Influence of electronically and sterically tunable cinnamate ligands on the spectroscopic properties and reactivity of bis(triphenylphosphine)platinum(0) olefin complexes

Article abstract of DOI:10.1021/om301021j

A total of 48 new bis(triphenylphosphine)(cinnamic acid ester)platinum(0) complexes were synthesized to examine electronic and steric influences on their behavior as inhibited precatalysts and to correlate this with ¹H, ¹³C, ¹⁹F, ³¹P and ¹⁹⁵Pt NMR spectroscopic, IR spectroscopic, and X-ray structural properties (9 X-ray structures included). The substituent at the 4-position of the phenyl group proved to be a valuable moiety in controlling the electronic properties of the olefin ligand and, therefore, the metal-ligand bond strength. Reactivity and NMR spectroscopic data correlate with the Hammett parameters of this substituent: in particular, the coupling constants ²J_PP and ¹J_PPt. The reactivity of the complexes was determined via NMR titration with triphenylphosphine (¹H NMR; triggering ligand substitution) and reaction with diphenylsilane (¹H and ²⁹Si NMR; triggering oxidative addition). The determined equilibria correlate with the electron density of the olefin. As one quintessence the reactivity can be predicted indirectly from the NMR ²J_PP coupling constants of the complexes, as was also found for the related Pd complexes.

Full text of DOI:10.1021/om301021j

Article
pubs.acs.org/Organometallics
Influence of Electronically and Sterically Tunable Cinnamate Ligands
on the Spectroscopic Properties and Reactivity of
Bis(triphenylphosphine)platinum(0) Olefin Complexes
Magnus R. Buchner,† Bettina Bechlars,‡ Bernhard Wahl,§ and Klaus Ruhland*,∥
†WACKER-Institut fur Siliciumchemie, Lehrstuhl fur Anorganische Chemie, Technische Universitat Munchen, Lichtenbergstraße
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4, 85747 Garching bei Munchen, Germany
̈
‡Lehrstuhl fur Anorganische Chemie, Technische Universitat Munchen, Lichtenbergstraße 4, 85747 Garching bei Munchen, Germany
§Lehrstuhl fur Anorganische Chemie mit Schwerpunkt Neue Materialien, Technische Universitat Munchen, Lichtenbergstraße
̈
̈
̈
̈
̈
̈
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4, 85747 Garching bei Munchen, Germany
̈
∥Lehrstuhl fur Chemische Physik und Materialwissenschaften, Universitat Augsburg, Universitatsstraße 1, 86135 Augsburg, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: A total of 48 new bis(triphenylphosphine)(cinnamic acid
ester)platinum(0) complexes were synthesized to examine electronic and
steric influences on their behavior as inhibited precatalysts and to correlate
1
this with H, 13C, 19F, 31P and 195Pt NMR spectroscopic, IR spectroscopic,
and X-ray structural properties (9 X-ray structures included). The sub-
stituent at the 4-position of the phenyl group proved to be a valuable
moiety in controlling the electronic properties of the olefin ligand and, therefore, the metal−ligand bond strength. Reactivity and
NMR spectroscopic data correlate with the Hammett parameters of this substituent: in particular, the coupling constants 2JPP and
1JPPt. The reactivity of the complexes was determined via NMR titration with triphenylphosphine (1H NMR; triggering ligand
substitution) and reaction with diphenylsilane (1H and 29Si NMR; triggering oxidative addition). The determined equilibria
correlate with the electron density of the olefin. As one quintessence the reactivity can be predicted indirectly from the NMR 2JPP
coupling constants of the complexes, as was also found for the related Pd complexes.
In these complexes electronic properties of the double bond
can be adjusted by the substituent in a position2 para to the olefin
moiety (R1). By variation of the ester group a fine tuning of the
electronics (R2) of the olefin and the steric demand3 (R3) of the
INTRODUCTION
■
Concerning homogeneous catalysis, Pd(0) and Pt(0) complexes
bearing phosphane or N-heterocyclic carbene ligands still belong
to the most important group of applied catalysts. The electronics
and sterics of the ligands control to a vast amount the reactivity of
these complexes. This is often related to spectroscopic
properties, such as the 1JPtP coupling constant, which correlates
with the electron density at the metal center and therefore with
reactivity.1
In this study we want to focus on the inhibiting properties of a
cinnamate ligand on the Pt(PPh3)2 moiety (Scheme 1). These
inhibiting properties (in particular in connection with Pt as a
catalyst for the hydrosilylation) are of great interest for
switchable catalysts. Our final target in this context is a cinnamate
ligand which binds firmly to Pt as an inhibitor and can be cleaved
off via photoassistance.
On the trail to this target, we examined the inhibiting pro-
perties of the cinnamate moiety itself (still without a photoactive
group), which is presented here.
We decided to do an extensive study on bis(triphenylphos-
phine)(cinnamic acid ester) complexes of platinum, as shown in
Table 1, in which the olefin compound as the most labile ligand
serves as an electronic and steric tuner, being released in the first
step of the catalytic cycle, and thus acting as a more or less
efficient catalyst inhibitor depending on the strength of the
interaction with the metal center.
1
ligand can be performed. H, 13C, 19F, 31P, and 195Pt NMR
spectroscopy were utilized as sensitive probes in solution, and the
results are compared to the single-crystal data of nine complexes
and are correlated with reactivity toward ligand substitution and
oxidative addition, two of the most important elemental steps in
catalysis.
