Rhodium complexes clusters

Formal silylcarbonylation and silylformylation reactions are mainly catalyzed by cobalt and rhodium complexes (clusters) yet, Chatani et al. [158] have found a new type of carbonylation of diynes with trialkylsilanes leading to catechols (Eq. 100). 

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]. 

Cluster or bimetallic reactions have also been proposed in addition to monometallic oxidative addition reactions. The reactions do not basically change. Reactions involving breaking of C-H bonds have been proposed. For palladium catalysed decomposition of triarylphosphines this is not the case [32], Likewise, Rh, Co, and Ru hydroformylation catalysts give aryl derivatives not involving C-H activation [33], Several rhodium complexes catalyse the exchange of aryl substituents at triarylphosphines [34] ... 

It is also remarkable that the rhodium complex catalyst in the supported ionic liquid showed long-term stability, being used for 18 batches without any significant loss of activity. It was confirmed that no rhodium metal clusters were formed. However, acetone could be used to remove the ionic layer and its presence led to a total loss of activity. 

Decomposition of the trimethylsilyl diazoacetate 133 with a ruthenium cluster in the presence of benzaldehyde and dimethylfumarate led to formation of the THE derivative 135 in 54% isolated yields. The ruthenium catalyst proved superior to all standard rhodium complexes for this transformation. 

Involvement of two nucleophilic nitrogen atoms is thus typical for the amino heterocycles. The mutual disposition of the pyridine and amine nitrogen atoms allows the formation of chelate structures for the cobalt complexes of purine, 221 and 222. Structures with the N, iV -five-membered metal cycles were proven for the tri- and tetranuclear complexes of silver ) with 8-aminoquinoline (223) (92IC4370), and polymeric copper- and rhodium-acetate clusters (224). Another coordination mode can be found in the complexes of 4-amino-3,5-bis(pyridin-2-yl)-l,2,4-triazole, (225 or... 

Under mild conditions, hydroformylation of olefins with rhodium carbonyl complexes selectively produces aldehydes. A one-step synthesis of oxo alcohols is possible using monomeric or polymeric amines, such as dimethylbenzylamine or anion exchange resin analog to hydrogenate the aldehyde. The rate of aldehyde hydrogenation passes through a maximum as amine basicity and concentration increase. IR data of the reaction reveal that anionic rhodium carbonyl clusters, normally absent, are formed on addition of amine. Aldehyde hydrogenation is attributed to enhanced hydridic character of a Rh-H intermediate via amine coordination to rhodium. 

Homogeneous Catalytic Oxidation with Phosphine-Substituted Complexes of Rhodium Carbonyl Clusters... 

With 5.1 as the precatalyst, the degradation products in the propylene hy-droformylation reaction are diphenylpropyl phosphine (Ph2PC3H7), benzene, and less soluble rhodium complexes. These complexes are tentatively formulated as phosphido clusters. Formation of all the products could be rationalized by the hypothetical catalytic cycle shown in Fig. 5.6. 

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