Lanthanide complexes hydrolysis

When not complexed, lanthanide ions have a high affinity for bone in vivo because they act as calcium ion mimics. Because the lanthanides undergo hydrolysis above a pH of 4, they readily form radiocolloids when not complexed, and are then taken up by the liver. This bone and liver uptake results in non-specific radiation doses to non-target (normal) tissues and organs and is undesirable.91 The polyaminocarboxylate class of ligands are considered to be the optimal choice for the basis of BFCAs for the+3 metal cations, including the lanthanides. It is essential that the... 

In addition to examining complexes of the four ligands for simple ester hydrolysis, Janda s group has taken these ligands and screened lanthanide complexes for... 

The best catalyst for the hydrolysis of 186 and 187 was found to be 184Gd. This system increased the rate of hydrolysis 127-fold. Differences in the rates of hydrolysis were found for the different phosphate esters tested. In addition to investigating the hydrolysis of simple phosphate esters, the lanthanide complexes were tested for their ability to hydrolyze double stranded DNA. In this reaction, Compound 184Gd gave the best rate acceleration. 

Hydrolytic catalysis by metal ions is also important in the hydrolysis of nucleic acids, especially RNA (36). Molecules of RNA that catalyze hydrolytic reactions, termed ribozymes, require divalent metal ions to effect hydrolysis efficiently. Thus, all ribozymes are metalloenzymes (6). There is speculation that ribozymes may have been the first enzymes to evolve (37), so the very first enzymes may have been metalloenzymes Recently, substitution of sulfur for the 3 -oxygen atom in a substrate of the tetrahymena ribozyme has been shown to give a 1000-fold reduction in rate of hydrolysis with Mg2+ but no attenuation of the hydrolysis rate with Mn2+ and Zn2+ (38). Because Mn2+ and Zn2+ have stronger affinities for sulfur than Mg2+ has, this feature provides strong evidence for a true catalytic role of the divalent cation in the hydrolytic mechanism, involving coordination of the metal to the 3 -oxygen atom. Other examples of metal-ion catalyzed hydrolysis of RNA involve lanthanide complexes, which are discussed in this volume. 

The most successful synthetic approach to structurally well defined lanthanide hydroxide complexes is the ligand-controlled hydrolysis approach [69-71]. The essence of this methodology is schematically shown in Figure 6.25. It makes use of the high propensity of lanthanide ions toward hydrolysis, but controlled and limited by certain supporting ligands. The scheme starts with a lanthanide complex whose coordination sphere constitutes both organic and aqua... 

The preparation, characterization, aqueous stability, and photophysical properties of NIR emitting lanthanide complexes with tetradentate chelating ligands 36 and 37 were described by Raymond and coworkers [61, 62]. In aqueous solution, the chelating ligand 36 or 37 forms stable complexes with Ln(III) ions, and sensitized NIR lanthanide luminescence was detected for the complexes with Pr(III), Nd(III), Ho(III), or Yb(III) ions. For [Ln(36)2] complexes, the luminescence decay curves were biexponential due to partial hydrolysis of the complexes or alternately the presence of a slowly exchanging equilibrium mixture with a hydrated form of the complexes. For [Ho(37)2] , the NIR band due to Fs -> I transition of the Ho(III)... 

K. Matsuura, M. Endo, M. Komiyama, Lanthanide Complex-Oligo-DNA Hybrid for Sequence-Selective Hydrolysis of RNA , J. Chem. Soc., Chem. Commun., 2019 (1994)... 

The formation of these hydroxo complexes manifests the high proneness of lanthanide to hydrolysis, while the lack of activities toward their... 

Alternatively, hydrolysis of presumably not so soluble lanthanide carboxylate complexes may be possible by carrying out the hydrolysis under hydrothermal conditions. It appears that such conditions (high temperature and high pressure) are particularly conducive to the formation of polynuclear lanthanide complexes, often with unpredictable but nevertheless interesting structure. Under ambient pressure, analogous synthesis generally produces complexes with carboxylate coordination only and without any involvement of hydroxo groups. 

Further elaboration of 4 can be accomplished to install assorted functional groups of relevance for proton-transfer relays (Figure 3.5). For example, hydrolysis provides the dicarboxylic acid-appended terpy 5 [32], reduction with borane and hydrolysis yields the aminomethyl-terpy 6 [33], addition of azide provides the tetrazole-substituted terpy 7 [32], and treatment with hydrazine yields 8 [34]. Compounds 5-7 have been utilized as ligands for studying luminescence in lanthanide complexes [32, 33], but to our knowledge, have not yet been described in the catalysis literature. 

A recent example has been described by Brown et al. who have studied the KR of p-nitrophenyl esters of the d- and i-N-tert-butoxycarbonyl derivatives of glutamine and phenylalanine with ethanol or methanol promoted by chiral lanthanide complexes, providing enantioselectivities of up to 99% ee [302]. On the other hand, an enantioselective hydrolysis of phenylalanine derivatives was reported in 1986, providing a perfect enantiomer discrimination (s> 1000), as a result of catalysis with a tripeptide [303]. In 2007, Maruoka et al. reported the KR of differently a,a-disubstituted a-siloxy aldehydes based on an asymmetric rearrangement into the corresponding chiral acyloins using axially chiral organoaluminium Lewis acids, which provided selectivity factors of up to 39.5... 

Figure 2 Hydrolysis of aqua lanthanide complexes via olation of the intermediate Ln-OH species, affording lanthanide hydroxide complexes...

Traces of water in our solutions of monomeric praseodymium cryptate are most probably responsible for the formation of the dimeric complex reported here. Partial hydrolysis of this complex takes place because the excess of (2.2.1) cryptand brings about a pH increase. Incomplete hydrolysis of a lanthanide macrocyclic complex has also been noted by Biinzli et al. [14] who prepared a dimeric praseodymium complex with 1,4,7,10,13-pentaoxacyclododecane (15-crown-5) by dehydrating in vacuo a monomeric species. The metal ions in this dimer are bridged by only one hydroxyl group and by three trifluoroacetate anions. The distance between the two praseodymium ions in the (2.2.1) cryptate reported here is 3.927(1) A this value compares very well with the values reported for the two other dinuclear lanthanide complexes mentioned above [13-14]. 

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