Lanthanide complexes of 2-hydroxynicotinic acid: synthesis, luminescence properties and the crystal structures of [Ln(HnicO)₂(μ-HnicO)(H₂O)] · nH₂O (Ln=Tb, Eu)

Abstract

New lanthanide complexes of 2-hydroxynicotinic acid (H₂nicO) [Ln(HnicO)₂(μ-HnicO)(H₂O)] · nH₂O (Ln=Eu, Gd, Tb, Er, Tm) were prepared. The crystal structures of the [Tb(HnicO)₂(μ-HnicO)(H₂O)] · 1.75H₂O (1) and [Eu(HnicO)₂(μ-HnicO)(H₂O)] · 1.25H₂O (2) complexes were determined by X-ray diffraction. The 2-hydroxynicotinate ligand coordinates through O,O-chelation to the lanthanide(III) ions as shown by X-ray diffraction and the infrared, Raman and NMR spectroscopy results. Photoluminescence measurements were performed for the Eu(III) and Tb(III) complexes. Lifetimes of 0.592 ± 0.007 and 0.113 ± 0.002 ms were determined for the Eu³⁺ and Tb³⁺ emitting states, ⁵D₀ and ⁵D₄, respectively. A value around 30% was found for the ⁵D₀ quantum efficiency. The energy transfer mechanisms between the lanthanide ions and the ligands are discussed and compared with those observed in similar complexes involving the 3-hydroxypicolinate ligand based on the luminescence of the respective Gd³⁺-based complexes.

Introduction

In the last few years, there has been intense research on the synthesis of photoactive lanthanide complexes [1], [2], [3]. Although trivalent lanthanide ions have weak absorption coefficients, in some cases highly luminescent lanthanide complexes have been prepared by chelating strongly absorbing organic ligands to the lanthanide ions [4], [5]. In such cases a synergistic effect occurs between the ligand and the lanthanide ion. This is often referred as the antenna-effect: the absorbed energy by the ligand, which has a broad intense absorption band, is transferred intra-molecularly to the lanthanide ion, emitting part of this energy within a narrow-wavelength region [3]. The possibility of having ligands that function as light-harvesting units in lanthanide complexes has been investigated [4], [5]. We have been particularly interested in the study of the photoluminescence behaviour of these type of lanthanide complexes associated to the study of the coordination modes of aromatic N and O donor ligands, such as 3-hydroxypicolinic acid [6], picolinic acid [7] and 2,6-dihydroxybenzoic acid [8]. Other lanthanide complexes with aromatic ligands containing nitrogen and oxygen donors have been reported, such as with nicotinate [9], [10], picolinate [11], dipicolinates [12], 2,2^′-bipyridine-6,6^′-dicarboxylate [13] and bis-pyridones [14].

The 2-hydroxynicotinic acid (H₂nicO) ligand used in the present work is a potential chelating ligand with different chelating modes [15] including N,O-chelation (of the monodeprotonated ligand, HnicO⁻, through the pyridine nitrogen and the deprotonated phenolate oxygen, forming a four-membered chelate ring) or O,O-chelation (through the carboxylate group and the deprotonated phenolate oxygen, forming a six-membered chelate ring, salicylato type chelation [16], [17], either of the twice deprotonated ligand, nicO²⁻, or, as found in this work, of the monodeprotonated HnicO⁻ ligand with a protonated pyridinic nitrogen). The ketonic tautomer of H₂nicO is stabilized by resonance and exists predominantly in a zwitterionic form (Scheme 1) in solutions at pH range 3–10.5 [18].

Few complexes of H₂nicO have been isolated, in an earlier study on second- and third-row transition metal complexes we reported the synthesis and characterization of solid Pd(II), Pt(II), Re(V), Mo(VI) and W(VI) complexes [15]. The crystal structures of the compounds trans-[PdCl(HnicO)(PPh₃)₂] · 0.75CH ₃CN and cis-[PtCl(HnicO)(PPh₃)₂] · 0.75CH₃OH · 0.5H₂O show the HnicO⁻ ligand coordinated to palladium or platinum through the nitrogen atom only [15]. Solution studies were reported for gallium(III) complexes [19], in which the 2-hydroxynicotinate ligand is bound by the deprotonated phenolate and carboxylate oxygens, the pyridine nitrogen being protonated. Other solution studies include complexes of V(IV) [20], Al(III) [21], [22], Cu(II) [23], [24] and Hg(II) [25].

Here the synthesis of the lanthanide(III) complexes [Ln(HnicO)₂(μ-HnicO)(H₂O)] · nH₂O (Ln=Eu, Gd, Tb, Er, Tm) is reported together with their characterization using vibrational and NMR spectroscopy. The chelation modes of the ligand to the different lanthanides are discussed on the basis of the X-ray crystal structures of [Tb(HnicO)₂(μ-HnicO)(H₂O)] · 1.75H₂O (1) and [Eu(HnicO)₂(μ-HnicO)(H₂O)] · 1.25H₂O (2), and spectroscopic results, and compared to our previous published data for analogous compounds [6]. The crystal structures of the Tb(III) and Eu(III) complexes, prove conclusively that the 2-hydroxynicotinate functions as a salicylato type ligand with O,O-chelation in the solid metal complexes. Spectroscopic data are consistent with the Gd(III), Er(III) and Tm(III) complexes having the same type of coordination. The local environment of the lanthanide ions in these compounds is further discussed on the basis of their luminescence properties.