Nucleophilicity of heteroaromatic N-oxides in coordination with Zn(II) tetraphenylporphyrinate and in the substitution reactions

Article abstract of DOI:10.1134/S1070363214010186

Linear correlations have been revealed between thermodynamic and kinetic parameters of the nucleophilic substitution and coordination reactions of zinc(II) tetraphenylporphyrinate with heteroaromatic Noxides in different solvents. Complex formation of zin

Full text of DOI:10.1134/S1070363214010186

ISSN 1070-3632, Russian Journal of General Chemistry, 2014, Vol. 84, No. 1, pp. 115–124. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © V.P. Andreev, P.S. Sobolev, D.O. Zaitsev, A.B. Il’yukhin, 2014, published in Zhurnal Obshchei Khimii, 2014, Vol. 84, No. 1,
pp. 121–131.
Nucleophilicity of Heteroaromatic N-Oxides
in Coordination with Zn(II) Tetraphenylporphyrinate
and in the Substitution Reactions
a
a
a
b
V. P. Andreev , P. S. Sobolev , D. O. Zaitsev , and A. B. Il’yukhin
a
Petrozavodsk State University, pr. Lenina 33, Petrozavodsk, Karelia, 185910 Russia
e-mail: andreev@petrsu.karelia.ru
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
Received February 14, 2013
Abstract—Linear correlations have been revealed between thermodynamic and kinetic parameters of the
nucleophilic substitution and coordination reactions of zinc(II) tetraphenylporphyrinate with heteroaromatic N-
oxides in different solvents. Complex formation of zinc(II) tetraphenylporphyrinate with n-donor ligands in
chloroform can serve as model system in spectroscopy studies of nucleophilicity and basicity of the compounds
capable of the n,ν type complexes formation.
DOI: 10.1134/S1070363214010186
This work extends the studies on correlation
between the stability constants of zinc(II) tetraphenyl-
porphyrinate complexes with various organic ligands
and physicochemical parameters of the same ligands
nucleophilic substitution reactions.
phenylporphyrinate coordination with heteroaromatic
pyridine-N-oxides (set I) and quinoline-N-oxides (set
II) (Table 1). Taking into account the tabulated data,
we will discuss some S reactions of the above-listed
N
ligands and their non-oxidized analogs (sets III and
IV). Noteworthily, N-oxides of 2,6-dimethyl-4-
We have recently demonstrated [1, 2] that
thermodynamic parameters of zinc(II) tetraphenyl-
porphyrinate and zinc(II) protoporphyrinate dimethyl
ester coordination with 3- and 4-substituted pyridines
in chloroform linearly depend on the values of the
bands shifts (Δλ) in electronic absorption spectra of
these metal porphyrinates upon interaction with the
nitropyridine (pK –0.86 [5]), 2,6-dichloropyridine,
a
and benzofuroxane (pK of the latter pair were not
a
found in the available literature; in the case of 2-
chloropyridine-N-oxide, pK was of –0.77 [6]) did not
a
change the electron absorption spectrum of zinc(II)
tetraphenylporphyrinate even when applied in the form
of the saturated solutions in chloroform; that was
evidently due to their extremely low nucleophilicity/
basicity (see Scheme 1).
pyridines as well as on pK of the ligands in water, and
a
on the Hammett constants σ of the substituents. The
linear correlation between changes of enthalpy and
entropy of the complexes formation confirms that the
process is “isoequilibrium” or “isothermodynamic”.
Linear correlations have been found as well between
thermodynamic and kinetic parameters of some
nucleophilic substitution reactions of pyridines and
complex formation reactions between zinc(II)
tetraphenylporphyrinate and the pyridines.
The measured stability constants K of zinc(II)
tetraphenylporphyrinate complexes with hetero-
aromatic N-oxides in chloroform were, strictly
speaking, the concentration ones. However, as the
initial concentrations of the complex forming com-
–5
pounds were low (2×10 mol/L of zinc(II) tetra-
–4
–2
phenylporphyrinate and 10 –10
mol/L of the
Pyridines nucleophilicity is usually in line with
reactivity of the corresponding pyridine N-oxides [3,
]. In view of that, in this work we analyzed the kinetic
and thermodynamic parameters of zinc(II) tetra-
ligands) and the ionic forms were absent in the
solution, the experimentally determined values should
have been fairly close to the thermodynamic stability
constants.
4
1
15

1
16
ANDREEV et al.
