Synthesis and structure of thienyl Fischer carbene complexes of PtIIfor application in alkyne hydrosilylation

Article abstract of DOI:10.1039/d1nj00791b

Transmetallation of group 6 thienylene Fischer carbene complexes to Pt^IIprecursors yielded new examples of neutral platinum(ii) bisethoxycarbene complexes with either 2-thienyl (T) or 5-thieno[2,3-b]thienylene (TT) carbene substituents. The use

Full text of DOI:10.1039/d1nj00791b

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Synthesis and structure of thienyl Fischer carbene
II
complexes of Pt for application in alkyne
Cite this: New J. Chem., 2021,
4
5, 6220
hydrosilylation†
a
a
a
Zandria Lamprecht,
Frederick P. Malan,
Simon Lotz
and
b
Daniela I. Bezuidenhout
*
II
Transmetallation of group 6 thienylene Fischer carbene complexes to Pt precursors yielded new examples
of neutral platinum(II) bisethoxycarbene complexes with either 2-thienyl (T) or 5-thieno[2,3-b]thienylene (TT)
carbene substituents. The use of analogous aminocarbene group 6 precursors proceeded to give isomeric
platinum(II) product mixtures where the resultant bisaminocarbene ligands displayed different orientations
due to restricted rotation around the Pt–aminocarbene bond caused by the sterically demanding TT sub-
Received 15th February 2021,
Accepted 15th March 2021
II
stituents. The well-defined Pt ethoxycarbene complexes were screened as catalyst precursors in the
benchmark hydrosilylation reaction employing phenylacetylene and triethylsilane substrates. Marked selec-
tivity for the b-E isomer (E)-triethyl(styryl)silane was observed, and the (pre)catalysts proved recyclable,
active in solvent-free reactions, and displaying a high alkyne functional group tolerance.
DOI: 10.1039/d1nj00791b
rsc.li/njc
II
8
ligands of W(0) FCCs to a Pt centre. The major products obtained
Introduction
from the reactions with group 6 carbene precursors are neutral
II
Acyclic heteroatom-stabilized carbene complexes of platinum(II) bisethoxycarbene complexes of Pt and cationic mononuclear
1–3
II
have been known since 1915, and yet the number of isolated trisaminocarbene complexes (Fig. 1e). In this study, Pt Fischer
examples are limited. In general, there are four methods available carbene complexes with (annulated) thiophene substituents, are
II
to prepare Pt Fischer carbene complexes (FCCs). Firstly, ligand synthesized for the first time employing the methodology of
modification of Pt isocyanide precursors with alcohols or primary carbene transfer, and their use as catalysts for the alkyne hydro-
II
amines produces the corresponding acyclic aminoxy- or diamino- silylation reaction is investigated. Hydrosilylation of terminal
II
4
carbene Pt complexes, respectively (Fig. 1a). Cationic alkoxy- alkynes is one of the leading methods to produce organosilanes,
II
9–18
carbene complexes can be prepared by reacting the Pt precursors and is catalysed by many transition metals including platinum.
with alkynes (Fig. 1b), to yield the p-bonded intermediates that Although Pt complexes containing N-heterocyclic carbene (NHC)
lead to carbene formation after alkyne modification, while neutral
II
5
bisalkoxycarbene complexes are obtained from the reaction of
6
SiMe
3
-substituted alkynes and H
2
PtCl
6
ꢀ6H
2
O (Fig. 1c). The first
transmetallation reactions from group 6 transition metal FCCs to
produce mononuclear Pt-biscarbene complexes employed
[
PtCl (MeCN) ] or PtCl precursors (Fig. 1d) and the products were
2 2 2
7
stabilized by amine-chelate formation.
II
We have recently prepared Pt multicarbene complexes (Fig. 1e)
by carbene transfer reactions of the ethoxy- and aminocarbene
a
Department of Chemistry, University of Pretoria, Private Bag X20, Hatfield 0028,
Pretoria, South Africa
b
Laboratory of Inorganic Chemistry, Environmental and Chemical Engineering,
University of Oulu, P. O. Box 3000, 90014 Oulu, Finland.
E-mail: daniela.bezuidenhout@oulu.fi
†
Electronic supplementary information (ESI) available: Synthesis details, NMR,
crystal data collection, structure refinement, crystal packing details and hydro-
silylation catalysis details. CCDC CSD 2061163–2061168. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/d1nj00791b
Fig. 1 Heteroatom-stabilized FCCs of platinum(II).
6
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9,12,18
ligands are commonly employed as catalysts for this reaction,
no Fischer carbene catalysts have been reported.
Results and discussion
Synthesis of platinum(II) FCCs
II
Pt multicarbene complexes were synthesized by transmetallation
reactions of the chromium or tungsten carbonyl FCC precursors
P1–P4, containing either a 2-thienyl (T) or a 5-thieno[2,3-b]thieny-
lene (TT) substituent (see Experimental section and ESI† for the
preparation of P1–P4), via a modified procedure of that reported
7
by Sierra and co-workers (Scheme 1). Transmetallation occured
by stirring a group 6 FCC with a platinum precursor for 24–30 h in
II
refluxing dichloromethane (DCM). The syntheses of Pt FCCs are
facile, but purification of the compounds proved challenging.
The complexes are light sensitive, insoluble in most solvents (e.g.
II
dichloromethane, chloroform, acetonitrile) and chromatographic Fig. 2 Pt Fischer carbene complexes 1–4.
II
purification processes are compromised. Four Pt biscarbene
complexes were successfully isolated, namely cis-[PtCl {C(OEt)-2-
2
C
C
4
H
3
S}
2
] (1), cis-[PtCl
] (3a/b) and cis-[PtCl
2
{C(OEt)-5-C
6
H
3
S
2
}
2
] (2), cis-[PtCl
H S }
6 3 2 2
2 2
{C(NH )-5- stereoisomers that are inseparable. The two isomers are 3a (yield
6
H
3
S
2
}
2
2
{C(NMe
2
)-5-C ] (4a/b) (Fig. 2). 64%) and 3b (yield 13%), obtained in the ratio 5 : 1 as deter-
1
The transmetallation reactions do not run to completion, even mined by integrating the H NMR spectrum resonances (ESI,†
with prolonged reaction times, and starting material is recovered Fig. S9).
from all reactions.
Compounds 4 were isolated from transmetallation reactions
The cis-biscarbene complexes of 3 and 4 have three isomeric using P4Cr/W along with Pt(COD)Cl or cis-[PtCl (NCMe) ] in DCM.
2
2
2
possibilities where the TT spacers and amino fragments have The preferred reaction conditions are employed by reacting P4Cr
various orientations to yield different geometric stereoisomers with Pt(COD)Cl in DCM, to produce 4 in 71% yield. Excess P4
2
Cr
(
Fig. 2) due to restricted rotation enforced by the sterically and Pt(COD)Cl
2
are removed by washing the reaction precipitate
demanding thieno[2,3-b]thienylene carbene substituent and the with minimal amounts of DCM, chloroform and ether. Two-out-
amino group with increased Ccarbene–N bond order. Evidence of-three possible geometric stereoisomers of the cis-biscarbene
for the formation of two out of the possible three isomers is complex of 4 are obtained.
observed for 3 and 4. In addition, the formation of a cationic Pt
The two isomers, 4a (yield 39%) and 4b (yield 8%), are
triscarbene complex ([PtCl{C(NMe )-5-C H S } ]Cl, 4d) is observed accompanied by the triscarbene complex (4d, yield 24%) in
2
6 3 2 3
1
for the reactions done with group 6 transition metal dimethyla- the ratio 5 : 1 : 3 as determined by integrating H NMR spectrum
minocarbene complexes. The orientation of the carbene sub- resonances (ESI,† Fig. S11). Compounds 4 only dissolve in
stituents in 4d is unknown.
