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

Abstract

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, TCNQF 2, and TCNQF 4 . 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 TCNQF 4 acceptor, a 1:1 compound formulated as ( 1 )(TCNQF 4 ) was obtained with a charge ρ ( 1 ) = +1 on 1, while with TCNQF 2, a 2:1 conducting salt formulated as ( 1 ) 2 (TCNQF 2 ) 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. C TTF –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 TCNQF 4 salt. It is shown that TTF oxidation also activates the TTF sp 2 hydrogen atom located α to the iodine atom, toward the preferential formation of C TTF –H α ···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.