Reaction between S4N4 and Ni2+ in dimethyl sulphoxide. Formation and characterization in solution of S4N2(NH2)2, a novel sulphur–nitrogen heterocycle precursor of [Ni(S2N2H)2]

Abstract

The complex [Ni(S₂N₂H)₂] undergoes a protanation reaction in dimethyl sulphoxide (dmso) solution upon addition of stoichiometric amounts of a strong acid, giving [Ni(S₂N₂H₂)(S₂N₂H)]⁺. The value of the proton association constant has been determined potentiometrically, log K₁= 3.3. Proton and ¹⁵N NMR studies reveal that the proton binds at one NH group, resulting in the formation of the bidentate co-ordinated ligand SN–S–NH₂. This metal complex is also formed very slowly at room temperature, in the reaction of S₄N₄ and Ni²⁺ in dmso. The reaction rate increases dramatically on addition of stoichiometric amounts of sodium acetate of NBu₄OH. The compound S₄N₄ does not react directly with the metal ion but with the trace amounts of water contained in the solvent, resulting in the formation of S₄N₂(NH₂)₂ which then reacts with Ni²⁺ ions to give the final metal complex. The novel sulphur–nitrogen heterocycle S₄N₂(NH₂)₂ has been characterized in solution by molecular exclusion chromatography, ion exchange, UV/VIS, IR and NMR (¹H and ¹⁵N) spectrophotometry. Comparison of the NMR spectra with those of the nickel(II) complex suggests that S₄N₂(NH₂)₂ consists of a non-planar S₄N₂ ring with two exocyclic NH₂ groups bonded to opposite sulphur atoms in the ring and on the same face. On the basis of this structure the formation of [Ni(S₂N₂H)₂] is easily understood.