Nitrogen is a vital element for the development of many basic building biomolecules which sustain living organisms in the planet. Although nitrogen, in form of the molecule N2, is largely present in the atmosphere (78%), plants and animals are not able to use it directly (nitrogen fixation) owing to the high chemical inertness and stability of the molecule. The most important nitrogen fixation process so far developed is the Haber–Bosch (HB) reaction, which was firstly invented by Fritz Haber and then industrially developed by Carl Bosch more than one century ago (Fig. 1).[1] The HB process converts nitrogen to ammonia by reaction with hydrogen using a metal-based catalysis under high temperatures (300-500 °C) and pressures (100-300 atm). Via the HB process more than 150 million metric tons (t) of ammonia are produced per year.[1] The produced ammonia via the HB synthesis represents today the main source for the direct or indirect production of fertilizers for food. It is estimated that about half of the planet's population bases their own food on the HB synthesis and that half of the nitrogen present in the human body originates from ammonia coming from the HB process. Ammonia is the second most important chemical in the world and is commonly used not only for the production of fertilizers, but also for pharmaceuticals and synthetic fibers. More recently, ammonia has gained much attention also as a hydrogen carrier due to its high hydrogen density and low liquefying pressure. It is therefore evident that such method is vital for life of living organisms and for industry and future society. Unfortunately, the HB process suffers from serious drawbacks in terms of energy demand and greenhouse gas emission. It is calculated that this process accounts for ca. 2% of the global energy (13% of the world’s electric energy) and is responsible for ca. 1.5% of the global CO2 emissions (more than 400 Mt) in the world. It is clear that the HB process is fully incompatible with national and international targets on energy saving and greenhouse gas emissions, and that greener alternatives are highly sought after. Over the past decades, metal-organic frameworks (MOFs) have emerged as an extensive class of crystalline materials with ultrahigh porosity and enormous internal surface areas. They are comprised of infinite networks of metal centers bridged by simple organic linkers through metal–ligand coordination bonds (Fig. 2a). These properties have made MOFs of utmost interest for a variety of applications including catalysis and photocatalysis,[2] drug delivery, gas capture, and gas separation.[3] However, the most common synthetic procedures so far pursued still rely on harsh reaction conditions consisting in heating the precursors for long reaction time under autogeneous pressure in often flammable and toxic volatile organic compounds (VOCs 

Deep Eutectic Solvents (DESs) represent an emerging class of green and bio-renewable solvents. They are eutectic mixtures of acids and bases (Lewis or Brønsted) exhibiting a melting point much lower than either of the individual components (Fig. 2b). They are readily prepared by mixing and heating two or three safe and nature-inspired components able to engage in reciprocal hydrogen bond interactions. Thus, no waste is generated in the process and no purification is required. In comparison with conventional VOCs, DESs show high thermal stability, non-flammability and practically no vapor pressure. In addition, DESs allow: (a) the easy isolation of the final products through distillation, precipitation or extraction, and (b) the fine tuning of their physical and chemical properties for a range of applications by simply varying their chemical constituents. Similarly to MOFs, the chemistry of DESs has experienced in the last decades an almost unparalleled growth, as evidenced by not only the sheer number of research papers published, but also the ever expanding scope of the research (Fig. 3).[4 ]

The use of DESs in the synthesis of MOFs is still in its infancy. Recent pioneering studies in this field have shown clear advantages as DESs can act not only as solvents, but also behave as structure-directing agents with one of their components being also present in the architectures, either as a ligand coordinated to the metal centers or in the pores of MOFs.[5] Removal of DES components from the MOF in turn revealed to be an effective strategy for preparing novel porous architectures.[6] DESs have also been employed for post-synthetic modification of MOFs to afford composites to be used in catalysis (Fig. 4).[7] The high proportion of active sites and porosity have also made MOFs highly promising as heterogeneous catalysts in C–C and C–heteroatom coupling reactions, however, always using VOCs jointly with precious metals such as Pd. MOF-catalyzed cross-coupling reactions still remain unexplored in DESs.[8]