Complexes of divalent transition metal ions with bis(aminomethyl)phosphinic acid in aqueous solution and in the solid state

Article abstract of DOI:10.1039/B305844A

Bis(aminomethyl)phosphinic acid, (NH₂CH₂)₂PO₂H (HL), was synthesized using a new procedure. Its coordination ability towards CO(II)/(III), Ni(II), Cu(II) and Zn(II) was studied both in solution and in the solid state. Because of the presence of two nitrogen atoms the ligand exhibits a higher overall basicity than common (aminoalkyl)phosphinic acids. Consequently, the values of the determined stability constants are comparable with those found for (aminoalkyl)phosphonic acids. NMR titrations of Zn(II) point to the interaction of phosphinate with the metal ion in a strong acid solution. The X-ray structures show several coordination modes in the solid state. All-trans-[MCl₂(H₂L-O)₂ (H₂O)₂]Cl₂, where M(II) are Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Ca(II), and Cd(II), crystallized from acid solutions. The central ion is octahedrally coordinated with two phosphinate oxygen atoms, two molecules of water and two chlorides in all-trans arrangement. Amine groups are protonated and non-coordinated. Participation of the donor groups in crystals isolated from neutral solutions depends on the metal ion. All donor atoms are coordinated in monomeric fac-N,N,O-trans-O,O'-[Co(L-N,N,O)₂]⁺. On the other hand, in the zinc(II) complex, two phosphinate oxygen atoms and two amine nitrogen atoms (trans to each other) of two different ligand molecules are coordinated in an equatorial plane and two amino groups of the two other ligand molecules are bound in axial positions. Thus, each molecule of the amino acid forms a five-membered N,O-chelate to one zinc(II) ion and the other amino group is bound to the neighbouring ion creating an infinite chain. Nickel(II) forms a trans-O,O′-[Ni(H₂O)₂-(L-N,N)₂] complex in which the metal ion is chelated by four amine nitrogen atoms forming two six-membered chelates in an equatorial plane and the octahedron is completed with two water molecules at the apical positions. The phosphinate group is not coordinated. The above results point to a relatively low coordination ability of the phosphinate group; however, due to its low pK_A, it is able to bind metal ions at lower pH than other coordinating groups do.

Full text of DOI:10.1039/B305844A

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Complexes of divalent transition metal ions with
bis(aminomethyl)phosphinic acid in aqueous solution and in the
solid state
Vojteˇch Kubícˇek, Pavel Vojtísˇek, Jakub Rudovský, Petr Hermann* and Ivan Lukesˇ
Department of Inorganic Chemistry, Universita Karlova (Charles University), Hlavova 2030,
128 40 Prague 2, Czech Republic. E-mail: petrh@natur.cuni.cz; Fax: ϩ420 22195 1253;
Tel: ϩ420 22195 1263
Received 23rd May 2003, Accepted 11th August 2003
First published as an Advance Article on the web 3rd September 2003
Bis(aminomethyl)phosphinic acid, (NH2CH2)2PO2H (HL), was synthesized using a new procedure. Its coordination
ability towards Co()/(), Ni(), Cu() and Zn() was studied both in solution and in the solid state. Because of the
presence of two nitrogen atoms the ligand exhibits a higher overall basicity than common (aminoalkyl)phosphinic
acids. Consequently, the values of the determined stability constants are comparable with those found for
(aminoalkyl)phosphonic acids. NMR titrations of Zn() point to the interaction of phosphinate with the metal ion
in a strong acid solution. The X-ray structures show several coordination modes in the solid state. All-trans-
[MCl2(H2L-O)2(H2O)2]Cl2, where M() are Mn(), Co(), Ni(), Cu(), Zn(), Ca(), and Cd(), crystallized from
acid solutions. The central ion is octahedrally coordinated with two phosphinate oxygen atoms, two molecules of
water and two chlorides in all-trans arrangement. Amine groups are protonated and non-coordinated. Participation
of the donor groups in crystals isolated from neutral solutions depends on the metal ion. All donor atoms are
coordinated in monomeric fac-N,N,O-trans-O,OЈ-[Co(L-N,N,O)2]ϩ. On the other hand, in the zinc() complex, two
phosphinate oxygen atoms and two amine nitrogen atoms (trans to each other) of two different ligand molecules are
coordinated in an equatorial plane and two amino groups of the two other ligand molecules are bound in axial
positions. Thus, each molecule of the amino acid forms a five-membered N,O-chelate to one zinc() ion and the other
amino group is bound to the neighbouring ion creating an infinite chain. Nickel() forms a trans-O,OЈ-[Ni(H2O)2-
(L-N,N)2] complex in which the metal ion is chelated by four amine nitrogen atoms forming two six-membered
chelates in an equatorial plane and the octahedron is completed with two water molecules at the apical
positions. The phosphinate group is not coordinated. The above results point to a relatively low
coordination ability of the phosphinate group; however, due to its low pKA, it is able to bind metal ions
at lower pH than other coordinating groups do.
complexes of (aminoalkyl)phosphonic acids with transition
metal ions is usually higher than that of aminocarboxylates,
Introduction
(Aminoalkyl)phosphonic or phosphinic acids as phosphorus
analogues of naturally occurring aminocarboxylic acids have
attracted much interest due to their biological activity.1,2
Because of the tetrahedral structure of the phosphonic
R–PO3H2 as well as phosphinic R2PO2H (R = alkyl, aryl) acids,
they mimic tetrahedral intermediates in hydrolysis of carb-
oxylic esters, amides and peptides. Therefore, aminophosphinic
acid derivatives exhibit pronounced biological effects; they
strongly inhibit metalloenzymes,2,3 e.g. matrix metallo-
proteinases and astacins, angiotensin I converting enzyme,
thrombin, endothelin converting enzyme, etc., or alter receptor
functions, e.g. of GABA receptors.2,4 (Aminoalkyl)phosphinic
acid peptides have been also suggested as compounds of inter-
est for a new drug design,2,5 e.g. analgetics, antibiotics or agents
against protozoa infections.
which is caused, at least in the first approximation, by enhanced
basicity of the ligands. However, coordination of the second
ligand molecule to the same metal ion is disfavoured due to the
bulkiness of the phosphonate grouping. For metal ions with
higher charges, phosphonates are clearly superior ligands due
to a better compensation of Coulombic repulsion by metal
ion mainly if more than one phosphonate group is coordin-
ated. Some of their properties are also evident in phos-
phonodipeptides and their complexes.9–11 It is possible to
compare complexation abilities of distereoisomers. We have
shown that the differences are higher than for corresponding
carboxylic peptides as a consequence of stronger interactions
between side chains in phosphonopeptides. Deprotonation of
peptide nitrogen atom and simultaneous coordination to
a metal ion was observed only in the case of Cu2ϩ with a higher
pKA corresponding to the reaction. This was explained by
electron delocalization in the phosphonate dianion.
On the other hand, little attention has been paid to co-
ordination properties of (aminoalkyl)phosphinic acids in
solution12 and in the solid state.13–20 The results indicate that,
compared to (aminoalkyl)phosphonic and aminocarboxylic
acids, the amino groups in (aminoalkyl)phosphinic acids are
weakly basic and the phosphinic acid groups are often strongly
acidic. Consequently, the complexes exhibit lower stability con-
stants. The influence of the substituent on the phosphorus atom
in the phosphinic acid group on coordination properties in
phosphinic acid analogues of glycine (H2N–CH2–P(R)O2H,
R = H, Me, Ph, tert-Bu) was investigated21 with the aim to
explain the influence of the phosphinic acid pendant groups in
The complexing behaviour of (aminoalkyl)phosphonic/
phosphinic acids is not so well understood as that of amino-
carboxylic acids.6 Simple (aminoalkyl)phosphonic acids have
been extensively studied in solution.7 In contrast, much less
papers have dealt with their complexes in the solid state.8 The
differences in basicity, charge, the electron-releasing effect and
the size of relevant donor groups result in the differences in
complex-forming properties of aminophosphonic and amino-
carboxylic acids in solution. Because of more basic character of
2Ϫ
the –PO3 group compared to –COOϪ group, there is a
possibility of monodentate co-ordination of the ligands via the
phosphonate group, thus leaving other donor groups easily
accessible to protonation, forming stable protonated complexes.
In the case of chelate formation, thermodynamic stability of
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 3 9 2 7 – 3 9 3 8
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polyazamacrocycles.22 Phosphinic acid analogues of Gly–Gly
with the same substituents on phosphorus atoms exhibit a
surprising behaviour.23 They form with Cu2ϩ very easily a violet
[Cu(phosphinodipeptide)2]2Ϫ species where the ligands are co-
ordinated only through amine and deprotonated amide nitro-
gen atoms. In contrast to Gly–Gly and Gly–Gly(P), the violet
complex is formed at a relatively low pH (around 9) even with
only a small excess of the ligand and the effect is greatly
influenced by the phosphorus atom substituent.
In continuation of our investigations of complexation of
transition metal ions with (aminoalkyl)phosphonic/(amino-
alkyl)phosphinic acids as well as with their dipeptides,10,11,21,23
we present here the results obtained with bis(aminomethyl)-
phosphinic acid. It is the simplest model for compounds
in which the phosphinic acid group is located inside the chain,
e.g. oligopeptide chain. Such compounds have been investi-
gated as strong inhibitors of a variety of enzymes.2 Recently,
several papers dealing with the complexation ability of poly-
dentate open-chain24 and macrocyclic25 ligands containing the
(–NHCH2)2PO2H moiety appeared. A preliminary conference
report of the work presented has been communicated.26
presence of extra HNO3 in the Ϫlog[Hϩ] range 1.8–11.9 (or till
metal hydroxides precipitation) using a PHM 240 pH-meter,
a 2 ml ABU 900 automatic piston burette and a GK 2401B
combined electrode (Radiometer). The initial volume was 5
cm3, the concentration of the ligand was 0.004 mol dmϪ3 and
ligand-to-metal ratios were 1.1 : 1, 2 : 1 and 4 : 1 (in the case
of Cu2ϩ also 0.007 mol dmϪ3 with the same HL : M ratios).
