Expanded ring N-heterocyclic carbene complexes of zero valent platinum dvtms (divinyltetramethyldisiloxane): Highly efficient hydrosilylation catalysts

Article abstract of DOI:10.1016/j.jorganchem.2010.08.045

The synthesis and characterisation of a series of new 6- and 7-membered NHC (N-heterocyclic carbene) complexes 7-12 of zero valent platinum dvtms (divinyltetramethyldisiloxane), [Pt(NHC)(dvtms)] are reported. A number of the complexes were investigated as catalyst precursors in the hydrosilylation of a range of unsaturated substrates, including alkynes, alkenes and a ketone with triethylsilane and bis(trimethylsiloxy)methylsilane (MD'M), at low catalyst loading (0.005 mol %); in general, the activity, and the selectivity for 1-functionalised product was found to be high.

Full text of DOI:10.1016/j.jorganchem.2010.08.045

Journal of Organometallic Chemistry 696 (2011) 188e194
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Journal of Organometallic Chemistry
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Expanded ring N-heterocyclic carbene complexes of zero valent platinum dvtms
(divinyltetramethyldisiloxane): Highly efficient hydrosilylation catalysts
*
Jay J. Dunsford, Kingsley J. Cavell , Benson Kariuki
School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and characterisation of a series of new 6- and 7-membered NHC (N-heterocyclic carbene)
complexes 7e12 of zero valent platinum dvtms (divinyltetramethyldisiloxane), [Pt(NHC)(dvtms)] are
reported. A number of the complexes were investigated as catalyst precursors in the hydrosilylation of
a range of unsaturated substrates, including alkynes, alkenes and a ketone with triethylsilane and bis
(trimethylsiloxy)methylsilane (MD’M), at low catalyst loading (0.005 mol %); in general, the activity, and
the selectivity for 1-functionalised product was found to be high.
Received 11 August 2010
Received in revised form
25 August 2010
Accepted 25 August 2010
Available online 6 September 2010
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
N-heterocyclic carbenes
Platinum
Hydrosilylation
1. Introduction
catalysts demonstrated high activities but are highly moisture
sensitive and demonstrate poor selectivity as the result of the
N-heterocyclic carbenes have long been recognised as important
ligands in organometallic chemistry and catalysis.1 Recently,
expanded-ring 6- and 7-membered NHCs, new additions to the
ligand armoury have been attracting increasing attention within
formation of colloidal platinum. More recently, studies by Markó
have demonstrated the excellent performance of 5-membered NHC
platinum dvtms (divinyltetramethyldisiloxane) complexes in
hydrosilylation.6 The application of in situ formed Pt(NHC) catalysts
in highly selective hydrosilylation of styrene has also been previ-
ously reported.7 The aim here is to investigate the utilisation of
the literature.2 Their strong
s-donating nature and sterically
imposing distribution of ring substituents2aec should make them
attractive stabilising ancillary ligands for a range of catalytic
transformations. However, to date little is known about their
application in catalysis.
the more sterically demanding and stronger
s e donor 6- and 7-
membered derivatives as ligands in this reaction.
We have previously reported a simple, versatile and high
yielding route to 6- and 7-membered NHC precursors2a based upon
the methodology of Bertrand.8 We have also described the
synthesis of Ag, Rh, Ir and Ni complexes of large ring NHCs.2a,c,d,f
Here we report the synthesis and characterisation of the first 6-
and 7-membered zero valent platinum dvtms complexes and their
catalytic performance in hydrosilylation using a range of substrates.
We first reported the syntheses and structures of a series of
7-membered formamidinium NHCs2a and their utilisation as
ligands in catalysis.2d In the catalytic reactions investigated to date
we have found that the large rings offer significant advantages over
the traditional 5-membered derivatives in some applications. To
further expand upon our understanding of these ligands we report
here on their use in catalytic hydrosilylation.
Catalytic hydrosilylation, defined as the addition of H-SiR3
compounds to unsaturated carbonecarbon bonds (and unsaturated
carboneoxygen bonds), is an important route to organosilicon
compounds.3 Furthermore, the reaction fulfils the principle of
‘green chemistry’ whereby if selectivity is high it provides an atom
economical route to such compounds.
2. Results and discussion
2.1. Synthesis of complexes
The formamidinium salts 1e6 were prepared under aerobic
conditions by the reaction of the appropriate N,N00-diarylamidine
with 1,3-dibromopropane or 1,4-diiodobutane in refluxing aceto-
nitrile in the presence of half an equivalent of potassium carbonate
(Scheme 1) as previously described by our group.2a
The first examples of highly active platinum hydrosilylation
catalysts were the Speier catalyst4 and the Karstedt catalyst.5 These
* Corresponding author.
E-mail address: cavellkj@cf.ac.uk (K.J. Cavell).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.08.045

J.J. Dunsford et al. / Journal of Organometallic Chemistry 696 (2011) 188e194
189
Table 1
Characteristic 195Pt NMR shifts of platinum (0) complexesa.
Complex
195Pt NMR (ppm)
[Pt(5-Mes)(dvtms)]6a
[Pt(6-Mes)(dvtms)]
[Pt(6-Xyl)(dvtms)]
[Pt(6-Tolo)(dvtms)]
[Pt(6-DIPP)(dvtms)]
[Pt(6-DE)(dvtms)]
ꢀ5339
ꢀ5347.7
ꢀ5345.3
ꢀ5311.7
ꢀ5377.7
ꢀ5335.3
a
Measured in CDCl3, unless otherwise noted.
Scheme 1. Synthesis of saturated N-aryl substituted expanded N-heterocyclic carbene
salts.
example, the observed shifts in [Pt(5-Mes)(dvtms)] and [Pt(6-Mes)
(dvtms)] 7 are ꢀ5339 ppm,6a and ꢀ5347.66 ppm respectively. On
the other hand the corresponding 13C NMR shifts for the N-CNHC-N
carbon are notably different (Table 2), with the [Pt(5-Mes)(dvtms)]
being 184.2 ppm6a and complex 7 211.5.
