Synthesis, spectroscopy, X-ray structure and redox behaviors of 4-(N-R-salicylideneimine)-2,6-diphenylphenols

Article abstract of DOI:10.1016/j.saa.2007.07.021

A series of sterically hindered 4-(N-R-salicylaldimine)-2,6-diphenylphenols (X), where R = H (1), 3-CH₃ (2), 5-CH₃ (3), 3-OCH₃ (4), 4-OCH₃ (5), 5-OCH₃ (6), 3-^tBu (7), 5-^tBu (8), 3,5-^tBu₂(9) and 5,6-benzo(10), were synthesized and their structure as well as redox behavior studied by analytical, spectroscopic [¹H, (¹³C) NMR, IR, UV-vis and mass spectrometry] and cyclic voltammetric (CV) techniques. Single crystal X-ray diffraction studies of 7 evidenced its existence as non-planar enol-imine tautomer structure, in which the phenol ring of the molecule is twisted around C-N single bond by 21.5(2)°. The packing structure of 7 is stabilized by C-H...π(Ph) and O...O and C...O intermolecular short contact interactions. The CV of X display rate is dependent on irreversible and quasi-reversible redox waves in the anodic and cathodic regions due to oxidation and reduction of phenolic and iminic groups, respectively. As evidenced by ESR and UV-vis study, chemical oxidation of X by PbO₂ and (NH₄)₂Ce(NO₃)₆ in MeCN and CHCl₃ generates stable phenoxyl radicals [(g ～ 2.005 and λ ～ 450 nm (1600-8200 M^-1 cm^-1)].

Full text of DOI:10.1016/j.saa.2007.07.021

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 70 (2008) 60–68
Synthesis, spectroscopy, X-ray structure and redox behaviors of
4-(N-R-salicylideneimine)-2,6-diphenylphenols
Veli T. Kasumova,∗, Hasan Turkmen , Ibrahim Uc¸ar ,
a
b
˙
¨
Ahmet Bulutb, Nurettin Yaylıc
a Chemistry Department, Faculty of Arts and Sciences, HU, Sanliurfa, Turkey
b Physics Department, Faculty of Arts and Sciences, OMU , Kurupelit, Samsun, Turkey
c Chemistry Department, Faculty of Arts and Sciences, KTU, Turkey
Received 2 February 2007; received in revised form 11 July 2007; accepted 12 July 2007
Abstract
A series of sterically hindered 4-(N-R-salicylaldimine)-2,6-diphenylphenols (X), where R = H (1), 3-CH3 (2), 5-CH3 (3), 3-OCH3 (4), 4-OCH3
(5), 5-OCH3 (6), 3-tBu (7), 5-tBu (8), 3,5-tBu2(9) and 5,6-benzo(10), were synthesized and their structure as well as redox behavior studied by
analytical, spectroscopic [1H, (13C) NMR, IR, UV–vis and mass spectrometry] and cyclic voltammetric (CV) techniques. Single crystal X-ray
diffraction studies of 7 evidenced its existence as non-planar enol–imine tautomer structure, in which the phenol ring of the molecule is twisted
around C–N single bond by 21.5(2)◦. The packing structure of 7 is stabilized by C–H· · ·␲(Ph) and O· · ·O and C· · ·O intermolecular short contact
interactions. The CV of X display rate is dependent on irreversible and quasi-reversible redox waves in the anodic and cathodic regions due to
oxidation and reduction of phenolic and iminic groups, respectively. As evidenced by ESR and UV–vis study, chemical oxidation of X by PbO2
and (NH4)2Ce(NO3)6 in MeCN and CHCl3 generates stable phenoxyl radicals [(g ∼ 2.005 and λ ∼ 450 nm (1600–8200 M−1 cm−1)].
© 2007 Elsevier B.V. All rights reserved.
Keywords: 2,6-Diphenylphenol-salicylaldimines; Redox behavior; X-ray crystal structure
1. Introduction
galactose oxidase and photosystem II (PSII) [5] has simulated
considerable interest and research activity in the molecular sys-
tems involving readily oxidizable bulky phenolic fragments
[6].
Salicylaldimines (SAs) continues to be a subject of current
interest, because they are widely used as a precursor for a design
of various metal complexes, in organic synthesis and flexible
multifunctional ligands as building blocks for constructing novel
supramolecular frameworks with interesting electronic, mag-
netic, optical and catalytic properties [1]. Their fundamental
properties such as photo- and thermo-induced proton transfer
effects suggest the possibility of using these compounds as ele-
ments for constructing the optical switches or optical memory
devices [2]. Furthermore, the subtle factors such as ␲· · ·␲ and
CH· · ·␲ interactions appearing in SA, have also been impor-
tant in a variety of molecular phenomena at physical, chemical
and biological levels [1c,3,4]. The realization of the widespread
occurrence of phenoxyl type tyrosyl radicals in the chemistry
of living system enzymes such as ribonucleotide reductase,
The coordination chemistry of SA ligands bearing readily
oxidizable terminal bulky 2,6-tBu2-1-hydroxyphenyl group on
the imine nitrogen, 4-(N-salicylaldimine)-2,6-tBu2phenol and
their transition metal complexes, which are oxidized with PbO2
producing stable phenoxyl radical complexes are reported by
one of us in 1981 [7a–c]. Although, the chemistry of bidentate
SA and other ligands having peripherally bonded sterically hin-
dered 2,6-tBu2-1-hydroxyphenyl arms and their transition metal
complexes are well studied, their 2,6-di-phenylphenol bearing
SA analogous are not reported in the literature. In continuation
to our systematic study of the chemistry of sterically demanded
redox active SA ligands and their transition metal complexes
[7,8], we present studies on synthesis, spectroscopy, structural
characterization and redox behavior of a new series of a bulky
SA (X) prepared on the basis of salicylaldehyde derivatives
and sterically hindered 2,6-di-phenyl-p-aminophenol in which
electron-donating tert-butyl groups on 2,6-di-tert-butylphenolic
∗
Corresponding author.
