Immobilization of Phosphines on Silica:  Identification of Byproducts via ³¹P CP/MAS Studies of Model Alkyl-, Aryl-, and Ethoxyphosphonium Salts

Abstract

Immobilizing bifunctional phosphines with ethoxysilane groups on silica often leads predominantly, and sometimes quantitatively, to P(V) side products that occupy space on the surface but cannot bind metal complexes. Although this side reaction is well-known, and dreaded because it leads to leaching of adsorbed catalysts that are not bound covalently, the exact nature of the surface-bound side product was not yet known. With the help of polycrystalline model compounds of the types [R3PEt]+X- and [R3POEt]+X- (R=alkyl, aryl; X=Cl-, Br-, I-, BF4-) and their solid-state NMR characteristics [delta(31P), CSA], it is demonstrated that the side product is an ethylphosphonium salt bound to the surface by a siloxide anion, [R3PEt]+[Si-O]-.

Diphenylphosphino functionalization of mesoporous silica using tripodal linker units

“…In addition to the PPh 2 (d À15 ppm) and OPPh 2 (d 40 ppm) peaks, a shoulder peak appeared near d 25 ppm in the 31 P CP/MAS spectrum. The peak near d 25 ppm might be due to the PeOeSi species formed by the interaction between the phosphorous group and the surface silanol group, similar to the findings of Blümel et al [17,18]. In the 29 Si CP/MAS spectrum, the peak assigned to the tetraalkyl-coordinated silicon (d 2 ppm) was detected, whereas the peak due to three trialkylmonooxygen-coordinated silicon atoms was negligible.…”

Highly Active Silica Gel-Supported Metathesis (Pre)Catalysts

“…It is know that two bonds dominate so we assume 6 to be the structure of the functionalized silica gel. [12] By refluxing the Grubbs-Herrmann catalyst [2] with 1 equivalent CuCl in dichloromethane with the functionalized silica gel in a ratio of 10 : 1 to the immobilized ligand, the immobilized (pre)catalyst was isolated, following filtration and drying as a stable green-colored silica gel 7a, with a loading of 0,034 mmol/g. [13] The product showed good stability on storing at þ 4 8C under nitrogen over two weeks without any sign of decomposition.…”

Confined growth of CdSe quantum dots in colloidal mesoporous silica for multifunctional nanostructures

“…The porous silica spheres were then transferred to a Se-TOP stock solution and formed a good dispersion. TOP molecules were found to play an important role in this modification, as they intensively coordinated with the oxygen from the silanol groups and rendered the silica particle hydrophobic characteristics and ensured particle dispersity in the solution [35,36]. Such coordination also took place on the mesopore framework, which helped to stabilize the Se precursors within the mesopores.…”