Rhodium complexes with alkali metals

This is in contrast with the rhodium system where alkali metal salts were reported to have no effect on methanol carbonylation (19). In spite of the promotion effect of lithium the nickel catalyst is not maintained in a soluble stable complex form. Precipitation of nickel iodide is common when one of the alkali metals is the only catalyst promoter. 

Alkyl chlorides are with a few exceptions not reduced by mild catalytic hydrogenation over platinum [502], rhodium [40] and nickel [63], even in the presence of alkali. Metal hydrides and complex hydrides are used more successfully various lithium aluminum hydrides [506, 507], lithium copper hydrides [501], sodium borohydride [504, 505], and especially different tin hydrides (stannanes) [503,508,509,510] are the reagents of choice for selective replacement of halogen in the presence of other functional groups. In some cases the reduction is stereoselective. Both cis- and rrunj-9-chlorodecaIin, on reductions with triphenylstannane or dibutylstannane, gave predominantly trani-decalin [509]. 

The solvents used in these rhodium-catalyzed reactions may also act as complexing agents for counterions of the anionic rhodium complexes. For example, tetraglyme is known to coordinate alkali metal cations. Such solvation decreases the possibility of the cation interacting with the anionic rhodium catalyst and lowering its activity or solubility. The crown ethers, such as [18]-crown-6... 

Binary rhodium(lV) compounds are confined to the purple red tetrafluoride and the black dioxide. The hydrated dioxide may be prepared by oxidizing rhodium(IIl) compounds, either with chlorine or electrochemically. Attempts to dehydrate this material lead to decomposition. No cationic rhodium(IV) complexes have been characterized unambiguously, but both [RhCle] -and [RhFe] are well established. The alkali metal salts of the hexafluororhodate(lV) ion are all isomorphous with their platinate(fV) analogs. 

Favorable thermodynamics (AG° (298K) <0) for obtaining acetylene diolates from M-M bonded complexes occurs when 2(M-0)>(M-M)+ 147 kcal while single metal units require that the M-0 bond energy exceed 78 kcal (Table I, entries u,v). This limiting type of CO coupling is best known for reactions of alkali metals with CO which form solid ionic acetylene diolate compounds.Rhodium macrocycle complexes have Rh-0 bond energies 50-60 kcal and thus are excluded as potential candidates for acetylene diolate formation. 

Rhodium(l) complexes HRh(CO)L3 of alkali metal phenylphosphinoalkyl-sulfonates (L = 6a, n = 3,4 6c) have been used as catalysts for hydroformylation of higher olefins (e.g., n-l-tetradecene) in mefhanoHc solution. The catalyst could be recovered with loss of activity by extraction of the isolated product with water. 

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). 

Stille has used the alkali-metal phosphide route to prepare polymer-attached optically-active diphosphine ligands. Either a monomer (e.g., 17) was copolymerized and treated with an excess of PPh2 or the monomer-diphosphine unit (e.g., 18) was synthesized before polymerization.In both cases, the polymeric materials were treated with a rhodium complex... 

Polynuclear anionic metal carbonyl compounds are usually prepared by reduction reactions of metal carbonyls M(CO) with such reducing agents as the alkali metals, NaBH4 in ethers, hydrocarbons, liquid ammonia, and similar solvents [see, for example, reactions (2.54), (2.55), (2.84), (2.89), (2.95>-(2.102), and (2.108)-(2.113)]. In alkali medium the metal carbonyls may be reduced by certain solvents (e.g., alcohols) or by the CO ligand itself, and in the presence of Lewis bases the carbonyls disproportionate to give anionic clusters. The mixed metal clusters containing platinum and rhodium are formed by reduction reactions of chloro complexes... 

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

The second development was the use of promoter salts (e.g., alkali, phospho-nium or ammonium salts) to stabilize the activated complex in the catalyst system [41b] and the use of co-catalysts with rhodium, such as base catalyst metals Ti, Zr,... 

Rhodium and iridium are unreactive metals they react with O2 or the halogens only at high temperatures (see below) and neither is attacked by aqua regia. The metals dissolve in fused alkalis. For Rh and Ir, the range of oxidation states Table 19.3) and the stabilities of the highest ones are less than for Ru and Os. The most important states are Rh(III) and Ir(III), i.e. which is invariably low-spin, giving diamagnetic and kineticaUy inert complexes (see Section 25.2). 

The metal is inert to aqua regia but Rh(OH)3 can be prepared by fusing rhodium with sodium bisulfate followed by water and alkali. Rhodium(O) complexes are derived from RhCb by direct reaction with CO to form Rh2(CO)g, and the clusters Rh4(CO)i2 and Rh6(CO)ie. 

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