Influence of Electronically and Sterically Tunable Cinnamate Ligands

Article pubs.acs.org/Organometallics

Influence of Electronically and Sterically Tunable Cinnamate Ligands on the Spectroscopic, Kinetic, and Thermodynamic Properties of Bis(triphenylphosphine)palladium(0) Olefin Complexes Magnus R. Buchner,† Bettina Bechlars,‡ Bernhard Wahl,

§

and Klaus Ruhland*,∥

†

WACKER-Institut für Siliciumchemie, ‡Lehrstuhl für Anorganische Chemie, and §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 detailed study of the influence of electronic and steric characteristics of cinnamic acid esters on the spectroscopic, kinetic, and thermodynamic properties of bis(triphenylphosphine)palladium(0) cinnamic acid ester complexes is presented (51 different new complexes included). These complexes show a dynamic behavior on the NMR spectroscopic time scale. Therefore, the rotational barriers of the olefin about the metal−olefin bond as well as the dissociation entropy and enthalpy of the olefin and the dissociation mechanism could be determined. These findings are interpreted together with the NMR spectroscopic, IR spectroscopic, and X-ray structural data (7 new structures included) concerning the influence of the different olefin ligands on the complex properties by means of Hammett plots. DFT calculations were performed to support the mechanistic conclusions.

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INTRODUCTION Zerovalent group 10 complexes are among the most applied catalyst classes in modern synthetic chemistry. Nevertheless, only relatively simple complexes (e.g., tetrakis(triphenylphosphine)palladium) are usually used. Even though detailed studies concerning the electronic properties of distinct ligand systems have been performed, no examinations have been conducted on subtle changes in the electronic and steric properties of cinnamate ligands to date. The focus of this examination is the correlation of directly measurable spectroscopic, kinetic, and thermodynamic properties with the electronics and sterics of the ligands. The analyzed systems, summarized in Figure 1, were chosen due to their broad tunability. In these complexes the electronic

The electronic influence was assigned via the Hammett parameter σp, while the steric demand was determined via the A values, which are compiled in Table 1. Table 1. Hammett and A2 Values OMe Me H Cl CF3 NO2

σP+/−

σI

σ°R

σ°R+/−

−0.27 −0.17 0 0.23 0.54 0.78

−0.78 −0.31 0 0.11 0.74 1.28

0.27 −0.05 0 0.46 0.42 0.65

−0.42 −0.13 0 −0.18 0.08 0.15

−1.02 −0.25 0 −0.36

A Me Et iPr tBu

1.70 1.75 2.15 4.50

0.6

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RESULTS AND DISCUSSION Synthesis. The cinnamic acid ester ligands used were synthesized in case of phenol esters and the tert-butyl ester3,4 either by reaction of the appropriate acid chloride with the alcohol in pyridine at 100 °C5,6 or by deprotonation of the alcohol with sodium hydride and subsequent salt metathesis with the acid chloride in nearly quantitative yield. The other alkyl esters were obtained by refluxing the cinnamic acid in the corresponding alcohol in the presence of catalytic amounts of H2SO4 in quantitative yield.7

Figure 1. Investigated cinnamic acid complexes: R1, R2 = OMe, Me, H, Cl, CF3,NO2; R3 = Me, Et, iPr, tBu.

properties of the double bond can be adjusted by the substituent in a position para1 to the olefin (R1). By variation of the ester group a fine tuning of the electronics (R2) of the olefin and the steric demand2 (R3) of the ligand can be performed. © 2012 American Chemical Society

σP

Received: September 9, 2011 Published: January 10, 2012 588

dx.doi.org/10.1021/om200849n | Organometallics 2012, 31, 588−601

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Scheme 1. Synthetic Routes to Bis(triphenylphosphine)palladium Cinnamates (R1 = OMe, Me, H, Cl, CF3, NO2)

Figure 2. ORTEP13 representation of PdPhCl−PhNO2 (top left), PdPhNO2−PhOMe (top right), PdPhNO2−Ph (bottom left), and PdPhNO2− PhNO2 (bottom right). Thermal ellipsoids are given at the 50% probability level. Hydrogen atoms are omitted for clarity.

diffusion of pentane into a toluene solution of the appropriate complex (Figure 2). Table 2 compares the most important structural data of the four complexes. The five atoms P1/P2, C1/C2, and Pd are almost in one plane in a trigonal geometry (olefin/P1/P2). The bond length Pd−C1 is always shorter than the bond length Pd−C2. At the same time, the bond length P2−Pd is always longer than the bond length P1−Pd, in accordance with what one would predict on the basis of trans influence. The C1−C2 double bond is elongated through the coordination to the Pd center. No indication is found by X-ray analysis for an interaction of the carbonyl function with the metal center. Although the carbonyl oxo atom is slightly directed toward the Pd center, the Pd−O distance is not shorter than the sum of van der Waals radii of Pd and oxygen. No simple influence of the substituents R1 and R2 on the structure parameters can be concluded from the X-ray data. These structures are compared to the structures of three chelating complexes in Figure 3 determined by us. In particular, the P1−Pd−P2 angle can be fixed at smaller angles in comparison to those for the bis(triphenylphosphine)

