STRUCTURAL AND PHOTOPHYSICAL PROPERTIES OF LANTHANIDE COMPLEXES WITH PLANAR AROMATIC TRIDENTATE NITROGEN LIGANDS AS LUMINESCENT BUILDING-BLOCKS FOR TRIPLE-HELICAL STRUCTURES

The ligand 2,6-bis(1-methylbenzimidazol-2-yl)pyridine (mbzimpy, 1) rea cts with lanthanide perchlorate in acetonitrile to give the successive complexes [Ln(mbzimpy)n]3+ (n = 1-3). Tris complexes were isolated fo r La, Eu, Gd, and Tb, while a bis complex only could be crystallized f or Lu. The crystal structure of [Lu(mbzimpy)2(H2O)(CH3-OH)] (ClO4)3.3C H3OH (12, LuC46H52N10O17Cl3, a = 13.140(3) angstrom, b = 22.007(4) ang strom, c = 18.927(7) angstrom, beta = 107.53(1)-degrees, monoclinic, P 2(1)/c, Z = 4) shows three uncoordinated ClO4- anions and a [Lu(mbzimp y)2(H2O)(CH3OH)]3+ cation where Lu(III) is eight-coordinated by two me ridional tridentate mbzimpy ligands, one methanol molecule, and one wa ter molecule, leading to a low-symmetry coordination sphere around the metal ion. The crystal structure of [Eu(mbzimpy)3](ClO4)3 [6, EuC63H5 1N15O12Cl3, a = 24.703(2) angstrom, c = 16.982(2) angstrom, trigonal, R3BAR, Z = 6) shows a mononuclear cation [Eu(mbzimpy)3]3+ with C3 symm etry where Eu(III) is nine-coordinated by three tridentate mbzimpy (1) which are wrapped around the metal ion to give an approximate trigona l tricapped prismatic arrangement of the nine nitrogen donor atoms lea ding to a pseudo-D3 symmetry. Luminescence studies of crystalline [Eu( mbzimpy)3] (ClO4)3 (6) confirm the high symmetry for the Eu(III) sites while H-1-NMR and luminescence measurements in solution indicate that the triple-helical structure of [Eu(mbzimpy)3]3+ is maintained in ace tonitrile. Spectrophotometric titrations show that the three expected successive complexes [Ln(mbzimpy)]3+, [Ln(mbzimpy)2]3+, and [Ln(mbzimp y)3]3+ are formed in solution for all the lanthanide ions studied (Ln = La, Nd, Eu, Gd, Tb, Ho, Yb, Lu), but that [Ln(mbzimpy)3]3+ complexes are significantly less stable for the heavier lanthanides (Ln = Ho, Y b, Lu) and for the tridentate ligands 2,6-bis(1-X-benzimidazol-2-yl)py ridine (X = propyl, pbzimpy, 2; X = 3,5-dimethoxybenzyl, dmbbzimpy, 3] which have bulky groups bound to the aromatic benzimidazole rings. Th e origin of these effects is discussed together with the use of these complexes as luminescent building blocks for the formation of triple-h elical structures.