New metal chelates with sterically hindered azo ligands: Synthesis and physicochemical properties

Article abstract of DOI:10.1007/s11173-005-0132-0

Metal chelates based on 3,5-di-(tert-butyl)-2-hydroxyazobenzene are synthesized and their physicochemical properties were studied. Four alternative structures and isomeric transformations between these structures were suggested on the basis of NMR data, electric dipole and magnetic moments, X-ray diffraction and quantum-chemical data. The change in stereochemistry of a complex through introduction of tert-butyl substituents was found to give unusual (for metal chelates with azo ligands (MN₂O₂)) dome-shaped isomers with the trans-planar coordination core at a vertex or with a distorted cis-structure.

Full text of DOI:10.1007/s11173-005-0132-0

Russian Journal of Coordination Chemistry, Vol. 31, No. 8, 2005, pp. 533–540. Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 8, 2005, pp. 563–571.
Original Russian Text Copyright © 2005 by Kogan, Lyubchenko, Shcherbakov, Ionov, Tkachev, Shilov, Aldoshin.
New Metal Chelates with Sterically Hindered Azo Ligands:
Synthesis and Physicochemical Properties
V. A. Kogan*, S. N. Lyubchenko*, I. N. Shcherbakov*, A. M. Ionov*,
V. V. Tkachev**, G. V. Shilov**, and S. M. Aldoshin**
*Rostov State University, Rostov-on-Don, Russia
**Institute of Problems of Chemical Physics, Russian Academy of Sciences,
Chernogolovka, Moscow oblast, 142432 Russia
Received April 20, 2004
Abstract—Metal chelates based on 3,5-di-(tert-butyl)-2-hydroxyazobenzene are synthesized and their physi-
cochemical properties were studied. Four alternative structures and isomeric transformations between these
structures were suggested on the basis of NMR data, electric dipole and magnetic moments, X-ray diffraction
and quantum-chemical data. The change in stereochemistry of a complex through introduction of tert-butyl sub-
stituents was found to give unusual (for metal chelates with azo ligands (MN2O2)) dome-shaped isomers with
the trans-planar coordination core at a vertex or with a distorted cis-structure.
Metal chelates with azo ligands have been for a long
The aim of this work was to synthesize metal che-
time in the focus of the researchers’ attention [1–5]. lates of type A (M = Ni(II) (A); Pd(II) (B), X = O, R =
This is explained by both their wide practical and theo- H (I), 4-CH3 (II), 4-OCH3 (III), 2,4,6-tris(CH3) (IV),
retical application in coordination chemistry. Recently, 2-OCH3 (V); R1 = 3,5-di-t-Bu) and to study the effect
of particular interest are compounds capable of iso- of tert-butyl groups on the structure and properties of
meric transformations [6], including azo compounds these compounds.
and their metal chelates [5, 7]. The latter include metal
chelates of type A
EXPERIMENTAL
R1
4
6
3
Synthesis of 2-hydroxy-N-phenylazobenzenes.
Equimolar quantities of 2-nitroso-4,6-di-tert-butyl-
phenol and of the corresponding substituted aniline
were heated in glacial acetic acid at 70–80°C for 3–5 h.
Then, water was added to the solution and the obtained
gummy product was chromatographed (with Al2O3 as
an adsorbent and hexane–benzene mixture as eluent).
The characteristics of the ligands formed (azo com-
pounds I–V) are given below.
5
X
2
M/2
1
N N
2
1
6
3
5
4
R
X = NH, O, S
(A)
R = H (I). The yield was 40%; mp = 131°C; orange needle-
whose capability of isomerisation depends essentially
on their structures. This is explained by the fact that due
to the ambidentate nature of the azo group [8], both
nitrogens have identical donor properties and can
equally participate in coordination. Therefore, both
five- and six-membered chelate cycles can form and
simultaneously occur in one molecule, which was
established by X-ray diffraction method [2, 9]. More-
over, it was shown that in addition to the isomeric trans-
formations in typeA metal chelates, the cis-trans-isom-
erism can also take place [10]. The variety of isomeric
transformations depends on the nature and ionic radius
of a metal (M) and, mainly, on the stereochemical con-
ditions produced by a ligand [11]. A slight change in
spatial effects can shift the isomeric equilibrium or
result in new isomers.
shaped crystals.