The corresponding palladium complexes were all synthesized by reaction of Pd(C3H5)Cp8 with 2 equiv of triphenylphosphine to generate a bis(triphenylphosphine)palladium(0) fragment9,10 in situ, via reductive elimination, in analogy to the synthesis of bis(tri-tert-butylphosphine)palladium(0).11 Subsequent addition of olefin gives the desired complexes in quantitative yield in this simple one-pot reaction (Scheme 1). The nitro cinnamic acid complexes were isolated as deep red, airstable solids, while all other palladium complexes are orange to yellow solids which decompose in air within minutes. All complexes, with the exception of nitro and trifluoromethyl cinnamic acid esters, decompose slowly, in the presence of oxygen within seconds, in solution. The sterically less shielded methyl ester complexes decompose within minutes in solution at room temperature. However, the degradation is slow enough at −25 °C for sufficient spectroscopic analysis. Despite the lability in solution all complexes are stable at ambient temperature in the solid state under an Ar atmosphere. X-ray Structures. Single crystals of PdPhCl−PhNO2, PdPhNO2−PhOMe, PdPhNO2−Ph, and PdPhNO2−PhNO2 suitable for X-ray diffraction analysis could be grown by slow 589


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Table 2. Structural Data of the Pd Complexes Discussed in the Text name PdPhNO2−PhNO2 PdPhNO2−Ph PdPhCl−PhNO2 PdPhNO2− PhOMe dppePdPhNO2−iPr dpppPdPhNO2−Et dppbPdPhNO2− PhOMe

C1C2 (Å)

C1−Pd (Å)

C2−Pd (Å)

P1−Pd (Å)

P2−Pd (Å)

P1−Pd−P2 (deg)

C1−Pd−P1 (deg)

C2−Pd−P2 (deg)

C3O (Å)

C2−C3 (Å)

C3−O (Å)

1.411(5) 1.422(2) 1.427(3) 1.413(5)

2.125(3) 2.136(2) 2.137(2) 2.132(3)

2.157(3) 2.151(2) 2.166(2) 2.140(3)

2.3288(8) 2.3184(4) 2.3249(6) 2.3216(8)

2.3416(9) 2.3310(4) 2.3449(6) 2.3334(8)

106.82(3) 111.29(1) 107.47(2) 112.31(3)

100.7(1) 103.86(4) 99.34(6) 103.13(9)

114.48(9) 106.20(4) 114.85(7) 105.87(9)

1.208(4) 1.202(2) 1.205(3) 1.202(4)

1.448(5) 1.467(2) 1.442(3) 1.460(4)

1.404(4) 1.382(2) 1.398(3) 1.377(4)

1.410(6) 1.422(4) 1.422(4)

2.131(4) 2.148(2) 2.124(2)

2.108(4) 2.126(2) 2.128(2)

2.293(1) 2.3117(9) 2.3062(2)

2.308(1) 2.3022(8) 2.2952(5)

85.82(4) 96.37(3) 104.63(2)

117.5(1) 113.83(6) 107.32(6)

118.2(1) 110.58(6) 108.83(6)

1.220(5) 1.216(3) 1.204(3)

1.452(6) 1.469(3) 1.459(2)

1.355(5) 1.362(3) 1.381(3)

Figure 3. ORTEP13 representation of dppePdPhNO2−iPr (top left), dpppPdPhNO2−Et (top right), and dppbPdPhNO2−PhOMe (bottom). Thermal ellipsoids are given at the 50% probability level. Hydrogen atoms are omitted for clarity.

bis(triphenylphosphine)palladium(0) complexes of this study. No simple correlation was found between the CO stretching frequencies and the Hammett parameters σP, σI, and σ°R or the steric A parameter of R1, R2, and R3, either for the complexes or for the free ligands. Despite this lack of correlation, the CO stretching frequency shifts significantly to lower wavenumbers by 10−44 cm−1 upon coordination to the metals in comparison to the free ligand in all 48 cases, indicating a decrease in bond order for the carbonyl bond on the coordination of the olefinic double bond and pointing to the conclusion that mesomeric structure c in Scheme 2 plays a role in the electronic description of the complexes. No simple correlation was

complexes through the chelation and also the configuration of the two remaining phenyl substituents at each P atom is influenced. This causes a change in the Pd−C and Pd−P bond lengths. The smaller the bond angle P1−Pd−P2, the shorter the Pd−C2 bond length in comparison to Pd−C1, while the bonds Pd−P2 and Pd−P1 are about equally long in all cases. The chelating complexes will be important to gain more insight into the thermal behavior of the complexes and the dissociation mechanism of olefin ligand later in this paper. IR Spectroscopy. As a diagnostic indicator for the bonding situation in the Pd complexes, we measured the CO stretching frequency of the ester carbonyl function in the 48 590


