In search for tuneable intramolecular intermetallic interactions in polynuclear lanthanide complexes

Abstract

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 [Ln₂(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 ¹H 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 [Eu₂(L5)(NO₃)₆(H₂O)₂] (regular helix), which puts the two metals at a contact distance of 8.564(1) Å. In solution, fast rearrangements yield an average planar extended conformation of the spacer, which increases the intramolecular intermetallic contact distance by 30% in [Ln₂(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 [Ln₂(L5)(NO₃)₆] (Ln⋯Ln ≈ 12 Å) is comparable with that found in the highly charged triple-stranded helicate [Ln₂(L5)₃]⁶⁺ (Ln⋯Ln ≈ 9 Å). The origin of this effect and its consequences on programming stable polynuclear complexes is discussed.