The synthesis of the zero valent platinum NHC complexes 7e12
(Scheme 2) follows a protocol first described by Markó6a and
consists of suspending the NHC halide salt (1.1 eq.) in a solution of
commercially available Karstedt’s catalyst (1 eq. of platinum) in dry
t
toluene, followed by the addition of BuOK (1.4 eq.) and stirring at
ambient temperature for 17 h. This protocol yields the corre-
sponding [Pt(NHC)(dvtms)] complexes; single crystals suitable for
X-ray diffraction were isolated by slow crystallisation from
divinyltetramethyldisiloxane solution at ambient temperature.
Table 2
13C NMR data for CNHC in the amidinium salts and Pt0 complexes.a
CNHC ppm
Ref
5-Mes
[Pt(5-Mes)(dvtms)]
6-Mes
[Pt(6-Mes)(dvtms)]
6-Xyl
158.9
184.2
153.5
211.5
153.3
2b
6a
2a
2a
2a
2a
b
[Pt(6-Xyl)(dvtms)]
_
6-Tolo
153.7
[Pt(6-Tolo)(dvtms)]
6-DIPP
_b
152.8
210.7
153.3
216.6
[Pt(6-DIPP)(dvtms)]
6-DE
[Pt(6-DE)(dvtms)]
a
Values given in ppm, measured in CDCl3.
Not observed.
Scheme 2. Synthesis of platinum (0) dvtms complexes.
b
It was noted by Markó that the 5-membered NHC derivatives
Allowing that the 195Pt NMR shifts are determined by
-donating ability of the ancillary NHC ligand, and the extent of
demonstrated
a high tolerance to aerobic conditions, which
s
remains the case here as in both the solid and solution states they
remain indefinitely stable.
back donation from the platinum centre, the similarity in the 195Pt
shifts is somewhat surprising. The 195Pt NMR shifts in Table 1 would
seem to indicate that, either the expanded-ring NHC derivatives are
As previously noted,2b,c the NHC ligands increase in steric demand
with increasing ring size. This is very evident here and the combi-
nation of two bulky ligands (7-NHC and dvtms) meant that very low
yieldsof[Pt(7-NHC)(dvtms)] complexes wereisolated, andinthecase
of [Pt(7-DIPP)(dvtms)] and [Pt(7-DEP)(dvtms)] no complex could be
isolated at all. Currently we are investigating the synthesis of
complexes using less sterically demanding 7-NHC ligands.
not stronger
s-donors than their 5-membered counterparts, or
back-bonding from platinum to the NHC ligand and/or the coor-
dinated alkenes exactly counterbalances any increased donor
capacity, resulting in comparable 195Pt resonances. If the latter is
the case, it may be expected that this would be reflected in the Pt to
alkene bond lengths, and/or the C]C bond lengths of the Pt-
coordinated alkenes in the 6- and 7-membered derivatives.
However, distances appear to be essentially the same; for C]C
lengths for example, [Pt(5-Mes)(dvtms)] 1.436(7) and 1.419(7), 7
1.412(6) and 1.431(5), and 12 1.413(6) and 1.425(6). Furthermore,
there is a slight elongation of the PteCNHC bond lengths in the large
ring complexes compared to the 5-membered derivatives {[Pt(5-
Mes)(dvtms)] 2.046(4)6a}, suggesting that for larger rings there is
2.2. Characterisation
Upon formation of the platinum (0) dvtms complexes 1H NMR
spectra reveal the disappearance of the characteristic for-
mamidinium salt NCHN proton at ca 7.5 ppm, indicative of complex
formation. The 13C NMR spectra shows a pronounced downfield
shift of the N-CNHC-N carbon, for example from 153.5 ppm in the
formamidinium salt 1 to 211.5 ppm in the corresponding platinum
complex 7, which is again indicative of complex formation. The
195Pt NMR shifts of the complexes enable the electronic environ-
ment of the platinum centre to be probed in more detail. Markó
noted that the 195Pt shifts in [Pt(NHC)(dvtms)] complexes were
little
p* back-bonding from the platinum centre to the NHC.
However, it is possible that the significant steric pressure exerted
by the large-ring NHCs causes bond elongation, offsetting any
impact of donor strength or p* back-bonding.
The crystal structures of three [Pt(n-NHC)(dvtms)] complexes
have been determined. Selected bond lengths and angles for
complexes 7, 9 and 12 are provided in Table 3 and the structures for
7, 9 and 12 are provided in Figs. 1e3.
essentially down to two contributory factors: the s-donating ability
of the ancillary NHC ligand (the more electron rich the ligand the
more downfield the shift), and also the extent of back donation
from the platinum centre to the
p* orbital of the ancillary NHC
ligand and the alkene moieties leading to a decrease in electron
2.3. Hydrosilylation catalysis
density at the platinum centre and an up field shift.6a
The 195Pt NMR shifts of the 5- and 6-membered platinum (0)
dvtms derivatives were found to be comparable (Table 1). For
Regioselectivity in the hydrosilylation of unsaturated carbone
carbon compounds is an important consideration. Imidazolium

190
J.J. Dunsford et al. / Journal of Organometallic Chemistry 696 (2011) 188e194
Table 3
Selected bond lengths (Å) and angles (deg) in selected platinum (0) complexesa.
