High-valent metal ions

Hersleth H-P, Ryde U, Rydberg P et al (2006) Structures of the high-valent metal-ion haem-oxygen intermediates in peroxidases, oxygenases and catalases. J Inorg Biochem 100 460-476... 

Many compounds have been synthesized, characterized, and studied as models for proposed intermediates in various homogeneous catalytic reactions. Here we discuss two examples. Complex 2.4 is proposed as a model that shows the mode of interaction between an organic hydroperoxide and high-valent metal ions such as Ti4+, V5+, and Mo6+. This type of interaction is considered to be necessary for the oxygen atom transfer from the hydroperoxide to an alkene to give an epoxide (see Chapter 8). 

High-valent metal ions tend to promote deprotonation to form oxo-metal complexes and further desolvation to eventually yield simple metal oxides. Thus pure aqua complexes are found mainly with metals in oxidation states III and below. 

Of all polyalkyls, the longest known are the Pt(FV) species. The orange complex [Me3Pt(/x -I)]4, which has a cubane structure with octahedral platinum, was described by Pope and Peachey in 1907-1909. Some of its reactions (Eqs. 15.15-15.17 L = NH3, en, py, PMC3) illustrate how the chemistry resembles that for aqueous high-valent metal ions, such as the Co(III) Werner compounds that we looked at in Chapter 1. 

In classical Werner complexes, such as [Co(NH3)6], a relatively high valent metal ion binds to the lone pairs of electronegative donor atoms, typically, O, N, or halide. The M-L bond has a marked polar covalent character, as in L M-NH3, where L represents the other ligands present. The M-NH3 bond consists of the two electrons present in lone pair of free NH3, but now donated to the metal to form the complex. 

The vanadium complex 2.73 has a peroxo ligand. An analogue with an alkylperoxo (RO ") ligand that has a similar strucmre is also known. The oxidation state of vanadium in 2.73 is 5+. As we will see later, the alkyl peroxo analogue of 2.73 is a model (see Section 3.4) that mimics the mode of activation of organic hydroperoxides by high-valent metal ions. 

Nitrogen protonation or incorporation of high oxidation state metal ions facilitates nucleophilic substitution (especially at Craeso) whereas N-deprotonation or coordination to low valent metal ions enhances the electrophilic reaction (especially at Cme50). The effects of electron-donating substituents (alkyl) are less than those of electron-withdrawing groups (especially n acceptors, N02, COR). 

The main inspiration for the formation of nanoparticles from single amino acids arose from the fact that in nature, many particles are formed from short chain peptides that contain a cysteine or histidine amino acid. These short chain peptides are principally used as a form of detoxification from the effects of high heavy metal ion concentrations in vivo. From these biological examples, the chemistry of these peptides has been expanded to form novel zero-valent and semiconductor nanoparticles under in vitro conditions (Figure 2). 

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