Macrobicyclic bipyridinates

Bkouche-Waksman, I. Guilhem, J. Pascard, C. Alpha, B. Deschenaux, R. Lehn, J.-M. 110. Crystal structures of the lanthanum(III), europium(III), and terbium(III) cryptates of tris(bipyridine) macrobicyclic ligands. Helv. Chim. Acta 1991, 74,1163-1170. 

The one-stage condensation of tren with 5,5 -diformyl-2,2 -bipyridine made it possible to also obtain the macrobicyclic Schiff base, whose hydrogenation on the palladium catalyst yielded the clathrochelate tris-bipyridine tranbpy ligand 1 (Scheme 91) [195]. 

In addition, the NMR spectra made it possible to determine the molecular Di symmetry for the [Ru(tabpy)]2 cation in solution [190] and the Cs symmetry for the corresponding semiclathrochelate complex as well as to reveal dynamic effects in macrobicyclic tris-phenanthrolinates and tris-bipyridinates [211]. 

Photoluminescence of tris-bipyridine europium and terbium(III) clathrochelates in aqueous solutions and in the solid state has been studied in the temperature range from 4.2 to 300 K [212, 374, 390-394]. It has been assumed that luminescence is favoured by (a) the inertness of clathrochelates (b) the complete encapsulation of the rare earth metal ion in macrobicyclic ligand cavities, preventing deactivation of these luminescent ions owing to interaction with the solvent molecules and (c) the possibility of energy transfer from the ligand chromophore to the luminescent ion. 

Fig. 69. Temperature dependence of luminescence intensities for clathrochelate europium (1) and terbium(tll) (2) tris-bipyridinates and the initial macrobicyclic ligand (3).

Terbium clathrochelate showed green emission of very high intensity. The emission spectrum contains the D4 —> Fj transition bands of the encapsulated terbium ion. The same but less intense emission spectrum was observed at higher temperatures. The luminescence quantum yield is close to 1 at 4.4 K and is approximately 0.05 at room temperature [390]. The decrease in intensity of the terbium(III) ion luminescence starts at 100 K (higher than that of free macrobicyclic tris-bipyridine ligand and lower than that of the corresponding europium(III) compound, Fig. 69). It may be... 

The luminescence properties of macrobicyclic mixed bipyridine-bipyrazine and macrocyclic polybipyrazine europium(III) complexes were reported in Ref. 380. A noticeable enhancement in the lifetime and fluorescence quantum yield for the macrobicyclic [Eu(bpy.bpy.bpz)]3+ cation was observed in comparison with other europium(III) clathrochelates and attributed to the introduction of the bpz fragment. Among the four studied europium(III) complexes, only the macrobicyclic one is stable in dilute aqueous solution [380]. 

Pioneering work in using a lanthanide ion for emission was reported by Lehn and co-workers (107). Those studies are based on 2,2 -bipyridine (bpy), 1,10-phenanthroline (phen), or 3,3 -bis(isoquinoline) (biqn) units incorporated into macrobicyclic structures conforming to the design as in Fig. 18(fc). When a lanthanide ion such as Eu(III) or Tb(lll) is added, they show strong emission that is red shifted by interacting with the triplet state of the heterocyclic units. 

More recently Lehn (60) has reported a number of photoactive macrobicyclic ligands (60)-(64), containing anthracene, bipyridine and phenanthro-line groups (Figure 2.19). Direct access to the Na Br complexes of the... 

The synthesis of ligands bearing mixed type of chelating groiq)s such as amide-bipyridine or amide-phenantroline (e.g. 8) or having a macrobicyclic structures such as 9, allows a good compromise between stability and liuninescence properties of the corresponding complexes with lanthanide ions. 

Due to its exertional complexing abilities, 2,2 -bipyridine is predestinated for being incorporated in macrocyclic host systems. Thus, a series of macrobicyclic ligands containing three bipyridine units could be obtained. They exhibit unique properties with regard to selective molecular recognition, complex stability and photophysical processes. 

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