Rhodium complexes monomeric

Rhodium complexes with chelating bis(oxazoline) ligands have been described to a lesser extent for the cyclopropanation of olefins. For example, Bergman, Tilley et al. [32] have prepared a family of bis(oxazoline) complexes of coordinatively unsaturated monomeric rhodium(II) (see 20 in Scheme 13). Interestingly, the use of complex 20 in the cyclopropanation reaction of styrene afforded mainly the cis cyclopropane cis/trans = 63137), with 74% ee and not the thermodynamically favored trans isomer. No mechanistic suggestions are proposed by the authors to explain this unusual selectivity. 

The rhodium complexes cis- and irans-[Rh(eda)2(N02 )X]+, where X = N02, or ON , photoreact (182) with dioxygen in aqueous solution to form monomeric [Rh(eda)2(H20)(02)]2+ and dimeric complexes. These complexes act as one-electron oxidizing agents, oxidizing 1 to I2 and Fe11 to Fe111. 

The same pattern of dimer stabilization at the expense of monomeric or more highly polymerized forms is also evident in the behavior of these rhodium complexes toward oxidation. Treatment of the dimer l with iodine, bromine, or trifluoromethyl disulfide results in trans-annular, oxidative-addition to form the brown dimers, 3, via reaction (4)(3., 6.). As inspection of Figure 1 shows... 

A Pauson-Khand type reaction of enynes, where the CO source is an aldehyde, has been reported by Morimoto and coworkers. This CO-transfer carbonylation system was carried ont with monomeric or dimeric rhodium complexes supported by monodentate or chelating phosphine ligands (e g. [RhCl(cod)]2/dppe or dppp [RhCl(CO)PPh3]). This reaction is snccessfiil for a series of enynes and aldehydes (Scheme 28). 

The major part of the spin density is delocalized over the hydrocarbon framework of the trop ligand. Inspection of previously reported monomeric rhodium complexes with... 

Site-site interactions have been studied with rhodium complexes because of their importance as hydrogenation and hydroformylation catalysts. ds-Complexes were observed in IR spectra of macroporous resins f731. Equation 15 shows one example. Only a dimeric complex was observed in solution, tended X-ray absorption fine structure (EXAFS) spectra indicate that (polystyrylmethyl)diphenylphosphine forms two different complexes with RhBr(PPh3)3, depending on Ae degree of cross-linking (741. At high P/Rh ratios a dimeric structure was observed with a 2% cross-linked gel resin and 0.22 mmol of Rh/g, and a monomeric structure was observed with a macroporous 20% cross-linked resin and 0.14 mmol of Rh/g. 

Catalysis by imprinted surfaces has been extended to transition metal catalyst hydrolysis and hydrogenation [70-73]. For the catalytic hydrogenation of alkenes, the dimeric and monomeric rhodium complexes were attached to silica surfaces as shown in Fig. 23. 

Blnuclear Catalysts Stanley et al. have shown how a rhodium complex that is a poor catalyst in monomeric form becomes very active and selective when connected in a binuclear system with a methylene bridge as shown below. Linear to branched ratios as high as 27 to 1 can be achieved. A rhodium hydride is believed to attack a RhCOR group at the neighboring site in the product forming step. This shows how the proximity of two metals can provide useful chemical effects without their being permanently connected by a metal-metal bond. 

Once the rhodium complex was obtained, it was linked through the primary hydroxyl group to the polymeric support. The coupling was carried out by reacting the polymer with 3 equiv of the monomeric Rh complex in the presence of 4 equiv of DCC and 40 mol% of iV,A/ -dimethylaminopyridine (DMAP) at room temperature. 

The hydrophilic polymer-based rhodium complex depicted below was examined in four consecutive runs in the hydroformylation of 1-octene using an aqueous two-phase system (50 bar syngas, 100 C, 2 h) [56]. The n-regioselectivity was influenced by the polymeric backbone, leading to an lib ratio of 72 28 after the first run. After the fourth run, the catalytic system reached a TOF of 2360 h which is comparable to that found with a corresponding monomeric rhodium complex (2400 h , lib = 40 60). 

Particularly interesting is the unusually stable five-coordinate monomeric divalent rhodium complex, [Rh (H)(GO)(PPh3)3], produced by bulk oxidative electrolysis or chemical oxidation of Rh (H)(CO)(PPh3)3 in... 

Table 9. Summary of Rh Chemical Shift Data for Monomeric and Dimeric Rhodium Complexes"...

The rhodium(II) compound is a diamagnetic dimer with oxygen it forms a paramagnetic monomeric 02 adduct, probably a superoxide complex represented as (porph)Rh3+02. 

When irradiated in the presence of norbornadiene and high-pressure synthesis gas, rhodium chloride is converted to a catalyst which is active for a variety of reactions. /2A/. The salt is probably converted photochemically to the rhodium norbornadiene complex 9. This dimer may undergo a consecutive photoreaction to give the monomeric hydrido complex 10, which is the actual catalyst for polymerisation, hydrogenation, and hydroformylation reactions. 

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