Semiconducting Manganese-Bipyridyl Coordination Polymer via a Manganese–Metal-Nanoparticle Approach

The coordination polymer [Na₂Mn₂(4,4'-bipy)₇(2,2'-bipy)₄] is prepared by a novel redox approach using Mn(0) nanoparticles (2.4 ± 0.3 nm in size) as the starting material. The Mn(0) nanoparticles are then reacted with 2,2'-bipyridine (2,2'-bipy) and 4,4'-bipyridine (4,4'-bipy) in toluene at 80 °C. The title compound is composed of _∞¹[Mn(2,2'-bipy)(4,4'-bipy)_3/2] chains and double-stranded _∞¹[Na₂(2,2'-bipy)₂(4,4'-bipy)₄] chains forming an interpenetrating coordination network with five noncharged bipy ligands and six anionic [bipy]^- ligands. Such composition and coordination with bipy as sole ligand and anion are observed for the first time. Structure and composition are validated by X-ray diffraction based on single crystals and powders, XANES spectroscopy, infrared spectroscopy, and elemental analysis. Optical spectroscopy of the black-red, shiny metallic crystals shows absorption below 350 nm with a band gap <1.5 eV. Power-voltage curves indicate semiconducting behavior with a positive temperature gradient and silicon-like conductivity (3 × 10^-4 S/m, 25 °C). In sum, the redox approach using Mn(0) nanoparticles allows for preparation of a Mn-coordinated interpenetrating network with 2,2'-bipy and 4,4'-bipy as sole ligands and semiconducting properties. The redox approach generally offers the option to realize further bipyridyl networks with other metals and semiconducting or metallic properties.