 1. Electrochemical data.
Complex 1 gave a well-defined anodic wave at a peak
potential of 1.13 V, followed by another ill-defined wave around
1.5 V (Figure 6 and Table 1). Further anodic processes around 2
Complex
Ep (V)[a]
1.13, 1.5
1.33, ca. 2.4
ca. 1.6[b], 2.12
> 2.0
Ph3PAu(CF2)4AuPPh3 (1)
Cy3PAu(CF2)4AuPCy3 (10)
IPrAu(CF2)4AuIPr (11)
Ph3PAu(CF2)6AuPPh3 (18)
Cy3PAu(CF2)6AuPCy3
Ph3PAu(n-C4F9)
V
were overlapped with the solvent oxidation wave. No
corresponding reduction events were observed when the scan
direction was reversed, which suggests that the observed
oxidations are irreversible processes. The cyclic voltammogram
of 10 was similar, showing a well-defined oxidation wave at 1.33
V, followed another one at ca. 2.4 V. In contrast, 11 showed a
weak shoulder around 1.6 V, followed by a well-defined peak at
2.12 V. The oxidation of the other studied complexes occurred at
significantly higher potential values. For instance, the first-
oxidation waves of LAu(n-C4F9) (L = Ph3P, IPr) were observed at
2.5
2.09[c]
IPrAu(n-C4F9)
2.00
2.09 and
2
V, respectively, while dinuclear complexes
LAu(CF2)6AuL (L = PPh3 (18) or Cy3P) did not show any
detectable oxidation event at potentials lower than 2 V. In
summary, complexes 1 and 10 show significant oxidation events
at potentials lower than those of dinuclear complexes with longer
(CF2)n chains or bulkier L ligands, whose oxidation potentials are
similar to those of mononuclear complexes LAu(n-C4F9) (L =
PPh3, IPr).
[a] Vs FeCp2+/FeCp2. [b] Weak shoulder. [c] The observed shoulder appeared
as a peak after subtraction of a blank voltammogram (see Figure S61).
Cyclic voltammetry studies in dinuclear AuI complexes (µ-
XZ2)Au2Cl2 (X = CH2, Z = PPh2 or 3-mesitylimidazol-1-yl-2-
ylidene;[23] X = N(i-Pr), Z = PPh2[26]) have shown that the
oxidation potentials of these complexes were 0.12–0.32 V lower
than those of structurally similar mononuclear complexes. We
have observed larger differences (DE ³ 0.9 V) between 1 and
5
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Ph3PAu(n-C4F9) or Ph3PAu(CF2)6AuPPh3, or between 10 and
Cy3PAu(CF2)6AuPCy3. These larger differences can be
attributed to a destabilization of the HOMO of the folded
conformers of 1 and 10 induced by the stronger aurophilic
interactions.
diadduct (TEMPO)2(µ-C4F8)[21] was observed (6% in the
photodecomposition, traces in the thermal decomposition). This
suggests that homolysis of the Au–CF2 bonds takes place during
the decomposition of 6. To test if the observed
octafluorocyclobutane could be formed by decomposition of a
complex of the type F, [Au{k2-(CF2)4}I(PPh3)] was heated (100
ºC, toluene) or irradiated (402 or 310 nm, CD2Cl2). However, no
octafluorocyclobutane was detected in these experiments.
Theoretical studies on binuclear AuII complexes suggest that
the LUMO of these complexes has Au–Au and Au–halogen
antibonding character.[30,45,57,58] In line with this, photoexcitation
of 6 could lead to weakening of the Au–Au bond facilitating its
homolysis to give radical intermediates (Scheme 9), which would
eliminate a molecule of Ph3PAuCl. The resulting diradical (G),
could undergo an intramolecular coupling, to give auracycle 8.
This would constitute the main decomposition pathway, but G
could also give Ph3PAuCl and C4F8. Diadduct (TEMPO)2(µ-
(CF2)4) would be produced in the presence of TEMPO.
Mechanistic considerations
Reaction pathways leading to the observed products are
outlined in Scheme 8. Chlorination of the initial dinuclear AuI
complex would give a dinuclear AuII complex (B) or a mixed-
valent complex (C). Products of the type B were formed when
the starting complex has a (CF2)4 chain and PPh3, PMe3, dppf or
binap as neutral ligands. In contrast, complexes of the type C
are formed if the gold centers are bridged by a longer chain, as
in 18, or if L is a bulky ligand, as in 11. In these cases, C
undergoes further chlorination to form dinuclear AuIII complexes
(D). Reductive elimination with formation of a C–Cl bond from
the AuIII centers would afford LAuCl and products of the type E.
Complexes B evolve by photo- or thermally activated elimination
of LAuCl to give F. Remarkably, disproportionation of B to give
C was not observed,[42] whereas oxidation of complexes B with
PhICl2 was sluggish (observed for 6 and 7).
