Deprotonation Induced Ligand Oxidation in a NiII Complex of a Redox Noninnocent N1-(2-Aminophenyl)benzene-1,2-diamine and Its Use in Catalytic Alcohol Oxidation

Article abstract of DOI:10.1021/acs.inorgchem.6b00646

Two nickel(II)-complexes, [Ni^II(H₃L)₂](ClO₄)₂ ([1](ClO₄)₂) and [Ni^II(HL)₂] (2), containing the redox-active tridentate ligand N¹-(2-aminophenyl)benzene-1,2-diamine (H₃L) have been synthesized. Complex [1](ClO₄)₂ is octahedral containing two neutral H₃L ligands in a facial coordination mode, whereas complex 2 is a singlet diradical species with approximately planar configuration at the tetracoordinate metal atom with two pendant NH₂ side arms from each of the coordinated ligands. Both complexes are found to be chemically interconvertible; complex [1]²⁺ gets converted to complex 2 when exposed to base and oxygen via simultaneous deprotonation and oxidation of the coordinated ligands. Molecular and electronic structures of the isolated complexes are scrutinized thoroughly by various spectroscopic techniques, single crystal X-ray crystallography, and density functional theory. The observed dissociation of a ligand arm upon oxidation of the ligand was exploited to bring about catalytic alcohol oxidation using coordinatively saturated complex [1](ClO₄)₂ as a catalyst precursor. Both the complexes [1](ClO₄)₂and 2 were tested for catalytic oxidation of both primary and secondary alcohols.

Full text of DOI:10.1021/acs.inorgchem.6b00646

Article
pubs.acs.org/IC
Deprotonation Induced Ligand Oxidation in a NiII Complex of a
Redox Noninnocent N1‑(2-Aminophenyl)benzene-1,2-diamine and Its
Use in Catalytic Alcohol Oxidation
Rina Sikari,†,# Suman Sinha,†,# Upasona Jash,† Siuli Das,† Paula Brandao,§ Bas de Bruin,‡
̃
and Nanda D. Paul*,†
†Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Botanic Garden, Howrah 711103, India
§Departamento de Química, CICECO-Instituto de Materiais de Aveiro,Universidade de Aveiro, 3810-193 Aveiro, Portugal
‡Homogeneous Catalysis Group, van ’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH
Amsterdam, The Netherlands
S
* Supporting Information
ABSTRACT: Two nickel(II)-complexes, [NiII(H3L)2](ClO4)2 ([1]-
(ClO4)2) and [NiII(HL)2] (2), containing the redox-active tridentate
ligand N1-(2-aminophenyl)benzene-1,2-diamine (H3L) have been
synthesized. Complex [1](ClO4)2 is octahedral containing two neutral
H3L ligands in a facial coordination mode, whereas complex 2 is a singlet
diradical species with approximately planar configuration at the
tetracoordinate metal atom with two pendant NH2 side arms from each
of the coordinated ligands. Both complexes are found to be chemically
interconvertible; complex [1]2+ gets converted to complex 2 when exposed to base and oxygen via simultaneous deprotonation
and oxidation of the coordinated ligands. Molecular and electronic structures of the isolated complexes are scrutinized thoroughly
by various spectroscopic techniques, single crystal X-ray crystallography, and density functional theory. The observed dissociation
of a ligand arm upon oxidation of the ligand was exploited to bring about catalytic alcohol oxidation using coordinatively
saturated complex [1](ClO4)2 as a catalyst precursor. Both the complexes [1](ClO4)2and 2 were tested for catalytic oxidation of
both primary and secondary alcohols.
Scheme 1. Conversion of Diamine/Aminophenol Ligands to
the Tridentate Congeners
INTRODUCTION
■
Exploring the chemistry of transition metal ions with redox
noninnocent ligands has been an active research area over the
past years.1 Redox noninnocent ligands not only offer
interesting (ambiguous) electronic structures to its complexes,
but can also participate in electron transfer processes during
bond activation processes, particularly in catalysis.2
Among the various redox noninnocent ligands available, the
diamine3 and the aminophenol4 ligand systems have been
studied most thoroughly. Other than exploring the ambiguous
electronic structures of its complexes, the ligand centered redox
events have also been utilized to bring about useful chemical
transformations. Consequently, keeping the basic redox active
diamine/aminophenol part intact, several other new ligands
were designed and synthesized.5,6 Attaching an additional
donor-moiety to phenyl ring (B) (Scheme 1) of diamine/
aminophenol ligands produces tridentate pincer type ligands.
