Influence of Electronically and Sterically Tunable Cinnamate Ligands

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 für Siliciumchemie, Lehrstuhl für Anorganische Chemie, Technische Universität München, Lichtenbergstraße 4, 85747 Garching bei München, Germany ‡ Lehrstuhl für Anorganische Chemie, Technische Universität München, Lichtenbergstraße 4, 85747 Garching bei München, Germany § Lehrstuhl für Anorganische Chemie mit Schwerpunkt Neue Materialien, Technische Universität München, Lichtenbergstraße 4, 85747 Garching bei München, Germany ∥ Lehrstuhl für Chemische Physik und Materialwissenschaften, Universität Augsburg, Universitätsstraße 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 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 2JPP and 1 JPPt. 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.

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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 properties of the cinnamate moiety itself (still without a photoactive group), which is presented here. We decided to do an extensive study on bis(triphenylphosphine)(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. © 2013 American Chemical Society

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 ligand can be performed. 1H, 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.

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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 1643

dx.doi.org/10.1021/om301021j | Organometallics 2013, 32, 1643−1653

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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

R1/R2

σP

R3

A

OMe Me H Cl CF3 NO2

−0.27 −0.17 0.00 0.23 0.54 0.78

Me Et iPr tBu

1.70 1.75 2.15 4.50

whereas all other complexes decompose slowly with the formation 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)

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-nitrosubstituted complexes were synthesized from (PPh3)2PtC2H46 in benzene, which resulted in the desired products in good to excellent yields (Scheme 2). All platinum complexes were isolated as white or creamcolored 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,

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

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 1 H 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 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.

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 compounds (Table 3). The P−Pt−P angle is, except for PhMe-Ph at Table 2. Selected Bond Lengths (Å)

Pt[PhOMe-PhOMe] Pt[PhMe-Ph] Pt[Ph-PhMe] Pt[Ph-Ph] Pt[PhNO2-PhOMe] Pt[PhNO2-PhMe] Pt[PhNO2-Ph] Pt[PhNO2-PhNO2] Pt[PhNO2-iPr]

1

2

3

4

5

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)

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Table 3. Selected Bond Angles (deg)

Pt[PhOMe-PhOMe] Pt[PhMe-Ph] Pt[Ph-PhMe] Pt[Ph-Ph] Pt[PhNO2-PhOMe] Pt[PhNO2-PhMe] Pt[PhNO2-Ph] Pt[PhNO2-PhNO2] Pt[PhNO2-iPr]

α

β

γ

δ

∑

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)

360.1 360.5 361.2 360.4 360 360.1 360 360 360

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 highfield 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 Hammett parameters of the substituents R1 (σR1) and R2 (σR2). As expected, a correlation of σR1 and σR2 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

Table 4. Selected Torsion Angles (deg)

θ Pt[PhOMe-PhOMe] Pt[PhMe-Ph] Pt[Ph-PhMe] Pt[Ph-Ph] Pt[PhNO2-PhOMe] Pt[PhNO2-PhMe] Pt[PhNO2-Ph] Pt[PhNO2-PhNO2] Pt[PhNO2-iPr]

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)

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) σR2 and (c, d) σR1. 1646


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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). 13 C 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 13 C 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 chemical shift on coordination with the Hammett parameters σR1 and σR2 observed, where the influence of R2 is again much less pronounced than that of R1 (Figure 6). A similar behavior was found by us for the analogous Pd complexes. 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). 31 P 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 spectra. A slight linear trend is found for the dependence on σR2 (Figure 7a,b) and a two-regime development is observed for σR1 (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 found a strong correlation between the 2JPP 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

Figure 6. Changes of 13C NMR shifts (according to amount) for the CHCO signals of the Pt[PhR1-PhR2] complexes on coordination of the olefin ligand dependent on (a) σR2 and (b) σR1.

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. 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]). 1647


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Figure 7. Dependence of 31P NMR shifts of the Pt[PhR1-PhR2] complexes on (a, b) σR2 and (c, d) σR1.

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 related complexes.1,10 In contrast, the 1JXPt coupling constants are strong indicators for the Pt−X bonding mode (oxidation state, trans influence, coordination number),12 in particular in related complexes. Figure 9 depicts the correlation of 1JP1Pt and 1 2 JP Pt with the Hammett parameters of R1 and R2 in the Pt[PhR1PhR2] complexes. The 1JP1Pt coupling constant is always larger than the 1JP2Pt coupling constant. As can be seen from Figure 9a, there is a linear correlation between 1JP1Pt and σR2 with positive slope (see Table 3 in the Supporting Information for correlation equations). There also exists a linear correlation between 1JP1Pt and σR1, but with a negative and steeper slope (Figure 9c). The complementary behavior concerning the sign of the slope is found for 1JP2Pt. In this case the slope is negative with σR2 (Figure 9b) and positive with σR1 (Figure 9d). The steepnesses of the slopes for the two P atoms are almost the same; just the sign inverts.

