Oxidation redox noninnocent” ligands

In this chapter and in an upcoming review (Part II), the chemistry of mononuclear paramagnetic organometallic complexes of the second- and third-row groups 9(VIIIB) and lO(VlII) platinum metals (Rh, Ir, Pd, Pt) are summarized. In this chapter, the higher valent species (Rh(II), Ir(II), Rh(IV), Ir(IV), Pd(ni), Pt(III) and complexes with oxidized redox noninnocent ligands) are described. In the next chapter, the lower valent species (Rh(0), lr(0), Pd(I), Pt(I), and complexes with reduced redox noninnocent ligands will be discussed. 

Ghosh S, Baik M-H. Redox properties of Tanaka s water oxidation catalyst redox noninnocent ligands dominate the electronic structure and reactivity. Inorg Chem. 2011 50 5946-5957. 

Another approach to achieve a two-electron transformation such as oxidative addition or reductive elimination for d f metals is to introduce redox noninnocent ligands. Instead of changing the oxidation state of the metal center, the electrons stored in the ligand fragment become involved in... 

In spite of the absence of C—H bond activation by an oxidative addition mechanism mediated by f-elements, these two approaches, use of a bimetallic system or a redox noninnocent ligand, bring new opportunities to discover novel types of C—H bond activation utilizing f-element metal complexes, as discussed below. It is interesting to note that, in most cases, strong reducing conditions are appHed. 

The redox noninnocence of the 2-mercapto-3,5-di-tert-butylaniline ligand has recently been investigated with nF ions. The spectroelectrochemistry of the complex displays a range of electron transfers where the monocation, the neutral species, and the mono- and dianions have been characterized. In a related manner, Wieghardt and coworkers have reported the first example of a stable N, O-coordinated o-iminobenzoquinone via air oxidation of the initial nF complex with 2-anilino-4,6-di-tert-butylphenol (94). The analogous o-iminobenzosemiquinonate 7r-radical complex was also isolated for this system and earlier for the bis-(o-immobenzosemiquinonate)nickel(II) complex. ... 

Enedithiolate anions are redox-active ligands (see equation 1), capable of coordinating with a variety of metals as neutral dithioketones (1), thioketone radical thiolate monoanions (2) and ene-l,2-dithiolate dianions (3). The complexes are named dithiolenes irrespective of their oxidation state to remind one that they are noninnocent ligands . 

While many of the species discussed in this chapter are metal-centered radicals, it is clear that formation of ligand-centered radicals by oxidation of redox noninnocent radicals can also play an important role. This includes oxidation of the known redox noninnocent dithiole-thionate-, indole-, and catechol-based ligands, as well as the rather unexpected formation of aminyl radicals upon oxidation of Rh NR2) amido species. 

On the basis of these observations, the electron-transfer catalysis in the activation of aromatic C—H bonds was proposed to proceed via two possible pathways. Outher-sphere oxidation, which yields [Ir (Cp )Me2(PR3)] " and inner-sphere oxidation via the Cp ligand yielding another type of iridium(IV) radical cation that easily converts to a so-called tucked-in intermediate (Fig. 35). Both intermediates were claimed to be capable of subsequent activation of aromatic C—H bonds. Apparently, the redox and chemical noninnocence of the Cp ligand plays a crucial role in at least part of the observations. 

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