Facile Reversible Benzophenone Insertion into Rare‐Earth Metal Pyrazolate Complexes

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 [a]

Emergence of a New [NNN] Pincer Ligand via Si−H Bond Activation and β‐Hydride Abstraction at Tetravalent Cerium

“…In a previous study we found that the reaction between 1 and benzophenone affords Me 2 pz‐functionalized diphenylmethoxy ligand moieties via a cascade of benzophenone association/dissociation and nucleophilic attack by the Me 2 pz co‐ligands, which is facilitated by weakening of the ketone C=O double bond upon coordination to cerium(IV) . The present ligand transformation may bear some similarity, being triggered by the nucleophilic attack of the “activated” Si−H functionality by a pyrazolato ligand.…”

Effective and Reversible Carbon Dioxide Insertion into Cerium Pyrazolates

“…As a consequence of the equilibrium between potential alternative oligomers formed in the presence of only one equivalent of CO 2 per cerium, the interpretation of the ambient‐temperature NMR spectrum was difficult. This was already experienced for the insertion of benzophenone into the Ce−N(Me 2 pz) bond . Upon heating to 90 °C, CO 2 was liberated and [Ce(Me 2 pz) 4 ] re‐formed (see Figure S14).…”

“…The Ce À N and Ce À O bond lengths average 2.528 and 2.255 , respectively, thus matching the values found in the benzophenoneinserted product [Ce(Me 2 pz) 2 (pdpm) 2 ] (Ce1ÀN1 2.564 , Ce1ÀO1 2.173 ; pdpm = (3,5-dimethylpyrazol-1-yl)diphenylmethanolate). [23] Other homoleptic Ce IV complexes, [Ce-(L) 4 ] (with L as a donor-functionalized alkoxy ligand engaged in a 5-membered chelate to cerium), also have similar Ce À O bond lengths as those in 2, thus highlighting a common chelating coordination motif. [24,25] Seemingly, no delocalization occurs across the O=CÀO fragment, which exhibits average CÀO bond lengths of 1.207 (terminal) and 1.291 (bridging).…”

“…This was already experienced for the insertion of benzophenone into the CeÀN(Me 2 pz) bond. [23] Upon heating to 90 8C, CO 2 was liberated and [Ce(Me 2 pz) 4 ] re-formed (see Figure S14). After cooling to ambient temperature, a partial reinsertion of CO 2 was observed, as was found in the VT NMR experiments on 2·toluene and 2·thf.…”

“…[20] Pyrazolates (pz), on the other hand, are dinitrogen-derived Cp counterparts, where the putative N À CO 2 bond may tolerate a more reversible insertion process, as seen for other CO 2 -heteroatom bonds (Scheme 1 e). [21,22] As the tetravalent [Ce(Me 2 pz) 4 ] 2 complex was recently shown to undergo reversible insertion of ketones into the CeÀN bond, [23] we extended the study toward CO 2 . Quantitative insertion of CO 2 into the Ce À N(Me 2 pz) bond was observed for both tetravalent and trivalent cerium Me 2 pz complexes, and intriguingly the insertion process was found to be fully reversible.…”