Ab initio MO study of silver ion complexation in [2.2.2]cyclophane pi-prismands

Ab initio Hartree-Fock and DFT MO calculations have been used to study the conformations of [2,2,2]cyclophane pi-prismands and the formation of ct-com plexes with silver ion and [2.2,2]cyclophanes. The lowest energy cyclophane conformations have been calculated up to the HF/6-31+G* level of theory. T he silver pi-complexes of the lowest energy conformations have been calcula ted with HF/3-21G* and B3LYP/3-21G* levels of theory. The nature of bonding in silver ion pi-complexes has been studied with natural bond orbital anal ysis (NBO), Energies of the calculated cyclophanes and complexes, together with formation energies of those complexes, have also been discussed. The r esults obtained have been compared to X-ray crystal structures whenever suc h structures were available. Calculated and experimental X-ray structures a greed reasonably well given provision to the rather small 3-21G* basis set and the omission of triflate anion in the calculation model. The NBO analys is showed that when Ag+ is bonded to the cyclophane cavity, the bonds are f ormed by sigma-donation and d-pi*-back-donation between the silver ion and the hydrocarbon skeleton, resulting in hexahapto (3 x eta(2)) overall pi-bo nding. This is in agreement with the well-known "bonding-back-bonding" sche me in transition metal carbonyl complexes. In this case the sigma-donation from hydrocarbon to silver ion is the main contribution to the metal-cyclop hane bonding. One dihapto (eta(2)) bonding to one aromatic ring in the pres ent pi-prismands relates by strength to a single strong hydrogen bond, whic h is up to 100 kJ/mol; thus the strength of this 3 x eta(2) bonding in thes e [2,2.2]cyclophane pi-prismands can be compared to three simultaneous stro ng hydrogen bonds.