Shaping the cavity of the macrocyclic ligand in metallocalix[4]arenes: The role of the ligand sphere

The coordination form of calix[4]arene ligands and therefore the cavity of the macrocyclic ligand can be controlled by other ligands in transition metal calix[4]arene complexes, if strong directing coligands such as oxo groups are used. This paper describes the synthesis and characterization of the d(0) transition metal complexes [Cax(OMe)(2)O2TiCl2] I (monoclinic, space group P2(1)/c, lattice constants a = 21.639(4), b = 20.152(3), c = 12.750(3) Angstrom, beta = 95.68(3), V = 5532.6(19) Angstrom (3)) and [Cax(OMe)(2)O2MoO2] 2 (monoclinic, space group P2/c, lattice constants a = 12.433(3), b = 16.348(3), c = 24.774(5) Angstrom, beta = 99.15(3), V = 4971.6(17) Angstrom (3)). Whereas in I the calix[4]arene ligand adopts an elliptically distorted cone conformation, the macrocyclic ligand binds in a paco-like conformation to the metal center of 2, in the solid state and in solution. This was predicted by density functional theory calculations on models of different isomers of 1 and 2: cis,cone-1',2', trans,cone-1',2', and cispaco-1',2'. According to these calculations, the energetic difference of 72.9 kJ/mol between both cis-dioxomolybdenum compounds is quite pronounced in favor of the cis,paco isomer, and 28.0 kJ/mol for the titanium compounds in favor of the cis,cone isomer.