Spin solitons in halogen-bridged one-dimensional mixed-valence complexes, [MII(en)2][MIVBr2(en)2](ClO4)4 (M: Pt, Pd) studied by 1H NMR and EPR

Abstract

Temperature and Larmor frequency dependences of ¹H NMR spin–lattice relaxation time (T₁) in 1-D mixed-valence complexes [M(en)₂][MBr₂(en)₂](ClO₄)₄ (M: Pt, Pd; en: ethylenediamine) were observed in the ranges 90–330 K and 15–60 MHz. EPR spectra observed for the Pd complex showed the presence of paramagnetic Pd^III sites of a temperature independent concentration of 10⁻⁴ per Pd site, and a motional narrowing upon heating attributable to thermally activated motions of these sites. The NMR T₁ data giving an ω^1/2 frequency dependence in the range 100–150 K were successfully analysed by applying the Devreux's treatment of rapidly 1-D diffusing electron spins. Comparing the present results with the electrical conductivity data, the diffusing paramagnetic spins could be explained by kink-type neutral solitons analogous to the spin soliton model reported for trans-polyacetylene. Diffusion rates of 10¹²–10¹³ and 10¹³–10¹⁴ rad s⁻¹ were estimated in Pt and Pd complexes, respectively, below ca. 150 K. This diffusion in the present 1-D structure complexes was shown to be much easier than that in [Pd(chxn)₂][PdBr₂(chxn)₂]Br₄ (chxn: 1R,2R-cyclohexanediamine) with 2-D hydrogen bond networks.