A Polyaromatic Terdentate Binding Unit with Fused 5,6-Membered Chelates for Complexing s‑, p‑, d‑, and f‑Block Cations

The
polyaromatic terdentate ligand 6-(azaindol-1-yl)-2,2′-bipyridine
(L7) combines one 5-membered chelate ring with a fused
6-membered chelate ring. It is designed to provide complexation properties
intermediate between 2,2′;6′,2″-terpyridine (L1) (two fused 5-membered chelate rings) and 2,6-bis­(azaindol-1-yl)­pyridine
(L6) (two fused 6-membered chelate rings). In polar organic
solvents, L7 displays remarkable affinities for the successive
fixation of two small univalent cations M = H+ or Li+, leading to stable [Mm(L7)]m+ (m = 1–2)
complexes. Upon reaction with M = Mg2+ or Zn2+ cations, the large positive charge densities borne by the metals
result in the successive cooperative complexation of two ligands to
give [M­(L7)n]n+ (n = 1–2). For small Sc3+, unavoidable traces of water favor the formation of the
protonated ligand at millimolar concentrations in acetonitrile, but
the use of larger Y3+ cations leads to [Y­(L7)n]n+ (n = 1, 2), for which stability constants of log­(β1,1Y,L7) = 2.9(5) and log­(β1,2Y,L7) = 5.3(4) are estimated. The
complexation behaviors are supported by speciations in solution, thermodynamic
analyses, and solution and solid-state structures.