Rh-R complex

Rhodium Porphyrins. Chemical syntheses of [CPDRh32 and (P)Rh(R) complexes are well known(4-11). Electrochemical techniques have also been used to synthesize dimeric metal-metal bonded [(TPP)RhJ 2 as well as monomeric metal-carbon a-bonded (TPP)Rh(R) and (0EP)Rh(R)(12-16). The electrosynthetic and chemical synthetic methods are both based on formation of a highly reactive monomeric rhodium(II) species, (P)Rh. This chemically or electrochemically generated monomer rapidly dimerizes in the absence of another reagent as shown in Equation 1. 

An electrosynthetic method was used to generate the 25 different (P)Rh(R) complexes listed in Table 1(14,16). Many of the complexes in this Table had not been previously reported, especially (P)Rh(RX). In all cases, bulk electrolysis of... 

Table I. The Redox Potentials and UV-vls Spectral Data for the Electrogenerated (TPP)Rh(R) Complexes in THE Containing 0.2 M TBAP a...

TPP)Rh(L)J+C1 in the presence of an alkyl halide leads to a given (P)Rh(R) or (P)Rh(RX) complex. The yield was nearly quantitative (>80X) in most cases based on the rhodium porphyrin starting species. However, it should be noted that excess alkyl halide was used in Equation 3 in order to suppress the competing dimerization reaction shown in Equation 1. The ultimate (P)Rh(R) products generated by electrosynthesis were also characterized by H l MR, which demonstrated the formation of only one porphyrin product(lA). No reaction is observed between (P)Rh and aryl halides but this is expected from chemical reactivity studles(10,15). Table I also presents electronic absorption spectra and the reduction and oxidation potentials of the electrogenerated (P)Rh(R) complexes. 

Scheme VIII. 10. Translormations of a Rh-R complex fomied from an alkane.

The radical nature of (P)Rh(R) formation from (P)Rh is evident in the reaction of CP)Rh with alkenes and alkynes. Reaction with HCaCR or H2C=CHR where R = n-CsH , n-C4H9, n-CsHu or n-CeHis results in (TPP)Rh(R) formation with loss of the C2 fragment . This unexpected product reflects the high stability of the (P)Rh(R) complex. 

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 . ... 

Scheme 10.2 Selection model of the Rh(l)-complex-catalyzed asymmetric hydrogenation to the (R)-amino acid derivative (according...

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