Structural and Charge Transport Properties of Bis[(5,10,15,20-tetramethylporphyrinato)nickel(II)] Perrhenate, [Ni(tmp)]₂[ReO₄]

Electrochemical oxidation of (5,10,15,20-tetramethylporphyrinato)nickel(II), Ni(tmp), in the presence of the perrhenate ion affords the new molecular conductor [Ni(tmp)]₂[ReO₄], which is composed of planar Ni(tmp) molecules stacked metal-over-metal. The two unique Ni(tmp) molecules within the cell are on sites of 4 symmetry, are separated by 3.355 (2) Å, and are rotated with respect to one another by 27.5 (1)° about the c axis (stacking direction). The ReO₄⁻anion lies on a site of 4 symmetry within a channel formed by adjacent porphyrin stacks. ESR measurements confirm the tmp ligand as the site of oxidation. Single-crystal room-temperature conductivity along the needle (crystallographic c) axis averages 90 Ω⁻¹cm⁻¹, typical for tmp-based conductors. The temperature dependence of the conductivity is thermally activated and has been fit to an expression describing a semiconductor with temperature-dependent carrier mobility; σ =_σ0T^−αexp(−E_g/kT) (E_g= 0.24 (2) eV, 3.5 < α < 18). Previously synthesized Ni(tmp)-based conductors exhibit metal-like conductivities at high temperatures. The semiconducting nature of the conductivity in the present compound is attributed to a small transfer integral between the π molecular orbitals on adjacent intrastack porphyrin rings as a consequence of the 27.5° rotation angle between independent rings. The compound crystallizes in space group C³_4hPA/n of the tetragonal system with two formula units in a cell of dimensions a = 16.710 (4) Å and c = 6.720 (2) Å (V = 1876 ų) at 125 K. Full-matrix least-squares refinement of 144 variables gives a final value of 0.085 for the R index on F²for 2742 unique observations and a value of 0.052 for the R index on F for the 1850 observations having F₀²3 3σ(F₀²).