Cobalt complexes of redox noninnocent azo-aromatic pincers. Isolation, characterization, and application as catalysts for the synthesis of quinazolin-4(3: H)-ones

Article abstract of DOI:10.1039/d0dt00394h

Herein we report the synthesis, characterization and catalytic application of three new cobalt(ii)-complexes of redox noninnocent arylazo ligands, 2-(phenylazo)-1,10-phenanthroline (L1a), 2-(4-chlorophenylazo)-1,10-phenanthroline (L1b) and 2,9-bis(phenyldiazo)-1,10-phenanthroline (L2) respectively. The reaction of L1a with CoIICl2·6H2O produced a μ-dichloro bridged binuclear cobalt(ii)-complex [CoII2(L1a)2Cl2] (1a) while the same reaction when carried out with 2-(4-chlorophenyl)azo-1,10-phenanthroline (L1b) and 2,9-bis(phenyldiazo)-1,10-phenanthroline (L2) ligands produced two new mononuclear five-coordinate cobalt(ii)-complexes 1b and 2 respectively. In complex 1a and 1b, the ligands L1a and L1b are coordinated to the cobalt(ii)-center in a tridentate mode utilizing all of its nitrogen donor sites while in complex 2 one of the azo-donor sites of the ligand L2 remain pendant. All these complexes were characterized using available spectroscopic techniques and DFT studies. We further explored the potential of these complexes as catalysts for the synthesis of pharmaceutically important organic compounds via the functionalization of alcohols. A variety of substituted quinazolin-4(3H)-ones were synthesized under aerobic conditions via the coupling of alcohols and 2-aminobenzamide using 1b as the catalyst. Mechanistic investigations revealed that both cobalt and the arylazo scaffold act synergistically during catalysis. This journal is

Full text of DOI:10.1039/d0dt00394h

View Article Online
View Journal
Dalton
Transactions
An international journal of inorganic chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: S. Sinha, S. Das, R.
Mondal, S. Mandal and N. D. Paul, Dalton Trans., 2020, DOI: 10.1039/D0DT00394H.
Volume 46
Number 10
14 March 2017
Pages 3073-3412
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Dalton
Transactions
An international journal of inorganic chemistry
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
rsc.li/dalton
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
ISSN 0306-0012
COMMUNICATION
Douglas W. Stephen et al.
shall the Royal Society of Chemistry be held responsible for any errors
N-Heterocyclic carbene stabilized parent sulfenyl, selenenyl,
and tellurenyl cations (XH+, X = S, Se, Te)
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
rsc.li/dalton

Page 1 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT00394H
Cobalt Complexes of Redox Noninnocent Azo-aromatic Pincers.
Isolation, Characterization, and Application as Catalyst for the
Synthesis of Quinazolin-4(3H)-ones
Suman Sinha,a Siuli Das,a Rakesh Mondal,a Sutanuva Mandal,*,b and Nanda D.
Paul*,a
aDepartment of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur,
Botanic Garden, Howrah 711103, India
E-mail: ndpaul2014@chem.iiests.ac.in; ndpaul@gmail.com.
bDepartment of Chemistry, Banwarilal Bhalotia College, GT Rd, Ushagram, Asansol, West
Bengal 713303, India
E-mail: sutanuva@gmail.com.

Dalton Transactions
Page 2 of 31
View Article Online
DOI: 10.1039/D0DT00394H
Abstract. Herein we report synthesis, characterization and catalytic application of three new
cobalt(II)-complexes of redox noninnocent arylazo ligands, 2-(phenylazo)-1,10-phenanthroline
(L1a), 2-(4-chlorophenylazo)-1,10-phenanthroline (L1b) and 2,9-bis(phenyldiazo)-1,10-
phenanthroline (L2) respectively. Reaction of L1a with CoIICl2.6H2O produced a µ-dichloro
bridged binuclear cobalt(II)-complex [CoII (L1a)2Cl2] (1a) while the same reaction when carried
2
out with 2-(4-chlorophenyl)azo-1,10-phenanthroline (L1b) and 2,9-bis(phenyldiazo)-1,10-
phenanthroline (L2) ligands, two new mononuclear five-coordinate cobalt(II)-complexes 1b and
2 respectively were obtained. In complex 1a and 1b, the ligands L1a and L1b are coordinated to
the cobalt(II)-center in a tridentate mode utilizing all of its nitrogen donor sites while in complex
2 one of the azo-donor sites of the ligand L2 remain pendant. All these complexes were
characterized using available spectroscopic techniques and DFT studies. We further explored the
potential of these complexes as catalysts for the synthesis of pharmaceutically important organic
compounds via the functionalization of alcohols. Variety of substituted quinazolin-4(3H)-ones
were synthesized under aerobic conditions via coupling of alcohols and 2-aminobenzamide using
1b as the catalyst. Mechanistic investigations revealed that both cobalt and the arylazo scaffold
act synergistically during catalysis.

Page 3 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT00394H
Introduction. Application of redox-active ligands in transition metal-catalyzed homogeneous
catalysis has got renewed attention in recent years due to the understanding of the fact that this
type of scaffolds, other than coordinating the metal ions and offering steric control, can act
synergistically during chemical transformations.1-6 Active participation of ligand-centered redox
processes allows to avoid the thermodynamically unfavorable high energy metal-centered redox
events7-12 or sometimes influence the substrate binding selectivity via controlling the Lewis
acidity/basicity of the metal ions.13,14 Ligand-centered radical type substrate activation often
allows accomplishing various chemical reactions that are otherwise tough or sometimes
impossible to achieve following the classical synthetic routes. Moreover, this class of ligands
allows to achieve catalytic reactions using cheap, earth-abundant base metals which are
otherwise known to be catalyzed by expensive noble metals like rhodium, iridium, palladium
etc.1-29
In recent times using transition metal complexes of redox-active ligands as catalysts, several
research groups reported various interesting chemical transformations. For example, Chirik and
co-workers reported hydrosilylation and hydroboration, hydrogenation, cycloaddition reactions
using 3d-base metal complexes of redox-active ligands.15-18 Heyduk and co-workers reported
catalytic nitrene transfer reactions using early transition metal catalysts featuring redox-active
scaffolds.19,20 van der Vlugt and co-workers reported catalytic C(sp3)–H amination reactions
using an iron complex featuring redox-active o-aminophenol-derived NNO scaffold.21,22 Zhang
and de Bruin co-workers independently reported several catalytic reactions involving cobalt
catalyzed carbene and nitrene radical intermediates.23-29
Among the various redox-active ligands, arylazo scaffolds have drawn significant attention
over the decades because of their rich redox properties.30-34 However, the chemistry of these

