Chromium(II) Complexes with Hydroxyethylidenediphosphonic Acid

Article abstract of DOI:10.1023/A:1009536824427

The complex formation of chromium(II) with hydroxyethylidenediphosphonic acid (HEDP, H₅L) in an aqueous solution is studied by spectrophotometry. The pH region is determined where the HEDP complexonates of chromium(II) are stable. The effect of pH on the complexation and the composition of the coordination sphere of the complex ions is shown. The optimum conditions for the practical application of HEDP as a stabilizing ligand are chosen. The instability constants for tetra-, tri-, di-, and monoprotonated and neutral chromium(II) hydroxyethylidenediphosphonate complexes were calculated, whose pK were found to be 1.21, 2.43, 6.87, 12.60, 19.40, respectively. A new complex of chromium(II), Na₄[Cr(H₂L)₂ · 2H₂O] · 4H₂O, was isolated from a solution and characterized using elemental, IR spectroscopic, and thermographic analyses.

Full text of DOI:10.1023/A:1009536824427

Russian Journal of Coordination Chemistry, Vol. 27, No. 1, 2001, pp. 38–40. Translated from Koordinatsionnaya Khimiya, Vol. 27, No. 1, 2001, pp. 42–45.
Original Russian Text Copyright © 2001 by Popova, Smotrina, Denisova, Aksenova.
Chromium(II) Complexes
with Hydroxyethylidenediphosphonic Acid
T. V. Popova, T. V. Smotrina, O. N. Denisova, and N. V. Aksenova
Mari State University, pl. Lenina 1, Ioshkar Ola, Russia
Received April 13, 2000
Abstract—The complex formation of chromium(II) with hydroxyethylidenediphosphonic acid (HEDP, H L)
5
in an aqueous solution is studied by spectrophotometry. The pH region is determined where the HEDP com-
plexonates of chromium(II) are stable. The effect of pH on the complexation and the composition of the coor-
dination sphere of the complex ions is shown. The optimum conditions for the practical application of HEDP
as a stabilizing ligand are chosen. The instability constants for tetra-, tri-, di-, and monoprotonated and neutral
chromium(II) hydroxyethylidenediphosphonate complexes were calculated, whose pK were found to be 1.21,
2
.43, 6.87, 12.60, 19.40, respectively. A new complex of chromium(II), Na [Cr(H L) · 2H O] · 4H O, was iso-
4 2 2 2 2
lated from a solution and characterized using elemental, IR spectroscopic, and thermographic analyses.
INTRODUCTION
chloride with zinc dust in an inert gas atmosphere fol-
lowed by the addition of sodium acetate [10].
Study into coordination compounds of metals with
unstable oxidation states is always of topical interest.
The electronic absorption spectra (EAS) of solu-
The search for information on these compounds makes tions of the chromium(II) acetate without HEDP and
it possible to assert that this interest does not wane with with a complexone added during the synthesis of the
time. The chromium(II) complexes with ligands of a acetate in an inert atmosphere were recorded on a
number of aliphatic amines, amino acids, aliphatic and
aromatic mono- and dicarboxylic acids and their deriv-
atives, complexones of polyamino- and polythiocarb-
oxylic acids, thiophene and pyridine derivatives, and
macrocyclic ligands are described in [1–9]. The authors
of these papers report on the instability of the chro-
mium(II) complexes in aqueous solutions; on their
increasing stability in noncoordinating solvents; and on
the high stability of some solid chromium(II) com-
plexes, which were synthesized in an inert gas atmo-
sphere. Data on compounds of chromium(II) with
phosphorus-containing ligands, in particular, with
Specord UV VIS spectrophotometer. They are pre-
sented in Fig. 1. After HEDP was introduced into the
chromium(II) acetate solution, the red color of the solu-
tions immediately changed to violet. The EAS of the
chromium(II) acetate with HEDP exhibits a high
hyperchromic effect (∆A = 0.4) and changes its shape
in the short-wave visible region. When the solution is
A
0
.7
hydroxyethylidenediphosphonic acid (HEDP, H L), are
5
2
0
.6
unavailable.
The purpose of this work was to study the complex-
0.5
0.4
2+
ation reaction between Cr ions and hydroxyeth-
ylidenediphosphonic acid and to confirm this through
comparative analysis of similar complexation pro-
3+
2+
cesses involving Cr and Cr ions. We also attempted
to establish the conditions under which the chro-
mium(II) complexonates with HEDP exist in an aque-
ous solution and in a solid state and to make a conclu-
sion on the possibility of stabilizing the unstable oxida-
tion state of chromium via complexation with HEDP.
0
0
0
.3
.2
.1
1
1
2
3
00
400
500
600
700 λ, nm
RESULTS AND DISCUSSION
Fig. 1. Electronic absorption spectra of chromium(II) ace-
tate (1) without and (2) with HEDP at cCr2+ = 5 × 10 mol/l.
The chromium(II) acetate used as a starting com-
pound was prepared by the reduction of chromium(III)
–3
1
070-3284/01/2701-0038$25.00 © 2001 åÄIä “Nauka /Interperiodica”