determine the electron density at the metal center and, thus, the stability and reactivity of the related complexes. The dependence of the 2JPP coupling constant on the electron density of the coordinating metal center is more pronounced for R1 than for R2 as would have been expected. Although sterics also influence the size of 2JPP, most likely via the bond angle P−Pt−P, the main contribution is by electronics (see Figure 6 in the Supporting Information). 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. The more downfield signal (P2) is observed as a doublet of doublets with Pt satellites, if R1 is CF3. 195 Pt 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 1648


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Table 5. 31P NMR Data for the Pt Complexes

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] Pt[Ph-Et] Pt[Ph-iPr] Pt[Ph-tBu] Pt[PhCl-Me] Pt[PhCl-Et] 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[PhOMe-PhOMe] Pt[PhOMe-PhMe] Pt[PhOMe-Ph] Pt[PhOMe-PhNO2] Pt[PhMe-PhOMe] Pt[PhMe-PhMe] Pt[PhMe-Ph] Pt[PhMe-PhNO2] Pt[Ph-PhOMe] Pt[Ph-PhMe] Pt[Ph-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]

Table 6. 195Pt NMR Results for the Pt Complexes

P1 (ppm)

P2 (ppm)

|2JPP| (Hz)

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] Pt[Ph-Et] Pt[Ph-iPr] Pt[Ph-tBu] Pt[PhCl-Me] Pt[PhCl-Et] 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[PhOMe-PhOMe] Pt[PhOMe-PhMe] Pt[PhOMe-Ph] Pt[PhOMe-PhNO2] Pt[PhMe-PhOMe] Pt[PhMe-PhMe] Pt[PhMe-Ph] Pt[PhMe-PhNO2] Pt[Ph-PhOMe] Pt[Ph-PhMe] Pt[Ph-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]

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. We point out that an expected complementary development to the 1 JPPt coupling constant was not found for the corresponding 1JPtC coupling constants (see Figures 8 and 9 in the Supporting Information).

Pt (ppm)

|1JP2Pt| (Hz)

|1JP1Pt| (Hz)

|1JC1Pt| (Hz)

|1JC2Pt| (Hz)

−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 −5044.0 −5043.2 −5042.1 −5047.6 −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 −5048.8 −5048.2 −5047.1 −5047.0

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

Reactivity (Ligand Exchange). To determine the reactivity of the complexes, basic ligand substitution and oxidative addition reactions were performed. 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 1649


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Figure 9. Dependence of 1JP1/2Pt values for the Pt[PhR1-PhR2] complexes on the Hammett parameters of (a, b) R1 and (c, d) R2.

Figure 10. Dependence of 1JP1/2Pt values for the Pt[PhR1-R3] complexes on the A parameters of R3: (a) P1; (b) P2.

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, PhOMePh, 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[PhOMePhNO2], 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). 1650


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Scheme 4. Equilibrium between Oxidative Addition of Ph2SiH2 and Cinnamate Coordination

the metal to olefin back-bonding is becoming more important with the electron-withdrawing character of R1, in accordance with chemical intuition.

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. 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)2PtH(SiHPh2) and the starting complex forms (Scheme 4). Hydrogen evolution was observed, and evidence for homocoupling of the silane was gained in 29Si NMR spectroscopy when an excess of silane was used or the complexes were added to the neat silane. From the signals in the 1H 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.

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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. 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. 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 the Hammett parameters of the cinnamate ligand. The 2JPP coupling constants correlate equally well linearly with the σR1 parameters of the cinnamate ligand for both metal centers (slope of Hammett plot: for Pd, −13 Hz; for Pt, −11 Hz).

Table 7. Equilibrium Constants for the Oxidative Addition of H2SiPh2 Pt[PhOMe-PhMe] Pt[PhMe-PhMe] Pt[Ph-PhMe] Pt[PhCl-PhMe] Pt[PhCF3-PhOMe] Pt[PhNO2-PhMe]

K

ln K

32.3 ± 1.6 12.9 ± 0.6 9.2 ± 0.5 3.1 ± 0.2 0.50 ± 0.03 0.05 ± 0.0025

3.56 2.56 2.22 1.12 −0.69 −3.08

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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. Triphenylphosphine, 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 1 H 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 resolution 0.5 Hz). Chemical shifts for 1H 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

Figure 12. Dependence of ln K of silane oxidative addition on the electronic properties of the ligand (R2 = 0.96).

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 1651


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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 Technische Universität München. ATR FT-IR spectroscopy was done with a Thermo Scientific Nicolet 380 Smart Orbit. X-ray single crystal parameters were obtained as follows. The single crystals were stored 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 and absorption effects. The structures were solved by a combination of direct methods and difference Fourier syntheses. All non-hydrogen atoms were refined with anisotropic displacement parameters, whereas all hydrogen atoms were refined with isotropic displacement parameters. Full-matrix least-squares refinements were carried out by minimizing P(Fo2 − Fc2)2 with the SHELXL-97 weighting scheme.46 The final residual electron density maps showed no remarkable features. Neutral atom scattering factors for all atoms and anomalous dispersion corrections for non-hydrogen atoms were taken from ref 47. 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. Detailed synthesis and characterization data for all 48 new complexes are given in the Supporting Information.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

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. Corresponding Author

*E-mail for K.R.: [email protected]. Notes

The authors declare no competing financial interest.

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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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REFERENCES

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Influence of Electronically and Sterically Tunable Cinnamate Ligands

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