RESULTS AND DISCUSSION
■
Synthesis and Stability of the Complexes. Reaction of
Pt(PPh3)34 with one equivalent of 4-nitrocinnamic acid esters in
toluene led to the desired bis(triphenylphosphine)(olefin)-
complexes (Pt[PhNO2-PhR2]) in quantitative yields.5 Released
triphenylphosphine can be removed by washing with
pentane. The same procedure was successful for 4-chloro- and
4-trifluoromethyl cinnamic acid phenol esters as well as for
cinnamic acid aryl ester ligands even though yields were lower,
caused by the comparably higher solubility of the product
complexes in pentane. Due to the higher electron density at the
double bond of 4-methylcinnamic acid, 10 equiv of ligand was
Received: November 5, 2012
Published: March 1, 2013
© 2013 American Chemical Society
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Scheme 1. Target Strategy for a Photo-Gated Pt Complex with Inhibiting Cinnamate Ligand
Table 1. Examination Field: Hammett and A Values
whereas all other complexes decompose slowly with the forma-
tion of platinum black. All complexes are stable in solution in the
absence of oxygen.
X-ray Structures. Single crystals of Pt[PhOMe-PhOMe],
Pt[PhMe-Ph], Pt[Ph-PhMe], and Pt[Ph-Ph] (Figure 1),
Pt[PhNO2-PhOMe], Pt[PhNO2-PhMe], Pt[PhNO2-Ph], and
Pt[PhNO2-PhNO2] (Figure 2), and Pt[PhNO2-PhiPr] (Figure 3)
R1/R2
σP
R3
A
OMe
Me
H
−0.27
−0.17
0.00
Me
Et
1.70
1.75
2.15
4.50
iPr
tBu
Cl
0.23
CF3
NO2
0.54
0.78
needed to shift the equilibrium to the product side. The extensive
amounts of pentane required to remove the excess ligand and the
increased solubility of the product led to very low yields via this
route. This synthetic approach failed completely when
4-methoxycinnamic acid was used and only mixtures of
Pt(PPh3)3 and product were isolated. In situ NMR experiments
showed the same trends mentioned above for the alkyl esters:
only the nitro complexes could be isolated via this route. With
all other ligands, even though 100% conversion was confirmed
by 1H NMR spectroscopy for 4-chloro, 4-trifluoro, and
cinnamic acid alkyl esters, only Pt(PPh3)3 was reisolated.
The reason is the high solubility of the non-nitro-substituted
alkyl ester complexes in pentane. On washing to remove
triphenylphosphine, Pt(PPh3)3 precipitated while the alkyl
ester complexes remained in solution, shifting the equilibrium
to the starting material side. Therefore, all non-nitro-
substituted complexes were synthesized from (PPh3)2PtC2-
H46 in benzene, which resulted in the desired products in good
to excellent yields (Scheme 2).
Figure 1. ORTEP7 representations of Pt[PhOMe-PhOMe] (top left),
Pt[PhMe-Ph] (top right), Pt[Ph-PhMe] (bottom left), and Pt[Ph-Ph]
(bottom right). Thermal ellipsoids are given at the 50% probability level.
Hydrogen atoms are omitted for clarity.
suitable for X-ray analysis could be grown by slow diffusion of
pentane into a toluene solution of the appropriate complex.
Selected bond lengths are given in Table 2.
The length of the olefin bond is between 1.430 and 1.458 Å
and therefore consistent with the olefin in known complexes
such as Pt(PPh3)2C2H4 and its nickel analogue.8 The C(Ar)−Pt
bond is, with the exception of Ph-PhMe, PhNO2-PhMe, and
PhNO2-iPr, slightly shorter than the C(CO)−Pt bond. The same
trend was found by us for the analogous Pd complexes and was
All platinum complexes were isolated as white or cream-
colored solids (yellow in the case of nitro cinnamic acid esters)
and could be handled in air for a short period of time but should
be stored under an inert gas atmosphere for longer periods. Only
the nitro cinnamic acid derivatives are air stable in solution,
Scheme 2. Synthetic Routes to Bis(triphenylphosphine)(cinnamates)platinum(0)
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115.0°, in the range of 107.7−110.8°. The deviation from the
ideal 120° is caused by the greater steric demand of the olefin.
P1−Pt−C(Ar) angles are smaller than the C(CO)−Pt−P2 angle
due to the greater steric demand of the carboxyl group,
supporting the notion that a steric influence via the R3 moiety
is effective. All complexes show almost perfect coplanarity
concerning the olefin carbon atoms, platinum, and both
phosphorus atoms. The torsional angles of the olefin are, at
145.9−155.7° significantly smaller than the ideal 180° for
noncoordinated (E)-olefins due to the hybridization change of
the olefin carbon atoms as a consequence of the π back-bonding
of platinum (Table 4).
