Understanding, controlling and programming cooperativity in self-assembled polynuclear complexes in solution.

Deviations from statistical binding, that is cooperativity, in self-assembled polynuclear complexes partly result from intermetallic interactions DeltaE(M,M), whose magnitudes in solution depend on a balance between electrostatic repulsion and solvation energies. These two factors have been reconciled in a simple point-charge model, which suggests severe and counter-intuitive deviations from predictions based solely on the Coulomb law when considering the variation of DeltaE(M,M) with metallic charge and intermetallic separation in linear polynuclear helicates. To demonstrate this intriguing behaviour, the ten microscopic interactions that define the thermodynamic formation constants of some twenty-nine homometallic and heterometallic polynuclear triple-stranded helicates obtained from the coordination of the segmental ligands L1-L11 with Zn(2+) (a spherical d-block cation) and Lu(3+) (a spherical 4f-block cation), have been extracted by using the site binding model. As predicted, but in contrast with the simplistic coulombic approach, the apparent intramolecular intermetallic interactions in solution are found to be i) more repulsive at long distance (DeltaE(1-4)(Lu,Lu)>DeltaE(1-2)(Lu,Lu)), ii) of larger magnitude when Zn2+ replaces Lu3+ (DeltaE(1-2)(Zn,Lu)>DeltaE(1-2)(Lu,Lu) and iii) attractive between two triply charged cations held at some specific distance (DeltaE(1-3)(Lu,Lu)<0). The consequences of these trends are discussed for the design of polynuclear complexes in solution.

A simple chemical tuning of the effective concentration: selection of single-, double-, and triple-stranded binuclear lanthanide helicates.

The replacement of terminal 2-benzimidazol-6-carboxypyridine (two internal rotational degrees of freedom) with 2-benzimidazol-8-hydroxyquinoline (one internal rotational degree of freedom) into segmental bis-tridentate ligands in going from L2 and [L3-2 H](2-) to [L12 b-2 H](2-) does not significantly affect the structures of the resulting binuclear lanthanide triple-stranded helical complexes [Ln(2)(L2)(3)](6+), [Ln(2)(L3-2 H)(3)], and [Ln(2)(L12 b-2 H)(3)] (palindromic helices, intermetallic contact distance approximately 9 A, helical pitch approximately 1.4 nm per turn). However, their thermodynamic assemblies are completely different in solution, as evidenced by the spectacular decrease of the effective concentrations by two orders of magnitude for [L12 b-2 H](2-). This key parameter in the [Ln(2)(L12 b-2 H)(n)] (n=2, 3) complexes is further abruptly modulated along the lanthanide series (Ln=La to Lu), which provides an unprecedented tool for 1) tuning the number of ligand strands in the final helicates, 2) selectively coordinating lanthanides in the various complexes, and 3) controlling the ratio of lanthanide-containing polymers over discrete assemblies.

In search for tuneable intramolecular intermetallic interactions in polynuclear lanthanide complexes.

Reaction of unsymmetrical tridentate 2-benzimidazolyl-6-carboxamidopyridine binding units in the ligands (b) and with neutral Ln(NO(3))(3) (Ln is a trivalent lanthanide) gives mononuclear [Ln((b))(NO(3))(3)(solvent)] and binuclear [Ln(2)(L5)(NO(3))(6)(solvent)(2)] complexes. The crystal structures of (b) and [Eu((b))(NO(3))(3)(CH(3)CN)] unravel the conformational change of the tridentate binding units required for its coordination to the metal, a process responsible for the change in electronic absorption spectra and in (1)H NMR spectra recorded in acetonitrile solution. In the solid state, the bis-tridentate ligand shows variable helical conformations of its central diphenylmethane spacer in its uncoordinated form (amphiverse helix) and in its complexed form in [Eu(2)(L5)(NO(3))(6)(H(2)O)(2)] (regular helix), which puts the two metals at a contact distance of 8.564(1) A. In solution, fast rearrangements yield an average planar extended conformation of the spacer, which increases the intramolecular intermetallic contact distance by 30% in [Ln(2)(L5)(NO(3))(6)(H(2)O)(2)]. Surprisingly, the thermodynamic analysis of the complexation processes in solution points to unusual, and to some extent non-predicted charge effects because the intramolecular intermetallic repulsive interaction measured in the neutral complex [Ln(2)(L5)(NO(3))(6)] (Ln...Ln approximately 12 A) is comparable with that found in the highly charged triple-stranded helicate [Ln(2)(L5)(3)](6+) (Ln...Ln approximately 9 A). The origin of this effect and its consequences on programming stable polynuclear complexes is discussed.

