Structural, electrical, magnetic, and spectroscopic properties of ring-oxidized molecular metals produced by iodination of metal-free and nickel tetrabenzporphyrins

Detailed studies of the structure, conductivitity, magnetoresistance, optical spectra, and magnetic properties (susceptibility, EPR) for the new molecular metal tetrabenzporphyrin iodide (H-2(tbp)I) and the electrical, spectral, and magnetic properties of Ni(tbp)I are reported. Paramagnetic transition-ion impurities were carefully excluded during the synthesis of H-2(tbp)I and Ni(tbp)I, and both materials show much higher, metal-like conductivites than previously seen for less-pure Ni(tbp)I. Comparison of the specular reflectance data for Ni(tbp)I and H-2(tbp)I allows a distinction between purely ring pi-transitions and metal-involved charge-transfer transitions, and the spectra fix the energy levels of the pi orbitals involved in conduction. Transport, magnetic, and optical properties show that both H-2(tbp)I and Ni(tbp)I are ring-based conductors that have metal-like conductivities, varying as similar to 1/T, down to ca. 30-40 K. However, the remaining level of defects is higher in the tbp conductors than in H-2(pc)I, and whereas the latter is metallic down to the mK temperature range, the defects in the (tbp) compounds localize the conduction electrons at similar to 10 K (Ni(tbp)I) and similar to 30 K (H-2(tbp)I), leading to transport through one-dimensional variable-range hopping. EPR g-values for H-2(tbp)I and Ni(tbp)I are close to that for the free electron and are nearly temperature-independent. The line widths for both samples are extremely narrow and also are nearly temperature-independent. These results show that Ni(tbp)I does not display doubly-mixed valence, as thought earlier: Paramagnetic impurities significantly altered the EPR signals of the prior samples. H-2(tbp)I crystallizes in the space group P4/mcc with cell constants of a = 14.173(10) Angstrom and c = 6.463(4) Angstrom. Full-matrix least-squares refinement of 63 variables gave an R index of 0.061 on F-0(2).