Chemistry of azopyrimidines: Synthesis, spectral characterization, electrochemistry and X-ray crystal structure of bis[2-(arylazo)pyrimidine] complexes of copper(I)

Article abstract of DOI:10.1016/S0277-5387(99)00085-6

2-(Arylazo)pyrimidines (aapm, 3) have been synthesized by condensing nitrosoaromatics with 2-aminopyrimidine. They yield cationic bis-chelated complexes with copper(I), Cu(aapm)₂⁺ and are isolated as perchlorate salts. The complexes are 1:1 electrolytes in MeOH and exhibit intense MLCT transitions in the visible region. The N=N stretch in copper(I) complexes shows a large shift to lower frequency (approx. 1315 cm^-1) from the free ligand value (approx. 1425 cm^-1) due to d(Cu)→π^*(aapm) back bonding. The complexes show highly resolved symmetrical ¹H NMR spectra. In MeOH the Cu(aapm)₂²⁺/Cu(aapm)₂⁺ couple appears at E_1/2 at approx. 0.7 V versus SCE at 298 K. The structure has been confirmed by X-ray crystallography. Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(99)00085-6

Polyhedron 18 (1999) 1909–1915
Chemistry of azopyrimidines
Synthesis, spectral characterization, electrochemistry and X-ray crystal
structure of bis[2-(arylazo)pyrimidine] complexes of copper(I)
Prasanta Kumar Santraa, Debasis Dasa, Tarun Kumar Misraa, Ramkrishna Roya,
b
Chittaranjan Sinhaa , , Shie-Ming Peng
*
aDepartment of Chemistry, The University of Burdwan, Burdwan 713 104, India
bDepartment of Chemistry, National Taiwan University, Taipei, Taiwan
Received 3 December 1998; accepted 2 March 1999
Abstract
2-(Arylazo)pyrimidines (aapm, 3) have been synthesized by condensing nitrosoaromatics with 2-aminopyrimidine. They yield cationic
bis-chelated complexes with copper(I), Cu(aapm)12 and are isolated as perchlorate salts. The complexes are 1:1 electrolytes in MeOH and
exhibit intense MLCT transitions in the visible region. The N=N stretch in copper(I) complexes shows a large shift to lower frequency
(approx. 1315 cm21) from the free ligand value (approx. 1425 cm21) due to d(Cu)→p (aapm) back bonding. The complexes show highly
*
1
resolved symmetrical H NMR spectra. In MeOH the Cu(aapm)221 /Cu(aapm)21 couple appears at E1 / 2 at approx. 0.7 V versus SCE at 298
K. The structure has been confirmed by X-ray crystallography.
1999 Elsevier Science Ltd. All rights reserved.
Keywords: Azopyrimidines; High potential copper(I) complexes; Distorted tetrahedral structure
1. Introduction
ring significantly modify the p-acidity and regulate the
physical and chemical properties of the compounds [27–
29]. Ligands consisting of one N-heterocyclic ring with a
pendant nitrogen donor from azo function, known as
(arylazo)heterocycles have been used to stabilize low
oxidation states [7–10,30–41] like Cu(I), Ru(II), Os(II),
Re(II),Co(II), Fe(II), etc.
Recent years have witnessed a great deal of interest in
the synthesis of the complexes of copper with a-diimine
type ligands because of interdependence of their coordina-
tion geometry and their redox and photochemical be-
haviour [1–5]. Copper can adopt variable valence states
and the ability is reflected in its redox properties [6–10].
The monovalent copper (d10) chemistry has drawn special
attention because of its instability, unusual structural
features, utility in solar energy and supramolecular de-
vices, catalytic activity in photo-redox reactions and the
biological relevance of high potential copper complexes
[6–22].
Proper combination of the steric crowding and p-acidity
in a well designed ligand are the most important pre-
requisites for high stability of copper(I) complexes [7–
10,18–21]. The use of heterocyclic N-donor ligands to
stabilize low valent metal-redox states is widely studied in
coordination chemistry [23–26]. The number of hetero
atoms, ring size and the substituents in the heterocyclic
There are two studies available in the literature on the
stabilization of copper(I) by azoheterocycles; the first
study appeared in 1983 from Datta and Chakravorty [7]
which looked at copper(I) complexes of 2-
(arylazo)pyridine (aap, 1); the second study has appeared
recently from our group [8–10], which looked at copper(I)
complexes of N(I)-alkyl-2-(arylazo)imidazoles (RaaiX, 2).
