Mixed-valence molecular magnets based on [TCNQ]₃^{2 -} building block: Structures, magnetic properties and DFT analyses

Two radical compounds, [CNQl]₂[(TCNQ)₃] (1) and [CNPy]₂[(TCNQ)₃] (2) where CNQl⁺ = 1-(4′-cyanobenzyl)quinolinium and CNPy⁺ = 1-(4′-cyanobenzyl)pyridinium, have been prepared and characterized structurally. In crystals of both 1 and 2, there are two crystallographic independent TCNQ species (labeled as TCNQ (A) and TCNQ (B)) in an asymmetric unit. The dihedral angle between the mean-molecular-planes of unit A and B, defined by the corresponding quinodimethane groups in TCNQ, is 0.9° in 1 versus 68.2° in 2. The unit A and B stack in the pattern ...ABA...ABA... along b-axis in 1, while in two homogeneous types (...AA... and ...BB...) along b-axis in 2, respectively. The typical magnetic characteristics in both 1 and 2 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} versus T deviate from linearity. The IR spectra clearly show the existence of a mixed-valence state in both 1 and 2, namely, TCNQ (A) and (B) units possess the different negative charge. Even if 1 and 2 have the same molar ratio between countercation and TCNQ species (Cation: TCNQ = 2:3), their EPR spectra are significantly different. An isotropic signal with g = 2.0066 is observed in 1, while two sets of signals in 2, in which one is isotropic with g = 2.0062 and the other possesses five-fold hyperfine splitting structure that arise from the coupling between spins of electron and four equivalence nuclear of H-atoms in a quinodimethane group, with g = 2.0067 and A = 0.25 Gs. Calculations for charge distribution in TCNQ (A) and TCNQ (B) units were performed and the results disclosed the TCNQ (A) and TCNQ (B) units may be assigned to TCNQ^-1 and TCNQ⁰ in 1, respectively, and TCNQ^-δ (where partial charge δ = 0.62-0.72) in 2.