Binding mode transition metal complexes

For the purposes of this chapter, which focuses on comparisons of isocyanide binding in transition metal complexes and isocyanide adsorption on metal surfaces, we first summarize known modes of isocyanide binding to one, two and three metals in their complexes. In such complexes, detailed structural features of isocyanide attachment to the metals have been established by single-crystal X-ray diffraction studies. On the other hand, modes of isocyanide attachment to metal atoms on metal surfaces are proposed on the basis of comparisons of spectroscopic data for adsorbed isocyanides with comparable data for isocyanides in metal complexes with known modes of isocyanide attachment. 

In Fig. 1 some of the most common metal-dioxygen binding modes found for mononuclear and dinuclear transition metal complexes are diagrammed. Note that the terms superoxo and peroxo ultimately... 

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... 

This is a very common binding mode for transition metal complexes, found for a variety of metals including members of all three periods of d-block elements as well as early metals... 

Binding Modes of N-Heterocycles in Transition Metal Complexes. 

Transition metal complexes of 6-mercaptopurines have been characterized with three binding modes N-only, S(6)-only and [N(7)-S(6)l chelation. Two additional binding modes, the [N(l)-S(6)] chelate and the [N(3)-N(9)] chelate, have also been proposed, but not characterized. 

Now we may examine in detail the interaction of one class of metal complexes with nucleic acids, how these complexes bind to polynucleotides, the techniques used to explore these binding interactions, and various applications of the complexes to probe biological structure and function. Tris(phenanthroline) metal complexes represent quite simple, well-defined examples of coordination complexes that associate with nucleic acids. Their examination should offer a useful illustration of the range of binding modes, reactivity, techniques for study, and applications that are currently being exploited and explored. In addition, we may contrast these interactions with those of other transition-metal complexes, both derivatives of the tris(phenanthroline) family and also some complexes that differ substantially in structure or reactivity. 

The reactivity of Qo comparable to that of electron deficient conjugated olefins is nicely reflected by reactions with transition metal complexes. A variety of single crystal structures and spectroscopic studies show that the complexation of transition metals to the fullerene core proceeds in a dihapto manner or as hydrometalation reactions rather than in rf- or ] -binding mode. This was elegantly demonstrated by the reaction of Cgg with ruthenium complexes (Scheme 8) [144]. A variety of iridium complexes ( ] -Cgo)Ir(CO)Cl(PR R R )2 were synthesized by allowing Cgg to react with different Vaska-type complexes Ir(CO)Cl(PR R R )2 [145]. ] -Complex formation was also observed upon reaction of Cgo with other Ir [146] as well as Rh [147] complexes. Hydro-metallation was obtained with Cp2Zr(H)Cl [140]. 

In sharp contrast, the late transition metals are more coordinative to soft carbon-carbon multiple bonds rather than hard oxygen. These binding modes are further classified into mono- and bi-dentate coordinations, depending on the ligands on the metal catalysts, or the substituent pattern in the Claisen diene systems and solvents employed. Bi-dentate coordination of the Claisen substrate is advantageous over the weak mono-dentate coordination of the Claisen rearrangement product, y,d-unsaturated carbonyl compounds, to release the metal complex allowing the catalytic cycle. Furthermore, enantiodiscrimination by chiral late transition metal complexes is based on the discrimination of two enantiotopic diene faces in the enantiomeric six-membered transition states. 

Tetrahydrofuran (THF), acetone, water, and ethanol are much less strongly ligating and are widely used. Early transition metal complexes can be very sensitive to solvents containing labile protons, but this depends on the case. All of these solvents can act as weak ligands, and their complexes can be synthetically useful. Ketones usually bind in the mode via O, as in 4.22, but can also bind in the V mode via both C and O, as in 4.23 (Eq. 4.S9). The latter is favored by low steric hindrance and by a strongly back-donating... 

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