Ligation sphere

A rather unique strategy which often works very well for anion signaling involves the recoordination that can take place in the ligating sphere of a metal ion in a stable complex with a fluorescent probe upon anion attack. The basic... 

Fig. 8. Molecular model of Protein 86 heme complex showing the proposed His-Met ligation sphere. Reprinted from Ref (137) copyright 1997 with permission of Cambridge University Press.

On the basis of these properties, the three approaches summarized in Figure 3 have been elaborated specifically for the introduction of photoactive M—— M cores into layered phosphate host structures. These are (1) the direct intercalation of solvated M—— M cores into layered phosphates wherein the phosphate groups of the layers form the ligation sphere for the bimetallic core (2) acid-base reaction of specially functionalized ligands on the bimetallic core with protons from the layers and (3) replacement of the phosphate groups with functionalized phosphonates that offer well-defined coordination sites for the M—M core. We now discuss each of these methodologies. 

ENDOR studies revealed at least one nitrogen from histidine residues is present in the ligation sphere (Hendrich et al., 1992) of mixed valence... 

The last step, the release of acetaldehyde, can be interpreted as a reductive elimination to give the hydrate of acetaldehyde (Eqs. (9.19) and (9.20)) where Eq. (9.19) represents merely the completion of the complex ligation sphere at the central Pd by a solvent molecule. A reductive elimination is a common reaction of group 8 metal compounds. For this case, it has been first proposed in [20]. Keith etal. [21] derived such a pathway but chloride assisted from theoretical considerations. The barrier heights of the transition states for other pathways, for example, P-hydride elimination, were found to be too high. The route according to Eq. (9.20) would also be valid for chloride-free Pd compounds. 

Figure 2. A, ligation sphere of the iron ions B, proposed desaturase reaction cycle.

Section 18.2). The latest generation of such catalysts (1 in Fig. 18.17) reproduces the key features of the site (i) the proximal imidazole ligation of the heme (ii) the trisi-midazole ligation of distal Cu (iii) the Fe-Cu separation and (iv) the distal phenol covalently attached to one of the imidazoles. As a result, binding of O2 to compound 1 in its reduced (Fe Cu ) state appears to result in rapid reduction of O2 to the level of oxides (—2 oxidation state) without the need for outer-sphere electron transfer steps [Collman et ah, 2007b]. This reactivity is analogous to that of the heme/Cu site of cytochrome c oxidase (see Section 18.2). 

To account for stereochemical results for the epoxidation of allyl alcohols, a slightly different intermediate has been proposed as shown in Fig. 6.9.16 The authors propose an intermediate (A) analogous to the intermediate in peracid oxidations. A small molecule of alcohol or water is coordinated to Ti with deprotonation and another is coordinatively ligated to Ti without deprotonation to achieve a pentacoordinated ligand sphere. During epoxidation, the allyl alcohol substrate is held in position by a hydrogen bond. 

It is noteworthy that, in the carboxamidate-ligated dirhodium(II) complexes (86), (87), and (89), the rhodium core is coordinated by four ligands and two N and two O atoms are bound to each rhodium center, constituting a unique coordination sphere (Figure 10).225... 

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