Stoichiometry of lanthanide( III ) complexes with tripodal aminophosphonic ligands – a new solution to an old problem †

The Eu 3+ and Gd 3+ complexes with an N -(methylene-2-pyridine)- N , N -di(methylenephosphonate) ligand (H 4 NP 2 py), an analogue of nitrilotri(methylphosphonic) acid (H 6 NTP), were synthesized and structurally characterized by X-ray single crystal di ﬀ raction. The determined crystal structures ([C(NH 2 ) 3 ] 5 [Ln (NP 2 py) 2 ]·12H 2 O) are the ﬁ rst example of a monomeric Ln 3+ complex encapsulated by two tripodal aminophosphonic ligands. Each of the NP 2 py anions coordinates to Ln 3+ through two oxygen atoms from each monodentate phosphonic group, amine nitrogen and pyridine nitrogen atoms, ﬁ lling thus 8 coordination sites of Ln 3+ . The luminescence properties of [C(NH 2 ) 3 ] 5 [Eu(NP 2 py) 2 ]·12H 2 O crystals were studied and compared with those of Eu – NP 2 py complexes in solution. Speciation analysis of Ln – NP 2 py complexes (Ln : NP 2 py = 1 : 2), performed by luminescence and potentiometric methods, showed that both [Ln(NP 2 py)] − and [Ln(NP 2 py) 2 ] 5 − species may exist in solution. However, the formation of the latter one occurs in alkaline solutions at pH as high as 8. By implementing the Speci ﬁ c Ion Interaction Theory (SIT) it was possible to calculate the thermodynamic stability constants of the [Eu(NP 2 py)] − and [Eu(NP 2 py) 2 ] 5 − complexes. The corresponding log β 0Eul and log β 0EuL2 values are 16.3 ± 0.11 and 19.5 ± 0.15, respectively.


Introduction
The chemistry of metal-phosphonates has attracted increasing attention over the last few decades due to the variety of pro-perties that can be introduced via a phosphonic moiety. 1 (Poly) aminopolyphosphonates are very strong complexing agents for alkaline earth, transition and rare earth metal ions. 2 Owing to the high basicity of the oxygen atoms, 3,4 phosphonates interact with cations more strongly than their carboxylic analogues and are able to create more stable complexes. 5,6 Phosphonate complexes with luminescent, paramagnetic lanthanide ions 7,8 play a significant role as promising phosphors for new technologies and potential contrast agents in magnetic resonance imaging (MRI) 9 as well as paramagnetic shift reagents for biologically important cations (e.g. Na + ). 10 Owing to its good pharmacokinetics, biodistribution and bone-seeking properties the 153 Sm-EDTP complex, 1a,6e-h known as QUADRAMET®, was approved by the US FDA for use of painful skeletal metastases in humans. 1d, 11 In general, polyaminopolyphosphonic acids are the analogues of the nitrilotris(methylenephosphonic) acid (hereinafter referred to as NTP). To date the NTP ligand has found many applications 12 driven in many cases by coordination properties of this ligand in Ln 3+ complexes. 13 Because the majority of its applications are related to the NTP properties in aqueous solution, it seems particularly important to study its properties in this medium. However, the formation of the Ln 3+ complexes with ligands of the NTP type in solution remains still not fully understood. For example it is unclear whether bis(nitrilotris(methylenephosphonato))lanthanide complexes are † Electronic supplementary information (ESI) available: Table 1S. The fitting parameters for eqn (3), determined from the spectra of the 5 D 0 → 7 F 1,2,4 transitions of the [Eu(NP 2 py) 2 ] 5− complex in aqueous solution; Table 2S. Selected luminescent data of Eu-NP 2 py and Tb-NP 2 py