Metal complexes with azomethines containing the isomeric E-Z azo fragments

Article abstract of DOI:10.1134/S1070328410070018

Cobalt and zinc complexes with phenylazosalicylaldehyde azomethines were obtained by chemical and electrochemical syntheses. The B3LYP/6-311G(d, p) calculations showed that the azo group in both the ligands and the complexes are mainly in the E-form.

Full text of DOI:10.1134/S1070328410070018

ISSN 1070ꢀ3284, Russian Journal of Coordination Chemistry, 2010, Vol. 36, No. 7, pp. 479–489. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © A.D. Garnovskii, A.S. Burlov, A.G. Starikov, A.V. Metelitsa, I.S. Vasil’chenko, S.O. Bezugliy, S.A. Nikolaevskii, I.G. Borodkina, V.I. Minkin, 2010, pubꢀ
lished in Koordinatsionnaya Khimiya, 2010, Vol. 36, No. 7, pp. 483–493.
Metal Complexes with Azomethines Containing
the Isomeric E–Z Azo Fragments
,
A. D. Garnovskiia *, A. S. Burlova, A. G. Starikovb, A. V. Metelitsaa, I. S. Vasil’chenkoa,
S. O. Bezugliyb, S. A. Nikolaevskiia, I. G. Borodkinaa, and V. I. Minkina
,
b
a Research Institute of Physical and Organic Chemistry, Southern Federal University, RostovꢀonꢀDon, Russia
b Southern Scientific Center, Russian Academy of Sciences, RostovꢀonꢀDon, Russia
*Eꢀmail: garn@ipoc.rsu.ru
Received December 1, 2009
Abstract—Cobalt and zinc complexes with phenylazosalicylaldehyde azomethines were obtained by chemiꢀ
cal and electrochemical syntheses. The B3LYP/6ꢀ311G(d, p) calculations showed that the azo group in both
the ligands and the complexes are mainly in the Eꢀform.
DOI: 10.1134/S1070328410070018
Along with various photophysical effects that plicity has been studied mainly for isomerization of
change the magnetic properties of materials (selective complexes with stilbene [8, 9] and azobenzene ligands
photoinduced growth of magnetic domains, excitation [4]. Since LD LISCꢀtype reactions accompanied by a
of electrons resulting in crystal magnetization, radiaꢀ spin crossover can occur in solutions, the study of their
tionꢀinduced formation of paramagnetic centers in mechanism seems to be especially promising. We
crystals, etc.) [1, 2], photochemical transformations examined cobalt complexes with salicylaldimine
that enable reversible modulation of the magnetism of ligands containing the photochromic azobenzene
solids and solutions are of great interest for the prepaꢀ fragment.
ration of a new generation of energyꢀindependent
Despite permanent interest in metal complexes
capacious magnetic storage devices [3–5]. Among the
with various azo ligands [10–19], the literature data on
most important mechanisms behind photoinduced
metal complexes with the N=N group being outside
transformations that lead to reversible change in the
the coordination unit are scarce. Typical examples of
magnetism on the molecular level (spin crossover),
the latter include complexes with azomethines conꢀ
lightꢀinduced ones include a capture of photoexcited
taining the phenylazo group in the aldehyde fragment
states (LIESST) [1, 6], modifications of the crystal latꢀ
of chelated Schiff’s bases [20–24].
tice of the anionic fragment of paramagnetic comꢀ
plexes of ionic photochromes [7], and a spin crossover
induced by isomerization of the photochromic ligands
The goal of this study was to obtain metal comꢀ
plexes with azo ligands, perform quantumꢀchemical
calculations, and examine the absorption and emisꢀ
in d4 d7 metal complexes (LD LISC) [8].
–
Photoinduced ligand isomerization that changes sion spectra of azo azomethine ligands (I) and their
the character of the environment of the metal ion and metal complexes (II) promising for the study of the
causes the transition into the state of a different multiꢀ crossover effect.
R1
R1
N
N
N
O
N
OH
M(CH3COO)2
M/2
C N
H
C N
R2
R2
H
(1)
I
: R1 = H, R2 = Ph (a);
R1 = H, R2 = (CH2)17CH3 (
R1 = C2H5O, R2 = (CH2)11CH3 (
R1 = C2H5O, R2 = (CH2)17CH3 (
II: M = Co, R1 = H, R2 = Ph (a);
M = Zn, R1 = C2H5O, R2 = (CH2)17CH3 (
M = Co, R1 = C2H5O, R2 = (CH2)11CH3 (
M = Zn, R1 = C2H5O, R2 = (CH2)11CH3 (
M = Co, R1 = C2H5O, R2 = (CH2)17CH3 (
b);
b
);
c
d
);
).
c);
d
);
e).
