Direct Conversion of Bis(imidazolium) Precursors with Rare-Earth

The non-alkylated pro-ligands H2(L5^Fu)Br2 and H2(L6^Fu)Br2 react sluggishly with CeNʺ3 in THF at RT (Scheme 3.1.26). ¹H NMR spectra of the brown (methylene-bridged) or black-green (ethylene-bridged) suspensions are not particularly informative as it exhibits plenty of tiny broad resonances in the region expected for diamagnetic molecules (see SI, Figures 5.3.27-28). In addition, a singlet resonance attributable to HNʺ and two very broad singlets in the negative region are observed. The smaller resonance at ‒3.9 ppm is assigned to unreacted CeNʺ3, the bigger ones with a shoulder at ‒0.8 ppm (methylene-bridged) and ‒1.16 ppm (ethylene-bridged) indicate other CeNʺ-containing moieties, possibly additionally coordinated by Br or THF. ²⁹Si NMR spectra of both reaction solutions do not shed more light on possible products as only the singlets attributed to silicon grease (‒21.5 ppm), HNʺ (1.9 ppm) and unidentified decomposition products previously observed in reactions with KNʺ are detected.

Generally, similar observations were made when reactions were repeated in pyridine-d5 rather than THF.

Scheme 3.1.26. Reactions of H2(L5^Fu)Br2 and H2(L6^Fu)Br2 with CeNʺ3 at RT in THF-d8.

The results mentioned above could indicate that the success of the synthetic approach via CeNʺ3 might be impeded by bromide anions which could block coordination sites on cerium.

Additionally, the low solubility of non-alkylated imidazolium cations balanced by bromide is probably slowing down the reaction, allowing decomposition to occur at a competitive rate. To circumvent these obstacles the corresponding bis(imidazolium) tetraphenylborates H2(L5^Fu)(BPh4)2 and H2(L6^Fu)(BPh4)2 were treated with CeNʺ3 under analogous reaction conditions (Schemes 3.1.27-29).

The treatment of H2(L5^Fu)(BPh4)2 with CeNʺ3 in THF at RT yields a brown solution. ¹H NMR spectrum displays besides the resonances attributed to BPh4¯ a set of 7 resonances in the region expected for diamagnetic organic molecules (Figure 3.1.20). However, if the deprotonated pro-ligand should form a symmetric molecule a set of 6 resonances would be expected. Although the small triplet appearing at lower frequency (6.64 ppm) than the resonance attributed to para-CH of BPh4ˉ have the integral ratio of 1.0 compared to other signals, a matching integral ratio could be just a coincidence. In this case this resonance can be tentatively assigned to a BPh4¯ interacting with a cerium centre.

Scheme 3.1.27. Reactivity of H2(L5^R)(BPh4)2 with CeNʺ3 at RT in THF-d8.

The pattern of the remaining resonances is similar to the spectrum of L5^Fu* but the chemical shift values are quite different. Especially a very significant shift of singlet attributed to the methylene linker at 4.89 ppm is notable. Such a large shift to lower frequency could indicate

that the CH2-group is attached to carbon atoms instead of electronegative nitrogens.

Therefore, a possible shift of methylene group to C2 position yielding e.g. L5^Fu** could be the reason for this observation (Scheme 3.1.27). The overall deprotonation of the pro-ligand is confirmed by the formation of HNʺ and after 1 day at RT the amount of formed L5^Fu** matches nicely the required 2.0 equiv. of generated HNʺ. Additionally, a broad singlet at 1.96 ppm proves the consumption of CeN̎ʺ3 through the shift to higher frequency.

Figure 3.1.20. ¹H NMR spectrum of the reaction mixture obtained after the treatment of H2(L5^Fu)(BPh4)2

with CeNʺ3 in THF-d8 after 1 h at RT.

Interestingly, by comparing the ¹H NMR spectra of the same reaction solution at different points of time an unexpected trend for the shift of the BPh4ˉ resonances to lower frequencies is observed (Figure 3.1.21). Such differences could indicate a possible agnostic interaction of BPh4¯ which cerium centre (see Scheme 3.1.27), which was already previously observed for other rare earth cations and different ligand motifs.^[206] Additionally, slow disappearance of the resonances attributed to L5^Fu** indicates its instability at RT or further reactions leading to insoluble products. Further characterization by ²⁹Si and ¹³C NMR spectroscopy is not particularly informative. Besides the resonances attributed to HN" and silicon grease only smaller singlets caused by impurities are present in the positive region (7.57 and 15.73 ppm).

