Rhodium complexes binuclear species

These studies have Indicated that simple rhodium carbonyl complexes, e.g., mono- and binuclear species are Involved in the fragmentation and aggregation reactions of rhodium carbonyl clusters under high pressures of carbon monoxide and hydrogen. They indicate that it is possible to write formal equations for such reactions in the case of rhodium carbonyl anionic hydrido clusters (equation 25) and for the more particular situation when there are not hydrides present (equation 26)... 

The interaction of small, well defined, rhodium clusters, Rh and Rhs, with O2 has been investigated (220) by matrix infrared, and UV-visible, spectroscopy, coupled with metal/02 concentration studies, warm-up experiments, and isotopic oxygen studies. A number of binuclear O2 complexes were identified, with stoichiometries Rh2(02)n, n = 1-4. In addition, a trinuclear species Rhs(02)m, m = 2 or 6, was identified. The infrared data for these complexes, as well as for the mononuclear complexes Rh(02)x, = 1-2 (229), are summarized in Table XI. Metal-concentration plots that led to the determination of... 

Although the latter product is a solvated mononuclear [Rh(MeOH)2(diphos)]+ cation, in the solid state it is isolated as a binuclear complex of formula [Rh2 (diphos)2](BF4)2, in which each rhodium center is bonded to two phosphorus atoms of a chelating bis(diphenylphosphino)ethane ligand, and to a phenyl ring of the bis(diphenylphosphino)ethane ligand of the other rhodium atom. This dimer reverts to a mononuclear species on redissolving. The mechanism of hydrogenation of the prochiral alkene methyl(Z)-a-acetamidocinnamate, studied in detail by Halpern [31], is depicted in Scheme 1.7. 

Finally, copper(I), silver(I), and mercury(II) triazenides react with selected rhodium(I), iridium(l), and platinum(II) halide complexes to afford metal-metal bonded binuclear triazenide-bridged species 129). 

Few triazenide complexes reported to date are paramagnetic. Examples studied by ESR are restricted to the low-spin osmium(III) complexes OsX2(PhNNNPh)(PPh3)2 (3), the binuclear mixed-valence rhodium salts [ Rh(/z-ArNNNAr)(CO)(PPh3) 2][PFe] (43), and the low-spin d cobalt(II) species Co(ArNNNAr)(C5H5)L (178). 

A number of binuclear phosphine-ligand bridged complexes have been shown to function as catalysts or catalyst precursors. Here we review these cases briefly. More detailed coverage of hydrogenation and hydroformyla-tion using rhodium catalysts will appear in the next chapter. In all cases of catalytic activity shown by binuclear complexes, there is a serious question about the true identity of the catalytically active species. As with catalysis begun by metal carbonyl clusters, the possibility exists that a small amount of highly active mononuclear compound is the true catalyst. 

Kinetics of reactions of binuclear rhodium(II) complexes are perhaps rather fast for this chapter, but mention may be made of a kinetic determination of the rhodium-rhodium bond strength in an octaethylporphyrinato species and of kinetics of axial substitution in the tetraacetato complex [Rh2(OAc)4(OH2)2]. ... 

Quaterthiophene, a representative of the oligomers of thiophene [97JPC(A)4437] reacts with the labile 16-electron species [(triphos)RhH] to yield 252 (970M1517). The sulfur atom bonded to rhodium has expressed nucleophilic properties and attack by methyl iodide followed by NaBPlu gives the thioether 253. The latter reacts with [(triphos)RhH] to give the binuclear complex 254 that can be further methylated to give 255. 

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