Covalent attachment of a biomimetic Ru-(terpy)(bpy) complex on silica surface: Catalytic potential F. Papafotiou, K. Karidi, A. Garoufis ⇑ , M. Louloudi ⇑ Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece article info Article history: Available online 7 August 2012 Dedicated to Alfred Werner on the 100th Anniversary of his Nobel prize in Chemistry in 1913. Keywords: Ruthenium complex Grafting Biomimetic oxidation catalysis abstract The Ru-containing modified silica [Ru II (terpy)(4 0 Mebpy/4CONH(CH 2 ) 3 SiO 3/2 )Cl] + m zSiO 2 has been pre- pared by covalent attachment on silica surface of biomimetic [Ru II (terpy)(4-CO 2 H-4 0 -Mebpy)Cl] + complex through the formation of a pseudo-peptide bond. The catalytic ability of bio-derived silica for alkene oxi- dation with HOO t Bu has been evaluated exhibiting significant efficiency and, in some cases, showing increased activity compared vs. the corresponding ‘net’ [Ru II (terpy)(4-CO 2 H-4 0 -Mebpy)Cl] + complex. The data supported that the covalently attached ruthenium complex preserves the catalytic behaviour of the ‘net’ ruthenium complex indicating that the presented grafting process was successful. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Covalently anchoring of bio-inspired metal complexes on silica surface through the side chain functionalities offers alternative processes to develop bio-derived silica frameworks that can be uti- lized as catalytic materials for specific transformation in fine chemical production [1]. This synthetic strategy is grounded on the evidence that the active sites of metalloenzymes involve metal complexes which are efficient and selective catalysts in vivo as well as in vitro, in oxidation reactions of organic compounds [2–5]. Among the different oxidation reactions, the epoxidation of olefins is of major importance for organic synthesis. Nevertheless, the syn- thesis of epoxides catalyzed by transition metal complexes is still important on laboratory as well as on industrial scale [3,6–8]. Based on our recent work on catalytic epoxidations using manga- nese and iron biomimetic complexes ‘net’ and covalently attached on inorganic matrices [9–15], we were interested in using ruthe- nium-based catalytic materials in these reactions. The potential of ruthenium as epoxidation catalyst arises from its extensive redox chemistry and its propensity to form high-valent oxo com- plexes [16]. Ruthenium complexes as homogeneous catalysts offered inter- esting results, e.g. such as high yield and chemoselectivity [16–23], however, they become more useful if the separation process is tar- geted to recover them at the end of reaction. Metal-based catalysts can be covalently anchored on the surface of porous materials most effectively by post-grafting method [1,24,25] through anchoring of the organic ligands present [26–28]. This covalent route to immo- bilisation of homogeneous catalysts is believed to be the most suit- able, since adsorption and ionic methods lead to a decrease in catalyst stability [1]. Although ruthenium complexes with Schiff bases or polypyri- dine ligands have been thoroughly studied, rather few reports are available on immobilised ruthenium complexes and their application in oxidation catalysis. As example, we refer the work of Ram and co-workers [29–31], Reedijk and co-workers [32] and Che and co-workers [33]. More recently, ruthenium-salophen, and ruthenium-bipyridine complexes supported on polystyrene, or mesoporous silica FSM have been prepared and evaluated as oxidation catalysts of alkenes with NaIO 4 and TBHP respectively [34,35]. The biomimetic ruthenium complex, [Ru II (terpy)(4-CO 2 H-4 0 - Mebpy)Cl]Cl (Scheme 1), has been designed and served as precursor of polypyridylruthenium conjugated peptide/amino acid complexes [36]. This class of compounds has been shown remarkable photoin- duced nucleolytic activity cleaving the DNA phosphorodiesteric bonds [37]. In the present study, this Ru-complex has been immobilised on silica surface (Scheme 2). The applied method, depicted in Scheme 3, builds up the ruthenium complex on preformed silica. Thus, the ligand 4 0 -Mebpy-4-COOH (4 0 -methyl-2,2 0 -bipyridine-4-carboxylic acid) was attached onto a modified amino-propyl-silica via pseu- do-peptide bond formation, then reaction with Ru(terpy)Cl 3 yielded the supported ruthenium complex, [Ru II (terpy)(4 0 Mebpy/4CON- H(CH 2 ) 3 SiO 3/2 )Cl] + m zSiO 2 . The developed Ru-based material as well as the ruthenium complex [Ru II (terpy)(4-CO 2 H-4 0 -Mebpy)Cl]Cl, was 0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2012.07.094 ⇑ Corresponding author. E-mail addresses: agaroufi@cc.uoi.gr (A. Garoufis), mlouloud@uoi.gr (M. Louloudi). Polyhedron 52 (2013) 634–638 Contents lists available at SciVerse ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly