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

Ti-imido complex [TiCl(N t Bu)(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')(NPhPPh 2 )] ( 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 [Et 3 SiHSiEt 3 ][B(C 6 F 5 ) 4 ], yielding [Ti(NPN')(NPhPPh 2 )][B(C 6 F 5 ) 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 SbCl 3 , yielding [Sb(NPN')][B(C 6 F 5 ) 4 ] ( 5 ) and [TiCl 3 (NPhPPh 2 )] ( 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.