Competition between the C–H···N Hydrogen Bond and C–I···N Halogen Bond in TCNQFn (n = 0, 2, 4) Salts with Variable Charge Transfer

The formation of charge-transfer salts with an iodinated tetrathiafulvalene derivative, EDT-TTF-I (1), has been investigated with a series of TCNQ acceptors of different oxidative strengths, namely, TCNQ, TCNQF, TCNQF2, and TCNQF4. These series of compounds have been prepared in order to investigate the effect of the charge in these complexes on the C–I···NC halogen bond interactions which can take place between 1 and the various TCNQs. With the most oxidizing TCNQF4 acceptor, a 1:1 compound formulated as (1)­(TCNQF4) was obtained with a charge ρ(1) = +1 on 1, while with TCNQF2, a 2:1 conducting salt formulated as (1)2(TCNQF2) was characterized with ρ(1) ≈ +0.5. With TCNQ itself, both a 1:1 conducting phase, (1)­(TCNQ) with ρ(1) ≈ +0.5, and a 2:1 (1)2(TCNQ) compound with ρ(1) = 0 were crystallized. Transport and magnetic properties were rationalized, based on the ρ(1) value and band structure calculations. CTTF–I···NC halogen bond interactions were observed in the mixed valence [ρ(1) = +0.5] salts and even in the neutral [ρ(1) = 0] compound, while they are surprisingly absent from the full charge transfer TCNQF4 salt. It is shown that TTF oxidation also activates the TTF sp2 hydrogen atom located α to the iodine atom, toward the preferential formation of CTTF–Hα···NC hydrogen bonds, also present in the mixed-valence salts. These series provide an opportunity to evaluate the relative strength of competing C–H···N hydrogen and C–I···N halogen bonds and their relative sensitivity to charge.