Platinum(0) olefin complexes of a bulky terphenylphosphine ligand. Synthetic, structural and reactivity studies

Article abstract of DOI:10.1039/c5cc07308a

A novel terphenylphosphine PMe₂Ar^Dipp₂ (1) (Dipp = 2,6-ⁱPr₂C₆H₃) forms stable Pt(0) complexes with ethene and 3,3-dimethylbut-1-ene that behave as sources of the reactive Pt(PMe<

Full text of DOI:10.1039/c5cc07308a

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Platinum(0) olefin complexes of a bulky terphenylphosphine
ligand. Synthetic, structural and reactivity studies †‡
Received 00th January 20xx,
Accepted 00th January 20xx
Laura Ortega-Moreno, Riccardo Peloso, Celia Maya, Andrés Suárez, and Ernesto Carmona*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel terphenylphosphine PMe2ArDipp (1) (Dipp = 2,6-iPr2C6H3)
2
coordinative and electronic unsaturation of the Pt(0) centre is
partly compensated by weak Pt···Carene secondary
interaction11 that involves one of the flanking aryl rings of the
terphenylphosphine ligand. Some very efficient
forms stable Pt(0) complexes with ethene and 3,3-dimethylbut-1-
ene that behave as a source of the reactive Pt(PMe2ArDipp
a
2
)
fragment. The complexes are efficient catalysts for the selective
hydrosilylation of terminal alkynes.
hydrosilylations of alkynes catalysed by these complexes are
also communicated. Together with the Pd(0) analogues, Pt(0)-
PR3 complexes constitute one of the most important families
of organometallic catalysts,12 but in comparison with
palladium, platinum facilitates through its characteristic NMR
parameters relevant information on the electron density at the
metal centre and therefore on its reactivity.13
Tertiary phosphines and related phosphorus-donor ligands are
among the most important auxiliary ligands in organometallic
chemistry and homogeneous catalysis.1-4
In recent years bulky phosphines have been shown to
promote highly demanding chemical transformations and have
permitted the synthesis of high-value products, including many
relevant biomolecules.5 Buchwald-type heteroleptic dialkyl-
biarylphosphines, PR2Ar,5a,b are available for an ample range of
biaryl (Ar) structures but a limited set of bulky alkyl (R) groups
(mainly tBu and Cy).6 Rather puzzling, the analogous
dimethylphosphines, PMe2Ar, are very rare to the extent that
only one example seems to be known (Ar = [1,1’-biphenyl]-2-
yl, i.e. methyl Johnphos),5a as reported by Tyler and co-workers
in 2014.7 This is surprising considering the importance of PMe3
and its aryl derivatives, e. g. PMe2Ph.8,9
Terphenyl groups stand among the bulkiest ligands and
ligand substituents.14 The synthesis of the new
terphenylphosphine PMe2ArDipp
(
1
) was effected with the
procedure represented in Scheme 1 (for synthetic details see
‡)
2
the ESI . Phosphine
1 was isolated as a colourless, fairly air-
stable solid in yields close to 55%. It is soluble in common
organic solvents including aliphatic hydrocarbons. These
solutions have moderate stability in contact with air. In CD2Cl2,
it is characterised by a 31P{1H} resonance at -41.3 ppm.
To probe the coordination capability of the new phosphine
In contrast with the widely utilised biarylphosphines,
PR2Ar, information on the analogous terphenylphosphines,
PR2Ar’ (Ar’ = terphenyl substituent) is scarce.10 We report
1
, we aimed at its Pt(0) olefin complexes since they were
expected to be reactive species with valuable synthetic and
catalytic applications, acting, for example, as a source of the
reactive “Pt(PMe2ArDipp )“ fragment.
