Density functional study of tetraphenolate and calix[4]arene complexes of early transition metals

Density functional calculations have been performed on some calix[4]arenes complexes of early transition metals. Particular emphasis has been placed o n the comparison of the main properties of these metal complexes with the a nalogous metal complexes based on four monodentate phenolate ligands to stu dy the effect of the geometrical constraints imposed by the calixarenes fra mework on the electronic structure. The results show that the most stable g eometry of titanium and molybdenum tetraphenolates is pseudotetrahedral (sl ightly flattened for molybdenum) and that the distortion to a square planar coordination requires, respectively, 52.0 and 21.5 kcal mol(-1). However, a significant energy decrease is found when the four phenolate groups are b ent in the same hemisphere, reproducing the calix[4]arene geometry. Such a coordination determines the energy decrease of the unoccupied metal d orbit als of sigma and pi symmetry, which leads to an increase of the electron-ac cepting properties of these metal fragments.