Lanthanide Helicates Research Articles

The first self-assembled trimetallic lanthanide helicate: different coordination sites in symmetrical molecular architectures.

A tris-tridentate segmental ligand has been designed for the self-assembly of homotrimetallic triple-stranded lanthanide helicates possessing different coordination sites along the threefold axis.

Supramolecular coordination chemistry in aqueous solution: lanthanide ion-induced triple helix formation.

The self-assembly of dinuclear triple helical lanthanide ion complexes (helicates), in aqueous solution, is investigated utilizing laser-induced, lanthanide luminescence spectroscopy. A series of dinuclear lanthanide (III) helicates (Ln(III)) based on 2,6-pyridinedicarboxylic acid (dipicolinic acid, dpa) coordinating units was synthesized by linking two dpa moieties using the organic diamines (1R,2R)-diaminocyclohexane (chxn-R,R) and 4,4'-diaminodiphenylmethane (dpm). Luminescence excitation spectroscopy of the Eu3+ 7F0-->5D0 transition shows the apparent cooperative formation of neutral triple helical complexes in aqueous solution, with a [Eu2L3] stoichiometry. Eu3+ excitation peak wavelengths and excited-state lifetimes correspond to those of the [Eu(dpa)3]3- model complex. CD studies of the Nd(III) helicate Nd2(dpa-chxn-R,R)3 reveal optical activity of the f-f transitions, indicating that the chiral linking group induces a stable chirality at the metal ion center. Molecular mechanics calculations using CHARMm suggest that the delta delta configuration at the Nd3+ ion centers is induced by the chxn-R,R linker. Stability constants were determined for both ligands with Eu3+, yielding identical results: log K = 31.6 +/- 0.2 (K in units of M-4). Metal-metal distances calculated from Eu3+-->Nd3+ energy-transfer experiments show that the complexes have metal-metal distances close to those calculated by molecular modeling. The fine structure in the Tb3+ emission bands is consistent with the approximate D3 symmetry as anticipated for helicates.

The solution structure of supramolecular lanthanide triple helices revisited: application of crystal-field independent paramagnetic NMR techniques to mono- and di-metallic complexes†

Pyridine-containing tridentate binding units react with trivalent lanthanide ions, LnIII, to give C3-symmetrical nine-coordinate triple helical complexes in which subtle intramolecular interactions control the final structures. While X-ray crystal structures allow some rationalisation in the solid state, the access to solution structures by paramagnetic NMR is limited by the unpredictable variation of the crystal-field parameter along the lanthanide series. A recent technique which considers two different nuclei within the same complex overcomes this drawback and its application to triple helical lanthanide complexes of increasing sophistication is described. This new approach confirms the previously suggested geometrical change occurring in the monometallic complexes [Ln(L2)3]3+ for the heavier lanthanides. Its use in the case of the heterodimetallic triple-stranded helicates [LnCo(L4)3]6+ evidences a single isostructural series while the separation of contact and pseudo-contact contributions according to classical techniques gives intractable results. Finally, new equations are derived for homodimetallic complexes with large Ln⋯Ln separation and they are applied to the structural analysis of the triple-stranded helicates [Ln2(L5 − 2H)3]. The latter reveals that the dimetallic edifices display a single solution structure along the complete lanthanide series in contrast with a previous analysis considering invariant crystal-field parameters. The scope and limitations of this technique for supramolecular lanthanide complexes is discussed together with specific effects resulting in amplification of magnetic properties in polymetallic systems.

Lanthanide Helicates Self-Assembled in Water: A New Class of Highly Stable and Luminescent Dimetallic Carboxylates

The segmental ligand bis{1-ethyl-2-[6‘-carboxy)pyridin-2‘-yl]benzimidazol-5-yl}methane (LC) reacts with the entire series of LnIII ions to yield a new class of neutral lanthanide carboxylates, the triple-stranded dimetallic helicates [Ln2(LC-2H)3], which are stable in water in the pH range 4−13. A competitive titration with 1,4,7,10-tetraazacyclododecane-N,N‘,N‘‘,N‘‘‘-tetraacetic acid (dota) shows the EuIII helicate to have a stability comparable to that of [Eu(dota)]-. According to the X-ray structures of isotypical [Ln2(LC-2H)3]·20.5H2O (Ln = Eu, Tb) (C93H72Ln2N18O12·20.5H2O, monoclinic, P21/c, Z = 8), the helicates are comprised of two types of slightly different molecules (A, B) within the asymmetric unit. The three ligand strands are helically wrapped around the two nine-coordinate LnIII ions, leading to pseudo-D3 symmetries around the metals. The LnIII−LnIII distances lie in the ranges 8.81−8.83 A for molecules A and 9.04−9.07 A for molecules B, while the helical pitch amounts to 23.7−23.8 A (A) and...

Self-Assembled Dinuclear Lanthanide Helicates: Substantial Luminescence Enhancement upon Replacing Terminal Benzimidazole Groups by Carboxamide Binding Units.

