Syntheses, X-ray Crystal Structures, and Spectroscopic Properties of New Nickel Dithiolenes and Related Compounds

The direct addition of nickel powder to the reaction mixtures of 1,3-dialkyl-4,5-dioxoimidazolidine-2-thione (1) with the thionation Lawesson reagent produces [Ni^II(R₂timdt^-)₂] (R₂timdt = 1,3-dialkylmidazolidine-2,4,5-trithione). These complexes belong to a new class of nickel−dithiolenes, showing remarkably high absorption (ε ≈ 80 000 dm³ mol^-1 cm^-1, λ ≈ 1000 nm) in the near-infrared region (near-IR), accompanied by high photochemical stability that makes these complexes promising near-IR dyes. In the absence of nickel the reaction yields generally the compounds 4,5,6,7-tetrathiocino[1,2-b:3,4-b‘]diimidazolyl-1,3,8,10-tetraalkane-2,9-dithione (2) instead of the expected 1,3-dialkylimidazolidine-2,4,5-trithione. However, with bulky substituents on the nitrogen atoms, well-characterized reaction products have not been obtained until now. Only in the R = Prⁱ case, the new tetrathiocino isomer (4,5,9,10-tetrathiocino[1,2-b:5,6-b‘]diimidazolyl-1,3,6,8-tetraisopropane-2,7-dithione (3) in trace amounts and the bis(1,3-diisopropyl-2-thioxoimidazolin-4-yl) disulfide (4) were isolated from the reaction with the Lawesson reagent and P₄S₁₀, respectively. Reaction of 5 with different amounts of I₂ leads to a variety of products, and among them the following derivatives have been characterized:  [Ni^II(Prⁱ₂timdt^-)₂]·2I₂ (6) a neutral adduct in which each I₂ molecule interacts with each peripheral thione sulfur atom of 5; [Ni^II(Prⁱ₂timdt^-)₂]·2I₂·¹/₂I₂ (7), which differs from 6 with the presence of half diiodine as a guest; with a larger diiodine excess (starting from a 1:10 molar ratio) a partial oxidation of 5 is achieved and [Ni^II(Prⁱ₂timdt^-)₂] [Ni^II(I)₂(Prⁱ₂timdt)₂]·5I₂ (8) is formed, a ligand mixed-valence compound in which the square-planar complex 5 and the octahedral complex [Ni^II(I)₂(Prⁱ₂timdt)₂] (the ligand in neutral form) are bound by diiodine molecules in such a way that a sequence of 12 iodine atoms (S·I₂···I₂···I^-···I₂···I^-···I₂···I₂·S) is formed. Spectroscopic and X-ray diffractometric studies of 3 (monoclinic, space group P2₁/n, a = 13.104(6) Å, b = 15.091(6) Å, c = 6.067(7) Å, β = 103.00(2)°, Z = 2), 4 (monoclinic, space group C2/c, a = 18.717(5) Å, b = 8.846(5) Å, c = 14.475(5) Å, β = 97.78(2)°, Z = 4), 5 (orthorhombic, space group Pna2₁, a = 18.806(5) Å, b = 5.628(7) Å, c = 23.665(5) Å, Z = 4), 7 (triclinic, space group P1̄, a = 9.126(7) Å, b = 9.153(7) Å, c = 12.924(6) Å, α = 82.11(2), β = 70.14(2), γ = 72.51(2)°, Z = 1), and 8 (monoclinic, space group C2/c, a = 21.971(6) Å, b = 28.081(5) Å, c = 12.207(8) Å, β = 93.57(2)°, Z = 4) are given.