Oxophilicity, lanthanides

First, deprotonation of dimethyl phosphite accompanied by coordination of oxygen to the oxophilic lanthanide gives 33. Nucleophilic attack of P on the imine carbon along with N-coordination gives 34 proton transfer followed by product de-complexation regenerates the catalyst [33],... 

In contrast, Nd2(OCHiPr2)6 cannot be deoligomerized by these solvents, including pyridine. This is in accord with the lower nucleophilicity of the siloxide O-atom which is reflected in the less distinct tendency to bridge the oxophilic lanthanide centers [20]. 

The focal point is a carboxylate moiety an excellent ligand for the oxophilic lanthanides. The resulting isolation of the lanthanide ion and the antenna effects of the dendrons results in significantly enhanced lanthanide fluorescence." These kinds of species are reminiscent of the light harvesting and conversion devices we looked at in Chapter 11 and indeed the... 

Polyoxometalate anions, due to the large number of oxygen atoms, have also been used to control the hydrolysis of the oxophilic lanthanide ions (Pope, 2008). Hydrolysis of Y(III) or Yb(III) by carbonate in the presence of the trivacant Wells-Dawson polyoxoanion, a-[P2Wi505g] produced... 

D networks based on the lanthanides with their high coordination numbers (typically 7-11) are less predictable than those based on transition metals and the high oxophilicity of the lanthanide (III) ions means that they do not bind strongly to ligands such as pyridine derivatives. As a result lanthanide coordination networks are less common in the literature however, they represent tremendous potential in terms of ever more complex topologies. 

As a prelude to this section, intrinsic properties of the Ln-OR moiety will be passed in review. Alkoxide ligands are put into the category of hard ligands according the Pearson terminology and therefore optimally match the electropositive requirements of the hard lanthanide cations. The natural abundance of the lanthanide elements is the force behind the hard-soft relationship. The strongly electropositive and oxophilic character is expressed in high forma-... 

The oxophilic character of lanthanide is evident from the characterization of the complex (MeCp)3Yb4(/i-Cl)(,(/i3-Cl)(/i4-0)(THF)3 by the reaction of YbCb and YbOCl with MeCp in THF. The crystal structure of the complex shows an oxygen-centred tetrahedral arrangement of Yb atoms with p.-C bridging each edge and p -C 1 over the triangular face formed by the (MeCp)Yb units [137]. Yb-0(/x4-oxide) bond distance range is 2.13(1)— 2.29 (1) A. 

Niobium (formerly called columbium) and tantalum are Transition Metals having a considerable affinity for oxygen donor groups they are thus called oxophilic see Oxophilic Character). They occur as mixed-metal oxides such as columbites (Fe/Mn)(Nb/Ta)206 and pyrochlore NaCaNb206p. Their discovery in minerals extends back to the beginning of the nineteenth century, when they were believed to be identical and called tantalum. Rose showed that at least two different elements were involved in the minerals, and named the second one niobium. Their separation was resolved around 1866, especially by Marignac. These metals often display similar chemical behavior as a result of nearly identical atomic radii (1.47 A) due to the lanthanide contraction see Periodic Table Trends in the Properties of the Elements)... 

The large size and ionic, electropositive character of the lanthanides, properties which make the chemistry experimentally difficult, also make the lanthanides strongly electrophilic and oxophilic. These properties can also impart unusual chemistry. 

The common properties attributed to helper ligands in improving lanthanide-based detection techniques include enhanced stability, sensitivity, and selectivity. We will start with the effects of ancillary ligands on lanthanide photophysics and then explore the impacts of these ligands on sensor properties (Section II). Other factors influencing stability, such as sterics and oxophilicity, will be summarized in Section III. A brief discussion of future applications will follow (Section IV). 

Ligands bearing O, N, or S atoms as donors are used to incorporate both lanthanides and transition metals into the complexes, as lanthanides are hard Lewis acids and oxophilic. 

Due to their oxophilic nature, early transition metals show general properties which somehow recall those of the main group elements and lanthanides and afford O-bonded... 

The oxophilicity and coordination ability of the lanthanide elements turned out to be crucial for the attraction and activation of oxygenated functions which display pivotal components in important condensation and addition reactions [101]. Orga-nometallic systems such as fluorinated )5-diketonate and alkoxide complexes which contain highly polarized Ln-O-C linkages and are soluble in non-oxy-gen-containing solvents seem to be predestined for this type of homogeneous transformation (Structures 20-24). It must be assumed that other precatalysts (Ln"-derivatives or Ln -alkyls) underlie in situ formation of catalytically active Ln-O(alkoxide) moieties such as enolates when substrates such as ketones or aldehydes are involved. 

The utility of Sm in deoxygenative coupling of carbonyl compounds has already been discussed [38]. One study has been published on the reaction of acylphos-phonates with another lanthanide metal, Yb [46]. Ytterbium converted a substituted acylphosphonate under mild conditions (THF, HMPA, rt) to 1,2-diphenyl-2-oxoethyl phosphate and l-(phosphoryloxy)-l-phenylmethylphosphonate. The reactivity of Yb was explained in terms of its oxophilicity, leading initially to an acylytterbium complex. 

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