Long-range stereocontrol in the self-assembly of two-nanometer-dimensioned triple-stranded dinuclear helicates

A series of bisimine-bridged dicatechol ligands 2-H(4)-5-H(4) were synthesized and were used to prepare triple-stranded dinuclear helicate-type complexes with a length of up to more than 2 nm. X-ray structural analyses of Na(4)[(2)(3)V(2)], Na(4)[(3)(3)Ti(2)], Na(4)[(4)(3)Ti(2)], and Na(4)[(5)(3)Ti(2)], as well as temperature-dependent NMR investigations of Na(4)[(4)(3)Ti(2)] and Na(4)[(5)(3)Ti(2)] show that, in the case of the rigid linear ligands 2 and 3, and of the ligand 5, which possesses C(2h) symmetry in its idealized structure, homochiral helicates are diastereoselectively formed. Ligand 4, on the other hand, with idealized C(2v) symmetry, leads with surprisingly high selectivity to the formation of the heterochiral meso-helicate. This is attributed to the ability of ligand 4 to adopt a less-restricted conformation in the meso compound than in the helical complex. NMR investigations indicate that both complex units of Na(4)[(4)(3)Ti(2)] invert (LambdaDelta-->DeltaLambda) simultaneously, while in the case of Na(4)[(5)(3)Ti(2)] a stepwise racemization proceeds.     

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.     

Solid-state self-assembly of polymeric double helicates leading to linear arrays of silver(I) ions and reversible strand/double helix interconversion in solution

Reaction of a bent py-hyz-pym-hyz-pym 1 and of a linear py-hyz-py-hyz-pym 3 (py=pyridine; pym=pyrimidine; hyz=hydrazone) ligand strands with silver(I) tetrafluoroborate in CH(3)NO(2) generates double-helical dinuclear 2 and trinuclear 4 complexes. These complexes form polymeric, highly ordered solid-state structures, with wirelike, linear continuous or discontinuous polycationic Ag(n) (+) arrays with Ag--Ag distances of 2.78 to 4.42 A. Ligand 5, an isomer of 1, is found to yield a [2x2] grid-type complex 6. Titration experiments reveal the formation of linear rack-type dinuclear species from 1 and 5. Acid-base modulated, reversible interconversion between strand 1 and double helicate 2 may be achieved by using tren as a competing complexing agent (tren=N(CH(2)CH(2)NH(2))(3)). Progressive addition of silver(I) ions to a 1:1 mixture of 1 and 5 leads to the preferential formation of the double helicate 2 over the grid complex 6, illustrating a process of self-organisation with selection of the correct ligand.     

Enantioselective Assembly of a Ruthenium(II) Polypyridyl Complex into a Double Helix

Evolution can increase the complexity of matter by self‐organization into helical architectures, the best example being the DNA double helix. One common aspect, apparently shared by most of these architectures, is the presence of covalent bonds within the helix backbone. Here, we report the unprecedented crystal structures of a metal complex that self‐organizes into a continuous double helical structure, assembled by non‐covalent building blocks. Built up solely by weak stacking interactions, this alternating tread stairs‐like double helical assembly mimics the DNA double helix structure. Starting from a racemic mixture in aqueous solution, the ruthenium(II) polypyridyl complex forms two polymorphic structures of a left‐handed double helical assembly of only the Λ‐enantiomer. The stacking of the helices is different in both polymorphs: a crossed woodpile structure versus a parallel columnar stacking.     

Messages in molecules: ligand/cation coding and self-recognition in a constitutionally dynamic system of heterometallic double helicates

Double helicates are known to exhibit self-recognition characteristics determined by the coordination geometry of the metal involved as well as by the topicity of the ligands. Combining tridentate (terpyridine, T) or bidentate (bipyridine, B) subunits in a tritopic strand affords a set of ligands able to assemble by pairs to form double helicates, homo- or heterostranded, homo- or heterotopic, depending on the coordination properties of the metals involved. The four ligand strands, BBB, TTT, BBT, and TBT form constitutionally dynamic sets of double helicates with the metal ions Cu(I), Cu(II), and Zn(II); these helicates correspond to the correct coding of the BB, BT, and TT pairs for tetra-, penta-, and hexacoordinate Cu(I), Cu(II), and Zn(II) cations, respectively.     

Lanthanide class of a trinuclear enantiopure helical architecture containing chiral ligands: synthesis, structure, and properties

The pinene-bipyridine carboxylic derivatives (+)- and (-)-HL, designed to form configurationally stable lanthanide complexes, proved their effectiveness as chiral building blocks for the synthesis of lanthanide-containing superstructures. Indeed a self-assembly process takes place with complete diastereoselectivity between the enantiomerically pure ligand L(-) and Ln(III) ions (La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er), thus leading to the quantitative formation of a trinuclear supramolecular architecture with the general formula [Ln(3)(L)(6)(mu(3)-OH)(H(2)O)(3)](ClO(4))(2) (abbreviated as tris(Ln[L](2))). This class of C(3)-symmetrical compounds was structurally characterized in the solid state and solution. Electrospray (ES) mass spectrometric and (1)H NMR spectroscopic analyses indicated that the trinuclear species are maintained in solution (CH(2)Cl(2)) and are stable in the investigated concentration range (10(-2)-10(-6) m). The photophysical properties of the ligand HL and its tris(Ln[L](2)) complexes were studied at room temperature and 77 K, thus demonstrating that the metal-centered luminescence is well sensitized both for the visible and near-IR emitters. The chiroptical properties of tris(Ln[L](2)) complexes were investigated by means of circular dichroism (CD) and circularly polarized luminescence (CPL). A high CD activity is displayed in the region of pi-pi* transitions of bipyridine. CPL spectra of tris(Eu[(+)-L](2)) and tris(Tb[(+)-L](2)) present large dissymmetry factors g(em) for the sensitive transitions of Eu(III) ((5)D(0)-->(7)F(1), g(em)=-0.088) and Tb(III) ((5)D(4)-->(7)F(5), g(em)=-0.0806). The self-recognition capabilities of the system were tested in the presence of artificial enantiomeric mixtures of the ligand. (1)H NMR spectra identical to those of the enantiomerically pure complexes and investigations by CD spectroscopic analysis reveal an almost complete chiral self-recognition in the self-assembly process, thus leading to mixtures of homochiral trinuclear structures.