The coordination behavior along with its redox properties can
thus be tuned to some extent by choosing different substituents
in the phenyl arm.
iminobenzoquinone ligands with an additional thioether donor
function.6 Interestingly, the soft S-donor arm was found to be
involved in fluxional processes; often it coordinates to the
central metal ion and sometimes it remains pendant. Such type
of fluxional behavior shown by the coordinated ligands is of
interest in synthetic coordination chemistry and catalysis, in
particular because it can provide vacant coordination sites at the
metal during catalytic turnover, sites that are masked in the
Recently Heyduk and co-workers reported a few of such
tridentate redox noninnocent pincer and studied their
coordination chemistry with suitable metal ions.5a−e At the
same time Kaim and co-workers explored the coordination
behavior and redox properties of some hybrid redox-active
Received: March 16, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b00646
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 2. Deprotonation and Oxidation of Ligand H3L
ground state structures thus enabling stabilization of such
reactive unsaturated intermediates.
(see the Supporting Information (S1)). Complex [1](ClO4)2
showed an intense peak due to the molecular ion [1-H]+ at m/
z, 455 amu. Notably, the experimental spectral features of the
complexes correspond well to the simulated isotopic pattern for
the given formulation. A representative spectrum, that of [1]2+
along with the simulated spectrum, is shown in the Supporting
Information (Figure S1). The room temperature magnetic
moment μeff of complex [1]2+ is 2.68 μB, which is close to the
spin-only value of a metal-centered S = 1 spin system, as
expected for an octahedral NiII ion.9
Herein, we report the synthesis, isolation, and crystallo-
graphic characterization of the octahedral and square planar
nickel(II) complexes [1](ClO4)2 and 2, respectively. These
complexes differ not only in their geometry around the central
nickel(II) atoms, but also have different ligand oxidation states.
The fluxional coordination of the ligand is revealed by
reversible interconversion between [1]2+ and 2 upon exposure
to base and oxygen or protons and a reducing agent. Complex
[1](ClO4)2 is hexacoordinated, bound to two tridentate N1-(2-
aminophenyl)benzene-1,2-diamine ligands in a facial coordina-
tion mode. Complex [1](ClO4)2 upon deprotonation in the
presence of molecular oxygen as the oxidant transforms to its
square planar analogue 2 via oxidative dehydrogenation.
Complex 2 is a singlet diradical species with approximately
planar configuration around the tetracoordinate metal atom
with two pendant NH2 side arms from each of the coordinated
ligands. Beyond mere characterization of the complexes
isolated, catalytic alcohol oxidation reactions were carried out
to test the fluxional coordination behavior of the coordinated
ligand in complex [1](ClO4)2. The hexacoordinated Ni(II)
complex [1](ClO4)2 showed excellent catalytic activity in
oxidizing both primary and secondary alcohols, even though it
does not have vacant coordination sites available in its ground
state structure. Both complexes [1](ClO4)2 and 2 show
excellent activity in the catalytic oxidation of both primary
and secondary alcohols. Notably, catalytic alcohol oxidation
reactions are of contemporary interest because of their
relevance to chemical energy conversion using renewable
resources.7
The same reaction, however, in the presence of a strong base
like NaOH and oxygen produced the new, bluish-green colored
neutral complex 2 in 90% yield. Interestingly, complex 2
possesses an S = 0 ground state, as determined by magnetic
susceptibility measurements at room temperature. It displayed
1
sharp H NMR signals in the normal range for diamagnetic
compounds. The spectral pattern reveals that the two ligands in
the complexes are magnetically equivalent on the NMR time
scale. The aromatic protons of the coordinated ligand appear in
the region δ 6.3−7.5. The broad peak at δ 6.37 is assigned to
the -NH resonances. The most notable observation in the
spectrum is the appearance of a broad singlet at δ 3.91, which is
attributed to the uncoordinated -NH2 groups, one from each of
the two coordinated ligands. The intensity of the -NH signal
decreases upon shaking a solution containing a droplet of D2O.
The 1HNMR spectrum of 2 is shown in the Supporting
Information (Figure S3).
X-ray Crystal Structures. X-ray diffraction (XRD)-quality
single crystals of complex [1](ClO4)2 were obtained by slow
evaporation of its solution in dichloromethane-methanol (1:1)
solvent mixture under nitrogen atmosphere, and the structure
was solved. The molecular view of the cationic complex [1]2+
with partial atom numbering scheme is displayed in Figure 1,
and the Oak Ridge thermal-ellipsoid plot (ORTEP) with
RESULT AND DISCUSSION
■
The tridentate ligand, N1-(2-aminophenyl)benzene-1,2-dia-
mine, used in this study was prepared following the literature
available.8 The coordination chemistry of the corresponding
tris(amide) ligands having substitution in the two terminal -NH
arms is quite well-known, but the coordination chemistry of the
unsubstituted triamine (H3L) has been less explored. These
type of ligands are known for their redox noninnocent
behavior;5 in its completely deprotonated trianionic form it
offers catecholate type of binding [NNNcat]3−, whereas upon
coordination it can be oxidized by one electron to afford the
radical iminosemiquinone form, [NNNsq•]2−, and by a second
electron to afford the iminoquinone form, [NNNq]− (Scheme
2).