Dalton Transactions
Page 4 of 31
View Article Online
DOI: 10.1039/D0DT00394H
scaffolds was mainly confined to understanding the structure-bonding correlations35-39 and
compared to the other diamine, or aminophenol based redox-active ligands, the potential of
arylazo ligands in catalysis has been less explored. Recently, only a few reports have come up
where this class of complexes were used as catalysts for alcohol dehydrogenation and catalytic
C-N cross-coupling reactions.40-47 Therefore, the synthesis of new well-defined transition metal
complexes of arylazo scaffolds and studies of their catalytic activities is of significant
importance.
Herein we describe the synthesis and characterization of three new cobalt(II)-complexes of 2-
(arylazo)-1,10-phenanthroline (L1a/b) and 2,9-bis(phenyldiazo)-1,10-phenanthroline (L2) ligands.
Reaction of 2-(phenylazo)-1,10-phenanthroline (L1a) with CoIICl2.6H2O produced a µ-dichloro
bridged binuclear cobalt(II)-complex [Co2(L1a)2Cl2] (1a) while the same reaction when carried
out with 2-(4-chlorophenylazo)-1,10-phenanthroline (L1b) and 2,9-bis(phenyldiazo)-1,10-
phenanthroline (L2) ligands, two new mononuclear five-coordinate cobalt(II)-
complexes 1b and 2 respectively were obtained (Scheme 1). In complex 1a and 1b the
ligands L1a and L1b are coordinated to the cobalt(II)-ion in a tridentate mode utilizing all of its
nitrogen donor sites while in complex 2 one of the azo-donor sites of the ligand L2 remains
pendant (Figure 1). Beyond mere characterization, we explored the application of these
complexes in catalytic synthesis of quinazolin-4(3H)-ones via coupling of 2-aminobenzamides
and alcohols. All the complexes showed promising catalytic activity under air. Variety of multi-
substituted quinazolin-4(3H)-ones bearing different functional groups were prepared in
satisfactory yields under air using 1b as the catalyst. Spectroscopic characterization and
mechanistic investigations revealed that the arylazo moiety acts synergistically with the cobalt
ion during catalysis.

Page 5 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT00394H
Results and Discussion.
Synthesis and Characterization of Cobalt(II) Complexes. Three arylazo ligands, 2-
(phenylazo)-1,10-phenanthroline (L1a), 2-(4-chlorophenylazo)-1,10-phenanthroline (L1b) and
2,9-bis(phenyldiazo)-1,10-phenanthroline (L2) respectively were used in this work (Scheme 1).
The ligands, L1a and L1b, are two tridentate scaffolds, which differs only to substitution of the
phenyl ring of the arylazo moiety while the L2 has four nitrogen donor sites (Scheme 1). These
ligands were synthesized following a reported literature procedure.45,46
The reaction of hydrated CoIICl2.6H2O (17.0 mmol) with L1a (17.0 mmol) produced a neutral
light brown colored µ-dichloro bridged binuclear cobalt(II)-complex [CoII (L1a)2Cl2] (1a) in 90%
2
yield. A dark brown penta-coordinate cobalt(II)-monomer, [CoIIL1bCl2] (1b), was isolated in
92% yield from the reaction of CoIICl2.6H2O with the 2-(4-chlorophenylazo)-1,10-
phenanthroline (L1b) under identical reaction conditions. Reaction with the ligand L2 afforded a
new dark brown colored monomer [CoIIL2Cl2] (2) in 85% yield (Scheme 1). All the complexes
are paramagnetic. The room temperature magnetic moment μeff of complex 1a is 4.25 μB, 1b is
4.20 μB, and 2 is 4.35 μB which are consistent with the high-spin Co(II)-electronic configuration
having three unpaired electrons. These considerably higher magnetic moment values (spin-only
value for high-spin CoII(S = 3/2, µeff = 3.87µB) may be attributed to the spin-orbit coupling.48
These complexes were also characterized by IR spectroscopy. In the IR spectra of 1a and 1b, the
N=N stretching frequencies were observed at 1410 cm-1, while two types of N=N stretching
frequencies were observed at 1405 cm-1 and 1440 cm-1, respectively, for complex 2. Compared
to the N=N stretching frequency of the uncoordinated ligands L1a/b (1450 cm-1), and L2 (1455
cm-1), the lower N=N stretching frequencies observed in the IR spectra of 1a/b and 2 may be
attributed to Co(d()) to ligand(*) back donation.40-47 The resonance at 1440 cm-1 in 2 may be
attributed to the pendant azo-moiety.45

Dalton Transactions
Page 6 of 31
View Article Online
DOI: 10.1039/D0DT00394H
N
N
Cl
N
N
N
N
N
N
N
Cl
Cl
EtOH, Reflux
6 hour
CoIICl2.6H2O
CoII
CoII
+
N
N
Cl
1a
N
L1a
Cl
Cl
N
N
N
N
N
N
N
N
EtOH, Reflux
6 hour
Cl
CoIICl2.6H2O
CoII
Cl
+
L1b
1b
N
N
N
N
N
N
N
N
N
EtOH, Reflux
6 hour
Cl
Cl
CoIICl2.6H2O
CoII
N
+
N
N
L2
2
Scheme 1. Synthesis of the cobalt(II) complexes.
X-ray Crystallography. The identity of these complexes was confirmed by X-ray single-crystal
diffraction. X-ray quality crystals of complex 1a, 1b, and 2 were grown via slow evaporation of
their dichloromethane/hexane (5:1) solutions under air. The relevant crystallographic parameters
are given in Table S1 (see supporting information), and the thermal ellipsoid plots for the
molecular structure of complex 1a, 1b, and 2 are shown in Figure 1.
Figure
1.
ORTEP
diagram of 1a, 1b and 2 with partial atom numbering scheme. Ellipsoids are drawn at the 50%

Page 7 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT00394H
probability level. Hydrogen atoms are omitted for clarity.
The X-ray structure of complex 1a consists of a dimeric [Co2Cl2(L1a)2] unit with a central
planar Co2Cl2 ring. Geometry around each cobalt(II) center is a distorted octahedral. The
ligand L1a coordinates the central cobalt(II)-ion in a tridentate meridional fashion, and the rest of
the three sites are coordinated to three chlorido ligands out of which two are bridging in nature.
Two chlorido ligands bridge two cobalt(II) ions at a distance of 3.624(7)Å. The presence of a
crystallographic inversion center makes one half of the molecule identical to the other half. The
Co1-Cl2/Cl2A-Co1A dihedral angle is 95.98(5)°. The N=N bond lengths of both the ligands are
equal, c.a. 1.261(7)Å.
On the other hand, the X-ray structure of 1b and 2 revealed monomeric penta-coordinate
geometry around the cobalt(II) ion. In complex 1b, the ligand L1b coordinates the cobalt(II)-ion
in a tridentate mode with two chlorido ligands occupying the rest two sites. In compound 2, the
ligand L2 binds the cobalt(II)-ion in a tridentate manner using two N-donor atoms of the 1,10-
phenanthroline moiety and one N-donor atom of the azo-chromophore while the other azo-arm
remains uncoordinated at a distance of 4.598(2) Å from the cobalt(II) center. The geometry of
both 1b and 2 are in-between squire pyramid and trigonal bipyramid (τ (index of trigonality) =
0.45).49 In complex 1b, N=N bond length is 1.257(3)Å, while two distinctly different N=N bond
lengths were observed in compound 2. The coordinated N=N bond length is 1.265(2)Å, whereas
the non-coordinated N=N length is 1.246(3)Å.40-45 Compared to the free ligands, the slightly
elongated N=N bond distances of the coordinated azo-chromophores may be attributed to the
Co(d()) to ligand(*) back donation as reflected in the IR spectra of these complexes (see
above).40-47