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Scheme 2

found between this shift and the Hammett parameters of R1 or R2. NMR Spectroscopy. All bis(triphenylphosphine)palladium cinnamic acid ester complexes except for the 4-nitrocinnamic acids show line broadening of the signals in 1H, 13C, and 31P NMR spectroscopy due to the rotation of the olefin about the metal−olefin bond axis, which will be discussed in detail below. To get more insight into the electronic situation of the coordinated olefin, we have evaluated the development of the chemical shifts Δδ of protons and carbons in the Ar−C1H1 C 2H 2−C 3O moiety relative to the free ligand as well as the chemical shift δ of the two phosphorus atoms as a function of the substitution pattern of the cinnamic ester rests R 1, R 2, and R 3. Additionally the 2JPP and 2JCP as well as the 3 JHH and the 3JPH coupling constants were used as diagnostic criteria to get more insight into the electronic situation of the complex. 1 H NMR. The olefinic protons are both shifted to high field in 1H NMR spectroscopy upon coordination of the olefin (ΔδH negative). The Hammett plots of ΔδH1 and ΔδH2 show decisively different influences of the substituents R1, R2, and R3 on the change in chemical shifts of the olefinic protons and allow us to understand in greater detail the electronic situation of the [cinnamic acid ester]Pd(PPh3)2 complex (Figures 4 and 5). For H2 a linear correlation is found between the Hammett parameter of R1 and the chemical shift ΔδH. The slope of the Hammett plot ρ is very similar in all eight cases (ρ = −0.53 ± 0.04, squared correlation coefficients between 0.96 and 0.98). R2 and R3 only slightly influence the chemical shift ΔδH2 with more electron withdrawing groups R2 having a stronger influence than less electron withdrawing groups. For H1 a decisively different behavior is found and must be divided into two areas. For σp values of R1 lower than about 0.05, the Hammett plot develops similarly to the case of H2 with the slope ρ = −0.44 ± 0.07 taking into account all eight plots. For σp values of R1 higher than about 0.05 the slope of the Hammett plot decreases dramatically to about −0.17 ± 0.02, becoming less dependent on R1. We conclude from these data that mesomeric structures b and c in Scheme 2 (Michael-analogous interaction) play a role in the electronic description of the complexes, in particular for electron-releasing substituents R1 (although the main contribution will be by mesomeric structure a in accordance with the well-known Dewar−Chatt−Duncanson model). In mesomeric structure a the ligand electronics at H1 and C1 is less perturbated in comparison to the free ligand than in b and c (thus, Δδ is small for high Hammett parameters at position 1). For position 2 (H2 and C2) the electronic situations are similar

Figure 4. Change in chemical shift ΔδH on coordination of the olefinic proton H1 as a function of σp of R1 for different values of R2 (black) and R3 (gray).

in a and c and in both cases are dependent on the Hammett parameter of R1. Signals for both H1 and H2 show a 3JHH coupling to each other with coupling constants of ca. 10 Hz (independent of the substitution pattern of the olefin), which are significantly smaller than those observed for the noncoordinated olefins with about 16 Hz (also independent of the substitutent pattern at the cinnamic ester). This is in accordance with the X-ray results, which indicate a distortion of 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. 145°, determined via the Karplus equation, in accordance with the X-ray results.12 Coupling is also observed to both phosphorus nuclei via the palladium with coupling constants of ca. 6 Hz for the H2 proton and ca. 7 Hz for the H1 proton. The similarity of both coupling constants is presumably caused by the dynamic character of the complexes. Therefore, both olefinic protons show a multiplicity of a doublet of pseudotriplets (dpt) and not the expected doublet of doublets of doublets (ddd). With exception of the 4-nitrocinnamate complexes, coupling could only be observed at low temperatures (246 K). No significant change of these coupling constants was observed as a function of the substitution pattern of the olefinic ligand, either in the complexes or in the free ligands in the case of 3JHH. 13 C NMR. The same trends concerning the shifts between coordinated and free ligand of C1 and C2 are present in 13C NMR spectroscopy. The signals arising from the olefinic carbon nuclei are shifted upfield in comparison to those for the noncoordinated ligand. Due to the dynamic behavior no coupling constants could be determined at ambient temperature, except for the 4-nitro and CF3 cinnamates, and in some 591