Reaction of Ni(ClO4)2 (0.387 mmol) with the ligand H3L
(0.774 mmol) in boiling methanol in a Schlenk tube under
argon atmosphere produced a new, almost colorless dicationic
complex [NiII(H3L)2](ClO4)2 ([1](ClO4)2) in 85% yield.
Elemental analysis along with positive-ion electrospray
ionization mass spectra convincingly support its formulation
Figure 1. Molecular view of complex [NiII(H3L)2](ClO4)2 ([1]-
−
(ClO4)2). The ClO4 counterions are omitted for clarity (i means the
symmetry operation i = −x + 1, −y, z).
B
DOI: 10.1021/acs.inorgchem.6b00646
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Summary of Selected Experimental and Calculated Bond Lengths (Å) and Bond Angles (deg) of Complexes [1]2+, 2,
and [2]−
[1]2+
2
[2]−
X-ray
DFT
X-ray
DFT
DFT
Bonds
Ni−N1
Ni−N8
Ni−N15
N1−C2
N8−C7
N8−C9
N15−C14
C2−C3
C2−C7
C3−C4
C4−C5
C5−C6
C6−C7
2.115(2)
2.139(2)
2.116(2)
1.447(3)
1.463(3)
1.450(3)
1.447(4)
1.389(4)
1.387(4)
1.388(4)
1.383(5)
1.386(4)
1.391(4)
2.1400
2.2115
2.1982
1.4539
1.4740
1.4556
1.4589
1.3979
1.3939
1.3976
1.3939
1.3980
1.4000
1.8262(19)
1.8578(19)
1.8404
1.8920
1.8550
1.9199
1.341(3)
1.355(3)
1.427(3)
1.3395
1.3548
1.4237
1.3540
1.3743
1.4102
1.414(4)
1.423(4)
1.362(4)
1.406(4)
1.371(4)
1.413(4)
1.4197
1.4462
1.3810
1.4189
1.3793
1.4207
1.4106
1.4395
1.3941
1.4037
1.3967
1.4082
Bond Angles
N1−Ni−N8
N8−Ni−N15
81.26(9)
78.62(9)
80.528
75.663
84.18(8)
89.945
83.351
complete atom numbering scheme is submitted in the
Supporting Information (S4, S5). The metrical parameters
are collected in Table 1.
The molecular structure of complex [1](ClO4)2 consists of
one [Ni(H3L)2]2+ cation and two ClO4 anions leading to the
molecular formula [NiII(H3L)2](ClO4)2. The geometry of the
cation [1]2+ is a distorted octahedron comprising two facially
coordinating ligands. Notably, none of the neutral σ-donating
amine donors is deprotonated, and all six donors are bound to
the central Ni-ion with their nitrogen lone-pairs. The bond
distances are consistent with the nonoxidized neutral form of
the ligand as revealed by density functional theory (DFT)
studies. The average C−N and Ni−N bond distances are
1.455(3) Å and 2.123(2) Å, respectively. These Ni−N distances
are typical of a Ni(II) center bound to neutral nitrogen donor
atoms, as shown by a search carried in the Cambridge Structure
Database (web CSD version 1.1.7)10 for compounds containing
the NiN6 fragment.
The packing diagram of complex [1](ClO4)2 is shown in the
Figure 2. The crystal structure of this complex is stabilized by a
two-dimensional (2-D) network of N−H···O hydrogen bonds
in which layers of ClO4− alternate with layers of [NiII(H3L)2]2+.
The geometric parameters of the hydrogen bonds are listed in
Table 2.
Figure 2. Perspective view of 2-D network of N−H···O hydrogen
bonding interactions along the [100] direction displaying cationic
−
layers of [NiII(H3L)2]2+ separated by ClO4 anions (C−H hydrogen
atoms have been omitted for clarity).
