Structural and photophysical properties of heterobimetallic 4f-Zn iminophenolate cryptates

Lanthanide complexes with the Schiff base axial macrobicyclic ligand L-1 react with Zn(II) nitrate in the presence of CaH2 to yield Ln(III)-Zn(II) heterodinuclear cryptates with the formula [Ln(NO3)(L-1-3H)Zn](NO3).xH(2)O-yMeOH. The macrobicyclic receptor L-1 is an azacryptand N[(CH2)(2)N=CH-R-CH=N-(CH2)(2)](3)N (R = 1,3-(2-OH-5-Me-C6H2)). The crystal structures of the Pr(III), Yb(III), and Lu(III) complexes, chemical formulas [Ln(NO3)(L-1-3H)Zn](NO3). xSolv (monoclinic, C2/c, Z = 8), as well as that of [Zn-2(L-1-3H)](NO3).H2O (15) (triclinic, P (1) over bar, Z = 2), have been determined by X-ray crystallography. The ligand is helically wrapped around the two metal ions, leading to pseudo-C-3 symmetries around the metals. The Ln(III)-Zn(II) distances lie in the range 3.3252(13) to 3.2699(14) Angstrom, while the Zn(II)-Zn(II) distance in 15 amounts to 3.1037(18) Angstrom. The three five-membered chelate rings of the ligand backbone coordinating the Ln(III) ion adopt a (lambdalambdadelta)(5) (or (deltadeltalambda)(5)) conformation while the three pseudochelate rings formed by the coordination of the ligand to the Zn(II) ion adopt a (lambda'lambda'lambda')(5) (or ((delta'delta'delta')(5)) conformation. Thus in the solid state the conformation of the three cations is Lambda(deltadeltalambda)(5)(delta'delta'delta')(5) or its enantiomeric form Delta(lambdalambdadelta)(5)(lambda'lambda'lambda')(5). In solution, the helicates present a time-averaged C-3 symmetry, as shown by H-1 NMR, and the conformation of the cations is described as Lambda(deltadeltadelta)(5)(delta'delta'delta')(5) (or Delta(lambdalambdalambda)(5)(lambda'lambda'lambda')(5)). The photophysical properties of the cryptates depend on the nature of the Ln(III) ion, and (L-3H)(3-) is revealed to be a good sensitizer for Eu(III) and Tb(III) at low temperatures, but the emission at room temperature is limited by the low energy of the ligand (3)pipi(*) state. While Eu(III) is most effectively sensitized by the ligand triplet state, the Tb(III) (D-5(4)) sensitization occurs via the singlet state. The quantum yield of the metal-centered luminescence in the Eu-Zn cryptate amounts to 1.05% upon ligand excitation. The low energy of the ligand (3)pipi(*) state allows efficient sensitization of the Nd(III) and Yb(III) cryptates, which emit in the near-infrared.