Metal valence structures and magnetic interactions in halogen-bridged 1-D Ni-Pd mixed-metal complexes studied by ¹³C and ¹H solid state NMR

¹³C NMR spectra at room temperature and the temperature dependences of 1H T1 in the solid state were measured in [Ni _1-xPd_xX(chxn)₂]X₂ (X: Cl, Br; chxn: 1R,2R-cyclohexanediamine; 0.0 < x < 1.0), where antiferromagnetically coupled paramagnetic -X-Ni³⁺-X-Ni³⁺-X- chains were formed at x = 0.00, while the mixed-valence -X-Pd²⁺-X-Pd⁴⁺-X- state was made at x = 1.00. ¹³C signals at α-carbons in chxn coordinating to Pd atoms showed a doublet assignable to Pd²⁺ and Pd⁴⁺ in x = 1.00, while, with a slight decrease of x from 1.00, a clear broadening and a shift to low-field of signals, indicating conversion into the averaged Pd³⁺ state, were observed. This can be explained by the fluctuation of the Pd valency caused by neighboring paramagnetic Ni ³⁺ sites introduced in small amounts in the 1-D chain. The x dependences of the chemical shifts of β- and γ-carbons are also attributable to the effect from a partial mixing of the paramagnetic Pd ³⁺ sites. The values of ¹H T₁ and its temperature dependence observed in the ranges of 100-300 K and x < 0.13 for [Ni_1-xPd_xBr(chxn)₂]Br₂ could be explained by a model of strong exchange interactions between Ni³⁺ and Pd³⁺ as well as Ni³⁺ sites. Gradual changes in the T ₁ value and slope with increasing x from 0.00 to 0.93 are attributable to the variation of the exchange interaction value, which depends upon the number of Ni-Ni pairs.