Grafting of bulky rare earth metal complexes onto mesoporous silica MCM-41†

Abstract

Heteroleptic yttrium bis(dimethylsilyl)amide complexes featuring chelating N,N′-bis(3,5-di-tert-butylsalicylidene)ethane-1,2-diamine (H₂L¹) and trans-1,2-bis(2,4,6-triisopropylbenzenesulfonamido)cyclohexane (H₂L²) ligands were immobilized on mesoporous silica MCM-41 according to a heterogeneously performed silylamide route. FTIR Spectroscopy revealed a novel grafting sequence involving amine elimination and subsequent metal siloxide formation and surface silylation. Nitrogen adsorption/desorption isotherms of the resulting hybrid materials obtained at 77.4 K indicated peculiarities of the immobilization process such as the presence of differently modified mesopores. According to preliminary catalytic investigations, the hybrid materials exhibit promising catalytic activity in a hetero Diels–Alder reaction (Danishefsky transformation). Molecular model complexes were synthesized via amine elimination reactions with triphenylsilanol and fully characterized by means of spectroscopy (FTIR, ¹H, ¹³C, ²⁹Si NMR). Mononuclear [YL¹(OSiPh₃)(thf)(CH₃CN)] adopts a slightly distorted pentagonal bipyramidal geometry in the solid state, with the (planar) L¹ and acetonitrile ligands located in the equatorial plane.