Synthesis and characterization of nickel-group bis(dithiocroconate) complexes and dicyanomethylene-substituted analogues

Abstract

Two series of nickel-group metal bis(dithiolene) complexes with ligands 4,5-disulfanylcyclopent-4-ene-1,2,3-trionate (L¹) and 2-dicyanomethylene-4,5-disulfanylcyclopent-4-ene-1,3-dionate (L²) have been prepared and characterized: [NBu₄ⁿ]₂[M(Lⁱ)₂](M = Ni, Pd or Pt; i= 1 or 2). Oxidation of the dianion complexes yielded paramagnetic monoanions with ESR spectra indicative of a delocalized b_3g HOMO (highest occupied molecular orbital), like that previously found for comparable bis(dithiolene) complexes. The intense low-energy visible absorptions and multiple, reversible reductions exhibited by the dianions likewise suggest that the LUMO (lowest unoccupied molecular orbital) is a ligand-based a_u(π*) orbital, rather than the b_1g(d_xy) orbital as commonly found for such complexes. The stabilization of the ligand-based LUMO in this case is attributed to the strongly electron-withdrawing character of the ligand substituents. Iodination of [NBu₄ⁿ]₂[Pd(L²)₂] in CH₂Cl₂ solution yielded the novel iodine inclusion compound [NBuⁿ₄]₂[Pd(L²)₂]·I₂, which crystallizes in space group P with Z= 1, a= 10.792(3), b= 13.995(5), c= 10.737(3)Å, α= 105.48(1), β= 115.15(1) and γ= 76.51(2)° at 25 °C. The I₂ molecules are associated with the complex anions through short [3.1696(9)Å] S ⋯ I contacts; however, the observed I–I distance [2.7354(4)Å] indicates that the degree of charge transfer associated with this interaction is small.