The synthesis, X-ray structure analysis, and photophysical characterization of 4-[(E)-2-(4-cyanophenyl)diazenyl]-morpholine

Article abstract of DOI:10.1002/jhet.513

A novel triazene, 4-[(E)-2-(4-cyanophenyl)diazenyl]-morpholine (1) was prepared via a diazonium ion coupling reaction between 4-aminobenzonitrile and morpholine. The x-ray structure of 1 was determined and evidenced π delocalization in the triazene subuni

Full text of DOI:10.1002/jhet.513

January 2011
The Synthesis, X-ray Structure Analysis, and Photophysical
Characterization of 4-[(E)-2-(4-Cyanophenyl)diazenyl]-
morpholine
215
Taylor Chin,a Ashley Phipps,a Frank R. Fronczek,b and Ralph Isovitscha*
aDepartment of Chemistry, Whittier College, 13406 Philadelphia Street, Whittier, CA 90601
bDepartment of Chemistry, Louisiana State University, Baton Rouge, LA 70803
*E-mail: risovits@whittier.edu
Received January 27, 2010
DOI 10.1002/jhet.513
Published online 2 September 2010 in Wiley Online Library (wileyonlinelibrary.com).
A novel triazene, 4-[(E)-2-(4-cyanophenyl)diazenyl]-morpholine (1) was prepared via a diazonium
ion coupling reaction between 4-aminobenzonitrile and morpholine. The x-ray structure of 1 was deter-
mined and evidenced p delocalization in the triazene subunit. The room temperature absorption spec-
trum of 1 in acetonitrile was dominated by an intense triazene-centered p!p* transition at 325 nm.
Compound 1 was observed to be luminescent, with an emission maximum at 434 nm in room tempera-
ture acetonitrile solution. The emission spectrum of 1 in propionitrile glass at 77K exhibited a narrowed
emission band with a maximum at 449 nm. Broad emission from 400–700 nm with poorly resolved
vibrational structure was observed from solid 1 at room temperature.
J. Heterocyclic Chem., 48, 215 (2011).
INTRODUCTION
nitrile group, which can be transformed into other func-
tional groups or act as a metal ligand. In this work, we
present the synthesis of 1 and its crystal structure. The
photophysical characterization of 1, the first for a tria-
zene prepared from morpholine, is also detailed.
Triazenes are a diverse class of compounds that pos-
sess three consecutively bonded nitrogen atoms. The
first nitrogen atom in a triazene is bonded to the second
by a double bond, which is bonded to the third by a sin-
gle bond, for example, RN¼¼NANR0R00. The classic syn-
thesis of triazenes involves the coupling of an amine to
an aryl diazonium ion [1,2]. A recent and versatile syn-
thetic procedure yields triazenes from the reaction of or-
ganic azides with N-heterocyclic carbenes [3].
RESULTS AND DISCUSSION
Compound 1 was prepared in good yield (64%). It
exhibited the predicted spectroscopic characteristics,
except for the 13C-NMR spectrum. The expected resonan-
ces for the aromatic ring and the nitrile substituent were
observed, as well as three broad singlets at 45.1, 51.3, and
66.4 ppm for the morpholine ring. The assignment of these
singlets to the carbons of the morpholine ring was sup-
ported by the HMQC NMR spectrum of 1, which showed a
connection between the broad carbon resonances and the
morpholine protons. Three signals were observed instead
of the expected two because the partial p character of the
NꢀN single bond inhibited its rotation, which resulted in
the magnetic nonequivalence of the morpholine carbons
[11,12]. The identity of 1 was corroborated by an elemental
analysis that yielded acceptable values and a EI-HR-MS
that had an [MþH]þ peak at m/z ¼ 217.1081, which was
ꢁ 1.5 ppm less than the calculated value.
Triazenes have been utilized in a variety of research areas.
In organic synthesis, they have been used as protecting groups
for aromatic amines, as linkers in solid-phase peptide synthe-
sis and as synthons for novel heterocyclic compounds [4–6].
Triazenes have also been investigated as antitumor agents
and incorporated into novel conductive aromatic polymers
[7,8]. More recently, triazenes have been employed as easily
prepared and handled equivalents for the in situ generation of
reactive diazonium ions, which can participate in palladium-
catalyzed reactions that form conjugated materials [9].
Our research utilizes triazenes as diazonium ion
equivalents to prepare luminescent stilbenoid metal-or-
ganic materials via the method of Sengupta et al [10].
Compound 1 (Scheme 1) is a logical synthon for our
research, because it is easily prepared from economical
and readily available reagents. It also has a versatile
Compound 1 was recrystallized from petroleum ether
to yield yellow needles that were suitable for X-ray
C
V
2010 HeteroCorporation

