Lanthanides complex-substrate interaction

Lanthanide ions continue to be very widely used as shift reagents for substrate molecules (Section III.F). There is, however, considerable interest in the NMR properties of the lanthanide complexes themselves. The isotropic shifts of the latter are invariably dominated by dipolar interactions between the lanthanide ion and the nucleus under question. Bleaney (156) has predicted that the dipolar shift will be dominated by a component which varies as T around room temperature whereas Horrocks et a/. (157,158) predict a more complex temperature dependence. Hill et al. (159) have examined the temperature dependence of some tetraethylammonium tetrakis-N,N-diethyldithiocarbamato-lanthanate(iii) salts, Et4N Ln(dtc)4, in an attempt to clarify the situation. The observed temperature dependences are complex and both contact and dipolar contributions had to be considered in the form ... 

The Pfeiffer effect, the outer-sphere interaction of a chiral substrate with a rapidly interconverting racemic solution of a chiral lanthanide complex, can be investigated by measurement of the luminescence dissymmetry factor (the ratio of circularly polarized luminescence to total luminescence) for Eu or Tb " complexes. Thus the racemic D chiral complexes [M(dpa)3], where M = Eu or Tb, interact in an outer-sphere manner with the following optically active spiecies cationic chiral transition metal complexes, ascorbic acid, aminocarboxylates, tartrates, amines and phenols. Association constants can be obtained from limiting values of the dissymmetry factors. In some cases, inner-sphere complexation can be demonstrated, as judged by changes in the general nature of the circularly polarized luminescence spectrum and pH irreversibility of the complexation. 

Fig. 16. Two strategies for site-selective scission of DNA and RNA (A) covalent strategy in which lanthanide complexes are covalently linked near the target phosphodiester linkages by using oligonucleotides that are complementary with the DNA and RNA substrates, and (B) non-covalent strategy , in which the target phosphodiester linkage is activated by some non-covalent interactions and dilferentiated from the others in the substrate in terms of intrinsic reactivity. The black ribbons show the oligonucleotides (or their equivalents) used for the artificial enzymes...

The properties of the lanthanide elements and their organometallic complexes described in the previous section explain in part why organo-met lic chemists in the past found lanthanide chemistry much less interesting than transition metal chemistry. The highly ionic, trivalent organolanthanide complexes appeared to have little potential to interact with the small-molecule substrates that provide such a rich chemistry for the transition metals neutral unsaturated hydrocarbons, H2, CO, phosphines, etc. The two-electron oxidation reduction cycles so important in catalytic transition metal chemistry in 18 16 electron complexes seemed... 

The interaction of a lanthanide metal with a substrate such as 3-hexyne could occur in several ways 60) by it complex formation, by oxidative addition into a C—H bond, or by reduction involving radical species. Subsequent lanthanide metal vapor studies were designed to test some of these possibilities. Co-condensation of lanthanide metal vapor with reagents containing acidic hydrogen atoms, e.g., terminal alkynes, demonstrated that oxidative addition of C—H was a viable reaction [Eqs. (32) and (33)] 51). These reactions also provided access to a new class of... 

This article has reviewed the synthesis and reactivity towards small molecules of a range of U(lll) cyclooctatetraene and pentalene complexes. It is evident that in many cases the uranium centre is capable of tt back-bonding through the 5/ orbitals additionally, a C - C agostic interaction between a bound substrate and a U(IV) centre has been observed. Clearly uranium is capable of bonding with a degree of covalency , and this is perhaps why the reduction chemistry of U(III) is so rich and diverse, and not simply an iteration of low-valent lanthanide chemistry. 

Lanthanide ions have been covalently attached to either the primary or secondary side of cyclodextrins through a diethylenetriaminepentaacetic acid (DT-PA) ligand [29]. Adding Dy(III) to the cyclodextrin-DTPA derivatives can enhance the enantiomeric resolution in the spectra of substrates that form host-guest complexes with the cyclodextrin. Enhancements in enantiomeric resolution are larger with the secondary derivative, which is consistent with the observation that enantiodiscrimination with cyclodextrins usually involves interactions with the hydroxyl groups on the secondary opening of the cavity. 

There is a similarity between high oxidation state early transition metal complexes and those of lanthanides. In both cases the substrates are activated by direct interaction with small, highly electropositive... 

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