4.4 Experimental Section
6.2.3 Cyclic Voltammetry
To further illuminate the electronic properties of the nickel center in this type of polymerization catalysts, cyclic voltammetry measurements were conducted. The focus was put on the investigation of the electron density of the nickel depending on the substitution pattern of the corresponding ligand.
Figure 6.7 depicts the complexes which were selected for the investigation of their electronic properties. CF31-pyr and Me1-pyr were selected as extreme cases with very electron withdrawing and electron donating substituents, respectively. The two mixed complexes
(MeCF3)1-pyr and Me/CF31-pyr contain both electron donating as well as electron withdrawing substituents but differ in their constitution. In addition to these four terphenyl amine based complexes, also the two catalyst precursors Fur5-pyr and Thio5-pyr were investigated since they gave remarkable results in ethylene oligomerization experiments.
Figure 6.7: Complexes used for cyclic voltammetry measurements.
All complexes showed one oxidation process during cyclic voltammetry measurements in the potential range expected for a four coordinate square planar Ni(II)/Ni(III) pair.
Measurements were carried out using different scan rates from 25 to 2,000 mV s-1 and the half-wave potential was found to be independent of the scan rate. The oxidation of all complexes is only partially reversible which is attributed to a relatively fast decomposition of the Ni(III)-species. Cyclic voltammograms of the corresponding ligands did not show any redox
transitions at the potentials observed for the complex. Therefore, the oxidation and reduction processes during cyclic voltammetry measurements are assumed to be metal centered. EPR measurements conducted with the chemically oxidized Me1-pyr (in situ oxidation with [1,1’-diacetylferrocenium]SbF6) did not provide further information concerning the exact location of the electron within the complex structure since no EPR signal could be detected.
Representative referenced cyclic voltammograms of all complexes are provided in the experimental section of this chapter.
In Table 6.6 the half-wave potentials of the six different complexes determined from the cyclic voltammograms are listed. Complex Me1-pyr bearing electron donating substituents was found to be oxidized most easily as evidenced by the lowest (most negative) E1/2 value. Electron withdrawing substituents hinder the oxidation of the nickel center which is displayed by the highest half-wave potential of E1/2 = 306 mV found for CF31-pyr as compared to E1/2 = -33 mV found for Me1-pyr. This translates to a higher electron density of the nickel atom in the electron donating methyl substituted complex Me1-pyr compared to that of CF31-pyr. This result follows the expected behavior and strongly supports the hypothesis of an electronic influence of the remote substituents on the active nickel center.
Table 6.6: Half-wave potentials of selected catalysts according to cyclic voltammetry measurements.
entry complex E1/2 [mV]a substituents are intermediate to the most extreme values of the fully methyl and CF3-substituted complexes. In agreement with their oligomerization behavior observed in Chapter 6.2.2, producing oligomers with intermediate properties, the electron density of the nickel center also appears to be inbetween those of CF31-pyr and Me1-pyr for both complexes. Though the symmetrical mixed complex (MeCF3)1-pyr exhibits a slightly lower half-wave potential than the asymmetrical complex Me/CF31-pyr (91 mV vs. 164 mV), the electronic influence of the CF3 and methyl substituents seem to level each other out in both complexes. For the two complexes
Fur5-pyr and Thio5-pyr bearing electron rich motifs on the salicylaldiminato ligand with
coordinating motifs, the redox potentials of E1/2 = 84 and 76 mV, respectively, are in the range of the mixed complexes and significantly higher than the one of the electron rich complex Me1-pyr.
Given that the electronic nature of the remote substituents correlates with polymer molecular weight on the one hand and the half-wave potential on the other hand, the half-wave potential of the complexes was found to correlate with the molecular weight of the polyethylene they produce under given reaction conditions (Figure 6.8, orange). Complexes with a higher half-wave potential produce higher molecular weight polymer and vice versa. This does not only hold true for the two benchmark complexes CF31-pyr and Me1-pyr but also for the two mixed complexes and those bearing coordinating motifs, having half-wave potentials and molecular weights in between the extreme values.
Figure 6.8: Correlation of the catalysts half-wave potential vs. polymer molecular weight and degree of branching. a molecular weight of polymers synthesized at 40 °C and 20 bar of ethylene; * polymer obtained at 40 °C and 40 bar of ethylene according to ref. 53. Dashed lines are merely a guide to the eye.
For the four different terphenyl amine based nickel salicylaldiminato complexes, also the overall degree of branching of the polymers correlates with the catalysts’ half-wave potential.
This is in agreement with a higher propensity of a more electron rich nickel center for β-hydride elimination as fundamental step for both chain transfer and chain walking. While this experimental correlation (linear) is even quantitative, Fur5-pyr and Thio5-pyr with coordinating motifs also fit the trend but not quantitatively (red box in Figure 6.8). Both exhibit half-wave potentials similar to the one of (MeCF3)1-pyr (E1/2 = 84 and 76 mV vs. 91 mV) but produce polymers with a three times higher degree of branching. In summary, these CV studies show that the nature of the remote substituents impacts the electronic nature of the nickel center, as observed by the different ease of oxidation of the nickel catalyst precursors.