7
9
12
PteCNHC
2.073 (4)
2.120
2.139
2.072 (5)
2.132
2.149
2.070 (4)
2.124
2.140
b
PteCC¼C
PteCC¼C
c
C
C
C
NHCeN
C¼CeCC¼C
C¼CeCC¼C
1.347 (5)
1.412 (6)
1.431 (5)
116.3 (3)
81.56
1.339 (7)
1.435 (10)
1.423 (9)
116.56 (4)
88.53
1.358 (5)
1.413 (6)
1.425 (6)
117.1 (3)
85.90
d
d
N-CNHC-N
Torsion (ꢁ)
a
Abbreviations: CNHC, carbene carbon.
b
c
Refers to the distance of platinum to the terminal carbon.
Refers to the distance of platinum to the internal carbon.
Refers to the distance between olefinic carbons.
d
Fig. 3. ORTEP ellipsoid plot at 30% probability of the molecular structure of 12. Solvent
molecules have been omitted for clarity. Selected bond angles (Å) and angles (deg) for
12: C(5)-Pt(1), 2.070(4); C(5)-N(1), 1.358(5); C(5)-N(2), 1.348(5); C(1)-N(1), 1.496(5); C
(4)-N(2), 1.485(5); C(14)-N(1), 1.436(5); C(23)-N(2), 1.448(5); C(6)-C(7), 1.413(6); C(10)-
C(11), 1.425(6); C(6)-Pt(1), 2.126; C(10)-Pt(1), 2.124; C(7)-Pt(1), 2.145; C(11)-Pt(1),
2.140; C(11)-Si(1), 1.839(4); C(7)-Si(2), 1.845(4); N(1)-C(5)-N(2), 117.1(3); N(1)-C(5)-Pt
(1), 119.3(3); N(2)-C(5)-Pt(1), 123.7(3); C(5)-Pt(1)-C(6), 93.9; C(5)-Pt(1)-C(10), 95.9; C
(5)-Pt(1)-C(7), 131.7; C(5)-Pt(1)-C(11), 134.3.
The application of expanded-ring NHC ancillary ligands in
hydrosilylation would seem to be an ideal progression. A spectator
ligand with increased basicity and providing increased steric
protection of the platinum centre would be expected to enhance
catalytic activity, by stabilising the active catalyst, and by promoting
reductive elimination. The large N-CNHC-N angles in expanded-ring
6- and 7-membered NHC’s force the N-aryl substituents toward the
platinum centre essentially blocking two faces of the metal coor-
dination sphere.
Fig. 1. ORTEP ellipsoid plot at 30% probability of the molecular structure of 7. Solvent
molecules have been omitted for clarity. Selected bond angles (Å) and angles (deg) for
7: C(4)-Pt(1), 2.073(4); C(4)-N(1), 1.348(5); C(4)-N(2), 1.347(5); C(22)-N(2), 1.434(5); C
(13)-N(1), 1.443(5); C(1)-N(2), 1.481(5); C(3)-N(1), 1.486(5); C(5)-C(6), 1.412(6); C(9)-C
(10), 1.431(5); C(5)-Pt(1), 2.120; C(9)-Pt(1), 2.132; C(6)-Pt(1), 2.139; C(10)-Pt(1), 2.135;
C(6)-Si(1), 1.840(4); C(10)-Si(2), 1.851(4); N(1)-C(4)-N(2), 116.3(3); N(2)-C(4)-Pt(1),
123.6(3); N(1)-C(4)-Pt(1), 120.1(3); C(4)-Pt(1)-C(5), 95.7; C(4)-Pt(1)-C(9), 93.5; C(4)-Pt
(1)-C(6), 134.1; C(4)-Pt(1)-C(10), 131.8.
Three of the [Pt(n-NHC)(dvtms)] complexes 7, 9 and 10 were
tested in hydrosilylation, probing the effect of steric hindrance on
catalytic activity and regioselectivity (Scheme 3).
Scheme 3. The range of steric influence investigated in hydrosilylation.
2.3.1. Hydrosilylation of 1-octyne
The hydrosilylation of 1-octyne yields three possible
regioisomers (Scheme 4) but in the case of platinum-based catalysts
the
b
-(Z) isomer, I is not produced leaving the usually predominant
-isomer, III.
b-(E) isomer, II and the
a
Fig. 2. ORTEP ellipsoid plot at 30% probability of the molecular structure of 9. Solvent
molecules have been omitted for clarity. Selected bond angles (Å) and angles (deg) for
9: C(9)-Pt(1), 2.072(5); C(9)-N(1), 1.339(7); C(9)-N(2), 1.345(7); C(20)-N(2), 1.442(8); C
(13)-N(1), 1.461(7); C(12)-N(2), 1.496(7); C(10)-N(1), 1.484(6); C(1)-C(2), 1.423(7); C(5)-
C(6), 1.435(7); C(1)-Pt(1), 2.127; C(5)-Pt(1), 2.132; C(2)-Pt(1), 2.155; C(6)-Pt(1), 2.149; C
(2)-Si(1), 1.844(5); C(6)-Si(2), 1.848(5); N(1)-C(9)-N(2), 116.6(4); N(1)-C(9)-Pt(1), 122.5
(3); N(2)-C(9)-Pt(1), 120.7(4); C(9)-Pt(1)-C(1), 99.22; C(9)-Pt(1)-C(5), 89.15; C(9)-Pt(1)-
C(2), 138.00; C(9)-Pt(1)-C(6), 128.21.
Scheme 4. Product distribution in the hydrosilylation of 1-octyne.
based [Pt(5-NHC)(dvtms)] complexes have been employed by
Markó in comprehensive studies, which demonstrate high regio-
selectivity and activity for a range of substrates and silanes.6 These
studies indicated the main contributory factors in the catalytic
activity and regioselectivity of [Pt(5-NHC)(dvtms)] complexes are
steric hindrance imparted by the N-aryl groups, the rate of
formation of the monoligated platinum complex, and its relative
stability once formed.
