Study of complexation of alkyl-substituted 2,2'-dipyrrolylmethene with lanthanide salts by electronic spectroscopy

Article abstract of DOI:10.1134/S0036023610060161

It has been found with the use of electronic spectroscopy that the reaction of alkyl-substituted 2,2'-dipyrrolylmethene with La(III), Pr(III), Sm(III), Dy(III), Ho(III), Er(IIII), and Yb(III) salts leads to the formation of tris(dipyrrolylmethene) complexes. Based on electronic absorption spectroscopy data and formation constants of lanthanide complexes, together with the earlier data, analysis of the nature of M-L bonds in 2,2'-dipyrrolylmethene chelates with p, d, and f elements has been performed. A correlation was found between the complex stability and the polarizing action of the complex-forming ion on the chromophore π system of dipyrrolylmethene, which allows one to classify these heterocyclic ligands with probes highly sensitive to the change in the ratio between the ionic and covalent contributions to coordination interactions. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S0036023610060161

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2010, Vol. 55, No. 6, pp. 932–936. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © E.V. Rumyantsev, A. Desoky, E.V. Antina, 2010, published in Zhurnal Neorganicheskoi Khimii, 2010, Vol. 55, No. 6, pp. 991–995.
PHYSICAL METHODS OF INVESTIGATION
Study of Complexation of AlkylꢀSubstituted 2,2'ꢀDipyrrolylmethene
with Lanthanide Salts by Electronic Spectroscopy
E. V. Rumyantseva, A. Desokya, and E. V. Antinab
a Ivanovo State University of Chemistry and Technology, pr. Engel’sa 7, Ivanovo, 153000 Russia
b Institute of Solution Chemistry, Russian Academy of Sciences, Akademicheskaya 1, Ivanovo, 153045 Russia
eꢀmail: evr@isuct.ru
Received September 22, 2008
Abstract—It has been found with the use of electronic spectroscopy that the reaction of alkylꢀsubstituted
2,2'ꢀdipyrrolylmethene with La(III), Pr(III), Sm(III), Dy(III), Ho(III), Er(IIII), and Yb(III) salts leads to
the formation of tris(dipyrrolylmethene) complexes. Based on electronic absorption spectroscopy data and
formation constants of lanthanide complexes, together with the earlier data, analysis of the nature of M–L
bonds in 2,2'ꢀdipyrrolylmethene chelates with
found between the complex stability and the polarizing action of the complexꢀforming ion on the chroꢀ
mophore system of dipyrrolylmethene, which allows one to classify these heterocyclic ligands with probes
p, d, and f elements has been performed. A correlation was
π
highly sensitive to the change in the ratio between the ionic and covalent contributions to coordination interꢀ
actions.
DOI: 10.1134/S0036023610060161
Rareꢀearth metal complexes with organic ligands
provide wide possibilities for synthesis of compounds
with various physicochemical properties. They can be
used as molecular and ionic precursors, catalysts, anaꢀ
lytical agents, thermosensitive dyes, etc. [1, 2]. Lanꢀ
thanide complexes with linear oligopyrroles, a diverse
group of polydentate ligands in which the chelating
unit is dipyrrolylmethene, are of considerable interest
for obtaining new functional materials. Dipyrrolylꢀ
methenes are chromophore molecules in which the
Me
Me
Et
Me
Me
N
NH
HL
Me
Dimethylformamide (DMF) was used as a solvent,
which was dictated by rather high solubilities of the
reaction components and available data in the solꢀ
high mobility of the conjugated
π
system is successꢀ vation and dissociation of lanthanide salts in this
solvent [6].
fully combined with the propensity to form a sixꢀ
membered chelating rings with metals [3]. In effect,
this leads to the appearance of optimal photophysical
and spectral properties of metal complexes with dipyrꢀ
rolylmethenes, which have already used for developing
liquid and solid laser dyes [4], photosensitizers [5],
and other materials.
EXPERIMENTAL
3,3'4,5,5'ꢀPentamethylꢀ4'ꢀethylꢀ2,2'ꢀdipyrrolylmeꢀ
thene (HL) was synthesized, purified, and identified
as described in [7].
Lanthanide salts of chemically pure grade were
recrystallized for aqueous solutions with addition of
the corresponding ammonium salts (NH4Cl, NH4Br,
NH4NO3) to suppress hydrolysis. The composition of
Despite considerable progress that has been made
in the last two decades in the area of the coordination
chemistry of linear dipyrrolylmethenes, none of the
works has addressed their lanthanide complexes. In
this context, the synthesis and study of such complexes
is of practical and theoretical interest. The primary
task is to study complexation reactions of dipyrrolylꢀ
methenes with lanthanide ions and determine the staꢀ
bility of the resulting coordination compounds. In this
paper, we report the results of studying the coordinaꢀ
tion interactions of La(III), Pr(III), Sm(III), Dy(III),
Ho(III), Er(III), and Yb(III) salts with alkylated
dipyrrolylmethene (HL),
the resulting hydrates EX3
= 3–6) was confirmed by elemental analysis and
· n
H2O (X = Cl–, Br–, NO–3 ;
n
thermogravimetry. Anhydrous salts were obtained by
dehydration of the corresponding hydrates on heating
in vacuum as described in [8].
DMF of chemically pure grade was dried over
molecular sieves 4 Å and distilled at reduced pressure.
The water content, determined by the Fisher method,
was no higher than 0.02%.
932