Dalton Transactions
Page 8 of 31
View Article Online
DOI: 10.1039/D0DT00394H
Cyclic Voltammetry and DFT Studies. To understand the redox properties, cyclic
voltammograms of 1a, 1b, and 2 were recorded in CH3CN solutions at 30°C; potentials are
referenced to the saturated Ag/AgCl electrode. Voltammograms are displayed in SI. In our
previous work, we observed that the arylazo ligand, L1a undergoes one reversible reduction at
−0.89 V and one irreversible reduction at −1.34 V, while the cyclic voltammogram of L2 exhibits
two reversible reductive waves at −0.96 and −1.11 V.45 The cobalt(II)-
complexes 1a, 1b and 2 showed multiple irreversible ligand centered reductions in the potential
range -0.32 to -1.27.45 The irreversibility observed during reduction processes may be attributed
to the dissociation of one chlorido ligand upon reduction. Compared to the free ligands, the
significant anodic shift in the reduction potentials indicates that the ligand centered reduction
becomes much facile upon coordination to the Co(II)-centers as was observed previously upon
coordination to other metal transition metal ions.40-47
To understand the electronic levels associated with these redox processes, DFT calculations
were performed using the Gaussian 09 series of programs with B3LYP hybrid functional
(G09/B3LYP).50-52 In the calculations, the LANL2DZ53 basis function has been employed for
Co, and 6-31G* basis set for carbon, hydrogen, nitrogen, and chlorine.54
Geometry optimization of the complexes 1b and 2 reveals that the cobalt ion in these
complexes is high-spin, having an electronic structure description of [CoII{(L1b)}0Cl2] (1b), and
[CoII{(L2)}0Cl2] (2) respectively. High-spin Co(II)-ion (S = 3/2) is coordinated to the unreduced
azo-aromatic ligands L1b or L2, respectively. Broken-symmetry55-57 calculations performed to
identify the presence of ligand-centered azo-anion radicals, however, converged back to the high
spin Co(II)-ion (S = 3/2) configuration. The spin density analysis shows that in complex 1b, the
Co(II)-center posses +2.63 (α spin), and in complex 2 the Co(II)-center posses +2.65 (α spin)

Page 9 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT00394H
(Figure 2). Molecular orbital analysis revealed that the LUMOs are ligand-based mostly
distributed over the azo-chromophore which in turn support the engagement of the azo-
chromophore during reduction processes.40-47 This indeed highlights the possibility of utilization
of the arylazo scaffolds as electron reservoir during multi-electron chemical transformations.40-47
Figure 2. Spin density plot of 1b (left) and 2 (right).
Catalytic Synthesis of Quinazolin-4(3H)-ones. With these characterization data in hand, we
next shifted our focus to explore the application of these complexes towards the catalytic
synthesis of biologically active quinazolin-4(3H)-one scaffolds via functionalization of
inexpensive and sustainable alcohols. It is worth mentioning that quinazolin-4(3H)-ones
represent one of the privileged building blocks found in a myriad of naturally occurring
molecules, biologically active compounds, and pharmaceuticals.58-60 The classical synthetic
approaches developed so far for the construction of quinazolin-4(3H)-one skeletons mostly
proceed via time-consuming multi-step synthetic procedures, require strong oxidants, generates
copious amount of waste and mostly require expensive heavier metals as catalysts.61-70
Therefore, synthesis of substituted quinazolin-4(3H)-ones via functionalization of sustainable
alcohols using inexpensive and abundantly available cobalt catalysts under aerobic conditions
would be of great importance.71-75
Optimization of Reaction Conditions. Our initial catalytic synthesis of quinazolin-4(3H)-one

Dalton Transactions
Page 10 of 31
View Article Online
DOI: 10.1039/D0DT00394H
begin with the reaction of benzylalcohol (3a) with 2-aminobenzamide (4a) using complex 1b as
the catalyst. Several control reactions were carried out varying different reaction parameters such
as solvents, bases, temperature, reaction time, catalyst loading, etc. to obtain the optimal reaction
conditions (Table 1). The coupling of 3a and 4a proceeds well under air in toluene, xylene in the
presence of an inorganic base like KOH or NaOH. A significant decrease of yield was observed
in MeOH, acetonitrile, DMF, etc. 2-phenylquinazolin-4(3H)-one (5aa) was isolated in the
highest yield of 90% at 120°C in the presence of 4.0 mol% of 1b and 0.5 equiv. of NaOH (Table
1, entry 4). Significant decreases of yield of 5aa was observed upon lowering of temperature
below 120°C; however, the yield of 5aa did not increase significantly upon increasing the
catalyst loading or temperature. No progress of the catalytic reaction was observed in the
absence of NaOH.

Page 11 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT00394H
Table 1. Optimization of Reaction Conditions.a-d
O
N
O
OH
NH
NH2
NH2
Co-Catalyst
+
Base, solvent,
temperature (°C)
3a
5aa
4a
Entry
Catalyst(mol%)
Solvent
Base
Temperature (°C) Yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
1b (4.0 mol%)
1b (4.0 mol%)
1b (4.0 mol%)
1b (4.0 mol%)
1b (4.0 mol%)
1b (4.0 mol%)
1b (4.0 mol%)
1b (4.0 mol%)
1b (4.0 mol%)
1b (4.0 mol%)
1b (4.0 mol%)
1a (4.0 mol%)
2 (4.0 mol%)
1b (4.0 mol%)
1b (2.5 mol%)
-
xylene
xylene
xylene
xylene
xylene
xylene
toluene
acetonitrile
ethanol
THF
KOtBu
NaOtBu
KOH
NaOH
K3PO4
NEt3
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
-
120
120
120
120
120
120
120
120
120
120
120
120
120
100
120
140
140
140
75
71
88
90
trace
NR
83
trace
trace
52
trace
84
80
65
73
NR
NR
trace
DMF
xylene
xylene
xylene
xylene
xylene
xylene
xylene
1b (4.0 mol%)
CoCl2.6H2O
(10.0 mol%)
NaOH
19
20
Co(ClO4)2.6H2O
(10.0 mol%)
CoCl2(phen)2
(10.0 mol%)
xylene
xylene
NaOH
NaOH
140
140
trace
trace
aStoichiometry: benzylalcohol (3a) (1.0 mmol); 2-aminobenzamide (4a) (1.0 mmol),
bsolvent: 5.0 mL; cunder air; dIsolated yields after column chromatography.
In the presence of only base, 5aa was obtained in trace amounts. Interestingly all three
complexes, 1a, 1b, and 2, act as an efficient catalyst in this reaction. However, the
complex 1b was found to be slightly more effective (Table 1, entries 4, 12, 13). The commonly
available cobalt(II) salts such as Co(ClO4)2.6H2O, CoCl2.6H2O are found to be ineffective (Table
1, entries 18, 19). To be more sure about the involvement of our catalyst, we performed the

