Rhodium complexes, equilibrium concentrations

For the supported catalyst it is expected that the ligand does not leach since it is chemically bonded to the carrier. In contrast, the rhodium metal bound to the ligand is subject to leaching due to the reversible nature of the complex formation. The amount will depend on the equilibrium between rhodium dissolved in the organic phase and that bound to the ligand. When an equilibrium concentration of 10 ppb Rh is attained, the yearly loss of Rh for a 100 kton production plant will be about 1 kg Rh per year. Compared to the reactor contents of rhodium (see Table 3.9, 70 kg Rh) this would result in a loss of 1.5% of the inventory per year, which would be acceptable. 

The equilibrium concentrations of Rh2(CO)s and Rh4(CO)i2 were determined by infrared spectroscopy by monitoring the absorbance of the band at 1886.8 cm-1, which corresponds to the stretching of the bridging carbonyls of the tetrarhodium complex. More details of the experimental procedure can be found in the original papers. For our purpose, it is enough to say that the equilibrium concentrations of the rhodium complexes were quite low (< 10-3 mol dm-3), but the same was not true for the CO concentration ( 2 mol dm-3 see... 

In the low-temperature region, the P-31 spectra of all of the solutions exhibited the doublet arising from the triphenylphosphine coordinated with rhodium. This doublet has a chemical-shift value of + 39.8 ppm (relative to 85% H3P04) and a coupling constant,/P Rh, of 155 Hz. As such, it arises undoubtedly from triphenylphosphine bound to rhodium in a trigonal bipyramidal complex, (Ph3P)3Rh(CO)H. None of the spectra showed any indication of the trans-bisphosphine complex that is formed by dissociation of the tris-phosphine complex. The equilibrium concentration of (Ph3P)2Rh(CO)H was too low for detection by NMR under all of the experimental conditions used in the present studies. 

Modification of Metal-Metal Bonding in Rhodium Complexes by a Bridging Pi phosphine. The yellow, planar complexes, (RNC)i,Rh+, undergo novel self-association reactions in concentrated solution to form the blue or violet dimers, (RNC)8Rh22+, via reaction (1) (1,2). The equilibrium constants for this reaction are strongly... 

These catalytic reactions of dihydrosilanes make possible the use of asymmetric catalysts to produce chiral silicon compounds. Introduction of a chiral ligand L on the rhodium complex will not change the validity of the kinetic Scheme 12. However, in this case complexes 56 and 57 will be diastereomeric and their equilibrium concentrations will be different. The ratio of the substituted silanes will be close to k, [56] k2 [57]. 

In the hydroformylation of w-1-hexene the rhodium concentration was varied at a low P/Rh ratio of 20 1 to 40 1. By increasing the rhodium concentration from 50 to 400 ppm, the conversion rate rises from 33 to 44% under standard conditions. This relatively minor effect must be due to the fact that a high rhodium concentration implies a high concentration of Na-TPPTS, which has a negative effect on the solubility of 1-hexene in the aqueous phase (salt effect). On the other hand, the salt effect shifts the equilibrium of the rhodium complexes toward the phos-phine-rich complex 2. Hence, the n/iso ratio is improved substantially. 

Examples are listed in Table 8.7 for various numbers of bonds (x) between the double bonds. For the compounds with x = 6, the formation of the 7-membered ring is the preferred reaction. For x >6, the polymer is the favoured product. For x = 4 there is a remarkable variation in behaviour with the catalyst no reaction is observed with the molybdenum carbene catalyst, but with the rhodium complex there is 86% conversion of substrate in 72 h to products consisting of about 5% of cyclic dimer , 4% of cyclic trimer and 91% of linear oligomers (M = 1815). In the early stages of reaction the products are mainly the cyclic species but these undergo ROMP once their equilibrium concentration has been exceeded. With the ruthenium complex as initiator the kinetics of ROMP are less favourable and the products after 72 h consist of 25% cyclic dimer, 17% cyclic trimer and 58 % of linear oligomers (Marciniec 1995a). 

The effect of iodide and acetate on the activity and stability of rhodium catalysts for the conversion of methanol into acetic acid have been studied. Iodide salts at low water concentrations (<2 M) promote the carbonylation of methanol and stabilize the catalyst. Alkali metal iodides react with methylacetate to give methyl iodide and metal acetate the acetate may coordinate to Rh and act as an activator by forming soluble rhodium complexes and by preventing the precipitation of Rhl3. A water-gas shift process may help to increase the steady-state concentration of Rh(I). The labile phosphine oxide complex (57) is in equilibrium with the very active methanol carbonylation catalyst (58) see equation (56). 