0
0
0
Table 1. Stability constants (K), thermodynamic parameters of the complex formation (ΔG , ΔH , ΔS ), shifts of I, II, and the
Soret absorption bands (Δλ) upon the complex formation, σ and σPyO constants of the substituents, and basicity (pK
a
) of the
ligands in water at 25°C; see complexes description in the text
0
0
0
pK
a
(25°C)
7–9]
σ
[12]
σ
PyO
Δλ
Соре
–ΔH ,
ΔS ,
–1 –1
ΔG ,
Ligand
K
298
ΔλII
Δλ
I
[
[7]
–0.603
–0.24
–
kJ/mol
J mol K
kJ/mol
Ie
2470±63
1353±24
1272±24
816±35
609±14
134±2.5
1650±27
1311±30
1142±25
1040±25
1030±11
454±9
2.05
1.29
–
–0.268
–0.17
–
10.2
9.3
–
14.5
13.8
13.8
13.3
12.6
10.0
14.1
13.6
13.4
13.0
13.5
12.4
10.6
14.1
13.6
13.2
14.7
14.0
13.3
12.7
11.8
9.3
17.0
15.7
15.4
15.0
13.8
12.0
16.3
15.8
15.6
15.2
15.8
15.0
13.0
16.1
15.7
15.5
17.4
16.3
15.7
15.3
14.6
12.1
15.9
12.9
16.0
14.7
14.0
12.3
16.9
16.2
15.9
18.2
16.3
13.94±0.26
19.0±0.8
–19.35
–17.85
–17.70
–16.60
–15.88
–12.13
–18.35
–17.77
–17.44
–17.20
–17.18
–15.15
–12.93
–18.10
–17.59
–17.32
–21.34
–19.90
–19.20
–18.91
–18.27
–13.86
–17.63
–12.87
–14.81
–14.46
–14.06
–10.42
–15.01
–14.43
–13.68
–20.2
Ib
13.88±0.21 13.47±0.75
Ig
13.66±0.03
13.43±0.39
13.81±0.37
13.74±0.15
13.90±0.31
13.83±0.25
13.81±0.22
13.77±0.44
13.82±0.31
13.88±0.29
13.59±0.22
13.83±0.28
13.69±0.16
14.04±0.26
13.90±0.29
13.61±0.26
13.81±0.34
13.96±0.24
13.74±0.11
13.50±0.24
11.91±0.03
11.85±0.29
12.05±0.22
12.18±0.24
12.02±0.32
13.23±0.1
10.6±0.6
7.1±0.6
Ia
0.79
0.36
0
0
8.8
8.4
5.3
–
Ii
0.227
0.778
(–0.21)
(–0.12)
(–0.06)
(–0.03)
–0.069
0.37
0.206
1.19
Ih
–1.7
–5.71±0.4
15.16±0.8
13.23±0.7
12.47±0.4
11.7±0.8
11.4±0.65
4.6±0.41
–2.1±0.2
14.22±0.9
13.40±0.7
11.0±0.6
25.3±1.2
21.2±0.7
18.1±0.95
16.55±0.6
15.00±0.7
1.41±0.46
19.2±0.45
5.40±0.4
9.50 ±0.5
7.90±0.5
7.23±0.6
It
1.43 [10]
(–0.40)
(–0.26)
(–0.16)
(–0.11)
–0.139
Ir
1.25 [10]
–
Ip
1.10 [10]
–
Iv
1.05 [10]
–
Ic
1.08
9.1
7.8
6.0
–
Ik
–
а
а
Iz
185±6
–0.97
0.709
1.151
Iu
1495±29
1215±25
1090±24
5522±136
3085±73
2326±72
2069±32
1600±31
267±9
–
–
–
Is
–
–
–
–
Iq
–
–
–
–
IIe
IIb
IIa
IIj
IIi
IIh
Id
–
1.44
0.86
0.232
0.47
–1.39
1.02
–0.967
–
–0.268
–0.603
10.4
9.8
9.2
–
–0.17
–0.24
0
0
–
–
0.227
0.206
–
0.778
1.19
–
–
1237±32
189±5
–
–
-
9.0
5.8
–
13.3
10.4
13.1
12.6
12.3
9.6
Iy
–
IId
IIx
IIw
IIy
IIu
IIs
IIq
IIIa
IVa
396±6
–
344±7
–
–
–
–
–
–
–
0
0
–
–
292±4
–
–
–
67.2±1.6
429±6
–
–
–
11.80±0.11 –4.21±0.21
–
–
–
15.4
14.4
14.0
15.0
14.7
12.03±0.15
12.36±0.39
12.15±0.34
17.30±0.17
14.58±0.14
10.0±0.6
7.26±0.63
5.90±0.41
8.9±0.6
340±8
–
–
–
251±6
–
–
–
3517±150
114±3
5.29
4.81 [11]
–
9.8
8.5
–
-9.81±0.6
–11.73
a
Σσ and ΣσPyO, respectively.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 1 2014

NUCLEOPHILICITY OF HETEROAROMATIC N-OXIDES
117
Scheme 1.
X
X
X
X
Y
Y
N
N
O
N
O
N
_
_
_
_
Ia Ie, Ig Ii, Ik, IIa, IIb, IId, IIe, IIh IIj ,I IIa, IIIb, IIIf, IIIk IIIo IVa
_
_
Ip Iw, Iy, Iz
IIq, IIs, IIu, IIw IIy
Y = H, X = H (a), 4-Me (b), 3-Me (c), 2-Me (d), 4-MeO (e), 4-NMe
2
(f), 4-N
CH=CH) (p), 2-(С CH=CH) (q), 4-(4-MeOС
CH=CH) (t), 2-(4-NMe CH=CH) (u), 4-(4-ClС CH=CH) (v); Y = 2-Me,
(y); Y = 3-Me, X = 4-NO (z).
