A Unified Synthetic Approach to Porous Hybrid Single-Site Metallosilicates

  Martin Kejík,^1 Zdeněk Moravec,^1 Lucie Šimoníková,^1 Aleš Stýskalík,^1 Craig E. Barnes,^2 Jiří
                                             Pinkas^1

^1Masaryk University, Department of Chemistry, CZ-61137 Brno, Czech Republic, 380095@mail.muni.cz

^2 University of Tennessee, Department of Chemistry, Knoxville, TN 37996-1600, USA


A generalized low-temperature non-hydrolytic sol-gel strategy to produce uniformly dispersed
metallosilicate sites in highly porous hybrid silicate matrices is reported. The readily available
spherosilicate molecular building block (Me[3]Sn)[8]Si[8]O[20] is cross-linked in two steps by
irreversible condensation reactions with high-valence d- and p-block chlorides and alkyl-metals in
toluene or THF to form statistically connected rigid amorphous networks. Initially, a limited
amount of the metal site precursor (e.g., TiCl[4], VOCl[3], Py-AlCl[3], Et[3]N-AlMe[3],
[Me[4]N^+][AlCl[4]^-], ZnEt[2], SbCl[3], Ph[3]SbCl[2], …) is introduced to an excess of the
building block to achieve full condensation and uniformity. A minimum site separation of ≈1 nm is
ensured by the bulky building block. In the second step, a limited amount of a ditopic hybrid
linker (ClMe[2]SiCH[2]CH[2]SiMe[2]Cl or ClMe[2]Si(C[6]H[4])(C[6]H[4])SiMe[2]Cl) is added to produce
extensively cross-linked gels while maximizing the probability of full condensation. The byproducts
of the condensations (Me[3]SnCl or Me[3]SnR) are inert and volatile and they do not interfere with
subsequent steps. Removal of all volatiles under vacuum affords pure materials, while the mass of
the lost byproducts can be used to monitor condensation by gravimetric techniques with great
precision. Since a kinetically-driven approach and mild conditions are used, it is possible to
produce a variety of complex sites incorporating organic ligands and sensitive moieties
(Py-Al(OSi≡)[3], Sb(OSi≡)[3], Ph[3]Sb(OSi≡)[2]). The prepared xerogels were characterized by IR and
MAS NMR spectroscopies, gas adsorption, gravimetry, STEM/EDS, ICP-OES, and TG/DSC. The fundamental
limits of the system and the interplay of parameters, such as site loading, linker flexibility,
stoichiometry, and solvent effects, were explored and correlated with structure, condensation, and
porosity. A method of molecular texture elucidation using the linker as a two-point correlation
probe was developed. Additionally, a series of quantum mechanical DFT calculations were conducted
to help explain observed behaviors and to make predictions for the stability and reactivity of
additional, as-of-now untested, precursors as well as creating a baseline for future calculations.

The financial support by GAČR Junior 20-03636Y is gratefully acknowledged. CIISB research
infrastructure project LM2018127 funded by MEYS CR is gratefully acknowledged for the financial
support of the measurements at the Josef Dadok National NMR Centre.