Tetradentate ligand influences

Since biological copper ion is often coordinated by the imidazole group of histidine residues, typical copper complexes designed to model enzyme 

The ligand changes effect large variations in their dioxygen reactivity The reaction of [Cu(TMPA)(MeCN)] with O2 in propionitrile at low temperatures initially generates an unstable copper(II)-superoxo compound (A.max 410 nm, 4,000 M cm 747 nm s 1,000 M cm ) and then 

More recently, we modified the TMPA ligand in terms of its electronic donating ability to the copper center and studied the effects on dioxygen 

34 X 10 M s and 4.10 x 10 M s with increasing as the complex is made more electron rich. The equilibrium constant for end-on peroxo formation K at 183 K) for the complex with R = MeO is roughly one order of magnitude greater than for the complex with R = H. 

A dramatic ligand effect was observed for the oxygenation reaction of [Cu(NMe2-TMPA)] formation of the superoxo compound occurs within the mixing timescale, and peroxo formation is delayed for two reasons 1) the equilibrium is considerably larger than for R = MeO and H, and 2) the rapid rate (according to k- depletes Cu as most of it is bound as Cu - 

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As for V(III), trigonal prismatic complexes, and trigonally distorted octahedral complexes are not uncommon for V(IV) [56]. The ligand set exerts a stronger influence on the redox potentials, whether the coordination geometry is octahedral or trigonal prismatic, at least to a first approximation. A series of compounds made with tridentate and tetradentate ligands with N- and O-donor atoms such as the azophenol derivative shown in Fig. 8 has both reversible oxidations and reductions. 

A complex (-503 ppm) is readily formed when one acetate is replaced with the pyridyl functionality (2-Cyr). These complexes have octahedral coordination in the solid state, and unlike the complexes described above, this coordination is retained in aqueous solution [49], The ligand is complexed in a tetradentate fashion, and here, as similarly observed for the analogous amino alcohol complexes, the presence of pyridine in the coordination sphere has little influence on 51V NMR chemical shifts. Other complexes with similar tetradentate ligands have also been found to retain octahedral coordination when in aqueous medium [54],... 

The tetradentate ligand (110) has been prepared by Wietzke and co-workers.168 It resembles (108) with three pyridine groups replaced by softer donors (pyrazine groups), in order to study the influence of the electronic configuration of the ligand on the complexation of /-elements. The ligand (110) is, unlike (108), a selective complexant of actinidcs(III).1 For tris[(2,2 -bipyridin-6-yl)-methyl]amine, see ref. 169. 

C. Tetradentate vs. tridentate ligand influences in dioxygen reactivity 145... 

C. Tetradentate vs. Tridentate Ligand Influences in Dioxygen Reactivity... 

Kruger, H.-J., Peng, G. and Holm, R. H. (1991) Low-potential nickel(III,II) complexes - new systems based on tetradentate amidate thiolate ligands and the influence of ligand structure on potentials in relation to the nickel site in [NiFe]-hydrogenases. Inorg. Chem., 30, 734-42. 

The selectivity of peptide motifs for certain metals comes from the coordinating contribution from amino acid side chains, the common coordination number of the metal, hardness/softness of the metal ion, ligand field stabilisation effects and the hardness/softness of any coordinating side chains of the amino acid sequence. An example of the influence of side chains and the importance of the position of the side chain comes from the tripeptides Gly-Gly-His, also known as copper binding peptide. The side chain imidazole ring of the His residue has a very efficient nitrogen donor (the imidazole N), which can form a tetradentate chelate ring for coordination as in Scheme 10.3. 

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