In contrast to the structure of complex [1]2+, the single-
crystal X-ray diffraction analysis of complex 2 revealed a
distorted square planar environment around the Ni center with
a pendant NH2 side arm from each of the two coordinating
ligands at (averaged) nonbonding distances of 3.679(3) Å
(Figure 3). The bond distances in complex 2 are similar to
those observed in similar NiII complexes containing two
partially oxidized o-diiminobenzosemiquinonato ligands.11
The average C−N and Ni−N distances in 2 are 1.348(3) and
1.84175(19) Å, respectively. In similar bidentate di-imine type
of ligands, C−N distances are considered to be an excellent
indicator of the charge and the oxidation state of the
coordinated ligand.12 The iminosemiquinone oxidation state
is characterized by C−N distances of 1.36 Å. Moreover, the
one-electron oxidized o-diiminobenzosemiquinonato ligands
also display partial localization of double bond character within
the aryl rings. In agreement with the available literatur-
e,5a−c,11,12 in 2, the partial double bond localization is also
evident (Table 1). The DFT calculations reasonably reproduce
the structures of [1]2+ and 2. DFT studies indeed confirm the
presence of two one-electron semiquinone radicals in complex
2 (cf. below). Given the diamagnetic nature of 2, as revealed by
the magnetic susceptibility and NMR spectroscopic measure-
ments (vide supra), the two unpaired electrons of these ligand
radicals must be quite strongly antiferromagnetically coupled as
observed in similar systems.5a−c,11,12
Cyclic Voltammetry and Electron Paramagnetic
Resonance (EPR). Cyclic voltammograms of [1]2+ and 2
were recorded in CH3CN solutions containing 0.1 M
[NEt4](ClO4) at 25 °C. The potentials are referenced to the
saturated Ag/AgCl electrode. The results are summarized in
C
DOI: 10.1021/acs.inorgchem.6b00646
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 2. Hydrogen Dimensions of Compound [1]2+
D−H···A
H···A/ Å
D···A/ Å
D−H···A/o
N(1)−H(1A) ···O(102) [1/2 + x, 1/2 + y, 1/2 + z]
N(1)−H(1B) ···O(104)
N(8)−H(8)···O(101) [1/2 − x, 1/2 − y, 1/2 + z]
N(15)−H(15A) ···O(101) [1 − x, −y, z]
N(15)−H(15B) ···O(103)
2.32(3)
2.16(3)
2.09(2)
2.60(3)
2.09(4)
3.135(3)
3.091(4)
2.940(3)
3.437(3)
2.993(4)
146(2)
168(3)
150(2)
150(3)
162(3)
couple under identical experimental conditions (for irreversible
waves) and by exhaustive electrolyses of 2 at −0.9 V for the
reversible waves. However, controlled-potential coulometry at
+0.5 V produced a brown solution that immediately
decomposes to give a mixture of unidentified products. The
peak potential separations in the CV, ΔE = Ep,ox − Ep,red
observed for the first reversible reduction wave is 85 mV which
is close to that observed for the ferrocenium/ferrocene couple
at a scan rate of 50 mV s−1. In contrast, ΔE for the 2e− wave
centered at 0.34 V using the same conditions is more than 230
mV at 25 °C. Moreover, Ip for the oxidative redox couple (at
0.34 V) is significantly (∼2.5 times) higher than that of the
single-electron reductive wave. Similar NiII-complexes reported
by others are also known to exhibit such type of single step
two-electron redox events.11c,13 On the basis of our
experimental results coupled with the available literature
data,11c,13,14 we assign the first anodic response at 0.34 V as a
two-electron process. Notably, the simpler analogue of complex
2, the one without the additional aryl amine arm shows a one-
electron reversible oxidation at −0.07 V and a one-electron
reversible reduction at −1.21 V along with two additional quasi
reversible responses at 0.21 and −1.89 V respectively.3b
ii
Figure 3. Molecular view of complex 2 NiII(HL)2
symmetry operation ii = −x, −y, −z + 1).
(
means the
Table 3. The voltammogram of 2 is displayed in Figure 4 and
that of [1]2+ is shown in the Supporting Information (S6).
a b c
, ,
Table 3. Cyclic Voltammetry Data
of [1]2+ and 2
c
c
compound oxidation E1/2, V (ΔEp, mV) Reduction E1/2, V (ΔEp, mV)
[1]2+
0.68, 1.03, 1.37, 1.68
0.34 (230)
To obtain more insight into the nature of the electronic
levels associated with the reversible reduction process at −0.7
V, electrogenerated monoanion [2]− was studied by EPR
spectroscopy in CH2Cl2/0.1 M TBAH at 77 K. The one-
electron reduced complex [2]−, generated by exhaustive
electrolysis of [2], displayed a single line at g = 2.005
characteristic for a ligand radical complex. No resolved
hyperfine couplings were observed at 77 K. These results are
in agreement with DFT calculations, which show that both the
highest occupied and lowest unoccupied molecular orbitals
(HOMO, HOMO−1 and LUMO, LUMO+1) of complex 2 are
primarily ligand-centered. Therefore, both oxidation and
reductions are expected to occur at the ligands rather than at
the metal. The one-electron ligand centered reduction in 2
produces the monoanionic complex [2]− with an electronic
structure [(L•−)NiII(L2−)]. However, the DFT results reveal a
spin-delocalized alternative electronic structure with two
2
−0.70 (85), −1.3 (140)
a
Solvent: CH3CN; supporting electrolyte Bu4NClO4 (0.1 M), and
b
c
reference electrode SCE. Solute concentration ca. 10−3 M. E1/2
=
0.5(Epa + Epc), where Epa and Epc are anodic and cathodic peak
potentials, respectively; ΔEp = Epa − Epc; scan rate = 50 mV s−1.
equivalent ligands L1.5‑
.
