Esters mixed ligand complexes

An interesting series of reactions were carried out starting from bis (cyclopentadienyl)nickel and secondary phosphite esters (Scheme 24).1713 Finally, the mixed-ligand complex [Pt (MeO)2PO 2diphos] can act by itself as a ligand towards nickel perchlorate giving the mixed metal oligomer (225).1714... 

Activation parameters have been determined58 for the hydrolysis of a-amino acid esters in mixed ligand complexes (Table 6). For base hydrolysis of the complex [Cu(nta)-(NH2CH2C02Et)], AH = 20.5 kJ mol-1 and AS = -138 J K 1 mol-1. Catalysis in this system is primarily due to a substantial decrease in AH (by ca. 21 kJ mol1) compared with the free ligand. A detailed discussion of the activation parameters is available.58... 

A number of recent studies have been carried out with palladium(II) complexes,85"87 of the types (18) and (19). These mixed ligand complexes are formed in solution by the addition of the a-amino acid ester to the corresponding diaqua species, for example [Pd(en)(OH2)2j2+. The kinetics of hydrolysis of the ester ligand in such mixed ligand complexes have been studied in detail. Nucleophilic attack by both OH- and OH2 occurs. Typical kinetic data are summarized in Table 15. 

Substantial rate accelerations are observed in these systems for base hydrolysis. Thus for the ethylenediamine complex (18) rate increases of 4x 104 for GlyOEt to 1.4 x 107 for ethyl picolinate are observed.85 These rate accelerations are consistent with the formation of carbonyl-bonded species (18). The effects with methyl L-cysteinate and methyl L-histidinate are much less marked as such ligands can give mixed ligand complexes which do not involve alkoxycarbonyl donors. Thus in the case of methyl L-histidinate the complex (20) is formed. For these latter two esters only relatively small rate accelerations of 20-100 occur. For the chelate ester complexes, the ratios of kcm/kH2o fail within the range 3.8 x 109 to 3.2 x 1011. Such values for the relative nucleophilicity of water and hydroxide ion are comparable with those previously noted for copper(II) complexes.82... 

The kinetics of hydrolysis of the ester ligand in the mixed ligand complexes has been studied in detail, and nucleophilic attack by both OH and OH2 has been shown to occur [23], The hydrolysis data obtained are summarised in Table 7.5. 

It is seen from these studies that when the donor ligand atoms are involved in the chromophore and they are more strong donors, such as sulphur or selenium, the acceptor molecule attacks the sulphur atom(14). In the initial stage of the reaction, mixed-ligand complexes and esters or ligand disulphides(14) are obtained. 

An interesting correlation has been observed53 between the formation constant XCuL of the metal complex and its catalytic activity in a mixed ligand with an amino acid ester. Large values of XCUL (equation 13) lead to lower base hydrolysis rates in the ternary complex. The Lewis... 

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