Rhodium salts

The well-known Adams platinum oxide can be prepared conveniently by the procedure of Adams et al. (2). Platinum oxides prepared in this way usually contain some traces of sodium, which in certain reactions can have an adverse effect. The sodium can be removed by washing with dilute acid (53). The Nishimuri catalyst (30% Pt, 70% Rh oxides) can be prepared by the same procedure as for platinum oxide or with variations from platinum and rhodium salts (64,65,66). This catalyst has much merit. It is usually most useful when hydrogenolysis is to be avoided (67,85,86). 

Rhodium was discovered in 1803 by the eminent Norfolk scientist W.H. Wollaston he dissolved platinum metal concentrates in aqua regia and found that on removing platinum and palladium he was left with a red solution. From this he obtained the salt Na3RhCl6, which yielded the metal on reduction with hydrogen. The rose-red colour (Greek rhodon) of many rhodium salts gave the element its name. 

Name from rhodon (Greek = rose) some rhodium salts are pink Properties... 

The stoichiometric reaction of low-valent rhodium salts with l, -diynes to afford rhodacyclopentadiene complexes is well established and has been reviewed.73 733 The first rhodium-catalyzed reductive cyclization of a non-conjugated diyne has been reported only recently.74 743 The stereochemical outcome of the rhodium-catalyzed hydrosilylation-cyclization is dependent upon the choice of catalyst. Whereas reductive cyclization of 1,6-diyne 54a catalyzed by Rh4(CO)i2 provides modest yields of the Z-vinylsilane 54c, exposure of 54a to Wilkinson s catalyst... 

Hydroaminomethylation is a simple, efficient and atom-economic method to synthesize various amines. This one-pot reaction consists of three consecutive steps in the first step a hydroformylation of an olefin is performed followed by the reaction of the resulting aldehyde with a primary or secondary amine to give the corresponding enamine or imine. Lastly, this intermediate is hydrogenated to the desired secondary or tertiary amine (Fig. 11) [33-39]. In most cases rhodium salts or complexes are used as the homogeneous catalyst in the hydroaminomethylation. 

Kabalka and co-workers reported the direct cross-coupling of cinnamyl alcohols with aryl- and vinylboronic acids using simple rhodium salts in an environmentally benign ionic liquid medium (Eq. 6) [30]. The ability to utilize allylic alcohols, without activation, is significant from the viewpoint of atom economy, yet challenging due to the poor leaving group ability of hydroxide. 

Rhodium chloride is used to prepare other rhodium salts. 

Heteroatom-containing enediynes undergo a carbotricyclization process on exposure to catalytic quantities of rhodium salts and phenyldimethylsilane under a carbon monoxide atmosphere to produce the corresponding fused tricyclic compounds (Equation 140) <2000JA2385>. 

All three metals of this group give rise to ammino-derivatives the compositions of which, however, differ considerably. The ammino-derivatives of ruthenium mostly contain a nitroso-group as well as ammonia the ammino-derivatives of rhodium salts closely resemble the cobalt -ammines and the ammino-derivatives of palladium salts correspond to the ammino-derivatives of platinum salts. 

Acido-pentammino-rhodium salts, purpureo-salts,[Rh (NH3) 5 R] R 2. 

Aquo-pentammino-rhodium salts, roseo-salts, [Rh(NH3)5H20]R3. 

The sodium pyrophosphate, [Rh(NH3)6]NaP207.12H20, is obtained as a silky white crystalline precipitate on the addition of sodium pyrophosphate to a dilute solution of neutral hexammino-rhodium salt. It crystallises in small hexagonal prisms or rhombic plates and is almost insoluble in water. 

Ammonia may be replaced by ethylenediamine in the hexammino-salts, giving rise to the triethylenediamino-rhodium salts. These are completely analogous to the triethylenediamino-salts of cobalt. 

Rh en3][NO,.C10H14O]3 is prepared by treating an aqueous solution of triethylenediamino-rhodium chloride with a solution of sodium d-camphor nitronate, when a sparingly soluble precipitate of the 1-salt separates. From the mother-liquor the d-salt is obtained. These two salts serve as the starting material for the preparation of the optical isomers of the triethylenediamino-rhodium salts. 

The acid sulphate, [Rh(NH3)5N02]S04.TI2S04, crystallises in long white needles. These decompose on heating, leaving a residue of rhodium. The nitrito-salts, like all the rhodium salts, resemble those of cobalt and chromium, but are more stable, the nitrate being particularly stable. This fact Jorgensen attributes partly to the nature of the central atom.1... 