The segmental ligands bis{1-alkyl-2-[6'-(N,N-diethylcarbamoyl)pyridin-2'-yl]benzimidazol-5-yl}methane (alkyl = methyl (L(5)), ethyl (L(6))) react with lanthanide perchlorates (Ln = La, Eu, Gd, Tb) in acetonitrile to yield the f-f dinuclear homotopic triple-stranded helicates [Ln(2)(L(i)())(3)](6+) (i = 5, 6) under thermodynamic control. The crystal structure of [Tb(2)(L(6))(3)](ClO(4))(3)(MeCN)(2)(THF)(0.5)(EtOH)(0.5) (11a, C(124)H(145)N(26)O(31)Cl(6)Tb(2), triclinic, P&onemacr;, Z = 2) shows the wrapping of the ligands about a pseudo-C(3) axis passing through the metal ions. The Tb ions are 9-coordinate in facial pseudo-tricapped trigonal prismatic sites and are separated by 9.06 Å. (1)H-NMR and ES-MS data establish that the triple helical structure is maintained in solution. Spectrophotometric titrations (Ln = La, Eu) indicate log beta(23) = 24-25 and the formation of a 2:2 complex [Ln(2)(L(5))(2)](6+) (log beta(22) = 19-20). Quantum yield determination in acetonitrile shows that the terminal N,N-diethylcarboxamide groups in L(5) favor efficient intramolecular L(5) --> Eu(III) energy transfers leading to strong Eu-centered red luminescence, 50 times as intense as the luminescence observed when the carboxamide groups are replaced by substituted benzimidazole units in [Eu(2)(L(4))(3)](6+). Resistance toward hydrolysis also results from the use of carboxamide groups, and no quenching of luminescence is observed for [Eu(2)(L(5))(3)](6+) in moist acetonitrile up to 2.5 M water. The crucial role played by carboxamide groups for the control of structural, electronic, and photophysical properties is discussed. Replacing perchlorates by triflates allows the isolation of the dinuclear double-stranded helicate [Eu(2)(L(6))(2)(CF(3)SO(3))(4)(H(2)O(2))(2)](CF(3)SO(3))(2)(MeOH)(2)(H(2)O)(5)(.5), whose crystal structure (13a, C(85)H(106)Eu(2)F(18)N(16)O(30)S(6), monoclinic, C2/m, Z = 2) reveals a side-by-side arrangement of the two strands and 9-coordinate Eu ions linked through hydrogen-bonded water molecules.

In Search for Mononuclear Helical Lanthanide Building Blocks with Predetermined Properties: Lanthanide Complexes with Diethyl Pyridine‐2, 6‐Dicarboxylate

AbstractThe ligand diethyl pyridine‐2, 6‐dicarboxylate (L5) reacts with LnIII in acetonitrile to successively give the complexes [Ln(L5)i]3+ (Ln = La to Lu, i = 1‐3). Spectroscopic investigations (ES‐MS, UV/Vis, NMR) show that the 1:3 complexes [Ln(L5)3]3+ have poor stability in solution and exist as a mixture of rapidly interconverting conformers. Variable‐temperature NMR data show that the helical P→M interconversion and dynamic on‐off equilibria of the ester side arms both control the observed average structure in solution. Contrary to similar lanthanide building blocks possessing benzimidazole or carboxamide side arms, [Eu(L5)3]3+ has a sizable quantum yield in anhydrous acetonitrile; this has been attributed to an improved ligand → EuIII energy transfer resulting from a good energetic match between the ligand‐ and metal‐centered excited states. Pure 1:3 complexes cannot be isolated in the solid state, but crystalline 1:2 complexes [Ln(L5)2](TfO)3.nH2O have been prepared. The X‐ray crystal structure of [Eu(L5)2(TfO)2(OH2)]TfO (1) reveals two meridionally tricoordinated ligands L5, but the long Eu‐O(ester) bonds imply only weak interactions between the carbonyl groups of the ester side arms and EuIII, providing a limited protection of the metallic site. The photophysical studies show that nonacoordinate EuIII in 1 binds an additional water molecule to give a decacoordinate complex in the solid state, thus confirming the accessibility of the metallic site for further complexation.

In Search for Mononuclear Helical Lanthanide Building Blocks with Predetermined Properties: Triple‐stranded Helical Complexes with N,N,N',N'‐tetraethylpyridine‐2,6‐dicarboxamide

AbstractThe ligand N,N,N',N'‐tetra‐ethylpyridine‐2,6‐dicarboxamide (L9) reacts with trivalent lanthanide ions (LnIII) to give stable mononuclear triple‐stranded helical complexes [Ln(L9)3]3+ (Ln = La to Lu). The crystal and molecular structures of [La(L9)3](ClO4)3.2.5C2H5CN (8) and [Eu(L9)3](TfO)3.2THF (9) show that the three ligand strands are each meridionally tricoordinated to produce a pseudo‐tricapped trigonal prismatic arrangement of the nine donor atoms (six amide O and three pyridine N) around LnIII. The distortions in the LaIII coordination sphere of 8 are more significant than for EuIII in 9, and the photophysical studies on Eu‐doped (2%) La, Gd, and Lu complexes confirm a better structural match of L9 for the heavy LnIII ions. The separation of contact and pseudo‐contact contributions to the induced lanthanide paramagnetic NMR shifts in [Ln(L9)3]3+ shows that the triple‐helical structure is maintained in acetonitrile, but a minor structural change relative to that observed in the solid state occurs between TbIII and ErIII leading to two distinct isostructural series for Ln = La‐Tb and Ln = Er‐Lu. The origin of this effect together with its consequences for the dynamic helical (P→M) interconversion and stability of [Ln(L9)3]3+ in solution are discussed. A detailed investigation of the emission properties of [Ln(L9)3]3+ (Ln = Eu, Tb) shows that mixed pyridine‐carboxamide ligands can be used to simultaneously optimize the structural and photophysical properties in mononuclear triple‐helical lanthanide building blocks.