To what extent can a conjugation between two pairs of peri-nitro and peri-amino groups be realized through the naphthalene core?

Article abstract of DOI:10.1002/poc.3114

Through-conjugation for a wide range of 1,8-diamino-4,5-dinitronaphthalenes (N-acylated, N-alkylated, N,N′-bridged, N-heterocyclic, and N-deprotonated compounds) was for the first time quantified in solution by means of ultraviolet-visible and proton nucl

Full text of DOI:10.1002/poc.3114

Research Article
Received: 17 December 2012,
Revised: 06 March 2013,
Accepted: 07 March 2013,
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/poc.3114
To what extent can a conjugation between two
pairs of peri-nitro and peri-amino groups be
realized through the naphthalene core?
Valery A. Ozeryanskiia*, Ekaterina A. Filatovaa, Alexander F. Pozharskiia,
Dmitrii A. Shevchuka and Vladimir I. Sorokina
Through-conjugation for a wide range of 1,8-diamino-4,5-dinitronaphthalenes (N-acylated, N-alkylated, N,N0-bridged,
N-heterocyclic, and N-deprotonated compounds) was for the first time quantified in solution by means of ultraviolet–
visible and proton nuclear magnetic resonance spectroscopy and compared with that of the simpler naphthalene and
benzene push-pull systems. Surprisingly, an extent of conjugation in 1,8-diamino-4-nitro- and 1,8-diamino-4,5-
dinitronaphthalenes measured in dimethyl sulfoxide is commensurable. On the whole, the repulsive peri-interactions
between the amino groups in systems with N-alkylated and N-deprotonated amino groups are more favorable for an
effective D-p-A charge transfer than in N,N0-bridged compounds (perimidines, 2,3-dihydroperimidines and perimidin-
2-ones). The best electron donors from peri-positions are pyrrolidin-1-yl and methylamido groups. The conclusions
obtained from solution studies were deepened by solid- state X-ray experiments for a number of push–pull naphthalenes,
including 6,7-dinitroperimidine N-anion and two representatives of 4,5-diaminonaphthalene-1,8-dicarbaldehydes. In
particular, they helped to trace changes in the bond order redistribution and twisting of the naphthalene core. The latter
reaches a record value of 27ꢀ for 4,5-dinitro-1,8-di(pyrrolidin-1-yl)naphthalene. Copyright © 2013 John Wiley & Sons, Ltd.
Keywords: 1,8-diamino-4,5-dinitronaphthalenes; 4,5-diaminonaphthalene-1,8-dicarbaldehydes; arylamide anions; peri-interaction;
push–pull donor–acceptor system
their 1,4,5,8-tetrasubstituted analogs 1.[17,18] Meanwhile, in such
derivatives with the doubled fragment of the benzene type, the
INTRODUCTION
Organic molecules of D-p-A type with p-donor and p-acceptor
substituents on the opposite ends of the conjugated chain are
extensively studied compounds because of their nonlinear
optical[1–3] and some other[2,4,5] interesting properties including
application as molecular sensors.[6] For example, interaction
between +M (D) and ꢁM (A) substituents, called a through-
conjugation, is known to exist in para-disubstituted benzenes.[7,8]
The true structure of such push–pull molecules is a linear
combination of the neutral aromatic and localized zwitterionic forms
(Scheme 1).[7,9] Depending on the donor–acceptor strength of
groups A and D, the contribution of the quinoid resonance structure
may be significant, but, owing to high benzene ring aromaticity,
it is always counterbalanced with the benzenoid structure so
that the intramolecular charge transfer is never complete.[9,10]
Because aromaticity of naphthalene is markedly lower than
that of benzene,[11] push–pull naphthalene systems are rather
attractive, and some of them were in a focus of recent investigations
devoted to intramolecular charge transfer processes,[12] designing
new fluorophores,[13,14] p-extended structures,[15] and colorimetric
indicators.[16] Many of the explored systems are 1,4-di-, 1,5-di-, or
1,4,5-trisubstituted naphthalenes, and much less is known about
efficiency of through-conjugation and the degree of electron trans-
fer via the aromatic p-system may be higher, in particular because
of a possibility of resonance interaction between D and A groups
at positions 1,5 and 4,8, which is impossible in the benzene series.
* Correspondence to: Valery A. Ozeryanskii, Department of Organic Chemistry,
Southern Federal University, Zorge 7, 344090 Rostov-on-Don, Russian Federation.
E-mail: vv_ozer2@sfedu.ru
a V. A. Ozeryanskii, E. A. Filatova, A. F. Pozharskii, D. A. Shevchuk, V. I. Sorokin
Department of Organic Chemistry, Southern Federal University, Zorge 7, 344090,
Rostov-on-Don, Russian Federation
Scheme 1. Direction of through-conjugation in para-D-p-A benzenes
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V. A. OZERYANSKII ET AL.
In the present work, we have synthesized and for the first
time studied the possible limits of the through-conjugation in a
number of 1,8-diamino-4,5-dinitronaphthalenes 2 and closely
related molecules 3 and 4. A set of solid reasons was put behind
this choice (A = NO2, D = NR2): (i) The difference between the
Hammett sp constants for these groups is high;[19] (ii) The
nitro-aromatics are an extremely important class of compounds
with varied applications, and nitro-amino push–pull systems are
frequently in the center of theoretical[9,20] and experimental[9,15]
investigations; and (iii) Easy and wider changes in the degree
of resonance interaction are achievable; thus, in addition to
common ways of the amino group modification, a possibility
for internitrogen bridging appears. Noteworthy is that effective
conjugation of the amino and nitro groups in systems such as
2 is usually accompanied by molecular distortion caused by
bulkiness of the adjacent peri-substituents and their electrostatic
repulsion (see bipolar structure 2a). Although the degree and
the mode of this distortion are often unpredictable,[18,21] most
of them are reduced to a strong twisting of the naphthalene
core, expressed via a torsion angle ABCD in structure 5 (up to
23–26ꢀ[18]). Consequently, we also resolved the X-ray structures
of nine new molecules of this and related types.