Titrations were carried out at least four times, each consisting
of about 40 points. An inert atmosphere was ensured by con-
stant passage of argon saturated with the vapor of the solvent
used in measurements. The water ion product, pKw = 13.78, and
stability constants of M2ϩ–OHϪ systems were taken from ref.
29. The protonation constants βn calculated are concentration
constants and are defined by βn = [HnL]/([H]n × [L]) (pK3 = log
β1; pK2 = log β2 Ϫ log β1; pK1 = log β3 Ϫ log β2); the stability
constant are defined by βpqr = [MpHqLr]/[M]p × [H]q × [L]r. The
constants (with standard deviations) were calculated with pro-
gram OPIUM (ref. 30). The program minimises the criterion of
the generalised least-squares method using the calibration
function
E = E0 ϩ S × log[Hϩ] ϩ jA × [Hϩ] ϩ jB × Kw/[Hϩ]
Experimental
where the additive term E0 contains the standard potentials of
the electrodes used and contributions of inert ions to the liquid-
junction potential, S corresponds to the Nernstian slope, the
value of which should be close to the theoretical value and jA ×
[Hϩ] and jB × [OHϪ] terms are contributions of the Hϩ and
OHϪ ions to the liquid-junction potential. It is clear that jA and
jB cause deviation from a linear dependence between E
and Ϫlog[Hϩ] only in strong acid and strong alkaline solutions.
The calibration parameters were determined from titration of
standard HNO3 with standard KOH solutions before any titra-
tion of the ligand to give a pair of calibration titration/titration
used for calculations of the constants. The calibration pro-
cedure of the glass electrode in the wide pH region described
above permitted determination of log β with the statistical
standard deviation 0.01 (even outside the titration pH region;
see Supplementary material in ref. 22b). However, the real
accuracy could be lower and we estimate it to be within 0.05
log units or even decreased for species with low abundance
(5–15%).
General
Reactions with pyrophoric bis[(trimethysilyl)oxy]phosphine
((Me3SiO)2PH) were performed under an argon atmosphere.
PhtNCH2Br27 (Pht is phtaloyl) and Na3[Co(CO3)3]ؒ3H2O28
were prepared using literature procedures. Other chemicals
were from commercial sources (Lachema, Aldrich, Fluka
or Acros). Dichloromethane was dried over P2O5. Water for
determination of protonation constants was purified using the
Milli-Q (Millipore) purification system. TLC was performed on
Silufol silica gel sheets (Kavalier, Votice, Czech Republic) or on
Merck 1.05554 F254 sheets in a propan-2-ol–25% aq. NH3–
water (7 : 3 : 3) mixture using ninhydrin or UV detection.
Elemental analyses were carried out in the Institute of Macro-
molecular Chemistry, Academy of Sciences of the Czech
Republic (Prague). Melting points were determined using a
Kofler hot-stage apparatus (Boetius) and are uncorrected.
NMR spectra were recorded on a Varian Unity Plus at 400
MHz for 1H, 169 MHz for 31P and 100 MHz for 13C with
internal references TMS for DMSO-d6 solutions and t-BuOH
for D2O solutions and external reference 85% H3PO4. Thermo-
gravimetric (TG) analysis was performed at the Institute
of Chemical Technology (Prague) on a Stanton Redcroft
apparatus at 25–300 ЊC in air (10 ЊC minϪ1). Sulfonate (Dowex
50, Fluka) and carboxylate (Amberlite CG50/N1, Rohm &
Haas) cation exchangers were used for ion-exchange column
chromatography.
NMR titrations
Measurements of 1H and 31P{1H} NMR spectra in dependence
on pH (from 5 M HCl up to precipitation of metal hydroxide)
were done at ligand concentration ∼0.1 mol dmϪ3 in the pres-
ence or absence of ZnCl2 (ratio HL : Zn = 2 : 1, room temper-
ature). pH of the samples was adjusted with dilute aqueous
KOH and aqueous HCl. Deuterium oxide (containing 1%
t-BuOH and 15% H3PO4 as an external standards) in an
inserted glass capillary was used for frequency lock and the
water signal in 1H NMR was presaturated.
Chemicals and stock solutions for potentiometric titrations
A stock solution of nitric acid (∼0.03 M) was prepared by pass-
ing a solution of recrystallised potassium nitrate through a
Dowex 50W-8 column in the Hϩ form because of traces of NO
and NO2 present in the concentrated acid. Carbonate-free KOH
solution (∼0.2 M) was standardized against potassium hydro-
gen phthalate and HNO3 solution against the ∼0.2 M KOH
solution. Samples of the ligand (HLؒHCl) for preparation of
stock solutions were dried to constant weight at room temper-
ature in a desiccator over P2O5. Analytical concentration of the
ligand determined from the weights of the dried ligand was
in accordance with its concentration determined together with
the refinement of protonation constants using the OPIUM
program.
UV-VIS spectra
Metal chloride hydrate (M = Ni(), Cu()) and HLؒHCl (two
equivalents) were dissolved in water and pH of the sample was
carefully adjusted with dilute KOH and aqueous HCl solutions
to a value where the maximum of [M(L)2] species should be
present according to distribution diagrams. The Co() complex
was dissolved in water (without adjusting pH). Spectra were
recorded on a UV 300 (Pye Unicam) in the 200–1100 nm range
at room temperature.
Crystal structure determinations
The diffraction-quality single crystals of all-trans-[MCl2-
(H2L)2(H2O)2]Cl2 complexes were grown from aqueous solution
by slow evaporation at room temperature. Single crystals
of complexes (trans-O,O-[Ni(L-N,N)2(H2O)2]ؒ2H2O, [Zn(L)2]ؒ
Potentiometric titrations
Titrations were carried out in a vessel thermostatted at 25
0.1 ЊC, at ionic strength I(KNO3) = 0.1 mol dmϪ3 and in the
D a l t o n T r a n s . , 2 0 0 3 , 3 9 2 7 – 3 9 3 8
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2H2O) and ligand (HLؒHCl, its solution pH was adjusted with
HCl to 2–3) were obtained from aqueous solutions by vapour
diffusion of EtOH. Single crystals of fac-N,N,O-trans-O,OЈ-
[Co(L-N,N,O)2](ClO4)ؒH2O were obtained from concentrated
aqueous solution on standing in a closed vial. The selected
crystals were mounted on glass fibres in random orientation
by epoxy glue for measurements at room temperatures and by
silicone fat for low-temperature measurements. Diffraction
data were collected using a Nonius Kappa CCD diffractometer
at 293(2) and at 150(1) K (Cryostream Cooler (Oxford Cryo-
system)) and analyzed using the HKL program package (ref.
31). The structures were solved by the direct methods, and
refined by full-matrix least-squares techniques (SIR92,32
SHELXL9733). Final geometric calculations were carried out
with a recent version of the PLATON program.34 Scattering
factors for neutral atoms used were included in the SHELXL97
program. Hydrogen atoms were localized in all structures
(with the exception of two hydrogen atoms of a water molecule
in the cadmium() complex) on the difference maps and refined
isotropically. Tables 1 and 2 give pertinent crystallographic
data.
Mp 290–295 ЊC (dec.); TLC (ninhydrin): Rf = 0.6; δH (D2O)
3.24 [d, J(PH) 9.8 Hz, CH2P]; δP (D2O) 21.1 (q, J(PH) 9.8 Hz).
(Found C, 15.33; H, 6.17; N, 16.69; Cl, 22.13. Calc. for
C2H10ClN2O2P (160.54): C, 14.96; H, 6.28; N, 17.45; Cl,
22.08%). Thermogravimetry: Decomposition starts at 50 ЊC
with loss of HCl followed by full decomposition of the
compound.
Complexes of HL with divalent metal ions from strong acid
solutions (general procedure)
Bis(aminomethyl)phosphinic acid hydrochloride (1.2 mmol)
and metal() chloride hydrate (0.6 mmol) were dissolved in 6 M
HCl (10 ml). After dissolution of the solids, the hydrochloric
acid was removed in vacuum. The solid residue was dissolved in
water (10 ml) and the solution was evaporated again. The dis-
solution and evaporation was repeated three times. The com-
plex was then dissolved in water (5 ml) and crystallised by free
evaporation. Crystals were filtered off, washed with ethanol and
dried in a desiccator over P2O5. Yields and analytical data are
given in Table 3.
Example of thermogravimetric decomposition (all-trans-
[CoCl2(H2L)2(H2O)2]Cl2): Loss of two water molecules pro-
ceeds at 170–230 ЊC. Decomposition of the complex starts at
290 ЊC.
CCDC reference numbers 211306 (Ca), 211312 (Mn), 211308
(Co), 211313 (Ni), 211310 (Cu), 211315 (Zn), and 211307 (Cd)
for all-trans-[MCl2(H2L)2(H2O)2]Cl2 (M = Ca, Mn, Co, Ni, Cu,
Zn and Cd); 211314 (trans-O,O-[Ni(L-N,N)2(H2O)2]ؒ2H2O);
211316 ([Zn(L)2]ؒ2H2O); 211311 (HLؒHCl); and 211309
(fac-N,N,O-trans-O,OЈ-[Co(L-N,N,O)2](ClO4)ؒH2O).
Complexes of HL with Ni(II), Cu(II) and Zn(II) from
neutral/alkaline solutions (general procedure)
See http://www.rsc.org/suppdata/dt/b3/b305844a/ for crystal-
lographic data in CIF or other electronic format.
