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

In order to correlate d-electrons of copper(II) ions with pi-electrons of Ni(dmit)(2) (dmit = 2-thioxo-1,3- dithiole-4,5-dithiolate) acceptors, we prepared the d-L(pi)-pi type of charge- transfer salt [Cu(Metri) (2)][Ni(dmit)(2)](2)(Me- tri = methyl-substituted tribenzo[b, f, j][1, 5, 9] triazacycloduodecine), where the pi-conjugated system of ligand Me-tri (L(pi)) is expected to behave as a mediator to link the d- and the pi-electrons. In the crystal, Ni(dmit)(2) anions with S = 1/2 pi-spin form face-to-face dimers, and divalent Cu( Me- tri)(2) cations exist with their L(pi) 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 antiferromagnetic 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(pi) orbital is involved. Assuming charge- transfer interactions of this L(pi) orbital with the unoccupied pi-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.