Table 1. Proton NMR and UV–Vis data for p-nitroanilines 6–10
Compound
1H NMR (DMSO-d6)
lmax (lge)
(nm)
(DMSO)
3
H-3
H-2 Δd a J2,3
8.21 7.82 0.39 8.95 332 (4.11)
7.95 6.60 1.35 9.16 389 (4.05)
8.01 6.60 1.41 9.47 400 (4.23)
8.05 6.76 1.29 9.48 408 (4.33)
7.41 5.97 1.44 9.95 484 (4.40)
3
The spin–spin coupling constants J2,3 (structure 2a, proton
NMR, nuclear magnetic resonance; UV–Vis, ultraviolet–visible;
DMSO, dimethyl sulfoxide.
nuclear magnetic resonance [1H NMR] spectroscopy) and the
position of the long-wave absorption band (lmax, ultraviolet–
visible [UV–Vis] spectroscopy) were chosen as the criteria of
p-conjugation and intramolecular charge transfer. These param-
eters are known to be sensitive to the double bond localization
[C(2)–C(3) aromatic bond], which will increase on going from
2 to 2a.[20,22] Finally, dimethyl sulfoxide (DMSO) was used
as the solvent in all measurements because of the solubility
demands and the ability of this solvent to solubilize ionic species
(N-anions) and stabilize zwitterionic forms of type 2a.
aΔd = d(H-3) ꢁ d(H-2).
analog).[23] Anion 10 was generated through the addition of
powdered KOH (3–5 molar excess) to the corresponding solutions;
spectral readings were performed in regular (0.5–2 h) intervals to ensure
the ionization is complete.
Model a-nitronaphthalenes and related compounds 11 (dark blue-violet
crystals with mp = 258–259 ꢀC from MeCN),[24] 12,[25] 13,[26] and 14[27]
were prepared in accord with the previously published procedures.
Nitronaphthalenes 15 and 24 were obtained by nitration (1.6 equiv HNO3
in concentrated [conc.] H2SO4, ꢁ30 ꢀC, 10 min) of 1,8-di(pyrrolidin-1-yl)
naphthalene[28] following the procedure described for the nitration of
1,8-bis(dimethylamino)naphthalene.[27] The compounds were separated
by chromatography on Al2O3 with CH2Cl2 elution, which gave first 15
(main purple fraction) followed by 24 (red fraction).
a
EXPERIMENTAL
General
The 1H NMR spectra were recorded on a Bruker (Rostov-on-Don, Russian
Federation) DPX-250 (250 MHz) spectrometer with the solvent as the internal
standard (d/ppm, nJ/Hz). UV–Vis spectra were obtained in DMSO on a Varian
Cary-50 Probe (Rostov-on-Don, Russian Federation) spectrometer. Thin layer
chromatography was carried out on Al2O3 with Brockmann activity III and on
silica gel (70–230 mesh, MERCK, Germany). The progress of reactions and
the purity of products were monitored by thin layer chromatography
on Al2O3 and Silufol plates using the development with iodine vapor.
The melting points were measured in sealed capillaries and are
uncorrected. The solvents were purified and dried by standard methods.
4-Nitro-1,8-di(pyrrolidin-1-yl)naphthalene (15)
Yield 19%, dark maroon leaflets with mp = 170–171ꢀC (EtOAc). Found:
C, 69.27; H, 6.88; N, 13.42%. Calcd for C18H21N3O2: C, 69.43; H, 6.80;
N, 13.49%. 1H NMR (CDCl3): 1.90 (m, 8H), 3.13 (m, 4H), 3.28 (m, 2H), 3.54
(m, 2H), 6.33 (d, 1H, H-2, 3J = 9.47Hz), 6.63 (dd, 1H, H-7, 3J = 7.90 Hz,
4J = 0.95Hz), 7.42 (dd, 1H, H-6, 3J = 7.90 Hz, 3J = 8.53 Hz), 8.41 (m, 2H,
H-3,5). Perchlorate 15ꢂHClO4: beige crystals with mp = 180–182 ꢀC
(decompose [decomp.], MeCN). 1H NMR (CD3CN): 2.18 (m, 4H), 2.35
(m, 4H), 3.42 (m, 4H), 3.82 (m, 4H), 7.95 (m, 2H, H-2,6), 8.04 (dd, 1H,
H-7, 3J = 7.74 Hz, 4J = 1.11 Hz), 8.32 (d, 1H, H-3, 3J = 8.48 Hz), 8.44
(dd, 1H, H-5, 3J = 8.85 Hz, 3J = 1.11 Hz), 19.22 (br s, 1H, NH).
Preparation of starting materials
The compounds used in this work are listed in Tables 1–3.
Common p-nitroanilines 6, 7, 9 were recrystallized from EtOH.