E-mail address: vkasumov@harran.edu.tr (V.T. Kasumov).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.07.021

V.T. Kasumov et al. / Spectrochimica Acta Part A 70 (2008) 60–68
61
R-salicylaldimines and PbO2 under high vacuum (10−3 to
10−4 mmofHg)ina25 mlvesselequippedwitha3–4 mmquartz
tubes at one end for taking ESR spectra. Cyclic voltammetry
was performed using a standard three-electrode configura-
tion with a platinum working and counter electrodes and an
Ag/AgCl reference electrode. Crystals of 7 (R = 3-tBu) suit-
able for X-ray analysis were obtained by crystallization from
methanol. The m/z (%) data main peaks extracted from ESI
MS spectra for some X was as follows. LC–MS/MS m/z (%)
peaks for 2: 380(80) [M + H]+, 291(15), 242(20), 154(42),
149(1 0 0), 108(35); for 3: 380(1 0 0) [M]+, 363(8), 272(4),
221(4), 118(20); for 4: 395(1 0 0) [M]+, 380(7), 366(7), 352(7),
254(17), 248(17), 222(3), 137(3), 107(3); for 5: 396(1 0 0)
[M + H]+, 380(20), 248(15), 152(57), 130(20); for 6: 396(1 0 0)
[M + H]+, 380(6), 302(12), 262(12), 138(4); for 7: LC–MS/MS
m/z (%); m/z = 422(1 0 0) [M]+, 380(4), 221(3), 148(8), 120(8),
116(22); for 8: 422(1 0 0), 395(2), 302(2), 221(2).
Scheme 1.
residue was replaced by electron-withdrawing phenyl arms as
shown in Scheme 1.
2. Experimental
2.1. Physical property measurements
XRD data are collected on a STOE IPDSII image plate detec-
˚
The experimental procedures and instruments used for ana-
lytical, spectroscopic [1H, (13C) NMR, IR, UV–vis and ESR]
and X-ray crystallographic measurements as well as chemical
and electrochemical oxidations were carried out as described
recently [7c,d]. The electrospray mass spectra were carried out
on a Micromass Quattro LC–MS/MS in methanol solution.
tor using Mo K␣ radiation (λ = 0.71019 A) from a suitable single
crystal mounted on a glass fiber. Crystallographic data for com-
pound 7, 4-(N-3-tBu-salicylideneimine)-2,6-di-phenylphenol,
are given in Table 2. Data collection: Stoe X-AREA [12]. Cell
refinement: Stoe X-AREA [12]. Data reduction: Stoe X-RED
[12]. The structure was solved by direct methods using SIR97
[13] and anisotropic displacement parameters were applied to
non-hydrogen atoms in a full-matrix least-squares refinement
based on F2 using SHELXL-97 [14]. All the hydrogen atoms of
methylandmethylenewerepositionedgeometricallyandrefined
by a riding model with Uiso 1.2 times that of attached atoms.
Molecular drawings were obtained using ORTEP-III [15].
2.2. Preparation of compounds
All the solvents were reagent grade and used without further
purification. The reagent grade chemicals, 2,6-di-phenylphenol,
2,4-di-t-butylphenol, salicylaldehyde, CH3O–, CH3–, C(CH3)3-
substituted salicylaldehydes, 2-hydroxynaphthaldehyde and
tetraethylammonium tetrafloroborat (TEATFB) were pur-
chased from Aldrich Chemical Co. The reagents 3,5-
tBu2-salicylaldeyde [9] and 2,6-di-phenyl-p-aminophenol [10]
were synthesized according to published procedures. 2,6-
Di-phenylphenol-salicylaldimines (X) were prepared using
standard procedures involving the condensation of equimolec-
ular amounts of appropriate salicylaldehyde derivatives with
p-amino-2,6-di-phenylphenol as described earlier [11]. The
analytical and physical data for X compounds are pre-
sented in Table 1. The oxidation of X were carried out
by mixing of degassed solutions of N-2,6-di-phenylphenol-
3. Results and discussion
3.1. UV–vis, IR, 1H NMR, 13C NMR and mass
spectroscopic characterization of X
The X SA was obtained as yellow solids in good yields and it
is not air sensitive. Their structure were identified by satisfactory
microanalyses, UV–vis, IR, H NMR, 13C NMR and ESI MS
1
techniques, as well as X-ray structural descriptions of 7 [R = 3-
C(CH3)3] indicates its existence as the non-planar enol–imine
Table 1
Physical and analytical data for X compounds
Compound
Yield (%)
mp (◦C)
Formula
Elemental analysis, found/calcd. (%)
C
H
N
1
2
3
4
5
6
7
8
9
92
91
89
93
91
93
89
88
78
95
124
110
113
164
160
177
204
135
169
210
C
25H19NO2
C26H21NO2
C26H21NO2
83.18/3.83
5.53/5.24
5.48/5.57
5.80/5.57
5.56/5.35
5.48/5.35
5.58/5.35
6.57/6.46
6.59/6.46
7.63/7.38
5.31/5.09
3.65/3.83
3.56/3.68
3.42/3.68
3.13/3.54
3.33/3.54
3.21/3.54
2.78/3.32
2.88/3.32
2.72/2.93
3.44/3.37
81.68/82.25
81.68/82.25
79.91/78.92
77.57/78.92
77.50/78.92
81.96/82.63
81.06/82.63
81.84/82.98
82.58/83.29
C26H21NO3
C26H21NO3
C
26H21NO3
C29H27NO2
29H27NO2
C33H35NO2
29H21NO2
C
10
C

62
V.T. Kasumov et al. / Spectrochimica Acta Part A 70 (2008) 60–68
Table 2
1-hydroxyphenyl bearing SA analogs. In particularly, in the
Crystal data and structure refinement for compound 7
complexations of X with Cu(II) unlike their 2,6-ditertbutylated
analogs [7b,c], no any oxidative C–C coupling of uncoordinated
X ligands were observed.
