Hydroxo complexes rhodium

The cleavage of polynuclear hydroxo-bridged rhodium(III) and iridium(III) complexes into the corresponding mononuclear fragments has been reported in only a few instances, but the well-established tendency of mononuclear complexes of these metal ions to undergo substitution reactions with retention of configuration indicates the possibility of analytical and synthetic applications such as described above for chromium (III). 

Rhodium(III) tetra(/ -sulfonatophenyl)porphyrin [(TSPP)Rh] aquo and hydroxo complexes react with a series of alkenes in water to form /3-hydroxyalkyl coordination compounds. Addition reactions of (TSPP)Rh-OH to unactivated terminal alkenes CH2=CHR invariably occur with both kinetic and thermodynamic preferences to place rhodium on the terminal carbon to form (TSPP)Rh-CH2CH(OH)R complexes. Acrylic and styrenic alkenes initially react to place rhodium on the terminal carbon to form [Rh]-CH2CH(OH)R as the kinetically preferred isomer, but subsequently proceed to an equilibrium distribution of regioisomers where [Rh]-CH(CH2OH)R is the predominant thermodynamic product. Equilibrium constants for reactions of the diaquo rhodium(III) compound [(TSPP)Rhm(H20)2]3 in water with a series of terminal alkenes that form /9-hydroxyalkyl complexes were directly evaluated and used in deriving thermodynamic values for addition of the Rh-OH unit to alkenes. The AG° for reactions of the Rh-OH unit with alkenes in water was found to be approximately 3 kcalmol-1 less favourable than the comparable Rh-H reactions in water.100... 

Hydrothermal methods, for molecuarlar precursor transformation to materials, 12, 47 Hydrotris(3,5-diisopropylpyrazolyl)borate-containing acetylide, in iron complex, 6, 108 Hydrotris(3,5-dimethylpyrazolyl)borate groups, in rhodium Cp complexes, 7, 151 Hydrotris(pyrazolyl)borates in cobalt(II) complexes, 7, 16 for cobalt(II) complexes, 7, 16 in rhodium Cp complexes, 7, 151 Hydrovinylation, with transition metal catalysts, 10, 318 Hydroxides, info nickel complexes, 8, 59-60 Hydroxo complexes, with bis-Cp Ti(IV), 4, 586 Hydroxyalkenyl complexes, mononuclear Ru and Os compounds, 6, 404-405 a-Hydroxyalkylstannanes, preparation, 3, 822 y-Hydroxyalkynecarboxylate, isomerization, 10, 98 Hydroxyalkynes, in hexaruthenium carbido clusters, 6, 1015 a-Hydroxyallenes... 

Similarly, reference is made to another more recent review (41) of the author s contributions to the chemistry of rhodium, iridium, rhenium, osmium and ruthenium alkoxo, and hydroxo complexes, following a similar earlier publication (40) on the alkoxo derivatives of platinum metals. Reference may also be made to the first X-ray structural study (265, 266) of a siloxy derivative, [(cod)Rh(/i-OSiMe)3]2. 

Rhodium hydroxo complexes have been considered as active intermediates (see Figure 10.2). 

Kalinina, V.E. and Lyakushina, V.M. (1977) Catalytic properties of hydroxo-complexes of rhodium(IV) in the oxidation of cop-per(II) tellurate by hypobromite. Russ. 

Tye JW, Hartwig IF (2009) Computational studies of the relative rates for migratory insertions of alkcmes into square-planar, methyl, -amido, and -hydroxo complexes of rhodium. J Am Chem Soc 131(41) 14703-14712... 

Hayashi and co-workers established the catalytic cycle of the asymmetric conjugate addition in 2002 [16]. An example is outlined in Scheme 3.4 for the reaction of phenylboronic acid 2m with 2-cyclohexenone la. The reaction has three main intermediates hydroxo-rhodium (A), phenylrhodium (B), and oxa- j-allylrhodium (C) complexes. They are related in the catalytic cycle by (1) transmetallation of a phenyl group from boron to hydroxo-... 

Hayashi proved the validity of this catalytic cycle by the observation of aU three intermediates and their respective transformations using NMR experiments (Scheme 3.5) [16]. Transmetallation of a phenyl group from boron to rhodium takes place by addi-hon of phenylboronic acid 2 m to hydroxo-rhodium complex 16 in the presence of tri-phenylphosphine to generate the phenylrhodium complex 17. The reaction of 17 with 2-cyclohexenone la gives oxa- j-allylrhodium 18, which is converted immediately into hydroxo-rhodium complex 16 upon addition of water, liberating the phenylation product 3 am. In this NMR study, triphenylphosphine was used to stabilize the phenylrho-dium(I) complex. In the absence of triphenylphosphine, the characterization of the phenyl-rhodium species was unsuccessful. 

