Metal complexes of thiophenes

The synthesis and chemistry of metal complexes of thiophenes have been reported including the electrophilic additions to osmium-thiophene complexes <9902988> and nucleophilic additions to ruthenium-thiophene complexes <99JOMC242>. The selectivity for the insertion of ruthenium into 3-substituted thiophenes was studied <99CC1793>. For example, treatment of 3-acetylthiophene (84) with Ru(cod)(cot) led to a regioselective 1,2-insertion of ruthenium giving thiaruthenacycle 85. 

Catalytic hydrodesulfurization (HDS) is a very important industrial process that involves removal of sulfur from crude oils by high-temperature ( 400°C) treatment with hydrogen over Co- or Ni-promoted Mo or W catalysts supported on alumina. In an attempt to determine the mechanism of this process, many transition metal complexes of thiophene, a sulfur-containing heterocycle that is particularly difficult to desulfurize, have been prepared and their reactivities studied in order to compare their behavior with those of the free thiophenes that give H2S and C4 hydrocarbons under HDS conditions (88ACR387). Thiophene can conceivably bind to the catalyst surface by either cr-donation via a sulfur electron pair or through a variety of -coordination modes involving the aromatic system... 

Three major topics have dominated research activity on thiophenes since 1996 the design and synthesis of dithienylethene molecules for application as photochromic systems (Section 3.10.2.1.3) reactions brought about under transition metal catalysis (Section 3.10.2.11) and the synthesis, characterization, and reactivity of a plethora of transition metal complexes of thiophenes (Section 3.10.6). All three had received brief mention in CHEC-11(1996) (Sections 2.10.2.2.3, 2.10.4.7.3, and 2.10.6, respectively), but together account for almost one-third of the chapter now. In addition, shorter sections have been introduced to cover the following topics one-electron oxidation of thiophenes (Section 3.10.2.2) electrochemical reactions at cathodes (Section 3.10.2.7.5) sulfur-extrusion and sulfur-transfer reactions (Section 3.10.2.10) and reactivity of silicon-linked substituents (Section 3.10.4.5). 

The reactivity of specific transition metal complexes of thiophenes is discussed in Section 3.10.6. Apart from these, there are several other reactions of thiophenes and benzo[3]thiophenes that take place in the presence of transition metals. Although it is quite likely that these reactions also proceed through the initial formation of complexes with the metals, no specific information is available on the isolation, characterization, and further transformation of such intermediates. These reactions are discussed in this section. 

Several excellent reviews are available on the reactivity of organotransition metal complexes of thiophenes <20010M1259, 2000CCR63, 1998ACR109>. In addition, Sadimenko has provided a compendium of all the known syntheses and reactions of the organometallic derivatives of thiophene and benzo[ ] thiophene <2001AHC(78)1>. 

In the following sections, the reactivity of the transition metal complexes of thiophenes has been classified from an organic chemist s perspective. 

One of the most successful preparative routes for r -bonded metal complexes of thiophenes has been the chemical or electrochemical reduction of the corresponding 18e 1)5 Ru, Rh, and Ir precursors [58-64] upon addition of two extra electrons to complexes like Cp Ir( ri -Th), the thiophenic ligand necessarily transforms from a 6e-donor into a 4e-donor situation in order to avoid oversaturation at the metal center, as exemplified for Cp Ir(2,5-Me2T) in Eq. 2.16. 

The first example of a metal complex of the dithiaporphyrin (400) has been reported. Ligand (400) reacts with [Ru(cod)Cl2] to give tra 5-[RuCl2(400)] which has been characterized in solution using NMR spectroscopy and electrochemical methods, and in the solid state by X-ray diffraction. The latter reveals that the two thiophene rings in coordinated ligand (400) are tilted out of the mean porphyrin plane. 

Removal of thiophene impurities from petroleum feedstocks is accomplished by a process called hydrodesulfurization (HDS) which involves the insertion of metals into the thiophene ring between the C-S bond. In order to better understand the mechanism of this reaction, different groups have utilized selenophene model systems due to the enhanced NMR characteristics of Se. Metal complexes of selenophenes that have been studied include rhodium <19970M2751>, molybdenum <2006POL499>, manganese <20010M3617, 19950M332>, chromium... 

In conclusion, this first Chapter attempts to provide the non specialist with and a general basis for understanding the importance and the major advances and drawbacks of heterogeneous HDS and HDN reactions, as well as the principal challenges that need to be addressed in the future. It is also meant to provide the context in which the organometallic chemistry related to these processes will be discussed in the rest of the book. Chapter 2 will be devoted to the description of metal complexes of the thiophenes, their syntheses, structures, bonding characteristics and reactivity patterns. 

Thus it appears from the knowledge gained from the chemistry of metal complexes of n-bonded thiophenes that n adsorption on highly (triply) unsaturated metal sites, which has been frequently invoked in the heterogeneous literature, may in itself be a peripheral situation, rather than a crucial phenomenon directly related to the actual HDS reactions taking place, since a number of other pathways involving the more abundant singly or doubly CUS are available under desulfurization conditions. 

A very convenient method for the synthesis of (158) consists in the side-chain bromination of 2-thienyl ethyl ketone followed by reaction with sodium methoxide in methanol and with methanesulphonyl chloride in pyridine, which gave (157) in 88% yield. Treatment of (157) with calcium carbonate led to 1,2-rearrangement of the thienyl group to give (158). Metal complexes of s> -thiophen-2-aldoxime, thiophen-2-carbaldehyde 2-benzothiazolylhydrazone, and thiophen-2-carbaldehyde thiosemicarba-zone have been studied. 

Surprisingly, the electrooxidation of metal complexes of protoporphyrin-IX dimethyl ester, possibly via the vinyl groups, leads to the deposition of electroactive porphyrin films on the electrode surface [107-109], The electrochemical polymerization of pyrrole, thiophene and amine metal-substituted complexes is described in more detail in Section 6.3. 

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