479

480
GARNOVSKII et al.
Special interest in ligands
is due to the presence of the
isomerizable azo group in their molecules [25–28]:
I
and their complexes II 1.63 (m, 2H, CH2), 3.62 (t, 2H, CH2, 3J = 7.0 Hz),
4.11 (quintet, 2H, CH2, 3J = 6.9 Hz), 6.90–6.98
(m, 3H, HCAr), 7.79–7.98 (m, 4H, HCAr), 8.37 (s,
1H, HC=N), 14.35 (br.s, 1H, OH).
E⎯Z photoꢀ and thermoꢀ
R
R
IR (ν,
cm–1): 3100–3400 (s, OH), 1636 (vs, C=N),
R
N N
N N
1500 (m, N=N), 1285 (m, Ph–O).
R
(2)
For C27H39N3O2
E
Z
anal. calcd. (%)
Found (%):
:
С 74.10;
С 74.18;
Н 8.98;
Н 9.76;
N 9.60.
N 9.58.
R = Ar, Het
1ꢀ[5ꢀ(4ꢀEthoxyphenylazo)salicylidene]aminooctaꢀ
decane (Id). The yield was 80%, yellow crystals, Tm
79–80 C.
=
EXPERIMENTAL
°
5ꢀPhenylazosalicylaldehyde and 5ꢀ(4ꢀethoxyphenylꢀ
azo)salicylaldehyde were prepared as described in [24].
Synthesis of azomethines Ia–Id. A solution of an
appropriate amine (aniline, dodecylamine, and octaꢀ
decylamine) (0.1 mol) in methanol (10 ml) was added
to a solution of arylazosalicylaldehyde (0.1 mol) in
methanol (10 ml). The reaction mixture was refluxed
on a water bath for 1 h. On cooling, the precipitates of
azomethines that formed were filtered off and recrysꢀ
tallized from acetonitrile.
1H NMR (
δ
, ppm): 0.88 (t, 3H, CH3, 3
J
= 6.7 Hz),
1.25–1.57 (m, 34H, 17CH2), 3.63 (t, 3H, CH3, 3J
=
7.0 Hz), 4.11 (quintet, 2H, CH2, 3
J = 7.0 Hz), 6.97–
7.94 (m, 7H, HCAr), 8.39 (s, 1H, HC=N), 14.37 (br.s,
1H, OH).
IR (ν,
cm–1): 3100–3400 (s, OH), 1636 (vs, C=N),
1500 (m, N=N), 1285 (m, Ph–O).
For C33H51N3O2
5ꢀPhenylazosalicylideneaminobenzene (Ia). The
anal. calcd. (%):
Found (%):
С 75.96;
С 75.87;
Н 9.85;
Н 9.64;
N 8.05.
N 8.16.
yield was 68%, yellow needles,
1H NMR (
, ppm): 7.26–8.07 (m, 13H, HCAr),
8.76 (s, 1H, HC=N), 13.90 (s, 1H, OH).
IR (
, cm–1): 3100–3400 (s, OH), 1638 (s, C=N),
Tm = 140–141°C.
δ
ν
Chemical synthesis (CS) of complexes IIa–IId
[29]. A solution of Co(CH3COO)2 · 4H2O or
Zn(CH3COO)2 · 2H2O (1 mmol) in methanol (10 ml)
was added to a solution of an appropriate azomethine
Ia–Id (2 mmol) in methanol (10 ml). The reaction
mixture was refluxed on a water bath for 1 h. The preꢀ
cipitates that formed were filtered off, recrystallized
from MeOH–CHCl3 (2 : 1), and dried in a vacuum
desiccator.
1495 (w, N=N), 1289 (m, Ph–O).
For C19H15N3O
anal. calcd. (%): С 75.73;
Found (%): С 75.84;
Н 5.02;
Н 5.12;
N 13.94.
N 14.02.
1ꢀ(5ꢀPhenylazosalicylidene)aminooctadecane (Ib):
yellow crystals, 60% yield, Tm = 60–61 C.
1H NMR ( , ppm): 0.88 (t, 3H, CH3, 3
1.18–1.36 (m, 30H, 15CH2), 1.73 (quintet, 2H, CH2,
3J = 7.4 Hz), 3.64 (t, 2H, CH2, 3
= 6.6 Hz), 7.04
°
δ
J = 6.6 Hz),
Electrochemical synthesis (ES) of complexes IIa–
IId was carried out according to a standard procedure
by the reaction of azomethines Ia–Id with cations of
the anode dissolved during electrolusis [30, 31].