Dalton Transactions
Page 12 of 31
View Article Online
DOI: 10.1039/D0DT00394H
catalytic reaction using Co(1,10-phen)2Cl2 as the catalyst.76 However, only a trace amount of the
desired product, 5aa, was obtained using Co(1,10-phen)2Cl2 as the catalyst under our optimal
conditions (Table 1, entry 20).
Substrate Scope. To explore the versatility of the present cobalt-catalyzed quinazolin-4(3H)-
ones synthesis, we investigated the substrate scope, both for alcohols and 2-aminobenzamides
(Tables 2, 3). Firstly, we explored the functional group tolerance of a wide variety of benzyl
alcohols. Reactions were performed under the optimized conditions between 4a with benzyl
alcohols (3a-s) containing various electron-donating, withdrawing, and heterocycle groups.
Reactions proceeded well in the presence of both electron-donating and withdrawing
substituents on the benzyl alcohols, and the desired quinazolin-4(3H)-ones were isolated in
satisfactory yields. In the presence of electron-donating -methyl, -methoxy or tert-butyl groups,
the coupling of the respective benzyl alcohols (3b-h) with 4a proceeds smoothly under the
optimized conditions producing the corresponding quinazolin-4(3H)-ones (5ba-5ha) in 65-79%
yields (Table 2, entries 2-8). Benzyl alcohols have electron-withdrawing halogens at ortho-,
para- or meta- positions yielded the desired quinazolin-4(3H)-ones (5ia-5oa) in 70-75% isolated
yields (Table 2, entries 9-15). Strongly electron-withdrawing -CF3 group was also tolerated
under the optimal reaction conditions affording the corresponding quinazolin-4(3H)-one, 5pa in
76% isolated yield (Table 2, entry 16). Heteroaryl alcohols like 2-thiophenemethanol (3q), 2-
pyridinemethanol (3r) yielded the desired quinazolin-4(3H)-ones, 5qa and 5ra in 68 and 63%
yields respectively (Table 2, entries 17, 18). Aliphatic alcohols are also compatible as the
coupling partner; however, the desired quinazolin-4(3H)-ones are obtained in moderate yield
after 48h.

Page 13 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT00394H
Table 2. Substrate Scope with Various Alcohols.a-d
O
O
N
1b
(4.0 mol%)
OH
NH2
NH2
NH
+
Xylene, NaOH (0.5
equiv.)
R
120 oC, 30h
4a
5aa - 5sa
3a-s
Benzylalcohols Bearing Electon Donating Groups
O
O
O
O
3
4
2
1
NH
NH
NH
NH Me
Me
N
N
N
N
5aa
90%
5ba
72%
5ca
71%
Me
5da
75%
O
O
O
5
O
N
8
6
7
NH
NH OMe
NH
NH
OMe
N
N
N
tBu
OMe
5ea
65%
OMe
5ga
5ha
79%
5fa
70%
78%
Benzylalcohols bearing Electron Withdrawing Groups
O
N
O
O
O
11
12
9
10
NH
NH
NH Cl
NH
Cl
N
N
N
5ja
68%
5ia
72%
5ka
71%
5la
70%
Cl
F
O
O
O
O
16
14
15
13
NH
NH
NH
NH
Br
Br
N
N
N
N
CF3
5na
70%
5oa
75%
5pa
Br
5ma
69%
76%
Benzylalcohols bearing Heterocyclic Functionalities
Aliphatic Alcohol
O
O
O
19
17
18
NH
NH
NH
N
S
N
N
N
5qa
68%
5ra
63%
5sa
23%
aStoichiometry: benzyl alcohol (1.1 mmol); 2-aminobenzylamine (1.0 mmol); NaOH (0.5 equiv).
bUnder air; cSolvent: 5.0 mL; dIsolated yields after column chromatography.
Substrate scope was further explored using various substituted 2-aminobenzamides (4b-f) as
the coupling partners (Table 3, entries 1-6). Reactions proceeded well with 2-aminobenzamides
containing electron-donating and withdrawing functionalities. Desired quinazolin-4(3H)-ones
were isolated in acceptable yields even in the presence of both electron-withdrawing and
donating groups on 2-aminobenzamide or in presence multiple electron-donating groups (Table

Dalton Transactions
Page 14 of 31
View Article Online
DOI: 10.1039/D0DT00394H
3, entries 4-6). Reaction also proceeds with 3-aminonaphthalene-2-carboxamide (4g), the desired
quinazolin-4(3H)-one, 5ag was isolated in 76% yield (Table 3, entry 7).
Table 3. Substrate Scope with Various Amides.a-d
O
O
N
1b
OH
NH2
NH2
NH
(4.0 mol%)
+
Xylene, NaOH
120o, 30h
R
R
3a,b
NH
5ab-ag,kf
4b-g
O
1
3
2
O
4
O
O
Me
MeO
NH
NH
NH
N
N
N
N
MeO
O
Me
Br
OMe
5ab
81%
5ac
75%
5ad
83%
5ae
85%
O
N
O
N
5
7
6
Me
Me
NH
NH
NH
N
Br
Br
Cl
5af
82%
5kf
75%
5ag
76%
aStoichiometry: benzyl alcohol (1.1 mmol); 2-aminobenzamide (1.0 mmol); base (0.5 equiv);
bUnder air; cSolvent: 5.0 mL; dIsolated yields after column chromatography.
Mechanistic Investigation. To unveil the mechanistic pathway and to explore the plausible
intermediates involved in the above-reported cobalt catalyzed synthesis of quinazolin-4(3H)-
ones, we performed some control experiments under the optimized reaction conditions. Since we
performed the catalytic reactions at 120°C, firstly, we studied the stability of our catalyst at a
higher temperature. Notably, under both aerobic and inert conditions, all the catalysts¸ 1a, 1b,
and 2 were found to be stable at 120°C in xylene and other common organic solvents. Even
when we heated a solution of these complexes at 150°C, we did not observe any decomposition.
Next, to check the homogeneous nature of the catalytic reactions, as well as to confirm the
involvement of 1b, we performed the mercury poisoning test. Both the dehydrogenation of
alcohols as well as the dehydrogenative coupling of alcohols with benzamides proceeded well in
the presence of mercury. In fact, the dehydrogenative coupling of 3a with 4a yielded 5aa in 89%

Page 15 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT00394H
yield (see Experimental Section for details). These experimental results indeed rule out the
possibility of involvement of some in-situ formed cobalt nano-particles and confirm the active
participation of 1b during these catalytic reactions.
Once we confirmed the active participation of 1a in these catalytic reactions, next, we paid
attention to exploring the plausible pathways of cobalt catalyzed conversion of alcohols to the
corresponding carbonyls. To probe the formation of any organic radical during 1b catalyzed
dehydrogenation of alcohols, we performed the dehydrogenation of 3a in the presence of a
radical inhibitor 2,2-diphenyl-1-picrylhydrazyl (DPPH). Contrary to our previous results on iron-
catalyzed dehydrogenation of alcohols,45 the present cobalt-catalyzed dehydrogenation of benzyl
alcohol remains unaffected in the presence of DPPH (Scheme 2). Under the optimized
conditions, in the presence of DPPH, benzaldehyde (3a') was isolated in 90% yield as the
dehydrogenated product of 3a (Scheme 2). Moreover, the coupling of 3a and 4a also proceeds
well in the presence of DPPH yielding 5aa in almost identical yield (Scheme 2). These
experimental observations point to the non-involvement of any organic radical during this cobalt-
catalyzed transformations. To further confirm it, we performed the dehydrogenation of
cyclobutanol (3t) under our optimal conditions. Formation of cyclobutanone (3t') as the sole
dehydrogenated product of 3t further strengthen the fact that 1b-catalyzed dehydrogenation of
alcohols does not involve any ketyl radical intermediate; instead, it proceeds via two-electron
hydride transfer pathway (Scheme 2).40,44