DMSO and partly in chlorinated solvents (e.g. DCM and CDCl
3
),
Compounds 3 were synthesized using P3
W
(see ESI,† Fig. S1) hence the more insoluble 4d could be purified by washing the
and Pt(COD)Cl
evidence for the isolation of two-out-of-three possible geometric short silica gel filter using acetone as eluent produced only the
decomposition product (NMe )C(O)-5-C (5, yield 24%).
2
in DCM. The cis isomer 3 was obtained with precipitate with a variety of solvents. In an attempt to purify 4, a
2
6 3 2
H S
II
The solubility of Pt FCCs decrease (1 4 2) with an increase in
the number of thiophene units in the annulated spacer (1 o 2). A
decrease in solubility is also seen when changing the heteroatom
substituent from ethoxy- (2) to amino- (3, 4).
II
The mechanism for obtaining a Pt mononuclear biscar-
2
bene complex from Pt(COD)Cl , requires the substitution of
4
the Z -COD ligand, leaving two cis empty coordination sites on
the metal. A stepwise mechanism is likely followed were the COD
2
ligand remains partially coordinated (Z -COD), still bound to the
metal through one of the two double bonds of cyclooctadiene, as
1
9,20
the first carbene ligand coordinates.
Coordination of the
II
second carbene ligand, to the Pt metal, completely displaces the
COD ligand. Further substitution would require that the trans
effect of a carbene ligand is relatively stronger than that of a
8
Scheme 1 Fischer carbene ligand transfer to a platinum(II) metal centre.
chloro ligand.
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Spectroscopic characterization
bond to take up a position where the amino substituent is on
the opposite side of the sulphurs in the TT ring. The greatest
difference is observed for the NH and H4 proton resonances of
3 2 2 3 2
NMR spectra were recorded in CDCl , CD Cl and (CD ) SO,
1
depending on the solubility of the compounds. H NMR data
are summarized in Table 1. The aromatic proton adjacent to the
carbene carbon is most affected by the coordination of the
carbene fragment. In 1 this is seen in the resonance of H3 and
even more so in H4 of 2 (ESI,† Fig. S6 and S7). Due to
conjugation in the thiophene ring of 1, H5 is also shifted
significantly downfield. The methylene resonances of Pt
ethoxycarbene complexes appear as broadened peaks instead
of quartets. One such broad peak/unresolved quartet is
3a and 3b. Two significantly different NH resonances are
observed, hence H-bonding interactions are suspected. Because
three geometric stereoisomers are possible for 3 (Fig. 2), 3b is
expected to be one of the minor isomers. A third isomer 3c,
with both amino substituents on opposite sides of the sulphurs
II in the TT rings, was not observed.
In solution, the TT spacers may rotate and the major product
(3a) is expected to be the energetically favoured compound
1
where the amino groups are on the same side as the sulphurs
of the TT-rings. This is also the favoured orientation for group 6
observed in the H NMR spectrum of 1 and two in the case of
2, resonating ca. 0.6 ppm apart (Fig. S6 and S7, ESI†). Due to the
2
1,22
transition metal aminocarbene complexes.
Compound 3b is
bulky nature of thienylenes and the cis-orientation of the two
carbene ligands in the square planar structure of the platinum
complexes, the peak broadening is ascribed to restricted rotation
of the OEt group around the carbene carbon, more pronounced in
presumed to be the result of both intra- and intermolecular NH
interactions and H-bonding interactions with polar solvents
(DMSO-d6 or THF), causing restricted rotation in the molecule.
2
2
(larger TT spacer) compared to 1 (T spacer). The broad signals of Single crystal X-ray diffraction confirms the molecular structure
are duplicated indicating different electronic environments for of this minor isomer 3b (vide infra), and is obtained due to the
the ethoxy substituents. Recording the NMR spectrum of 2 in a preferential crystallisation of this compound. The possibility of a
II
different solvent, e.g. CD
not significantly improve the resolution of the spectra.
2 2
Cl
or at a lower temperature (2 1C) did cationic triscarbene monochlorido Pt complex for 3, similar
8
to those reported earlier by us, is excluded due to the 1 : 1
1
In the H NMR spectrum of 3a, a double duplication of the integration of the carbene ligand’s protons. Similarly, the trans-
carbene ligand resonances is observed in a 1 : 1 ratio (ESI,† Fig. biscarbene isomer would be expected to yield only one set of
S9), for the compound 3b. The two cis-biscarbene isomers are duplicate signals and is therefore also ruled out as the possible
not separable. The difference between the two stereoisomers 3a product of 3b.
1
and 3b can be visualized by restricting rotation around the
H NMR spectra of mixtures of 4a, 4b and 4d in CDCl were
3
Pt–Ccarb bond. In 3a a structure is assigned whereby each recorded (ESI,† Fig. S11). DMSO was employed as a deuterated
1
amino substituent is on the same side as the sulphurs in the solvent for H NMR spectroscopy of the least soluble, purified
adjacent TT ring, of the two carbene ligands (Fig. 2). The 1 : 1 fraction 4d (ESI,† Fig. S12). Poor resolution and peak broadening
ratio of the aromatic proton resonances of 3b indicates that the are observed for 4d, ascribed to small differences in chemical
molecule constitutes of two different carbene ligands. In 3b one shifts of protons as more than one of the same carbene ligand
ligand has the same conformation as the TT substituents in 3a, is present in the macromolecule, along with the impact of
but the other TT substituent is rotated around the Ccarb–CTT bulky carbene substituents restricting rotation in the molecule.
1
Table 1 H NMR chemical shifts (d) of the Pt FCCs
0
0
a
e
Complex
H3
H4
H5
H4
H5
OEt
2 2
NH /NMe
b
1
8.69
8.68
7.28
7.31
9.08
9.03
8.26
8.56, 8.46
8.06
8.00
7.61
8.09
8.13
5.60, 1.59
5.54, 1.59
5.77, 5.18, 1.59
5.73, 5.21, 1.58
c
1
b
2
7.35
7.39
7.40
7.46, 7.43
7.35
7.45
7.29
7.41
7.26, 6.78
7.44
7.49
7.66
7.73, 7.72
7.45
7.73
7.38
7.70
7.37, 6.87
c
2
d
3
3
4
4
4
4
4
a
b
a
a
b
b
d
10.77, 10.61
11.42, 11.18
4.22, 3.86
4.06, 3.77
3.30
4.15, 3.65
4.10, 3.63, 3.38, 3.30
d
b
d
b
d
7.80
7.65, 7.37
df
a
Proton chemical shifts for the ethoxy fragment are reported with the first value being the chemical shift of the methylene group, and the second
b
c
the chemical shift of the methyl group. In the case of 2 there are two values for the methylene groups. NMR data recorded in CDCl
3
.
