Differential supercapacitor and Schottky diode behaviours in two new isostructural coordination polymers based on redox active metal ions

Two new isostructural 2D coordination polymers, denoted 1(Mn) and 2(Fe), have been synthesized using the layer diffusion method. These polymers, {[Mn(PDA)(4-bpdb)(H2O)2]·4-bpdb}n and {[Fe(PDA)(4-bpdb)(H2O)2]·4-bpdb}n, incorporated 1,4-phenylenediacetate (PDA) and 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (4-bpdb) ligands. Structural analysis through the single-crystal X-ray study revealed that compound 1(Mn) and compound 2(Fe) exhibited a one-dimensional connectivity along the a-axis between metal ions (Mn(II)/Fe(II)) and 1,4-phenylenediacetate ligands and formed a 2D layered structure interconnected with 4-bpdb. These layers are arranged in an AAA… pattern along the b-axis, forming a three-dimensional supramolecular structure. Non-bonded 4-bpdb molecules occupy voids within this arrangement, stabilized by hydrogen bonds and π⋯π interactions. The redox-active metal centers (Mn2+/Fe2+) present in the coordination polymers hold promise for electrochemical studies. Supercapacitor assessment revealed an intriguing discrepancy: compound 1(Mn) demonstrated nearly 2.5 times greater specific capacitance than compound 2(Fe) in a 1 M H2SO4 medium, despite their isostructural nature. However, cyclic stability testing showed a reverse trend, with compound 2(Fe) exhibiting excellent stability, retaining 98% capacity after 5000 cycles compared to 1(Mn)'s 78%. Conductivity measurements indicated that compound 1(Mn) possesses conductivity 10 times superior to that of compound 2(Fe). Notably, both compounds exhibited potential for Schottky diode fabrication. Overall, this study highlights the influence of specific metal ions on distinctive supercapacitor characteristics and conductivity within isostructural coordination polymers.