IR Spectroscopy. The CO stretching frequency of the
carbonyl moiety is dependent on the electron density and the
resulting bond order at the carbonyl itself and therefore on the π
back-bonding from the metal to the conjugated CC double
bond. A correlation of the electron density of the olefin and the
carboxylic CO IR wavenumber was expected between the
Hammett parameter σ of R1 and R2 and the FT-IR data, but it was
not observed, similar to the case for the analogous Pd
complexes.9 Despite the lack of correlation to the olefin’s
electron density, the CO stretching frequency shifts to lower
wavenumbers on coordination to the metals by 20 to 30 cm−1 for
all 48 complexes, again in accordance with the findings for the
analogous Pd complexes.9
Figure 2. ORTEP7 representations of Pt[PhNO2-PhOMe] (top left),
Pt[PhNO2-PhMe] (top right) ,Pt[PhNO2-Ph] (bottom left), and
Pt[PhNO2-PhNO2] (bottom right). Thermal ellipsoids are given at the
50% probability level. Hydrogen atoms are omitted for clarity.
1H NMR Spectroscopy. An exemplary spectrum is given in
Figure 1 in the Supporting Information. All platinum complexes
show a static behavior in 1H, 13C, 31P, and 195Pt NMR
spectroscopy on the spectral time scale, in contrast to the case
for the analogous Pd complexes,9 indicating a stronger bond of
the Pt to the coordinated double bond in comparison to Pd.
All cinnamate complexes of platinum show the characteristic
high-field shift of the olefinic protons in comparison to the free
ligand (ca. 3.8−4.3 ppm for the olefinic proton closer to the
carboxyl group (CHCO) and ca. 4.3−4.6 ppm for that next to the
aryl ring (CHCAr)). The signal assignment is based on
comparison with the free ligand. Both signals show a coupling
to each other with coupling constants of 8−9 Hz, which are
significantly smaller than those observed for E-substituted
protons of the free ligand (about 16 Hz), as expected because
of the coordination of the platinum which distorts the planar
geometry of the double bond by forcing all substituents of the
olefin away from the metal. The coupling constants indicate a
H−CC−H torsion angle of ca. 150°, determined via the
Karplus equation.10 This is in accordance with our X-ray analysis
results. A coupling is also observed to both phosphorus nuclei via
the platinum with a cis coupling constant of about 4 Hz and a
Figure 3. ORTEP7 representations of Pt[PhNO2-iPr]: (left) side view;
(right) top view. Thermal ellipsoids are given at the 50% probability
level. Hydrogen atoms are omitted for clarity.
ascribed to the special bonding mode of the Michael type double
bond to the metal center.9
Both C−Pt bond lengths are in the range of 2.106−2.136 Å
and are comparable to those in other known platinum olefin
complexes (such as 2.116(9) and 2.106(8) Å in Pt(PPh3)2C2H4
and 2.10(3) and 2.12(3) Å in Pt(PPh3)2C2(CN)4). The P1−Pt
distance is in all complexes shorter than the P2−Pt distance
(a consequence of the stronger trans influence of the aryl group
in comparison with the ester moiety).
The C(Ar)−Pt−C(CO) angle is close to 40° for all com-
pounds (Table 3). The P−Pt−P angle is, except for PhMe-Ph at
Table 2. Selected Bond Lengths (Å)
1
2
3
4
5
Pt[PhOMe-PhOMe]
Pt[PhMe-Ph]
1.439(0)
1.443(4)
1.430(0)
1.443(3)
1.445(0)
1.444(0)
1.458(0)
1.445(0)
1.444(3)
2.132(0)
2.121(3)
2.128(0)
2.108(2)
2.110(0)
2.122(0)
2.116(0)
2.106(0)
2.123(2)
2.140(0)
2.133(3)
2.127(0)
2.136(2)
2.117(0)
2.115(0)
2.133(0)
2.116(0)
2.122(2)
2.291(0)
2.292(1)
2.283(0)
2.284(1)
2.293(0)
2.290(0)
2.292(0)
2.292(0)
2.288(1)
2.279(0)
2.278(1)
2.278(0)
2.272(1)
2.274(0)
2.273(0)
2.273(0)
2.274(0)
2.284(1)
Pt[Ph-PhMe]
Pt[Ph-Ph]
Pt[PhNO2-PhOMe]
Pt[PhNO2-PhMe]
Pt[PhNO2-Ph]
Pt[PhNO2-PhNO2]
Pt[PhNO2-iPr]
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Table 3. Selected Bond Angles (deg)
α
β
γ
δ
∑
Pt[PhOMe-PhOMe]
Pt[PhMe-Ph]
39.4(0)
39.7(1)
39.3(0)
39.8(1)
40.0(0)
39.9(0)
40.1(0)
40.0(0)
39.8(1)
109.5(0)
105.6(1)
112.5(0)
109.0(1)
106.0(0)
105.6(0)
106.3(0)
106.1(0)
109.7(1)
110.5(0)
115.0(0)
107.7(0)
109.0(0)
110.5(0)
110.4(0)
109.3(0)
110.8(0)
107.7(0)
100.7(0)
100.2(1)
101.7(0)
102.6(1)
103.5(0)
104.2(0)
104.3(0)
103.1(0)
102.8(1)
360.1
360.5
361.2
360.4
360
Pt[Ph-PhMe]
Pt[Ph-Ph]
Pt[PhNO2-PhOMe]
Pt[PhNO2-PhMe]
Pt[PhNO2-Ph]
Pt[PhNO2-PhNO2]
Pt[PhNO2-iPr]
360.1
360
360
360
Table 4. Selected Torsion Angles (deg)
Both protons also couple to 195Pt which is indicated through the
presence of 195Pt satellites (about 50−60 Hz).