N-Methyl-p-nitroaniline
8
was synthesized from MeNH2ꢂHCl and
4,5-Dinitro-1,8-di(pyrrolidin-1-yl)naphthalene (24)
p-nitrochlorobenzene in refluxing pyridine, as described in the literature
(this method may be also applied for the synthesis of N,N-dimethyl
derivative 9; p-nitrochlorobenzene may be substituted by its bromo
Yield 3%, red crystals sparingly soluble in EtOAc, CHCl3, MeCN, EtOH,
acetone, toluene, or DMSO, good solubility in CH2Cl2. The compound turns
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CONJUGATION IN 1,8-DIAMINO-4,5-DINITRONAPHTHALENES
Table 3. Proton NMR and UV–Vis data for dinitronaphthalenes
16–27 and dialdehydes 28 and 29
Table 2. Proton NMR and UV–Vis data for a-picryl-11 and
a-nitronaphthalenes 12–15
Compound
1H NMR (DMSO-d6)
lmax (lge)
(nm) (DMSO)
Compound
1H NMR (DMSO-d6)
lmax (lge)
(nm)
(DMSO)
Δda
3J2,3
H-3
H-2
3J2,3
H-3
H-2
7.21
6.95 0.26
8.30
350 (4.13)
450 (3.75)
8.33
6.97
8.77
439 (4.00)
8.40
8.40
6.76 1.64
9.00
9.08
8.34
6.88
8.85
420 (4.09)
6.54 1.86
465 (4.15)
8.49
8.25
7.17
6.70
9.00
9.06
414 (4.18)
439 (4.23)
8.26
8.25
6.82 1.44
9.16
9.59
484 (4.19)
530 (4.28)
6.50 1.75
8.25
8.18
6.77
6.63
9.10
9.10
446 (3.25)
448 (4.06)
NMR, nuclear magnetic resonance; UV–Vis, ultraviolet–visible;
DMSO, dimethyl sulfoxide.
dark above 290 ꢀC, but no melting is observed up to 360 ꢀC. Found: C, 60.39;
H, 5.28; N, 15.33%. Calcd for C18H20N4O4: C, 60.66; H, 5.66; N, 15.72%. 1H
NMR (CDCl3): 1.98 (m, 4H), 2.24 (m, 4H), 2.71 (m, 2H), 3.16 (m, 2H), 3.37 (m,
2H), 3.55 (m, 2H), 6.44 (d, 2H, H-2,7, 3J = 9.12Hz), 8.16 (d, 2H, H-3,6,
1
3J = 9.12Hz). H NMR (DMSO-d6): 1.59 (m, 2H), 1.91 (m, 2H), 2.00 (m, 2H),
2.11 (m, 2H), 2.54 (m, 2H), 3.06 (m, 2H), 3.40 (m, 2H), 3.70 (m, 2H), 6.64
(d, 2H, H-2,7, 3J= 9.28Hz), 8.10 (d, 2H, H-3,6, 3J = 9.28 Hz).
Several dinitronaphthalenes were prepared in accord with the
published data: 16,[29] 18,[26] and 23.[30] Syntheses of other symmetrical
peri-disubstituted derivatives and their properties are as follows.
8.17
6.76
9.12
557 (3.76)
6,7-Dinitro-1H-perimidine-2-one (17) and 4,9-dinitro-1H-perimidine-2-
one (17-ortho)
(Continues)
To a suspension of 1H-perimidine-2-one (1.84 g, 10 mmol) in glacial AcOH
(75 mL), a solution of conc. HNO3 (d = 1.5, 1.05 mL, 25 mmol) in AcOH
(25 mL) was added dropwise at 5–10 ꢀC for 15 min with stirring. After
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V. A. OZERYANSKII ET AL.
Table 3. (Continued)
Table 4. Selected X-ray geometrical characteristics [C(2)–C
(3) and N(1)–C(1) bond lengths, torsion angles, ’, be-
tween acceptors and aromatic rings, molecular twisting,
∠ABCD, internitrogen distances N(1). . .N(2)] for studied
push–pull molecules and model compounds
Compound
1H NMR (DMSO-d6)
lmax (lge)
(nm) (DMSO)
H-3
H-2
3J2,3
Compound rC(2)– rN(1)– ’ ∠ABCD
rN(1). . .
CCDC
ref. code
8.11
6.85
9.19
476 (3.75)
(o) (o)
(Å)
C(3)
C(1)
N(2)
c
(Å)a (Å)b
7
8
9
11
1.382 1.355
1.371 1.358
1.375 1.358
2
2
3
—
—
—
—
—
—
NANILI02
FUXNAN
DIMNAN01
NUPJIR
1.386 1.386 65d 15
2.838
14
15
1.366 1.371 23 15 2.859
1.371 1.348 16 3.001
1.393 1.386 36
ZOSKAT
8.10
7.98
6.64
6.38
9.28
9.30
494 (4.15)
e
6
e
e
16ꢂDMF
6
4
2.357 (2.989)f
2.348
(2.983)
2.320
16ꢂMeCN 1.382 1.383 39
e
e
17-ortho 1.406 1.376
6
1
22
23
24
1.368 1.369 22 13 2.392
(3.117)
1.380 1.367 28 23 2.917
(2.976)
1.379 1.352 32 27 3.079
(3.020)
2.387
1.377 1.356 12 24 3.033
520 sh. (3.62)
SORYEE
e
e
e
28
29
35
36
1.374 1.364 13
4
1.407
1.405
—
—
43
39
6
6
(2.946)
(3.094)
DNTNAP01
e
7.93
7.78
6.36
6.14
9.32
9.77
568 (3.96)
DMF, dimethylformamide; CCDC, Cambridge Crystallographic
Data Centre.
aAverage values for two bonds in the case of molecules 2
and 3.
bAverage values for two bonds in the case of molecule 2.
cAverage values in the case of two acceptors.
dFor picryl group.
598 sh. (3.48)
eThis work.
fValues in parentheses are for nitro nitrogens.
additional stirring for 1 h at 20 ꢀC, the red-brown reaction mass was
gradually mixed with water (400 mL), and the residue was filtered off,
washed with water, and dried in air. The products were washed with
hot toluene (150 mL) to leave peri-dinitro isomer 17, which is sparingly
soluble in common organic solvents. Recrystallization of the compound
from AcOH gave 17 (43–60% in a set of runs) as orange-brown crystals
not melted up to 360 ꢀC (darken above 305 ꢀC). Found: C, 48.05;
H, 2.10; N, 20.41%. Calcd for C11H6N4O5: C, 48.19; H, 2.21; N, 20.43%.