Azomethines Ia–Id (2 mmol) were dissolved in acetoꢀ
nitrile (10 ml) and [Et4N]ClO4 (0.01 g) was added as a
conductive agent. A current (40 mA, 15 V) was passed
through the solution for 1 h. The precipitates of the
resulting complexes were filtered off, recrystallized
from MeOH–CHCl3 (2 : 1), and dried in a vacuum
desiccator. The melting temperatures and spectroꢀ
scopic characteristics of the chemically and electroꢀ
chemically obtained complexes are identical.
J
(d, 8H, HCAr, 3
J = 9.0 Hz), 7.43–7.53 (m, 3H, HCAr),
7.99 (dd, 1H, HCAr, 3J = 9.0 Hz, 4J = 2.4 Hz), 8.40
(s, 1H, HC=N), 14.46 (s, 1H, OH).
IR (ν,
cm–1): 3100–3400 (s, OH), 1636 (vs, C=N),
1494 (m, NN), 1287 (m, Ph–O).
For C31H47N3O
anal. calcd. (%)
:
С 77.94;
С 77.98;
Н 9.92;
Н 9.84;
N 8.80.
N 8.78.
Found (%):
1ꢀ(5ꢀ(4ꢀEthoxyphenylazo)salicylidene)aminododeꢀ
cane (Ic). The yield was 75%, yellow crystals, Tm
69–70 C.
1H NMR (
1.19–1.22 (m, 18H, 9CH2), 1.46 (t, 3H, CH3, 3
=
°
Bis(5ꢀphenylazosalicylideneaminobenzolato)cobalt (IIa).
The yield was 75%, red crystals, Tm = 252–253°C,
= 7.0 Hz), μeff = 4.31 μB (293 K).
δ
, ppm): 0.85 (t, 3H, CH3, 3
J
J
= 6.7 Hz),
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 36
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METAL COMPLEXES WITH AZOMETHINES CONTAINING
481
IR (
ν,
cm–1): 1604 (vs, C=N), 1496 (m, N=N),
Bis{1ꢀ[5ꢀ(4ꢀethoxyphenylazo)salicylidene]aminoꢀ
1319 (m, Ph–O).
octadecanato}cobalt (IIe). The yield was 63%, a redꢀ
brown powder,
Tm = 135–136°C, μeff = 4.86 μB (293 K).
For C38H28N6O2Co
IR (
ν,
cm–1): 1620 (s, C=N), 1500 (m, N=N),
anal. calcd. (%)
:
С 69.20; Н 4.28; N 12.74; Со 8.93.
(CS) С 69.14; Н 4.21; N 12.67; Со 8.84.
(ES) С 69.15; Н 4.30; N 12.70; Со 8.96.
1326 (m, Ph–O).
Found (%):
For C66H100N6O4Co
anal.calcd.(%)
:
С 72.03; Н 9.16; N 7.64; Со 5.35.
(CS) С 72.15; Н 9.14; N 7.74; Со 5.47.
(ES) С 72.08; Н 9.10; N 7.58; Со 5.39.
Found (%):
Bis[1ꢀ(5ꢀphenylazosalicylidene)aminooctadecanato]zinc
(IIb). The yield was 95%, yellow crystals, Tm = 85–
86°
C.
1H NMR (
1.18–1.29 (m, 60H, 30CH2), 1.58–1.65 (m, 4H,
2CH2), 3.61 (t, 4H, 2CH2, 3
= 7.2 Hz), 6.93 (s, 2H,
HCAr, 3
= 9.16 Hz), 7.40–7.52 (m, 6H, HCAr), 7.83–
8.03 (m, 8H, HCAr), 8.34 (s, 2H, 2HC=N).
IR (
cm–1): 1614 (s, C=N), 1496 (m, N=N),
Electronic absorption spectra were recorded on a
Varian Carry 100 spectrophotometer; emission spectra
were recorded on Shimadzu RFꢀ5001 PC spectrofluoꢀ
δ
, ppm): 0.87 (t, 6H, 2CH3, 3
J = 6.7 Hz),
J
1
rimeter. H NMR spectra were recorded on a Varian
J
Unityꢀ300 instrument (300 MHz) in CDCl3 with
2
internal stabilization of the H polar resonance line.
