Designed Synthesis of a Radical Cation Salt of Ni(tmp): Structural, Magnetic, and Charge-Transport Properties of Bis [(5,10,15,20-Tetramethylporphyrinato)Nickel(II) ] Hexafluorophosphate

Electrochemical oxidation of (5,10,15,20-tetramethvlporphyrinato)nickel(II), Ni(tmp), in the presence of hexa-fluorophosphate ion affords the new molecular conductor [Ni(tmp)]₂[PF₆], which is comprised of partially oxidized (+1/2) S₄-ruffled Ni(tmp) molecules stacked metal-over-metal. The PF₆^-anions lie within channels formed by adjacent porphyrin stacks on sites that minimize the porphyrin-hydrogen to-fluorine contacts. The choice of the PF₆^-anion, as being likely to produce an ordered crystal with this degree of partial oxidation, was prompted by an examination of the crystal structures of similar metallomacrocyclic conductors. ESR studies confirm the tmp ligand as the site of oxidation. Single-crystal room-temperature conductivity along the needle (crystallographic c) axis averages 75 Ω^-1cm^-1, somewhat lower than that observed for Ni(tmp)I and for most porphyrinic conductors containing larger porphyrins. The temperature dependence of the conductivity is metallic from room temperature to 205 K and decreases rapidly as the temperature is decreased further. Single-crystal X-ray diffraction measurements at 220 and 150 K demonstrate that this change in behavior does not result from a structural transition at 205 K. The compound crystallizes in space group D_4h⁴-P4/nnc of the tetragonal system with two formula units in a cell of dimensions a = 16.685 (1) Å and c = 6.994 (1) Å (volume = 1947 ų) at 220 K. The two Ni(tmp) molecules in the unit cell are separated by 3.497 (1) Å and are rotated with respect to one another by 34.7° (3)°. Full-matrix least-squares refinement of 76 variables gives a final value for the R index on F² of 0.127 for 1845 unique observations and a value of 0.045 for the R index on F for the 1243 observations having F₀²> 3σ(F₀²).