Electronic Structures and UV−Vis Spectra. To get a
better insight into the electronic structures of the synthesized
complexes, the geometries of the complexes [1]2+ and 2, were
optimized, using DFT (B3LYP and LANL2DZ on Ni; 6-31G*
on C, H, N; see Experimental Section). The structural
parameters of the DFT optimized geometries match quite
well to the experimental metrical parameters. Selected
calculated bond lengths and angles are compared with the
corresponding experimental ones in Table 1.
Figure 4. Cyclic voltammogram of 2 in CH2Cl2.
Complex [1]2+ showed multiple irreversible oxidative waves
in the potential range 0.5−1.7 V. Interestingly, after one or two
scans the color of the solution changes to brownish-blue, and
the initial voltammogram could not be reproduced. Complex 2,
on the other hand, showed two one-electron reductive
responses in the potential range −0.7−1.3 V and a two-
electron oxidative response at 0.34 V. Notably, the first
reductive response is reversible, while the second one is quasi-
reversible. The one-electron stoichiometry of the redox
response has been confirmed by comparison of the current
height with the redox response of the ferrocene/ferrocenium
The most stable structure of [1]2+ proves to be a triplet state
(S = 1), as expected for an octahedral nickel(II) complex. The
fac coordination mode of the H3L ligand proved to be more
stable than the mer coordination mode, also in DFT geometry
optimization (energy difference 9.46 kcal mol−1). Spin density
D
DOI: 10.1021/acs.inorgchem.6b00646
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 5. Spin density plots of [1]2+ (A); 2 (BS, S = 0) (B); and [2]− (C).
analysis indeed confirms the presence of two unpaired electrons
on the Ni center, as expected for octahedral NiII. Complex 2, on
the other hand, is computed to be a singlet diradical complex
with an approximately planar configuration around the tetra-
coordinate metal atom with the two iminosemibenzoquimone
ligand radicals coupled antiferromagnetically. This is in good
agreement with all experimental data described above. Broken
symmetry density functional calculations (BS, S = 0), clearly
shows that the two coordinated ligands are open shell, π-radical
monoanions (L•−), as shown computationally for analogous
systems.3a,11−13,15 The selected bond parameters along with the
spin density analysis reveal the iminosemiquinone (L•−)
oxidation state of the coordinated ligands with the valence
configuration of [NiII(L•−)2]. One of these two ligands remains
in the spin-up manifold, while the second one is in the spin-
down manifold, yielding the observed antiferromagnetically
coupled diamagnetic singlet (S = 0) ground state. Notably, the
broken symmetry S = 0 state was found to be energetically
more stable than the triplet S = 1 state by ∼2.6 kcal mol−1.
Geometry optimizations were also carried out on [2]−, i.e.,
the one-electron reduced redox partner of 2. Optimized
structural parameters for the computed ground state of [2]−
are collected in Table 1, and the spin density plot is depicted in
Figure 5. Notably, upon one-electron reduction, the remaining
unpaired electron gets delocalized over the two ligands, as is
clear from the spin density plot in Figure 5. Similar
observations were reported for related systems.3a,11−13,15 This
result is consistent with the experimentally observed ligand
centered EPR data of complex [2]−. Analysis of the molecular
orbitals reveals that both the occupied and unoccupied MOs of
the complexes [1]2+ and 2 are mainly ligand-based, with a small
contribution from the metal center in all cases (S7, S8).
The electronic spectrum of complex [1](ClO4)2 was
recorded in acetonitrile and that of complex 2 was recorded
in dichloromethane. The experimental spectral results are
collected in Table 4, and the corresponding spectra are shown
Figure 6. Electronic spectra of [1]2+ (black), 2 (blue) in CH3CN and
CH2Cl2 respectively.
feature is similar to those reported for closely related NiII
complexes containing two antiferromagnetically coupled o-
imonobenzosemiquinoate radical ligands.3b,6a,11
Deprotonation Induced Interconversion. The [1]2+
→
2 transformation exemplifies an unprecedented interconversion
of a hexa-coordinated NiII complex to a coordinatively
unsaturated square planar NiII complex via simultaneous
deprotonation and one-electron oxidation of each of the
coordinated ligands. To get more insight into this redox-
induced dissociation of a ligand arm upon oxidation, several
reactions were carried out under continuous spectroscopic
monitoring.
Deprotonation/Reprotonation Studies of [1]2+ and 2.
