Homogeneous amine-borane dehydrogenation catalyzed by Rare-Earth and 3d transition metal complexes.

Project leaders

Andrea Rossin, Elena Shubina

Agreement

RUSSIA - RFBR-suspended - Russian Foundation for Basic Research

Call

CNR/RFBR triennio 2018-2020 2018-2020

Department

Chemical sciences and materials technology

Thematic area

Chemical sciences and materials technology

Status of the project

New

Research proposal

The transition from a global economy based on the unsustainable exploitation of fossil fuels to a new lifestyle utilizing green energy sources underpins the use of hydrogen. Nowadays, we witness the development of a hydrogen-based technology; however, its widespread application is still hampered by a number of factors that need to be optimized simultaneously, including low production cost, fast energy recharging, large gravimetric and volumetric energy storage densities, low operational temperature etc. At that, different applications may be envisaged, in a matter of compromise among different approaches to H2 production: thermolysis of solid state carriers, hydrolysis (alcoholysis) or catalytic dehydrogenation. Among the chemical compounds which easily release hydrogen on demand, there are B-N compounds (amine-boranes, AB). They have attracted considerable interest, as they offer an appropriate balance of volumetric and gravimetric energy densities. The simplest example, ammonia-borane (NH3BH3, 19.3 max.% wt. H) is non-flammable and stable under standard conditions; its dehydrogenation/hydrolysis mediated by organometallic catalysts offers the potential to tune both rate and extent of H2 release. Over the past ten years, relatively expensive transition metals (Ir, Rh, Ru, Pd) have been exploited as homogeneous catalysts when combined with suitable ligand sets. As the final application of these catalysts is the transportation sector, the development of cost-effective catalytic systems based on more Earth abundant metals (e.g. Mn, Fe, Co, Ni, Cu, Zn) are warmly encouraged to make the process really sustainable. The transition to 3d and main group catalysts is not trivial, because of the difference in their coordination chemistry and (consequently) catalytic performance. With their lower M-H bond strengths and greater tolerance for coordinative unsaturation, first row transition metals may even be better suited than heavier metals to act as hydride donors, giving a wider application perspective. However, at present the efficiency of such catalysts is rather limited and needs further exploration.

Previous work carried out by the Italian and Russian teams (CNR-RFBR project 2015-2017) has provided experimental evidence for (i) the advantage of the presence of a hemilabile basic functionality within the ligand skeleton to improve the H2 release extent and (ii) the formation of metal hydrides [M(H)n] and tetrahydroborates [M(BH4)n] as catalyst "resting states" along the dehydrogenation pathway. Proton and hydride transfer are the key elementary steps of the reaction mechanism. Their balance influences the dehydrocoupling pathway and the overall catalyst performance. Lewis acidity of the core metal promotes L.BH3 coordination and hydride transfer to the activated substrate. At the same time, the presence of a basic site (ligand heteroatom or a metal-bound hydride/chloride) triggers a proton transfer from the NH-group of AB. Further development of a well-performing catalytic system surely goes through a good balance of Lewis acidic and basic properties.

Known for their Lewis acidity and versatility of coordination modes, lanthanides (Ln) could also be a class of metallic elements to consider for this application; however, they have been scarcely used for H2 production from amine-boranes to date. The few examples appearing in the literature though have provided evidence of their great potential as homogeneous catalysts for this reaction and gave hints that the coordination and activation of "amine-borane" fragments largely depends on the properties of the metal involved: whilst sigma-(B-H) borane complexes are prevalent for late-transition metals, early-transition and main-group metals tend to generate amido-borane derivatives [M(NH2BH3)]. However, neither substrate variation nor reaction conditions optimization has been performed for the few complexes described in the literature.

For this project proposal, the catalysts to be studied are homogeneous in nature and will consist in lanthanide and 3d metal complexes supported by polydentate ligands, including (NCP), (NNN) and (NCN) systems. Novel polytopic symmetrical (NNN)/(NCN) and unsymmetrical (NCP) ligands have been recently designed and prepared at ICCOM-CNR and will be used to prepare novel metal complexes as homogeneous single-site catalysts. Their structure, reactivity and interaction with AB will be jointly studied at the INEOS and ICCOM laboratories. Variable temperature spectroscopic [multinuclear (1H, 13C, 31P, 11B) NMR, IR, UV-Visible] measurements and quantum-chemical calculations to gain insights into the AB-metal complex interaction and overall energetics [evaluation of the reaction thermodynamics (DG) and kinetics (DG#, Transition States)] will be performed to study the reaction mechanism. H2 formation and evolution may occur after an initial dihydrogen bonding (DHB) interaction between the hydridic and protic H atoms in the system. The data on structure and properties of various reaction intermediates will be obtained, together with the kinetic and thermodynamic parameters of each step. Attempts to isolate the reaction intermediates identified during these studies will be made (if stable at ambient conditions). The metal atom influence on structure, vibrational frequencies, electron density distribution and reactivity of the starting catalysts will be acquired.

Research goals

The objective of this bilateral CNR/RFBR project is the preparation of novel homogeneous AB dehydrogenation catalysts and the analysis of their catalytic performance, using a comprehensive approach based on the combination of experimental and theoretical methods. A special emphasis will be put on the assessment of the role of the intermolecular dihydrogen bonding that precedes the H2 release and of other non-covalent interactions that influence the reaction pathway. In addition, the effect of the ligands type and substituents, as well as that of the metal on the H2 release temperatures, kinetics and yields will be rationalized, with the aim of creating second-generation systems with improved efficiency. The project involves the Istituto di Chimica dei Composti Organometallici of Florence (ICCOM) of the National Research Council (CNR) and the Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences (INEOS) of Moscow. All the participants are largely experienced in this research field, and have a sound track of highly positive international collaborations including the participation, either as Coordinating or Participating team, in numerous international projects (4th RFP, 5th RFP, 6th RFP, INTAS, NATO, GDRI). This collaboration has already produced several joint publications and joint presentations at International and National meetings (see the attached participants CVs for the detailed list).

Last update: 08/07/2025