The ¹H NMR spectrum acquired after the treatment of the related alkylated precursor H2(L5^tFu)(BPh4)2 with CeN"3 under the same reaction conditions shows similar features including the significant shift of the resonance attributed to the methylene-bridge (4.62 ppm) and a broad singlet at –1.95 ppm assigned to Ce-coordinated Nʺ group(s) (see SI, Figure

5.3.29). Therefore, it is quite likely that the parent H2(L5^Fu)(BPh4)2 and alkylated H2(L5^tFu)(BPh4)2 pro-ligands react in the same manner (Scheme 3.1.27).

Figure 3.1.21. Comparison of the ¹H NMR spectra of the reaction mixture obtained after the treatment of H2(L5^Fu)(BPh4)2 with CeNʺ3, allowed to stand for several days at RT.

Scheme 3.1.28. Reaction of H2(L6^Fu)(BPh4)2 with CeNʺ3 at RT in THF-d8.

Interestingly, due to lower solubility of ethane-1,2-diyl-bridged non-alkylated congener H2(L6^Fu)(BPh4)2 in THF it does not react with CeN"3 although an intense change in colour of the solution form yellow over violet to red-brown suggest a possible electrostatic interaction of BPh4¯with Ce(III) coordinated by N"-groups (Scheme 3.1.28). Also the shift of the BPh4 anions to lower frequency by simultaneously losing the resolution further supports this assumption (Figure 3.1.22). ²⁹Si NMR spectrum show similar to the reactions above only the presence of grease, HN" and an impurity at 7.57 ppm. H2(L6^Fu)(BPh4)2 also shows the same behaviour towards CeN"3 in pyridine-d5.

Figure 3.1.22. Comparison of the ¹H NMR spectra of the reaction mixture at different time intervals obtained after the treatment of H2(L6^Fu)(BPh4)2 with CeN"3 at RT in THF-d8.

As expected, in contrast to H2(L6^Fu)Br2, H2(L6^tFu)Br2 slowly reacts with CeN"3 at RT in pyridine due to increased solubility (Scheme 3.1.29). ¹H NMR spectrum of the obtained orange-brown solution displays a set of resonances indicating an asymmetric product derived from deprotonated pro-ligand. Also the treatment of H2(L6^tFu)(BPh4)2 with CeNʺ3 in pyridine-d5 yields the same organic product but in a much cleaner and quicker reaction (for ¹H NMR spectrum see Figure 3.1.23). The presence of some CeN"-fragments is evident through the appearance of two broad singlets in the negative chemical shift spectral window. The most striking feature is a complex multiplet at 3.5 – 4.0 ppm assigned to the ethane-1,2-diyl-bridge, which implies diastereotopic environments and thus significant steric congestion. The amount of generated HN" is a little larger than required for a full deprotonation of one equivalent of the pro-ligand.

Nevertheless, considering the certain degree of decomposition always present in such reactions, it can be considered as a clean reaction, especially since also the amount of BPh4ˉ matches the stoichiometry of generated asymmetric L6^tFu-derivative as well. Further characterization by ¹³C NMR spectroscopy did not shed more light on the molecular structure of the compound due to onset of the decomposition of the obtained material.

Therefore, all data indicate either a formation of asymmetric organic product or asymmetrical coordination of the ligand to the cerium centre. It is difficult to definitely exclude one option from another, but due to appearance of the resonances in the region expected for diamagnetic molecules as well as the good resolution of these signals a formation of an undesired

asymmetric organic product (L6^tFu)* e. g. as suggested in Scheme 3.1.29, is more likely. The resonances attributed to (L6^tFu)* disappear within 5 d storage of the solution at RT.

Scheme 3.1.29. Schematic representation of the reaction of H2(L6^tFu)X2 with CeN"3 in pyridine at RT.

Figure 3.1.23. ¹H NMR spectrum of the reaction mixture obtained after the treatment of H2(L6^tFu)(BPh4)2

with one equivalent of CeN"3 in pyridine-d5 at RT.

In summary, it has been shown that in many cases the pKB value of CeN"3 is not sufficient enough to be used as internal base for non-alkylated precursors, especially if the bis(imidazolium) cations are balanced by bromide. In other cases, where a definitive reaction is observed, the experimental data rather suggest a formation of various diamagnetic organic compounds or alkali metal bonded decomposition products, usually arising from rearrangement of the NHC alkyl groups, rather than novel cerium NHC compounds. Possibly,

both synthesis and work-up at lower temperatures, the use of other cerium precursors with more sterically congested ancillary ligands and lower pKB values such as Li[Ce{N(i-Pr)2}4(THF)]

could allow the isolation of Ce complexes.