Hydrido complexes rhodium

Steckhan et al. demonstrated convincingly that systems that fulfil these conditions are tris(2,2 -bipyridyl)rhodium complexes [57] and, more effectively, substituted or unsubstituted (2,2 -bipyridyl(pentamethylcyclopentadienyl)chloro or aquo rhodium complexes [58] (see Scheme 1). Electrochemical reduction of these complexes at potentials between —680 mV and —840 mV vs. SCE leads to the formation of hydrido rhodium complexes. Strong catalytic effects observed in cyclic voltammetry and preparative electrolysis indicate a very fast hydride transfer from the hydrido complex to NAD(P) under formation of only 1,4-NAD(P)H and the starting complex as shown in the following reaction scheme [58] ... 

Allyl rhodium complexes deposited on silica gel activate methane. The products of the reaction are allyl and hydrido rhodium complexes, as well as propylene and small amounts of butene and butane. 

The carbonylation of methane is catalyzed by RhCl(P(CH3)3)3 under irradiation to yield acetaldehyde (eq. (49)) (11). The utilization of dense CO2 as a stable and CH4-miscible reaction medium is the key to accomplish the reaction. The reaction presumably proceeds via the oxidative addition of the C—H bond to the rhodium center. The C—H oxidative addition product obtained from RhCl(P(CH3)3)3 and benzene was successfully isolated, and the structure was unambiguously determined by X-ray analysis (compound B, eq. (50) (76). The resulting complex is a (phenyl)(hydrido)rhodium complex, as expected and gave benzene and benzaldehyde upon treating with CO. 

Scheme 1.23 Formation of a phosphine-modified hydrido rhodium complex from RhCKCOHPPhjij.

Within four qrdes, the best catalyst almost completely lost its activity. The authors speculated about oxidation during recycling and decomposition of the hydrido rhodium complex in the presence of water. 

The interesting complex chemistry of rhodium has been rather neglected this is probably because most of the synthetic methods for obtaining complexes have been tedious. In general, substitutions of chlorine atoms bonded to rhodium by other ligands are slow, and products have usually been mixtures. The situation is now changing, since novel catalytic approaches to rhodium complexes have been developed.1 The catalysis in the present synthesis involves the rapid further reaction of the hydrido complex formed from l,2,6-trichIorotri(pyridine)rho-dium(III) in the presence of hypophosphite ion. 

An excess of ligand, including CO, will often inhibit isomerisation. HCo(CO)4, an unstable hydrido-carbonyl complex, belongs to the examples of catalysts also active in an atmosphere of CO. This is the only homogeneous catalyst being commercially applied, albeit primarily for its hydroformylation activity. Higher alkenes are available as their terminal isomers or as mixtures of internal isomers and the latter, the cheaper product, is mainly converted to aldehydes/alcohols by hydroformylation technology. Later we will see that the isomerisation reaction also plays a pivotal role in this system. Since 1990 several catalysts based on rhodium, platinum and palladium have been discovered that will also hydroformylate internal products to terminal aldehydes. 

Optimized reaction conditions call for the use of Wilkinson s catalyst in conjunction with the organocatalyst 2-amino-3-picoline (60) and a Br0nsted add. Jun and coworkers have demonstrated the effectiveness of this catalyst mixture for a number of reactions induding hydroacylation and C—H bond fundionalization [25]. Whereas, in most cases, the Lewis basic pyridyl nitrogen of the cocatalyst ads to dired the insertion of rhodium into a bond of interest, in this case the opposite is true - the pyridyl nitrogen direds the attack of cocatalyst onto an organorhodium spedes (Scheme 9.11). Hydroamination of the vinylidene complex 61 by 3-amino-2-picoline gives the chelated amino-carbene complex 62, which is in equilibrium with a-bound hydrido-rhodium tautomers 63 and 64. 

Choukroun, R.. Iraqi, A.. Gervais, D., Daran, f.C. and Jeannin, Y. (1987) A semibridging hydrido zirconium-rhodium complex A possible way to catalytic hydrogen transfer on do-de systems. Organometallics. 6. 1197-1201. 

