Search

This work illustrates the use of basic statistical mechanics for rationalizing the loading of linear multitridentate polymers with trivalent lanthanides, Ln(III), and identifies the specific ionic sizes of europium and yttrium as promising candidates for the further design of organized heterometallic f–f′ materials. Using [Ln(hfac)3] (hfac = hexafluoroacetylacetonate) as lanthanide carriers, the thermodynamically controlled formation of Wolf type-II lanthanidopolymers [{Ln(hfac)3}m(L4)] is modeled with the help of two simple microscopic descriptors: (i) the intrinsic affinity of Ln(III) for the tridentate binding sites fN3(Ln) and (ii) the intermetallic interactions ΔE1–2(Ln,Ln) operating between two occupied adjacent sites. Selective complexation (fN3La << fN3Eu > fN3(Y)) modulated by anticooperative interactions (ΔE1–2(La,La) ≃ ΔE1–2(Eu,Eu) > ΔE1–2(Y,Y) ≈ 0) favors the fixation of Eu(III) in semiorganized lanthanidopolymers [{Eu(hfac)3}m(L4)] displaying exploitable light-downshifting.

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 [M(m)(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(β(1,1)(Y,L7)) = 2.9(5) and log(β(1,2)(Y,L7)) = 5.3(4) are estimated. The complexation behaviors are supported by speciations in solution, thermodynamic analyses, and solution and solid-state structures.

The successive protonation steps occurring in 2,2′;6′,2″-terpyridine (L1) are characterized by a strong affinity for the first entering proton (ΔG(connect)(H,L1) = −17 kJ/mol) followed by allosteric anticooperativity (ΔE(interaction)(H,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; ΔG(connect)(H,L2) = −28 kJ/mol and ΔE(interaction)(H,H,L2) = 7 kJ/mol). On the contrary, 2,6-bis(8-quinolinyl)pyridine (L3) is less eager for the initial protonation (ΔG(connect)(H,L3) = −10 kJ/mol), but the fixation of a second proton in [H2L3]2+ is driven to completion by positive cooperativity (ΔE(interaction)(H,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.