Synthesis, Structure, Magnetism, and Spectroscopic Properties of Some Mono- and Dinuclear Nickel Complexes Containing Noninnocent Pentane-2,4-dione Bis(S-alkylisothiosemicarbazonate)-Derived Ligands

Deprotonation of [Ni^II(H₂L¹)]I·0.5CH₃OH (1), where (H₂L¹)^- represents the ligand pentane-2,4-dione bis(S-methylisothiosemicarbazonate(−), in an ethanol solution of aqueous ammonia under argon affords air-sensitive red-brown [Ni^II(HL¹)]·C₂H₅OH (2). Air oxidation of 2 in ethanol/NH₃ yields the paramagnetic green-black dinuclear species [Ni(L¹−L¹)Ni] (3) where the ligand (L¹−L¹)^6- is formed by oxidative C−C coupling at the methine carbon atoms of (L¹)^3-. In this formulation the nickel ions are trivalent. Reaction of 3 in ethanol with concentrated HCl affords red diamagnetic [Ni^II(H₂L¹−L¹H₂)Ni1I^I]Cl₂·0.5C₂H₅OH (4). This reaction is a 2 electron reduction of 3 with concomittant protonation at the ligand. Deprotonation of 4 in C₂H₅OH/ammonia under argon gives red-brown [Ni^II(HL¹−L¹H)Ni^II]·C₂H₅OH (5). Replacement of the S-methyl groups in (H₂L¹)^- by two dodecyl groups, (CH₂)₁₁CH₃, gives the ligand (H₂L²)^-; the complexes [Ni^II(H₂L²)I (6) and their dinuclear, oxidized forms [Ni(L²−L²)Ni] (7) have been prepared analogously. The molecular structure of 3 (C₁₈H₂₈N₁₂Ni₂S₄) has been determined by X-ray crystallography. Crystal data for 3:  space group P1̄ with a = 11.475(2) Å, b = 13.615(1) Å, c = 18.207(6) Å, α = 83.87(1)°, β = 88.36(1)°, γ = 72.58(1)°, V = 2698.7(10) ų, and Z = 2. The structure refinement converged to R = 0.066 for 8865 unique reflections. Electronic and NMR spectra, the magnetism, and electrochemistry of new compounds are reported. It is shown that 3,4-diacetyl-2,5-hexanedione-tetrakis(S-alkylisothiosemicarbazonates) are noninnocent ligands in 3 and 7.