STUDY OF COMPLEXATION
933
483
A
483
Α
440
1.0
1.0
0.5
0
2
1
440
0.5
1
2
0
350
360
410
460
510
, nm
400
450
500
550
, nm
λ
λ
Fig. 2. Changes in the electronic absorption spectra of the
Fig. 1. Changes in the electronic absorption spectra of a
0
–5
˜
reaction mixture at nHo(NO ) = 1.33
×
10 mol/L as a
solution of HL (c0HL = 1.33
×
10 mol/L) as a function of
–5
3
3
function of c0HL
,
mol/L: (1)
3.29
×
10 and (
2)
5.88
×
10
.
0
–6
–5
–7
–6
˜
nErCl
,
mol/L: (
1)
8.1
×
10 and (
2)
9.33
×
10
.
3
Complexation reactions were studied by spectroꢀ leads to the only complexation product. In this case,
photometry. Electronic absorption spectra of solutions
of ligand and their metal complexes were recorded in
the range 350–800 nm on an Akvilon SFꢀ103 spectroꢀ
photometer (Russia) controlled by personal computer
with the Spectr 1.0 software. Studies were carried out
in quartz cells with a path length of 2 and 10 mm
placed in a temperatureꢀcontrolled (298.15 K) Peltier
cell.
an increase in the HL concentration is accompanied
by the simultaneous increase in the intensity of the
absorption bands at 440 and 482–483 nm. In the
molar ratio diagram at сM = const, a wellꢀpronounced
bend is observed at сL
bend is observed at сM
dence of the formation of the single stable complex
ML3] by the reaction
/сM = 3, and at сL = const, the
/сL = 1/3 (Fig. 3), which is eviꢀ
[
To determine the composition, equilibrium conꢀ
centrations, and apparent molar extinction coeffiꢀ
cients of the metal complexes, the molar ratio method
was used. The composition–property diagrams were
obtained by plotting as ordinates the measured
absorption of the nascent metal complex minus the
absorption of the ligand (HL) at a given wave length.
The reagent concentrations were varied in the range
10–4–10–6 mol/L. Under these conditions, the
absorption caused by lanthanide complex solvates was
negligible.
M3+ + 3HL = [ML3] + 3H+.
Thus, the above data point to the substitution of
three dipyrrolylmethene ligands for all acido ligands in
the initial lanthanide salt, which leads to the formation
of tris(dipyrrolylmethene) complexes [ML3].
Me
Me
Me
Et
Me
Me
N
M
N
Me
Me
N
N
RESULTS AND DISCUSSION
Et
Me
Me
The changes in the electronic absorption spectra at
various ligandꢀtoꢀlanthanide salt concentration ratios
point to the formation of one metal complex species.
An increase in the lanthanide salt concentration at the
constant ligand concentration in a solution leads to
the appearance and gradual buildup of the intensity of
one absorption maximum at 483 nm with concurrent
decrease of the ligand band at 440 nm (Fig. 1). In the
nearꢀUV region, a weak charge transfer band is
observed (372–377 nm). The use of the molar ratio
method with variation in the ligand concentration at
N
N
Me
Me
Me
Et
Me
Me
Me
[ML ]
3
The apparent equilibrium constants (
К
) were calꢀ
culated from the spectral data obtained at сL
/сM > 3 by
the constant metal salt concentration (Fig. 2) also the equation
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 6 2010

