Dimers rhodium carboxylate

The most common oxidatiou states and corresponding electronic configurations of rhodium are +1 which is usually square planar although some five coordinate complexes are known, and +3 (t7 ) which is usually octahedral. Dimeric rhodium carboxylates are +2 (t/) complexes. Compounds iu oxidatiou states —1 to +6 (t5 ) exist. Significant iudustrial appHcatious iuclude rhodium-catalyzed carbouylatiou of methanol to acetic acid and acetic anhydride, and hydroformylation of propene to -butyraldehyde. Enantioselective catalytic reduction has also been demonstrated. 

Figure 2.38 The effect of varying the relative energies of the metal and ligand orbitals upon the final molecular orbital scheme for a dimeric rhodium carboxylate. (Reprinted from Coord. Chem. Rev., 50, 109, 1983, with kind permission from Elsevier Science S.A., P.O. Box 564,...

No report involving incorporation of rhodium into a quadruple bond has been published. For the sake of completeness the dimeric rhodium carboxylates are noted here. The Rh2(02CCH3)4 dimer can be conveniently synthesized from rhodium(III) chloride and sodium acetate (215), although routes from... 

Figure 2.35 The lantern structure adopted by dimeric rhodium(II) carboxylates.

As corroborated by deuterium labeling studies, the catalytic mechanism likely involves oxidative dimerization of acetylene to form a rhodacyclopen-tadiene [113] followed by carbonyl insertion [114,115]. Protonolytic cleavage of the resulting oxarhodacycloheptadiene by the Bronsted acid co-catalyst gives rise to a vinyl rhodium carboxylate, which upon hydrogenolysis through a six-centered transition structure and subsequent C - H reductive elimina-... 

Electronic Effects. A graphic demonstration of how purely-electronic effects control the mode of coordination of the sulfoxide ligand is demonstrated in DMSO adducts of rhodium car-boxylates, [Rh2(02CR)i (DMS0)2l (11,12). The dimeric structure of the rhodium carboxylates with bridging carboxylate groups has been confirmed (3) and these readily add donor ligands L in the axial... 

The dimeric rhodium(II) compounds become enantioselective catalysts if they contain optically active ligands. For this the anions of optically active carboxylic acids seem to be most appropriate. The complex in which the Rhz unit is clamped by four mande-late anions was synthesized and structurally characterized some time ago. [7] As a catalyst, however, this complex results in only small enantiomeric excesses. [8] The reason for this is probably that the asymmetric centers lie in a plane between the two Rh atoms and are thus too far away from the coordination sites directed to the outside, at which the catalysis occurs. Chiral substituents at the nitrogen atoms of carboxamide anions would be considerably closer to these reaction centers, and... 

Endres and Maas prepared dimeric rhodium(II) carboxylate complexes 24a-24c from the sodium salts 25a-25c and rhodium(III) chloride hydrate in ethanol [Eq. (7)] [29, 30]. 

Figure 5.2 Proposed intermediates from rhodium carboxylate dimer and iminoiodinane.

An interesting series of complexes with antitumour activity is that of the rhodium carboxylates, whose structure is a dimer containing bridging carboxylates ... 

Similar to the intramolecular insertion into an unactivated C—H bond, the intermolecular version of this reaction meets with greatly improved yields when rhodium carbenes are involved. For the insertion of an alkoxycarbonylcarbene fragment into C—H bonds of acyclic alkanes and cycloalkanes, rhodium(II) perfluorocarb-oxylates 286), rhodium(II) pivalate or some other carboxylates 287,288 and rhodium-(III) porphyrins 287 > proved to be well suited (Tables 19 and 20). In the era of copper catalysts, this reaction type ranked as a quite uncommon process 14), mainly because the yields were low, even in the absence of other functional groups in the substrate which would be more susceptible to carbenoid attack. For example, CuS04(CuCl)-catalyzed decomposition of ethyl diazoacetate in a large excess of cyclohexane was reported to give 24% (15%) of C/H insertion, but 40% (61 %) of the two carbene dimers 289). 

