Tiempos de transferencia mínimos.

To date there are limited examples of rare earth complexes with 2-oxazolylphenol ligands, totalling only three papers.102,103,104 The earliest of the three papers by Hubert-Pfalzgraf et al. investigated using these complexes in metal organic chemical vapour deposition (MOCVD) with a view to generating thin films of cerium (IV) oxide.102 Berg et al. conducted a structural study of these complexes, providing the only detailed structural analysis to date.103 Shibasaki et al. most recently investigated heteroleptic lanthanum complexes with 2-oxazolylphenols and their use in anti-selective Mannich reactions.104

Hubert-Pfalzgraf et al. investigated two tetrakis 2-oxazolylphenol cerium complexes and their applications for liquid injection MOCVD (Fig. 3.11). 102 Both complexes were noticeably less volatile than other cerium precursors despite having similar molecular weights. This is most likely due to π-π stacking between the phenyl substituents. TGA-GC/MS and mass spectrometry (EI) were used to investigate the decomposition pathways of the two complexes. It was interesting to note the stark differences between the two complexes despite their structural similarities. Ultimately it was concluded that complex 2, Ce (iPrHoxaz)4, was

suitable as a potential precursor for CeO2 deposition under an inert atmosphere. Complex 1,

Ce (Me2oxaz)4 was unsuitable as a precursor in non-oxidative conditions. The difference

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Figure 3.11: Proposed structure of cerium tetrakis oxazolylphenolate complexes102

Berg et al. synthesised tris allox complexes with a range of lanthanides with the general formula Ln(Allox)3 (Ln=La, Ce, Sm, Er, Y).103 These complexes were well characterized by 1H NMR

spectroscopy in solution and single crystal XRD in the solid state for the Ce and Er complexes. The purpose of the study was to understand the structural changes of the tris(oxazoline- phenoxide) complexes both in the solid state and in solution. The addition of the allyl substituents was primarily incorporated to see whether coordination of the tethered alkene can occur, but also to provide additional steric bulk.

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Figure 3.12: Synthesis of oxazolylphenolate complexes by Berg et al. 103

Crystal structures for the cerium and erbium complexes were obtained and they were very similar to each other. Both demonstrated distorted octahedral coordination geometry, with a mer arrangement of the three coordinating allox ligands. This showed the allyl groups to be non-coordinating, as the closest contact between the metal and the terminal or internal alkene carbons was 4Å.

Solution 1H NMR spectra of the complexes in d8-toluene showed a set of 9 resonances

consistent with an average C3v symmetry. Berg et al. noted that Ln(Allox)3 complexes can exist

in two different geometries, facial and meridional (Fig. 3.13), and that the differing symmetries between these two forms could be distinguished by 1HNMR spectroscopy. The facial form would be C3 symmetric and as such corresponding chemical shifts on the ligands would be

equivalent leading to a total of 12 chemical shifts. The meridional form would have C1

symmetry rendering all corresponding chemical shifts to be inequivalent and giving 36 chemical shifts. As such, 9 chemical shifts would indicate a facial geometry for the complexes;

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however crystal structure data strongly indicated meridional complexes. This strongly implies the two geometries are interconverting and the chemical shifts are being thermally averaged in the 1HNMR spectra.

Figure 3.13: Facial (fac) and Meridional (mer) isomers

Berg et al. attribute this fluxionality to the Bailar (trigonal) or Ray-Dutt (rhombic) twists.105 The Bailar twist cannot interconvert between mer and fac isomers, but it can scramble between sites of the same geometry (for example fac-Δ and fac-Λ) (Fig. 3.14 and Fig. 3.15). Ray-Dutt twists can interconvert between all four isomers (mer-Δ, mer-Λ, fac- Δ and fac- Λ) (Fig. 3.16).103

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Figure 3.15: Bailar twist interconversion of mer-Λ (on the left) to mer-Δ (on the right)

Figure 3.16: Ray-Dutt twist interconversion of fac-Δ (on the left) to mer-Λ (on the right)

Berg et al. notes that it is not possible to distinguish between these two processes as the fac

isomer isn’t visible at low temperatures. Consequently, it is not possible to rule out that these complexes may be just fluxional mer complexes showing thermal averaging in 1H NMR. However taking into account the work of other groups and the highly fluxional nature of rare earth co-ordination, they believe it is highly likely that both processes are occurring simultaneously.105

It should be noted that as the allox ligand is achiral its mer-Δ and mer-Λ complexes (as well as

fac-Δ and fac-Λ complexes) are enantiomers of each other and therefore indistinguishable by

1_{H NMR spectroscopy. By comparison the chiral ligands used in this study will have Δ and Λ}

forms of the two geometries that will be diastereisomers of each other; hence all forms (mer- Δ, mer-Λ, fac-Δ, fac-Λ) are inequivalent, resulting in very complicated NMR spectra.

Shibasaki et al. have thus far been the only group to investigate the catalytic potential of rare earth complexes of 2-oxazolylphenols.104 The anti-selective Mannich-type reaction between

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N-substituted imines and trichloromethyl ketone (an ester donor equivalent) was effectively catalysed by a heteroleptic lanthanum complex in good yield and ee (Fig. 3.17).

Figure 3.17: Anti-selective direct catalysed Mannich type reaction104

The process is notable due to its use of a Lewis base to catalyse a Brønsted base catalysed process. The catalyst was formed in situ and as such the active form of the catalyst was not characterised; however probable transition states were proposed (Fig. 3.18). This model strongly favours the formation of the less sterically hindered intermediate, resulting in anti- selectivity.

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