The coordination and polymerisation of cyclic 1,3-dienes by gold(i) cations

Article abstract of DOI:10.1039/c1cc16193h

Cyclopentadiene and 1,3-cyclohexadiene are readily polymerised by [LAu][X]. With specific ancillary ligands polymerisation was suppressed and a molecular species involving an Au(i) cation η²-bonding CpH was isolated and whose structure was probed in both the solid state and in solution.

Full text of DOI:10.1039/c1cc16193h

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COMMUNICATION
The coordination and polymerisation of cyclic 1,3-dienes by
gold(I) cationsw
Rajashekharayya A. Sanguramath, Sanjib K. Patra, Michael Green and
Christopher A. Russell
Received 5th October 2011, Accepted 21st November 2011
DOI: 10.1039/c1cc16193h
Cyclopentadiene and 1,3-cyclohexadiene are readily polymerised
by [LAu][X]. With specific ancillary ligands polymerisation was
suppressed and a molecular species involving an Au(I) cation
g2-bonding CpH was isolated and whose structure was probed
in both the solid state and in solution.
(presumably AgCl) was necessary to avoid the formation of
1
additional unidentified products (by both 31P and H NMR).
The solid was redissolved in a minimum amount of CH2Cl2 and
layered with n-hexane which yielded colourless crystals of 1 (Fig. 1)
which contained the desired [(Z2-CpH)Au(P(But)2(o-biphenyl))]+
cation.z
In the solid state, Au binds asymmetrically across one of the
double bonds of CpH in a Z2 fashion (B4% slippage towards
–CH2 end). The coordinated CQC bond (1.358(5) A) is
elongated compared to the uncoordinated bond (1.332(6) A).
The exponential growth of gold chemistry in the past decade
has been exemplified by the myriad of cyclisation reactions
using alkynes and allenes as the reactive substrate;1 however,
the extension of Au(I) catalysis to activate alkenes and dienes
has been comparatively limited, possibly due to their propensity
for side reactions.2
Mꢀ ꢀ ꢀC interactions with the o-biphenyl unit of
a
M–P(But)2(o-biphenyl) unit are well documented; for gold(I),
an Auꢀ ꢀ ꢀC distance of o 2.95 A was suggested as representing
a significant interaction.11 In 1, the shortest Auꢀ ꢀ ꢀC contact is
3.05 A and the P1–Au1–(CQCcentroid) angle (172.61) deviates
only slightly from linear, suggesting minimal interaction with
the o-biphenyl unit. In contrast to the solid-state structure, the
room temperature 1H NMR of 1 showed only two resonances
for the alkene protons (d = 6.41 and 6.58 ppm) suggesting a
fluxional process may be operating. Upon cooling, one of the
peaks (6.41 ppm) broadened and began to de-coalesce into two
peaks, whereas the other peak (6.58 ppm) did not de-coalesce
even at ꢁ90 1C but did become broader (see ESIw). These
observations are consistent with our recently proposed gliding
mechanism for a [LAu]+ cation over an acyclic 1,3-diene
backbone.6a
Recent work has sought to probe the coordination behaviour
of cationic gold(I) complexes of the type [LAu]+ (L = phosphine,
NHC) with alkynes,3 allenes,4 alkenes5 and acyclic 1,3-dienes6
by careful reactions and detailed characterisation of isolated
complexes. One notable omission from this list is the cyclic
dienes6b which have a proven record as a useful class of sub-
strate in gold catalysis. For example, cyclic dienes in general
and conjugated cyclic dienes in particular have been used as
substrates in Au(I) catalysed intramolecular hydroammination
of 1,3-dienes to form hexahydroindoles and octahydrocyclo-
hepta pyrroles,7 cyclisation of unactivated 6,8-diene-1-ynes to
form carbotetracycles,8 annulations of phenols with dienes9
and addition of active methylenes to dienes to form functio-
nalised carbocycles.10 Despite their utility as synthons for
many Au(I) catalysed transformations, experimental evidence
regarding their coordination chemistry is lacking.6b Herein we
seek to address this issue.
