Metal complexes of ethane and propane frame-substituted oligophosphonic and oligophosphonocarboxylic acids

Article abstract of DOI:10.1080/10426500701441465

The stability constants of the complexes of one oligophosphonic acid (ethane-1,1,2-triphosphonic acid 1), five phosphonocarboxylic acids (ethane-1,2-diphospho-no-1-carboxylic acid 2, propane-1,1-diphosphono-2,3- dicarboxylic acid 3, ethane-1,2-diphosphono-1,2-dicarboxylic acid 4, ethane-1-phosphono-1,2-dicarboxylic acid 5, and propane-1-phosphono-1,2,3- tricarboxylic acid 6), and ethane-1,1,2-tricarboxylic acid 7 with Cu2 +, Ni2 +, Zn2 +, Cd2 + and Ca2 + were determined by pH-metric titrations. The six ligands of type HNL (N = 4 - 6) form mononuclear MLHn (n = 0, 1, , N-1) complexes with chelate structure. The calcium complexes seem to be less stable than the transition metal complexes.

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Metal Complexes of Ethane and
Propane Frame-Substituted
Oligophosphonic and
Oligophosphonocarboxylic
Acids
a
b
c
Cs. Bogdán , G. Péczely & G. Hägele
a
Institute of Chemistry, Eötvös Loránd University ,
Budapest, Hungary
b
Department of Inorganic Chemistry , University of
Pécs , Hungary
c
Department of Inorganic and Structural Chemistry ,
Heinrich Heine University , Düsseldorf, Germany
Published online: 23 Aug 2007.
To cite this article: Cs. Bogdán , G. Péczely & G. Hägele (2008) Metal
Complexes of Ethane and Propane Frame-Substituted Oligophosphonic and
Oligophosphonocarboxylic Acids, Phosphorus, Sulfur, and Silicon and the Related
Elements, 182:10, 2337-2350, DOI: 10.1080/10426500701441465
To link to this article: http://dx.doi.org/10.1080/10426500701441465
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Phosphorus, Sulfur, and Silicon, 182:2337–2350, 2007
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701441465
Metal Complexes of Ethane and Propane
Frame-Substituted Oligophosphonic
and Oligophosphonocarboxylic Acids
∗
Cs. Bogd a´ n
Institute of Chemistry, E o¨ tv o¨ s Lor a´ nd University, Budapest, Hungary
G. P e´ czely
Department of Inorganic Chemistry, University of P e´ cs, Hungary
G. H a¨ gele
Department of Inorganic and Structural Chemistry, Heinrich Heine
University, D u¨ sseldorf, Germany
The stability constants of the complexes of one oligophosphonic acid (ethane-
1
,1,2-triphosphonic acid 1), five phosphonocarboxylic acids (ethane-1,2-diphospho-
no-1-carboxylic acid 2, propane-1,1-diphosphono-2,3-dicarboxylic acid 3, ethane-
,2-diphosphono-1,2-dicarboxylic acid 4, ethane-1-phosphono-1,2-dicarboxylic
acid 5, and propane-1-phosphono-1,2,3-tricarboxylic acid 6), and ethane-1,1,2-
1
2
+
2+
2+
2+
2+
tricarboxylic acid 7 with Cu , Ni , Zn , Cd and Ca were determined by
pH-metric titrations. The six ligands of type HNL (N = 4 – 6) form mononuclear
MLHn (n = 0, 1, . . . , N–1) complexes with chelate structure. The calcium complexes
seem to be less stable than the transition metal complexes.
Keywords Oligophosphonates; phosphonocarboxylates; protonated metal complexes;
stability constant
INTRODUCTION
Organophosphonate and phosphonocarboxylate compounds are widely
used in a broad variety of applications. Their ability to prevent
Received April 26, 2007; accepted April 29, 2007.
∗
Currently at KFKI Research Institute for Particle and Nuclear Physics, Budapest,
Hungary.
Cs. Bogd a´ n is indebted to Prof. Ferenc Gaizer (Prof. Emeritus of the Institute of Chem-
istry, E o¨ tv o¨ s Lor a´ nd University, Budapest, Hungary), whose help, hints and stimulating
suggestions contributed to the current work.
Address correspondence to Cs. Bogd a´ n; KFKI Research Institute for Particle and Nu-
clear Physics of the Hungarian Academy of Sciences; Konkoly Thege Mikl o´ s u´ t 29-33,
Budapest H-1121, Hungary. E-mail: bogdan@rmki.kfki.hu
2337