For C20H26N2O
anal. calcd. (%): C, 77.34;
Found (%): C, 77.01;
H, 8.44;
H, 8.24;
N, 9.02.
N, 8.96.
R = 4-CH3 (II). The yield was 57%; mp = 147°C; orange
powder.
For C21H28N2O
anal. calcd. (%): C, 77.72;
Found (%): C, 77.51;
H, 8.70;
H, 8.44;
N, 8.63.
N, 8.36.
R = 4-OCH3 (III). The yield was 38%; mp = 145°C; yellow
powder.
For C21H28N2O2
anal. calcd. (%): C, 74.06;
H, 8.29;
N, 8.33.
1070-3284/05/3108-0533 © 2005 Pleiades Publishing, Inc.

534
KOGAN et al.
Table 1. Elemental analysis of Ni(II) (A) and Pd(II) (B) azo complexes
Content (found/calcd.), %
H
Empirical
formula
Compound
R
mp, °C
Yield, %
C
N
IA
H
C40H50N4O2Ni
C40H50N4O2Pd
C42H54N4O2Ni
C42H54N4O2Pd
C42H54N4O4Ni
C42H54N4O4Pd
C46H62N4O2Ni
C46H62N4O2Pd
C42H54N4O4Ni
C42H54N4O4Pd
270
250
243
230
233
265
226
254
269
234
70.23/70.84
65.45/66.18
71.43/71.42
66.83/66.91
68.23/68.32
64.23/64.17
72.43/72.46
68.23/68.19
68.30/68.32
64.30/64.17
7.45/7.37
6.68/6.95
7.75/7.71
7.45/7.22
7.39/7.38
7.00/6.93
8.15/8.20
7.65/7.72
7.20/7.38
6.86/6.93
7.60/7.26
6.63/7.72
7.87/7.94
7.57/7.43
7.57/7.59
7.20/7.13
7.37/7.35
6.87/6.92
7.65/7.59
7.65/7.13
78
50
68
40
60
47
75
43
70
44
IB
H
IIA
IIB
IIIA
IIIB
IVA
IVB
VA
VB
4-CH3
4-CH3
4-OCH3
4-OCH3
2,4,6-tris-CH3
2,4,6-tris-CH3
2-OCH3
2-OCH3
boiled for 30 min. The ligand and Ni acetate were dis-
solved in absolute methanol, while Pd acetate was dis-
solved in acetone. In the case of the Ni complexes, a
solution of KOH (1 mmol) in 1 ml of methanol was
added to the reaction mixture after it was heated. The
precipitates were isolated and washed several times
with small amounts of absolute methanol and dried at
80°C. The results of elemental analysis, the yield of the
product, and melting points of compounds are given in
Table 1.
Found (%):
C, 74.51;
H, 8.00;
N, 8.10.
R = 2,4,6-tris(CH3) (IV). The yield was 40%; mp = 84°C;
orange powder
For C23H32N2O
anal. calcd. (%): C, 78.29;
Found (%): C, 78.51;
H, 9.15;
H, 8.95;
N, 7.94.
N, 7.50.
R = 2-OCH3 (V). The yield was 35%; mp = 120°C; yellow
powder.
