Phosphonated Podand Type Ligand for the Complexation of Lanthanide CationsPhosphonated Podand Type Ligand for the Complexation of Lanthanide Cations

Abstract

A new tripodal ligand, based on a central nitrogen atom tris‐functionalized with 6‐methylene‐2‐pyridyl phosphonic acid was synthesized and characterized, in particular by its X‐ray crystal structure. The coordination behaviour of the tripod with lanthanide cations in aqueous solutions was studied by means of UV/Vis electronic absorption spectroscopy and steady‐state and time‐resolved luminescence spectroscopy, revealing the formation of a [LnL] complex followed by polynuclear species with 2:1 and 3:1 metal/ligand stoichiometries. The [LnL] complexes (Ln = Eu, Tb and Yb) were isolated and characterized and the solid‐state structure of the Eu complex was determined by X‐ray diffraction analysis on monocrystals, revealing the observation of dimeric species, in which the Ln3+ cations are firmly held in the cavity formed by the three pyridylphosphonate arms, the coordination of the cations being completed by a water molecule and a phosphonate function of the second complex, allowing for the formation of the dimers, which are further stabilized by π–π stacking interactions between one pyridyl unit of each adjacent monomer. The spectroscopic properties of the complexes in aqueous solutions were studied, showing an impressive 16 µs excited state lifetime for the Yb complex in D2O, despite the presence of a water molecule in the first coordination sphere.

pH‐Dependent Hydration Change in a Gd‐Based MRI Contrast Agent with a Phosphonated Ligand

“…For Gd2L,t he link with the Na + cation occurs through as ingle oxygen atom of ap hosphonate function bound to Gd2, while for Gd1 and Gd3, the neighboring Na + cation is coordinated by two O atoms of two phosphonatef unctions coordinatedt ot he Gd atom. As already observedf or other phosphonated-based Ln complexes such as the Eu [20a] or Gd [20b] complex of DOTP (DOTP = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylenephosphonic acid)), or for the Eu complex of ligand L, [10] the complexes form layers in which the GdL complexes are sandwiched between layers of Na + cations ( Figure S5 and S6). Such structures result in the presence of numerous water molecules for the solvation of the complexes on the one hand and from the formation of clusters by the counterions on the other hand.…”

“…The [LnL]c omplexes were obtained by mixinge quimolar amountso ft he ligand and the Ln salts in aqueous solutions followed by neutralization. [10] In acidic aqueous solutionc ontaining formic acid, the ES/MS spectrum of the Gd complex ( Figure S2) unambiguously confirms the presence of the GdL complex with peaks at m/z 685.96 and 707.95 units having the perfect expected isotopicd istribution for [Gd(L-3 H)H] + and [Gd(L-3 H)Na] + ( Figure S3). In addition, one can also notice the presenceo fp eaks corresponding to monocharged dimeric speciesa t1 370.91 (attributed to [(Gd(L-3 H)) 2 H] + )a nd 1391.90 m/z units (attributed to [(Gd(L-3 H)) 2 Na] + )a sw ell as a doubly charged dimer at 696.96 ([(Gd(L-3 H)) 2 HNa] 2 + )a nd peaks corresponding to [(Gd(L-3 H)) 2 H 2 ] 2 + and [(Gd(L-3H)) 2 Na 2 ] 2 + partly hidden by the signals of the corresponding monomers, but faintly evidenced by the appearance of peaks separated by 0.5 m/z units ( Figure S4).…”

“…Eu, Tb and Yb complexes were synthesized according to reported procedures. [10] Full experimental details concerning the synthesis of the Gd and Lu complexes, potentiometric and spectrophotometric studies, crystallography,D FT and ab initio calculations and NMR measurements can be found in the Supporting Information, with additional data concerning UV/Vis spectra of L;E S/MS characterization of GdL;s tacking interactions in GdL;E volution of the lifetime of the Eu complex in H 2 Oa nd D 2 Oa safunction of pH (pD);2 D-DOSY spectra of LuL at different pH;p lot of the observed paramagnetic shift of YbL and splitting of the Kramer's doublet in GdL (10 Figures);C rystal data and structure refinement for the TbL and GdL complexes;C alculated bond distances ()o ft he metal coordination environments of the Gd complex; 1 Ha nd 31 PNMR shifts (ppm) for YbL and LuL complexes;O ptimized Cartesian coordi-…”

“…Ta ble 2s ummarizes some of the main characteristics of the LnL complexes (Ln = Gd and Tb, this work, Ln = Eu ref. [10]). Considering that the refinement process of the structures introduces the possible squeezing of some Ha toms of the water molecules, some hydroxide anionsm ay be hidden by the fitting procedure and it is quite difficult to ascertain the charge of the complexes.…”

“…[7] However,i tw as soon recognized that the coordination behavior of phosphonated ligands, and particularly 2-pyridine-phosphonic acid, [8a-c] may be affectedb yt he formation of polynuclear speciesi ns olution, [8d-f] and this could be advantageously used to engineer the formation of complex heteropolynuclear edifices. [9] In an effort to design new polyphosphonated ligandsf or Ln coordination,w er ecently reported the synthesis of ligand L (Scheme1), [10] at ripodal ligand based on three 6-phosphono-2-methylene-pyridyl arms grafted on ac entral Na tom. Ligand L forms very stable [LnL]c omplexes with lanthanide cationsi n aqueous solutions, with additional polynuclear species in the presenceo fa ne xcesso fc ations.…”