Rhodium temperature dependence

Herrmann et al. reported for the first time in 1996 the use of chiral NHC complexes in asymmetric hydrosilylation [12]. An achiral version of this reaction with diaminocarbene rhodium complexes was previously reported by Lappert et al. in 1984 [40]. The Rh(I) complexes 53a-b were obtained in 71-79% yield by reaction of the free chiral carbene with 0.5 equiv of [Rh(cod)Cl]2 in THF (Scheme 30). The carbene was not isolated but generated in solution by deprotonation of the corresponding imidazolium salt by sodium hydride in liquid ammonia and THF at - 33 °C. The rhodium complexes 53 are stable in air both as a solid and in solution, and their thermal stability is also remarkable. The hydrosilylation of acetophenone in the presence of 1% mol of catalyst 53b gave almost quantitative conversions and optical inductions up to 32%. These complexes are active in hydrosilylation without an induction period even at low temperatures (- 34 °C). The optical induction is clearly temperature-dependent it decreases at higher temperatures. No significant solvent dependence could be observed. In spite of moderate ee values, this first report on asymmetric hydrosilylation demonstrated the advantage of such rhodium carbene complexes in terms of stability. No dissociation of the ligand was observed in the course of the reaction. 

Figure 10.7 shows the temperature dependence of CO oxidation rate on a rhodium surface, as reported by Bowker et al. It shows that the rate of reaction maximizes when both reactants, adsorbed CO and O, are present in comparable quantities at a temperature where the activation barrier of the reaction can be overcome. 

Attard GA, Price R, Alakl A. 1995. Electrochemical and ultra-high-vacuum characterization of rhodium on Pt(l 11)—A temperature-dependent growth mode. Surf Sci 335 52-62. 

In most cases the catalytically active metal complex moiety is attached to a polymer carrying tertiary phosphine units. Such phosphinated polymers can be prepared from well-known water soluble polymers such as poly(ethyleneimine), poly(acryhc acid) [90,91] or polyethers [92] (see also Chapter 2). The solubility of these catalysts is often pH-dependent [90,91,93] so they can be separated from the reaction mixture by proper manipulation of the pH. Some polymers, such as the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers, have inverse temperature dependent solubihty in water and retain this property after functionahzation with PPh2 and subsequent complexation with rhodium(I). The effect of temperature was demonstrated in the hydrogenation of aqueous allyl alcohol, which proceeded rapidly at 0 °C but stopped completely at 40 °C at which temperature the catalyst precipitated hydrogenation resumed by coohng the solution to 0 °C [92]. Such smart catalysts may have special value in regulating the rate of strongly exothermic catalytic reactions. 

This mechanism clearly implicated alkane complexes as precursors to C-H activation but the IR absorptions of [Cp Rh(CO)Kr] and [Cp Rh(CO)(C6Hi2)] were not resolved and were presumed to be coincident. The temperature dependent data gave values of AH = 18 (or 22) kj mol for the unimolecular C-H (or C-D) activation step representing a normal kinetic isotope effect, kn/fco 10- However, an inverse equilibrium isotope effect (K /Kq 0.1) was found for the slightly exothermic pre-equilibrium displacement of Kr by CoHn/C Dn implying that C6Dj2 binds more strongly to the rhodium center than does C Hn-... 

Figure 13 illustrates the temperature dependence of the spectra of the solution derived by reacting 1 mol of tris(triphenylphosphine)-rhodium(I) carbonyl hydride with 6 mol of dppp. The maintenance of the narrow line width of the intense doublet signal at 16.1 ppm shows that the bicyclic complex does not dissociate up to 90°C. On the other hand, broadening signals of the complex spectrum of the monocyclic complex and that of the singlet signal of free dppp indi-... 

The lifetime of the rhodium precatalyst depends on the rate at which the metal complex HRh(CO)(TPPTS)3 and the excess ligand TPPTS undergo decomposition. The catalyst lifetime is considerably increased by occasional addition of extra ligand. In general an increase in the reaction temperature and/or CO pressure results in a decrease in the catalyst lifetime. 

A study of the temperature dependence of the NMR spectrum of a solution of norbornadiene and [(C7Hg)RhCl]2 indicates the formation at low temperature of the five-coordinate rhodium complex (186) (596). The kinetics of norbornadiene exchange suggest that the slow reaction... 

With respect to succeeding membrane separation it was found that generally an increase of the molecular mass of the amines leads to improved retention of rhodium, of phosphorus ligand and, last but not least, of the amines. It can be demonstrated that an increase in molecular mass does have a contradictoryeffect on the overall efficiency. A high amount of permeate corresponds to a lower flux of permeate due to the higher concentration of compounds within the retentate (osmotic pressure). Traces of amine in the permeate are the result of a very low temperature-dependent dissociation of the ammonium salts into amine and free acid according to eq. (7). 

Fig. 5.9. Temperature dependence of the CO oxidation rate over rhodium surfaces (from Bowker et...

Keywords Rhodium, Ruthenium, Diphosphines, Enamides, Substrate chelation, Pressm e and temperature dependence. Dihydrides, Alkyl hydrides. Transient NMR... 

The inverse temperature dependence of the water-solubility of nonionic tenside phosphines at the Tp was applied by Bergbreiter et al. [41] in the hydrogenation of allyl alcohol using water-soluble rhodium catalysts modified with the smart ligand 15 in aqueous media. In this case, on heating the sample to 40-50°C the reaction stopped but on cooling to 0 °C hydrogenation was resumed in the aqueous phase (cf. Section 4.6.3). 

The thermoregulated phase-transfer function of nonionic phosphines has been proved by means of the aqueous-phase hydrogenation of sodium cinnamate in the presence of Rh/6 (N =32, R = n-CsHu) complex as the catalyst [16]. As outlined in Figure 2, an unusual inversely temperature-dependent catalytic behavior has been observed. Such an anti-Arrhenius kinetic behavior could only be attributed to the loss of catalytic activity of the rhodium complex when it precipitates from the aqueous phase on heating to its cloud point. Moreover, the reactivity of the catalyst could be restored since the phase separation process is reversible on cooling to a temperature lower than the cloud point. 

The poly(alkylene oxide)-bound phosphine ligands 1 and 2, as well as a cationic rhodium(I) complex of 1, were demonstrated to possess inverse temperature-dependent solubility. The effects of these solubility properties on catalysis have been demonstrated in the hydrogenation of allyl alcohol in water. An approximately 20-fold decrease in rate is observed when the temperature is raised from 0 °C to 40-50°C [9a], This unusual temperature dependence has been termed smart behavior . 

Over palladium and rhodium, the propane distributions are independent of temperature and highly unsymmetrical some 60% of the total propane is propane-ds in each case. The distributions can be expressed as the sum of two random distributions of H and D atoms from pools of differing composition. The proportions of the various deuteropropanes formed over platinum are temperature-dependent but can be similarly treated. 

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