Formula
C29H27NO2
Formula weight
Temperature (K)
Wavelength (Mo K␣)
Crystal system
Space group
421.52
297(2)
0.71073
Triclinic
P − 1
Characteristic IR absorption frequencies of X are given
in Table 5. The strong bands observed in the region of
1613–1618 cm−1 and a broad absorption at ca. 2600–2700 cm−1
in the IR spectra of X are attributed to C N and intramolecu-
lar hydrogen bonded νOH (O–H. . .N), respectively (Table 5).
It is interesting that the characteristic νOH stretching of pheno-
lic hydroxyl group except CH3O substituted 4–6 which exhibit
absorptions at 3377, 3430 and 3386 cm−1, respectively, were
appeared in the range of 3505–3535 cm−1. The observed lower-
ing of νOH in 4–6 suggests the existence of the intermolecular
CH3O· · ·H–O interactions in these compounds.
Unit cell dimensions
a, b, c (A)
α, β, γ (◦)
Volume (A )
Z
˚
9.1780(8), 10.6136(9), 12.5465(11)
89.154(7), 71.657(6), 80.076(7)
1141.70(17)
2
3
˚
Calculated density (g cm−3
μ (mm−1
F (0 0 0)
Crystal size (mm)
θ range (◦)
)
1.2262(2)
0.076
448.0
)
The electronic spectra of X recorded in ethanol (Table 5)
and MeCN (Table 6) are significantly different. As can be seen
from Table 6 in MeCN, except 10 no absorption features at
longer wavelength (λ > 380 nm) for all SA compounds were
observed. Except 10 the similar spectral patterns also were
observed even in the case of higher concentrated THF solutions
of SA (X) in the range of 260–267, 315–335 and 351–375 nm
regions. The absorptions <380 nm are assigned to intraligand
␲ → ␲* and n → ␲* transitions in the enol–iminic tautomers.
The low intense bands (ε = 102–558 M−1 cm−1) in the range
of 420–467 nm region (Table 5) detected in EtOH solutions is
attributed to n → ␲* transitions in the dipolar zwitterionic or
ketoamine tautomeric structures of X [16].
0.18 × 0.28 × 0.34
1.95, 27.98
Index ranges
−12 ≤ h ≤ 12, −13 ≤ k ≤ 13, −16 ≤ l ≤ 16
Reflections collected
Independent reflections
Reflections observed (>2σ)
Absorption correction
Refinement method
Data/restrains/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
13539
5420 (Rint = 0.047)
3474
Integration
Full-matrix least-squares on F2
5420/0/305
1.030
0.043
0.150, −0.140
Largest differe−n3ce in peak
˚
and hole (A
)
tautomeric form in solid state and in non-polar and low polar
solvents. The assignments of 1H NMR and 13C NMR spectra of
X are presented in Tables 3 and 4. The m/z data extracted from
electrospray mass spectroscopic (ESI MS) spectra of some X
compounds are presented in Experimental section. ESI MS spec-
tra of X scanned m/z from 500 to 100 exhibit well-resolved peaks
corresponding to the parent molecular ions ([M]+ and [M + H]+)
and cationic fragment ions due to losses of Ph, OCH3, biphenyl
and other groups (see Section 2). The representative ESI MS
spectrum of 5 and assignment of main cationic fragment ions
peaks is presented in Fig. 1. The m/z values of the parent molec-
ular ions are in accord agreement with the proposed formula in
Scheme 1. Our preliminary examination indicates that the com-
plexation reactivity’s of X with Cu(II), Co(II), Ni(II), Pd(II)
and Zn(II) metal ions are different from those for 2,6-tBu2-
3.2. Crystal structure description of 7
Themolecularstructuresof7, withtheatom-labelingscheme,
are shown in Fig. 2, selected bond lengths and angles are given
in Table 7. The molecular structure of 7 can be described
as consisting of three structural fragments; tert-butyl- and
phenyl-substituted phenol cores divided by central part, which
contains C N double bond. The central part of the molecule
is characterized by pseudoaromatic six-membered chelate ring,
which contains OH–C C–C N (phenol–imine tautomer) het-
erodienic moiety. In 7, the phenol–imine tautomer is favored
over the keto-amine form in the solid state. This is evi-
˚
dent from the observed O2–C21 bond distance of 1.352(2) A,
which is consistent with an O–C single bond character in
Table 3
1H NMR spectral data of X compound (δ ppm)
Compound
OH/NH
CH
N
OH
OCH3
Sal.PhH
PhH (m)
CH3
tBuH (s)
1
2
3
4
5
6
7
8
9
13.40
13.68
13.13
13.73
13.83
12.90
14.06
13.15
13.72
15.65
8.66
8.67
8.65
8.72
8.59
8.66
8.69
8.70
8.71
5.49
5.47
5.48
5.48
5.45
5.50
5.44
5.41
5.45
5.56
–
–
–
3.92
3.84
3.80
–
–
–
6.87–7.03 m
