Chromium Amino-bis(phenolate) Complexes as Catalysts for Ring-Opening Polymerization of Cyclohexene Oxide

The chromium­(III) amino-bis­(phenolate) “ate” complex, {Na­[CrCl2L1]}, 1, where L1 = 6,6′-((1,4-diazepane-1,4-diyl)­bis­(methylene))­bis­(2,4-di-tert-butylphenolato) and its derivatives 2–4 (where the phenolate substituents and the diamino linker were varied) were prepared. Single crystals obtained by recrystallization of 1 were shown by X-ray diffraction to be a related octahedral complex, 1′, where one of the O-donors in 1′ is protonated, resulting in a monoanionic ligand and affording a neutral Cr­(III) dichloride complex. The ligand [L1] chelates in a meridional fashion and anionic chloride donors reside in a trans axial orientation. Cr­(III) amino-bis­(phenolate) complexes 1–4 showed good activity toward ring-opening polymerization (ROP) of cyclohexene oxide (CHO) without the use of an added cocatalyst, producing high-molecular weight poly­(cyclohexene oxide) where 1 was the most active catalyst. Arrhenius data obtained for the ROP of CHO using 1 revealed an activation barrier of 64.0 ± 9.8 kJ mol–1 and the nonlinear polymer molecular weight growth versus conversion relationship suggests a step-growth polymerization mechanism.