934
A483
RUMYANTSEV et al.
A
Δ
2.0
1.5
1.0
0.5
(a)
2
3
1
0
1
2
3
4
5
cM
сL/
Δ
A483
1.5
(b)
350
450
550
λ
, nm
Fig. 4. Typical electronic absorption spectra of solutions of
dipyrrolylmethene complexes with
) [ErL3], ( ) [ZnL ], and ( ) [BF L] (
[9, 10], respectively).
f
2
ꢀ,
d
ꢀ
, and elements:
p
1.0
0.5
(
1
2
3
2
and 3 taken for
2
(table), average of the series of concentration logK.
values, can be considered standard.
Previously [3, 9], for
dꢀmetal complexes with
dipyrrolylmethenes, it was noticed that the thermodyꢀ
namic stability of chelates increases with an increase
in the degree of polarization of the
π
system of the
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7
0.8
сM/cL
chromophore by the auxochrome estimated from the
bathochromic shift of the strong band in the electronic
absorption spectrum of the complex as compared with
the free ligand. Comparison of these results with our
data allows us to analyze the nature of the M–L bonds
in the dipyrrolylmethene complexes. Figure 4 shows
typical spectra of complexes of 2,2'ꢀdipyrrolylꢀ
Fig. 3. Molar ratio diagrams for the system HL–ErCl
–
3
0
0
˜
DMF at (a) nErCl = const and (b) cHL = const.
3
methenes with
strongest polarization of the
mophore appears at the maximal overlap of the orbitꢀ
als of the central atom and the donor nitrogen atoms of
the ligand, which is realized for B(III) complexes with
dipyrrolylmethene [10]. A decrease in the contribuꢀ
p
ꢀ,
d
ꢀ
, and
f
elements. As is seen, the
system of the chroꢀ
[ML3][H+]3
[M3+][HL]3 (cM0 3+ − [ML ])[HL]3
27[ML3]4
π
K =
=
,
3
where [ML3] and [HL] are the equilibrium concentraꢀ
tions of the metal complex and ligand (mol/L) directly
tion of the orbitals of the dꢀmetal ion to the formation
cM0 3+
determined from electronic spectra, and
initial concentration of the lanthanide salt (mol/L).
is the
of the complex with dipyrrolylmethene leads to the
hypsochromic shift of the intraligand band. The smallꢀ
est bathochromic shift is observed for the complexes
with lanthanides, which points first of all to the preꢀ
dominance of the ionic character of coordination
In the working concentration ranges, the changes
in the ionic strength of solutions caused by the comꢀ
plexation reaction are less than
1 ×
10–4 mol/L. At
°
K
bonds and is confirmed by the dependence of log
0
constant
, the values of the complexation equilibꢀ
cM3+
of formation of [ML3] complexes on the inverse radius
of Ln(III) (Fig. 5). The monotonic character of this
dependence is evidence of strengthening of the M–L
rium constants are independent of the salt concentraꢀ
°
values
tion in the solution. Therefore, the log
K
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 6 2010