7.26–7.61
7.42–7.61
7.42–7.63
7.42–7.63
7.42–7.63
7.42–7.63
7.35–7.62
7.42–7.62
7.28–7.58
7.35–7.82
–
2.31
2.32
–
–
–
–
–
–
–
–
–
–
–
–
1.47
1.32
7.19–7.23, 6.83
7.16–7.26, 6.93 d
6.87–7.02, 7.26
6.48 d, 4.24 m
6.88, 6.97, 7.76
6.86 t, 7.20–7.21
7.28–7.39, 6.60 d
7.20, 7.21 d
1.32, 1.49
–
10
9.39, 8.11
–
8.1d, 7.22 d, 7.28 d
–

V.T. Kasumov et al. / Spectrochimica Acta Part A 70 (2008) 60–68
63
Table 4
13C NMR chemical shifts δ (ppm) for X in CDCl3
Compound
CH
N
Ar–C
CH3
OCH3
C(CH3)3
C(CH3)3
1
160.9
160.7, 148.5, 141.1, 136.9, 132.1, 131.9, 129.6, 129.2,
128.9, 127.9, 122.5, 119.2, 118.9, 117.1
159.3, 148.5, 141.3, 136.9, 133.8, 129.7, 129.6, 129.3,
129.0, 128.1, 126.2, 122.6, 118.5
158.8, 158.5, 141.5, 137.0, 133.7, 132.1, 129.6, 129.3,
129.0, 128.1, 122.6, 119.0, 117.0
151.2, 148.6, 148.4, 141.1, 136.3, 129.7, 129.3, 129.0,
128.1, 123.6, 122.6, 119.2, 118.5, 114.5
160.0, 148.2, 141.3, 137.1, 133.3, 129.6, 129.3, 128.9,
128.0, 122.4, 107.1
155.3, 152.2, 148.6, 141,3, 136.9, 129.7, 129.3, 129.0,
128.1, 122.6, 120.1, 118.9, 117.9, 115.1
160.5, 148.4, 141.2, 137.6, 137.0, 130.4, 130.1, 129.6,
129.3, 129.0, 128.1, 122.6, 119.2, 118.3
158.7, 148.4, 141.6, 141.4, 136.9, 130.3, 129.6, 129.2,
128.9, 128.4, 127.9, 122.5, 118.5, 116.7
158.11, 148.26, 141.30, 140.41, 136.99, 136.82, 129.63,
129.22, 128.90, 127.91, 127.62, 126.60, 122.52, 118.36
148.3, 138.7, 136.8, 136.0,133.1, 129.9, 129.3, 129.1,
128.2, 127.9, 127.3, 123.4, 121.8, 119.0
–
15.6
20.4
–
–
–
–
2
3
4
5
6
7
8
9
161.0
160.9
161.0
163.7
160.6
161.5
161.3
161.8
154.2
–
–
–
–
–
–
56.2
–
–
–
55.5
55.9
–
–
–
–
–
–
–
34.9
29.4
–
–
33.9
31.4
–
–
35.1, 34.1
–
31.5, 29.4
–
10
–
–
the enol–imine tautomer form; in addition, the N1–C19 dis-
tance of 1.280(2) A is in accordance to lengths of double C N
the substituted phenyl and tert-butyl groups will probably causes
the phenol moieties to rotate out conjugation with each other,
thus leading to a loss of planarity. The dihedral angle of two
phenols mean plane (C1/C6 and C20/C25) is 24.23(6)◦. The
dihedral angles between the phenol (C1/C6) and the phenyl
groups (C13/18 and C7/C12) mean planes are 69.75(5)◦ and
46.88(6)◦ while that between the phenyls (C13/18 and C7/C12)
mean planes is 49.62(6)◦. In the chelate ring two other C–C
˚
bonds, as in N-(m-hydroxyphenyl)-5-methoxysalicylaldimine
˚
˚
dO–C = 1.362(2) A and dC N = 1.279(2) A [17]. Salicylaldimine
part of structure preserves planarity by intramolecular hydro-
gen bond formation, whereas the phenyl-substituted phenol
ring twisted around N1–C1 single bond [torsion angles of
C19–N1–C1–C6 and N1–C19–C20–C21 are 21.5(2)◦ and
−1.3(2)◦, respectively]. Thesetorsionanglessuggestdelocaliza-
tion of electron density between the aromatic phenols. However,
˚
bonds are different; C19–C20 bond distance [1.452(2) A] is
˚
longer than C20–C21 bond distance [1.407(2) A].
Fig. 1. Electrospray mass spectrum of a methanol solution of 5.

64
V.T. Kasumov et al. / Spectrochimica Acta Part A 70 (2008) 60–68
Table 5
IR (cm−1) and electronic absorption spectral data for X compound
Compound
IR spectra
νCH
Electronic spectra λmax (log ε, M−1 cm−1
)
νOH
N
1
2
3
4
5
6
7
8
9
1618
1613
1618
1614
1617
1617
1614
1618
1615
1619
3532
3505
3508, 3534
3377
3430
3386
3533
3539
3535
208(4.78), 267(4.53), 350a, 355(4.36), 440 nm (2.44)
210(4.7), 225(4.7), 268(4.6), 322(4.4), 354(4.4), 447(2.6)
210(4.7), 233(4.73), 267(4.42), 313a, 332a, 447a(2.33)
209(4.7), 230(4.83), 269(4.65), 339(4.28), 453(2.73)
210(4.67), 235(4.66), 348(4.53), 419(3.78), 456(2.77)
209(4.7), 235(4.7), 348(4.5), 343(4.3), 369(4.3), 468(2.1)
230(4.72), 268(4.63), 330(4.42), 355(4.46), 447(2.07)
208(4.64), 232(4.63), 267(4.49), 358(4.29), 447(2.37)
230(4.76), 271(4.60), 336(4.38), 361(4.40), 480a
234(4.69), 316(4.15), 396(4.19), 467(3.91)
10
3524
aShoulder.
dimer of 7 in the crystal unit cell. For the C23–H23· · ·Ai con-
tact, the distance between atom H23 and the centre of the phenyl
ring (ring Ai: C13–C18; symmetry code (i) −x, 1 − y, 1 − z) is
The six-membered pseudoaromatic chelate OH–C C–C N
ring is formed by heteronuclear intramolecular hydrogen bond
O–H· · ·N which is enhanced by delocalization of the ␲-
conjugated system (resonance assisted hydrogen bond, RAHB).
