Ab initio modeling of the endohedral reactivity of polyoxometallates .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 investigated by means of ab initio Hartree-Fock calculations on the model 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 analysis shows that most of the stabilization should be attributed to an electrostatic origin, including direct coulombic interaction, polarization of the guest molecule and counter polarization of the vanadate host. The stabilization energy due to orbital interactions and charge transfer is increasing with the acceptor potentialities of the R substituent: H < CH3 < C6H5 Mulliken population analysis indicates that two opposite charge flows should be considered for R not equal H: (i) a net donation from the cyanide pi orbitals to the surrounding V-V d shells and (ii) a back donation from the eight oxygen atoms at the rim of the vanadate basket to the acceptor orbitals of R. Both electrostatic and charge transfer stabilization energies should therefore be attributed to the dual potentialities (acidic/basic; donor/acceptor) of the R-CN molecules duly activated by a convenient positioning with respect to the 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 relatively acidic character of the accessible region located inside the vanadate 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 class of hollow polyoxometallate clusters could provide a mechanism explaining the stabilization and the self-assembly of ''electronically inverse'' host-guest systems.