For C21H28N2O2
The identification of complexes IA–VA, IB–VB was
performed by a standard procedure of cryoscopic
method using Beckman thermometer for benzene solu-
tions of the complexes. Cryoscopic constant equal to
5.1 K was used for benzene. The dipole moments of the
complexes were measured at 25°C in benzene, while
the magnetic susceptibility was measured for powders
as described in [12]. The values of the molar mass, elec-
tric dipole moments (m), and effective magnetic
moments (meff) for the obtained complexes are listed in
Table 2.
anal. calcd. (%): C, 74.06;
Found (%): C, 74.31;
H, 8.29;
H, 7.95;
N, 8.23.
N, 8.10.
Synthesis of Ni (IA–VA) and Pd (IB–VB) metal
chelates. Hot solutions containing stoichiometric quan-
tities of the corresponding metal acetate (0.5 mmol)
and spatially screened 2-hydroxy-N-phenylbenzene
(1 mmol) were poured together and the mixture was
Table 2. Selected physicochemical parameters of com-
plexes IA–VA, IB–VB
1
The H NMR spectra were recorded on a Bruker
DPX 250 instrument (250 MHz) in CDCl3, the chemi-
cal shifts (d) were measured with respect to tetramethyl
silane.
Com-
Molar mass
Dipole mo-
µeff, µB
pound (found/calcd.), g/mol
ment µ, D
IA
688/677
711/725
720/705
745/753
740/737
760/785
770/761
790/809
725/737
760/785
Diamagnetic
Diamagnetic
Diamagnetic
Diamagnetic
Diamagnetic
Diamagnetic
2.91
1.42
1.79
X-ray diffraction analysis of complex IA. The unit
cell parameters of a crystal and a three-dimensional set
of intensities were obtained from dark opaque single
crystals of low quality on automated four-circle KUMA
diffractometer (MoKα radiation, graphite monochro-
mator): a = b = 23.770(3), c = 17.224(3) Å, space group
IB
IIA
IIB
IIIA
IIIB
IVA
IVB
VA
VB
R3c. The intensities of 5827 reflections were measured
in a range of angles θ ≤ 26° by ω/2θ scan mode. After
elimination of systematic absences and averaging
intensities of equivalent reflections, the working array
of the measured F2 (hkl) consisted of 3672 independent
reflections, of which 1399 reflections with F2 > 4σ(F2)
Diamagnetic
3.08
1.25
1.90
Diamagnetic
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 8 2005

NEW METAL CHELATES WITH STERICALLY HINDERED AZO LIGANDS
Table 3. H NMR spectra of ligands I–V, Ni (IA–VA) and Pd (IB–VB) complexes (δ, ppm (J, Hz))
535
1
Com-
pound
CH3
M (H)
H
R
t-Bu
Aromatic protons
OH
(OCH3)
I
H
H
H
9H,
9H,
H, 7.438 s. 3H, 7.45– H, 7.782 s. 2H, 7.862 H, 13.680
1.372 s.
1.467 s.
(2.3)
7.56 m.
(2.3)
m.
s.
IA
Ni
Pd
H
9H,
9H,
5H 7.2–
7.35 m.*
2H, 8.427
d. (7.33)
0.989 s.
1.276 s.
IB
9H,
9H,
H, 7.224 d. H, 7.360 d. 3H, 7.32– 2H, 8.361
(2.7) (2.7) 7.55 m. d. (8.23)
0.793 s.
1.242 s.
II
4-CH3
9H,
9H,
3H,
2.419 s.
2H, 7.294 H, 7.408 d. H, 7.754 d. 2H, 7.762 H, 13.728
d. (8.5) (2.6) (2.6) d. (8.5) s.
1.361 s.
1.456 s.
IIA
IIB
III
IIIA
IIIB
IV
Ni
Pd
H
4-CH3
9H,
9H,
3H,
2H, 7.080 H, 7.274 d. H, 7.329 d. 2H, 8.296
d. (8.1) (2.7) (2.7) d. (8.1)
1.001 s.
1.277 s.
2.339 s.
4-CH3
9H,
0.803 s.
9H,
1.239 s.
3H,
2.406 s.