The toluene extract was evaporated to dryness, and the residue was
chromatographed on silica gel with EtOAc elution. The first mobile fraction
gave ortho-dinitro isomer 17-ortho (7–9% in a set of runs) as brown needles
with mp above 260 ꢀC (decomp., EtOAc). Found: C, 48.12; H, 2.07; N, 20.29%.
7.99
6.79
8.59
424 (3.62)
7.78
6.65
8.76
470 (4.24)
1
Calcd for C11H6N4O5: C, 48.19; H, 2.21; N, 20.43%. H NMR (DMSO-d6): 7.57
(d, 2H, H-6,7, 3J=9.47Hz), 8.35 (d, 2H, H-5,8, 3J= 9.47 Hz), 10.98 (br s, 2H, NH).
1,8-Di(methylamino)-4,5-dinitronaphthalene (21)
NMR, nuclear magnetic resonance; UV–Vis, ultraviolet–visible;
This was prepared using the following modified procedure.[31] A yellow
suspension of compound 18 (0.40 g, 1.3 mmol), KOH (7.4 g, 0.13 mol,
100 equiv), and methanol (360 mL) was refluxed for 48 h until a clear dark
red solution is received. After that, the solvent was distilled off, the
DMSO, dimethyl sulfoxide.
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CONJUGATION IN 1,8-DIAMINO-4,5-DINITRONAPHTHALENES
1,2,2,3-Tetramethyl-6,7-dinitro-2,3-dihydro-1H-perimidine (20)
A mixture of compound 21 (0.05 g, 0.18 mmol), acetone (30 mL), and
trifluoroacetic acid (0.05 mL) was refluxed for 10 h until the starting
compound is consumed (monitoring with Al2O3–acetone). After that,
the brown-red solution was evaporated to dryness in vacuum, and the
residue was recrystallized from acetone. This gave compound 20
(0.03 g, 53%) as orange-brown crystals, darken above 280 ꢀC (decomp.
at 312 ꢀC). Found: C, 57.08; H, 5.16; N, 17.59%. Calcd for C15H16N4O4:
C, 56.96; H, 5.10; N, 17.71%. 1H NMR (CDCl3): 1.55 (s, 6H, C–Me), 3.10
(s, 6H, N–Me), 6.52 (d, 2H, H-4,9, 3J = 8.94Hz), 8.22 (d, 2H, H-5,8, 3J = 8.94Hz).
Dinitro compounds with the NH groups (16, 17, and 17-ortho, in some
extent 21) are easily dissolved in aqueous Na2CO3 solutions with changing
in color, which imply that these derivatives are relatively strong NH-acids.
Dialdehydes 28 and 29 were obtained by POCl3/dimethylformamide
(DMF) formylation of the corresponding peri-diamines.[32]
1,2,2,3-Tetramethyl-2,3-dihydro-1H-perimidine-6,7-dicarbaldehyde (28)
Temperature 0 to 20 ꢀC, yield 33%. Dark yellow-brow crystals with
mp = 190–191 ꢀС (toluene). Other data see ref.[32]
(a)
4,5-Di(pyrrolidin-1-yl)naphthalene-1,8-dicarbaldehyde (29)
Temperature ꢁ5 to 0 ꢀC, yield 12%. Dark yellow crystals with mp = 195–
196ꢀС (decomp., toluene). Found: C, 74.27; H, 6.73; N, 8.54%. Calcd for
(b)
Figure 1. (a) Single-crystal X-ray structure of nitronaphthalene 15 (30%
probability level, hydrogen atoms are omitted for clarity). (b) View along
the naphthalene central bond showing out-of-plane bending of the two
pyrrolidino groups
mixture was diluted with water (700 mL) and neutralized with conc. HCl
to pH 3, and the precipitate thus formed was filtered off, washed with
water, and then dried at room temperature. This gave dinitro derivative
21 (0.23 g, 64%) as small dark red crystals, melted at 263–264 ꢀC
(lit. mp = 235 ꢀC.[31]
)
(a)
(b)
1,3-Dimethyl-6,7-dinitro-2,3-dihydro-1H-perimidine (19)
A
mixture of compound 21 (0.05 g, 0.2 mmol), paraform (0.12 g,
4.0 mmol), methanol (25 mL), and several drops of conc. HCl (0.2 mL)
was refluxed for 48–72 h until the starting compound is consumed (mon-
itoring with Silufol–EtOAc). After that, the mixture was evaporated to
dryness, and the residue was washed with water (10 mL) and then dried
at room temperature. Chromatography (silica gel, acetone, first yellow
fraction) gave compound 19 (0.01 g, 19%) as small brown-orange
rhombs with mp = 263–265 ꢀC (acetone). Found: C, 54.29; H, 4.17;
N, 19.22%. Calcd for C13H12N4O4: C, 54.17; H, 4.20; N, 19.43%. 1H NMR
(CDCl3): 3.13 (s, 6H, N–Me), 4.47 (s, 2H, NCH2N), 6.49 (d, 2H, H-4,9,
3J = 8.93 Hz), 8.23 (d, 2H, H-5,8, 3J = 8.93 Hz).
Figure 2. (a) Single-crystal X-ray structure of dinitroperimidine
dimethylformamide (DMF) solvate 16ꢂDMF (30% probability level,
intermolecular H-bonding with DMF molecule is shown). (b) View along
the naphthalene central bond showing the twisting of the nitro groups
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V. A. OZERYANSKII ET AL.
C20H22N2O2: C, 74.51; H, 6.88; N, 8.69%. 1H NMR (CDCl3): 1.60 (m, 2H),
1.94 (m, 4H), 2.19 (m, 2H), 2.69 (m, 2H), 3.18 (m, 2H), 3.39 (m, 2H),
3.56 (m, 2H), 6.53 (d, 2H, H-3,6, 3J= 8.53Hz), 7.92 (d, 2H, H-2,7,
3J = 8.53 Hz), 9.78 (s, 2H, CHO).
