In search for tuneable intramolecular intermetallic interactions in polynuclear lanthanide complexes

Reaction of unsymmetrical tridentate 2-benzimidazolyl-6-carboxamidopyridine binding units in the ligands L4(b) and L5 with neutral Ln(NO3)(3) (Ln is a trivalent lanthanide) gives mononuclear [Ln(L4(b))(NO3)(3)(solvent)] and binuclear [Ln(2)(L5)(NO3)(6)(solvent)(2)] complexes. The crystal structures of L4(b) and [Eu(L4(b))(NO3)(3)(CH3CN)] 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 H-1 NMR spectra recorded in acetonitrile solution. In the solid state, the bis-tridentate ligand L5 shows variable helical conformations of its central diphenylmethane spacer in its uncoordinated form (amphiverse helix) and in its complexed form in [Eu-2(L5)(NO3)(6)(H2O)(2)] (regular helix), which puts the two metals at a contact distance of 8.564(1) angstrom. 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)(NO3)(6)(H2O)(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)(NO3)(6)] (Ln center dot center dot center dot Ln approximate to 12 angstrom) is comparable with that found in the highly charged triple-stranded helicate [Ln(2)(L5)(3)](6+) (Ln center dot center dot center dot Ln approximate to 9 angstrom). The origin of this effect and its consequences on programming stable polynuclear complexes is discussed.