Dalton Transactions
Page 16 of 31
View Article Online
DOI: 10.1039/D0DT00394H
1) Progress of Reaction in Presence of Radical Scavenger DPPH
DPPH (2.0 equiv.)
OH
1b (3.0 mol%)
O
i)
NaOH (0.5 equiv)
Toluene, 80oC, 4h
(3a', 90%)
(3a)
O
O
NH
O
NH2
toluene
ii)
+
reflux, argon
NH2
N
H
(3a')
(4a)
(6)
O
O
N
DPPH (2.0 equiv.)
1b (3.0 mol%)
NH
NH
iii)
NaOH (0.5 equiv)
Xylene, 130oC, 24h
N
H
(6)
(5aa, 92%)
2) Dehydrogenation of Radical Clock Substrate (Cyclobutanol)
O
OH
1b (3.0 mol%)
CHO
+
NaOH (0.5 equiv)
Toluene, 80oC, 4h
(3t')
(3t'')
(3t)
Cyclobutanol
Cyclobutanone
(2e- hydride transfer product)
4-hydroxybutanal: Not Found
(1e- HAT product)
Scheme 2. Feasibility of radical vs. non-radical pathways during 1b-catalyzed dehydrogenation
of alcohols.
Next, we performed the dehydrogenation of 1-phenylethanol (3u) stoichiometrically under an
inert atmosphere in the presence of 0.5 equiv. NaOH, and 1.0 equiv. of 1b and analyzed the
reaction mixture using IR spectroscopy (See SI). Interestingly, instead of the N=N stretching
observed at 1405 cm−1 for the parent complex 1b (see above), the reaction mixture showed N−H
stretching at 3029 and 3064 cm−1 respectively indicating the conversion of the azo-chromophore
to its two-electron reduced hydrazido analogue.40,44,45 Moreover, when the same stoichiometric
reaction was performed using deuterated 1-phenylethanol (3u-D2), characteristic N−D stretching
frequencies were observed at 2124 cm−1 (see SI). 40,44,45 These experimental findings points to the
direct involvement of the azo/hydrazo redox couple during the dehydrogenation of alcohols. The
azo-chromophore acts as the acceptor of hydride formed during the dehydrogenation of alcohols
and facilitates the dehydrogenation reactions.40,44 However, when we performed the same
reactions under aerobic conditions, we did not observe any N−H stretching; instead, we observed

Page 17 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT00394H
the formation of H2O2. Even when we exposed the reaction mixture, obtained from the
stoichiometric dehydrogenation of 3a, to the air, we observed H2O2 formation, which indicates
that the hydrazido intermediate formed during stoichiometric dehydrogenation of 3a under inert
conditions, react with dioxygen to form H2O2.40,44,45 The active involvement of the N=N
chromophore, further strengthened from the fact that the dehydrogenation of 3a, as well as the
dehydrogenative coupling, did not proceed well in the presence of [Co(1,10-phen)2Cl2] as the
catalyst (Table 1, entry 20). To further understand the involvement of the coordinated azo-
chromophore, we repeated the stoichiometric dehydrogenation of 3a under inert conditions
using 2 as the catalyst and analyzed the reaction mixture using IR spectroscopy. In line with the
results obtained with 1b, N-H stretching was observed at 2934, and 2973 cm−1 and an N=N
stretching was observed at 1410 cm−1 pointing to the non-involvement of the pendant azo-
chromophore present in 2 during these dehydrogenation reactions.
Finally, we performed the dehydrogenation of 3u in the presence of an easily reducible
substrate like 4-methoxybenzaldehyde (3f') to explore the possibility of transfer hydrogenation
(Scheme 3).71 Under argon atmosphere, dehydrogenation of 3u in the presence of 3f' did not
proceed; only a trace amount of acetophenone (3u') was observed on the TLC plate (Scheme 3).
On the other hand, under air, acetophenone was obtained in 95% yield, and H2O2 was detected as
the hydrogenated product of the molecular oxygen (Scheme 3).40-45 However, in both cases, we
did not observe any transfer hydrogenated product of 3f'.

Dalton Transactions
Page 18 of 31
View Article Online
DOI: 10.1039/D0DT00394H
Transfer Hydrogenation Experiment: To Probe the Evolution of H2
i) Under Argon Atmosphere
H
O
O
O
O
H
OH
1b (3.0 mol%)
H
H
+
+
+
a)
NaOH (0.5 equiv)
Toluene, 80oC, 4h
MeO
MeO
MeO
(3u)
(3f')
(3u')
(3f')
(3f)
Easily reducible
Trace
Unreacted Transfer hydrogenation product
Not observed
O
O
N
O
O
NH
OH
1b (3.0 mol%)
NaOH (0.5 equiv)
Xylene, 130oC, 24h
H
H
NH
+
+
+
b)
MeO
MeO
MeO
N
H
(3f')
Easily reducible
(5aa)
Trace
(3f')
(3f)
6
(
)
Unreacted Transfer hydrogenation product
Not observed
ii) Under Aeroric Condition
H
O
O
O
O
H
OH
H
1b (3.0 mol%)
NaOH (0.5 equiv)
Toluene, 80oC, 4h
H
+
+
+
a)
MeO
(3f)
MeO
MeO
(3u')
(3f')
(3u)
(3f')
90%)
Unreacted Transfer hydrogenation product
Easily reducible
Not observed
O
O
O
N
O
OH
NH
H
1b (3.0 mol%)
NaOH (0.5 equiv)
Xylene, 130oC, 24h
NH
H
+
+
b)
+
MeO
N
H
MeO
MeO
6
(3f')
Easily reducible
(5aa)
90%
(3f')
(3f)
(
)
Unreacted Transfer hydrogenation product
Not observed
Scheme 3. Transfer hydrogenation reactions using 1b.
Next, we paid attention to understand the plausible mechanism for the dehydrogenation of
the intermediate 2,3-dihydro-2-phenylquinazolin-4(1H)-one (6).71 Similar to the
dehydrogenation of alcohol, under the optimal reaction conditions, the pre-formed 6 undergoes
dehydrogenation to form 5aa in 90% yield, and H2O2 was detected as the corresponding
hydrogenated product of molecular oxygen (Scheme 4). Under argon atmosphere,
dehydrogenation of 6 did not proceed, and only a trace amount of 5aa was observed on the TLC
plate (Scheme 4). Notably, dehydrogenation of 6 also remained unaffected in the presence of
DPPH (Scheme 2). Moreover, the reaction mixture obtained from the stoichiometric
dehydrogenation of 6, under argon atmosphere, showed N-H stretching at 3025 and 3060 cm−1
respectively indicating the active participation of the arylazo scaffold during 1b-catalyzed
dehydrogenation of 6 as discussed above during 1b-catalyzed dehydrogenation of 3a.40,44,45

Page 19 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT00394H
H
H
O
O
1b (3.0 mol%)
i)
+
H2O2
NaOH (0.5 equiv)
2.0
1.5
1.0
Toluene, 80oC, 4h
90%
Air
-
I3
H2O2
+
KI
Absorption peak
at 350 nm
O
O
N
NH
ii)
1b (3.0 mol%)
NH
+
H2O2
0.5
0.0
N
H
NaOH (0.5 equiv)
Xylene, 130oC, 24h
(6)
Air
92% (5aa)
350
400
450
500
550
600
-
I3
H2O2
+
KI
nm
Absorption peak
at 350 nm
−
Scheme 4. Detection of H2O2: Identification of absorption Peak of I3 .
A plausible mechanistic cycle for the 1b-catalyzed synthesis of quinazolin-4(3H)-ones is
shown in Scheme 5. In the presence of a base, alcohols undergo deprotonation and
coordinate 1b to form a cobalt-alkoxy intermediate A which upon β-H elimination produces a
cobalt hydride species (B) and aldehyde. Subsequent transfer of the cobalt-bound hydride to the
azo-chromophore leads to the formation of the intermediate C. Next, the intermediate D is
formed via the mono-protonation of C . Finally, the intermediate D reacts with oxygen and
transforms back to A and generates H2O2. The aldehyde, thus formed, undergo
cyclocondensation with 2-aminobenzamide to form 2,3-dihydro-2-arylquinazolin-4(1H)-one
intermediates, which upon further 1b-catalyzed dehydrogenation afforded quinazolin-4(3H)-
ones.