systems; Table 3.The complex formation constants of NP 2 Py with Sm 3+ , Eu 3+ , Gd 3+ and Tb 3+ at 25°C, I = 0.1 mol dm −3 (NaClO 4 ), in 1 : 1 and 1 : 2 molar ratios Ln : L; Table 4S. Stability constants of NP 2 Py with Eu 3+ at 25°C in different ionic strengths (KCl); Fig. 1S. Phosphorescence spectrum of [C(NH 2 ) 3 ] 5 [Gd(NP 2 py) 2 ]·12H 2 O crystals at 77 K; Fig. 2S. The plot of luminescence quantum yields of Tb : NP 2 py 1 : 1 and 1 : 2 systems versus pH; Fig. 3S. UV spectra of the NP 2 py ligand and Tb : NP 2 py complex at various pH; Fig. 4S. Plot of q H2O versus pH of solution for the Eu-NP 2 py complex with Eu : NP 2 py ratios of 1 : 1 and 1 : 2. The Δq H2O is the difference between q H2O for 1 : 1 and 1 : 2 solutions at the same pH; Fig. 5S. Plot of q H2O versus pH of solution for the Tb-NP 2 py complex with Tb : NP 2 py ratios of 1 : 1 and 1 : 2; Fig. 6S. Species distribution curves of H 6 NP 2 py acid (L). Charges omitted for brevity; Fig. 7S. Species distribution curves for the Ln 3+ -N 2 Py complexes as a function of pH for a 1 : 1 Ln : L molar ratio (A) and 1 : 2 Ln : L (B). Equimolar complexes are marked in blue, biscomplexes in red color, I = 0.1 M NaClO 4 , 25°C. CCDC 1541894 and 1541895. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7qi00191f formed in solution at all. [14][15][16] Moreover no crystal structures of monomeric Ln 3+ complexes with tripodal aminophosphonic ligands have been reported so far. The high negative charge of the NTP ligand (−6) and the presence of rather bulky phosphonic groups are probably the main factors destabilizing the [Ln(NTP) 2 ] 9− complexes in solution.
The aim of this paper is to study the chemical properties of a modified NTP ligand -N-(methylene-2-pyridine)-N,N-di (methylenephosphonic) acid (hereinafter referred to as NP 2 py -Scheme 1) to establish finally whether, [Ln(NP 2 py) 2 ] 5− complexes may be indeed formed in solution. 17 We have focused our attention on the stoichiometry and stability of species in the solution of the Ln-NP 2 py system by implementing various physical methods such as X-ray crystallography, luminescence spectroscopy, potentiometry as well as a theoretical approach based on Specific Ion Interaction Theory (SIT). 18 Because coordination of aminopolyphosphonates to Ln 3+ cations has not been completely clear, we believe that the presented data may be helpful in understanding the interaction of this important class of ligands with lanthanide ions in solution, in general.

Crystal structures
The crystal structures of compounds of the following formula [C(NH 2 ) 3 ] 5 [Ln(NP 2 py) 2 ]·12H 2 O (Ln = Eu or Gd) were determined. The crystals of both compounds are isostructural and belong to the P1 space group. The asymmetric unit comprises one [Ln(NP 2 py) 2 ] 5− complex anion, five guanidinium cations out of which one is disordered and twelve water molecules out of which two are disordered. The monomeric [Ln(NP 2 py) 2 ] 5− complex anions are completely deprotonated and their negative charge is compensated by [C(NH 2 ) 3 ] + cations.
Two anionic [NP 2 py] 4− ligands are bound to the Ln 3+ cation. Each of them is linked with the Ln 3+ by two oxygen atoms from both monodentate phosphonic groups, one tertiary nitrogen atom and one pyridine nitrogen atom, filling thus four coordination sites of the Ln 3+ cation. The molecular structure of the [Eu(NP 2 py) 2 ] 5− complex is presented in Fig. 1. The selected Ln-L bond lengths are presented in Table 1.