3
(g), 4-NO
2
(h), 4-Cl (i), 4-Br (j), 3-COOEt
(
(
k), 4-CONHNH
2
(m), 3-CONH
2
(n), 4-CN (o), 4-(С
6
H
5
6
H
5
6 4
H
CH=CH) (r), 2-
4-MeOС CH=CH) (s), 4-(4-NMe
6
H
4
2
6
С H
4
С
2 6
H
4
6 5
H
X = 4-Cl (w), 4-Br (x), 4-NO
2
2
Similarly to the case of pyridines [1, 2], the steric
factor being of minor significance, the logarithms of
stability constant of zinc(II) tetraphenylporphyrinate
complex with pyridine-N-oxides and quinoline N-
oxides (IIIa > IIa > Ia > aniline > IVa) in chloroform
were linearly correlated with pK of the respective
a
ligand in water, with the Hammett constant σ
of the respective substituent, and with the bands shift
(Δλ) of the electron absorption spectra (Table 1 and
Fig. 1).
(
а)
(b)
log K
log K
σ
pK
a
(
c)
log K
ΔλII
Fig. 1. Stability of the complexes of zinc(II) tetraphenylporphyrinate with 3- and 4-substituted pyridines [1] (1, triangles), pyridine-
N-oxides (2, diamonds), quinoline-N-oxides (3, squares), and anilines [13, 14] (4, crosses) in chloroform at 298 K as functions of
(
a
a) pK of the ligands in water, (b) the Hammett constant, and (c) absorption band II shift upon complexation.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 1 2014

1
18
ANDREEV et al.
The log K – pK and log K – σ data were described
the mechanism of their nucleophilic substitution
a
with different linear equations within the sets of
reactions S could be assumed. At close basicity, the
N
pyridines, pyridine-N-oxides, and quinoline-N-oxides
somewhat higher nucleophilicity of quinoline-N-oxides
(
lines 1–3 in Figs. 1a and 1b). However, the log K – Δλ
(log K = 0.387pK + 2.68, n = 4, r = 0.997) as
a
dependences coincided in the cases of pyridines and
compared to that of pyridine-N-oxides (log K =
pyridine-N-oxides (sets 1 and 2 in Fig. 1c); for
0.339pK + 2.68, n = 10, r = 0.991) and of pyridines
a
example, in the case of band II, log K = 0.272Δλ –
(log K = 0.189 pK + 2.57, n = 7, r = 0.991; in the case
II
a
0
.623, n = 18, r = 0.992). In the cases of all the
of quinoline, the data point was much lower than the
fitting lines 1–3 in Fig. 1a) was seemingly due to
special features of the above-mentioned heterocycles
dynamic polarization upon interaction with acceptors
of various Pearson hardness [15] in the protic and
datasets, except for log K – pK data for pyridines, the
lines slopes were close in the cases of ligands of
different classes.
a
The higher activity in the nucleophilic substitution
reactions (“supernucleophilicity”) of pyridine-N-
oxides as compared to pyridines of close basicity is
usually explained by better steric availability of the
N→O group oxygen atom [3, 4] as compared to that of
+
aprotic solvents (H in water and zinc(II) tetraphenyl-
porphyrinate in chloroform).
The slopes of the log K – σ fitting lines (Fig. 1b)
were not significantly different between the sets of
pyridines (log K = –1.01σ + 3.56, n = 7, r = 0.994),
pyridine-N-oxides (log K = –1.09σ + 2.99, n = 7, r =
nitrogen atom in pyridines. However, the basicity of
pyridines in water was much more sensitive (pKa
Py
=
–
5.33σ + 5.23) to the nature of heteroaromatic ring
0
.98), quinoline-N-oxides (log K = –1.15σ + 3.14, n =
6, r = 0.98), and anilines (log K = –0.85σ + 2.32, n =
4, r = 0.97). As σ values depended exclusively on the
PyO
substituents than that of pyridine-N-oxides (pKa
=
–
3.35σ + 0.93); hence, the above-mentioned trend might
1
be due to difference in hydration of the bases and their
protonated forms [1]. Therefore, in order to correctly
compare the complex formation ability of pyridines
and the corresponding N-oxides, the zinc(II) tetra-
phenylporphyrinate complexes stability should have
been measured in the same medium (chloroform was
chosen in this work due to availability of experimental
nature of aromatic rings substituents, it was concluded
that their coordination to zinc(II) tetraphenylporphy-
rinate in chloroform was not very sensitive to the
(
O,N) donor center type and to the aromatic system of
the ligand.