As can be seen from Table 4 the hydrosilylation of 1-octyne by
triethylsilane catalysed by complexes 7 and 10 proceeds at a similar
rate with an almost identical product distribution suggesting the
greater steric hindrance imparted by the diisopropylphenyl func-
tionality over that of the mesityl group makes little difference to
activity and none in regioselectivity. However, complex 9, which is

J.J. Dunsford et al. / Journal of Organometallic Chemistry 696 (2011) 188e194
191
Table 4
The catalytic hydrosilylation of 1-octyne by triethylsilanea.
c
No.
Complex
Time (h)
Yield %b
TOF (hꢀ1
12500
)
Selectivity I:II:IIId
0:89.5:10.5
1
2
3
4
5
6
7
7
9
9
10
10
3
24
3
24
3
82.7
100
0
e
e
0
86.5
100
9090
0:89.1:10.9
24
Scheme 5. Product distribution in the hydrosilylation of 1-octene.
a
Reaction conditions: alkyne/silane ratio 1.0, 0.005 mol% catalyst, solvent is
o-Xylene, T ¼ 80 ꢁC.
b
Table 6
The catalytic hydrosilylation of 1-octene by MD0Ma.
Total GCeMS yield based upon consumption of 1-octyne using n-decane as
internal standard.
c
Turn-over frequency, determined at 25% conversion, defined as moles of
c
No.
Complex
Time (h)
Yield %b
TOF (hꢀ1
2293
)
Selectivity I:II:III:IVe
1-octyne converted per mole of catalyst per hour.7
d
Selectivity of products determined by 1H NMR and GCeMS after 24 h.
1
2
3
4
5
6
7
7
9
9
10
10
3
24
3
24
3
61.8
100
100
100
4.6
1.3:98.7:0:0
0.8:99.2:0:0
1.1:98.9:0:0
9615
77.3d
the least hindered, displays no activity at all. This may be explained
by a catalyst deactivation pathway previously proposed by Markó
et al.6a in which two terminal alkynes coordinate to the platinum
centre, thus forming a new stable complex and resulting in
complete catalyst deactivation.
24
100
a
Reaction conditions: alkene/silane ratio 1.0, 0.005 mol% catalyst, solvent is
o-Xylene, T ¼ 72 ꢁC.
b
Total GCeMS yield based upon consumption of 1-octene using n-decane as
internal standard.
Upon changing the silane to bis(trimethylsiloxy)methylsilane
(MD’M), a more challenging silane for hydrosilylation (Table 5), the
regioselectivity remains identical between complexes but a marked
difference in the catalytic activity between 7 and 10 may be
observed. The mesityl derivative 7 is shown to be much more
effective after 3 h than the diisopropylphenyl derivative 10, which is
an unexpected outcome considering the usual trend of the most
hindered ancillary ligands displaying the greatest catalytic activity.
However, the extremely hindered coordination sphere in 10 is
perhaps too congested for the complex to function effectively with
such a bulky silane. Again, for complex 9 no activity is observed,
which reinforces the view that the complex undergoes permanent
deactivation through coordination of two 1-octyne monomers.
c
Turn-over frequency, determined at 25% conversion, defined as moles of
1-octene converted per mole of catalyst per hour7.
d
Turn-over frequency calculated at 3h.
e
Selectivity of products determined by 1H NMR and GCeMS after 24 h.
previous 1-octyne-MD0M case this observation may be attributed to
the fact that the coordination sphere of the most sterically encum-
bered complex 10 (and to a lesser extent 7) is just too congested for
the MD’M substrate.
Substituting the less challenging triethylsilane for the silane
MD’M yields a highly unexpected result. From GCeMS analysis the
1-octene undergoes isomerisation to 2-, 3- and 4-octene with no
hydrosilylation products observed. The reason for isomerisation
rather than hydrosilylation is unclear and is the subject of further
investigations.
Table 5
The catalytic hydrosilylation of 1-octyne by MD0Ma.
2.3.3. The catalytic hydrosilylation of 2-hexyne
The hydrosilylation of an internal alkyne, 2-hexyne, has also
been investigated (Scheme 6) with both triethylsilane (Table 7) and
MD’M (Table 8).
c
No.
Complex
Time (h)
Yield %b
TOF (hꢀ1
9090
)
Selectivity I:II:IIIe
0:75.4:24.7
1
2
3
4
5
6
7
7
9
9
10
10
3
24
3
24
3
75
100
0
e
e
0
18.7
100
310d
0:74.9:23.1
24
a
Reaction conditions: alkyne/silane ratio 1.0, 0.005 mol% catalyst, solvent is
o-Xylene, T ¼ 80 ꢁC.
b
Total GCeMS yield based upon consumption of 1-octyne using n-decane as
internal standard.
c
Scheme 6. Hydrosilylation of 2-hexyne with MD’M.
Turn-over frequency, determined at 25% conversion, defined as moles of
1-octyne converted per mole of catalyst per hour.7
d
Turn-over frequency calculated at 3 h.
Table 7
e
Selectivity of products determined by 1H NMR and GCeMS after 24 h.
The catalytic hydrosilylation of 2-hexyne with triethylsilanea.
c
No.
Complex
Time (h)
Yield %b
TOF (hꢀ1
1336
)
Selectivity I:IIe
49.4:50.6
1
2
3
4
5
6
7
7
9
9
10
10
3
24
3
24
3
32.6
100
14.4
100
28.7
100
2.3.2. Hydrosilylation of 1-octene
The hydrosilylation of 1-octene with triethylsilane and MD’M as
substrates has also been investigated (Scheme 5), yielding
a number of interesting outcomes. With the more demanding
MD’M silane excellent regioselectivity is observed, in all cases
119.7d
961.5
50.0:50.0
49.8:50.2
24
favouring the
isomer, I. Products III and IV were not observed.
b
-(E) isomer, II (Table 6) in a ratio of 99:1 over the
a
-
a
Reaction conditions: alkene/silane ratio 1.0, 0.005 mol% catalyst, solvent is
o-Xylene, T ¼ 80 ꢁC.
b
Total GCeMS yield based upon consumption of 2-hexyne using n-decane as
An interesting observation here is the inverse trend regarding
catalytic activity and associated steric hindrance of the complexes;
complex 9 being superior to the more sterically demanding
complexes 7 and 10. In all three cases 100% conversion was obtained
after 24 h, indicating catalyst stability is not a problem. As in the
internal standard.
c
Turn-over frequency, determined at 25% conversion, defined as moles of
2-hexyne converted per mole of catalyst per hour.
d
Turn-over frequency calculated at 3 h.
e
Selectivity of products determined by 1H NMR and GCeMS after 24 h.