Chlorides were removed from HLؒHCl (0.30 g, 1.88 mmol) on
Dowex 50 (Hϩ-form) with water elution. The free amino acid
was eluted with 5% aqueous ammonia. The HL-containing
solution was evaporated to dryness in vacuum and this was
repeated three times to remove excess ammonia. The residue
was dissolved in water (2 ml) and added to the freshly precipi-
tated and washed M(OH)2 (0.94 mmol). After dissolution of
the hydroxide, the solution was filtered through a plug of cotton
wool. pH of this solution was carefully adjusted (glass elec-
trode) to the required value mentioned below using 10% aque-
ous solutions of NaOH or HClO4. The complexes crystallized
on EtOH vapour diffusion in a large closed vial.
trans-O,OЈ-[Ni(L-N,N)2(H2O)2]ؒ2H2O: pH of the solution
was adjusted to 9.25. Blue needles, losing water of crystalliz-
ation on long standing. Single crystals were selected from the
bulk. Yield 0.27 g (83%). (Found: C, 13.2; H 5.94. Calc. for
C4H16N4NiO4P2ؒ4H2O (376.89): C 12.8, H 6.42%). Thermo-
gravimetry: complete dehydration occurs at 110–120 ЊC. The
anhydrous form is stable up to 350 ЊC. UV-VIS: Ϫlog[Hϩ] = 9.3:
λmax (ε) = 597 (12), 366 nm (18 molϪ1 dm3 cmϪ1).
Bis(phthalimidomethyl)phosphinic acid
In argon atmosphere, the dried NH4H2PO2 (2.0 g, 0.024 mol)
was suspended in (Me3Si)2NH (20 ml) and the mixture was
heated at 110 ЊC (bath temperature) under a small flow of argon
for 6 h. The mixture containing pure (Me3SiO)2PH was cooled
to room temperature and dry dichloromethane (60 ml) was
added. After cooling the solution in a water-ice bath, a solution
of PhtNCH2Br (12.0 g, 0.05 mol) in dry CH2Cl2 (100 ml) was
dropped into the solution during 5 min. The reaction mixture
was stirred at room temperature for 12 h under argon and then
MeOH (20 ml) was added to hydrolyse silyl esters. After 10 min,
the reaction mixture was diluted with 50 ml of CH2Cl2 and
extracted with 6 M HCl (100 ml). After some time, a fine white
crystalline solid precipitated in the organic phase. It was filtered
off, washed with EtOH (5 ml) and dried in a desiccator over
P2O5. The aqueous phase was re-extracted with CH2Cl2 (2 ×
50 ml). The organic layers were collected, dried with anhydrous
Na2SO4 and the solvent was removed in vacuum to give addi-
tional product. Both the fractions were recrystallized together
from hot EtOH to give 7.1 g of product (76%).
[Cu(L)2]ؒ2H2O: pH of the solution was adjusted to 8.50.
Light blue microcrystalline powder. Yield 0.30 g (93%). (Found:
C 13.2; H 5.42. Calc. for C4H16CuN4O4P2ؒ2H2O (345.72): C
13.9, H 5.83%). Thermogravimetry: dehydration occurs in the
range of 150–180 ЊC. The anhydrous form is stable up to 220 ЊC.
UV-VIS: Ϫlog[Hϩ] = 8.2: λmax (ε) = 616 (83), 273 nm (4100
molϪ1 dm3 cmϪ1).
Mp 280–285 ЊC (dec.); TLC (ninhydrine): Rf
= 0.8;
δH (DMSO-d6) 4.07, [4 H, d, J(PH) 7.6 Hz, 2 CH2P]; 7.8–8.0, [8
H, m, 2 C6H5]; δP (DMSO-d6): 33.4 [q, J(PH) 7.6 Hz]. (Found C,
54.18; H, 3.90; N, 6.67. Calc. for C18H13N2O6P (384.24): C,
56.26; H, 3.41; N, 7.29%).
[Zn(L)2]ؒ2H2O: No pH adjustment. Colorless crystals, losing
water of crystallization on long standing. Single crystals were
picked up from the bulk. Yield 0.35 g (92%). Thermogravi-
metry: dehydration starts at 50 ЊC followed by decomposition.
No region of the anhydrous form was observed.
Bis(aminomethyl)phosphinic acid hydrochloride (HLؒHCl)
Bis(phthalimidomethyl)phosphinic acid (5.90 g, 0.015 mol) was
suspended in a mixture of 6 M HCl (200 ml) and EtOH (50 ml).
The mixture was refluxed for 12 h and partly evaporated after
cooling. Crystalline phthalic acid was filtered off, washed with
cold water and the filtrate was purified on a cation exchange
resin column (Hϩ, Dowex, 3 × 20 cm) by elution with water
followed by 5% aqueous ammonia. The fractions containing
product were combined, the solvent was evaporated in vacuum
and excess of ammonia was removed by repeated evaporation
with water. The residue was dissolved in 1 M aqueous HCl
(5 ml) and the product was precipitated by slow addition of
excess EtOH. Yield 2.20 g HLؒHCl (92%).
fac-N,N,O-trans-O,OЈ-[Co(L-N,N,O)2](ClO4)ؒH2O
The chloride anion from HLؒHCl (0.25 g, 1.56 mmol) was
removed on Dowex 50 with water elution. The amino acid was
eluted with 5% aqueous ammonia. The HL-containing solution
was evaporated to dryness in vacuum and this was repeated
three times to remove excess ammonia. The residue was dissol-
ved in water (2 ml). The Na3[Co(CO3)3]ؒ3H2O complex (0.24 g,
0.78 mmol) was suspended in water (5 ml) and the ligand
D a l t o n T r a n s . , 2 0 0 3 , 3 9 2 7 – 3 9 3 8
3929

D
Table 1 Experimental data for the X-ray difraction studies of all-trans-[MCl2(H2L-O)2(H2O)2]Cl2 (C4H24Cl4MN4O6P2). All complexes are monoclinic, space group P21/n (no. 14), Z = 2 with M in special position. Data
were obtained at room temperature (293(2) K), with the exception of M = Ca and Ni (150(1) K); λ = 0.71070 Å using CCD rotation scans
Parameter
Ca
Mn
Co
Ni
Cu
Zn
Cd
Formula weight
Crystal dimension/mm
Shape
Colour
a/Å
b/Å
c/Å
α/Њ
468.09
482.95
486.94
0.5 × 0.5 × 0.4
Prism
486.72
491.55
493.38
0.6 × 0.3 × 0.16
Prism
Colorless
8.2290(2)
7.8630(2)
13.3150(3)
90
540.41
0.3 × 0.3 × 0.52
Plate
Colorless
8.3050(2)
7.9080(2)
13.5910(3)
90
0.7 × 0.35 × 0.4
Irregular
Colorless
8.4190(2)
7.9000(2)
13.6270(3)
90
0.3 × 0.35 × 0.3
Irregular
Pale pink
8.3320(4)
7.9230(4)
13.3980(8)
90
0.5 × 0.35 × 0.4
Irregular
Pale green
8.2110(2)
7.8530(2)
13.2350(5)
90
0.2 × 0.35 × 0.4
Irregular
Pale blue
8.1620(2)
8.0250(2)
13.3400(3)
90
Violet
8.2740(2)
7.9080(2)
13.2520(4)
90
β/Њ
102.423(1)
90
885.11(4)
1.756
103.325(4)
90
860.65(4)
1.864
103.028(2)
90
844.77(4)
1.914
103.042(2)
90
831.39(5)
1.944
104.285(1)
90
846.75(4)
1.928
103.033(2)
90
839.35(4)
1.952
102.386(1)
90
871.83(4)
2.059
γ/Њ
V/Å3
Dc/g cmϪ3
µ/mmϪ1
1.17
1.60
1.86
2.03
2.14
2.32
2.08
F(000)
484.0
3.58–27.50
494.0
3.01–28.33
498.0
3.16–27.54
500.0
3.16–27.53
502.0
3.15–27.48
504.0
3.14–27.50
540.0
3.07–27.54
θ range of data collection/Њ
Index ranges h/k/l
0
10/0
0
11/–10
0
10/0
0
10/0
0
10/0
0
10/0
0
10/Ϫ10
10/Ϫ17
3942
0.0250
1887
17
10/Ϫ17
5048
0.0362
1852
16
10/Ϫ17
3838
0.0291
1841
16
10/Ϫ17
3803
0.0439
1640
16
10/Ϫ17
3872
0.0259
1814
17
10/Ϫ17
3841
0.0247
1803
16
10/Ϫ17
14605
0.0278
1886
17
Reflections collected
Rσ
Reflections observed [I > 2σ(I )]
Independent reflections
Rint
1991
2038
1922
1910
1933
1922
1997
0.0102
0.0194
0.3798
1991/0/146
1.148
0.0197
0.0545
0.000
0.0245
0.0400
0.1402
2038/0/146
1.217
0.0291
0.0833
0.000
0.0258
0.0339
0.4540
1922/0/146
1.113
0.0268
0.0761
0.001
0.0321
0.0659
0.0922
1910/0/146
1.069
0.0367
0.1040
0.003
0.0172
0.0351
0.3536
1933/0/146
1.055
0.0208
0.0636
0.001
0.0165
0.0189
0.5553
1922/0/146
1.060
0.0190
0.052
0.0514
0.0309
11.2365
1997/0/138
1.303
0.0467
0.1370
0.001
Coeffs. in weighting schemea
Data/restrains/parameters
Goodness-of-fit on F 2
Final R, RЈ indices [I ≥ 2σ(I )]b
Maximum shift/esd
0.000
0.401/Ϫ0.357
Largest diff. peak and hole/e Å3
0.372/Ϫ0.262
0.596/Ϫ0.461
0.401/Ϫ0.487
0.750/Ϫ1.182
0.479/Ϫ0.604
3.48/Ϫ0.87
a w = 1/[σ2(F0 ) ϩ (A × P)2 ϩ B × P], where P = (F02 ϩ 2Fc2)/3 (SHELXL97, ref. 33). b R = Σ|F0 Ϫ Fc|/Σ|Fc|, RЈ = [Σw(F02 Ϫ Fc2)2/Σw(F0 )2]1/2 (SHELXL97, ref. 33).
