Complexes Containing an M-C Bond in Aqueous Catalysis

Compounds containing one or more metal-carbon a- or n- bonds (organometallic complexes in a strict sense) are stiU not so frequently used as catalysts in aqueous phase, despite the fact that their applications in aqueous systems are increasing. 

The reaction of nitriles with water to form amides was also found to be catalyzed by complex 1 [23]. This is a commercially relevant reaction since amides are used as lubricants, detergent additives, drug stabilizers, etc. [24]. In aqueous solution this complex was found to be in equilibrium with [Cp 2Mo(OH)(H20] (2), and this monomer was proposed to be the active hydration catalyst The hydration was proposed to occur by an intramolecular attack of a hydroxide ligand on a coordinating nitrile. The reaction occurred under mild conditions, preventing the autocatalytic formation of acrylamide. Although this system allows the hydration of simple and functionalized nitriles, including acrylonitrile, which is hydrated exclusively at the C N position, its major drawback resides in the fact that the reactions are subject to product inhibition. 

With the work by Grubbs et al. [27] and Herrmann et al. [28], the use of ruthenium carbene complexes as homogeneous catalysts for the ROMP (Ring-Opening Metathesis Polymerization) of olefins was estabhshed (see Section 2.4.4.3). The development of catalysts that can catalyze hving polymerization in water was an important goal to achieve, especially for applications in biomedicine. In this context, two water-soluble ruthenium carbene complexes (3 and 4) have been reported that act as initiators for the living polymerization of water-soluble monomers in a quick and quantitative manner [29]. 

The compound [Cp Ir(H20)3] (Cp = ri -CjMej) was found to be an efEcient catalyst precursor for the hydrogenation of water-soluble compounds [25]. Such organometallic aqua complexes are very interesting, since their properties change drastically with variations of the solution pH, due to deprotonation of the aqua ligands [26]. The active catalyst was foimd to be a binuclear i-hydride complex [(Cp Ir)2(p-H)(ii-OH)( i-HCOO)] (5) that could be isolated and characterized. 

The pH-dependent transfer hydrogenation of water-soluble carbonyl compounds in the presence of a hydrogen donor (HCOONa) was studied for a different series of carbonyl compounds, namely straight-chain aldehydes, cyclic aldehydes, ketones, aldehyde-acids and keto-acids. The reaction rates were found to be higher for aldehydes than for ketones. Furthermore there was a strong pH dependence of the reaction, the best results being obtained at pH 3.2. This value corresponds to the higher yield of the catalyst formed from the catalyst precursor and HCOONa. 

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