Synthesis, Staructure and Redox Properties of Cu(II) Chelate Complexes on the Basis of 2-(Hydroxyphenyl)-1H-benzo[d]imidazol-1-yl Phenol Ligands

Article abstract of DOI:10.1002/ejic.202100184

A series of 2-(hydroxyphenyl)-1H-benzo[d]imidazols bearing readily oxidizable 2,6-di-tert-butyl-phenol substituents and bis-chelate Cu(II) complexes based on these ligands were synthesized. The structure of the compounds was characterized by NMR, IR, mass

Full text of DOI:10.1002/ejic.202100184

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Synthesis, Staructure and Redox Properties of Cu(II) Chelate
Complexes on the Basis of 2-(Hydroxyphenyl)-1H-benzo[d]
imidazol-1-yl Phenol Ligands
Eugeny P. Ivakhnenko,*[a] Pavel A. Knyazev,[a] Yulia G. Vitkovskaya,[a] Leonid D. Popov,[b]
Konstantin A. Lyssenko,[c] Oleg P. Demidov,[d] Andrey G. Starikov,[a] Gennady S. Borodkin,[a]
and Vladimir I. Minkin[a]
A series of 2-(hydroxyphenyl)-1H-benzo[d]imidazols bearing
readily oxidizable 2,6-di-tert-butyl-phenol substituents and bis-
chelate Cu(II) complexes based on these ligands were synthe-
sized. The structure of the compounds was characterized by
NMR, IR, mass spectroscopy and X-ray crystallographic analysis
and redox behavior explored using ESR spectra and cyclic
voltammetry. Under oxidation with PbO2 in toluene solution
both the ligands and Cu(II) complexes form stable phenoxyl-
type radicals. As shown by ESR measurements and DFT
calculations, no exchange interaction exists between unpaired
electrons located on metal and oxygen centers in the biradical
Cu (II) complexes. The ligands and complexes reveal antioxidant
radical scavenging activity measured spectrophotometrically
and using ESR and DPPH-tests.
Introduction
tially hindered phenol moieties efficiently inhibiting oxidative
stress. Metal coordination changes the redox potential and
additionally affects its antioxidant capacity.[11] Therefore, the
goals of the present work consisted in the synthesis, structural
characterization of Cu(II) complexes with the 2,6-di-tert-butyl-
phenol-containing ligands and comparative study of redox
activity and stability of the corresponding phenoxyl radicals
formed in the oxidation. As the appropriate ligands bearing 2,6-
di-tert-butylphenol pendants we have chosen 2,6-di-tert-butyl-
Metal complexes with ligands functionalized with readily
oxidizable 2,6-di-tert-butylphenol moieties have drawn signifi-
cant interest as active antioxidants, antiphlogistic specimens,
radioprotectors and absorbers of various reactive oxygen
species (ROS).[1–4] The central metal ions of such type coordina-
tion compounds, especially those having unpaired electrons as
Cu(II) ions, contribute to the production of phenoxyl radicals
and substantially modify their stability and reactivity.[1,5,6]
Copper is an essential element responsible for the function of
many cellular enzymes, such as cytochrome c oxidase, NADH
dehydrogenase 2, tyrosinases and proteins.[7,8] The previous
studies[9,10] revealed particularly high antioxidant and antiaggre-
gant properties of benzimidazole derivatives containing spa-
4-(2-(2-hydroxyphenyl)-1H-benzo[d]imidazol-1-yl)phenols
3
known to exhibit pronounced antioxidant and antiplatelet
activities.[9,10] The compounds were synthesized in good yields
by the reaction of quinone imines 1 with various derivatives of
salicylic aldehyde (Scheme 1) in general accordance with the
[a] Prof. Dr. E. P. Ivakhnenko, P. A. Knyazev, Y. G. Vitkovskaya,
Dr. A. G. Starikov, G. S. Borodkin, Prof. Dr. V. I. Minkin
Institute of Physical and Organic Chemistry
Southern Federal University
194/2 Stachki st., Rostov-on-Don,
344090, Russian Federation
E-mail: ivakhnenko@sfedu.ru
https://twitter.com/sfedu_official
[b] L. D. Popov
Chemistry Department
Southern Federal University
7 Zorge st., Rostov-on-Don,
344090, Russian Federation
[c] Dr. K. A. Lyssenko
A.N. Nesmeyanov Institute of Organoelement Compounds
Russian Academy of Sciences
28 Vavilov st., Moscow,
119991, Russian Federation
[d] Dr. O. P. Demidov
North Caucasus Federal University
1 Pushkin st., Stavropol,
355017, Russian Federation
Scheme 1. Synthesis of 1-(3,5-di-tert-butyl-4-hydroxyphenyl)-2-hydroxyaryl-
benzimidazoles 3 (a-R1 =R2 =R3 =R4 =H; b-R1 =R2 =H; R3 =R4 =t-Bu; c-
R1 =R2 =CH3; R3 =R4 =H; d-R2 =OCH3; R1 =R3 =R4 =H; e-R1 =COOC2H5;
R2 =R3 =R4 =H).
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202100184
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procedures described for some of the previously obtained Results and Discussion
compounds of this group.[12–17]
The structures of the newly prepared compounds (1b–e,
3b–e) were established on the basis of the data of IR-, mass-
and H NMR spectroscopy and elemental analyses. The com-
pounds are stable in their enol OH forms 3. The DFT B3LYP-6/
311+ +G(d,p) calculation with the account taken for solvation
(toluene) showed that the keto NH form of the parent
hydroxyphenylbenzimidazol 3a is energy disfavored by
3.3 kcal/mol. Molecular and structure of this compound featur-
ing its enol form was determined using X-ray crystallography
and shown in Figure 1.
Synthesis and structures of Cu(II) complexes
1
Reaction of 2,6-di-tert-butyl-4-(2-(2-hydroxyphenyl)-1H-benzo[d]
imidazol-1-yl)phenols 3 with Cu(II) acetate readily occurs at
short-term reflux of the methanol solutions of the components
to give well crystallized bis-chelate complexes 4 in good or
moderate yields (Scheme 2). Molecular structures of two
prepared complexes were determined using X-ray crystallog-
raphy and are pictured in Figure 2 and Figure 3.