ν
,
IR spectra of powdery complexes
I and II were
1326 (m, Ph–O).
recorded on a Varian 3100ꢀFTIR Excalibur instruꢀ
ment by frustrated total internal reflection. The magꢀ
netic moments of solid samples were determined by
the Faraday method at room temperature.
For C62H92N6O2Zn
anal. calcd. (%)
:
С 73.09; Н 9.10; N 8.25; Zn 6.42.
(CS) С 73.13; Н 9.19; N 8.18; Zn 6.49.
(ES) С 73.05; Н 9.14; N 8.20; Zn 6.50.
Found (%):
Calculations were performed by the DFT method
(B3LYP/6ꢀ311++G(d, p)) using the Gaussian03 proꢀ
gram [32]. As demonstrated in [33–35], these funcꢀ
tional and basis set reproduce well the energy characꢀ
teristics of intramolecular rearrangements in transiꢀ
tion metal complexes. To find stationary points on the
potential energy surface (PES), we fully optimized the
Bis{1ꢀ[5ꢀ(4ꢀethoxyphenylazo)salicylidene]aminꢀ
ododecanato}cobalt (IIc). The yield was 75%, a redꢀ
brown powder,
Tm = 130–131°C, μeff = 4.84 μB (293 K).
IR (ν,
cm–1): 1616 (s, C=N), 1499 (m, N=N), molecular geometries and calculated the force conꢀ
1320 (m, Ph–O).
stants. The structures that refer to the energy minima
on the PES were found by a minimum energy path
proceeding along the gradient line from the saddle
point to its adjacent stationary (saddle or minimum)
point [36]. The graphical images of the molecular
structures (Figs. 1, 2) were plotted with the ChemCraft
program; the Cartesian coordinates of the atoms from
the quantumꢀchemical calculations were used as input
parameters for this program.
For C54H76N6O4Co
anal. calcd. (%)
:
С 69.58; Н 8.22; N 9.02; Со 6.32.
(CS) С 69.46; Н 8.28; N 9.15; Со 6.42.
(ES) С 69.60; Н 8.30; N 9.00; Со 6.38.
Found (%):
Bis{1ꢀ[5ꢀ(4ꢀethoxyphenylazo)salicylidene]aminꢀ
ododecanato}zinc (IId). The yield was 89%, yellow
crystals,
1H NMR (
1.18–1.21 (m, 36H, 18CH2), 1.46 (t, 6H, CH3, 3J
7.9 Hz), 1.62–1.65 (m, 4H, 2CH2), 3.60 (t, 4H, 2CH2,
3J = 7.0 Hz), 4.11 (quintet, 4H, 2CH2, 3J = 6.9 Hz),
6.91–7.00 (m, 6H, HCAr), 7.78–7.99 (m, 8H, HCAr),
8.32 (s, 2H, 2HC=N).
T
m = 130–131
°C.
RESULTS AND DISCUSSION
δ
, ppm): 0.84 (t, 6H, 2CH3, 3
J = 6.7 Hz),
1
Elemental analysis (C, H, N, and M) and IR, H
NMR, and electronic absorption spectroscopy of
=
azomethines
azo azomethines
I
and their metal chelates II revealed that
exist as iminophenol tautomers in
I
both solution and the solid state and that their reacꢀ
tions with Co(II) and Zn(II) acetates give the comꢀ
IR (ν,
cm–1): 1618 (s, C=N), 1499 (m, N=N),
1326 (s, Ph–O).
plexes ML2 (where HL are ligands
When performing quantumꢀchemical calculations,
we considered not only the relative stabilities of the E⎯Z
I).
For C54H76N6O6Zn
tautomers (2) of free and coordinated Schiff’s bases
containing the phenylazo substituent but also the posꢀ
sibility of a square–tetrahedron equilibrium for saliꢀ
cylideneaminate [30, 31].
anal. calcd. (%)
:
С 69.10; Н 8.16; N 8.95; Zn 6.97.
(CS) С 69.18; Н 8.13; N 8.87; Zn 6.82.
(ES) С 69.07; Н 8.20; N 9.00; Zn 7.05.
Found (%):
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 36
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482
GARNOVSKII et al.
