Synthesis, characterization and study on the dissimilar reactivity of a Ni(II)-bis(iminosemiquinone) complex core to ligand-appended hemilabile –CH2OH and –CH2NH2 units

Article abstract of DOI:10.1016/j.ica.2019.119340

Noninnocent ligand H₃L^CH₂^OH reacted with NiCl₂·6H₂O under aerial atmosphere and provided corresponding complexes 1 {[Ni^II(HL^CH₂^OH)₂]

Full text of DOI:10.1016/j.ica.2019.119340

Journal Pre-proofs
Research paper
Synthesis, Characterization and Study on the Dissimilar Reactivity of a Ni(II)–
Bis(Iminosemiquinone) Complex Core to Ligand–Appended Hemilabile –
CH2OH and –CH2NH2 Units
Prasenjit Sarkar, Samir Ghorai, Ganesh Chandra Paul, Mahmuda Khannam,
Surajit Barman, Chandan Mukherjee
PII:
S0020-1693(19)31106-5
https://doi.org/10.1016/j.ica.2019.119340
ICA 119340
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
30 July 2019
26 October 2019
2 December 2019
Please cite this article as: P. Sarkar, S. Ghorai, G. Chandra Paul, M. Khannam, S. Barman, C. Mukherjee, Synthesis,
Characterization and Study on the Dissimilar Reactivity of a Ni(II)–Bis(Iminosemiquinone) Complex Core to
Ligand–Appended Hemilabile –CH2OH and –CH2NH2 Units, Inorganica Chimica Acta (2019), doi: https://doi.org/
10.1016/j.ica.2019.119340
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Synthesis, Characterization and Study on the
Dissimilar Reactivity of Ni(II)–
a
Bis(Iminosemiquinone) Complex Core to Ligand–
Appended Hemilabile –CH2OH and –CH2NH2 Units
Prasenjit Sarkar,‡ Samir Ghorai,‡ Ganesh Chandra Paul, Mahmuda Khannam, Surajit Barman,
and Chandan Mukherjee*
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati, 781039, Assam,
India; Corresponding author, email: cmukherjee@iitg.ac.in
‡ Contributed equally.
Abstract
Noninnocent ligand H3LCH OH reacted with NiCl26H2O under aerial atmosphere and provided
2
corresponding complexes 1 {[NiII(HLCH OH)2]0} and 1a {[NiII(LCHO)2]0} in the presence of Et3N
2
and NaOH, respectively. Ligand H4LCH NH2 provided complex 2 {[NiII(HL3]0} in the presence of
2
NiCl26H2O and Et3N under air. All the complexes were characterized by various spectroscopic
1
and spectrometric techniques, which included FT-IR, mass, UV–vis/NIR, H NMR, cyclic
voltammetry and single crystal X–ray diffraction methods. Molecular structural analyses on
complexes 1 and 1a revealed that both four-coordinate complexes were neutral in charge and
consisted of two ligand–centered iminosemiquinone radicals and a Ni(II) ion. While, the
hemilabile –CH2OH group remained unaffected and did not make any coordination with the
central Ni(II) ion in complex 1, two-electron oxidation of each –CH2OH group to –CHO group
was realized in complex 1a. In complex 2, the ligand backbone was an innocent tetradentate
salen unit, which was formed by a complete modification of the initial noninnocent ligand
H4LCH NH2. To find out the mechanistic path for the formation of complex 2, ligand H3L1 and
2
H4L2 were also introduced and it was presumed that transimination took place in the ligand
backbone during complex 2 formation.
Keywords
Redox noninnocent ligands; Ni(II)radical complexes; antiferromagnetic coupling; cyclic
voltammetry; oxidation of alcohols.
1

Introduction
Redox noninnocent ligands-coordinated transition metal complexes are widely under
investigation as redox catalysts. [1-6] Bidentate 2–aminophenol derivatives (H2LAP; Scheme 1)
have been recognized as noninnocent ligands [7-15] and have been identified to exist in its
various oxidation states in the corresponding coordination metal complexes as shown in Scheme
1. Some of the complexes have been exclusively studied as redox catalysts. [1-6] For instances:
radical(s)–containing Cu(II) complexes have been employed for the aerial oxidation of primary
alcohols to the corresponding aldehydes as well as for the aziridination of vinylic substrates; [16,
17] using four–coordinate Pd(II)-aminophenolate and five-coordinate Fe(III)-iminosemiquinone
complexes, C–H amination reactions of several azide compounds are reported to occur via the
initial nitrene radical intermediates formation [18, 19]; Desage–El Murr and coworkers have
reported an electron transfer from a Ni(II)–coordinated iminosemiquinone unit to [CF3]+ and the
consequent generation of [CF3] radical, which undergoes C–C bond formation with either
indole– or vinyl silyl ether–based substrates in a catalytic reaction. [20]
Scheme 1. Top: the possible various oxidation states of 2-aminophenol derivatives. AP =
amidophenolate; ISQ = iminosemiquinone; and IBQ = iminobenzoquinone. Bottom: Ligands
with the designation of the substrate unit and the noninnocent unit.
2

