Cluster lanthanide alkoxides clusters

Question 4.16 Lanthanide alkoxide clusters do not contain metal-metal bonds. Transition metal carbonyl clusters frequently do. Comment. 

Presence of small nonalkoxide ligands such as oxo, hydroxo, hydrido, halo, and chalcogenido groups is commonly observed in lanthanide alkoxide clusters. These small anionic species are important in facilitating the formation and maintaining the structural integrity of the cluster structures. 

In addition to the frequent incorporation of 0x0 and hydroxo groups into cluster core structures, halo ligands have also been found in many lanthanide alkoxide clusters. An early example is [Nd6(/i6"Cl)(/i3-OPr )2 (/i-OPr )9(OPr )6] (Andersen et al., 1978), a hexanuclear cluster obtained from the reaction of NdCla with NaOPr in HOPr. The six neodymium... 

Sheng, H., Xu, R, Yao, Y, Zhang, Y, and Shen, Q. (2007) Novel mixed-metal alkoxide clusters of lanthanide and sodium synthesis and extremely active catalysts for the polymerization of e-caprolactone and trimethylene carbonate. Inorganic Chemistry, 46, 7722-7724. 

Clusters of lanthanide alkoxides, aryloxides, and macrocyclic polyaryloxides... 

The second t)q)e of cluster compounds of the lanthanide alkoxides features one or several small-unit, inorganic ligands including oxo, hydroxo, halo, and other groups as part of the cluster core structure. Frequently, a combination of such ligands is foimd in the same cluster species. In a sense, this "central spherical charge density" drives the assembly of the polynuclear complexes without their participation, no clusters are formed or species of completely different structures result. 

Monomeric lanthanide alkoxides are generally imstable, unless they are stabilized by bulky substituents (20). Spassky and co-workers have shown that clustered lanthanide alkoxides Ln5(/i-0)(0-i-C3H7)i3 can initiate ROP of DL-lactide in dichloromethane (56). The following order of reactivity (La Sm Yb) has been reported. Low polydispersity is seen even at high conversion and long reaction time, except in case of lanthane because of exceedingly high reactivity and low selectivity. ROP of -CL is more complex, and transesterification reactions can occur before the monomer conversion is complete. 

As a consequence monomeric complexes are obtained much more easily. Also, the tendency to bridge lanthanide centers is less distinct and, for example, a small cluster chemistry as it exists for alkoxides, e.g. OiPr [13] and OtBu derivatives [14], is not yet known. However, the Ln-N(amide) bond is less strong than the Ln-O(alkoxide) bond, and even comparable to Ln-C(alkyl) bonds, which has an effect in synthetic chemistry. This has been confirmed by the determination of absolute bond disruption enthalpies D by means of calorimetric titrations for the representative systems Cp Sm X (X = OrBu, D = 82.4 kcalmol-1 NMe2, 48.2 CH(SiMe3)2, 47.0) [15]. 

In very acidic solutions, bismuth(III) exists in the form of the nonaaquo ion [Bi(H20)9] +, which is similar to the aquo complexes of the lanthanide ions, but partial hydrolysis of bismuth(III) salts leads to the formation of bismuth oxo clusters. The core structure of these complexes is often based upon a Bie octahedral core with oxide, hydroxide, or alkoxide functions bridging the edges and/or faces of the octahedron. The [Bi6(OH)i2] + ion (11) has been studied spectroscopically. In oxo clusters, the octahedron is face-bridged by eight oxo or alkoxide functions (12). Such core structures have been found in the hydrolysis of bismuth nitrate or perchlorate. ... 

The assembly of clusters is the result of a complex interplay of several factors. It is evident that deprotonation of the alkoxide pendant arms of the chelating ligand promotes the ability of alkoxide sites to form bridges to lanthanide ions. The nature of the product is dependent on the relative Lewis acidity of the lanthanide ions, the harder Lewis acids being more able to displace the hydroxyl protons from the protonated ligand. 

The first step in RNA hydrolysis is the intramolecular nucleophilic attack of the phosphorus atom by the 2 -OH of ribose. This step is activated by the coordination of the phosphodiester linkage in RNA to the lanthanide(III) ion in the bimetallic cluster [R 2(OH)2] , since the electrons are withdrawn by the metal ion from the phosphorus atom. This electron withdrawal promotes the electrophilicity of the P atom, although it is not so drastic as the effect achieved by the Ce(IV) in DNA hydrolysis (cf. sect. 5). Furthermore, the hydroxide ion bound to another lanthanide(III) ion in the bimetallic cluster functions as a general base catalyst, and enhances the electrophilicity of the 2 -OH by removing its proton. Alternatively, the 2 -OH is directly coordinated to this metal ion, and its dissociation to alkoxide ion is facilitated. In this way, both the nucleophilic center (the oxygen in the 2 -OH) and the electrophilic center (the phosphorus atom) are simultaneously activated by the bimetallic cluster, and thus the intramolecular nucleophilic attack proceeds efficiently. 

---