The suppression of columnar π-stacking in 3-adamantyl-1-phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl

Article abstract of DOI:10.3390/molecules21050636

3-Adamantyl-1-phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl(4) crystallizes as chains of radicals where the spin bearing benzotriazinyl moieties are isolated from each other. Magnetic susceptibility studies in the 5-300 K temperature region indicate that

Full text of DOI:10.3390/molecules21050636

molecules
Article
The Suppression of Columnar π-Stacking in
3-Adamantyl-1-phenyl-1,4-
dihydrobenzo[e][1,2,4]triazin-4-yl
Christos P. Constantinides 1,*, Andrey A. Berezin 1, Georgia A. Zissimou 1, Maria Manoli 1,
Gregory M. Leitus 2 and Panayiotis A. Koutentis 1,
*
1
Department of Chemistry, University of Cyprus, P. O. Box 20537, Nicosia 1678, Cyprus;
berezin.andrey@ucy.ac.cy (A.A.B.); georgia_zissimou@hotmail.com (G.A.Z.); manoli.maria@ucy.ac.cy (M.M.)
Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel;
gregory.leitus@weizmann.ac.il
2
*
Correspondence: constantinides.christos@ucy.ac.cy (C.P.C.); koutenti@ucy.ac.cy (P.A.K.);
Tel.: +357-22892783 (P.A.K.)
Academic Editor: Fawaz Aldabbagh
Received: 18 April 2016; Accepted: 10 May 2016; Published: 14 May 2016
Abstract: 3-Adamantyl-1-phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl (4) crystallizes as chains of
radicals where the spin bearing benzotriazinyl moieties are isolated from each other. Magnetic
susceptibility studies in the 5–300 K temperature region indicate that radical
4 demonstrates typical
paramagnetic behavior stemming from non-interacting S = ½ spins.
Keywords: 1,2,4-benzotriazinyls; hydrazyls; adamantyl; paramagnet; organic radicals;
crystal packing
1. Introduction
1,4-Dihydrobenzo[e][1,2,4]triazin-4-yls a sub-class of the hydrazyl family of radicals has
attracted considerable attention owing to their robust thermal, moisture and air stability. The first
1,2,4-benzotriazinyl radical was reported by Blatter in 1968 (Blatter radical
from electrochemical studies by Neugebauer [ ], this stable radical remained relatively obscure.
Nevertheless, the recent development of improved syntheses of 1,2,4-benzotriazinyls [ 10] have
led to numerous new derivatives, some of which display a range of low dimensional magnetic
properties [11 22], while others have been used as radical initiators in polymer chemistry [23 25], and
as sensors of picric acid [18].
1, Chart 1) [1], and, aside
2–5
6–
–
–
1
2
Ph
N1
1
2
3
(R = Ph, R = H)
8
R2
1
2
(R = F3C, R = H)
7
N 2
1
2
(R = Ph, R = fur-2-yl)
4
6
R1
4 (R1 = 1-Ad, R2 = H)
3
N
5
Chart 1. Structures of 1,2,4-benzotriazin-4-yls 1–4 and IUPAC numbering of the ring system.
Solid-state
for two derivatives radicals within the
singlet ground states: (a) 1-phenyl-3-trifluoromethyl-1,4-dihydrobenzo[e]-[1,2,4]triazin-4-yl
) demonstrates an abrupt fully reversible spin transition at 58(2) K between a diamagnetic
low temperature phase and paramagnetic high temperature phase 16 and (b)
σ
or
π
dimerization is suppressed in the majority of these radicals, nevertheless,
π-stacked columns associate leading to dimers with
(2
a
[ ]
7-(fur-2-yl)-1,3-diphenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl (3) has a singlet ground state
below 100 K and a thermally accessible triplet state [13].
Molecules 2016, 21, 636; doi:10.3390/molecules21050636
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The extensive delocalization of the singly occupied molecular orbital (SOMO) orbital density
over the benzo-fused hydrazonyl and the N1-Ph (Figure 1), indicates that positive orbital overlap
and
solid-state packing motifs have been reported [11
date, 1,2,4-benzotriazinyls stack to form 1-D columns. The propensity of 1,2,4-benzotriazinyls
π-stacking, can be achieved in a variety of different association modes, and a number of different
–
22]. In the majority of crystals structures reported to
π
to form columns could potentially be overcome by strategic introduction of bulky substituents
in the periphery. To test this hypothesis we prepared a 1,2,4-benzotriazinyl substituted with an
adamantyl group at C-3. Herein, we report the crystal structure and magnetic properties of the
3-adamantyl-1-phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl (4).
Figure 1. SOMO of Blatter radical 1 calculated at the UB3LYP/6-311+G(d,p).
2. Results and Discussion
2.1. Synthesis, Electrochemical and EPR Data
Radical
available 1-benzylidene-2-phenylhydrazine (
nitrobenzene and K2CO3 in DMSO at ca. 100 ˝C for 1 day gave 2-benzylidene-1-(2-nitrophenyl)-1-
phenylhydrazine ( ) which upon treatment with excess NH2OH HCl in pyridine and heating to
ca. 80 ˝C for 1 day released 1-(2-nitrophenyl)-1-phenylhydrazine (
) in 80% yield. Acetylation and
4
was previously prepared via a multistep route (Scheme 1) starting from the readily
5
) [10]. Reaction of the benzylidene with 1-fluoro-2-
5
6
¨
7
subsequent mild reduction of the nitro group followed by an acid-mediated cyclodehydration gave
the fused triazine which upon alkali treatment afforded the desired adamantly-substituted radical 4.
Ph
N
Ph
N
K2CO3 (1.1 equiv)
DMSO, 100 oC, 1 d
Ph
90%
NH2OH.HCl (10 equiv)
pyridine, , 1 d
80%
F
H
N
Ph
NH2
+
N
Ph
N
NO2
NO2
NO2
5
6
7
i) Sn, AcOH, 0.5 h at 20 o
C
Ph
Ph
N
O
ii) ca. 118 oC, 10 min
1-AdCOCl, Et3N (1.1 equiv)
DCM, 0-20 oC, 4-24 h
58%
N
N
N
R1 iii) 2 M NaOH, 12 h, 20 o
C
H
79%
N
R1
NO2
1
1
8
4
(R = Adamantyl)
(R = Adamantyl)
Scheme 1. Synthesis of the 3-adamantyl-1,2,4-benzotriazinyl 4.
The redox behavior of radical
4
is typical of 1,2,4-benzotriazinyls exhibiting two fully reversible
22]. Worthy of note is that the
waves corresponding to the 1/0 and 0/+1 processes (Figure 2, left) [11
´
–

