Rh and Ir

The chemistry of pincer complexes of the group 9 has been dominated by the study of the derivatives with rhodium and iridium. To date, the reactivity of cobalt pincer compounds with carbon monoxide has not been reported, mainly due to the fact that the number of pincer complexes derived from cobalt is reduced [17]. Thus, the interest in the Rh and Ir PCP pincer complexes is mainly owing to the fact that they have been proved efficient in the catalytic dehydrogenation of aliphatic C—H bonds this is particularly true in the case of the iridium PCP pincer complexes [3j. 

Quinone methides (QMs) have been postulated as intriguing transients in various biochemical transformations and are of general importance in modern organic chemistry [21]. However, most of these compounds have only been observed in dilute solutions thus, in an effort to explore the chemistry of this sort of compounds, in 1998, Milstein and coworkers reported [22] the synthesis and reactivity of PCP-type complexes containing QM moiety (46). This compound was shown to be remarkably stable toward the reactions with nucleophiles and electrophiles. And to probe the effect of the electron density in the rhodium atom on the stability of the methylene 

PhjSnCo (PMe ) 2 with anthracene yields Ph2SnCo(ri -anthracene) (PMe ) 2  

The fragment HCoL, generated by laser flash photolysis of HCoL. (L = diethylphenylphosphonite) exhibits a bimolecular rate 

1 1 adducts with a variety of olefins adduct stediility is 

Substitution of (allyl)Fe(CO),N0 with optically active phosphine (L ) yields separable (allyl)Pe(CO)(NO)L dlastereo-Isomers which are configurationally stable at the Fe atom up to 

Very basic phosphines undergo addition to the allyl 2 57 

Allylic halides undergo oxidative addition to CpRu(C0) X to 

---

Propane, 1-propanol, and heavy ends (the last are made by aldol condensation) are minor by-products of the hydroformylation step. A number of transition-metal carbonyls (qv), eg, Co, Fe, Ni, Rh, and Ir, have been used to cataly2e the oxo reaction, but cobalt and rhodium are the only economically practical choices. In the United States, Texas Eastman, Union Carbide, and Hoechst Celanese make 1-propanol by oxo technology (11). Texas Eastman, which had used conventional cobalt oxo technology with an HCo(CO)4 catalyst, switched to a phosphine-modified Rh catalyst ia 1989 (11) (see Oxo process). In Europe, 1-propanol is made by Hoechst AG and BASE AG (12). 

For maintenanee of optimum values pH at definition of Rh and Ir used the aeetie buffer mixes, prepared with addition of isopropanole. Colour of eomplexes develops within 2 h at 65 5°C or during 5 min under influenee of mierowave radiation. 

The effect of the CFSE is expected to be even more marked in the case of the heavier elements because for them the crystal field splittings are much greater. As a result the +3 state is the most important one for both Rh and Ir and [M(H20)6] are the only simple aquo ions formed by these elements. With rr-acceptor ligands the +1 oxidation state is also well known for Rh and Ir. It is noticeable, however, that the similarity of these two heavier elements is less than is the case earlier in the transition series and, although rhodium resembles iridium more than cobalt, nevertheless there are significant differences. One example is provided by the +4 oxidation state which occurs to an appreciable extent in iridium but not in rhodium. (The ease with which Ir, Ir sometimes occurs... 

The pentafluorides of Rh and Ir may be prepared by the deliberate thermal dissociation of the hexafluorides. They also are highly reactive and are respectively dark-red and yellow solids, with the same tetrameric structure as [Rup5]4 and [Osp5]4 (p. 1083). 

Ternary hydrides of Rh and Ir containing the octahedral anions have been... 

Like Rh and Ir, all three members of this triad have the fee structure predicted by band theory calculations for elements with nearly filled d shells. Also in this region of the periodic table, densities and mps are decreasing with increase in Z across the table thus, although by comparison... 

Thiophene, benzo[d]thiophene and dibenzo[d,organosulfur compounds formed at Rh- and Ir-mediated thiophene ring opening 97SL643. 

A series of Rh and Ir complexes is also basic enough to add boron halides, although the metal-boron bonds are weak, e.g., dppe and PPh3-Rh complexes serve as electron-pair bases when they react with BF3 and BCI3 . Table 1 summarizes the results. 

Many late transition metals such as Pd, Pt, Ru, Rh, and Ir can be used as catalysts for steam reforming, but nickel-based catalysts are, economically, the most feasible. More reactive metals such as iron and cobalt are in principle active but they oxidize easily under process conditions. Ruthenium, rhodium and other noble metals are more active than nickel, but are less attractive due to their costs. A typical catalyst consists of relatively large Ni particles dispersed on an AI2O3 or an AlMg04 spinel. The active metal area is relatively low, of the order of only a few m g . ... 

The surface-catalyst composition data for the silica-supported Ru-Rh cuid Ru-Ir catalyst are shown in Figure 1. A similcir plot for the series of silica-supported Pt-Ru bimetallic catalysts taken from ref. P) is included for comparison purposes. Enthalpies of sublimation for Pt, Ru, Rh and Ir are 552, 627, 543, and 648 KJ/mole. Differences in enthalpies of sublimation (a<75 KJ/mole) between Pt and Ru cind between Rh and Ru are virtually identical, with Pt euid Rh having the lower enthalpies of sublimation. For this reason surface enrichment in Pt for the case of the Pt-Ru/Si02 bimetallic clusters cannot be attributed solely to the lower heat of sublimation of Pt. Other possibilities must also be considered. 

Chiral monodentate carbene complexes of Rh and Ir of the type [MCl(l,5-COD) (NHC)] (M = Rh, Ir) with the ligands 7-9 (Fig. 2.1) have been stndied as catalysts for the enantioselective hydrogenation of methyl-2-acetamido acrylate. Even though the activities were high, the enantiomeric excesses (ee) were poor [7, 8]. 

This regioselectivity is opposite to the one observed by the non-catalysed additions of BH3 THF or 9-BBN to the same alkene, or those catalysed by Rh and Ir catalysts. Chiral NHC ligands (generated from 84) on Cu under the same conditions proceed with high enantioselectivity (enantiomeric ratio 99 1) [71] (Scheme 2.12). 

The Rh and Ir complexes 85-88 (Fig. 2.14) have been tested for the intramolecular hydroamination/cyclisation of 4-pentyn-l-amine to 2-methyl-1-pyrroline (n = 1). The reactions were carried out at 60°C (1-1.5 mol%) in THF or CDCI3 The analogous rhodium systems were more active. Furthermore, the activity of 87 is higher than 85 under the same conditions, which was attributed to the hemilabihty of the P donor in the former complex, or to differences in the trans-eSects of the phosphine and NHC ligands, which may increase the lability of the coordinated CO in the pre-catalyst [75,76]. 

Carbonyls containing four metal atoms in the cluster are formed by Co, Rh, and Ir in which the metal atoms are arranged in a tetrahedron with 12 CO ligands, nine in terminal positions and three that are bridging. These structures are shown in Figure 21.4. 

The reason for preferential axial site occupancy by mono-dentate ligands is probably related to results of recent CNDO calculations on [Co CO) ] (15) which show that the axial carbonyls are least involved in back-bonding. Extrapolation to Rh - and Ir -derivatives seems reasonable and finds some support from n.m.r. measurements, which show that the n.m.r. chemical shift of the axial carbonyl is always at higher field than the radial carbonyl in both rhodium (16) and iridium derivatives. (12)... 

---