Preparation and characterization of (tetrabenzoporphyrinato)cobalt(II) iodide, a ring-oxidized molecular conductor
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Oxidation of (tetrabenzoporphyrinato)cobalt(II), Co(tbp), with iodine affords Co(tbp)I. The structure comprises metal-over-metal columnar stacks of partially (one-third) oxidized Co(tbp) groups surrounded by chains of I3- ions. Co(tbp)I crystallizes in the tetragonal space group D24k-P4/mcc with a = 14.129 (3) A, c = 6.334 (1) A, V = 1249 A3, and Z = 2. In contrast to (phthalocyaninato)cobalt(II) iodide, Co(pc)I, which shows metal oxidation and is a metal-spine conductor, conductivity and thermoelectric power measurements on Co(tbp)I show that oxidation occurs at the macrocycle; the charge carriers are holes in the five-sixths-filled band comprising overlapping p-π orbitals from adjacent tbp rings. Reflectance spectroscopy measurements on single crystals provide an explanation of why the tbp macrocycle is more readily oxidized: the p-π orbitals are higher in energy for Co(tbp)I than for Co(pc)I. Ring oxidation of Co(tbp) leaves one spin on each Co2+ ion. However, Co(tbp)I does not exhibit an EPR signal associated with either the carriers or the Co2+ (S=1/2) ions, and the magnetic susceptibility data yield a Curie constant of 0.039 (emu K)/mol, far less than expected for a linear chain of S = */2 spins. In addition, Co(tbp)I does not have a measurable magnetoconductance, as would be expected from a system with conduction electrons scattered by reorientable local moments. This is in contrast to Cu(pc)I, which also is ring oxidized with the metal ion and carriers forming a spin-coupled system, but which shows both the full Curie constant (C = 0.4 (emu K)/mol) for the Cu2+ (S=1/2) ions and an EPR signal. Thus, spins in the Co2+(dr2 orbitals of a partially oxidized Co(tbp) stack must exhibit antiferromagnetic coupling through direct or carrier-mediated interactions, or most likely both, that is much stronger than in Cu(pc)I. © 1992, American Chemical Society. All rights reserved.
author list (cited authors)
Liou, K., Newcomb, T. P., Heagy, M. D., Thompson, J. A., Heuer, W. B., Musselman, R. L., ... Ibers, J. A.