In complexes 4, the central Cu(II) ions four-coordinated to
two N atoms and two O atoms from benzimidazole ligands
adopt planar, slightly tetrahedrally distorted configuration. The
two CuÀ N, as well as the CuÀ O bond lengths, are nearly
equivalent. The 3,5-di-tert-butylphenol rings are diverted from
°
the plane of the rest of the molecules by the angles about 60
which sharply reduces their π-conjugation with the core of the
molecule and restrict delocalization of unpaired electron of the
phenoxyl radicals produced from the complexes.
The B3LYP-6311+ +G(d,p) computed structure of 4a (Fig-
ure 9) found at the doublet potential energy surface also reveals
chair-like distortion of the bis-chelate coordination site and well
reproduces the experimental geometry (Figure 2).
Central Cu(II) ion of complexes 4 is coordinatively unsatu-
rated and readily form additional bonds with electron donor
centers of external agents. Thus, solution of 4a in chloroform
leads to the formation of a six-coordinated Cu(II) adduct
4a·(CHCl3)2. Stability of this adduct caused by the Cu····Cl
binding contacts agrees with the recent findings indicating that
transition metal complexes can act as halogen bond
acceptors.[18,19] Solution of 4a in dimethylsulfoxide also affords a
stable pentacoordinated Cu(II) adduct 4a·DMSO. The XRD
molecular structures of these compounds are shown in Figure 4
and Figure 5.
Figure 1. The shape of a molecule of 2,6-di-tert-butyl-4-(2-(2-hydroxyphenyl)-
5-methoxy-1H-benzo[d]imidazol-1-yl)phenol 3d. Selected bond lengths (Å):
C4À O11 1.3677(15), N37À C5 1.3930(18), N37À C8 1.3786(17), N37À C23
1.4401(15), N39À C8 1.3301(17), N39À C24 1.3766(19), C18À O43 1.347(2). The
°
dihedral angle (rotation about the C23À N37 axis is equal to 57.05 .
Scheme 2. Synthesis of bis-chelate Cu(II) complexes of 1-(3,5-di-tert-butyl-4-
hydroxyphenyl)-2-hydroxyarylbenzimidazole ligands.
Figure 3. View of the molecular structure of complex 4e. The dihedral angles
°
(rotation about the C34À N61 and C57À N3 bonds are equal to 65.76 and
°
Figure 2. View of the molecular structure of complex 4a. The dihedral angle
50.51 , respectfully. Selected bond lengths (Å): Cu4À O5 1.8880(16), Cu1À O7
°
(rotation about the C14À N2 bond is equal to 60.59 . Selected bond lengths
1.8990(16), Cu4À N9 2.0138(17), Cu4À N45 1.9669(16), C88À O2 1.361(3),
C83À O8 1.363(2).
(Å): Cu1À O1 1.9212(11), Cu1À N1 1.9652(12), C17À O2 1.386(2).
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Figure 6. Experimental (a) and calculated (b) spectra of radical 5a (toluene,
300 K).
Figure 4. View of the molecular structure of complex 4a·(CHCl3)2. The
°
dihedral angle (rotation about the C14À N2 bond is equal to 86.91 . Selected
strates coupling of the unpaired electron with two equivalent
meta-protons of the phenoxyl ring and imine nitrogen atom
(14N, S=1). Spectral parameters of this and other observed
stable radicals 5 are listed in Table 1. It is worth of noting that
isotropic g-values and spin coupling constants remain practi-
cally equal for all studied radicals, which is explained by the
diversion of the unpaired electron containing moieties from the
common plane and cutting off their conjugation with the
benzimidazole π-system (see Figures 2–4).
bond lengths (Å): Cu1À O1 1.9212(11), Cu1À N1 1.9652(12), C17À O2 1.386(2).
ESR spectra of complex 4a are shown in Figure 7. It contains
four hyperfine structure lines indicated coupling the unpaired
electron with the copper nucleus (S=3/2). The more intense
highest-field line features additional hyperfine structure caused
by coupling the unpaired electron of copper with two
equivalent paramagnetic 14N nuclei (S=1) (Figure 7, red).
Addition of PbO2 to a toluene solution of 4a leads to oxidation
of a phenol hydroxyl group to produce the triradical 6. The ESR
spectrum of 6 shown in Figure 8 represent superposition of
signals of non-interacting unpaired electrons related to Cu(II)
Figure 5. View of the molecular structure of complex 4a·DMSO. The dihedral
angles (rotation about the C14À N2 and C14AÀ N2A bonds are equal to
°
°
59.11 and 57.07 , respectfully. Selected bond lengths (Å): Cu1À O1 1.925(2),
Cu1À O1A 1.945(2), Cu1À N1 1.984(3), Cu1À N1A 1.999(3), Cu1À O1S 2.285(2),
O2À C17 1.369(4), O2AÀ C17A 1.375(4).
Free radicals
Table 1. ESR spectral parameters of phenoxyl radicals 5.
Antioxidative properties of α-tocopherol related to scavenging
secondary radicals are determined by the formation of α-
tocopheroxyl radical,[4,20] which led to intense studies of various
synthetic analogues-precursors of phenoxyl radicals. In this
direction, significant attention was given to antioxidant activ-
ities of benzimidazol derivatives of sterically crowded
phenols.[2,3,6,15] We have found that oxidation of hydroxyphenyl-
benzimidazoles 3 with PbO2 in toluene or chloroform solutions
readily affords phenoxyl radicals stable at room temperature in
the absence of dioxygen for at least 5–7 days (Scheme 3). ESR
spectrum of phenoxyl radical 5a shown in Figure 6 demon-
Radical
aN
aH
g
Solvent
3,5
5a
5b
5c
5d
5e
1.30
1.56
1.22
1.30
1.32
1.78
1.70
1.70
1.73
1.82
2.005
2.005
2.004
2.005
2.005
Toluene
Toluene
Chloroform
Chloroform
Chloroform
Figure 7. ESR spectrum of Cu(II) complex 4a in toluene solution (red):
aCu =70.8 G; aN1,2 =10.7 G; g=2.12 and of a powder sample (blue):
aCu =172.8 G; g=2.27).
