Preparation, characterization, and low-temperature transition of (triazatetrabenzoporphyrinato)copper(I) iodide, a new porphyrinic conductor with local moments coupled to itinerant charge carriers

Abstract

We present the preparation and characterization of a new metallomacrocycle-based molecular conductor, (triazatetrabenzoporphyrinato)copper(II) iodide, Cu(tatbp)I. The material crystallizes with two formula units in space group Dih-P4/mcr of the tetragonal system in a cell of dimensions a = 13.998 (5) A and c = 6.426 (3) 8, (120 K). The structure has been refined to a value of R ( F ) of 0.085 for 862 data and 65 variables. This material is isostructural with (phthalocyaninato)copper(II) iodide, Cu(pc)I, and has analogous physical properties. However, Cu(tatbp) exhibits subtle metrical and electronic differences from Cu(pc) because the tatbp macrocycle has a methine carbon in place of one bridging nitrogen atom of pc. These differences produce distinct charge-transport and magnetic properties for each of the iodinated compounds. Cu(tatbp)I is a ring-oxidized organic conductor with metallic behavior that contains a dense array of localized Cu2+ moments embedded in the "Fermi sea" of carriers. I3C NMR spectroscopy at 13 MHz of enriched and natural-abundance I3C nuclei in Cu(tatbp)I show a distribution of positive and negative spin densities. The temperature dependence of the shifts identifies their origin as an isotropic contact hyperfine interaction transferred from Cu(I1). EPR and magnetic susceptibility measurements show that the local and itinerant spin systems are coupled and that the local moments are exchange-coupled to one another by direct and carrier-mediated mechanisms. This unusual situation results in two novel transitions of the coupled systems: for T < T, = 20 K, g,, increases anomalously as T is decreased; for T < Tb 6-8 K the EPR line width begins to broaden sharply, but the signal of Cu(tatbp)I remains detectable to T < 2 K. Comparisons of X-band and Q-band data show that gl is dependent on magnetic field at liquid-helium temperatures. Magnetic moments localized on the Cu2+ metal spine of Cu(tatbp)I also have a dramatic effect on the conductivity and dielectric constant. From -90 to 5 1 0 K, both four-probe (27 Hz) and microwave (13 GHz) conductivities decrease by 3 orders of magnitude and both slightly increase with magnetic field. For T < 6 K the microwave conductivity is enhanced in comparison to the four-probe conductivity and decreases with field; over the same range the dielectric constant increases with field. These effects correlate with a relaxation of the local moments observed in EPR spectroscopy and reflect a dielectric loss associated with an unusual coupling between magnetic and dielectric properties. (1) (a) Ogawa, M. Y.; Martinsen, J.; Palmer, S. M.; Stanton, J. L.; Tanaka, J.; Green, R. L.; Hoffman, B. M.; Ibers, J. A. (3) (a) Quirion, G.; Poirier, M.; Ogawa, M. Y.; Hoffman, B. M. Solid State Commun. 1987,64,613-616. (b) Quirion, G.; Poirier, M.; Liou, K. K.; Ogawa, M. Y.; Hoffman, B. M. Phys. Rev. B 1988, 37, 4272-4275. (4) Martinsen, J.; Stanton, J. L.; Greene, R. L.; Tanaka, J.; Hoffman, B. M.; Ibers, J. A. J . Am. Chem. SOC. 1985, 107, 6915-6920. (5) (a) Martinsen, J.; Greene, ...

Stabilizing g-States in Centrosymmetric Tetrapyrroles: Two-Photon-Absorbing Porphyrins with Bright Phosphorescence

“…It is important to note that the central metal, whether it is Zn, Pd or Pt (but not Ni), has only a minimal effect on the macrocycle structures, while the dominant distortion force is due to the interactions between the adjacent benzo- and meso -aryl groups. Therefore, it is reasonable to assume that the equilibrium structures of Pt and Pd complexes of meso -unsubstitited porphyrin 7 (Pt- 7 and Pd- 7 ) should be as planar as that of Zn- 7 or of other structurally characterized meso -unsubstututed π-extended porphyrins. − Similarly, one can expect that 11 should be distorted as much as 10 (Figure S3, p S17). Indeed, an earlier published tetraaryltetraphthalimidoporphyrin was found to be saddled, although with somewhat lower total our-of-plane distortion ( D oop = 2.27 Å) …”

Structural, Electrical, Magnetic, and Spectroscopic Properties of Ring-Oxidized Molecular Metals Produced by Iodination of Metal-Free and Nickel Tetrabenzporphyrins

“…We have extensively studied quasi one-dimensional molecular metals based on porphyrin molecules. − When the central metals and ligands are changed, the electrical, magnetic, and spectral properties are significantly modified. For example, negative thermoelectric power for Co(pc)I (pc = phthalocyanine) suggests that Co(pc)I is a metal-centered conductor where the Co d z 2 orbital is the path for conduction electrons .…”

Thermally activated magnetic interactions in a one-dimensional semiconductor. Structural, charge-transport, and magnetic studies of bis[(tetramethylporphyrinato)copper] perrhenate

“…Similar behavior is seen in Cu(tatbp)!. 3 We now report the synthesis and properties of [Cu(tmp)]2-[Re04], a ligand-oxidized conductor. Strong intramolecular spin exchange between the localized Cu(II) moments and the electrons in the partially occupied -band is again observed in this compound.…”