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

The segmental tris-tridentate ligand L7 reacts with stoichiometric quantities of Ln(III) (Ln=La-Lu) in acetonitrile to give the complexes [Ln(2)(L7)(3)](6+) and [Ln(3)(L7)(3)](9+). Formation constants point to negligible size-discriminating effects along the lanthanide series, but Scatchard plots suggest that the self-assembly of the trimetallic triple-stranded helicates [Ln(3)(L7)(3)](9+) is driven to completion by positive cooperativity, despite strong intermetallic electrostatic repulsions. Crystallization provides quantitatively [Ln(3)(L7)(3)](CF(3)SO(3))(9) (Ln=La, Eu, Gd, Tb, Lu) and the X-ray crystal structure of [Eu(3)(L7)(3)](CF(3)SO(3))(9).(CH(3)CN)(9).(H(2)O)(2) (Eu(3)C(216)H(226)N(48)O(35)F(27)S(9), triclinic, P1, Z=2) shows the three ligand strands wrapped around a pseudo-threefold axis defined by the three metal ions rigidly held at about 9 A. Each metal ion is coordinated by nine donor atoms in a pseudo-trigonal prismatic arrangement, but the existence of terminal carboxamide units in the ligand strands differentiates the electronic properties of the terminal and the central metallic sites. Photophysical data confirm that the three coordination sites possess comparable pseudo-trigonal symmetries in the solid state and in solution. High-resolution luminescence analyses evidence a low-lying LMCT state affecting the central EuN(9) site, so that multi-metal-centered luminescence is essentially dominated by the emission from the two terminal EuN(6)O(3) sites in [Eu(3)(L7)(3)](9+). New multicenter equations have been developed for investigating the solution structure of [Ln(3)(L7)(3)](9+) by paramagnetic NMR spectroscopy and linear correlations for Ln=Ce-Tb imply isostructurality for these larger lanthanides. NMR spectra point to the triple helical structure being maintained in solution, but an inversion of the magnitude of the second-rank crystal-field parameters, obtained by LIS analysis, for the LnN(6)O(3) and LnN(9) sites with respect to the parameters extracted for Eu(III) from luminescence data, suggests that the geometry of the central LnN(9) site is somewhat relaxed in solution

The Solution Structure of Rhombic Lanthanide Complexes Analyzed with a Model-Free and Crystal-Field Independent Paramagnetic NMR Method

The model-free approach has been extended with the derivation of a novel three-nuclei crystal-field independent method for investigating isostructurality in nonaxial (i.e., rhombic) complexes along the lanthanide series. Application of this technique to the heterotrimetallic sandwich complexes [LnLu2(TACI-3H)2(H2O)6]3+, which possess a single C2v-symmetrical paramagnetic center, unambiguously evidences isostructurality for Ln = Pr−Yb, while the variation of the second-rank crystal-field parameters and along the series prevents reliable structural analyses with the classical one-nucleus equation. Extension toward polymetallic magnetically noncoupled rhombic lanthanide complexes in [Ln2Lu(TACI-3H)2(H2O)6]3+ (two paramagnetic centers with Cs microsymmetry) and [Ln3(TACI-3H)2(H2O)6]3+ (three paramagnetic centers with C2v microsymmetry) requires only minor modifications of the original three-nuclei equation. Isostructurality characterizes [Ln2Lu(TACI-3H)2(H2O)6]3+ (Ln = Pr−Yb), while [Ln3(TACI-3H)2(H2O)6]3+ exhibit a structural change between Eu and Tb which results from the concomitant contraction of the three metallic centers. Particular attention has been focused on (i) the stepwise increase of contact (i.e., through-bond) and pseudocontact (i.e., through-space) contributions when the number of paramagnetic centers increases, (ii) the assignment of 13C resonances in the strongly paramagnetic complexes [Ln3(TACI-3H)2(H2O)6]3+ (Ln = Tb−Yb) for which reliable T1 measurements and {1H−13C} correlation spectra are not accessible, and (iii) the combination of crystal-field dependent and independent methods for analyzing the paramagnetic NMR spectra of axial and nonaxial lanthanide complexes

