Synthesis, Characterization and Redox Behaviour of 1,2,4 - Dithiazolium Salts and their Radicals.

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Shotonwa, Ibukun O.
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Lethbridge, Alta. : University of Lethbridge, Dept. of Chemistry
This thesis describes the synthesis, characterization and redox behaviour of three potential candidates for molecular conductors with the aim of determining whether or not a stable neutral species is accessible. The target compounds are 3,5-diphenyl-1,2,4-dithiazolium perchlorate salt 3+, 3-diethylamino-5-phenyl-1,2,4-dithiazolium perchlorate salts 1+ and 3,3,5,5-tetramethyl-1,2,4-dithiazolium hexafluorophosphate salt 14+. Crystals of 3+ are monoclinic, space group P21/c, a = 8.9194 (11) Å, b = 10.8047 (14) Å, c = 15.422 (2) Å, β = 102.91 (2) º, V = 1448.67 (3) Å3, Z = 4 and R = 6.79 %. Crystals of 1+ are monoclinic, space group P21/n, a = 7.4531 (9) Å, b = 12.0752 (15) Å, c = 17.09 (2) Å, β = 102.5 (10) º, V = 1501.6 (3) Å3, Z = 4 and R = 2.9 %. Crystals of 14+ are monoclinic, space group P21/c, a = 9.344 (3) Å, b = 19.100 (7) Å, c = 7.383 (3) Å, β = 101.815 (5) º, V = 1289.74 (8) Å3, Z = 4 and R = 12.1 %. They all possess flat central NC2S2 rings with almost coplanar aryl and alkylamino substituents. The redox-active properties of these compounds were investigated via cyclic voltammetry and rotating disk electrode (RDE) voltammetry in CH2Cl2 and CH3CN solutions containing 0.4 and 0.1 M [nBu4N][PF6] on platinum and glassy carbon electrodes. Cyclic voltammetry revealed the three compounds to show uniform behaviour with chemically and electrochemically irreversible 0/+1 reduction processes with strong offset reoxidation waves that show dependence on scan rates, solvents and substituent size. Diffusion coefficients were estimated by RDE voltammetry in CH3CN/0.1 M [nBu4N][PF6]: 3, 8.48(2) × 10-6 cm2/s; 1, 4.51(4) × 10-5 cm2/s; 14, 4.30(2) × 10-5 cm2/s. The electronic structures of cationic, neutral, anionic and dianionic species of 3, 1, and 14 are herein discussed in the light of density functional theory (DFT) calculations. Digital simulations of experimental CVs detected an E1E2C mechanism (open anion model) that displayed slow electron transfer rates for both the reductive and offset oxidation processes coupled with the determination of rate constants for the follow up first order homogenous chemical step. An alternate mechanism is the E1C2E2C2’ (dimer model) wherein the neutral radicals act as “spectator” molecules in the electrochemical-chemical cycle without having any effect on the equilibrium. Both mechanisms had very common parameters (E^0,k_s,k_f and K_eq) extracted from them in addition to giving clues for the non-detection of neutral radicals via SEEPR experiments. On the basis of energy requirements, the open anion model is favoured while on the basis of extent of fits between theoretical and experimental CVs, the dimer model is better favoured.
Electrochemistry , Density Functional Theory , X-ray Crystallography , Simulation