15-16
2
herein
the
use
of
a
newly
prepared
2,6-di-iso-
= 2,6-(2,6-
A convenient entry
dimethylterphenylphosphine, PMe2ArDipp (Dipp
2
=
into this system was provided by the Pt(II) complex
PtCl2(PMe2ArDipp ), that was obtained as an air-stable orange
ArDipp
i
propylphenyl, i.e. 2,6- Pr2C6H3 and therefore
iPr2C6H3)2C6H3) for the stabilization of low-coordinate Pt(0)
2
2
solid in yields of around 90% from commercial PtCl2 and the
phosphine. Under an atmosphere of C2H4 (1.5 bar) the Pt(II)
precursor was reduced to the desired complex
Pt(C2H4)2(PMe2ArDipp ) (2a) by a variety of reducing agents (Na,
PMe ArDipp
2
olefin complexes such as Pt(olefin)(
stands for CH2=CH2 and CH2=CHtBu. Formally, these complexes
), where olefin
2
2
are two-coordinate fourteen-electron species, but the
K, sodium amalgam, etc.) of which Zn powder proved to be the
most convenient from an experimental point of view (Scheme
2). Even though the bis-ethylene complex 2a formed cleanly in
the presence of an excess of ethylene, under an atmosphere of
dinitrogen solutions of 2a slowly underwent C2H4 dissociation
with formation of the apparently two coordinate mono-
ethylene complex Pt(C2H4)(PMe2ArDipp ) (2b). For this reason
2
work-up of reaction mixtures gave always an admixture of 2a
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and 2b, which in the presence of C2H4 converted cleanly and
quantitatively into the bis-ethylene complex 2a, permitting its
isolation in analytically pure form. In the solid state the
conversion of 2a into 2b was very slow under ordinary
conditions. Considerable decomposition occurred at 50 °C
data collected for single crystals of
3
revealed the presence of
two diastereomers in the crystal lattice. Figure 1 contains data
for the molecules arbitrarily designated as
structural information for the other stereomer (
‡. Molecules
A
,
whereas
B
) can be
A
approach a two-coordinate
found in the ESI
under high vacuum (10-4 torr) preventing full characterization geometry with a Pt
−P bond that is shorter than in the trigonal
of 2b, for which the tethered κ1-P,ƞ1-arene coordination mode planar complex 2a (2.193(2) vs. 2.2866(6) Å) and it is in the
represented in Scheme 2 was proposed by analogy with the lower end of the range of the Pt-PR3 distances for compounds
structurally characterised 3,3-dimethylbut-1-ene (from now on listed in the CSD.17 Olefin coordination in
3
is characterised by
tert-butylethene, tbe) complex analogue,
3
.
nearly identical Pt−C bonds (ca. 2.08 Å, av. value), once more
shorter than in 2a (roughly 2.12 Å) and by a C-C bond of length
1.40(1) Å, comparable to that found in other CH2=CHtBu
transition metal complexes.18 At variance with the linearity
expected for a two coordinated complex, the angle subtended
by the phosphorus atom and olefin centroid at platinum is of
nearly 142°. This deviation is caused by the existence of a weak
Pt···Carene interaction with C27 (2.314(4) Å) that
counterbalances in part the coordinative and electronic
unsaturation of the Pt(0) centre. Since the next shortest
platinum contact (to C32) spans 2.627(6) Å, the Pt···Carene
electronic interaction can be defined as ƞ1.11
Dipp
Dipp
iPr
iPr
iPr
iPr
P
PCl3
2 Mg(Me)Br
MgBr
PX2
−78 ºC
Et2O, −78 ºC
[CuCl]
Dipp
Dipp
(X = Cl, Br)
PMe2ArDipp
2
1
( )
Scheme 1 Synthesis of the terphenylphosphine PMe2ArDipp
2
.
iPr
iPr
iPr
iPr
P
P
− C2H4
Pt
Pt
iPr
Room temperature NMR spectra of mixtures of the ethene
iPr
iPr
)
r
+ C2H4
iPr
a
b
C
º
5
.