The electron density on the double bond is the main
parameter determining the strength of the metal−olefin bond
in these related complexes. The more electron poor the olefin is,
the stronger the back-bonding into the olefin’s π* orbital.
Therefore, the σ character of the olefinic C−C bond is more
pronounced, leading as a main factor to the high-field shift of the
olefinic proton signals in NMR spectroscopy. Thus, this high-
field shift relative to the free ligand can be considered an indirect
indicator of the organometallic bond strength. A simple way to
describe the electron density at the double bond is to use the
θ
Pt[PhOMe-PhOMe]
Pt[PhMe-Ph]
148.5(0)
151.6(3)
155.7(0)
147.9(2)
145.9(0)
147.3(0)
148.0(0)
146.0(0)
150.0(2)
Pt[Ph-PhMe]
Pt[Ph-Ph]
Pt[PhNO2-PhOMe]
Pt[PhNO2-PhMe]
Pt[PhNO2-Ph]
Pt[PhNO2-PhNO2]
Pt[PhNO2-iPr]
1
2
1
2
Hammett parameters of the substituents R (σR ) and R (σR ).
1
2
As expected, a correlation of σR and σR with the change in
chemical shift of both olefin signals on coordination is observed
in the 1H NMR spectra (Figure 4). The changes of the chemical
shift on coordination increase with more electron-withdrawing
groups in the cinnamate ligand, where the effect of R1 is more
pronounced than for R2. This is also in accordance with chemical
trans coupling constant of ca. 8−9 Hz. Therefore, both olefinic
protons show a multiplicity of a doublet of pseudotriplets (dpt)
and not the expected doublet of doublets of doublets (ddd).
Figure 4. Changes of 1H NMR shifts on coordination for the olefinic signals of the Pt[PhR1-PhR2] complexes dependent on (a, b) σR and (c, d) σR .
2
1
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Figure 5. Changes of 1H NMR shifts (according to amount) on coordination for the olefinic signals of the Pt[PhR1-R3] complexes dependent on A(R3).
For R1 the same principle correlation as for the Pt[PhR1-PhR2]
complexes can be observed for Pt[PhR1-PhR3] (see Figure 2 in
the Supporting Information).
13C NMR Spectroscopy. An exemplary spectrum is given in
Figure 3 of the Supporting Information). The signals of both
olefinic carbon atoms are, as expected, shifted to higher field in
13C NMR spectroscopy on platinum coordination relative to the
free ligand. The signal of the carbon (CHCO) next to the
carboxyl carbon is observed at ca. 48−49 ppm, while that closer
to the aryl group (CHCAr) is at ca. 59−62 ppm. Both carbon
atoms show a coupling to the two phosphorus atoms with a cis
coupling constant of ca. 5 Hz and a trans coupling constant of
ca. 30 Hz. Therefore, both signals are observed as doublets of
doublets. Both signals exhibit 195Pt satellites indicating additional
coupling to the metal center. This coupling constant (190−
210 Hz) will be further discussed in the section about 195Pt NMR
spectroscopy. The signals of the carboxylic carbon and the
aromatic carbon next to the double bond are both observed as
doublets of doublets with platinum satellites due to coupling via
the olefin to the metal and via the olefin and platinum to both
phosphorus nuclei, but no significant change in NMR shift is
observed. Coupling to only one 31P atom is observed for the
carbon atom in a position ipso to R1 when R1 is OMe or NO2.
Only for the CHCO carbon is a clear correlation of the change in
1
chemical shift on coordination with the Hammett parameters σR
and σR observed, where the influence of R2 is again much less
2
Figure 6. Changes of 13C NMR shifts (according to amount) for the
CHCO signals of the Pt[PhR1-PhR2] complexes on coordination of the
pronounced than that of R1 (Figure 6). A similar behavior was
found by us for the analogous Pd complexes.
2
1
Steric changes in R3 have only a small influence on the change
in chemical shift of the olefinic carbons (see Figure 4 in the
Supporting Information).
olefin ligand dependent on (a) σR and (b) σR .
intuition, and one can conclude that the changes in chemical shift
of the olefinic protons can be used as an easily available measure
for the strength of coordination of this ligand.
31P NMR Spectroscopy. An exemplary spectrum is given in
Figure 5 in the Supporting Information. The two phosphorus
nuclei can be observed as two distinct signals in the 31P NMR
The ethyl and isopropyl ester moieties in the alkyl cinnamate
complexes possess diastereotopic groups which show two
distinct signals on coordination, respectively. For the ethyl
esters two doublets of quartets are observed for the methylene
moiety, one of which is shifted to high field by about 3.7 ppm,
while two separated signals are observed for the two methyl
groups of the isopropyl group. When the steric demand
(quantified by the steric A parameter of the alkyl group R3) of
the ester moiety is increased, the changes of 1H shifts decrease for
both olefinic protons, indicating that sterics also influence the
chemical shift of these protons (Figure 5) and that the
coordination is less strong for more sterically crowded
substituents, in accordance with the X-ray analysis results
(compare Pt[PhNO2-iPr] with Pt[PhNO2-Ph]).
2
spectra. A slight linear trend is found for the dependence on σR
(Figure 7a,b) and a two-regime development is observed for σR
1
(Figure 7c,d).
Both signals are split into doublets due to 2JPP coupling. The
coupling constants are at ca. 30−45 Hz within the typical range of
cis couplings (Table 5). For the analogous Pd complexes we
2
found a strong correlation between the JPP coupling constants
measured and the Hammett parameters of the para substituents
of the coordinated cinnamate derivative.
