Mechanistic investigation of the reaction with H 2 O

In the previous part, it was demonstrated that the reaction of complex 7 gave complex 12 in presence of water. Complex 12 involved an intramolecular DHB. This part was focusing on the identification of intermediates during the hydrolysis as illustrated in Figure 4.23. According to the titration shown in Figure 4.12, two ¹H NMR signals for pyrazolate C−H signal were observed during the reaction. One corresponded to complex 7, the other to the complex 12. The concentration of the intermediate was so low that its detection was not possible. The hypothesis of an instable Ni^II(η²−H2) intermediate (cf discussion in the introduction) resulting from the protonation of Ni−H with H2O was a possible hypothesis.

The detection of an intermediate was achieved by using the alternative complex 7B for the hydrolysis (Figure 4.24). Complex 7B was obtained by the same synthetic procedure described for complex 7 (Figure 3.4). However, the procedure was stopped before the addition of the [2,2,2]

cryptand. The cryptand was a necessary element for the crystallization and the full characterization of the complex, but not for the inherent stability of the anionic dihydride complex. The ¹H and ³¹P NMR spectroscopic properties of complex 7B were very similar to complex 7 (compare Figure 4.25 with Figure 3.7). Complex 7B displayed a single ³¹P NMR signal at 85.2 ppm, which was slightly downfield shifted compared to complex 7 (84.1 ppm). The presence of a single ³¹P NMR signal confirmed the purity of the sample.

Figure 4.23. Mechanistic problem for the reaction of complex 7 to give complex 12.

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Figure 4.24. Two different complexes for the study of hydrolysis with water.

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Complex 7B (Figure 4.24) was used for the mechanistic analysis of the reaction with H2O. One equivalent of H2O was added to an NMR sample of complex 7B dissolved in THF-d⁸ at 258 K. A colour change from dark violet to dark brown was observed. The temperature was kept at 258 K for the NMR measurement. A colour change was not observed in the case of complex 7 with H2O. The ¹H and ³¹P NMR spectra of this experiment were represented in Figure 4.26 and in Figure 4.28 respectively.

New ¹H NMR signals were observed and clearly indicated the formation of a new product. A single signal for a pyrazole C−H was detected at 6.60 ppm. Three downfield signals at 7.81 ppm, 7.42 ppm and 7.26 pmm were consistent with protons of an aromatic pyridine (compared with complex 2, chapter 1). Three other upfield signals (6.33 ppm, 5.83 ppm and 5.46 ppm), were consistent with protons of a dearomatized pyridine (cf chapter 2). Two hydride signals in the upfield region were detected. The hydride signals and the signals in the aromatic region were almost equally integrating for 1. The -CH signal of a deprotonated side arm was found at 2.81 ppm. A signal for a -CH2 group was overlapping with the signal of the solvent. It was indirectly detected by a NOESY correlation (Figure 4.27, yellow shaded area). As a link between protonation and aromaticity of the pyridine, the -CH2 group correlated with the proton of the aromatic pyridine (yellow shaded area), while the -CH group correlated with the proton of the dearomatized pyridine (green shaded area).

Figure 4.25. ¹H and ³¹P NMR (top right) spectra of complex 7B in THF-d⁸.

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Figure 4.26. ¹H NMR spectrum obtained after addition of H2O to complex 7B resulting in the formation of complex 9 in THF-d⁸ at 258 K

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Figure 4.27.NOESY NMR spectrum obtained after addition of H2O to complex 7B resulting in the formation of complex 9 in THF-d⁸ at 258 K. Black asterix represented

an uncomplete consumption of 7B.

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The ¹H NMR experiment suggested the formation of the asymmetric complex 9. Complex 9 carried one proton more than complex 7B. This proton was located on the -CH2 group. The protonation of the side arm in complex 7B led to rearomatization of the corresponding pyridine, which induced dramatic NMR changes for all nuclei. Two main ³¹P NMR signals were observed on the spectrum shown in Figure 4.28 and confirmed this hypothesis. The two NMR signals were having the same intensity. The signal at 84.4 ppm was similar to the signal at 85.2 ppm in complex 7B (Figure 4.26). The other signal was observed at 93.4 ppm. The deshielding effect seen in the second ³¹P NMR signal was likely induced by the rearomatization of the pyridine (positive inductive effect).

Complex 9 was not further analysed as it was only stable in solution at 258 K. The spectroscopic data collected from those experiments was not enough to confirm the structure of complex 9.

However, NMR clearly evidenced the presence of an asymmetric intermediate featuring two inequivalent hydrides. The asymmetry was likely coming from one side arm of the ligand being protonated. Even if the acidity of water was likely low in THF, it appeared reasonable that the CH arm in complex 7B was basic enough to deprotonate H2O. This reaction gave rise to complex 9 likely with formal KOH elimination. Eventually, the first step of the reaction mechanism for the formation of complex 9 to 12 was proposed (Figure 4.29).

Figure 4.28. ³¹P NMR spectrum obtained after addition of H2O to complex 7B resulting in the formation of complex 9 in THF-d⁸ at 258 K. Black asterisk indicated the formation of complex 12.

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Complex 9 was stable at 258 K but further reacted at 278 K. The transformation of complex 9 was monitored by ¹H and ³¹P NMR spectroscopy at 278 K. When the temperature of the sample was increased from 258 K to 278 K, new ¹H NMR signals slowly formed. Several intermediates were probably involved in the reaction, but their concentration was too low, and their identification remained difficult. A ¹H NMR signal at 4.55 ppm was detected which was likely due to formation of H2. A major complex formed after 665 min and displayed very similar ¹H and ³¹P NMR signals (Figure 4.31 respectively Figure 4.32) compared to complex 12 (cf part 4.2).

Thus, it was possible that complex 12B formed. Complex 12B could not be crystalized and the characterization of the complex was not further pursued. Complex 12 and complex 12B were different from each other by a [2,2,2] cryptand molecule. Complex 7 led to complex 12 in presence of water. It was suggested that complex 7B led to complex 12B in presence of water.

When the NMR sample was removed from the NMR spectrometer, the colour of the solution was no more brown but bordeaux, akin to complex 12.

In conclusion the monitoring by NMR spectroscopy of the reaction of complex 9 to give complex 12 could not evidence a clear mechanistic pathway. The last step of the reaction was of critical interest for this work as it formed H2. To be transformed into complex 12, complex 9 had to eliminate one hydride and one hydrogen on the side arm. It was tempting to suggest that one hydride and one hydrogen of the -CH2 group of complex 9 would combine into H2 with a concomitant dearomatization of a pyridine moiety (Figure 4.30). This possibility will be discussed in the next part with labelling experiments.

Figure 4.29.New mechanistic problem for the reaction of complex 9 to complex 12.

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Figure 4.31. Monitoring the reaction of complex 9 en route to 12B by ¹H NMR spectroscopy in THF-d⁸ at 278 K. ( ) was correlated to the -CH resonance of the pyrazole in complex 7B. ( ) was correlated to the Ni-H signal of 7B. ( ) was the signal of silicon grease contained in the deuterated solvent.

Figure 4.30. Mechanstic problem: Did the combination between a hydride of complex 9 and the hydrogen atom of the -CH2 group led to the H2 observed by ¹H NMR spectroscopy?

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Figure 4.32. Monitoring the reaction of complex 9 en route to 12B by ³¹P NMR spectroscopy in THF-d⁸ at 278 K. ( ) represented residual amounts of complex 7B which was not hydrolysed and ( ) represented the formation of complex 11 (Figure 4.42).