To the best of our knowledge there are no reported examples
of mononuclear metal Z2-CpH complexes (although there are a
few reports with Cp derivatives12). The ‘‘mainstream’’ transition
metals commonly form Z4 complexes with CpH; in addition,
dinuclear bis-Z2 complexes have been observed where separate
metal centres coordinate the two double bonds.13
As a preliminary investigation we chose to react cyclopenta-
diene (CpH, 1.5 equiv.) with a gold cation generated from the
slurry of [P(But)2(o-biphenyl)AuCl]/AgSbF6 (1 equiv.) in
CH2Cl2 at room temperature (Scheme 1). The reaction was
complete within 5 min. (as monitored by 31P NMR) and
immediate dilution of the reaction and filtration of the precipitate
In continuing our investigation, we sought to probe the role
of the phosphine ligand coordinated to the gold centre. Thus
we treated an excess of CpH with a mixture of But3PAuCl/
AgSbF6 in CH2Cl2. After 16 h at room temperature, 1H NMR
spectroscopy showed polycyclopentadiene (vide infra) as the
School of Chemistry, University of Bristol, Cantock’s Close, Bristol,
UK BS8 1TS. E-mail: Chris.Russell@bris.ac.uk;
Fax: +44 (0)117 929 0509; Tel: +44 (0)117 928 7599
w Electronic supplementary information (ESI) available: Syntheses
and experimental data for all complexes and materials. CCDC
847613 and 847614. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1cc16193h
Scheme 1 Synthesis of cyclopentadiene complex 1.
c
1060 Chem. Commun., 2012, 48, 1060–1062
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Scheme 2 Synthesis of polycyclopentadiene.
The reaction was optimised for high substrate conversion by
screening potential Au(I) salts, Ag(I) salts and solvents
(Table 1). As is evident from Table 1, the polymerisation
reaction was sensitive to the nature of the ancillary ligand
on Au(I), the counter ion and the solvent. Near quantitative
conversion of CpH was observed when more Lewis acidic
[LAu]+ cations with the [SbF6]ꢁ counter anion were employed
as the precatalyst (entries 3 and 4). The reaction failed to occur
upon employing strongly electron donating phosphines as
ancillary ligands (entries 5 and 6). The Ph3PAuCl/AgSbF6
combination was found to give the highest conversion to the
polymers. Control experiments using either the Au(I) or Ag(I)
source alone gave no conversion.
Fig. 1 Thermal ellipsoid plot of the cation of 1 shown at 50%
probability level; all hydrogen atoms and the [SbF6] counter anion
are omitted for clarity. Selected bond lengths (A) and angles (1):
Au1–C1
2.279(3),
Au1–C2
2.311(3),
Au1–P1
2.2908(6),
C1–C2 1.358(5), C3–C4 1.332(6), C2–C3 1.469(5), C1–C5 1.495(4),
C4–C5 1.494(5); P1–Au1–C1 167.15(9), P1–Au1–C2 156.90(9),
C1–Au1–C2 34.40(12).
major product and a crystalline material, 2, containing the
[(Z2-(CpH)2)Au(PBut3)]+ cation as a minor product. A rational
synthesis of 2 was achieved by the direct reaction of dicyclo-
pentadiene with a stoichiometric amount of But3PAuCl/
AgSbF6. The structure of 2 (Fig. 2) was confirmed by X-ray
crystallography showing the Au(I) cation binding to one of the
CQC bonds in an endo fashion.
The reaction either becomes very slow (entry 13) or shuts
down (entry 11) when coordinating solvents were used. Very
low conversion of CpH was observed with toluene as a solvent
presumably due to both the Au(I) and Ag(I) sources being
insoluble in this solvent, which in turn makes it difficult to
generate the proposed active catalyst system [R3PAu]+[SbF6]ꢁ.
CH2Cl2 proved to be the best solvent for the reaction.
