Reactions of H 3 (L2)Br with REE Precursors

Due to positive results obtained in deprotonation studies with H3(L2)Br and alkali metal bis(trimethylsilyl)amides (see Chapter 2, Section 2.1.1.2), a series of experiments probing the propensity of generated M(L2) (M = Na, K) for transferring NHC to various REE precursors were performed. Hereby M(L2) was formed in situ at various reactions conditions and subsequently transferred to a solution or suspension of respective REE precursor.

In the first tentative experiments the transmetallation reactions were conducted at RT in THF using NaN" or KN" as bases and CeCl3(THF)2. Unfortunately, at RT these reactions yield merely organic decomposition products and/or decomposition products bonded to alkali metals. Therefore, analogue to the 1,2-shift of N-(3,5-di-tert-butyl-2-hydroxybenzyl) substituent

observed in Na(L2) by Kawaguchi^[171], it is likely, that a rearrangement of the ligand framework occurs also in these cases bevor even the ligand can react with CeCl3(THF)2. Notably, also Shen observed the same type of reactivity of her bidentate N-(3,5-di-tert-butyl-2-hydroxybenzyl) functionalised NHC ligand in two step transmetallation reaction in situ with n-BuLi and YbCl3.^[98]. Therefore, further transmetallation reactions with H3(L2)Br were conducted at –78 °C.

Scheme 3.1.5. In situ transmetallation reaction of H3(L2)Br with NaN" and CeCl3(THF)2. The structure of the possible Ce NHC complex is a suggestion based on NMR data.

The reaction of in situ formed Na(L2) with CeCl3(THF)2 at –78 °C (Scheme 3.1.5) yields a dark brown residue whose ¹H NMR spectrum shows the presence of well-defined resonances indicating the formation of one major species as well as some minor ones (Figure 3.1.8). The appearance of partly very broad resonances in the region unusual for diamagnetic organic molecules clearly suggests a coordination of organic ligands to Ce(III) centre. The limited number of resonances also indicates a formation of a compound with high intrinsic symmetry.

The singlets at 1.24 and –5.01 ppm can be tentatively assigned to t-Bu groups. Further resonances 11.06, 9.10, 8.94 and 6.14 ppm are probably attributed to the aryloxo-moieties and both chemically inequivalent methylene linkers. Imidazol-2-ylidene backbone protons resonate as broad small singlets at 18.35 and 14.35 ppm. The integral ratios of the resonances attributed to imidazole-backbone and the t-Bu group at –5.01 ppm does not exactly match the rest of the ligand framework, probably due to high paramagnetic influence on these group, which might lead to overestimating or underestimating the integral values. Additional analysis of the crude product by ²⁹Si NMR spectroscopy lead to expected detection of grease, HN" as well as NaN". Further smaller singlets in the positive region at 7.41, 7.58, 15.97 and 17.73 ppm indicate a formation of terminal –OSiMe3 sites^{[172, 177]}, which might be caused by at least partial silylation of aryloxo-substituents.

Figure 3.1.8. ¹H NMR spectrum of the crude product obtained in an in situ transmetallation reaction of Na(L2) and CeCl3(THF)2 at –78 °C in THF. The spectrum was recorded in THF-d8 at RT.

Unfortunately, to date it was not possible to obtained crystalline material for further analysis neither by slow diffusion of pentane into saturated solution of the crude product in THF at RT nor by crystallization in a THF/toluene mixture at –26 °C. Due to lack of crystallographic characterization the exact molecular structure of the obtained complex remains unknown.

Since the reaction did not proceed cleanly, the formation of Na[Ce(L2)2] is still possible (Scheme 3.1.5). The complexes of the type M(THF)[REE(L2)2] (M = alkali metal) have been already observed for other rare earth metals from other synthetic methods.^[99]

The alternative precursor Ce(BH4)3(THF)4 was treated with K(L2) at –78 °C, as it is expected to react via elimination of KBH4 in a transmetallation reaction (Scheme 3.1.6). ¹H NMR spectrum of the obtained brown residue indicates a relatively clean conversion displaying only a limited number of the resonances that could be attributed to desired Ce NHC species as well (Figure 3.1.9). The broad singlets at 20.42, 6.04 and 6.48 ppm could be tentatively assigned to Ce(BH4)nX-fragments. The other resonances could be attributed to the rest of the ligand framework with some left-over resonances indicating a formation of a side-product. Further characterization by ²⁹Si NMR spectroscopy confirms a clean reaction as only the presence of HN" and silicon grease is detected.