Dalton Transactions
Page 20 of 31
View Article Online
DOI: 10.1039/D0DT00394H
N
N
N
N
Co
Cl
Cl
Cl
PhCH2OH
HCl
NaOH
O
NH2
O
NH2
O
O
NaOH
H2O
NH
N
N
NaOH
NH
N
N
N
N
R
N
Co
N
1b
- HCl
N
R
Cl
N
H
Co
ArCHO
H2O2
O2
N
Cl
O
R
Cl
Ar
R
A
(
)
H2O2
O2
NH
Ar
Cl
O
E
(
)
N
N
N
N
NH
N
N
N
N
N
N
N
Co
Co
H
N
Co
N
NH
Co
Cl
Cl
O
Cl
H
Cl
Cl
Cl
H
R
Cl
HN
Ar
F
(
)
B
(
)
D
(
)
N
Ar
NH
Cl
O
(H)
N
NH
N
N
N
NH
N
Co
Cl
O
PhCH2OH
N
Co
Cl
Cl
NH
G
(
)
Cl
N
C
(
)
H
R
Scheme 5. Proposed Mechanism.
Conclusion. In summary, we reported the synthesis and characterization of three new cobalt(II)-
complexes coordinated to 2-(phenylazo)-1,10-phenanthroline (L1a), 2-(4-chlorophenylazo)-1,10-
phenanthroline (L1b) and 2,9-bis(phenyldiazo)-1,10-phenanthroline (L2) ligands. We explored
the potential of these well-defined cobalt(II)-complexes to act as an efficient catalyst during the
synthesis of quinazolin-4(3H)-ones under air. All the complexes showed promising catalytic
activity and variety of substituted quinazolin-4(3H)-ones were prepared under air starting from
inexpensive and easily accessible alcohols and amides. Mechanistic investigations revealed that
cobalt and arylazo moiety act in a synergistic way during the catalysis and allow avoiding
energetically demanding two-electron cobalt centered redox processes. Overall, the present work
describes the potential of simple and economically affordable base metal complexes as efficient

Page 21 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT00394H
homogeneous catalysts to bring about useful chemical transformations, which are usually carried
out expensive heavy metals.
Experimental Section.
General Information. CoCl2.6H2O, and Co(ClO4)2.6H2O were purchased from Sigma-Aldrich.
All other reagents and chemicals were purchased from commercial sources and used without
further purifications. Tetrabutylammonium perchlorate was prepared and recrystallized as
reported before.40 Solvents were dried prior to use following the standard methods. All the other
starting materials and reagents used in this work were purchased from commercial suppliers and
were used as received. Analytical TLC was done using Merck 60 F254 silica gel plate (0.25mm
thickness) and column chromatography was performed using Merck 60 silica gel (60-120 mesh).
A Perkin Elmer 240C elemental analyzer was used elemental analysis. Bruker Avance
300/400/500 MHz and JEOL 400 MHz spectrometers were used for recording 1H NMR spectra,
and TMS was used as the internal standard. Electrochemical experiments were carried out using
a PC-controlled AUT.MAC204 electrochemistry system. Room-temperature magnetic moment
measurements were carried out with a Gouy balance (Sherwood Scientific, Cambridge, U.K.).
Uncorrected melting points are reported using a XT4A melting point apparatus. Caution!
Perchlorates have to be handled with care and appropriate safety precautions.
Synthesis of [CoII (L1a)2Cl2] (1a). In a 100 mL round bottom flask 50 mg (0.17 mmol) of the
2
.
ligand (L1a) was dissolved in 15 mL ethanol and 40.7 mg (0.17 mmol) CoIICl2 6H2O was added
to it. The orange color of the solution immediately changes to reddish brown upon addition of
the metal salt. The reaction mixture was refluxed for six hours in a heating mantle. After the
completion of the reaction, the reaction mixture was passed through a filter paper and a dark
brown color precipitate was collected. The complex [CoII (L1a)2Cl2] was purified by fractional
2

Dalton Transactions
Page 22 of 31
View Article Online
DOI: 10.1039/D0DT00394H
crystallization using dichloromethane/hexane solvent mixture. Its yield and characterization data
are as follows: Yield 90%. UV/Vis: λmax/nm (ε, M-1cm-1) 404 (15,734), 342 (10,292), 300 (9,422),
237, (20,749). IR (KBr cm-1): 1605 (ν C=N), 1410(ν N=N) Anal. Calcd for C36H24Cl4Co2N8: C,
52.20; H, 2.92; N, 13.53. Found: C, 52.05; H, 3.10; N, 13.62.
Synthesis of [CoL1bCl2] (1b). This was synthesized following the same procedure as described
for 1a. Its yield and characterization data are as follows: Yield 92%. UV/Vis: λmax/nm (ε, M-1cm-1)
396 (16,524), 335 (13,372), 235 (25,712). IR(KBr cm-1): 1600 (ν C=N), 1410 (ν N=N). Anal.
Calcd for C18H11Cl3CoN4: C, 48.19; H, 2.47; N, 12.49. Found : C, 48.05; H, 2.63; N, 12.39.
Synthesis of [CoL2Cl2] (2). It was also synthesized as following the same procedure as 1a/b,
taking 2,9-bis-2-phenyldi-azo-(1,10-phenanthroline) (L2) as the ligand. Its yield and
characterization data are as follows: Yield 85%. UV/Vis: λmax/nm(ε, M-1cm-1) 376 (25,410), 321
(27,514), 233 (23,998). IR(KBr cm-1): 1605 (ν C=N), 1405 (ν N=N (coordinated)), 1440 (ν N=N
(pendant)). Anal. Calcd for C24H16Cl2CoN6: C, 55.62; H, 3.11; N, 16.22. Found : C, 55.50; H,
3.25; N, 16.08.
General Procedure for the Catalytic Synthesis of Quinazolin-4(3H)-ones. Under aerobic
conditions, a mixture of CoL1bCl2 (1b) (4.0 mol%), NaOH (0.5 mmol), and 2-aminobenzamide
(1.0 mmol) were taken in an oven dried 50 mL round bottom flask. Alcohols (1.1 mmol)
dissolved in 5.0 mL xylene was added to the round bottom flask using a syringe. The reaction
mixture was then refluxed in an oil bath under aerobic conditions at 120°C for 30h. After the
reaction was complete, the reaction mixture was concentrated under vacuum and purified by
flash silica column chromatography using petroleum ether and ethyl acetate as the eluent.