H, 7.206 d. 2H, 7.246 H, 7.346 d. 2H, 8.244
(2.7) d. (8.5) (2.7) d. (8.5)
4-OCH3
4-OCH3
4-OCH3
2,4,6-
9H,
1.358 s.
9H,
1.455 s.
3H,
3.877 s.
2H, 6.998 H, 7.386 d. H, 7.732 d. 2H, 7.843 H, 13.643
d. (8.9)
2H, 6.830 H, 7.269 d. H, 7.330 d. 2H, 8.419
d. (8.9) (2.7) (2.33) d. (8.9)
2H, 6.944 H, 7.200 d. H, 7.338 d. 2H, 8.374
d. (8.9) (2.675) (2.7) d. (8.9)
(2.7)
(2.7)
d. (8.9)
s.
Ni
Pd
H
9H,
1.017 s.
9H,
1.282 s.
3H,
3.816 s.
9H,
0.846 s.
9H,
1.245 s.
3H,
3.828 s.
9H,
9H,
1.471 s.
3H,
2.324 s.
2H, 6.955 H, 7.34 d. H, 7.747 d.
H, 13.629
s.
tris-CH3 1.367 s.
s.
(2.7)
(2.7)
6H,
2.413 s.
IVA
IVB
Ni
Pd
2,4,6-
9H,
9H,
9H,
2H,
H, 7.205 d. H, 7.359 d.
tris-CH3
1.2 s.*
1.256 s.
2.238 s. 6.756 s.* (2.3) (2.3)
2,4,6-
tris-CH3 0.829 s.
9H,
9H,
1.231 s.
3H,
2.283 s.
2H,
6.870 s.
H, 7.369 d. H, 7.405 d.
(2.7) (2.7)
6H,
2.499 s.
V
H
2-OCH3
9H,
1.361 s.
9H,
1.469 s.
3H,
2H,
2H, 7.36– H. 7.757 d. H, 7.850 q. H, 14.228
4.013 s. 6.99–7.09 7.45 m.
m.
(2.3)
(2.3)
s.
VA
VB
Ni
Pd
2-OCH3
2-OCH3
9H,
9H,
Broad signal
1.2 s.*
1.227 s.
9H,
1.356 s.
9H,
1.463 s.
3H,
3.975 s.
H, 6.084 d. 2H, 6.22– H, 7.321 d. H, 8.096 d.
(7.0)
6.35 m.
H, 6.395 d.
(8.47)
(2.3)
(2.3)
were used in the further calculations. The structure was
solved by the direct method and refined by the full-
matrix least-squares method on F2 with the SHELXL97
RESULTS AND DISCUSSION
In order to establish the structures of metal chelates
IA–VA and IB–VB, their magnetic properties, dipole
program [13] in anisotropic approximation to R = 9.8%. moments, NMR spectra, quantum-chemical calcula-
tions and preliminary results of X-ray diffraction anal-
The electronic and spatial structures of type I
ysis were compared. As follows from Table 2, two Ni
bis(chelate) conformers were calculated in terms of the
density functional theory (DFT) in the augmented basis
complexes are paramagnetic, which makes the study of
their NMR spectra difficult. The NMR spectra of the
of three-exponential functions with PBE functional [14]. initial ligands and metal chelates are given in Table 3.
The calculations were performed with the PRIRODA
program for Intel-compatible processors.
1
The H NMR spectra of the ligands exhibit signals
from protons of two nonequivalent t-Bu groups, methyl
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 8 2005

536
KOGAN et al.
and methoxy substituents (R) in the aryl fragment at the fact that the spectra of the complexes contain signals
azo group, aromatic protons, and the OH group.