The solution structure of homotrimetallic lanthanide helicates investigated with novel model-free multi-centre paramagnetic NMR methods

The combination of one contact and three pseudo-contact contributions to the NMR hyperfine paramagnetic shift of each proton in the triple-stranded helicates [Ln3(L1)3]9+(Ln = Ce–Yb except Pm, Gd) produce intractable 1H NMR spectra whose assignment is limited by the large electronic contribution to the nuclear relaxation processes. The detailed analysis of the NMR spectra for the diamagnetic complexes [Ln3(L1)3]9+(Ln = La, Lu, Y) shows that the triple-helical structure found in the solid state is maintained in solution. Extension of the classical one-nucleus crystal-field dependent model-free method for paramagnetic D3-symmetrical homotrimetallic lanthanide complexes possessing two different metallic sites (i.e. two second-rank crystal-field parameters: B20central and B20terminal) allows (i) the complete interpretation of the paramagnetic signals for Ln = Ce–Yb and (ii) the detection of a concomitant abrupt change of the contact terms Fi and of the pseudo-contact terms Si=B20centralG1i+B20terminal(G2i+G3i) occurring near the middle of the lanthanide series. The derivation and application of a novel three-nuclei crystal-field independent method eventually demonstrates that the helicates [Ln3(L1)3]9+ adopt a single D3-symmetrical structure along the complete lanthanide series in solution, which ascribes the discontinuity observed for Si to a concomitant decrease of the two crystal-field parameters. Comparison with structural models is limited by the extreme sensitivity of the structural factors Cikl and Dikl to minor geometrical variations affecting the wrapping of the ligand strands, but calculations of the geometrical factors Gmi(m= 1–3) for [Ln3(L1)3]9+ in solution confirm the formation of a regular triple-helical structure. Generalization of this novel three-nuclei method for addressing the solution structure of rhombic lanthanide complexes is discussed

Analysis of paramagnetic NMR spectra of triple-helical lanthanide complexes with 2,6-dipicolinic acid revisited : a new assignment of structural changes and crystal-field effects 25 years later

Variable-temperature 1H and 13C NMR measurements of the D3-symmetrical triple-helical complexes [Ln(L1-2H)3]3- (L1 = pyridine-2,6-dicarboxylic acid; Ln = La−Lu) show evidence of dynamic intermolecular ligand-exchange processes whose activation energies depend on the size of the metal ion. At 298 K, the use of diastereotopic probes in [Ln(L3-2H)3]3- (L3 = 4-ethyl-pyridine-2,6-dicarboxylic acid) shows that fast intramolecular P M interconversion between the helical enantiomers occurs on the NMR time scale. Detailed analyses of the paramagnetic NMR hyperfine shifts according to crystal-field independent techniques demonstrate the existence of two different helical structures, one for large lanthanides (Ln = La−Eu) and one for small lanthanides (Ln = Tb−Lu), in complete contrast with the isostructurality proposed 25 years ago. A careful reconsideration of the original crystal-field-dependent analysis shows that an abrupt variation of the axial crystal-field parameter r 2 parallels the structural change leading to some accidental compensation effects that prevent the detection of structural variations according to the classical one-nucleus method. Crystal structures in the solid state and density functional theory calculations in the gas phase provide structural models that rationalize the paramagnetic NMR data. A regular triple-helical structure is found for small lanthanides (Ln = Tb−Lu) in which the terdentate chelating ligands are rigidly tricoordinated to the metals. A flexible and distorted structure is evidenced for Ln = La−Eu in which the central pyridine rings interact poorly with the metal ion. The origin of the simultaneous variation of structural parameters and crystal-field and hyperfine constants near the middle of the lanthanide series is discussed together with the use of crystal-field-independent techniques for the interpretation of paramagnetic NMR spectra in axial lanthanide complexes

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