Functionalization of large-pore periodic mesoporous silicas: metal silylamide and isopropoxide molecular grafting and secondary surface ligand exchange

Abstract

Divalent metal silylamide complexes of zinc and cobalt and trivalent aluminium isopropoxide were successfully grafted onto large-pore hexagonal channel-like SBA-15 and cubic MCM-48-like periodic mesoporous silica (PMS) KIT-6, to afford hybrid materials Zn[N(SiMe₃)₂]₂@SBA-15, Co[N(SiMe₃)₂]₂(thf)@SBA-15/KIT-6 and {Al(OCHMe₂)₃}@SBA-15 with well-defined surface species (SiO)₂M(thf)_x, SiOM[N(SiMe₃)₂](THF)_x (x = 0, M = Zn; x = 1, M = Co), SiOSiMe₃ and (SiO)₂Al(OCHMe₂) and SiOAl(OCHMe₂)₂, respectively. Surface silylamido ligand exchange with 1,4-benzenediol, 1,4-benzenedicarboxylic acid, and biphenyl-4,4′-dicarboxylic acid gave hybrid materials of type SiOML(solvent)_x and (SiO)₂M(solvent)_x (L = O(C₆H₄)OSiMe₃, O₂C(C₆H₄)CO₂SiMe₃, O₂C(C₆H₄)₂CO₂SiMe₃; M = Zn and Co) revealing in situ ligand silylation as a side reaction. Surface isopropoxide/carboxylato ligand exchange generated species (SiO)₂AlL(DMF)_x and SiOAl(OCHMe₂)_2−yL_y (L = O₂C(C₆H₄)CO₂H, y ≤ 2) devoid of any secondary functionalization (such as formation of ester groups), offering a potential approach to PMS-encapsulated metal–organic frameworks. All featured surface species were verified by ¹H, ¹³C and ²⁹Si magic angle spinning (MAS) NMR and diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy as well as elemental analysis. Additionally, preservation of the long-range ordered mesostructures was proven by powder X-ray diffraction (PXRD), nitrogen physisorption, and transmission electron microscopy (TEM).