Reaction of the pro-ligand ?,?,?’,?’-tetra(3,5-di-tert-butyl-2-hydroxyphenyl-p-)xylene-para-tetraphenol
(p-L1H4) with two equivalents of [NbCl5] in refluxing toluene afforded, after work-up, the complex
{[NbCl3(NCMe)]2(?-p-L1)}·6MeCN (1·6MeCN). When the reaction was conducted in the presence
of excess ethanol, the orange complex {[NbCl2(OEt)(NCMe)]2(?-p-L1)}·312
MeCN·0.614toluene
(2·312
MeCN·0.614toluene) was formed. A similar reaction using [TaCl5] afforded the yellow complex
{[TaCl2(OEt)(NCMe)]2(?-p-L1)}·5MeCN (3·5MeCN). In the case of the meta pro-ligand, namely ?,?,?’,?’tetra
(3,5-di-tert-butyl-2-hydroxyphenyl-m-)xylene-meta-tetraphenol (m-L2H4) only the use of [Nb(O)-
Cl3(NCMe)2] led to the isolation of crystalline material, namely the orange bis-chelate complex {[Nb-
(NCMe)Cl(m-L2H2)2]}·312
MeCN (4·312
MeCN) or {[Nb(NCMe)Cl(m-L2H2)2]}·5MeCN (4·5MeCN). The molecular
structures of 1–4 and the tetraphenols L1H4 and m-L2H4·2MeCN have been determined. Complexes
1–4 have been screened as pre-catalysts for the ring opening polymerization of ?-caprolactone, both
with and without benzyl alcohol or solvent present, and at various temperatures; conversion rates were
mostly excellent (>96%) with good control either at >100 °C over 20 h (in toluene) or 1 h (neat).