Mechanistic Studies of Catalytic Polyethylene Chain Growth in the Presence of Water
Andreas Berkefeld and Stefan Mecking∗ General procedures and materials
All experiments were carried out under argon of 99.998 % purity using standard Schlenk line tech- niques or in a glovebox under nitrogen. All glassware was dried carefully by treatment with a heat gun under high vacuum or in an oven at 90◦C overnight. Solvents were dried and distilled under argon prior use. Ethers were dried over Na/ benzophenone, pentane and CH2Cl2 over CaH2. The deuterated solvents [D8] THF, CD2Cl2 and CDCl3 were purchased from Eurisotop, dried and vacuum – transfered prior to use. CD2Cl2 and CDCl3 were distilled from CaH2, [D8] THF from a NaK – alloy. Ethylene (99.95 %) supplied by Praxair was used as received.
1H NMR experiments were performed on a Varian 400 spectrometer. Chemical shifts were referenced to residual proton resonances of the solvents. In general, low temperature NMR experiments were carried out with prior temperature determination using a sample of neat methanol.
1D1H spectra ofα–diimine(dimethyl)palladium(II) were acquired at room temperature on a Bruker 250 spectrometer operating at 250 MHz.
Synthetic Procedures
The diimine ligand [1], MgMe2 [2], [H(OEt2)2][BArF4] [3, 4], [(cod)PdCl2] [5, 6] (cod = 1.5–cyclooctadiene) and [(α–diimine)PdMe(OEt2)][BArF4] [7] were prepared according to literature procedures. A new efficient one – pot synthesis of [(α–diimine)PdMe2] [7] was developed and is described here.
Synthesis of α–diimine(dimethyl)palladium(II)
A 100 mL Schlenk flask was charged with 0.57 g (2 mmol) of [(cod)PdCl2] and 0.11 g (2 mmol) of dimethyl magnesium in the glovebox. After cooling to –80◦C, 40 mL of diethyl ether were added via canula. The resulting yellow suspension was stirred and allowed to warm to –30◦C. After filtration and washing of the residual solid with diethyl ether at –10◦C the resulting light brown solution was cooled again to –80◦C. Afterwards, a solution of 0.81 g (2 mmol) of the diimine in 10 mL of diethyl ether was added via canula. After warming to room temperature overnight a brown solid precipitated. The suspension was chilled again to –30◦C in order to complete precipitation. The resulting red brown solid was isolated and washed with a small amount of pentane. Finally, it was dissolved in 10 mL of methylene chloride. The solvent was reduced to approximately 2 to 3 mL and the deep red solution was layered with 20 mL of pentane. Crystallisation at room temperature yielded 0.52 g (0.96 mmol, 48%) of dark purple crystals.
1H NMR (CDCl3, 250 MHz, 25◦C): δ 7.17 (m, 6H, Harom), 3.00 (septet, 3J (H,H) = 6.83 Hz, 4H, ArCH(CH3)2), 1.89 (s, 6H, (H3C)CN), 1.26 (d, 3J (H,H) = 6.77 Hz, 12H, ArCH(CH3)2), 1.07 (d,
3J (H,H) = 6.94 Hz, 12H, ArCH(CH3)2), –0.35 (s, 6H, Pd(CH3)2) ppm.
1
Konstanzer Online-Publikations-System (KOPS)
NMR – Scale Experiments at Various Temperatures
NMR tubes were charged with solid reagents in a glove box. They were sealed with rubber septa and transferred to a cold bath at –80◦C outside of the box. Tubes were wrapped with Nescofilm after each addition. Solvent, liquid reagents and ethylene were added using gas tight 10 or 1000µL Hamilton syringes. Outside the probe of the NMR spectrometer, the samples were kept at –80◦C.
The generally used amount of 1–OEt2 was 15 mg (10.3 µmol) dissolved in 700 µL of [D8] THF. To calculate the volume of an equivalent amount of ethylene a pressure of 1 atmosphere and ideal gas conditions were assumed. One equivalent corresponds to 250 µL ethylene or 0.18 µL of degassed water, respectively. At the beginning of each experiment, a 1H NMR spectrum was acquired at –60◦C to confirm the complexes purity. Afterwards, the probe of the NMR was warmed to the desired temperature and another 1H NMR spectrum was taken to determine appropriate Shim, Lock and Receiver Gain values. Three seconds were set as the relaxation delay. 10.85 µs turned out to be most appropriate as pulse width in order to obtain an exact 90◦ pulse. 8 transients led to proper spectra with a reasonable signal to noise ratio.
