We present the preparation and characterization of a new metallomacrocycle-based molecular conductor, (triazatetrabenzo-porphyrinato)copper(II) iodide, Cu(tatbp)I. The material crystallizes with two formula units in space group D24h-P4/mcc of the tetragonal system in a cell of dimensions a = 13.998 (5) A and c = 6.426 (3) A (120 K). The structure has been refined to a value of R(F2) 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.13 C NMR spectroscopy at 13 MHz of enriched and natural-abundance13 C 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 hyperfime interaction transferred from Cu(II). 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 < Ta≃ 20 K, g1 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 g1 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 ≤10 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry