Mutual effects of equatorial and axial ligands on the stability of praseodymium(III) and samarium(III) phthalocyanine complexes

Article abstract of DOI:10.1134/S0036023615090119

The nature of axial ligand in lanthanide(III) phthalocyanine complexes markedly affects their physicochemical properties. The role of axially coordinated anion in the stability of coordination sphere was determined by the example of praseodymium(III) and samarium(III) phthalocyanine complexes (X)LnPc (X = Cl^-, Br^-, AcO^-). The synthesis and the results of dissociation kinetics study of the complexes under the action of AcOH in ethanol solution were presented. Stability parameters for the complexes, kinetic equations, and activation parameters for the dissociation of coordination centers were determined. Effect of the axial ligand on the strength of macrocycle binding and the stoichiometric mechanism of complex dissociation reaction was considered. The state of axial bond in the complexes was shown to determine the value of reaction order toward acid (1 or 2) and the nature of rate-limiting step of the reaction: with participation of molecule or ionized in the axial direction complex.

Full text of DOI:10.1134/S0036023615090119

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2015, Vol. 60, No. 9, pp. 1123–1128. © Pleiades Publishing, Ltd., 2015.
Original Russian Text © T.N. Lomova, 2015, published in Zhurnal Neorganicheskoi Khimii, 2015, Vol. 60, No. 9, pp. 1231–1237.
PHYSICAL METHODS
OF INVESTIGATION
Mutual Effects of Equatorial and Axial Ligands on the Stability
of Praseodymium(III) and Samarium(III) Phthalocyanine Complexes
T. N. Lomova
Krestov Institute of Solution Chemistry, Russian Academy of Sciences, Akademicheskaya ul. 1, Ivanovo, 153045 Russia
eꢀmail: tnl@iscꢀras.ru
Received October 27, 2015
Abstract—The nature of axial ligand in lanthanide(III) phthalocyanine complexes markedly affects their
physicochemical properties. The role of axially coordinated anion in the stability of coordination sphere was
determined by the example of praseodymium(III) and samarium(III) phthalocyanine complexes (X)LnPc (X
–
–
–
=
Cl , Br , AcO ). The synthesis and the results of dissociation kinetics study of the complexes under the
action of AcOH in ethanol solution were presented. Stability parameters for the complexes, kinetic equaꢀ
tions, and activation parameters for the dissociation of coordination centers were determined. Effect of the
axial ligand on the strength of macrocycle binding and the stoichiometric mechanism of complex dissociation
reaction was considered. The state of axial bond in the complexes was shown to determine the value of reacꢀ
tion order toward acid (1 or 2) and the nature of rateꢀlimiting step of the reaction: with participation of molꢀ
ecule or ionized in the axial direction complex.
DOI: 10.1134/S0036023615090119
The phthalocyanine complexes of rare earth eleꢀ tion (X)LnPc (X = Cl , Br , AcO–) (Scheme 1) in ethꢀ
–
–
ments of all known structures—acido phthalocyanine anol medium under the action of acetic acid.
(
X)LnPc (1 : 1 complexes), doubleꢀdecker PcLnPc⋅
.
and PcLnHPc, and tripleꢀdecker PcLnPcLnPc comꢀ
plexes of sandwich type, as well as complexes of the
latter type of higher stoichiometry—are of large scienꢀ
tific and practical interest. These compounds are used
to study important theoretical issues such as lanꢀ
thanideꢀinduced shift and structure of molecules in
X
N
N Ln
N
N
N
N
N
solution [1], magnetoꢀchiral dichroism [2],
π π
–
N
interactions and selfꢀorganization in different media,
forced association and aggregation [3]. The sandwichꢀ
type complexes provided a basis for the development
of optically active materials [4], molecule magnets [5–7],
fieldꢀeffect transistors [8, 9], and selfꢀorganizing sysꢀ
tems [10]. The coordination sphere of 1:1 acido phthꢀ
alocyanine complexes (X)LnPc is much less stable
than that of sandwich complexes [11, 12], therefore
the complexes themselves are not used in material
design. However, rare earth monophthalocyanine
Scheme 1. Acido phthalocyanine complexes of lanthaꢀ
nides(III) (X)LnPс, Ln = Pr, Sm; X = Cl , Br , AcO .
–
–
–
EXPERIMENTAL
Complexes (X)LnPc were prepared from Li Pc and
2
complexes, like doubleꢀdecker complexes, attract the corresponding Ln salt by procedure similar to that
⋅
described in [21]. Li Pc and LnX3
⋅
6H O ((AcO) Sm
interest as precursors in the molecular design of tripleꢀ
decker heteronuclear and heteroleptic complexes
2
2
3
2
H O) in 1 : 2 molar ratio were introduced into
2
DMSO, the mixture was heated to reflux, thermoꢀ
stated at boiling temperature for 15–20 min, and
cooled. Water was added to the reaction mixture in
[
13–15]. The binding strength of the axial acido ligand
in complex is of fundamental importance. There are
only few works [16–20] that study this issue at quantiꢀ
tative level, however, almost all of them deal with axiꢀ
ally coordinated rare earth porphyrin complexes.
volume ratio (1–2.5) : 35. The precipitate of H Pc was
2
separated by filtration. The filtrate was diluted with a
twofold volume of water. The resultant precipitate of
Therefore in this work, we report the synthesis of (X)LnPc complex was filtered off, washed with water,
praseodymium(III) and samarium(III) phthalocyaꢀ and dried in air. Yield of the complexes is about 90%.
nine complexes and chemical kinetics study of coordiꢀ Figure 1 and Table 1 show electronic absorption specꢀ
nation sphere stability for the complexes of composiꢀ tra (EAS).
1123

