Transmetallation Reactions in Situ

Although Li(L3^Mes) can be readily isolated, due to its unknown exact molecular structure, it was generated and used in situ to ensure stoichiometric control. Therefore, to test the propensity of Li(L3^Mes) for transferring bis(NHC) ligand to REE, the chosen REE precursors were treated with a solution of Li(L3^Mes) formed in situ at 0 °C or at RT (e.g. as seen in Scheme 3.1.9).

No reaction of in situ formed Li(L3^Mes) with CeN"3 is observed at RT in benzene (Scheme 3.1.9). Also ¹H NMR spectrum of the residue obtained after the stirring of the same reaction mixture for 18 h at 60 °C shows no sign of any transfer of NHC ligand to the cerium centre. No reaction was observed in benzene with CeCp3 either (Scheme 3.1.10).

Scheme 3.1.9. Reaction conditions used for the treatment of CeNʺ3 with Li(L3^Mes).

Scheme 3.1.10. Reaction conditions used for the treatment of CeCp3 with Li(L3^Mes).

However, the use of the metal precursor Ce(BH4)3(THF)4 results in the formation of various diamagnetic products (Scheme 3.1.11). The ¹H NMR spectrum of the crude product shows no remaining resonances attributable to the Li NHC adduct but instead numerous smaller resonances, especially in the aliphatic region (see Figure 5.3.17, SI). A set of resonances attributable to a single major product is assignable to a product similar to 1-mesitylimidazole, but with different chemical shift values. Therefore, since no resonances in the region expected for 1,1’-(hydroxyethane-1,1-diyl) bridge are detected, a possible main product could be 2-mesitylimidazole formed by 1,2-shift of the N-mesityl-substituent and degradation of the bridge.

Both processes have been already observed for alkali metal NHC adducts described in

Chapter 2. Moreover, a coordination of imidazoles via nitrogen atoms to cerium or lithium cation would be likely in the presence of these metals. Such reactivity would be not surprising, as Shen observed on several occasions a coordination of imidazole moieties to REE and lithium as a result of the degradation of a NHC ligand due to thermally induced decomposition.^[97-98] In the case of the reaction described here the observed chemical shifts rather suggest a coordination to Li.

Scheme 3.1.11. Reaction of Ce(BH4)3(THF)4 with Li(L3^Mes) formed in situ.

Since Li(L3^Mes) is not a useful reagent in reactions with respect to the tested cerium precursors, different rare earth metals have been considered. Due to its higher Lewis acidity yttrium is expected to form stronger M–CC bonds. Moreover, the diamagnetic nature of Y nucleus facilitates the characterization of the products. Therefore, a solution of Li(L3^Mes) was treated with a suspension of YCp3 in benzene (Scheme 3.1.12). The reaction of Li(L3^Mes) with YCp3 is clearly visible by slow consumption of REE precursor and change of the colour of the solution from yellow-orange to intense pink. Unfortunately, the ¹H NMR spectrum of the crude product displays extremely high number of overlapping resonances in aromatic and aliphatic region (SI, Figure 5.3.18). Although two resonances attributed to a possibly intact linker between two imidazole-2-ylidenes are visible, they do not correlate with the number and intensity of other resonances suggesting that in most cases the decomposition of the ligand framework must have been occurred. Furthermore, no doublet expected for a Y–C bond is found in the ¹³C NMR spectrum. However, ⁷Li NMR spectrum indicates a possible formation of LiCp due to the presence of a broad singlet at 6.96 ppm. This would suggest a transfer of the ligand to Y ion in a salt elimination reaction. Finally, ²⁹Si NMR sheds no more light on the possible products as only the formation of HN" is evident. Further characterization of the residue obtained after the filtration of the pink solution by ¹H and ⁷Li NMR spectroscopy in pyridine-d5 only confirms the formation of LiCp.

Scheme 3.1.12. Reaction of YCp3 with Li(L3^Mes) formed in situ.

Although it was not possible to isolate the corresponding potassium bis(NHC) adduct due to its higher instability in comparison to Li(L3^Mes), a possibility of generation of [K(L3^Mes)] in situ for subsequent use as NHC transfer reagent to a cerium centre was considered as an alternative due to apparent inertness of Li(L3^Mes) towards some cerium precursors.

Scheme 3.1.13. Generation of [K(L3^Mes)] in situ and subsequent reaction with CeN"3.

Because of the previously mentioned thermal instability of K bis(NHC)s (see Chapter 2, Section 2.1.2.2), H3(L3^Mes)Cl2 was allowed to react for only 30 min with KN" in THF at RT (Scheme 3.1.13). A subsequent rapid transfer of [K(L3^Mes)] solution to a solution of CeN"3 in THF resulted in isolation of a crude product, whose ¹H NMR spectrum is hardly quantifiable (SI, Figure 5.3.19). However, some broad resonances and well as resonances in the negative region including a broad singlet attributed to bis(trimethylsilyl)amide groups coordinated to Ce at –1.72 ppm indicate a possible formation of desired compounds. Unfortunately, after attempting to wash the crude product with hexane only various diamagnetic decomposition products as well as 1-methylimidazole are detected in the washing solution as well as in the remaining residue. This behaviour is similar to the degradation pathway of alkali metal adducts of 1,1’-(2-hydroxyethane-1,1-diyl)-bridged functionalised bis(NHC) described in Chapter 2, Section 2.1.2.2, suggesting that a potential REE ligand complex is either highly air-sensitive and/or thermally unstable. Similar results were obtained by treating H3(L3^Mes)(BPh4)2 with KN"

and subsequent transfer of the resulting solution to CeCp3.

Since neither potassium nor lithium adducts of 1,1’-(2-hydroxyethane-1,1-diyl) bridge modified N-mesityl functionalised bis(NHC)s show particularly promising behaviour towards transmetallation to REEs, the use of an analogous isopropyl-functionalised pro-ligand was considered. In the reaction of H3(L3^i-Pr)Cl2 with LiN" in THF it was possible to recover the bis(imidazolium) chloride out of Li(L3^i-Pr) by hydrolysis with HCl (see Chapter 2. Section 2.1.2.2), therefore proving that Li(L3^i-Pr) is quite stable with respect to ligand rearrangement.

Therefore, to test the reactivity of isopropyl wingtip modified precursors in two step reactions with LiN" and cerium borohydride, a synthetic protocol similar to the reactions with H3(L3^Mes)X2

has been chosen (Scheme 3.1.14). Unfortunately, similar to the cases described above the formation of 1-isopropylimidazole as main species is observed in ¹H NMR spectrum of the crude product, although the appearance of a broad singlet at –0.15 ppm attributed to LiBH4[201]

supports a successful salt elimination reaction (SI, Figure 5.3.20).

Scheme 3.1.14. Generation of [Li(L3^i-Pr)] in situ and subsequent reaction of this intermediate with Ce(BH4)3(THF)4.

In conclusion, although the pro-ligands H3(L3^R)X2 tend to form at least meta-stable alkali metal NHC adducts the synthetic approach to corresponding REE NHC complexes via two-step transmetallation reaction proved to be unsuccessful. The reason for it is either the inertness of Li(L3^Mes) towards many precursors and the decomposition of the ligand yielding substituted imidazoles if the reaction occurs.

3.1.2.2 Direct Conversion of Bis(imidazolium) Precursors with Rare Earth Amides and