1.379
1.407
1.419
1.254
1.412
1.410
1.382
1.404
1.434
1.334
1.421
1.410
0.997
1.716
1.336
1.421
1.391
1.401
1.454
1.246
0.996
1.721
1.391
1.401
1.431
1.455
1.284
1.285
1, Cs
4, C1
1.360
1.439
1.418
1.258
1.450
1.253
1.362
1.448
1.402
1.374
1.431
1.250
1.254
1.470
1.783
1.031
1.420 1.472
1.439
1.424
1.776
1.032
1.420
1.395
1.374
1.325
1.396
1.325
5,
C1
2,
Cs
1.370
1.427
1.418
1.256
1.426
1.372
1.291
1.406
1.432
1.267
1.215
1.425
1.383
1.425 1.291
1.445
1.446
1.384
1.407
1.424
1.248
1.273
1.210
1.427
1.407
1.424
1.303
1.304
6, C1
3, Cs
1.379
1.408
1.387
1.400
1.332
1.221
1.407
1.332
1.422
1.456
1.284
0.998
1.709
1.441
7, C1
Fig. 1. DFTꢀcalculated (B3LYP/6ꢀ311++G(d, p)) geometrical parameters of structures 1–7 (the bond lengths are cited in angꢀ
stroms, also in Figs. 2 and 3).
Initially, we studied the structures and relative staꢀ (with consideration to the zeroꢀpoint energy of harꢀ
bilities of different forms of the ligands (R = H). Our monic vibrations). Therefore, structures and are in
calculations showed that structure , in which the azo dynamic equilibrium with each other. The most stable
group has the configuration and the proton of the conformation of the ligand (structure ) is thermoꢀ
chelate ring is localized on the O atom, is characterꢀ dynamically less favorable than structure by
ized by the lowest total energy and, accordingly, is a 16.3 kcal/mol. Apparently, structures and are also
1
2
1
E
Z
4
1
Z
4
5
global minimum (Fig. 1, Table 1).
At the same time, tautomer is less stable than the
iminophenol form by less than 2 kcal/mol. The calꢀ
culated barrier to the interconversion between these
structures, which occurs by intramolecular proton this ligand is induced by photoirradiation only, it was
transfer through transition state , is 2.3 kcal/mol interesting to estimate the barrier to this rearrangeꢀ
in equilibrium because of a small (2.7 kcal/mol) barꢀ
rier to their prototropic isomerization involving tranꢀ
2
sition structure
6.
1
Although E⎯Z isomerization of the azo group of
3
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 36
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2010

METAL COMPLEXES WITH AZOMETHINES CONTAINING
483
1.373
1.418
1.418
1.304
1.423
1.386
1.864
1.886
1.256
1.431
1.434
1.294
1.408
1.407
8
,
C2h
1.368
1.431
1.418
1.256
1.425
1.406
1.287
1.870
1.888
1.381
1.437
1.418
1.412
1.308
9
,
C2v
1.369
1.418
1.256
1.291
1.422
1.406
1.430
1.383
1.940
2.000
1.443
1.432
1.412
1.302
10,
C2
1.376
1.433
1.247
1.417
1.428
1.305
1.420
1.386
1.864
1.886
1.431
1.435
1.407
1.294
11
,
C2
1.372
1.432
1.248
1.421
1.291
1.427
1.939
1.383
1.427
1.442
1.433
1.411
2.000
1.302
12, C2
Fig. 2. DFTꢀcalculated (B3LYP/6ꢀ311++G(d, p)) geometrical parameters of structures 8–12.
ment under thermal conditions. Theoretical isomerꢀ tion path allows an activation energy decrease to
28 kcal/mol.
ization of azobenzene has been discussed in [37–39].
DFT B3LYP calculations showed that the isomerizaꢀ
tion on the groundꢀstate PES is prevented until a barꢀ
rier of 40 kcal/mol is surmounted. An analysis of this
isomerization by the CASSCF method on a tripletꢀ
The calculated barrier to the isomerization of
structure
1 proceeding through transition state 7 is
38.5 kcal/mol. This agrees well with the aforemenꢀ
tioned calculations and suggests that the reaction is
excited PES revealed that the spinꢀforbidden transiꢀ impossible under thermal conditions. Therefore, the
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 36
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484
GARNOVSKII et al.
*
Table 1. Total energy Etot, the relative energy ΔE, the total energy Etot, ZPE corrected to the zeroꢀpoint energy of harmonic viꢀ
brations, and the relative energy ΔEZPE corrected to the zeroꢀpoint energy of harmonic vibrations in structures 1–7 calculated
by the DFT method (B3LYP/6ꢀ311++G(d, p)); λ is the number of imaginary Hessian eigenvalues
Structure,
symmetry
Etot, au
Δ
E,
kcal/mol
E
tot, ZPE, au
ΔEZPE
,
kcal/mol
λ
1
2
3
,
,
,
Cs
Cs
Cs
C1
–741.64541
–741.64240
–741.63748
–741.61921
–741.61538
–741.61071
–741.58180
0.0
1.9
–741.42901
–741.42630
–741.42526
–741.40298
–741.39950
–741.39879
–741.36761
0
0
0
1
0
0
1
1
1.7
2.3
5.0
4,
16.4
18.8
21.8
39.9
16.3
18.5
19.0
38.5
5, C1
6
,
,
C1
C1
7
* ZPE is the zeroꢀpoint energy of harmonic vibrations.
