Coordination chemistry of the calix[6]arene system: application in polymerisation catalysis and beyond

This perspective looks at structurally characterized examples of metal-based calix[6]arene complexes, focusing on the coordination chemistry adopted and their applications, primarily in polymerisation catalysis. In particular, we look at our attempts to utilize the ability of the calix[6]arene system to accommodate multiple metal centres, and where appropriate, we compare the catalytic behaviour with that of other metallocalix[n]arenes formed from commercially available calix[n]arenes, i.e. the smaller but more common calix[4]arene and the larger calix[8]arene system. We also discuss how synthetic efforts are now targeting calix[6]arenes for which a change of the bridge can provide additional possibilities, i.e. heteroatom bridged tetrahomodioxacalix[6]arenes (containing -CH₂OCH₂- bridges) and tetrahomo diazacalix[6]arenes (containing -CH₂N(R)CH₂- bridges, R typically is methyl). The outlook for the future, in terms of the use of calix[6]arene-based catalysts is also discussed.