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

Abstract

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.

INSIGHTS into the structures adopted by titanocalix[6 and 8]arenes and their use in the ring opening polymerization of cyclic esters

“…4). In one molecule there is a ladder structure, like 4, made up of four Ti(IV) ions and phenolate oxygens (from the L 2 ligand) and bridging oxygens, O(3), O(7), O (9), and O (10). The latter two are oxo dianions.…”

“…In the case of tungstocalix [6 and 8]arenes, we observed how different sized calixarene rings and their associated conformations can drastically affect the catalytic activity for the ROP of ε-caprolactone (ε-CL). 10 More recently, McIntosh et al reported preliminary studies on the use of the complex [Ti 4 (L 2 ) (On-Pr) 8 (THF) 2 ] as a catalyst for the ROP of rac-lactide (r-LA) at 130°C. 11 Other titanocalix[n]arene work in this area employs the de-tert-butylated n = 4 system (1,3-di-n-propylcalix[4]arene), with well-behaved ROP of rac-lactide observed when employing either microwave radiation or heat; the former method was beneficial to the rate of polymerization at the expense of control.…”

Use of titanocalix[4]arenes in the ring opening polymerization of cyclic esters

“…10 We have also investigated the use of metallocalix[n]arenes for the ROP of cyclic esters such as ε-caprolactone, and have reported how, for a series of tungstocalix [6 and 8]arenes, different sized rings and their associated conformations can drastically affect the catalytic activity. 11 Molybdo-and tungstocalix [4] arenes were found to be less active. 12 Herein, we have screened several titanocalix [4]arenes, namely [TiCl 2 LIJO) 2 -IJOR) 2 ] (type I: R = Me (1), n-Pr (2) and n-pentyl (3)), the dimeric compound {[TiLIJO) 3 IJOR)] 2 IJμ-Cl) 2 } (R = n-decyl (4)), and [TiIJNCMe)ClIJLIJO) 3 IJOR))]•MeCN (type II, R = Me, 5•MeCN, R = n-propyl 6 and n-pentyl 7) for the ROP of ε-CL, δ-VL and r-LA (the copolymerization of ε-CL and r-LA was also investigated) (Chart 1).…”

Use of Metal Catalysts Bearing Schiff Base Macrocycles for the Ring Opening Polymerization (ROP) of Cyclic Esters

“…In order to synthesize such bimetallic catalysts, suitable pro-ligand frameworks need to be available, whose coordination chemistry is well suited to binding more than one metal simultaneously in close proximity. Macrocycles are ideal in this respect, and we and others have had some success with the use of calix[n]arenes in ROP [4][5][6][7][8][9].…”