Allene-derived gold and platinum complexes: Synthesis and first applications in catalysis

Article abstract of DOI:10.1039/d0dt00665c

We report here the synthesis, full characterisation and first application in catalysis of novel Au(i), Au(iii) and Pt(ii) carbene-type complexes formed from bis(pyridyl)allenes. The catalytic activity of the new Au(i)-complexes in the cyclisation of 1,6-enynes, a benchmark reaction for new Au and Pt complexes, was comparable to Au(i)-state-of-the-art catalysts used in these reactions. Reactions with the new Au(iii)- and Pt(ii)-complexes occurred under milder conditions than those reported with AuCl₃ and PtCl₂,.

Full text of DOI:10.1039/d0dt00665c

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DOI: 10.1039/D0DT00665C
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Allene-derived gold and platinum complexes: synthesis and first
applications in catalysis.
Hanna K. Maliszewska,a David L. Hughes,a and María Paz Muñoz*a
§Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We report here the synthesis, full characterisation and first containing a pyridine and a phosphine oxide substituents with
application in catalysis of novel Au(I), Au(III) and Pt(II) carbene-type Au(I) complexes to give Au(I)-carbene-type complexes from the
complexes formed from bis(pyridyl)allenes. The catalytic activity of nucleophilic attack of the pyridine into the allene (Figure 1,
the new Au(I)-complexes in the cylisation of 1,6-enynes, benchmark bottom).9 However, in terms of generality and metal used, the
reaction for new Au and Pt complexes, was comparable to Au(I)- scope for the formation of this type of allene-derived metal
state-of-the-art catalysts used in these reactions. Reactions with carbene complexes and their potential catalytic activity is
the new Au(III)- and Pt(II)-complexes ocurred under milder completely unexplored.
conditions than the reported with AuCl3 and PtCl2.
Here we report our results on the use of bis(pyridyl)allenes
as precursors of novel Au(I), Au(III) and Pt(II) carbenes, as well
as the preliminary results on the catalytic activity of the new
complexes in benchmark reactions for Au and Pt catalysis
(Figure 1, this work).
The allene functional group is known to act as a ligand in
transition metal complexes through its double bonds,1 but
when decorated with two donor units on each of the terminal
substituents (LB in Figure 1), it becomes a new attractive ligand
scaffold with increased coordination sites. Since 2008 only a
handful of such structures have emerged, incorporating mainly
phosphorus-based groups capable of complexation to various
metal centres (Figure 1, previous work). For example, bis-
phosphine allene ligands have been reported in 1:1 (ligand to
metal) complexes with Rh(I),2 Pd(II) and Pt(II).3 Complexes of
bis-phosphine allene ligands with Au(I) gave 1:2 or polymeric
complexes.3
Previous work:
R
R
R
R’
LB
R’
LB
M
R’
LB
•
•
•
or
LB
LB
M
LB = Lewis Base
LB
M
M
tBu
H3C
H3C
-
PF6
tBu
Ph
•
•
•
Ph
Ph
•
Ph
N
Ph2P
Cl
M = Pd(II), Pt(II)
PPh2
Ph2P
AuCl
PPh2
AuCl
M
N
Ar
Ar
P
P
Cu(I)
Rh
Cl
Ar
Ar
O
O
Krause 2008
Fensterbank 2016
Pyridines in general constitute one of the most established and
well performing classes of ligands for transition metal
complexes, but their use in allene-containing ligands is limited.
There is one reported example of bis(pyridyl)allene-metal
complex in a 1:1 complex with Cu(I) (detected by H NMR,
Figure 1) and a 2:2 complex was crystallised with Ag(I).4
In all of these complexes, the allenic structure shows carbene
character at the central carbon (as bent allenes)5 but retains its
chemical integrity. However, very few catalytic reactions have
been reported with some of these allene-metal complexes,2,3,6
and progress in this field is slow, probably due to the ability of
metals to activate the allene1,7 towards nucleophilic attack,8
which disrupts the allene backbone. In this context,
Fensterbank et al. reported in 2015 the reaction of allenes
Ready 2011
MeO2C
MeO2C
This work:
M
R
R’
R
•
R’
M
LB
1
First Catalytic
Screening
LB
N
N
MeO2C
MeO2C
M = Au(I), Au(III)
LB = 2-methyl pyridine
For M = Au(I) and LB = CH2P(Ph2)=O, see Fensterbank 2015
Figure 1. Previous work on allene-based metal complexes and summary of this work.