STUDY OF COMPLEXATION
935
bonds with a decrease in the Ln(II) ionic radius, as it
follows from the simple Fajans polarization theory
[11]. Nevertheless, the contribution of the 4f orbitals
to the formation of coordination bonds in lanthanide
complexes with dipyrrolylmethenes cannot be
ignored. This is reflected by the deviation from the
Characteristics of electronic spectra (λmax, nm (logε ) and
formation constants (logK° ) of the complexes of 3,3',4,5,5'ꢀ
pentamethylꢀ4'ꢀethylꢀ2,2'ꢀdipyrrolylmethene (HL) with
La(III), Pr(III), Sm(III), Dy(III), Ho(III), Er(IIII), and
Yb(III) in DMF at 298.15 K
Complex
[LaL ]
λ
max, nm (logε
)
logK°
°
K –1/r plot, which is a result of
straight line of the log
a superposition of the covalent contribution and the
effect of steric hindrances [12]. Despite the fact that
elucidation of this problem as applied to the dipyrroꢀ
lylmethene complexes calls for additional studies, the
following circumstance is noteworthy. Two linear segꢀ
372 (3.63), 482 (4.74)
374 (3.65), 483 (4.93)
375 (3.71), 483 (4.95)
375 (3.65), 483 (4.95)
375 (3.65), 483 (4.95
376 (3.65), 483 (4.95)
377 (3.71), 483 (5.00
1.37 0.10
1.49 0.09
1.56 0.25
1.84 0.30
2.02 0.25
2.57 0.51
2.93 0.64
3
[PrL ]
3
[SmL ]
3
°
K –1/r curve. The
ments can be discerned in the log
[DyL ]
3
intersection of these straight lines gives the limiting
radius of the Ln ion (rlim = 0.91 Å); for larger lanꢀ
thanide ions, the electrostatic interaction of M3+ with
[HoL ]
3
[ErL ]
the L– anion remains almost constant and the log
K
3
°
values are constant. This fact is consistent with the
conclusions in [12] for lanthanide complexonates.
[YbL ]
3
The formation of [ML3] complexes is characterized
by the constants that are 8–13 orders of magnitude
of the dipyrrolylmethene ligands to coordination
interactions, they are suitable probes that indicate the
ratio between the ionic and covalent contributions to
smaller than the corresponding values for the
dꢀmetal
complexes [3, 9], which is also a direct result of the
predominantly ionic character of the bond between the M–L bonds.
the central Ln atom and the dipyrrole ligand. Taking
A crucial result of the ionic character of the bond in
Ln complexes is a high and variable coordination
number of the central atom. Our findings and data on
into account the correlation between the stability and
the strength of the polarizing action of the metal ion
on the chromophore system of the dipyrrole ligand
(Fig. 4), we should note that electronic spectroscopy
not only reveals the character of coordination bonds
but also allows one to predict the stability of complexes
in series of structurally related compounds. Thus,
dipyrrolylmethene complexes with triply charged
d
metals [13] point to the possibility of arrangement of
three dipyrrolylmethene ligands in the octahedral
coordination sphere. Taking into account the strucꢀ
tural rigidity of dipyrrolylmethene molecules, we can
assume that, in the [ML3] complexes, the structure of
the coordination polyhedron is also close to octaheꢀ
dral and involved DMF molecules as extra ligands
(according to [6], the number of DMF molecules
coordinated to the lanthanide ion changes depending
on the nature of the lanthanide from 7 to 9).
because of the high sensitivity of the
π
ꢀelectron system
log
3.5
K
Yb
Er
3.0
2.5
2.0
1.5
1.0
Thus, our results are the benchmark for studying
dipyrrolylmethene complexes with lanthanides as
functional components of new materials.
Ho
Dy
ACKNOWLEDGMENTS
Sm
Pr
La
This work was supported by the Council for Grants
of the President of the Russian Federation for Support
of Young Scientists (grant no. MKꢀ313.2009.3), as
well as by the Analytical Departmental Target Proꢀ
gram “Development of Scientific Potential of the
Higher School (2009–2010)” (project no. 2.1.1/827)
and the Federal Target Program “Scientific and Scienꢀ
tific and Pedagogical Shots of Innovative Russia” for
2009–2013 (contract no. 02.740.11.0253).
r
lim = 0.91 Å
0.9
1.0
1.1
1.2
1/
r
Fig. 5. Plot of logK° of formation of the tris(dipyrrolylꢀ
methene) complexes of lanthanides in DMF vs. the inverse
3+
radius of the M ion (Å).
r
is the limiting radius of the
lim
Ln ion (see text for details).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 6 2010

936
RUMYANTSEV et al.
7. A. S. Semeikin, S. A. Syrbu, M. B. Berezin, and
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