Also, for the Pt complexes in this paper this correlation was
confirmed as shown in Figure 8 (see Table 3 in the Supporting
Information for correlation equations). Thus, for both metal
centers this easily available experimental value can be used to
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Figure 7. Dependence of 31P NMR shifts of the Pt[PhR1-PhR2] complexes on (a, b) σR and (c, d) σR .
2
1
Figure 8. Dependence of 2JPP values for the Pt complexes Pt[PhR1-PhR2] on (a) R2 and (b) R1.
Unfortunately, the chemical shift in 195Pt NMR spectroscopy
cannot be expected to correlate unequivocally with the oxidation
state or the electron density at Pt in a simple manner, even for
determine the electron density at the metal center and, thus, the
stability and reactivity of the related complexes. The dependence
2
of the JPP coupling constant on the electron density of the
related complexes.1,10 In contrast, the JXPt coupling constants
1
coordinating metal center is more pronounced for R1 than for R2
are strong indicators for the Pt−X bonding mode (oxidation
as would have been expected. Although sterics also influence the
state, trans influence, coordination number),12 in particular in
2
size of JPP, most likely via the bond angle P−Pt−P, the main
related complexes. Figure 9 depicts the correlation of 1JP Pt and
1
contribution is by electronics (see Figure 6 in the Supporting
Information).
1
JP Pt with the Hammett parameters of R1 and R2 in the Pt[PhR1-
2
Both signals show platinum satellites due to direct interaction
with the metal center. These coupling constants (about 3700 Hz
for P1 and 4100 Hz for P2) allow distinguishing which P signal
belongs to which P atom in the complex structure (trans
influence; Table 6), and this will be discussed in the context of
the 195Pt NMR spectroscopy results.
PhR2] complexes.
1
1
1
2
The JP Pt coupling constant is always larger than the JP Pt
coupling constant. As can be seen from Figure 9a, there is a
1
1
2
linear correlation between JP Pt and σR with positive slope
(see Table 3 in the Supporting Information for correlation
equations). There also exists a linear correlation between 1JP Pt
1
The more downfield signal (P2) is observed as a doublet of
doublets with Pt satellites, if R1 is CF3.
and σR , but with a negative and steeper slope (Figure 9c). The
1
complementary behavior concerning the sign of the slope is
1
195Pt NMR Spectroscopy. An exemplary spectrum is given
in Figure 7 in the Supporting Information. The signals in the 195Pt
NMR spectra are at about −5050 ppm in the typical range for
bis(triphenylphosphine)(olefin)platinum(0) complexes (Table 6).11
found for JP Pt. In this case the slope is negative with σR
2
2
1
(Figure 9b) and positive with σR (Figure 9d). The steepnesses
of the slopes for the two P atoms are almost the same; just the
sign inverts.
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Table 5. 31P NMR Data for the Pt Complexes
Table 6. 195Pt NMR Results for the Pt Complexes
P1 (ppm)
P2 (ppm)
|2JPP| (Hz)
|1JP Pt
|
|1JP Pt
|
|1JC Pt
|
|1JC Pt
|
2
1
1
2
Pt (ppm)
(Hz)
(Hz)
(Hz)
(Hz)
Pt[PhOMe-Me]
Pt[PhOMe-Et]
Pt[PhOMe-iPr]
Pt[PhOMe-tBu]
Pt[PhMe-Me]
Pt[PhMe-Et]
28.4
28.1
28.3
27.5
28.2
28.5
28.6
28.6
29.1
28.4
28.5
27.2
27.6
27.6
27.3
26.8
27.5
27.4
27.2
26.7
26.8
26.7
27.2
26.6
27.6
27.6
27.5
27.0
28.0
27.3
27.9
26.8
28.0
28.0
27.9
27.5
26.8
26.8
26.8
26.3
26.7
26.7
26.6
26.2
26.7