NMR data
fragments of 3a and 3b. In the case of 4 the chemical
Broadened resonances observed. The first values reported being the chemical shifts of the two
carbene fragments opposite each other and the second the chemical shifts of the carbene fragment trans to a chloro ligand.
d
e
recorded in CD
2
Cl
2
.
NMR data recorded in (CD ) SO. Proton chemical shifts for the NH
3
2
2
f
shifts represent the methyl groups of NMe
2
.
6
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1
3
This is not uncommon especially for cationic triscarbene com- Table 2
C NMR chemical shifts (d) of the Pt FCCs
II
plexes of Pt with two carbene ligands trans to each other and
b
0
0
Complex C2
C3
C4
C5
C4
C5
Ccarb OEt
8
one carbene ligand trans to Cl.
a
1
2
3
150.5 145.4 130.4 142.5
235.2 80.5, 14.8, 14.8
140.2 153.6 122.1 131.9 233.2 80.1, 14.9
124.2 150.8 121.0 130.9 196.5
Two of the three geometric isomers of the cis-biscarbene
complex (4a/b, Fig. 2) are identified in the reaction mixture. The
sharp, resolved signals of the biscarbene complexes are clearly
distinguishable from the broadened signals of the triscarbene
a
c
c
c
c
d
a
a
b
NMR data recorded in CD
fragment are reported with the first value being the chemical shift of
Cl
2 2
.
Carbon chemical shifts for the ethoxy
complex (Fig. S11, ESI†). The major cis-biscarbene isomer (4a) the methylene group, and the second the chemical shift of the methyl
c
d
group. Assignments could not be made unambiguously. NMR data
recorded in (CD SO.
has the NMe
in the TT spacers and the minor isomer (4b) has the NMe
fragments rotated to the same side as the sulphur atoms in the
2
fragments rotated away from the sulphur atoms
3 2
)
2
TT spacers. A single crystal of 4a could be isolated to confirm resonance in 1 is duplicated. Broad signals are obtained for the
the molecular structure (vide infra). Hydrogen bonding interac- carbene carbon and C4 resonance of 2 (ESI,† Fig. S8).
tions closer than 2.6 Å are absent in the solid state structure of
4
Compound 3b was isolated as a mixture with 3a as the major
a and did not affect the rotations in the molecule. The third component and no C NMR data were obtained for this
1
3
isomer 4c with two different orientations of the NMe
2
frag- complex, or the insoluble 4d. 2D NMR spectroscopy was
ments, would result in duplicated proton resonances in a 1 : 1 employed to assign especially the aromatic resonances for the
ratio (similar to 3b), and is not observed. The individual proton complex mixtures (ESI†).
resonances of 4a are more downfield compared to 4b (Fig. S11,
Molecular structures
ESI†). Two significantly different NMe resonances are observed
2
for 4a in CDCl , ca. 0.3 ppm apart, that are more downfield Crystallizations of the platinum(II) complexes 1, 2, 3b and 4a,
3
compared to the single resonance of 4b at 3.30 ppm (Table 1). In precursor P4
CD SO both NMe
fragments of 4a and 4b appear as two from saturated DCM solutions of the compounds, layered with
signals, respectively.
hexane. The structures of P4 and 5 are reported in the ESI.†
W
and the oxidized ester-product 5 were carried out
(
)
3 2
2
W
A cationic triscarbene complex structure for 4d is further Due to the poor solubility of 3, the compound was crystallized
supported by the poorer solubility of the complex in deuterated from a large amount of THF layered with hexane. Single crystal
solvents and the upfield shifts of the TT proton resonances X-ray diffraction studies confirmed the molecular structures
when compared with the corresponding biscarbene complexes. (Fig. 3). Selected bond lengths, angles and torsion angles are
In (CD ) SO the 2 : 1 ratio of the two carbene ligands trans to reported in Table 3, employing the atom numbering scheme as
3
2
each other and one carbene ligand trans to Cl are observed from used in the NMR assignments. The single crystal X-ray diffraction
proton integration 2 : 3 (2H + 1H) : 2 : 1 : 1 in the aromatic region data set for 2 is of too poor quality to publish, but the molecular
(
Fig. S12, ESI†), for the three thienothienylene carbene sub- structure is included in the ESI,† Fig. S19 to prove atom con-
stituents. The resonances of the two trans carbene ligands are nectivity and to use as proof of the molecule’s conformation. The
chemically equivalent and more downfield than the third square planar cis-Pt dichloride fragment of 1 and 2 is attached to
carbene ligand (trans to Cl). The three NMe fragments in 4d two thiophenes and two [2,3-b]-TTs respectively, each through an
2
resonate as four peaks (Table 1), with the two trans NMe2 ethoxycarbene carbon.
signals appearing as two broad signals (0.47 ppm apart) more
In 3b and 4a, the square planar cis-Pt dichloride fragment is
trans to attached to two [2,3-b]-TT spacers, in the case of the former
2
downfield compared to the two broad signals of NMe
Cl (0.08 ppm apart).
1
3
C NMR data are summarized in Table 2. The carbene
carbon chemical shifts of the Pt ethoxycarbene complexes
1 and 2) are at 235.2 and 233.2 ppm, respectively, comparable
to the Pt ethoxy-FCC ([PtCl {C(OEt)-p-C H NMe } ], 238.5 ppm
in CD Cl ). Compared to the carbene carbon signals of PtCl
2
II
(
II
2
6
4
2 2
8
2
2
bisalkoxycarbene complexes with aliphatic carbene substituents
6
(
Fig. 1c, ca. 278 ppm in CDCl
3
) and mononuclear Pt-bisethoxy-
carbene complexes with cyclic amine substituents (Fig. 1d, ca. 198
7
3
ppm in CDCl ), 1 and 2 are shifted significantly upfield and
downfield, respectively. The carbene carbon chemical shift of 2 is
ca. 36 ppm downfield from the corresponding signal in the
analogue aminocarbene complex 3a (196.5 ppm), measured in
(
(CD ) SO); commensurate with the strong shielding effect of the
3 2
amino-substituent of the carbene carbon atom, compared to an
8
ethoxy substituent.
Carbon chemical shifts are very similar in 1 and 2, with the
Fig. 3 The molecular structures of 1, 3b and 4a with the atomic displace-
carbene carbon resonance slightly more downfield in 1. The methyl ment ellipsoids shown at the 50% probability level.