Bis(pyridyl)allenes 1a-b were prepared according to
modified literature procedures, the key step of the synthesis
being a Pd-catalysed SN2’ substitution on propargylic ester
precursors using organozinc derivatives.4 Having in mind the
catalytic utility of Au(I) and Au(III) complexes, we attempted the
coordination studies of the ligands with both.
a. School of Chemistry, University of East Anglia, Norwich Research Park, Norwich,
NR4 7TJ, UK. E-mail: m.munoz-herranz@uea.ac.uk
Allene 1a was reacted with 1 equiv. of (Me2S)AuCl at rt and
Electronic Supplementary Information (ESI) available: general experimental details, after 15 min the quantitative formation of 2a was observed
synthesis of allenes 1a and 1b, full characterisation of all compounds, NMR spectra,
kinetic data on isomerisation of 2b’ to 2b”, and details of the crystal structure
analyses of complexes 2a, 3a, 2b’’, 3b’’ and 5. See DOI: 10.1039/x0xx00000x
(Scheme 1). Activation of the allene moiety by Au(I) promotes
the attack of one of the pyridine nitrogen atoms on to the
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terminal carbon atom of the allene, thus forming an indolizine- structures are based on an indolizine-type bicyclic VcioewreArwticilethOntlhinee
type core of the new Au species, where the Au is bonded to the Au bonded to the formal allene centralDcOaI:rb10o.1n0,3n9/eDig0DhTb0o0u6r6in5Cg
former central carbon atom of the allene. 2a resembles the stereogenic centre. The geometry around Au centres is
structures of isolated Au(I)-carbenes in which conjugation of close to linear for Au(I)- and square planar for Au(III)-
heteroatoms, in this case from one of the pyridines, to the compounds. The Au(I)-C bond distances of 2a and 2b’’ were
electron deficient former central carbon of the allene, found to be in the lower range of previously reported for Au(I)
contribute to stabilisation,10 and can be represented in its two carbenes, 1.986(3) Å and 1.997(3) Å, respectively, supporting
resonance forms of zwitterionic vinyl-Au11 and neutral carbene, partial carbene character.16 The carbene versus vinyl-Au
with the remaining pyridine ring pendant on a newly created character was further probed by several attempts to
stereogenic centre.12
functionalise the complexes but Au complexes proved to be
1a was also reacted with a Au(III) salt in a refluxing ethanol quite robust. For example, reaction of complex 2b’’ with NTf2H
with the precipitation of yellow powder identified as 3a, sharing led to decomposition after long times, and isolation of Au(III)
most of its structural features with the Au(I) analogue (Scheme complex 3b’’ in 10-15% yield,17 but we could not identify any
1).13 Both compounds were fully characterised including X-ray protodemetalation products. Other attempts to functionalise
diffraction analysis (vide infra, see ESI).
the complex by oxidation failed, recovering 2b’’ unaltered,18
which further supports its dual carbene/vinyl metal character.
(Me2S)AuCl
1.0 equiv.
AuCl
ClAu
Ph
AuCln
Ph
Ph
AuCln
Ph
Ph
Ph
N
tBu
CH2Cl2, rt
15 min
tBu
Ph
•
Ph
(Me2S)AuCl
1.0 equiv.
rt, 48 h
N
N
N
N
N+
CH2Cl2, rt
15 min
N
Ph
N
tBu
N
2b’:2b’’ 1:0.12 (quanti.) to 2b’’
•
N
HAuCl4.H2O
1.0 equiv.
2b’
2b’’
1a
N
Cl3Au
AuCl3
HAuCl4.H2
1.0 equiv.
O
Ph
N
2a (Au(I)), n = 1, quant.
3a (Au(III)), n = 3, 42%
tBu
Cl-
Ph
Ph
tBu
•
EtOH, reflux, 3h
tBu
1b
+
+
MeOH, reflux,
130 min
N
N
N
N
N
Au
Cl Cl
N
3b’
3b’’
3b’:3b’’, 0.17:1 (20%)
4b (60%)
1.986(3)
Scheme 2. Reactions of allene 1b with Au(I) and Au(III) complexes.
1.987(3)
Au(III) carbenes are less documented in the literature, but
when comparing our structures with more prevalent Au(III)-
NHC carbenes the Au-C distances of 2.004(2) for 3a and 2.020(2)
for 3b’’ are in the mid-range of previously reported values,19
and slightly shorter than in reported vinyl-Au(III) complexes.13a
During our studies on Pt complexes with allene-containing
ligands we serendipitously discovered formation of Pt(II)
complex 5 in the process of a reduction of bis(pyridyl)allene-
Pt(IV) complex 4c, analogous to the Au(III) complex 4b, with
ascorbic acid in aqueous media. Structure of 5 was
unambiguously confirmed by X-ray crystallography (Scheme 3,
see ESI). The expected Pt(II) equivalent of 4c was not observed.
