Role of Alkyl Substituent and Solvent on the Structural, Thermal, and Magnetic Properties of Binary Radical Salts of 1,2,3,5-Dithia- or Diselenadiazolyl Cations and the TCNQ Anion

Abstract

The synthesis, structural, thermal, and magnetic properties of a series of simple binary organic salts based on the radical anion of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 4-(N-alkylpyridinium-3-yl)-1,2,3,5-dithiadiazolyl (DTDA), 1R (R = Et, Pr, Bu), radical cations and their heavier selenium analogues (DSDA), 2R, are described. Single-crystal X-ray structural analyses reveal that short alkyl substituents on the pyridinium moiety of DTDA/DSDA cations lead to crystallization of isostructural acetonitrile (MeCN) solvates 1Et·MeCN, 1Pr·MeCN, 2Et·MeCN, and 2Pr·MeCN with trans-cofacial DTDA radical cation and eclipsed-cofacial TCNQ radical anion dimers. A slight increase in the substituent chain length to butyl affords the solvate 1Bu·0.5MeCN or the nonsolvate 1Bu. The nonsolvate 1Bu can be exclusively isolated using propionitrile (EtCN), whereas the isostructural selenium analogue 2Bu crystallizes from MeCN. The crystal packing in 1Bu·0.5MeCN and 1Bu/2Bu is distinctively different: rare one-dimensional (1D) columnar π-stacks of evenly spaced TCNQ radical anions with periodic distortions along the vertical stacking direction and cis-cofacial DTDA dimers in 1Bu·0.5MeCN vs discrete, non-eclipsed-cofacial TCNQ dimers and trans-antarafacial DTDA/DSDA dimers in 1Bu/2Bu. The nonsolvated structure 1Pr with trans-cofacial DTDA and non-eclipsed-cofacial TCNQ dimers can be isolated from EtCN. Single-crystal and powder X-ray diffraction methods confirmed a thermally driven, irreversible, single-crystal-to-single-crystal structural transformation between 1Pr·MeCN and 1Pr. Thermogravimetric analyses of all nonsolvated salts show varied, yet robust, thermal behavior, while the thermal behavior of the solvates is consistent with more facile lattice solvent loss from structures with longer N-alkyl chains. Variable-temperature magnetic susceptibility measurements indicate that all structures are diamagnetic at low temperatures. However, thermally populated magnetic states could be observed for 1Et·MeCN, 1Et·EtCN, 1Pr·MeCN, 1Bu·0.5MeCN, 1Bu, and 2Bu at higher temperatures. This can be correlated with desolvation and structural changes that lead to the generation of weakly antiferromagnetically coupled non-eclipsed-cofacial TCNQ dimers, in agreement with results from density functional theory (DFT) calculations.