Zinc and Cobalt Coordination Polymers Based on the Redox-Active Linker 4,4′-(Phenazine-5,10-diyl)dibenzoate: Structures and Electrochemical Properties

The novel coordination polymers [Zn(PZDB)(DEF)₂]_n (Zn-PZDB) and [Co(HPZDB)₂(DEF)₂]_n (Co-HPZDB) (H₂PZDB = 4,4′-(phenazine-5,10-diyl)dibenzoic acid, DEF = N,N-diethylformamide) are synthesized solvothermally from the metal nitrate salts and the linear H₂PZDB linker with the redox-active phenazine-5,10-diyl core. Zn–PZDB is composed of zigzag chains with the Zn ion tetrahedrally coordinated by two diethylformamide (DEF) molecules and two carboxyl O-atoms from the bridging PZDB²⁻ linker. The crystal structure of Co-PZDB represents a two-dimensional (2D) coordination grid. The Co²⁺ ion is octahedrally coordinated by two DEF molecules and four carboxyl oxygen atoms of the semi-deprotonated HPZDB⁻ linker molecules. Yellow Zn-PZDB and red Co-PZDB turn green upon air exposure, which is due to linker oxidation, forming the (H)PZDB⁺ radical cation. UV/Vis/NIR spectroelectrochemistry reveals that Me₂PZDB undergoes two reversible one-electron oxidations, producing characteristic absorption bands. Similar spectroscopic changes are observed upon oxidation of Zn-PZDB and Co-HPZDB. Chemical oxidation with SbCl₅ yields the same color changes together with electron paramagnetic resonance signals typical of ligand-based radical cations. These findings indicate that the PZDB linker is the primary redox site. Ligand oxidation is followed by disintegration of the coordination polymers.