Copper five-coordinate centers

Type II copper enzymes generally have more positive reduction potentials, weaker electronic absorption signals, and larger EPR hyperfine coupling constants. They adopt trigonal, square-planar, five-coordinate, or tetragonally distorted octahedral geometries. Usually, type II copper enzymes are involved in catalytic oxidations of substrate molecules and may be found in combination with both Type I and Type III copper centers. Laccase and ascorbate oxidase are typical examples. Information on these enzymes is found in Tables 5.1, 5.2, and 5.3. Superoxide dismutase, discussed in more detail below, contains a lone Type II copper center in each of two subunits of its quaternary structure. 

Rate constants for the replacement of water by azide or thiocyanate from the five coordinate (2tyr 2his, 1H20) copper center in Fusarium galactose oxidase decrease with increasing pH, due to the greater difficulty of displacing OH- (312). 

Leznoff and co-workers reported the structural characterization and the polymorphic nature of a gold(I)-copper(II) coordination polymer, [Cu Au(CN)2 2(dmso)2] 00 [125]. In a green polymorphic form it possessed a 1-D chain structure with five-coordinate copper(II) centers, while a 2-D corrugated sheet structure with six-coordinate copper(II) centers was characterized in the blue polymorph, both of which are linked together to form a 3-D structure via aurophilic interactions. These two polymorphs exhibited virtually identical vapochromic behavior towards water, MeCN, dioxane, dmf, pyridine and ammonia, with the formation of [Cu Au(CN)2 2(solvent)x]oc,. 

Cycloaddition Reactions. Bis(oxazoline) copper complex 2 catalyzes the dipolar cycloaddition reaction between electron deficient nitrones and electron rich alkenes. While exo.endo selectivities are marginal, products can be obtained in as high as 94% enantiomeric excess (eq 19). Based on the stereochemical outcome of the reaction, a five-coordinate intermediate has been postulated in which both the nitrone (as a bidentate ligand) andl alkene are coordinated to the Cu center. 

Copper(I) complexes are typically two-, three-, or four-coordinate. The metal center is d °, diamagnetic, and thus has no EPR spectrum. Copper(II) complexes are typically four- or five-coordinate, d, and have one unpaired electron.The interaction of the unpaired electron with the nuclear spin of copper (/ = I) causes characteristic hyperfine splitting of gn into four components in the EPR, and the value of is found to be near to but greater than... 

The fifth and the sixth parts will be devoted to recent work based on templating metal centers other than copper(I). Five-coordinated systems constructed around Zn + will be considered in the fifth section, whereas the use of octahedral M(diimine)3 + as templates will be the topic of the sixth and last chapter. 

Figure 8 The copper(I) rotaxane by Sauvage et al the four-coordinate (upper panel) and the five-coordinate arranganent (center panel). Principle of the photochemicaUy and chemically triggered rearrangement of the rotaxane. (Reproduced from Ref. 115. American Chemical Society, 2001.)...

The Near-IR/vis absorption spectrum of complex 73 in water-NaOH solution was fully consistent with a trigonal-bipyramidal geometry of the copper(II) ion and the EPR spectrum is in agreement with a five-coordination sphere with the cation bound to the four nitrogen atoms of the tren unit and to one solvent molecule, a structure similar to that proposed for the parent complex 71 soluble in organic solvents. Thus it appears that the water solubilization retains the geometry of the complex around the metal center, but it was found that the redox properties were... 

Addition of aliphatic amines (L) to [Cu(S2CNEt2)2] has been studied by ESR spectroscopy (1697). Initial formation of five-coordinate adducts is proposed, while over longer periods addition of more base affords complexes of the type [Cu(S2CNEt2)2L2], which are believed to contain four coordinate, square-planar, copper centers, with a trans disposition of monodentate dithiocarbamate ligands. 

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