Density functional analysis of the trigonal uranyl equatorial coordination in hexahomotrioxacalix[3]arene-based macrocyclic complexes.

Representative models of the different geometries observed for uranyl complexes with fully deprotonated p-R-hexahomotrioxacalix[3]arene (R = tert-Bu, Me) ligands, among which is the rare trigonal geometry, have been investigated using all-electron scalar relativistic density functional theory (DFT). Optimized structures of complexes incorporating triethylammonium (HNEt3+) and 4-methylpiperidinium (HMePi+) cations are in close agreement with experimental crystal diffraction data. Possible explanations for the structural differences between these uranyl complexes are discussed in terms of varying degrees of bonding between uranium and oxygen atoms from the ether and phenoxide groups. In particular, molecular orbital analysis highlights the central role of 5f-2p hybridization in the U-O bonding.