Pyrrole-Based Ti(III) and Ti(IV) PNP Pincer Complexes: Insertion of Ketones into the Ti(IV)-Phosphorus Bond

The synthesis, characterization, and reactivity of pyrrole-based
Ti(III) and Ti(IV) PNP pincer complexes are described. [P(NH)P-iPr] (1) reacts with [TiCl4(THF)2] at room temperature in the presence of NEt3 to
afford the Ti(IV) complex [Ti(PNPiPr)(Cl)3]. This complex reacts with acetone and cyclopentanone to
give complexes [Ti(PNOacet-iPr)(Cl)3] and [Ti(PNOcyclo-iPr)(Cl)3], respectively. Insertion of the ketone into the Ti(IV)-P
bond took place, forming a new tridendate PNO-ligand. Treatment of
[TiCl3(THF)3] with the lithium salt of [P(NH)P-iPr] afforded, upon workup, complex [Ti(PNP-iPr)(Cl)2(THF)], a paramagnetic complex with an μeff value of 1.8(1) μB which corresponds to
one unpaired electron and a formal oxidation state of +III. This compound
does not react with ketones. A mechanistic proposal based on DFT calculations
is presented. Ketone insertion proceeds via an associative reaction
initiated by ketone coordination at the metal center, followed by
the opening of the five-membered chelate ring, and finally an intramolecular
nucleophilic attack of the noncoordinated phosphine arm at the carbonyl
atom of the ketone. All complexes were characterized by X-ray crystallography.