CHROMIUM(II) COMPLEXES
39
kept in air, its color gradually changes from violet through
A
blue to green and absorption bands appear in its spectrum
with maxima at 460 and 660 nm. These bands are charac-
teristic of the HEDP complexonates of chromium(II) [11],
which is evidence of an oxidation reaction.
1
0
.7
0
.6
2
In weak acid or weak alkaline media, with no expo-
sure to mechanical forces, the violet solutions of the
chromium(II) HEDP complexonates remain stable in
air over the course of 2 to 3 h. The EAS of the chro-
mium(II) HEDP complexonates measured at different
intervals starting from the time of their preparation at
constant pH and concentration of Cr are presented in
Fig. 2. The mechanical stirring of the solutions consid-
erably increases the rate of the chromium (II) oxida-
tion. Even with an optimum pH 6–7, continuous stir-
ring of the solution reduced the time interval within
which the complex is stable to 3–5 min.
0.5
0
0
.4
.3
3
5
2
+
0
0
.2
.1
5
4
3
1
2
4
3
5
1
2
4
3
00
400
500
600
700 λ, nm
The stability of solutions of the chromium(II)
HEDP complexonates stored in an atmosphere of CO2
was found to be 25–30 times lower than that in air,
though the initial solutions of chromium(II) acetate are
Fig. 2. Electronic absorption spectra of solutions of the
chromium(II) complex with HEDP recorded (1) 1, (2) 4,
(3) 5, (4) 8, and (5) 12 h after their preparation; cCr2+ = 5 ×
more stable in CO2.
10−3
mol/l, pH 6.0. The spectrum of the solution of the chro-
–
2
mium(III) HEDP complex at cCr3+ = 1.3 × 10 mol/l (pH
.0) is indicated by the dotted line.
The solution concentration and pH of a medium also
produce a substantial effect on the stability of the chro-
mium(II) HEDP complexonates. A decrease in the con-
centration and pH makes the solutions of chromium(II)
with HEDP unstable, which is undoubtedly due to complex formation of chromium(II) and chromium(III)
hydrolytic processes and the increasing lability of the in aqueous solutions [8, 11, 13–15].
coordination sphere of the protonated complexes.
5
The EAS of the chromium(II) complex differs sub-
stantially from that of a similar chromium(III) HEDP
complexonate and has a doublet band with maxima at
320 and 350 nm. Complex formation is accompanied
by hypsochromic effects in addition to the hyperchro-
mic effect (∆λ = 40 and 10 nm), although the batho-
chromic shift is registered for similar reactions of chro-
mium(III). This is undoubtedly the result of the differ-
ent electronic configurations and energy states of the
chromium atom with different oxidation states.
Study of the optical density of the solutions as a
function of pH at the unchanged concentration of the
2+
Cr ions shows that the reaction between chromium(II)
acetate and HEDP starts at pH 2 and is complete at pH
5
.5 with the formation of the neutral HEDP complex-
onate. The composition and pKinst of the expected
Cr(II) HEDP complexonates were calculated from the
A = f(pH) dependence using the Rossotti method. For
tetra-, tri-, di-, and monoprotonated and neutral Cr(II)
HEDP complexonates, the pKinst are equal to 1.21 ±
The complexes were isolated in the solid state by salt-
0
.10, 1.43 ± 0.10, 5.84 ± 0.20, 10.68 ± 0.30, and 19.40 ± ing out with ethanol from concentrated aqueous solu-
.20, respectively. The reduction of the chromium(II) tions with the initial component ratios Cr(II) : HEDP =
0
chloride with zinc dust in a solution affords zinc chlo- 1 : 1 and 1 : 2 at pH 7. For this purpose, the chromium(II)
ride, which at pH 2–5 forms a complex with HEDP acetate was prepared in the solid state and repeatedly
with a ratio of Zn : HEDP = 1 : 1 that does not absorb washed with water saturated with CO (to remove zinc
2
in the visible spectral region [12]. Therefore, the reac- compounds) and ethanol. Then, HEDP and alkali were
tion of the complex formation of chromium(II) with added to provide the required pH of the medium.
HEDP was run with a ligand excess.
The synthesized complexes are fine bright lilac (1 :
) and blue (1 : 1) powders (the chromium(III) com-
2
By making use of the data on complexation in the
plexes with a similar composition are green). We found
that the reducing properties of the solid chromium(II)
HEDP complexonates are far less substantial than those
of aqueous solutions. These powders are stable in air in
the absence of moisture, whereas minor amounts of
moisture change the color to green.
Cr(III)–HEDP–H O system [11], one can distinguish
2
several specific features of the complex formation of
chromium(II) with HEDP as compared to similar reac-
tions of chromium(III). The chromium(II) hydroxyeth-
ylidenediphosphonate complexes were formed at a
high rate, whereas complexation in the Cr(III)–HEDP
system was very slow as a result of the inertness of the
A nonstoichiometric mixture of the Cr(II) com-
starting chromium(III) aqua complexes, which corre- pounds was isolated from a solution with an initial 1 : 1
lates well with the published data on the kinetics of the ratio of components. We failed to identify these com-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 27 No. 1 2001

