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.