2
338
Cs. Bogd a´ n et al.
precipitation of calcium and magnesium salts at substoichiometric
concentrations renders them effective in water treatment for scale
1
,2
inhibition. Phosphonocarboxylates are used extensively in detergents
3
as detergent builders and/or sequestering agents. These materials are
also applied as corrosion inhibitors in industrial cleaning and in per-
oxy bleach stabilization. The aqueous bath for corrosion-resistant steel
coating contains organophosphonic acid, ester, salt and/or polymer.4
5
Organophosphonates are also applied in flame-resistant polymers,
6
as stabilizers or masking agents in photographic processing and in
actinide separation methods. In the early 1990s, these compounds have
7
been identified as promising candidates to create “structurally tailored”
and microporous materials,8 f1or catalysis and for the electrochemical
,9
10
remediation of polluted soils.1
The potent biological activity of carboxyalkylphosphonates and
aminoalkylphosphonates makes them useful as components of micro-
fertilizers and pesticides in agriculture.12
Phosphonates play also a significant role in medicine and pharma-
cology. Arylphosphonates inhibit protein-tyrosine and serine/threonine
phosphatases.1 Phosphonocarboxylates show antiviral properties,
3
14
1
5
inhibit the intestinal and renal phosphate transport as well as the
sodium-dependent phosphate reabsorption.
1
6−18
Bisphosphonates in-
9
9m
hibit bone reabsorption and are used as complexing agents for
Tc
for bone imaging.19 Propane-1,1-diphosphono-2,3-dicarboxylic acid was
found to play a major role in this field, since it stabilizes the radiophar-
maceutical solutions.20
Development of technical applications of organophosphonates and
phosphonocarboxylates requires reliable protonation and complex sta-
bility constants, that permits equilibrium modeling and prediction
of the underlying molecular processes. Since most oligophosphonates
resist biodegradation,1 chemical speciation calculations and predic-
tion of their fate in aqueous environment is of primary impor-
2
1
21,22
tance. Compared to phosphinates, aminoalkylphosphonates
and
alkyl phosphonocarboxylates,2
1−23
considerably less attention has been
devoted so far to the metal-binding properties of short-chain and
oligophosphonocarboxylates. To the best of our knowledge, only in one
2
4
2+
publication data on the Ca complexes of propane-1,1-diphosphono-
,3-dicarboxylic acid (3) are reported: CaL, CaLH Ca2LH and CaLH2
2
complexes were substantiated in the pH > 4 interval at TCa/TL = 0.5.
We have reported the protonation constants of 2 oligophosphonic and
5
phosphonocarboxylic acids: the ethane or propane frame-substituted
25
polyacids 1–7 (see Table I for structures). These ligands, containing
2
3
−
−
several PO and COO groups in vicinal and/or geminal positions are
expected to be powerful complex-forming agents with a wide range of
24,26
possible coordination modes.
pH-Metric titrations were performed

Metal Complexes of Oligophosphonic Acids
2339
TABLE I Structures of the Polyacids 1–7
in order to determine the compositions and stability constants of their
metal ion complexes. Transition metals of biochemical importance
2
+
2+
2+
2+
(
Cu , Zn ) or toxicity (Ni , Cd ) were selected for our studies and
2
+
Ca was also investigated in view of possible practical (anti-scaling)
applications.
EXPERIMENTAL
Materials and Syntheses
The polyacids were prepared in our laboratories by a variety of known
methods, summarised briefly below. The effective concentration of all
ligands was determined by potentiometric titrations against standard-
ized NaOH and no protolytically active impurities could be detected.
The < 100% purity data below result from the known observation, that
hydrolysis of the alkyl esters of oligophosphonocarboxylic acids by aque-
ous HCl leads to highly hygroscopic parents acids.

