Modeling of the Tridentate Amorphous Silica Ligand

Abstract

The 3-fold coordination of NiII cations to amorphous silica is modeled performing density functional theory (DFT) calculations on framework model clusters of increasing size. Using the nT notation of zeolite structures where n is the number of T atoms (T = Si or Al), four model clusters with the following structures are investigated: (i) (Si2O3H4)2- (2T) is made of two vicinal silanolate groups (SiO-) bonded through an oxygen bridge. The valence requirements of silicium are satisfied by addition of terminal hydrogen atoms. (ii) (Si3O6H4)2- (3T) is a six-membered ring made of two silanolate groups and one silanol group (SiOH) bonded through oxygen bridges. (iii) (Si4O7H6)2- (4T) is an eight-membered ring with two vicinal silanolate groups in positions 1 and 3 and a silanol group in position 5, and (iv) (Si5O8H8)2- (5T) is a flexible ten-membered ring with two vicinal silanolate groups in positions 1 and 3 and an isolated silanol group in position 7. DFT calculations are performed in order to estimate the ability of each model cluster to reproduce the experimental characteristics of previously described silica-supported NiII(O)3 species: (i) three-coordinated NiII of distorted C3v close to D3h symmetry and (ii) Ni−O distances in agreement with EXAFS measurements. (Si5O8H8)2- is preferred to the (Si2O3H4)2- model proposed earlier. Because of its flexibility, the larger framework model is able to best reproduce the experimental geometry of the NiII site. This model cluster may be assimilated to two vicinal silanolates and one neighboring isolated silanol or siloxane bridge on the real silica surface.