Rhodium square-planar complexes

The SHAPES force field" has been implemented in CHARMM and used to examine the structures of several square planar rhodium complexes. This force field is based on angular overlap considerations and treats angular distortions for a variety of geometries. Spherical internal coordinates and Fourier potential functions form the basis for the description of these molecular shapes. The parameters for this force field were derived from normal coordinate analysis, ab initio calculations, and structure-based optimizations. The average rms deviation for bond lengths was 0.026 A, and the average rms deviation for bond angles was 3.2°. 

Figure 8.6 Transition state for the oxidative addition of benzaldehyde to the square-planar rhodium complex In the rhodium-catalyzed decarbonylatlon, calculated at the...

The most interesting work on the isocyanide complexes of the elements in this subgroup has been done with rhodium and iridium. For the most part, the work is involved with the oxidative addition reactions of d square-planar metal complexes. 

Remarkably, Claver et al. showed that in a square planar rhodium carbonyl chloride complex, two bulky phosphite ligands (65) were able to coordinate in a trans orientation.214 Diphosphite ligands having a high selectivity for linear aldehyde were introduced by Bryant and co-workers. Typical examples are (67)-(70).215,216... 

The mechanistic basis of iridium-complex-catalyzed enantioselective hydrogenation is less secure than in the rhodium case. It is well known that square-planar iridium complexes exhibit a stronger affinity for dihydrogen than their rhodium counterparts. In earlier studies, Crabtree et al. investigated the addition of H2 to their complex and observed two stereoisomeric intermediate dihydrides in the hydrogenation of the coordinated cycloocta-1,5-diene. The observations were in contrast to the course of H2 addition to Ms-phosphine iridium complexes [69]. 

Haegele et al. (269) have used exact isotope masses and isotope abundances together in determining the detailed fragmentation patterns of square planar rhodium (I) -diketonate complexes. They found that some species postulated by other workers were in error. High resolution is needed to distinguish the 28 mass units for loss of CO (27.9949) from C2H4 (28.0313) (269) or the 69 mass units for PF2 (68.9906) from CFa (68.9952) (90). 

Table 7.3 Binding constants for square planar rhodium-phosphine complexes in 1.

The synthesis of square planar rhodium (I) complexes, fran5-RhCl(RN=C=NR)(PCy3)2. where R=p-tolyl, is also reported. ... 

Note Abstraction of the bromide substituent creates a cationic, four coordinate and square planar rhodium(I) complex. 

Figure 5,27 Axial chirality in a square planar rhodium(l) carbene complex caused by an...

Using suitable reaction conditions, all four substituents on the square planar rhodium(I) starting material [Rh(cod)OEt]2 could be replaced by a l,3,7,9-tetramethylxanthine-8-ylidene ligand yielding a cationic tetrakisxanthine-8-ylidene rhodium(I) complex [102] (see Figure 6.44). 

A,A-dialkyldithiocarbamates produce stable, square-planar rhodium(II) complexes of the type [Rh(S2CNR2)2] (R = Me, Et) similar anionic hfr(maleonitriledithiolate) are also known. Neutral complexes are formed by the thiol containing amino acids cysteine, methyl cysteine, or penicillamine. It is noteworthy that these paramagnetic [Rh(aminoacid)2] complexes can easily be prepared from dimeric, diamagnetic [Rh(02CMe)2MeOH]2. ... 

To effectively model the asymmetric hydrogenation reaction, we must look at the mechanism carefully. The first step involves the displacement of solvent and the coordination of the enamide to produce the two diastereomers (Fig. 3) (17-20). It appears as though the enamide-coordinated diastereomers are in rapid equilibrium with each other through the solvento species (Fig. 4). This square planar rhodium(I) cation is then attacked by dihydrogen to form an octahedral rhodium(III) complex (Fig. 4). Hydrogen then inserts into the Rh-C bonds, and the product is reductively eliminated (Fig. 4). From a molecular mechanics standpoint we have three entities to model the square planar rhodium(I) solvento species and the two intermediates (square pyramidal dihydrogen complex and the octahedral dihydride). 

In order to model the square planar rhodium(I) complex we need to realize that the positions trans to the diphosphine may not be equivalent, since the diphosphine is chiral. Consider the [(diphosphine)Rh(norbornadiene)] as a model for the solvento species. In order to distinguish between the nonequivalent phosphorus atoms, we label them and P ,. Each olefin is 90° from one phosphorus atom... 

Figure 4 Attack of dihydrogen on the two diastereomers of [(S,S-CHIRA-PHOS)Rh(MAC)]+. Notice that the two diastereomers of [(S,S-CHIRAPHOS) Rh(MAC)]+ are in equilibrium via the solvento species. After hydrogen attacks the square planar rhodium(l) complex, an octahedral rhodium(lll) dihydride is formed. (Redrawn from Ref. 32.)...

Typical square-planar rhodium-olefin complexes such as acetylacetonates (48) have a stoichiometry of two coordinated olefins per metal-atom. Since chelating olefins are bidentate in their cationic rhodium biphosphine complexes, it would be surprising if bis-olefin complexes were never found under hydrogenation conditions. It seems clear, in fact, that they can be the major coordinated species under certain conditions. Thus examples of 2 1 rhodium enamide complexes with biz-diphenyl-phosphinopropane have been observed (49), although the majority of cases involve a8-unsaturated acids co-complexed with DIOP. 

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