from protons of only two nonequivalent t-Bu groups
suggests the symmetric (five- or six-membered) metal
chelate core or a rapid rearrangement of the six-mem-
bered cycles to five-membered cycles in the NMR time
In the case of complexes with the Ni2+ and Pd2+
ions, no signals are observed from the OH groups
(13.6–14.3 ppm), which indicates that the coordination
compounds contain deprotonated form of a ligand. The scale, which is doubtful. Indeed,
Bu-t
Bu-t
Bu-t
O
N
O
O
M/2
N
M/2
M/2
N
N
N
N
t-Bu
t-Bu
t-Bu
(6)
(5)
(5)
1
the H NMR spectrum of compound IA in nitroben- observed also in the second ligand). It is of interest to
zene-D5 remains almost unchanged in a temperature consider the orientation of tert-butyl groups with
interval 20–150°C, i.e., no isomeric transformation respect to the phenyl ring plane. In this case, the C(10)
occurs in this interval.
and C(14) atoms lie in this plane at the distances 2.88
and 2.86 Å from the C(4) atom, which, with account of
the positions of hydrogen atoms at these atoms, makes
it possible to conclude that the distances from the
hydrogen atom at C(4) to the nearest hydrogen atoms of
tert-butyl groups lie within 2.10–2.35 Å.
The value of µ for IA (Table 2) is comparable with
µ = 1.31 D for the analogous Ni chelate without the
t-Bu group and with the planar trans-structure of the
coordination core (MN2O2) [9]. Taking into account
diamagnetic properties of compound IA (Table 2), one
can conclude that it also has the planar trans-structure
of the coordination core. This conclusion can be rical structures of complex IA showed (Fig. 1) that the
applied to compounds IIA and IIIA.
At the same time, the values of µeff for compounds
IVA and VA correspond to two unpaired electrons,
which indicates the tetrahedral configuration of the
coordination core in the case of IVA (Table 2). Obvi-
ously, the same effect determines the high value of µeff
X-ray diffraction data and consideration of geomet-
bulky tert-butyl substituents in the ortho-position to the
phenyl oxygen produce substantial spatial hindrances
for the mutual arrangement of two ligands in the com-
plex in both cis- and trans-orientation. In this case, sev-
eral geometrical isomers and conformers can appear.
Their relative stability was determined by quantum-
for compound VA. However, in this case, the structure chemical calculations of the spatial and electronic
can be completed to octahedron due to the additional structures of the possible configurations of the Ni com-
coordination of the methoxy O atom, which was previ- plex with azo ligand IA. The structure determined by
ously observed for the analogous metal chelates with X-ray diffraction analysis is shown in Fig 1a, while the
azo ligands [12]. The same comparison shows that the molecular models of the alternative structures of com-
dipole moment 1.9 D for complexVA (Table 2) is insuf- plex IA constructed on the basis of the calculated
ficient for the effective pseudotetrahedral structure to atomic coordinates are presented in Fig. 1b–1d. Four
be realized.
X-ray diffraction data for complex IA showed that
alternative geometrical configurations were consid-
ered: IA-td i.e., the trans-configuration of coordination
core with C2 symmetry—dome-shaped conformation
(Fig. 1a); IA-tl, i.e., centrosymmetric trans-structure—
ladder conformation (Fig. 1b); IA-c, i.e., the cis-config-
uration of the coordination core (Fig. 1c), and IA-tetr,
i.e, the tetrahedral structure of the coordination core
(Fig. 1d). The former three configurations correspond
to minima on a singlet potential energy surface, while
the tetrahedral configuration, as was expected, on a
triplet surface.
the synthesized compound had six-membered chelate
cycles, while in the presence of the t-Bu group, the mol-
ecule has an unusual dome-shaped conformation with
the planar trans-coordination core at the vertex (Fig. 1a).
It is a new type of structural isomers, which was not yet
discovered for type A metal chelates. This isomer dif-
fers from the known isomers in the cis-position of the
ligand planes with respect to the coordination core.
