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

Abstract

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′)­(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.

PCP Pincer Complexes of Titanium in the +3 and +4 Oxidation States

“…In an attempt to isolate a low-coordinate Ti­(III) pincer complex, halide abstraction on 2 was attempted. In order to achieve this, [(SiEt 3 ) 2 (μ-H)]­[BAr F 4 ] (Ar F = C 6 F 5 ) was usedthis has been used to great effect to abstract halides from particularly strong M–Cl bonds (e.g., lanthanides). − Addition of [(SiEt 3 ) 2 (μ-H)]­[BAr F 4 ] to 2 in benzene resulted in the immediate formation of a brown oil, which could be extracted with fluorobenzene as a deep brown-red solution. A structural analysis found that only half an equivalent of chloride had been abstracted and a dimeric species could be isolated in the solid state, [(( tBu PCP)­TiCl) 2 (μ-Cl)]­[BAr F 4 ] ( 3 , Scheme ).…”

PCP Complexes of Titanium in the +3 and +4 Oxidation State

“…lanthanides). [26][27][28][29][30][31] Addition of [(SiEt3)2(µ-H)][BAr F 4] to 2 in benzene resulted in the immediate formation of a brown oil, which could be extracted with fluorobenzene as a deep brown-red solution. Structural analysis found that only half an equivalent of chloride had been abstracted and a dimeric species could be isolated in the solid state, [(( tBu PCP)TiCl)2(µ-Cl)][BAr F 4] (3, Scheme 3).…”

“…1 H NMR (C6D6): 6.90 (m, 1 H, Arp-H), 6.69 (d, 2 JHH = 5.7 Hz, 1 H, Ar-H), 3.37 (s, 4 H, PCH2), 1.28 (s, 36 H, CCH3). 31…”

“…The crude product could be dissolved in fluorobenzene (0.5 mL) and layered with hexane to afford brown needle single crystals of [{( tBu PCP)TiCl}2(μ-Cl)][BAr F 4] suitable for x-ray diffraction. No 1 H or 31…”