Analysis of correlation of the complexes stability
and their spectral properties (log K – Δλ data, the both
parameters were determined in chloroform medium)
revealed that behavior of 3- and 4-substituted pyridines
and pyridine-N-oxides could be fitted with the same
data). Unfortunately, the found pK values of the
a
studied compounds in chloroform were scarce;
therefore, we used the available data on basicity in
acetonitrile, another aprotic solvent. Analysis of the
equation (log K = 0.272Δλ – 0.623, n = 19, r =
data in the log K – pK (MeCN) coordinates revealed
II
a
0
.992). Evidently, in the cases of those ligands, the
that [in contrast to the log K – pK (H O) plots] the
a
2
steric effects on the complex formation and solvation
in the low-polar solvent were similar, and the
electronic spectra changes resulting from zinc(II) tetra-
phenylporphyrinate coordination were affected
exclusively by the electron-donating properties of
heteroatoms (oxygen or nitrogen) depending in turn on
their polarizability and electronegativity as well as on
the electronic effects of the substituents. In the case of
quinolines and quinoline-N-oxides, the surrounding of
nitrogen and oxygen atoms were much different, and
the log K – Δλ data were therefore fitted with other
equations (Fig. 1c).
pyridine complexes with zinc(II) tetraphenylporphy-
rinate were more stable than those of pyridine-N-
oxides of the same basicity, the exceptions being only
the ligands containing very strong electron-donating
substituents (for instance, 4-dimethylaminopyridine,
σ –0.84). To conclude, the term “supernucleophilicity”
should not be used to discuss the complex formation of
pyridine-N-oxides with zinc(II) tetraphenylporphy-
rinate. Moreover, the substituents in the aromatic rings
being the same, the stability constants in the case of
pyridine derivatives were always higher than those in
the cases of pyridine-N-oxides.
The fitting lines 2–4 in Figs. 1a–1c were almost
parallel. That fact could point at the similar
mechanisms of coordination and protonation of hetero-
aromatic N-oxides and anilines, therefore, similarity in
Conclusions on evaluation of the steric effects on
the zinc(II) tetraphenylporphyrinate complex forma-
tion with heteroaromatic N-oxides were as follows.
Contrary to the pyridines case [16], location of methyl
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 1 2014

NUCLEOPHILICITY OF HETEROAROMATIC N-OXIDES
119
substituent in pyridine-N-oxide slightly influenced the
complexes stability constant (pyridine-N-oxide, K =
log K
8
4
16 L/mol; 2-methylpyridine-N-oxide, K = 1237 L/mol;
-methylpyridine-N-oxide, K = 1353 L/mol, and 3-
methylpyridine-N-oxide, K = 1030 L/mol). Hence, the
hyperconjugation effect of Me group in the positions 2
and 4 (in the case of 3-substituted ligand that effect
was absent) affected the nucleophilicity of ligands
more than its inductive and steric effects. Besides, in
the cases of all the studied 2-, 3-, and 4-substituted
pyridine-N-oxides (including styryl derivatives and 3-
methyl-4-nitropyridine-N-oxide), the general equations
(
log K = 0.268Δλ – 0.556, n = 18 r = 0.99 and log K =
ΔλII
Fig. 2. Stability of the complexes of zinc(II) tetraphenyl-
porphyrinate with 4-substituted quinoline-N-oxides (1), 2-,
-, and 4-substituted pyridine-N-oxides (2), 4-substituted 2-
methylquinoline-N-oxides (3), and 2-styryl-substituted
quinoline-N-oxides (4) in chloroform at 298 K as function
of the absorption band II shift upon complex formation.
The symbols shape corresponds to that in Fig. 1.
II
0
.358pK + 2.66, n = 13, r = 0.994) were held
a
satisfactorily. Position of methyl group in the sub-
stituted quinoline-N-oxides affected more the studied
coordination process. In particular, the complex
stability constant was of K = 2326 L/mol in the case of
parent quinoline-N-oxide; that in the case of 4-
methylquinoline-N-oxide (K = 3085 L/mol) was higher
and that in the case of 2-methylquinoline-N-oxide (K =
3
Furthermore, we studied the structure of zinc(II)
tetraphenylporphyrinate complex with 4-chloroquino-
line-N-oxide by X-ray diffraction (Fig. 3). Data in
Table 2 confirm that it was the axial 1 : 1 molecular
complex.
3
96 L/mol) was much lower. Evidently, in the latter
case, steric hindrance was extremely significant due to
combined spatial influence of 2-Me and 8-H. Behavior
of quinoline-N-oxides bearing 2-styryl substituents
was somewhat special. Their coordination with zinc(II)
tetraphenylporphyrinate was characterized by signify-
cantly lower stability constants, but it caused much
stronger spectral changes (Δλ of chloroform solutions)
than that of 2- and 4-styrylpyridine-N-oxides. Hence,
on the contrary to the pyridine-N-oxides case (line 2 in
Fig. 2), the data corresponding to quinoline-N-oxides
was fitted by three different lines 1, 3, and 4 (Fig. 2).
That was probably due to special features of steric
interaction of hydrogen atom, methyl and styryl groups
in the position 2 of quinoline ring with the zinc(II)
tetraphenylporphyrinate macrocycle in the course of
the complex formation.
2
The dsp Zn atom was located in the porphyrin
cycle plane the molecule of zinc(II) tetraphenylpor-
phyrinate. In the course of 1 : 1 axial complex
3
2
formation, the Zn hybridization was changed to sp d
9]. The four hybrid orbitals were located in the
[
porphyrin cycle plane, and the other two orbitals (the
free and the ligand-binding) were normal to it.
