Sulfur Dioxide Oxidation Catalyzed by Photosensitized Ytterbium Diphthalocyanine
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Sulfur Dioxide Oxidation Catalyzed by Photosensitized Ytterbium Diphthalocyanine Rudolf Słota Æ Gabriela Dyrda Æ Krzysztof Szczegot
Received: 3 July 2008 / Accepted: 2 August 2008 / Published online: 9 September 2008 Ó Springer Science+Business Media, LLC 2008
Abstract Oxidation of SO2 into SO3 was found effectively catalyzed by photosensitized ytterbium diphthalocyanine, YbPc2 (Pc = phthalocyanine ligand, C32H16N8) when performed in air-saturated dimethylformamide (DMF) solution at 20 °C. The process follows according to a multi-step complex mechanism involving chemical induction as its driving force. Excitation energy of the sandwich molecular system in YbPc2 is used to promote the reaction by creating a reactive intermediate form of the ytterbium complex hosting up to eight SO2 molecules. The conversion of SO2 proved complete. Keywords Ytterbium diphthalocyanine Sulfur dioxide Photosensitization Homogeneous catalysis UV–Vis spectroscopy Excited states
1 Introduction Phthalocyanines are synthetic analogs of the porphyrins, a family derived from natural heterocyclic compounds, such as chlorophyll or heme. An extensive description of their chemistry and properties one may find in the basic works of Berezin [1], Leznoff and Lever [2] or Simon and Andre´ [3]. Lanthanide phthalocyanine double-decker complexes, LnPc2 (Fig. 1) were first developed by the Russian school in the 1960s, particularly by Kirin and Moskalev and
The work was presented during the conference #Catalysis for Society#, Krakow, May 11-15, 2008. R. Słota (&) G. Dyrda K. Szczegot Faculty of Chemistry, Opole University, ul. Oleska 48, 45 -095 Opole, Poland e-mail: [email protected]
co-workers (see e.g., [4]). They have been considered one of the most promising molecular materials featuring very interesting opto-electronic and catalytic properties and hence showing a great application potential. Generally, the chemistry of LnPc2 is determined by the electronic density distribution over the both phthalocyanine macrocycles coupled via the trivalent rare earth metal template. The part of the metal consists in controlling the strength of the whole bonding system. However, the core of this unique molecular setup is the electronic path of conjugated p bonds linking the C and N atoms inside the Pc ring (shaded, in Fig. 1). Its aromatic character is the key reason for the unusual thermodynamic durability of the phthalocyanine moiety, no matter whether in the metal-free compound (H2Pc) or any of its numerous metal complexes. Decomposition temperature of H2Pc, ZnPc, and YbPc2 was found 559, 636, and 676 °C, respectively [5]. Phthalocyanines, including LnPc2s, have always been considered attractive for catalytic applications [1, 2]. In particular, ideas involving photosensitized catalysis arouse considerable interest [6], especially as it proved possible to use solar light to activate such reactions [7]. The potential revealed by these compounds seems particularly interesting for projects focused on environment-related objectives, such as clean t
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