BEDT-TTF-Salze mit quadratischen Platinaten(II) als Gegenionen: [BEDT-TTF]4[Pt(CN)4] / BEDT-TTF-Salts with Square Platinates(II) as Counter Ions: [BEDT-TTF]4[Pt(CN)4]

Abstract

The compound [BEDT-TTF]4[Pt(CN)4], BEDT-TTF = Bis(4.5-ethylendithiolo)tetrathiafulvalen = C10H8S8, stoichiometry C44H32N4S32Pt, Mr = 1837.9 (g/mol), was obtained by electrocrystallization in at least three different modifications. Two of them could be identified by single crystal X-ray structure elucidation. 1 = β-modification, is isolated in black, lustrous platelets: triclinic, P 1̄, Z = 1, α = 9.7280(30) Å, b = 10.9237(27) Å, c = 16.5432(60) Å, a = 95.820(25)°, β = 98.296(26)°, γ = 115.215(24)°, V = 1547.56 A3, dc = 2.0 (g/cm3), room temperature, Rw = 0.082 for 7322 observed reflections. 2 = y-modification, is isolated in black not very regular crystals: monoclinic, C 2/c, Z = 4, a = 11.163(6) Å, b = 33.56(2) Å, c = 16.56(1) Å, β = 91.39(4)°, V = 6202.04 A3, dc = 1.97 (g/cm3), room temperature, Rw = 0.053 for 1404 observed independent reflections. Both solids contain sheets of BEDT-TTF cations separated by sheets of the counter anions. 1 contains two crystallographically independent BEDT-TTF moieties while 2 contains four independent donor units per cell. Of interest are the very different lengths of the central C=C bonds of the BEDT-TTF moieties which are assumed to reflect the charge density on these ions. In 1 these distances are 1.350(14) Å and 1.306(14) A, respectively, for the two different BEDT-TTF cations in the lattice, in 2 they are 1.372(34) A, 1.401(42) A and 1.289(54) Å respectively. The remarkable differences in these bond lengths indicate, according to published concepts, quite different charges on the different BEDT-TTF units. Two of the distances are shorter than in the neutral donor. Measurements of the specific resistance and the thermopower show that 1 is a metal with a room temperature conductivity of 10 S/cm. At around 200 K 1 becomes semiconducting. 2 turns out to be a semiconductor with a room temperature conductivity of 1—2 S/cm. Around 225 K a structural phase transition occurs. ESR measurements show an antiferromagnetic coupling between the spins in the solid. A third modification (δ) (semiconducting with a room temperature conductivity around 2 × 10-2 S/cm) was identified by ESR investigations.