Oxidation of Styrene to Benzaldehyde Catalyzed by Schiff Base Functionalized Triazolylidene Ni(II) Complexes

Abstract

Four new Schiff base functionalized 1,2,3-triazolylidene nickel complexes, [Ni-(L1NHC)2](PF6)2; 3, [Ni-(L2NHC)2](PF6)2; 4, [Ni-(L3NHC)](PF6)2; 7 and [Ni-(L4NHC)](PF6)2; 8, (where L1NHC = (E)-3-methyl-1-propyl-4-(2-(((2-(pyridin-2-yl)ethyl)imino)methyl)phenyl)-1H-1,2,3-triazol-3-ium hexafluorophosphate(V), 1, L2NHC = (E)-3-methyl-4-(2-((phenethylimino)methyl)phenyl)-1-propyl-1H-1,2,3-triazol-3-ium hexafluorophosphate(V), 2, L3NHC = 4,4′-(((1E)-(ethane-1,2-diylbis(azanylylidene))bis(methanylylidene))bis(2,1-phenylene))bis(3-methyl-1-propyl-1H-1,2,3-triazol-3-ium) hexafluorophosphate(V), 5, and L4NHC = 4,4′-(((1E)-(butane-1,4-diylbis(azanylylidene))bis(methanylylidene))bis(2,1-phenylene))bis(3-methyl-1-propyl-1H-1,2,3-triazol-3-ium) hexafluorophosphate(V), 6), were synthesised and characterised by a variety of spectroscopic methods. Square planar geometry was proposed for all the nickel complexes. The catalytic potential of the complexes was explored in the oxidation of styrene to benzaldehyde, using hydrogen peroxide as a green oxidant in the presence of acetonitrile at 80 °C. All complexes showed good catalytic activity with high selectivity to benzaldehyde. Complex 3 gave a conversion of 88% and a selectivity of 70% to benzaldehyde in 6 h. However, complexes 4 and 7–8 gave lower conversions of 48–74% but with higher (up to 90%) selectivity to benzaldehyde. Results from kinetics studies determined the activation energy for the catalytic oxidation reaction as 65 ± 3 kJ/mol, first order in catalyst and fractional order in the oxidant. Results from UV-visible and CV studies of the catalytic activity of the Ni-triazolylidene complexes on styrene oxidation did not indicate any clear possibility of generation of a Ni(II) to Ni(III) catalytic cycle.

Fe(ii) complexes of pyridine-substituted thiosemicarbazone ligands as catalysts for oxidations with hydrogen peroxide

“…Even so, some catalysts with a higher selectivity towards benzaldehyde using different oxidants have been reported. 25,27–31 The results indicate that the [Fe(TSC) 2 ] complexes show a lower activity than that found for some of the reported catalysts, but the capability of these complexes to carry out selective oxidative cleavage of styrene into benzaldehyde is remarkable. As for the thioanisole data, no meaningful differences are observed between the three Fe II complexes studied.…”

Co(ii/iii), Ni(ii) and Cu(ii) complexes with a pyrazole-functionalized 1,3,5-triazopentadiene: synthesis, structure and application in the oxidation of styrene to benzaldehyde

“…8,12,13 As presented in Table 1, the best catalytic results were obtained with the Cu(II) catalysts, complex 4 being the best catalyst for the selective conversion of styrene into benzaldehyde. Both the reaction time (3 h) and selectivity (96%, entry 9 in Table 1) in our system are usually higher than those reported for [Ni(L 1 NHC) 2 ](PF 6 ) 2 , (L 1 NHC = (E)-3-methyl-1-propyl-4-(2-(((2-(pyridin-2-yl)ethyl)imino)methyl)phenyl)-1H-1,2,3-triazol-3-ium hexafluorophosphate(V) (reaction time 6 h, selectivity 70%) 14 and [2-ampH] 4 [{Co(H 2 O) 5 }Mo 7 O 24 ]Á9H 2 O, (2-amp = 2-aminopyridine), (reaction time 24 h, selectivity 76%) 15 complexes.…”

Chemoselective Oxidation of Alkenes by Metal‐Organic Framework‐Supported Copper Catalyst

“…Several photocatalysts [21][22][23] and homogeneous catalysts based on heteropolyacids, [24] metal complexes with Schiff bases, [25][26][27][28] porphyrins, salen, polyoxo-or N-ligands, and polyoxometalates [29][30][31][32][33] have been developed for oxidation of styrene by O 2 , H 2 O 2 and tert-butylhydroperoxide (TBHP). However, due to the poor selectivity, difficulty in separation and recycling of these homogeneous catalysts, it is a matter of the utmost urgency to develop highly active, selective, and reusable heterogeneous catalysts for economical and eco-friendly oxidation of alkenes.…”