Switching Amination Pathways: How Ru(II) Oxidation State Favors Ru–O/N over Ru–C Bonded Intermediates

Employing the Ru(II)-complex ([Ru–H]) as an efficient catalyst for the reductive amination of 5-hydroxymethylfurfural (HMF) with H2 and NH3 to 2,5-bis(aminomethyl)furan (BAMF) in tert-amyl alcohol solution, the reaction mechanisms have been investigated at the M06/def2-TZVP, 6–311++G (d,p) theoretical level. The aromatic precatalyst [Ru–H] serves as the precursor to the catalytically active species, undergoing in situ conversion to the dearomatized complex [Ru-0], a process facilitated by the aldehyde group of HMF. In [Ru-0], the Lewis acidic Ru(II)-center facilitates the cleavage of multiple bonds (N–H, C–OH, H–H, and C–H), and the Lewis basic N-site in the ligand acts as a proton transfer-station, assisted by the Cl– anion as a stable mediator. The positive charge of the Ru(II)-center favors reaction pathways involving Ru–O or Ru–N bonded intermediates over those featuring Ru–C bonds. In contrast, the Ru(0)-center, with its metallic nature, preferentially accesses and stabilizes the Ru–C bonded pathway. The optimal reaction pathway proceeds through the reductive amination of the aldehyde group rather than the amination dehydration of the hydroxymethyl group. The rate-determining step is associated with the cleavage of the C–OH bond for the amination dehydration of aldehyde to imine. The findings of this study offer valuable theoretical insights for the rational design of novel catalysts targeting the reductive amination of aldehydes and hydroxymethyl-containing compounds.