A methods study of immobilization of PONOP pincer transition metal complexes on silica polyamine composites (SPC)

Immobilization of catalytically active transition metal complexes on silica polyamine composite (SPC) surfaces offers many advantages for applications in catalysis particularly for catalyst recovery and reuse. We report here the immobilization of PONOP pincer complexes of Ru, Rh, Ni and Pd on the poly(allylamine) SPC, BP-1 using the Mannich reaction. Three different methods have been investigated for synthesizing the PONOP pincer transition metal complexes on BP-1: 1) direct reaction of the preformed pincer complexes using a two step Mannich reaction; 2) immobilization of the PONOP ligand using the Mannich reaction followed by the addition of a transition metal compound of a given metal; 3) the stepwise construction of PONOP on BP-1 followed by addition of a transition metal compound. The immobilized complexes on BP-1 were characterized by FT-IR, solid-state CPMAS C-13 and P-31 NMR, as well as elemental analysis. Anchoring of the complexes on BP-1 was also evaluated by the metal loading data obtained from the digestion of the loaded composites followed by Atomic Absorption Spectroscopy (AAS) or Inductively Coupled Plasma Atomic Emission Spectroscopy (ICPAES). The results showed that method 1 works better for the loading of pincer complexes on the SPC than methods 2 and 3. In the case of the Ru and Ni pincer complexes reasonable agreement with the phosphorous analysis was realized, while for the Pd complex values were high relative to the loading predicted from the phosphorus analysis, indicating the formation of the Pd nanoparticles on the surface during immobilization. For the Rh and Ru immobilized complexes with methods 2 & 3, metal loading was lower than the phosphorous analysis and this is attributed to entrained triphenylphosphine from the starting rhodium and ruthenium complexes based on the C-13 and P-31 CPMAS NMR data. Solution experiments using the PONOP pincer ligand and the Ru(PONOP) complex with n-butyl amine were conducted to model the site of electrophilic aromatic substitution on the pyridine ring. It was found that substitution both meta-and para-to the nitrogen takes place and this helped in the interpretation of the solid-state data. (C) 2016 Elsevier B.V. All rights reserved.