The conversion of [1]2+ to 2 is associated with the
deprotonation (loss of four protons) of the coordinated ligands
along with one-electron oxidation of each of the coordinated
ligands, leaving the nickel(II) oxidation state of the metal
unchanged. Deprotonation studies, using NaOH as base, were
carried out in the presence of oxygen as well as under an inert
atmosphere. Upon addition of NaOH to an acetonitrile
solution of [1]2+ in the presence of molecular oxygen, the
initial color of the solution gradually changes to bluish-green,
and the color-intensity gradually increases over the time,
corresponding to formation of complex 2. UV−vis monitoring
of this reaction shows a gradual increase in absorption, which
maximizes at 845 nm, and the final spectral pattern matches
exactly with that of complex 2 (Figure 7). However, addition of
NaOH or other bases to the acetonitrile solution of [1](ClO4)2
in the absence of oxygen (oxidant) produces a mixture of
unidentified products. Similarly, complex [1](ClO4)2 when
exposed to air in the absence of base also does not undergo
conversion to 2, instead produced a mixture of unidentified
products. However, chemical oxidation [1](ClO4)2 with
ferrocenium cation or molecular iodine produce an intense
blue-colored solution. Our repeated attempts to isolate the blue
complex were however unsuccessful. These experimental
a
Table 4. UV−Vis NIR Data
compound
λmax, nm (ε, M−1 cm−1
)
[1](ClO4)2
494 (5969, b), 293 (17450, s), 230 (38224)
2
240 (18841), 276 (10938, s), 325 (5969, s), 845 (22724)
a
[1]2+ in CH3CN and 2 in CH2Cl2, b:broad; s:shoulder.
in Figure 6. Complex [1](ClO4)2 shows a broad weak intensity
peak at 494 nm and a intense absorption at 230 nm with
shoulder at 293 nm. On the other hand, complex 2 showed an
intense peak at 845 nm along with a mid intensity peak at 240
nm. The intense peak at 845 nm is assigned as a spin and dipole
allowed ligand-to-ligand charge transfer band. This spectral
E
DOI: 10.1021/acs.inorgchem.6b00646
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 7. (a) UV−vis monitoring of the deprotonation study of [1](ClO4)4 in the presence of air and NaOH in acetonitrile solution. (b) Schematic
representation of [1](ClO4)4 ↔ 2 interconversion.
findings indicate that for the conversion of [1]2+ to 2, both base
and oxygen are required for simultaneous deprotonation and
oxidation of the coordinated ligands. To get further insight of
the mechanism of the electron transfer process associated with
ligand oxidation, EPR studies were carried out immediately
after the addition of NaOH to [1]2+ in air. The EPR spectrum
obtained at 10 K reveals a sharp, single-line signal centered
around g = 2.013, strongly suggestive for a ligand centered
radical (Figure 7). Interestingly, the same solution, when
exposed to air in the presence of NaOH for several hours,
produces complex 2. Notably, the blue-colored complex
obtained by oxidation of [1]2+ with Fc+ in the absence of
base also showed an EPR signal characteristic for a ligand
radical complex (SI, S9), but reveals a different signal. The EPR
spectrum is clean and shows clearly resolved hyperfine
interactions when measured at RT. A satisfactory simulation
could be obtained assuming an isotropic g-value of 2.0033 and
hyperfine coupling with one unique nitrogen atom (21 MHz)
and two equivalent ones (16 MHz). As such the spectrum is
suggestive for formation of a square planar [(κ3-L•2−)-
NiII(NCMe)] complex. This would imply ligand dissociation
(e.g., [H5L]2+ formation from H3L acting as a base) occurs
upon oxidation of [1]2+ in the absence of added base, thus
explaining why the presence of base is important in the
conversion of [1]2+ to 2.
external stimuli like base and oxygen might affect the activity of
the complexes in catalytic reactions. Although complex
[1](ClO4)2, in its native state, is coordinatively saturated, we
still decided to test its activity in catalytic alcohol oxidation
reactions. Interestingly, and counterintuitively, complex [1]-
(ClO4)2 actually proved to be an efficient catalyst for the
catalytic oxidation of both primary and secondary alcohols
under aerobic conditions.
Initial experiments were focused on the evaluation of the
optimal reaction conditions for the catalytic alcohol oxidation
under aerobic conditions using benzyl alcohol as the model
substrate at 70 °C using [1](ClO4)2 as the catalyst.
Optimization of the reaction conditions revealed that the
reaction proceeded most efficiently in nonpolar solvents like
toluene, whereas reactions in solvents of high polarities such as
MeCN, dioxane, methanol, and DMF afforded poor yields
(Table 5, entries 10−14). Among the series of bases tested such
as K3PO4, K2CO3, Cs2CO3, LiOtBu, NaOtBu, KOtBu, NaOMe,
NaOH, and NaH, the best results were obtained with NaOH
and NaH (Table 5, entries 1−9). In further studies we focused
on the use of NaOH as the base. Lowering the reaction
temperature decreased the yield slightly; however, the same
yield can be achieved using longer reaction times. Lowering the
catalyst loading below 6 mol % leads to poor conversion of the
alcohols to the corresponding aldehydes or ketones. The
reaction conditions were also investigated to check the stability
of the catalytic system in air and the effect of open and closed
systems. Excellent conversion of alcohol to aldehyde or ketone
was achieved in the presence of oxygen. In a closed system with
limited oxygen present only poor conversion of alcohol to
aldehyde was observed.
