Self-assembly and photophysical properties of lanthanide dinuclear triple-helical complexes

Abstract

The dinucleating ligand bis[ 1- methyl-2-(6'- [ 1”-(3,5 imethoxybenzyl)benzimidazol-2~'-ylp]y rid-2'-y1)benzimidazol- 5-yllmethane (L) reacts with lanthanide perchlorates to give dinuclear 2:3 complexes [Lnz(L)#+ (Ln = La, Eu, Gd, Tb, and Lu). Detailed ES-MS, lH-NMR, luminescence, and spectrophotometric measurements in acetonitrile show that the cations [Ln2(L)#+ are produced by strict self-assembly and adopt a triple-helical structure in solution (pseudo-D3s ymmetry). The crystal structure of [EU~(L)~] C~O~)~((1C1H, E~UC2NC1)9s~H 177N39036_Cl6,a = 17.634- (3) A, b = 21.408(4) A, c = 29.437(7) A, a = 82.13(1)', /3 = 85.76(1)', y = 89.79(1)', triclinic, P1, Z = 2) shows a dinuclear pseudo-D3 triple- elical cation, [Eu2(L)#+, where the three bis(terdentate) ligands L are wrapped around the helical axis defined by the europium atoms. The Eu(II1) of each site is 9-coordinated by six nitrogen atoms of the benzimidazole units occupying the vertices and three nitrogen atoms of the pyridine units occupying the capping positions of a slightly distorted, tricapped trigonal prism. Luminescence studies of the crystalline complex [Eu~(L)~]- (ClO.&nsolv (n = 9, solv = CH,CN, 11; n = 2, solv = H20,6; n = 9, solv = H20,7) confirm the pseudo-D3 symmetry of the Eu(II1) sites in 11 and show that secondary interactions with water molecules in 6 and 7 destroy the trigonal symmetry. An efficient intramolecular energy transfer between the %A* excited state centered on L and the excited levels of Eu(II1) and Tb(II1) is observed (antenna effect) together with a dipole4ipolar Tb - Eu intramolecular energy transfer in the heterodinuclear-doped Eu-Tb compound. Stability constants and 1H NMR in acetonitrile show that the homodinuclear complexes [LI I ~ (L) ~ ]a~re+ l ess stable for the heavier lanthanides Tb and Lu. The origin of this effect is discussed together with the nonstatistical distribution of the different species observed when stoichiometric quantities of L (3 equiv) are mixed with Ln1(C104)3 (1 equiv) and L I I ~ ( C ~ O(1~ e)q~ui v) in solution (Ln1 # Ln2; Ln1 = La, Eu, Tb, and Lu; Ln2 = Tb and Lu)

Discriminating between lanthanide ions: self-assembly of heterodimetallic triple-stranded helicates

“…3 fully confirms the structural data obtained in solution, the three ligand strands being helically wrapped around the two metal ions and the three amide functions being coordinated to the heaviest metal ion, Eu III . Bond lengths and angles of the coordination polyhedron are standard for the two ions and compare well, after correction for ionic radius differences, with those found in the previously reported homodimetallic structures of [Eu 2 (L A ) 3 ] 6+ , 8 [Tb 2 (L B ) 3 ] 6+ , 9 and [Ln 2 (L C -2H) 3 ] (Ln = Eu, Tb, Yb). 10 In conclusion, we have presented here the first design of an unsymmetrical ditopic ligand coded for the supramolecular recognition of two different lanthanide ions.…”

The First Self‐Assembled Trimetallic Lanthanide Helicates Driven by Positive Cooperativity

“…[14] Above 200 K, the lifetimes of both sites in [Eu 3 (L7) 3 ] 9 sharply decrease to reach 1.7(1) ms at 295 K, because of thermally activated vibrational-quenching processes. Interestingly, the lifetime of the central EuN 9 sites is less affected at 295 K than that reported for the bimetallic model complex [Eu 2 (L1) 3 ] 6 (t 0.29(1) ms at 295 K); [13] …”

“…Factor analysis [23] points to the existence of four absorbing species (L7, [Ln 2 (L7) 3 ] 6 , [Ln 3 (L7) 3 ] 9 , and [Ln 3 (L7) 2 ] 9 ) in agreement with the ESI-MS results, and the spectrophotometric data can be fitted with nonlinear least-squares techniques [24] to the equilibria given in Equations (1) ± (3), which characterize the assembly process (the formation constants log(b Ln ij are collected in Table 1). 17.5 ± 20.0); [13] ring between the closely packed benzimidazole rings (vide infra, Figure 6) …”

“…(12)] and d para ij /C j versus hS z i j /C j [Eq. (13)] are expected to give linear correlations as long as the structural factors G m i (m 1 ± 3), the contact term F i , and the crystalfield parameters do not change along the lanthanide series. [17,39] …”

Lanthanide Homobimetallic Triple‐Stranded Helicates: Insight into the Self‐Assembly Mechanism

“…In spite of this difficulty, we have demonstrated that a careful ligand design based on the induced fit principle [20] leads to the strict selfassembly of lanthanide-containing triple-stranded helicates in organic media. [21,22] Ditopic pentadentate, hexadentate or nonadentate ligands derived from 2-pyridin-2-yl-1H-benzimidazole are indeed versatile building blocks and can be used to prepare bimetallic 3dϪ4f and 4fϪ4f assemblies, [23] as well as trimetallic 4fϪ4fϪ4f edifices. [24] Recently, we have also shown that grafting different substituents at the end of the hexadentate ligands results in ditopic hosts able to specifically recognize a pair of lanthanide ions in solution.…”