216
T. Chin, A. Phipps, F. R. Fronczek, and R. Isovitsch
Vol 48
Scheme 1
Table 1
Photophysical data for compound 1.
kmax,abs
kmax,em nm
(kexc nm)
Medium/Temperature
nm (e)a
CH3CN solution/r.t.
230 (9,200)
325 (20,100)
434 (300)
CH3CH2CN glass/77 K
solid state/r.t.
449 (300)
469 (350)
569
analysis. The ORTEP representation of 1 is shown in
Figure 1. The asymmetric unit consisted of two inde-
pendent molecules of 1 whose bond lengths and angles
were essentially identical, with the largest difference
between them being the magnitude of the torsion angle
between the N¼¼N double bond and the plane of the
phenyl ring, 26.42(16)ꢂ for one and 6.88(16)ꢂ for the
other. The NꢀN double bond was found to adopt an
(E)-configuration with a N(3)AN(2)AN(1) bond angle
of 114.72(10)ꢂ, which deviated from the optimal trigonal
planar geometry by approximately 6ꢂ. The N(2)AN(3)
group participated in p delocalization with the morpho-
line nitrogen N(1) that was evidenced by N(1)AN(2)
and N(2)AN(3) bond lengths of 1.3320(13) and
613
a Extinction coefficient units Mꢀ1cmꢀ1
.
listed in Table 1. The room temperature electronic
absorption and emission spectra of 1 in room tempera-
ture acetonitrile solution are presented in Figure 2.
The absorption spectrum of 1 in room temperature
acetonitrile was dominated by an intense absorption
band at 325 nm, which was assigned to a p!p* transi-
tion that arose from the triazene subunit. A less intense
absorption band at 230 nm was assigned to a p!p*
transition in the phenyl ring. Excitation of a dilute (2.0
ꢃ 10ꢀ5M) acetonitrile solution of 1 at room temperature
yielded an emission spectrum with a band that had a
maximum at 434 nm. The excitation of a dilute (2.0 ꢃ
10ꢀ5M) solution of 1 in propionitrile at 77 K yielded a
slightly red-shifted, narrower emission band with a max-
imum at 449 nm. Solid 1 at room temperature yielded
weak, broad emission from 400–700 nm with poorly
resolved fine structure that had maxima at 469, 569, and
613 nm. All observed emission was assigned to a p*!p
transition centered on the triazene moiety. The observed
absorption and emission properties of 1 agreed with
those found in the literature for similar compounds
[17,18].
˚
1.2748(14) A, respectively. These values are between
˚
literature value of 1.222 A for a N¼¼N double bond and
2
3
˚
1.420 A for a N(sp )AN(sp ) single bond [13]. The
bond angles of the morpholine nitrogen N(1) varied
from 114.73(10) to 122.63(10)ꢂ, which indicated some
degree of sp2 hybridization of N(1) and its participation
in p delocalization with N(2)AN(3). The morpholine
ring adopted
a chair-like conformation with the
N(2)AN(3) group in an equatorial position on N(1). The
structure of 1 was similar to the structures of related tri-
azenes [14,15].
While the photophysical characteristics of molecules
containing a NꢀN double bond (azo compounds) have
been extensively documented, information regarding the
photophysical properties of triazenes is limited [16]. The
experimental photophysical data for compound 1 are
Figure 2. Room temperature electronic absorption and emission spec-
tra of compound 1 in acetonitrile solution. (—) absorption spectrum;
(- - -) emission; spectrum (kexc ¼ 300 nm).
Figure 1. ORTEP view of 1. Ellipsoids are represented at the 50%
probability level.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