The bonds around the Ni atom have planar configura-
tion with tetrahedral distortion and trans-position of
The steric repulsion of ligands in the coordination
analogous bonds (the angles O(1)NiO(2) 171.6(3)°, sphere of a central atom can be minimized as a result of
N(2)NiN(4) 174.6(4)°). The chelate cycle is distorted the distortion of the geometrical structure of the coordi-
and has two bends, i.e., 9.3° along the N(2)···O(1) line nation core and of the ligand fragments, whereas the
and 23.8° along the N(1)···O(1) line toward one side possibility of formation of one or another structure and
relative to the metal cycle plane (note that both ligands its relative stability will be determined by availability
have analogous structures, and the same distortions are of low-energy distortions, which relax the interligand
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 8 2005

NEW METAL CHELATES WITH STERICALLY HINDERED AZO LIGANDS
537
C(38)
(a)
C(37)
C(39)
C(36)
C(40)
C(8)
C(7)
C(35)
C(9)
O(1)
C(10)
N(3)
C(21)
N(4)
C(3)
Ni
C(4)
C(2)
C(26)
C(33)
N(2)
C(20)
C(22)
C(5)
C(19)
O(2)
C(32)
C(1)
C(14)
C(25)
C(15)
N(1)
C(31)
C(34)
C(6)
C(23)
C(11)
C(12)
C(28)
C(18)
C(13)
C(24)
C(16)
C(27)
C(17)
C(29)
C(30)
C
H
(b)
H
H
H
C
C
H
H
C
C
C
C
C
C
H
C
C
C
H
H
H
C
C
C
N
N
C
C
H
O
H
H
C
H
Ni
H
H
H
C
H
O
C
H
C
C
C
H
C
N
H
N
C
C
H
H
C
H
C
C
H
C
C
C
H
H
C
C
C
H
C
C
C
H
H
Fig. 1. Structure and alternative molecular models of complexes IA: (a) IA-td, (b) IA-tl, (c) IA-c, and (d) IA-tetr.
repulsion. The analysis of vibrations of the planar struc-
As follows from the geometrical parameters of the
tures of Ni complexes for the coordination core shows coordination core (Table 4, Fig. 1), the least possibili-
that such distortions are the tetrahedral configuration of ties of such relaxation of the arising steric strains are
the metal atom bonds (angle θ differs from zero) and shown by the most probable “natural” trans-structure in
the bending of the chelate ring plane along the line of the ladder conformation (IA-tl) (Fig. 1b). One can
the donor atoms (angle ϕ).
clearly see close contacts of the phenyl substituent of
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 8 2005

538
KOGAN et al.
C
C
C
(c)
C
C
C
C
H
N
C
N
C
C
C
C
C
C
C
C
C
C
C
Ni
C
C
N
O
H
H
C
N
C
H
H
H
H
H
O
C
C
C
C
C
C
C
H
C
C
C
C
C
H
C
H
C
H
H
H
H
H
C
C
C
H
H
H
H
H
H
(d)
C
C
H
C
C
H
C
C
H
C
H
H
C
H
N
H
N
C
C
C
C
C
C
C
N
N
H
H
C
C
Ni
C
C
C
C
C
C
C
H
C
O
C
C
C
C
H
O
C
C
C
C
C
H
C
C
C
C
C
Fig. 1. Contd.