192
J.J. Dunsford et al. / Journal of Organometallic Chemistry 696 (2011) 188e194
Table 8
valent platinum dvtms and tested three examples in catalytic
hydrosilylation. The increased -donating nature and large
The catalytic hydrosilylation of 2-hexyne with MD0Ma.
s
No.
Complex
Time (h)
Yield %b
TOF (hꢀ1
1096
)
Selectivity I:IIe
46.1:53.8
c
N-CNHC-N angles of the expanded-ring NHC ancillary ligands
were expected to afford enhanced stability, activity and regio-
selectivity in the catalytic hydrosilylation of unsaturated carbone
carbon compounds at low catalytic loadings. From the results
presented here it can be noted that there are a number of factors
which govern the outcome of such hydrosilylation trans-
formations, for example the choice of silane in combination with
the catalyst system seems to be crucial, and the catalyst system
must be selected with this in mind. The steric demand of the
catalyst appears to be of less importance with regard to regio-
selectivity in the majority of cases. In the case of the hydro-
silylation of 1-octene with MD’M excellent activity and
regioselectivity have been observed along with some interesting
outcomes such as the isomerisation of 1-octene to internal
alkenes when using triethylsilane as substrate, the reason for
which is currently the subject of further investigations.
1
2
3
4
5
6
7
7
9
9
10
10
3
24
3
24
3
43.8
100
14.8
100
73.6
100
123.7d
7575
46.6:53.4
46.3:53.7
24
a
Reaction conditions: alkyne/silane ratio 1.0, 0.005 mol% catalyst, solvent is
o-Xylene, T ¼ 80 ꢁC.
b
Total GCeMS yield based upon consumption of 2-hexyne using n-decane as
internal standard.
c
Turn-over frequency, determined at 25% conversion, defined as moles of
1-octene converted per mole of catalyst per hour.7
d
Turn-over frequency calculated at 3 h.
e
Selectivity of products determined by 1H NMR and GCeMS after 24 h.
From the tables, it can be seen that in both cases there is very
little discrimination between isomers, although the II isomer is
slightly more favoured in the case of the more hindered MD’M
substrate (Table 8). This lack of selectivity is not unexpected; the
alkyl chains adjacent to the internal triple bond are similar in steric
impact and provide little discrimination in terms of preferred
orientation on coordination of the alkyne to the Pt centre. There-
fore, the size of the NHC ligand can have little influence in directing
attack of the substrate.
4. Experimental
4.1. General remarks
All manipulations were performed using standard Schlenk
techniques under an argon atmosphere, except where otherwise
noted. All complexes after their formation were treated under
aerobic conditions. Solvents of analytical grade were freshly
distilled using an MBraun SPS-800 solvent purification system.
Deuterated solvents for NMR measurements were distilled from
the appropriate drying agents under N2 immediately prior to use,
following standard literature methods.9 All other reagents were
used as received. NMR spectra were obtained on a Bruker Avance
AMX 400 or 500. The chemical shifts are given as dimensionless
In terms of activities, the best activity using triethylsilane as
substrate is obtained with complex 7 as the catalyst, whereas the
most active catalyst in the MD’M case is the 6-DIPP derivative 10.
2.3.4. The catalytic hydrosilylation of 2-cyclohexen-1-one
The hydrosilylation of the ketone, 2-cyclohexene-1-one has also
been investigated (Scheme 7). With the silane MD’M good catalytic
activity is observed (Table 9), which follows the trend in which
complexes providing the greatest steric hindrance being the most
active.
d
values and are frequency referenced relative to the peak for
TMS for 1H and 13C. Coupling constants J are given in hertz as
positive values regardless of their real individual signs. The
multiplicity of the signals is indicated as “s”, “d”, “t” or “m” for
singlet, doublet, triplet or multiplet, respectively. Mass spectra
and high-resolution mass spectra were obtained in electrospray
(ES) mode unless otherwise reported on a Waters Q-Tof micro-
mass spectrometer. GCeMS data was obtained on an Agilent
Technologies 6890N GC system with an Aligent Technologies
5973 inert MS detector with MSD.
Scheme 7. Hydrosilylation of 2-cyclohexen-1-one with triethylsilane.
Table 9
The catalytic hydrosilylation of 2-cyclohexene-1-one by MD0Ma.
c
No.
Complex
Time (h)
Yield %b
TOF (hꢀ1
15625
)
4.2. General protocol for the synthesis of halide salts
1
2
3
4
5
6
7
7
9
9
10
10
3
24
3
24
3
71.3
100
61.9
100
78.6
100
A mixture of 1 mmol of amidine, 0.5 mmol of K2CO3 and
1e2 mmol of dihalide in 25 ml of acetonitrile is heated under reflux.
At the end of the reaction, the volatiles are removed in vacuo, the
residue dissolved in dichloromethane and diethyl ether is slowly
added until the product begins to crystallise.
