A series of novel ion-pair complexes based on TCNQ: syntheses, crystal structures and properties

Abstract

Four novel ion-pair complexes, (AP)(TCNQ) (1), (CP)₂[TCNQ]₃ (2), (CMP)(TCNQ) (3) and (BQ)₂(TCNQ)₂·CH₃CN (4), where AP = 4-amino-1-benzylpyridinium, CP = 1-(2-chlorobenzyl)pyridinium, CMP = 1-(4-chlorobenzyl)-3-methylpyridinium, BQ = 1-(4-bromobenzyl)quinolinium and TCNQ = 7,7,8,8-tetracyanoquinodimethane, have been constructed and their X-ray single crystal structural analyses showed that the TCNQ anions and cations form a well-separated stacking column. The size and shape of the cations play a predominant role in influencing the arrangements of TCNQ anions and, consequently, in altering the magnetic properties. 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. As for 2, there exists TCNQ⁻¹ and TCNQ⁰ entries, two TCNQ (A) and one TCNQ (B) molecules stack into a triad, and the triads further develop into a column in a pattern ⋯ABA⋯ABA⋯ along the b-axis. In 3 and 4, the TCNQ anions are uniformly spaced to give a one-dimensional (1-D) chain structure through π⋯π interactions, which is firstly observed in the ion-pair system based on TCNQ and benzylpyridinium derivatives. The typical magnetic characteristics in both 1 and 2 are strong AFM coupling interaction within a TCNQ stack leading to the χ_MT values at room temperature, much smaller than expected spin-only value, and temperature dependent magnetic coupling behavior causing the plot of χ_MversusT to deviate from linearity. It is noteworthy that complexes 3 and 4 show a peculiar magnetic transition from paramagnetism in the high-temperature (HT) phase to the spin gap in the low-temperature (LT) phase.