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Table 3 Selected bond lengths (Å) and angles (1) of 1, 3b and 4a
a
a
a
Complex
1
3b
4a
Bond lengths
M–Ccarb
1.986(7), 1.945(9)
1.285(9), 1.34(1)
2.372(3), 2.373(2)
1.42(1), 1.39(1)
1.47(1), 1.43(1)
1.44(1), 1.40(1)
1.35(1), 1.36(2)
1.728(7), 1.77(1)
1.685(9), 1.69(1)
1.968(3), 1.964(2)
1.303(4), 1.297(4)
2.3874(7), 2.3765(8)
1.451(4), 1.449(4)
1.378(4), 1.383(4)
1.445(4), 1.414(4)
1.396(4), 1.375(4)
1.703(3), 1.712(3)
1.758(3), 1.747(3)
1.993(9), 1.98(1)
1.30(1), 1.29(1)
2.363(3), 2.383(2)
1.46(1), 1.49(1)
1.39(1), 1.37(2)
1.43(1), 1.42(1)
1.37(2), 1.36(2)
1.71(1), 1.73(1)
1.754(8), 1.731(9)
Ccarb–O/N
Pt–Cl
Ccarb–C2/C5
C2–C3
C3–C4
C4–C5
S–C2
S–C5
Bond angles
M–Ccarb–O/N
M–Ccarb–C2/C5
O/N–Ccarb–C2/C5
Torsion angles
M–Ccarb–C2/C5–C3/C4
O/N–Ccarb– C2/C5–C3/C4
Angle between two mean planes
126.0(6), 127.5(6)
121.1(6), 123.2(6)
113.0(7), 109.3(7)
119.1(2), 121.3(2)
122.8(2), 120.6(2)
118.1(3), 118.2(3)
122.8(7), 125.5(7)
117.2(7), 115.8(7)
119.9(9), 118.6(8)
14(1), 9(1)
5.4(4), ꢁ179.0(2)
ꢁ174.6(3), 1.1(5)
4.18, 2.88
ꢁ139(1), 116(1)
46(2), ꢁ64(1)
39.54, 61.34
11.15
-167.5(7), ꢁ170.7(8)
10.03, 9.76
72.75
b
c
Angle between two thienylene mean planes
87.10
a
b
First set of data reported for first carbene fragment and the second for the second carbene fragment. First mean plane drawn through C2, C3,
c
C4 and C5, and the second through M, Ccarb and O/N. Mean plane drawn through C2, C3, C4 and C5 of each thienylene.
through aminocarbene carbons and the latter through dimethyla- 1 and 3b, respectively, are approximately perpendicular to each
minecarbene carbons.
other.
The angles between the fragments (mean plane drawn
The ethoxycarbene complexes 1 and 2 have the anti-isomer
arrangement in the solid state with their ethyl groups and through C2, C3, C4 and C5 of each thienylene) are 72.75 and
2
3–26
metal moiety on the same side of the (M)Ccarb–O(Et) bonds.
p
87.101 individually (Fig. 4). The two carbene p -orbitals are also
In both structures the ethoxy substituents appear on the same approximately perpendicular to each other and hence do not
side as the thienylene spacer’s adjacent sulphur atom. In the compete for the same p-interaction-orbital as the Pt center. This
case of the aminocarbene complex 3b, the first amino group is would explain why steric factors play such a small part in these
on the same side as its thienylene spacer’s adjacent sulphur carbene ligands. This is in contrast to the two TT fragments in
atom and the second on the opposite side of its thienylene 4a where the spacers are almost parallel (11.151 angle between the
spacer’s adjacent sulphur atom. For P4 , the dimethylamine fragments). The averaged Pt–Ccarb bond lengths for Pt-carbene
W
group is on the same side as the sulphur atoms in the TT spacer complexes in this study, 1, 3b and 4a, are the same and
(ESI,† Fig. S19), contradictory to what is observed for 4a (Fig. 3 independent of the type of thienylene spacer or ethoxy/amino/
and 4) where the dimethylamine groups are remote from the dimethylamine group present in the molecule (1.966(8), 1.966(3)
II
sulphur atoms in the TT spacers. The thienylene spacer, carbene and 1.987(9) Å respectively). Compared to a Pt bisethoxycarbene
2 6 4 2 2
carbon, metal and amino fragment of 3b (for both carbene complex [PtCl {C(OEt)-p-C H NMe } ] with an averaged Pt–Ccarb
fragments) are approximately in the same plane. The angle bond length of 1.935 (16) Å, the former is slightly longer in
8
between a mean plane through C2, C3, C4 and C5 and a second length.
through Pt, Ccarb and N confirms this (Table 3). This is not the
case in 1 and 4a, and P4 and 5 (ESI†), where the angles between {C(NMe
the two mean planes deviate from linearity as much as 10–891. NMe
In Pt cationic tris-dimethylaminecarbene complexes ([PtCl-
+
ꢁ
W
2
)-p-C
6
+
H
4
NMe
2
}
3
] [W(CO)
5
Cl] and [PtCl{C(NMe
2
)-p-
ꢁ
C
6
H
4
2
}
3
] PF
6
) the carbene ligands trans to each other
These results are also supported by the torsion angles reported have longer averaged Pt–Ccarb bond lengths, 2.053(4) and
in Table 3 and Table S6, ESI.† The two T and two TT fragments in 2.055(4) Å respectively, compared to their singular carbene
ligand trans to Cl, 1.975(6) and 1.973(4) Å respectively. The Pt–Ccarb
bond lengths in Pt cationic tris-dimethylaminecarbene com-
plexes are longer compared to Pt biscarbene complexes (1, 3b,
8
4a and [PtCl {C(OEt)-p-C H NMe } ]).
2 6 4 2 2
The Ccarb–C2/C5 bond lengths of 1 are shorter compared to
b and 4a (Table 3), confirming that the ethoxycarbene complex
3
is more dependent on electron density from the thienylene
substituent to stabilize the carbene carbon compared to amino-
and dimethylaminecarbene complexes. For 3b, NHꢀ ꢀ ꢀS intra-
molecular interactions are observed (Fig. S20, ESI†) for the
sulfur of the thienylene spacer that is cis to the NH unit. This is
2
Fig. 4 Angles between the thienylene fragments of 1, 3b and 4a.
not possible if the sulphurs of the thienylene spacer are trans to
6
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the NH
2
unit, instead intermolecular hydrogen bonding inter- reactivity of the T- and TT-substituted carbene ligands. However,
actions occur between TT-H4 and NH with the oxygen of a co- the presence of the Fischer carbene ligand is required for catalytic
crystallized THF molecule (2.510 and 2.091 Å, respectively). activity, as determined from comparison to the performance of
Thus, two different orientated TT spacers are observed.
the platinum precursors, K PtCl (entry 12) and cis-[PtCl (NCMe) ]
2 4 2 2
(entry 13).
Catalytic hydrosilylation
Observation of plausible intermediates of platinum complexes
containing chelated acyclic aminocarbene ligands during hydro-
silylation catalysis, has been reported previously. These inter-
mediates represent either products of oxidative addition of the
H–Si bond of the hydrosilane to the catalyst precursor’s core or a
product of the 1,2-alkyne insertion into the Pt–H bond of the
catalyst core, already containing the silyl fragment (see Scheme S2,
The well-studied and documented a hydrosilylation model
reaction with phenylacetylene and triethylsilane is depicted in
9
9
–11,14,27,28
Scheme 2.
for the reaction; triethyl(1-phenylvinyl)silane (a-isomer), (E)-triethyl-
styryl)silane (b-E-isomer), (Z)-triethyl(styryl)silane (b-Z-isomer),
hydrogenated products (styrene and ethylbenzene) and a dehy-
Six possible products have been reported
(
II ESI†). K PtCl and cis-[PtCl (NCMe) ] did not perform as well as
2 4 2 2
drogenative silylation product (triethyl(phenylethynyl)silane). Pt
our platinum catalyst precursors, 2 and 1, that display pronounced
selectivity for one-to-two products. Performing the reactions neat
NHC catalysts display almost exclusive selectivity for the b-E or
1
1,14,27,29,30
a-isomers.