10869
15625
24
a
Reaction conditions: enone/silane ratio 1.0, 0.005 mol% catalyst, solvent is
o-Xylene, T ¼ 100 ꢁC.
b
Total GCeMS yield based upon consumption of 2-cyclohexen-1-one using
4.2.1. N,N0-Bis(2,6-diethylphenyl)-3,4,5,6-tetrahydro-pyrimidin-1-
ium-bromide 5
n-decane as internal standard.
c
Turn-over frequency, determined at 25% conversion, defined as moles of
2-cyclohexen-1-one converted per mole of catalyst per hour.7
Yield 57.06% (2.95 g). 1H NMR (CDCl3, 400 MHz, 298 K):
d 7.51
3
(1H, s, NCHN), 7.33 (2H, t, JHH ¼ 7.7 Hz, p-CH), 7.17 (4H, d,
3JHH ¼ 7.7 Hz, m-CH), 4.20 (4H, t, 3JHH ¼ 5.6 Hz, NCH2), 2.71 (4H, q,
A surprising result was noted when using triethylsilane as
substrate; all catalysts appear to have deactivated after approxi-
mately 40 min, with no further substrate conversion after that time.
3
3JHH ¼ 7.5 Hz, CH2CH3), 2.65 (2H, t, JHH ¼ 3.7 Hz, NCH2CH2), 2.60
(4H, q, 3JHH ¼ 7.5 Hz, CH2CH3), 1.24 (12H, t, 3JHH ¼ 7.6 Hz, CH2CH3).
13C NMR (CDCl3, 125 MHz, 298 K):
d 153.34 (1C, s, NCHN), 140.60
3. Conclusions
(2C, s, AreC), 137.59 (1C, s, AreC), 130.87 (1C, s, AreC), 127.51 (2C, s,
AreC), 48.15 (2C, s, NCH2), 23.99 (12H, s, CH2CH3), 19.59 (1C, s,
NCH2CH2), 15.23 (8H, s, CH2CH3). MS (ES): m/z 349.2644 (M e Br
C24H33N2 requires 349.2636).
In summary, we have isolated and characterised a series of
new 6- and 7-membered expanded-ring NHC complexes of zero

J.J. Dunsford et al. / Journal of Organometallic Chemistry 696 (2011) 188e194
193
4.3. General protocol for the synthesis of platinum (0) dvtms
complexes6a
3JHH ¼ 13.6, HC]CH2), 1.44 (2H, t, 3JHH ¼ 11.6, HC]CH2), 1.15 (24H,
m, 3JHH ¼ 5.6, CH(CH3)2), 0.00 (s, 6H, Si-CH3), ꢀ0.97 (s, 6H, Si-CH3).
13C NMR (CDCl3, 125 MHz, 298 K):
d 210.7 (1C, s NCN), 147.2 (1C, s,
The expanded-ring NHC salts (2.12 mmol) were suspended in
dry toluene (5 mL) followed by the subsequent addition of Kar-
stedt’s catalyst (21.2% of Pt in dvtms) (1.93 mmol). The solution was
CeAr), 146.3 (1C, s, CeAr), 129.8 (2C, s, CeAr), 125.5 (2C, s, CeAr),
52.4 (2C, s, HC]CH2), 45.5 (2C, s, NCH2), 37.8 (2C, s, HC]CH2), 30.3
(3C, s, C(CH3)2), 28.6 (3C, s, C(CH3)2), 24.9 (1C, s, NCH2CH2), 23.8
(2C, s, C(CH3)2), 3.4 (6C, s, Si-CH3eq), 0.0 (6C, s, Si-CH3ax).195Pt NMR
t
cooled to 0 ꢁC with stirring before subsequent addition of BuOK
(0.302 g, 2.7 mmol). The reaction mixture was stirred at room
temperature until the reaction was judged to be over by TLC (P.E/
Et2O 85:15). The mixture was filtered on a pad of CeligelÒ (SiO2/
Celite 1:1) and eluted with P.E/Et2O 85:15. Evaporation of the
combined filtrates furnished a white solid, which was washed with
(CDCl3, 107 MHz, rt):
C3H59N2OSi2Pt requires 785.3793).
d
ꢀ5377.70 ppm. MS (ES): m/z 785.3829 (M e
4.3.5. 1,3-Bis-(2,6-diethylphenyl)-3,4,5,6-tetrahydro-pyrimidin-1-
ium-platinum-(divinyltetramethyldisiloxane) 11
i
excess PrOH yielding a white crystalline solid.