complexes
2 revealed the presence of two types of
1
5
(
2
H4
,
F
C2
H
T
coordinated alkene molecules. They are characterised by
1
,
n
Z
diagnostic H resonances at 2.35 (2JHPt = 56 Hz) and 2.20 ppm
2a
u
,
2b
C
t
B
C
6
º
PtCl2(PMe2ArDipp
C
)
2
H
5
C
H
6
2
C
O
(2JHPt = 68 Hz), assigned to the bis- and mono-ethylene
complexes 2a and 2b, respectively, and by associated 13C{1H}
=
,
e
H 2
C
2
H
n
5
Z
n
,
e
2
=
u
l
º
C
C
T
H t
o
T
H
B
F
ArDipp
u
2
,
2
5
º
2
signals that appear at 38.5 (2a; JCPt = 152 Hz) and 42.5 ppm
P
iPr
C
iPr
Pt
(
2b
;
2JCPt = 270 Hz). In the 31P{1H} NMR spectrum, the
OC
Pt
CO
Pt
P
CO
C6H6, 25 ºC
tBu
molecules of 2a and 2b yield resonances with markedly
Pt
iPr
ArDipp
2
iPr
C
O
P
ArDipp
P
different δ
and 1JPPt coupling constant values at -16.2 (3265 Hz)
2
and 24.1 (4333 Hz), respectively. The 31P{1H} NMR parameters
3
4
for 2b are strikingly similar to those recorded for the
structurally characterised tbe complex 3 (23.9 ppm; 4457 Hz).
On this basis bidentate κ1P, ƞ-arene coordination of the
phosphine ligand in 2b can be proposed.
Scheme 2 Synthesis and olefin substitution reactions of ethylene and 3,3-dimethylbut-
1-ene complexes of the Pt(PMe2ArDipp2) fragment.
As represented also in Scheme 2, treatment of 2a (or of
2a
quantitatively
Pt(CH2=CHtBu)(PMe2ArDipp
/
2b mixtures) with an excess of tbe produced cleanly and
(by NMR) the new complex
). On preparative scale,
was best obtained by the Zn powder reduction of
PtCl2(PMe2ArDipp
2
)
(
3
a
compound
3
2) in the presence of the olefin. Complexes 2a
were characterised by elemental analysis, NMR
spectroscopy, and X-ray crystallography, while the composition
of 2b was mainly inferred from NMR studies of 2a 2b mixtures
and by comparison of the data with those of
Figures 1 shows an ORTEP view of the molecules of
and
3
/
3
.
3. The
molecular structure of 2a and selected bond distances and
angles for the two complexes can be found in Figures S1-S3
‡
(see the ESI ). For the sake of brevity we concentrate the
discussion on complex , as a unique example of two-
3
coordinate Pt(0)-olefin-phosphine complex. All Pt(0)L2
compounds listed in the Cambridge Structural Database (CSD)
contain phosphine or NHC ligands.17 Furthermore, only a Fig. 1 ORTEP view of the solid structure of 3 (molecule A) with ellipsoids drawn at the
30% probability level.