Due to the lack of solubility and the
(solvent free) with 2 (entry 14) and 1 (entry 15), led to reaction
ambiguity with regards to the purity/molecular structure of the
aminocarbene complexes 3 and 4, only the well-defined ethoxy-
FCCs 1 and 2 were evaluated as catalyst precursors for phenyla-
cetylene hydrosilylation with triethylsilane. The reactions in
Tables 4 and 5 were performed in toluene-d8, except for the
neat experiments (entries 14 and 15, Table 4). A stability test was
performed for 2 (entry 1, Table 4), in the absence of any
substrate. After heating the reaction tube with 2 for 6 hours at
completion. No yield could be calculated for these reactions, but
the product distribution could be determined as a ratio. Compared
to entries 9 and 11, respectively, slightly more b-E-isomer and less
b-Z-isomer formed. Complex 2 displayed high functional group
tolerance as substrate screening was investigated (Table 5). Com-
pared to hydrosilylation with phenylacetylene as substrate, entries
1–5 led to higher % yields of the hydrosilylation isomeric products.
Entry 7, carried out with N-Boc-propargylamine as substrate,
8
0 1C, no decomposition indicative of a Pt-catalyzed carbene self-
3
1,32
yielded less hydrosilylation isomeric products. No products are
obtained when using propargylamine and bis(trimethylsilyl)-
acetylene as substrates. Because of the high % yields obtained,
the reaction time of selected substrates was decreased with the
aim to decrease % yields to enable comparisons. The % yields
of entries 9 and 12 (Table 5) were insignificantly influenced by
the time change. In the case of internal alkyne, 3-hexyne (entry
10) and amine-functionalised alkyne, N-Boc-propargylamine
(entry 11), a decrease in activity is seen, more extreme in the
case of the latter. Selectivity of 2 for the b-E-isomer is better in
entries 1, 2, 4 and 5 (Table 5), compared to hydrosilylation with
phenylacetylene as substrate. The NMR spectrum of entry 5,
Table 5 is displayed in Fig. S27, ESI.† Complete selectivity for
the b-E-isomer is observed when the internal alkyne 3-hexyne is
used as substrate, as the expected a-isomer formation is not
possible. Selectivity for the b-E-isomer is poorer in the cases
where 3-TMSO-1-propyne (entry 3) and N-Boc-propargylamine
dimerization product is observed.
Optimisation of the hydrosilylation reaction conditions were
performed using 2 (entries 3–10, Table 4). The optimal reaction
conditions were found to be 0.3 mol% catalyst loading at 80 1C
for 2 hours (entry 9). Complete conversion was reached within
this time, compared to some of the most active catalysts where
9
6
hours are required. Pronounced selectivity for the b-E-isomer
II
was observed, as have been reported for most Pt catalysed hydro-
1
4,33
silylation reactions.
In the case of entry 9, product distribution
(b-E/a/b-Z) is determined as 74/23/3 (see NMR spectrum in Fig. S22,
ESI†). By-products from the reaction; styrene, ethylbenzene and
triethyl(phenylethynyl)silane, are reported in Table S8, ESI.†
Styrene formed (up to 7% yield) during most of the optimiza-
tion reactions, as long as the catalyst loading is high enough
(40.2 mol%) and the reaction time long enough (41 h). Ethyl-
benzene was not observed and triethyl(phenylethynyl)silane is
mostly observed in trace amounts (o1% yield). Changing the
catalyst precursor to 1 (entry 11, Table 4) leads to a slight
improvement in the % conversion and yield, but the selectivity
is not influenced. The insignificantly different behaviour leads to
the conclusion that there is virtually no difference between the
(entry 7) are used as substrates (TMSO = trimethylsulfoxide, Boc
=
tert-butoxycarbonyl). When the reaction time of selected
substrates is decreased, a slightly higher selectivity for the
1
b-E-isomer is observed. Assignments of H NMR signals (Table
S7, ESI†), for products obtained during hydrosilylation were
1
4,34–37
made according to literature reports: phenylacetylene,
38–41
42–45
46
38
1-hexyne,
3-hexyne,
3-TMSO-1-propyne, 5-hexyn-1-ol
47
and 5-chloro-1-pentyne. The only evidence of a dehydrogenative
silylation product is observed as a byproduct in the reaction
using phenylacetylene as a substrate (triethyl(phenylethynyl)-
2
35
silane, 1.23 (9H, t, J 7.9)).
Comparison of the catalytic performance of 2 and 1 with
II
recent examples of Pt NHC catalysts for this benchmark
II
hydrosilylation reaction, reveals superior selectivity of the Pt
FCCs 1 and 2 in the majority of cases.
11
11,14,27,29,30
Scheme 2 Hydrosilylation of alkynes using hydrosilane.
Similarly,
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Table 4 Optimization of hydrosilylation using phenylacetylene and triethylsilane (variation of catalyst loading, temp., time, and catalyst precursor: 1, 2,
cis-[PtCl (NCMe) ], K PtCl )
2 2 2 4
Catalyst loading
(mol%)
Product distribution
b-E/a/b-Z (%)
a
b
ꢁ1
Entry
Catalyst precursor
Time (h)
Temp. (1C)
Conv. (%)
Total% yield /TON/TOF (h
)
cd
1
2
—
2
2
2
2
2
2
2
2
1
2
—
1
6
6
6
6
6
6
6
3
2
1
2
2
2
2
2
80
80
80
80
80
80
40
80
80
80
80
80
80
80
80
0
0/0/0
1/0/0
0/0/0
ce
2
25
98
97
99
40
16
98
94
48
98
13
52
—
—
100/0/0
74/24/2
75/23/2
75/23/2
41/56/3
60/40/0
74/24/2
74/23/3
71/27/2
73/23/4
0/0/0
e
3
83/83/14
91/182/30
87/290/48
39/195/33
5/17/3
85/283/94
81/270/135
41/137/137
83/277/138
0/0/0
4
5
0.5
0.3
0.2
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
e
6
e
7
e
8
e
9
10
11
12
13
14
15
e
e
2 4
K PtCl
cis-[PtCl
2
1
2
(NCMe)
2
]
44/147/73
—
—
50/45/5
76/22/2
77/22/1
f
f
a
Conversion as determined by NMR integration based on the two substrates (phenylacetylene and triethylsilane) and referenced to internal
b
c
standard anisole. Yield as determined from NMR integration based on the limiting substrate (most of the time phenylacetylene). Experiments
not performed in duplicate. Substrate loading = 0. An unidentified precipitate was observed. Reactions were performed neat (no solvent).
Complete conversion was observed, however no yield could be calculated.
d
e
f
Table 5 Substrate functional group tolerance and product distribution the silane to the Pt metal centre is followed by chloride ligand
dependence on time (with 2 as catalyst precursor at 0.3 mol% catalyst
loading in toluene-d at 80 1C)
8
dissociation. Alkyne migratory 1,2-insertion occurs into the Pt–H
bond, followed by reductive elimination to give the b-E-isomer.