Yield: 68.09% (0.796 g). 1H NMR (CDCl3, 400 MHz, 298 K):
d 7.04
(2H, t, 3JHH ¼ 7.6 Hz, p-CH), 6.92 (4H, d, 3JHH ¼ 7.6, m-CH), 3.52 (4H, t,
3JHH ¼ 5.9 Hz, NCH2), 2.76 (4H, q, 3JHH ¼ 7.5 Hz, CH2CH3), 2.33 (2H, t,
3JHH ¼ 5.4 Hz, NCH2CH2), 1.76 (2H, t, 3JHH ¼ 11.1 Hz, HC]CH2), 1.22
(12H, t, 3JHH ¼ 7.6 Hz, CH2CH3), 0.99 (4H, d, 3JHH ¼ 11.58, HC]CH2),
ꢀ0.001 (6H, s, Si-CH3), ꢀ0.98 (6H, s, Si-CH3). 13C NMR (CDCl3,
4.3.1. 1,3-Bis-(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydro-
pyrimidin-1-ium-platinum-(divinyltetramethyldisiloxane) 7
Yield: 73.23% (0.900 g). 1H NMR (CDCl3, 400 MHz, 298 K):
d 6.63
(4H, s, m-CH), 3.38 (4H, t, 3JHH ¼ 5.6, NCH2), 2.29 (2H, t, 3JHH ¼ 4.8,
NCH2CH2), 2.23 (6H, s, o-CH3), 2.15 (6H, s, o-CH3), 2.03 (6H, s, p-
CH3),1.84 (2H, t, 3JHH ¼ 11.6, HC]CH2),1.43 (4H, d, 3JHH ¼ 13.6, HC]
CH2), 0.04 (6H, s, Si-CH3), ꢀ0.94 (6H, s, Si-CH3). 13C NMR (CDCl3,
125 MHz, 298 K): d 216.60 (1C, s, NCN),147.29 (1C, s, p-CH),146.16 (1C,
s, p-CH), 142.65 (1C, s, m-CH), 129.82 (1C, s, m-CH), 128.62 (1C, s, m-
CH), 127.79 (1C, s, m-CH), 50.67 (2C, s, HC]CH2), 45.34 (2C, s, NCH2),
36.99 (2C, s, HC]CH2), 26.73 (4C, s, CH2CH3), 26.11 (4C, s,
CH2CH3), 23.75 (1C, s, NCH2CH2), 17.02 (6C, s, CH2CH3), 16.73 (6C,
s, CH2CH3), 3.75 (6C, s, Si-CH3eq), 0.00 (6c, s, Si-CH3ax).195Pt NMR
125 MHz, 298 K): d 211.5 (1C, s, NCN), 127.8 (2C, s, CeAr), 126.7 (2C,
s, CeAr), 46.1 (2C, s, HC]CH2), 41.8 (2C, s, NCH2), 32.76 (2C, s, HC]
CH2), 23.9 (1C, s, NCH2CH2), 20.3 (1C, s, CH3), 19.4 (1C, s, CH3), 17.6
(2C, s, CH3), 0.00 (2C, s, Si-CH3eq), ꢀ4.27 (2C, s, Si-CH3ax). 195Pt NMR
(CDCl3, 107 MHz, 298 K):
d
ꢀ5335.29.
(CDCl3, 107 MHz, 298 K):
C30H47N2OSi2Pt requires 701.2854).
d
ꢀ5347.66. MS (ES): m/z 701.2864 (M e
MS (ES): m/z 729.3177 (M e C32H51N2OSi2Pt requires 729.3167).
4.4. General hydrosilylation procedure
4.3.2. 1,3-Bis-(2,6-dimethylphenyl)-3,4,5,6-tetrahydro-pyrimidin-
1-ium-platinum-(divinyltetramethyldisiloxane) 8
Starting materials (20 mmol), decane (internal standard)
(10 mmol) and o-Xylene (30 mL) were added to a 250 mL 2-neck
round bottom flask and equilibrated to reaction temperature for
2 h. Catalyst (0.005 mol %) in toluene stock solution was injected
after this equilibration period and the reaction followed at 20 min
intervals by GCeMS analysis. Samples were prepared for GCeMS
analysis by elution through a small column of activated charcoal in
HPLC DCM.
Yield: 64.22% (0.675 g). 1H NMR (CDCl3, 400 MHz, 298 K):
d 6.90
(2H, t, 3JHH ¼ 6.2, p-CH), 6.83 (4H, d, 3JHH ¼ 8.3, m-CH), 3.41 (4H, t,
3JHH ¼ 5.9, NCH2), 2.32 (6H, s, CH3), 2.22 (6H, s, CH3), 1.85 (2H,
t,3JHH ¼ 5.4, NCH2CH2), 1.45 (4H, d, 3JHH ¼ 13.7, HC]CH2), 1.06 (2H,
3
t, JHH ¼ 11.1, HC]CH2), 0.01 (6H, s, Si-CH3), ꢀ0.96 (6H, s, Si-CH3).
13C NMR (CDCl3, 125 MHz, 298 K):
d 137.39 (1C, s, p-CH), 137.34 (1C,
s, p-CH), 131.14 (1C, s, m-CH), 130.54 (1C, s, m-CH), 129.66 (1C, s, m-
CH), 129.46 (1C, s, m-CH), 49.89 (2C, s, HC]CH2), 45.96 (2C, s,
NCH2), 36.99 (2C, s, HC]CH2), 24.20 (1C, s, NCH2CH2), 21.03 (6C, s,
CH3), 20.71 (6C, s, CH3), 3.91 (6C, s, Si-CH3eq), 0.00 (6C, s, Si-CH3ax).
4.5. Identification of hydrosilylation products
195Pt NMR (CDCl3, 107 MHz, 298 K):
676.2748 (M e C28H42N2OSi2Pt requires 676.2748).
d
ꢀ5345.28. MS (ES): m/z
The hydrosilylation products were isolated by vacuum distil-
lation. The stereo- and regiochemistry of unsaturated products
were determined by 1H NMR based upon characteristic olefinic
4.3.3. 1,3-Bis-(2-methylphenyl)-3,4,5,6-tetrahydro-pyrimidin-1-
ium-platinum-(divinyltetramethyldisiloxane) 9
coupling constants. The two protons in the
b
-(E) isomer are
mutually trans with typical JHH coupling constants of between 18
and 20 Hz whereas the two protons in the -(Z) isomer are
mutually cis with typical JHH coupling constants of between 12
and 16 Hz. The two protons of the -isomer are geminal hence
3
Yield: 64.38% (0.714 g). 1H NMR (CDCl3, 400 MHz, 298 K):
b
3
d
7.21e6.92 (8H, m, AreCH), 3.68 (4H, t, 3JHH ¼ 4.8, NCH2), 2.44 (2H,
t, 3JHH ¼ 5.5, NCH2CH2), 2.41 (3H, s, CH3), 2.36 (3H, s, CH3), 2.22 (2H,
a
3
3
t, JHH ¼ 10.9, HC]CH2), 1.38 (4H, d, JHH ¼ 13.3, HC]CH2), 0.12
have much smaller coupling constants. The stereo- and regio-
chemistry of saturated products were determined by the multi-
plicity of peaks. The molecular weights of hydrosilylation products
were determined by the molecular ion peaks present in the
GCeMS analysis.