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Figure 5. Change in chemical shift ΔδH on coordination of the olefinic proton H2 as a function of σp of R1 for different values of R2 (black) and R3 (gray).

counterbalanced by olefin-to-metal electron donation (mesomeric structure a in Scheme 2), ΔδP1 and ΔδP2 are almost constant. For Hammett parameters higher than 0.1, where electron donation via R1 is inefficient and thus metal-to-olefin back-donation dominates (b and c in Scheme 2), ΔδP1 and ΔδP2 show a linear dependence on the Hammett parameter with ρ = 0.75 for P1 and ρ = 1.5 for P2 (Figures 8 and 9), because now electron density is provided partially by the P atoms to the Pd center rather than by the olefin ligand. This effect is more pronounced for P2 (trans to C1) than for 1 P . Also, the nitro group in the case of P2 shows a decisive influence in the R2 position on the chemical shift change of P2 in contrast to P1 (Scheme 2). To determine exact coupling constants and observe all signals, the rotation about the olefin−palladium bond had to be frozen. Therefore, spectra of all dynamic complexes were recorded at 246 K (in case of R1 = CF3 at 278 K). Due to cooling, the dynamic behavior was inhibited and all signals could be observed. At low temperature the 2JPP coupling shows a linear dependence in a Hammett plot for R1, as shown in Figure 10 with ρ = −13.00 ± 0.26 (squared correlation coefficients between 0.97 and 0.99). If R at the phenol becomes more electron-withdrawing (R2) or less sterically crowded (R3), the 2JPP value decreases continuously. Since the P−Pd−P bond angle remains almost constant in the 48 complexes at about 110°, the behavior of the 2JPP coupling constants mainly reflects the electron density at the Pd center, which decreases with more electron-withdrawing substituents at the cinnamic acid ester ligand due to more pronounced metal-to-olefin back-donation.

cases the olefinic carbon signals were not observable at all at ambient temperature. At low temperature both carbon atoms show a coupling to the two phosphorus atoms with a cis coupling constant of ca. 5 Hz for C2 and 3 Hz for C1 and a trans coupling constant of ca. 18 Hz for C2 and 25 Hz for C1. Therefore, both signals are observed as a doublet of doublets. The change of chemical shift of C1 on coordination supports the relevance of mesomeric structures b and c in Scheme 2 (Figure 6). As in the case of H1, at negative Hammett parameters there is observed a linear decrease of ΔδC1, while at positive Hammett parameters ΔδC1 remains almost constant. Again, and in analogy to H2, the behavior of C2 is different. Here for ΔδC2 a linear relation to the Hammett parameters is found throughout the complete region with ρ = −6.83 ± 0.26 (Figure 7, squared correlation coefficients between 0.96 and 0.99). The JCP coupling constants between the carbonyl carbon and the two phosphorus atoms are in a range between 2 and 4 Hz with no significant dependence on the substitution pattern of the cinamic acid ester. The size indicates that a direct interaction of this carbon with the metal center resulting in a 2J coupling (about 20 Hz in the trans case) is not detectable. 31 P NMR. Further support for the description in Scheme 2 is found by 31P NMR spectroscopy. Here concerning the Δδ values (measured relative to free PPh3 as external standard) behavior complementary to 1H and 13C NMR spectroscopy is observed (Figures 8 and 9). For Hammett parameters lower than 0.1, in which case metal-to-olefin back-donation is 592


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Figure 6. Change in chemical shift ΔδC on coordination of the olefinic carbon C1 as a function of σp of R1 for different values of R2 (black) and R3 (gray).

Figure 7. Change in chemical shift ΔδC on coordination of the olefinic carbon C2 as a function of σp of R1 for different values of R2 (black) and R3 (gray).

Rotational Barriers for the Cinnamate Ligands. 1H NMR. The aromatic signals of the two distinguishable PPh3 ligands are only resolved properly below 270 K (vide supra), and upon warming to temperatures up to 343 K, the signals broaden

and combine to only one signal in the center of the two signals at low temperature due to rotation about the metal−olefin bond (Figure 11; at temperatures higher than 343 K dissociation comes into play as well, shifting the centerd signal to lower field). 593


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Figure 8. Change in chemical shift ΔδP of P1 relative to free PPh3 as a function of σp of R1.

Figure 9. Change in chemical shift ΔδP of P2 relative to free PPh3 as a function of σp of R1.