Crystal data for 22 [16ꢂ[1,8-bis(dimethylamino)-2,7-dimethoxynaphthalene]
proton transfer complex]
(Obtained at ꢁ20 ꢀC as trihydrate by slow diffusion of Et2O into CH2Cl2
solution of the equimolar amounts of 16 and 1,8-bis(dimethylamino)-2,7-
dimethoxynaphthalene;[34] brownish purple needles with mp= 114–
116 ꢀC (decomp.)): C28H36N6O9 [C12H7N4O4ꢂC16H23N2O2ꢂ3H2O], M = 600.63,
space group P21/c (monoclinic), a = 10.5916(14), b = 19.828(3), c = 14.1938
(18) Å, b = 101.221(3)ꢀ, V= 2923.9(7) Å3, Z = 4, Dc = 1.364 g cmꢁ3, m(Mo-Ka) =
0.103 mmꢁ1, 100 K, 32 607 reflections collected, 7056 unique (Rint = 0.1662),
3305 reflections with I > 2s(I), 395 parameters, R1 = 0.0625, wR2 (all data) =
0.1070. CCDC reference number 911343.
3,4-Dinitro-7,10-diphenyl-8,9-diazafluoranthene (36)
A
mixture of 5,6-dinitroacenaphthylene[33] (242 mg, 1 mmol), 3,6-
diphenyltetrazine (240 mg, 1 mmol), and dry o-xylene (25 mL) was
refluxed for 48 h. The solvent was removed, and the rest was
chromatographed on Al2O3 with CHCl3 elution. Evaporation of the main
yellow fraction gave compound 36 (95 mg, 21%) as yellow-orange
crystals with mp = 251–253 ꢀC (decomp., benzene). Found: C, 70.03; H,
3.15; N, 12.34%. Calcd for C26H14N4O4: C, 69.95; H, 3.16; N, 12.55%. 1H
NMR (CDCl3): 7.68 (m, 6H, Ph), 7.84 (d, 2H, H-1,6, 3J = 7.72 Hz), 7.89 (m,
4H, Ph), 8.21 (d, 2H, H-2,5, 3J = 7.72 Hz). UV–Vis (CHCl3), lmax/nm (lge):
255 (4.58), 290 sh. (4.45), 420 (4.92).
Crystal data for 24
(Obtained from CH2Cl2): C18H20N4O4, M = 356.38, space group C2/c
(monoclinic), a = 13.0257(13), b = 9.7142(13), c = 13.0509(15) Å, b = 103.001
(3)ꢀ, V = 1609.1(3) Å3, Z = 4, Dc = 1.471 g cmꢁ3 m(Mo-Ka) = 0.106mmꢁ1
, ,
100 K, 8230 reflections collected, 2343 unique (Rint = 0.0457), 1696 reflec-
tions with I > 2s(I), 119 parameters, R1 = 0.0445, wR2 (all data) = 0.1022.
CCDC reference number 911344.
X-ray crystallography
X-ray measurements were conducted with Bruker SMART (Moscow, Russian
Federation) 1000 (for 15, 17-ortho, and 29; o/2θ-scanning), Bruker APEX II
(Moscow, Russian Federation) (for 16ꢂDMF, 16ꢂMeCN, 22, 24, and 28;
o-scanning) and KM-4 Kuma Diffraction (Moscow, Russian Federation)
(for 36; o/2θ-scanning) diffractometers (Mo-Ka line, graphite monochromator).
Atomic coordinates, bond lengths, bond angles, and thermal parameters for all
compounds have been deposited at the Cambridge Crystallographic Data
Centre (CCDC). These data can be obtained free of charge via www.ccdc.
cam.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge
CB2 1EZ; fax: +44 1223 335 033; or deposit@ccdc.cam.ac.uk). Any request to
the CCDC for data should quote the full literature citation and CCDC reference
numbers.
Crystal data for 28
(Obtained from CH2Cl2): C17H18N2O2, M = 282.33, space group Pna21
(orthorhombic), a = 20.036(2), b = 24.036(3), c = 11.4021(12) Å, V = 5490.9
(10) Å3, Z = 16, Dc = 1.366 g cmꢁ3, m(Mo-Ka) = 0.091 mmꢁ1, 100 K, 44 350
reflections collected, 10 231 unique (Rint = 0.0821), 6579 reflections with
I > 2s(I), 817 parameters, R1 = 0.0708, wR2 (all data) = 0.1572. CCDC refer-
ence number 911345. There are four independent molecules in the unit
cell with highly disordered CHO groups.
Crystal data for 29
(Obtained from toluene): C20H22N2O2, M = 257.33, space group P21/n
(monoclinic), a = 10.1751(6), b = 19.9254(11), c = 15.8690(9) Å, b = 95.633
Crystal data for 15
(1)ꢀ, V = 3201.8(3) Å3, Z = 8, Dc = 1.338 g cmꢁ3, m(Mo-Ka) = 0.087 mmꢁ1
,
(Obtained from MeCN): C18H21N3O2, M = 311.38, space group P212121
(orthorhombic), a = 8.0937(12), b = 10.6030(16), c = 18.010(3) Å, V = 1545.6
120 K, 36 695 reflections collected, 9241 unique (Rint = 0.0522), 4348
reflections with I > 2s(I), 433 parameters, R1 = 0.0508, wR2 (all data) =
0.0760. CCDC reference number 911346. There are two independent
molecules in the unit cell.
(4) Å3, Z = 4, Dc = 1.338 g cmꢁ3 m(Mo-Ka) = 0.089mmꢁ1
, , 120 K, 16 108
reflections collected, 3723 unique (Rint = 0.0731), 2440 reflections with
I > 2s(I), 208 parameters, R1 = 0.0513, wR2 (all data) = 0.0997. CCDC
reference number 911339.
