Enhanced Long-Range Si···N Interactions in Organosilicon Cations. A Theoretical Study

Abstract

For the model organosilicon cation H3N···SiH3+, potential curves have been calculated by density functional theory at the B3LYP level, by both MP2 and MP4 perturbation procedures and by a Coupled Cluster approach. Relative to the analogous potential curve for the uncharged adduct, H3N···SiH4, a considerable increase of the long-range Si···N interaction accompanied by bond shortening is predicted. For chemical comparison, the structure of the "double R2N-twister" dication salt, [C6H4(CH2NR2)2-Si-(R2NCH2)2C6H4]++Cl2-, reported in the literature as a non-crystalline solid, was optimized by DFT calculations and a minimum for the hexacoordinated organosilicon center with each two Si···N bonds of different lengths was located. To test the reliability of the quantum chemical procedure, in addition the analogous and structurally characterized dication salt, [(3-picoline)4SiH2]++[Cl-···(HCCl3)2]2, which contains a hexacoordinated Si center with 4 equivalent Si···N interactions, has been selected to reproduce its structure by DFT optimization. Both hexacoordinated, tetra-N-surrounded Si dications, substantiate the increase of long-range Si···N interactions as predicted by the potential curves for the model adduct H3N···SiH3+ based on highly correlated calculations.