Complexes of p-tert-butylcalix[5]arene with lanthanides: synthesis, structure and photophysical properties †

Abstract

Spectrophotometric pK a determination for p-tert-butylcalix [5]arene (H 5 L) in acetonitrile (pK a1 = 11.5 ± 0.7, pK a2 = 15.4 ± 1.0 at 298 K) evidenced both intra-and inter-molecular stabilisation of the deprotonated forms. Dimeric complexes [Ln 2 (H 2 L) 2 (dmso) 4 ] (Ln = Eu III , GdIII , or Tb III; dmso = dimethyl sulfoxide) were isolated from tetrahydrofuran (thf) in the presence of NaH as base. A single-crystal analysis of [Eu 2 (H 2 L) 2 (dmso) 4 ]ؒ 10thf showed the deformation of the cone conformation of the calixarene upon complexation and co-ordination of dmso molecules by inclusion through the hydrophobic cavity of the ligand. A photophysical investigation revealed a total quenching of the metal luminescence by a ligand-to-metal charge-transfer state in the case of Eu III while luminescence of Tb III is sensitised (quantum yield in thf: 5.1%). The temperature-dependent lifetime of Tb III is analysed in terms of a potential metal-to-ligand back-transfer process.The cyclic framework of calixarenes, 1 associated with the presence of phenol oxygen donor atoms, affords an interesting platform for the complexation of metal atoms, 2 while the hydrophobic cavity allows the inclusion of charged 3 and neutral 4 organic guests. The relative facility with which calixarenes can be partially or totally functionalised at the upper or lower rims, coupled with their easy large-scale synthesis, at least for the even members of the series (n = 4, 6 or 8), has opened further perspectives for their use in supramolecular chemistry. 5In particular, the hard acid character of the lower rim makes calixarenes interesting potential ligands for the complexation of trivalent lanthanide ions, either for extraction purposes 6 or for the design of efficient lanthanide-based luminescent devices.7-9 Antenna effects 10 can be generated directly by the phenol units or by lower or upper rim substitution.Previous work on the complexation of lanthanides with calix[n]arenes has mainly focused on the four-, six-and eightmembered systems. While p-tert-butylcalix [8] 19 who isolated water-soluble inclusion complexes between p-sulfonatocalix [5]arene with La, Gd, Eu, Tb and Yb and showed by X-ray crystallography that the interaction is either outer sphere or occurs exclusively via one sulfonato functionality, the metal centre acting often as a bridging brick. In this paper we present the isolation and the structural and photophysical properties of lanthanoid() (Ln = Eu, Gd or Tb) phenolate-bonded inner-sphere complexes with p-tert-butylcalix [5]arene (H 5 L). The reported data add valuable information on the interaction between lanthanoid() ions and p-tert-butylcalix [n]arenes and demonstrate that multiple bonds between the ligand and the metal ions can be generated only in the presence of a strong base. In addition, the spectrophotometric determination of the first two pK a values of the ligand is reported. Results and Discussion Acid-base behaviour of H 5 L in acetonitrileThe compound H 5 L was titrated at 298 K by Et 3 N in acetonitril...

Lanthanide-Containing Molecular and Supramolecular Polymetallic Functional Assemblies

“…20.6(6) 57 (3) 361 (8) calix [5]arenes 173,174 or with porphyrins and phthalocyanins 175 (but triple-decker complexes see the f-ion separation increased by the insertion of a ligand molecule between them 37,176 ), while cis-inositol L25 yields trimetallic complexes. 177 Lanthanide-templated reactions may also be used to produce polymetallic complexes, especially with cyclic Schiff bases.…”

Basics of Lanthanide Photophysics

“…where k 0 is the rate constant at 0 K (practically: at 4 K, or even at 77 K) which allows one to decipher which vibration is responsible for it; examples are presented in references [15] and [16]. The intrinsic quantum yield essentially depends on the energy gap DE between the emissive state of the metal ion and the highest sublevel of its ground, or receiving, multiplet.…”

Structural and photophysical properties of LnIII complexes with 2,2′-bipyridine-6,6′-dicarboxylic acid: surprising formation of a H-bonded network of bimetallic entities†

“…Due to the very low solubility of the complexes, the weak f-f absorption bands and, in the case of europium, the possible charge transfer band 15 could not be evidenced.…”