In continuation of our understanding on reactivity of transition metal–radical complexes,
we investigated two ligands H3LCH OH and H4LCH NH2, which were comprised of a noninnocent
2
2
2–amino–4,6–di–tert–butylphenol unit to generate a metal-coordinated ligand–based radical; and
either a benzyl alcohol or a benzyl amine hemilabile unit in the ligand backbone as the substrate
(Scheme 1). We intended to realize the reactivity difference between benzyl alcohol and benzyl
amine units under the same chemical and geometrical environment. In this context, we study
Ni(II) ions because Ni(II) would provide four–coordinate complexes. Thus, two axial positions
would be available for the hemilabile units to establish weak interactions and thereafter, the units
would undergo electron–transfer reactions with the Ni(II)–coordinated radical centers as
observed in other radical–containing metal complex catalyses. [3, 4, 21]
Noninnocent ligand H3LCH OH [22] reacted under air with NiCl26H2O and provided
2
four–coordinate, neutral Ni(II)–bis(iminosemiquinone) complexes 1 and 1a in the presence of
Et3N and NaOH, respectively. In complex 1a the ligand–centered benzyl alcohol unit underwent
oxidation to benzaldehyde unit. Additionally, complex 1, which contained benzyl alcohol in the
ligand backbone, could also be transformed into 1a in the presence of NaOH in CH3OH.
Contrary to ligand H3LCH OH, ligand H4LCH NH [23] provided complex 2 in the presence of
2
2
2
NiCl26H2O and Et3N under air. In the complex, the ligand backbone was an innocent
tetradentate salen unit, which was formed by a complete modification of the initial noninnocent
1
ligand H4LCH NH2. The complexes were characterized by FT–IR, mass, UV–vis/NIR, H NMR
2
spectroscopic techniques, and X–ray diffraction technique and reported in this work. In addition,
possible routes for the formation of complex 1a and 2 were also incorporated.
Experimental Section
Materials: All the chemicals and solvents were obtained from commercial sources and were
used as supplied, unless noted otherwise. The 3,5–di–tert–butylcatechol, 2–aminobenzonitrile,
2–aminobenzylamine, and 2–aminobenzylalcohol, were purchased from Sigma–Aldrich.
Solvents were obtained from Merck (India). Mass spectra were measured in HPLC-grade
acetonitrile solution.
3

Physical measurements: X–ray crystallographic data were collected using a SuperNova, (Single
source at offset, Eos) diffractometer, equipped with a sealed tube MoK radiation (  =
0.71073 Å). Structures were solved with the Superflip, structure solution program using Charge
Flipping and refined by direct methods using SHELXS–97 or SHELXS-2013 and with
fullmatrix least squares on F2 using SHELXL–97. [24] All the nonhydrogen atoms were
refined anisotropically. IR spectra were recorded on Perkin Elmer Instrument at normal
temperature by making KBr pellet (grinding the sample with IR–grade KBr). Mass spectral data
were obtained from Q-TOF MS Spectrometer. UV–Vis–NIR spectra were recorded on Perkin
Elmer, Lamda 750, UV–Vis–NIR spectrometer, preparing a known concentration of the samples
13
in HPLC Grade CH2Cl2 at room temperature using cuvette of 1 cm width. 1H–, and C–NMR
spectra of the ligand and complexes were recorded in ‘Varian Mercury plus 400 MHz NMR
machine at 298 K. X–band EPR spectra were examined on JEOL, JES–FA200 machine. Cyclic
voltammograms (CVs) of the complexes (1 mM) were being recorded on VersaSTAT 3
instrument in CH2Cl2 solutions containing 0.10 M [(nBu)4N]ClO4 as supporting electrolyte. A
glassy carbon working electrode, a platinum wire counter electrode, and a Ag/AgCl reference
electrode were used for the measurements. Ferrocene was used as an internal standard, and
herein, all the potentials are referenced versus the ferrocenium/ferrocene (Fc+/Fc) couple. The
number of electron transfer was determined by comparing peak current.
Table 1: Crystallographic parameters and refinement data for complexes 1, 1a and 2.
Complex 1
Complex 1a
Complex 2
C28 H31 N3 Ni O
484.27
Empirical formula
Formula weight
CCDC Number
C21 H27 N Ni0.50 O2 C42 H50 N2 Ni O4
354.79
705.55
1943217
1943218
964880
Crystal habit, colour
Block, green
Block, green
Needle, brown
Crystal size, mm3
Temperature, T
Wavelength, λ (Å)
Crystal system
0.35  0.34  0.30
102(3)
0.71073
0.33  0.30  0.28
293(2)
0.71073
0.46  0.22  0.18
296(2)
0.71073
triclinic
triclinic
monoclinic
Space group
'P -1'
'P -1'
‘C 2/c’
Unit cell dimensions
a = 5.7899(8) Å
b = 12.5127(9) Å
c = 13.9208(14) Å
a = 6.6721(3) Å
b = 14.0949(8) Å
c = 21.7775(10) Å
a = 25.810(2) Å
b = 6.9366(6) Å
c = 28.778(3) Å
α = 71.193(8)°, β = α = 104.148(4)°, β = α = γ = 90°, β =
80.081(10)°
92.361(4)°
96.065(8)°
γ = 81.402(8)°
γ = 93.747(4)°
4