The a-isomer is formed through 2,1-insertion of the alkyne
into the Pt–H bond, followed by reductive elimination. As the
formation of the b-Z-isomer is negligible, the possibility of
alkyne insertion into the Pt–Si bond, is discarded.
Finally, the recyclability of 2 was investigated. A catalytic
experiment was done in duplicate with 0.3 mol% catalyst
loading at 80 1C for 2 hours in toluene-d8. After reaction
completion/full conversion, substrates phenylacetylene and
triethylsilane were added again, with the same equivalents as
employed in the first batch reaction. This procedure was
repeated four times, and the reaction still ran to completion
and no decrease in activity was observed.
Total %
Product
b
Time Conv. yield /TON/ distribution
ꢁ
1
Entry Substrate
(h)
2
(%)
99
TOF (h
)
b-E/a/b-Z (%)
1
2
3
4
94/313/157 76/21/3
100/333/167 99/0/1
94/313/157 61/37/2
98/327/163 88/11/1
87/290/145 81/16/3
2
100
2
2
2
2
2
99
99
100
6
a
5
6
7
0/0/0
0/0/0
73
73/243/122 69/31/0
0/0/0 0/0/0
Conclusions
a
8
2
1
13
98
II
The number of carbene ligands that coordinate to the Pt
9
95/317/317 86/12/2
metal, depends on the steric and electronic properties of the
carbene ligand as determined by its substituents. In this study,
1
1
0
1
86
86/287/287 99/0/1
II
mostly neutral mononuclear Pt -biscarbene complexes were
obtained (1, 2, 3a/b and 4a/b) from carbene transfer via group
1
2
1
1
38
99
36/120/120 72/28/0
96/320/320 89/10/1
6
FCC precursors. In the case of both the Pt-biscarbene com-
1
plexes 3 and 4, two geometric stereoisomers are observed. The
formation of an insoluble cationic Pt triscarbene complex 4d
was only indicated for the transmetallation reaction from
dimethylaminocarbene complexes.
a
An unidentified precipitate was observed.
the activity of the FCCs compares well with the state-of-the-art NHC
complexes, as complete conversion is possible with 0.3 mol% ligands coordinated to a Pt metal centre, as molecular catalyst
This study reports the first utilization of Fischer carbene
II
II
catalyst loading at 80 1C for 2 hours, although some reports cite precursors for alkyne hydrosilylation reactions. The Pt FCCs 1
11,14
9
lower catalyst loading,
or higher turnover numbers (TONs).
and 2 were tested as catalyst precursors for the benchmark
Considering the evidence for vinylsilane product formation via reaction of phenylacetylene with triethylsilane. Both (pre)catalysts
the standard Chalk–Harrod mechanism, the same mechanism is exhibited good activity as complete conversion was reached in a
9,34
proposed for 2 and 1 (Scheme S2, ESI†).
Oxidative addition of short period of time, and a high functional group tolerance for the
6
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NJC
1
3
alkyne substrates. At optimal reaction conditions, product distri- CH
2
), 1.59 (6 H, t, J 7.1, CH
3
). d C(75.468 MHz; CD
2
Cl
2
; Me
4
Si)
),
tively. Notably, this selectivity is an improvement on that of known 14.8 and 14.8 (CH ). m/z (C H O Cl S Pt, 546.39 g mol ) calcu-
bution (b-E/a/b-Z) was determined as 73/23/4 and 74/23/3, respec- 235.2 (Ccarb), 142.5 (C5), 130.4 (C4), 145.4 (C3), 150.5 (C2), 80.5 (CH
2
ꢁ
1
3
14 16
2
2 2
II
ꢁ
Pt NHC complexes, with comparable activity. The possibility of lated: 736.9030, found: 737.1392 (30%, [M -{C(OEt)C H S} -H] ).
2
4 3 2
excluding solvent use in neat hydrosilylation reactions with 1/2 as
catalyst precursors, proved possible with the additional advantage
Synthesis of platinum(II) ethoxycarbene complexes of [2,3-b]-TT
of improved selectivity for the b-E-isomer, while 2 can be reused for The same reaction method as above was applied to excess P2Cr
at least four batch hydrosilylation reactions, without any evidence (0.17 g, 0.4 mmol) and Pt(COD)Cl (0.06 g, 0.2 mmol). Instead
2
of decomposition or decreased reactivity.
of using large amounts of ether, the product was repeatedly
washed with minimum amounts of chloroform, followed by a
single ether wash. Dissolved in DCM, the product (listen in
Experimental
ESI,† Table S3) was dried on MgSO , filtered and dried under
4
General
vacuum.
3
ꢁ1
ꢁ1
2
: lmax(CH
2 2
Cl )/nm 400 (e/dm mol cm 33140), 355 (43120).
All operations were carried out using standard Schlenk techniques
or vacuum line techniques under an inert atmosphere of nitrogen
or argon, using oven-dried glassware. Silica gel 60 (particle size
1
3
d H (400.13 MHz; CDCl
.2, H5 ), 7.35 (2H, s, br, H4 ), 5.77 and 5.18 (2H + 2H, s, br, CH
3
; Me
4
Si) 9.08 (2H, s, H4), 7.44 (2H, d, J
5
0
,4
0
0
0
5
2
),
1
1.59 (6H, s, br, CH
3
). d H (500.139 MHz; CD
2
Cl
2
; Me
4
Si) 9.03 (2H, s,
0.063–0.20 mm) was used as resin (stationary phase) for all column
3
0
0
H4), 7.49 (2H, d, J
5
0
,4
0
5.1, H5 ), 7.39 (2 H, s, br, H4 ), 5.73 and 5.21
chromatography separations.
1
(2H + 2H, s, br, CH ), 1.58 (6H, s, br, CH ). d H (400.13 MHz;
2
3
0
Chemical reagents and solvents
CD
(
CH
2
Cl
2
; Me
4
Si at 2 1C) 9.05 (2H, s, H4), 7.49 (2H, s, br, H5 ), 7.39
0
2H, s, br, H4 ), 5.73 and 5.14 (2H + 2H, s, br, CH ), 1.61 (6H, s, br,
2
THF and diethyl ether were distilled over sodium wire and
benzophenone under N (g) atmosphere, hexane over sodium
2
wire and DCM over CaH . Other chemicals were used as they were
commercially supplied by Sigma Aldrich and Strem Chemicals.
The n-BuLi used in syntheses was from a stock 1.6 M solution in
hexane.
Triethyloxonium tetrafluoroborate was prepared according to
literature procedure and stored in diethyl ether under Ar (g).
13
3
). d C (125.75 MHz; CD
2
Cl
2
4
; Me Si) 233.2 (br, Ccarb), 153.6
0
(
C5), 140.2 (br, C4), 152.3 and 148.3 (C3 and C2), 131.9 (C5 ), 122.1
2
0
ꢁ1
(C4 ), 80.1 (CH
2
), 14.9 (CH
3
). m/z(C18
16
H O
2
S
4
Cl
2
Pt, 658.56 g mol
)
ꢁ
calculated: 691.8747, found: 691.8950 (35%, [M + Cl] ).