As is seen the Eu-L and Gd-L bond lengths are similar within the error of bond length estimation. The Ln-L bond lengths increase in the order Ln-O < Ln-N ( py) < Ln-N (am) which reflects the sequence of ligand donor properties. 19 The coordination polyhedron of the Ln 3+ cation may be described as a distorted prism (Fig. 2). The Ln 3+ cation lies in the general position, but the approximate symmetry of the complex anion is C i , as shown in Fig. 2. The [Ln(NP 2 py) 2 ] 5− complexes are held together by a network of hydrogen bonds and electrostatic interactions. Owing to the high negative charge of the anionic [Ln(NP 2 py) 2 ] 5− complex the distance between the neighbouring anions is large and the anionic complexes are well separated from each other; the shortest Eu⋯··Eu' and Gd⋯··Gd' distances are 10.47(7) Å and 10.37(7), respectively.    Fig. 3.
Narrow peaks attributed to the f-f transitions of the Eu 3+ and broad band of the ligand ( 1 π → 1 π*, 1 n → 1 π*) are observed in the spectrum. The presence of the latter band indicates that the energy transfer from the ligand to the Eu 3+ states occurs.  Fig. 4. The emission intensities of the 5 D 0 → 7 F J transitions in relation to the 5 D 0 → 7 F 1 one are also included in this figure.
Five bands attributed to the electronic transitions from the excited 5 D 0 to the lower lying 7 F 0,1,2,3,4 states are observed in the spectrum. All these transitions are induced electric dipole (ED) transitions, except for the 5 D 0 → 7 F 1 , which is of magnetic dipole (MD) character. Thus the 5 D 0 → 7 F 1 transition may be used as a reference since its intensity does not depend on the chemical surrounding of Eu 3+ . 21 The 5 D 0 → 7 F 0 and 5 D 0 → 7 F 3 transitions are forbidden due to the selection rules. For this reason, the intensities of these transitions are usually very low as compared with those of 5 D 0 → 7 F 1,2,4 transitions. Since the ground 7 F 0 and the emitting 5 D 0 states are nondegenerate, the 5 D 0 ↔ 7 F 0 transitions consist of a single unsplit line for the given Eu 3+ surrounding. Therefore there is a one to one correspondence between the number of lines in the spectrum of this transition and the number of chemically distinct environ-ments of the Eu 3+ ion. The energy levels of the 7 F 1,2,3,4 multiplets and their crystal field (e) as well as vibronic (v) components found in the RT spectra are presented in Table 2.
Because 5 D 0 → 7 F 0,2,3,4 transitions are strictly forbidden in centrosymmetric systems, the presence of these transitions in the luminescence spectrum indicates that the C i symmetry of the [Eu(NP 2 py) 2 ] 5− complex in the crystal is approximate. The determined luminescence lifetime of the 5 D 0 state at RT is 1840 μs.
The next step of our studies was the analysis of the spectroscopic properties of the Eu-NP 2 py complex in aqueous solution.
Luminescence spectra of the Eu-NP 2 py and Tb-NP 2 py systems in solution Ln-NP 2 py complexes with the metal to ligand ratio 1 : 1. Taking into account Eu-L bond lengths it may be reasonably assumed that the encapsulation of the Ln 3+ cation by the NP 2 py ligand is achieved by the coordination of phosphonic and pyridine groups. The binding of the N(imino) atom to the   Table 2 The energy of the crystal field and vibronic lines of the selected multiplets for the 4f 6 configuration at RT. Δ CFS is the energy difference between the highest and lowest CF components of the 2S+1 L J multiplet Ln 3+ is forced by the former ones. In this context, it seems to be important to indicate how the coordination of phosphonate and pyridine groups depends on the pH of a solution. In order to find the binding mode of the NP 2 py ligand and the stoichiometry of the formed Ln-NP 2 py species, the luminescence spectra of Eu-NP 2 py and Tb-NP 2 py complexes in aqueous solution were recorded. The selected luminescence spectra of the Eu-NP 2 py complex at different pH are presented in Fig. 5. At pH 2.26 the intensity of the hypersensitive 5 D 0 → 7 F 2 transition (1.08) is similar to that of the magnetic dipole 5 D 0 → 7 F 1 transition and higher than the relative intensity for the aqua ion (0.68). The increase of pH brings about an increase of the relative intensity of the 5 D 0 → 7 F 2 transition from 1.08 ( pH = 2.26) to 2.74 ( pH = 10.5). It means that the NP 2 py ligand starts to coordinate at pH ∼2. 26. Under these conditions competing reactions undergo between the protonation and coordination of phosphonate groups to the Eu 3+ .