(6H, s, Si-CH3), ꢀ0.72 (6H, s, Si-CH3). 13C NMR (CDCl3, 125 MHz,
298 K):
d 136.06 (1C, s, AreC), 133.31 (1C, s, AreC), 133.21 (1C,
s,AreC), 132.74 (1C, s, AreC), 131.41 (1C, s, AreC), 130.77 (1C, s,
AreC),129.42 (1C, s, AreC), 128.94 (1C, s, AreC), 50.23 (2C, s, HC]
CH2), 42.67 (2C, s, NCH2), 34.56 (2C, s, HC]CH2), 20.46 (1C, s, CH3),
20.19 (1C, s, CH3), 19.95 (1C, s, NCH2CH2), 3.74 (2C, s, Si-CH3eq),
ꢀ0.47 (2C, s, Si-CH3ax).195Pt NMR (CDCl3, 107 MHz, 298 K):
4.6. X-ray crystallography
d
ꢀ5311.70. MS (ES): m/z 645.2228 (M e C26H39N2OSi2Pt requires
Data recorded on a Nonius Kappa CCD diffractometer equipped
with an Oxford Crysosystem cryostat. In general, the structures
were solved by direct methods. Hydrogen atoms were added at
calculated positions and refined using a riding model. Anisotropic
displacement parameters were used for all non-hydrogen atoms;
hydrogen atoms were given isotropic displacement parameters
equal to 1.2 or 1.5 times the equivalent isotropic displacement
parameter of the atom to which the hydrogen atom is attached.
Crystal and structure refinement data is shown in Table 10.
645.2239).
4.3.4. 1,3-Bis-(2,6-diisopropylphenyl)-3,4,5,6-tetrahydro-
pyrimidin-1-ium-platinum-(divinyltetramethyldisiloxane) 10
Yield: 59.87% (0.843 g). 1H NMR (CDCl3, 400 MHz, 298 K):
d
7.07
(2H, m, 3JHH ¼ 7.6, p-CH), 6.98 (4H, m, 3JHH ¼ 7.6, m-CH), 3.67 (2H,
3
3
m, JHH ¼ 5.2, CH(CH3)2), 3.46 (4H, t, JHH ¼ 6, NCH2), 3.21 (2H, m,
3JHH ¼ 6.8, CH(CH3)2), 2.34 (2H, t, 3JHH ¼ 5.8, NCH2CH2), 1.47 (4H, d,

194
J.J. Dunsford et al. / Journal of Organometallic Chemistry 696 (2011) 188e194
Table 10
References
Crystal data and structure refinement details for selected platinum (0) complexes.
12
C30H46N2OPtSi2 C26H39N2OPtSi2 C31H48N2OPtSi2
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7
9
Empirical formula
fw/g molꢀ1
Cryst syst,
space group
a/Å
(c) F.K. Zinn, M.S. Viciu, S.P. Nolan, Annu. Rep. Prog. Chem. Sect.
(2004) 231;
(d) K.J. Cavell, D.S. McGuinness, Coord. Chem. Rev. 248 (2004) 671;
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B 100
701.2864
Triclinic P-1
645.2228
Monoclinic
P21/c
16.6400 (4)
8.0170 (2)
19.9650 (6)
90
90.3850 (12)
90
2663.33 (12)
1.611
5.379
1288
Block,
colourless
0.19 ꢂ 0.15 ꢂ
0.08
3.00e27.39
10530/6010
715.98
Triclinic P-1
8.86200 (10)
10.8890 (2)
17.5950 (4)
95.9890 (10)
94.8930 (10)
113.0190 (10)
1539.35 (5)
1.514
4.660
708
Block,
colourless
0.40 ꢂ 0.25 ꢂ
0.25
2.28e27.42
9704/6866
9.0070 (8)
10.8920 (8)
17.6870 (9)
96.400 (6)
96.930 (6)
110.520 (7)
1591.0 (2)
1.495
4.510
724
Block,
colourless
0.40 ꢂ 0.35 ꢂ
0.10
2.54e27.49
10914/7246
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(2007) 4800;
b/Å
c/Å
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(e) M. Iglesias, D.J. Beetstra, K.J. Cavell, A. Dervisi, I.A. Fallis, B. Kariuki,
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M.K. Whittlesey, Organometallics 29 (2010) 991;
a
b
/deg
/deg
g
/deg
Vol/Ǻ3
Z, calc density (Mg mꢀ3
)
abs coeff (mmꢀ1
F(000)
)
Cryst
Cryst dimens/mm3
q
range (deg)
No. of reflns collected/
unique
Rint
No. of data/restraints/
parameters
(i) C.J.E. Davies, M.J. Page, C.E. Ellul, M.F. Mahon, M.K. Whittlesey, Chem.
Commun. 46 (2010) 5151.
0.0328
6866/0/346
0.0457
6010/0/306
0.0357
7246/0/344
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Final R indices
0.0303, 0.0746
0.0325, 0.0762
1.653, ꢀ1.896
0.0395, 0.0815
0.0594, 0.0877
1.368, ꢀ1.457
0.0347, 0.0854
0.0377, 0.0875
1.880, ꢀ2.367
[F2 < 2 (F2)]:
q
[5] B.D. Karstedt, Ger. Offen. (1973).
R indices (all data):
R1, wR2
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1881;
Largest diff peak and
hole/e Åꢀ3
(c) G. Berthon-Gelloz, J.-M. Schumers, G. De Bo, I.A. Markó, J. Org. Chem. 73
(2008) 4190;
(d) I.E. Markó, S. Stérin, O. Buisine, G. Mignani, P. Branlard, B. Tinant,
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Acknowledgements
We would like to acknowledge the Engineering and Physical
Sciences Research Council (EPSRC) for their financial support and
the Cardiff Catalysis Institute for JJD’s PhD studentship.