RAHB is formed between the phenol O–H group and imine
˚
3.103 A, the distance between atom H23 and the plane of ring A
i
◦
˚
is 3.018 A, and the C23–H23· · ·A angle is 141.42 . Dimers are
˚
N atom [O–H· · ·N (O1· · ·N1: 2.575(2) A)]. The N1· · ·O1 dis-
assembled by short contact between the two symmetry related
ii
tance in the intramolecular hydrogen bond is in accordance to
lengths of RAHB heteronuclear intramolecular hydrogen bonds
˚
O1 atoms [O1· · ·O1 : 3.735(2) A; symmetry code (ii) 2 − x, −y,
1 − z], forming polymeric chain in the ab plane. In addition there
˚
in ␤-enaminones and related heterodienes (2.5–2.6 A) [18]. The
is also strong short contact between symmetry related C28 and
˚
iii
ii
proton is localized near the oxygen atom dO–H = 0.96(2) A. The
˚
O2 atoms [C28 · · ·O2 : 3.508(2) A; symmetry code (iii) 1 − x,
OHN angle of 152.1(18) is typical for phenol–imine tautomer
structures.
1 − y, −z; which forms centrosymmetric dimer of (I).
In the extended structure, there are no intermolecular hydro-
gen bonding and ␲–␲ stacking interactions. The crystal packing
of 7 is formed via C–H· · ·␲ and short contacts. It can be seen
from Fig. 3 that the two molecules are joined by two C–H· · ·␲
interactions, which leads to the formation of a centrosymmetric
3.3. Chemical oxidation of X
One-electron oxidation of 1 with PbO2 at 300 K in deoxy-
genated PhMe/CHCl3 mixtures under vacuum leads to the
formation of poorly resolved phenoxyl radical signals centered
at g = 2.0045 with a line width of 5.5 G. One-electron oxidation
of 9 under similar conditions immediately leads to the formation
Table 6
Electronic spectral data for some X and X + ACN system in acetonitrile
Compound
Solvent
Electronic spectra, λ (nm) (log ε, M−1 cm−1
)
1
MeCN
MeCN
MeCN
MeCN
MeCN
230, 268, 329, 356, 376(sh)
264(sh), 283(sh), 340(sh), 441
232(3.71), 266(3.62), 317(3.39), 352(3.42)
231a, 262, 347, 450a
231(4.690), 265(4.55), 329(sh)(4.31),
355(4.36)
250a, 340a, 450(3.21)
229(4.83), 268(4.64), 325(4.49)
255(sh), 345(4.03), 447(3.43)
231(4.61), 267(4.43), 347(4.54)
274(4.45), 361(4.19), 485a (3.27)
235(4.87), 319(4.45), 340(4.40), 365(3.27)
232a, 255a, 365(4.03), 440(3.51)
226, 266, 352, 380a
271a, 330a, 390a, 416a, 480a
234a, 266(4.54), 330(4.33), 354(3.39)
254a, 272a, 347(4.03), 450a(2.87)
230, 268, 330,357
280, 310, 490a
232(4.61), 319(4.08), 384(4.13), 438(3.65),
459(3.58)
1 + ACN
2
2 + CAN
3
3 + CAN
4
4 + CAN
5
5 + CAN
6
6 + CAN
7
7 + CAN
8
8 + CAN
9
9 + CAN
10
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
10 + CAN
MeCN
311(4.18), 356(3.92)
Fig. 2. An ORTEP representation of the molecular structure of 7 shows the
atom-numbering scheme.
aA broad shoulder.

V.T. Kasumov et al. / Spectrochimica Acta Part A 70 (2008) 60–68
65
Table 7
◦
˚
Some selected bond distances (A) and bond angles ( ) for the compound 7
Bond distances (◦)
C1–C2
C2–C3
C3–C4
C4–O1
C1–N1
C3–C7
C7–C8
C8–C9
1.385(2)
1.391(2)
1.402(2)
1.3643(2)
1.418(2)
1.487(2)
1.389(2)
1.384(2)
C9–C10
C5–C13
C2–H2
1.369(3)
1.494(2)
0.93
0.93
0.96
1.378(2)
1.383(2)
1.360(3)
C19–N1
C21–O2
C26–C28
C22–C26
C21–C22
C22–C23
C19–C20
C20–C21
1.280(2)
1.351(2)
1.528(2)
1.539(2)
1.409(19)
1.390(2)
1.450(2)
1.406(2)
C8–H8
C28–H28A
C13–C14
C14–C15
C15–C16
Bond angles (◦)
C19–N1–C1
O1–C4–C5
C28–C26–C29
C28–C26–C27
C6–C1–N1
121.90(13)
120.19(13)
107.55(14)
109.66(14)
124.62(14)
122.62(13)
C4–O1–H1A
C21–O2–H2A
O2–C21–C20
O2–C21–C22
C12–C7–C8
109.5
109.5
119.74(12)
118.66(12)
118.03(15)
118.10(14)
C1–C2–C3
C3C4–C5
C2–C1–C6
C21–C20–C19
N1–C19–C20
C25–C20–C21
122.39(13)
120.95(13)
118.46(13)
121.70(13)
122.18(14)
119.04(13)
C4–C3–C7
C18–C13–C14
of the phenoxyl radical species displaying a well-resolved ESR
spectrum (g = 2.0045) with hfsc of 1.125 G at the central part of
the spectrum (Fig. 4). Both sides of this spectrum exhibits a low
intense of a broad satellite lines spacing of ca. 4 G, probably,
due to coupling to 13C (I = 1/2) isotopes in natural abundance
have been observed. We were unsuccessful to generate phenoxyl
radicals from other X by using of PbO2 as oxidizer. But our
preliminary studies revealed that the above compounds could
be readily oxidized by (NH4)2[Ce(NO3)6] (ACN) in MeCN,
CHCl3, EtOH and other solvents. Recently, we demonstrated
that ACN could be used as good oxidant for oxidation of some
sterically demanded SA and their metal complexes [7g,8d].
