Reduction of Rh

Electroreduction of the cationic Rh(IlI) complex [Rh(Por)(MeNFl2)2l in CH2CI2 followed by reaction with alkyl halides has been utilized to form a-alkyl products. The reaction scheme proposed for this reaction was one-electron reduction of Rh(lll) to form Rh(Por)-. This can either dimerize or attack the carbon atom of the alkyl halide RCH2X, the latter step involving elimination of either X- or 2t7.2.ix reactions of Co(ll) and Fe(II) porphyrins M(Por) with... 

Radiolytic reduction has been investigated as a means of producing transient Rh(II) porphyrin products, and as in the above study, the observed products were strongly dependent on pH and solvent. Radiolytic reduction of Rh(TMP)Cl in alcohol formed transient Rh(TMP)- which was prevented from dimerization by the bulky TMP ligand. In alkaline 2-propanol the product is [Rh(TMP)r. in weakly acidic 2-propanol the hydride Rh(TMP)H is formed, and in strongly acidic 2-propanol the alkylated rhodium(III) porphyrins Rh(TMP)CH3 and Rh(TMP) (C(CH 3)20H) are observed. The alkyl products result from reaction of Rh(TMP)-with CH3- and C(CH3)20H formed by radiolysis of the 2-propanol solvent. 

More initial rate data for monoene substrates have appeared using as catalysts RhCl(CO)P2, HRh(CO)P3 (110, 158), and Rh(NO)P3 (110, 157), where P is a tertiary phosphorus donor. Borohydride reduction of Rh(NO)(PPh3)2Cl gives an active catalyst (159). 

Rh(bpy)2]+, obtained by the in-situ reduction of [Rh(bpy)2Cl2]Cl with hydrogen in methanolic sodium hydroxide [43], can reduce a series of simple ketones under 1 bar H2 and at 30 °C [44]. Yields of over 98% were obtained in all cases with a SCR of up to 680 1. When a mixture of ketones and aldehydes was placed under such conditions, the ketones were found to be reduced preferentially, although unsaturated ketones were generally reduced to saturated ketones. 

Reactions (32) to (38) were proposed for hydrogen production at pH 7 in the presence of excess bipy and involve the further reduction of [Rh(bipy)3]2+ to [Rh(bipy)2]+ and free bipy (reaction 36) TEOA- (produced after proton loss from TEOA formed in reaction 34) was proposed as the source of the electron for the reduction to Rh1. This Rh1 species is then protonated to a Rh111 hydride species (reaction 37), which reacts with H+ to form hydrogen (reaction 38). Recoordination of a bipy ligand regenerates [Rh(bipy)3]3+ and completes the cycle. Reaction (38) can occur in the absence of platinum, although the rate of hydrogen production increases 20 fold in its presence. 

The TPR patterns of the freshly prepared catalysts contain two peaks. The assignment of these peaks is possible if we take a reference spectrum of the fully oxidized catalysts (Fig. 2.4, right panel). Comparison suggests that the first peak in the TPR pattern of the fresh Rh/SiC>2 catalyst is associated with the reduction of Rh-O bonds and the second with Rh-Cl. 

Although the structure of the active catalyst obtained in solution was uncertain, the Monsanto group suggested at the time that Wilkinson-type Rh(I) complexes might be involved. They did not speculate on how the reduction of Rh(HI) to Rh(I) might be accomplished, but one possibility is... 

It should be mentioned that MX fixing on chelating polymers leads to the formation of rather stable complexes. As a rule, no dissociation of the chelate node proceeds by the reduction of transition metal. This can be demonstrated by two characteristic examples. The reduction of Rh(III), fixed On polyamides with Dipy groups, to Rh(I) by the action of H2 proceeds [1 lh]... 

The findings of Sen and Halpern (36, see Reaction 7) suggest alternative pathways for the Rh-catalyzed co-oxidation, in which the phosphine is oxidized by liberated peroxide and the ketone is formed during the reduction of Rh(III) to Rh(I) (a Wacker cycle). Read and Walker (87) ruled out Wacker chemistry, since addition of water in their system (OH is needed for the reduction, Reactions 7 and 8) decreased ketone yield somewhat. More convincing evidence for the absence of Wacker type chemistry comes from isotope studies using H2018, work just published by Tang et al. (89). 