The protonation of the N py atom at low pH prevents the Eu-N py interaction and consequently may hinder the L-Eu energy transfer. Therefore the question arises whether the efficiency of an antenna effect as a function of pH may serve as a coordination probe for establishing the binding of pyridine to the Eu 3+ . In general the Tb 3+ compounds are more convenient to study the energy transfer process in comparison with those of Eu 3+ for which the ligand band may be imposed on the L-M charge transfer spectra of Eu 3+ . 22 Accordingly, to find the pH range in which the coordination of the pyridyl group to the Ln 3+ occurs, the luminescence spectra of the Tb-NP 2 py complex at various pH with an excitation wavelength of 266 nm were recorded (Fig. 6). The oscillator strengths of the 1 π → 1 π*, 1 n → 1 π* transition calculated from absorption spectra versus pH are also presented in the insert of this figure.
As can be seen the largest increase of the luminescence intensity of all f-f transitions is observed between pH 3 and 5.5. Simultaneously the oscillator strength of the ligand band monotonically decreases. These results indicate that the changes of the efficiency of the antenna effect may be used to study the coordination modes of chromophore groups to the Tb 3+ . The determined values of luminescence quantum yields It is also worth considering the changes of the energy of the ligand centered transitions, caused by coordination of the NP 2 py ligand to the Tb 3+ (Fig. 3S †). Because the 1 π → 1 π*, 1 n → 1 π* transitions are particularly sensitive to the interaction of the N pyridine atom with the metal cation, a remarkable bathochromic shift of the ligand band (∼550 cm −1 ) is observed in the spectra of the Tb-NP 2 py complex at pH above 6 as compared with the spectra of the free ligand (see Fig. 3S). The observed changes of the f-f and ligand bands above pH ∼ 3 support the conclusion that the coordination of the pyridyl group to the Tb 3+ occurs at pH between 3.0 and 5.5. The coordination of the NP 2 py ligand is completed above pH = 6.
Ln-NP 2 py complexes with the metal to ligand ratio 1 : 2. The luminescence spectra of the Eu-NP 2 py system with the Eu : L = 1 : 2 in solutions at different pH are presented in Fig. 7.   6 Luminescence spectra of the Tb-NP 2 py 1 : 1 complex at different pH. The plot of the oscillator strengths of the 1 π → 1 π*, 1 n → 1 π* transition versus pH is shown in the insert. As can be seen in Fig. 7 the spectral pattern of f-f bands strongly depends on pH. A qualitative comparison between the spectra of the crystal and solutions of the Eu : NP 2 py = 1 : 2 complex demonstrates that at pH = 10.5 [Eu(NP 2 py) 2 ] 5− is a predominant species. As the pH decreases, the CF components of individual multiplets of the [Eu(NP 2 py) 2 ] 5− complex vanish and below pH 8 they completely disappear. It may suggest that the formation of the [Eu(NP 2 py) 2 ] 5− complex starts at pH ∼ 8. The presence of additional bands (marked with an asterisk in the spectra in Fig. 7) may indicate that the other species, different to that found in the crystal, also exist in solution.
Owing to the simplicity of the 5 D 0 → 7 F 0 spectrum it was used to monitor the changes in the nearest coordination sphere of the Eu 3+ cation. The luminescence spectra of the 5 D 0 → 7 F 0 transition of solutions and the crystal are presented in Fig. 8.
