Hydroxides metal coordination

The conjugate base of the latter is the actual nucleophile. Thus the increase of activity up to pH 7.3 accounts for the deprotonation of this water molecule while the decrease in activity above this pH is related to the reduced ability of a phosphate to compete with a hydroxide for coordination to the metal center. This is a common feature of many catalysts with similar mechanisms. 

Similar methods of encapsulation are also observed in pillared clays, which were also introduced as catalysts as long ago as the early 1980s. The field has been thoroughly reviewed up to 2000 [65], Layered double hydroxide structures have also been used for the entrapment of metal coordination compounds [66],... 

Klingler and co workers—impact of metal coordination on the formate pathway. Klingler and coworkers114,125 carried out studies on the cation reactivity of formate in solution at 180 °C, 7.8 mol/L of H20 with the solvent triethylene glycol, and the formate concentration was varied. It was noted that formic acid produced from the carbonylation of water is catalyzed by hydroxide anion, and rapidly builds up to close to equilibrium concentration levels under these conditions, so that they had to use gold plated stainless steel autoclaves. They obtained a first order dependency of the water-gas shift reaction rate (i.e., H2 evolution rate) with formate concentration. As a control experiment, they added metal pentacarbonyl iron to the reactor, and observed no additional rate increase, indicating that the water-gas shift... 

A metal-coordinated hydroxide ion might act as a general base, abstracting the proton from the 2 -OH (Fig. 3b) or, alternatively, a metal ion might act as a Lewis acid to accelerate the deprotonation of 2 -OH by coordinating directly with the 2 -oxygen (Fig. 3d). 

Hydroxide ion coordinated to cobalt(IIl) has been observed to function as a nucleophile in both intermolecular and intramolecular reactions. The possibility that coordinated hydroxide is directly involved in the catalytic action of some metalloenzymes such as carbonic anhydrase has prompted a number of investigations of metal hydroxide reactivity towards organic substrates. Much of this chemistry has been reported and reviewed.21,23... 

The commonest reactions involve the displacement of halide by hydroxide or cyanide ion to yield co-ordinated phenols or nitriles. Once again, the metal may play a variety of different functions. The polarisation of the C-Cl bond is the most obvious, but stabilisation of the product may be of equal importance, as could the involvement of a metal coordinated nucleophile. The availability of a one-electron redox inter-conversion between copper(n) and copper(i) also opens up the possibilities of radical mechanisms involving homolytic cleavage of the C-Cl bond. All of these different processes are known to be operative in various reaction conditions. In other cases, organocopper intermediates are thought to be involved. 

As discussed in this chapter, there are three direct modes of activation that metal ions can provide for hydrolysis reactions, i.e., by Lewis acid, nucleophile and leaving group. In addition, metal coordinated water molecules can act as general acid catalysts and metal coordinated hydroxides can act as general base catalysts. We are mainly concerned with the three direct modes of activation since there is no particular advantage to using metal-based general acids and bases. 

Rives, V. and Ulibarri, M. A. (1999). Layered double hydroxides (LDH) intercalated with metal coordination compounds and oxometalates. Coord. Chem. Rev. 181, 61. 

Intramolecular hydrolysis by Co111-coordinated hydroxide in n s-[Co(0H) OP02(OC6H4NO2) (en)2] has also been demonstrated (Scheme 58a) and a phosphorane intermediate analogous to (163) is suggested. 16 The advantages of metal-coordinated hydroxide over alkoxide in facilitating such cyclizations and hydrolyses are discussed in this report. 

The first step is likely to be a so-called Hieber base reaction (0 in Scheme 1), describing a nucleophilic attack at metal-coordinated carbon monoxide by hydroxide. While eq. (3) is the prototypical example as first reported by Walter Hieber in 1932 [5], several other variants have since become known. For example, azides and alkyl/aryl anions attack metal carbonyls according to eqs. (4) and (5), yielding metal isocyanates (via a Curtius-type azide degradation step) and metal carbenes, respectively [6, 7]. 

Therefore, at neutral or slightly alkaline pH, the small decrease in efficiency of coordinated vs. free hydroxide ions is more than compensated for by the higher concentration of reactive species available (i.e., HO—M+ vs. HO ). Another common role for zinc enzymes is thus to provide a binding site at which the substrate can be attacked by the metal-coordinated hydroxide ... 

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