1124
LOMOVA
centration
n
were determined by the optimization of
A
0
τ
0
ln(C
C(X)LnPc)−τ and k −C
dependences,
1
.0
(X)LnPc
eff
AcOH
1
respectively, with the use of Microsoft Excel software.
The values of activation energies were determined by
optimization of the dependences in Arrhenius equaꢀ
tion coordinates, activation entropy was found from
the main equation of transition state theory transꢀ
formed to the view:
2
0.8
3
4
#
= 19.1log T
Δ
S
k
+
E/T
– 19.1log
T
– 205.
(1)
#
The average value of ΔS was found as arithmetic
#
0
0
0
.6
.4
.2
mean from the
Δ
S values for all temperatures of
kinetic experiment.
5
6
7
RESULTS AND DISCUSSION
8
UVꢀvis spectra of Pr and Sm complexes (Fig. 1,
Table 1) correspond to the literature data on absorpꢀ
tion spectra of lanthanide(III) phthalocyanines in
organic solvents [21]. Table 1 shows that the spectra
are weakly dependent on the nature of the axial acido
ligand. The position of longꢀwavelength maximum
changes markedly on variation of nature of coordinatꢀ
ing solvent, which may indicate the hexacoordinated
state of complexes in solutions with solvent molecule
in the first coordination sphere. The spectra in ethanol
do not change in time on heating in (X)LnPc concenꢀ
λ
, nm
–5
Fig. 1. UVꢀVis spectra of (Cl)SmPc in ethanol (
1
) and ethꢀ
equal to: 0 ( ),
).
tration range (1–10) 10 mol/L and obey Bouguer–
×
anol–15% AcOH mixture at 296 K and
), 6( ), 9( ), 12 ( ), 16 ( ), and 21 min (
τ
2
Lambert–Beer law, which indicates the absence of
dissociation and association processes of lanꢀ
thanide(III) phthalocyanines in this solvent.
3
(3
4
5
6
7
8
When 100% AcOH was added to ethanol solutions,
The rates of reactions of (X)PrPc and (X)SmPc
the complexes dissociate to form H Pc that remains
complexes with AcOH in ethanol medium were deterꢀ
mined by spectrophotometry. UVꢀVis spectra of soluꢀ
tions were recorded on an Agilent 8453 UVꢀVis and
Specord M400 spectrophotometers, complex concenꢀ
tration drop in kinetic experiments was determined on
a SFꢀ26 spectrophotometer equipped with thermoꢀ
stated cell. Temperature maintenance accuracy was
2
dissolved under experimental conditions. This conꢀ
clusion was drawn on the basis of transformation of
UVꢀVis spectra of complexes (Fig. 1, curves 2–8).
Strictly first order on complex concentration for all
studied (X)LnPc (Table 2) follows from the constancy
of keff values determined from kinetic curves in firstꢀ
order linear coordinates (see Experimental). The table
shows that dissociation rate increases for all complexes
with AcOH concentration. However, the dependences
of reaction rate on initial AcOH concentration for Pr
and Sm complexes are not the same:
±
0.1 K. Measurements were performed at = 672 nm.
λ
Acetic acid of reagent grade was dried by fractional
thawing, ethanol was dried by the standard procedure
22]. Water content in the dried solvents was 0.03%, as
[
measured by the Karl Fischer technique.
0
Rate constants (keff) at constant acid concentration
and independent of reagent concentrations kinetic
–
dC(X)PrPc
/
d
τ
=
kC(X)PrPc
C
,
(2)
(3)
AcOH
0
2
constants
k
along with reaction order toward acid conꢀ
–
dC(X)SmPc
/d
τ
=
kC(X)SmPc(CAcOH) .
Table 1. Position (
λ
, nm) of longꢀwavelength absorption band Q(0,0) in electronic spectra of (X)LnPc in solutionsa
max
Complex
Methanol
Ethanol
Complex
(Cl)SmPc
(Br)SmPc
AcO)SmPc
Methanol
Ethanol
DMSO
(
(
(
Cl)PrPc
Br)PrPc
AcO)PrPc
672.8
672.8
672
672.4
672.4
671.6
669.4
669.4
668.4
674
674
(
672.8
676.5
a
Weak bands at 604–606 nm and Soret band with maximum at 344 nm were also detected.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 60 No. 9 2015

