Rhodium dimerization

The long Pt-Pt bond (2.694 A) follows the trend observed in rhodium dimers as the number of bridging ligands decreases (Figure 3.98). 

However, the significant key difference for rhodium arises from the chemistry of the Rh(ll) dimer, [Rh(Por)]2, which exhibits a relatively low Rh—Rh bond strength. It undergoes homolytic dissociation and exists in equilibrium with the monomer, Rh(Por)- (Eq, (15)). The rhodium dimer can also exist in equilibrium with the hydride Rh(Por)H (Eq. (16)), and thus the hydride complex can exhibit the chemistry of the dimer, driven by formation of the Rh(Por)- monomer formed as in Eqs. (15) and (16). 

A simple method for the in-situ preparation of Wilkinson-type catalysts consists of the addition of the appropriate amount of the triarylphosphine to the rhodium dimers, [Rh(/<-Cl)(diene) 2 or Rh(//-Cl)(cyclooctene)2]2, according to Eqs. (4) and (5). The best results are usually obtained for a rhodium/phosphine ratio of 1 2. 

The dichlororuthenium arene dimers are conveniently prepared by refluxing ethanolic ruthenium trichloride in the appropriate cyclohexadiene [19]. The di-chloro(pentamethylcyclopentadienyl) rhodium dimer is prepared by refluxing Dewar benzene and rhodium trichloride, whilst the dichloro(pentamethylcyclo-pentadienyl)iridium dimer is prepared by reaction of the cyclopentadiene with iridium trichloride [20]. Alternatively, the complexes can be purchased from most precious-metal suppliers. It should be noted that these ruthenium, rhodium and iridium arenes are all fine, dusty, solids and are potential respiratory sensitizers. Hence, the materials should be handled with great care, especially when weighing or charging operations are being carried out. Appropriate protective clothing and air extraction facilities should be used at all times. 

The rhodium dimer has two Lewis acidic sites and thus the catalyst could coordinate to two substrate molecules under saturation kinetics, which would make the Michaelis-Menten plots complicated. This does not happen and the second site becomes less acidic once the other site is occupied by the substrate. What does happen, though, is that other Lewis bases compete with the substrate, as might be expected. The ligand dissociation reaction may be part of the rate equation of the process. Coordination of one Lewis base reduces already the activity of the catalyst. The solvent of choice is often anhydrous dichloromethane. The polar group may also be part of one of the substrates and in this instance one cannot avoid inhibition. 

Alternatively, the rhodium dimer 30 may be cleaved by an amine nucleophile to give 34. Since amine-rhodium complexes are known to be stable, this interaction may sequester the catalyst from the productive catalytic cycle. Amine-rhodium complexes are also known to undergo a-oxidation to give hydridorhodium imine complexes 35, which may also be a source of catalyst poisoning. However, in the presence of protic and halide additives, the amine-rhodium complex 34 could react to give the dihalorhodate complex 36. This could occur by associative nucleophilic displacement of the amine by a halide anion. Dihalorhodate 36 could then reform the dimeric complex 30 by reaction with another rhodium monomer, or go on to react directly with another substrate molecule with loss of one of the halide ligands. It is important to note that the dihalorhodate 36 may become a new resting state for the catalyst under these conditions, in addition to or in place of the dimeric complex. 

Asymmetric C-H insertion using chiral rhodium catalysts has proven rather elusive (Scheme 17.30). Dimeric complexes derived from functionalized amino acids 90 and 91 efficiently promote oxidative cychzation of suifamate 88, but the resulting asymmetric induction is modest at best ( 50% ee with 90). Reactions conducted using Doyle s asymmetric carboxamide systems 92 and 93 give disappointing product yields ( 5-10%) and negligible enantiomeric excesses. In general, the electron-rich carboxamide rhodium dimers are poor catalysts for C-H amination. Low turnover numbers with these systems are ascribed to catalyst oxidation under the reaction conditions. 

Several lines of inquiry have been explored to address key mechanistic issues in the rhodium-catalyzed C-H insertion of carbamates and sulfamates (Scheme 17.32) [99]. A pathway involving initial condensation between substrate 96 and PhI(OAc)2 to form iminoiodinane 97 was envisioned in the original design of this chemistry. Coordination of 97 to an axial site on the rhodium dimer would promote nitrene formation and the ensuing C-H insertion event Surprisingly, control experiments with PhI(OAc)2 and sulfamate 96 (or analogous carbamates) give no indication for a reaction between these two components. 

Rh(0H)3H20 (161, 236) and hexachlororhodate(III) have also been reported (19). The carboxylates are initially isolated as the solvent adduct, but heating under vacuum is adequate to prepare the anhydrous product. A variety of donors can occupy the terminal positions of the copper acetate type structure found in these rhodium dimers. No simple explanation of the bond order in these compounds seems adequate to describe the metal-metal attractive force in view of the number of electrons available and the prominent role of axial ligands in the total bonding scheme (200). 

In a logical continuation of this work, carbene addition to an iron-iron double bond has also been exploited for the simple synthesis of the first /. -methylene complex in the nitrosyl series. The readily available /x-nitrosyliron complex [(Tj5-C5H5)Fe(/u.-NO)]2 (26) exhibits the same structural features as the rhodium dimer 21 (157) and reacts with diazomethane in the temperature range -80-25°C to give the expected /z-methylene derivative 27 (Scheme 14) as a black, air-stable compound in... 

An almost unique variation on the -peroxo coordination mode is exhibited by a rhodium dimer, [RhCl(02)(PPh3)2]2,60 which is composed of two identical subunits which have the dioxygen moiety coordinated in the -peroxo mode. These subunits are linked, not by a chlorine bridge as in other rhodium complexes such as [RhCl(CO)2]2, but via the coordinated dioxygen group as shown in (3). 

Rhodium(i)-catalyzed ene-allene carbocyclization strategy is suggested for the formation of seven-membered heterocycles, azepines and oxepines. In particular, treatment of an allenyl allyl ether with a catalytic quantity of chlorodi(carbonyl)rhodium dimer affords 4-alkylidene-5-alkyl-2,3,4,5-tetrahydrooxepines (Equation 28) in 40-55% yields <20040L2161>. 

Limonene is used as a starting material for growth regulators of tobacco plants. Hydroaminomethylation is able to reduce the number of reactions steps for their production to one. The reaction takes place with yields up to 93% after 20 h with a rhodium dimer as a catalyst (Scheme 20). 

The analysis of the DOS profile fully confirms this hypothesis. In Figure 7 contributions of metal s and d shells to the total density of states for the host Pd2, Rh2 and PdO clusters are given. The contribution of a 5s shell to the occupied part of the spectrum of the palladium dimer is much smaller than for other systems. In the case of 4d shells (Figure 7b) it is the rhodium dimer molecule where the large part of the d spectrum lies above the Fermi level while for the palladium dimer spectrum the main d contribution keeps well below the Fermi level. 

Two mechanisms have been proposed for the last step of the catalytic cycle, reaction 6 (Scheme 6.1), a direct reaction of complex 7 with H2 and a reaction of the hydride complex 1 and the acyl complex 6 or 7 to give aldehyde plus a rhodium dimer ... 

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