2
340
Cs. Bogd a´ n et al.
Sodium diethyl phosphite reacted with vinylidene chloride in
◦
tetrahydrofurane solution at 100–105 C to form ethane-1,1,2-
trisphosphonic acid hexaethyl ester in good yield, which was subse-
quently converted by hydrolysis with concentrated HCl to ethane-
1
,1,2-trisphosphonic acid (1, purity 93.6%, which corresponds to a
2
7
monohydrate). This procedure was termed as Anomalous Michaelis-
2
8
Becker reaction, described in the patent literature and briefly men-
tioned in two further publications.29 For the sake of completeness, we
want to point out, that a more complex route to compound 1 was re-
ported by Saunders and Simpson.30
Sodium diethyl phosphite reacted with 2,3-dichloropropionic acid
ethyl ester at reflux temperature in a Michaelis-Becker type reaction31
to give 2,3-diphosphonopropionic acid pentaethyl ester (yield: 25.2%). It
is interesting to note, that this reaction was first studied by Kamai and
Kukhtin as early as 1957.32 We found, that 2,3-diphosphonopropionic
acid pentaethyl ester can also be prepared by Pudovik-type addi-
tion of two moles of HP(O)(OEt)2 to H–C≡C–COOEt (see below). The
pentaester was hydrolyzed by boiling in water for 5 h to give 2,3-
diphosphonopropionic acid (2, purity: 97.4%).
Some phosphonocarboxylic acid esters were prepared by addition of
diethyl phosphite to α-unsaturated carboxylic acid derivatives under al-
33
kaline (sodium ethylate) catalysis. This synthetic route was pioneered
3
4
by Pudovik. Addition to acetylenedicarboxylic acid gave ethane-
,2-diphosphono-1,2-dicarboxylic acid hexa-ethyl ester in 10.3% yield,
1
which was hydrolysed by conc. HCl to give ethane-1,2-diphosphono-1,2-
dicarboxylic acid 4 in 98.4% yield.
Pudovik-type addition to diethyl maleate gave ethane-1-phosphono-
,2-dicarboxylic acid tetraethyl ester in 28% yield, the hydroly-
1
sis of which under acidic conditions led to ethane-1-phosphono-1,2-
dicarboxylic acid 5 with a purity of 92.1%.
The propane-frame substituted phosphonocarboxylic acids 3 and 4
were prepared similarly.33 In particular, the hexaethyl ester of propane-
1,1-diphosphono-2,3-dicarboxylic acid 3 was synthesized by Michaelis-
type addition of methylenediphosphonic acid tetraethyl ester to maleic
acid diethyl ester in 38.4% yield. It was then hydrolysed to give propane
1
,1-diphosphono-2,3-dicarboxylic acid 3 with a purity of 96.4%. Phos-
phonoacetic acid diethyl ester was reacted with maleic acid dibutylester
to form the pentaester of propane-1-phosphono-1,2,3-tricarboxylic acid
(
1
yield: 40.3%). Its hydrolysis (in boiling water for 5 h), yielded propane-
-phosphono-1,2,3-tricarboxylic acid 6 with a purity of 97.6%. Both
Pudovik-type reactions were conducted in the presence of sodium
◦
methylate at 100–110 C.

Metal Complexes of Oligophosphonic Acids
2341
For the analytical studies presented below compound 4 was used as
a genuine mixture of meso and rac forms, while 6 contained the threo
and erythro forms. No attempts were made to separate the individual
stereoisomers.
All other chemicals were obtained from Merck and were of analytical
grade; bidistilled, carbonate-free water was used throughout this study.
Potentiometric Titrations
◦
The pH-metric measurements were performed at 25 ± 0.1 C and a con-
−3
stant total ionic strength of 0.5 mol·dm , adjusted by NaClO4. The
titration apparatus included a glass electrode, a separate reference
Ag/AgCl electrode with the inner KCl reference electrolyte replaced by
−
3
0
.5 mol·dm NaClO4, and an OP-204 digital pH-meter with a precision
−
3
of 0.1 mV (all from Radelkis, Hungary). 3–5 mmol·dm polyacid, 3–
−
3
2+
−3
1
0 mmol·dm (for Ca , 15–50 mmol·dm ) of the corresponding metal
−3
perchlorate and 0.01 mol·dm HClO4 were titrated under a nitrogen
atmosphere (99.9995% N2 from Linde, washed with NaOH and NaClO4
−
3
−3
solutions) with a 0.1 mol·dm NaOH titrant containing 0.4 mol·dm
35
NaClO4 (for further details, see ref. ).
The titration datasets were evaluated with the least-squares
MAXIPOT-F computer program.36 This program refines the selected
parameters by minimizing the sum of squared residuals in electromo-
ꢀ
2
tive forces (emf), [ (ꢀE) ]. In the course of the refinement, the optimum
values for the decadic logarithm of the concentration stability constants,
−q
−p
−r
βqpr = [MqLpHr][M] [L] [H] along with their error estimates were
calculated (ionic charges are throughout omitted). The previously deter-
−
1
−i
mined cumulative protonation constants β01i = [HiL][L] [H] , listed
+
in the rows H and the columns of the corresponding r value in Table II,
were held constant during the iterations. To account for differences in
ligand donor group basicities during comparison of metal-binding affini-
ties, basicity-adjusted stability constants22 were calculated as [log β110
–
β01n], where n is the number of donor groups characterized by proto-
nation constants. Similarly, the conditional stepwise stability constants
corresponding to the reaction:
M + HiL = MLHi
Ki = [MLHi][M]−1[HiL]−1
(1)
provide more reliable means to estimate the relative metal-binding abil-
2
2
ity of the protonated ligands than the log β values.
RESULTS AND DISCUSSION
The pH-metric measurements were planned in the pH range 1.8–11. In
certain systems, we could indeed carry out measurements up to pH∼11,

2
342

2343

2
344

2345

2
346
Cs. Bogd a´ n et al.
but in others the emf became unstable even at pH∼4. In these lat-
ter cases, the measurements were interrupted as the solutions became
opalescent or a precipitate was formed. We could not recognize any
clear-cut trend in connection with this phenomenon, but it seems that
compounds containing vicinal carboxylate groups can be titrated to a
2
+
higher pH range. On the other hand, the systems containing Ni bear
a less pronounced tendency to hydrolysis as compared to the systems
2
+
2+
2+
with Cu , Zn or Cd
.
The log β values together with their errors obtained by joint computer
evaluation of the parallel measurements are contained in Table II. The
distribution curves for the Ni2 complexes, calculated via the deter-
mined stability constants, are depicted as typical examples in Figure 1.
Although the ligands studied bear multiple functional groups allow-
ing the binding of more than one metal ion and although the titrations
were also performed at metal : ligand ratios higher than 1:1, the com-
puter evaluation did not support evidence for the existence of polynu-
clear complexes.
+
ML complexes are typically formed at pH > 6, but complexes of ad-
equate solubility were formed only with ligands holding more carboxy-
late than phosphonate groups. The double negative charge and the
more basic character of the phosphonate group allows the formation
of protonated complexes MLHn (n = 1–4). As shown in Figure 1 for
2
+
the Ni complexes, the MLHn species often become predominant in
the pH range corresponding to the individual protonation step leading
to LHn, established previously25 for the (binary) ligand-proton system.
The solubility of phosphonate complexes presumably decreases upon
charge neutralization concomitant with protonation. Several associa-
tion processes, including the formation of colloids, can take place even at
2
6
millimolar concentrations typical for potentiometric measurements,
which strongly affects the reliability of the obtained stability constants.
2
+
For metal ions with a greater tendency to hydrolysis (e.g. Cu and
Zn2 ), the formation of monohydroxo complexes could also be sub-
+
stantiated by the computer evaluation (see the log β11−1 values in
Table II). In the Cd2 -containing systems, precipitation occured even
in the acidic region and no hydrolysed species could be detected in
solution.
+
Considering the log β110 constants, the stability of the tricarboxylate
complexes is by far the lowest. This seems to be a primary conse-
7
quence of the weaker basicity of the carboxylate group as compared to
the phosphonate moiety. If the metal-binding affinity is corrected by the
adj
10
sum of the protonation constants (cf. the corresponding log β1 values
of ligands 7 and 6 in Table II), the loss in affinity greatly decreases (for
2
+
2+
2+
2+
Zn or Cd ) or even reverses (for Ni or Cu ).

Metal Complexes of Oligophosphonic Acids
2347
SCHEME 1 Percentage distribution of Ni(II) in its different complexes and
that of the total ligand in the Ni-free species as a function of pH. T
L
= TNi(II) =
−
3
−3
5·10 mol·dm
.
The stabilities of the d-metal complexes of the phosphonate com-
pounds studied here vary in a wide range. The sequence of stability
with different metal ions does not demonstrate a definite trend. Gener-
2
+
ally, the Ni complexes exhibit a slightly lower stability than the other
metal ions.
3

2
348
Cs. Bogd a´ n et al.
Comparison of the relative complex stability of different ligands on
the basis of uncomplexed metal ion concentration in the pH range up to
–5 (plots not shown here) also reveals that the most stable complexes
4
2
+
are formed with ligand 6. Here the value of pM (pM = −log [M ]) varies
between 6 and 7. The next strongest complex-forming compounds are
2
and 5, where the pM values are in the range of 4.5–5.5. The weakest
complexes are formed with ligands 4, 1, and 3, where the concentra-
tion of the uncomplexed metal ion varies between 20% and 40% in the
pH range < 5. These trends are corroborated by the magnitude of the
conditional and basicity-adjusted stability constants. Here, the trispho-
sphonate 1 forms relatively weak complexes, as expected. The stability
of the complexes decreases with the number of phosphonate moieties,
however, due to the stability decreasing effects of the larger spatial re-
quirement and the higher charge of the phosphonate group. This is also
reflected in the shift of the pH at the formation maximum of the cor-
2
3
−
−
responding NiL species to higher values upon PO /CO substitution
2
(
see Figure 1).
The levels of free metal ion concentration in the region of pH
–5 suggest the following sequences of increasing degree of metal ion
2
complexation:
Ni2 complexes :
Cu2 complexes :
Zn2 complexes :
Cd2 complexes :
+
7 ∼ 4 < 1 ∼ 3 ꢀ 2 < 5 ꢀ 6
7 ∼ 4 ∼ 1 < 3 ꢀ 2 < 5 ꢀ 6
7 ∼ 4 < 3 ∼ 1 ꢀ 2 ꢀ 5 ꢀ 6
7 ∼ 4 < 1 < 3 ꢀ 2 ꢀ 5 << 6
(2)
(3)
(4)
(5)
+
+
+
The above order is in good accordance with the metal-binding abil-
ity of the protonated ligands estimated by the relative magnitudes of
conditional stepwise stability constants (see Table I). In the series of
compounds studied here, the structural motif for outstanding metal
complex stability was proven to be the carboxylate and phosphonate
groups in geminal position. The log Ki > 4 values for these ligands in-
2
2
dicate the formation of at least one chelate ring. Compound 4 contains
−
2−
already two pairs of geminal COO /PO3 moieties, but here the sterical
demand of the charged donor groups leads to diminished metal-binding
affinity.
2
+
Ca forms complexes of significant lower stability as expected. Re-
liable formation constants could be evaluated only from titrations at
TCa/TL ≥ 5. The pCa values range from 1.5 to 1.7 and the following se-
quence of metal-binding affinity can be established:
Ca2 complexes : 7 < 4 < 1 < 5 = 2
+
(6)

Metal Complexes of Oligophosphonic Acids
2349
CONCLUSION
The investigated oligophosphonate/carboxylate ligands were shown to
form only mononuclear complexes even at higher metal : ligand ratios in
aqueous solution (the analysis of precipitates observed at higher pHs
was not attempted). The protonated complexes MLHi prevail in the
same pH range as LHi. The high conditional stability constants found
for several protonated complexes point to chelated structures. The in-
crease in log β stability constants with increasing number of phospho-
nate groups shows their higher ability to form complexes in a biased
fashion: log β values should be purified from effects of different basicity.
Since the greater charge density of the phosphonate(s) is overcompen-
sated by their higher sterical bulk and greater electrostatic repulsion,
the basicity-adjusted or the conditional stability constants indicate that
the gain in affinity to metals diminishes or vanishes, as reported pre-
2
2
viously in a similar comparison of aminocarboxylates/phosphonates.
2
−
The most stable complexes were observed for ligands holding PO and
3
−
CO groups in geminal position.
2
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