Scheme 3. Formation of phenoxyl radicals 5 by oxidation of 3 with PbO2 in
toluene or chloroform solutions.
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Cyclic voltammetry
Electrochemical properties of complexes 4a, 4b and their
ligands 3a and 3b were studied using cyclic voltammetry of
dichloromethane solutions (Figure 10).
Redox behavior of the complexes is characterized by two
irreversible oxidation waves, the first of which is caused by
oxidation of one of two hydroxy group with the potentials E1/
1 =0.34 V (4a) and E1/21 =0.37 B (4b) and the second one with
2
the potentials E1/22 =0.56 B (4a) and E1/22 =0.60 B (4b) is, most
likely, associated with oxidation of the second hydroxy group.
Parameters of the CV collected in Table 2 clearly evidence
significantly lower values of the potentials of oxidation of
phenol hydroxyl groups of Cu(II) complexes as compared with
those characteristic of their free ligands.
Figure 8. ESR spectrum of triradical 6 obtained by oxidation of complex 4a
in toluene solution with excess PbO2 (aCu =70.8 G; aN1,2 =10.7 G; g=2.12;
incut-aN =1.30 G; aH3,5 =1.78 G; g=2.005).
ion (aCu =70.8 G; aN1,2 =10.7 G; g=2.12) and free phenoxyl
radical (g=2.005; aN =1.30 G; aH3,5 =1.78 G). The ESR spectra for
complexes 4b–e are identical to the spectra of complex 4a.
Geometries and spin distributions of complex 4a, biradical
6a and triradical 6 calculated with the use of DFT B3LYP-6311
+ +G(d,p) are shown in Figure 9.
In accordance with experimental data (Figure 7, Figure 8)
the calculations showed that in spite of the conjugate π-
electron systems of all the species, no exchange interactions
exist between the unpaired electrons located at metal and
phenoxyl oxygen centers. Spin coupling values JCuO and JOO
calculated based on the broken symmetry approach[21,22] are
negligible (in the range 0–0.02 cmÀ 1).
Scavenging activity
The ability of 2-(hydroxyphenyl)-1H-benzo[d]imidazols and their
Cu(II) complexes to readily produce highly stable phenoxyl-type
radicals provide for the antioxidant properties of these com-
pounds. Their scavenging activity was tested using the standard
procedure involving the reaction of hydrogen atom transfer to
the
stable
free
radical
2,2-diphenyl-1-picrylhydrazyl
(DPPH).[11,23,24] The reaction can be spectrophotometrically
monitored by measuring decrease in absorption of the intense
longest wavelength band (lmax =517 nm, ethanol) of DPPH
(Figure S29–S32). The degree of the reduction of DPPH (“radical
inhibition”) was calculated according to the formula:[25]
Q ¼ 100 � ðA0-AcÞ=A0
where A0 is the initial absorbance and Ac is the value for added
sample concentration c.
Figure 10. Cyclic voltammograms of a) oxidation of ligand 3a (red) and
Cu(II) complex 4a (black) and b) of ligand 3b (red) and complex 4b (black).
Table 2. CV parameters of compounds 3a, 3b, 4a and 4b.
Compound
E1/21, [V]
E1/22, [V]
Figure 9. Optimized geometries and spin distribution in complex 4a and
radicals generated by its oxidation calculated by the DFT (B3LYP/6-311+
+G(d,p) method. Spin density distribution in biradical 6a is shown on the
right. Bond lengths are given in Angstroms, hydrogen atoms with the
exception of hydroxy groups are omitted for clarity.
3a
4a
3b
4b
0.58
0.34
0.54
0.37
0.56
0.60
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For the reaction of phenol 3a with DPPH proceeding at
ions and metal binding sites on the antioxidant activity of
organic radical scavengers.[6,7,26]
290 K for 1 hour the value Q290 =13.7%, whereas at 348 K
(Figure S29) the value Q348 increases to 37.8% and the
calculated time needed to consume 50% of the initial DPPH
concentration τ (50)348 =154.6 min (Figure S30). For a Cu(II) Experimental Section
complex 4a the corresponding values of Q and τ are the
following: Q290 =8.4%; Q 348K =19.3%; τ (50)348 =168.1 min.
Development of the reactions between the hydroxyphenyl-
containing ligands and Cu(II) complexes with DPPH was traced
by evolution of their ESR spectra. Figure 11 demonstrates
superposition of signals produced by DPPH (g DPPH =2.00,
a2NDPPH =8.94 G) and the phenoxyl radical 5a g5a =2.00, aN =
1.30 G, a2H =1.78 G formed through abstraction of a hydrogen
atom from 3a. Intensity of the first signal is gradually degraded
with time (few minutes), while that of the second one is
synchronously extinguished.
Materials and methods
All reagents and solvents were purchased from commercial sources
(Aldrich) and used without additional purification. The compounds
were characterized by 1H and 13C NMR, IR-, mass spectra and
elemental analysis. 1H NMR spectra were recorded on Varian
(300 MHz) spectrometers in CDCl3 solutions. The signals were
referred with regard to the signals of residual protons of deutero-
solvents (7.24 ppm),
δ values were measured with precision
0.01 ppm. Elemental analysis was performed by the classical micro-
analysis method. High-resolution mass spectra were recorded on a
Bruker maXis Impact UHR-TOF mass spectrometer (electrospray
ionization). Melting points were determined using a PTP (M)
apparatus and were left uncorrected. Control on the progress of
the reactions and the individuality of products of the reactions was
accomplished using thin layer chromatography with Silufol UV-245
wafers (benzene and methylene dichloride as eluents and iodine as
developer). IR spectra were recorded on a Varian Excalibur 3100 FT-
IR instrument using the attenuated total internal reflection
technique (ZnSe crystal). ESR measurements were carried out on X-
band spectrometer Bruker EMX Plus. The theoretical spectrum was
calculated using the Bruker WinEPR Simfonia v.1.25 program. The
cyclic voltammograms of benzimidazoles 3a,b and their complexes
4a,b were measured in 5 mM solution with the use of three-
electrode configuration (glassy-carbon working electrode, Pt coun-
ter electrode, Ag/Ag+ reference electrode (0.01 M AgNO3 in
CH3CN)) and potentiostat–galvanostat Elins P-45X. The ferrocene/
ferrocenium (Fc/Fc+) couple served as internal reference. Electronic
absorption spectra were recorded on a Varian Cary 5000 instrument
in the range of 400–700 nm. The dynamic DPPH test measured the
optical density of a solution with the same concentration of
antioxidant and DPPH radicals. Several measurements were made
of the same solution at regular intervals. Then a graph was plotted
as the percentage of absorbed radicals versus time. The constant τ
(50), the time required for the absorption of 50% DPPH radicals,
was determined from the graph.