Ammonia unites readily with iridium salts, giving rise to complex ammino-derivatives. The first compounds described appear to be ammines analogous to those of palladium and platinum, to which they were compared by Berzelius 8 and Skoblikoff.4 A further series were described by Claus 5 wliich he represented like those of ammino-rhodium salts, as they bore a marked resemblance to these. After Jorgensen had established the constitution of the ammines of rhodium, cobalt, and chromium salts, Palmaer gave similar constitution to the iridium compounds. 

Ammino-derivatives of Ruthenium Halides—Derivatives of Ruthenium Salts containing a Nitroso-group—Ammino-derivatives of Rhodium Salts— Ammino-derivatives of Palladium Salts. 

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 dimerization of 19 also occurs in the presence of rhodium salts with stable crystalline rhodium complexes of 19 that can be isolated.26 it is interesting to note that the thermal dimerization of 19 gives only a 25% yield of the cyclodimer 20.27 The metal-catalyzed dimerization of cyclohepta-l,2,3-triene in the presence of tetrakis(triphenylphosphane)nickel(0) gives tricyclic 4-radialene 21.28 The structure of this cydobutane was confirmed by X-ray diffraction. 

We thank the National Research Council of Canada for financial support, including a scholarship (R.H.M.), and Johnson Matthey Ltd. for loan of the ruthenium and rhodium salts. 

The mechanism for the reaction is believed to be as shown in Eq. 15.170 (start with CH3OH, lower right, and end with CHjCOOH, lower left).180 The reaction can be initiated with any rhodium salt, e.g., RhCl3, and a source of iodine, the two combining with CO to produce the active catalyst, IRItfCO y. The methyl iodide arises from the reaction of methanol and hydrogen iodide. Note that the catalytic loop involves oxidative addition, insertion, and reductive elimination, with a net production of acetic acid from the insertion of carbon monoxide into methanol. The rhodium shuttles between the +1 and +3 oxidation states. The cataylst is so efficient that the reaction will proceed at atmospheric pressure, although in practice the system is... 

The reaction of an alkere with carbon monoxide and hydrogen, catalyzed by cobalt or rhodium salts, to form an aldehyde is called hxdmfonnylaiion (or sometimes the oxo process) ... 

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. 

The cyclopropanation of alkenes, alkynes, and aromatic compounds by carbenoids generated in the metal-catalyzed decomposition of diazo ketones has found widespread use as a method for carbon-carbon bond construction for many years, and intramolecular applications of these reactions have provided a useful cyclization strategy. Historically, copper metal, cuprous chloride, cupric sulfate, and other copper salts were used most commonly as catalysts for such reactions however, the superior catalytic activity of rhodium(ll) acetate dimer has recently become well-established.3 This commercially available rhodium salt exhibits high catalytic activity for the decomposition of diazo ketones even at very low catalyst substrate ratios (< 1%) and is less capricious than the old copper catalysts. We recommend the use of rhodium(ll) acetate dimer in preference to copper catalysts in all diazo ketone decomposition reactions. The present synthesis describes a typical cyclization procedure. 

Catalyst Description. The LPO catalyst is a triphenylphosphine modified carbonyl complex of rhodium. Triphenylphosphine, carbon monoxide, and hydrogen form labile bonds with rhodium. Exotic catalyst synthesis and complicated catalyst handling steps are avoided since the desired rhodium complex forms under reaction conditions. Early work showed that a variety of rhodium compounds might be charged initially to produce the catalyst. Final selection was made on the basis of high yield of the catalyst precursor from a commodity rhodium salt, low toxicity, and good stability to air, heat, light, and shock. 

As regards catalysts not containing preformed metal carbon bonds such as rhodium salts active in butadiene polymerisation in aqueous emulsions or in protic solvents as well as other catalysts of this type (used in non-polar hydrocarbon media), there is a theoretical rather than practical interest paid to such catalysts. However, a few of them have activity and stereospecificity comparable with Ziegler Natta catalysts [27-35],... 

Rhodium salts such as Rh(N03)3.2H20 and RhCl3.3H20 exhibit catalytic activity for polymerisation of butadiene in protic solvents or aqueous emulsions and yield trans- 1,4-polybutadiene (>99%) [27,28,150-154]. Propagation active species are proved to be formed in such systems by the insertion of... 

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