Preparation of the first sulfur-containing cobalt and nickel complexes stabilised by the macrocyclic cyclam ligand; observation of S–H bond activation

Abstract

Cobalt and nickel complexes bearing the ligand 1,4,8,11-tetraazacyclotetradecane (cyclam/[14]aneN₄) besides S₂^2–, SH^– and S^2– anions were prepared for the first time by treatment of the corresponding metal perchlorates with sodium and potassium hydrogensulfide or disulfide in the presence of the macrocycle. The reaction of Co(ClO₄)₂·6MeCN with cyclam and SH^– led to the formation of the almost identical polymeric complexes [Co(SH)(cyclam)][ClO₄]·½DMF 1 and [Co(SH)(cyclam)][ClO₄]·MeOH 2, in which the Co atoms are bridged by SH groups to give unusual linear co-ordination polymers. These structures seem to be determined essentially by packing effects, which was verified by the synthesis and characterisation of [CoCl(cyclam)][ClO₄]·½DMF 3. Despite the replacement of the bridging SH^– groups by Cl^– anions, 3 has bond lengths and angles similiar to those of 1 and 2. In contrast, the reaction of the corresponding nickel salt under the same conditions afforded the trinickel compound [{Ni(µ-SH)(cyclam)}₂][Ni(SH)₂(cyclam)][ClO₄]₂ 4, which contains a mononuclear neutral Ni(SH)₂ and a binuclear cationic Ni₂(µ-SH)₂ fragment probably connected by hydrogen bridges. The thiol-bridged binuclear complex [{Co(µ-SH)(cyclam)}₂]²⁺ 5 and the isostructural cation [{Ni(µ-SH)(cyclam)}₂]²⁺ 6, both with SH^– counter ions, were prepared by addition of two molar equivalents of SH^– to cobalt perchlorate or by heating of 4, respectively. The reaction of [Ni(cyclam)][ClO₄]₂ with S₂^2– instead of SH^– yielded the disulfido complex [Ni₂(µ-S₂)(cyclam)₂][ClO₄]₂ 7, while with Co^II the binuclear sulfido-bridged complex [{Co(µ-S)(cyclam)}₂][ClO₄]₂ 8 was obtained. This was formed by cleavage of the S–S bond of the S₂^2– anion, and the enhancement of the oxidation state of cobalt from +2 to +3. Furthermore, it was also generated by reaction of complex 2 with azobenzene. Thus, the S–H bonds seem to be activated by the cyclam-stabilised cobalt(II) ion, and transfer of the hydrogen atoms to the unsaturated NN bond of azobenzene led to the formation of diphenylhydrazine.