Ethyleneglycol tungsten complexes of calix[6 and 8]arenes: synthesis, characterization and ROP of ε-caprolactone.

By varying the reaction conditions, the reaction of [W(eg)3] (eg = 1,2-ethanediolato) with p-tert-butylcalix[n]areneHn (n = 6 or 8) in refluxing toluene affords, following work-up, a number of products which have been fully characterized. From the reaction of p-tert-butylcalix[6]areneH6 with one or two equivalents of [W(eg)3], only the oxo-bridged complex {[W(eg)]2(μ-O)p-tert-butylcalix[6]arene} (1) could be isolated, whereas the use of four equivalents of [W(eg)3], in the presence of molecular sieves, afforded {[W(eg)2]2p-tert-butylcalix[6]areneH2}·2MeCN (2); molecules of 2 pack in bi-layers. Under similar conditions, use of one or two equivalents of [W(eg)3] and p-tert-butylcalix[8]areneH8 afforded {[W(eg)]2p-tert-butylcalix[8]arene}·MeCN (3) in which each tungsten centre was bound by four calixarene oxygens. By contrast, the small orange prisms resulting from the use of four equivalents of [W(eg)3] and p-tert-butylcalix[8]areneH8 were shown by synchrotron radiation to be a mixture of two isomers 4a/4b·3.5MeCN). In the major isomer {1,2-[W(eg)2]2p-tert-butylcalix[8]areneH4} (4a), two tungsten centres bind to neighbouring sets of phenolate oxygens, whereas in the minor isomer {1,3-[W(eg)2]2p-tert-butylcalix[8]areneH4} (4b), there is a protonated phenolic group between the two pairs of phenolate oxygens bound to tungsten; the major : minor ratio is about 83 : 17. Use of p-tert-butyltetrahomodioxacalix[6]areneH6 with two equivalents of [W(eg)3] resulted in the isolation of {[WO(eg)]2p-tert-butyltetrahomodioxacalix[6]areneH2} (5·0.83toluene·MeCN), in which each dimethyleneoxa bridge is bound to an oxotungsten(vi) centre. Complexes 1-5, together with the known complex [W(eg)p-tert-butylcalix[4]arene] (6), have been screened for their ability to ring open polymerize (ROP) ε-caprolactone; for 1, 2 and 5, 6 conversion rates were good (>88%) at 110 °C over 12 or 24 h, whereas the calix[8]arene complexes 3 and 4 under the same conditions were inactive.