Terpyridine–Ru Complexes Noncovalently Supported on Cobalt Magnetic Nanoparticles for Nitroarene Transfer Hydrogenation

This work reports the preparation and characterization of active and recyclable supported catalysts involving molecular Ru–terpyridine complexes and their ability to promote the transfer hydrogenation of nitroarenes in 2-propanol. The system proposed is based on the design of original pyrene-tagged terpyridine ligands, either monomeric “1_tpy” (one pyrene tag for one terpyridine) or multivalent “5_tpy” (one pyrene tag for five terpyridines), able to interact with the graphene surface of commercially available cobalt magnetic nanoparticles (MNPs) through π-stacking interactions. The existence of noncovalent interactions between the pyrene-tagged terpyridines and the MNP surface at 20 °C was assessed by fluorescence spectroscopy for the first time in 2-propanol, an organic medium. Interestingly, 1_tpy and 5_tpy were shown to be released upon temperature increase (80 °C) while the association was restored upon temperature decrease (20 °C). The reversibility of this phenomenon enabled to reach high activities in nitroarene transfer hydrogenation for the corresponding Ru complexes (catalytic species are free in refluxing 2-propanol) and high recyclabilities (noncovalent interactions are restored upon cooling). It is noteworthy that the multivalent ligand 5_tpy displaying, as expected, the higher terpyridine loading (0.15 mmol of tpy/g of MNPs vs 0.03 mmol for 1_tpy) also gave rise to the Ru catalyst with the best recyclability (8 consecutive cycles with no loss of activity against 5 cycles for the catalyst based on the monomeric ligand 1_tpy). In both cases, the ruthenium leaching was found to be low and the expected aniline met the requirements of the pharmaceutical industry (<10 ppm Ru) from the third run.