An interesting case is the reaction of 10 with one equivalent
of PhICl2, which gives mainly Cy3PAuCl and [Au{k2-
(CF2)4}Cl(PCy3)] (12). Since complexes of the types LAuCl and F
are typically formed by decomposition of dinuclear AuII
complexes, the main reaction product should be a complex of
the type B. The steric repulsions between the bulky PCy3 ligands
could render this intermediate unstable, provoking fast
decomposition to Cy3PAuCl and 12. Small amounts of products
of the type C or D, resulting from dichlorination of the Au centres,
were observed in this reaction.
F2C CF2
F2C CF2
F2C
CF2
F2C
CF2
hν
Cl Au
Ph3P
Au Cl
PPh3
Cl Au Au Cl
PPh3
F2C CF2
Ph3P
6*
6
F2C CF2
+
-Ph3PAuCl
Ph3PAuCl
TEMPO
F2C
F2C CF2
CF2
F2
C
F2C
Ph3P
F2C
Cl Au
PPh3
CF2
CF2
CF2
TEMPO
CF2
+
Au
Cl
C
TEMPO
F2
G
8
Ph3PAuCl
Scheme 9. Proposed decomposition mechanism for 6.
Cl
PhICl2
Au
Au
L
Au
L
Cl Au
L
Au Cl
L
Conclusions
L
F
B
+ LAuCl
We have reported the first macrocyclic complexes containing
two AuI centers linked by a perfluorocarbon chain, which adopt a
rare figure-eight conformation. The higher energetic barrier for
exchange between both enantiomeric conformers of [Au2{µ-
(CF2)4}(µ-diphosphine)] (diphosphine = dppf (2), binap (3))
respect to the acyclic analogues R3PAu(CF2)4AuPR3 (R = Ph (1)
or Me (5)) is originated by conformational constraints imposed
by the presence of the diphosphine ligands.
PhICl2
PhICl2
X
PhICl2
Cl Au Cl
Cl
Cl Au Cl
L
Au
L
Cl Au Cl Cl Au Cl
E
L
L
L
LAuCl
+
D
C
Scheme 8. Reactions leading to AuII, AuIII and mixed-valent complexes.
Complexes 1, 2, 3 or 5 react with halogenating agents to
give dinuclear AuII complexes which present unprecedented
structures. In the acyclic complexes 6 and 7, the bridging (CF2)4
chain allows the approach of both Au centres, but does not
impede their separation at relatively long distances. Therefore,
they constitute a new type of AuII complexes having semi-
supported AuII–AuII bonds.
CV studies suggest a marked lowering of the oxidation
potential of the dinuclear [AuI, AuI] complexes induced by the
presence of aurophilic interactions. This effect, together with the
presence of a (CF2)4 linker between the AuI centres and the low
steric hindrance of the neutral ligands, is beneficial for the
formation and stability of the [AuII–AuII] complexes. In contrast,
the presence of a longer (CF2)6 linker or bulky neutral ligands in
the starting [AuI, AuI] complexes induce the formation of [AuIII,
AuI] and [AuIII, AuIII] complexes.
We hypothesize that the close approach of the Au centers in the
starting dinuclear AuI complexes is beneficial for the formation of
complexes B in two ways. Firstly, it lowers the redox potential at
which the first oxidation occurs, as observed for 1 and 10.
Secondly, it would stabilize the initially formed [AuII, AuI] species,
which could undergo a further one-electron oxidation to give
[AuII–AuII] complexes (B). If the separation of the Au atoms is
large, the [AuII, AuI] intermediates would be oxidized to [AuIII,
AuI] species and then to [AuIII, AuIII] complexes.
The fast photodecomposition of complexes B and the
detection of octafluorocyclobutane are indicative of a radical
reaction. In agreement with this, the yield of the decomposition
of 6 was reduced in the presence of TEMPO from 100% to 77%
(photochemical) or 65% (thermal), and the radical-trapping
6
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10.1002/chem.202103153
Chemistry - A European Journal
FULL PAPER
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7
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10.1002/chem.202103153
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FULL PAPER
Entry for the Table of Contents
F2C CF2
F2C CF2
CF2
F2C
F2C CF2
F2C
F2C CF2
CF2
ClAuPR3
PhICl2
hν or
Au
Au Au
Cl Au Au
R3P
Cl
R3P
Cl
R3P
PR3
PR3
Aurophilic interactions promote the oxidation of AuI2 compounds to give complexes containing a AuII–AuII bond instead of AuIII2 or
mixed-valent species. The obtained AuII complexes undergo thermal or photochemical elimination of R3PAuX to give AuIII
perfluorinated auracyclic complexes through radical intermediates.
Institute and/or researcher Twitter usernames: @GqoLab; @JuanGilRubio1
8
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