Zinc 8-aminotrihydroquinolines appended with pendant N-diphenylphosphinoethyl arms as exceptionally active catalysts for the ROP of epsilon-CL

Five types of N-(2-(diphenylphosphino)ethyl)-2-R-5,6,7,8-tetrahydroquinolin-8-amines (R = H L1, Me L2, iPr L3, Cl L4, Ph L5) along with one example of an N-isopropyl-5,6,7,8-tetrahydroquinolin-8-amine (L6) have been prepared and employed as neutral ligand supports for zinc(II) chloride complexes, LZnCl2 (Zn1–Zn6). All zinc compounds were characterized by multinuclear NMR spectroscopy (1H/13C/31P) and elemental analysis, while the structures of Zn2 and Zn5 were further studied by single crystal X-ray diffraction: distorted tetrahedral geometries with the diphenylphosphine moieties non-coordinated were a feature. Pre-treatment of Zn1–Zn5 with two equivalents of either LiCH2SiMe3 or LiN(SiMe3)2 led to catalysts displaying good to exceptionally high activity for the ring opening polymerization (ROP) of ε-caprolactone (ε-CL) at 50 °C. For example, Zn4/2LiCH2SiMe3 displayed a TOF of 2.1 × 104 h−1 with 71% conversion seen within 10 minutes with the ε-CL : Zn molar ratio at 5000 : 1; the resulting polycaprolactone (PCL) comprised predominately cyclic structures along with lower levels of trimethylsilylmethyl-capped linear PCL. By comparison, Zn1–Zn5 on treatment with two equivalents of LiN(SiMe3)2 at the same high monomer to zinc ratio, led to catalysts exhibiting an order of magnitude higher activity. Zn1/2LiN(SiMe3)2 in the presence of BnOH exhibited a remarkable TOF of 1.35 × 105 h−1 with 90% monomer conversion within two minutes. In these cases, a distinct selectivity towards linear polymers capped with CH3O end-groups (derived from the quenching solvent) was observed with cyclic polyesters only observable with larger amounts of ε-CL. As a general observation, the 2-R substituent on the chelating ligand showed noticeable effects on the efficiency and on the molecular weight of the PCLs, with 2-Cl-containing Zn4 in combination with LiCH2SiMe3 and BnOH affording the highest molecular weight PCL of the study (9.57 × 104 g mol−1). Based on the microstructural properties of the PCL, distinct mechanisms (viz., coordination–insertion and ring expansion) have been proposed to account for the mode of propagation mediated by Zn/2LiCH2SiMe3 and Zn/2LiN(SiMe3)2. In addition, a zinc silylamido L1−HZn[N(SiMe3)2] (Zn7) species has been tentatively assigned (obtained via deprotonation of L1 with Zn[N(SiMe3)2]2), that proved inactive for the ROP of ε-CL but displayed good activity for ROP of rac-LA.