The ligands were synthesized from iminoacids (L-proline, sarcosine and ... L1, L2 and L3 is lower than that of two L-proline, sarcosine or imda ligands, even if ...
Complexation of some aminocarboxylate ligands containing in-chain phosphinate group
Tamás R. Varga, Róbert Király and Ernő Brücher* Department of Inorganic and Analytical Chemistry Lajos Kossuth University Debrecen, H-4010, POB. 21. HUNGARY
The stability constants of complexes formed with aminocarboxylate ligands containing in-chain phosphinate functional group are relatively high and indicate the participation of phosphinate group in complexation.
In recent years there is a considerable interest in the chemistry of ligands containing phosphinate functional group(s). The successful use of the acyclic and cyclic polyazapolycarboxylate ligands in medical diagnosis and therapy [1] promoted the synthesis and study of the phosphinate analogues [2]. The biological activity of some aminoalkyl-phosphinic acids initiated the synthesis and study of several new ligands and complexes [3]. These results along with and earlier studies [4] revealed that N-methylphosphinic acids are stronger acids than amino-carboxylic acids and the ability of phosphinate group for coordination is weaker than that of the carboxylate group. As a consequence, the complexes formed with phosphinate containing ligands are less stable than their carboxylate containing counterparts. In the complexes studied so far phosphinate groups possessed terminal position exclusively. However, they may also have an intra-chain position in a multidentate ligand, e.g., via the R,R’ alkyl groups of dialkyl-phosphinate groups, RR’POO-. Such acyclic ligands bearing the N-CH2-PO(OH)-CH2-N functionality were recently synthesized in our laboratory [5]. In order to obtain information on the complexation properties of the in-chain phosphinate group, we studied the equilibria of the complexes formed between the ligands L1, L2, L3 and L4 and the metal ions Mg2+, Ca2+, Cu2+ , Zn2+ , Cd2+ , La3+ , Nd3+ , Gd3+ and Lu3+. The results of these studies are of importance also considering that a macrocyclic ring containing the 1
RR’POO- group would be negatively charged without the attachment of other charged functional group(s). N 1
L : R1R2N =
R1
O N
P
N
R2
OH
R2
COOH
L2: R1 = CH3 , R2 = CH2COOH
R1
L3: R1, R2 = CH2COOH L4: R1R2N =
N
O
The ligands were synthesized from iminoacids (L-proline, sarcosine and iminodiacetic acid, H2imda) or morpholine, paraformaldehyde and phosphinic acid via Mannich-reaction [5]. The protonation constants of ligands and the stability constants of complexes were determined by pH-potentiometric titrations in 1.0 M KNO3 for L1, L2 and L4 and in 1.0 M Me4NCl for L3 at 25 °C. Titrations were carried out at 2-3 different concentrations of the ligands (210-3 - 110-2 M) and at 2-4 different metal to ligand concentration ratios (1:1, 1:2, 1:3 and 1:8). The H+ concentration was obtained from the pH data by the method suggested by Irving et al [6]. The protonation and stability constants are defined as KiH = [HiL]/[Hi-1L][H+] and KML = [ML]/[M][L]. The equilibrium constants were calculated from the titration data using PSEQUAD [7]. The number of data points, used for the equilibrium calculations, was between 90 and 220. The standard deviation values calculated by the software are given in parentheses in Tables 1 and 2. The protonation of the ligands L1, L2 and L3 can be characterised with four protonation constants. The first two protonation constants can presumably be assigned to the two nitrogen atoms, while the third and fourth ones to that of the carboxylate groups. The protonation constant of the phosphinate group is so low (log KH 1) that it cannot be determined by pHpotentiometry. 2
The effect of the phosphinate group on the complexation properties of ligands L1, L2 and L3 can be expressed by comparing the stability constants of their complexes with those of the bis(L-prolinate), bis(sarcosinate) and bis(iminodiacetate) complexes which contain the same donor atoms but the phosphinate group. In determining the stability constants the basicity of ligands, which is characterised by the sum of the protonation constants, is of high importance. For a comparison we use the log KiH values for L1, L2 and L3 and the 2 log KiH values for L-proline, sarcosine and imda. These data in Table 1. indicate that the basicity of L1, L2 and L3 is lower than that of two L-proline, sarcosine or imda ligands, even if the protonation constant of the phosphinate group is taken into consideration. The ligand L4 contains two nitrogen donors beside the phosphinate group but the coordination of nitrogens is strongly hindered. As a result, the formation of complexes between L4 and Ca2+, Zn2+ and Cd2+ could not be detected by pH-potentiometry. The stability constant of the Cu2+-complex formed with L4 is very low; log KML = 1.6 (0.2). Lanthanide(III)-ions form somewhat more stable complexes. The formation of the Nd3+-complex was detected by spectrophotometry. Beside the absorption band of the Nd3+.aq at 427.1 nm (2P1/2 4I9/2 transition) in the presence of the ligand L4 a new band appears at 428.4 nm. The stability constants obtained by pH-potentiometry at 1:8 metal to ligand ratio are log KNdL = 2.0 (0.07) and log KLuL = 2.4 (0.04). These data indicate that the ligand,coordinating through only its phosphinate group, is very weakly bound to some first row transitional metal ions, while its coordination to lanthanide(III)-ions possessing 3+ charge is somewhat stronger. The ligands L1, L2 and L3 form complexes with 1:1 metal to ligand ratio. Cu2+, Zn2+ and Cd2+ ions also form protonated complexes with all these ligands. Lanthanide(III) ions form protonated complexes with L3 but for comparison we use only the stability constants of the ML complexes.
