Rhodium complexes mononuclear

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 hydridocomplex under high pressure conditions, has been proven. Even the equilibria with the precursor cluster Rh2(CO)8 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]. 

From both sources, the rhodium(II) mononuclear complex has a variety of options open to it. In the absence of other reagents, for example, it simply dimerizes and forms the metal-metal bonded species. However, it is quite a reactive species, and, among other things, it abstracts halogen atoms, for example, from organic halides [Scheme III]. 

A moderate pressure (>5 atm.) of CO in the reaction system leads to the selective formation of 29, while alkynes undergo rhodium-catalyzed hydrosilylation with a hydrosilane to afford vinylsilanes in the absence of CO. The presence of the rhodium complex is crucial for the smooth progression of siiyiformyiation, regardless of the presence of mononuclear or polynuclear complexes. This generalization is supported by the studies of many others [15]. The most important feature of this reaction is the excellent regioselectivity, which favors the formylation of the internal sp-carbon of the acetylenic bond of terminal... 

A. Synthesis of Mononuclear Cyclopentadienyl and Indenyl Rhodium Complexes of OFCOT... 

D. Structure and Solution Dynamics of a Mononuclear Indenyl Rhodium Complex of OFCOT... 

The five-coordinate acyl complex may not be mononuclear. Using a rhodium complex in presence of a diamine, alkenes were converted to alcohols according to reaction (84),... 

The structure of 73 was substantiated spectroscopically (5 nB = 82.5 ppm) and confirmed crystallographically. A centrosymmetric chloro-bridged dimeric structure with tricoordinate copper centers was observed. The pendant character of the borane is apparent from the rather long the B- Cu (3.05 A) and B- Cl (4.06 A) distances associated with a trigonal planar geometry around boron. Compared to related mononuclear palladium and rhodium complexes (see Section IV.B), the presence of the chloro-bridge disfavors the formation of Cl-B interactions (coordination mode F) in 73. 

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... 

Collman et al. prepared halopropyl rhodium complexes by the reaction of dihalopropanes with a macrocyclic rhodium(I) complex [Eq. (29)] (180). The ratio of mononuclear to binuclear product was found to be solvent dependent, with the variations in product ratios in different solvents being related to the solubility of the Rh(I) macrocycle. 

Other examples are now recognized where a mononuclear compound that is catalytically active under homogenous conditions at one temperature can also act as a source of a highly active nanoparticulate metal catalyst at higher temperature. This was recently demonstrated for a rhodium complex in hydrogenation reactions [C. M. Hagen, J. A. Widegren, P. M. Maitlis and R. G. Finke, J. Am. Chem. Soc., 2005, 127, 4423]. 

The reaction of M3(CO)12 with both open-chain and cyclic poly-alkenes has attracted some attention, especially in the case of Ru3(CO)i2. In most of the examples reported, the organic fragment bonds to the metal framework in such a way as to interact with more than one of the three metal atoms (68-77). There are some exceptions to this general statement, however. One is the reaction of Ru3(CO)j 2 with cyclopentadiene, in which a mononuclear complex is obtained (78). In other cases, tetranuclear and hexanuclear compounds are obtained (79 81). Cluster breakdown has also been observed in the case of a rhodium complex upon reaction with ethylene (55) as shown in Fig. 3. 

Additional evidence that reaction paths analogous to the reactions of mononuclear and dinuclear rhodium complexes exist on palladium is shown by the effect of triphenylphosphine on the stereoselectivity of the hydrogenation of di-f-butylacetylene (DTBA). In the absence of PPhs but on a highly dispersed... 

In the gas phase reaction of Rh with c-CaH, among other processes, elimination of C2H4 has been observed and the resulting metal ion was shown to have the structure of a methylidene-rhodium complex (238) instead of a hydrido-methylidene species (239) . RhCHa (238) reacts readily with H2 and CH4 and represents the first example of methane activation by a cationic mononuclear transition-metal complex in the gas phase. Reactions of both Rh = CH2 and its analogues Fe-CH2 and C0-CH2 with cyclic hydrocarbons were studied, and it is assumed that in the initial step metallacycloalkanes are generated (Scheme 35). 

In support of this suggestion, when [Rh2(CO)2( r-SP AN-POP) (p-Cl)2] was used as a catalyst precursor, the catalytic reaction rate was four times higher than when a 1 1 (molar) diphosphine Rh ratio was used. In a subsequent computational investigation, the oxidative addition reactions of Mel with di-rhodium complexes, [Rh(CO)(PR3)( r-Cl)]2 (R = H, Me) and that with mononuclear [Rh(CO)(PH3)2Cl] and [Rh(CO)2I2] were compared on the basis of DFT calculations [96]. Calculated activation parameters for nucleophilic attack by rhodium on Mel showed good agreement with experimental results. 

While mononuclear rhodium complexes give a-branched aldehydes 6, the hydroformylalions of a- and /3-pinene in the presence of Rh6(CO)1(, both give (/J)-10-formylpinane (7) as the major product (d.r. up to 83 16)18. Under the reaction conditions an equilibration of a- to /J-pinene is established (a//J 8 1). 

The X-ray structure of 12 (Fig. 5) shows that the anion has a trimetallic core of rhodium atoms, doubly capped by two para-tolylimido ligands. Meanwhile, the cation is the known mononuclear rhodium complex [Rh(CO)(dppm)2]+ contaminated with another known cationic complex [Rh(02)(dppm)2]+. The metal-metal separations within the anion, in the range 2.840(2)-2.898(2) A, can be considered as suitable for metal-metal bonds. 

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