Complexes of the type [Rh(TPP)(RX)] [RX = C H X (n = 3-5, X = Cl or Br n = 3-6, X = I) TPP = dianion of tetraphenylporphyrin] were prepared by Anderson et al. (179). The nature of RX was found to determine the overall electrochemical behavior for the reduction of [Rh(TTP)(RX)]. For some complexes, specifically those where X = Br and I, the bound alkyl halide could be reduced without cleavage of the metal-carbon bond. This resulted in the electrochemically initiated conversion of [Rh(TPP)(RX)] to a [Rh(TPP)(R)] complex. The E. value for this reduction was dependent on the chain length and halide of the RX group and followed the trend predicted for alkyl halides. The reduction of the bound RX occured at Ei values significantly less negative that those for reduction of free RX under the same solution conditions. 

The hydrido complex is formed as an intermediate in the electrochemical reduction of [Rh(di-phos)2]Cl,42 but is itself oxidized in the system (equation 75).215... 

Closely tied to the photochemistry of these complexes is their behavior in the presence of reducing agents. Reduction of [Rh(bipy)3]3+ with BH4 or Zn amalgam was reported to yield [Rh(bipy)2]+.4 1... 

The kinetics of the reduction of [Rh(bipy)2Cl2]+ in alkaline aqueous ethanol (under H2) revealed a two stage process an initial induction period, during which time Rh1 species formed, and a faster, autocatalytic region. The kinetics could be fit to the expression d[Rh ]/df = k[Rhm]2[Rh1]. The reaction rate was retarded by the addition of excess bipy, suggesting the suppression of a dissociation equilibrium, and the rate constant varies with [OH- ], but npt with [Cl- ]. A five-step mechanism for the autocatalytic process was proposed, involving the formation (via an unspecified mechanism) of [Rh(bipy)2]+, followed by the oxidative-addition of H2 to [Rh(bipy)2]+, and chloro-bridged dimeric and trimeric intermediates.823... 

The electrochemical reduction of [Rh(bipy)2Cl2]+ and [Rh(bipy)3]3+ in room temperature acetonitrile solutions has been studied by DeArmond and co-workers and is represented in Scheme 29. For both complexes, they claim that the initial one-electron reduction is followed by a moderately fast elimination of a ligand (Cl- or bipy) if the cyclic voltammetry scans are sufficiently rapid, this reduction is reversible.824 After ligand labilization, the two paths merge, and there is evidence for two additional one-electron reductions, leading to [Rh(bipy)2] and [Rh(bipy)2]-. 

At natural pH (around 5), reduction of [Rh(bipy)3]3+ with these radicals leads to H2, with an efficiency of about 25%. The dihydrogen precursor is presumed to be the colorless RHm-hydride for [Rh(bipy)2]+ is stable in an 02 free environment, and at pH 10 a suspension of [Rh(bipy)2]C104 is stable for at least three months, and does not produce any dihydrogen.825... 

The current status of our understanding in this area has been succinctly summarized by Hoffman, who commented that. . reduction of [Rh(bipy),]3+ in aqueous solution yields a very rich chemistry.. . Both [Rh(bipy)3]2+ and [Rh(bipy)2]+ are involved in highly complex interlocking ligand-labilization, acid-base, redox, and aggregation reactions .825... 

An interesting series of substitution/redox reactions involving both dimeric Rh11 and monomeric Rh111 complexes of OEP has been reported.408,409 In methanolic solution, H2 reacts with [Rh(OEP)Cl] to generate [Rh(OEP)H] and HC1 borohydride reduction of [Rh(OEP)Cl] leads to a species identified as [Rh (OEP)]-, which forms the same hydrido species when acidified (v H at 2220 cm, ... 

Reduction of [Rh(DMG)2Cl2]- in basic aqueous ethanol (to an unspecified Rh1 complex) is autocatalyzed, with a rate expression of the form rate = fc[RhIII][Rh1].914 The rate of reduction is not affected by added chloride ion, but varies as the inverse fourth ( ) power of [OH-]. A mechanism, involving deprotonation of the DMG ligands, and Cl--bridged RhIU—Cl—Rh1 intermediates, has been proposed.914... 

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