Advantages of Covalent Immobilization of Metal‐Salophen on Amino‐Functionalized Mesoporous Silica in Terms of Recycling and Catalytic Activity for CO₂ Cycloaddition onto Epoxides

Abstract

Dedicated to the memory of Professor Michel CheNi II and Mn III Schiff base complexes (Salophen-Ni and Salophen-MnCl) bearing a pending carboxylic group were prepared and characterized. Both complexes were grafted onto a mesoporous amino-functionalized SBA-15 silica, by formation of an amide function between the propylamine groups of the support and the carboxylic acid functions of the salophen ligand (corresponding respective to 1.30 wt.% of Ni and 1.06 wt.% of Mn). The co-catalytic behaviour of the free and grafted complexes was then evaluated in the CO 2 cycloaddition reaction onto styrene oxide, using tetra-butylammonium bromide ( nÀ Bu 4 NBr) as the main catalyst. In homogeneous conditions, the Mn III Schiff base complex and the Ni II one, to a lesser extent, behave as efficient co-catalysts for this reaction (styrene conversion of 100 % and 65 % respectively after 3 h at 120°C, under 15 bars of CO 2 ). Upon immobilization at the surface of the aminofunctionalized SBA-15, we showed that the co-catalytic activity of the less efficient one, i. e. Ni 2 + salophen complex, could be enhanced (reaching a full conversion after 7 h), hence highlighting a potential synergistic effect between the unused amine functions of the support and the Ni 2 + salophen cocatalyst. Both salophen complexes were successfully re-used in homogeneous conditions or after their immobilization without any appreciable loss of activity. This work is only a first step towards a completely heterogeneous catalytic system, in which the tetraalkylammonium halide catalyst and the metal-salophen co-catalyst will both be covalently anchored on the same support.

Chromium-Salophen as a Soluble or Silica-Supported Co-Catalyst for the Fixation of CO2 Onto Styrene Oxide at Low Temperatures

“…This was accompanied by a concomitant decrease in pores volume (from 0.69 for {NH 2 }-SBA-15 to 0.44 cm 3 g −1 for {Salophen-tBu-Cr}-SBA-15) and mean pores size values (from 5.7 nm for {NH 2 }-SBA-15 to 5.2 for {Salophen-tBu-Cr}-SBA-15). These data are compatible with those observed in the case of manganese and nickel complexes grafted on silica SBA-15 described elsewhere (Balas et al, 2021). Furthermore, {Salophen-tBu-Cr}-SBA-15 was characterized by a high hexagonal structuration of its porosity (presence of the (100), (110), and ( 200) reticular planes characteristic of the SBA-15 structure, see Supplementary Figure S8).…”

“…In the continuity of our previous work (Balas et al, 2021), the preparation, the characterization, and the immobilization of a Cr 3+salophen complex bearing a -CO 2 H function (N,N'-bis(3,5-di-tertbutylsalicylidene)-1-carboxy-3,4-phenylene-diamine-chromium (III)chloride) onto a mesoporous SBA-15 silica through amide bonding was thus addressed in the present manuscript. This compound showed excellent co-catalytic performances in the cycloaddition reaction of CO 2 onto styrene oxide, both in its soluble form or after heterogenization.…”

“…The presence of a -COOH function on the central phenyl group of the salophen ligand allowed to covalently graft the complex at the surface of an amino functionalized SBA-15 silica ({NH 2 }-SBA-15), according to our previous works on the anchoring of hybrid polyoxometalates (Villanneau et al,2013;Bentaleb et al, 2015) and as shown also for the Mn and Ni derivatives (Balas et al, 2021). This occurred through the formation of a covalent amide bond using N,N'-dicyclohexylcarbodiimide (DCC) and 1-hydroxy-1Hbenzotriazole (HOBT) coupling agents, in dichloromethane under refluxing conditions, affording Salophen-tBu-Cr@{NH 2 }-SBA-15.…”

Nickel(II)-Based Building Blocks with Schiff Base Derivatives: Experimental Insights and DFT Calculations

“…Schiff bases usually coordinate to a metallic ion through the imine nitrogen atom and are able to complex most metals in the periodic table with distinct oxidation states, coordination numbers and geometries [4][5][6][7], thus finding a plethora of applications within numerous scientific areas [8,9]. Their diverse applications, associated with their structural flexibility, made them essential compounds that have extensively been used in homogenous and heterogeneous catalysis [10][11][12][13][14], chemical sensing [15][16][17][18], nonlinear optics (NLO) [19][20][21][22], beside remarkable pharmacological and biological activities [23][24][25][26][27][28], to name a few.…”

DataSheet1.pdf

“…From this study, it appeared that the chromium (III) salophen complex behaved clearly as a better homogeneous co-catalyst compared to the corresponding manganese (III) and nickel (II) complexes (Balas et al, 2021). Indeed a higher yield of styrene carbonate was observed at 80 °C for Salophen-tBu-Cr (92% after 7 h) compared to Salophen-tBu-Mn and Salophen-tBu-Ni (respectively, 48% for Mn and 26% for Ni) during the same reaction time (Table 1, entries 8 and 9).…”