Two well separated 5 D 0 → 7 F 0 peaks in the spectrum of the solution of the Eu : NP 2 py = 1 : 2 system at pH 10.5 are observed. The comparison of the energy of these peaks with those in the crystal and the solution of Eu : NP 2 py = 1 : 1 at pH = 10.5 enabled us to attribute the higher energy peak at 17 330 cm −1 to the [Eu(NP 2 py) 2 ] 5− species, while the lower energy peak centered at 17 286 cm −1 to the [Eu(NP 2 py)] − species. Both 5 D 0 → 7 F 0 peaks are separated by 56 cm −1 . This difference is probably caused by the change of the number of donor atoms and/or number of water molecules coordinated to the Eu 3+ (see section "Number of coordinated H 2 O molecules to the Eu 3+ cation" in the ESI †).
Quantitative analysis of the EuL + L ⇆ EuL 2 reaction. In the next step we focused our attention on a quantitative description of the reaction (1) expressed by the equilibrium constant K 12 ½EuðNP 2 pyÞ À þ NP 2 py 4À ⇆ ½EuðNP 2 pyÞ 2 5À ð1Þ K 12 ¼ ½EuðNP 2 pyÞ 5À 2 ½EuðNP 2 pyÞ À Á½NP 2 py 4À ð2Þ To determine the K 12 constant we used the method based on the fitting of the spectra of the crystals to those of solutions. 23 The spectrum of the crystal under study served us as a model of 1 : 2 species while the spectrum of Eu-NP 2 py 1 : 1 at pH = 10.5 was used as the spectrum of a pure 1 : 1 complex. However owing to the dynamic effects, 23,24 the spectra of f-f transitions in the solution are broadened in comparison to the spectra of the crystal. For this reason the simulated spectra of the [Eu(NP 2 py) 2 ] 5− species derived from the spectra of the crystal were artificially broadened. The equation that was used to calculate the simulated spectrum of the [Eu(NP 2 py) 2 ] 5− species in solution is as follows: where: J calcintensity at a given value of λ; zthe number of peaks for the given 5 D 0 → 7 F J transition; J maxthe maximum intensity of the peak; λ maxthe wavelength of the peak maximum; Δλthe half-width of a peak; A 12 and B 12 are adjustable parameters connected with artificial broadening of bands.
The final set of parameters used to calculate the simulated spectrum of the [Eu(NP 2 py) 2 ] 5− species is presented in Table 1S (ESI †). The simulated and experimental spectra of compounds under study are shown in Fig. 9.
The average molar fractions of [Eu(NP 2 py) 2 ] 5− and [Eu(NP 2 py)] − species are ∼81% and ∼19%, respectively. These results are consistent, within the error of experimental limits (±10%), regardless of the transition taken for the calculation.  The estimated averaged value of log K 1-2 for the reaction (1) is about 4.4 ± 0.5.
It should be noted that the presented speciation analysis for the case of f-elements is not always feasible, because the complexes existing in equilibrium are usually characterized by similar spectral patterns as it was shown previously by Horrocks et al. for the case of lanthanide complexes with NTP. 15 To support the presented results the analysis was extended by potentiometric measurements.

Potentiometric study
Protonation of the ligand. We performed potentiometric titrations of a free ligand in order to determine its acid-base properties. The fully protonated form of the H 4 NP 2 py possesses six dissociable protons [H 6 L] 2+ (see Fig. 6S †) two at each phosphonic group, one at the pyridine moiety and one on the central tertiary N amine atom. However, it was possible to determine only four protonation constants (K 1 -K 4 ) because the K 5 and K 6 values corresponding to the dissociation of one proton from each of the PO 3 H 2 groups can be determined only for solutions at pH below 2. This is beyond the available range for standard potentiometric titrations, and the log K 5 and log K 6 values were determined previously by the NMR method. 17 Table 4.