26.1
26.0
25.6
28.6
28.7
28.7
28.7
28.4
28.5
28.6
28.6
29.1
28.4
28.5
28.5
28.0
28.1
28.3
28.3
28.8
28.0
28.2
28.2
27.2
27.5
28.5
28.5
28.3
28.3
28.3
27.8
28.9
28.2
29.0
27.7
28.8
28.8
28.8
28.3
27.9
27.9
27.9
27.3
27.8
27.8
27.7
27.2
28.0
27.3
27.3
26.8
45.6
47.6
46.6
48.7
44.4
45.4
46.4
47.2
42.6
43.8
44.7
45.5
41.0
41.8
42.8
43.8
38.2
39.0
40.0
40.7
33.8
34.9
35.7
36.5
44.8
44.6
44.5
41.4
43.4
43.3
43.3
40.2
41.8
41.6
41.3
38.6
39.7
39.7
39.6
36.9
37.0
36.9
36.5
34.1
32.9
33.0
32.6
30.2
Pt[PhOMe-Me]
Pt[PhOMe-Et]
Pt[PhOMe-iPr]
Pt[PhOMe-tBu]
Pt[PhMe-Me]
Pt[PhMe-Et]
Pt[PhMe-iPr]
Pt[PhMe-tBu]
Pt[Ph-Me]
−5058.1
−5053.8
−5048.7
−5079.2
−5061.8
−5057.6
−5051.9
−5052.7
−5067.7
−5063.6
−5059.1
−5060.2
−5067.0
−5063.5
−5058.8
−5059.9
−5070.7
−5066.6
−5061.3
−5062.4
−5065.1
−5057.3
−5051.3
−5052.3
3613.5
3631.5
3654.1
3682.4
3632.2
3649.7
3670.8
3728.6
3637.8
3654.3
3673.8
3730.3
3645.1
3661.1
3676.7
3729.6
3684.4
3699.4
3737.7
3761.4
3730.2
3735.7
3773.7
3811.1
3625.4
3624.9
3622.5
3582.5
3642.2
3641.1
3643.7
3598.7
3645.0
3645.1
3642.0
3604.4
3650.2
3650.2
3647.0
3610.7
3683.4
3679.0
3674.5
3648.5
3741.8
3741.9
3737.1
3681.7
4199.4
4188.7
4171.7
4144.9
4176.9
4167.1
4151.7
4095.2
4149.9
4139.6
4122.8
4067.2
4119.5
4110.3
4098.5
4047.8
4077.2
4067.6
4031.3
4003.4
4027.3
4026.5
3992.9
3957.7
4218.0
4218.8
4225.6
4275.2
4197.8
4197.0
4206.7
4246.2
4168.4
4169.8
4176.1
4219.6
4138.6
4139.8
4146.1
4187.5
4088.8
4102.1
4109.1
4145.4
4032.3
4033.1
4038.7
4095.1
192.2
194.7
200.9
205.9
195.7
202.6
203.9
218.6
196.8
208.4
212.8
218.6
199.3
205.0
198.2
200.6
200.1
204.3
209.1
218.4
208.0
193.8
218.5
218.6
193.7
188.9
193.2
157.0
186.0
198.9
197.6
186.0
198.3
199.1
196.0
188.3
206.3
205.1
199.9
175.0
197.5
201.0
209.8
193.8
206.0
209.0
201.0
201.4
197.4
194.8
194.5
196.0
197.3
195.2
195.2
192.2
199.1
205.4
196.6
195.2
197.2
196.2
186.4
190.2
197.1
199.9
186.9
199.9
191.6
190.7
188.1
175.8
163.5
193.1
197.2
200.8
189.0
195.7
196.8
200.3
197.6
198.4
194.5
199.8
201.0
199.5
199.7
206.2
195.4
194.2
211.3
206.5
187.6
184.4
180.2
187.6
Pt[PhMe-iPr]
Pt[PhMe-tBu]
Pt[Ph-Me]
Pt[Ph-Et]
Pt[Ph-iPr]
Pt[Ph-Et]
Pt[Ph-iPr]
Pt[Ph-tBu]
Pt[Ph-tBu]
Pt[PhCl-Me]
Pt[PhCl-Me]
Pt[PhCl-Et]
Pt[PhCl-Et]
Pt[PhCl-iPr]
Pt[PhCl-iPr]
Pt[PhCl-tBu]
Pt[PhCF3-Me]
Pt[PhCF3-Et]
Pt[PhCF3-iPr]
Pt[PhCF3-tBu]
Pt[PhNO2-Me]
Pt[PhNO2-Et]
Pt[PhNO2-iPr]
Pt[PhNO2-tBu]
Pt[PhCl-tBu]
Pt[PhCF3-Me]
Pt[PhCF3-Et]
Pt[PhCF3-iPr]
Pt[PhCF3-tBu]
Pt[PhNO2-Me]
Pt[PhNO2-Et]
Pt[PhNO2-iPr]
Pt[PhNO2-tBu]
Pt[PhOMe-PhOMe]
Pt[PhOMe-PhMe]
Pt[PhOMe-Ph]
Pt[PhOMe-PhNO2]
Pt[PhMe-PhOMe]
Pt[PhMe-PhMe]
Pt[PhMe-Ph]
Pt[PhOMe-PhOMe] −5044.0
Pt[PhOMe-PhMe]
−5043.2
−5042.1
Pt[PhOMe-Ph]
Pt[PhOMe-PhNO2] −5047.6
Pt[PhMe-PhOMe]
Pt[PhMe-PhMe]
Pt[PhMe-Ph]
−5047.6
−5046.4
−5052.0
−5045.1
−5054.2
−5053.4
−5052.5
−5051.0
−5055.6
−5054.9
−5053.8
−5053.0
−5058.6
−5057.8
−5056.6
−5055.9
Pt[PhMe-PhNO2]
Pt[Ph-PhOMe]
Pt[Ph-PhMe]
Pt[PhMe-PhNO2]
Pt[Ph-PhOMe]
Pt[Ph-PhMe]
Pt[Ph-Ph]
Pt[Ph-Ph]
Pt[Ph-PhNO2]
Pt[PhCl-PhOMe]
Pt[PhCl-PhMe]
Pt[PhCl-Ph]
Pt[Ph-PhNO2]
Pt[PhCl-PhOMe]
Pt[PhCl-PhMe]
Pt[PhCl-Ph]
Pt[PhCl-PhNO2]
Pt[PhCF3-PhOMe]
Pt[PhCF3-PhMe]
Pt[PhCF3-Ph]
Pt[PhCF3-PhNO2]
Pt[PhNO2-PhOMe]
Pt[PhNO2-PhMe]
Pt[PhNO2-Ph]
Pt[PhNO2-PhNO2]
Pt[PhCl-PhNO2]
Pt[PhCF3-PhOMe]
Pt[PhCF3-PhMe]
Pt[PhCF3-Ph]
Pt[PhCF3-PhNO2]
Pt[PhNO2-PhOMe] −5048.8
Pt[PhNO2-PhMe]
−5048.2
−5047.1
−5047.0
Pt[PhNO2-Ph]
Pt[PhNO2-PhNO2]
Figure 10 depicts the behavior of the Pt[PhR1-R3] complexes.
