Two-step electrochemically directed synthesis of Pr(4)N(TCNQ)(n) (n=1,2): Preparation, structure, and properties of a magnetically isolated dimer and a quasi-one-dimensional chain

Solid-state electrochemistry of a tetracyanoquinodimethane (TCNQ)-modified electrode in contact with a tetrapropylammonium cation (Pr(4)N(+)) electrolyte showed two electron-transfer steps to give Pr(4)N(TCNQ) 2 (1) and Pr(4)N(TCNQ) (2) rather than the traditional one-electron step to directly give Pr(4)N(TCNQ). Two thermodynamically stable Pr(4)N(+-) TCNQ stoichiometries, 1 and 2, were synthesized and characterized. The degree of charge transfer (rho) calculated from the crystal structure is -0.5 for the TCNQ moieties in 1 and -1.0 for those in 2. Raman spectra for Pr(4)N(TCNQ) 2 show only one resonance for the extracyclic C=C stretching at 1423 cm(-1), which lies approximately midway between that of TCNQ at 1454 cm(-1) and TCNQ- at 1380 cm(-1). Both the magnetic susceptibility and EPR spectra are temperature-dependent, with a magnetic moment close to that for one unpaired electron per (TCNQ) 2 unit in 1, whereas 2 is almost diamagnetic. Pressed discs of both complexes show conductivity (1-2 x 10(-5) Scm(-1)) in the semiconductor range. For 1, the position of zero current for the steady-state voltammograms implies 50 of TCNQ and 50 TCNQ(0) is present in solution, thereby supporting a dissociation of (TCNQ)(2)(-) in solution, but is indicative of only TCNQ(-) being present for 2.