For reductive cleavage of the acyl-Rh complex, it is generally accepted that molecular hydrogen is the hydrogen donor as shown in Scheme 7.1. However, this step could be effected alternatively by another hydrido-Rh complex, that is, RC(=0)MLn + LmMH — RC(=0)H + LmM-MLn. It is strongly indicated that this bimolecular reductive cleavage involving two rhodium species is operative under certain reaction conditions and catalyst systems [54-58]. 

During the course of our studies (30-32) of the synthesis and structures of rhodium acetylide and hydrido-acetylide complexes, we developed (32) a step-wise route to rrans-bis(acetylides) of the general form mer-rra/tf-[Rh(PMe3)3(H)(C=CR) (OCR )]. Unfortunately, scrambling processes have thus far precluded the preparation of the unsymmetrically substituted complexes (R R ) in the absence of... 

A similar pattern has always been discussed for rhodium, with hydri-dotetracarbonylrhodium H-Rh(CO)4 as a real catalyst species. The equilibria between Rh4(CO)i2 and the extremely unstable Rh2(CO)s were measured by high pressure IR and compared to the respective equilibria of cobalt [15,16]. But it was only recently that the missing link in rhodium-catalyzed hydroformylation, the formation of the mononuclear hydrido complex under high pressure conditions, has been proven. Even the equilibria with the precursor cluster Rh2(CO)s could be determined quantitatively by special techniques [17]. Recent reviews on active cobalt and rhodium complexes, also ligand-modified, and on methods for the necessary spectroscopic in situ methods are given in [18,19]. 

Rhodium(i).—Group VII Donors. Hydrido-phosphine complexes. The trifluorophos-phine complex [RhH(PF3)(PPh3)3], an analogue of the well-known homogeneous catalyst [RhH(CO)(PPh3)3], has been synthesized by the displacement process (27).117 Similarly, white crystalline [RhH(PF3)2(PPh3)2] was obtained using a two-molar... 

We have already seen in Section 2.2.2 that metal-alkyl compounds are prone to undergo /3-hydride elimination or, in short, /3-elimination reactions (see Fig. 2.5). In fact, hydride abstraction can occur from carbon atoms in other positions also, but elimination from the /8-carbon is more common. As seen earlier, insertion of an alkene into a metal-hydrogen bond and a /8-elimination reaction have a reversible relationship. This is obvious in Reaction 2.8. For certain metal complexes it has been possible to study this reversible equilibrium by NMR spectroscopy. A hydrido-ethylene complex of rhodium, as shown in Fig. 2.8, is an example. In metal-catalyzed alkene polymerization, termination of the polymer chain growth often follows the /8-hydride elimination pathway. This also is schematically shown in Fig. 2.8. 

Both in situ infrared and multinuclear NMR under less severe conditions have been used to gain mechanistic insights. For the hydroformylation of 3,3-dimethyl but-l-ene, the formation and hydrogenolysis of the acylrhodium species Rh(C()R)(C())4( R=CH2CH2Bur) can be clearly seen by IR. NMR spectroscopy has also been very useful in the characterization of species that are very similar to the proposed catalytic intermediates. We have already seen (Section 2.3.3, Fig. 2.7) NMR evidence for equilibrium between a rhodium alkyl and the corresponding hydrido-alkene complex. There are many other similar examples. Conversion of 5.3 to 5.4 is therefore well precedented. In the absence of dihydrogen allowing CO and alkene to react with 5.1, CO adducts of species like 5.6 can be seen by NMR. Structures 5.11 and 5.12 are two examples where the alkenes used are 1-octene and styrene, respectively. 

The scheme reduces to its most simple form when carbon monoxide is the only ligand present in the system, because equilibria of mixed ligand/carbon monoxide complexes do not occur. The kinetics of the hydroformylation reaction using hydrido rhodium carbonyl as the catalyst was studied by Marko [20]. For 1-pen-tene the rate expression found is ... 

The homogeneous catalyst has been prepared in alcoholic media and is a cation formed by loss of chloride. The procedure is described here for production of the neutral hydrido species HRh[( + )-diop]2, which is a slower catalyst than the in situ species for asymmetric hydrogenation but is equally effective in terms of optical yields. The method follows that of Levison and Robinson6 for synthesis of hydrido(triphenylphosphine)rhodium complexes. 

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