Crystal data for 36
Crystal data for 16ꢂDMF
(Obtained from acetone): C26H14N4O4, M = 446.41, space group P21/n
(monoclinic), a = 16.778(3), b = 7.692(2), c = 17.044(3) Å, b = 111.96(3)ꢀ,
V = 2040.0(7) Å3, Z = 4, Dc = 1.453 g cmꢁ3, m(Mo-Ka) = 0.101 mmꢁ1, 293 K,
4028 reflections collected, 3059 unique (Rint = 0.0522), 2128 reflections
with I > 2s(I), 364 parameters, R1 = 0.0367, wR2 (all data) = 0.0938. CCDC
reference number 911348.
(Obtained from DMF): C15H15N5O5, M= 345.32, space group Cc (monoclinic),
a= 14.8622(18), b= 14.6665(17), c= 7.4433(9) Å, b = 110.373(2)ꢀ, V= 1521.0
(3) Å3, Z=4, Dc =1.508gcmꢁ3, m(Mo-Ka) = 0.116 mmꢁ1, 100 K, 8217 reflections
collected, 3566 unique (Rint = 0.0402), 3022 reflections with I> 2s(I), 229 para-
meters, R1 = 0.0534, wR2 (all data) = 0.1248. CCDC reference number 911340.
Crystal data for 16ꢂMeCN
RESULTS AND DISCUSSION
(Obtained from MeCN): C12H8N4O4ꢂMeCN, M = 313.28, space group Cc
(monoclinic), a = 13.156(7), b = 14.790(7), c = 7.272(4) Å, b = 106.750(13)ꢀ,
V = 1355.0(12) Å3, Z = 2, Dc = 1.536 g cmꢁ3, m(Mo-Ka) = 0.117 mmꢁ1, 100 K,
6870 reflections collected, 2949 unique (Rint = 0.0404), 2502 reflections
with I > 2s(I), 210 parameters, R1 = 0.0405, wR2 (all data) = 0.0969. CCDC
reference number 911341.
Solution measurements
We have first studied sterically unhindered p-nitroanilines repre-
sented by structures 6–9 including N-anion 10, which, similar to
other N-anions described in this work, were generated in solution
with the help of KOH/DMSO system (Table 1). As seen, the selected
criteria (3J2,3 constants and lmax of long-wave absorption bands)
are indeed in line with the expected electron-donating trend of
the amino groups:
Crystal data for 17-ortho
(Obtained from DMF): C11H6N4O5, M = 274.20, space group Pꢁ1 (triclinic),
a = 4.9626(6), b = 13.4805(16), c = 15.946(2) Å, a = 90.016(3)ꢀ, b = 95.406
(3)ꢀ, g = 90.012(3)ꢀ, V = 1062.0(2) Å3, Z = 4, Dc = 1.715 g cmꢁ3, m(Mo-Ka) =
NHAc < NH2 < NHMe < NMe2 < NMeꢁ
0.140 mmꢁ1, 295 K, 11262 reflections collected, 5355 unique (Rint
=
0.0327), 2974 reflections with I > 2s(I), 361 parameters, R1 = 0.0490, wR2
(all data) = 0.1087. CCDC reference number 911342. There are two
independent molecules in the unit cell.
This sequence is in accord with their Hammett constants, sp,[19]
as well as with other experimental and theoretical parameters.[8,35]
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J. Phys. Org. Chem. (2013)

CONJUGATION IN 1,8-DIAMINO-4,5-DINITRONAPHTHALENES
At the same time, no strict correlation is observed between the
3J2,3 constants and the dH-3 and dH-2 chemical shifts as well as with
the difference between them (Δd values). Nevertheless, a general
tendency of decreasing dH-3 and dH-2 values and increasing the
Δd parameter is present as the electron donor ability of amino
group grows.[8] A position of the NMeꢁ group in the aforemen-
tioned raw as the strongest electron donor is confirmed by the
corresponding 3J2,3 constant (~10Hz), which leaves no doubt of a
significant contribution of quinoid structure 10a into the reso-
nance hybrid.
(a)
(b)
On going from p-nitroanilines 3 to simple naphthalene
derivatives 4, along with more extended p-conjugated system,
two kinds of repulsive peri-interactions appears: between the
amino groups and between the nitro group and the H-5 hydrogen.
Hence, the trend found in the benzene series is not expected to re-
peat here in detail.
Figure 3. (a) Single-crystal X-ray structure of cyclic urea 17-ortho (30%
probability level, intramolecular H-bonds are shown). (b) View along
the naphthalene central bond showing almost complete flatness of the
molecule
Table 2 indicates that there is no dependence of dH-3 (dH-2) or Δd
values on the extent of through-conjugation in molecules 4. How-
ever, irrespectively of the substituents in the naphthalene core, a
correlation exists when 3J2,3 and lmax descriptors are considered.
The picryl group, taken as the extended variant of a nitro function,
turned out to be the worst acceptor from the naphthalene a posi-
tion (cf. 11 and 14). This is clearly due to the acoplanarity of the
trinitrophenyl substituent and the naphthalene moiety (see the
next section) so that virtually only the inductive component of
the electron-withdrawing effect of this group persists. Meanwhile,
the coplanarity of functional groups alone is not enough, as the
record-holders in Table 2 are compounds 14 and 15 with bulky
dialkylamino groups but not the expected dihydroperimidine 13.
On the whole, the following quite good relationships are found
3
between the J2,3 and lmax values (Tables 1 and 2; n, number of
points, R2, square correlation coefficient):
3
ð4ꢁnitroanilinesÞ J2;3 ¼ 136:56lmax ꢁ 880:54; n ¼ 5;
(1)
R2 ¼ 0:9485
3
ð1; 8ꢁdiaminoꢁ4ꢁRꢁnaphthalenesÞ J2;3 ¼ 142:01lmax ꢁ 826:01; (2)
n ¼ 5; R2 ¼ 0:9925
(a)
Comparative studies of dinitronaphthalene and dinitroperimidine
derivatives 16–27 seemed especially interesting along with peri-
dialdehydes 28, 29 having less sterically demanded[18] CHO groups.
