Rhodium complexes dinuclear activation

Several dinuclear rhodium complexes such as the above-mentioned [Rh2(OAc)4] have been used as hydrogenation catalysts [22, 23]. Maitlis and coworkers have studied the chemistry and catalytic activity of the [Rh(C5Me5)Cl2]2 complex and related complexes. Kinetic studies suggested that cleavage into monomer occurs in the most active catalysts [90]. 

A series of studies deals with the catalytic activity of the dinuclear thiolate-bridged rhodium complex [ Rh( i-S Bu)(CO)(TPPTS) 2] in the hydroformylation ofpropene, 1-hexene and 1-octene (Scheme 4.4) [76-80]. Turnover frequencies up to 3100 h" were detected. 

It is tempting to speculate that only for the dinuclear complexes of iridium is there initial formation of a dihydrido-complex, which subsequently reacts with an alkene to form an alkyl intermediate, or with an alkyne to form an alkenyl intermediate. If such is the case, the activity of dinuclear rhodium complexes must depend on initial formation of an alkyne or alkene complex, which would then react with hydrogen. There exists some evidence for such a scheme. The successive hydrogenation of alkynes and alkenes " suggests that activation of an alkene is inhibited by an alkyne, probably by preferential coordination of the latter. Further, complexes (VII, X = H) or (IX) do not alone react with hydrogen, but do so after reaction with an alkyne (acetylene or phenylactylene). ... 

Dinuclear rhodium complex with 36 was active in the addition reactions of carbenes (Scheme 4.22). Moreover, Ihe yield decreased for the stericaUy hindered olefins compared with rhodium acetate (II) due to steric factors. Furthermore, this complex preferably catalyzed the inclusion of carbene to the CH bond of aromatic fragment [82],... 

When the corresponding rhodium complexes are tested in methanol carbonylation, the activity obtained is ca. 2.5 times higher than the one obtained with the Monsanto system under the same conditions. The stability of the systems is evident by the ability to perform several runs without any noticeable loss in activity. Erom the residue of the reaction, when ligand 82b is employed, apart from the corresponding [RhI(CO)82b] complex, the dinuclear isomeric compounds 85a and 85b were isolated and characterized by X-ray diffraction (Scheme 10). 

Recently, Severin reported the Kharasch addition of several mixed rhodium(I)-ruthenium(II) and rhodium(III)-ruthenium(II) catalysts (see Part 3, Sect. 8.5). It was also demonstrated that 0.5 mol% of dinuclear rhodium(I) complex 434 having a bridging THDP ligand is an active catalyst for the Kharasch addition of BrCCl3 to simple olefins 433 in water [228]. The yields of adducts 435 ranged from 81 to 92%. 

Increasing the number of electrons reduces the activation of N2, because the electrons occupy the orbitals which are bonding with respect to the NN bond, and actually stabilize it. In agreement with this prediction dinitrogen is sufficiently activated to be reduced by protonation by dinuclear complexes of titanium(II), zirco-nium(Il), niobium(III), tantalum(III), molybdenum(IV), and tungsten(IV), whereas it is not reduced by protonation by certain d -d complexes, such as those of molybdenum(O), ruthenium(II), or rhodium(I). Apparently dinuclear complexes M-N=N-M in which M has the d electronic configuration can be intermediates in dinitrogen reduction in protic media, particularly if they represent part of polynuclear complexes (vide infra). 

A parallel can be found in the work of Stanley et al. who described very efficient and selective dinuclear rhodium(I) complex 13 for the hydroformylation of a-olefins (Scheme 13) [58-60]. This bimetallic catalyst in its racemic form is much faster and more selective toward linear products than its monometallic analogues. The enhanced activity is attributed to the formation tmder catalytic conditions of highly active Rh(II) dimer species 14 with a covalent Rh-Rh bond. 

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