Four novel ion-pair compounds consisting of 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical: Syntheses, crystal structures and properties

Four novel ion-pair compounds, [IBzmethylPy][TCNQ] (1), [IBzdimethylPy][TCNQ] (2), [BrBzdimethylPy][TCNQ] (3) and [CNBzdimethylPy][TCNQ] (4) where [IBzmethylPy]Br = 1-(4-iodobenzyl)-3-methylpyridinium bromide, [IBzdimethylPy]Br = 1-(4-iodobenzyl)-3,5-dimethylpyridinium bromide, [BrBzdimethylPy]Br = 1-(4-bromobenzyl)-3,5-dimethylpyridinium bromide and [CNBzdimethylPy]Br = 1-(4-cyanobenzyl)-3,5-dimethylpyridinium bromide, have been prepared and structurally characterized. The difference of substituent group of pyridine ring leads to exhibit different packing structures. Two neighboring cations of [IBzmethylPy]⁺ overlap into a dimer via π-π stacking between benzene rings in compound 1, while the cation are connected by hydrogen bonds and van de Waals interactions to form a 3D supramolecular architecture containing 1D channels in 2-4. A strong dimer of [(TCNQ)₂]^2- is formed, and the adjacent dimers slide to each other along the direction of the shorter molecular axis of TCNQ^- in 1-4. The typical magnetic characteristics in 1-4 are: (1) strong AFM coupling interaction within a TCNQ stack leads to the χ_MT values at room temperature much smaller than expected spin-only value; (2) temperature dependent magnetic coupling behavior causes the plot of χ_M^-1 m versus T deviate from linearity; (3) The temperature dependences of magnetic susceptibility show spin gap of singlet-triplet feature. The best simulations to susceptibility data in the range 1.8-300 K yielded the parameters Δ/k_B = 1723 K and zJ′/k_B = -11 K for 1; Δ/k_B = 1689 K and zJ′/k_B = -7 K for 2; Δ/k_B = 1682 K and zJ′/k_B = -6 K for 3; Δ/k_B = 1684 K and zJ′/k_B = -8 K for 4. The paramagnetic behavior of compounds 1-4 is proven by their electron paramagnetic resonance spectra with the characteristic bands of the radicals at 2.0063, 2.0067, 2.0060, and 2.0064, respectively.