In 31P NMR spectrospcopy the same temperature-dependent rotation can be detected (Figure 12). The rotational barriers of the cinnamates about the olefin− palladium axis can be determined via line shape analysis14 and are compiled in Table 3. The rotational barrier increases with the electron deficiency of the olefin. An overall linear trend is observed for σp of R1 (Figure 13). Using the basic Hammett description

electron-releasing groups R1 while for electron-withdrawing groups b and c must be less important to unimportant (Figure 14). At low temperatures a distinct dependence of the rotation rate is only found for substituents R1 with electron-releasing properties (σp < 0), while the rotational rate for substituents with electron-withdrawing properties shows a much less pronounced dependence on the Hammett parameter σp (Scheme 2). At 311 K an approximate linear development over the complete range of Hammett parameters is found, approaching the reaction constant ρ = −3.17 (Scheme 2). Dissociation Thermodynamics. 1H NMR. No sharp signals are obtained upon heating beyond the coalescence point of ca. 320 K.

log kR /kH = σp we find further evidence for a decisive contribution of mesomeric structures b and c (in which rotation is facilitated) in the case of 594


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Figure 10. 2JPP as a function of the Hammett parameter σp of R1 with different R2 (black) and R3 (gray).

Figure 11. 1H NMR spectra of PdPh−PhMe as a function of the temperature (aromatic signals of the PPh3 ligands).

The equilibrium constant for the observed dissociation/ coordination equilibrium in toluene depicted in Figure 15 is given by

This is caused by the partial dissociation of the ligand, which leads to an additional equilibrium, causing a shift of the olefinic signals in 1H NMR spectroscopy to lower field upon higher temperature and a second line-broadening effect (Figure 15). Above ca. 360 K the compounds start to decompose slowly.

K = [B][C]/[A] 595


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Figure 12. 31P NMR spectra of PdPh−PhMe as a function of the temperature.

Table 3. Rotational Barriers Determined by Line Shape Analysis

R2 PdPhOMe−PhOMe PdPhMe−PhOMe PdPh−PhOMe PdPh−PhMe PdPh−Ph PdPh−PhNO2 PdPh−Me PdPh−Et PdPh−tBu PdPhCl−PhOMe PdPhCF3−PhOMe PdPhNO2−PhOMe

EA (olefin) (kJ/mol)

0.96 0.97 0.93 0.93 0.94 0.91 0.99 0.99 0.98 0.95 0.93 0.98

49.8 58.1 54.2 57.6 57.7 66.4 60.3 61.8 60.5 63.7 68.4 70.8

± ± ± ± ± ± ± ± ± ± ± ±

5.9 5.7 8.5 6.9 8.1 12.6 3.6 3.7 5.0 8.3 13.6 5.8

Figure 13. Rotational barriers as a function of σp of R1, R2 = OMe.

The chemical shift of the olefin in the pure complex is δA, and that of the noncoordinated ligand is δB at ambient temperature. The value of δA was temperature-corrected by independent measurements of the undissociated complex at lower temperatures (Figure 15; Supporting Information). The dissociation enthalpies and entropies can then be obtained from a van’t Hoff plot and are given in Table 4 together with the quality of the linearity R2. The dissociation enthalpy increases with electron deficiency of the double bond, as expected, since the metal-to-olefin backbonding and thus the bond strength increases (Figure 16). The dissociation entropy increases as well with increasing Hammett parameter, as shown in Figure 17. We explain this by the fact that at low Hammett parameters the rotation of the olefin is less frozen than for high Hammett parameters (as supported by

As shown in the Supporting Information, this results in

K = (1 − XA )2 [A 0]/(2XA (1 + XA ))

XA = [A]/([A]+[B] + [C]) with the average position of the signal14 in 1H NMR spectroscopy

δ = (nA δA + nBδB)/(nA + nB) and thus

XA =

2

δ − δB δA − δB δ − δB − δA − δB 596


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Table 4. Dissociation Enthalpies and Entropies Extracted from van’t Hoff Plots

R2 PdPhOMe−PhOMe PdPhMe−PhOMe PdPh−PhOMe PdPh−PhMe PdPh−Ph PdPh−PhNO2 PdPh−Me PdPh−Et PdPh−iPr PdPh−tBu PdPhCl−PhOMe PdPhCF3−PhOMe PdPhNO2−PhOMe dppePdPhNO2−iPr dpppPdPhNO2−Et dppbPdPhNO2−PhOMe

Figure 14. Plot using the basic Hammett equation concerning the rotation about the metal−cinnamate ligand axis at different temperatures.

the activation energies for the rotation). Thus, for high Hammett parameters on dissociation this rotation in addition to the translational degrees of freedom is set free, resulting in a greater dissociation entropy at higher Hammett parameters. In both cases (enthalpies and entropies) the Hammett plot supports the importance of mesomeric structures b and c in Scheme 2 for the pathway of dissociation, if σp is used as the fitting parameter, and this will be further supported by the DFT calculations below. An overall linear behavior is found if for −CF3 and −NO2 the σP− values are chosen instead of the σP values. 31 P NMR. The two signals in 31P NMR start to broaden at ca. 270 K and then transit through coalescence at ca. 320 K and finally re-form a new sharp signal at ca. 330 K. However, this signal broadens again at higher temperature and massively shifts