one ligand and of the tert-butyl substituents of another slightly deviate from a common plane, the donor atoms
ligand. Thus, the distortion such as the tetrahedral con- are almost at the vertices of a square, whose center
figuration of the coordination core is blocked in this accommodates the Ni ion (angle θ = 0°). The most
case. All five atoms forming the coordination core only essential distortion of the coordination center—the
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NEW METAL CHELATES WITH STERICALLY HINDERED AZO LIGANDS
539
Table 4. Geometrical parameters of alternative structures of complex IA (θ is dihedral angle, ϕ is the angle of bending of
a chelate cycle plane along the line of donor atoms
Distance, Å
Twisting along
Structure
IA-tl
θ, deg
ϕ, deg
Angle NNiO, deg
N–N bond, deg
Ni–O
Ni–N
1.860
1.863
1.873
1.897
1.887
1.888
1.866
1.935
0.0
21.7
24.3
78.9
30.0
26.6
22.4
2.2
89.9
92.1
91.6
92.4
16
IA-td
IA-c
6.3
6.2
2.2
IA-tetr
Table 5. Electron distribution in coordination core of complex IA
Atomic charges
Structure
Bond order
Ni
O
N(1)
N(2)
O–Ni
Ni–N
N=N
O–C
IA-tl
IA-td
IA-c
0.683
0.766
0.725
–0.642
–0.662
–0.647
–0.418
–0.418
–0.424
–0.276
–0.290
–0.288
0.45
0.45
0.39
0.45
0.46
0.55
1.39
1.46
1.47
1.12
1.14
1.16
bending of the molecular plane along the line of donor explained by the structure of frontier orbitals of the che-
late-forming ligands
atoms equal to 30° (angle ϕ) is the ladder formation,
which is the result of the t-Bu interaction with the
phenyl ring of another ligand. Therefore, relaxation of
a steric strain in structure IA-tl involves the higher-
energy distortions of the ligands. The spatial repulsion
of the phenyl substituents at the donor N atom of the
tert-butyl groups results in a noticeable twisting of the
ligand fragments about the double azo bond (up to 16°)
and in significant deviation of azo group from the aro-
matic ring plane. The N atom of azo group deviates
from this plane by 17°.
Y
X
–
determined by the relative electronegativity of the
donor X and Y atoms of a ligand.
A less electronegative terminal heteroatom makes
the greatest contribution to HOMO (of the sigma type)
of an anion, which makes the bond of this atom to the
coordinating ion more stable in the cis-structure, but
less stable in the trans-structure, since particularly this
MO participates in bonding with unoccupied d orbital
of the transition metal ion with d8 configuration. In the
case of the ligand under consideration, this role is
played by the donor N atom of the azo group.
The comparison of the calculated total energies
(Table 6) confirms the conclusion that the most unfa-
vorable is the ladder structure IA-tl. It is 11.7 kcal/mol
less stable than the cis-structure. The energy difference
In the cis-structure I-c and in the dome-shaped
structure I-td, the spatial strains can be removed by
mutual rotation of ONiN fragments (angle θ in Table 4)
on chelate cycles, which is maximum for the cis-struc-
ture and equal to 24.3° and 21.7° for IA-c and IA-td,
respectively. The possibility of such rotation leads to a
sharp decrease in twisting along the double bond N=N
of the ligand fragments to ≈6° and to almost coplanar
location of azo group and aromatic nucleus of the
ligands (the deviation does not exceed 6°).
Table 6. The calculated dipole moments, total energies (Etot)
and the energies relative to the most stable isomer IA (Erel)
As was expected, the electron density distributions
in the coordination core of the cis- and trans-structures
differ significantly (Table 5). In the trans-structures,
independent of the realized conformation of a complex,
the Ni2+ ion bonds with the donor N atoms of the azo
group are 0.02 Å longer and less populated than in the
cis-chelate. The bonds with the donor O atoms show the
opposite tendency (the distance in the cis-complex is
0.01 Å longer than in the trans-complex). This is
Structure
Etot, au
Erel, kcal/mol*
µ, D
IA-tl
IA-td
IA-c
–3443.782659
–3443.798423
–3443.801276
–3443.786669
11.7
1.7
0.0
9.2
0.0
0.8
3.4
IA-tetr
* E is the difference in total energies of conformers with respect
rel
to E(IA-c).
