Protonation and Complexation Properties of Polyaromatic Terdentate Six-Membered Chelate Ligands

The successive protonation steps occurring in 2,2′;6′,2″-terpyridine
(L1) are characterized by a strong affinity for the first
entering proton (ΔGconnectH,L1 = −17 kJ/mol)
followed by allosteric anticooperativity (ΔEinteractionH,H,L1 = 6 kJ/mol), a behavior mirrored by 2,6-bis­(azaindolyl)­pyridine
(L2) despite the extension of the chelate ring size from
five members (L1) to six members (L2; ΔGconnectH,L2 = −28 kJ/mol and ΔEinteractionH,H,L2 = 7 kJ/mol). On the contrary, 2,6-bis­(8-quinolinyl)­pyridine
(L3) is less eager for the initial protonation (ΔGconnectH,L3 = −10 kJ/mol), but the fixation of
a second proton in [H2L3]2+ is
driven to completion by positive cooperativity (ΔEinteractionH,H,L3 = −5 kJ/mol). Because of its unusual ability
to adopt a cis–cis conformation with a large affinity for the
entering protons, L2 has been selected for exploring
the reactivity of a terdentate fused six-membered chelate with labile
metallic cations possessing increasing electrostatic factors along
the series Mz+ = Li+ < Mg2+ ≈ Zn2+ < Y3+. Spectroscopic, thermodynamic, and structural studies demonstrate
that covalency is crucial for stabilizing the complexes [Zn­(L2)n]2+.
With the highly charged Y3+ cation, hydrolysis drastically
competes with ligand complexation, but anhydrous conditions restore
sufficient selectivity for the successful coordination of neutral
fused six-membered polyaromatic terdentate chelates with large 4f-block
cations.