Amido phosphine complexes of zinc: synthesis, structure, and catalytic ring-opening polymerization of ε-caprolactone

Abstract

A series of diarylamido phosphine ligands of the type N-(2-dihydrocarbylphosphinophenyl)-2,6-dialkylanilide 1a-d have been prepared and employed to investigate the coordination chemistry of zinc. Protonolysis of ZnMe2 with one equivalent of N-(2-diphenylphosphinophenyl)-2,6-dimethylaniline (H[1a]) produced a mixture of [1a]ZnMe (2a) and Zn[1a]2 (4a), whereas that involving ZnEt2 gave exclusively the three-coordinate [1a]ZnEt (3a). In contrast, treatment of ZnR2 (R = Me, Et) with N-(2-diphenylphosphinophenyl)-2,6-diisopropylaniline (H[1b]), N-(2-diisopropylphosphinophenyl)-2,6-dimethylaniline (H[1c]), or N-(2-diisopropylphosphinophenyl)-2,6-diisopropylaniline (H[1d]) under similar conditions generated quantitatively the corresponding three-coordinate zinc methyl 2b-d and zinc ethyl 3b-d. The bis-ligand complexes 4a,b,d were isolated by either protonolysis of alkyls 2-3 with one equivalent of H[1] or metathesis of ZnX2 (X = Cl, OAc) with the corresponding lithium derivatives 5. Attempts to prepare [1a-d]ZnX (X = Cl, OAc) were not successful regardless of stoichiometry of the starting materials employed. Alcoholysis of zinc alkyls 2-3 led undesirably to protonation on the amido nitrogen donor of 1, highlighting perhaps its higher basicity than alkyls. The reaction of ZnCl2 with H[1c] generated the phosphorus-bound adduct {H[1c]ZnCl(mu-Cl)}2 (6c). Interestingly, attempts to deprotonate 6c with n-BuLi produced unexpectedly the alkylated product [1c]Zn(n-Bu) (7c) instead of [1c]ZnCl; analogous reactions employing NEt3 led to Lewis base substitution to give H[1c] and [ZnCl2(NEt3)]2. Structural characterization of all new compounds was achieved by multi-nuclear NMR spectroscopy (1H, 13C, 31P, and 7Li) and X-ray crystallography (2c-d, 3c, 4d, 5c-d, and 6c) where appropriate. On the basis of the NMR and X-ray data, in combination with the synthetic investigations, the steric nature of these amido phosphine ligands is recognized to follow the order of 1a < 1b < 1c < 1d. Interestingly, zinc alkyls 2-3 are all active initiators for catalytic ring-opening polymerization of ε-caprolactone whereas the bis-ligand complexes 4 are not.

Coordination Chemistry and Reactivity of Zinc Complexes Supported by a Phosphido Pincer Ligand

“…[40,41] The crystal structure of 3 is shown in Figure 1. Zinc displays a nearly trigonal-planar coordination geometry (the sum of the bond angles about the metal center is close to 3608) [32,34,42,43] with three phosphorus atoms (P2, P3, and P1) belonging to two different PPP molecules acting as bidentate and monodentate ligands, respectively. This coordination geometry is quite distorted due to the constraint imposed by the bidentate pincer ligand (P2-Zn1-P3 85.76(6)8).…”

“…[30,31] Since phosphine derivatives of group 12 metals are quite rare, we decided to extend the use of this class of ligands to the coordination chemistry of zinc. Some examples of crystallographically characterized zinc complexes were reported by Darensbourg et al [32] and more recently by Liang et al [33,34] In all cases phosphine donors are in combination with hard phenoxide or amide additional donors. In this context, the bis(2-diphenylphosphinophenyl)phosphine pincer ligand shows an original coordination environment in which an unusual phosphido anionic donor is flanked by phosphine neutral donors.…”

Synthesis, Structure, and Ring‐Opening Polymerization Catalysis of Zinc Complexes Containing Amido Phosphinimine Ligands

“…[51,52] No sign of the formation of the bis-2 complex was detected (NMR spectroscopy evidence) even at elevated temperatures (e.g. 100°C), a result that is markedly different from that found with 1, [42,43] but ascribable to the increased steric hindrance for the amido phosphinimine ligands because of the larger chelating rings. The formation rates of 3-4 appear to increase in the order a Ͻ b Ͻ c, consistent with that expected from a steric standpoint.…”

“…The preparation of three-coordinate zinc complexes that are free of coordinating solvents is of current interest. [14,42,43,[52][53][54][55][56][57] The successful isolation of 3c and 4c from THF solutions is noteworthy in view of the relatively small steric size of 2c. In general, these alkyls are thermally stable under an inert atmosphere, even at elevated temperatures (100°C, Ͼ24 h), but extremely sensitive to moisture, producing exclusively H [2] as indicated by 31 …”

“…[42] Herein, we describe the synthesis, structural characterization, and ROP activities of zinc amido phosphinimine complexes. Differences in the structure and reactivity of these compounds relative to those of their parent amido phosphane [42,43] and β-diketiminate derivatives [12,13] are discussed.…”