Volume, V (Å3)
Z
935.62(18)
2
1978.18(17)
2
5123(8)
8
Calculated
Mg·m3
density, 1.259
1.185
1.256
Absorption coefficient, 0.562
µ (mm1)
0.531
0.781
F(000)
380
752
2048

range for data 2.65 to 24.99
2.72 to 24.99
3.04 to 25.00
collection
Limiting indices
–6 ≤ h ≤ 6, –14 ≤ k ≤ –6 ≤ h ≤ 7, –16 ≤ k ≤ –29 ≤ h ≤ 30, –8 ≤ k
14,
9,
≤ 7,
–16 ≤ l ≤ 16
6216/5759
0.0285]
–22 ≤ l ≤ 25
[R(int)= 13348/
–29 ≤ l ≤ 34
6933 9287/4511 [R(int)=
0.0565]
Reflection
collected/unique
[R(int)= 0.0468]
Completeness to 
99.7% ( = 25.00°)
min. 0.821/0.845
99.9% ( = 25.00°)
0.839/0.862
99.9% ( = 25.00°)
0.869/0.814
Max.
and
transmission
Refinement method
‘SHELXL-2013 '
‘SHELXL-2013 '
6933/0/457
1.068
Full–matrix least–
squares on F2
4511/0/304
Data/restraints/paramet 3293/0/230
ers
Goodnessoffit on F2
1.046
1.030
Final
[I>2sigma(I)]
R indices (all data)
R
indices R1 = 0.0823, wR2 = R1 = 0.0667, wR2 = R1 = 0.0557, wR2 =
0.1987 0.1399 0.1166
R1 = 0.1150, wR2 = R1 = 0.1256, wR2 = R1 = 0.0994, wR2 =
0.2278
1.073 and -0.486 e·Å3
0.1737
0.1439
0.436 and -0.354 0.302 and -0.440
e·Å3 e·Å3
Largest diff. peak and
hole
Syntheses
Synthesis of [C42H54N2NiO4]; 1; {[NiII(HLCH OH)2]0}: A mixture containing ligand H3LCH OH
2
2
(0.329 g, 1.0 mmol) and NiCl26H2O (0.119 g, 0.5 mmol) in acetonitrile (15mL) was stirred for

10 minutes under air at room temperature (30 C). Triethylamine (0.2 mL) was added to the
reaction mixture. After that, the resulting solution was refluxed for 2 h. The solution was cooled
to room temperature. Then the reaction mixture was filtered to obtain a green solid. The obtained
residue was recrystallized from dichloromethane:acetonitrile (3:1) solvent mixture by slow
solvent evaporation that led to the X–ray quality single crystal. Yield: 0.183g, 0.26 mmol, 52%.
Fourier transform infrared (FTIR) (KBr pellet, cm–1): 3556, 3437, 2957, 2905, 2867, 1631, 1533,
1
1459, 1359, 1321, 1257, 1228, 1199, 1174, 1107, 1040, 914, 745. H NMR (CDCl3, 399.85
t
t
MHz):  0.99 (s, 18H, Bu), 1.06 (s, 18H, Bu), 2.07 (s, 1H, -OH), 2.31 (s, 1H, -OH), 4.72–5.01
5

