Rhodium complexes oxidative addition reactions

The reversibility of the rhodium reaction indicates that the reaction is not as energetically favorable as for the iridium analogues. The rate of addition to Rh(I) is more affected by the nature of the phosphine than for iridium. For these rhodium complexes the addition reaction was autocatalytic, the formation of the acetyl product speeded the reaction. Oxidative addition of C2H5I or C4H9I did not occur. An alternate proposal for the preparation of the acyl product has been offered . 

As shown in Scheme 168, oxidative addition reactions with either methyl chloride or methyl iodide proved successful and yielded the corresponding octahedral rhodium(III) complexes. ... 

The most interesting work on the isocyanide complexes of the elements in this subgroup has been done with rhodium and iridium. For the most part, the work is involved with the oxidative addition reactions of d square-planar metal complexes. 

Cluster or bimetallic reactions have also been proposed in addition to monometallic oxidative addition reactions. The reactions do not basically change. Reactions involving breaking of C-H bonds have been proposed. For palladium catalysed decomposition of triarylphosphines this is not the case [32], Likewise, Rh, Co, and Ru hydroformylation catalysts give aryl derivatives not involving C-H activation [33], Several rhodium complexes catalyse the exchange of aryl substituents at triarylphosphines [34] ... 

In order to get a catalytic cycle it is necessary that the metal sulfide intermediate can react with hydrogen to form the reduced metal complex (or compound) and H2S. For highly electropositive metals (non-noble metals) this is not possible for thermodynamic reasons. The co-ordination chemistry and the oxidative addition reactions that were reported mainly involved metals such as ruthenium, iridium, platinum, and rhodium. 

Many related complexes of iridium and rhodium undergo the oxidative addition reaction of alkanes and arenes [1]. Alkane C-H bond oxidative addition and the reverse reaction is supposed to proceed via the intermediacy of c-alkane metal complexes [4], which might involve several bonding modes, as shown in Figure 19.5 (for an arene the favoured bonding mode is r 2 via the K-electrons). 

M. F. Lappert, and A. J. Oliver, A Three-Fragment Oxidative Addition Reaction as a Route to Transition Metal Carbene Complexes Imidoyl Halides and Rhodium(I) Compounds as Precursors for Rhodium(III) Carbenes, J. Chem Soc., Chem. Comm. 1972, 274-275. 

This is the most populous class of rhodium(I) complex. Interest in this type of complex has undoubtedly been inspired by the important catalytic applications of [RhCKPPhj ].10-20 All complexes of this stoichiometry are 16-electron compounds and are formally coordinately unsaturated. However, for steric reasons they do not readily add a fifth ligand and pentacoordinate rhodium(I) complexes are rare. Many of the complexes undergo important oxidative addition reactions. The rhodium(III) products from these reactions are described in Section 48.6. 

There remain a number of other five-coordinate complexes whose physical properties are shown in Table 66. Of these complexes only the AsCy3 complex has been prepared from a rhodium(III) source (equation 194). The remainder have been prepared from oxidative addition reactions of [RhCl(PPh3)3], The arylazo complexes have structure (75),948 whilst the sulfinato complexes are bound through sulfur.950... 

Polydentate ligands form both rhodium(I) and rhodium(III) complexes. Both oxidation states will be considered in this section since most of the rhodium(I) complexes readily undergo oxidative addition reactions. 

The rhodium(III) complexes can be prepared either by oxidative addition to the corresponding rhodium(I) complexes or by direct reaction of the ligands with rhodium(ITI) salts. Normally the reducing properties of tertiary polyphosphines ensure that rhodium(I) complexes are formed hence the rhodium(III) complexes of these ligands have been prepared via oxidative addition reactions. However, the sterically hindered ligand (105) fails to reduce hydrated rhodium trichloride even when allowed to react with the latter in refluxing ethanol (equation 260).235... 

