Influence of the Increasing Number of Organic Radicals on the Structural, Magnetic, and Electrochemical Properties of the Copper(II)-Dioxothiadiazole Family of Complexes

Abstract

The preparation, structures, and electrochemical and magnetic properties supported by density functional theory (DFT) calculations of three new copper(II) compounds with [1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 1,1-dioxide (td) and its radical anion (td^·-) are reported: {[Cu^IICl(td)](μ-Cl)₂[Cu^IICl(td)]} (1), which incorporates only neutral td ligands; [Cu^IICl(td^·-)(td)]·2MeCN (2), which comprises one neutral td and one radical td^·-; and PPN[Cu^IICl(td^·-)₂]·2DMA (3), where Cu^II ions are coordinated by two radical anions td^·- (DMA, dimethylacetamide; PPN⁺, the bis(triphenylphosphine)iminium cation). All three compounds show interesting paramagnetic behavior with low-temperature features indicating significant antiferromagnetic coupling. The magnetic properties of 1 are dominated by Cu^II···Cu^II interactions (J_CuCu) mediated through the Cl^- bridges, while the magnetic properties of 2 and 3 are governed mainly by the td^·-···td^·- (J_tdtd) and Cu^II-td^·- (J_Cutd) exchange interactions. The structure of 2 features only two major magnetic coupling pathways enabling the fitting of experimental data with J_tdtd = -36.0(5) cm^-1 and J_Cutd = -12.6(2) cm^-1 only. Compound 3 exhibits a complex network of magnetic contacts. Attempt to approximate its magnetic behavior using only a local magnetic contacts model resulted in J_tdtd = -5.6(1) cm^-1 and two J_Cutd constants, -12.4(2) and -22.6(4) cm^-1. The experimental fitting is critically compared with the results of broken symmetry density functional theory (BS DFT) calculations for inter- and intramolecular contacts. More consistent results were obtained with the M06 functional as opposed to popular B3LYP, which encountered problems reproducing some of the experimental intermolecular exchange interactions. Electrochemical measurements of 2 and 3 in MeCN showed three reversible nearly overlapping redox peaks appearing in a narrow potential range of -600 to -100 mV vs Fc/Fc⁺. Small differences between the redox events suggest that such compounds may be good candidates for new switchable materials, where the electron transfer between the metal and the ligand center is triggered by temperature, pressure, or light (valence tautomerism).