2
2

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Table 2 Experimental data for the X-ray difraction studies of HLؒHCl, trans-O,O-[Ni(L-N,N)2(H2O)2]ؒ2H2O, [Zn(L)2]ؒ2H2O and fac-N,N,O-trans-
O,OЈ-[Co(L-N,N,O)2](ClO4)ؒH2O
Parameter
HLؒHCl
[Ni(L)2(H2O)2]ؒ2H2O
[Zn(L)2]ؒ2H2O
[Co(L)2](ClO4)ؒH2O
Empirical formula
Formula weight
C2H10ClN2O2P
160.54
C4H24N4NiO8P2
376.92
C4H20N4O6P2Zn
347.55
C4H18ClCoN4O9P2
422.54
T/K
150(1)
0.3 × 0.2 × 0.2
Prism
Colorless
Monoclinic
P21 (no.4)
4.9780(2)
7.6100(3)
8.3910(3)
90
293(2)
0.1 × 0.4 × 0.07
Needle
293(2)
0.1 × 0.2 × 0.4
Plate
Colorless
Monoclinic
P21/c (no.14)
5.6990(1)
17.2390(3)
6.2940(2)
90
293(2)
Crystal dimension/mm
Shape
Colour
Crystal system
Space group
a/Å
b/Å
c/Å
0.2 × 0.3 × 0.3
Irregular
Brown orange
Monoclinic
P21/n (no.14)
10.2410(3)
9.4680(2)
15.1850(5)
90
Blue
Monoclinic
P21/c (no.14)
8.030(1)
6.397(1)
14.353(2)
90
α/Њ
β/Њ
94.750(3)
90
316.78(2)
2
104.814(9)
90
712.8(2)
2
90.351(2)
90
618.34(2)
2
104.466(2)
90
1425.68(7)
4
γ/Њ
V/Å3
Z
Dc/g cmϪ3
1.683
1.756
1.867
1.969
λ/Å
0.71070
0.77
0.71070
1.62
0.71070
2.27
0.71070
1.67
µ/mmϪ1
F(000)
168.0
3.62–27.47
396.0
3.40–25.12
360.0
3.45–34.99
Ϫ9
25/Ϫ9
13538
0.0339
2414
2670
0.0406
0.0327; 0.2042
2670/0/120
1.055
0.0284
0.0742
864.0
3.49–27.54
θ range of data collection/Њ
Index ranges h/k/l
0
6/Ϫ9
10
0
9/0
9/Ϫ25
10
0
13/0
9/Ϫ10
1443
0.0356
1354
1442
—
7/Ϫ16
1261
0.0719
1014
1256
0.0168
0.0926; 0.5343
1256/0/136
1.084
0.0604
0.1576
16
12/Ϫ19
6554
0.0310
2898
3278
0.0245
0.0455; 0.5651
3278/0/291
1.051
0.0311
0.0827
19
Reflections collected
Rσ
Reflections obsd. [I > 2σ(I )]
Independent reflections
Rint
Coeffs. in weighting schemea
Data, restrains, parameters
Goodness-of-fit on F 2
Final R, RЈ indices [I ≥ 2σ(I )]b
0.0211; 0.1474
1442/0/113
1.055
0.0242
0.0602
Maximum shift/esd
0.001
0.286; Ϫ0.349
0.001
0.986; Ϫ0.859
0.001
0.722; Ϫ0.744
0.000
0.606; Ϫ0.518
Diff. peak and hole/e Å3
2
2
2
2
a w = 1/[σ2(F0 ) ϩ (A × P)2 ϩ B × P], where P = (F0 ϩ 2Fc2)/3 (SHELXL97, ref. 33). b R = Σ|F0 Ϫ Fc|/Σ|Fc|, RЈ = [Σw(F0 Ϫ Fc2)2/Σw(F0 )2]1/2
(SHELXL97, ref. 33).
Table 3 Yields and analyticala data for all-trans-[MCl2(H2L)2(H2O)2]Cl2 complexes
Complex
Analysis (%)
Yieldb (%)
C
H
N
Cl
[CaCl2(H2L)2(H2O)2]Cl2
[MnCl2(H2L)2(H2O)2]Cl2
[CoCl2(H2L)2(H2O)2]Cl2
[NiCl2(H2L)2(H2O)2]Cl2
[CuCl2(H2L)2(H2O)2]Cl2
[ZnCl2(H2L)2(H2O)2]Cl2
[CdCl2(H2L)2(H2O)2]Cl2
13.5 (10.3)
10.3 (10.0)
10.4 (9.9)
10.5 (9.9)
10.2 (9.8)
10.3 (9.7)
9.3 (8.9)
5.33 (5.17)
4.46 (5.01)
4.58 (4.97)
4.81 (4.97)
4.57 (4.92)
4.46 (4.90)
4.10 (4.48)
14.9 (12.0)
11.3 (11.6)
11.4 (11.5)
11.5 (11.5)
11.1 (11.4)
11.2 (11.4)
10.0 (10.4)
26.3 (30.3)
29.0 (29.4)
28.7 (29.1)
28.8 (29.1)
28.7 (28.9)
28.3 (28.7)
26.2 (26.2)
45
60
72
28
65
52
81
a Required values are given in parentheses. b Yields strongly depend on the volume of mother solution left over the crystals due to a high solubility of
the complexes.
Results and discussion
solution was added. The mixture was left for a week in the
refrigerator (5 ЊC). After that 70% aqueous HClO4 (0.07 g) was
added and the mixture was left in the refrigerator for another
few weeks. The deep-red solution was purified on an Amberlite
CG50 column (2 × 20 cm). Cobalt() complex and HClO4 were
eluted with water and the free ligand and cobalt() species were
trapped on the column. The cobalt()-containing fraction was
evaporated to a small volume (ca. 2 ml) and the solution was
left to crystallize in a closed flask. After two weeks, red-violet
crystals were filtered and dried in air, yield 0.03 g (12% based on
the starting cobalt complex). Found: C, 11.9; H 4.46. Calc. for
C4H16ClCoN4O8P2ؒH2O (421.54): C 11.4, H 4.29%. Thermo-
gravimetry: dehydration starts at 50 ЊC and is complete at
140 ЊC. The anhydrous form is stable up to 280 ЊC. UV-VIS:
Ϫlog[Hϩ] = 6.5: λmax (ε) = 510 (70), 368 (154), 207 nm (14600
molϪ1 dm3 cmϪ1).
Synthesis of the ligand and its crystal structure
The HL ligand was synthesised by Maier in a multi-step syn-
thesis some time ago.35 Here we used a different, simpler and
scalable approach based on the Arbuzov reaction of intermedi-
ate (Me3SiO)2PH which is now widely used in the synthesis of
phosphinic acids.36 (Scheme 1). The pyrophoric phosphine is
synthesised in situ by reaction of ammonium hypophosphite
and (Me3Si)2NH.37 Alkylation of the phosphine with alkyl halo-
genides leads to mono- and disubstituted phosphinic acids.
However, the reaction is not usually used for the synthesis of
monoalkylphosphinic acids. Even if the phosphine : RX ratio is
1 : 1, mixture of the starting phosphine, monoalkylphosphinic
and dialkylphosphinic acid is obtained.38 Here, the use of an
excess of the halogenide gave a high yield of the protected
D a l t o n T r a n s . , 2 0 0 3 , 3 9 2 7 – 3 9 3 8
3931

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Table 4 Bond lengths (Å) and angles (Њ) in the crystal structure of
HLؒHCl
Bond or angle
Length or angle
P(1)–O(11)
P(1)–O(12)
P(1)–C(11)
P(1)–C(12)
1.505(1)
1.498(1)
1.832(2)
1.831(2)
O(11)–P(1)–O(12)
O(11)–P(1)–C(11)
O(2)–P(1)–C(1)
O(12)–P(1)–C(11)
O(12)–P(1)–C(12)
C(11)–P(1)–C(12)
118.29(7)
108.6(1)
108.1(1)
108.6(1)
108.6(1)
103.7(1)
Scheme 1
ligand. Deprotection led to the desired ligand isolated as a
stable monohydrochloride salt. The ligand can be prepared as
free base but this form decomposes during several days in the
solid state as well as in aqueous solution.
In the solid state, HLؒHCl is protonated on both nitrogen
atoms (Fig. 1a). Deprotonated phosphinate shows almost the
same P–O(1) and P–O(2) bond lengths (Table 4), which are
Table 5 Protonation constants of HL (βn) and its stability constants
(βpqr) with Co2ϩ, Ni2ϩ, Cu2ϩ and Zn2ϩ a
Ion
Hϩ
Species
log β
intermediate between P᎐O and P–O bond lengths. Structure
᎐
HL
8.509(6)
(H2L)ϩ
15.580(7)
16.35(2)
10.37(6)
4.074(5)
6.79(7)
parameters around the phosphorus atom are almost the
same as those found in the structure of (aminomethyl)-
(H3L)2ϩ
Co2ϩ
[Co(HL)]2ϩ
[Co(L)]ϩ
Ϫ
39a
methylphosphinic acid (ϩNH3CH2P(Me)O2
)
and recently
published bis(N-morpholinomethyl)phosphinic acid ([(OC4-
H8NHCH2)2PO2]ClؒH2O39b where only O–P–O (115.9Њ) and
C–P–C (101.7Њ) angles (aminomethylphosphinic acid) and
C–P–C (101.5Њ) angle (morpholinophosphinic acid) are slightly
different. The structure is stabilized by strong intermolecular
hydrogen bonds N–H ؒ ؒ ؒ O (2.96–2.98 Å). The presence of
N–H ؒ ؒ ؒ Cl hydrogen bonds can be also proposed as the ClϪ
are close to the nitrogen atoms (3.17–3.24 Å). The hydrogen
[Co(L)2]
[Co(OH)(L)]
[Co(OH)2(L)]Ϫ
[Ni(HL)]2ϩ
[Ni(L)]ϩ
Ϫ5.1(1)
Ϫ14.84(2)
11.0(1)
Ni2ϩ
Cu2ϩ
5.580(7)
9.41(2)
[Ni(L)2]
[Ni(OH)(L)2]Ϫ
[Cu(HL)]2ϩ
[Cu(L)]ϩ
Ϫ1.46(4)
12.71(5)
7.64(1)
[Cu(HL)(L)]ϩ
[Cu(L)2]
19.79(3)
13.05(2)
3.07(4)
[Cu(OH)(L)2]Ϫ
[Cu(OH)2(L)2]2Ϫ
[Zn(HL)]2ϩ
[Zn(L)]ϩ
Ϫ8.66(3)
10.6(1)
Zn2ϩ
4.12(2)
[Zn(L)2]
7.53(6)
Ϫ1.07(9)
Ϫ10.6(1)
[Zn(OH)(L)2]Ϫ
[Zn(OH)2(L)2]2Ϫ
a Values in parenthesis are standard deviations given by OPIUM.
bond-stabilized structure is formed by layers of ligand mole-
cules intercalated in layers of chloride anions (Fig. 1b).
Solution studies
The ligand possesses three protonation sites and we were able to
determine all the corresponding protonation (dissociation) con-
stants (Table 5). Basicities of amino groups (pKA 8.51 and 7.07)
show that the one value is higher and the other is lower than
those determined for simple (aminomethyl)phosphinic acids
(8.07–8.43).21 Nevertheless, the found basicity is much lower
than that of glycine (9.56)40 or of (aminomethyl)phosphonic
acid (10.05).40 The values indicate that the electron-withdraw-
ing effect of phosphinate efficiently decreases the amine basicity
even in the case of two amino groups in a molecule. In com-
parison with propane-1,3-diamine (10.52 and 8.74),40 the con-
stants observed for HL are more close to each other. This can
be explained by the presence of the negatively charged phos-
phinate group between the positively charged amino groups,
releasing electrostatic repulsion. The phosphinate group is pro-
tonated only at low Ϫlog[Hϩ] and the value is comparable with
those determined for the phosphinic acid analogues of glycine21
and Gly–Gly.23 The diprotonated form of the ligand is stable in
a wide region of Ϫlog[Hϩ] up to physiological pH.
In all the systems studied the expected species [M(L)]ϩ,
[M(L)2] and protonated [M(HL)]2ϩ were found. In addition,
hydroxo species [Co(OH)(L)] and [Co(OH)2(L)]Ϫ were observed
Fig. 1 Molecular structure of the (H2L)+ cation with the atom
numbering scheme (a) and crystal packing of HLؒHCl as seen along the
a axis (b).
D a l t o n T r a n s . , 2 0 0 3 , 3 9 2 7 – 3 9 3 8
3932

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Table 6 Derived stability constants and constants of similar ligandsa
pKA or log K
c
Ion
Hϩ
Equilibrium
HL
Gly(P)b
Gly(Pt-Bu
)
Glyd
HL
H2L
H3L
H ϩ L
H ϩ HL
H ϩ H2L
8.51
7.07
0.77
6.30
4.07
2.72
10.05
5.39
0.4
7.34
4.45
3.64
8.427
1.204
9.56
2.36
Co2ϩ
Ni2ϩ
Cu2ϩ
[Co(HL)]
Co ϩ L
H ϩ [Co(L)]
[Co(L)]
3.17
4.67
3.79
[Co(L)] ϩ L
[Co(L)2]
[Ni(HL)]
Ni ϩ L
[Ni(L)] ϩ L
H ϩ [Ni(L)]
5.4
5.58
3.83
6.44
5.25
3.73
[Ni(L)]
[Ni(L)2]
3.62
2.65
5.78
4.80
[Cu(HL)]
[Cu(HL)(L)]
H ϩ [Cu(L)]
H ϩ [Cu(L)2]
5.07
6.74
7.64
5.41
4.44
Cu ϩ L
[Cu(L)] ϩ L
[Cu(L)]
[Cu(L)2]
8.12
6.50
5.37
0.90
8.12
6.88
Zn2ϩ
[Zn(HL)]
Zn ϩ L
[Zn(L)] ϩ L
H ϩ [Zn(L)]
[Zn(L)]
6.5
4.12
3.41
6.72
5.00
4.96
4.23
[Zn(L)2]
a Charges are omitted for simplicity. b Gly(P) (aminomethyl)phosphonic acid (ref. 40). c Gly(Pt-Bu) = aminomethyl(t-butyl)phosphinic acid (the most
basic of the simple alkyl(aminomethyl)phosphinic acids, ref. 21). d Gly glycine (ref. 40).
Fig. 2 Distribution diagrams of (a) Co2+–HL (cL = 0.004 mol dmϪ3, cCo = 0.001 mol dmϪ3); (b) Ni2+–HL (cL = 0.004 mol dmϪ3, cNi = 0.002 mol
dmϪ3); (c) Cu2+–HL (cL = 0.004 mol dmϪ3, cCu = 0.002 mol dmϪ3) and (d) Zn2+–HL (cL = 0.004 mol dmϪ3, cZn = 0.001 mol dmϪ3).
in the system with Co(), while, in the Zn() and Cu() systems,
the [M(OH)(L)2]Ϫ and [M(OH)2(L)2]2Ϫ were identified and in
Ni() the [Ni(OH)(L)2]Ϫ was only confirmed. As expected, the
formation of the hydroxo complexes is the easiest for zinc()
and disadvantageous for nickel().
Complexation starts in dependence on a metal ion from
Ϫlog[Hϩ] = 4 (Cu()) to 6 (Co()) with formation of both the
protonated [M(HL)]2ϩ and non-protonated [M(L)]ϩ species
(Fig. 2). The [M(HL)]2ϩ is present only in a very low concen-
tration, except for the Cu2ϩ system where its abundance is about
20% (M : L ratio 1 : 1). On the basis of comparison of pKA of
the ligand and the protonated complexes, it is evident that in
the [M(HL)]2ϩ species one amino group is coordinated and the
other is protonated (Tables 5 and 6). In [M(L)]ϩ, coordination
of both the amino groups is assumed. Their stability constants
decrease in the expected order Cu() > Ni() > Zn() > Co()
and are lower than that found for propylene-1,3-diamine,40
because of lower basicity of the amino groups. In systems with
the molar ratios metal : ligand = 1 : 2 and higher, species [M(L)2]
were found with high abundances. Their stability constants also
increase in the expected order. In these species, coordination of
four amino groups from two ligand molecules is also assumed.
Positions of the absorption bands in the visible region for
[Ni(L)2] (366 and 597 nm) and [Cu(L)2] (616 nm) correspond
to those observed for the complexes with common amino
acids. This points to the mixed amine and (phosphinate-O)/
H2O coordination spheres in both the species with four nitrogen
atoms presumably in the equatorial plane (see below for the
structure of Ni() complex). The phosphinate groups could be
also coordinated. On the other hand, easy formation of the
D a l t o n T r a n s . , 2 0 0 3 , 3 9 2 7 – 3 9 3 8
3933

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hydroxo-species [M(OH)(L)2]Ϫ and [M(OH)2(L)2]2Ϫ at pH ∼8
would indicate coordination of water molecules instead of the
phosphinate group in the neutral region.
To confirm or to exclude the phosphinate coordination to the
central ion, 31P and 1H NMR titrations of the ligand solution in
the absence or presence of Zn() in molar ratio M : L = 1 : 2
were carried out; the results are shown in Fig. 3. A comparison
of 31P NMR titration curves of both the solutions shows no
differences in the chemical shifts in the range from strong
acid to pH about 4. The small differences in the pH range 4–8
correspond to the deprotonation and coordination of amino
Scheme 2
interaction of the negatively charged phosphinate with positive
charge of metal ion, instead of the protonated amine, does not
result in any significant changes in shielding of phosphorus
nuclei. On the other hand, as shown in Scheme 2 the methylene
hydrogen atoms in non-coordinated species are directed outside
the phosphinate group interacting with amino group(s). If the
phosphinate group is bound to a positively charged metal ion,
the protonated amino group turns due to the charge repulsion.
Then, the methylene hydrogen atoms are close to the metal ion
and thus are influenced with its charge. The orientation of
methylene group in the structure of all-trans-[CoCl2(H2L-O)2-
(H2O)2]Cl2 is also shown in Fig. 4. The X-ray structure
investigation of all-trans-[MCl2(H2L-O)2(H2O)2]Cl2 (see
below) shows the ionic interaction of metal ions with the
phosphinate group leaving both amino groups protonated
and thus supports the suggested coordination in acid region
for all the divalent metal ions studied. This fact is relevant
for biological studies as similar phosphinate inhibitors were
found to bind to a metal ion in the active centre of enzymes.42
The HL is a tridentate ligand and, thus, both the amino and
phosphinate groups are available to form two five-membered
rings in the coordination sphere of [M(L)]ϩ and [M(L)2]
species with divalent ions. However, the X-ray structures (see
below) indicate that this coordination arrangement seems to be
disadvantageous due to the phosphinate size and its rigidity
and, therefore, HL can be coordinated only through amino
groups which form a six-membered ring or through a single
amino and phosphinate groups, which form only one five-
membered ring.
1
groups. In contrast to the 31P, the H NMR spectra are more
1
sensitive. The changes in H NMR spectra in dependence on
pH of the free ligand (Fig. 3) correspond at pH < 1 to the
deprotonation of phosphinic acid group and at pH > 6 to
deprotonation of amino groups. In the presence of Zn(), we
can see analogous changes due to deprotonations; however, the
deprotonations are slightly shifted to the acid region. The chem-
ical shifts also moved to lower values in comparison with the
free acid. There is no difference in the chemical shifts between
the solution of the free ligand and its solution in the presence of
zinc() in 2 M and 6 M HCl and, thus, only in this region the
phosphinic acid group is fully protonated and non-coordinated.
The large differences between the values observed for the free
ligand and its solution with Zn() pH = 0–5 indicate co-
ordination of the phosphinate group. The changes above pH 5
correspond to the coordination of amino groups.
Fig.
4
Molecular structure of all-trans-[CoCl2(H2L-O)2(H2O)2]2+
cation with atom numbering scheme.
Considering the hard/soft character of the ions and ligand, it
is well known that the phosphonate/phosphinate group is a
hard donor. This follows from correlation of the stability con-
stants found for carboxylic and phosphonic amino acids
because the relative stabilization (ratio of stability constants of
these amino acids) decreased in the order Cu() > Zn() ≈
Co() > Ni().6 The highest relative stabilization of copper()
is due mainly to a high basicity of the amino group(s) and a
similar affinity to the phosphonate group to that of zinc()
and cobalt().6 The nickel() ion is relatively soft and shows a
much lower affinity to hard phosphinate as follows from a
comparison of the other ions studied. Stability constants
determined for [Cu(L)]ϩ, [Cu(L)2], [Ni(L)]ϩ and [Ni(L)2] are
relatively high and well compared with those for other phos-
phorus-containing amino acids (Table 4). Therefore, we assume
a dominant coordination only through both the nitrogen atoms
and formation of six-membered chelates (see also UV-VIS
Fig. 3 NMR titration of HL in the absence (full circles ᭹) or presence
of Zn() (0.5 equiv., open squares ᮀ).
Explanation of the mentioned low sensitivity of phosphorus
nuclei and high sensitivity of methylene hydrogen atoms to co-
ordination results from the following considerations. It is
known that a deprotonated phosphinate or phosphonate group
interacts with a protonated amino group inside of a molecule
of both the acids and the largest changes in 31P NMR shifts
are caused by deprotonation of amino groups.41 Thus, the
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Table 7 Geometry of the centrosymmetric coordination shell of M() (inversion center in atom M) and of the phosphinate group in all-trans-
[MCl2(H2L-O)2(H2O)2]2ϩ cations (interatomic distances in Å, bond angles in Њ)
Parameter
Ca
Mn
Co
Ni
Cu
Zn
Cd
M–Cl(1)
M–O(3w)
M–O(1)
2.6818(3)
2.355(1)
2.3020(9)
2.5173(5)
2.222(1)
2.165(1)
2.4536(4)
2.125(2)
2.101(1)
2.3968(6)
2.090(2)
2.081(1)
2.3402(4)
2.382(1)
1.9724(9)
2.4096(3)
2.151(1)
2.1150(9)
2.536(4)
2.336(4)
2.282(4)
O(1)–M–Cl(1)
O(1)–M–O(3w)
Cl(1)–M–O(3w)
88.38(2)
91.85(3)
93.00(3)
87.78(3)
92.99(5)
90.25(4)
87.76(3)
92.59(5)
89.68(4)
87.26(4)
92.94(6)
90.47(4)
87.58(3)
94.60(4)
89.69(2)
87.23(3)
93.27(4)
89.96(3)
87.0(1)
93.9(2)
91.3(1)
M–O(1)–P
140.90(5)
142.04(7)
142.58(7)
141.54(9)
141.56(6)
141.26(6)
138.7(2)
P–O(1)
P–O(2)
P–C(1)
P–C(2)
1.5111(9)
1.4988(9)
1.828(1)
1.825(1)
1.509(1)
1.498(1)
1.829(2)
1.829(2)
1.512(1)
1.497(1)
1.823(2)
1.824(2)
1.512(2)
1.501(2)
1.824(2)
1.828(2)
1.5169(9)
1.499(1)
1.826(1)
1.828(1)
1.513(1)
1.498(1)
1.827(1)
1.827(1)
1.517(4)
1.486(4)
1.823(6)
1.824(2)
O(1)–P–O(2)
O(1)–P–C(1)
O(1)–P–C(2)
O(2)–P–C(1)
O(2)–P–C(2)
C(1)–P–C(2)
117.53(5)
107.52(5)
107.68(5)
108.88(5)
108.77(5)
105.85(6)
117.19(7)
107.77(7)
107.95(7)
108.39(7)
109.09(7)
105.88(8)
117.14(7)
108.15(7)
108.17(7)
107.98(8)
109.05(7)
105.78(8)
116.81(9)
108.42(8)
108.25(9)
109.22(9)
107.68(7)
105.9(1)
115.80(6)
109.33(6)
107.44(6)
108.52(6)
108.54(6)
106.86(7)
117.06(6)
108.07(6)
108.18(6)
107.91(6)
109.05(6)
106.04(7)
116.7(2)
107.9(2)
108.0(3)
108.8(3)
108.8(2)
106.1(3)
spectra). This coordination mode was found in the structure of
trans-O,O-[Ni(L-N,N)2(H2O)2] (see below). In [Zn(L)]ϩ and
[Co(L)]ϩ, the same considerations lead to the suggestion that
the ligand is coordinated mainly by a single amino group and
the phosphinate group. Such an explanation is supported by
isolation of a zinc() complex where the ligand bridges the
metal ions through the other amino group (see below).
defined solids and the copper() complex was obtained only as
a microcrystalline powder.
Molecular structures of the nickel() and zinc() complexes
are shown in Figs. 5 and 6, respectively. Selected bond lengths
and angles are listed in Table 8. In the centrosymmetric trans-
O,O-[Ni(L-N,N)2(H2O)2], the metal ion is chelated by four
amino nitrogen atoms of two ligand molecules forming two
six-membered N,N-chelate rings in the equatorial plane and the
octahedron is completed with two water molecules in apical
positions (Fig. 5). The phosphinate group is not coordinated,
P–O(11) and P–O(12) lengths are virtually the same and both
oxygen atoms form hydrogen bonds to the coordinated mole-
cule of water in the neighbouring molecule, O(12) ؒ ؒ ؒ O(13w) =
2.65 Å, and to the solvate water, O(11) ؒ ؒ ؒ O99 = 2.75 Å.
Synthesis of complexes and their crystal structures
From acid solutions obtained by dissolution of HLؒHCl
and MCl2, where M are the metals studied above (Co, Ni,
Cu and Zn), complexes of the general formula all-trans-
[MCl2(H2O)2(H2L-O)2]Cl2 easily crystallize. The same com-
plexes were obtained with other divalent ions such as Ca(),
Mn() and Cd(). The X-ray structural investigation has shown
that all the complexes are isostructural. The octahedral co-
ordination sphere of the metal ions is formed by two oxygen
atoms (mutually trans) of two phosphinate groups, two mole-
cules of water (mutually trans) and two chloride anions. The
nitrogen atoms were found protonated and non-coordinated.
A representative molecular structure is shown in Fig. 4 and
selected bond lengths and angles are listed in Table 7. From
the table, it is clear that the bond lengths M–OP, M–Ow
and M–Cl increase with the increasing ionic radius of the
metal ions, except for M–Ow in the Cu() complex due to the
Jahn–Teler distortion. This structural motif with protonated
amines and coordinated phosphinates was also observed in the
manganese() complex of (aminomethyl)methylphosphinic
acid (HLЈ) all-trans-[MnCl2(HLЈ-O)2(H2O)2] (ref. 14) and in the
tetraaqua complex of Co() with piperazine-1,4-diylbis-
(methylphosphinic) acid.19 In an analogous complex of HLЈ,
the Mn–O(1) distance is slightly shorter (2.141 Å) and the
O(1)–P–O(2) angle is less open (113.8Њ).14 Surprisingly, the
bromide analog of the above complex of HLЈ is polymeric
with –Mn–(O–P–O)2–Mn– eight-membered rings and non-
coordinated bromide anions.13
Fig. 5 Molecular structure of trans-O,O-[Ni(L-N,N)2(H2O)2] with
atom numbering scheme.
Zinc() in polymeric [Zn(L)2] forms the octahedral co-
ordination sphere with the centre of symmetry on the central
ion (Fig. 6). The coordination sphere consists of two phos-
phinate oxygen atoms O(11) and two amino nitrogen atoms
N(11) (trans to each other) of two different ligand molecules in
the equatorial plane forming a five-membered N,O-chelate. The
octahedron is completed with two additional amino nitrogen
atoms N(12) of the two other ligand molecules which are bound
to two neighbouring zinc() ions forming an indefinite chain.
The bond lengths in the expected range are listed in Table 8.
The P–O(11) (to the coordinated oxygen) and P–O(12) (to the
non-coordinated oxygen) bond lengths are practically the same.
This indicates only ionic interaction of phosphinate with the
central ion. The Zn–O(11) length (2.157 Å) in this complex is
slightly longer than the length of the corresponding bond pres-
ent in “acidic” complex (2.115 Å, Table 7). The O(12) shows
three weak hydrogen bonds (2.77–2.93 Å) to the molecule of
Crystals suitable for X-ray structure determination of the
Ni() and Zn() complexes with coordinated amines were pre-
pared in a reproducible way only after carefully adjusting the
pH to a value that was estimated from the distribution
diagrams. It was necessary to remove foreign anions and, there-
fore, the complexes were prepared from the zwitterionic form of
the ligand and freshly precipitated metal hydroxides. We tested
this and modified procedures also for the preparation of Co()
and Cu() complexes. For Co(), we were unable to get any
D a l t o n T r a n s . , 2 0 0 3 , 3 9 2 7 – 3 9 3 8
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Table 8 Bond lengths (Å) and angles (Њ) in trans-O,O-[Ni(L-N,N)2(H2O)2]ؒ2H2O, [Zn(L)2]ؒ2H2O and fac-N,N,O-trans-O,OЈ-[Co(L-N,N,O)2](ClO4)ؒ
H2O
Parameter
[Ni(L)2(H2O)2]ؒ2H2O
[Zn(L)2]ؒ2H2O
[Co(L)2](ClO4)ؒH2O
M–N(11)
M–N(12)
M–O(11)
M–N(21)
M–N(22)
M–O(21)
2.136(5)
2.124(5)
2.132(1)
1.982(2)
1.978(2)
1.898(1)
1.968(2)
1.982(2)
1.914(1)
2.264(1)c
2.110(4)a
2.1566(9)
b
b
b
b
b
b
N(11)–M–N(12)
N(11)–M–O(11)
N(11)–M–N(21)
N(11)–M–N(22)
N(11)–M–O(21)
N(12)–M–O(11)
N(12)–M–N(21)
N(12)–M–N(22)
N(12)–M–O(21)
O(11)–M–N(21)
O(11)–M–N(22)
O(11)–M–O(21)
N(21)–M–N(22)
N(21)–M–O(21)
N(22)–M–O(21)
M–O(11)–P(1)
92.9(2)
89.63(4)
84.98(4)
87.13(8)
88.37(8)
177.80(7)
93.63(8)
92.50(7)
89.64(7)
92.41(8)
177.93(7)
89.72(7)
88.74(7)
92.30(7)
178.89(6)
86.92(8)
90.38(6)
88.33(6)
104.52(7)
103.53(7)
91.9(2)a
b
b
b
b
b
b
93.2(2)a
91.43(4)c
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
M–O(21)–P(2)
P(1)–O(11)
P(1)–O(12)
P(1)–C(11)
P(1)–C(12)
1.506(4)
1.501(4)
1.801(6)
1.810(6)
1.531(1)
1.5123(9)
1.821(1)
1.823(1)
1.546(1)
1.482(2)
1.815(2)
1.818(2)
O(11)–P(1)–O(12)
O(11)–P(1)–C(11)
O(11)–P(1)–C(12)
O(12)–P(1)–C(11)
O(12)–P(1)–C(12)
C(11)–P(1)–C(12)
118.1(2)
108.6(3)
107.8(2)
107.6(3)
108.2(2)
105.9(3)
116.81(5)
106.50(5)
108.59(5)
109.47(6)
108.64(5)
106.34(6)
116.70(8)
101.98(9)
101.82(9)
116.2(1)
114.8(1)
103.3(1)
b
b
b
b
b
b
b
b
P(2)–O(21)
P(2)–O(22)
P(2)–C(21)
P(2)–C(22)
1.541(1)
1.492(1)
1.809(2)
1.815(2)
b
b
b
b
b
b
b
b
b
b
b
b
O(21)–P(2)–O(22)
O(21)–P(2)–C(21)
O(21)–P(2)–C(22)
O(22)–P(2)–C(21)
O(22)–P(2)–C(22)
C(21)–P(2)–C(22)
116.82(8)
101.9(1)
102.87(9)
115.2(1)
115.1(1)
103.0(1)
a Coordinated H2O molecule. b Centrosymmetric coordination shell. c Nitrogen atom from further ligand molecule in the endless polymeric chain.
Fig. 6 Molecular structure of [Zn(L)2] with atom numbering scheme.
water and to the amino group of a neighbouring molecule. For
comparison, the solid-state structure of zinc() complex with
(aminomethyl)methylphosphinic acid (HLЈ), [Zn(HLЈ-O)2Cl2]
consists of indefinite chains formed by zinc atoms and bridging
ligands coordinated through oxygen atoms only (to different
zinc atom).15 The amine group is protonated and a tetrahedral
D a l t o n T r a n s . , 2 0 0 3 , 3 9 2 7 – 3 9 3 8
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arrangement of Zn() is completed by two chloride anions. The
Zn–O bond distances in the complex of HLЈ are significantly
shorter (1.970 and 1.928 Å).15
complex. From this point of view, the –N–CH2–P(O)(OH)–
CH2–N– should be selective for complexation of small ions.
A typical feature of complexation for most phosphinic and
aminophosphinic acids is the formation of the eight-membered
[M–(O–P–O)2–M] ring, in which two phosphinate groups link
two metal ions forming dimeric16,21,43 or polymeric motifs.21,44
This eight-membered ring is also formed in complexes with co-
ordinated amines.17,21,43 The absence of the ring in the structures
presented is surprising, probably corresponding with suppres-
sion of the coordination ability of the phosphinate group
between two aminomethyl groups capable of forming a six-
membered chelate ring. From acidic solutions, complexes with
another coordinating mode were prepared. Coordination
through phosphinate oxygen atoms confirms the ability of the
phoshinic acid group to bind metal ions even at strongly acidic
pH. The results presented in this paper help in understanding
the behaviour of phosphorus acid enzyme inhibitors as well
as of more complex ligands such as macrocycles containing
phosphorus acid pendant arms.
The expected coordination sphere formed by two molecules
of the ligand coordinated to the same ion through phosphinate
and also through both amino groups were observed only for
Co(). The fac-N,N,O-trans-O,OЈ-[Co(L-N,N,O)2](ClO4)ؒH2O
was prepared by the reaction of the cobalt() carbonate com-
plex with ligand in the presence of HClO4. Unfortunately, a
large proportion of trivalent cobalt was reduced to the divalent
one during the reaction. The complex was purified on a carb-
oxylic cation exchanger. Its water eluate was concentrated and
the desired complex was obtained on slow evaporation in air.
In the fac-N,N,O-trans-O,OЈ-[Co(L-N,N,O)2]ϩ, the ligand is
facially coordinated with all nitrogen atoms in an equatorial
plane (Fig. 7). The octahedron is axially flattened due to shorter
Co–O bond lengths (1.898 and 1.913 Å) than Co–N ones
(1.968–1.982 Å) (Table 8). Bond angles around the central ion
are deformed because the ligand is too small to reach vertexes
of a regular octahedron (up to 7Њ). As expected, the P–O length
of non-coordinated oxygen is significantly shorter than the
other. Perchlorate anions are highly disordered and, like water
molecules, participate in the network of intermolecular hydro-
gen bonds stabilizing the crystal structure. In the UV-VIS spec-
trum of the fac-N,N,O-trans-O,OЈ-[Co(L-N,N,O)2]ϩ, two bands
at 368 nm (1T2g) and 510 nm (1T1g) only were found. Surpris-
Acknowledgements
We thank Dr J. Ederová (Institute of Chemical Technology,
Prague) for TG measurements. This work was supported by the
Ministry of Education of the Czech Republic (grant CEZJ
07/98:143100011), the Grant Agency of the Czech Republic
(grant 203/02/0493 and 203/03/0168) and EU COST D18.
1
ingly, no splitting or significant broadening of the T1g level,
typical for trans isomers, was observed.45 It corresponds with
the short Co–O bond lengths mentioned above and, therefore,
the geometry is more similar to N6 or cis-N4O2 species. The
parameters 10 Dq and B, 21 500 and 475 cmϪ1, are in the range
observed for similar Co() complexes of macrocyclic ligands
derived from cyclam with two methylphosphonate pendant
arms.46
References
1 P. Kafarski and B. Lejczak, Phosphorus, Sulfur Silicon Relat. Elem.,
1991, 63, 193.
2 Aminophosphonic and Aminophosphinic Acids. Chemistry and
Biological Activity, ed. V. P. Kukhar and H. R. Hudson, Wiley,
Chichester, 2000 and references cited therein.
3 J. Buchardt, M. Ferreras, C. Krog-Jensen, J.-M. Delaissé, N. T.
Foged and M. Meldal, Chem. Eur. J., 1999, 5, 2877; V. Dive,
J. Cotton, A. Yiotakis, A. Michaud, S. Vassiliou, J. Jirácˇek, G.
Vazeux, M.-T. Chauvet, P. Cuniasse and P. Corvol, Proc. Natl. Acad.
Sci. USA, 1999, 96, 4330.
4 J. Ong, D. I. B. Kerr, H. Bittiger, P. C. Waldmeier, P. A. Baumann,
N. G. Cooke, S. J. Mickel and W. Froestel, Eur. J. Pharmacol.,
1998, 362, 27.
5 H. Chen, F. Noble, B. P. Roques and M.-C. Fournié-Zaluski, J. Med.
Chem., 2001, 44, 3523.
6 T. Kiss and I. Lázár, in Aminophosphonic and Aminophosphinic
Acids. Chemistry and Biological Activity, ed. V. P. Kukhar and H. R.
Hudson, Wiley, 2000, ch. IX, pp. 285–326; I. Lukesˇ, J. Kotek,
P. Vojtísˇek and P. Hermann, Coord. Chem. Rev., 2001, 216–217, 287.
7 T. Kiss, J. Balla, G. Nagy, H. Kozlowski and J. Kowalik, Inorg. Chim.
Acta, 1987, 138, 25; T. Kiss, E. Farkas and H. Kozlowski, Inorg.
Chim. Acta, 1989, 155, 281; E. Matczak-Jon and W. Wojciechowski,
Inorg. Chim. Acta, 1990, 173, 85; E. Matczak-Jon, B. Kurzak,
W. Sawka-Dobrowolska, P. Kafarski and B. Lejczak, J. Chem. Soc.,
Dalton Trans., 1996, 3455; E. Matczak-Jon, B. Kurzak, W. Sawka-
Dobrowolska, B. Lejczak and P. Kafarski, J. Chem. Soc., Dalton
Trans., 1998, 161; B. Kurzak, E. Matczak-Jon and M. Hoffmann,
J. Coord. Chem., 1998, 43, 243; E. Matczak-Jon, W. Sawka-
Dobrowolska, D. Dobrzynska and M. Hoffmann, Polyhedron, 1999,
18, 2169.
