Template Synthesis of NPN′ Pincer-type Ligands at Titanium Using an Ambiphilic Phosphide Scaffold

Ti-imido complex [TiCl­(NtBu)­(BIPP)] [1; BIPP = bis­(iminophosphoranyl)­phosphide ligand] reacts with terminal alkynes R–CCH (R = phenyl, isopropenyl, cyclopropyl, and 2-pyridyl) via P–P bond cleavage of the BIPP ligand. The resulting complexes [TiCl­(NPN′)­(NPhPPh2)] (2a–d) contain a pincer-type NPN′ phosphide ligand that incorporates the terminal alkyne and the imido ligand from complex 1. Complexes 2a–d feature two chiral centers (Ti and P) with interdependent absolute configurations; thus, they are formed stereoselectively. Complex 2a (R = phenyl) undergoes chloride abstraction with [Et3SiHSiEt3]­[B­(C6F5)4], yielding [Ti­(NPN′)­(NPhPPh2)]­[B­(C6F5)4] (3). Complex 3 is a moderately active and stereoselective initiator for the ring-opening polymerization of rac-lactide. Complex 3 activates the CO bond of 4-iodobenzaldehyde to give complex 4 as a single diastereomer despite the presence of three chiral centers. Complex 3 undergoes transmetallation with SbCl3, yielding [Sb­(NPN′)]­[B­(C6F5)4] (5) and [TiCl3(NPhPPh2)] (6) selectively. The bonding situation in 3 and 5 was analyzed using Bader’s atoms in molecules and the electron localization function, showing that the nitrogen atoms of the NPN′ ligand are electronically similar, and that the metal–phosphide interaction is more polar in the case of titanium.