In the complex of zinc(II) tetraphenylporphyrinate
with 4-chloroquinoline-N-oxide, the coordination
number of zinc equaled five, with tetragonal pyramid
as coordination polyhedron (Fig. 3a). Zinc atom was
off the base plane (determined by the porphyrin
nitrogen atoms) by 0.28 Å and was located at 2.13 Å
from oxygen atom. That was typical of such zinc coor-
dination; for example, in the zinc(II) tetraphenylpor-
phyrinate complex with 3-nitroaniline (Cambridge
Structural Database reference code HAMLAI [17]) the
Probably, the measured stability constants of
pyridine-N-oxides and quinoline-N-oxides complexes
with zinc(II) tetraphenylporphyrinate corresponded to
the n,ν type 1 : 1 molecular complexes with the dative
O–Zn bond. In particular, the linear correlation
off-plane distance was of 0.25 Å. The L ligand (4-
2
between pK in water and log K in chloroform (Table 1,
chloroquinoline-N-oxide) was disordered in such a
a
Fig. 1a) along with the X-ray diffraction data for the
isoquinoline-N-oxide complex with zinc(II) tetra-
phenylporphyrinate [9] confirmed that protonation and
coordination of ligands I and II with metal
porphyrinates proceeded via the same center, the
N→O group oxygen atom.
way (Fig. 3b) that L atoms of the both positions were
located practically in the same plane (average
deviation from the mean plane passing through all the
2
non-hydrogen atoms of L was of 0.03 Å, the largest
2
deviation being of 0.12 Å). The angle between the
planes of porphyrin and the ligands was of 26.58°.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 1 2014

1
20
ANDREEV et al.
(
a)
(
b)
Fig. 3. (a) Structure of the 1 : 1 molecular complex of zinc(II) tetraphenylporphyrinate with 4-chloroquinoline-N-oxide I (one of the
positions of disordered ligand L is shown) and (b) scheme the disordering.
2
We failed to find any compounds containing the L2
ligand in the Cambridge Structural Database [17]
version 5.32, August 2011). The geometry parameters were
close to those of [Zn(2-methylquinoline-1-oxide) Cl ];
in particular, the N→O bond length coincided with the
data reported on the of n,ν type molecular complexes
formed by heteroaromatic N-oxides [18].
styryl fragment (evidently being weaker electron
acceptor than the N→O group and being conjugated
with the latter) became the electron-donating group as
well, due to its +M-effect (σPyO –0.11). However, its
effect was weaker than that of unsubstituted styryl
group (σPyO –0.16).
(
2
2
As the structure of zinc(II) tetraphenylporphyrinate
Geometry of zinc(II) tetraphenylporphyrinate com-
complexes with heteroaromatic N-oxides was similar
plex with 4-chloroquinoline-N-oxide most likely
corresponded to the sp hybridization state of oxygen
because the angle between the plane of heterocycle and
the plane of the NOZn fragment ruled out the
conjugation of oxygen atom with heterocyclic ring as
well as with the metal ion.
to that of the S reactions transition states [5] (the N-
N
3
oxides acting as nucleophile), we evaluated the
correlation between the parameters characterizing
those processes. Examples of linear correlations
between rate constants of five nucleophilic substitution
reactions log k in acetonitrile and dichloromethane
with log K and Δλ of the corresponding zinc(II)
tetraphenyl-porphyrinate complexes with 3- and 4-
substituted pyridine-N-oxides are collected in Table 3.
Using the above-derived linear equations of log K =
PyO
f(σ ), we calculated the σ constants of styryl
substituents (Table 1, in parentheses). From the result
it followed that upon coordination of zinc(II) tetra-
phenylporphyrinate with the Ip, Ir, It, and Iv ligands,
Hence, the rate constant of nucleophilic substitution
reactions in different organic solvents could be
calculated from the K and Δλ values using the
nucleophilicity/basicity scale offered in [16]; reverse
calculation was possible as well. Importantly, similar
trends were revealed in the cases of 2-, 3-, and 4-
methylpyridine-N-oxides coordination with zinc(II)
the electronic effects of NMe and OMe groups in p-
2
position of benzene rings were weakened by the –
CH=CHPh conjugated system. Hence, the σ values of
p
p-Me NC H CH=CH and p-MeOC H CH=CH were of
2
6
4
6
4
–
0.21 and –0.12, respectively. The chlorine atom of the
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 1 2014

NUCLEOPHILICITY OF HETEROAROMATIC N-OXIDES
121
Table 2. Selected geometry parameters of complex I (bond lengths,
Å and bond angles, deg)
tetraphenylporphyrinate as well as in the cases of their
SN reactions [24]. Therefore, we concluded that the
steric effects of methyl group were insignificant.
However, that was not the case of styryl derivatives
a
Bond
d, Å
n = 1
n = 2
1
3
4
1
2
Zn –N
2.049(4)
[23]. Stability of zinc(II) tetraphenylporphyrinate
1
Zn –N
2.057(4)
2.066(3)
2.075(4)
complexes with compounds It and Iu was almost
equal, but the corresponding equilibrium constants of
acyl group transfer [Table 3, reaction (5)] differed by
almost an order of magnitude. Evidently, that was due
to more strict steric demands for formation of
pentacoordinated SN reactions transition state than
those in the case of 1 : 1 n,ν complex with planar
molecule of metal porphyrin.