From Figure 10 it can be concluded that sterics also influence the
bonding mode of the olefin ligand.
Reactivity (Ligand Exchange). To determine the reactivity
of the complexes, basic ligand substitution and oxidative addition
reactions were performed.
We point out that an expected complementary development
The synthesis of Pt[PhMe-PhR2] from Pt(PPh3)3 required
10 equiv of olefin ligand, and this route failed completely for
PhOMe-PhR and all alkyl complexes with the exception of
4-nitro cinnamates (vide supra). Therefore, an NMR titration
1
to the JPPt coupling constant was not found for the
1
corresponding JPtC coupling constants (see Figures 8 and 9 in
the Supporting Information).
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Figure 9. Dependence of 1JP Pt values for the Pt[PhR1-PhR2] complexes on the Hammett parameters of (a, b) R1 and (c, d) R2.
1/2
Figure 10. Dependence of 1JP Pt values for the Pt[PhR1-R3] complexes on the A parameters of R3: (a) P1; (b) P2.
1/2
Scheme 3. Equilibrium between Coordination of Triphenylphosphine and Cinnamate
(1H NMR spectroscopy) of a Pt(PPh3)3 solution in benzene-d6
was performed with PhOMe-PhOMe, PhOMe-PhMe, PhOMe-
Ph, and PhOMe-PhNO2 (Scheme 3).
Only the systems Pt[PhOMe-PhMe] and Pt[PhOMe-Ph]
could be used for quantitative NMR titration (exemplary spectra
are given in Figure 10 in the Supporting Information). Due to
overlap of the methoxy signals in Pt[PhOMe-PhOMe] with
those of the free ligand, this titration could not be analyzed
quantitatively for this ligand. The same accounts for Pt[PhOMe-
PhNO2], due to the low solubility of the ligand, resulting in
coordination ratios of over 100%. The substitution reaction is
very fast, and its kinetics cannot be followed by simple NMR
measurements. The results are shown in Figure 11. As expected,
the cinnamate complex concentration increases with the amount
Figure 11. NMR titration of Pt(PPh3)3 with cinnamate ligands. Dashed
lines are calculated with Kex = 0.28 (black) and Kex = 0.41 (gray).
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Scheme 4. Equilibrium between Oxidative Addition of Ph2SiH2 and Cinnamate Coordination
of ligand. Even with 10 equiv conversions of only ca. 80% could
be obtained. Higher concentrations of free ligand could not be
investigated because of a saturation of the solution with
cinnamate. On the basis of these results the equilibrium constant
at 25 °C of this reaction for Pt[PhOMe-Ph] can be estimated as
0.41 and for Pt[PhOMe-PhMe] as 0.28.
the metal to olefin back-bonding is becoming more important
with the electron-withdrawing character of R1, in accordance
with chemical intuition.
CONCLUSIONS
■
Characterizing 48 new bis(triphenylphosphine)(cinnamate)Pt
complexes in detail we were able to shed light on the bonding and
structural situation of them. We were able to correlate electronic
effects of the cinnamate ligand on the change in 1H chemical shift
of the olefinic protons on coordination of the olefin ligand and in
particular on the coupling constants if bonds of the inner
coordination sphere are involved with the Hammett parameters
of the substituents at the cinnamate ligand.
Reactivity (Oxidative Addition). Since the oxidative
addition of silane is one of the key steps in hydrosilylation, the
dependence of the insertion of platinum into the Si−H bond on
the properties of the cinnamate ligand was investigated.13−16 On
the addition of 1.1 equiv of Ph2SiH2 to the olefin complexes
the olefin dissociates and an equilibrium between (PPh3)2-
PtH(SiHPh2) and the starting complex forms (Scheme 4).
Hydrogen evolution was observed, and evidence for
homocoupling of the silane was gained in 29Si NMR spectros-
copy when an excess of silane was used or the complexes were
added to the neat silane.
The reactivity toward ligand substitution and oxidative
addition was also examined and, in the case of oxidative addition,
could be correlated linearly with the Hammett parameters of the
modified cinnamate ligands with a considerable negative slope,
allowing us to predict the inhibition power of these olefin ligands
for the catalytic cycle.
1
From the signals in the H NMR spectrum the equilibrium
constants were determined (exemplary spectra are given in
Figure 11 in the Supporting Information).
The equilibrium constant K (Table 7) decays exponentially
with increasing electron density, as expected, and this is shown
in Figure 12.
