Solid‐State NMR Characterization of Wilkinson’s Catalyst Immobilized in Mesoporous SBA‐3 Silica

Abstract

The Wilkinson's catalyst [RhCl(PPh(3))(3)] has been immobilized inside the pores of amine functionalized mesoporous silica material SBA-3 and The structure of the modified silica surface and the immobilized rhodium complex was determined by a combination of different solid-state NMR methods. The successful modification of the silica surface was confirmed by (29)Si CP-MAS NMR experiments. The presence of the T(n) peaks confirms the successful functionalization of the support and shows the way of binding the organic groups to the surface of the mesopores. (31)P-(31)P J-resolved 2D MAS NMR experiments were conducted in order to characterize the binding of the immobilized catalyst to the amine groups of the linkers attached to the silica surface. The pure catalyst exhibits a considerable (31)P-(31)P J-coupling, well resolvable in 2D MAS NMR experiments. This J-coupling was utilized to determine the binding mode of the catalyst to the linkers on the silica surface and the number of triphenylphosphine ligands that are replaced by coordination bonds to the amine groups. From the absence of any resolvable (31)P-(31)P J-coupling in off-magic-angle-spinning experiments, as well as slow-spinning MAS experiments, it is concluded, that two triphenylphosphine ligands are replaced and that the catalyst is bonded to the silica surface through two linker molecules.

Heterogenized Wilkinson‐Type Catalyst for Transfer Hydrogenation of Carbonyl Compounds

“…Grünberg et al studied the molecular configuration of phosphane ligands in immobilized Wilkinson's catalysts on amino-n-propyl-functionalized mesoporous silica of the type SBA-3. [19] The direct-polarization 31 P NMR spectrum in Figure 3 for the Wilkinson-type catalyst immobilized/tethered on silica gel, is very similar to the Figure 3. Direct-polarization 31 P NMR spectrum of silica-bound Wilkinson-type catalyst, the large peak corresponds to main signals from diphenylphosphane groups, those marked with (*) from spinning sidebands.…”

“…1 H}-31 P cross-polarization spectrum detected by Grünberg et al [19] and those presented by Merckle et al…”

“…On the other hand, cyano compounds and many halogenated aromatic compounds were compatible with the hydrogen transfer conditions and gave high yields after 8 h (Entries 13-16). Benzaldehydes gave the corresponding benzyl alcohols in good to excellent yields (Entries [19][20].…”

Wilkinson‐Type Immobilized Catalyst on Diamond Nanoparticles for Alkene Reduction

“…Considering that Wilkinson's catalyst and related trisphosphine Rh‐complexes exhibit chemical shifts between 20 and 80 ppm, and that the signals of the metal‐bound phosphorus atoms in the Rh complex 2 (Scheme B) derived from catechol phosphine 1 appear in the same region (chemical shifts are 53 and 27 ppm for the P,O‐chelating and monodentate ligands, respectively), the broad resonance at 25 ppm could be assigned to Rh‐bound phosphines, whereas the narrower line at 53 ppm might evidence the presence of P,O–Rh chelates. To confirm this proposed assignment and further interpret the 31 P MAS NMR spectrum, 2 D homonuclear J ‐resolved MAS NMR experiments were performed, allowing the measurement of cis ‐ and trans ‐P−Rh−P 2 J coupling constants in the solid state . As shown in Figure A, the 31 P 2 D J ‐resolved MAS spectrum clearly indicates the existence of a strong 2 J P−P coupling of approximately 540 Hz for two broad contributions centered at approximately 25 and 50 ppm.…”

“…As shown in Figure A, the 31 P 2 D J ‐resolved MAS spectrum clearly indicates the existence of a strong 2 J P−P coupling of approximately 540 Hz for two broad contributions centered at approximately 25 and 50 ppm. This J coupling constant is larger than the value reported for Wilkinson's catalyst but remains consistent with a 2 J P−P trans coupling. Moreover, the corresponding 31 P through‐bond double‐quantum single‐quantum (DQ–SQ) MAS NMR correlation spectrum (Figure B) shows intense cross‐correlation peaks between these two broad peaks at 25 and 50 ppm, suggesting the presence of Rh complex with a molecular structure similar to that depicted in Scheme .…”