Synthesis of platinum(II) aminocarbene complexes of [2,3-b]-TT
The same reaction method as above was applied to excess P3
(0.11 g, 0.22 mmol) and Pt(COD)Cl (0.03 g, 0.1 mmol). A cream
brown reaction mixture was obtained when minimum amounts
of DCM were added. The product was washed repeatedly with
DCM and ether using cannula filtration. The product was dried
under vacuum, dissolved in THF and filtered through Celite
4
8
W
4
9
2
2
Boron trifluoride etherate was distilled before use. Pt(COD)Cl ,
50
21,51–53
cis-[PtCl
2
(NCMe)
2
], and group 6 FCC precursors
[Cr(CO)5-
{
{
C(OEt)-2-C H S}] P1 , [Cr(CO) {C(OEt)-5-C H S }] P2 , [W(CO) -
C(OEt)-5-C H S }] P2 , and [W(CO) {C(NH )-5-C H S }] P3 were
4
3
Cr
5
6
3
2
Cr
5
6
3
2
W
5
2
6
3
2
W
prepared according to literature procedures. The synthesis and
characterization of [Cr(CO) {C(NMe )-5-C }] P4Cr and [W(CO)
C(NMe )-5-C }] P4 are reported in the ESI.†
and MgSO
isolated are listed in ESI,† Table S4.
4
before being put up for crystals. The products
5
2
6
H
3
S
2
5
-
{
2
6 3
H S
2
W
ꢁ1
3
a: nNH(KBr pellet)/cm 3294s, br (n ) and 3210s, br (n ).
as s
1
d H (500.139 MHz; (CD ) SO; Me Si) 10.77 and 10.61 (2H + 2H,
3
2
4
3
0
Synthesis and characterization of
Fischer carbene complexes
Synthesis of platinum(II) ethoxycarbene complexes of
thiophene
s, br, NH
2
), 8.26 (2H, s, H4), 7.66 (2H, d, J
5.3, H4 ). d C(125.75 MHz; (CD
5
0
,4
0
5.3, H5 ), 7.40 (2
SO; Me Si) 196.5
3
0
13
H, d, J
4
0
,5
0
3
)
2
4
(Ccarb), 150.8 (C5), 124.2 (C4), 146.0 and 145.8 (C3 and C2),
0
0
ꢁ1
130.9 (C5 ), 121.0 (C4 ). m/z (C14
H
10
N
2
Cl
2
S
4
Pt, 600.49 g mol
)
ꢁ
calculated: 677.7935, found: 677.7938 (9%, [M + Br] ).
1
Excess P1Cr (3.00 g, 9.0 mmol) and Pt(COD)Cl (1.53 g, 4.1 mmol)
were dissolved in 20 mL dried DCM (degassed for 15 min) and 2H, s, br, NH ), 8.56 and 8.46 (2H, s, H4), 7.73 and 7.72 (2H, d, J
allowed to reflux under an inert atmosphere at 50 1C for 24–30 5.3, H5 ), 7.46 and 7.43 (2 H, d, J
3b: d H (500.139 MHz; (CD ) SO; Me Si) 11.42 and 11.18 (2H +
3 2 4
2
3
0
0
2
5 ,4
0
3
0
5.3, H4 ).
,5
0
0
4
hours. The solvent was removed under vacuum and the reaction
mixture dissolved in a minimal amount of DCM. The product
was cannula filtered and triturated with hexane. The product was
Synthesis of platinum(II) dimethylamine-carbene complexes of
2,3-b]-TT
[
repeatedly washed with large amounts of dry ether to separate The same reaction method was applied to excess P4Cr (0.17 g,
unreacted Pt(COD)Cl from 1 (listed in ESI,† Table S2), before 0.45 mmol) and Pt(COD)Cl (0.056 g, 0.15 mmol). The solvent
being dried under vacuum. was removed under vacuum and the reaction mixture washed
: d H(400.13 MHz; CDCl ; Me Si) 8.09 (2 H, d, J 4.7, H5), with minimal amounts of DCM, chloroform and ether. The
2
2
1
3
1
3
4
5,4
3
3
7
(
.28 (2 H, dd, J 4.7, 3.7, H4), 8.69 (2 H, d, J 3.7, H3), 5.60 product (listed in ESI,† Table S5) was dried under vacuum.
4 H, s, br, CH
3,4
1
2
), 1.59 (6 H, t, J 6.8, CH
3
). d H(300.13 MHz; Filtering the product through silica gel, with acetone as eluent,
Si) 8.13 (2 H, dd, J5,4 5.0, J5,3 0.9, H5), 7.31 (2 H, causes the product to decompose to the corresponding organic
3
4
CD
2
Cl
2
; Me
4
3
3
dd, J 5.0, 3.8, H4), 8.68 (2 H, d, J3,4 3.8, H3), 5.54 (4 H, s, br, ester product 5.
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a: d H (300.13 MHz; CDCl
Paper
1
4
3
; Me
4
Si) 8.06 (2H, s, H4), 7.45 Characterization and analytical techniques
3
0
3
0
(
6
2H, d, J 5.3, H5 ), 7.35 (2H, d, J 5.3, H4 ), 4.22 and 3.86 (6H +
Ultraviolet spectra could not be obtained for 1, 3, 4 and 5 (these
compounds are yellow to colourless when diluted to 0.01 mM in
0 mL of DCM and 3 and 4 are insoluble in DCM). 4d did not
ionize during mass spectrometric (MS) analysis. Dimerization
of 1 through bridging chlorido ligands was observed in the MS
of the sample.
Nuclear magnetic resonance spectroscopy. NMR spectra
were recorded on Bruker AVANCE 500, Ultrashield Plus 400
AVANCE 3 and Ultrashield 300 AVANCE 3 spectrometers, at
5 1C. The H NMR spectra were recorded at 500.139, 400.13 or
00.13 MHz, and the C NMR spectra at 125.75, 100.613 or
5.468 MHz, respectively. CDCl , CD Cl and (CD ) SO were
3 2 2 3 2
used as solvent. For the samples measured in CDCl , chemical
shifts (reported as d (ppm) downfield from Me Si) are referenced
at 7.26 ppm for dH and 77.0 ppm for dC. The CD Cl reference
signals are at 5.32 ppm for dH and 54.0 ppm for dC, and the
CD ) SO reference signals are at 2.50 ppm for dH and 39.5 ppm
for dC. An additional low temperature H NMR spectrum of 2
was measured in CD Cl at 2 1C. The toluene-d8 reference signal
is at 2.09 ppm for dH (hydrosilylation reactions). Coupling
constants (J) are reported in Hz. Preparation of the samples
was carried out under Ar(g) and the NMR tubes were sealed
before data collection. The H NMR data are reported in the
format: chemical shift (integration, multiplicity, coupling
constant, assignment) and the C NMR data in the format:
chemical shift (assignment), in the order of assignments. The
spectral coupling patterns are: s – singlet, d – doublet, t – triplet
and br – broad.