With regards to the formation of 2 it was unclear if the
dimerisation of CpH was induced by metal complexation or
whether complexation followed (4+2) dimerisation. However,
the observation of oligomerisation was of special interest and
we sought to study the reaction in greater detail. Thus we
treated CpH with a catalytic amount of Ph3PAuCl/AgSbF6
(2 mol%) in CH2Cl2 at room temperature which after only
15 min. led exclusively (1H NMR) to polycyclopentadiene
formed via competing 1,2 and 1,4-addition reactions
(Scheme 2).14 To our knowledge, there is only a single report
of a gold-catalysed olefin polymerisation, that of the Au(III)-
mediated polymerisation of styrene.15 Thus the current report
contains the first examples of olefin polymerisation using a
gold(I) precatalyst. In addition, the substrates employed
produce hydrocarbon polymer products which are of great
technological interest as new materials with high glass-
transition temperature and low dielectric constant.14,16
Analysis of the polymer (Table 2) showed that a high
molecular weight polymer with reasonably good molecular
weight distribution (PDI) was obtained when a bulky biaryl-
dialkylphosphine gold(I) source was employed as the catalyst
(entry 5). A pronounced change in polymer chain length was
observed upon changing the counter ion (entry 1). The poly-
mer chain length decreased with increase in catalyst loadings
(entries 2–4). The isolated polymer showed a glass transition
temperature Tg of 68 1C (Mn = 26.8 kDa), which is consistent
with literature value (Fig. 3).14b
Table 1 Au(I)-catalysed CpH polymerisationa
Entry Au(I)
Ag(I)
Solvent % conversionb
1
2
Ph3PAuCl
CH2Cl2
0
0
AgSbF6
3
4
5
6
7
8
9
10
11
12
13
14
Ph3PAuCl
Me2SAuCl
Cy3PAuCl
But3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
(MeO)3PAuCl
Ph3PAuCl
Ph3PAuCl
Ph3PAuCl
AgSbF6 CH2Cl2 495
AgSbF6 CH2Cl2 495
AgSbF6 CH2Cl2
AgSbF6 CH2Cl2
AgOTf CH2Cl2
AgBF4 CH2Cl2
0
0
86
10c
0
Ag2O
CH2Cl2
AgSbF6 CH2Cl2
AgSbF6 THF
0
0
AgSbF6 Toluene
AgSbF6 MeNO2
5
5
11
(P(But)2(o-biphenyl))AuCl AgSbF6 CH2Cl2
Fig. 2 Thermal ellipsoid plot of the cation of 2 shown at 50%
probability level; all hydrogen atoms and [SbF6]ꢁ anion are omitted
for clarity. Selected bond lengths (A) and bond angles (1): Au1–C1 2.278(7),
Au1–C2 2.301(7), C1–C2 1.351(11), C8–C9 1.394(11), Au1–P1 2.2964(16),
C1–Au1–C2 34.3(3), P1–Au1–C1 164.5(2), P1–Au1–C2 160.2(2).
a
CpH (1.2 mmol) was added to a CH2Cl2 (2 mL) solution of 2 mol%
Au(I)/Ag(I) and stirred for 15 min. in the absence of light at room
temp. Determined by 1H NMR spectroscopy. The kinetics of this
b
c
reaction were slow. After 2 h, conversion was 495%.
c
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Chem. Commun., 2012, 48, 1060–1062 1061

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Table 2 Molecular weight distribution of the polycyclopentadienea
Crystal data for 2: C22H39AuPF6Sb, M = 767.22, monoclinic, a =
8.4126(11) A,
b = 13.2481(19) A, c = 12.0197(15) A, b =
103.296 (8)1, V = 1303.7(3) A3, T = 100 K, space group P21,
Z = 2, m(Mo-Ka) = 6.768 mmꢁ1, 14 619 reflections measured, 5644
independent reflections (Rint = 0.0378). The final R1 [I 4 2s(I)] was
0.0321 and the final wR(F2) was 0.0470 (all data). The final R1 value
was 0.0321 (I 4 2s(I)). The final wR(F2) value was 0.0666 (I 4 2s(I)).
The goodness of fit on F2 was 0.945. CCDC 847 614.