Reaction rates and equilibrium constants were determined from 1H NMR signal intensities at –20 and –10◦C. It is known from the literature that ethylene insertion occurs above ca. –40◦C [7].
Exchange equilibrium of water vs. ethylene at –60◦C
1–OH2 (Figure S1) was generated from the diethyl ether complex, 1–OEt2, in an NMR tube via addition of 10 eq. of water and was converted stepwise to the ethylene complex,1–C2H4, by addition of increasing amounts of ethylene. In order to exclude ethylene insertion, this experiment was performed at –60◦C. Starting with half an equivalent, the amount of ethylene was increased to 15 equivalents in 7 steps. After each ethylene addition, the tube was shaken briefly and transferred to the probe. Integral values accounting for the water and ethylene complex as well as the ones for free ethylene and water were noted. Another value was determined after waiting for 30 minutes at –60◦C to confirm that the equilibrium had already been reached. The following signals were employed to calculate the equilibrium constant:
1H NMR ([D8] THF, 400 MHz, –60◦C): δ 5.393 (C2H4), 3.276 (H2O), 0.287 (Pd(CH3)(H2O)) and 0.234 (Pd(CH3)(C2H4)) ppm.
1D EXCY NMR: A stacked plot of the resulting spectra is depicted in Figure S2. The experiment was performed by irradiating on the frequencies of free ethylene and free water, spectra 1 and 2, respectively. Correlations of the signals assigned to free and coordinated water is unambiguous.
Hence, the experiment ascertains that the aforementioned signals do indeed refer to coordinated water. The obtained exchange dynamics of the respective known [7] ethylene signals confirm that the method operates correctly.
Polyethylene chain growth in the presence of water
An appropriate amount of water was added to a freshly prepared solution of 1–OEt2 in [D8] THF.
After shaking the tube thoroughly it was transferred immediately to the NMR probe and a1H NMR
Figure S1. 1H NMR spectrum of 1–OH2 in [D8] THF, 12 eq. of water, at –60◦C
Figure S2. EXCY NMR spectrum at –60◦C: Irradiation on 1. free H2O and 2. free C2H4([D8] THF;
≤2 eq. ethylene; 12 eq. water)
spectrum was acquired.
1H NMR ([D8] THF, 1.5 equivalents of water, 400 MHz, –60◦C): δ 7.87 (br s, 8H, Hortho, BArF4 – counterion), 7.69 (s, 4H, Hpara, BArF4 – counterion), 7.47 – 7.30 (m, 6H, Harom), 6.06 and 6.02 (s and br s, a total of 2H, Pd(CH3)(H2O)), 3.37 (quart, 4H, O(CH2CH3)2, free ether), 3.138 (2x septet overlaying, 2H each, ArCH(CH3)2), 2.33 and 2.28 (s, 3H each, H3C)CN), 1.42 and 1.38 (d,
3J (H,H) = 6.4 Hz, 6H each, ArCH(CH3)2), 1.23 (2x d overlapping, 12H, ArCH(CH3)2), 1.12 (t, 6H, O(CH2CH3)2, free ether), 0.28 (s, 3H, Pd(CH3)(H2O)) ppm. The resonance of free water was observed to occur at 4.10 ppm.
The chemical shifts of water are dependent on the temperature to a great extent. The other signals of the complex underwent only a very small change in their chemical shift.
Clearly changed signals in NMR spectra due to an increased amount of water: 1H NMR([D8] THF, 14 eq. of water, 400 MHz, –60◦C):δ6.32 (br s, 2H, Pd(CH3)(H2O)), 3.20 (br, free water, overlapping with quartet due to free ether). Upon warming to –20 and –10◦C the signals corresponding to coordinated water shifted to 6.07 and 6.00, respectively. Signals due to free water shifted to 2.00 and 2.90, respectively.
As the second step 16 eq. of ethylene were added. After shaking the tube thoroughly it was transferred to the NMR probe and the rate of ethylene insertion into the Pd–Me bond was studied, that is the first order disappearance of the Pd–Me singlet signal vs. time.
1H NMR ([D8] THF, 14 equivalents of water, 400 MHz, –20◦C): δ 7.82 (br s, 8H, Hortho, BArF4 – counterion), 7.62 (s, 4H, Hpara, BArF4 – counterion), 7.50 – 7.25 (m, 6H, Harom), 6.08 (br, Pd(CH3)(H2O)), 4.58 (s, Pd(CH3)(C2H4)), 3.38 (quart, 4H, O(CH2CH3)2, free ether), 3.12 (2x septet overlapping, 4H, ArCH(CH3)2), 2.54 and 2.42 (s, (H3C)CN, Pd(CH3)(C2H4) species), 2.31 and 2.27 (s, (H3C)CN, Pd(CH3)(OH2) species), 1.5 – 1.0 (2x 4d and 1x t, 6H, signals overlapping, ArCH(CH3)2of both olefin and water complex, O(CH2CH3)2of free ether), 0.34 (s, Pd(CH3)(OH2)), 0.28 (s, Pd(CH3)(C2H4)) ppm, respectively. Note the resonance of free excess ethylene and water to occur at 5.38 and 2.98 ppm.
