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

The chromium(III) amino-bis(phenolate) "ate" complex, {Na[CrCl(2)L1]}, 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.