The catalytic cycle involves the same fundamental reaction steps as the rhodium system oxidative addition of Mel to Ir(I), followed by migratory CO insertion to form an Ir(III) acetyl complex, from which acetic acid is derived. However, there are significant differences in reactivity between analogous rhodium and iridium complexes which are important for the overall catalytic activity. In situ spectroscopy indicates that the dominant active iridium species present under catalytic conditions is the anionic Ir(III) methyl complex, [IrMe(CO)2l3] , by contrast to the rhodium system where the dominant complex is [Rh(CO)2l2] - PrMe(CO)2l3] and an inactive form of the catalyst, [Ir(CO)2l4] represent the resting states of the iridium catalyst in the anionic cycles for carbonylation and the WGSR respectively. At lower concentrations of water and iodide, [Ir(CO)3l] and [Ir(CO)3l3] are present due to the operation of related neutral cycles . 

Good yields of certain rhodium(III) perfluoroalkyl derivatives have been obtained in oxidative addition reactions of perfluoroalkyl iodides (R I) with rhodium (I) trifluorophosphine complexes (method C). 

Switching from palladium to rhodium, we encounter some very interesting chemistry. Zeng et al. [302] reacted the tiidentate PCP phosphino functionalised imidazolium salt with silver(I) oxide and subsequently transferred the carbene to rhodium(I) (see Figure 3.100). Careful selection of the rhodium precursor complex and reaction conditions enables tetrahedral, square bipyramidal and octahedral rhodium(I) and rhodium(III) complexes to be formed. As the authors explained, the activation of the C-Cl bond in methylene chloride in an oxidative addition reaction on rhodium(I) resulting in a rhodium(in) complex requires an electron rich rhodium(I) complex. The presence of a NHC ligand is advantageous in this respect. 

The majority of the pentacoordinate cationic complexes contain at least one hydrido ligand, although some are known to contain bulky neutral ligands. The latter may be regarded as the rhodium(III) analogs of the tricoordinate rhodium(I) complexes (see Section 4.1 above), from which most have been prepared by oxidative addition reactions. 

Organometallic compounds of rhodium have the metal center in oxidation states ranging from +4 to -3. but the most common oxidation states are +1 and +3. The Rh(I) species have a d electron configuration and both four coordinated square planar and five coordinated trigonal bipyramidal species exist. Oxidative addition reactions to Rh(I) form Rh(III) species with octahedral geometry. The oxidative addition is reversible in many cases, and this makes catalytic transformations of organic compounds possible. Presented here are important reactions of rhodium complexes in catalytic and stoichiometric transformations of organic compounds. 

In support of this suggestion, when [Rh2(CO)2( r-SP AN-POP) (p-Cl)2] was used as a catalyst precursor, the catalytic reaction rate was four times higher than when a 1 1 (molar) diphosphine Rh ratio was used. In a subsequent computational investigation, the oxidative addition reactions of Mel with di-rhodium complexes, [Rh(CO)(PR3)( r-Cl)]2 (R = H, Me) and that with mononuclear [Rh(CO)(PH3)2Cl] and [Rh(CO)2I2] were compared on the basis of DFT calculations [96]. Calculated activation parameters for nucleophilic attack by rhodium on Mel showed good agreement with experimental results. 

No rationale is provided for the particular catalytic efficiency of the [Rh(CO)2Cl]2 for this transformation, or for the suppression of catalytic activity in the presence of ligands that improve the reactivity in closely related reactions [176], Nevertheless, this reaction is likely to share a common mechanism with other heterocyle arylation processes that include heterocycle coordination, C-H insertion and tautomerization to the rhodium(I) NHC complex, oxidative addition to the bromoarene, and reductive elimination of the arylated heterocycle (Scheme 20). 

Calculations for the oxidative addition reactions between methane and the whole sequence of second row transition metal atoms from yttrium to palladium have been carried out [53]. The lowest barrier for the C-H insertion has been found for the rhodium atom. Palladium has the lowest methane elimination barrier. In another paper [54], the formation of complexes [CHi-Fc] (17 = +1, 0, -1) and the oxidative addition of methane to Fe has been studied by using the MINDO/SR-UHF method. The potential energy curves for the oxidative addition were calculated for a symmetry (structure VI-14). 

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