MUTUAL EFFECTS OF EQUATORIAL AND AXIAL LIGANDS
1125
Table 2. Rate constants keff for dissociation of phthalocyanine complexes with praseodymium(III) and samarium(III)
under the action of AcOH in ethanol
3
–1
keff
×
10 , s
0
C
,
mol/L
AcOH
293.2 K
298.2 K
303.2 K
(
Cl)PrPc
Br)PrPc
0.07
0.14
0.43
0.71
1.056 ± 0.001
2.855 ± 0.02
10.0 ± 0.7
1.593 ± 0.001
3.58 ± 0.09
12.6 ± 0.7
21 ± 1
2.363 ± 0.003
4.50 ± 0.03
15.9 ± 0.9
27 ± 2
16.0 ± 0.9
(
0.07
0.14
0.43
0.71
1.025 ± 0.001
2.117 ± 0.002
8.8 ± 0.5
1.251 ± 0.001
3.02 ± 0.08
11 ± 1
1.521 ± 0.002
4.33 ± 0.07
15 ± 1
14 ± 1
19 ± 1
24 ± 1
(AcO)PrPc
0.07
0.14
0.43
0.71
0.996 ± 0.001
1.936 ± 0.002
7.3 ± 0.3
1.014 ± 0.001
2.845 ± 0.02
9.3 ± 0.6
1.480 ± 0.001
4.2 ± 0.2
12.2 ± 0.7
22 ± 2
13 ± 1
17 ± 1
3
–1
keff
×
10 , s
0
C
,
mol/L
AcOH
298.2
K
308.2
K
318.2 K
(Cl)SmPc
2.13
2.84
3.55
2.7 ± 0.1
4.2 ± 0.2
6.8 ± 0.4
4.7 ± 0.1
7.4 ± 0.2
10.5 ± 0.6
6.4 ± 0.1
12.0 ± 0.8
20 ± 1
(Br)SmPc
2.13
2.84
3.55
2.0 ± 0.1
3.1 ± 0.1
5.0 ± 0.2
3.9 ± 0.1
5.9 ± 0.3
8.5 ± 0.6
6.2 ± 0.1
9.7 ± 0.3
16.5 ± 0.8
(AcO)SmPc
2.13
2.84
3.55
1.85 ± 0.09
2.9 ± 0.1
4.5 ± 0.2
3.6 ± 0.1
5.55 ± 0.3
8.9 ± 0.5
6.1 ± 0.2
9.2 ± 0.4
15.0 ± 0.7
The orders of dissociation reactions toward AcOH
concentration experimentally determined from the
dependences obtained by variation of temperature and
axial ligand nature (exemplified by Figs. 2 and 3) were
found to be 1.07–1.20 and 1.75–2.27 for praseodyꢀ
mium and samarium complexes, respectively. Table 3
exhibits activation parameters and the secondꢀ and
k1
2+
–
(
X)PrPc + AcOH
(X)Pr + H Pc + AcO , (5)
2
K1
+
–
(
X)SmPc + AcOH
[(AcOH)SmPc] X , (6)
+
–
[(AcOH)SmPc] X + AcOH
(7)
k2
3+
–
–
Sm + H Pc + 2AcO + X .
thirdꢀorder rate constants
k
that are independent of
2
concentrations. Form the considered spectral and
kinetic data for the total reaction of complex dissociaꢀ
tion (Eq. (4)), one can write stepwise transformation
schemes (5), (6), and (7) that adequately reflect the
experimental kinetic equations of view (2) and (3):
In this scheme, the reaction of Pr complex dissociꢀ
ation proceeds in one slow stage with constant k1 that
equals to in the experimental rate equation (2). In
the case of Sm complexes, there is a quasi equilibrium
with slow oneꢀside stage (7) that can be described by
k
3+
–
–
(
X)LnPc + 2AcOH = Ln + X + H Pc + 2AcO ,(4) kinetic equation (8).
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 60 No. 9 2015

