Rhodium complexes base hydrolysis

The rate law, rate constants, and activation parameters have been established for the base hydrolysis of the fm 5 -[Rh(en)2(OH)I]+ cation. It appears from these results that the intrinsic kinetic trans effect of a hydroxo-ligand is very similar to that of chloride in this type of rhodium(iii) complex.Base hydrolysis of c/5 -[Rh(bipy)2Cl2]+ or of its phen analogue is normally very slow, but is considerably catalysed by the presence of reducing agents such as ethanol or hydrazine. " This behaviour is reminiscent of substitution at trfl -[Rh(py)4Cla]+. " ... 

An interesting asymmetric transformation is the asymmetric conjugate addition to a-acetamidoacryhc ester 30 giving phenylalanine derivative 31, which has been reported by Reetz (Scheme 3.10) [10]. The addition of phenylboronic acid 2m in the presence of a rhodium complex of l,T-binaphthol-based diphosphinite ligand 32 gave a quantitative yield of 31 with up to 11% enantiomeric excess. In this asymmetric reaction the stereochemical outcome is determined at the hydrolysis step of an oxa-7r-aUylrhodium intermediate, not at the insertion step (compare Scheme 3.7). 

The base hydrolysis of [Co(NHg)5(02CCF3)] + and the analogous complexes of rhodium, iridium, and chromium(IIT) appears to involve the concerted attack of two hydroxide ions—one bonding to the acyl carbon atom of the trifluoroacetato group, and the other deprotonating the first 58, 129). 

Complexes of trifluoromethanesulfonate anion with cobalt(III) are labile oward substitution under mild conditions, and they have proved to be useful synthetic precursors to a variety of aminecobalt(III) complexes. The pentaammine-(trifluoromethanesulfonato-O)rhodium(III) ion, which is readily prepared from [Rh(NH3)5Cl]Cl2 in hot CF3SO3H, is also versatile as a synthetic precursor. " Its synthesis and solvolysis to give essentially quantitative yields of the penta-ammineaqua- and hexaamminerhodium(III) ions are described below. The aqua complex has previously been prepared by the base hydrolysis or Ag -induced aquation of [Rh(NH3)5Cl]Cl2 in water, but the present method presents a cleaner and more rapid alternative. The methods for preparation of the [RhCNHj) ] ion have evolved from the procedure of J0rgensen. They involve prolonged reaction of [Rh(NH3)5Cl]Cl2 with ammonia in a pressure vessel at elevated temperature. The solvolysis of [Rh(NH3)5(0S02CF3)](CF3S0j)2 in liquid ammonia is a simple, high-yield, and rapid alternative. 

Two-catalyst procedures have also been developed, which use both acid and base catalysis. For example, Schumann et al. (77) used a two-stage procedure to speed up entrapment of air-sensitive rhodium complexes. The procedure involved acid hydrolysis to form an initial sol, which was doped with metal complex, and base was added to induce condensation. The effect of this two-step procedure on gel structure was not examined The rhodium complex was effectively trapped within the silicate matrix, however, and was an active catalyst for hydrogenation of styrene and nitrobenzene. 

The decomposition products are L Rh(H20)2 and NO3 throughout the pH range 1-11. Except for a small (20%) amount of NO3 produced initially from L RhO " and NO2 in an outer-sphere process (Scheme 10), the rate of NO3 formation is identical, within error, to the rate of disappearance of the rhodium complex. This reaction is base-catalyzed, typical for hydrolysis of inorganic Rh(HI) complexes. Finally, the complex is unreactive toward reductants such as Fe q and ABTS , and thus almost certainly not a peroxynitrito complex, which would be expected to behave as an oxidant. On the basis of these arguments, the absorbance decrease at 240 nm in Pig. 14 is assigned as hydrolysis of L (H20)Rh(N0s). The kinetic treatment yielded the acidity constant = 1.2 x 10 and a rate constant = 1.7 x 10 s for the hydrolysis of the conjugate base, L (H0)Rh(N03)+. 

Hydrolysis of [Rh(NH3)5(N02)] and of cis-[Rh(NH3)4(N02)2] results in replacement of ammonia, but hydrolysis of trans-[Rh(NH3)4(N02)2] results in replacement of a nitro ligand. This pattern parallels that established earlier for the analogous cobalt(III) complexes, and derives from the dominance of the trans effect of NO2. Activation parameters for the cobalt(III) and rhodium(III) complexes are compared. It is concluded that ammonia loss occurs by an Id mechanism at both metals, but that loss of nitrite has more associative character in the case of rhodium(III). " Evidence from aquation, base hydrolysis, and ligand replacement studies gives an indication of chemically nonequivalent coordination sites in [Rh(tren)Cl2]. ... 

Cuprous chloride tends to form water-soluble complexes with lower olefins and acts as an IPTC catalyst, e.g., in the two-phase hydrolysis of alkyl chlorides to alcohols with sodium carboxylate solution [10,151] and in the Prins reactions between 1-alkenes and aqueous formaldehyde in the presence of HCl to form 1,3-glycols [10]. Similarly, water-soluble rhodium-based catalysts (4-diphenylphosphinobenzoic acid and tri-Cs-io-alkylmethylam-monium chlorides) were used as IPTC catalysts for the hydroformylation of hexene, dodecene, and hexadecene to produce aldehydes for the fine chemicals market [152]. Palladium diphenyl(potassium sulfonatobenzyl)phosphine and its oxide complexes catalyzed the IPTC dehalogenation reactions of allyl and benzyl halides [153]. Allylic substrates such as cinnamyl ethyl carbonate and nucleophiles such as ethyl acetoactate and acetyl acetone catalyzed by a water-soluble bis(dibenzylideneacetone)palladium or palladium complex of sulfonated triphenylphosphine gave regio- and stereo-specific alkylation products in quantitative yields [154]. Ito et al. used a self-assembled nanocage as an IPTC catalyst for the Wacker oxidation of styrene catalyzed by (en)Pd(N03) [155]. 

A more detailed understanding of the conjugate-base mechanism for the hydrolysis of kinetically inert amine complexes of cobalt(III) and rhodium(III) is possible if liquid ammonia is used as a solvent. Such techniques allow the separation of parameters for the pre-equilibrium step ( Tcb) and the rate-determining step. A polyamine complex generally contains more than one potentially acidic proton and each proton is characterized by its own acidity constant (Kcb)- exchange rates for... 

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