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The stability constants of the complexes formed with the ligands L1 and L2, containing two nitrogen and two carboxylate oxygen donor atoms beside the phosphinate group, are shown in Table 2. The log KML values for the Zn2+ and Cd2+ are comparable with the log 2 values of the bis-complexes formed with the corresponding amino acids (for Cd2+ we use the log 2 value obtained for the glycine, since the Cd2+-L-proline and Cd2+-sarcosine systems have not been studied yet). However, if we take into account the lower basicity of L1 and L2, the stability constants indicate that the phosphinate group has a significant contribution to the log KML values. The Cu2+ ion forms complexes of lower stability with L1 and L2 than with two proline or sarcosine ligands. This is partly due to the lower basicity of L1 and L2, but the tetragonal structure of the Cu2+-complexes also hiddens the coordination of the phosphinate group. The visible spectra of the Cu2+-complexes are very similar. The maxima of the absorption bands are at 612, 619 and 613 nm for the CuL1, CuL2 and Cu2+-bis(L-prolinate), respectively. These data suggest that the ligands L1 and L2 are coordinated to the Cu2+ ion similary to the amino acids, i.e., through two nitrogen and two carboxylate oxygen donors. The position of the two amino acid residues in L1 and L2 can only be cis, while the structure of the Cu2+-amino acid complexes is generally trans (though in some cases cis structure was also detected in solution) [8]. In the formation of the complexes of lanthanides with amino acids both the structures (monodentate and bidentate coordination) and equilibria (hydrolysis of Ln3+ ions) are strongly disputed [9]. The formation of the complexes with ligands L1 and L2 starts at around pH 4 but it slows down in the pH range 5.5-6, probably because of the formation of some hydroxo complexes. However, if the titration points obtained in this narrow pH range are omitted from the calculations, the data fit quite well. The stability constants of the complexes formed with the ligands L1 and L2 are higher than those of the bis-complexes of the amino acids, showing the important role of the phosphinate group in complexation. 4
The stability constants of the complexes formed with the ligands L3 (which can be regarded as an analogue of EDTA) are relatively high. The log KML values reported in Table 2. are significantly higher than the log 2 values of the bis-(imda) complexes (the log KML values are closer to those of the EDTA complexes), which illustrates the importance of the >POOgroup in complex formation. The downfield shift of the 1HNMR signals of the ligands L1, L2 and L3 in the presence of Zn2+, La3+ and Lu3+ also indicate the complex formation. Simultaneously, the signals of L1 and L2 undergo significant broadening. The acetate methylene protons in the complexes ZnL3 and LuL3 give AB multiplets, due to the long life-time of the metal-nitrogen bonds on the NMR time-scale and the relatively rigid structure of these complexes. As a conclusion, we pointed out that the stability constants of the complexes of ligands L1, L2 and L3 indicate the participation of the weakly basic in-chain phosphinate group in complex formation. The coordination of phosphinate group is stronger to hard, trivalent metals than to divalent ions, particularly if the coordination number is 6 or higher than 6. The contribution of the >POO- group to the stability constants increases as the number of the charged functional groups of the multidentate ligand increases, since the coordinaton of the other donor atoms results in the binding of the in-chain phosphinate group. Acknowledgements This work was supported by the Ministry of Culture and Education (FKFP 0448/1997). The authors are grateful for the technical help of Judit Vanka and Béla Rózsa.
Table 1. Protonation constants of ligands 5
ligand
log K1H
log K2H
log K3H
log K4H
nloga KiH
L1
10.92 (0.006)
7.35 (0.01)
2.0 (0.015)
1.3 (0.022)
21.57
L-prolineb
10.38
1.90
L2
10.14 (0.005)
5.93 (0.008)
sarcosineb
9.99
2.20
L3
9.83 (0.008)
6.44 (0.012)
2.67 (0.02)
imdab
9.34
2.61
1.82
L4
6.96 (0.008)
5.03 (0.009)
24.56 2.19 (0.011)
1.5 (0.013)
19.76 24.38
2.20 (0.034)
21.04 23.9 11.99
a) n = 1 for L1, L2 L3 and L4 and n = 2 for L-proline, sarcosine and imda b) Ref. [10], I = 0.1 M, 25 C
Table 2. Stability constants of complexes
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log KML L1
log 2
L2
L3
imda
prolinea sarcosineb glycinec
Mg2+
3.26 (0.06)
1.94 (0.04)
7.96 (0.006)
4.85
Ca2+
2.54 (0.05)
2.23 (0.05)
8.74 (0.026)
7.36
2.44c
Cu2+
15.36 (0.03)
14.08 (0.06)
17.88 (0.032)
16.54
16.35a, 14.59b
Zn2+
10.01 (0.08)
8.8 (0.2)
16.46 (0.011)
12.52
9.69a, 8.33b
Cd2+
8.58 (0.06)
8.23 (0.03)
14.16 (0.08)
10.12
7.71c
La3+
6.89 (0.01)
7.38 (0.05)
13.97 (0.012)
9.97
6.15c
Nd3+
7.50 (0.07)
7.9 (0.1)
15.60 (0.025)
11.39
6.40c
Gd3+
7.6 (0.14)
7.8 (0.08)
16.24 (0.023)
12.07
6.96c
Lu3+
7.9 (0.16)
7.91 (0.03)
17.21 (0.050)
10.12
7.01c
a) L-proline Ref. [10], b) sarcosine Ref. [10], c) glycine Ref. [11]
References
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In recent years a number of papers have been published on the complexation properties of cyclic and acyclic polyaza-polyphosphinate ligands. In all these ligands phosphinate functional groups are in terminal position. Here we report the complexation properties of some ligands with phosphinate group in in-chain position. The in-chain phosphinate group can be a part of such a macrocyclic ring, that has a negative charge without the attachment of other functional groups. In this respect we are confident, that the reported results are important for the scientist in the large area of this part of coordination chemistry.
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