Highly dispersed Pd-SBA15 materials from tris(tert-butoxy)siloxy complexes of Pd(II)

Two novel tris(tert-butoxy)siloxy palladium(II) complexes of the form (4,4′-di-tert-butyl-2,2′-bipyridyl)Pd[OSi(O^tBu) ₃](R) were synthesized (1, R = OSi(O^tBu)₃ and 2, R = CH₃). The structures of these compounds were determined by multinuclear NMR spectroscopy and single crystal X-ray diffraction. The solid-state thermolytic chemistry of 1 and 2 was studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). These compounds were covalently grafted onto the surface of mesoporous silica (SBA15) under mild, nonaqueous conditions to generate supported Pd(II) centers. Reactions of 1 and 2 with the surface Si-OH groups occurs selectively through the -OSi(O ^tBu)₃ ligand with elimination of HOSi(O ^tBu)₃. The new materials, designated Pd(1)SBA15 and Pd(2)SBA15, were characterized using N₂ porosimetry, TGA, powder X-ray diffraction (PXRD), X-ray absorption spectroscopy (XAS), and transmission electron microscopy (TEM). The coordination environments of the supported Pd centers were investigated using Fourier-transform infrared (FTIR) spectroscopy, diffuse reflectance UV-vis (DRUV-vis) spectroscopy, and XAS. Comparison with the molecular precursors 1 and 2 revealed that the supported Pd centers share many of the same structural and spectroscopic characteristics. The supported Pd centers were robust in inert atmosphere up to the decomposition temperatures of 1 and 2 (150-200 °C). The catalytic behavior of the PdSBA15 materials in the semihydrogenation of 1-phenyl-1-propyne was studied and compared to that of 1 and 2 in solution; the supported materials exhibited marked enhancements in stability and selectivity to (Z)-1-phenyl-1-propene.