Inert Carbon Free Radicals. 14. Synthesis, Isolation, and Properties of Two Strongly π−π Interacting Mixed-Valence Compounds: The Perchloro-4,4‘-ethynylenebis(triphenylmethyl) Anion Radical Potassium (18-Crown-6) Salt and the Perchloro-α,α,α‘,α‘-tetraphenyl-p-xylylene Anion Radical Tetrabutylammonium Salt

Abstract

The perchloro-4,4‘-ethynylenebis(triphenylmethyl) anion radical (11) potassium 18-crown-6 salt and perchloro-α,α,α‘,α‘-tetraphenyl-p-xylylene anion radical (15) tetrabutylammonium salt have been isolated in pure form as solids perfectly stable in the air and moisture, withstanding temperatures up to 200 °C. Their physical and chemical properties have been studied using UV−vis, NIR, IR, ESR, cyclic voltammetry, magnetic susceptibility, and electrical conductivity and compared with other already reported and structurally related perchlorotriphenylmethyl systems in which an intramolecular “spin-charge exchange” takes place. In contrast with those compounds, the acetylene anion radical 11 shows an unusual band in the vis−NIR region (754 nm, 37.7 kcal/mol), not present in its corresponding biradical 10 and dianion 9, and the Thiele anion radical 15 shows a typical, but bathochromically shifted ca. 10 kcal/mol, radical band and an unusual NIR band at 1342 nm (21.2 kcal/mol) of high intensity. Application of Hush's theory of mixed-valence class III compounds to these unusual bands for PTM compounds, gives the values of ca. 18 and 10 kcal/mol for the resonance energy of 11 and 15 respectively. The close coincidence of these theoretical values with those obtained by comparing the UV−vis spectra of 11 and 15 with the PTM anion radical systems without resonance interaction can be regarded as a preliminary test of the validity of Hush's theory of organic mixed-valence class III compounds. The cyclic voltammograms of anion radicals 11 and 15 indicate that their precursors, the biradical 10 and perchloro-Thiele hydrocarbon 16 have suitable redox potentials as acceptor components of electrically conducting charge-transfer complexes. 11 and 15 present magnetic properties similar to those of PTM radicals, but as a consequence of the “spin charge resonance” they show a halving of the hyperfine coupling constants. More spectacular differences are shown in the electrical conductivity of 11 and 15 with values of 10-4 and 10-6 S cm-1, respectively, which can be a consequence of their molecular design as suggested the ΔE1/2 differences (6.4 vs 12 kcal/mol). These conductivity values are in the range of the corresponding TCNQ salts.