Page 23 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT00394H
Hg0 Poisoning Experiment. Under the optimized aerobic conditions, catalyst 1b (4.0 mol%),
NaOH (0.5 equiv.), benzyl alcohol (3a, 1.0 mmol) and 2-aminobenzamide (4a, 1.0 mmol) were
added to a round bottom flask containing a magnetic stir bar. To it mercury (50 equiv.) and 5.0
mL xylene was added and refluxed in stirring condition at 120°C for 30h. Once the reaction is
complete, the solvent of the resulting mixture was concentrated under reduced pressure and the
organic part was extracted with diethyl ether and subjected to column chromatography using a
mixture of hexane and ethyl acetate as eluent. Isolated yield of 5aa is 89%.
X-ray Crystallography. X ray quality crystals of complexes 1a, 1b, and 2 were grown via slow
evaporation of their dichloromethane/hexane (5:1) solutions under air. The X-ray single crystal
data of 1a, 1b, and 2 were collected with monochromated Mo-Kα radiation (λ = 0.71073 Å) on a
Bruker SMART Apex II diffractometer equipped with a CCD area detector at 294(2) K. The
crystals were positioned at 60 mm from the CCD and the spots were measured using 10s
counting time. Data reduction was carried out using the SAINT-NT software package.77 Multi-
scan absorption correction was applied to all intensity data using the SADABS program.78 The
structure was solved by a combination of direct methods with subsequent difference Fourier
syntheses and refined by full matrix least squares on F2 using the SHELX-2013 suite.79 All non-
hydrogen atoms were refined with anisotropic thermal displacements. The crystal data together
with refinement details are given in Table S1 (See Supporting Information). In complex 2, the
cobalt center was found to be severely disordered, for which the cobalt sites were treated with a
split model.
Computational Details. Gaussian 09 program49 with B3LYP hybrid functional (G09/B3LYP)
was used to perform DFT calculations and assign all the electronic structures.50,51 Optimized
geometry were obtained without applying any geometric constraints. Within the calculations the

Dalton Transactions
Page 24 of 31
View Article Online
DOI: 10.1039/D0DT00394H
quasi-relativistic effective core pseudo potential proposed by Hay and Wadt, LANL2DZ52
pseudo potential, and the corresponding optimized set of basis function has been employed for
Cu. For carbon, hydrogen, nitrogen, and chlorine 6-31G* basis set53 was used. Broken-symmetry
calculations were performed following the formalism as introduced by Noodleman to achieve the
spin-polarized symmetry-broken solutions.54-56
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The research was supported by DST-SERB (Project: CRG/2019/001737). S. S. thanks IIESTS,
S. D. thanks UGC and R. M. thanks CSIR for fellowship support. Financial Assistance from
IIESTS is acknowledged. SAIF facility of IIESTS is duly acknowledged for single crystal X-ray
facility.
References
1
2
3
4
5
P. J. Chirik, K. Wieghardt, Science., 2010, 327, 794−795.
D. L. J. Broere, R. Plessius, J. I. van der Vlugt, Chem. Soc. Rev., 2015, 44, 6886−6915.
O. R. Luca, R. H. Crabtree, Chem. Soc. Rev., 2013, 42, 1440−1459.
V. Lyaskovskyy, B. de Bruin, ACS Catal., 2012, 2, 270−279.
B. de Bruin, P. Gualco, N. D. Paul, Redox Non-Innocent Ligands. In Ligand Design in
Metal Chemistry; John Wiley & Sons, Ltd: Chichester, UK, 2016; pp 176−204.

Page 25 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT00394H
6
7
J. I. van der Vlugt, Chem. Eur. J. 2019, 25, 2651 – 2662.
R. Sikari, S. Sinha, S. Das, A. Saha, G. Chakraborty, R. Mondal, N. D. Paul, J. Org.
Chem., 2019, 84, 4072-4085.
8
9
R. Sikari, S. Sinha, G. Chakraborty, S. Das, N. P. van Leest, N. D. Paul, Adv. Synth. Catal.,
2019, 361, 1–13.
G. Chakraborty, R. Sikari, S. Das, R. Mondal, S. Sinha, S. Banerjee, N. D. Paul, J. Org.
Chem., 2019, 84, 2626-2641.
10 A. L. Smith, K. I. Hardcastle, J. D. Soper, J. Am. Chem. Soc., 2010, 132, 14358−14360.
11 R. P. Yu, J. M. Darmon, J. M. Hoyt, G. W. Margulieux, Z. R. Turner, P. J. Chirik, ACS
Catal., 2012, 2, 1760−1764.
12 Y. Huang, M. -E. Moret, R. J. M. Klein Gebbink, Eur. J. Org. Chem., 2014, 3788.
13 M. R. Ringenberg, S. L. Kokatam, Z. M. Heiden, T. B. Rauchfuss, J. Am. Chem. Soc.,
2008, 130, 788–789.
14 A. M. Allgeier, C. A. Mirkin, Angew. Chem. Int. Ed., 1998, 37, 894–908.
15 J. V. Obligacion, P. J. Chirik, Nat. Rev. Chem., 2018, 2, 15–34.
16 J. V. Obligacion, J. M. Neely, A. N. Yazdani, I. Pappas, P. J. Chirik, J. Am. Chem. Soc.,
2015, 137, 5855-5858.
17 N. G. Léonard, P. J. Chirik, ACS Catal., 2018, 8, 342–348.
18 S. K. Russell, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc. 2011, 133, 8858–8861.
19 A. I. Nguyen, R. A. Zarkesh, D. C. Lacy, M. K. Thorson, A. F. Heyduk, Chem. Sci. 2011,
2, 166–169.
20 A. I. Nguyen, K. J. Blackmore, S. M. Carter, R. A. Zarkesh, A. F. Heyduk, J. Am. Chem.
Soc., 2009, 131, 3307−3316.

Dalton Transactions
Page 26 of 31
View Article Online
DOI: 10.1039/D0DT00394H
21 B. Bagh, D. L. J. Broere, V. Sinha, P. F. Kuijpers, N. P. van Leest, B. de Bruin, S.
Demeshko, M. A. Siegler, J. I. van der Vlugt, J. Am. Chem. Soc., 2017, 139, 5117–5124.
22 D. L. J. Broere, N. P. van Leest, B. de Bruin, M. A. Siegler, J. I. van der Vlugt, Inorg.
Chem., 2016, 55, 8603−8611.
23 V. Lyaskovskyy, A. I. O. Suarez, H. Lu, H. Jiang, X. P. Zhang, B. de Bruin, J. Am. Chem.
Soc., 2011, 133, 12264-12273.
24 N. D. Paul, A. Chirila, H. Lu, X. P. Zhang, B. de Bruin, Chem. Eur. J., 2013, 19, 12953-
12958.
25 C. te Grotenhuis, B. de Bruin, Synlett., 2018, 29, 2238-2250.
26 A. S. Karns, M. Goswami, B. de Bruin, Chem. Eur. J., 2018, 24, 5253–5258.
27 K. Lang, S. Torker, L. Wojtas, X. P. Zhang, J. Am. Chem. Soc., 2019, 141, 12388–12396.
28 Y. Hu, K. Lang, J. -R. Tao, M. K. Marshall, Q. -G. Cheng, X. Cui, L. Wojtas, X. P. Zhang,
Angew. Chem. Int. Ed., 2019, 58, 2670–2674.
29 Z. Tang, S. Mandal, N. D. Paul, M. Lutz, P. Li, J. I. van der Vlugt, B. de Bruin, Org. Chem.
Front., 2015, 2, 1561-1577.
30 S. Samanta, P. Ghosh, S. Goswami, Dalton Trans., 2012, 41, 2213-2226.
31 W. Kaim, Inorg. Chem., 2011, 50, 9752−9765.
32 W. Kaim, B. Schwederski, Coord. Chem. Rev., 2010, 254, 1580−1588.
33 N. D. Paul, U. Rana, S. Goswami, T. K. Mondal, S. Goswami, J. Am. Chem. Soc., 2012,
134, 6520−6523.
34 S. Goswami, D. Sengupta, N. D. Paul, T. K. Mondal, S. Goswami, Chem. - Eur. J., 2014,
20, 6103−6111.

Page 27 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT00394H
35 P. Ghosh, S. Samanta, S. K. Roy, S. Demeshko, F. Meyer, S. Goswami, Inorg. Chem.
2014, 53, 4678−4686.
36 S. Joy, T. Krämer, N. D. Paul, P. Banerjee, J. E. McGrady, S. Goswami, Inorg. Chem.,
2011, 50, 9993−10004.
37 N. D. Paul, T. Krämer, J. E. McGrady, S. Goswami, Chem. Commun. 2010, 46,
7124−7126.
38 B. Sarkar, W. Kaim, J. Fiedler, C. Duboc, J. Am. Chem. Soc., 2004, 126, 14706−14707.
39 S. Das, S. Sinha, U. Jash, R. Sikari, A. Saha, S. K. Barman, P. Brandão, N. D. Paul, Inorg.
Chem., 2018, 57, 5830-5841.
40 S. P. Rath, D. Sengupta, P. Ghosh, R. Bhattacharjee, M. Chakraborty, S. Samanta, A.
Datta, S. Goswami, Inorg. Chem., 2018, 57, 11995-12009.
41 D. Sengupta, R. Bhattacharjee, R. Pramanick, S. P. Rath, N. Saha Chowdhury, A. Datta, S.
Goswami, Inorg. Chem., 2016, 55, 9602-9610.
42 R. Mondal, S. Sinha, S. Das, G. Chakraborty, N. D. Paul, Adv. Synth.Catal. 2019, 361, 1–8.
43 S. D. McCann, S. S. Stahl, J. Am. Chem. Soc., 2016, 138, 199−206.
44 S. Das, S. Sinha, D. Samanta, R. Mondal, G. Chakraborty, P. Brandaõ, N. D. Paul, J. Org.
Chem., 2019, 84, 10160-10171.
45 S. Sinha, S. Das, R. Sikari, S. Parua, P. Brandaõ, S. Demeshko, F. Meyer, N. D. Paul,
Inorg. Chem., 2017, 56, 14084−14100.
46 S. Sinha, R. Sikari, V. Sinha, U. Jash, S. Das, P. Brandão, S. Demeshko, F. Meyer, B. de
Bruin, N. D. Paul, Inorg. Chem., 2019, 58, 1935−1948.
47 D. Sengupta, P. Ghosh, T. Chatterjee, H. Datta, N. D. Paul, S. Goswami, Inorg. Chem.,
2014, 53, 12002−12013.

Dalton Transactions
Page 28 of 31
View Article Online
DOI: 10.1039/D0DT00394H
48 C. Rajnák, J. Titiš, O. Fuhr, M. Ruben, R. Boča, Inorg. Chem., 2014, 53, 8200-8202.
49 A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn, G. C. Verschoor, 1984, 1349-1356.
50 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman,
G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M.
Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
Nakai, T. Vreven, J. A. Jr. Montgomery, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd,
E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, N. J. Millam, M.
Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich,
A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian
09, revision B.01; Gaussian, Inc.: Wallingford, CT, 2010.
51 A. D. Becke, J. Chem. Phys., 1993, 98, 5648−5652.
52 C. Lee, W. Yang, R. G. Parr, Phys. Rev. B: Condens. Matter Mater. Phys., 1988, 37,
785−789.
53 P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Phys.
Chem., 1994, 98, 11623−11627.
54 P. J. Hay, W. R. Wadt, J. Chem. Phys., 1985, 82, 270−283.
55 L. Noodleman, J. Chem. Phys. 1981, 74, 5737−5743.
56 L. Noodleman, E. R. Davidson, Chem. Phys. 1986, 109, 131−143.
57 V. Bachler, G. Olbrich, F. Neese, K. Wieghardt, Inorg. Chem. 2002, 41, 4179−4193.

Page 29 of 31
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT00394H
58 U. A. Kshirsagar, Org. Biomol. Chem., 2015, 13, 9336-9352.
59 I. Khan, A. Ibrar, N. Abbas, A. Saeed, Eur. J. Med. Chem., 2014, 76, 193–244.
60 I. Khan, S. Zaib, S. Batool, N. Abbas, Z. Ashraf, J. Iqbal, A, Saeed, Bioorg. Med. Chem.,
2016, 24, 2361–2381.
61 D. J. Connolly, D. Cusack, T. P. O’Sullivan, P. J. Guiry, Tetrahedron, 2005, 61, 10153–
10202.
62 R. J. Abdel-Jalil, W. Voelter, M. Saeed, Tetrahedron Lett., 2004, 45, 3475−3476.
63 M. Bakavoli, A. Shiri, Z. Ebrahimpour, M. Rahimizadeh, Chin. Chem. Lett., 2008, 19,
1403−1406.
64 R. J. Abdel-Jalil, H. M. Aldoqum, M. T. Ayoub, W. Voelter, Heterocycles, 2005, 65,
2061−2070.
65 F. C. Jia, Z. W. Zhou, C. Xu, Y. D. Wu, A. X. Wu, Org. Lett., 2016, 18, 2942−2945.
66 Z. Zhang, M. Wang, C. Zhang, Z. Zhang, J. Lu, F. Wang, Chem. Commun., 2015, 51,
9205-9207.
67 J. Chen, K. Natte, A. Spannenberg, H. Neumann, P. Langer, M. Beller, X. F. Wu, Angew.
Chem., Int. Ed., 2014, 53, 7579−7583.
68 T. Xu, H. Alper, Org. Lett., 2015, 17, 1569−1572.
69 J. Zhou, J. Fang, J. Org. Chem., 2011, 76, 7730−7736.
70 S. M. A. H. Siddiki, K. Kon, A. S. Touchy, K. Shimizu, Catal. Sci. Technol., 2014, 4,
1716−1719.
71 S. Parua, S. Das, R. Sikari, S. Sinha, N. D. Paul, J. Org. Chem., 2017, 82, 7165−7175.
72 R. Sharma, R. A. Vishwakarma, S. B. Bharate, Adv. Synth. Catal. 2016, 358, 3027−3033.
73 S. Guo, Y. Li, L. Tao, W. Zhang, X. Fan, RSC Adv., 2014, 4, 59289−59296.