Formate dehydrogenase activity by a Cu(II)-based molecular catalyst and deciphering the mechanism using DFT studies

Due to the requirement to establish renewable energy sources, formic acid (FA), one of the most probable liquid organic hydrogen carriers (LOHCs), has received great attention. Catalytic formic acid dehydrogenation in an effective and environmentally friendly manner is still a challenge. The N3Q3 ligand (N3Q3 = N,N-bis(quinolin-8-ylmethyl)quinolin-8-amine) and the square pyramidal [Cu(N3Q3)Cl]Cl complex have been synthesised in this work and characterised using several techniques, such as NMR spectroscopy, mass spectrometry, EPR spectroscopy, cyclic voltammetry, X-ray diffraction and DFT calculations. This work investigates the dehydrogenation of formic acid using a molecular and homogeneous catalyst [Cu(N3Q3)Cl]Cl in the presence of HCOONa. The mononuclear copper complex exhibits catalytic activity towards the dehydrogenation of formic acid in H₂O with the evolution of a 1 : 1 CO₂ and H₂ mixture. The activation energy of formic acid dehydrogenation was calculated to be E_a = 86 kJ mol^-1, based on experiments carried out at various temperatures. The Gibbs free energy was found to be 82 kJ at 298 K for the decomposition of HCOOH. The DFT studies reveal that [Cu(N3Q3)(HCOO^-)]⁺ undergoes an uphill process of rearrangement followed by decarboxylation to generate [Cu(N3Q3)(H^-)]⁺. The initial uphill step for forming a transition state is the rate-determining step. The [Cu(N3Q3)(H^-)]⁺ follows an activated state in the presence of HCOOH to liberate H₂ and generate the [Cu(N3Q3)(OH₂)]²⁺.