Base free transfer hydrogenation using a covalent triazine framework based catalyst

Transfer hydrogenation (TH) reaction – the addition of hydrogen to an unsaturated group of an organic molecule from a source other than H2 – has been gaining a lot of attention as it is an appealing alternative to direct hydrogenation. The reasoning behind it is the elimination of pressurised hydrogen and high pressure equipment use. Besides, a conventional hydrogenation catalyst is rarely selective – any present unsaturated or oxidised functional group is exposed to reduction, resulting in, most of the time, a fully hydrogenated product or a mixture of products. Exceptions for highly chemoselective hydrogenation catalysts exist, such as noble metal nanoparticles supported on metal oxides or encapsulated inside metal–organic frameworks. In contrast with conventional hydrogenation, TH allows the reaction to be performed selectively, aiming for a specific unsaturated bond and leaving the rest of the original molecule intact. A conventional TH catalyst is a transition metal complex; among different metals, iridium is the most active one. Ir complexes involving a N-heterocycle carbene ligand, or halfsandwich complexes with a Cp* ligand are typical examples. Moving from soluble organometallic compounds to materials that remain solid under the reaction conditions, facilitates separation and enables recycling. There exist a number of works on heterogeneous TH systems, including magnetic nanoparticle-, polymer-, graphene-, carbon nanotube-, silica-, zeoliteor oxide-supported catalysts. Recently, Porous Organic Frameworks (POFs) have been gaining attention within catalytic and other applications. Covalent Triazine Frameworks (CTFs), a subclass of POFs, are highly porous and stable solids made by trimerisation of aromatic nitriles. CTFs are rich in nitrogen functionalities, and their distribution can be varied by using different building blocks. Starting from a pyridine containing building unit, quasibipyridine moieties become available in the final material. Bipyridine is a widely-applied ligand in organometallic chemistry; its presence within a framework enables anchoring of a transition-metal complex. Following this approach, a number of molecular heterogeneous catalysts were developed for a range of different catalytic reactions by several research groups. Recently, we developed a CTF based catalyst which is highly active in the reversible formic acid (FA) dehydrogenation reaction. Applying different conditions, the same catalyst is active in both hydrogen production from FA and hydrogen storage by reaction with carbon dioxide. Both these reactions