Scheme 3.1.6. In situ transmetallation reaction of H3(L2)Br with KN" and Ce(BH4)3(THF)4. The structure of the possible Ce NHC complex is a suggestion based on NMR data.

Further purification was conducted by the addition of toluene to the solution of crude product in THF at –26 °C. The formation of a fine, colourless precipitate is observed, which is no longer soluble in THF. Therefore, an elimination of KBH4 is likely. ¹H NMR spectrum of the remaining solution still displays resonances attributed to the crude product, however, it also shows the formation of additional cerium NHC compounds with less symmetry and also some possible decomposition products. Further attempts to obtain crystalline material by slow diffusion of pentane into the solution of crude product in THF at RT failed unfortunately.

Besides the previously observed decomposition behaviour by 1,2-rearragement and elimination of one of the N-substituents in the alkali metal NHC adducts (see Chapter 2), another possible decomposition pathway is a migration of other nucleophilic ligands on the metal to carbene. Such rearrangement was observed for similar backbone saturated N-(3,5-di-tert-butyl-2-hydroxyphenyl) mono(NHC) Group IV chloro benzyl (Bn) complexes.^[200]

Interestingly, a migration of Bn-group to carbene was especially favoured for Ti, as the rearrangement product was already obtained in a reaction at –35 °C. For heavier analogues, Zr and Hf, the targeted carbene complexes obtained under the same reaction conditions were stable at RT. However, in contrast to very stable Hf complex the corresponding Zr compound could be converted to the rearrangement product by heating it to 60 °C. Therefore, similar to this rearrangement behaviour the same process could occur in K[Ce(L2)(BH4)2], which could produce eliminated KBH4 as a result of migration of BH3 to carbene and subsequently the dimerization or oligomerization due to vacations of the coordination sites.

Figure 3.1.9. ¹H NMR spectrum of the crude product isolated after treating Ce(BH4)3(THF)4 suspension in THF with in situ formed K(L2) at –78 °C in THF. The spectrum was recorded in THF-d8 at RT.

In summary, similar to the reactions with CeCl3(THF)2 a formation of a K[Ce(L2)(BH4)2] is conceivable (Scheme 3.1.6), albeit a more symmetric but impure compound K(THF)[Ce(L2)2], analogue to the compounds isolated by Shen^[99] (see Introduction, Scheme 1.3.8), could exist.

Furthermore, a formation of more complicated oligomeric structures is possible as well.

Unfortunately, due to internal rearrangement processes leading to elimination of KBH4 further characterization of the complexes was so far unsuccessful.

Reactions of H3(L2)Br with Li[Ce{N(i-Pr)2}4](THF)

REE amides have been previously used for the synthesis of REE bearing bis(N-(3,5-di-tert-butyl-2-hydroxybenzyl) NHC ligands by Shen.^[98] Based on this study the reactivity of Li[Ce{N(i-Pr)2}4](THF) with H3(L2)Br was tentatively explored. Hereby a reaction of H3(L2)Br with Li[Ce{N(i-Pr)2}4](THF) is visible within minutes at RT in THF (Scheme 3.1.7), as a dark solution containing large amounts of HN(i-Pr)2 is formed (Figure 3.1.10). Furthermore, ¹H NMR spectrum shows the presence of LDA as well as a set of paramagnetically affected resonances. The broad singlets at 19.12, 2.23, 0.63, 0.17 ppm and a multiplet –7.70 –7.0 ppm can be tentatively assigned to terminal or bridging amides coordinated to Ce(III). The remaining resonances cleanly match the expected integral ratio for C2-deprotonated ligand.