1126
LOMOVA
0
AcOH
0
log
C
–1.2 –1.0 –0.8 –0.6 –0.4 –0.2
0.4
0.5
logC
AcOH
–
–
–
1.5
2.0
2.5
–1.5
1
2
3
–
2.0
2.5
1
logkeff
2
3
–
logkeff
0
Fig. 2. logkeff on log
C
plots for (X)PrPc complexes at
AcOH
2
98 K. X = Cl (
1
), Br (
2), AcO (3). Approximation reliꢀ
0
2
ability
R
= 0.995–0.999.
Fig. 3. logkeff on logCAcOH plots for (X)SmPc complexes
at 318 K. X = Cl (
1
), Br (
2), AcO (3). Approximation reliꢀ
2
ability
R = 0.984–0.999.
0
–
dC
+
–
/dτ = k C C
+
–
.
(8)
2
AcOH
[
(AcOH)SmPc] X
[(AcOH)SmPc] X
Thus, in the course of dissociation of (X)SmPc comꢀ
Taking into account equilibrium (6), equation (8)
is reduced to form (9):
+
–
plexes, outerꢀsphere complex [(AcOH)SmPc] X reacts
in the rateꢀlimiting step, whereas (X)PrPc complexes
undergo elementary slow dissociation reaction at the
Pr–N bond in initial state with ligand X within the first
coordination sphere. It is characteristic that the kinetꢀ
ics of dissociation reaction is determined by the metal
–
dC
+
–
/dτ = –dC X)SmPc/dτ
(
[
(AcOH)SmPc] X
(9)
0
2
=
k K C(X)SmPc(C
) .
2
1
AcOH
The comparison of equation (9) with experimental
equation (3) leads to expression for experimental rate cation in (X)LnPc and is the same for all its axial comꢀ
constant: k2K1 plexes. This means that the acido ligand in Pr comꢀ
k
=
.
Table 3. Kinetic parameters for dissociation of phthalocyanine complexes with praseodymium(III) and samarium(III)
under the action of AcOH in ethanol
10 , s mol–1
4
–1
L
n
E
, kJ/mol (R2
)
–
S#, J/mol K
Δ
Complex
Cl)PrPc
T
, K
293
98
03
k
×
(
(
(
(
(
(
256
315
389
221
294
390
192
264
1.17
1.12
1.07
1.16
1.17
1.18
1.13
1.20
1.13
1.78
1.58
2.27
1.70
1.54
1.91
1.75
1.75
1.75
29.3 (0.997)
182.59 ± 0.05
141.23 ± 0.1
146.7 ± 0.2
244 ± 2b
2
3
293
2
3
293
2
3
298
3
3
298
3
3
298
3
Br)PrPc
41.8 (1)
98
03
AcO)PrPc
Cl)SmPca
Br)SmPc
AcO)SmPc
40.5 (0.9928)
20 (0.49)b
98
03
333
6.91
14.1
11.45
5.48
12.0
14.2
4.84
9.43
08
18
37.8 (0.8922)
46.8 (0.9973)
187 ± 1
08
18
158.6 ± 0.3
08
18
3
15.9
a
b
2
2
The dimension of (X)SmPc dissociation rate constants: L /(s mol ).
A rough value with unsatisfactory correlation coefficient.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 60 No. 9 2015

