Crystal structure and properties of the 7,7,8,8-tetracyano-p-quinodimethanide salt of the 1,1′-bis-(p-cyanophenyl)-4,4′-bipyridinium dication

Abstract

The crystal structure of [1,1′-bis-(p-cyanophenyl)-4,4′-bipyridinium]²⁺(7,7,8,8-tetracyano-p-quinodi-methanide)₄^2–, CPP–TCNQ₄, its electrical conductivity, and the properties of the TCNQ salts of five para-substituted dications are reported. CPP–TCNQ₄ is triclinic, space group P, with a= 7.558(5), b= 13.501(4), c= 15.155(4)Å, α= 102.8(2), β= 100.60(4), γ= 98.43(4)°, U= 1 458.5 ų, D_m= 1.36 Mg m^–3, Z= 1, D_c= 1.34 Mg m^–3. The structure was solved by direct methods and refined to a final R of 0.048 for 2 338 observed reflections. The radical anions stack, plane-to-plane, in columns parallel to b, the mean inter-TCNQ spacings being 3.28, 3.36, and 3.00 Å between TCNQ(A)–TCNQ(A′), TCNQ(A)–TCNQ(B), and TCNQ(B)–TCNQ(B′), respectively. Adjacent columns form sheets, parallel to (001), successive sheets being interleaved along c by the CPP dications. The electrical conductivity along b, the direction of stacking, varies as σ=σ₀exp(–E_a/kT) where σ_{300 K} 0.1 S cm^–1 and E_a 0.10 eV.