Reaction between S4N4and 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.