Gold(ii) in redox-switchable gold(i) catalysis

Article abstract of DOI:10.1039/C9CC00283A

Gold(ii) species catalyse the cyclisation of N(2-propyn-1-yl)benzamide to 2-phenyl-5-vinylidene-2-oxazoline without halide abstraction while the saturated gold(i) complex is inactive. Redox-switching between gold(ii) and gold(i) turns catalytic turnover o

Full text of DOI:10.1039/C9CC00283A

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Gold(II) in Redox-Switchable Gold(I) Catalysis
a
a
a
a
b
Philipp Veit, Carla Volkert, Christoph Förster, Vadim Ksenofontov, Steffen Schlicher, Matthias
b
a
Bauer and Katja Heinze *
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Gold(II) species catalyse the cyclisation of N(2-propyn-1- switchable catalysis, RSC) without additional halide
2
6–28
yl)benzamide to 2-phenyl-5-vinylidene-2-oxazoline without halide scavengers.
Indeed, RSC is a strongly expanding field in
abstraction while the saturated gold(I) complex is inactive. Redox- homogeneous catalysis.26–40 The redox-switching event occurs
switching between gold(II) and gold(I) turns catalytic turnover on either at a remote redox-active unit, e.g. a Fc unit located on
26–28,30,33,34,36–38
and off.
the ligand
or directly at the catalytic metal
III
IV 32
centre, e.g. Ce /Ce . In gold(I) catalysis, activation by
oxidation of the remote Fc unit increases the Lewis acidity of
Molecular gold complexes have attracted great attention as
(
pre-)catalysts in homogeneous catalysis, especially in the
I
26–28
the Au ion.
Thus, carbene ligands with low donor ability
1–3
activation of alkynes.
(Pre-)catalysts and intermediates
should be beneficial for high catalytic activity in gold catalysis.
The Tolman electronic parameter (TEP) of the Fc-MICs
increases from a very high donicity (TEP  2046 cm ) to a lower
2054 cm ) upon oxidation of the Fc unit.
neutral carbene ligand :C(Fc)OEt already possesses a similar
donor strength (TEP = 2054 cm ) as the oxidised Fc-MICs.
Consequently, the oxidised carbene ligand [:C(Fc)OEt] should
have an even lower donor ability and could enable high catalytic
feature gold in its favoured oxidation states +I and +III.4–7 In
photocatalysis however, transient mononuclear gold(II) key
intermediates have been postulated but remained elusive so
41
–1
–1
27,28
one (TEP

The
8–14
far.
Apart from very few examples, the chemistry of
mononuclear gold(II) complexes is underdeveloped due to the
favoured disproportionation or dimerisation of gold(II).15–19
In gold(I) catalysed reactions, gold(I) halide complexes are
usually employed as precatalysts. Typically, silver(I) salts,
–1
27,28,42
+
activity with AuCl[C(Fc)OEt] 1 as precatalyst (Scheme 1).
20
21
copper(II) triflate or alkali metal salts activate the precatalyst
by halide abstraction. Chlorido gold(I) complexes with N-
2
2
23,24
heterocyclic, or acyclic diamino
carbene ligands are
successfully employed in a plethora of catalytic applications in
conjunction with halide scavengers.1–7 The group of Peris
recently reported a chlorido gold(I) complex bearing a redox-
active ferrocenyl-imidazolylidene ligand. The presence of an
oxidant increases the catalytic performance of the complex. Yet,
this effect was ascribed to the formation of cationic protonated
25
complexes under the reaction conditions. On the other hand,
the group of Sarkar demonstrated in seminal studies that
ferrocenyl-substituted mesoionic carbene (Fc-MIC) ligands
enable ON/OFF switching of the catalytic performance of gold(I)
catalysts AuCl(Fc-MIC) by oxidation/reduction cycles (redox-
Scheme 1 Switchable gold-catalysed cyclisation of N(2-
propyn-1-yl)benzamide to 2-phenyl-5-vinylidene-2-oxazoline.
Compared to Fc-MICs with redox-active Fc substituents in a
position relative to the carbene donor, the Fc moiety in