Molecules 2016, 21, 636
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0/+1
oxidation potential of E1/2
= 0.08 V vs Fc/Fc+ couple makes radical
4
the best 1,2,4-benzotriazinyl
donor reported to date. The solution EPR spectrum of radical 4 (Figure 2, right) exhibits the typical
1,2,4-benzotriazinyl seven-line multiplets consistent with coupling of the unpaired electron with
the three similar but slightly nonequivalent 14N nuclei. The hyperfine coupling constants (hfcc),
determined by the simulation of the EPR spectrum, are aN(1) = 7.51, aN(2) = 4.94 and aN(4) = 5.13 G
with g = 2.0040.
Figure 2. Cyclic voltammetry (left) and solution EPR spectrum (right) of radical 4.
2.2. X-ray Studies
Suitable single crystals of radical
of a concentrated c-hexane solution. Radical
two molecules in the asymmetric unit. These two molecules are oriented in a dihedral angle of 56.9˝
4
for X-ray diffraction studies were obtained by slow cooling
4
adopted the orthorhombic space group Pna21 with
with respect to each other (Figure 3). The intramolecular geometry of radical
1,2,4-benzotriazinyls [11–22].
4 is typical of other
Figure 3. Asymmetric unit (left) and crystallographic atom numbering for 3-adamantyl-1,2,4-
benzotriazinyl 4 (right). Hydrogen atoms omitted for clarity.
Each molecule in the asymmetric unit forms a linear chain of radicals at regular distances.
Radicals in the chain running parallel to c-axis (Figure 4, top) have two close contacts, one between
neighboring adamantyls [dC46 . . . C19 = 4.297(5)˝] and one between adamantyl and benzotriazinyl
[dC20 . . . C1 = 3.828(5)˝]. Radicals within this chain are related by a 2-fold screw axis with direction
(0, 0, 1) at 0,0,z and screw component (0, 0,
½). Radicals in the second chain running parallel to a-axis
(Figure 4, bottom) are connected via a weak hydrogen contact between neighboring benzotriazinyls
[dN6 . . . C34 = 3.585(4)˝]. This contact may propagate an antiferromagnetic interaction between the
positive spin density on N6 and the negative spin density on C34. Carbon C34 has minimal orbital