All these molecules (Table 3) reveal no signs of desymmetrization in
solution, showing a set of two doublets in the aromatic region of
1
their H NMR spectra as in the case of p-nitroanilines. A priori this
could not be expected for perimidine 16 and mono-N-deprotonated
diamine 21 (anion 25). Obviously, in these molecules, fast
intermolecular (for 16) and intramolecular (for 25) prototropism lead
to the signal averaging in the spectra. The [NHN]ꢁ hydrogen bond in
25 appears at dH 9.6 ppm compared with dH 7.6 ppm from the two
NH protons in 21. Other proton NMR parameters of monoanion
25 are merely intermediate between those of diamine 21 and
dianion 27.
(b)
Figure 4. (a) Single-crystal X-ray structure of dinitro substituted
perimidine N-anion 22 (30% probability level, cationic part and water
molecules are not shown). (b) View along the naphthalene central bond
with the nitro groups directed to the viewer
J. Phys. Org. Chem. (2013)
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V. A. OZERYANSKII ET AL.
in our hands was dinitrodiamine 21, which we managed to
convert into 19, 20, 25, and 27.
(a)
Interestingly, anti-analog of 30, compound 31 was claimed
3
to possess very high J2,3 value (10 Hz),[36] but its structure
was lately disproved as being in fact the 2,6-dinitro isomer
32.[26] In addition, the D-p-A molecules of type 33 are much
worse platforms for effective p-conjugation. For example,
dinitrodiamide 34 forms almost colorless crystals from DMF
3
and has J2,3 = 9 Hz[36] (we measured 8.1 Hz).[26]
3
We have selected the J2,3 NMR parameter as the main
(b)
Figure 5. Single-crystal X-ray structure of dinitronaphthalene 24 (30%
probability level, hydrogen atoms are omitted for clarity). (a) Side view
and (b) view along the naphthalene central bond showing severe twist-
ing of the aromatic moiety
Positions of other NH fragments to mention are dH(NH): 12.6
(16), 11.6 (17), and 11.0 ppm (17-ortho). Isomers 17 and 17-or-
tho were obtained by us on dinitration of 1H-perimidine-2-one,
an easily available naphthalene-fused urea. Although less solu-
ble isomer 17 prevailed, it was not easy to distinguish between
them because of close physical appearance and 1H NMR pat-
terns. We, therefore, subjected 17-ortho to X-ray diffraction
analysis, which confirmed the arrangement of functional groups
and revealed strong intramolecular H-bonding (see the next
section).
(a)
Unfortunately, we were unable to prepare for this work
1,8-diamino-4,5-dinitronaphthalene (30), the still unknown
parent system of type 2. Treatment of perimidine-2-one 17
with the excess of KOH in DMSO under short reflux
(~190 ꢀC) or at ambient temperature for 45 days gave noth-
ing but dianion 26, which is stable under these conditions
(this approach, however, quite easily transforms 18 into
21). No hydrolysis was observed on dissolution of 17 in
conc. H2SO4 (24 h at 22 ꢀC). Therefore, the simplest structure
(b)
Figure 6. (a) Single-crystal X-ray structure of dialdehyde 28 (30% prob-
ability level, hydrogen atoms are omitted for clarity). (b) View approxi-
mately along the naphthalene central bond with the CHO groups
directed to the viewer
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CONJUGATION IN 1,8-DIAMINO-4,5-DINITRONAPHTHALENES
frame of reference to arrange the nitro and formyl derivatives
in accordance with the degree of conjugation (Table 3) and
at the same time have excluded compounds 16, 17, 22,
and 26 from consideration. The problem is that the NMR con-
centrations are generally by 102–103 times higher than those
usually employed for UV–Vis measurements. As a result, for
rather strong NH-acids 16 and 17 (they are soluble in aqueous
Na2CO3), a partial ionization in DMSO is possible causing dis-
crepancies in the UV–Vis trend observed for the neutral com-
pounds. Already ionized species such as 22 and 26 also fall
out of the correlation demonstrating anomalous bathochromic
shift of the long-wave absorption bands if compared with the
compounds close in 3J2,3 constants. This can be assigned to some
extra delocalization of the negative charge via the internitrogen
bridge (the m-C-2 atom). With these exceptions, the following cor-
relation was found for the rest of dinitronaphthalenes:
Tables 2 and 3 clearly demonstrate that, if the diamino
parts of the push–pull molecules are similar, the p-acceptor
ability of the two peri-NO2 groups in 2 is almost the same
as that of one a-NO2 group in 4 with slight predominance
of the latter (compare pairs 12/18, 13/19, 14/23, and 15/
24). Hence, as representatives of the R2N-p-NO2 systems, mo-
tifs 2 and 4 are close in function and, in the terms of UV–Vis
absorption, both surpass their benzene counterparts 3 be-
cause of the intrinsically longer conjugation chains. As to
3
the J2,3 criterion, despite of the notable contribution of reso-
nance forms 26a and 27a, the bond order redistribution in 2
and 4 is not as effective as in 3 (see the next section).
ð1; 8ꢁdiaminoꢁ4; 5ꢁdinitronaphthalenesÞ
(3)
3J2;3 ¼ 235:78lmax ꢁ 1695:7; n ¼ 8; R2 ¼ 0:9656
As it was already mentioned, neutral perimidines 16–20,
unless they are converted into anionic species 22 and 26, are
generally less effective for through-conjugation than peri-
diamines 21, 23, and 24. This conclusion is spread on
dialdehydes 28 and 29 (Table 3). On the whole, the best donors
from peri-positions are pyrrolidin-1-yl and methylamido groups.
As seen, the correlation is quite good, and the worse points are
those corresponding to anions 25 and 27.