(4H, -CH2), 6.17 (s, 1H, Ar-H), 6.23 (s, 1H, Ar-H), 6.91 (s, 2H, Ar-H), 7.26 (s, 2H, Ar-H), 7.35–
13
7.51 (m, 4H, Ar-H), 7.60–7.62 (m, 2H, Ar-H) ppm. C NMR (CDCl3, 100 MHz):  148.87,
146.34, 142.28, 135.42, 129.67, 129.18, 127.49, 125.64, 121.69, 121.55, 119.04, 114.20, 64.76,
35.02, 34.40, 31.64, 29.58 ppm. ESI–MS (CH3CN) m/z for [C42H54N2NiO4]+: Calcd, 708.34;
Found, 708.35. UV–Vis–NIR (CH2Cl2) max, nm (, M–1cm–1): 890(35350), 590(2450),
470(3050).
Synthesis of [C42H50N2NiO4]; 1a; {[NiII(LCHO)2]0}: To a stirred solution of ligand H3LCH OH
2
(0.329 g, 1.0 mmol) and NiCl26H2O (0.120 g, 0.5 mmol) in methanol (5mL), was added a
methanolic solution of NaOH (0.060 g in 20 mL methanol). Resulting solution was stirred for 1 h
at room temperature (30 C), caused a green precipitation. The precipitate was collected through
filtration, washed with methanol. Recrystallization of this solid from dichloromethane:methanol
(3:1) solvent mixture by slow solvent evaporation technique, provided the crystalline compound
suitable for the single crystal X–ray diffraction analysis. Yield: 0.144 g, 0.20 mmol, 41%. FTIR
(KBr pellet, cm–1): 3075, 2955, 2900, 2866, 2751, 1691, 1592, 1571, 1534, 1522, 1473, 1450,
1437, 1361, 1323, 1286, 1257, 1228, 1199, 1174, 1155, 1106, 1095, 1025, 999, 919, 880, 873,
1
854, 837, 818, 672, 647, 636, 612, 590, 563, 543. H NMR (CDCl3, 399.85 MHz):  0.96 (s,
18H. tBu), 1.06 (s, 18H, tBu), 6.30 (d, 2H, J = 8.0 Hz, Ar-H), 6.92 (s, 2H, Ar-H), 7.57–7.64 (m,
6H, Ar-H), 8.08 (s, 2H, Ar-H), 10.75 (s, 1H, -CHO), 10.97 (s, 1H, -CHO) ppm. 13C NMR
(CDCl3, 100 MHz):  190.22, 189.63, 172.85, 172.63, 155.63, 155.29, 151.37, 151.25, 147.77,
141.16, 133.79, 133.46, 128.37, 127.13, 126.85, 112.74, 112.59, 34.95, 34.39, 30.41, 29.07 ppm.
ESI–MS (CH3CN) m/z for [C42H50N2NiO4]+: Calcd, 704.31; Found, 704.32. UV–Vis–NIR
(CH2Cl2) max, nm (, M–1cm–1): 900(42400), 590(2800), 388(7150).
Synthesis of [C28H31N3NiO]; 2; {[NiII(HL3]0}: Ligand H4LCH NH (0.326 g, 1.00 mmol) was
2
2
treated with NiCl26H2O (0.239 g, 1 mmol) in acetonitrile (15 mL) in the presence of
triethylamine (0.2 mL). The resulting reaction mixture was allowed to reflux for 3 h. After
cooling to room temperature (30 C), it was further stirred for 1.5 h. After that, the reaction
mixture was filtered and washed with acetonitrile to get a red solid. Solid residue was
recrystallized from dicloromethane:acetonitrile (3:1) solvent mixture. Yield: 0.065 g, 0.13 mmol,
27%. FTIR (KBr pellet, cm–1): 3346, 3056, 2945, 2900, 2860, 1617, 1599, 1560, 1530, 1476,
6

1446, 1414, 1342, 1326, 1290, 1264, 1221, 1178, 1154, 1121, 850, 834, 746, 730, 677, 538, 457.
1H NMR (CDCl3, 399.85 MHz):  1.35 (s, 9H, tBu), 1.49 (s, 9H, tBu), 5.97 (s, 1H, N-H), 6.51 (t,
J = 7.2 Hz, 1H, Ar-H), 7.03 (d, J = 8.4 Hz, 1H, Ar-H), 7.15 (t, J = 6.8 Hz, 1H, Ar-H), 7.18 (s,
1H, Ar-H), 7.40 (t, J = 7.6 Hz, 2H, Ar-H), 7.46 (s, 1H, Ar-H), 7.55 (t, J = 7.8 Hz, 1H, Ar-H),
7.69 (d, J = 8.0 Hz, 1H, Ar-H), 7.82 (d, J = 7.6 Hz, 1H, Ar-H), 8.56 (s, 1H, –C=C-H), 8.73 (s,
1H, –C=C-H) ppm. 13C NMR (CDCl3, 100 MHz):  163.08, 155.59, 152.73, 144.76, 143.68,
139.20, 138.38, 136.47, 133.33, 133.17, 132.62, 131.23, 124.79, 124.39, 124.31, 121.36, 120.23,
116.18, 114.60, 108.78, 35.14, 34.40, 31.89, 29.43 ppm. ESI–MS (CH3CN) m/z for
[C28H31N3NiO + H]+: Calcd, 484.19; Found, 484.20. UV–Vis–NIR (CH2Cl2) max, nm (, M–
1cm–1): 518(25100), 390(40200), 326(42050).
Results and Discussion
Scheme 2. Schematic representations for the syntheses of the complexes.
3,5Ditertbutylcatechol in a condensation reaction with an equimolar amount of
2aminobenzonitrile or 2aminobenzylalcohol in hexane, in the presence of triethylamine (Et3N)
7

