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

Abstract

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(PNP iPr)­(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.

Pendulum-like hemilability in a Ti-based frustrated Lewis Trio

“…5 . 40–42 This also resulted in the formation of complex 5 based on NMR analysis (see Fig. S62–S63 † ), confirming the speciation in the solution matched that observed in the solid-state.…”

Synthesis and characterization of pyrrole-based group 4 PNP pincer complexes

“…We have recently shown [ 24 ] that [P(NH)P- i Pr] ( 1 ) reacts with [TiCl 4 (THF) 2 ] in the presence of base to yield the Ti(IV) complex [Ti(PNP i Pr )(Cl) 3 ] ( 2 ). If the same reaction is performed with [MCl 4 (THF) 2 ] (M = Zr, Hf), instead of monomeric analogs, the dimeric complexes [Zr(PNP i Pr )(μ-Cl)(Cl) 2 ] 2 ( 2 ) and [Hf(PNP i Pr )(μ-Cl)(Cl) 2 ] 2 ( 3 ) featuring two bridging chloride ligands are obtained in 76 and 80% isolated yields (Scheme 1 ).…”

“…With respect to group 4 metals pyrrole-based PNP pincer complexes are rare [ 22 , 23 ]. We have recently described several Ti(IV) and Ti(III) PNP complexes [ 24 ] which were shown to undergo ketone insertion reactions into a Ti(IV)-P bond thereby forming new complexes with tridendate PNO-ligands.…”

Synthesis and characterization of pyrrole-based group 4 PNP pincer complexes

“…We have recently shown [24] that [P(NH)P-iPr] (1) reacts with [TiCl 4 (THF) 2 ] in the presence of base to yield the Ti(IV) complex [Ti(PNP iPr )(Cl) 3 ] (2). If the same reaction is performed with [MCl 4 (THF) 2 ] (M = Zr, Hf), instead of monomeric analogs, the dimeric complexes [Zr(PNP iPr )(µ-Cl)(Cl) 2 ] 2 (2) and [Hf(PNP iPr )(µ-Cl) (Cl) 2 ] 2 (3) featuring two bridging chloride ligands are obtained in 76 and 80% isolated yields (Scheme 1).…”

“…The computational results presented have been achieved in part using the Vienna Scienti c Cluster (VSC). Calculations were performed using the Gaussian 09 software package [36] with the PBE0 functionals without symmetry constraints, the Stuttgart/Dresden ECP (SDD) basis set to describe the electrons of titanium, zirconium and hafnium and a standard 6-31G** basis for all other atoms as already described previously [24].…”