Activated Zinc Complexes Supported by a Neutral, Phosphinimine-Containing Ligand: Synthesis and Efficacy for the Polymerization of Lactide

Abstract

A general approach for the synthesis of cationic zinc complexes supported by neutral ancillary ligands has been developed and exploited to prepare [LH][B(C6F5)4] (L = 4-(2,6- i Pr2-C6H3NPPh2)dbf), 1a, and [LH][SO3CF3], 1b) using [HNMe2Ph][B(C6F5)4] and triflic acid, respectively. Reactions of 1a and 1b with diethylzinc afforded the corresponding complexes [LZnEt][B(C6F5)4], 2a, and [LZnEt(OSO2CF3)], 2b. Both zinc complexes have exhibited notable activities for the catalytic polymerization of l-lactide.

Synthesis and Single Crystal X‐ray Structures of Cationic Zinc β‐Diketiminate Complexes

“…Structurally characterized cationic complexes containing coordinatively saturated zinc atoms are well known,17 whereas the number of structurally characterized threefold coordinated cationic zinc complexes was limited for a long time to Bochmann cationic zinc diazadiene complexes (Scheme , type I ). [15b, f] Recently, Hayes et al synthesized threefold coordinated cationic zinc complexes based on mono(phosphinimine)dibenzofuran and bis(phosphinimine)dibenzofuran pincer‐type ligands (type II ), which also contained weakly coordinating anions 18,19. Unfortunately, these cationic alkylzinc complexes of the type [LZnR] + (R = Me, Ph) were found to be almost inactive for the polymerization of lactide.…”

Synthesis and Structural Diversity of ZnCl2 and Zn(C6F5)2 Complexes of a Phosphinimine Ligand

“…Two derivatives of this architecture have been utilized, where the choice of N‐aryl group was either 2,4,6‐trimethylphenyl (Mes), L Mes , or 2,6‐di iso propylphenyl (Dipp), L Dipp , which vary significantly in steric bulk. The preparation of L Dipp was reported in a previous study,7e but this is the first account of the smaller analogue L Mes (Scheme ).…”

“…This afforded the ligand in 69 % yield as an analytically pure powder that is indefinitely stable at ambient temperature when stored in an inert atmosphere. The diagnostic 31 P{ 1 H} NMR resonance appears at δ = –15.3, which is upfield of the corresponding L Dipp resonance by approximately 2 ppm 7e. While the aromatic region of the 1 H NMR spectrum is complicated, the mesityl groups provide excellent spectroscopic handles, with singlet signals appearing at δ = 2.30 and 2.21 ppm for the para ‐ and ortho ‐CH 3 groups, respectively.…”

“…Single crystals of L Mes were obtained and the crystal structure was determined (Figure 1), which serves to verify the connectivity of the molecule. The P–N bond length is in the expected range for a formal bond order of two [P(1)–N(1) = 1.547(3) Å], and is similar to that measured in the previously reported crystal structure of L Dipp [1.559(2) Å],7e as well as other neutral phosphinimines 9.…”

“…While the C 6 F 5 substituents of complexes 3 and 4 are not particularly amenable to derivatization, attempts were made to remove a C 6 F 5 group using the protic salt [HNMe 2 ][B(C 6 F 5 ) 4 ]. It was found, however, that upon reaction of 3 with this reagent, the acidic proton was simply transferred to the phosphinimine nitrogen with displacement of Zn(C 6 F 5 ) 2 ,resulting in formation of the previously reported compound [ L Dipp H][B(C 6 F 5 ) 4 ] 7e. Reaction of complex 4 with this same reagent produced a similar result.…”