under air, provided ligand H2LCN or H3LCH OH in good yields.[25, 22] The reduction of H2LCN
2
using LiAlH4 in dry THF under inert atmosphere yielded the expected ligand H4LCH NH2. [23] In
2
a 1.0:0.5 ratio reaction between ligand H3LCH OH and NiCl26H2O in acetonitrile in the presence
2
of Et3N under air provided complex 1. Upon changing the base to sodium hydroxide (NaOH),
complex 1a was formed under the same reaction condition in methanol (Scheme 2). In complex
1a, the alcohol group was being oxidized to the aldehyde group. It is to note that complex 1a can
also be prepared starting from complex 1 by reacting with 3 equivalent amounts of NaOH in
CH3OH. Ligand H4LCH NH upon reacting with an equivalent amount of NiCl26H2O in the
2
2
presence of Et3N under air provided complex 2 (Scheme 2). Noteworthy, the ligand backbone in
complex 2 was completely different than the initially used H4LCH NH ligand scaffold. The
2
2
relocation in the position of 2,4ditertbutyl phenol unit from aniline N atom to benzyl amine
N atom was warranted.
In the infrared (IR) spectrum of complex 1 a band at 3540 cm1 was found, which implied
the presence of free –OH group in the complex. Interestingly, in complex 1a, the band was
absent and a new band at 1693 cm1 was found due to the (C=O) stretching of the –CHO group.
The IR spectrum of complex 2 exhibited a band at 3346 cm–1 due to asymmetric (N–H)
stretching. The asymmetric, symmetric, and bending overtone bands for (C–H) stretches of
tert–butyl groups were found at 2945, 2860, and 2900 cm–1, while the corresponding bending
vibrational band arose at 1476 cm–1. Two bands appeared at 1617, and 1599 cm–1 for two
different (C=N) stretching units. The phenolate (C–O) band appeared at 1264 cm–1.
The electrospray ionization (ESI) mass spectra of all the complexes were measured in the
positive mode in CH3CN solution. Complex 1 and complex 1a showed a 100% molecular ion
peak at m/z = 708.35, and 704.32, respectively, corresponded to the [M]+, while, complex 2
showed a 100% molecular ion peak at m/z = 484.20 that corresponded to [M + H]+ (M stands for
molecular mass). Isotope distribution pattern examination of the observed mass peaks revealed
the composition as: C42H54NiN2O4 for complex 1; C42H50NiN2O4 for complex 1a; and
C28H31N3NiO for complex 2 and supported the formation of the expected metal complexes.
8

Crystallographic Study
Figure 1. ORTEP diagrams for complexes 1 [A], 1a [B], and 2 [C] were drawn at 40% thermal
probability level. H atoms except attached with C13 (1, 1a); O2 (1); and N3 (2) were omitted for
clarity.
The molecular structures of all the complexes were investigated by X-ray single crystal
diffraction measurements. Complex 1, and complex 1a crystallized in the triclinic space group
P–1, while, complex 2 crystallized in the monoclinic space group C 2/c. In complexes 1 and 2, a
9

unit cell is comprised of one asymmetric unit, while, in complex 1a two asymmetric molecules
are present in a unit cell. Nevertheless, the bond distances are almost equal in both the
asymmetric molecules. Thus, herein, only one asymmetric unit will be presented. Molecular
structures of complex 1, complex 1a, and complex 2 along with the atom labeling scheme are
shown in Figure 1. Selected bond distances and bond angles are summarized in Table S1.
Both complex 1 and complex 1a were neutral in charge and four-coordinate, where the
central Ni atom was coordinated by two N atoms and two O atoms of two deprotonated
amidophenolate ligands. The N1–Ni1–N1i and O1–Ni1–O1i bond angles were 180 (4 = 0) [22]
and represented a perfect square planar coordination geometry around the Ni atom. The Ni1N1,
and Ni1O1 bond distances were 1.842(4) [1]; 1.833(3) [1a] Å and 1.836(3) [1]; 1.821(3) [1a] Å
(parenthesis indicates the complex), respectively. The bond distances were in accord with the
previously reported similar Ni complexes with +II physical oxidation state. [27] Thus, herein, the
oxidation state of the central Ni atom has been assigned as +II. The C13–O2 = 1.403(9) Å in
complex 1 showed the single bond character, while the short bond distance was 1.239(7) Å and
consolidated a double bonded C=O unit in complex 1a. Therefore, the oxidation of benzyl
alcohol group in complex 1 to benzaldehyde group in complex 1a was crystallographycally
characterized.
Chart 1. [28–36]
It has been well established that the oxidation state of a metal ion–coordinated 3,5–di–
tert–butyl groups–containing 2–amidophenolate unit can be determined by analyzing the
systematic changes in CPh–CPh, CPh–NPh, and CPh–OPh bond distances (Chart 1) [CPh = phenyl
ring belonging a carbon atom, NPh,= the nitrogen atom attached to a phenyl ring (i.e. aniline N
atom), and OPh = the oxygen atom attached to a phenyl ring (i.e. phenol O atom)]. In the
10

