Mononuclear rhodium species

Scheme 4.3 Equilibrium between dinuclear and mononuclear rhodium species...

In the case of BDPP with a bite angle of 90°, the high-pressure NMR and high-pressure IR studies showed the structures of the hydrido dicarbonyl diphosphine resting state as an axial-equatorial BPT. Similar behavior was observed for the furanoside diphosphines. Dinuclear rhodium species in equilibrium with the mononuclear pentacoordinate rhodium hydride carbonyl diphosphines have been found for these ligands. The position of this equilibrium depends on the hydrogen concentration and the ligands. The rate... 

This knowledge that the dominant rhodium species in solution are mononuclear can be combined with the results of a brief kinetic study 100) that yields an order in the total rhodium concentration of very nearly one. The conclusion is that the active species in this catalyst system are mononuclear,... 

The diruthenium(O) complexes [Ru2(CO) J,-(RO)2PN(Et)P(OR)2 2] (3, R = Me, Pr) (Scheme 2) have been reported to react with dihydrogen affording l,2-dihydrides.[13] Despite the formal similarity between these compounds and the previously described iridium and rhodium dppm derivatives, they display different coordination environments around the metal atoms. This could be related to the fact that, unlike the typical planar structure of most mononuclear d species, the unsaturated zerovalent ruthenium fragments adopt a structure which resembles a trigonal bipyramid with an empty equatorial site. [14]... 

As reported in the same paper, it is likely that this unstable rhodium cluster is converted into the mononuclear rhodium-hydride species HRh(CO)x (x = 3,4), which are usually considered as the true catalyst system in the reaction mixture. These compounds represent extremely unstable intermediates, which would certainly recombine to form higher nuclearity rhodium species if alkene is not present in the reaction mixture. This mechanism is proposed for all the hydroformylation experiments carried out in the presence ofRh4(CO)iz... 

Sometimes, also polynuclear clusters such as Rh4(CO)j2 or Rh6(CO)26 were submitted to the formation of rhodium catalysts [18]. Metallic rhodium embedded in inorganic materials (carbon, AI2O3) was tested for mini-plant manufacturing. In this context, the frequently phosphorus ligands [PPhj, P(OPh)3] were added with the intention to detach rhodium from the heterogeneous layer (activated rhodium catalyst = ARC) [19, 20] More recently, ligand (Xantphos, PPhj, BIPHEPHOS)-modified or unmodified rhodium(O) nanoparticles were used as catalyst precursors for solventless hydroformylation [21]. It is assumed that under the reaction conditions these metal nanoparticles decompose and merge into soluble mononuclear Rh species, which in turn catalyze the hydroformylation. 

The interaction of small, well defined, rhodium clusters, Rh and Rhs, with O2 has been investigated (220) by matrix infrared, and UV-visible, spectroscopy, coupled with metal/02 concentration studies, warm-up experiments, and isotopic oxygen studies. A number of binuclear O2 complexes were identified, with stoichiometries Rh2(02)n, n = 1-4. In addition, a trinuclear species Rhs(02)m, m = 2 or 6, was identified. The infrared data for these complexes, as well as for the mononuclear complexes Rh(02)x, = 1-2 (229), are summarized in Table XI. Metal-concentration plots that led to the determination of... 

Although the latter product is a solvated mononuclear [Rh(MeOH)2(diphos)]+ cation, in the solid state it is isolated as a binuclear complex of formula [Rh2 (diphos)2](BF4)2, in which each rhodium center is bonded to two phosphorus atoms of a chelating bis(diphenylphosphino)ethane ligand, and to a phenyl ring of the bis(diphenylphosphino)ethane ligand of the other rhodium atom. This dimer reverts to a mononuclear species on redissolving. The mechanism of hydrogenation of the prochiral alkene methyl(Z)-a-acetamidocinnamate, studied in detail by Halpern [31], is depicted in Scheme 1.7. 

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]. 

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