8 G. I. Sinyavskaya, Koord. Khim., 1991, 17, 1599; G. I. Sinyavskaya,
Koord. Khim., 1992, 18, 899; P. Fenot, J. Darriet, C. Garrigou-
Lagrange and A. Cassaigne, J. Mol. Struct., 1978, 43, 49;
T. Glowiak, W. Sawka-Dobrowolska, B. Jezowska-Trzebiatowska
and A. Antonow, Inorg. Chim. Acta, 1980, 45, L105; T. Glowiak,
W. Sawka-Dobrowolska, B. Jezowska-Trzebiatowska and
A. Antonow, J. Crystallogr. Mol. Struct., 1980, 10, 1; S. Drumel,
P. Janvier, D. Deniaud and B. Bujoli, J. Chem. Soc., Chem.
Commun., 1995, 1051; S. J. Hartmann, E. Todorov, C. Cruz and
S. C. Sevov, Chem. Commun., 2000, 1213.
Fig. 7 Molecular structure of fac-N,N,O-trans-O,O’-[Co(L-N,N,O)2]+
with atom numbering scheme.
Conclusions
Comparing molecular structures of [Zn(L)2], [Ni(L)2], and
[Co(L)2]ϩ species, we found three different coordination modes
in dependence on the properties of the central ions. We assume
that the ligand HL is too small and rigid to possess the octa-
hedral apexes and to easily form two chelate rings. The rigidity
is only overcome in [Co(L)2]ϩ due to the covalent contribution
of small Co() and short Co–O and Co–N bonds. In the Zn()
and Ni() complexes, the M–O and M–N bond lengths are
larger (see Table 8) and, therefore, an analogous coordination
sphere is not advantegous. Both the ions differ in their soft/hard
character and this results in different coordination ability
towards phosphinate. Zinc() forms polymeric structure and
the phosphinate group is non-coordinated in the nickel()
9 M. Hariharan, R. J. Motekaitis and A. E. Martell, J. Org. Chem.,
1975, 40, 470; T. Kiss, E. Farkas, H. Kozlowski, Z. Siatecki,
P. Kafarski and B. Lejczak, J. Chem. Soc., Dalton Trans., 1989, 1053.
10 P. Hermann and I. Lukesˇ, J. Chem. Soc., Dalton Trans., 1995, 2605;
P. Hermann, I. Lukesˇ, P. Vojtísˇek and I. Císarˇová, J. Chem. Soc.,
Dalton Trans., 1995, 2611.
D a l t o n T r a n s . , 2 0 0 3 , 3 9 2 7 – 3 9 3 8
3937

View Article Online
11 L. Bláha, I. Lukesˇ, J. Rohovec and P. Hermann, J. Chem. Soc.,
Dalton Trans., 1997, 2621; I. Lukesˇ, L. Bláha, F. Kesner, J. Rohovec
and P. Hermann, J. Chem. Soc., Dalton Trans., 1997, 2629.
12 I. Lukesˇ, P. Hermann and P. Pech, Collect. Czech. Chem. Commun.,
1989, 54, 653; T. Kiss, E. Farkas, M. Jezowska-Bojczuk,
H. Kozlowski and E. Kowalik, J. Chem. Soc., Dalton Trans., 1990,
377; K. B. Yatsimirski, L. B. Tsymbal and E. I. Sinyavskaya,
Zh. Neorg. Khim., 1990, 35, 117; T. Kiss, M. Jezowska-Bojczuk,
H. Kozlowski, P. Kafarski and K. Antczak, J. Chem. Soc.,
Dalton Trans., 1991, 2275; I. Lukesˇ, K. Bazakas, P. Hermann and
P. Vojtísˇek, J. Chem. Soc., Dalton Trans., 1992, 939.
“OPIUM” is available (free of charge) on http://www.natur.cuni.cz/
∼kyvala/opium.html.
31 Z. Otwinovski and W. Minor, HKL Denzo and Scalepack Program
Package, Nonius BV, Delft 1997;; for a reference see: Z. Otwinovski
and W. Minor, Methods Enzymol., 1997, 276, 307.
32 A. Altomare, M. C. Burla, M. Camalli, G. Cascarano,
C. Giacovazzo, A. Guagliardi and G. Polidori, J. Appl. Crystallogr.,
1994, 27, 435.
33 G. M. Sheldrick, SHELXL-97, Program for refinement of crystal
structures, University of Göttingen, Germany, 1997.
34 A. L. Spek, PLATON—A Multipurpose Crystallographic Tool;
see: http://www.cryst.chem.uu.nl/platon/.
13 T. Glowiak and W. Sawka-Dobrowolska, Acta Crystallogr., Sect. B,
1977, 33, 2698.
35 L. Maier, J. Organomet. Chem., 1979, 178, 157.
14 T. Glowiak and W. Sawka-Dobrowolska, Acta Crystallogr., Sect. B,
1977, 33, 2763.
36 M. Bujard, V. Gouverneur and C. Mioskowski, J. Org. Chem., 1999,
64, 2119.
37 K. Issleib, M. Mögelin and A. Balszuweit, Z. Anorg. Allg. Chem.,
1985, 530, 16; E. A. Boyd, M. Corless, K. James and A. C. Regan,
Tetrahedron Lett., 1990, 31, 2933.
38 S. Chen and J. K. Coward, J. Org. Chem., 1998, 63, 502.
39 (a) T. Glowiak and W. Sawka-Dobrowolska, Acta Crystallogr.,
Sect. B, 1977, 33, 1522; (b) F. Kasˇpárek, Z. Trávnícek, V. Bekárek
and J. Marek, Pol. J. Chem., 2002, 76, 1389.
ˇ
15 Z. Zák, J. Kozˇísˇek and T. Glowiak, Z. Anorg. Allg. Chem., 1981, 477,
221.
16 W. Sawka-Dobrowolska and T. Glowiak, Acta Crystallogr., Sect. C,
1983, 39, 345.
17 T. Glowiak, Acta Crystallogr., Sect. C, 1986, 42, 62.
18 J. F. Britten, C. J. L. Lock and Z. Y. Wang, Acta Crystallogr., Sect. C,
1993, 49, 881.
19 I. Lukesˇ, I. Císarˇová, P. Vojtísˇek and K. Bazakas, Polyhedron, 1995,
14, 3163.
20 M. P. Lowe, S. J. Rettig and C. Orvig, J. Am. Chem. Soc., 1996, 118,
10446.
40 A. E. Martell and R. M. Smith, Critical Stability Constants, Plenum
Press, New York, 1974–1989, vol. 1–6; NIST Standard Reference
Database 46 (Critically Selected Stability Constants of Metal
Complexes), version 5.0, 1994.
21 J. Rohovec, I. Lukesˇ, P. Vojtísˇek, I. Císarˇová and P. Hermann,
J. Chem. Soc., Dalton Trans., 1996, 2685.
22 (a) K. Bazakas and I. Lukesˇ, J. Chem. Soc., Dalton Trans., 1995,
1133; (b) J. Rohovec, M. Kývala, P. Vojtísˇek, P. Hermann and
I. Lukesˇ, Eur. J. Inorg. Chem., 2000, 195.
23 M. Lukásˇ, M. Kývala, P. Hermann, I. Lukesˇ, D. Sanna and
G. Micera, J. Chem. Soc., Dalton Trans., 2001, 2850.
24 T. R. Varga, R. Király and E. Brücher, ACH–Models Chem., 1999,
136, 431; L. Xu, S. J. Rettig and C. Orvig, Inorg. Chem., 2001, 40,
3734.
25 B. Song, T. Storr, S. Liu and C. Orvig, Inorg. Chem., 2002, 41, 685;
S. Aime, C. Cavallotti, E. Gianolio, G. B. Giovenzana, G. Palmisano
and M. Sisti, Tetrahedron Lett., 2002, 43, 8387.
41 T. G. Appleton, J. R. Hall, A. D. Harris, M. A. Kimlin and
I. J. McMahon, Aust. J. Chem., 1984, 37, 1833; K. Popov,
E. Niskanen, H. Rönkkömäki and H. J. Lajunen, New J. Chem.,
1999, 23, 1209; J. Kotek, P. Vojtísˇek, I. Císarˇová, P. Hermann,
P. Jurecˇka, J. Rohovec and I. Lukesˇ, Collect. Czech. Chem.
Commun., 2000, 65, 1289.
42 F. Grams, V. Dive, A. Yiotakis, I. Yiallouros, S. Vassiliou,
R. Zwilling, W. Bode and W. Stöcker, Nat. Struct. Biol., 1996, 3,
671.
43 F. Cecconi, C. A. Ghilardi, P. Gili, S. Midollini, P. A. L. Luis,
A. D. Lozano-Gorrin and A. Orlandini, Inorg. Chim. Acta, 2001,
319, 67; J. Rohovec, P. Vojtísˇek, I. Lukesˇ, P. Hermann and J. Ludvík,
J. Chem. Soc., Dalton Trans., 2000, 141.
26 V. Kubícˇek, P. Hermann, I. Lukesˇ, J. Kotek, P. Vojtísˇek and
I. Císarˇová, J. Inorg. Biochem., 2001, 86, 301.
27 G. W. Pucher and T. B. Johnson, J. Am. Chem. Soc., 1922, 44, 817.
28 H. F. Bauer and W. C. Drinkard, Inorg. Synth., 1966, 8, 202.
29 C. F. Baes Jr. and R. E. Mesmer, The Hydrolysis of Cations, Wiley,
New York, 1976.
44 J. S. Haynes, K. W. Oliver, S. J. Rettig, R. C. Thompson and
J. Trotter, Can. J. Chem., 1984, 62, 891; W. V. Cicha, J. S. Haynes,
K. W. Oliver, S. J. Rettig, R. C. Thompson and J. Trotter, Can. J.
Chem., 1985, 63, 1055.
45 A. B. P. Lever, Inorganic Electronic Spectroscopy, Elsevier,
Amsterdam, 1984, pp. 473–505.
30 M. Kývala and I. Lukesˇ, International Conference, Chemometrics
’95, p. 63, Pardubice, Czech Republic, 1995; full version of
46 J. Kotek, I. Císarˇová, P. Hermann, I. Lukesˇ and J. Rohovec,
Inorg. Chim. Acta, 2001, 317, 324.
D a l t o n T r a n s . , 2 0 0 3 , 3 9 2 7 – 3 9 3 8
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