0.96 0.99 0.96 0.99 0.99 0.96 0.99 0.99 0.99 0.99 0.83 0.99 0.97

ΔH (kJ/mol)

ΔS (J/(mol K))

56.3 ± 5.6 111 ± 62.8 ± 2.7 125 ± 66.6 ± 7.2 132 ± 73.6 ± 2.1 161 ± 71.3 ± 3.2 153 ± 66.6 ± 7.1 127 ± 77.2 ± 3.2 174 ± 85.3 ± 4.0 194 ± 72.1 ± 1.3 162 ± 80.6 ± 1.7 189 ± 92.2 ± 24.2 206 ± 137.3 ± 7.8 317 ± 183.8 ± 17.2 436 ± no dissociation up to 390 K no dissociation up to 390 K no dissociation up to 390 K

17 8 21 6 10 21 10 12 4 5 71 23 51

to the high-field region of the spectrum (Figure 12). This is due to the formation of an equilibrium with bis(triphenylphosphine)palladium, formed through dissociation of olefin. In the temperature range in which no line broadening occurs, the signals in the 31P NMR spectra are shifted to lower field with increasing temperature (Figure 18) and the 2Jpp coupling constants increase. When line broadening starts, the signals move toward each other and the coupling constant decreases as expected (Figure 19).

Figure 15. 1H NMR spectra of PdPh−PhMe as a function of the temperature (olefinic proton signals). 597


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Figure 18. 31P NMR shifts of PdPh−PhMe as a function of the temperature.

Figure 16. Dissociation enthalpies as a function of σp of R1 (gray), R2 = OMe. If for R1 = −CF3 and R1 = −NO2 σP− instead of σP is used, an overall linear relation is found (black).

Figure 19. 31P NMR 2JPP values of PdPh−PhMe as a function of the temperature. Figure 17. Dissociation entropies as a function of σp of R1, R2 = OMe. If for R1 = −CF3 and R1 = −NO2 σP− instead of σP is used, an overall linear relation is found (black).

Scheme 3

Dissociation and Rotation Mechanism. On the basis of the collected data we can draw a more detailed picture for both the dissociation and the rotation mechanism of the cinnamate ligand (Scheme 3). We propose that both pathways are connected insofar as the cinnamate ligand rotates to a position perpendicular to the P− Pd−P plane and this is accompanied by an increase in the bond angle P−Pd−P for both processes, although the reaction pathways might not be identical in both cases. If the thermal energy is large enough, from this reaction coordinate the ligand can be dissociated. If not, the ligand only rotates about the ligand−metal axis. This hypothesis is supported by the fact that the complexes with chelating bis(phosphine) ligands do not show any dissociation of the cinammate ligand up to 390 K, since in this case the two P donors cannot adopt the necessary 180° angle P− Pd−P. Also, the rotation about the cinammate−metal axis in the chelate complexes is decisively slower than for the

bis(triphenylphosphine) complexes, and the most restricted dppe complex (Table 2, Figure 3) shows the smallest rotational rates, followed by the dppp complex and the dppb complex (Table 5). 598


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Table 5. Rotational Rates for Several Complexes at Various Temperatures rotational rate (Hz) temp (K) 292 301 311 321

dppePdPhNO2− dpppPdPhNO2- dppbPdPhNO2iPr −Et −PhOMe 2.3 3 3.4

2.7 3.3 3.9

2.5 3.5 4.1

PdPhNO2− PhOMe 3.7 3.9 9.1 14.7

DFT Calculations. To support our mechanistic proposal, we performed DFT calculations for PdPh−Ph at the B3PW91/ LanL2DZ level. We employed the frozen-coordinate function on P−Pd−P at different stages (lowest energy: 114.3, 120, 130, 140, 160, 180°) and calculated the lowest energy structure connected with the frozen angle. At 150° the transition state for the rotation of the cinnamic acid ester ligand could be located. At 157°, according to the calculations, the transition state for the dissociation is found (both transition states were verified by examining the imaginary vibration mode; in the case of TSrot the rotation of the olefin was accompanied by a bending of the P−Pd−P angle, and in the case of TSdiss the bending of the P− Pd−P angle was accompanied by an increased distance of the olefin from the metal center; Figure 20). Figure 21 shows the development of calculated enthalpies for the frozen structures at different P−Pd−P angles and mirrors nicely the qualitative picture given in Scheme 3. An approximate dissociation enthalpy of 43 kJ/mol (experimental 71 kJ/mol) and rotational barrier EA of 29 kJ/mol (experimental: 58 kJ/mol) is found.