substituents in azobenzene have only a slight effect on pseudotetrahedral forms. The existence of “square–
the barrier to its isomerization. An analysis of the geoꢀ tetrahedron” equilibrium in such cobalt complexes
metrical parameters of structure
7 showed that the
was first reported in 1966 [40, 41]. The Gibbs energy
difference between the configurational isomers deterꢀ
mined from NMR spectra depends on the substituents
at the C atoms and does not exceed 3 kcal/mol. It was
interesting to consider the possibility of a similar spinꢀ
crossover isomerization of the complexes under study
N=N bond length in the azo group is shortened to
1.22 Å and the C=N bond length in the phenylazo
group is shortened to 1.33 Å. Apparently, these changes
are responsible for such a high barrier to the isomerꢀ
ization.
and estimate the influence of the
the azo group on this process. For this purpose, we
undergo rapid isomerization associated with the tranꢀ performed a quantumꢀchemical study of the stabilities
sitions between the lowꢀspin planar and highꢀspin of different complex forms for model structures III
E⎯Z isomerization of
It is known that cobalt complexes with
βꢀaminoviꢀ
nyl ketonates (R = H) in noncoordinating solvents
:
N
R
N
O
N
N
O
Co
N
R
N
III: R = H, CH3
According to calculations of bischelate complexes sponding to the minimum on a quartet PES. In strucꢀ
III (R = H), structure on a doubletꢀexcited PES is ture 10, the coordination polyhedron is a pseudotetraꢀ
8
hedron. A small energy difference between structures
and 10 (0.9 kcal/mol) suggests a dynamic equilibꢀ
rium between them in noncoordinating solvents.
A theoretical study of complexes with the conforꢀ
of the highꢀspin complex led to structure 10 correꢀ mation of the azo group showed that the energy differꢀ
characterized by the lowest energy and corresponds to
a lowꢀspin planar transꢀconfiguration (Fig. 2). cisꢀIsoꢀ
8
mer
structure
9
has also a minimum energy but is less stable than
by more than 10 kcal/mol. Consideration
8
Z
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METAL COMPLEXES WITH AZOMETHINES CONTAINING
485
Table 2. Total energy Etot, the relative energy ΔE, the total energy Etot, ZPE corrected to the zeroꢀpoint energy of harmonic viꢀ
brations, and the relative energy ΔEZPE corrected to the zeroꢀpoint energy of harmonic vibrations in structures 8–16 calculated
by the DFT method (B3LYP/6ꢀ311++G(d, p))
Structure, symꢀ
Multiplicity
E
tot, au
Δ
E,
kcal/mol
E
tot, ZPE, au
ΔEZPE, kcal/mol
metry
R = H
8
,
C2
2
2
4
2
4
–2864.96081
–2864.94559
–2864.95741
–2864.90778
–2864.90416
–2864.54582
–2864.52905
–2864.54445
–2864.49332
–2864.49172
h
9
,
C2
9.6
10.5
0.9
v
10
11
12
,
,
,
C2
C2
C2
2.1
33.3
35.5
32.9
33.9
R = CH3
13
,
C2
C2
4
2
4
2
–2943.59296
–2943.58618
–2943.53986
–2943.53326
–2943.12549
–2943.11613
–2943.07292
–2943.06368
14,
4.3
33.3
37.5
5.9
33.0
38.8
h
15
,
C2
C2
16
,
ence between lowꢀspin planar structure 11 and highꢀ forms, regardless of the substituents in the chelate ring
spin structure 12 is of the same order of magnitude as
that between the aforesaid transꢀisomers and 10
Therefore, they can also exist in equilibrium with each
other. We calculated that the complex with the ꢀazo
and the spin state. At the same time, replacement of
the functional groups at the donor N atom (R = H by
R = CH3) changes the groundꢀstate multiplicity in the
Co(II) bischelates studied.
8
.