Conclusion
To sum up, we have synthesized and determined molecular
structures of a series of novel chelate ligands, 2-hydroxyphenyl-
1H-benzo[d]imidazols functionalized with readily oxidizable 2,6-
di-tert-butyl-phenol substituents. Two bis-chelate Cu(II) com-
plexes based on these ligands and two their stable crystal
solvates were structurally characterized by X-ray crystallogra-
phy. Chemical oxidation of the ligands and complexes with 2,6-
di-tert-butylphenol pendant fragments studied by means of ESR
method reveals the formation of highly stable phenoxyl type
radicals, particularly stable in the case of biradicals produced by
the Cu(II) complexes. This feature, as well as the substantially
lower values of the potentials of oxidation of phenol hydroxyl
groups of Cu(II) complexes as compared with those of their free
ligands (Table 2, Figure 10) witness enhanced antioxidative
properties of metal complexes
4 inherent in promising
scavengers for ROSs. This finding provides an additional support
to the earlier observations of positive effects rendered by metal
The datasets of reflections from single crystals were obtained on an
automated diffractometer Bruker SMART APEX II CCD diffractometer
[λ(MoKα)=0.71072 Å, ω-scans] (complex 4a) and on an Agilent
SuperNova diffractometer using microfocus X-ray source (complex
4e) and Atlas S2 CCD detector. The collection of reflexes,
determination and refinement of unit cell parameters were
performed with the aid of the specialized CrysAlisPro 1.171.38.41
software suite.[27] The structures were solved by using ShelXT
program,[28] structure refinement was also performed with the
ShelXL program.[29] Positions of hydrogen atoms were calculated
geometrically and included in the refinement as riding groups.
Molecular graphics were rendered and prepared for publication
with the Olex2 software suite.[30] Selected crystal data of com-
pounds 3d, 4a and 4e are given in Supporting Information. The
complete X-ray diffraction datasets were deposited at the Cam-
bridge Crystallographic Data Center.
The density functional theory calculations were performed using
the Gaussian 16 program package[31] with the UB3LYP hybrid
functional[32] and the 6-311+ +G(d,p) basis set including diffuse
and polarization functions at all atoms. The stationary points on the
potential energy surfaces (PESs) were located by full geometry
optimization with calculation of force constant matrices indicating
Figure 11. ESR spectrum featuring the formation of radical 5a (peaks
appearing in the central part of the spectrum) upon interaction of 3a with
DPPH (toluene, T=290 K, g DPPH =2.00, a2NDPPH =8.94 G; g 5a=2.00, aN =1.30 G,
a2H =1.78 G).
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the correspondence of the structures obtained to the minima on
for C21H28N2O2: C, 74.08; H, 8.29; N, 8.23; found: C, 74.05; H, 8.30; N,
8.24.
the PES and checked for the stabilities of DFT wave function.
Exchange coupling of unpaired electrons in the paramagnetic
centres was estimated using the “broken symmetry” (BS)
approach.[21] The exchange coupling constants J (in cmÀ 1) were
calculated with the use of generalized spin-projection method
developed by Yamaguchi.[22] For the search of the BS states, the
Gaussian 16 procedure (Guess=Fragment=N) has been employed.
This technique implies a possibility to assume spin states and signs
along with charges for separate parts of a molecule on the stage of
generation of the initial guess. Structural visualizations presented in
Figure 9 were prepared using the ChemCraft software[33] with the
calculated atomic coordinates as input parameters.
Ethyl
4-amino-3-((3,5-di-tert-butyl-4-oxocyclohexa-2,5-dien-1-
ylidene)amino)benzoate 1d. Red crystals. Yield 55%. M.p. 177–
178 C. IR (nujol, cmÀ 1): 3466 (m), 3357 (m), 2954 (s), 2921 (s), 2854
°
(s), 1687 (s), 1662 (s), 1639 (w), 1621 (m), 1602 (m), 1581 (w), 1522
(w), 1498 (w), 1487 (w), 1456 (m), 1416 (w), 1390 (w), 1364 (m), 1345
(w), 1316 (m), 1294 (m), 1253 (m), 1223 (w), 1192 (w), 1155 (w),
1107 (w), 1082 (w), 1025 (w), 938 (w), 926 (w), 904 (w), 884 (w), 831
(w), 818 (w), 796 (w), 767 (w), 737 (w), 659 (w), 642 (w), 612 (w), 562
(w). 1H NMR (300 MHz, CDCl3) δ: 1.23 (s, 9H, tert-Bu); 1.32 (s, 9H, tert-
Bu); 1.32 (t, 3H, CH3); 4.30 (q, 2H, CH2); 4.49 (s, 2H, NH2); arom.: 6.76
(d, 1H, J=8.4 Hz), 7.04 (d, 1H, J=2.5 Hz), 7.09 (d, 1H, J=2.5 Hz),
7.36 (d, 1H, J=1.9 Hz), 7.77 (dd, 1H, J=8.4, 1.9 Hz). 13C NMR
(600 MHz, CDCl3) δ: 14.40 (OÀ CH2À CH3); 29.45, 29.48 (tert-Bu:
CÀ CH3), 35.35, 35.84 (tert-Bu: CÀ CH3); 60.44 (OÀ CH2À CH3); arom.:
114.28, 119.57 (CÀ N=), 121.95, 123.14, 129.97, 134.15 (CÀ NH2),
134.64, 146.01 (CÀ COOEt); 152.75, 153.59 (C-tert-Bu); 158.40 (C=NÀ );
166.39 (COOEt) 187.78 (C=O). HRMS (ESI) m/z: [M+H]+ calcd for
C23H31N2O3 383.2329; found 383.2338. Anal. calcd. for C23H30N2O3: C,
72.22; H, 7.91; N, 7.32; found: C, 72.25; H, 7.90; N, 7.31.