Entry Precatalyst
PDI Mn (kDa)
1
2
3
4
5
2 mol% Ph3PAuCl/AgOTf
3.42 2.1
4.00 28.9
2.89 26.8
3.74 17.6
2 mol% Ph3PAuCl/AgSbF6
5 mol% Ph3PAuCl/AgSbF6
10 mol% Ph3PAuCl/AgSbF6
5 mol% (P(But)2(o-biphenyl))AuCl/AgSbF6 2.03 71.4
1 (a) A. Furstner and P. W. Davies, Angew. Chem., Int. Ed., 2007,
¨
a
CpH (1.2 mmol) was added to a CH2Cl2 (2 mL) solution of catalyst
and stirred for 15 min. in the absence of light at room temp.
46, 3410; (b) A. S. K. Hashmi, Chem. Rev., 2007, 107, 3180;
(c) D. J. Gorin, B. D. Sherry and F. D. Toste, Chem. Rev., 2008,
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49, 5232.
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2010, 132, 9292; (b) D. Garayalde, K. Kruger and C. Nevado,
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C. A. Russell, Chem. Commun., 2009, 3877; (b) T. J. Brown,
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6451; (c) T. J. Brown, M. G. Dickens and R. A. Widenhoefer,
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To further assess the scope of Au(I) catalysed polymerisation,
1,3-cyclohexadiene was treated with 5 mol% of Ph3PAuCl/
AgSbF6 in CH2Cl2. After 2 h, 1,3-cyclohexadiene had poly-
merised to yield (1H NMR) polycyclohexadiene quantitatively
with a combination of both 1,2 and 1,4-addition (Scheme 3).17
The polycyclohexadiene from these reactions had a relatively
narrow molecular weight distribution (PDI 1.41) and moderate
size with Mn of 5.2 kDa. A glass transition temperature of 125 1C
was recorded, in agreement with that in the literature.17a,e
6 (a) R. A. Sanguramath, T. N. Hooper, C. P. Butts, M. Green,
J. E. McGrady and C. A. Russell, Angew. Chem., Int. Ed., 2011,
50, 7592; (b) R. E. M. Brooner and R. A. Widenhoefer, Organo-
metallics, 2011, 30, 3182.
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T. G. Gray, Organometallics, 2008, 27, 28; (b) P. Perez-Galan,
Fig. 3 (a) GPC curve (RI response, THF as eluent) of the polymer
obtained with 5 mol% (P(But)2(o-biphenyl))AuCl/AgSbF6 catalyst
system (b) DSC scan for a polycyclopentadiene at a scan rate of
10 1C minꢁ1
.
N.
A. M. Echavarren, Chem.–Eur. J., 2010, 16, 5324.
Delpont,
E.
Herrero-Gomez,
F.
Maseras
and
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Stone, J. Organomet. Chem., 1982, 232, 279; (b) J. Vicente,
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´ ´
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Scheme 3 Polymerisation of 1,3-cyclohexadiene.
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We thank the University of Bristol (M.G., C.A.R., R.A.S.)
and the European Union (Marie Curie Fellowship for S.K.P.)
for financial support. We thank Umicore AG & Co. KG for
the generous donation of HAuCl4.
Notes and references
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z Single crystals of complexes 1 and 2 suitable for X-ray diffraction
were grown from saturated CH2Cl2 solutions layered with n-hexane (1)
or Et2O (2) followed by storage at 4 1C (1) and ꢁ18 1C (2) for 48 h.
Crystals were mounted in inert oil and transferred to the cold gas
stream of the diffractometer. Structures were solved using SHELXS
and refined using SHELXL.18
Crystal data for 1: C25H33AuF6PSb, M = 797.21, monoclinic, a =
9.6951(2) A, b = 20.5032(4) A, c = 13.9332(3) A, b = 104.6250(10)1,
V = 2679.91(10) A3, T = 100 K, space group P21/c, Z = 4,
m(Mo-Ka) = 6.589 mmꢁ1, 46 190 reflections measured, 6223 indepen-
dent reflections (Rint = 0.0224). The final R1 [I 4 2s(I)] was 0.0188
and the final wR(F2) was 0.0491 (all data). The goodness of fit on F2
was 1.021. CCDC 847 613.
c
1062 Chem. Commun., 2012, 48, 1060–1062
This journal is The Royal Society of Chemistry 2012