Modeling of the tridentate amorphous silica ligand

The 3-fold coordination of Ni-II cations to amorphous silica is modeled per forming density functional theory (DFT) calculations on framework model clu sters 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 fo llowing structures are investigated: (i) (Si2O3H4)(2-) (2T) is made of two vicinal silanolate groups (SiO-) bonded through an oxygen bridge. The valen ce requirements of silicium are satisfied by addition of terminal hydrogen atoms. (ii) (Si3O6H4)(2-) (3T) is a six-membered ring made of two silanolat e groups and one silanol group (SiOH) bonded through oxygen bridges. (iii) (Si4O7H6)(2-) (4T) is an eight-membered ring with two vicinal silanolate gr oups in positions 1 and 3 and a silanol group in position 5, and (iv) (Si5O 8H8)(2-) (5T) is a flexible ten-membered ring with two vicinal silanolate g roups 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 describ ed silica-supported Ni-II(O)(3) Species: (i) three-coordinated Ni-II of dis torted C-3v close to D-3h symmetry and (ii) Ni-O distances in agreement wit h EXAFS measurements. (Si5O8H8)(2-) is preferred to the (Si2O3 H-4)(2-) mod el proposed earlier. Because of its flexibility, the larger framework model is able to best reproduce the experimental geometry of the Ni-II site. Thi s model cluster may be assimilated to two vicinal silanolates and one neigh boring isolated silanol or siloxane bridge on the real silica surface.