These NMR experiments provided great insight into the catalytic reaction. Whereas the catalytic reaction requires a high reachon temperature of 100 °C, aU three transformations in Scheme 3.5 proceed at 25 °C. In the same paper [16], Hayashi answered the question of why the catalytic reaction does not take place at the lower temperature. An outline of the reason is illustrated in Scheme 3.6. In the catalytic reaction, Rh(acac)(BINAP) is involved as a significant intermediate, because Rh(acac)(C2H4)2 is used as the rhodium precursor. It was confirmed that the hydroxo-rhodium complex is immediately converted into Rh(acac) (BINAP) by the reaction with 1 equiv. acetylacetone at 25 °C, in which the transmetallation from boron to rhodium is very slow at the same temperature. Thus, the acetylacetonato Hgand inhibits the catalytic reaction (Scheme 3.6, path a). 

HYDROXO-BRIDGED COMPLEXES OF CHROMIUM(III), COBALT(III), RHODIUM(III), AND IRIDIUM(III)... 

Some representative spectral data are given in Table VI. The spectroscopic properties of /x-hydroxo-/i-peroxo and -hydroxo/x-superoxo dicobalt(III) complexes have recently been reviewed by Fallab and Mitchell (119) and seem reasonably well rationalized in terms of the molecular orbital (MO) models formulated by Lever and Gray (120). The spectra of -hydroxo -/x-peroxo and /i-hydroxo-/i-superoxo complexes of rhodium(III) show features similar to those of the cobalt(III) species (121-124). 

There are several examples of well-characterized tri- and tetranu-clear hydroxo-bridged complexes of chromium(III) and cobalt(III). Penta- and hexanuclear aqua chromium(III) complexes have been prepared in solution, but their structure and properties are unknown. Oligomers of nuclearity higher than four have not been reported for cobalt(IIl), with the exception of some hetero-bridged heteronuclear species (193, 194). There appear to be no reports of rhodium(III) or iridium(III) complexes of nuclearity higher than two. 

The cis/trans isomerization reaction, Eq. (24), has been applied in the preparation of salts of the cis isomers of the chromium(III) complexes with L3 = (NH3)3 or tacn (319). For these species Eq. (24) equilibrium is shifted to the right, while the corresponding equilibria with the diaqua or dihydroxo species, respectively, are shifted to the left (Table X). The increased stability of the cis aqua hydroxo species can be explained in terms of intramolecular hydrogen bond formations (Section VI,C). As mentioned above, the corresponding cobalt(III) and rhodium(III) complexes have been isolated as salts only in the case of the trans-(H20)L3M(0H)2ML3(H20)4+ cations, but it seems very probable that their cis isomers could be prepared by reaction Eq. (24). 

The activation parameters for the ethylenediamine complexes of rhodium(III) and iridium(III) are also in keeping with an essentially dissociative mechanism. The observation that AHt(k-t) is larger than AHt(k 2) for iridium(III) has been rationalized in terms of stabilization of the aquahydroxo species by intramolecular hydrogen bond formation. Similarly, the observation for the rhodium(III) system that AHl(k ) < AHt(k-2) for ammonia, whereas A// (, ) AHt(k 2) for ethylenediamine may, in part, by rationalized in terms of the observed differences in the degree of intramolecular hydrogen bond stabilization of the aqua hydroxo species in the two systems [ZCH(en) > J h(NH3) see Table XXI]. 

Preliminary studies seem to indicate that acid hydrolysis of corresponding rhodium(III) complexes leads to peroxo bridge cleavage rather than hydroxo bridge cleavage (124). 

Structural, thermodynamic, and kinetic studies have shown that hydroxo-bridged polynuclear complexes of (diromium(III), cobalt(III), rhodium(III), and iridium(III) have many general features in common. Structurally, the four metal ions exhibit an almost identical pattern, and in particular the occurrence of many well-characterized oligomers... 

The decomposition of the peroxometallacycle (72a) or (72b) occurs in a way different from that previously shown to occur in the epoxidation of alkenes by molybdenum-peroxo complexes (equation 26). The three possibilities are shown in equations (58)-(60) and involve (a) a [C-/6, C-a] hydride shift which directly produces the methyl ketone and the rhodium-oxo complex, or the hydroxo species from (72b equation 58) (b) a [C-/3,0-/3] hydride shift which gives enol (equation... 

A kinetic study of the Rh-BINAP-catalysed 1,4-addition of phenylboronic acid using reaction calorimetry revealed that the catalytically inactive dimeric hydroxo- rhodium complex [Rh(OH) (/J)-BINAP ]2 (191) is the resting state (Scheme 6). A negative non-linear effect in eeprod and an amplified reaction rate were predicted and observed in the present reaction system that is characterized by the preferential formation of the homochiral (191) dimer.240... 

Hydroxo-Bridged Complexes of Chromium(III), Cobalt(III), Rhodium(III), and Iridium(III) Johan Springborg... 

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