handful of transition metal complexes of tert-butylethylene
have been characterised by X-ray crystallography.18 They
include a Pt(II) derivative18d but no Pt(0) species. The X-ray
2 | J. Name., 2012, 00, 1-3
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Variable temperature NMR studies (CD2Cl2) of complexes
2
Finally, we also deemed of interest to examine the catalytic
in the addition of hydrosilanes to terminal
alkynes (Table 1).23 Initial catalytic tests were performed using
and
3 disclosed their solution dynamic behaviour. Observation behaviour of 3
for the olefinic 1H and 13C{1H} resonances of coupling to the 31
P
and 195Pt nuclei in the temperature range studied (from -80 to the benchmark reaction of phenylacetylene with triethylsilane
25 °C) ruled out any dissociative mechanism for the observed in C6D6, with a catalyst loading of 0.1 mol% (entry 1). Complex
site exchanges. The NMR data obtained for these complexes
are similar to those reported for other Pt(0)-olefin-PR3 vinylsilane (97%), accompanied by minor amounts of the
complexes.12, 15a, 19 Pertinent details are provided in the ESI‡. It isomer. A minimum TOF value of 4000 h-1 was found. Next,
is worth noting, however, that only one stereomer of
exists other hydrosilanes and alkoxysilanes24 (entries 2-4) were
in solution between 80 and 25 °C. On this basis, it seems studied in the hydrosilylation of phenylacetylene. The latter
probable that stereomers and of complex exist in silicon derivatives added to phenylacetylene to yield the -(E)-
3 attained high selectivity towards the corresponding β-(E)
α
3
−
A
B
3
β
solution at 25 °C in a fast equilibrium on the laboratory time isomers with 83 and 70% selectivity respectively, although a
scale that favours strongly one of them. Unfortunately, NOESY lower catalytic activity was found. Finally, the reaction of
experiments did not permit discriminating the solution HSiCl3 with phenylacetylene was tested, providing the
structure. Work on related complexes is in progress to clarify corresponding
this issue. The analysis of the resonances pertaining to the two
β-(E)-vinylsilane with 88% selectivity (entry 5).
Dipp phosphine substituents in complex
3
indicates a fluxional
G‡
Table 1 Catalytic hydrosilylation of terminal alkynes catalysed by 3.[a]
0.1 mol% 3
process that exchanges the two rings. An approximate
ꢀ
SiR3
value of 12.6 Kcal·mol−1 can be estimated for this process from
the calculated rate at the coalescence temperature (ca. 0 °C).
The temperature dependence of these resonances is absent in
the spectra recorded for complex 2a at different temperatures
H
SiR'3
R
H
C6D6, 80 ºC
R
Conversion
(%)
Selectivity
Alkyne
Silane
(% E)
and can be ascribed to a fast rotation around the P−C bond to
the central aryl ring (Scheme 3) that exchanges the
PhC≡CH
1
HSiEt3
>99 (0.25 h)
97
2
3
PhC≡CH
PhC≡CH
PhC≡CH
PhC≡CH
HSiPhMe2
HSi(OEt)3
HSi(OTMS)3
HSiCl3
>99 (0.5 h)
>99 (5 h)
91
83
77
88
96
80
95
98
78
coordinated and the free flanking aryl substituents.
Next,
we assayed the reactivity of complexes
2 and 3
4
87 (22 h)
toward carbon monoxide. Though Pt(P^P)(olefin) complexes
may undergo reversible exchange of the alkene ligand with
CO,20 in situ generation of the coordinatively unsaturated
5
>99 (4 h)
>99 (0.25 h)
>99 (4 h)
>99 (0.25 h)
>99 (0.25 h)
>99 (2 h)
6
(p-MeO-C6H4)C≡CH
(p-Br-C6H4)C≡CH
1-Hexyne
HSiEt3
7
HSiEt3
Pt(PMe2ArDipp
2
) fragment was expected to lead instead to a
8
HSiEt3
Pt3(μ-CO)3(PMe2ArDipp
)
3
2
tBuC≡CH
Me3SiC≡CH
HSiEt3
trinuclear
accordance with expectations, bubbling CO into a benzene
solution of complexes or , at 25 °C, rapidly resulted in the
42-electron cluster.21 In
9
10
HSiEt3
2
3
11
12
HSiEt3
HSiEt3
>99 (2 h)
92
44
formation of an orange precipitate that was identified by
spectroscopy and X-ray crystallography as the triplatinum
EtO2CC≡CH
>99 (23 h)
complex
4 represented in Scheme 2. In agreement with the