dianionic, fully reduced 2–amidophenolate [(AP)2–] form, the aromaticity of the C6 ring prevails
and CPh–NPh, and CPh–OPh exhibit single bond character (Chart 1). In case of one–electron
oxidized iminosemiquinone {(ISQ1–)} and two–electron oxidized iminobenzoquinone {(IBQ0)}
forms, the aromaticity of the C6 ring collapses and a quinoid–type distortion appears.
Additionally, in the (IBQ0) form the CPh–NPh, and CPh–OPh are double bonds, while, the bonds
are of intermediate between single and double bonds in (ISQ)1– form [Chart 1].
The CC bond distances of the tert–butyl–containing phenyl rings were not within the
range of 1.390 ± 0.001 Å as would be for a fully reduced 2–amidophenolate system. An
alternating short–long–short bond distances {C3–C4 = 1.377(6) [1], 1.374(5) [1a] Å; C4–C5 =
1.433(6) [1] , 1.431(6) [1a] Å; C5–C6 = 1.380(6) [1] , 1.359(6) [1a] Å} followed by three long
bonds (Table 2) were found. Such type of quinoid–type distortion indicated oxidation of the
coordinated–ligands in the complexes. The C1–N1 and C2–O1 bond distances were 1.350(6) [1];
1.347(5) [1a] Å and 1.316(6) [1]; 1.316(5) [1a] Å, respectively, and were in between their single
bond and double bond characters. Thus, the coordinating ligands exist in their one–electron
oxidized iminosemiquinone (ISQ1–) form. Hence, X-ray structural analysis concluded that both
complex 1 and complex 1a are Ni(II)–bis(iminosemiquinone) complexes.
In neutral complex 2, the central Ni1 atom was four–coordinate with a slightly distorted
square planar geometry ( = 0.11). [37, 38] The dihedral angle between N2–N1–N3 and N1–
Ni1–O1 planes was 8.3 and indicated a small twist around the Ni atom in the molecule. An
asymmetric environment around Ni1 atom was reflected by the Ni1–O1 = 1.858(2), Ni1–N1 =
1.867(3), Ni1–N2 = 1.876(3), and Ni1–N3 = 1.830(3) Å bond distances. The difference in the
bond distances was due to the different nature (hybridization) of the coordinating atoms. The C–
C bond distances of the tert–butyl groups–containing C6 phenyl ring were almost all within 1.39
 0.02 Å range and were in accord with the phenyl C=C bond distances. [39–41] Furthermore,
both C1–N1 = 1.417(5), O1–C2 = 1.321(4) Å bond distances implied their single bond character.
These bond distances along with the double bond characterizing N1–C7 = 1.301(4) Å bond
confirmed an iminophenolate form of the tert–butyl groups–containing phenyl ring where the
PhO1– charge was located on the phenolate oxygen atom. The C–C bond distances in the C6 ring
comprised of C15–to–C20 atoms showed a substantial elongation in C15–C16 = 1.422(6), C20–
C15 = 1.426(5), and C19–C20 = 1.424(6) Å bond distances compared to the C16–C17 =
11

1.357(6), C17–C18 = 1.398(6), C18–C19 = 1.365(6) Ǻ and indicated a quinoid–type distortion in
the C6 ring. This distortion was due to the delocalization of amide negative charge over the
iminosalicylidene moiety as evidenced by short N3–C20 = 1.327(5), and C14–C15 = 1.408(5) Å
bond distances compared to their corresponding single bond characterizing values [42–44]
followed by an elongated C14–N2 = 1.324(5) Å bond distance compared to its double bond
characterizing value (1.28 Å).[45, 46] This type of alternative shortening and elongation is
common in salen–type complexes where the ligand is found to be in its fully reduced form.[38–
40] Hence, from the X–ray structural analysis it was clear that the neutral asymmetric complex 2
was Ni(II) salen–type with two imine N1 and N2 atoms, one amide N3 atom, and one phenolate
O1 atom in its coordination environment.
Four–coordinate and square planar Ni(II)–bis(iminosemiquinone) complexes 1 and 1a
contained two radical-centered unpaired electrons. The electrons could render an S = 1 or S = 0
ground state owing ferromagnetic and antiferromagnetic coupling among them, respectively. In
1
order to elucidate the electronic ground state of the complexes, H–NMR measurements were
performed (Figure S4–S5). NMR spectra as similar as that of ligand NMR spectra for both the
complexes concluded diamagnetic nature with S = 0 electronic ground state. Thus, an
antiferromagnetic coupling among the radical centers prevailed and the coupling was propagated
via filled metal-centered t2g orbitals.
Electrochemical Study
Cyclic voltammograms (CVs) for complexes 1, 1a and 2 were recorded in CH2Cl2, at a
glassy carbon working electrode, and a Pt counter electrode under argon atmosphere. The CVs
are depicted in Figure 2, Figure S16, Figure S17, Figure S18 and electron transfer potentials are
presented in Table 2. Complex 1 showed one reversible two-electron oxidation, one reversible
one-electron reduction and one irreversible reduction waves at -0.160, -0.965, and -1.640 V,
respectively. Unlike complex 1, complex 1a exhibited two one-electron reversible oxidation
waves at -0.133 V and 0.032 V, and one one–electron reversible reduction and one irreversible
reduction processes at -0.936 V and -1.594 V, respectively. A possible structural change during
one-electron oxidation, i.e., the formation of a five-coordinate species by the coordination of an
aldehyde unit in 1a+, could be the reason for two one–electron processes. On increasing the scan
rate from 50 mV/s to 500 mV/s, the peak separation between the two oxidation processes
12

