Synthesis, X-ray crystal structure and properties of a novel tetranuclear unsymmetrical (μ-SO 4) copper(II) complex: relevance to SO 2 fixation

• Published in: Inorganica Chimica Acta Vol. 358; no. 4; pp. 997 - 1004
• Main Authors: Fernandes, Christiane, Bortoluzzi, Adailton J., Szpoganicz, Bruno, Schwingel, Erineu, Neves, Ademir
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.03.2005
• Subjects: Crystal structure; SO 2 fixation; Tetranuclear copper(II) complex; Unsymmetrical ligand; μ-O,O′ and μ-O,O sulfate bridges; SO 2 fixation; Tetranuclear copper(II) complex; μ-O,O′ and μ-O,O sulfate bridges; Unsymmetrical ligand; Crystal structure
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/j.ica.2004.11.040

Reported here is the reversible fixation of SO 2 through a dinuclear Cu-hydroxo complex. A new tetranuclear Cu II complex with unusual (μ-SO 4) coordination was obtained and reactivity studies indicated that a bound Cu–OH is most probably the active species in the SO 2 fixation process. The synthesis, by fixation of SO 2, the unusual crystal structure, and the spectral and redox properties of the new μ - SO 4 2 - compound [Cu 4(TPPNOL) 2(μ-SO 4) 2](ClO 4) 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 SO 2 fixation was monitored by changes in the electronic spectra which indicated the formation of the intermediate hydroxo complex [Cu 2(TPPNOL)(OH) 2] +, in basic medium, which, we propose, acts as the nucleophile in the SO 2 fixation mechanism.