1
Zn –N
1
Zn –N
1
1n
Zn –O
2.126(11)
1.332(9)
2.130(7)
1.356(10)
1.741(10)
1.324(11)
1.400(11)
1.419(14)
1.355(15)
1.388(16)
1.363(15)
1.419(14)
1.483(17)
1.371(17)
1.401(14)
1
5n
O –N
1
n
47n
Cl –C
1.737(8)
5
n
45n
53n
46n
N –C
1.336(10)
1.381(10)
1.412(11)
1.352(13)
1.407(13)
1.373(11)
1.418(11)
1.456(13)
1.397(13)
1.384(10)
1.385(12)
It was natural to expect the linear correlation
between thermodynamic parameters of the studied
nucleophilic reactions and complex formation. We
calculated thermodynamic parameters of their forma-
tion from the measured values of stability constants of
zinc(II) tetraphenylporphyrinate complexes with
heteroaromatic N-oxides at different temperatures
5n
N –C
4
4
4
4
5n
6n
7n
8n
C
C
C
C
C
C
C
C
C
–C
47n
48n
49n
53n
50n
51n
52n
53n
–C
–C
–C
–C
–C
–C
–C
–C
(
273–313 K) (Table 1).
0
48n
49n
ΔH values in all the cases were negative. In the
cases of 3- and 4-substituted pyridine-N-oxides, 2-
styryl substituted pyridine-N-oxides, and 4-substituted
5
0n
quinoline-N-oxides, the values were close (–13.50 to
0
–
14.04 kJ/mol, ΔH
–13.78±0.15 kJ/mol, n = 22,
51n
average
series A). In the cases of N-oxides of 2-
5
2n
1.383(14)
methylpyridine, 2-methylquinoline, and 2-styryl-
0
a
12
1
11
5n 1n
1
quinoline, ΔH values were close as well, however, the
Bond angles, ω: O Zn O 10.2(4), n = 1; N O Zn 113.5(11),
n = 1; 116.7(11), n = 2.
average value was different (–11.80 to –12.36 kJ/mol,
0
ΔH
= –12.04±0.18 kJ/mol, n = 9, series B).
average
Hence, within the both series of heteroaromatic N-
tetraphenylporphyrinate coordination with pyridines
oxides (similarly to 3- and 4-substituted anilines with
0
[
1, 2], ΔS was linear with the ligand basicity (and with
0
ΔH
of –14.6±0.14 kJ/mol [1]) the coordination
0
average
ΔH ) as well, but the values were more variable (–27
to +54 J mol K ).
with zinc(II) tetraphenylporphyrinate was isoenthalpic.
–1
–1
0
Noteworthily, in the case of pyridines, ΔH values
0
were variable and linear with the ligand basicity [1].
The ΔS decrease with the decreasing of electron-
donating ability of the heteroaromatic N-oxide ligands
0
0
ΔS as well as ΔG values of formation of zinc(II)
(
similarly to the case of primary anilines [14]) could be
tetraphenylporphyrinate complexes with N-oxides of
- and 4-substituted pyridines (ΔS = 6.67pK +4.96,
n = 11, r = 0.998) and quinolines (ΔS = 7.12pK +
1.47, n = 4, r = 0.999) in chloroform (Fig. 4) were
proportional to the ligands basicity and Hammett
constants σ. Thus, despite enthalpy control of coor-
dination process (ΔS gave the major contribution to
ΔG ), the selectivity of complexes formation
at least partially due to the gradual increase of p-
character of the lone-electron pair of nitrogen atom
with shifting the electron density towards benzene ring
0
3
a
0
a
1
3
2
(
and, therefore, the change of sp hybridization into sp
one).
0
As expected, the stability constants of zinc(II)
tetraphenylporphyrinate complexes increased with
decreasing temperature. The plots of ΔG as function of
T (Fig. 5) revealed that at approximately 40 K the
complexes stability should have become independent
0
(
dependence of K value on the N-oxides structure
similarly to the case of anilines [13]) was determined
by the entropy effects. In the case of zinc(II)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 1 2014

1
22
ANDREEV et al.
Table 3. Nucleophilic substitution reactions with lgk values linearly (r = 0.97–0.999) correlated with lgK and Δλ of complex
formation; see detailed description in the text
Reaction
Reaction
Conditions
no.