4
0
POPOVA et al.
pounds. A single complex Na [Cr(H L) (H O) ] · 4H O
The bands within 640–650 cm–1 were assigned to
4
2
2
2
2
2
was isolated from a solution with a ratio of Cr(II) : the stretching vibrations of the Cr–O bonds. The spec-
HEDP = 1 : 2. Its composition was found from elemen- trum of the Cr(II) complex also contains a broad band
–
1
tal, IR spectroscopic, and thermographic analyses.
at 1180 cm , which is absent in that of the Cr(III)
HEDP complexonate. This band is assigned to the
vibration ν(P=O). This suggests that the coordination
For Na [Cr(C P O H ) (H O) ] · 4H O
4
2
2
7
5 2
2
2
2
of the PO group by the Cr(II) atom occurs with the
anal. calcd. (%): Cr, 7.9; P, 18.84; H O(cryst.), 10.94.
3
2
retention of the localized π(P=O) bond. In the IR spec-
Found (%):
Cr, 7.74; P, 18.86; H O(cryst.), 10.92.
2
–1
trum of HEDP, the band of π(P=O) lies at 1200 cm .
The coordination of the PO group in the Cr(III) com-
3
The thermolysis of the isolated chromium(II) com-
plex proceeds in several stages. In the 70–210°ë tem-
perature interval, the sample mass loss (11.00%) was
accompanied by the endoeffect (Tmax = 137°ë), which
was due to the removal of 4 moles of water of crystalli-
zation and agrees well with the theoretical value
plexonate is accompanied by the alignment of all PO
bonds, and the band of ν(P=O) in its IR spectrum dis-
appears. In our opinion, this difference in the IR spectra
also confirms the oxidation state of chromium(II) in the
synthesized compound.
(
10.94%). Further increase in temperature to 380°ë
resulted in an additional mass loss of 6.00% (∆mtheor
.14%), caused by the removal of two moles of water
=
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4
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–
1
nic groups at 915 and 960 cm , which indicates that the
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1
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1
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–1
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d
3
–1
–1
ν (PO ) = 1065 cm , and ν (PO ) = 1110 cm .
s
2
as
2
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 27 No. 1
2001