Figure 21. Calculated enthalpies for different P−Pd−P angles (#: transition state).

This is accompanied by a rotation of the coordinated double bond toward 45° relative to the P−Pd−P axis, as depicted in figure 22, and supports perfectly the proposed mechanism in Scheme 3. In the course of increasing the P−Pd−P angle, the C1−C2 distance decreases continuously to the value of the free cinnamate ligand (1.358 Å; fFigure 23).

■

CONCLUSION Examining 51 new cinnamic acid ester complexes of Pd(0), we were able to work out a detailed picture of the electronic interaction between the double bond of the ligand and the

Figure 20. Calculated transition states for the rotation (left) and the dissociation (right) of the olefin ligand. 599


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a Bruker AMX 400 instrument. 1H NMR (400 MHz) and 13C NMR (100 MHz) chemical shifts are given relative to the solvent signal for CDCl3 (7.26 and 77.2 ppm), C6D6 (7.15 and 128.1 ppm), and C7D8 (2.09 and 20.4 ppm);41,42 19F NMR (377 MHz) used neat CFCl3 and 31 P NMR (162 MHz) used 85% H3PO4 as external standards. FAB-MS analysis was carried out on a Finnigan MAT-90 instrument in a p-nitrobenzyl alcohol matrix under bombardment with ionized xenon. ESI-MS measurements were conducted on a Finnigan LCQ isntrument in acetonitrile. Microanalytical data were obtained in the microanalytical laboratory of the Technische Universität München. ATR FT-IR spectroscopy was done with a Thermo Scientific Nicolet 380 Smart Orbit instrument. X-ray single-crystal parameters were obtained as follows: 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 detection system (APEX II, κ-CCD) at the window of a rotating anode (Bruker AXS, FR591) and graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å). Raw data were corrected for Lorentz, polarization, and (arising from the scaling procedure) 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 leastsquares refinements were carried out by minimizing P(Fo2 − Fc2)2 with the SHELXL-97 weighting scheme.43 The final residual electron density maps showed no remarkable features. Neutral atom scattering factors for all atoms and anomalous dispersion corrections for nonhydrogen atoms were taken from ref 44. General Procedure for the Synthesis of Phenol Cinnamates. Route I. A 0.50 g amount of acid chloride and 1 equiv of alcohol were dissolved in 2.5 mL of pyridine and heated to 100 °C for 1 h. The warm reaction solution was poured into a mixture of 175 mL of 0.5 M hydrochloric acid and 100 g of crushed ice and extracted three times with diethyl ether, in the case of 4-nitrocinnamic acid with ethyl acetate. The combined organic layers were washed twice with saturated NaHCO3 solution and once with brine, dried over Na2SO4, and filtered, and the solvent was removed under reduced pressure. The spectroscopic properties of known substances were in accord with the literature. Route II. A 2.49 mmol amount of the alcohol was added to 1 equiv (2.49 mmol, 59.7 mg) of NaH in 5 mL of thf at 0 °C, and the mixture was warmed to ambient temperature. After 15 min 1 equiv (2.49 mmol) of the appropriate acid chloride was added and stirring continued overnight. The reaction mixture was quenched with 15 mL of water and extracted three times with Et2O. The combined organic layers were washed twice with saturated NaHCO3 solution and once with brine, dried over Na2SO4, and filtered, and the solvent was removed under reduced pressure to give the anticipated ester. General Synthetic Procedure for the Complexes. A 30.0 mg portion (141 μmol) of (η3-allyl)(η5-cyclopentadienyl)palladium(II) and 74.0 mg (282 μmol) of triphenylphosphine were dissolved in toluene and added to 141 μmol of the ligand, and the mixture was stirred for 1 h. After removal of the solvent in vacuo, the residue was stirred with pentane overnight. The solvent was decanted off and the residue washed once with pentane to give the product in microanalytical grade. DFT Calculations. The DFT calculations were performed using the program suite Gaussian 03.45 All molecular geometries were fully optimized. The frozen-coordinate function implemented in the program suite was used for the P−Pd−P angle. The DFT method used includes Becke’s three-parameter hybrid exchange functional in combination with the correlation functional of Perdew and Wang (B3PW91). Geometry calculations and frequency calculations were performed using the valence double-ζ LANL2DZ basis set. Transition states were proven to be one-dimensional saddle points by frequency analysis.

Figure 22. Calculated rotational angle of the coordinated double bond relative to the P−Pd−P axis at different P−Pd−P angles.

Figure 23. Calculated C1−C2 distance as a function of P−Pd−P angle.

metal center. The rotational kinetics of the cinnamic acid ester as well as the dissociation thermodynamics of this ligand were determined as a function of the Hammett parameters of the substituents at the cinnamic acid ester. A mechanism for the rotation and the dissociation is proposed which connects the two processes. The mechanism could be supported by DFT calculations.

