AB-INITIO MODELING OF THE ENDOHEDRAL REACTIVITY OF POLYOXOMETALATES .1. HOST-GUEST INTERACTIONS IN [RCN-SUBSET-OF(V12O32)(4-)] (R=H, CH3, C6H5)

The origin of the host-guest interaction energy between R-CN molecules (R = CH3, C6H5) and the basket-like cavitand (V12O32)(4-) has been in vestigated by means of ab initio Hartree-Fock calculations on the mode l system [HCN subset of(V12O32)(4-)] and on the two observed complexes . The computed stabilization energies range from -12.8 to -14.4 kcal . mol(-1) for the three considered systems. Decomposition energy analys is shows that most of the stabilization should be attributed to an ele ctrostatic origin, including direct coulombic interaction, polarizatio n of the guest molecule and counter polarization of the vanadate host. The stabilization energy due to orbital interactions and charge trans fer is increasing with the acceptor potentialities of the R substituen t: H < CH3 < C6H5 Mulliken population analysis indicates that two oppo site charge flows should be considered for R not equal H: (i) a net do nation from the cyanide pi orbitals to the surrounding V-V d shells an d (ii) a back donation from the eight oxygen atoms at the rim of the v anadate basket to the acceptor orbitals of R. Both electrostatic and c harge transfer stabilization energies should therefore be attributed t o the dual potentialities (acidic/basic; donor/acceptor) of the R-CN m olecules duly activated by a convenient positioning with respect to th e vanadate sites with opposite properties. The complementarity of the electrostatic properties of the host and guest molecules is evidenced by the computed distributions of the electrostatic potential. The rela tively acidic character of the accessible region located inside the va nadate host, as compared to the external, basic side is attributed to the topological features defining concavity. A generalization of that correlation between concavity/convexity and acidity/basicity to the cl ass of hollow polyoxometallate clusters could provide a mechanism expl aining the stabilization and the self-assembly of ''electronically inv erse'' host-guest systems.