MUTUAL EFFECTS OF EQUATORIAL AND AXIAL LIGANDS
, К–1
1127
0.00332
0.00336
0.00340 1/T
–3.2
–3.3
–3.4
–3.5
–3.6
–3.7
–3.8
–3.9
1
1/T
, К–1
.00315 0.00325 0.00335
2
3
0
–
–
6.4
6.6
lnkT
–6.8
–
–
–
–
7.0
7.2
7.4
7.6
T
4
ln
k
Fig. 4. lnkT on 1/
T plots for complexes (Cl)PrPc (1), (Br)PrPc (2), (AcO)PrPc (3), and (AcO)SmPc (4).
plexes is bound stronger than the macrocycle, whereas the higher stability. This rule retains also for the Pr
Sm complexes show opposite picture. This is obviꢀ complexes, in spite of participation of axially unꢀionꢀ
ously caused by the decrease of kinetic stability of Pr ized complex form in slow reaction (5). This may be
complexes as compared with Sm complexes (Table 3) caused by the cis effect of axial and macrocyclic
and related decrease of AcOH concentration in ligands, which is well tractable in the context of sugꢀ
kinetic experiment (Table 2) to the level at which the gested reaction schemes of reactions (5) and (6), (7)
bond of Pr with donor atom of the axial ligand remains for Pr and Sm complexes, respectively. Namely: the
unꢀionized (Eq. (5)).
harder axial acido ligand displacement into second
coordination sphere (rate constant incorporates K1
k
)
The considered above stoichiometric mechanisms
of complex dissociation are based on the assumption
of integer first (Pr complex) and second (Sm complex)
reaction orders toward AcOH concentration. Howꢀ
ever, as noted above, the obtained in experiment reacꢀ
tion orders differ considerably from integer values and
decrease when temperature rises as well (Table 3). This
fact indicates more complex real mechanism of dissoꢀ
ciation reaction. It is likely that both considered reacꢀ
tion routes are realized in different extent depending
on the structure of coordination sphere of complex
involved in rateꢀlimiting step and temperature.
the higher samarium complex stability, the worse coorꢀ
dination conditions for attacking AcOH molecule to
the sixth coordination position the higher praseodyꢀ
mium complex stability.
(Cl)LnPc > (Br)LnPc > (OAc)LnPc.
(10)
This conclusion is confirmed by the close order of
dependences in Fig. 4 revealed on the treatment (see
Experimental) of activation energies
E values (Table 3)
for the complexes of such a different stability: the
2
98
kinetic stability of (X)SmPc complexes (
k ) is by facꢀ
–
–
tors of 54.6, 53.7, and 45.6 higher (for X = AcO , Br
,
–
The kinetic stability of complexes of both cations and Cl , respectively) as compared with (X)PrPc
with the macrocyclic ligand is low sensitive to the (Table 3). The same energetic expenses in the routes to
nature of the axially bound acido ligand X (Table 3). transition states [(X)PrPc AcOH]# and [(AcOH)Sm Pc
+
298
decreases in the series (10) by AcOH]# at the same X for complexes, whose stability
The magnitude of
k
the factors of 1.19 and 1.43 for Pr and Sm complexes, differs by one and half order of magnitude, is compenꢀ
respectively, i.e., in agreement with the electronegativꢀ sated by solvatation contribution. First, transition
ity parameters of donor atom X. For the same lanꢀ states with polarized Ln–N and O–H bonds for all six
thanide cation, the stronger bond to the axial ligand complexes are solvated stronger than the initial reacꢀ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 60 No. 9 2015

1128
LOMOVA
tants, which is reflected in the negative
Δ
S# values
for more stable Sm complexes
and for chloride axial complexes (Cl)PrPc and
Cl)SmPc have larger negative values in comparison
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1
1
1
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1
1
1
1
1
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ACKNOWLEDGMENTS
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This work was supported by the Russian Academy
of Sciences (the program no. 8 of the Presidium of the
RAS “Development of Methods for the Preparation of
Chemical Compounds and the Design of New Mateꢀ
rials”) within the directive “Directed Synthesis of 21. N. B. Subbotin, L. G. Tomilova, N. A. Kostromina,
Substances with Prescribed Properties and the Design
of Functional Materials on Their Basis” and by the
Russian Foundation for Basic Research (project 22. NonꢀAqueous Solvent Systems, Ed. by T. C. Waddington
no. 15ꢀ03ꢀ00646). The author thanks Dr. L. G. Andriꢀ
anova for performance of experiment.
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Translated by I. Kudryavtsev
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 60 No. 9 2015