E
group is more stable by ~33 kcal/mol than the complex
with the Zꢀazo group (Table 2); this value equals the
double energy difference between the different forms
of the ligands. Such a considerable energy difference
The electronic absorption spectra of the most staꢀ
ble
azomethine fragment in DMF show unsymmetrical
absorption bands at 336–369 nm 23560–
34290 l mol–1 cm–1) and less intense bands as a shoulꢀ
Eꢀisomers of azobenzenes I (3) containing the
(
ε
between the structures with the Zꢀ and
precludes thermal equilibrium in solutions of comꢀ
plexes III
Replacement of the substituent at the N atom (III
Eꢀazo groups
der at 430–455 nm (
(Table 3, Fig. 4).
ε
5120–9270 l mol–1 cm–1)
.
:
R = Me) stabilizes the highꢀspin pseudotetrahedral
configuration of the complex (structure 13, Fig. 3),
while lowꢀspin planar structure 14 is less stable by
5.9 kcal/mol. Similar results were obtained for the
complex with the Zꢀazo group. The calculated energy
difference between structures 15 and 16 (5.8 kcal/mol)
The intense absorption bands belong to the ππ∗
type and the less intense longerꢀwavelength bands
belong to the π∗ꢀtype [42]. Introduction of the
azomethine fragment mainly shifts the ππꢀtype band
to the longer wavelengths compared to its position for
unsubstituted azobenzene, which results in partial
ꢀ
n
provides evidence for a highꢀspin form.
overlap of the ππ∗ꢀ and nπ∗ꢀtype bands (Fig. 4).
Thus, one can conclude that the
E⎯Z isomerization
Like most of the earlier examined azobenzenes, the
of the azo group in complexes III does not influence
the relative stabilities of their highꢀ and lowꢀspin
Zꢀisomers of compounds
I do not fluoresce [43].
N N
N
O
H
h
ν
R1
N
h
ν, Δ
N
R2
R1
O
(3)
N H
R2
(E
)
(Z)
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486
GARNOVSKII et al.
1.370
1.428
1.418
1.257
1.420
1.405
1.295
1.932
1.440
1.385
1.410
1.439
2.014
1.296
13, C2
1.373
1.418
1.256
1.417
1.301
1.423
1.423
1.387
1.864
1.963
1.407
1.406
1.434
1.295
14, C2h
1.373
1.432
1.248
1.295
1.419
1.426
1.425
1.386
1.931
1.439
1.440
1.409
2.015
1.296
15,
C2
1.377
1.420
1.388
1.405
1.433
1.247
1.416
1.303
1.863
1.423
1.435
1.428
1.963
1.294
16, C2
Fig. 3. DFTꢀcalculated (B3LYP/6ꢀ311++G(d, p)) geometrical parameters of structures 13–16.
Irradiation of solutions of compounds
wavelengths of both the ππ∗ꢀ and π∗ꢀtype bands
induced isomerization characteristic of azobenꢀ
zenes, which made the longerꢀwavelength bands less
intense and caused a bathochromic shift of the ππ∗
I
at the
After the irradiation was stopped, the absorption
spectra of compounds returned to their original
shapes because of thermal isomerization. The
ꢀisomers of substituted azobenzenes are substanꢀ
ꢀisomer of unsubstituted
derivative (Table 3). The characteristic relaxation
times in Nꢀalkyl derivatives Ib–Id is more than one
order of magnitude than those in Nꢀphenyl azobenꢀ
I
n
E⎯Z
E⎯Z
Z
I
tially less stable than the
Z
ꢀ
type bands and a slight hypsochromic shift of the
type bands [44] (Table 3, Fig. 5).
nπ∗ꢀ
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METAL COMPLEXES WITH AZOMETHINES CONTAINING
487
Table 3. Spectral absorption and kinetic characteristics of azobenzenes I and their metal complexes II in DMF at 293 K
Photostationary
state
Characteristic times
of the thermal relaxation processes
Initial form
, E
Compound
λmax, nm
εmax
,
l mol–1 cm–1
λmax, nm
τ293
(
Z), s*
Ia
336
455 sh
363
30890
5120
332
0.1
Ib
23560
8660
350
363
363
302
396
367
385
365
375
1.1
1.6
1.7
1.6
430 sh
369
Ic
29970
8070
440 sh
369
Id
34290
9270
440 sh
317
438
Azobenzene
17000
500
×
105 [40]**
IIa
IIb
IIc
IId
IIe
398
449 sh
381
60770
44560
66790
31710
61620
46570
64280
30510
53180
128
0.8
9.7
431 sh
399
23
214
0.6
8.4
23
480
445 sh
380
430
393
Notes: * The lifetime of the Zꢀform.