Self-assembly of the first discrete 3d-4f-4f triple-stranded helicate.

The connection of an additional bidentate chelating unit at the extremity of a segmental bis-tridentate ligand in L5 provides an unprecedented sequence of binding sites for the self-assembly of heterometallic 3d-4f triple-stranded helicates. Thorough thermodynamic and structural investigations in acetonitrile show the formation of intricate mixtures of complexes when a single type of metal (3d or 4f) is reacted with L5. However, the situation is greatly simplified when Zn(II) (3d-block) and Lu(III) (4f-block) are simultaneously coordinated to L5, thus leading to only two identified species: the target C(3)-symmetrical trinuclear triple-stranded d-f-f helicate HHH-[ZnLu(2)(L5)(3)](8+) and a tetranuclear double-stranded complex [Zn(2)Lu(2)(L5)(2)](10+). Interestingly, the removal of Zn(II) from the former triple-helical complex has only a minor effect on the coordination of Lu(III), and translational autodiffusion coefficients show a simple reduction of the length of the molecular rigid cylinder from L = 2.7 nm in HHH-[ZnLu(2)(L5)(3)](8+) to L = 2.3 nm in HHH-[Lu(2)(L5)(3)](6+). Finally, the complete thermodynamic picture provides five novel stability macroconstants containing information about short-range (ca. 9 A) and long-range (ca. 18 A) intramolecular intermetallic d-f and f-f interactions.

Linear polynuclear helicates as a link between discrete supramolecular complexes and programmed infinite polymetallic chains.

The contribution of the solvation energies to the assembly of polynuclear helicates reduces the free energy of intermetallic repulsion, DeltaE(MM), in condensed phase to such an extent that stable D(3)-symmetrical tetranuclear lanthanide-containing triple-stranded helicates [Ln(4)(L4)(3)](12+) are quantitatively produced at millimolar concentrations, despite the twelve positive charge borne by these complexes. A detailed modelling of the formation constants using statistical factors, adapted to self-assembly processes involving intra- and intermolecular connections, provides a set of five microscopic parameters, which can be successfully used for rationalizing the stepwise generation of linear bi-, tri- and tetranuclear analogues. Photophysical studies of [Eu(4)(L4)(3)](12+) confirm the existence of two different binding sites producing differentiated metal-centred emission at low temperature, which transforms into single site luminescence at room temperature because of intramolecular energy funelling processes.

Tuneable intramolecular intermetallic interactions as a new tool for programming linear heterometallic 4f-4f complexes.

Statistical mechanics predicts that the design of pure organized heteropolymetallic chains of metal ions bound to linear receptors depends on controlled deviations from the mixing rule DeltaE(MiMj) = 1/2 (DeltaE(MiMi) + DeltaE(MjMj)), whereby DeltaE(MiMj) is the intramolecular intermetallic interaction between neighboring metal i and metal j along the receptor. A thorough investigation of linear polymetallic trivalent lanthanide triple-stranded helicates shows that such deviations are amplified by an increase in the nuclearity of the final complexes and are thus easily evidenced in the tetranuclear heterobimetallic helicates [La(4-y)Lu(y)(L6)3](12+) (y = 0-4). The chemical and physical origins of this unprecedented behavior are discussed together with its practical consequences for programming pure heteropolymetallic 4f-4f complexes.