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) reacts with lanthanide perchlorate in acetonitrile to give the successive complexes [Ln(mbzimpy)n]3+ (n = 1-3). Tris complexes were isolated for La, Eu, Gd, and Tb, while a bis complex only could be crystallized for Lu. The crystal structure of [Lu(mbzimpy)2(H2O)(CH3-OH)] (ClO4)3.3CH3OH (12, LuC46H52N10O17Cl3, a = 13.140(3) angstrom, b = 22.007(4) angstrom, c = 18.927(7) angstrom, beta = 107.53(1)-degrees, monoclinic, P2(1)/c, Z = 4) shows three uncoordinated ClO4- anions and a [Lu(mbzimpy)2(H2O)(CH3OH)]3+ cation where Lu(III) is eight-coordinated by two meridional tridentate mbzimpy ligands, one methanol molecule, and one water molecule, leading to a low-symmetry coordination sphere around the metal ion. The crystal structure of [Eu(mbzimpy)3](ClO4)3 [6, EuC63H51N15O12Cl3, a = 24.703(2) angstrom, c = 16.982(2) angstrom, trigonal, R3BAR, Z = 6) shows a mononuclear cation [Eu(mbzimpy)3]3+ with C3 symmetry where Eu(III) is nine-coordinated by three tridentate mbzimpy (1) which are wrapped around the metal ion to give an approximate trigonal tricapped prismatic arrangement of the nine nitrogen donor atoms leading 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 acetonitrile. Spectrophotometric titrations show that the three expected successive complexes [Ln(mbzimpy)]3+, [Ln(mbzimpy)2]3+, and [Ln(mbzimpy)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, Yb, Lu) and for the tridentate ligands 2,6-bis(1-X-benzimidazol-2-yl)pyridine (X = propyl, pbzimpy, 2; X = 3,5-dimethoxybenzyl, dmbbzimpy, 3] which have bulky groups bound to the aromatic benzimidazole rings. The origin of these effects is discussed together with the use of these complexes as luminescent building blocks for the formation of triple-helical structures.