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 8 2005

540
KOGAN et al.
for the dome-shaped trans-isomer IA-td and cis-isomer using only the results obtained. However, the main con-
IA-c is not large and equals 1.7 kcal/mol (Table 6). This clusion can be drawn even on the basis of the available
is comparable with the solvation effects, and one can data: the introduction of t-Bu substituents gives new
believe that the formation of one or another structure unusual structural isomers for type A metal chelates.
will depend on the conditions of the synthesis or isola-
tion of the reaction products. In other words, two iso-
meric complexes of a given compound can be obtained,
ACKNOWLEDGMENTS
in principle, provided that they are separated by a high
energy barrier. In addition, these isomers differ notice-
ably in polarity: the dipole moment of the cis-structure
is 3.4 D, as compared to 0.8 D for the trans-isomer
IA-td (Table 6).
This work was supported by the Russian Foundation
for Basic Research (project no. 04-03-32978a).
REFERENCES
1. Alcock, N.W. Spencer, R.C., et al., J. Chem. Soc., Sect.
A, 1968, no. 10, p. 2383.
2. Kogan, V.A., Kharabaev, N.N., Osipov, O.A., and
Kochin, S.G., Zh. Strukt. Khim., 1981, vol. 22, no. 1,
p. 126.
For comparison, let us calculate the structure of a
high-spin (triplet) state of the Ni complex compound
(IA-tetr). Its structure corresponds to almost regular tet-
rahedral configuration of the coordination core (the
angle between the ONiN planes of the ligands is equal
to 79.9°). With almost lacking steric repulsion in the
coordination sphere of a metal, the chelate cycles are
flattened (the bending along the O–N line is only ~2°),
and the twisting along the azo bond is reduced. The
energy of this state is 9 kcal/mol higher than that of the
cis-isomer. Thus, the cis-structure IA-c and the trans-
structure IA-td are separated by a sufficiently high tran-
sition barrier, which gives additional evidence of the
possibility of isolation of both isomeric complexes. It is
not excluded that the transitions between these isomers
can be induced by photochemical processes.
3. Das, S., Hung, C-H., and Goswami, S., Inorg. Chem.,
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Zh. Obshch. Khim., 1975, vol. 45, no. 2, p. 362.
The above analysis of the alternative structures can
be applied also to the Pd complexes IB, IIB, and IVB,
which, judging from their µ values (Table 2), have in
general the same structures as the Ni complexes.
9. Aldoshin, S.M., Alekseenko, V.A., Atovmyan, L.O.,
et al., Koord. Khim., 1975, vol. 1, no. 8, p. 1075.
10. Kogan, V.A., Kochin, S.G., Antsyshkina, A.S., et al.,
Mendeleev Commun., 1999, no. 2, p. 82.
11. Kogan, V.A., Abstracts of Papers, XX Mezhdunarodnaya
Chugaevskaya konferentsiya po koordinatsionnoi khimii
(XX Int. Chugaev. Conf. on Coordination Chem.), Ros-
tov-on-Don, 2001, p. 101.
If there is equilibrium between isomers IA-c and
IA-td, then, with account of the dipole moments (Table 2),
this equilibrium is almost shifted toward a low-polar
structure IA-td. An overestimated dipole moment in a
solution of complex IB, as compared to IA (Table 2), is
likely to be exactly due to an increase in the quantity of
the polar structure of isomer IB-c, since Pd has a larger
12. Kogan, V.A., Shcherbak, S.I., and Osipov, O.A., Zh.
Obshch. Khim., 1971, vol. 41, no. 1, p. 165.
ionic radius. A decrease in the dipole moment of IVB 13. Sheldrick, G.M., The SHELX97 Manual, Göttingen
(Germany): Univ. of Göttingen, 1997.
14. Perdew, J.P., Burke, K., and Ernzerhof, M., Phys. Rev.
Lett., 1996, vol. 77, nos. 18–28, p. 3865.
as compared to IB suggests the reduction of its fraction
in equilibrium. Unfortunately, poor solubility of the
complexes does not allow determination of the dipole
moments for the whole series of compounds, and there-
fore, it is difficult to make unambiguous conclusions
15. Laikov, D.N., Chem. Phys. Lett., 1997, vol. 281, nos. 1–3,
p. 151.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 8 2005