Cryptands lanthanide complexes

Until very recently, studies of the use of luminescent lanthanide complexes as biological probes concentrated on the use of terbium and europium complexes. These have emission lines in the visible region of the spectrum, and have long-lived (millisecond timescale) metal-centered emission. The first examples to be studied in detail were complexes of the Lehn cryptand (complexes (20) and (26) in Figure 7),48,50,88 whose luminescence properties have also been applied to bioassay (vide infra). In this case, the europium and terbium ions both have two water molecules... 

The first cage lanthanide complexes studied for their photophysics were the simple 2.2.1 cryptands. The lack of a strongly absorbing chromophore, and easy approach of solvent molecules meant that their luminescence properties were disappointing in comparison to many recently studied complexes. The Lehn cryptand (L53) (Scheme 6... 

Recently, two cryptands and their lanthanide complexes have been synthesized which include either a bipyridyl (L56) or pyridyl (L57) chromophore (89). These have proved effective at populating the lanthanide excited states. Aqueous luminescence lifetimes of up to... 

Numerous macrocyclic and macropolycyclic ligands featuring subheterocyclic rings such as pyridine, furan or thiophene have been investigated [2.70] among which one may, for instance, cite the cyclic hexapyridine torands (see 19) [2.39] and the cryptands containing pyridine, 2,2 -bipyridine (bipy), 9,10-phenanthroline (phen) etc. units [2.56,2.57,2.71-2.73]. The [Na+ c tris-bipy] cryptate 20 [2.71] and especially lanthanide complexes of the same class have been extensively studied [2.74, 2.75] (see also Sect. 8.2). 

Liu, Y., Zhang, H. Y., Bai, X. P., Wada, T., and Inoue, Y. (2000) Molecular design of crown ethers. 21. Synthesis of novel double-armed benzo-15-crown-5 lariats and their complexation thermodynamics with light lanthanoid nitrates in acetonitrile, J. Org. Chem. 65, 7105-7109 see also Danil de Namor, A. F. D., Chahine, S., Jafou, O., and Baron, K. (2003) Solution thermodynamics of lanthanide-cryptand 222 complexation processes, J. Coord. Chem 56, 1245-1255 Israeli, Y., Bonal, C., Detellier, C., Morel, J. P., and Morel-Desrosiers, N. (2002) Complexation of the La(III) cation by p-sulfonatocalix[4]arene - A La-139 NMR study, Canad. J. Chem. 80, 163-168. 

Stability constants for lanthanide complexes with crown ethers and cryptands. 

Fig. 4.38. Stability constants in water at 298 K and = 0.1 M, for lanthanide complexes with the (2.2.1) cryptand and the bibracchial lariat ethers (2.1)DA and (2.2)DA. From data reported by J.-C.G. Biinzli, in Handbook on the Physics and Chemistry of Rare Earths, eds K.A. Gschneidner Jr., L. Eyring, Vol. 9, Ch. 60, North Holland,...

Cryptands have been somewhat deceptive for both coordination chemistry (Sastri et al., 2003) and photophysical properties of the resulting lanthanide complexes despite some commercial uses (Mathis, 1998), in particular of Lehn s Eu cryptate with cryptand 23a (fig. 28). The latter has been tested for the sensitization of the NIR luminescence of Nd and Yb. Characteristic emission from these two ions is seen upon excitation of the bipyridyl chro-mophores at 355 nm. Emission from Yb is reported to be much more intense than the one from Nd and the authors propose that the excitation mechanism depicted in fig. 9 is operative in this case since no transient absorption corresponding to the formation of the triplet state could be detected (Faulkner et al., 2001). Analysis of lifetime measurements in both water (r( F5/2) = 0.52 ps) and deuterated water (5.21 ps) gives a hydration number q = 1.5. Since fitting the luminescence decays to a double exponential function did not improve noticeably the resulting fit, the authors concluded that the non-integer value does not reflect an equilibrium between two different hydration states but, rather, that the distance of close approach of two water molecules is longer note that comparable experiments on Eu and Tb ... 

Obviously, many structural variations are possible in the design of ferrocene oxa-aza cryptands. Some of the oxa-azaferrocene cryptands form alkali metal and lanthanide complexes [90]. FAB mass spectrometry experiments have shown that the cryptands have strong selectivity for the potassium cation compared with Li+, Na, or Cs" " [94], In these complexes the macrocycle functions as a host, but in Mg + complexes the cation is coordinated by the amide carbonyl groups [95]. In the lanthanide complexes the metallocene moiety acts as an efficient center for radiationless deactivation of the lanthanide excited state [96]. 

Although not particularly designed to be a pH-sensitive luminescent probe, Beeby et al. described a phenanthridine-carrying water-soluble Yb complex, in which the photosensitisation mechanism is switched as a functi(m of pH [93], A crown ether-modified neodymium(III) cryptand was proposed for barium ion detection [94], but as with many prototype cation sensors, the probe only works in acetonitrile. Moreover, the complex necessitates ultraviolet excitation, which removes one of the main advantages of using NIR luminescent lanthanide complexes. 

J. REBIZANT, M. R. SPIRLET, P. P. BARTH LEMY, and J. F. DESREUX / SoUd State and Solution Structures of the Lanthanide Complexes with Cryptand (2.2.1) Crystallographic and NMR Studies of a Dimeric Praseodymium (2.2.1) Cryptate Containing Two p-Hydroxo Bridges... 

Abstract. A dimeric lanthanide cryptate was obtained by the addition of an excess of cryptand (2.2.1) to a slightly hydrated solution of the monomeric praseodymium (2.2.1) perchlorate complex in acetonitrile. This new lanthanide compound is centrosymmetric and displays the space group P2 /n. The encryptated metal ions are nine-coordinated, they are bonded to all the heteroatoms of a (2.2.1) ligand and they are linked to each other by two p-hydroxo bridges. The hydroxyl groups are relegating the cryptands to both end of the dimer and the praseodymium ions are less effectively accomodated in the macrocylic internal cavities than in the case of the monomeric Pr(2.2.1) complex. The formation of both the monomeric and the dimeric lanthanide complexes is readily observed by proton NMR. 

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]. 

Reblzant, J., M. R. Splrlet, P. P. Barthelemy and J. F. Desreux - Solid state and solution structures of the lanthanide complexes with cryptand (2.2.1)... 

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