From Lindqvist and Keggin ions to electronically inverse hosts: Ab initio modelling of the structure and reactivity of polyoxometalates

This review reports the ab initio Hartree-Fock and DFT calculations which h ave been carried out recently in our two groups in order to investigate the electronic structure of polyoxometalate clusters, either totally oxidized like (V10O28)(6-) or partly reduced such as [PMo12O40(VO)(2)](5-). The appr oach of protons or cationic groups to the external coating of oxygen atoms and their preferred site of fixation can be predicted from the topology of the computed distribution of molecular electrostatic potentials (MEP). The MEP distribution can also be used to get a better understanding of the form ation of inclusion and encapsulation complexes. Hemispherical carcerands li ke (V12O32)(4-) develop a dipolar field in the accessible part of their cav ity susceptible to attract small molecules with a permanent dipole like RCN (R=CH3, C6H5). The "electronically inverse" host anions known to encapsula te anionic species are formed in solution by means of a template mechanism which tends to maximize the electrostatic potential at the place of the gue st anion. A correlation is provided between the topology of the host and it s MEP distribution which explains, from simple geometric considerations, th e differences between electronically normal and electronically inverse host s, and shows that the host cage tends to get adapted not only to the shape of the guest molecule, but also to its electrostatic potential distribution . (C) 1998 Elsevier Science S.A. All rights reserved.