Molecules 2016, 21, 636
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density in the SOMO, however, through spin polarization, it possesses significant negative spin density.
The radical chains are related by two glide planes, one perpendicular to (0, 1, 0) with glide component
(
π
½
, 0, 0) and one perpendicular to (1, 0, 0) with glide component (0,
-stacked radicals was observed. The spin bearing benzotriazinyl moieties do not interact with each
other. The related 3-(tert-butyl)-1-phenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl was shown to crystallize
in 1-D columns without interactions between the benzotriazinyl moieties [ 20]. This radical
demonstrated typical paramagnetic behavior as it followed the Curie-Weiss law with θ = ´0.3 K [5].
½, ½). No formation of columns by
π
-
π
5,
Figure 4. Solid-state packing of radical
along a-axis (bottom).
4 demonstrating the formation of chains along c-axis (top) and
2.3. Magnetic Susceptibility Studies
Variable-temperature magnetic-susceptibility measurements on radical
4 were obtained using
a SQUID magnetometer in the temperature region 5–300 K in an applied field of 0.4 T. Data were
collected in both warming and cooling modes with no significant differences in sample susceptibility.
The temperature dependence of χT is shown in Figure 5.
Figure 5. Temperature dependence of χT upon heating from 5 to 300 K. Inset: Curie-Weiss behavior,
C = 0.377 emu K¨mol´1 and θ = ´0.61 K.

