Heterolytic cleavage transition metal complexes

One of the important properties of dihydrogen ligands, particularly in charged transition metal complexes, is their ability to nndergo heterolytic cleavage [9]. In addition, protonation of transition metal hydrides with acids is a common method for preparation of transition metal dihydrogen complexes ... 

Four-coordinate Rh(I) hydroxide and related complexes have been demonstrated to initiate aromatic C—H activation (Equation (11.13)).97,100 In contrast to the proposed mechanisms for the Ru(II) and Ir(III) reactions, mechanistic studies for the Rh(I) systems suggest the possibility of an initial exchange between RO (via Rh—OR heterolytic cleavage), coordination of the substrate undergoing C—H activation, and deprotonation by free RO (Scheme 11.47). Alkane C—H activation by late transition metal complexes via 1,2-CH addition across metal-heteroatom bonds has yet to be demonstrated. 

Xhalo can be expressed as the product of the equilibrium constants for the electron transfer between metal complexes (Xet), the elearon afSnity of the halogen (Kea)< and the equilibrium constant for the heterolytic cleavage of the Mt" -X bond Kx=Knaiido)f which measures the halidophilicity of the deactivator. This means that the activity of a catalyst in an ATRP reaaion depends not only on the redox potential but also on the halidophilicity of the transition metal complex. For complexes that have similar halidophilicity, the redox potential can be used as a very good measure of catalyst activity in an ATRP. "... 

Eq. (8) requires determination of the two-electron oxidation potential of L M by electrochemical methods. When combined with the two-electron reduction of protons in Eq. (9), the sum provides Eq. (10), the AGh- values of which can be compared for a series of metal hydrides. Another way to determine the AGh-entails the thermochemical cycle is shown in Scheme 7.3. This method requires measurement of the K of Eq. (11) for a metal complex capable of heterolytic cleavage of H2, using a base (B), where the pK., of BH+ must be known in the solvent in which the other measurements are conducted. In several cases, Du-Bois et al. were able to demonstrate that the two methods gave the same results. The thermodynamic hydricity data (AGh- in CH3CN) for a series of metal hydrides are listed in Table 7.4. Transition metal hydrides exhibit a remarkably large range of thermodynamic hydricity, spanning some 30 kcal mol-1. 

Although hydroxyl radical is commonly assumed to be the most toxic of the oxygen radicals (with little direct evidence), other direct reactions are more likely to be important for understanding the cytotoxicity of peroxynitrite. A second oxidative pathway involves the heterolytic cleavage of peroxynitrite to form a nitronium-like species (N02 ), which is catalyzed hy transition metals (Beckman et al., 1992). Low molecular weight metal complexes as well as metals bound in superoxide dismutase and other proteins catalyze the nitration of a wide range of phenolics, including tyrosine residues in most proteins (Beckman et al., 1992). 

Coordination chemistry has become a powerful tool for the control and the living nature of radical polymerization [79,80]. Various examples show that the role of initiator and counter radical can be played by organometallic species with an even number of electrons. Besides aluminum complexes used by Matyjaszewski, several other transition metals, metallocenes, and organolan-thanides with various ligands have been studied in controlled radical polymerization [79-97]. In some cases, a controlled polymerization was achieved [81,83-85,87,90-94,97]. However, the mechanism of the polymerization is not always known and it may happen that heterolytic cleavage of the active bond... 

Although alkoxide hgands are often stable and fairly unreactive ancillary ligands, particularly when bonded to early transition metals, the C-0 bond in an alkoxide ligand can undergo special types of reactions when a related oxo complex is accessible [114]. Two mechanisms of both homolytic and heterolytic fashions for C-O bond cleavage in alkoxide hgands have been discussed. 

Peroxides of transition metals are themselves active intermediates in heterolytic and homolytic liquid-phase catalytic oxidation reactions of alkenes, aromatic hydrocarbons and alkanes. Heterolytic oxidations are characterized by a requirement for a free coordination volume near the transition metal atom. Homolytic oxidations proceed via M-O bond cleavage in peroxo complexes. 

The combination of hard oxophilic early transition metals and soft nucleophilic late transition metals with opposite functionalities, provided they do not inhibit one another, is a priori ideal for promoting cooperative effect. A proof of concept can be found in the stoichiometric reactivity of early—late heterobimetallic complexes featuring a metal-metal bond [76]. It has been shown that such complexes are good candidates to realize the heterolytic cleavage of a bond in polar and apolar substrates. An illustrative example by Bergman et al. is the reaction of the Zr-lr complex 20 with carbon dioxide which leads to the rupture of the metal—metal bond (Scheme 18) [77]. The CO2 insertion occurs in the expected fashion with the CO2 bridging the two metals, the carbon atom coordinated to iridium, and the oxygen atom on the zirconium center. 

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