Situated in the middle of the second row of the transition-metal series, ruthenium lies at the heart of the Periodic Table. This central location bestows upon ruthenium properties that are common to both its early- and late-transition-metal cousins. Harnessing the best of both world, ruthenium benefits from a confluence of desirable properties, amounting to a winning combination for catalysis. Borrowing the high reactivity of elements to its left and the less oxophilic and Lewis acidic nature of those to its right, it results in a special array of properties, which led to refer to ruthenium as an element for the connoisseur. Of all the elements of the Periodic Table, ruthenium has the widest scope of oxidation states (from 2 in Ru(CO)4 2 to +8 in RuO4), and various coordination geometries in each electron configuration, which is in contrast to the narrow scope of oxidation states and simple square planar structure of palladium. For instance, in the principal lower oxidation states of 0, II, and III, ruthenium complexes normally prefer trigonal-bipyramidal and octahedral structures, respectively. Such a variety of ruthenium complexes has great potential for the exploitation of novel catalytic reactions and synthetic methods; however, as a consequence of the difficulties of matching the catalysts and substrates, ruthenium chemistry has lagged behind palladium chemistry by almost decade. Indeed, until the 1980s the reported useful synthetic methods using ruthenium catalysts were limited to a few reactions which include oxidations with RuO4, hydrogenation reactions, and hydrogen transfer reactions. As the coordination chemistry of ruthenium complexes has progressed, specific characters of ruthenium have been made clear. Ruthenium is relatively inexpensive in comparison with the other Group 8 transition metals such as rhodium, and a wide variety of ruthenium complexes have been prepared. RuCl3Â·nH2O is frequently used as the starting material in the preparation of most of ruthenium complexes. The ruthenium complexes can be roughly divided into five groups according to their supporting ligands: carbonyl, tertiary phosphines, cyclopentadienyl, arene/dienes, and carbenes. These ligands have proven to serve effectively as the activating factors such as generation of coordinatively unsaturated species by the liberation of ligands, and stabilization of reactive intermediates. It has been understood that the precise control of coordination sites and redox sequences of the intermediacies are especially important in the case of ruthenium to design specific organic transformations. Moreover, ruthenium complexes also demonstrate a variety of useful characteristics, which include low redox potential, high electron transfer ability, high coordination ability to heteroatoms, Lewis acid acidity, unique reactivity of metallic species and intermediates such as oxo-metals, metallacycles, and metal carbene complexes. Therefore, a large number of novel, useful reactions have begun to be developed using catalytic amounts of ruthenium complexes. The great influence of ruthenium chemistry on organic synthesis in recent years has now elevated the metal's importance to the same level as palladium, or even higher. Indeed, some ruthenium-catalyzed reactions have become industrial processes, with typical examples including a combination of the ruthenium-catalyzed asymmetric hydrogenation of 2-benzamidomethyl-3-oxobutanate via kinetic resolution and the ruthenium-catalyzed oxidation of (1R',3S)-3-[1'-(tert-butyldimethylsilyloxy)ethyl]azetidin-2-one. The latter process provides an important industrial scheme for the synthesis of 4-cetoxyazetidinone, which is a versatile and key intermediate in the synthesis of carbapenem antibiotics. Grubb's ruthenium carbene complexes [...] have also been used for industrial ring-opening metathesis polymerization (ROMP). Recent progress in the ruthenium carbine complex-catalyzed carbon-carbon double bond formation for organic synthesis is outstanding, and has become extremely important. The ruthenium catalysts involve hydrogenation, oxidation, various carboncarbon bond formations, CH activation, carbonylation, isomerization, bond cleavage reaction, metathesis reaction, and miscellaneous nucleophilic and electrophilic reactions. The organometallic chemistry of half-sandwich n6-arene-ruthenium(II) complexes has been widely developed in the last few years, due to their wide range of potential applications as catalyst precursors for hydrogen transfer, alkene polymerization, ring opening metathesis polymerization and olefin oxidation. Arene ruthenium have been also extensively investigated for their potent antibacterial and anticancer activity. Moreover arene ruthenium compounds belong to a well-established family of robust metal-organic molecules (metal = Ru(II), Rh(III) and Ir(III)) very soluble and stable in water, with many potential advantages such as alleviation of environmental problems associated with the use of organic solvents, industrial applications with the introduction of new biphasic processes and metal-mediated organic syntheses in water. The pi-ligated arene confers great stability to Ru in +2 oxidation state, and the characteristic ''piano stool'' structure offers the possibility to vary the additional donors via substitution of halide(s) with a variety of sigma-donors ranging from phosphines to beta-diketones, to aliphatic as well as aromatic amines. Our work regards the synthesis and characterization of new complexes of formula [Ru(arene)(ligand)Cl], containing diverse kinds of ligands, namely 4-acyl-5-pyrazolonate ligands and scorpionate ligands (bis-, tris- and tetrakis(pyrazolyl)borates, bis-(pyrazolyl)alkanes, bis(pyrazolyl)acetates and tris(pyrazolyl)methanesulfonates) with arene on Ru being cymene, benzene or hexamethylbenzene (HMB). We have been studied the reactivity of the [Ru(cymene)(ligand)Cl] complexes toward neutral and anionic monodentate donor ligand and toward exo-bidentate ditopic donor ligands, as we were interested in evaluating the coordination modes of the employed ligands system, the nuclearity of the resulting complexes and the geometries around the metal. We have also placed special emphasis on the neglected solution properties of the complexes, together with the structural characterization at the crystalline state. We have been studied the redox properties by cyclic voltammetry and controlled potential electrolysis, which, on the basis of their measured RuII/III reversible oxidation potentials, have allowed the ordering of the ligands according to their electron-donor character. Accomplished with these studies, we have also performed some DFT calculations in order to investigate the contribution of acylpyrazolonate or scorpionate ligand orbitals to the HOMOs, with respect to the contribution of the ancillary monodentate ligands on Ru. Additionally, for selected complexes containing scorpionate ligands, we have been studied the reactivity and the interionic structure through an integrated experimental approach based on NOE and PGSE (pulsed field gradient spin-echo) NMR experiments as function of the concentration. As an extension of our studies we have also been investigated the catalytic properties of selected neutral and ionic (arene)Ru(II) bis(pyrazol-1-yl)alkane complexes with respect to oxidation of a number of olefins by dihydrogen peroxide.
|Titolo:||(Arene)Ruthenium(II) Coordination Chemistry with Chelating and Tripodal Oxygen- and Nitrogen-Donor Ligands: Synthesis, Characterization, Reactivity and Application|
|Data di pubblicazione:||2009|
|Appare nelle tipologie:||Tesi di dottorato (Pregresso)|