Anchoring of the azo function in a heterocycle backbone is
interesting because of the photochromatic, redox active,
pH responsive and photo-redox activity of this functional
group [42,43]. With this background we have initiated to
search for new azoheterocycles of pyrimidine (3). It is
chosen because of its higher p-acidity [27–29] than
conventional widely used pyridine bases and its biochemi-
cal importance [44–47]. In coordination chemistry the
meta-related nitrogen in pyrimidine has played an im-
portant role for connecting different metals, transmitting
*Corresponding author. Fax: 191-342-644-52.
E-mail address: bdnuulib@giasclol.usnl.net.in (C. Sinha)
0277-5387/99/$ – see front matter
PII: S0277-5387(99)00085-6
1999 Elsevier Science Ltd. All rights reserved.

1910
P.K. Santra et al. / Polyhedron 18 (1999) 1909–1915
anti-ferromagnetic interactions and for obtaining magnetic
systems of high nuclearity [48–51].
In this report we describe the synthesis, spectral, redox
studies and single crystal X-ray structure of copper(I)com-
plexes of 2-(arylazo)pyrimidine.
mass), 2-(m-tollylazo)pyrimidine (m-tapm) (35%, gummy
mass), 2-( p-tollylazo)pyrimidine ( p-tapm) (28%, gummy
mass) and 2-( p-chloroazo)pyrimidine ( p-Clpapm) (40%,
semi solid) were prepared similarly.
2.5. Preparation of complexes
2. Experimental
2.5.1. Bis[2-(phenylazo)pyrimidine]copper(I) perchlorate
(Cu(papm)2(ClO4 ) (4)
2.1. Materials
Nitrogen gas was bubbled through an orange red solu-
tion of the ligand papm (0.2 g, 1.09 mmol) in dry MeOH
and [Cu(MeCN)4](ClO4) (0.4 g, 1.22 mmol) was added at
room temperature. The solution color turned to violet and
was stirred for 2 h. The solution volume was then reduced
to half by nitrogen bubbling. The dark crystalline mass that
separated was filtered off and was recrystallized from
methanol. The crystals were dried in vacuo and the yield
was 0.18 g (68%).
2-Aminopyrimidine was obtained from Aldrich.
[Cu(MeCN)4](ClO4) was prepared as previously described
[7]. Propylene carbonate (AR) was purchased from Al-
drich. All other chemicals and solvents were used as
previously described [8–10].
2.2. Instrumentation
All other complexes were prepared similarly and the
yield ranged from 62–75%.
Microanalytical (C,H,N) data were obtained from a
Perkin Elmer 2400 CHNS/O elemental analyzer. Spectro-
scopic data were obtained with the use of the following
instruments: UV–Vis spectra, Shimadzu UV-160A; IR
spectra (KBr disk, 4000–200 cm21), JASCO 420 FT-IR;
1H NMR spectra, Brucker 300 MHz FT-NMR spectrome-
ters. Electrochemical measurements were carried out with
the use of a computer controlled PAR model 270 VER-
SASTAT, using a platinum disk working electrode. The
solution was iR compensated and the results were collected
at 298 K. The reported results are referenced to the
saturated calomel electrode (SCE) in acetonitrile.
2.6. X-ray crystal structure and analysis
Single crystals of [Cu(papm)2]ClO4 (4), suitable for
X-ray diffraction were grown by slow diffusion of hexane
into dichloromethane solution at 298 K (Table 1). The
crystal size was 0.1530.4030.50 mm3. Diffraction mea-
surements were carried out on a Nonius CAD-4 fully
automated four circle diffractometer with graphite-mono-
˚
chromated (Mo Ka) radiation (l50.7107 A) at 298 K.
2.3. Synthesis of ligands
Table 1
Crystallographic data for [Cu(papm)2](ClO4) (4)
2-(Arylazo)pyrimidines (aapm, 3) were synthesized by
condensing 2-aminopyrimidine with nitrosoaromatics. A
representative case is detailed in Section 2.4. Available
information [32–38] on 2-(arylazo)pyridine was served as
a guideline for setting experimental conditions.
Empirical formula
fw
C20H16N8ClO4Cu
531.39
Monoclinic
C 2/c
Crystal system
Space group
˚
a, A
20.541 (2)
19.012(7)
11.893(2)
106.58(1)
4451.3(19)
8
298
0.7107
1.586
11.473
0.884
308
6.0
5.3
˚
b, A
˚
c, A
2.4. 2-(Phenylazo)pyrimidine (papm)
b, 8
V, A
3
˚
To a nitrogen flushed solution of 2-aminopyrimidine
(2.0 g, 2.1 mmol) in dry benzene (30 ml) in the presence
of molecular sodium (1 g), nitrosobenzene (2.5 g, 2.3
mmol) was added in small portions for 2 h under refluxing
conditions. The reaction was further continued for 3 h and
cooled to room temperature. A dark red solution was then
filtered and the filtrate was water washed (20 ml33). It
was then evaporated to reduce the volume to 5 ml and
chromatographed on silica gel (60–120 mesh) column
(3031 cm). A light yellow band was eluted by petroleum-
spirit (60–80) and rejected. A red band was then eluted
with benzene and collected. Evaporation of solvent gave a
‘gummy’ mass of the ligand. The yield was 1.2 g (31%).
2-(o-Tollylazo)pyrimidine (o-tapm) (30%, gummy
Z
T, K
˚
l, A
rcalcd g cm23
m(Mo Ka), cm21
a
Transm coeff.
Params refined
Rf
b
%
%
c
Rw
GOFd
2.31
a Maximum value normalized to 1.
b Rf 5o(uF0u2uFcu/ouF0u.
c Rw 5o W(uF0u2uFcu)2 /o W/uF0u2; W215s2(uF0u1guF0u2; g5
0.000100.
d The goodness-of-fit is defined as [oW(uF0u2uFcu)2 /(n0 2nv], where no
and nv denote the numbers of data and variables, respectively.

P.K. Santra et al. / Polyhedron 18 (1999) 1909–1915
1911
The unit cell was determined and refined using setting
angles of 25 reflections with 2u angles in the range of
19.00–30.268. The unit cell dimensions are listed in Table
4. Data were collected by u22u scans in the angular range
of 2.0–50.08. Three check reflections were measured after
every hour during data collections to monitor crystal
stability and showed no significant intensity variation. Of
the 3910 unique reflections 2518 with I.2s(I) were used
for the structure solution. Data reductions and structure
refinement were performed using NBCVAX packages. The
structure was solved by the Patterson method. All non-
hydrogen atoms were located from subsequent difference
Fourier maps. Tables containing full listings of atom
coordinates, anisotropic thermal parameters and hydrogen
atom locations are available as Supplementary data.
3. Results and discussion
Fig. 1. Single crystal structure of [Cu(papm)2]1 showing the atom
numbering scheme. For clarity, all hydrogen atoms have been omitted.
3.1. Ligands and complexes
2-(Arylazo)pyrimidines (aapm, 3) are used as ligands.
These were synthesized by condensing nitrosoaromatics
with 2-aminopyrimidine. The ligands are new and act as
N,N9-chelating molecules. The donor centres are ab-
breviated as N(1)(pyrimidine),N. and N(azo),N9. The atom
numbering scheme is shown in the structure of aapm (3).
V
21 cm2 mol21 suggesting a 1:1 ratio type electrolytic
nature of the compounds.
3.2. Structural studies
X-ray quality crystals were obtained by slow diffusion
of hexane into a CH2Cl2 solution of complex (4). The
solid state structure of the [Cu(aapm)2]1 complex cation is
shown in Fig. 1. The bond lengths and angles within the
coordination sphere of the copper ion are listed in Table 2.
The asymmetric unit contains one complex cation,
[Cu(aapm)2]1. The molecular packing diagram along the
c-axis (Fig. 2) shows that ClO24 is shared by two unit
cells. Two chelate rings Cu, N(1), C(4), N(3), N(4) and
Cu, N(5), C(14), N(7), N(8) separately constitute two
Table 2
˚
Selected bond distances (A) and angles (8) and their estimated standard
deviations for [Cu(papm)2]ClO4 (4)
˚
Distances (A)
Angles (8)
Cu–N (1)
Cu–N (5)
Cu–N (4)
Cu–N (8)
N(3)–N(4)
N(7)–N(8)
–
–
–
–
–
–
–
–
2.010(4)
2.038(4)
2.000(4)
1.981(5)
1.257(7)
1.260(6)
–
–
–
–
–
–
–
–
N(1)–Cu–N(4)
N(5)–Cu–N(8)
N(4)–Cu–N(8)
N(1)–Cu–N(5)
N(1)–Cu–N(8)
N(4)–Cu–N(5)
Cu–N(4)–C(5)
Cu–N(1)–C(1)
Cu–N(4)–N(3)
Cu–N(8)–N(7)
Cu–N(5)–C(11)
Cu–N(5)–C(14)
Cu–N(8)–C(15)
Cu–N(1)–C(4)
79.06(19)
79.20(20)
131.93(20)
119.13(18)
127.65(18)
126.25(18)
125.5(4)
133.3(4)
118.7(4)
119.5(4)
132.6(4)
The ligands react smoothly with [Cu(MeCN)4ClO4] in
boiling MeOH in the ratio 1:2 to yield cationic
[Cu(aapm)2]1 (4–8). The complexes were isolated as their
perchlorate salt.
Microanalytical data and iodometric determination of
copper support the ligand to metal in a 2:1 ratio. The
composition has also been confirmed by a previously
published method [52] at 560 nm spectrophotometrically.
Magnetic susceptibility measurements showed that the
compounds are diamagnetic (d10) in nature. They are
soluble in polar organic solvents. The molar conductance
(LM) of the complexes lies between 90 and 110
109.1(4)
126.7(4)
111.0(4)

1912
P.K. Santra et al. / Polyhedron 18 (1999) 1909–1915
elongated and may be due to coordination of the azo–N
function. The Cu–N(azo) (N(azo): N(4), N(8); 2.000(4),
˚
1.981(5)A) is shorter than the Cu–N(pym) (N(pym): N(1),
˚
N(5); 2.010(4), 2.038(4) A) bond distances. The short-
ening may be due to greater p-back bonding,
*
dp(Cu)→p (azo) [32–38]. The average chelate [N(1)–
Cu–N(4), N(5)–Cu–N(8)] angle is 79.18 and is the origin
of the structural distortion from tetrahedral geometry. This
is also reflected by other angular parameters around Cu(I)
in the coordination sphere. N(1)–Cu–N(8) and N(4)–Cu–
N(5) are 127.65(18) and 126.65(18)8, respectively. These
are certainly closer to the tetrahedral value (109.88) and
expectedly deviated from the square planar angle (1808).
Therefore, the overall coordination geometry about Cu(I)
is corroborated with the distorted tetrahedral structure.
3.3. Spectral studies
Fig. 2. Packing in the unit cell viewed along the c-axis.
Arylazopyrimidines exhibit n(N=N) and n(C=N) at 1420–
1430 and 1600–1610 cm21, respectively. Pyrimidine ring
stretching modes appear at the usual position [47].
[Cu(aapm)2](ClO4), the n(N=N) appears at 1310–1325
cm21 and is red shifted by 60–70 cm21. This has been
˚
planes (mean deviation 0.009 and 0.037 A, respectively)
and the dihedral angle is 878. This suggests distorted
tetrahedral CuN4 coordination. Pendant phenyl rings are
individually planar and slightly deviated from the planarity
of the respective chelate rings (dihedral angle 3.08 for
C5 –C10 ring with first chelate ring and 6.88 for C15 –C20
ring with the second chelate ring). These two pendant
phenyl rings are inclined at an angle of 84.18. Each ligand
coordinates in a bidentate fashion with Cu–N(pym) (Cu–
N(1)/Cu–N(5)) bond lengths comparable to other CuI-
complexes of heterocyclic N-donor ligands [53–55]. The
N=N bond distance (N(3)–N(4), 1.257(7); N(7)–N(8),
*
attributed to the presence of d(Cu)→p (aapm) back
bonding [7–10]. All the complexes exhibit a structureless
band approx. 1090 cm21 corresponding to n(ClO sug-
)
gesting lack of significant perchlorate coordination4 in the
solid state [53–55].
The electronic spectra of the complexes were recorded
in MeOH solution in the range 900–220 nm. The spectral
data are given in Table 3. The absorptions below 400 nm
are due to intraligand charge transfer transitions (in
comparison to free ligand values) and are not considered
further. The visible range of the spectrum is dominated by
metal-to-ligand charge transfer (MLCT) transition which is
a characteristic feature of the copper(I) complexes when
˚
1.260(6) A) are crystallographically indistinguishable. The
N–N distance is not available in the free ligand, however,
data available in some free azo ligands suggest that it is
˚
nearly 1.25 A [32–38]. Thus, the N–N distance is slightly
Table 3
Microanalytical, UV–Vis and redox data
Compound
Elemental analysesa
UV–VIS spectral datab
Cyclic voltammetric datac,
C
H
N
Cu
lmax /nm
E0, V(DEp, mV)
(103´/M21 cm21
)
Cu(II)/Cu(I)
0.69(100)
Ligand reductions
[Cu(papm)2](ClO4) (4)
45.11
3.09
20.96
12.10
708(0.87), 560(3.35),
362(12.56)
20.35(100), 20.81(120),
21.21(130)
(45.20)
47.17
(47.23)
47.08
(47.23)
47.31
(47.23)
39.91
(3.01)
3.52
(3.58)
3.64
(3.58)
3.62
(3.58)
2.38
(21.09)
20.14
(20.04)
20.12
(20.04)
19.92
(20.04)
18.73
(11.96)
11.07
(11.36)
11.74
(11.36)
11.78
(11.36)
10.65
[Cu(o-tapm)2](ClO4) (5)
[Cu(m-tapm)2] (ClO4) (6)
[Cu( p-tapm)2] (ClO4) (7)
[Cu( p-Clpapm)2] (ClO4) (8)
800(2.49), 635(5.21),
572(14.18), 370(18.74)
790(2.22), 630(4.11),
567(15.79), 368(16.66)
713(0.47), 564(1.59),
377(6.91)
0.66(90)
0.63(100)
0.64(100)
0.76(90)
20.41(80), 20.88(110),
21.34(140)
20.38(75), 20.77(100),
21.40(150)
20.34(68), 20.72(120),
21.38(140)
20.24(70), 20.58(80),
21.21(140)
720(1.05), 556(3.40),
381(9.68)
(40.00)
(2.33)
(18.67)
(10.58)
a Calculated values are in parentheses.
b Solvent: MeOH.
c Solvent: propylene carbonate; supporting electrolyte TBAP (0.1 M), solute concentration |1023 M, Pt-bead working electrode, scan rate 0.05 V s21
,
SCE reference and Pt-wire auxiliary electrode.

P.K. Santra et al. / Polyhedron 18 (1999) 1909–1915
1913
bonded with a conjugated organic chromophore [6–21]. A
similar situation is observed [7–10] in Cu(aap)12 and
Cu(aai)12 complexes. This is likely for a tetrahedrally
distorted CuN14 coordination sphere. A single crystal X-ray
structural study supports this behavior (vide supra). The
spectral data in Table 3 reveal that [Cu(o-tapm)2](ClO4)
(5) and [Cu(m-tapm)2](ClO4) (6) exhibit three MLCT
transitions of high intensity while others show two observ-
able transitions of relatively low intensity. This may be
due to the steric effect provided by o-Me/m-Me in the aryl
ring leading to stronger tetrahedral distortion and improved
stabilization of copper(I).
undisturbed with a small upfield shifting of the 12-H
signal. It is noted that all the protons exhibit only one
signal (singlet or multiplet) for each proton. This suggests
that both the chelate rings in the complexes are magneti-
cally equivalent at least on the NMR time scale and
complexes contain the effective C2-axis which is also
supported by tetrahedral CuN4 geometry from the solid
state structural study.
3.4. Redox studies
The redox behavior of the complexes in propylene
carbonate solution was examined under a nitrogen environ-
ment cyclic voltammetrically at a platinum disk working
All the chelates display highly resolved 1H NMR
spectra in CDCl3. The spectral data are collected in Table
4. The proton numbering pattern is shown in structure 3.
Assignment of individual proton resonance are made by
spin–spin interaction, comparative integration, chemical
shift and changes therein on substitution.
electrode
using
tetrabutylammonium
perchlorate
(NBu4ClO4) as a supporting electrolyte and the potentials
are reported with reference to the SCE. The results were
collected and are given in Table 2. The complexes
[Cu(aapm)2](ClO4) undergo a quasi-reversible oxidation–
reduction reaction at approx. 0.63–0.76 V versus SCE (at
50 mV s21 scan rate). The response is attributed to the
copper(II)/copper(I) couple (Eq. (1)). The quasi-reversible
character is
The spectra of the ligand is clearly divided into two
portions; the downfield part is due to pyrimidine protons
[47] (4–6-H) and the upfield signals refer to azoaryl
protons (8–12-H). Aryl protons are affected by substitu-
tion; for 10-methyl substitution ( p-tapm) the signals are
shifted upfield owing to inductive electron release by the
methyl group and the reverse effect is seen for 10-Cl
substitution in p-Clpapm due to the electron withdrawing
effect of the –Cl group with respect to papm. The signal
movement is corroborated with the electronic effect of the
substituents. 8-Me in o-tapm appears at 2.7 ppm whereas
9-Me and 10-Me in m-tapm and p-tapm appear at 2.5 and
2.4 ppm, respectively. The characteristic signal in m-tapm
is the appearance of a singlet for 8-H.
[Cu(aapm)2]21 1 e2á[Cu(aapm)2]1
(1)
accounted from the DEP (Epa 2Epc) values under the
conditions of measurements [7–10]. The redox data is
correlated with the electron donating and withdrawing
effects of the substituents in the azopyrimidine backbone.
Ligand reductions are observed as negative to SCE; three
consecutive reduction waves are observed at the negative
side to SCE. These potentials may be due to reductions of
azoimine functions in the ligand frame [7] analogous to
azopyridine systems. The LUMO of the ligand can accom-
modate up to two electrons and the reduction may be
represented by Eq. (2)
In complexes the protons are perturbed irregularly. The
data in Table 3 reveal that the pyrimidine protons are
mostly affected compared to aryl protons. The 6-H signal
is downfield shifted by approx. 1 ppm while 4- and 5-H are
upfield shifted. Aryl protons (8–11-H) remain almost
Table 4
1H NMR spectraa of aapm and [Cu(aapm)2](ClO4)
Compound
d, ppm (J, Hz)
4-Hb
5-H
6-Hb
8-H
9-H
10-H
11-H
12-Hb
R
papm
o-tapm
m-tapm
p-tapm
p-Clpapm
(4)
(5)
(6)
(7)
(8)
8.24(7.0)
8.18(7.0)
8.20(8.0)
8.20(7.4)
8.30(7.8)
8.11(7.0)
7.98(8.0)
8.00(7.0)
7.95(7.4)
8.12(8.0)
8.10d
8.24(7.0)
8.18b(7.0)
8.20(8.0)
8.20(7.4)
8.30(7.8)
9.20(7.0)
9.15(8.0)
9.11(7.0)
9.04(8.0)
9.22(8.0)
7.83b(9.0)
–
7.45d
7.45d
7.45d
7.83(9.0)
7.75(8.0)
7.72(8.0)
7.80b(9.0)
7.88b(8.0)
7.63(8.0)
7.63(8.0)
7.60(8.0)
7.54(8.0)
7.70(7.4)
–
8.09d
7.20b(8.0)
–
7.50c(8.0)
7.50c(8.0)
7.32c(8.0)
7.31b(8.0)
7.54b(8.0)
7.50d
2.61
2.54
2.43
–
2.64
2.64
2.51
2.45
–
8.11d
7.58d
7.32b(8.0)
–
8.10d
7.80b(9.0)
7.88b(8.0)
7.63b(8.0)
–
7.31b(8.0)
7.54b(8.0)
7.50d
8.14d
–
7.85c(9.0)
7.72c(8.0)
7.72c(8.0)
7.74c(7.0)
7.85c(7.0)
7.50d
7.28b(7.0)
–
7.38c(8.0)
7.42c(8.0)
7.38c(8.0)
7.17b(8.0)
7.58b(8.0)
7.43e
7.21b(8.0)
7.54b(8.0)
7.70b(7.4)
7.17b(8.0)
7.58b(8.0)
–
–
a In CDCl3, temp. 295 K.
b Doublet.
c Triplet.
d Multiplet.
e Singlet.

1914
P.K. Santra et al. / Polyhedron 18 (1999) 1909–1915
2
2
–N=N–1áe [–NiN–]21áe [–N–N–]5
(2)
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*
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