The Organic Metal (Me₂-DCNQI₎₂Cu: Dramatic Changes in Solid-State Properties and Crystal Structure Due to Secondary Deuterium Effects

Although (R¹,R³-DCNQI)₂Cu salts (1) are isomorphic (space group I4₁/a), they behave differently upon cooling: The group M salts remain metallic down to the lowest temperatures, whereas the group M-I salts show phase transitions to semiconductors at temperatures T_M-I = 160–230 K. With regard to the steric requirements of the methyl group, 1a (CH₃/CH₃/H₂) is expected to belong to group M-I, but it remains metallic even at 0.4 K (σ = 500 000 S cm⁻¹)- Deuterated 1a, however, namely 1c (CH₃/CD₃/H₂), 1d (CD₃/CD₃/H₂), and 1e (CD₃/CD₃/D₂), undergo sharp phase transitions at 58, 73, and 82 K, respectively, the lowest phase-transition temperatures observed so far for copper salts 1 at ambient pressure. Thereby, conductivities drop by 6–8 orders of magnitude within a few kelvins, transforming 1c-e from three-dimensional into strictly one-dimensional systems. This unprecedented strong secondary deuterium isotope effect is also reflected in significant changes in ESR signals and magnetic susceptibilities. Structure determinations by X-ray analysis of 1a and 1d at various temperatures (20 K, 156 K, and room temperature) reveal characteristic differences of the crystal structures above and below the phase-transition temperature T_M-I for both compounds. The very special structural features of the crystals are related to their 7-fold diamondoid superstructure, which includes infinite superhelices in the stacks of ligands and copper ions. A phase diagram of (R¹,R³-DCNQI)₂Cu salts (1) is derived from a comparison of temperature-dependent structural and conductivity data, which shows that the conductive properties of the salts depend dramatically on the N-Cu-N angle α_co.. A minimal threshold value (α_co ≈ 126.4°) is evaluated which determines the phase behavior of 1 upon cooling or applying pressure. In order to understand their unique properties as compared to those of other DCNQI metal salts, a new approach to DCNQI copper salts is presented in terms of a “two conductors in one” concept, which is a unique combination of a common one-dimensional (anisotropic) conduction path along segregated stacks and a three-dimensional (isotropic) conduction path within a Robin-Day class III network of delocalized mixed-valent metal ions bridged by organic spacers allowing electron transfer.