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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),15 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; benzene-d6 and toluene-d8 were dried and degassed by stirring over sodium potassium alloy, purified by condensation, and stored under argon.16 [Pd(C3H5)Cl]2 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 and used without further purification. Pd(C3H5)(C5H5),8 the acid chlorides, and alkyl cinnamates were synthesized according to literature procedures.7,17−40 1 H NMR, 13C NMR, and 31P NMR measurements were performed on a Bruker Avance 400 spectrometer and 19F NMR measurements on 600


Organometallics

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(24) Novokreshchennykh, V. D.; Mochalov, S. S.; Shabarov, Y. S. J. Org. Chem. USSR 1982, 18, 262−268. (25) Zheng, J.; Shen, Y. Synth. Commun. 1994, 24 (14), 2069−2073. (26) Garćıa, H.; Iborra, S.; Miranda, M. A.; Primo, J. Heterocycles 1985, 23 (8), 1983−1989. (27) Walter, R. Ber. Dtsch. Chem. Ges. A/B 1925, 58 (10), 2303− 2310. (28) Belozwetow. J. Gen. Chem. USSR 1966, 36, 1212. (29) Cevasco, G.; Thea, S. J. Org. Chem. 1994, 59 (21), 6274−6278. (30) Lohar, J. M.; Dave, J. S. Mol. Cryst. Liq. Cryst. 1983, 103, 143− 153. (31) Nagy, O. B.; Reuliaux, V.; Bertrand, N.; Mensbrugghe, A. V. D.; Leseul, J.; Nagy, J. B. Bull. Soc. Chim. Belg. 1985, 94, 1055−1074. (32) Pardin, C.; Pelletier, J. N.; Lubell, W. D.; Keillor, J. W. J. Org. Chem. 2008, 73 (15), 5766−5775. (33) Charette, A. B.; Janes, M. K.; Lebel, H. Tetrahedron: Asymmetry 2003, 14 (7), 867−872. (34) Bairwa, R.; Kakwani, M.; Tawari, N. R.; Lalchandani, J.; Ray, M.; Rajan, M.; Degani, M. S. Bioorg. Med. Chem. Lett. 2010, 20 (5), 1623− 1625. (35) Okutome, T.; Kawamura, H.; Taira, S.; Nakayama, T.; Nunomura, S.; Kurumi, M.; Sakurai, Y.; Aoyama, T.; Fujii, S. Chem. Pharm. Bull. 1984, 32 (5), 1854−865. (36) Masllorens, J.; Moreno-Manas, M.; Pla-Quintana, A.; Roglans, A. Org. Lett. 2003, 5 (9), 1559−1561. (37) Gerig, J. T.; McLeod, R. S. Can. J. Chem. 1975, 53 (4), 513− 518. (38) Skraup, S.; Beng, E. Ber. Dtsch. Chem. Ges. A/B 1927, 60 (4), 942−950. (39) Concellón, J. M.; Pérez-Andrés, J. A.; Rodríguez-Solla, H. Angew. Chem., Int. Ed. 2000, 39 (15), 2773−2775. (40) Parrish, J. P.; Jung, Y. C.; Shin, S. I.; Jung, K. W. J. Org. Chem. 2002, 67 (20), 7127−7130. (41) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. J. Org. Chem. 1997, 62 (21), 7512−7515. (42) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. Organometallics 2010, 29 (9), 2176−2179. (43) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64 (1), 112− 122. (44) Fuess, H.; Hahn, T.; Wondratschek, H.; U. Müller., Shmueli, U.; Prince, E.; Authier, A.; Kopsky, V.; Litvin, D. B.; Rossmann, M. G.; Arnold, S. H. E.; McMahon, B. Complete Online Set of International Tables for Crystallography; Springer-Verlag: Berlin, 2007; Vols. A−G. (45) Frisch, M. J. et al. Gaussian 03, Revision B.01; Gaussian, Inc., Pittsburgh, PA, 2003.

ASSOCIATED CONTENT

S Supporting Information *

Text, tables, and figures giving detailed characterization data for all new compounds, NMR chemical shifts of the complexes, IR ν(CO) data for the complexes and the free ligand, calculation procedure for the dissociation equilibrium constants, van’t Hoff plots for the dissociation equilibrium of the complexes, an Arrhenius plot for the rotation of the olefin in the complexes, and general experimental data for the X-ray analysis. 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: [email protected].

ACKNOWLEDGMENTS We thank Stefan Rudel for his synthetic work within his research practical. For financial support we thank the WACKER Chemie AG.

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REFERENCES

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