Werner, Daniel, Deacon, Glen B., Junk, Peter C., and Anwander, Reiner (2017) Facile reversible benzophenone insertion into rare-earth metal pyrazolate complexes. European Journal of Inorganic Chemistry, 2017 (28). pp. 3419-3428.

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Abstract

Treatment of the homoleptic Ce-IV pyrazolate complex [Ce(Me(2)pz)(4)](2) (Me(2)pz = 3,5-dimethylpyrazolato) with benzophenone (bp) led to the formation of an Me(2)pz-substituted diphenylmethoxy-(N,O)-chelating ligand (pdpm), possibly metal-templated through initial coordination of bp to the cerium atom and subsequent bp insertion into the Ce-N(Me(2)pz) bond. This coordination/insertion process was shown to be reversible, leading to a complex sequence of equilibria involving multiple degrees of insertion/de-insertion and association/dissociation. The dependency on temperature and the amount of bp of all equilibria was revealed, with insertion/association of bp being favored at low temperatures and de-insertion/dissociation preferentially occurring at elevated temperatures. Such sets of equilibria were also observed for the treatment of trivalent complexes [Ln(Me(2)pz)(3)(thf)](2) (Ln = La, Ce, Lu) with bp. Through structural analysis, the trivalent complexes were shown to be less effective in the bp-to-pdpm conversion than the Ce-IV derivative, giving direct evidence of how an increase in rare-earth Lewis acidity aids in ketone anchorage and concomitant conversion. The observed equilibria seem to also apply to the more illustrious organocerium systems. The conversion of bp into the corresponding tertiary alcohol by the routinely employed reagent CeCl3/nBuLi is the most selective when termination of the reaction by hydrolysis is performed at lower temperatures, with a reagent ratio bp/CeCl3/nBuLi of 1:1:1.

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