With progressing ethylene insertion the signals due to the Pd–Me species decreased. Furthermore, a new set of signals due to a Pd(C2n+1H4n+3) species began to form:
1H NMR ([D8] THF, 14 equivalents of water, 400 MHz, –20◦C): δ 5.98 (br s, Pd(C2n+1H4n+3)(H2O)), 4.53 (s, Pd(C2n+1H4n+3)(C2H4)), 2.54 and 2.41 (s, (H3C)CN, Pd(C2n+1H4n+3)(C2H4)), 2.31 and 2.25 (s, (H3C)CN, Pd(C2n+1H4n+3)(H2O)), 1.43 (d, 3J (H,H)
= 6.8 Hz, 6H, ArCH(CH3)2), 1.38 (d, 3J (H,H) = 6.0 Hz, 6H, ArCH(CH3)2), 1.24 (d, 3J (H,H) = 6.4 Hz, 6H, ArCH(CH3)2), 1.20 (d,3J (H,H) = 6.4 Hz, 6H, ArCH(CH3)2) ppm. The resonance of free excess water occurred at 2.97. Also some free ligand resulting from decomposition (2.06 ppm) was observed.
The corresponding signals due to a Pd(C2n+1H4n+3) species at –10◦C were as follows:
1H NMR ([D8] THF, 10 equivalents of water, 400 MHz, –10◦C): δ 5.90 (br s, Pd(C2n+1H4n+3)(H2O)), 2.55 and 2.41 (s, (H3C)CN, Pd(C2n+1H4n+3)(C2H4)), 2.31 and 2.25 (s,
Figure S3. Snapshot of catalyst’s resting states after complete conversion of the Pd–Me species at –60◦C ([D8] THF, initially 16 eq. ethylene, 10 eq. water)
(H3C)CN, Pd(C2n+1H4n+3)(H2O)) ppm. The presence of free ethylene was observed to accelerate decomposition of palladium species even at low temperatures to form a black precipitate. The resonances due to decomposition occurred at 2.73 (septet) and 2.08 (singlet) ppm. When excess ethylene had been fully consumed, no further decomposition was observed.
In one experiment the insertion was stopped by rapid cooling of the NMR probe to –80◦C. After- wards a 1H NMR spectrum revealed the equilibrium between the water and ethylene coordinated Pd(C2n+1H4n+3) species at –60◦C (Figure S3).
1H NMR([D8] THF, 14 equivalents of water, 400 MHz, –60◦C): δ 6.22 (s, Pd(C2n+1H4n+3)(H2O)), 4.52 (s, Pd(C2n+1H4n+3)(C2H4)), 2.57 and 2.42 (s, (H3C)CN, Pd(C2n+1H4n+3)(C2H4)), 2.32 and 2.25 (s, (H3C)CN, Pd(C2n+1H4n+3)(H2O)) ppm. The resonance of free excess ethylene and free water occur at 5.39 and 3.23 ppm, respectively.
Equilibrium constant of water vs. ethylene exchange
Water vs. olefin binding equilibrium constants were determined at various temperatures (olefin insertion also occurs at T ≥ –30◦C) from integral intensities of the four species involved, 1–
OH2+ C2H4⇀↽1–C2H4+ OH2, K1 and 2–OH2+ C2H4⇀↽2–C2H4+ OH2, K2, respectively. The concentrations were determined directly from the1H NMR spectra.
In principal, the equilibrium constants can be determined from a single measurement for a given temperature. To increase accuracy and to confirm concentration independence, K values were determined from measurements at various amounts of added water and ethylene. At –60◦C no
insertion of ethylene occurs whereas at –20◦C and above ethylene insertion will occur.
With respect to K = [Pd(C2n+1H4n+3)(C2H4)][H2O]/[Pd(C2n+1H4n+3)(OH2)][C2H4], the nominator is plotted in dependence of the denominator, and the resulting data points were fitted linearly (Figure S4 and Figure S5). Figure S4 concerns the methyl species at –60◦C, whereas Figure S5 represents the results at –20◦C of both the methyl and the alkyl (R>Me) species. The slope of the linear fit corresponds to the averaged value of the equilibrium constant.