diminished by 50 mV and supported the postulation. The CV of complex 2 represented
irreversible electron transfer (oxidation) processes at 0.367 and 0.591 V. A quasi–reversible
reduction occurred at -1.918 V. This could be the reduction of Ni(II) to Ni(I). [47]
Figure 2. Cyclic voltammograms of complex 1 (A); and 1a (B); measured at 100 mV/s.
Table 2. Voltammetric redox processes for complexes 1, 1a, and 2.
Complex
E0, V
Ered2
red1
ox1
ox2
E1/2
E1/2
E1/2
1
–1.640
–1.594
–0.965
–0.936
–1.918
–0.160a
-0.133
0.367b
1a
2
0.032
0.591b
a: 2e– process; b: irreversible processes.
13

Figure 3. UV–Vis–NIR oxidation study of (A) 1; (B) 1a using acetylferrocenium
hexafluorophosphate as the oxidant and ESR oxidation study of (C) 1 and (D) 1a using
acetylferrocenium hexafluorophosphate. X–band EPR spectra in CH2Cl2 solution. Conditions:
temperature = 25 C; microwave frequency = 9.446 GHz; modulation frequency = 100 kHz;
modulation amplitude = 100.0 G; and microwave power = 0.995 mW.
Complex 1 and complex 1a were oxidized chemically in CH2Cl2 using acetylferrocenium
hexafluorophosphate as the oxidant. Upon addition of 1 equivalent amount of the oxidant the
ligand-to-ligand change transfer band [27, 48, 49] at 890 nm [1] and 900 nm [1a] diminished in
intensity, while, a broad band, which could be assigned as ligand–based –* charge transfer
transitions of C=O and C=N units of an iminobenzoquinone moiety, [28] manifested at 516 nm
[1+] and 506 nm [1a+]. Addition of another equivalent of the oxidant to the solutions, the ligand-
to-ligand charge transfer band greatly reduced in intensity for complex 1+ and vanished for
14

complex 1a+. Further increase in the intensity of the broad band (506 nm) indicated the
formation of iminobenzoquinone unit upon the oxidation. While, 12+ and 1a2+ species were
diamagnetic and did not show any X–band EPR spectrum, both 1+ and 1a+ rendered radical-
based EPR signals {giso = 1.998[1+], and giso = 1.995[1a+]}. Thus, 1+ and 1a+ were Ni(II)-
(iminosemiquinone)(iminobenzoquinone) species. In the case of complex 1, which exhibited a
two-electron oxidation process, assumed that in the presence of one equivalent amount of the
oxidant 50% 12+ species was formed in the solution, initially. Thus formed species oxidized 1 by
an electron and finally 1+ existed in the solution, which provided the radical-based EPR signal.
Reactivity Study
Formation of two different complexes 1, and 1a from ligand H3LCH OH depending on the
2
employed base (Et3N or NaOH) for the deprotonation of the ligands, implied that H3LCH OH
2
behaved as bidentate ligand in the presence of triethylamine, while, as a tridentate ligand in the
presence of NaOH during the complexation process with NiCl26H2O. Employment of a strong
base like NaOH, ligand centered benzylic alcohol group (–CH2OH) might be deprotonated to
benzyloxide group (–CH2O–) and consequently, coordinated with the Ni(II) center. This caused
lowering of the energy of benzylic C–H bonds enormously and easily oxidized to the aldehyde
group by the coordinated–iminosemiquinone radical units as proposed for Cu–containing
mononuclear metalloenzyme Galactose Oxidase. [50–52] Regeneration of Ni(II)–
bis(iminosemiquinone) occurred by air. To find out the process of oxidative conversion of –
CH2OH to –CHO group, complex 1 was exposed to react with a methanolic solution of NaOH
under air: eventually, complex 1a was isolated. Therefore, by changing the basic strength, ligand
flexibility (bidentate and tridentate nature of the ligand) can be altered, which controlled the
different reactivity and thereafter, different product formation of H3LCH OH ligand in the presence
2
of NiCl26H2O.
15