Further characterization by ¹³C, HSQC or COSY 2D NMR was inconclusive. According to ¹H NMR spectroscopy the compound is stable at RT for at least 5 d. A crystallization by slow diffusion of pentane into a solution of the crude product in THF at RT yields no crystalline

material. Also a crystallization attempt in THF/toluene mixture at –26 °C is unsuccessful as a formation of crystalline material could not be induced. Therefore, it is unclear if in this case a Ce NHC complex or other Ce(III) complex with N-bonded imidazoles resulting form 1,2-rearragement of the ligand have been formed.^[97]

Scheme 3.1.7. Reaction of H3(L2)Br with Li[Ce{N(i-Pr)2}4](THF) in THF-d8 at RT. The Ce NHC product is suggested based on NMR data.

Figure 3.1.10. ¹H NMR spectrum of the reaction mixture obtained after the treatment of H3(L2)Br with Li[Ce{N(i-Pr)2}4](THF) in THF-d8 at RT.

To facilitate salt elimination Li[Ce{N(i-Pr)2}4](THF) was treated with H3(L2)Br in benzene at RT as well. However the obtained ¹H NMR spectrum displays even more complicated resonance pattern as corresponding reaction in THF (Figure 3.1.11). The unmistakable influence of paramagnetic cerium centre is evident and therefore it can be concluded that also in this case

a Ce NHC compound has been synthesized. Similarly to the reaction of Li[Ce{N(i-Pr)2}4](THF) with H2(L1^Me)Br mentioned above, a formation of a complicated oligomeric structures is highly likely. Interestingly, a ⁷Li resonance at 16.11 ppm suggest an incorporation of Li into molecular structure further supporting the assumption that complicated oligomeric structure has been formed. Unfortunately, to date, no single crystals suitable for SC-XRD could be obtained by slow diffusion of pentane into the solution of the crude product in benzene. Further crystallization, especially at low temperatures, should be therefore conducted.

Figure 3.1.11. ¹H NMR spectrum of the reaction mixture obtained after the treatment of H3(L2)Br with Li[Ce{N(i-Pr)2}4](THF) at RT in C6D6.

Since all the reactions described above were carried out using incorrect stoichiometry the addition of an additional equivalents of LDA was carried out in an attempt to yield higher yields and possibly more easily identifiable compounds due to higher intrinsic symmetry. Such a procedure has been previously successfully applied by Shen for other rare earth metals.^[99]

Therefore a mixture of 2.0 eq. of H3(L2)Br, 1.0 eq. of Li[Ce{N(i-Pr)2}4](THF) and 2.0 eq. of LDA was used in a one-pot reaction in THF (Scheme 3.1.8.). Unfortunately, even at –26 °C the reaction proceeds instantaneously and vigorously upon addition of cold THF. A fast cooling down of the reaction to –78 °C and subsequent slow warming up until RT was undertaken, but ideally, this reaction should be repeated at –78 °C.

Correspondingly, the recorded ¹H NMR spectrum of the crude product showed a formation of structurally complicated hardly quantifiable cerium-organo species on the one hand as well as a formation of decomposition products on the other hand (SI, Figure 5.3.16). Analogue to the

previously reported reactivity of N-(3,5-di-tert-butyl-2-hydroxybenzyl) mono(NHC) pro-ligand 79 with other lithium tetrakis(diisopropylamide) rare earth metallates,^[97] due to a very exothermic reaction the decomposition and rearrangement of the ligand to N-bonded imidazoles and to bis(aryloxo)-ligands are conceivable (Scheme 3.1.8). Any further attempts to purify the mixture by different crystallization methods (slow diffusion of pentane into a solution of the crude product in toluene or benzene at RT, crystallization out of toluene/pentane or toluene/DME/pentane solvent mixture at –26 °C) were unsuccessful. Therefore, due to high reactivity of LDA this synthetic protocol needs further modification including the lowering of the reaction temperatures as well as a step-wise addition of LDA.

Scheme 3.1.8. Reaction of H3(L2)Br with Li[Ce{N(i-Pr)2}4](THF) and LDA.

In summary, all of the experiments described in this section indicate a highly promising approach for targeting the corresponding REE NHC compounds. Further experiments towards isolation and crystallization should be therefore performed. Moreover, all synthetic protocols should be further modified to minimize competing rearrangement reactions e. g. by lowering the reaction temperature.

3.1.2 1,1’-(2-Hydroxyethane-1,1-diyl)-Bridge Functionalised Pro-Ligands H3(L4^R)X2