Thermally Activated Magnetic Interactions in a One-Dimensional Semiconductor. Structural, Charge-Transport, and Magnetic Studies of [Cu(tmp)]₂[ReO₄]

Electrochemical oxidation of (5,10,15,20-tetramethylporphyrinato)copper(II), Cu(tmp), in the presence of the perrhenate ion affords the new molecular conductor [Cu(tmp)]₂[ReO₄]. The compound is composed of partially oxidized (+1/2) Cu(tmp) cations stacked metal-over-metal and surrounded by chains of ReO₄ anions. Single-crystal, room-temperature conductivity along the needle (crystallographic c) axis averages 40 Ω^-1 cm^-1, lower than the values previously reported for Ni(tmp)-based conductors. The conductivity is thermally activated and may be fit to an expression that describes a semiconductor with temperature-dependent carrier mobility: σ = σ₀T^α exp(-ΔE/k_BT) (ΔE = 0.126 (2) eV, α = 3.8 (2)). Magnetic studies show that the local Cu(II) moments and the itinerant π-carrier electrons participate in strong spin exchange. This intramolecular Cu-π coupling is shown to mediate Cu-Cu spin exchange along the linear chains of Cu(IĨ) ions found in this compound. These magnetic interactions depend on the thermally activated carrier concentration. The angular variation of the line width indicates that exchange is highly one-dimensional. The compound crystallizes in space group Ch,-P4/n of the tetragonal system with two formula units in a cell of dimensions a = 16.774 (9) Å and c = 6.746 (4) Å (V = 1898 ų) at 110 K. Full-matrix least-squares refinement of 144 variables gives a final value of 0.069 for the R index on F² for 4695 unique observations and a value of 0.043 for the R index on F for the 2582 observations having F_o² > 3σ(F_o²).