January 2011
The Synthesis, X-ray Structure Analysis, and Photophysical Characterization of
4-[(E)-2-(4-Cyanophenyl)diazenyl]-morpholine
217
˚
Ka radiation (k ¼ 0.71073 A). A total of 24,607 measured
reflections with ymax ¼ 27.1ꢂ yielded 4,727 unique data. The
structure was solved by direct methods, and structure refine-
ment was carried out using SHELXL-97 [19]. All hydrogen
atoms were visible in difference maps, but were placed in cal-
culated positions in the refinement.
The synthesis of 1 was attractive for its operational
simplicity, good yield and its use of inexpensive starting
materials. Determination of the x-ray structure of 1
revealed two independent molecules in the asymmetric
unit as well as p delocalization of the nitrogen atoms
that comprise the triazene moiety. Compound 1 was
found to be luminescent both at room temperature and
at 77 K. The presented characterization of the photo-
physical properties of 1 was the first noted for a triazene
synthesized from morpholine.
Single-crystal X-ray diffraction data for 1.C11H12N4O,
˚
FW ¼ 216.25, triclinic, P1, a ¼ 9.2329(15) A, b ¼ 10.438(2)
ꢂ
ꢂ
˚
˚
A, c ¼ 11.853(2) A, a ¼ 87.977(8) , b ¼ 70.664(10) , c ¼
3
83.027(12) , V ¼ 1069.9(3) A , Z ¼ 4, qcalc ¼ 1.343 Mg/m3,
l ¼ 0.09 mmꢀ1, F(000) ¼ 456, Rint ¼ 0.024 for 3,445
observed reflections with I > 2rI), R1/wR2 ¼ 0.042/0.119,
Goodness-of-fit on F2 ¼ 1.03, 289 parameters.
ꢂ
˚
CCDC 756812 contains the supplementary crystallographic
data for this article. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via
www.ccd.cam.ac.uk/data_request/cif.
EXPERIMENTAL
The synthetic procedure was carried out using standard tech-
niques. Solvents and reagents were used as received. 1H- and
13C-NMR spectra were recorded on a JEOL ECX 300 MHz
spectrometer using TMS as the internal standard. The IR spec-
trum was recorded as a KBr disk on a JASCO 460 FT-IR. Ele-
mental analysis was done by M-H-W Laboratories of Tucson,
Arizona. Mass spectrometry was provided by the Washington
University Mass Spectrometry Resource with support from the
NIH National Center for Research Resources (Grant No.
P41RR0954).
Acknowledgments. The authors would like to thank the Fletcher
Jones Foundation for funds that allowed the purchase of the fluo-
rometer. Whittier College is acknowledged for the funds that sup-
ported this research. Dr. Andrew Maverick is acknowledged for
helpful discussion and encouragement.
REFERENCES AND NOTES
Emission and absorption spectra were recorded at room tem-
perature and at 77 K in spectrophotometric grade acetonitrile
and propionitrile (99% purity) utilizing a HoribaJobinYvon
FluoroMax-4 fluorometer and a Hewlett Packard 8453 diode
array spectrometer. All solutions were deoxygenated with ar-
gon before luminescence measurements. All emission spectra
were corrected for detector response utilizing a correction
curve supplied by the fluorometer manufacturer.
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HCl and heated on a hot water bath to yield a white suspen-
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dropwise over 10 min; a yellow-orange solid formed. The mix-
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(C¼¼C), 1494 (N¼¼N) cmꢀ1
;
1H-NMR (deuteriochloroform): d
3 3
3.87 (br. s, 8H), 7.49 (d, 2H, J ¼ 8.6 Hz), 7.61 (d, 2H, J ¼
8.9 Hz); 13C NMR (deuteriochloroform): d 45.1, 51.3, 66.4,
108.9, 119.4, 121.3, 133.2, 153.5; 196.3; EI-HR-MS: m/z for
[MþH]þ ¼ 217.1081, Calcd. m/z for [MþH]þ ¼ 217.1089.
Anal. Calcd. for C11H12N4O: C, 61.25; H, 5.88, N, 26.16.
Found: C, 61.13; H, 5.61; N, 25.93.
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systems Cryostream chiller and graphite-monochromated Mo
´ ´
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet