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.

氨基双(酚盐)合铬(III)‘ate’配合物{Na[CrCl₂L¹]}（记为1），其中L¹=6,6′-((1,4-二氮杂环庚烷-1,4-二基)双(亚甲基))双(2,4-二叔丁基苯酚根)，及其衍生物2~4（其酚盐取代基与二氨基连接臂均发生改变）已被成功合成。通过对1进行重结晶得到的单晶经X射线衍射（X-ray diffraction）表征，确认为一种相关的八面体配合物1'：其中1'的一个O供体位点被质子化，由此生成单阴离子配体，并得到中性的二氯化铬(III)配合物。配体[L¹]以面式（meridional）方式螯合，阴离子氯离子供体则处于反式轴向取向位置。氨基双(酚盐)合铬(III)配合物1~4在无需添加助催化剂的条件下，对环氧环己烷（cyclohexene oxide, CHO）的开环聚合（ring-opening polymerization, ROP）展现出良好的催化活性，可制备高分子量聚环氧环己烷，其中配合物1的催化活性最高。采用配合物1催化环氧环己烷开环聚合得到的阿伦尼乌斯（Arrhenius）数据显示，其活化能垒为64.0 ± 9.8 kJ·mol⁻¹；聚合物分子量随转化率的非线性增长关系表明该聚合反应遵循逐步增长聚合机理。