1
H, s, NCH ). d H (300.13 MHz; (CD ) SO; Me Si) 8.00 (2 H, s,
3 3 2 4
3
0
3
0
H4), 7.73 (2 H, d, J 5.3, H5 ), 7.45 (2H, d, J 5.3, H4 ), 4.06 and
1
ꢁ1
3
.77 (6H + 6H, s, NMe ). m/z (C H N Cl S Pt, 656.59 g mol )
2 18 18 2 2 4
ꢁ
ꢁ
calculated: 688.7982, found: 688.9038 (12%, [M + Br-NMe
2
-H] ),
calculated: 661.9028, found: 661.9876 (10%, [M + Br-2Cl-2H] ).
1
4
b: d H (400.13 MHz; CDCl
3
; Me
4
Si) 7.61 (2H, s, H4), 7.38 (2H,
3
0
3
0
1
d, J 5.3, H5 ), 7.29 (2 H, d, J 5.3, H4 ), 3.30 (12H, s, NMe
2
). d H
3
(
5
500.139 MHz; (CD
.2, H5 ), 7.41 (2H, d, J 5.2, H4 ), 4.15 and 3.65 (6H + 6H, s,
3
)
2
SO; Me
4
Si) 7.80 (2H, s, H4), 7.70 (2H, d, J
0
3
0
1
2
3
7
NMe2).
13
1
4d: d H (300.13 MHz; (CD ) SO; Me Si) 7.65 (2H, s, br, H4
3 2 4
0
0
(trans to carbene ligand)), 7.37 (3H, s, br, H5 (trans to carbene
3
ligand) + H4 (trans to Cl)), 7.26 (2H, s, br, H4 (trans to carbene
0
0
4
ligand)), 6.87 (1H, s, br, H5 (trans to Cl)), 6.78 (1H, s, br, H4
trans to Cl)), 4.10 and 3.63 (6H + 6H, s, br, NMe (trans to
(trans
2
2
(
2
carbene ligand)), 3.38 and 3.30 (3H and 3H, s, br, NCH
to Cl)).
3
(
3
2
1
ꢁ
1
5
: nCO(hexane)/cm 1639m, br (CQO stretching vibration).
1
2
2
d H(300.13 MHz; CDCl ; Me Si) 7.48 (1H, s, H4), 7.36 (1H, d,
3
4
3
0
3
0
J
5
0
,4
0
5.3, H5 ), 7.21 (1H, d,
). d C (75.468 MHz; CDCl
J
4
0
,5
0
5.3, H4 ), 3.21 (6H, s, br,
; Me Si) 164.1 ((NMe )C(O)),
45.9 (C5), 121.8 and 121.8 (C4), 141.1 and 140.2 (C3 and C2),
1
3
NMe
2
3
4
2
1
1
2
1
0
0
28.6 (C5 ), 120.3 (C4 ), 38.0 (br, NMe
11.3 g mol ) calculated: 212.0204, found: 212.0264 (84%,
2
9 9 2
). m/z (C H NOS ,
ꢁ1
1
3
+
+
[
M + H] ), calculated: 234.0023, found: 234.0091 (100%, [M + Na] ).
Catalytic hydrosilylation reactions
First-order analysis was carried out to assign signals of the
H NMR spectra. Additional 2D [ H, H] COSY NMR experi-
A dry high pressure NMR tube fitted with a J. Young valve, was
charged with the (pre)catalyst precursor, alkyne, triethylsilane
and internal standard in toluene-d8 under an atmosphere of
argon, before heating. Each reaction contained triethylsilane
1
1
1
ments were done where confirmation of the proton assign-
ments were required. Assigning the carbon chemical shifts,
1
3
obtained from proton-decoupled C NMR spectra, was possi-
(40.0 mL, 0.25 mmol) and 0.2 mL toluene-d8. Anisole was used
1
13
1
13
ble with the assistance of 2D [ H, C] HSQC and 2D [ H, C]
HMBC NMR experiments (see ESI,† Section S3). Standard
Bruker pulse programs were used in the experiments.
as internal standard (27 mL, 0.25 mmol) and was integrated in
the proton NMR spectra to represent three protons (area: 3.34–
3.31 or 3.36–3.30 ppm). No triethylsilane was added in entry 1
Fourier-transform infrared spectroscopy. Infrared spectro-
scopy was performed on a Bruker ALPHA FT-IR spectrophot-
ometer with a NaCl cell, using dried hexane as solvent. Insoluble
samples were measured in the solid state (KBr pellets). KBr pellets
are pressed from dry homogeneously powdered KBr containing
sample in 0.2–1% concentration. The absorptions were measured
(
Table 4) and entries 14 and 15 (Table 4) were done solvent free.
After a predetermined time at a certain temperature (80 1C,
except entry. 7, Table 4, was performed at 40 1C), the NMR data
are collected and analysed. Reaction conditions applied to
individual reactions are reported in Table S8, ESI.† After the
reactions were completed the solutions appeared pale yellow,
ꢁ
1
indicating the presence of hydrosilylation isomeric products from 400–4000 cm . The IR data are reported in the format:
that formed.
absorption intensity (assignment) in the order of highest to lowest
A descriptive example for a performed catalytic run (entry 9, wavenumber. The wave intensities are: w – weak, m – medium,
Table 4) is as follows: 0.3 mol% of 2, phenylacetylene (27.5 mL, s – strong, sh – shoulder and br – broad.
0
0
.25 mmol), triethylsilane (40.0 mL, 0.25 mmol), anisole (27.0 mL,
.25 mmol) and 0.2 mL toluene-d8 was added to a high were performed on a Waters Synapt G2 high definition mass
High-resolution mass spectrometry. Mass spectral analyses
s
pressure NMR tube, under an atmosphere of argon. The sealed spectrometer (HDMS) that consists of a Waters Acquity Ultra
s
NMR tube was then placed in an oil bath set at 80 1C for Performance Liquid Chromatography (UPLC ) system hyphe-
2
hours. The NMR tube is allowed to reach RT before NMR nated to a quadrupole-time-of-flight (QTOF) instrument. Data
spectra were collected. Catalytic reactions were performed in acquisition and processing was carried out with MassLynxTM
ꢁ1
duplicate, unless stated otherwise, and the averaged results (version 4.1) software. A leucine encephalin solution (2 pg mL
,
reported.
m/z 555.2693) was used as an internal lock mass control standard
6
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NJC
to compensate for instrumental drift and ensure good mass Acknowledgements
accuracy. The internal control was directly infused into the
The authors gratefully acknowledge the National Research Foun-
dation, South Africa (NRF 105740; NRF 105529, NRF 87788 and
NRF 9772) and Sasol Technology R&D Pty. Ltd. (South Africa) for
financial support.
source through a secondary orthogonal electrospray ionization
ESI) probe to allowing intermittent sampling. Flow injection
(
ꢁ1
analysis (FIA, 0.4 mL min flow rate) with the injection volume set
at 5 mL. Samples were made up in ultra-purity liquid chromatogra-
ꢁ
1
phy methanol to an approximate concentration of 10 mg mL . The
methanol was spiked with 0.1% formic acid and used throughout
the 1 min run. The capillary voltage for the ESI source was set at 2.8
kV and 2.6 kV for positive and negative mode ionization, respec-
tively. The source temperature was set at 110 1C, the sampling cone
voltage at 25 V, extraction cone voltage at 4.0 V and cone gas
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