Interestingly, it was a phenyl substituent that underwent
carbocyclisation to the allene. Complex 5 exhibits an hexacyclic
core with one of the central five-member rings being a
platinacycle formed by the chelation of Pt by the pyridine N1
atom and former central carbon atom C9 of the allene ligand.
The Pt atom is found in an approximately square planar
geometry completed by two chloride ligands. The free
pyridinium ring is a donor of a good intramolecular hydrogen
bond N(22)-H(22)...Cl(1) at 2.33(9) Å.20
2a
3a
Scheme 1. Reactions of allene 1a with Au(I) and Au(III) complexes and ORTEP
representation of the X-ray structures of complexes 2a and 3a (Hs omitted).
The non-symmetric allene 1b was also subjected to
coordination with Au(I) and Au(III) under similar reaction
conditions (Scheme 2). Interestingly, reaction with Au(III) gave
a mixture of complex 4b (60%), in which the Au is coordinated
to the two pyridines with the allene backbone intact,14 with
complexes 3b’:3b’’ that crashed out of methanol solution as a
0.17:1 mixture that did not change over time in solution (20%).
Good quality crystals of 3b’’ allowed full characterisation by X-
ray crystallography (see ESI). On the other hand, the Au(I)
complex was generated quantitatively in 15 min as a 1:0.12
mixture of 2b’:2b’’ that slowly equilibrated in solution to >99%
of 2b’’ (kisom. = 6*10-4 s-1). The equilibration process between the
kinetic (2b’) and thermodynamic (2b”) product could be viewed
as a reversible cyclisation/ring opening reaction with the AuCl
fragment moving across the allene core via the carbene/vinyl-
Au intermediate.15 Depending on the time of reactions involving
2b as catalyst, the kinetic (2b’) or thermodynamic (2b”) complex
can be identified as the active catalyst. Alternatively, the kinetic
isomer 2b’ may be first allowed to equilibrate fully to the
thermodynamic one, 2b’’, that can be used pure in catalytic
reactions. Good quality crystals of 2b’’ allowed full
characterisation by X-ray crystallography (see ESI).
tBu
tBu
Ph
N
L-ascorbic acid
10 equiv.
H2O, 25oC
•
N
Pt
Cl
Cl
N
PtIV
Cl
Cl
Cl
Cl
HN
5, 89%
4c
Solid state analysis of Au(I) and Au(III) complexes pointed to
many structural similarities within the class and with the
pyridine-phosphine complex reported by Fensterbank.9 The
Scheme 3. Reduction of Pt(IV)-complex 4c to Pt(II)-complex 5 and ORTEP
representation of the X-ray structure of 5.
2 | J. Name., 2012, 00, 1-3
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Au-vinyl and carbene complexes are commonly proposed
intermediates in Au-catalysed addition to allenes.21 Besides,
NHC (N-heterocyclic carbene)-Au complexes are very efficient
catalysts due to the -donor properties of the carbenes, as
compared with other ligands, e.g. phosphines.22 However, the
use of Au-non-stabilised carbenes as direct catalysts are
scarcely reported.23 The catalytic activity of our new class of
metal carbenes was assessed in the cycloisomerisation and
alkoxycyclisation of 1,6-enynes, benchmark reactions for
evaluation of novel Au and Pt complexes.24 Enyne 6 was
subjected to a reaction with 2 mol% of the catalyst in CH2Cl2 at
rt, standard conditions for reactions using [R3PAu+] complexes,
with a reaction time arbitrarily set at 60 min for comparison
purposes (Scheme 4). For reactions with a nitrogen tether
enyne derivative, see ESI. It was determined in initial
experiments that reactions with neutral complexes proceeded
very sluggishly or not at all, so all reactions were carried out in
the presence of AgNTf2 to generate cationic species.
Although NHC-Au complexes have not been widely used in
the cycloisomerisation of 1,6-enynes, carbene-type complexes
2a and 2b (as a mixture of 2b’:2b’’, 1:0.12) efficiently catalysed
the selective formation of skeletal rearrangement product 7a
with similar reactivity and selectivity to [R3PAu+] complexes.
Complex 2b’’ gave similar results to the mixture 2b’:2b’’, where
2b’ is the major isomer (Scheme 4).