Quinone imines
General procedures
Solution of 0.220 g (1.0 mmol) of 2,6-di-tert-butyl-p-benzoquinone,
1.0 mmol of the corresponding o-phenylenediamine and catalytic
amounts of trifluoroacetic acid in 30 ml of toluene was refluxed for
4 hours with Dean-Stark attachment until the release of water
stops. After cooling the reaction mixture was purified by column
4-((2-amino-5-nitrophenyl)imino)-2,6-di-tert-butylcyclohexa-2,5-
°
dienone 1e. Dark red crystals. Yield 69%. M.p. 182–183 C. IR (nujol,
cmÀ 1): 3450 (w), 3435 (w), 3348 (m), 3296 (w), 3184 (w), 3109 (w),
3070 (w), 2957 (s), 2925 (s), 2855 (m), 1650 (m), 1633 (m), 1614 (m),
1588 (w), 1569 (w), 1494 (w), 1463 (w), 1377 (w), 1364 (w), 1308 (m),
1267 (w), 1250 (w), 1197 (w), 1168 (w), 1151 (w), 1099 (w), 1080 (w),
1024 (w), 956 (w), 935 (w), 902 (w), 883 (w), 825 (w), 805 (w), 751
chromatography (carrier-Al2O3, eluent
product.
– toluene) to give the
4-(2-aminophenyl)imino)-2,6-di-tert-butylcyclohexa-2,5-dienone
1a. Yield 80%. Physical and spectral data are identical to those
described in [13]. 13C NMR (600 MHz, CDCl3) δ: 29.48, 29.49 (tert-Bu:
CÀ CH3), 35.30, 35.76 (tert-Bu: CÀ CH3); arom.: 115.57, 117.88, 121.41,
122.20, 128.11, 135.71 (CÀ N=), 134.94, 141.60 (CÀ NH2); 152.33,
153.27 (C-tert-Bu); 157.60 (C=NÀ ); 187.90 (C=O).
1
(w), 737 (w), 722 (w), 685 (w), 661 (w), 644 (w), 615 (w), 578 (w). H
NMR (300 MHz, CDCl3) δ: 1.23 (s, 9H, tert-Bu), 1.32 (s, 9H, tert-Bu);
4.76 (s, 2H, NH2); arom.: 6.74 (d, 1H, J=8.7 Hz), 7.01 (s, 2H), 7.59 (d,
1H, J=2.7 Hz), 7.99 (dd, 1H, J1 =8.7 Hz, J2 =2.7 Hz). 13C NMR
(600 MHz, CDCl3) δ: 29.43, 29.45 (tert-Bu: CÀ CH3), 35.45, 36.00 (tert-
Bu: CÀ CH3); arom.: 113.47, 116.89, 121.06, 124.10, 133.51 (CÀ N=),
134.21, 147.38 (CÀ NH2); 138.56 (CÀ NO2), 153.62, 154.86 (C-tert-Bu);
159.80 (C=NÀ ); 187.45 (C=O). HRMS (ESI) m/z: [M-H]À calcd for
C20H24N3O3 354.1823; found 354.1822. Anal. calcd. for C20H25N3O3: C,
67.58; H, 7.09; N, 11.82; found: C, 67.60; H, 7.10; N, 11.81.
4-(2-amino-4,5-dimethylphenyl)imino)-2,6-di-tert-butylcyclohexa-
°
2,5-dienone 1b. Violet crystals. Yield 75%. M.p. 154–155 C. IR
(nujol, cmÀ 1): 3440 (w), 3344 (w), 2998 (w), 2954 (s), 2925 (s), 2855
(m), 1644 (m), 1626 (m), 1586 (w), 1560 (w), 1522 (w), 1493 (w),
1456 (w), 1413 (w), 1376 (w), 1361 (w), 1332 (w), 1311 (w), 1288 (w),
1268 (w), 1249 (w), 1221 (w), 1197 (w), 1168 (w), 1107 (w), 1075 (w),
1022 (w), 1003 (w), 938 (w), 887 (w), 886 (w), 863 (w), 822 (w), 733
1
(w), 671 (w), 570 (w). H NMR (300 MHz, CDCl3) δ: 1.24 (s, 9H, tert-
Bu); 1.32 (s, 9H, tert-Bu); 2.14 (s, 3H, CH3); 2.20 (s, 3H, CH3); 3.97 (s,
2H, NH2); arom.: 6.44 (s, 1H), 6.61 (s, 1H); 7.04 (d, 1H, J=2.5 Hz), 7.13
(d, 1H, J=2.5 Hz). 13C NMR (600 MHz, CDCl3) δ: 18.73 (CH3), 19.69
(CH3); 29.51, 29.53 (tert-Bu: CÀ CH3), 35.25, 35.73 (tert-Bu: CÀ CH3);
arom.: 117.12, 122.51, 133.81 (CÀ N=), 140.12 (CÀ NH2), 135.14;
137.30 (CÀ CH3), 122.85 (CÀ CH3); 151.77, 152.56 (C-tert-Bu); 156.54
(C=NÀ ); 187.94 (C=O). HRMS (ESI) m/z: [M+H]+ calcd for C22H31N2O
339.2431; found 339.2434. Anal. calcd. for C22H30N2O: C, 78.06; H,
8.93; N, 8.28; found: C, 78.10; H, 8.91; N, 8.28.
Benzo[d]imidazol-1-yl phenols. General procedure
A solution of a equimolar amounts (0.16–1.36 mmol) of p-quinone
imine 1 and 2-hydroxy-3,5-di-(tert-butyl)benzaldehyde (compounds
3b) or salicylic aldehyde (compounds 3a,c–e) in toluene (5–30 mL)
was stirred at the refluxing temperature for 3–8 h. (CF3COOH was
used as the catalyst). The reaction mixture was cooled to the room
temperature and the formed deposit filtered off. The crude
products were purified by column chromatography sorbent Al2O3,
eluent toluene).
