Synthesis, structure, immobilization and solid-state NMR of new dppp- and tripod-type chelate linkers

Synthesis, Immobilization, MAS and HR‐MAS NMR of a New Chelate Phosphine Linker System, and Catalysis by Rhodium Adducts Thereof

“…Crystals suitable for X-ray structure determination [17] (Figure 1) could be grown from an acetonitrile solution. As with the X-ray structure of the analogous compound EtOSiA C H T U N G T R E N N U N G (CH 2 PPh 2 ) 3 [8] the orientation of the phosphine groups is not preorganized for coordination, but dominated by the crystal packing. As expected, the triflate counteranion occupies the sterically least crowded space close to the methyl group of the phosphonium moiety.…”

“…One conventional way of binding metal complexes to oxide surfaces is, for example, using ethoxysilane containing linkers, [2][3][4][7][8][9]11,24] such as (EtO) 3 Si-…”

“…The ethoxysilane linker EtOSiA C H T U N G T R E N N U N G (CH 2 PPh 2 ) 3 with the shortest possible alkyl chains is not sensitive in solution, but it obviously has too short a distance between the phosphine and the reactive ethoxysilane groups and decomposes on a silica surface, while EtOSiA C H T U N G T R E N N U N G [(CH 2 ) 2 PPh 2 ] 3 survives. [8] Alkyl chains with 7 or 11 methylene groups between the phosphine and ethoxysilane groups, however, led to mono-and bidentate linkers with reduced oxygen sensitivity, that can cleanly be immobilized on oxide supports. [9] Furthermore, results with Wilkinson-type catalysts tethered to the support by linkers with long alkyl chains show that they display higher activity and better recycling characteristics than catalysts bound by shorter linkers, [4] even when the latter are chelating and the former only monodentate.…”

“…[7a, 8,9,11] Here, we present a new class of chelating tridentate phosphine ligands with long alkyl chains that are immobilized via electrostatic interactions of phosphonium moieties with a silica surface. They are characterized by solid-state NMR, and their leaching has been studied.…”

“…[9] But even for the linker 17i with the shortest possible alkyl chain, the HR-MAS signals show sub- stantially narrower lines, in accordance with the case of a covalently bound tripod-type ethoxysilane containing linker. [8] When the surface coverage with linker is lowered to 25% of the maximal coverage, typically the signal half-widths increase. For example, the half-width for the phosphonium (phosphine) moiety of 11i increases from 92 (73) to 278 (189) Hz.…”