The d-L(π)-π type of charge-transfer salt [Cu(Me-tri)2][Ni(dmit)2]2: Interaction between copper(ii) d- and Ni(dmit)2 π-electrons via π-conjugated macrocyclic ligands

Abstract

In order to correlate d-electrons of copper(II) ions with π-electrons of Ni(dmit)₂ (dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate) acceptors, we prepared the d-L(π)-π type of charge-transfer salt [Cu(Me-tri)₂][Ni(dmit)₂]₂(Me-tri = methyl-substituted tribenzo[b, f, j][1, 5, 9]triazacycloduodecine), where the π-conjugated system of ligand Me-tri (L(π)) is expected to behave as a mediator to link the d- and the π-electrons. In the crystal, Ni(dmit)₂ anions with S = 1/2 π-spin form face-to-face dimers, and divalent Cu(Me-tri)₂ cations exist with their L(π) planes almost parallel to the dimers. From magnetic susceptibility measurements, it was revealed that spins on the Ni(dmit)₂ dimer relax from a triplet to a singlet state with decreasing temperature, and copper spins remain uncoupled below 75 K. The presence of weak anti-ferromagnetic interactions between the copper spins was confirmed below 25 K, although the copper ions are quite far apart. These phenomena are explained in terms of superexchange coupling via anti-parallel spins on the Ni(dmit)₂ dimer. As evidence to support this, the EPR spectrum at 4 K had narrow linewidth without hyperfine structure. At 75 K, the EPR signal is isotropic, which means that the Jahn–Teller distortion of Cu(Me-tri)₂ is fluxional. Compared with the results for [Cu(Me-tri)₂](BF₄)₂, Cu(Me-tri)₂ in the Ni(dmit)₂ salt was found to maintain fluxionality even at lower temperature. Semi-empirical molecular orbital calculations suggest that the distortion of Cu(Me-tri)₂ is more properly described as a pseudo-Jahn–Teller effect, in which the magnetic L(π) orbital is involved. Assuming charge-transfer interactions of this L(π) orbital with the unoccupied π-orbital of the Ni(dmit)₂ dimer, reasonable explanations are provided both for the superexchange coupling and for the persistence of the fluxional distortion at lower temperatures.