4-((2-amino-5-methoxyphenyl)imino)-2,6-di-tert-butylcyclohexa-
2,6-di-tert-butyl-4-(2-(2-hydroxyphenyl)-1H-benzo[d]imidazol-1-
yl)phenol 3a. Physical and spectra data is identical to those
described in [13, 15]. 13C NMR (600 MHz, CDCl3) δ: 30.17 (tert-Bu:
CÀ CH3), 34.62 (tert-Bu: CÀ CH3); arom.: 110.59, 112.55, 117.64, 117.94,
118.56, 123.15, 123.48, 124.29, 127.24, 128.51, 131.25, 136.65,
138.04, 139.89; 150.94 (C=NÀ ); 154.40 (CÀ OH), 159.76 (CÀ OH).
°
2,5-dienone 1c. Violet crystals. Yield 43%. M.p. 72–73 C. IR (nujol,
cmÀ 1): 3468 (w), 3368 (w), 3317 (w), 2956 (s), 2921 (s), 2853 (s), 1614
(m), 1505 (w), 1485 (w), 1463 (m), 1455 (m), 1377 (w), 1362 (w),
1338 (w), 1312 (w), 1296 (m), 1261 (m), 1213 (w), 1170 (w), 1139 (w),
1078 (w), 1025 (w), 962 (w), 936 (w), 915 (w), 885 (w), 823 (w), 807
1
(w), 742 (w), 723 (w), 652 (w), 634 (w), 617 (w), 568 (w). H NMR
(300 MHz, CDCl3) δ: 1.25 (s, 9H, tert-Bu); 1.32 (s, 9H, tert-Bu); 3.79 (s,
3H, OÀ CH3); 4.26 (s, 2H, NH2); arom.: 6.29–6.34 (m, 2H), 6.60 (d, 1H,
J=8.6 Hz); 7.03 (d, 1H, J=2.6 Hz), 7.12 (d, 1H, J=2.6 Hz). 13C NMR
(600 MHz, CDCl3) δ: 29.52, 29.54 (tert-Bu: CÀ CH3), 35.22, 35.72 (tert-
Bu: CÀ CH3); 55.34 (OÀ CH3); arom.: 100.44, 104.03, 122.36, 123.09,
129.81 (CÀ N=), 135.22, 144.50 (CÀ NH2); 151.40, 152.48 (C-tert-Bu);
155.50 (C=NÀ ); 160.82 (CÀ OCH3) 188.06 (C=O). HRMS (ESI) m/z: [M
+H]+ calcd for C21H29N2O2 341.2224; found 341.2229. Anal. calcd.
2,6-di-tert-butyl-4-(2-(3,5-di-tert-butyl-2-hydroxyphenyl)-1H-ben-
zo[d]imidazol-1-yl)phenol 3b. Yellowish crystals. Yield 35%. M.p.
226 C. IR (nujol, cmÀ 1): 3624 (m), 2997 (w), 2956 (s), 2930 (s), 2854
°
(s), 1772 (w), 1618 (w), 1591 (w), 1559 (w), 1541 (w), 1518 (w), 1498
(w), 1468 (m), 1461 (m), 1441 (m), 1401 (w), 1391 (w), 1371 (w),
1361 (w), 1325 (w), 1313 (w), 1277 (m), 1261 (w), 1236 (m), 1202 (w),
1153 (w), 1117 (w), 1060 (w), 1026 (w), 1006 (w), 944 (w), 931 (w),
899 (w), 880 (w), 848 (w), 825 (w), 809 (w), 775 (w), 750 (w), 716 (w),
1
678 (w), 639 (w), 623 (w), 575 (w). H NMR (300 MHz, CDCl3) δ: 0.97
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(s, 9H, tert-Bu), 1.43 (s, 18H, tert-Bu), 1.49 (s, 9H, tert-Bu); 5.48 (s, 1H,
Cu(II) complexes
OH); arom.: 6.98–7.77 (m, 8H); 14.04 (s, 1H, OH). 13C NMR (600 MHz,
CDCl3) δ: 29.60, 30.17, 31.26 (tert-Bu: CÀ CH3), 33.94, 34.58, 35.37
(tert-Bu: CÀ CH3); arom.: 110.51, 111.67, 118.26, 122.24, 122.91,
123.09, 124.52, 125.78, 129.55, 137.01, 137.27, 138.20, 138.75,
139.70; 152.21 (C=NÀ ); 154.35 (CÀ OH), 156.48 (CÀ OH). HRMS (ESI)
m/z: [M-H]À calcd for C35H45N2O2 525.3487; found 525.3477. Anal.
calcd. for C35H46N2O2: C, 79.81; H, 8.80; N, 5.32; found: C, 79.86; H,
8.82; N, 5.30.
General procedure
Solution of a benzo[d]imidazol-1-yl-phenol 3 (0.1 mmol) in meth-
anol (10 mL) was added with methanol solution (5 mL) of half
equivalent amount (0.05 mmol) of monohydrate of Cu(CH3COO)2
and the common solution was heated to boiling for 1 h. The
reaction mixture was cooled to the room temperature and allowed
to stand for 6–10 h. The formed deposit filtered off and purified by
crystallization from methanol.
2,6-di-tert-butyl-4-(2-(2-hydroxyphenyl)-5,6-dimethyl-1H-benzo
[d]imidazol-1-yl)phenol 3c. White powder. Yield 36%. M.p. 227–
229 C. IR (nujol, cmÀ 1): 3632 (w), 3585 (w), 2954 (s), 2923 (s), 2854
°
Bis(2,6-di-tert-butyl-4-(2-(2-hydroxyphenyl)-1H-benzo[d]imidazol-
(s), 1629 (w), 1587 (w), 1508 (w), 1486 (m), 1467 (m), 1440 (m), 1416
(w), 1378 (w), 1331 (w), 1298 (w), 1257 (w), 1238 (w), 1201 (w), 1172
(w), 1159 (w), 1119 (w), 1097 (w), 1035 (w), 999 (w), 948 (w), 903
(w), 853 (w), 824 (w), 758 (w), 723 (w), 683 (w), 658 (w), 618 (w), 577
1-yl)phenol) copper(II) 4a. Light green powder. Yield 35%. M.p.