Use of Au(III) carbenes 3a-b (Scheme 4) resulted in a full
conversion of the enyne at room temperature, in milder
conditions than AuCl3 complexes, which normally require the
reaction to be carried out in toluene at 80 oC. The reaction using
Cat. (3 mol%)
Conv. = >99%V; (iQeuwant.A) rticle Online
Cat. = 2a
MeO2
MeO2
C
MeO2
MeO2
C
C
AgNTf2 (4 mol%)
Cat. = 2b (2b’:2b’’, 1:0.12) Conv. = >99%; (Quant.)
C
DOI: 10.1039/D0DT00665C
MeOH, 35-50 o
24h
C
Cat. = 3a
Cat. = 3b (3b’:3b’’, 0.5:1) Conv. = 91%; (91%)
Cat.= 5 Conv. = 71%; (54% + 16% hydroxycyclisation)
Conv. = >99%; (90% + 7% of 7a)
10a
OMe
6
(0.1 mmol, 1 eqiv.)
Scheme 5. Alkoxycyclisation reaction of enyne 6. (Isolated yields in brackets).
In conclusion, bis(pyridyl)allenes are effective precursors for
novel Au(I), Au(III) and Pt(II) complexes by nucleophilic attack of
one of the subtituents into the metal-activated allene. Although
the new complexes could be initially described as vinyl-metal
structures, X-Ray analysis showed that the new metal carbon
bonds are in the same range as previously described metal-
carbene complexes. The catalytic activity of these metal
carbene-type complexes has been tested for the first time and
probed against benchmark reactions for novel Au and Pt
catalysts. Au(I) complexes 2 showed the more promising
catalytic activity, in similar efficiencies to commonly used
[R3PAu+] complexes. Au(III) and Pt(II) complexes (3 and 5
respectively) were also active in the tested reactions under mild
conditions. Further screening in more challenging reactions and
their asymmetric variant are currently undergoing in our
laboratory.
Conflicts of interest
“There are no conflicts to declare”.
Acknowledgements
complex 3a yielded a mixture of isomeric products 7a and 7a’, Funding by the University of East Anglia is gratefully
the latter being predominant.
acknowledged. Authors would like to thank Dr Abdul-Sada
Pt(II)-complex 5 was less reactive than the Au complexes, (University of Sussex) for HRMS analysis and Stephen Boyer
however, it selectively promoted formation of the Alder-ene (London Metropolitan University) for elemental analysis.
type compound 7b under milder conditions than the commonly
employed PtCl2 that needs toluene at 80 oC to give good results
(Scheme 4).
Notes and references
All reactions performed can achieve full conversion of the SM at
room temperature, with prolonged reaction times or increased
catalyst loading, which might be an advantage when carrying
out reaction with Pt(II) and Au(III) complexes, that normally
require harsher temperature conditions.
1
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MeO2C
Cat. (2 mol%)
MeO2C
MeO2C
2
3
MeO2C
MeO2C
MeO2C
AgNTf2 (2.5 mol%)
MeO2C
MeO2C
CH2Cl2 (0.2M),
25 oC, 1h
7b
6
7a
Conv. = >99%; 7a (77%)
7a'
(0.1 mmol, 1 eqiv.)
Cat. = 2a
Cat. = 2b (2b’:2b’’, 1:0.12) Conv. = 83%; 7a (83% with traces of hydroxycyclisation product)
4
5
Cat. = 2b’’
Cat. = 3a
Conv. = >99%; 7a (Quant. with 5% of unidentified product)
Conv. = >99%; 7a:7a’= 0.6:1 (79%)
Cat. = 3b (3b’:3b’’, 0.5:1) Conv. = >99%; 7a (77%)
Cat. = 5 Conv. = 57%; 7a:7b = 0.33:1 (52%)
Scheme 4. Cycloisomerisation reaction of enyne 6. (Isolated yields in brackets).
All catalysts were also found to be active in the
alkoxycyclisation reaction of 6 with 3 mol% catalyst loading in
6
7
o
MeOH at rt to 50 C, standard reaction conditions used with
[R3PAu(I)+], [NHC-Au+] and PtCl2 catalysts (Scheme 5). The best
results were obtained with Au(I) complexes 2a-b resulting in
quantitative formation of addition/cyclisation product 10a.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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4 | J. Name., 2012, 00, 1-3
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Dalton Transactions
MeO2
MeO2
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C
R
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N
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MLn
Catalysis
N
MeO2
C
MeO2
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Novel metal carbene complexes made from
bis(pyridyl)allenes have promising catalytic activity.