Scheme 3. Showing a mechanistic proposal for the conversion of 1a from 1.
16

Scheme 4. The mechanistic proposal for the formation of H3L3 from ligand H4LCH NH2, and
2
ligand H4L2.
On the other hand, the formation of complex 2 from the initially used ligand H4LCH NH
2
2
via the in situ generation of ligand H4L2 and H3L1 and their corresponding Ni(II) complexes is
presented in Scheme 4. Ligand H4LCH NH initially reacted with NiCl26H2O in the presence of
2
2
triethylamine and presumed to provide a diradical–coordinated square planar Ni(II) complex,
[NiII(H2LCH NH )2] (Scheme 4). Mass spectrometric analysis (Figure S11) of the reaction
2
2
solution (CH3CN), obtained after immediate mixing of the reactants H4LCH NH2, NiCl26H2O,
2
and Et3N under air, exhibited a mass peak at m/z = 706.39 that corresponded to
[NiII(H2LCH NH )2] species and supported its formation. The N atom from the free amine group
2
2
of benzylamine then attacked at the imine carbon atom present in [NiII(H2LCH NH )2] species.
2
2
This resulted in the formation of an aryl migrated diradical–coordinated octahedral Ni(II)
complex [NiII(H2L2)2] and complete modification of the initially used ligand backbone.
17

[NiII(H2L2)2] underwent further two–electron aerial oxidation and provided two equivalents of
organic ligand H3L1, which upon coordination with Ni(II) ion provided [NiIIH2L1] species
(Scheme 4). The generated species then succumbed to ligand imine hydrolysis and provided 2–
aminobenzaldehyde (A) and 4,6–di–tert–butyl–2–aminophenol (B). The coordination of imine N
atom to Lewis acidic Ni(II) center activated the imine carbon atom by increasing its
electrophilicity and hence, the hydrolysis occurred favorably. Thus in situ generated species A,
and B upon condensation produced ligand H3L3 and eventually, complex 2 was formed. The
formation of complex 2 by using both H4L1 [19] and H3L1 [19] ligands supported the formation
of the moieties as the intermediate metal–chelating units during the conversion of H4LCH NH2 to
2
H3L3 under the reaction condition.
Conclusions
Ligand H3LCH OH in the reaction with NiCl26H2O in the presence of triethylamine
2
(Et3N), as the base, under air, provided a regular four–coordinate Ni(II)–bis(iminosemiquinone)
complex with a square planar geometry. In the complex, two –CH2OH units from two ligands
remained far from the central Ni(II) ion and no Ni–O(alcohol) interaction was noticed. However,
the deprotonation of the alcohol units by using stronger base NaOH, four–coordinate Ni(II)–
bis(iminosemiquinone) complex was formed along with the oxidation of the alcohol units to
aldehyde units. This implied a weak interaction between Ni(II) and O(alcoholate) atoms and
thereafter, oxidation of benzyl alcoholate units took place with the participation of two metal–
coordinated ligands–based radicals and air. Unlike ligand H3LCH OH, ligand H4LCH NH2 provided a
2
2
non–radical–containing four–coordinate salen–based Ni(II) complex. During the complex
formation a complete relocation in the ligand backbone occurred where the 2,4–di–tert–
butylphenol unit was migrated to the benzyl amine part from the aniline part. Hence, herein,
instead of oxidation, a transimination took place.
Supplementary material: IR, NMR, Mass, CV and UV–vis/NIR, spectra of the complexes (1,
1a and 2); bond distances and bond angles tables for the complexes.
18

ACKNOWLEDGEMENT
This project is supported by SERB [EMR/2015/002491], Govt of India. GCP, MK and PS
acknowledge Indian Institute of Technology Guwahati (IITG) for their doctoral fellowship. SG
thanks CSIR–UGC for fellowship. FIST programme, DST, India, The Department of Chemistry,
and CIF, IIT Guwahati are thankfully acknowledged for instrumental facility.
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24

Graphical Abstract
A Ni(II)–bis(iminosemiquinone) complex core exhibited GOase–like reactivity by oxidizing the
ligand–based benzyl alcohol moiety to the corresponding aldehyde moiety in the presence of
NaOH. However, a transimination was realized for ligand–based benzyl amine moiety.
25

HIGHLIGHTS
 Oxidation of ligand–based benzyl alcohol units to benzaldehyde units by a Ni(II)–
bis(iminosemiquinone) complex core.
 Transimination reaction in the presence of a ligand–based benzyl amine substituent in a
Ni(II)–bis(iminosemiquinone) complex core.
 X–ray analyses of the complexes were done.
 Mechanistic investigation for the oxidation and the transimination were carried out.
Conflict of Interest
Authors have no Conflict of Interest.
Graphical Abstract
26

A Ni(II)–bis(iminosemiquinone) complex core exhibited GOase–like reactivity by oxidizing the
ligand–based benzyl alcohol moiety to the corresponding aldehyde moiety in the presence of
NaOH. However, a transimination was realized for ligand–based benzyl amine moiety.
27