1
1
2
1
2
R C
5
H
4
N→O + R X → R C
5
H
4
NOR X
Acetonitrile,
25°С [19]
1
2
R = H, 4-Me, 4-MeO; R = Me, Et, PhCH
2
; X = I, Br
1
2
1
2
2
3
R C
5
H
4
N→O + R X → R C
5
H
4
2
NOR X
Acetonitrile,
25°С [20]
1
R = H, 4-Cl, 4-Me, 4-MeO; R = Me, Et, i-Pr; X = I, Br
1
2
1
2
R C
5
H
4
N–OAc + R C
5
H
4
N→O → R C
5
H
4
N→O + R C
5
H
4
N–OAc
Acetonitrile,
дихлорметан,
1
2
R , R = H, 4-Cl, 4-Me, 4-MeO
2
5°С [21, 22]
1
+
–
4
2
2
+
–
4
1
4
5
Me
2
NCO(R C
5
H
4
N→O) BPh
+ R C
5
H
4
N→O ⇄Me
O, 4-(4-Me
2
NCO(R C
5
H
4
N→O) BPh
+ R C
5
H
4
N→O
Acetonitrile,
1
2
R , R = H, 4-Me, 4-MeO, 3-Me, 4-NC
4
H
8
2
NC
6
H
4
CH=CH)
25°С [10]
+
–
AcCl + RC
BzCl + RC
R = H, 4-Cl, 4-Me, 4-MeO, 4-(4-NMe
-(4-Me NC CH=CH), 4-C CH=CH), 2-Me, 4-NC
5
H
4
N→O ⇄ RC
N→O ⇄ RC
5
H
4
N –OAc + Cl
Acetonitrile,
Dichloromethane,
15–35°С
+
–
5
H
4
5 4
H N OBz + Cl
2
6 4 2
C H CH=CH), 4-Me N,
2
2
6
H
4
6
H
4
4
H
8
O
[23–25]
of the ligand structure (ΔG being of –13.78 and
12.04 kJ/mol in the cases of ligands series A and B,
respectively), provided that the trends revealed in the
case of solutions were still operative. Similar
conclusions followed from the analysis of corres-
ponding data in the cases of aniline [14] and pyridine
value (Σσ of 0.7–0.8 in the case of series A) ΔG of the
process should be temperature-independent; indeed,
that was observed in the case of 4-nitroquinoline-N-
oxide (σ = 0.778) (Fig. 5).
–
According to the X-ray diffraction data collected on
crystals [26], zinc(II) tetraphenylporphyrinate com-
plexes with heteroaromatic N-oxides and anilines were
of molecular 1 : 1 n,ν-type with the angle between
metal porphyrin plane and the ligand aromatic ring of
[
1] ligands; in the latter case the critical temperature
was of at 196 K.
The slopes of ΔG plots as function of T with
various Hammett constants suggested that at certain Σσ
2
4°–34°. In the similar complexes, pyridines were
oriented almost perpendicularly (with the angle of
0°–89°) to the porphyrin plane. Seemingly, those
8
0
–1
–1
ΔS , J mol K
structural differences caused the different in thermo-
dynamics and kinetics of pyridines coordination to
zinc(II) tetraphenylporphyrinate as compared with that
of anilines and heteroaromatic N-oxides.
To conclude, heteroaromatic N-oxides behavior in
the SN reactions as well as in the coordination
processes (in particular, the linear correlations between
log K, log k, pK , and σ constants) was quite similar to
a
that of the non-oxidized analogs. However, thermo-
dynamics of Zn(II) tetraphenylporphyrinate complexes
formation revealed some differences between the
ligands. In the cases of pyridines, the process was
isoequilibrium, whereas in the cases of pyridine-N-
oxides and quinoline-N-oxides it was isoenthalpic
pK
a
0
Fig. 4. ΔS of formation of the complex between zinc(II)
tetraphenylporphyrinate and 3- or 4-substituted pyridine-N-
oxides (1) and quinoline-N-oxides (2) in chloroform as
(
similarly to the cases of anilines). The latter group of
ligands could be divided into two series according to
the ΔH values. Series A included 3-and 4-substituted
a
function of pK of the ligands in water at 25°C.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 1 2014

NUCLEOPHILICITY OF HETEROAROMATIC N-OXIDES
123
0
ligands and pyridine-N-oxides containing 2-styryl
substituents. Series consisted of 2-methyl-
substituted ligands and quinoline-N-oxides containing
ΔG , kJ/mol
B
2
-styryl substituents.
The special features of the second series was
seemingly due to different steric hindrances in the
cases of 2-styrylpyridine N-oxides (planar molecules)
3
and 2-methyl derivatives (with sp carbon atom) and 2-
styrylquinoline-N-oxides (with 8-H and 2-styryl
2
substituents preventing the location of two sp oxygen
orbitals in the heterocycle plane).
When planning the research, we expected that
analysis of thermodynamic parameters of nucleophilic
substitution reactions of heteroaromatic N-oxides
would explain the difference in thermodynamic
behavior of pyridine-N-oxide and heteroaromatic N-
oxides. However, the scarce data on pyridine-N-oxides
Т, K
Fig. 5. ΔG of formation of the complex between zinc(II) tetra-
phenylporphyrinate and pyridine-N-oxides or quinoline-N-
oxides in chloroform as function of temperature. (1) 4-
NPyO, (2) 4-NQO, and (3) 3-COOEtPyO.
[
20–25] (for quinoline-N-oxides, no data was found)
and the formed precipitate was centrifuged off, washed
with acetone (3×2 mL), and dried in air.
revealed that in all the cases the linear correlations
between ΔH and ΔS or between ΔG and ΔS were
#
#
#
#
fulfilled, meaning that the reactions were isokinetic.
