Titanium complexes bearing oxa- and azacalix[4, 6]arenes: structural studies and use in the ring opening homo-/co-polymerization of cyclic esters.

Abstract

Reaction of excess [Ti(OiPr)4] with p-tert-butyltetrahomodioxacalix[6]areneH6 (L1H6) afforded, after work-up (MeCN), the complex [Ti2(OiPr)2(MeCN)L1]·3.5MeCN (1·3.5MeCN), whilst the oxo complex [Ti4(μ3-O)2(H2O)(L1)2]·MeCN (2·MeCN) was isolated via a fortuitous synthesis involving the use of two equivalents of [Ti(OiPr)4]. Reactions of p-methyl-dimethyldiazacalix[6]areneH6 (L2H6) with [TiF4] (four equivalents), [TiCl4(THF)2] (two equivalents) or [TiBr4] (>four equivalents) resulted in the titanium-based azacalix[n]arene complexes [Ti4F14L2H2(H)2]·2.5MeCN (3·2.5MeCN), [Ti2X4(H2O)2OL2H2(H)2] (X = Cl (4·5MeCN), Br (5·4.5MeCN) and [Ti4Br12L2(H)2(MeCN)6]·7MeCN (6·7MeCN), respectively. Reaction of four equivalents of [TiF4] with L3H4 (L3H4 = p-methyl-dimethyldiazacalix[4]areneH4) afforded the product [Ti2F2(μ-F)3L3(H)2(SiF5)]·2MeCN (7·2MeCN). These complexes have been screened for their potential to act as pre-catalysts in the ring opening polymerization (ROP) of ε-caprolactone (ε-CL), δ-valerolactone (δ-VL) and rac-lactide (r-LA). Generally, the titanium complexes bearing oxacalixarene exhibited better activities than the azacalixarene-based pre-catalysts. For ε-CL, δ-VL and r-LA, moderate activity at 130 °C over 24 h was observed for 1-6. In the case of the co-polymerization of ε-CL with r-LA, 1-6 afforded reasonable conversions and high molecular weight polymers; 7 exhibited lower catalytic performance due to low solubility. None of the complexes proved to be active in the polymerization of ω-pentadecalactone (ω-PDL) under the conditions employed herein.