The spectral changes for X + ACN systems were monitored
by UV–vis spectroscopy during the progress of the oxidation of
X in solvents. As soon as ACN and X were soluble in MeCN the
oxidation of the ligands was carried out with one equiv. molar
amount of ACN. The results of these experiments show that upon
mixing of equiv. molar amount of X SA and ACN in MeCN,
their yellow color simultaneously or for 5–10 min turns to red
along with the disappeared initial long wavelength absorption
bands at ca. 325–380 nm of non-oxidized X, and a new maxi-
mum peak appears around 440–490 nm for oxidized X (Table 6).
On standing the red color of these oxidized systems remains
unchanged for ca. 2–3 in air and then gradually converted to light
yellow. Note that the observed a strong absorption at ca. 430 nm
detectedinthespectraoftheoxidizedXcompounds(Table6)are
characteristic for phenoxyl type radicals (PhO•+H, PhO•) gen-
erated by chemical or electrochemical one-electron oxidation of
Fig. 3. View of the packing in 7 along the axis shows the possible intermolecular short contact interactions O· · ·O, C· · ·O and C–H· · ·␲ contacts (dashed lines) at
10% probability displacement ellipsoids.

66
V.T. Kasumov et al. / Spectrochimica Acta Part A 70 (2008) 60–68
Scheme 2.
representative voltammograms of X are presented in Fig. 5. It
is interesting to note that a single quasi-irreversible reductive
wave observed for 5–8 in the initial first scan with E1/2 at
−0.96(5), −0.98(6), −0.97(7) and −0.99(8) V (Fig. 5a), is
converted to another CV waves in the subsequent second
and third scanning (Fig. 5b). This behavior suggests that the
initially reduced species are unstable or they are involved
to side irreversible chemical process on the timescale of the
electrochemical experiment [19]. The anodic and cathodic
redox processes for the presented X compounds are strongly
dependent upon scan rate (Fig. 5e and f). The analyses of the
voltammograms indicate that with the increasing of sweep rates
from 0.02 to 1.2 V s−1 of the values of the peak potentials,
the peak currents and the standard potential determined as
(Epc + Epa)/2 are increased. They also tend to become diffusion
controlled as evident from the linearity of the ipc versus ν1/2 plot
passing through the origin. The ipc/ipa ratios are less (0.4–0.7)
than unity and increases as the scan rate is increased. The CV of
Fig. 4. ESR spectrum of radical generated from 1 in toluene at 300 K. Exper-
imental conditions: frequency, 9.52 GHz; microwave power, 5 dB; modulation
amplitude, 0.8 G; gain, 630; scan range, 100 G.
PhOH fragments [6b,d]. The absorption at ca. 440–490 nm is
assigned to a ␲ → ␲* transition of the phenoxyl radical ligand,
X•, as described in UV–vis spectra of the oxidized bulky phenol
bearing ligands and their complexes, in which the existence
of phenoxyl radicals were demonstrated by ESR spectroscopy
[6,7e,g,8d].
We have found that some OCH3 and t-Bu substituted
ligands (6–8), when they are oxidized by ACN in CHCl3,
even in the presence of atmospheric O2 condition, their ESR
spectra at 300 K exhibit low resolved radical signal with g-
factors of 2.003–2.004 and line widths, ꢀH = 6.0–6.7 G. For
other ligands under similar conditions no radical species were
detected. It is interesting that in the chemical oxidation of N-
alkyl(cycloalkyl)-3,5-tBu-salicylaldimines and their complexes
under above oxidative conditions, to generate uncomplexed and
coordinated phenoxyl radical species which display absorption
band around 720 nm [7e,8d].
t
the OCH3 and Bu substituted X exhibit irreversible oxidation
peak centered at ca. 0.0 V (Fig. 5b and e). The CV waves
observed in the range of −1.85 to 0.0 V region are tentatively
assigned to the reduction of the imine moiety of the molecule
as suggested by Dmitreva et al. [20] and Scott [21] is shown in
Scheme 2.
The second step is chemically irreversible because of fast
protonation of the dianion (Eq. (3)) where DH is any proton
donor in solution. Oxidation of the resulting monoanion is often
observed at the potential of more positive than the first reduc-
tion peak [21]. The electrochemical reduction of SA involves
hydrogenation of CH N bond, leading to saturated amines
[19,20]. The overall two-electron process gives rise to a sin-
gle reduction wave, but for some N-benzylideneanilines, a wave
splitting is invariably observed [19–21]. This behavior suggests
that the latter waves are originated from the two-electron reduc-
tion intermediates in Scheme 2 or of their conjugate bases [20].