X-ray diffraction data on compound
I
(
C H ClN OZn, M = 857.67) was collected with
We expect that the evaluation of limits of analogies
between complex formation and chemical reactions of
the same organic compounds will clarify the
underlying mechanisms.
53 34
5
Bruker APEX2 diffractometer [34] (Mo-Kα radiation,
graphite monochromator, φΦ and ω scanning, room
temperature). Red crystal of 0.15×0.09×0.04 mm.
Triclinic, P-1 space group, a = 10.3815(10) Å, b =
EXPERIMENTAL
1
3.0004(12) Å, c = 15.8764(15) Å, α = 104.196(2)°,
3
β = 95.096(2)°, γ = 101.090(2)°, Z = 2, V = 2017.6(3) Å ,
dcalc = 1.412 g/cm , μ = 0.724 mm , F(000) = 884,
2
1
Pyridine-N-oxides and quinoline N-oxides [9, 27,
3
–1
2
8] and styryl derivatives [29–31] were prepared as
θmax = 50°, –12 ≤ h ≤ 12, –15 ≤ k ≤ 15, –18 ≤ l ≤ 18;
6756 reflections were collected in total, 7111 of them
described in the above-cited works, the products
physical constants agreed with the reference data.
being independent (Rint = 0.0768), 3822 independent
reflections with I > 2σ(I) were registered. Absorption
was accounted for semiempirically according to the
equivalents (SADABS software [35]).
Electron absorption spectra were recorded SF 2000–
2 spectrophotometer. Stability constants of zinc(II)
tetraphenylporphyrinate complexes with pyridine-N-
oxides in chloroform (previously estimated from KFK-
0
The structure was solved by combination of direct
method and the differential Fourier syntheses (all
calculations were carried out in SHELXS-97 and
SHELXL-97 programs [36]). The L2 ligand was
disordered between two positions in the 3 : 2 ratio. The
population value was refined in the course of isotropic
refining of structure with fixed thermal parameters of
L2 atoms and was left constant in further refinement.
Hydrogen atoms of ligands were located from
geometry considerations. The structure was refined by
3
photocolorimeter data [32]) were evaluated with
higher accuracy and processed according to [1].
Thermodynamic constants of complex formation were
determined graphically using the equation below (as
the first approximation, in the narrow temperature
range of 273–313 K the ΔH and ΔS values were
considered constant [33]).
0
0
ln K
T
= –ΔH298/RT + ΔS298/R.
Synthesis of [ZnL L ], L : 5,10,15,20-tetra-
1
2
1
2
phenylporphyrin, L : 4-chloroquinoline-N-oxide (I).
means of full-matrix mean squares method (F ) in the
2
Acetone solutions of zinc(II) tetraphenylporphyrinate
anisotropic-isotropic (O, N, and C atoms of the
–
5
(
5×10 mol/L) and 4-chloroquinoline-N-oxide (5.5×
disordered L
2
ligand) approximation accounting for
–5
1
0
mol/L), saturated at boiling point, were mixed,
hydrogen atom in rider model with thermal parameters
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 1 2014

1
24
ANDREEV et al.
osnovy organicheskoi khimii (Theoretical Basis of
2
0% higher than the equivalent thermal parameters of
Organic Chenmistry), Leningrad: Khimiya, 1991.
16. Andreev, V.P., Nizhnik, Ya. P., and Lebedeva, N.Sh.,
Russ. J. Org. Chem., 2008, vol. 44, no. 6, p. 906.
17. Allen, F.H., Acta Cryst. (B), 2002, vol. 58, p. 380.
the adjacent carbon atoms. In the course of refinement,
the SAME card of SHELXL-97 program was used for
the ligand L . Refining results (548 parameters,
2
4
3 limitations on the bond lengths) were as follows:
R = 0.0566, wR = 0.1071 over the reflections with I >
18. Andreev, V.P., Vapirov, V.V., Nizhnik, Ya.P., Ale-
1
2
shina, L.A., and Semenova, T.A., Russ. J. Gen. Chem.,
2
σ(I); R = 0.1296, wR = 0.1351 over all reflections,
1 2
2
008, vol. 78, no. 5, p. 973.
GOF = 0.957, maximum and minimum peaks in the
3
1
2
2
2
2
9. Popov, A.F., Matveev, A.A., Koblik, I.V., Savelova, V.A.,
and Matvienko, V.N., Zh. Org. Khim., 1996, vol. 32,
no. 4, p. 609.
0. Matveev, A.A., Koblik, I.V., Popov, A.F., Savelova, V.A.,
and Matvienko, V.N., Russ. J. Org. Chem., 1998,
vol. 34, no. 2, p. 271.
1. Rybachenko, V.I., Chotyi, K.Yu., Kovalenko, V.V., and
Shreder, G., Russ. J. Gen. Chem., 2001, vol. 71, no. 5,
p. 6786.
2. Rybachenko, V.I., Chotyi, K.Yu., Kovalenko, V.V., and
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no. 10, p. 1695.
zero Fourier synthesis 0.404 and –0.410 e/Å . Struc-
ture data were deposited in a Cambridge banc of
structure data, CCDC 846681.
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