Because of our earlier results with Pd, a comparison of the
behavior between Pt and Pd was possible with this ligand system
for the first time. The Pt complexes are static on the spectral
NMR time scale, while the analogous Pd complexes show a
dynamic behavior concerning the coordination of the cinnamate
ligand. For both metal centers a shift to lower wavenumbers by
20−30 cm−1 was observed for the CO vibration in IR
spectroscopy, which could not be correlated systematically with
Table 7. Equilibrium Constants for the Oxidative Addition of
H2SiPh2
K
ln K
2
the Hammett parameters of the cinnamate ligand. The JPP
Pt[PhOMe-PhMe]
Pt[PhMe-PhMe]
Pt[Ph-PhMe]
32.3 1.6
12.9 0.6
9.2 0.5
3.56
2.56
1
coupling constants correlate equally well linearly with the σR
parameters of the cinnamate ligand for both metal centers (slope
of Hammett plot: for Pd, −13 Hz; for Pt, −11 Hz).
2.22
Pt[PhCl-PhMe]
Pt[PhCF3-PhOMe]
Pt[PhNO2-PhMe]
3.1 0.2
1.12
0.50 0.03
0.05 0.0025
−0.69
−3.08
EXPERIMENTAL SECTION
■
General Procedures. All manipulations and experiments were
performed under argon using standard Schlenk techniques and in a
glovebox filled with argon unless otherwise stated. Pentane was dried
and degassed using a two-column drying system (MBraun),17 benzene
and toluene were distilled from sodium, and pyridine was distilled from
potassium hydroxide. All solvents were stored under an argon
atmosphere. CDCl3 was used as received from Deutero GmbH, and
benzene-d6 and toluene-d8 were dried and degassed by stirring over
sodium potassium alloy, purified by condensation, and stored under
argon.18 K2PtCl4 was used as received from ABCR. Triphenylphos-
phine, cinnamic acid, phenol, 4-methoxyphenol, 4-nitrophenol, and
p-cresol were purchased from Merck and 4-methoxycinnamic acid,
4-methylcinnamic acid, and 4-nitrocinnamic acid from Aldrich; all were
used without further purification. Pt(PPh3)3,4 Pt(PPh3)2C2H4,6 the acid
chlorides, and the alkyl cinnamates were synthesized according to
literature procedures.9,19−43
1H NMR, 13C NMR, and 31P NMR measurements were performed
on a Bruker Avance 400 spectrometer and 19F NMR and 195Pt NMR
measurements on a Bruker AMX 400 spectrometer. 1H NMR (400 MHz;
digital resolution 0.2 Hz) and 13C NMR (100 MHz; digital
Figure 12. Dependence of ln K of silane oxidative addition on the
electronic properties of the ligand (R2 = 0.96).
1
resolution 0.5 Hz). Chemical shifts for H and 13C NMR are given
relative to the solvent signal for C6D6 (7.15 and 128.1 ppm)/C7D8 (2.09
and 20.4 ppm);44,45 19F NMR (377 MHz) used neat CFCl3, 31P NMR
(162 MHz; digital resolution: 0.5 Hz) used 85% H3PO4, and 195Pt NMR
The inversion point of the equilibrium (K = 1, ln K = 0) was
determined at σ0 = 0.35 0.08. The slope is −5.8, indicating that
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(85 MHz; digital resolution 0.6 Hz) used Na2PtCl6 in D2O as external
standards. FAB-MS analysis was carried out on a Finnigan MAT-90 in a
p-nitrobenzyl alcohol matrix under bombardment with ionized xenon.
ESI-MS were conducted on a Finnigan LCQ in acetonitrile. Micro
analytical analysis was performed in the micro analytical lab of the
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Technische Universitat Munchen. ATR FT-IR spectroscopy was done
̈
̈
with a Thermo Scientific Nicolet 380 Smart Orbit. X-ray single crystal
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under perfluorinated oil, transferred into a Lindemann capillary, fixed,
and sealed. Preliminary examination and data collection were carried out
on an area detecting system (APEX II, κ-CCD) at the window of a
rotating anode (Bruker AXS, FR591) with graphite-monochromated
Mo Kα radiation (λ = 0.71073 Å). Raw data were corrected for Lorentz
and polarization and, arising from the scaling procedure, for latent decay
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all hydrogen atoms were refined with isotropic displacement parameters.
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2
P(Fo − Fc2)2 with the SHELXL-97 weighting scheme. The final
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General Synthetic Procedures for the Complexes. Route I. A
100 mg (0.10 mmol) portion of Pt(PPh3)3 and 0.10 mmol of the
appropriate ligand were dissolved in 20 mL of toluene and stirred for 1 h.
After removal of the solvent in vacuo the residue was stirred with 25 mL
of pentane overnight. The solvent was decanted off and the residue
washed four times with 25 mL of pentane to give the product.
Route II. A 75.0 mg (0.10 mmol) portion of Pt(PPh3)2C2H4 was
dissolved in 5 mL of benzene, and this solution was added to 0.10 mmol
of the appropriate ligand and the mixture stirred for 1 h. After
lyophilization the residue was stirred in pentane overnight. The solvent
was decanted off and the obtained powder dried in vacuo to yield the
desired complex in microanalytical grade.
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Detailed synthesis and characterization data for all 48 new complexes
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ASSOCIATED CONTENT
* Supporting Information
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■
S
Figures, tables, text, and CIF files giving detailed synthesis and
characterization data for 48 new complexes and crystallographic
data for the structure studies in the paper. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for K.R.: klaus.ruhland@physik.uni-augsburg.de.
■
Notes
The authors declare no competing financial interest.
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1625.
ACKNOWLEDGMENTS
■
We thank Michael Zeilinger for his synthetic work within his
research practice. For financial support we thank the WACKER
Chemie AG.
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