** Benzene, 308 K.
zene Ia. The
thermal reverse, prevents complete photoinduced
conversion of the ꢀisomers into the ꢀforms because
of considerable overlap of their absorption bands. Irraꢀ
diation leads to a photostationary state in which the
ratio of the isomers depends on the wavelength and
intensity of the exciting light.
E–Z isomerization, along with its rapid complexes is generally biexponential; the characterisꢀ
tic times of the thermal reactions for the cobalt comꢀ
plexes are longer than the corresponding values for the
zinc complexes (Table 3). It can be seen from the preꢀ
sented data that both the time constants of the photoꢀ
induced relaxation processes in the complexes are one
to two orders of magnitude higher than the time conꢀ
stant of the thermal E–Z isomerization in the ligands.
The photostationary states in the ligands and the comꢀ
plexes are appreciably different: the absorption spectra
of the complexes differ only slightly prior to irradiation
and after establishing the photostationary state, while
spectral changes in the ligands are substantial. Thereꢀ
fore, a reverse photoreaction can bring the molecular
system to the initial state, the reverse photoreactions
being more efficient for the cobalt complexes (cf.
Figs. 6, 7).
E
Z
The electronic absorption spectra of Co (IIa, IIc,
and IIe) and Zn complexes (IIb and IId) with azomeꢀ
thine ligands containing a photochromic azobenzene
fragment show longerꢀwavelength peaks at 381–
399 nm (Table 3, Fig. 4). These bands are unsymmetꢀ
rical, having a shoulder at 431–449 nm. The influence
of the metal nature manifests itself in the longerꢀwaveꢀ
length range for the cobalt complexes compared to the
zinc ones. Neither cobalt nor zinc complexes were
found to fluoresce.
Irradiation of complexes II at the wavelengths of
their longerꢀwavelength absorption bands induced
photochromic transformations resulting in the
decreasing intensities and hypsochromic shifts of the
peaks (Figs. 6, 7). The established photostationary
state for the zinc complexes is more strongly shifted
toward the photoproducts than for the cobalt comꢀ
To find out whether complexes II can exhibit a
ligandꢀinduced crossover effect, lowꢀtemperature
magnetochemical studies (with and without UV irraꢀ
diation) of Co(II) and Fe(II, III) complexes are
plexes. The kinetics of the relaxation processes in the required.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 36
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488
GARNOVSKII et al.
, l mol–1 сm–1
80000
D
1.0
ε
8
1
5
0.8
0.8
0.4
0.2
6
1
2
60000
40000
20000
0
7
3
4
2
3
4
300 350 400 450 500 550 600 650 700
, nm
0
300
350
400
450
500
550
600
λ
λ, nm
Fig. 5. Electronic absorption spectra of azobenzene Id in
DMF ( ) prior to and after the irradiation with UV light
= 365 nm) for (
Fig. 4. Electronic absorption spectra of arylazosalicylaldiꢀ
1
mines (
1)
Ic, (
2)
Id, (3)
Ib, and (
4)
Ia and their metal comꢀ
IIb in DMF at 293 K.
(
λ
2
) 20, (
3
) 40, and (
4
) 70 s (
c
= 2.51 ×
–5
plexes (5)
IIa, (6)
IIc, (7)
IIe, and (
8)
10 mol/l, 293 K).
D
0.7
1.2
1.0
0.8
0.6
0.4
0.2
0
0.6
0.5
0.4
0.3
0.2
0.1
0
300
350
400
450
500
550
600
300
350
400
450
500
550
600
, nm
λ
, nm
λ
Fig. 6. Electronic absorption spectra of cobalt complex IIc
irradiated with UV light ( = 365 nm) in DMF at 293 K
10 mol/l). The spectra were recorded with
10ꢀs intervals.
Fig. 7. Electronic absorption spectra of zinc complex IIb
irradiated with UV light ( = 365 nm) in DMF ( = 1.71
10 mol/l, 293 K). The spectra were recorded with 2ꢀs
intervals.
λ
λ
c
×
–5
–5
(c
= 8.90
×
2. Einaga, Y., J. Photochem. Photobiol., C, 2006, vol. 7,
ACKNOWLEDGMENTS
This work was supported by the Russian Foundaꢀ
nos. 2–3, p. 69.
tion for Basic Research (project no. 09ꢀ03ꢀ
12109ofi_m).
3. Minkin, V.I., Izv. Akad. Nauk, Ser. Khim., 2008, no. 4,
p. 673.
4. Hasegawa, Y., Kume, S., and Nishihara, H., Dalton
Trans., 2009, no. 2, p. 280.
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