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 on the comparison of the main properties of these metal complexes with the analogous metal complexes based on four monodentate phenolate ligands to study the effect of the geometrical constraints imposed by the calixarenes framework on the electronic structure. The results show that the most stable geometry of titanium and molybdenum tetraphenolates is pseudotetrahedral (slightly flattened for molybdenum) and that the distortion to a square planar coordination requires, respectively, 52.0 and 21.5 kcal x mol(-1). However, a significant energy decrease is found when the four phenolate groups are bent in the same hemisphere, reproducing the calix[4]arene geometry. Such a coordination determines the energy decrease of the unoccupied metal d orbitals of sigma and pi symmetry, which leads to an increase of the electron-accepting properties of these metal fragments.