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The inverse of molar susceptibility (1/
χ
) followed the Curie-Weiss law (Figure 5, inset) with
´1
C = 0.377 emu K¨mol and
θ
=
´
0.61 K. On cooling from 300 K down to ca. 50 K,
χT remained stable
with a value of 0.375 emu K
¨
mol´1 expected for an S =
½
paramagnet. Below 50 K, χT gradually
decreased as antiferromagnetic interactions along the a-axis chain (weak hydrogen dN6 . . . C34 contacts)
became dominant.
3. Experimental Sections
Synthetic procedure: The synthesis of radical 4 was published previously [10].
Instrumental analyses: Cyclic voltammetry (CV) measurements were performed on a Princeton
Applied Research Potentiostat/Galvanostat 263A apparatus (Oak Ridge, TN, USA). The concentration
of the benzotriazinyl radical
4 used was 1 mM in CH2Cl2. A 0.1 M CH2Cl2 solution of
tetra-butylammonium tetrafluoroborate (n-Bu4BF4) was used as electrolyte. The electrolyte was dried
˝
for four days in the vacuum oven at 100 C prior to the use. The reference electrode was Ag/AgCl and
the scan rate was 50 mV/s. Ferrocene was used as an internal reference; the E1/2(ox) of ferrocene in this
system was 0.352 V [26]. EPR spectra were recorded on a Bruker EMXplus X-band EPR spectrometer
(Bruker, Billerica, MA, USA) at room temperature in dilute solution of CH2Cl2. For the EPR spectrum,
the microwave power was in the region 5–70 mW with modulation frequencies of 50 or 100 kHz and
modulation amplitudes of 0.5–1.0 Gpp. Simulations of the solution spectrum was made using Winsim
software [27]. X-ray data of
diffractometer, equipped with a CCD area detector utilizing Mo-K
4
(CCDC 1474192) [28] were collected on an Oxford-Diffraction Supernova
radiation ( = 0.71073 Å). A
α
λ
suitable crystal was attached to glass fibers using paratone-N oil and transferred to a goniostat where
they were cooled for data collection. Unit cell dimensions were determined and refined by using 4562
˝
(3.85 ď θ ď 27.68 ) reflections. Empirical absorption corrections (multi-scan based on symmetry-related
measurements) were applied using CrysAlis RED software [29]. The structures were solved by direct
method and refined on F2 using full-matrix least squares using SHELXL97 [30
,31]. Software packages
used: CrysAlis CCD [29] for data collection, CrysAlis RED [29] for cell refinement and data reduction,
WINGX for geometric calculations [32] and Mercury 3.1 (CCDC, Cambridge, UK) [33]. The non-H
atoms were treated anisotropically. The hydrogen atoms were placed in calculated, ideal positions
and refined as riding on their respective carbon atoms. Magnetic properties were studied by using a
Quantum Design SQUID MPMS2 field-shielded magnetometer (Quantum Design Inc., San Diego, CA,
USA). The DC (direct current) magnetic moment was measured for 43 mg sample of radical 4, placed
in gelatin capsules held by polyethylene straw. The magnetic susceptibilities were measured in the
temperature range of 5–300 K in an applied field of 0.4 T. Data were collected in both warming and
cooling modes with no significant differences in sample susceptibility.
Crystal refinement data (
a = 18.5886(10) Å, b = 19.3115(9) Å, c = 9.9148(6) Å,
Z = 8, T = 100(2) K, ρcalcd = 1.278 g
cm´3, 2θmax = 25. Refinement of 470 parameters on 5357
4
): C23H24N3, M = 342.45, Orthorhombic, space group Pna21,
α
= 90˝, = 90˝, = 90˝, V = 35.59(2) Å3,
β
γ
¨
independent reflections out of 12978 measured reflections (Rint = 0.0438) led to R1 = 0.04761 [I > 2σ(I)],
wR2 = 0.1154 (all data), and S = 1.050 with the largest difference peak and hole of 0.227 and
´0.176 ˆ 10´3, respectively.
4. Conclusions
Introduction of the bulky substituent adamantyl to the C-3 position of the 1,2,4-benzotriazinyl
framework successfully inhibited the formation of π-stacked radical columns, but instead of promoting
higher dimensional frameworks, only isolated radicals were obtained. This led to non-interacting
S = ½ spins and a typical paramagnetic behavior. Our results indicate that there is a fine balance to be
reached between steric perturbation and π-stacking interactions if materials with interesting magnetic
and transport properties are to be developed.
Acknowledgments: The authors thank the University of Cyprus (medium-size grants), the Cyprus Research
Promotion Foundation (grants no. ΥΓEIA/BIOΣ/0308(BIE)/13, NEKΥΠ/0308/02 and ANABAΘMIΣH/0308/32

Molecules 2016, 21, 636
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and the following organizations in Cyprus for generous donations of chemicals and glassware: the State General
Laboratory, the Agricultural Research Institute, the Ministry of Agriculture, MedoChemie Ltd. and Biotronics Ltd.
Furthermore, we thank the A. G. Leventis Foundation for helping to establish the NMR facility in the University
of Cyprus.
Author Contributions: Christos P. Constantinides and Panayiotis A. Koutentis have contributed to the conception,
drafting and critical revision of the submitted manuscript. Andrey A. Berezin and Georgia A. Zissimou have
contributed to the synthesis of the radical and collection of data. Maria Manoli has acquired the X-ray diffraction
data and solved the structure. Gregory M. Leitus has acquired the magnetometry data.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Sample of the radical is available from the authors.
©
2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).