1
a.Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg
University of Mainz, Duesbergweg 10–14, 55128 Mainz, Germany. E-Mail:
katja.heinze@uni-mainz.de
located in the
 position might have an even larger impact on
the ligand’s donor properties. Furthermore, the redox
b.
Department Chemie and Center for Sustainable Systems Design (CSSD), University
+
potentials for the Fc/Fc oxidation of :C(Fc)OEt and Fc-MICs are
of Paderborn, Warburger Straße 100, D-33098 Paderborn, Germany.
2
8,42
very different,
opening the possibility of different valence
†
Electronic Supplementary Information (ESI) available: [Experimental procedures,
+
+
spectral details, (TD-)DFT calculations]. See DOI: 10.1039/x0xx00000x
isomers for cationic gold complexes [AuCl(Fc-MIC)] and
1 and
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consequently a different reactivity. For comparison of [AuCl(Fc- room temperature and hence not observed at 295 K. The
View Article Online
0
/+
0/+
+
DOI: 10.1039/C9CC00283A
MIC)] and
1 , we studied the gold catalysed cyclisation of absence of an axial Fc resonance also at 77 K may be ascribed
50
III
I
N(2-propyn-1-yl)benzamide
oxazoline (Scheme 1).
Pre-catalyst AuCl[C(Fc)OEt]
from W(CO)
to
2-phenyl-5-vinylidene-2- to fast spin relaxation. Then, the Fe Au isomer slowly
20,26–28
II
II
converts to a Fe Au isomer almost quantitatively suggesting
is accessible by transmetalation43 that significant reorganisation takes place after the initial Fc/Fc+
[C(Fc)OEt] to AuCl(SMe ) and fully characterised oxidation. Some gold(II) might also form directly starting from
1
4
4
5
2
1
13
by H and C NMR, IR and UV/Vis spectroscopy, LIFDI mass minor amounts of a dimeric species (1)
2
.
spectrometry and cyclic voltammetry (ESI, Fig. S1 – S9). The
F
F
cyclic voltammograms of
4 4 6 5
1 with [nBu N][BAr ] (Ar = C F ) as
supporting electrolyte show a reversible one-electron oxidation
wave at E
½
= 0.58 V and 0.60 V in CH
2
Cl
2
and THF, respectively
+
(vs. FcH/FcH ; ESI, Fig. S7, S9). These processes occur at
significantly higher potential than the oxidation of Fc-MIC
2
6–28
gold(I) complexes.
nBu ] to [nBu
N][BArF
approximately 100 mV similar to Fc-MIC gold(I) complexes.
Changing the electrolyte from
6
[
4
4
4
N][PF ] lowers the oxidation potential by
27
Irreversible or quasireversible carbene reduction waves are
additionally observed at negative potentials (ESI, Fig. S6 – S9).
Interestingly, small oxidation waves appear at potentials 140 to
3
10 mV lower than the reversible oxidation wave. Potential and
intensity depend on the solvent and the electrolyte but not on
the batch of employed. Hence, this is an intrinsic property of
in a given environment. We tentatively ascribe this wave to
the oxidation of dimers (1) present to a minor extent in
1
1
2
solution. In fact, dimers and oligomers of AuCl(carbene)
complexes with small carbene ligands (but not for the Fc-MIC
2
6–28
45,46
ligands
) held together by aurophilic interactions
form in
43
…
the solid state. The presence of Au Au contacts in solution has
been suggested using diffusion-ordered spectroscopy
(
DOSY).47–49 1H DOSY experiments of
W(CO)
mainly monomeric nature of
1
in dichloromethane with
[C(Fc)OEt] as internal monomeric reference confirm the
as suggested by the
5
1
electrochemical experiments (ESI, Fig. S10). However,
…
dimerisation via Au Au contacts in the minor species (1)
2
should
hardly affect oxidation of the remote Fc substituents but rather
the gold centres. Hence, we investigated the chemical oxidation
of 1 by a suitable oxidant (Magic Blue, [N(p-C H Br) ][SbCl ] and
6 4 3 6
ammonium cerium(IV) nitrate, CAN) using X-band EPR
spectroscopy in more detail. Irrespective of the oxidant and
solvent (THF, CH
around g = 2 is detected for the
2
Cl
2
/DMF) employed, a broad resonance
/oxidant mixture both at 77
1
and 295 K (Fig. 1a, ESI, Fig. S11 – S16). Hence, the EPR resonance
+
derives from
1
. The observation of a signal already at room
temperature and the lacking axial symmetry in frozen solution
+
39,50,51
rules out the assignment of this resonance to a Fc ion.
On
the other hand, gold(II) species have been reported to possess
Fig. 1 a) EPR spectra of 1/Magic Blue in THF over time at 295 K,
b) doubly integrated intensity of the EPR resonance versus time
plot and c) conversion versus time plot for the cyclisation
g tensors with small g anisotropy and hyperfine coupling to the
197
3
15–18
gold nucleus ( Au: I = /
2
).
Due to the non-resolved
coupling(s), only the giso value of 2.017 is given which fits to
catalysed by
resonance increases in intensity over time at room temperature Gratifyingly, geometry optimisations using Density Functional
converging to a limiting value after 5 h. Quantification of the Theory (DFT) calculations on [1][SbCl ] converged to two
EPR resonance after 5 h by double integration and calibration electromers, namely a gold(I)/iron(III) and a gold(II)/iron(II)
ESI, Fig. S17, S18) confirms that the EPR resonance accounts for isomer in a contact ion-pair with the counter ion coordinating
5 % of the paramagnetic species (Fig. 1b). Hence, we suggest the gold with an Au-Cl(SbCl ) distance of 2.47 Å (Fig. 2a). The
that Magic Blue first oxidises the Fc unit in
yielding the cationic spin density in the gold(II) isomer is mainly shared between the
1/Magic Blue in dichloromethane (Scheme 1).
15-18
reported values for gold(II) complexes.
Furthermore, the
6
(
7
5

5
1
III
I
+
Fe Au electromer of
1
. Ferrocenium is typically EPR-silent at gold ion and the chlorido ligand (Fig. 2a). The original linear
2
| J. Name., 2012, 00, 1-3
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I
coordination of the Au center with a 177° C-Au-Cl angle is re-appear at typical diamagnetic values during a similar time
View Article Online
DOI: 10.1039/C9CC00283A
+
expanded yielding a 3+1 coordination of the gold(II) ion with a span suggesting the re-reduction of an initially formed Fc
2° C-Au-Cl angle in the Au species. This bending might raise species (ESI, Fig. S21). With these spectroscopic correlations, we
II
9
the activation barrier for the intramolecular electron transfer propose that a gold(II) centre slowly forms from the initially
I
III
II II
from Au to Fe giving the Au Fe isomer (Fig. 2a).
present ferrocenium ion. The gold(II) centre presumably
features a C-Au-Cl angle close to 90° (Fig. 2b), which enables
substrate coordination in contrast to the initially present
linearly coordinated gold(I) centre. The exact nature of the
52
active gold(II) catalyst remains speculative, yet three and four
1
7,18
coordinate
gold(II) species are conceivable with
coordination of counter ions, coordination of the substrate (via
the amide oxygen atom) or aggregation via chlorido ligands and
aurophilic interactions (Fig. 2). Attempts to get more insight into
the structure of the active species using Mößbauer (iron) and X-
3
ray absorption near edge structure spectroscopy at the gold L -
edge were inconclusive as removing the solvent leads to the
formation of elemental gold and several iron(II) and iron(III)
species (ESI, Fig. S22 – S25). In the absence of coordinating
solvents (DMF, THF) or substrates, a dark precipitate forms (ESI,
Fig. S26). Obviously, stabilising ligands (solvent, substrate) are
required to keep the gold(II) catalyst active. The majority of the
catalyst remains active for some time as a second batch of
substrate is converted to the product with similar TOF but
without induction period after a full initial conversion (ESI, Fig.
Fig. 2 DFT calculated optimised geometries and spin densities
III
I
II
II
of Fe Au and Fe Au isomers of a) [1][SbCl
6
] and b)
S27). The presence of [nBu
ESI, Fig. S28) suggesting that free coordination sites or only
weakly coordinating donor ligands are required to enable
catalysis. Activation of using AgSbF as halide scavenger
initiates a conventional gold(I) catalysis with a smaller TOF =
4
N]Cl (42 eq) inhibits the catalysis
+
[
(
1
+substrate] . Distances given in Å, hydrogen atoms omitted
(
except for the propargyl amide unit), contour value at 0.01 a.u..
]– to Au (Fig. 2a, right) shifts the spin
6
1
6
Coordination of [SbCl
density to give gold(II). A similar impact of ion coordination is
observed in gold(II) porphyrin/gold(III) porphyrin radical anion
–
3 –1
3.210 s but without induction period (ESI, Fig. S29).
1
7
valence isomers.
Conceptually similar, heterobimetallic
IV III
molybdenum(IV)/ferrocenium complexes Mo Fe undergo
intramolecular electron transfer to yield Mo Fe
V
II
3
9,40
electromers.
Fully analogously, rhodium(I) complexes
bearing a ferrocenyl phosphine-NHC ligand are first oxidised at
the Fc unit, while the second oxidation is rhodium centered
initiating a further intramolecular electron transfer from
+
35
+
rhodium to Fc . In these examples, the Fc/Fc redox couple
acts as intermediate parking position for a positive charge.
DFT calculations suggest that coordination of oxygen
nucleophiles such as amides (e.g. N(2-propyn-1-yl)benzamide)
+
to the gold centre of
1
instead of counter ions also stabilises
II II
I
III
the Au Fe valence isomer over the Au Fe isomer (Fig. 2b). In
the gold(II) isomer the C-Au-Cl angle is compressed to 93° and
the triple bond of the substrate coordinates to gold(II) (Fig. 2b).
Fig. 3 Conversion versus time plot of the cyclisation (Scheme 1)
during ON/OFF redox switching using Magic Blue (1.00/1.05 eq;
ON) and FeCp* (1.05/1.10 eq; OFF).
2
Hence, we employed the gold(I) complex
the cyclisation reaction of N(2-propyn-1-yl)benzamide to the Addition of decamethylferrocene FeCp*
oxazoline in dichloromethane (Scheme 1). Neither
nor oxidant catalytic turnover, while re-oxidation using Magic Blue re-starts
alone yield the heterocyclic product, while the mixture
/Magic the catalysis (Fig. 3). This second oxidation initiates catalysis
Blue (1 mol-%) quantitatively converts the substrate to the almost instantaneously without induction period and with a
1 and 1/Magic Blue in
2
as reductant stops
1
1
–
3
–1
product within a few hours (Fig. 1c, TOF = (8.3