The catalytic activity of rhodium diacetate compounds in the decomposition of diazo compounds was discovered by Teyssie in 1973 [12] for a reaction of ethyl diazoacetate with water, alcohols, and weak acids to give the carbene inserted alcohol, ether, or ester product. This was soon followed by cyclopropanation. Rhodium(II) acetates form stable dimeric complexes containing four bridging carboxylates and a rhodium-rhodium bond (Figure 17.8). 

Figure 17.8. Rhodium(II) carboxylate dimer, only one bridging carboxylate drawn...

Before turning to specific results we will have a look at the properties of rhodium(II) acetates/carboxamidates as catalysts for reactions with diazocompounds as the substrates via carbenoid intermediates. Rhodium(II) has a d7 electron configuration, forming the lantern type dimers with bridging carboxylates. The single electrons in the respective dz2 orbitals form an electron... 

Intermolecular amination experiments described by Muller using 02NC,5H4S02N=IPh (NsN=IPh) as the nitrene source underscore the value of certain rhodium(II) catalysts for C-H insertion (Scheme 17.5) [12, 34—36]. In accord with Breslow s finding, dirhodium carboxylates were demonstrated to catalyze the amination of allylic, benzylic, and adamantyl substrates. Notably, structurally related tetracarboxamide dimers fail to give... 

Rhodium(II) carboxylate dimers and their carboxamide counterparts have been demonstrated to be exceptionally useful catalysts for carbene transfer processes involving diazocarbonyl substrates [1]. Doyle s seminal work identified Rh2(OAc)4 as the catalyst of choice for a variety of cyclopropanation, C-H insertion, and ylide rearrangement transformations using diazoketones or diazoesters [2]. Important contributions by Taber [3], Padwa [4], and Davies [5] further established the superior catalytic activity of dirho-dium catalysts and the excellent selectivity of rhodium-[Pg.417]

Rhodium(II) forms a dimeric complex with a lantern structure composed of four bridging hgands and two axial binding sites. Traditionally rhodium catalysts faU into three main categories the carboxylates, the perfluorinated carboxylates, and the carboxamides. Of these, the two main bridging frameworks are the carboxylate 10 and carboxamide 11 structures. Despite the similarity in the bridging moiety, the reactivity of the perfluorinated carboxylates is demonstrably different from that of the alkyl or even aryl carboxylates. Sohd-phase crystal structures usually have the axial positions of the catalyst occupied by an electron donor, such as an alcohol, ether, amine, or sulfoxide. By far the most widely used rhodium] 11) catalyst is rhodium(II) acetate [Rh2(OAc)4], but almost every variety of rhodium] 11) catalyst is commercially available. 

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). 

Iridium complexes having oxygen ligands are not nearly as extensive as those having nitrogen. Examples include acetylacetonates [Ir(P(C(5H5)3)2 (acac)H2] [64625-61-2], aqua complexes Ir(OH2)6]3+ [61003-29-0], nitrato complexes [Ir(0N02)(NH3),J2 [42482 42-8], and peroxides IrCl(P(C6I fy)3)2(02-/-(>/ I I9)2(CO) [81624-11-5]. Unlike rhodium, very few Ir(II) carboxylate-bridged dimers have been claimed and [Ir,2(OOCCI I3)4 has not been reported. Some Ir(T) complexes exhibit reversible oxidative addition of 02 to form Ir(III) complexes. That chemistry has been reviewed (172). 

The dimeric tetraacetato bridged Rh2(OCOCH3)4 has been obtained by the interaction of ammonium chlororhodate(III) or rhodium (III) hydroxide with acetic acid.1-3 Other (car-boxylato)rhodium(II) compounds were prepared directly in a similar way or from the acetate by exchange.2,3 Halo car-boxylates (RCOO, R = CC13, CF3, CH2C1, etc.) were prepared also by interaction of rhodium trichloride with the appropriate sodium salt in ethanol.4 The carboxylatcs are normally first isolated as a solvent adduct, e.g., [Rh(OCOR)2-C2H5OH]2 but are easily converted to the unsolvated complex. The acetate is readily prepared in a modification of this last procedure. A similar method is satisfactory for the preparation of other lower carboxylates as well as halo carboxylates. 

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