Oxidation of 1,4,5,8,9,12,13,16-octamethyltetrabenzporphyrinatonickel(II), Ni(OMTBP), by elemental iodine yields two distinct complexes, Ni(OMTBP)(l3)p, where p = 0.36 and 0.97. The two complexes were characterized by resonance Raman and EPR spectroscopy and by static magnetic susceptibility and four-probe electrical conductivity measurements. Additionally the crystal and molecular structure of the p = 0.36 complex was determined from a study of both ordinary and diffuse X-ray scattering. The compound Ni(OMTBP)(l3)o,36 crystallizes in space group D11-P42/nbc of the tetragonal system with four formula units in a cell of dimensions a = 21.011 (9) Å and c = 7.556 (6) Å. The structure solution, the first for a tetrabenzporphyrin, was obtained by standard methods and refined to a conventional R index of 0.050, based on 687 significant observations. The predominant feature of the crystal structure is columns of S4 ruffled macrocycles stacked with large intermolecular spacings along the c axis. Chains of severely disordered linear I3- units parallel the stacking axis, between adjacent macrocycle columns, g values are near the free electron value and indicate that the unpaired spin density is ligand, not metal based; closer analysis reveals a minor contribution from the iodide chain. The symmetry of the g and line-width tensors indicates that in the p = 0.97 material the macrocycles also stack along c with the linear I3- ions paralleling this axis. The magnetic susceptibility of the p = 0.36 crystals is Curie-like, indicating negligible interactions between spins. The more highly oxidized p = 0.97 material exhibits a small but noticeable coupling between spins (J < 50 cm-1). The temperature-dependent electrical conductivity of each material exhibits qualitatively the same response: an activated conductivity occurs at temperatures less than a value Tm; above Tm the conductivity decreases in a metal-like manner from the maximum am. Values of Tm and for the p = 0.36 and 0.97 complexes are respectively 300 K, 12 S cm-1 and 340 K, 3 S cm-1. The existence of high conductivity but large intrastack spacings emphasizes the importance of partial oxidation in achieving high conductivity in a molecular crystal. This well-documented combination of a metal-like conductivity but a Curie-law susceptibility is unprecedented, and is discussed in terms of intuitive models of charge transport in molecular crystals.
ASJC Scopus subject areas
- Colloid and Surface Chemistry