Radical Ion Salts of 2,3-dichloro-5,6-dicyanobenzoquinone and Metallocenes. A Reexamination of Their Magnetic and Spectroscopic Properties

Article abstract of DOI:10.1021/ja00275a036

On the basis of UV-vis, infrared, and Moessbauer spectroscopies and electron spin resonance we have established that the anion of the salts formed from 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) and cobaltocene or decamethylferrocene is not the benzohydroquinone anion, (1-), as previously postulated, but rather the (1-) radical anion.A full S = 1/2 spin/DDQ (g = 2.00517, a₁₄ = 0.59, a₁₃ = 4.72, 4.31, and 2.78 G) is observed by ESR for in solution, whereas as previously reported it possesses < 1 percent S = 1/2 spin/DDQ in the solid state.As the temperature is increased the ESR of the solid signal increases and the behavior is fit to a singlet-triplet behavior (E_a = 0.25 eV).To account for this a structure based on a weak 2(2-) dimer, as reported, for example, for , is proposed.In contrast, a full S = 1/2 spin/DDQ is observed for in either solution or the solid.The magnetic susceptibility characterizes this latter compound as Curie-like for T > 2 K.The (1-) exhibits several characteristic spectroscopic signatures CN = 2220 cm^-1 (s) and ν_C=C 1580 cm^-1 -1 ν_CO absorption present in DDQ is absent>; UV-vis: λ_max with ε > 4000 M^-1*cm^-1 are found at 17000, 18300, 19700, 21900, 23150, 28800, 37000, 38900, and 40500 cm^-1>.The S = 1/2 (1-) in the (1+) salt is monomeric as the complex is isostructural to (1+)(1-).Ab inito molecular orbital calculations on DDQ and (1-) are in good agreement with the observed structural, ESR, and UV-vis data.The (1-) is stable in solution, and the absorptions in the 18000-25000 cm^-1 region are assigned to the first internal transition (2A₂ <-- 2B₁).The calculated spin and charge densities for (1-) show that the majority of the charge resides on the N, O, and Cl, while the majority of the spin resides on the C=O and CN p? orbitals.