To check the reversibility of this interconversion, complex 2
was also subjected to reduction and reprotonation. Chemical
reduction of complex 2 by cobaltocene in absolutely inert
atmosphere in alcoholic solvents yielded complex [1]2+. The
complex [1]2+ thus obtained via chemical reduction of 2 was
isolated, and its electronic spectral data indeed matches exactly
with that of pure [1]2+. This chemical reduction is quite fast
(nearly instantaneous).
Several control experiments were carried out to gain more
insight into the catalytic reactions. No product was obtained in
the presence of only ligand and base. Interestingly, no product
Alcohol Oxidation Catalysis. We were curious to check
how the control of fluxional coordination upon exposure to
F
DOI: 10.1021/acs.inorgchem.6b00646
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 5. Optimization of Reaction Conditions Using
[1](ClO4)2 and 2 as the Catalyst
Table 6. Oxidation of Primary and Secondary Alcohols to
Aldehydes and Ketones Catalyzed by [1](ClO4)2 and 2
a b c
, ,
a e
−
entry
solvent
base
time
yield (isolated)
1
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
THF
K3PO4
K2CO3
Cs2CO3
LiOtBu
NaOtBu
KOtBu
NaOMe
NaOH
NaH
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
16
50
2
30
3
35
4
20
5
30
6
35
7
40
8
95
9
95
10
11
12
13
14
15
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
trace
trace
trace
NR
NR
NR
95
acetonitrile
dioxane
methanol
DMF
DCM
d
16
toluene
a
Stoichiometry: benzyl alcohol (0.3 mmol); catalyst (0.02 mmol);
b
base (0.45 mmol, 1.5 equiv). Isolated yields after column
chromatography. Reaction temperature: 70 °C. Complex 2 was
c
d
used as the catalyst.
was obtained in the presence of a radical scavenger like
TEMPO.
Complex [1](ClO4)2, under the catalytic reaction conditions
(in the presence of base and oxygen), is believed to be
converted to the corresponding square planar complex 2 (see
above) which then acts as the actual catalyst in the catalytic
alcohol oxidation reactions. The catalytic activity of complex 2
was also investigated under the same optimized reaction
conditions as described above, and indeed complex 2 catalyzes
alcohol oxidation and is even a bit faster than complex
[1](ClO4)2 (see Table 5, entry 16).
To explore the substrate scope and the versatility of the
catalytic methodology developed above, several alcohols with
different electronic properties and functional groups were
tested. Excellent yields were obtained with alcohols containing
both electron donating as well as electron withdrawing groups.
Secondary alcohols are also suitable for oxidation to the
corresponding ketones. Reactions of methyl phenyl alcohol or
diphenyl alcohol produced acetophenone and benzophenone in
high yields (Table 6, entry 5, 6) under the same optimized
reaction condition with both [1](ClO4)2 and 2 as the catalyst.
It is noteworthy to mention that catalytic reactions with
complex 2 is comparatively faster, and almost identical yields
were obtained in shorter reaction times.
a
Stoichiometry: alcohol (0.3 mmol); catalyst (0.02 mmol) base
b
(NaOH) (0.45 mmol, 1.5 equiv). Isolated yields after column
c
d
chromatography. Reaction temperature: 70 °C. For catalyst [1]2+,
e
reaction time = 24 h; for catalyst 2, reaction time = 16 h. Isolated
yields with catalyst 2 is reported in the parentheses “()”.
Complex 2 converts to six-coordinate [1]2+ upon chemical
reduction with cobaltocene in alcohol solvents. The fluxional
coordination of H3L was used to carry out catalytic alcohol
oxidation reactions using complex [1]2+, where the coordina-
tion sites were originally masked in its ground state structure.
Both the complexes [1]2+ and 2 were found to act as efficient
catalysts in alcohol oxidation reactions. Both primary and
secondary alcohols were found to be converted to the
corresponding aldehydes and ketones in excellent yields.
Mechanistic investigations and catalytic oxidations reactions
involving other non benzylic alcohols are in progress and will
be reported in due course.
EXPERIMENTAL SECTION
■
CONCLUSION
Materials. Ni(ClO4)2 and all alcohols were purchased from Sigma-
Aldrich. All other reagents and chemicals were purchased from
commercial sources and used without further purifications. Tetrabu-
tylammonium perchlorate was prepared and recrystallized as reported
before.16 Caution! Perchlorates have to be handled with care and
appropriate safety precautions.
