Metal complexes, and hydrogenation

Ligand substitutions of 18-electron complexes can also occur by radical-chain processes initiated by atom abstraction. These radical chains occur through 17-electron intermediates that imdei o facile associative substitutions. Substitutions of metal carbonyl hydrides, halides, and stannyl complexes by this mechanism are all known. These reactions are particularly prevalent in first-row metal hydrides because the M-H bond is weaker than the M-H bond in second- and third-row metal complexes, and hydrogen atom abstraction is one step of the radical chain. However, they have also been proposed to occiu with third-row metal-hydride complexes... 

Du XZ, Miao W, Liang YQ (2005) IRRAS studies on chain orientation in the monolayers of amino acid amphiplriles at the air-water interface depending on metal complex and hydrogen bond formation with the headgroups. J Phys Chem B 109(15) 7428-7434. doi 10.1021/ Jp0441700... 

Du XZ, Liang YQ (2000) Roles of metal complex and hydrogen bond in molecular structures and phase behaviors of metal N-octadecanoyl-L-alaninate Langmuir-Blodgett films. J Phys Chem B 104(43) 10047-10052. doi 10.1021/Jp002016h... 

The direct connection of rings A and D at C l cannot be achieved by enamine or sul> fide couplings. This reaction has been carried out in almost quantitative yield by electrocyclic reactions of A/D Secocorrinoid metal complexes and constitutes a magnificent application of the Woodward-Hoffmann rules. First an antarafacial hydrogen shift from C-19 to C-1 is induced by light (sigmatropic 18-electron rearrangement), and second, a conrotatory thermally allowed cyclization of the mesoionic 16 rc-electron intermediate occurs. Only the A -trans-isomer is formed (A. Eschenmoser, 1974 A. Pfaltz, 1977). 

The Af-HjO diagrams present the equilibria at various pHs and potentials between the metal, metal ions and solid oxides and hydroxides for systems in which the only reactants are metal, water, and hydrogen and hydroxyl ions a situation that is extremely unlikely to prevail in real solutions that usually contain a variety of electrolytes and non-electrolytes. Thus a solution of pH 1 may be prepared from either hydrochloric, sulphuric, nitric or perchloric acids, and in each case a different anion will be introduced into the solution with the consequent possibility of the formation of species other than those predicted in the Af-HjO system. In general, anions that form soluble complexes will tend to extend the zones of corrosion, whereas anions that form insoluble compounds will tend to extend the zone of passivity. However, provided the relevant thermodynamic data are aveiil-able, the effect of these anions can be incorporated into the diagram, and diagrams of the type Af-HjO-A" are available in Cebelcor reports and in the published literature. 

This chapter will provide an overview of the development and use of early transition-metal complexes in hydrogenation, and in consequence has been divided into several sections. Section 6.2 will focus on the mechanistic differences in the hydrogenation reaction between early and late transition metals. The following three sections will describe the various systems based on Group IV (Sec-... 

Aldehydes may sometimes pose a problem in transfer hydrogenations catalyzed by transition metals. They can poison the catalyst or decarbonylate, forming CO, which may coordinate to the metal complex and result in a change in activity (Scheme 20.26) [65, 66]. 

Thus far, we have discussed the transition metal complex-catalyzed hydrogenation of C=C, C=0, and C N bonds. In this section, another type of transition metal complex-mediated reaction, namely, the hydroformylation of olefins, is presented. 

Most network structures involving crown ethers are simple hydrogen bonded chains where the crown forms second sphere coordination interactions with a complex ion. These are known for [18]crown-6, [15]crown-5 and [12]crown-4 hosts with a variety of metal complexes [17-25]. For instance when combined with the small [12] crown-4, the perchlorate salts of Mn(II), Ni(II) and Zn(II) form polymeric chain structures with alternating crown ethers and [M(H20)6]2+ cations [19]. Similarly the larger [18]crown-6 forms simple linear chains with metal complexes and cations such as fra s-[Pt(NH3)2Cl2] [20], [Cu(NH3)4(H20)]2+ (Fig.2) [21],and [Mg(H20)5(N03)] + [22],... 

The sol-gel entrapment of the metal complexes [Ru(p-cymene)(BINAP)Cl]Cl and the rhodium complexes formed in situ from the reaction of [Rh(COD)Cl]2 with DlOP and BPPM has been reported by Avnir and coworkers [198]. The metal complexes were entrapped by two different methods the first involved addition of tetramethoxysilane to a THF solution of the metal complex and triethylamine, while the second method was a two-step process in which aqueous NH4OH was added to a solution of HCl, tetramethoxysilane and methanol at pH 1.96 followed by a THF solution of the appropriate metal complex. The gel obtained by each method was then dried, crushed, washed with boiling CH2CI2, sonicated in the same solvent and dried in vacuo at room temperature until constant weight was achieved. Hydrogenation of itaconic acid by these entrapped catalysts afforded near-quantitative yields of methylsuccinic acid with up to 78% e.e. In addition, the catalysts were found to be leach-proof in ethanol and other polar solvents, and could be recycled. 

A limitation of aluminum-containing ionic liquids arises from their moisture sensitivity. Moreover, most transition metal complexes and organic reactants are not unreactive in the presence of the chloroaluminate compounds. These ionic liquids react with water in a highly exothermic manner, with the formation of hydrogen chloride and a white precipitate. 

Hofmeier H, El-ghayoury A, Schenning APH, Schubert US (2004) New supramolecular polymers containing both terpyiidine metal complexes and quadruple hydrogen bonding units. Chem Commun 318-319... 

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