Rhodium/ions/salts

A somewhat unusual copper catalyst, namely a zeolite in which at least 25% of its rhodium ions had been exchanged by Cu(II), was active in decomposition of ethyl diazoacetate at room temperature 372). In the absence of appropriate reaction partners, diethyl maleate and diethyl fumarate were the major products. The selectivity was a function of the zeolite activation temperature, but the maleate prevailed in all cases. Contrary to the copper salt-catalyzed carbene dimer formation 365), the maleate fumarate ratio was found to be relatively constant at various catalyst concentrations. When Cu(II) was reduced to Cu(I), an improved catalytic activity was observed. 

Rhodium, incorporated in the silver halide grains, decreases sensitivity and increases contrast. This action has been attributed to depression of latent image formation because of deep electron trapping by the trivalent rhodium ion (183-185). Eachus and Graves (184) showed that rhodium, probably as a complex, acts as a deep trap for electrons at room temperature. Weiss and associates (186) concluded that the rhodium salts introduce deep traps for both electrons and holes. Monte Carlo simulation showed that the photographic properties could be accounted for in this way over a wide range of exposure times. 

There seem to be very few complexes in which there are two rhodium ions in different oxidation states. The best-authenticated examples are the [RhH2(PR3)2]2 complexes (R = NMe2, PfOPr1 ]. The dimethylamino complex has been shown to have the structure (129) and contains both rhodium(I) and rhodium(III).1287 The other example is provided by the [Rh(02CMe)2]+ ion in [Rh(02CMe)2]2C104. This ion retains the classic lantern structure of the rhodium(II) carboxylato complexes, but the salt contains both rhodium(II) and rhodium(III).1288... 

Despite the above similarities, many differences between the members of this triad are also to be noted. Reduction of a trivalent compound, which yields a divalent compound in the case of cobalt, rarely does so for the heavier elements where the metal, univalent compounds, or hydrido complexes are the more usual products. Rhodium forms the quite stable, yellow [Rh(H20)6] " ion when hydrous Rh203 is dissolved in mineral acid, and it occurs in the solid state in salts such as the perchlorate, sulfate and alums. [Ir(H20)6] + is less readily obtained but has been shown to occur in solutions of in cone HCIO4. 

Rh(C204)3 was resolved by Werner as the strychnine salt but other ions, such as Coen3+ and Niphen3+, have been used more regularly for this [84], The dinuclear rhodium(II) acetate is described in section 2.8.2 the dinuclear structure is retained on one-electron oxidation, but when ozone is used as the oxidant, a compound with a trinuclear Rh30 core is formed, analogous to those formed by Fe, Cr, Mn and Ru. (It can also be made directly from RhCl3.)... 

It is thought that these trialkonolamine borates may enhance the reactivity of the rhodium carbonyl anions by minimizing their tendency to form contact ion pairs in solution under the reaction conditions employed. The same patent discloses that ammonium salts and salts of Groups I and II, especially cesium and ammonium carboxylate salts, function as promoters (63). 

Because salts of the [Rh(CO)2X2] ion are not only simple to prepare but rather stable species under ambient conditions, the reaction of [Rh(CO)2X2] ions with methyl iodide can be readily studied. Infrared spectroscopy at room temperature (15) reveals that an acetyl complex of rhodium(III) is the first detectable species after reaction. This species was isolated as its trimethylphenylammonium salt, and the structure of this material has been determined by X-ray crystallography (16). The... 

SIMS spectra of the RhCf salt in Fig. 9.1 show clear molecular peaks characteristic of rhodium coordinated by chlorine. In particular the RhCl2 signal is very intense. As explained in Chapter 4, there is little doubt that molecular cluster ions from compounds other than alloys are the result of a direct emission process. Hence, Fig. 9.1 implies that if a sample contains rhodium atoms with more than one chlorine ligand, SIMS is capable of detecting this combination with high sensitivity. 

A mechanistic study by Haynes et al. demonstrated that the same basic reaction cycle operates for rhodium-catalysed methanol carbonylation in both homogeneous and supported systems [59]. The catalytically active complex [Rh(CO)2l2] was supported on an ion exchange resin based on poly(4-vinylpyridine-co-styrene-co-divinylbenzene) in which the pendant pyridyl groups had been quaternised by reaction with Mel. Heterogenisation of the Rh(I) complex was achieved by reaction of the quaternised polymer with the dimer, [Rh(CO)2l]2 (Scheme 11). Infrared spectroscopy revealed i (CO) bands for the supported [Rh(CO)2l2] anions at frequencies very similar to those observed in solution spectra. The structure of the supported complex was confirmed by EXAFS measurements, which revealed a square planar geometry comparable to that found in solution and the solid state. The first X-ray crystal structures of salts of [Rh(CO)2l2]" were also reported in this study. 

Reactions of ruthenium catalyst precursors in carboxylic acid solvents with various salt promoters have also been described (170-172, 197) (Table XV, Expt. 7). For example, in acetic acid solvent containing acetate salts of quaternary phosphonium or cesium cations, ruthenium catalysts are reported to produce methyl acetate and smaller quantities of ethyl acetate and glycol acetates (170-172). Most of these reactions also include halide ions the ruthenium catalyst precursor is almost invariably RuC13 H20. The carboxylic acid is not a necessary component in these salt-promoted reactions as shown above, nonreactive solvents containing salt promoters also allow production of ethylene glycol with similar or better rates and selectivities. The addition of a rhodium cocatalyst to salt-promoted ruthenium catalyst solutions in carboxylic acid solvents has been reported to increase the selectivity to the ethylene glycol product (198). 

Metal ions in catalytic amounts exercise a profound influence on the course of the oxidation. In the absence of metal ions, the peracetic acid oxidation of 3-nitroaniline produces 3,3 -dinitroazoxybenzene. In the presence of traces of cupric ions and, to a lesser extent, in the presence of small quantities of iron, nickel, and rhodium salts, only 3,3 -dinitroazobenzene is formed. The oxidation of toluidines and aminophenols usually leads to tarry products [32]. 

The reactions of dihydrobilin (1,19-dideoxybiladiene-a, c) with transition metals are strongly influenced by the nature of the metal ion. Thus with Mn(OAc)3 or FeClj the corresponding metallocorrolates have been obtained in high yield, in the presence of chromium or ruthenium salts the reaction product isolated has been the metal free macrocycle, while coordination of rhodium requires the presence of an axial ligand such as a phosphine, arsine or amine [21]. Neutral pentacoordinated rhodium complexes have thus been obtained. Although analysis of the electronic spectra of the reaction mixtures demonstrated that cyclization of the open-chain precursor and formation of metallocorrolates occur even in the absence of extra ligands, no axially unsubstituted rhodium derivative has been reported. 

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