The Conducting Spin-Crossover Compound Combining Fe(II) Cation Complex with TCNQ in a Fractional Reduction State

The radical anion
salt [Fe­{HC­(pz)3}2]­(TCNQ)3 demonstrates
conductivity and spin-crossover (SCO) transition associated with Fe­(II)
complex cation subsystem. It was synthesized and structurally characterized
at temperatures 100, 300, 400, and 450 K. The compound demonstrates
unusual for 7,7,8,8,-tetracyanoquinodimethane (TCNQ)-based salts quasi-two-dimensional
conductivity. Pronounced changes of the in-plane direct-current resistivity
and intensity of the electron paramagnetic resonance (EPR) signal,
originated from TCNQ subsystem, precede the SCO transition at the
midpoint T* = 445 K. The boltzmannian growth of the
total magnetic response and structural changes in the vicinity of T* uniquely show that half [Fe­{HC­(pz)3}2] cations exist in high-spin state. Robust broadening of the EPR
signal triggered by the SCO transition is interpreted in terms of
cross relaxation between the TCNQ and Fe­(II) spin subsystems.