The valorization of CO2 to formic acid, which is discussed as a liquid hydrogen carrier, non-toxic source of carbon monoxide and represents an important industrial chemical with a growing demand, has challenged chemists for several decades. Since the direct hydrogenation of CO2 to formic acid is entropically unfavorable and thermodynamically restrained (ΔrG θ 298 K = 32.9 kJ mol -1 ), the integration of deprotonating bases, solvating solvents or esterification agents is often employed, turning the reaction exergonic. However, the resulting isolation of the interacting formate causes a high energy demand for downstream processing and sets prerequisites for the design of the crucial catalytic system. The catalytic species should be easily separated from the reaction solution prior to the thermal downstream process in order to inhibit the reverse decomposition of formate at reduced pressures and elevated temperatures. Initially, the development of suitable active catalysts was mainly driven by the design of molecular transition metal phosphine complexes. Due to their insufficient separability, alternative catalytic approaches were developed, aiming for a heterogeneous nature of the catalyst. Starting from classical carbon supported transition metal nanoparticles, the promoting effect of heteroatom doping has been highlighted by several studies, using nitrogen enriched carbon support materials. Corresponding to the homogeneous metal phosphine interactions, basic sites of the support stabilize the metal particles and induce pronounced electronic effects on the metal, correlating with enhanced catalytic activities. For a more specific tailoring of the ligating sites and accordingly the electronic properties of the metal, the anchoring of molecular ligand sites on silica materials has bridged the gap between molecular and solid catalysis. This heterogenization strategy on solid surfaces covered the immobilization of amines , phosphines , N-heterocyclic carbenes , bidentate P-N ligands or pincer ligands . The resulting catalysts were active in the synthesis of formates, although they often suffer from a lack of intrinsic stability and require complex multi-step preparation procedures. Instead of covalently binding ligands to a surface, porous organic polymers provide fixed ligand sites that are part of the framework structure. Their preparative versatility enables a tailored formation of functionalized macroligands, echoing the design strategies of molecular catalysis. Yang et al. reported the first example of an applicable porous organic polymer, derived from the crosslinked Tröger's base. However, the ruthenium (Ru)-loaded framework was barely active in the CO2 hydrogenation and required the addition of the molecular triphenylphosphine (PPh3) ligand, resulting in significant leaching issues. Inspired by molecular bidentate phenanthroline complexes, Yoon's group incorporated this ligand structure into a macroligand and varied the ligating microenvironment of the metal species. They embedded heptazine, phenantroline, NNN pincer or bipyridine units into the frameworks, acting as coordination sites for Ru or iridium (Ir) species. The range of nitrogen-functionalized frameworks for CO2 hydrogenation was later extended by studies on melamine-based polymers, N,P-containing polymers and pyridine-derived covalent triazine frameworks , the latter being examined in the continuous hydrogenation of CO2 to methyl formate. Moreover, different N-heterocyclic carbenes have been subjected to incorporation approaches and served as fixed donor centers for Ir complexes. Referring to the predominance of soft and strongly ligating phosphines in homogeneous catalysis, polyphosphines have been intensively studied for the immobilization of metal species. Kann et al.