Rhodium monovalent

The transition-metal catalyzed decomposition of thiirene dioxides has been also investigated primarily via kinetic studies103. Zerovalent platinum and palladium complexes and monovalent iridium and rhodium complexes were found to affect this process, whereas divalent platinum and palladium had no effect. The kinetic data suggested the mechanism in equation 7. 

The two catalyst components are rhodium and iodide. Under the circumstances CO and water reduce Rhl3 to monovalent rhodium. A large... 

Metallation of dioxacorrole with Rh(CO)2Cl 2 gives a mono-Rh complex in which the monovalent rhodium atom is coordinated by two corrole nitrogens (Scheme S4).244... 

Transition metal ions, within the zeolite framework, may undergo a reductive carbonylation to give mononuclear monovalent carbonyl coumpounds M(I)(CO) and ultimatly to give zerovalent polynuclear carbonyl clusters. The rhodium(I)and iridium(i)carbonyIs were identified using spectroscopic and volumetric methods, the zerovalent rhodium and Iridium clusters M (CO)j were also synthetized in the zeolite matrix and their structure investigated using IR, NMR and spin labelling methods. 

Equilibration with carbon monoxide at room temperature and low pressure (a few torr ) yielded the rhodium(I)-dicarbonyl compound (13) in addition to the Rh(I)(C0) paramagnetic complexe (11). The structure of this complex was elucidated by ESCA and UV measurements (13) which showed that the trivalent rhodium was indeed reduced to the monovalent state and by infrared spectroscopy which provided evidence for a gem dicarbonyl (14). Use of 1 1 C0 ... 

A fully hydrated zeolite was characterized by a clean doublet at 2090-2030 cm-. Evacuation of excess water restored the low frequency components of the doublet. Thus the presence of one or more water molecules as ligands of the monovalent rhodium significantly altered its back-donating potential towards the H of the CO molecule and modified the bond angle as well (table 2). 

As the VCO absorption bands due to the dicarbonyl decreased an IR band at 2340 cm due to CO. developed gradually together with a set of absorptions around 2100-2000 cm- due to new linear carbonyls and absorptions around 1800 cm- presumably due to bridged carbonyls (14). CO- appearance was interpreted as an indication of the further reduction of the monovalent rhodium either by CO or via the water gas shift reaction producing H- which is reported to occur on monoculear monovalent carbonyls (19, 20). As rhodium I was reduced to the zerovalent state, the observed VCO bands were ascribed to Rh (C0)j2 compound, in view of the excellent agreement between the observed frequencies and those reported for Rh (C0)j- in CI C or nujol or when adsorbed on fully dehydrated zeolite. 

Reaction pathways There is general agreement as to the nature of the catalyst precursor (26. 30). It is well admitted that even under CO low pressure Rh(III)-Y was reduced to the monovalent rhodium dicarbonyl attached to the zeolite framework via one or more oxide ions irrespective of the rhodium introduction procedure onto the zeolite. 

The following step could be the reductive elimination of the acetyl halide to react with methanol. The growth of IR bands in the 1710-1685 cm- domain might be interpreted as due to CH COI accumulation and possibly further reaction with substrates present in the medium. Nevertheless readdition of CO restored the monovalent rhodium dicarbonyl thus indicative that somehow CH COI was eliminated. 

To date, only one example of a monovalent metallocorrole has been reported. It was reported in 1976 by Grigg, et al. and involves a rhodium corrole, which was obtained in 36% yield as the result of reacting free-base diethyl-hexamethyl corrole 2.6 with Rh2(CO)4Cl2. Unlike the trivalent corrole complex 2.134, obtained earlier by the treatment of a dideoxybiladiene-ac with this same metal salt, the complex isolated in this instance analyzed as being the monovalent Rh(CO)2Corrole, 2.157 (Scheme 2.1.42). This complex was later prepared in 72% yield,although it was... 

Pracejus was fascinated by the idea of functionalizing cellulose as the cheapest chiral material and to use it in this form as a carrier for monovalent rhodium for asymmetric hydrogenation. Rh was shown by Wilkinson to be useful as a catalyti-cally active central metal in phosphane complexes. However, the catalytic activities of the new cellulose immobilized complexes in the hydrogenation of unsaturated amino acid precursors were low and the enantioselectivities did not exceed 35%... 

Rhodium carbonyl clusters in Y-type zeolite were generated from Rh(III)-Y and CO, and characterized by IR in the contribution of LeFebvre et al. [629], who identified monovalent rhodium dicarbonyl, Rh(I)(CO)2 as well as polynuclear carbonyls, Rh4(CO)i2 and the most stable RhsfCOlig. The latter exhibited a pronounced activity for and selectivity to formation of alcohols and C2-C5 hydrocarbons. 

Likewise, monovalent and trivalent rhodium were known as good dimerization catalysts for ethylene (37,38) and indeed rhodium exchanged zeolite Y appeared as an efficient and selective catalyst in ethylene dimerization under mild conditions (O-20°C, 200 Torr of ethylene). By contrast HY was inactive under similar temperature and pressure conditions and at 200°C polymerization and cracking were observed (36). Thus again the dimerization is not acid-catalyzed. The active species is however uncertain but most probably not metallic, perhaps trivalent or monovalent (36,39). 

However, whereas in solution, rhodium catalysts were shown to be the most active, in zeolites, iridium appeared to be an order of magnetitude more active than rhodium. Moreover, CH3CI could be used as the promoter while it could not be in solution. This is due, probably, to the fact that this molecule is strongly polarized by the electrostatic field and, possibly, to the modification of the redox properties of the monovalent precursor complex. This hypothesis was confirmed by the observation that the Si/Al ratio has a large influence on the activity of the rhodium complex (66). 

Mechanistically, the Monsanto process actually involves two interrelated catalytic cycles, one organometallic-based on rhodium ionic species, and an organic one where iodide should be considered as the true catalyst (Scheme 5). The two main components (rhodium and iodide) can be added in many forms. Under the reaction conditions Rhl3 is reduced by H2O and CO to monovalent rhodium active species 58, and methanol is converted to the iodo form, Mel. 

Rhodium can occur as a cation in the monovalent, trivalent and tetravalent states. Although the latter of these is likely to be unstable in water, two studies by Kalinina etal. (1977) and Kalinina and Lyakushina (1977) have reported stability constants for the oxo-ion, RhO " ". There is no supporting evidence for either of the species proposed or the form in which rhodium(lV) exists, and, as such, the data are not accepted. A solubility constant for rhodium(l) oxide is available (van Muylder and Pourbaix, 1974), although it is not clear from where this datum was originally sourced. The reported solubility is log /Csio = 3.31, but no confirmatory evidence has been presented for this solubility. Rhodium(III) is the dominant aqueous form of the element, and some stability constant data are available for the first monomeric hydrolysis species RhOH " ". Data are not available for any other species. Shaimon (1976) has indicated that the ionic radius of Rh is 0.665 A. 

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