Synthesis, structure and spectroscopic properties of a new class of polymerisable nickel dithiolenes

The synthesis, structure and spectroscopic properties of a new class of asymmetric nickel dithiolene complex containing pendent thiophene units [Ni(R₂pipdt)(b-3ted)] are described {R = Bz (3), ⁱPr (4); pipdt = piperazine-3,2-dithione; b-3ted = bis(3-thienyl)-1,2-ethylenedithiolene}. X-ray crystallography of both neutral species shows planar nickel dithiolene units and the thiophene groups twisted out of the molecular plane, with the R group having a large effect on the packing structure. DFT geometry calculations show good agreement with experimental data for 3 and 4 and confirm that the low energy absorption, assigned to a HOMO → LUMO transition, is inter-ligand charge-transfer in character. Solid state calculations show greater band dispersion in 4, which corresponds to a closer packing in the crystal lattice. Cyclic voltammetry of both complexes displays three reversible processes, corresponding to an interconversion between cationic, neutral, anionic and dianionic species, with tuning of the redox potentials by variation of the R group. Electrochemical polymerisation of 3 shows incorporation of intact dithiolene units into a polymer film with redox functionality similar to that of the monomer, whereas co-polymerisation of 3 with thiophene yields a highly conductive film with redox functionality similar to polythiophene and enhanced absorption across the near IR.