The CV of X recorded in the anodic potential range from 0.0
to 1.75 or −1.5 to +1.85 V versus Ag/AgCl exhibits various
3.4. Electrochemistry of X
The electrochemical electron transfer behavior of X was
studied by cyclic voltammetry technique using a Pt working
electrode in acetonitrile (MeCN) in the potentials ranging
from −1.85 to +1.85 V versus Ag/AgCl at 293 K under N2
containing Et4NBF4 (0.05 M) as the background electrolyte.
The main electrochemical data are given in Table 8 and some
Table 8
Cyclic voltammetric potentials (in V vs. Ag/AgCl) for some X SA in MeCN
E11/2
ꢀE
Ep2a/Ep2c
E12/2
ꢀE
Ep3a/Ep3c
E
E
pc
ir
ir
Compound
Ep1a/Ep1c
pa
2
3
4
5
6
7
8
9
10
−0.23/−0.67
−0.63/−1.05
−0.85/−1.31
−1.05/−0.56
−1.04/−0.61
−0.74/−1.15
−0.59/−1.18
−0.89/−1.03
−1.36/−1.02
−0.45
−0.84
−1.08
−0.81
−0.84
−0.94
−0.89
−0.96
−1.19
0.44
0.42
0.46
0.49
0.40
0.41
0.59
0.14
0.34
1.26/1.07
1.08/0.88
0.08/−0.02
1.67/1.44
1.06/0.94
1.28/1.04
1.29/1.16
−0.44/−0.73
1.39/1.08
1.16
0.98
0.03
1.56
1.0
1,16
1.23
−0.58
1.24
0.19
0.20
0.10
0.23
0.12
0.24
0.13
0.29
0.31
–
–
–
–
–
–
1.01
0.72, 1.52
1.03
1.08, 1.31
0.9, 1.1, 1.5
−0.36, 1, 16, 1.67
0,02, 1.05
–
−0.28
0.43
−0.76
–
0.28
–
0.33
0.39
–
−0.22/−0.38
–
−0.34/−0.44
0.05

V.T. Kasumov et al. / Spectrochimica Acta Part A 70 (2008) 60–68
67
Fig. 5. Representative CV of X: 6 (a) initial; (b) third scan waves; (c) 6 from −1.5 to +1.85 V range; (d) 5 from 0.0 to 1.85 V; 3 anodic waves with scan rates of
0.4 V s−1; (e) 0.02 V s−1; (f) all other CV recorded at a scan rate of 0.1 V s−1 in MeCN containing [Net4][BF4] at 290 K.
quasi-reversible and irreversible oxidation process (Fig. 5c–f
and Table 8). The observed I–III curves in this voltammograms
were assigned to an irreversible anodic oxidation process of
phenolate/phenoxyl couples. This scan rate dependent behav-
ior appears typical of CE process. Taking into account that the
investigated X compounds involve two different oxidasible phe-
nolic OH and their oxidation could generate PhO•, and because
that this radical is easier to oxidize than the parent phenol and
undergoes a one-electron oxidation to produce the phenoxonium
ion, [PhO•+] [22], the complexity of the anodic process is not
surprise.
quasi-reversible redox waves in the anodic and cathodic regions
due to the oxidation and reduction of phenolic and iminic
groups, respectively.
Supplementary data
Crystallographic data (excluding structure factors) for the
structure in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre as the supplementary
publication no. CCDC 632165. Copies of the data can
be obtained free of charge, on application to CCDC, 12
Union Road, Cambridge, CB12 1EZ, UK, fax: +44 1223
366 033, e-mail: deposit@ccdc.ac.uk or on the web www:
http://www.ccdc.cam.ac.uk.
4. Conclusions
A
series of new redox-active bulky 4-(N-R-
Acknowledgement
salicylideneimine)-2,6-di-phenylphenols chelating compounds
have been synthesised and characterized by structural and
spectroscopic techniques, as well as their chemical and elec-
trochemical electron transfer properties were studied. The
presented compounds have non-planar enol–imine tautomer
structure in solid state as evidenced by an X-ray crystallo-
graphic study of one of the salicylaldimines, 7. As evidenced
by EPR and UV–vis studies of the chemical oxidation of
the presented compounds to generate stable phenoxyl radical
species. Their CV display rate dependent on irreversible and
Financial support of this research by the Harran University
Research Fund (Grant No. HUBAK 385) is gratefully acknowl-
edged.
¨
References
[1] (a) D. Philip, J.F. Stoddart, Angew. Chem. Int. Ed. Engl. 35 (1996) 1154;
(b) H. Adams, F.J. Carver, C.A. Hunter, J.C. Morales, E.M. Seward, Angew.
Chem. Int. Ed. Engl. 35 (1996) 1542;

68
V.T. Kasumov et al. / Spectrochimica Acta Part A 70 (2008) 60–68
(c) J.-M. Lehn, Supramolecular Chemistry, Concepts and Perspectives,
VCH, Weinheim, 1995.
[2] (a) M. Cohen, Y. Hirshberg, G.M. Schmidt, J. Chem. Soc. B (1964) 205;
(b) D.H. Heinert, A.E. Martell, J. Am. Chem. Soc. 85 (1963) 188;
(c) G.M.J. Schimidt, in: D. Ginsburg (Ed.), Solid State Photochemistry,
Verlag Chemie, Weinheim, New York, 1976;
[8] (a) V.T. Kasumov, Z. Naturforsch. 56b (2001) 263;
(b) V.T. Kasumov, Spectrochim. Acta Part A 57 (2001) 1649;
(c) V.T. Kasumov, F. Ko¨ksal, Z. Anorg. Allg. Chem. 627 (2001) 2553;
(d) V.T. Kasumov, F. Ko¨ksal, A. Sezer, Polyhedron 24 (2005) 1203.
[9] J.F. Larrow, E.N. Jacobsen, Y. Gao, Y. Hong, X. Nie, C.M. Zepp, J. Org.
