Synthesis, X-ray crystal structure and properties of a novel tetranuclear unsymmetrical (μ-SO4) copper(II) complex: relevance to SO2 fixation

compound [Cu4(TPPNOL)2(μ-SO4)2](ClO4)2 (1) [HTPPNOL (N,N,N′-tris-(2-pyridylmethyl)-1,3-diaminopropan-2-ol)] are reported. In 1, the copper(II) ions are bridged by the alkoxo oxygen atoms of the HTPPNOL ligand and by exogenous sulfate bridges. The structure of 1 consists of a centro-symmetric tetranuclear core or a “Dimer of Dimers” complex, in which a μ-O,O′ sulfate oxygen atom is further coordinated to the copper centre of another similar dinuclear unit through a μ-O,O, sulfate bridge resulting in a tetranuclear arrangement. Thus, the dinuclear units are linked by two μ-O,O sulfate bridges. The simultaneous presence of two distinct coordination modes for the sulfate group in this structure is rare and 1 represents the first coordination compound presenting μ-O,O′ and μ-O,O type structures. The SO2 fixation was monitored by changes in the electronic spectra which indicated the formation of the intermediate hydroxo complex [Cu2(TPPNOL)(OH)2]+, in basic medium, which, we propose, acts as the nucleophile in the SO2 fixation mechanism.