Substituent effects on pancake bonding: solution-phase dimerization equilibria of phenyl- and fluorophenyl-dithiadiazolyl radicals.

Substituent effects on the dimerization of 1,2,3,5-dithiadiazolyl (DTDA) radicals have been studied extensively in the solid state, yet crystal packing forces obscure the intrinsic electronic contribution to pancake bond strength. Here we present the first pairwise solution-phase comparison of 4-phenyl-1,2,3,5-dithiadiazolyl (1) and 4-(perfluorophenyl)-1,2,3,5-dithiadiazolyl (2) using temperature-dependent UV-vis spectroscopy, multi-frequency continuous-wave EPR spectroscopy and Mims-ENDOR spectroscopy. van't Hoff analysis in toluene yields dimerization enthalpies of ΔH = -28.9 kJ mol-1 for 1 and ΔH = -19.5 kJ mol-1 for 2, showing that perfluorination of the aryl substituent weakens the π*-π* pancake bond by approximately 9 kJ mol-1. CW-EPR analysis reveals a small decrease of about 2% in the 14N hyperfine coupling upon perfluorination (room-temperature Aiso = 14.6 vs. 14.4 MHz; 80 K Azz = 39.6 vs. 38.7 MHz). Q-band Mims-ENDOR further resolves the individual ring-substituent couplings and shows that the 19F isotropic hyperfine constants of 2 (Aiso up to 5.7 MHz) exceed the 1H couplings of 1 (Aiso ≤ 2.0 MHz) by up to a factor of five, providing direct experimental evidence for a redistribution of SOMO spin density from the DTDA heterocyclic core onto the perfluorinated aryl ring. DFT calculations qualitatively reproduce the observed hyperfine trends and provide a positional assignment of the individual ring-substituent couplings. The influence of the solvent dielectric environment on the dimerization thermodynamics is additionally assessed by systematic variation of the solvent composition. Both radicals were obtained by a novel metal- and solvent-free route in which elemental sulfur itself reduces the dithiadiazolylium chloride during sublimation.