From Molecular Crystals to Electronic Devices: Investigating Structure-Property Relationships of Iron(III) and Cobalt(II) Coordination Complexes.

Metal-organic semiconductors based on earth-abundant 3d transition metals offer great potential for optoelectronic devices, yet establishing direct structure-property relationships between specific supramolecular networks, electronic structure, and charge transport remains a major challenge. In this work, we address this gap through a combined experimental and theoretical investigation of two new coordination complexes, [Fe-(N 3 L 1 )-(HN 3 L 1 )]-Cl4.4-(H2O) (complex 1) and [Co-(HN 3 L 1 )-(H2O)2Cl]-Cl2.3-(H2O) (complex 2), derived from the previously unexplored 4-imidazole-2,6-di-(pyrazinyl)-pyridine ligand. Single-crystal X-ray diffraction reveals extensive supramolecular architectures stabilized by π···π stacking and hydrogen bonding involving lattice water molecules and chloride counterions. Hirshfeld surface analysis and 2D fingerprint plots quantify the dominant intermolecular contacts, while molecular electrostatic potential, Quantum Theory of Atoms in Molecules, and NCI plot calculations confirm significant anion···π and lone-pair···π interactions in the solid state. Optical absorption studies yield band gaps of 2.02 and 2.16 eV for complexes 1 and 2, respectively, positioning them for visible-light applications. Fabricated Schottky barrier diodes (ITO/sample/Au) demonstrate rectifying behavior, with device 1 exhibiting a rectification ratio of approximately 66 and a conductivity of 9.2 × 10-5 S m-1, outperforming device 2 (rectification ratio ∼17, conductivity 1.14 × 10-5 S m-1). Periodic DFT calculations reveal that the superior electrical performance of complex 1 stems from localized intra-gap states associated with the deprotonated imidazolyl moiety, which facilitate charge transport through hopping mechanisms. These findings elucidate the crucial role of molecular packing and ligand deprotonation in tuning charge transport in functional metal-organic materials.