Mechanistic insights into formic acid dehydrogenation promoted by Cu-amino based systems

Catalytic Dehydrogenation of Formic Acid Promoted by Triphos-Co Complexes: Two Competing Pathways for H2 Production

In this study, we reported the synthesis and structural characterization of a triphos-CoII complex [(κ3-triphos)CoII(CH3CN)2]2+ (1) and a triphos-CoI-H complex [(κ2-triphos)HCoI(CO)2] (4). The facile synthetic pathways from 1 to [(κ3-triphos)CoII(κ2-O2CH)]+ (1') and [(κ3-triphos)CoI(CH3CN)]+ (2), respectively, as well as the interconversion between [(κ3-triphos)CoI(CO)2]+ (3) and 4 have been established. The activation energy barrier, associated with the dehydrogenation of a coordinated formate fragment in 1' yielding the corresponding 2 accompanied by the formation of H2 and CO2, was experimentally determined as 23.9 kcal/mol. With 0.01 mol % loading of 1, a maximum TON ∼ 1735 within 18 h and TOF ∼ 483 h-1 for the first 3 h could be achieved. Kinetic isotope effect (KIE) values of 2.25 (kHCOOH/kDCOOH) and 1.36 (kHCOOH/kHCOOD) for the dehydrogenation of formic acid and its deuterated derivatives, respectively, implicate that the H-COOH bond cleavage is likely the rate-determining step. The catalytic mechanism proposed by density functional theory (DFT) calculations coupled with experimental 1H NMR and gas chromatography-mass spectrometry (GC-MS) analysis unveils two competing pathways for H2 production; specifically, deprotonating a HCOO-H bond by a proposed Co-H intermediate C and homolytic cleavage of the CoII-H moiety of C, presumably via a dimeric Co intermediate D containing a [Co2(μ-H)2]2+ core, to yield the corresponding 2 and H2.

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 Ea = 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+.

Advances in Nonprecious Metal Homogeneously Catalyzed Formic Acid Dehydrogenation

Formic acid (FA) possesses a high volumetric concentration of H2 (53 g L−1). Moreover, it can be easily prepared, stored, and transported. Therefore, FA stands out as a potential liquid organic hydrogen carrier (LOHC), which allows storage and transportation of hydrogen in a safe way. The dehydrogenation to produce H2 and CO2 competes with its dehydration to give CO and H2O. For this reason, research on selective catalytic FA dehydrogenation has gained attention in recent years. Several examples of highly active homogenous catalysts based on precious metals effective for the selective dehydrogenation of FA have been reported. Among them are the binuclear iridium-bipyridine catalysts described by Fujita and Himeda et al. (TOF = 228,000 h−1) and the cationic species [IrClCp*(2,2′-bi-2-imidazoline)]Cl (TOF = 487,500 h−1). However, examples of catalytic systems effective for the solventless dehydrogenation of FA, which is of great interest since it allows to reduce the reaction volume and avoids the use of organic solvents that could damage the fuel cell, are scarce. In this context, the development of transition metal catalysts based on cheap and easily available nonprecious metals is a subject of great interest. This work contains a summary on the state of the art of catalytic dehydrogenation of FA in homogeneous phase, together with an account of the catalytic systems based on non-precious metals so far reported.

Mechanistic Insight into Bis(amino) Copper Formate Thermochemistry for Conductive Molecular Ink Design

Increasing attention has been given to amine-copper formate complexes for their use as low-cost printable conductive inks. The structure of amine ligands coordinated to copper centers has been reported to dictate the properties of copper molecular inks, such as stability and printability, thereby influencing the copper reduction pathway during the thermolysis. Yet, the underlying mechanism by which formate is oxidized when complexed with amine ligands is still not fully understood. Here, we propose a mechanistic pathway of copper formate dehydrogenation and decarboxylation and examine the critical role that amine ligands play in their thermal decomposition by employing first-principles electronic structure computations and experimental analyses of thermolysis reactions. Based on the computational characterization of the relevant reaction pathways for a number of primary and secondary amines as well as pyridine ligand complexes, we are the first to show that the hydrogen bonds formed between the amine ligand and formate are the key factors governing the activation energy, providing a design principle for the synthesis of organic ligands that can tune the height of the reaction barriers of the dehydrogenation and decarboxylation reactions. The calculations, confirmed by NMR studies, show that the reduction of Cu(II) to Cu(I) occurs in concert with the release of H₂ via the dimerization of Cu(II) hydride. This result suggests that the monomeric elimination of H₂ is not favorable for the Cu(II) to Cu(I) reduction and thus identifies dimeric amino copper formate as an important intermediate for copper reduction whose thermodynamic stabilities are also dictated by the nature of the amine ligands.