[a] Reactions were carried out at 80 °C in C6D6 using 1.35 mmol of alkyne ([akyne]
= 2.7 M), 1.35 mmol of silane, 0.07 mmol of hexamethylbenzene (internal NMR
standard) and 0.1 mol% of 3. Conversion and selectivity were determined by 1H
proposed C3h molecular symmetry, only one IR band was
recorded in CH2Cl2 at 1780 cm-1 due to the stretching vibration
of the bridging CO ligands, that were also responsible for a
weak 13C NMR resonance at 182.6 ppm. But the most
NMR spectroscopy. Only formation of β-(E) and α isomers were observed
4
is its intricate 31P{1H} NMR
To complete this study, a series of terminal alkynes were
hydrosilylated with HSiEt3 in the presence of complex . In the
case of arylacetylenes, significant dependence of the
characteristic NMR feature of
3
spectrum that shows the expected complexities inherent to
the mixture of isotopologues foreseen for this formulation, as
accurately described many years ago by Moor, Pregosin, and
Venanzi.22 The NMR parameters and other structural data
a
catalytic activity with the electronic effects of the para-
substituents was evidenced (entries 6 and 7). Thus, the
reaction rate of the (p-bromophenyl)acetylene was
significantly lower than that of the phenyl- and (p-
methoxyphenyl)acetylene. On the other hand, hydrosilylation
of alkyl-substituted terminal alkynes was accomplished with
high activity and E-selectivity (entries 8 and 9), whereas a
notable selectivity was obtained in the addition of HSiEt3 to
trimethylsilylacetylene (entry 10). The hydrosilylation of
alkynes containing an additional C=C bond was also examined
‡.
collected for this complex are given in the ESI
A
B
A
iPr
Pr
i
iPr
P
iPr
iPr
iPr
iPr
P
P
iPr
tBu
tBu
Pt
Pt
Pt
tBu
iPr
iPr
iPr
iPr
B
B
A
(entry 11). Selective Si−H addition to the acetylenic bond took
place leaving the C=C functionality untouched. Also, complex
3
Scheme 3 Proposed mechanism for the fast exchange of the flanking aryl rings of
complex 3 (C6D6 at 25 °C).
catalysed the hydrosilylation of ethyl propiolate, although a
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Conejero, R. Peloso, J. López-Serrano, C. Maya and E. Carmona,
Chem. Eur. J., 2015, 21, 8883.
reduced activity and significantly reduced selectivity was found
(entry 12).
11 A. Falceto, E. Carmona and S. Alvarez, Organometallics, 2014,
33, 6660.
12 M. R. Buchner, B. Bechlars, B. Wahl and K. Ruhland,
Organometallics, 2013, 32, 1643.
In summary, low-coordinate Pt(0) olefin complexes of the
newly designed air stable, bulky terphenylphosphine
PMe2ArDipp2, (
1), have been investigated. The lability of the
olefin ligands confer to these complexes rich substitution
chemistry as well as high and selective catalytic reactivity in
the addition of hydrosilanes to terminal alkynes.
13 (a) W. Baratta, S. Stoccoro, A. Doppiu, E. Herdtweck, A. Zucca
and P. Rigo, ,Angew. Chem. Int. Ed., 2003, 42, 105; (b) J. G.
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analysis: organic compounds and metal complexes, VCH,
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Inorg. Chem., 2007, 46, 11277; (b) M. Carrasco, I. Mendoza, M.
Faust, J. López-Serrano, R. Peloso, A. Rodríguez, E. Álvarez, C.
Maya, P. P. Power and E. Carmona, J. Am. Chem. Soc., 2014,
136, 9173; (c) M. Carrasco, I. Mendoza, E. Álvarez, A. Grirrane,
C. Maya, R. Peloso, A. Rodríguez, A. Falceto, S. Álvarez and E.
Carmona, Chem. Eur. J., 2015, 21, 410.
This work was possible thanks to the financial support (FEDER
contribution and Subprograma Juan de la Cierva) received
from the Spanish Ministry of Science and Innovation (Projects
CTQ2010-15833, CTQ2013-45011-P and Consolider-Ingenio
2010 CSD2007-00006) and the Junta de Andalucía (Grant FQM-
119 and Projects P09-FQM-5117 and FQM-2126). L.O.-M.
thanks the Spanish Ministry of Education (grant BES-2011-
048035).
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