Electrical Conducting Bis(Oxalato)platinate Complex with Direct Connection of Cu^II Ions

Reactions of K_1.62[Pt(ox)₂]·2H₂O and [Cu(bpy)(H₂O)₃](NO₃)₂ yielded partially oxidized one-dimensional (1D) bis(oxalato)platinates of [Cu(bpy)(H₂O)_n]₆[Pt(ox)₂]₇·7H₂O (n = 2, 3, or 4) (1) and [Cu(bpy)(H₂O)_n]₈[Pt(ox)₂]₁₀·8H₂O (n = 3 or 4) (2). The average oxidation numbers of the platinum ions in 1 and 2 are +2.29 and +2.40, respectively. Complexes 1 and 2 crystallize in the triclinic P1̄ and monoclinic C2/c space groups, respectively, and the [Pt(ox)₂]^n- anions are stacked along the crystallographic b axis with 7-fold periodicity for 1 and 10-fold periodicity for 2. In 1, an oxalato ligand in the platinum chain directly coordinates to a paramagnetic [Cu(bpy)(H₂O)₃]²⁺ ion, whereas no such direct coordination was observed for 2. The electrical conductivity of 2 at room temperature along the platinum chain is approximately 3 orders of magnitude smaller (σ_∥ = 1.3 × 10^-3 S cm^-1) than that of 1 (σ_∥ = 0.9−0.5 S cm^-1), and the activation energies of 1 and 2 are 29 and 67 meV, respectively. The longest inter-platinum distances in 1 and 2 are 2.762 and 3.0082 Å, respectively, and this is responsible for the lower electrical conductivity of 2. An X-ray oscillation photograph taken along the b axis of 1 reveals the 7-fold periodicity in the 1D chain, consistent with the period of the Peierls distortion estimated from the degree of partial oxidation. The semiconducting state of 1 can therefore be regarded as a commensurate Peierls state. The magnetoresistance of 1 at ambient pressure indicates no interaction between conduction electrons in the platinum chain and local spins of the paramagnetic Cu^II ions. Application of hydrostatic pressures of up to 3 GPa enhances electrical conduction, as is often seen as the usual pressure effect on the electrical conductivity, which is due to enhanced orbital (Pt-5d_z²) overlap by pressure application.