1.1)

10 s ; similar TOF as during the first cycle. Hence, the catalytic mixture
/oxidant forms catalytically active gold(II) species which can be
Interestingly, an reversibly switched off and on by reduction and oxidation,
induction period of ca. 2.5 h is observed (Fig. 1c). This time scale respectively. The catalytically active species, however, still
matches the development of the gold(II) EPR resonance (Fig. 1). relates structurally to the precatalyst, as 84 5 % of are
Complementary, the paramagnetically broadened and shifted recovered after ca. 63 % conversion and quenching with FeCp*
ESI, Fig. S19, S20). This is significantly faster than observed with
1
26–28
the oxidised Fc-MIC gold complexes.

1
2
1
1
H NMR resonances of the initial mixture of
1
and Magic Blue according to H NMR spectroscopy (ESI, Fig. S30).
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9 K. Heinze, Angew. Chem. Int. Ed., 2017, 56, 16126.
1
2
In summary, we report a redox-switchable gold-catalysed
cyclisation with the gold centre of the precatalyst
switching between highly active, presumably coordinatively
unsaturated gold(II)
View Article Online
0 L. Hettmanczyk, D. Schulze, L. Suntrup and B. Sarkar,
DOI: 10.1039/C9CC00283A
1 reversibly
Organometallics, 2016, 35, 3828.
2
1 R. Pretorius, M. R. Fructos, H. Müller-Bunz, R. A. Gossage, P.
17,18,52
and inactive, coordinatively
J. Pérez and M. Albrecht, Dalton Trans., 2016, 45, 14591.
saturated gold(I) centres. Our studies complement previous 22 N. Marion and S. P. Nolan, Chem. Soc. Rev., 2008, 37, 1776.
2
3 L. M. Slaughter, ACS Catal., 2012, 2, 1802.
24 V. P. Boyarskiy, K. V. Luzyanin and V. Y. Kukushkin, Coord.
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5 S. Ibáñez, M. Poyatos, L. N. Dawe, D. Gusev and E. Peris,
Organometallics, 2016, 35, 2747.
work on gold(I) pre-catalysts activated by redox-active
26-28,53
ligands.
The very high activity may be due to the
electrophilic nature of the gold(II) ion with free coordination
sites. To prevent aggregation or disproportionation, the gold(II)
centre must be stabilised by further donor ligands (counter ions, 26 L. Hettmanczyk, S. Manck, C. Hoyer, S. Hohloch and B.
2
Sarkar, Chem. Commun., 2015, 51, 10949.
27 L. Hettmanczyk, L. Suntrup, S. Klenk, C. Hoyer and B. Sarkar,
Chem. Eur. J., 2017, 23, 576.
substrates) which should not coordinate too strongly. The
+
+
Fc/Fc couple temporarily stabilises
1 via valence isomerisation
preventing aggregation or disproportionation of the
2
8 S. Klenk, S. Rupf, L. Suntrup, M. van der Meer and B. Sarkar,
unsaturated gold(II) centre in the absence of donor ligands.
Organometallics, 2017, 36, 2026.
Detailed studies on Au complexes with carbene ligands using 29 A. M. Allgeier and C. A. Mirkin, Angew. Chem. Int. Ed., 1998,
I
I/II
37, 894.
the Au switch are ongoing.
3
3
3
0 C. K. A. Gregson, V. C. Gibson, N. J. Long, E. L. Marshall, P. J.
Financial support from the Center for INnovative and Emerging
MAterials (CINEMA) and the Forschungsinitiative Rheinland-
Pfalz (LESSING) is gratefully acknowledged. Parts of this
research were conducted using the supercomputer Mogon and
advisory services offered by Johannes Gutenberg University
Mainz (www.hpc.uni-mainz.de), which is a member of the AHRP
and the Gauss Alliance e.V.. PETRA III (Hamburg, Germany) is
acknowledged for provision of beamtime.
Oxford and A. J. P. White, J. Am. Chem. Soc., 2006, 128
,
7
410.
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3 R. Savka, S. Foro, M. Gallei, M. Rehahn and H. Plenio, Chem.
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Conflicts of interest
There are no conflicts to declare.
4 K. Arumugam, C. D. Varnado, S. Sproules, V. M. Lynch and C.
W. Bielawski, Chem. Eur. J., 2013, 19, 10866.
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