Thermodynamics of the condensation of the Si8O20(SnMe3)8 building block with M–X (M = B, Al, Si, P, Ti, V, Zn, Sn, Sb, X = Cl, Me, Et) precursors by DFT-D3 calculations

Abstract

Synthesis of porous metallosilicate materials from siloxane oligomers is a promising approach for constructing well-defined structures at a molecular level. Here, we use quantum chemistry DFT methods and demonstrate a computationally cheap method for screening potential precursors for synthesizing porous metallosilicates. We estimate the thermodynamic parameters of condensation reactions of the octakis(trimethyltin)spherosilicate Si₈O₂₀(SnMe₃)₈ (CUBE) building block with metal chlorides and alkyl metals. These reactions represent the initial steps in the non-hydrolytic synthesis of metallosilicate gels containing potentially uniform single-site metal centers. Our main emphasis was on the spontaneity and irreversibility of the condensation and the computational screening of potential metal center sources. The precursors previously reported in successful condensations with CUBE, such as AlCl₃, [AlCl₄]⁻, Si–Cl compounds, PCl₃, TiCl₄, and VOCl₃, are shown to undergo sufficiently irreversible reactions, as are the untested precursors BCl₃, VCl₄, and POCl₃. Interestingly, AlMe₃ proves to be twice as exoergic as AlCl₃. The first chloride in Cp₂TiCl₂ reacts readily, but the second may be partially reversible. SbCl₃ and Ph₃SbCl₂ are borderline cases, and the reversibility of their condensations might pose a problem. SnCl₄ was found unsuitable as a precursor to stannosilicates. It should be possible to prepare zincosilicates from ZnEt₂, but not from ZnCl₂, as the affinity of Zn for Cl⁻ is so high that in the presence of a source of Cl⁻, zincosilicate structures will dissolve back to CUBE and ZnCl₂. The oxophilicity of the metal in the precursor is the main factor in the driving force for the condensation with CUBE. Alkyl metals and lighter elements are more prone to the reaction than the corresponding metal chlorides and heavier analogs. The propensity of [SnMe₃]⁺ to bind to Cl⁻ in preference to CUBE has a supporting effect. At low temperatures, the condensation is slightly disfavored, while at the experimentally used temperature of 100 °C, this process contributes over 20 kJ mol⁻¹ of the additional driving force and helps to complete the condensation. The reliability of B3LYP-D3 and PBE0-D3, together with the CBS extrapolation scheme, is also evaluated in calculations.