Proving the individual steps of the catalytic cycle in rhodium-catalyzed hy-droformylation is much less elaborate than in the cobalt case. Therefore, the nature of complexes involved in the catalytic cycle has been deduced mostly from the kinetics of the aldehyde formation and from spectroscopy of the reaction solutions. In situ infrared and NMR spectroscopy revealed so far only the main resting states of the catalyst being the pentacoordinate hydride- and the pen-tacoordinate acylrhodium complexes. The concentration of the postulated active intermediates in equilibrium with the resting states is obviously too low for direct observation. The main support of the involvement of 16-electron complexes is the negative effect of carbon monoxide and phosphine concentration on the rate of aldehyde formation. 

Rhodium,3 osmium4 and ruthenium5 based catalyst systems are affected by nitrile in a similar way. This arises from the relatively high affinity of complexes of these metals towards nitrile group coordination.11 The resulting equilibrium between free catalyst and catalyst with bound nitrile reduces the effective catalyst concentration and hence reaction rate for a given set of conditions. 

The experiments were conducted in a down-flow tubular reactor with continuous feed and product withdrawal. For phosphine resins, establishment of equilibrium was exhibited by the fact that rhodium concentrations in solution were proportional to percent loading. The concentration was also dependent on solvent. As the solvent polarity increased, rhodium concentration increased. Typical concentrations in the effluent were 0.2-2.0 X 10-5 A/ Rh for reaction at 85°C, 1500 psi 1/1 H2/CO. An increase in CO pressure increased the concentration of rhodium in solution, and an increase in temperature sharply decreased the metal concentration. These are understood as factors that influence the equilibrium between phosphine and carbonyl complexes. 

The reaction is first order in rhodium catalyst concentration, first order in dihydrogen pressure and has an order of minus one in carbon monoxide pressure. In our Scheme 6.1 this would be in accord with a rate-determining step at the end of the reaction sequence, e.g. reaction 6. Since the reaction order in substrate is zero, the rhodium catalyst under the reaction conditions predominates as the alkyl or acyl species any appreciable amount of rhodium hydride occurring under fast pre-equilibria conditions would give rise to a positive dependence of the rate of product formation on the aUcene concentration. The minus one order in CO suggests that the acyl species rather than the alkyl species is dominant under the reaction conditions. The negative order in CO is explained [29,30] by equilibrium 7. The saturated complex loses CO, and subsequently the unsaturated 16-electron species reacts with H2 to give aldehyde and rhodium hydride (reaction 6). 

Some additional information on the nature of the active catalyst was furnished by the experiments at diflFerent rhodium concentrations (Table II, No. 2, 3, and 4). As can be seen, the reaction rate showed a small fractional order with respect to catalyst concentration within the range investigated. This suggests that most of the rhodium is present in the form of a metal cluster complex and the active catalyst is a mononuclear species present in small concentration and in equilibrium with... 

Mobil group (14) who examined olefin hydroformylation catalyzed by Rh complexes supported on P-CgH4P(n-Bu)9- The catalyst resins contained 2-3 meq/g phosphorus and 0.2 meq/g Rh. Catalytic reactions were carried out in a tubular down-flow reactor. They observed that the rhodium concentrations in solution over the phosphine resins were proportional to the percent loading of the metal, indicating an equilibrium was established. As solvent, substrate or product polarity increased the rhodium concentration in solution increased. A higher concentration of rhodium was also noted when there was a higher CO partial pressure and/or the temperature was decreased as shown in Table 1. 

Rhodium. In addition to the studies on the alkene hydrogenation catalyst [RhCl(PPhs)3] (see Vol. 3), some other very detailed studies on this system have appeared. It is now apparent that the complexes [RhClLJ [L = PPh, or P(p-tolyl)3] do not dissociate to a spectroscopically detectable extent. However, despite its low concentration a species of composition [RhClLJ must be postulated, as previously reported, in order to explain the kinetics of H2 reactions with solutions of [RhOLs]. The complexes [RhClLa], howev, are in equilibrium with the dimers, [RhClLJa. The dimer reacts with H2 to form (47) and is also readily cleaved with ethylene or L, but not cyclohexene, to form [RhCl(C2H4)L2] or [RhClLJ. ... 

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