N‐Heterocyclic Tridentate Aromatic Ligands Bound to [Ln(hexafluoroacetylacetonate)₃] Units: Thermodynamic, Structural, and Luminescent Properties

Abstract

Herein, we discuss how, why, and when cascade complexation reactions produce stable, mononuclear, luminescent ternary complexes, by considering the binding of hexafluoroacetylacetonate anions (hfac(-)) and neutral, semi-rigid, tridentate 2,6-bis(benzimidazol-2-yl)pyridine ligands (Lk) to trivalent lanthanide atoms (Ln(III)). The solid-state structures of [Ln(Lk)(hfac)(3)] (Ln=La, Eu, Lu) showed that [Ln(hfac)(3)] behaved as a neutral six-coordinate lanthanide carrier with remarkable properties: 1) the strong cohesion between the trivalent cation and the didentate hfac anions prevented salt dissociation; 2) the electron-withdrawing trifluoromethyl substituents limited charge-neutralization and favored cascade complexation with Lk; 3) nine-coordination was preserved for [Ln(Lk)(hfac)(3)] for the complete lanthanide series, whilst a counterintuitive trend showed that the complexes formed with the smaller lanthanide elements were destabilized. Thermodynamic and NMR spectroscopic studies in solution confirmed that these characteristics were retained for solvated molecules, but the operation of concerted anion/ligand transfers with the larger cations induced subtle structural variations. Combined with the strong red photoluminescence of [Eu(Lk)(hfac)(3)], the ternary system Ln(III)/hfac(-)/Lk is a promising candidate for the planned metal-loading of preformed multi-tridentate polymers.

Chemical Potential of the Solvent: A Crucial Player for Rationalizing Host–Guest Affinities

“…(6) predicts some obvious deviations from the Langmuir isotherm as soon as the product HG jj eq c q ÀÁ DG S is not negligible and thus varies within the "useful dynamic range" of the target guest. [24] The increase in lipophilicity of the host in going from L1 to L3 is expected to facilitate the inter- Ca EDTA ðÞ jj eq Ca jj eq EDTA jj unbound tot monitored for the complexationofE DTAwith Ca 2 + in aqueousb uffer at pH 6.2 and 298 K. [18] q Ca ¼ Ca jj bound = EDTA jj tot corresponds to the occupancy factor. (6) casts some doubts on al arge part of the intrinsic affinities andc ooperativity factorsd educed from standard analyses of the Langmuir binding isotherms in biology and in chemistry (Eqs.…”

“…2017, 23,16787 -16798 www.chemeurj.org 2017 Wiley-VCHV erlag GmbH &Co. KGaA, Weinheim pretation of free energy change DG S produced by differential surfacesolvation according to Eq. The tridentate ligand 2,6-bis-(1-isopentyl-5-bromo-benzimidazol-2-yl)pyridine (1), [24] 3-nitro-4-isopentylamino-1-bromobenzene (5) [24] and [La(hfa) 3 (diglyme)] [26] were prepared according to literature procedures. [23c] Experimental Section Chemicals were purchased from Sigma-Aldrich and Acros and used without further purification unless otherwise stated.…”

Synthesis, Structure, Photoluminescence, and Electroluminescence of Four Europium Complexes: Fabrication of Pure Red Organic Light‐Emitting Diodes from Europium Complexes

“…Furthermore, the complexes also exhibit wider absorption windows than does tta, with the red edge extending up to 400 nm This is mainly due to the polarization of the Eu 3+ ion, which results, in part, from the ligand‐to‐metal charge transfer, thereby decreasing the energy gap. Nevertheless, according to recent studies on [Ln( L )(hfac) 3 ], the association constants are in the range 4 < log β < 6 in aprotic solvents . Therefore, the electronic spectra (Figures and ) recorded in dilute solutions might correspond to the photophysical data of the Eu( L )(hfac) 3 adduct and adduct decomplexation.…”

“…Of these, the development of N‐heterocyclic tridentate aromatic ligands is particularly interesting. Several studies of these ligands report the criteria for the formation of stable luminescent complexes by binding N‐heterocyclic tridentate aromatic ligands to Ln(hfac) 3 by cascade complexation (hfac = hexafluoroacetylacetonate) . More recently, N‐heterocyclic tridentate ligands have been used as the building units of multi‐tridentate polymers for loading Ln(hfac) 3 .…”

One‐step synthesis of a thienylenepyridazinylenethienylene‐based coil‐rod‐coil copolymer with enhanced emission and improved fluorescence stability

“…Heterocyclic aromatic structures always displayed unique features in photochemistry and photophysics, coordination chemistry, self-assembly, and so forth. [5][6][7] Pyridazine, a sixmember aromatic ring with two nitrogen atoms neighboring to each other, has been widely studied as the ligand in preparing the many metal complexes as well as the supramolecular metal coordinating units. 6,[8][9][10][11][12][13] In addition, pyridazine is often used as a structural component of compounds for biorelated studies, such as antinociceptive drugs, 14 antibacterial agents, 15 anti-inflammatory compounds, 16 and antihypertensive medicines.…”