Chem. 59 (1994) 1939.
(d) E. Hadjoudis, M. Vittoakis, I.M. Mavridis, Tetrahedron 43 (1987) 1345;
(e) S.T. Kobayahsi, T. Inabe, J. Phys. Chem. A 101 (1997) 644.
[3] (a) N. Yoshida, K. Ichikawa, M. Shiro, J. Chem. Soc., Perkin Trans. 2
(2000) 17;
[10] B.R. Osterby, R.D. McKelvey, J. Chem. Educ. 73 (1996) 260.
[11] V.T. Kasumov, F. Koksal, Trans. Metal. Chem. 28 (2003) 888.
[12] Stoe and Cie X-AREA (Version 1.18) and X-RED32 (Version 1.04), Stoe
and Cie, Darmstadt, Germany, 2002.
(b) P.E. Kruger, N. Martin, M. Nieuwenhuyzen, J. Chem. Soc., Dalton
Trans. (2001) 1966.
[4] M. Nishio, M. Hirota, Y. Umezawa, The CH/␲ Interaction—Evidence,
Nature and Consequences, Wiley–VCH, New York, 1998.
[5] (a) C. Stevens, Z.B. Ogel, J.N. Keen, K.D.S. Yadav, P.F. Knowles, Nature
350 (1991) 87;
[13] A. Altomare, M.C. Burla, M. Camalli, G.L. Cascarano, C. Giacovazzo, A.
Guagliardi, A.G.G. Moliterni, G. Polidori, R. Spagna, J. Appl. Cryst. 32
(1999) 115.
[14] G.M. Sheldrick, SHELXL-97, University of Gottingen, Germany, 1997.
[15] M.N. Burnett, C.K. Johnson, ORTEPIII Report ORNL-6895, Oak Ridge
National Laboratory, Tennessee, USA, 1996.
(b) N. Ito, S.E. Phllipps, K.D.S. Yadav, P.F. Knowles, J. Mol. Biol. 238
(1994) 794;
[16] (a) H.H. Jaffe, S.J. Yeh, R.W. Gardner, J. Mol. Spectrosc. 2 (1958) 120;
J. Charette, Spectrochim. Acta 23A (1963) 597.
[17] Z. Popovic´, G. Pavlovic´, D.M. Calogovic´, V. Roje, J. Mol. Struct. 23 (2002)
´
(c) M.M. Wittaker, P.J. Kersten, N. Nakamura, J. Sanders-Loehr, E.S.
Schwezer, J.W. Wittaker, J. Biol. Chem. 271 (1996) 681;
(d) J. Stubbe, W.A. van der Donk, Chem. Rev. 98 (1998) 705.
[6] (a) A. Sokolwski, H. Leitbecher, T. Weyhermu¨ller, R. Schnepf, E. Bothe,
E. Bil, K. Wieghardt, J. Biol. Inorg. Chem. 2 (1997) 1407;
(b) J. Mu¨ller, T. Weyhermu¨ller, E. Bill, P. Hildebrandt, L. Ould-Moussa, T.
Glaser, K. Wieghardt, Angew. Chem. Int. Ed. Engl. 37 (1998) 616;
(c) J.-L. Pierre, Chem. Soc. Rev. 29 (2000) 251;
615.
[18] P. Gilli, V. Bertolasi, V. Ferretti, G. Gilli, J. Am. Chem. Soc. 122 (2000)
1262.
[19] (a) A.A. Isse, A. Maye, E. Vianello, J. Electroanal. Chem. 431 (1997)
249;
(b) A.A. Isse, E. Gennarao, Vianello, Electrochim. Acta 42 (1997)
2065;
(d) B.A. Jazdewski, W.B. Tolman, Coord. Chem. Rev. 200–202 (2000) 633.
[7] (a) A.A. Medjidov, V.T. Kasumov, H.S. Mamedov, Russ. J. Coord. Chem.
7 (1981) 66;
(c) A.A. Isse, E. Vianello, A.M. Abdurrahman, J. Chem. Soc., Perkin Trans.
2 (1996) 597.
[20] (a) V.N. Dmitreva, N.I. Maltseva, V.D. Bezuglyi, B.M. Krasovitskii, Zh.
Obshch. Khim. 37 (1967) 372;
(b) A.A. Medjidov, V.T. Kasumov, H.S. Mamedov, V.S. Aliyev, Dokl.
USSR 283 (1981) 866;
(c) M.K. Guseyinova, V.T. Kasumov, A.A. Medjidov, H.S. Mamedov, J.
Struct. Chem. 22 (1981) 90;
(d) V.T. Kasumov, A.A. Medjidov, Russ. J. Coord. Chem. 21 (1995) 783;
(e) V.T. Kasumov, A.A. Medjidov, N. Yaylı, Y. Zeren, Spectrochim. Acta
Part A 60 (2004) 3037;
(f) V.T. Kasumov, A. Bulut, F. Ko¨ksal, M. Aslanog˘lu, I. Uc¸ar, C. Kazak,
Polyhedron 25 (2006) 1133.
(b) L.V. Konenko, V.D. Bezuglyi, V.N. Dmitreva, Zh. Obshch. Khim. 38
(1968) 2153.
[21] (a) J.M.W. Scott, W.H. Jura, Can. J. Chem. 45 (1967) 2375;
(b) J.E. Kuder, H.W. Gibson, D. Wychick, J. Org. Chem. 40 (1975) 875.
[22] (a) E.R. Altwicker, Chem. Rev. 67 (1967) 475;
(b) J.A. Richards, P.E. Whistan, D.H. Evans, J. Electroanal. Chem. 63
(1975) 311;
(c) F.G. Chang, J. Am. Chem. Soc. 113 (2002).