The most basic constant log K 1 = 10.63(2) ( Table 4) corresponds to the protonation of the nitrogen of the tertiary amino group and is two logarithmic units higher than that of H 6 NTP acid. 14,[25][26][27] The values of this constant for NTP, determined by potentiometric methods in 0.1 M ionic strengths in different electrolytes at 25°C were found to be: 12.7(1), 14 12.8(2), 25 12.20(6), 26 12.5. 27 Such a decrease of the highest protonation constant of H 4 NP 2 py is clearly caused by substitution of one of the phosphonic groups by the pyridine moiety. The same phenomenon has been noticed by Kurzak et al. 28 in N-3-picolyliminodi(methylenephosphonic) acid (NP 3 py); the log K value of the tertiary amine was found to be 10. 40(1) in 0.2 M KCl. The following deprotonation steps of H 4 NP 2 py release the protons from both phosphonic groups; log K 2 = 6.74(4), log K 4 = 2.23(5) and the pyridine moiety; log K 3 = 5.61(4) ( Table 4). Such deprotonation of H 4 NP 2 py is consistent with 31 P NMR results 17 and based on those data it was concluded that the third and fourth dissociation steps occur at pH between 2 and 3 (log K 4 ) and 4-7 (log K 3 , log K 2 ) and deprotonation of phosphonic groups is completed at pH above 8.2. The observed changes for the chemical shifts of pyridyl protons suggested that deprotonation of pyridine nitrogen occurs between pH 5 and 6, thus the pK 3 value was assigned to pyridine nitrogen. This conclusion is confirmed by the observed changes in the UV-vis spectra of the NP 2 py ligand. The spectrophotometric titration curve is shown in Fig. 10.
As was mentioned above, the 1 π → 1 π* and 1 n → 1 π* band observed in the UV spectrum of the NP 2 py ligand is particularly sensitive to the protonation of pyridine N (Fig. 3S †). A remarkable decrease of the ε value is observed between pH 5 and 7. This unequivocally confirms the attribution of log K 3 to the protonation of the pyridine moiety.
The minimalization of the sum equal to P ðε exp À ε calc Þ 2 by using eqn (4) enabled us to determine the β n values which are converted into K n values (Table 5).
As is seen from Table 5 the values of the respective log K n determined from different methods are the same, within the experimental error. Finally the calculated values of log K n were used to determine the speciation distribution curve, which is presented in Fig. 6S. † Formation constants. Potentiometric titrations of the Ln-NP 2 py system (where Ln = Sm 3+ , Eu 3+ , Gd 3+ and Tb 3+ ) were performed in order to determine the stoichiometry and stability constants of the formed complexes. The potentiometric measurements were performed for two Ln : NP 2 py molar ratios 1 : 1 and 1 : 2. The calculated stability constants of LnL = [Ln(NP 2 py)] − and LnL 2 = [Ln(NP 2 py) 2 ] 5− complexes (at 25°C, I NaClO 4 = 0.1 M) are collected in Table 3S † and selected values are shown in Table 6.
The species distribution curves are depicted in Fig. 7S. † The potentiometric measurements of Eu : NP 2 py = 1 : 1 and 1 : 2 systems at different ionic strengths (KCl) were also performed and the determined stability constants are given in Table 4S. † Based on potentiometric data it was possible to calculate log K 12 for reaction 2, which is equal to 5.27 and similar within the limit of experimental error to that estimated from the spectroscopic data (4.4 ± 0.5).
The determined stability constants for Eu-NP 2 py systems were used to calculate the average ligand number (n) as a function of pH (Fig. 11). The data for the Eu-NTA (1 : 2) complex (where NTA is nitrilotriacetic acid) are also included in this figure for comparison purposes. 29a As is seen in Fig. 11 the coordination of the NP 2 py ligand to the Eu 3+ starts at about pH 3 for 1 : 1 and 1 : 2 complexes. In the case of the Eu : NP 2 py 1 : 2 system the formation of the [Ln(NP 2 py) 2 ] 5− complex starts at about pH 8 and it is consistent with the spectroscopic data described above. The difference between Eu-NTA and Eu-NP 2 py complexes with the molar ratio 1 : 2 is clearly seen. The coordination of two NTA ligands occurs at low pH ∼ 2 29 and is completed at pH ∼ 6. As far as Eu-NP 2 py of the 1 : 2 system is concerned, the [Eu(NP 2 py)] − complex is a predominating species within the pH range 3-9 and the formation of the [Eu(NP 2 py) 2 ] 5− complex takes place at pH above 8. This may be explained qualitatively by considering the formal charge of reacting species. In the case of the Ln-NTA system the neutral LnNTA complex reacts with the H n NTA ligand to form finally [Ln(NTA) 2 ] 3− species with the formal charge −3. The formal charge of the Ln(NP 2 py) 2 complex is −5 and the formed complex is not as stable as [Ln(NTA) 2 ] 3− . Out of the considered complexes, the [Ln(NTP) 2 ] 9− one is the most negatively charged, therefore this may suggest that the stability of the complexes should change in the following order: Ln(NTA) 2 > Ln(NP 2 py) 2 > Ln(NTP) 2 . Such an assumption is consistent with potentiometric results derived from Sawada et al. 14  Extrapolation of [Eu(NP 2 py)] − and [Eu(NP 2 py) 2 ] 5− stability constants to zero ionic strength The SIT approach (Specific Ion Interaction Theory) 18 was used for the extrapolation and correction of equilibrium data to the infinite dilution standard state. This simple method provides good estimations of activity coefficients and is recommended for use by the OECD Nuclear Energy Agency. 30 The extrapolation of the formation constants to ionic strength equal to zero requires the estimation of activity coefficients of all species that participated in chemical reactions. This may be performed by the SIT model. For the reactions: Eu 3þ þ mL 5À ⇄ ½EuL m À4mþ3 : ð6Þ Table 5 The comparison of the protonation constants of the NP 2 py ligand using different methods 31 P NMR 17 UV-vis Potentiometry log K (HL) 10.9(9) 11.0(9) 10.63(2) log K 2 (H 2 L) 6.7(9) 6.6(9) 6.74(6) log K 3 (H 3 L) 5.5(9) 5.6(9) 5.61(8) log K 4 (H 4 L) 2.3(9) 2.7(9) 2.23(9) log K 5 (H 5 L) 0.98(5) -log K 6 (H 6 L) 0.70(5) -- 13.36 (7) 13.47 (10) 17.86(9) Fig. 11 The average ligand number n as a function of pH for Eu-NTA (1 : 2 ratio) as well as for Eu-NP 2 py complexes (1 : 2 and 1 : 1 ratios).
Linear regressions of eqn (10) and (11), using the experimental values of log β(I m ) and I m give the thermodynamic values of log β 0 HnL and log β 0 EuLm (Table 7). In general, these data may be useful in finding a reliable, thermodynamic model of the complexation reaction of aminophosphonic ligands with lanthanides particularly that, to the best of our knowledge, there are no stability constants of lanthanide/actinide phosphonates extrapolated to the zero ionic strength. This is the reason for which we are unable to refer our data to other ones.

Summary and conclusions
The compounds of the formula [C(NH 2 ) 3 ] 5 [Ln(NP 2 py) 2 ]·12H 2 O (where Ln = Eu, Gd) with new tripodal NP 2 py ligands were synthesized and their crystal structure was determined. The crystal comprises monomeric [Eu(NP 2 py) 2 ] 5− anions. Both ligands are bonded with the Eu 3+ cation by two oxygen atoms from monodentate phosphonic groups, one tertiary nitrogen atom and one pyridine nitrogen atom, filling thus the eight coordination sites of the Eu 3+ cation. The approximate C i symmetry of the [Eu(NP 2 py) 2 ] 5− complex is reflected in the luminescence spectra, in which the intensity of the magnetic dipole 5 D 0 → 7 F 1 transition dominates over the intensities of the remaining 5 D 0 → 7 F 0,2,3,4 transitions.