Structural considerations
One of the obvious consequences of intramolecular charge
transfer in the D-p-A push–pull molecules is the bond length
redistribution on going from the non-through-conjugated
(aromatic) to the fully conjugated (quinoid) structure. This
transition requires both the donor and acceptor groups to be
coplanar with the arylidene p-bridge.[8,37,38] Whereas this require-
ment is nicely fulfilled for simple 4-nitroanilines (structural data
relevant for the illustration of the key bonds length alternation
and acoplanarity, in particular ’ values, are available for 7–9, see
(a)
(b)
Figure 7. (a) Single-crystal X-ray structure of dialdehyde 29 (30% prob-
ability level, hydrogen atoms are omitted for clarity). (b) View along the
naphthalene central bond showing twisting of the molecule
Figure 8. Single-crystal X-ray structure of pyridazine 36 (30% pro-
bability level, hydrogen atoms are omitted for clarity)
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V. A. OZERYANSKII ET AL.
Scheme 2. Bond alternation and some properties of dication 37
Table 4 for details), hindered nitroaromatics[39] display notable
twisting of the nitro groups (picryl group in the case of 11) and
even aromatic moieties (Figs. 1–8). Thus, for parent 1,8-
dinitronaphthalene 35 and its more complex derivative 36 (in
the latter, the nitro groups are by 5% more distant, Table 4), the tor-
sion angles ’ are 43ꢀ and 39ꢀ, respectively (compare with ’ = 36ꢀ
for dinitroperimidine 16, Fig. 2).
Moreover, di(pyrrolidino)naphthalene 24 demonstrates record
twisting of the naphthalene core (27ꢀ, Fig. 7), the highest value reg-
istered so far for 1,4,5,8-tetrasubstituted naphthalenes.[18,42] At the
same time, there is no doubt that the internal twisting of molecules
15, 24, and 29 (a conjugation-induced chirality) is preserved in so-
lution, because the CH2 fragments in their room temperature
1H NMR spectra are displayed as six to eight multiplets (four multi-
plets for compound 15, see the Experimental part). Hence, we may
conclude that the pyrrolidin-1-yl groups, even being placed in ste-
rically hindered positions, are potent electron-donating substitu-
ents if compared with the NMe2 functions.[43]
Finally, the resonance p-interaction between the electron
donor and acceptor groups in the solid decreases on going from
4 to 2, yet all structures demonstrate a strong bond alternation
within the naphthalene skeleton.
It should be emphasized further that ’ values in naphthalene
derivatives may come to the level of benzenes 3 (that is close to
coplanarity) in two distinct cases: (i) intramolecular hydrogen
bonding (as in the case of flat H-bonded molecule 17-ortho, Fig. 5)
and (2) strong through-conjugation (as in the case of pyrrolidino
derivative 15, Fig. 3), which is accompanied by notable distortion
of the aromatic p-system expressed by ∠ABCD and rN(1). . .N(1)
(compare data for 14 and 15, Table 4). Obviously, this molecular
distortion is energetically favorable because of decreasing intra-
molecular Coulombic repulsion and steric strain.
CONCLUSIONS
In summary, using simple and adequate criteria, we have quan-
titatively compared the through-conjugation in 4-nitroanilines
and 1,8-diamino-4-nitronaphthalenes with that of 12 1,8-
diamino-4,5-dinitronaphthalenes, the widest set of sterically
hindered A2,D2-naphthalenes published to date. The selected
descriptors (3J2,3 constants and lmax of long-wave absorption
bands) reach their maximum in case of pyrrolidin-1-yl
derivatives and methylamido anions, that is, for compounds
possessing the highest degree of p-conjugation. The data
received (both in solution and in the solid state), apart from
the estimation of peri-interaction phenomena for the nitro and
amino groups, may be of help to foresee further steps enhanc-
ing the donor–acceptor interaction in D-p-A arenes along with
the possible limits in structure variations of organic compounds.
Transition from 2 to 2a also requires a contraction of the C(b)–
C(b0) aromatic bond in the naphthalene fragment [uniformly
marked here as C(2)–C(3)], which should be in close relation with
3
the J2,3 values determined for the same bond. This is however
only roughly true because of the different media (crystal field
and solution) and the different atoms used to extract the rC(2)–C(3)
3
and J2,3 parameters (carbon and hydrogen atoms, respectively).
Anyhow, a general trend for the shrinkage of this bond up to
1.37 Å in benzene 3 and naphthalene 4 push–pull molecules carry-
ing the most electron-donating amino groups (NHMe, NMe2, and
pyrrolidino) is easily seen (Table 4). Thus, all the C(2)–C(3) bonds
are notably shorter than those in the nonconjugated
peri-dinitronaphthalenes 35 and 36 or in compounds with the
nitro groups placed in other positions (11, 17-ortho). However,
the rC(2)–C(3) values in 14, 15, and 22 (1.37 Å) followed by 23 and
24 (1.38 Å) are still rather far from the purely quinoid bonds in
naphthalene-1,4,5,8-tetrone (rC(2)–C(3) = 1.32 Å)[40] but quite close
to that in dication 37, a product of two-electron oxidation of
1,4,5,8-tetrakis(dimethylamino)naphthalene (Scheme 2).[41]
In line with the shortening of the C(2)–C(3) bonds are the
lengths of the N(1)–C(1) bonds, also reflecting the resonance
interaction ofthe amino nitrogens with the aromatic rings (Table 4).
Again, in the solid, N,N0-bridged compounds, even in the anionic
form, possess longer rN(1)–C(1) distances than the N,N-dialkylated
(compare 16 and 22 with 23 and 24, see also dialdehydes 28
and 29). The rN(1)–C(1) values become minimal (~1.35 Å) for
compounds with the pyrrolidino groups (15, 24, and 29).
Acknowledgement
Financial support from the Russian Foundation for Basic
Research (project no. 11-03-00073) is gratefully acknowledged.
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