Rhodium RhCl

In 2000 Xumu Zhang and co-workers reported a bisphosphine complex of rhodium, [RhCl(dppb)]2, which, when used with silver hexafluoroantimonate, in some cases offers a rate advantage over the Wilkinson s catalyst/silver triflate system [28], allowing the reaction to be conducted at room temperature (Tab. 13.5). Scott Gilbertson and coworkers reported a related example, employing [Rh(dppe)(CH2Cl2)2] SbFg as a catalyst for the [5+2] reaction (Tab. 13.5, entry 5) [29]. 

Braun reaction, 0-73). In a similar reaction, treatment of N-alkyl-substituted amides with chlorotris(triphenylphosphine)rhodium RhCl(PPh3)3 or certain other catalysts give nitriles and the corresponding alcohols.419... 

Method C. 100 mg (0.4 mmol) of bis[/z-chloro l,2,5,6-> 4-cyclooctadiene (l,5) rhodium], [RhCl(>74-C8H12)]23, is dissolved in 10 mL of methylene chloride and 0.68 g (1.2 mmol) of tppts in 10 mL of water is added. The two-phase system is stirred intensively for 30 min. Then the reaction mixture is transferred to a separation funnel. The aqueous phase is separated and the organic phase extracted twice, in each case with 5 mL of water. The combined aqueous phases are then washed twice with methylene chloride (2x5 mL). The water solution is transferred into a round-bottomed flask and the solvent is removed in a vacuum (10 2 torr). The raw product obtained is sufficiently pure for most reactions. Depending on the stoichiometry, small amounts of tppts or (>74-C8H12)Rh2(/r-Cl)2(tppts)2 are present, which can be removed by column chromatography on Sephadex G-15. Yield 740 mg (93%) red glass-like solid. 

Since this discovery, a whole range of transition metals have been exploited, with ruthenium (Ru3(CO)12), rhodium ([RhCl(CO)2]2), iridium ([Ir(COD)Cl]2), molybdenum (Mo(CO)6) and iron (Fe(CO)4(NMe3)) all having been successfully employed as catalysts for the reaction.28,29,36,63 65... 

The use of silver fluoroborate as a catalyst or reagent often depends on the precipitation of a silver haUde. Thus the silver ion abstracts a CU from a rhodium chloride complex, ((CgH )2As)2(CO)RhCl, yielding the cationic rhodium fluoroborate [30935-54-7] hydrogenation catalyst (99). The complexing tendency of olefins for AgBF has led to the development of chemisorption methods for ethylene separation (100,101). Copper(I) fluoroborate [14708-11-3] also forms complexes with olefins hydrocarbon separations are effected by similar means (102). 

The Wilkinson hydrogenation cycle shown in Figure 3 (16) was worked out in experiments that included isolation and identification of individual rhodium complexes, measurements of equiUbria of individual steps, deterrnination of rates of individual steps under conditions of stoichiometric reaction with certain reactants missing so that the catalytic cycle could not occur, and deterrnination of rates of the overall catalytic reaction. The cycle demonstrates some generally important points about catalysis the predominant species present in the reacting solution and the only ones that are easily observable by spectroscopic methods, eg, RhCl[P(CgH 2]3> 6 5)312 (olefin), and RhCl2[P(CgH )2]4, are outside the cycle, possibly in virtual equiUbrium with... 

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

Tbe discovery of the catalytic properties of [RhCl(PPh3)3] naturally brought about a widespread search for other rhodium phosphines with catalytic activity. One of those which was found, also in Wilkinson s laboratory, was trans-[Rh(CO)H(PPh3)3 which can conveniently be... 

Tertiary phosphine complexes [42] are the most important rhodium(I) compounds. RhCl(PPh3)3 ( Wilkinson s compound ), a hydrogenation catalyst, is the most important, but they exist in a range of stoichiometries. Synthesis follows several routes ... 

An example of a rhodium(I) complex with a tridentate phosphine is shown in Figure 2.16 it is formed by the usual route, reaction of the phosphine with [RhCl(cycloocta-1,5-diene)]2. 

Aldehydes, both aliphatic and aromatic, can be decarbonylated by heating with chlorotris(triphenylphosphine)rhodium or other catalysts such as palladium. The compound RhCl(Ph3P)3 is often called Wilkinson s catalyst.In an older reaction, aliphatic (but not aromatic) aldehydes are decarbonylated by heating with di-tert-peroxide or other peroxides, usually in a solution containing a hydrogen donor, such as a thiol. The reaction has also been initiated with light, and thermally (without an initiator) by heating at 500°C. 

Scheme 3 Synthesis of rhodium polysulfido complexes from [RhCl(cod)2]...

The preparations of a number of rhodium(I) complexes of isocyanides, some of them new, have been described. The newtetrakis(methyl isocyanide) complex, [Rh(CNCH3)4], was isolated as salts of various anions from reactions of RhClj -3H20 or [(l,S-CgH,2)RhCl]2 and this isocyanide ligand (11), and several [Rh(CNR)4]+ alkyl and aryl isocyanide complexes (R= Bu, Pr, /)-C6H4C1, /.-CSH4CH3, and P-C6H4OCH3) have... 

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