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, 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 center dot center dot center dot 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 rho((1)) = +1 on 1, while with TCNQF(2), a 2:1 conducting salt formulated as (1)(2)(TCNQF(2)) was characterized with rho((1)) approximate to +0.5. With TCNQ itself, both a 1:1 conducting phase, (1)(TCNQ) with rho((1)) approximate to +0.5, and a 2:1 (1)(2)(TCNQ) compound with rho((1)) = 0 were crystallized. Transport and magnetic properties were rationalized, based on the rho((1)) value and band structure calculations. CTTF-I center dot center dot center dot N C halogen bond interactions were observed in the mixed valence [rho((1)) = +0.5] salts and even in the neutral [rho((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 alpha to the iodine atom, toward the preferential formation of CTTF-H-a center dot center dot center dot 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 center dot center dot center dot N hydrogen and C-I center dot center dot center dot N halogen bonds and their relative sensitivity to charge.