Rate constant of free vs. coordinated water exchange
According to the literature [8], rate constants, kC, of exchange processes such as Pd(R)(L) + L’⇀↽Pd(R)(L’) + L (L = H2O, R = CH3, C2n+1H4n+3)
at the coalescence temperature TC can be estimated with respect to the relationship kC=π∆ν/√ 2
= 2.22 ∆ν (a first order process in Pd(R)(OH2) being assumed, using proton resonances of approx- imately equal intensity of exchanging water (3.6 eq. excess water)) in which ∆ν corresponds to the distance of the signals in units of Hz. Within experimental accuracy exchange rate constants of Pd–Me and higher alkyl water species with free excess water were determined to be equal.
Water dependence of the insertion rate of ethylene
The following first order rate law of ethylene consumption in the presence of water was used to determine ki:
d([P d(CH3)(C2H4)] + [P d(CH3)(H2O)])
dt =−kapp([P d(CH3)(C2H4)] + [P d(CH3)(H2O)]) The observed apparent rate constant kapp is related to the insertion rate constant ki according to:
kapp = 1 1 + K1[C[H2O]
2H4]
ki
The concentrations of water and ethylene are assumed to be constant. This is a valid approximation for the first insertion step because an excess of more than 10 eq. of ethylene was added prior to the reaction’s start. The equilibrium constants were determined as described above. Plots of the concentration of the Pd–Me species vs. time are depicted in the following figures, S6 and S7, at –20◦C and –10◦C, respectively.
0 2 4 6 8 10 12 0
10 20 30 40 50 60
[Pd(CH 3)(C 2H 4)][H 2O]
[Pd(CH3)(OH2)][C2H4] K = 4.74 ± 0.16
Figure S4. Exchange equilibrium at –60◦C ([D8] THF; 10 eq. water; stepwise addition of 0 to 17 eq. ethylene; scales in arbitrary units)
0 2 4 6 8 10 12 14 16 18 20 22
0 5 10 15 20 25 30 35 40
[Pd(CH 3)(C 2H 4)][H 2O] or [Pd(C 2n+1H 4n+3)(C 2H 4)][H 2O]
[Pd(CH3)(H2O)][C2H4] or [Pd(C2n+1H4n+3)(H2O)][C2H4] K (R = Me) = 1.78 ± 0.07 (x)
K (R > Me) = 1.51 ± 0.09 (+)
Figure S5. Exchange equilibrium at –20◦C ([D8] THF; 12 eq. water; initially 16 eq. ethylene; scales in arbitrary units)
0 400 800 1200 1600 2000 2400 2800 10
20 30 40 50 60 70 80
kapp = (1.76 ± 0.04) 10-3 s-1 (+) kapp = (1.52 ± 0.06) 10-3 s-1 (x)
[Pd(CH3)(C2H4)]+[Pd(CH3)(OH2)]
Time / s
Figure S6. Mono exponential fit of two independent sets of data points obtained at –20◦C ([D8] THF; initial eq. ethylene: x = 16, + = 12; eq. water: x = 12, + = 3.6; ordinate scale in ar- bitrary units)
0 100 200 300 400 500 600 700 800 900 1000 -20
0 20 40 60 80 100 120 140 160
[Pd(CH 3)(C 2H 4)]+[Pd(CH 3)(OH 2)]
Time / s
kapp = (6.74 ± 0.16) 10-3 s-1
Figure S7. Mono exponential fit to a set of data points obtained at –10◦C ([D8] THF; initial 16 eq. ethylene; 10 eq. water; ordinate scale in arbitrary units)
References
[1] H. tom Dieck,Z. Naturforschung, B: Chemical Sciences 1981,36, 823 – 832.
[2] Andersen; WilkinsonInorg. Synth. 1979,19, 262 – 265.
[3] D. L. Reger, C. A. Little, J. J. S. Lamba, K. J. Brown,Inorg. Synth. 2004, 34, 5 – 8.
[4] M. Brookhart, B. Grant, A. F. Volpe,Organometallics 1992, 11, 3920 – 3922.
[5] M. F. Rettig, P. M. Maitlis, Inorg. Synth. 1977, 17, 134 – 137.
[6] l. Chatt, L. M. Vallarino, L. M. Venanci,J. Chem. Soc. 1957, 3413 – 3416.
[7] L. K. Johnson, C. M. Killian, M. Brookhart, J. Am. Chem. Soc. 1995, 117, 6414 – 6415.
[8] H. Friebolin, Ein– und zweidimensionale NMR – Spektroskopie: eine Einf¨uhrung, 3rd edn., Wiley – VCH, Weinheim 1999.
