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- i Pr] ( 1 ) reacts with [TiCl 4 (THF) 2 ] at room temperature in the presence of NEt 3 to afford the Ti(IV) complex [Ti(PNP i Pr )(Cl) 3 ]. This complex reacts with acetone and cyclopentanone to give complexes [Ti(PNO acet - i Pr)(Cl) 3 ] and [Ti(PNO cyclo - i Pr)(Cl) 3 ], respectively. Insertion of the ketone into the Ti(IV)-P bond took place, forming a new tridendate PNO-ligand. Treatment of [TiCl 3 (THF) 3 ] with the lithium salt of [P(NH)P- i Pr] afforded, upon workup, complex [Ti(PNP- i Pr)(Cl) 2 (THF)], a paramagnetic complex with an μ eff value of 1.8(1) μ B 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.