Syntheses, Structures And Properties Of Ion-pair Charge-transfer Compounds

The synthesis of new and designed crystal structure is part of a major strand of modern chemistry, in which a variety of means have been used in the attempt to design and control the products and crystal structures. The study of utilizing crystal-oriented syntheses to generate functional materials has recently attracted considerable attentions. Segregated-stack compounds with strong intermolecular contacts along the stacks probably posses a wide range of ground states at low temperatures (such as superconducting, spin density wave, Peierls insulators, spin-Peierls insulators and so on). The magnetic [M(mnt)2]^- monoanion (M = Ni, Pd and Pt, mnt = maleonitriledithiolato) are endowed to an advantage to form face-to-face stack due to their extended electronic structure and planar molecular geometry. Employing benzylpyridinium derivatives ([RBzPy]⁺, R represents a substituent group) with flexible molecular configuration as counter-cation to control arrangements of [M(mnt)2]^-monoanion, a series of ion-pair compounds with segregated stacks cations and anions have been constructed, some of which exhibit a spin-Peierls-like transition. TCNQ" radical ion shows a planar molecular and extended electrical structures that are similar to [M(mnt)2]^-monoanion, and has been extendedly employed to build molecular solids with low-dimensional conducting and magnetic feature, in which the electronic transport and magnetic coupled interactions can be achieved throughÏ€.. .Ï€interactions between the radical along the stacking direction. Such characteristic of TCNQ^- radical ion stimulates us to extend our research to the series of [RBzPy][TCNQ] compounds in order to gain more insight into the relationship between intermolecular cooperation interactions and magnetic properties of the compound with low-dimensional structural feature. In this paper, six ion-pair charge- transfer compounds consisting of 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical and derivatives of benzylpyridinium or benzyl quionlinium with molar ratio of 1:1 (1-3),2:1(4) and 2:3(5,6) were prepared and characterized structurally. In crystals of 1 and 3, TCNQ^- monoanion and corresponding cations form segregated-stacks, and the stacks are regular in 1 while irregular in 3. In crystal of 2, TCNQ^- monoanion form isolatedÏ€-dimers. In crystals of 4,5 and 6, there are two crystallographic independent TCNQ species. In crystal of 4, they possess almost equal fractional negative charge (-0.5), while in crystals of 5 and 6, the valences of TCNQ are -1 and 0, respectively. Compound 1 and 4 are paramagnetic, their magnetic behaviors do not fit well with 1D magnetic chain models. Very weak paramagnetic features of 2 and 3 arise from the magnetic impurities and the inherent magnetic features for the two compounds should be diamagnetic. The typical magnetic characteristics in both 5and 6 are: (1) strong antiferromagnetic(AFM) coupling interaction within a TCNQ stack leads to the x_mT values at room temperature much smaller than expected spin-only value; (2) temperature dependent magnetic coupling behavior causes the plot of x_m^-1 versus T deviate from linearity.