>330 C. IR (nujol, cmÀ 1): 3624 (m), 3101 (w), 3060 (w), 2953 (s),
°
2924 (s), 2854 (s), 1603 (m), 1547 (m), 1498 (m), 1470 (s), 1435 (m),
1421 (m), 1377 (w), 1366 (w), 1338 (m), 1307 (w), 1292 (w), 1280 (w),
1263 (m), 1254 (m), 1232 (m), 1201 (w), 1151 (w), 1137 (w), 1117
(w), 1103 (w), 1031 (w), 1018 (w), 941 (w), 890 (w), 870 (w), 850 (w),
816 (w), 756 (w), 740 (m), 703 (w), 684 (w), 659 (w), 626 (w), 568 (w).
Anal. Calc. for C54H58CuN4O4: C, 72.82; H, 6.56; N, 6.29; Found: C,
72.80; H, 6.48; N, 6.24.
1
(w). H NMR (300 MHz, CDCl3) δ: 1.43 (s, 18H, tert-Bu); 2.34 (s, 3H,
CH3), 2.40 (s, 3H, CH3); 5.49 (s, 1H, OH); arom.: 6.46–7.21 (m, 8H);
13.85 (s, 1H, OH). 13C NMR (600 MHz, CDCl3) δ: 20.22 (CH3), 20.55
(CH3); 30.20 (tert-Bu: CÀ CH3), 34.62 (tert-Bu: CÀ CH3); arom.: 110.67,
112.85, 117.57, 117.88, 118.60, 124.34, 127.01, 128.75, 130.91,
132.20, 132.85, 135.16, 137.96; 150.12 (C=NÀ ); 154.31 (CÀ OH),
159.60 (CÀ OH). HRMS (ESI) m/z: [M-H]À calcd for C29H33N2O2
441.2548; found 441.2540. Anal. calcd. for C29H34N2O2: C, 78.70; H,
7.74; N, 6.33; found: C, 78.65; H, 7.74; N, 6.31.
Bis(2,6-di-tert-butyl-4-(2-(3,5-di-tert-butyl-2-hydroxyphenyl)-1H-
benzo[d]imidazol-1-yl)phenol) copper(II) 4b. Brown-gray powder.
Yield 74%. M.p. 238–240 C. IR (nujol, cmÀ 1): 3670 (w), 3638 (m),
°
3585 (w), 2953 (s), 2920 (s), 2853 (s), 1614 (m), 1512 (m), 1469 (s),
1456 (m), 1441 (m), 1404 (w), 1378 (m), 1362 (w), 1335 (w), 1316
(m), 1293 (w), 1260 (m), 1238 (m), 1201 (w), 1154 (w), 1119 (w),
1066 (w), 1025 (w), 1010 (w), 953 (w), 932 (w), 903 (w), 885 (w), 868
(w), 834 (w), 815 (w), 791 (w), 759 (w), 748 (w), 726 (w), 692 (w), 638
(w), 616 (w), 569 (w). Anal. Calc. for C70H90CuN4O4: C, 75.40; H, 8.14;
N, 5.02; Found: C, 75.46; H, 8.17; N, 5.05.
2,6-di-tert-butyl-4-(2-(2-hydroxyphenyl)-5-methoxy-1H-benzo[d]
imidazol-1-yl)phenol 3d. Light green crystals. Yield 28%. M.p. 193–
195 C. IR (nujol, cmÀ 1): 3543 (w), 3530 (w), 3090 (w), 3066 (w), 3039
°
(w), 2997 (w), 2953 (s), 2921 (s), 2855 (m), 1600 (m), 1541 (w), 1522
(w), 1504 (w), 1486 (m), 1463 (m), 1454 (m), 1440 (m), 1420 (m),
1389 (w), 1376 (w), 1352 (w), 1330 (w), 1293 (w), 1278 (w), 1266 (w),
1253 (w), 1237 (m), 1197 (w), 1167 (m), 1109 (m), 1038 (w), 955 (w),
934 (w), 916 (w), 888 (w), 843 (w), 819 (w), 791 (w), 779 (w), 742 (w),
695 (w), 656 (w), 639 (w), 624 (w), 577 (w). 1H NMR (300 MHz, CDCl3)
δ: 1.42 (s, 18H, tert-Bu); 3.89 (s, 3H, OÀ CH3); 5.49 (s, 1H, OH); arom.:
6.48–7.26 (m, 9H); 13.73 (s, 1H, OH). 13C NMR (600 MHz, CDCl3) δ:
30.18 (tert-Bu: CÀ CH3), 34.62 (tert-Bu: CÀ CH3); 55.95 (OÀ CH3); arom.:
100.69, 111.01, 112.70, 113.62, 117.63, 117.91, 124.22, 127.04,
128.59, 131.09, 131.34, 138.00, 140.62; 150.96 (C=NÀ ); 154.35
(CÀ OH); 156.97 (CÀ OCH3); 159.61 (CÀ OH). HRMS (ESI) m/z: [M-H]À
calcd for C28H31N2O3 443.2340; found 443.2322. Anal. calcd. for
C28H32N2O3: C, 75.65; H, 7.26; N, 6.30; found: C, 75.60; H, 7.27; N, 6.31.
Bis(2,6-di-tert-butyl-4-(2-(2-hydroxyphenyl)-5,6-dimethyl-1H-ben-
zo[d]imidazol-1-yl)phenol) copper(II) 4c. Brown powder. Yield
78%. M.p. >350 C. IR (nujol, cmÀ 1): 3429 (w), 2955 (s), 2921 (s),
°
2853 (s), 1601 (w), 1556 (w), 1475 (m), 1466 (m), 1440 (m), 1403 (w),
1377 (w), 1338 (w), 1316 (w), 1253 (m), 1239 (m), 1201 (w), 1173 (w),
1160 (w), 1133 (w), 1122 (w), 1105 (w), 1036 (w), 1023 (w), 995 (w),
944 (w), 930 (w), 892 (w), 858 (w), 837 (w), 761 (w), 722 (w), 705 (w),
692 (w), 648 (w), 616 (w), 574 (w). Anal. Calc. for C58H66CuN4O4: C,
73.58; H, 7.03; N, 5.92; Found: C, 73.64; H, 7.00; N, 5.87.
