New 3D Ni and Co redox-active metal–organic frameworks based on ferrocenyl diphosphinate and 4,4′-bipyridine ligands have been synthesized, characterized by single crystal X-ray diffraction and spectroscopic techniques and explored as stable electrocatalysts capable of meeting two important parameters: the overpotential and Tafel slope (TS) in the hydrogen evolution reaction (HER). The electrochemical studies suggest that the reaction kinetics of a Ni-MOF (1) catalyst is more favorable than that of a Co-MOF (2) catalyst. Particularly, Ni-MOF exhibits better HER performance with an overpotential of 350 mV at a current density of 10 mA cm⁻², a small TS of 60 mV dec⁻¹ and superior long-term durability (of up to 10 000 cycles), ranking it among the most active non-noble metal-based molecular electrocatalysts. The introduction of a 4,4′-bpy linker in 2 significantly changes the catalytic properties in an organic or aqueous environment compared to 1D cobalt polymers based on ferrocenyl diphosphinate. For Co-MOF 2, there is a significant decrease in the overvoltage by ∼440 mV in comparison with the 1D Co polymer in an organic medium and by 50 mV in an aqueous medium. The TS changes from 120 to 65 mV dec⁻¹ when moving from 1D CofcdHp to a 3D structure of 2. Thus, a 4,4′-bpy linker reduces the overvoltage and gives more favorable HER kinetics (lower TS). These results provide important guidelines for the rational design of non-precious metal electrocatalysts.

New redox active 1D helical coordination polymers M(fcdHp) (M(II) = Zn(1), Co(2)) have been obtained by utilizing the 1,1′-ferrocenylenbis(H-phosphinic) acid (H2fcdHp) ligand and Zn or Co nitrate salts. Complexes 1 and 2 are isomorphic, crystallizing in the chiral space groups P4₁22 and P4₃22, respectively. Their redox, electrocatalytic and other properties are described. These compounds incorporated into carbon paste electrodes and exhibited reversible redox reactions, arising from the ferrocenyl moiety. These coordination polymers are efficient as electrocatalysts for the reduction of protons to hydrogen. Using N,N-dimethylformamidium ([DMF(H)⁺]) as the acid in the acetonitrile solution, Co CP (2) displays a turnover frequency of 300 s⁻¹, which is among the fastest rates reported for any CP electrocatalyst in CH₃CN. This high rate of catalytic reaction comes at the cost of the 820–840 mV overpotential at the potential of catalysis. As the hydrogen evolution reaction (HER) catalysts, the CPs exhibited in 0.5 M H₂SO₄ the overpotential η₁₀ of 340 or 450 mV, onset overpotential of 220 or 300 mV (vs. RHE), Tafel slope of 110 or 120 mV dec⁻¹, correspondingly for 1 and 2, and considerable long-term stability for the HER.

A new redox active coordination polymer [Co(H₂O)₂(Fc(PHOO)₂)·2H₂O]_n (Fc = ferrocene) was synthesized by the diffusion method and characterized by single-crystal X-ray diffraction. Phosphinate fragments formed infinite chains connected through ferrocene fragments to 2D PCP associated with cobalt atoms. Additional strong hydrogen bonding via the participation of coordinated and lattice water molecules resulted in the formation of a 3D network. The polymer exhibited water-induced crystal-to-amorphous transformation with chromotropism, as confirmed by spectroscopic techniques, elemental analysis, TGA and XRPD. The explosion of the amorphous phase [Co(Fc(PHOO)₂)]_n to form water vapor readily led to its conversion into the initial crystalline [Co(H₂O)₂(Fc(PHOO)₂)·2H₂O]_n with a reverse change of color. The dehydrated form of the polymer selectively sorbed water and could be used as a material with the function of an express dehydrator. Moreover, a modified electrode could qualitatively determine the water content in the solvent at a concentration of 0.05%. Both hydrated and dehydrated forms were evaluated as electrode materials for supercapacitors. The [Co(Fc(PHOO)₂)]_n (dehydrated form) electrode exhibited high specific capacitance and excellent cycling stability. Its maximum specific capacitance was 2517 F g⁻¹ at a current density of 2 A g⁻¹, and the specific capacitance retention was about 90.1% after 1000 cycles.