The luminescence spectra of Eu-NP 2 py and Tb-NP 2 py complexes in aqueous solutions at different pH with various Ln : L ratios were studied in order to find the coordination mode of the NP 2 py ligand and the stoichiometry of complexes. The NP 2 py ligand starts to coordinate through phosphonic oxygen atoms with Ln 3+ at pH ∼ 2. The coordination of the pyridyl group to the Tb 3+ occurs at pH between 3.0 and 5.5. The binding of the NP 2 py ligand is completed above pH = 6.
This study has demonstrated that both The potentiometric measurements were applied to determine the stability constants of complexes under study. The stability constants are between 11.83(5) for [Sm(NP 2 py)] − and 13.47(7) for [Tb(NP 2 py)] − and are slightly lower than those obtained for Ln-NTP complexes. 16 Thus, the substitution of one phosphonic group by a pyridyl moiety slightly affects the stability of the [Ln(NP 2 py)] − complex in comparison with the [Ln(NTP)] 3− one. 16 On the other hand the log β 0 LnL2 of the [Ln(NP 2 py) 2 ] 5− complexes are about 5-6 orders smaller in comparison with those reported for [Ln(NTP) 2 ] 9− . 16 It should be noted, however, that the existence of [Ln(NTP) 2 ] complexes in solution has not been proved unequivocally. [14][15][16] Potentiometric titrations at various ionic strengths were also performed in order to find the thermodynamic stability constants of [Eu(NP 2 py)] − and [Eu(NP 2 py) 2 ] 5− complexes. Such data are, in principle, preferable as a basis for discussion on the factors governing the stabilities of the complexes. The extrapolation of the formation constants to zero ionic strength under standard conditions was performed by the SIT procedure. latter are characterized by the thermodynamic stability constants: log β 0 EuNTA = 13.34 and log β 0 EuðNTAÞ 2 = 22.66. Taking into account all these data one may conclude that the high negative charge of a ligand may stabilize the LnL complex, but simultaneously destabilizes the LnL 2 one.

Materials
All chemicals used were of analytical grade. The stock solutions of anhydrous Ln 3+ chlorides (Aldrich) were standardized against EDTA using xylenol orange as an indicator. N-(methylene-2-pyridine)-N,N-di(methylenephosphonate) as a diammonium dihydrogen compound was synthesized as described previously. 17 Crystal preparation The luminescence quantum yields for solutions of the Tb 3+ -NP 2 py complexes were determined in relation to fluorescein in 0.1 M NaOH (Φ = 93%), by using eqn (13): where Q stands for the quantum yield, n is the refraction index (n X = 1.38, n S = 1.33), the integrals were taken for the emission spectra corrected for the instrument response, ε is the molar absorption coefficient, cconcentration, and dthickness of the cuvette (1 cm). The subscripts X and S stand for the sample and the standard, respectively. The pairs of the sample and the standard spectra used in the above equation were taken at the same instrument setting (slit widths, photomultiplier voltage etc.). The slits were 1 nm for the excitation and 1 nm for the emission recording.

X-Ray crystal analysis
The crystallographic measurements were performed on an Xcalibur, Sapphire2 four-circle diffractometer with graphitemonochromatized MoKα radiation. The data for the crystal were collected at 100(2) K using the Oxford Cryosystems cooler and were analytically corrected for absorption with the use of the CrysAlis RED program of the Xcalibur software. The structures were solved routinely by using Patterson synthesis. The C-and N-bonded hydrogen atoms were placed in positions calculated from the geometry. The final refinement was anisotropic for all non-H atoms. The computations were performed with the SHELXS97 31 and SHELXL97 programs, 32 and the molecular graphics was prepared with XP-Interactive Molecular Graphics. 33 High residual peaks suggest that the C50 guanidine cation is partially distorted, however it was not possible to resolve this distortion. A summary of the conditions for the data collection and the structure refinement parameters is given in Table 8.