Bis(2,6-di-tert-butyl-4-(2-(2-hydroxyphenyl)-5-methoxy-1H-benzo
[d]imidazol-1-yl)phenol) copper(II) 4d. Dark green crystals. Yield
Ethyl-1-(3,5-di-tert-butyl-4-hydroxyphenyl)-2-(2-hydroxyphenyl)-
60%. M.p. >300 C. IR (nujol, cmÀ 1): 3616 (w), 2954 (s), 2923 (s),
°
1H-benzo[d]imidazole-6-carboxylate 3e. White powder. Yield
2855 (s), 1619 (w), 1599 (w), 1554 (w), 1470 (m), 1422 (w), 1377 (w),
1337 (w), 1316 (w), 1256 (w), 1237 (w), 1205 (w), 1153 (w), 1119 (w),
1105 (w), 1036 (w), 964 (w), 937 (w), 904 (w), 837 (w), 804 (w), 788
(w), 755 (w), 723 (w), 653 (w), 622 (w). Anal. Calc. for C55H59CuN4O5:
C, 71.83; H, 6.47; N, 6.09; Found: C, 71.80; H, 6.42; N, 6.13.
28%. M.p. 208–209 C. IR (nujol, cmÀ 1): 3572 (w), 3520 (m), 3393 (w),
°
3053 (w), 2956 (s), 2919 (s), 2854 (s), 1773 (w), 1706 (s), 1664 (w),
1612 (w), 1582 (w), 1559 (w), 1541 (w), 1505 (w), 1480 (m), 1455 (m),
1436 (m), 1411 (m), 1378 (w), 1367 (w), 1321 (w), 1292 (s), 1255 (m),
1238 (m), 1201 (w), 1158 (w), 1128 (m), 1116 (m), 1099 (m), 1028
(w), 962 (w), 942 (w), 929 (w), 915 (w), 888 (w), 845 (w), 833 (w), 810
(w), 773 (w), 749 (m), 699 (w), 672 (w), 654 (w), 631 (w), 616 (w), 577
(w). 1H NMR (300 MHz, CDCl3) δ: 1.36–1.40 (m, 3H, CH3); 1.43 (s, 18H,
tert-Bu); 4.36 (q, 2H, CH2, J=7.1 Hz); 5.56 (s, 1H, OH); arom.: 6.50–
8.05 (m, 9H); 13.57 (s, 1H, OH). 13C NMR (600 MHz, CDCl3) δ: 14.35
(OÀ CH2À CH3); 30.15 (tert-Bu: CÀ CH3), 34.65 (tert-Bu: CÀ CH3); 61.02
(OÀ CH2À CH3); arom.: 112.09, 112.93, 117.84, 118.02, 118.16, 124.25,
124.61, 125.73, 127.46, 127.95, 131.95, 136.44, 138.25, 143.26
(CÀ COOEt); 153.37 (C=NÀ ); 154.73 (CÀ OH), 160.03 (CÀ OH); 166.90
(COOEt). HRMS (ESI) m/z: [M-H]À calcd for C30H33N2O4 485.2446;
found 485.2431. Anal. calcd. for C30H34N2O4: C, 74.05; H, 7.04; N,
5.76; found: C, 74.09; H, 7.03; N, 5.75.
Bis(ethyl-1-(3,5-di-tert-butyl-4-hydroxyphenyl)-2-(2-hydroxyphen-
yl)-1H-benzo[d]imidazole-6-carboxylate) copper(II) 4e. Dark green
small rhombic crystals. Yield 70%. M.p. >350 C. IR (nujol, cmÀ 1):
°
3581 (w), 2955 (s), 2923 (s), 2854 (s), 1749 (w), 1707 (m), 1616 (w),
1601 (w), 1548 (w), 1500 (w), 1470 (m), 1437 (w), 1420 (w), 1392 (w),
1376 (w), 1336 (w), 1317 (w), 1288 (w), 1271 (w), 1247 (m), 1208 (w),
1157 (w), 1130 (w), 1099 (w), 1030 (w), 965 (w), 935 (w), 892 (w),
865 (w), 851 (w), 824 (w), 774 (w), 754 (w), 727 (w), 678 (w), 652 (w),
639 (w), 577 (w). Anal. Calc. for C60H66CuN4O8: C, 69.65; H, 6.43; N,
5.41; Found: C, 69.71; H, 6.44; N, 5.50.
Deposition Numbers 2045191 (3d), 1838301 (4a), 1838303
(4a·(CHCl3)2), 1838309 (4a·DMSO), and 2045192 (4e) contain the
supplementary crystallographic data for this paper. These data are
provided free of charge by the joint Cambridge Crystallographic
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Research was financially supported by Ministry of Science and
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Conflict of Interest
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Keywords: Cu(II) complexes · Phenoxyl radicals · Biradicals ·
Triradicals · DFT calculations
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Manuscript received: March 4, 2021
Revised manuscript received: April 15, 2021
Accepted manuscript online: April 22, 2021
Eur. J. Inorg. Chem. 2021, 1–9
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FULL PAPERS
A series of 2-hydroxyphenyl-1H-
benzo[d]imidazols bearing readily ox-
idizable 2,6-di-tert-butyl-phenol sub-
stituents and bis-chelate Cu(II)
complexes based on these ligands
were synthesized. Under oxidation
both the ligands and Cu(II)
Prof. Dr. E. P. Ivakhnenko*, P. A.
Knyazev, Y. G. Vitkovskaya, L. D.
Popov, Dr. K. A. Lyssenko, Dr. O. P.
Demidov, Dr. A. G. Starikov, G. S.
Borodkin, Prof. Dr. V. I. Minkin
1 – 9
Synthesis, Staructure and Redox
Properties of Cu(II) Chelate
Complexes on the Basis of 2-(Hy-
droxyphenyl)-1H-benzo[d]imidazol-
1-yl Phenol Ligands
complexes form stable phenoxyl-
type radicals. As shown, no exchange
interaction exists between unpaired
electrons located on metal and
oxygen centers in the Cu(II)
complexes.