Oxidation of Rh

On oxidation of Rh(S2CNMe2)3, an unusual dimer is formed (Figure 2.54) with different rhodium environments the Rh2(S2CNMe2)5 has no metal-metal bond (Rh-Rh 2.556 A) [105],... 

Attempts to obtain Rh(IV) complexes by oxidation of Rh(Et2oxidation potential of the starting compound is as high as 1.06 V in acetonitrile. The characterisation of the obtained Rh(Et2tftc)3BF4 is, however, far from complete. 

In addition to the polymeric rhodium catalysts previously discussed, monomeric rhodium systems prepared from [Rh(CO)2Cl]2 by addition of strong acid (HC1 or HBF4) and Nal in glacial acetic acid have also been shown to be active homogeneous shift catalysts (80). The active species is thought to be an anionic iodorhodium carbonyl species, dihydrogen being produced by the reduction of protons with concomitant oxidation of Rh(I) to Rh(III) [Eq. (18)], and carbon dioxide by nucleophilic attack of water on a Rh(III)-coordinated carbonyl [Eq. (19)]. 

Addition of vinyl chloride to a reaction mixture containing some mildly basic electron donor such as dimethylacetamide (DMAC) converts the Rh1 complex to an active catalyst. It was found (/0) that this was not due to direct oxidation of Rh by vinyl chloride, but rather to the oxidation of Rh1 by HC1 generated by the interaction between vinyl chloride and dimethylacetamide catalyzed by Rh1 ... 

In the oxidized hydrocarbon, hydroperoxides break down via three routes. First, they undergo homolytic reactions with the hydrocarbon and the products of its oxidation to form free radicals. When the oxidation of RH is chain-like, these reactions do not decrease [ROOH]. Second, the hydroperoxides interact with the radicals R , RO , and R02. In this case, ROOH is consumed by a chain mechanism. Third, hydroperoxides can heterolytically react with the products of hydrocarbon oxidation. Let us consider two of the most typical kinetic schemes of the hydroperoxide behavior in the oxidized hydrocarbon. The description of 17 different schemes of chain oxidation with different mechanisms of chain termination and intermediate product decomposition can be found in a monograph by Emanuel et al. [3]. 

The activation of hydrocarbons on the catalyst surface was also discussed in the literature [255]. There are no clear experimental evidences of this activation with free radical generation [270]. However, examples of dimer (RR) formation as a result of oxidation of RH on the surface of Mn02 are known [270],... 

Catalytic surface is active toward hydroperoxide and decomposes it to free radicals. The free radicals initiate the chain oxidation of RH in the liquid phase. 

Since peroxyl radicals are also removed as a result of disproportionation, the reaction conditions are quasi-stationary for which the equality Vj = 2fc7[InH] [R02 ] I 2A 6[R02 ]2 is more appropriate. In this case, the rate of initiated chain oxidation of RH is equal to ... 

As already noted (see Chapter 4), autoxidation is a degenerate branching chain reaction with a positive feedback via hydroperoxide the oxidation of RH produces ROOH that acts as an initiator of oxidation. The characteristic features of inhibited autoxidation, which are primarily due to this feedback, are the following [18,21,23,26,31-33] ... 

The mechanisms by which an inhibitor adds to an oxidized hydrocarbon exerts its influence may differ depending on the reaction conditions. If the rate constants of the elementary reactions of RH, InH, R02 , In, ROOH, and 02 are known, the kinetics of the inhibited oxidation of RH can mathematically be described for any conditions. However, such an approach fails to answer questions how the mechanism of inhibited oxidation is related to the structure and reactivity of InH, RH, and R02 or what inhibitor appears the most efficient under the given conditions, and so on. At the same time, these questions can easily be clarified in terms of a topological approach whose basic ideas are the following [43-45,70-72] ... 

Inhibited oxidation of RH occurs through a relatively large number of reactions (see Chapter 4), but the primary mechanism of inhibited oxidation is determined by a few key reactions. For example, cumene is oxidized in the presence of p-cresol at 320-380 K... 

Equations (in the form T = A/B) for the Bounday Mechanisms of the Phenol-Inhibited Oxidation of RH [69]... 

It has already been noted in the Introduction that the WGS reaction occurs in competition with methanol carbonylation. The mechanism of the WGS reaction involves oxidation of Rh(I) to Rh(III) by reaction with HI, as shown in Scheme 2 [25]. 

LFER and negative A5 and AV values indicate an associative mechanism with strongly polar character to the activated complex for Mel oxidations of Rh(I) P-diketonates. 

Just as intermediates such as [RhMe(CO)2l3] are not generally observed under the usual working conditions of MeOH carbonylation to AcOH, no intermediate species are observed directly for Rh catalysed water gas shift. However, the first step in oxidation of [Rh(CO)2l2] to [Rh(CO)2l4] , and thus a key step in the Rh/HI catalysed water gas shift reaction, the formation of [RhH(CO)2l3] , has been reported by Bunel, who followed the reaction by NMR (Eq. (50)) [50]. 

This transition may exhibit a critical behavior when, at a certain concentration of inhibitor known as the critical concentration [InH]cr, the dependence of the induction period on [InH] drastically changes, so that di-/d[InH] at [InH] > [InH]cr becomes much higher than dr/d[InH] at [InH] < [InH]cr. In the literature this problem has been treated only with reference to mechanisms II, III, and VIII [61-68], while all the known mechanisms of inhibited oxidation of RH will be envisaged here (see earlier) [69]. The equations for the chain length, critical antioxidant concentration [InH]cr, stationary concentration of hydroperoxide [ROOH]st, and induction period are given in Table 14.3 and Table 14.4. 

Fig. 21. Half wave potentials of the oxidations of Rh(OMC)PPh3, Co(OMC)PPh3, Rh(TPP)CH3 and Co(TPP)CH3 in dichloromethane. Taken from Ref. [49] with permission...

The energetic basis for the electron-transfer oxidation includes the thermodynamic potential of oxidation (E°ox) for the electron transfer from RH in Eq. (7). Such an electron detachment is commonly effected at an electrode, by an oxidant, or with light. The oxidation is driven electrochemically by the anodic electrode potential, which matches the E°m value. Likewise, the driving force in the chemical oxidation of RH is provided by the redox potential (fi°ed) of the electron acceptor or oxidant (A) according to Eq. (5). 

The structure of Rhi2(CO)25(C2), reported in Fig. 10, is a further example of carbide stabilization (13). This is the most irregular cluster as yet characterized like Rh8(CO)i9C, it has no symmetry element. This cluster is also derived chemically from the oxidation of [Rh COVC]2-. The 2 central carbide atoms are definitely bonded together (1.47 A) and lead to... 

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