Cyclopentadienyl–Ru(II)–Pyridylamine Complexes: Synthesis, X-ray Structure, and Application in Catalytic Transformation of Bio-Derived Furans to Levulinic Acid and Diketones in Water

Abstract

A series of cationic half-sandwich cyclopentadienyl-ruthenium(II)-pyridylamine complexes, [(η⁵-C₅H₅)Ru(κ²-L)(PPh₃)]⁺ (L = N_amine-substituted pyridylamine ligands) ([Ru]-1-[Ru]-6), along with the analogous cyclopentadienyl-ruthenium(II)- N-isopropylpyridylimine complex [(η⁵-C₅H₅)Ru(κ²-L)(PPh₃)]⁺ (L = N-isopropylpyridylimine) ([Ru]-7), have been synthesized in good yields. Structural identities of all the complexes have been authenticated by ¹H, ¹³C, and ³¹P NMR, mass spectrometry, and X-ray crystallography. The synthesized complexes exhibited high catalytic activity for the transformation of the bio-derived furans, 2-furfural (furfural), 5-methyl-2-furfural (5-MF), and 5-hydroxymethyl-2-furfural (5-HMF) to levulinic acid (LA) and the diketones, 3-hydroxyhexane-2,5-dione (3-HHD), 1-hydroxyhexane-2,5-dione (1-HHD), and hexane-2,5-dione (HD) in water. Efficient transformation of furfural to LA over a range of η⁵-Cp-Ru-pyridylamine complexes is substantially affected by the N_amine-substituents, where a η⁵-Cp-Ru- N-propylpyridylamine complex ([Ru]-2) exhibited higher catalytic activity in comparison to other η⁵-Cp-Ru-pyridylamine and η⁵-Cp-Ru-pyridylimine complexes. The relative catalytic activity of the studied complexes demonstrated a substantial structure-activity relationship which is governed by the basicity of N_amine, steric hindrance at N_amine, and the hemilabile nature of the coordinated pyridylamine ligands.