Ligand stereochemistry

The way by which all the factors involved influence the course of a reaction varies from case to case, and prediction is largely empirical. For catalytic processes, the actual species acting as catalyst is often unknown because coordination number, type of ligands, stereochemistry of the complex, and formal charge are difficult to establish in the reaction medium. Often many species are present, and the most active may be the one having the lowest coordination number and being present in a concentration so low that it cannot be detected spectroscopically. Only kinetic studies can provide evidence for such species. 

When iodosylbenzene and one equivalent of triflic anhydride are mixed in dichloromethane, a (/ -phenylene)bisiodine(III) species, similar to that obtained from iodosylbenzene with two equivalents of triflic acid (structure 11, equation 23)38,63, is generated134. The addition of terminal alkynes to such mixtures affords (p-phenylene)bisiodonium ditriflates with one phenyl and one TfOC(R)=CH— ligand (stereochemistry unspecified)134 see equation 173. 

A. Sironi, The Ligand Stereochemistry of Transition Metal Carbonyl Clusters, in Metal Clusters in Chemistry , eds. P. Braunstein, L. A. Oro, and P. R. Raithby, Wiley-VCH Verlag GmbH, Wienbeim, 1999, Vol. 2, p. 937. 

A. Sironi, Inorg. Chem., 31, 2467 (1992). Transition Metal Carbonyl Clusters. A Molecular. Mechanics Approach to Ligand Stereochemistry. 

Type 2 copper centers are not uniform in ligand or ligand stereochemistries. One common feature is, however, that in the active enzyme, one coordination site is always free to bind oxygen. The most common ligand in type 2 copper centers is histidine. Tyrosine (often modified), methionine, and cysteine occur as well. There are three histidines and a modified tyrosine in amine oxidase and lysyl oxidase [28]. In diamine oxidase, two of the histidine residues have probably been replaced by cysteines [29]. In galactose oxidase, the copper ion is coordinated by two tyrosines, two histidines and an acetate ion [30]. Dopamine-/J-hydroxylase contains two differently coordinated copper ions per functional unit. One is coordinated by three histidines and a methionine and the other by two histidines and another, yet unknown, ligand [ 31 ]. Last but not least, the type 2 copper ion in Cu,Zn-superoxide dismutase is coordinated by four histidine residues, one of which connects the copper ion to the zinc ion, the second metal ion in the active site of the enzyme [32,33] (Fig. 6). 

In all three proteins, the type 1 copper is coordinated in a distorted tetrahedron in which the Cu2+ ion is situated 0.35 (Pcy), 0.43 (Paz), and 0.40 A (Acy) above the plane formed by both histidines and cysteine, towards the methionine residue [20]. In azurin, the distance between the methionine ligand and copper ion is markedly larger than for the members of the plastocyanin family, resulting in a more trigonal pyramidal conformation [20,21], a ligand stereochemistry which also occurs in halocyanin [18,94], Other factors influence the redox potentials as well, e.g., the hydrophobicity of the region surrounding the copper center. However, so little is known about the collaboration between the various factors that it is currently not possible to predict accurately the probable redox potentials of type 1 copper proteins. 

The Ligand Stereochemistry of Transition Metal Carbonyl Clusters... 

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