Physical Measurements. UV−vis spectra were recorded in a
Jasco spectrometer. 1H NMR spectra were recorded on a Bruker
Avance 400/500 MHz spectrometer, and SiMe4 was used as the
internal standard. A PerkinElmer 240C elemental analyzer was used to
collect microanalytical data (C, H, N). ESI mass spectra were recorded
■
The present report represents an unprecedented redox-induced
interconversion of two NiII-complexes supported by a redox
noninnocent tridentate ligand H3L. The ligand N1-(2-
aminophenyl)benzene-1,2-diaminein complex 1 shows fluxional
coordination behavior upon exposure to oxygen and base. The
coordinated ligands in complex [1]2+, in the presence of base
and oxygen, undergoes simultaneous deprotonation and
oxidation to produce the square planar neutral complex 2,
where two of the amine arms of the ligand become pendant.
G
DOI: 10.1021/acs.inorgchem.6b00646
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 7. Crystal Data and Structure Refinement Parameters of Complexes [1](ClO4)2 and 2
1
2 (no disorder)
empirical formula
formula weight
crystal system
space group
a (Å)
C24H26Cl2N6NiO8
C24H22N6Ni
453.18
656.12
tetragonal
monoclinic
P21/c
P−421/c
13.6483(5)
8.8868(7)
9.1899(6)
13.2495(11)
98.474(3)
1070.26(14)
2
b (Å)
13.6483(5)
c (Å)
14.6410(8)
β (deg)
90
volume
2727.3(2)
Z
4
Dc (Mg m−3
)
1.598
1.406
μ/(mm−1
)
0.967
0.930
F(000)
1352
472
theta min−max (deg)
index ranges
2.110−29.172
−18 < h < 18, −18 < k < 18, −20 < l < 20
2.317−27.935
−11 < h < 11, −12 < k < 9, −17 < l < 17
reflections collected
unique reflections, [Rint]
data/restraints/parameters
goodness-of-fit on F2
final R indices
46739
9735
3693[0.0490]
3693/5/206
1.037
2554[0.0322]
2554/0/156
1.098
R1, wR2 [I > 2σI]
0.0292, 0.0621 [3289]
0.0380, 0.0655
0.0535, 0.1490 [2021]
0.0687, 0.1582
1.50/−0.66
R1, wR2 (all data)
largest diff. peak and hole
0.56/−0.26
on a micromass Q-TOF mass spectrometer (serial no. YA 263). All
electrochemical measurements were performed using a PC-controlled
PAR model 273A electrochemistry system. Cyclic voltammetric
experiments were performed under nitrogen atmosphere using a Ag/
AgCl reference electrode, with a Pt disk working electrode and a Pt
wire auxiliary electrode, in acetonitrile or dichloromethane containing
supporting electrolyte, 0.1 M Et4NClO4 or 0.1 M Bu4NClO4,
respectively. A Pt wire gauge working electrode was used for
exhaustive electrolyses. Room temperature magnetic moment
measurements for [1]2+ were carried out with Gouy balance
(Sherwood Scientific, Cambridge, U.K.). EPR spectra in the X-band
were recorded with a Bruker EMX plus spectrometer.
Syntheses. The ligand H3L was prepared by following the reported
procedure.8 All other chemicals were of reagent grade and used as
received. Solvents were purified and dried prior to use. Synthesis and
handling of air-sensitive materials were carried out under an inert
atmosphere by using standard Schlenk technique.
Synthesis of [1](ClO4)2. Under a argon atmosphere, Ni(ClO4)2
(100 mg, 0.387 mmol) and the ligand H3L (154 mg, 0.774 mmol)
were added to a flame-dried Schlenk tube. The tube was capped with a
Teflon screw cap, evacuated, and backfilled with argon. The screw cap
was replaced with a condenser fitted with a argon balloon. Twenty
milliliters of deoxygenated methanol was added to the Schlenk tube
already containing Ni(ClO4)2 and H3L. The Schlenk tube was then
placed in an oil bath and heated to reflux for 4 h under argon
atmosphere. During this time the solution turned dull violet, and
colorless crystalline compound was formed. The solvent was decanted
and the complex [1](ClO4)2 was crystallized by slow evaporation of its
dichloromethane/methanol solution under inert atmosphere. Its yield
and characterization data are as follows: Yield 90%. UV/vis: 494(5969,
b), 293 (17450, s), 230(38224), IR (KBr cm−1): 3317, 3294, 3251
[ν(NH/NH2)]. Anal. Calcd for C24H26N6NiCl2O8: C 43.94, H 3.99, N
12.81; Found: 43.91, H 4.03, N: 12.85. ESI-MS, m/z 455[1 - H]+.
Synthesis of [Ni(L•‑)2] (2). Under an aerobic atmosphere,
Ni(ClO4)2 (100 mg, 0.387 mmol), the ligand H3L (154 mg, 0.774
mmol), and NaOH (32 mg, 0.8 mmol) were added to a flame-dried
Schlenk tube. Subsequently 20 mL of dry methanol was added to the
Schlenk tube already containing Ni(ClO4)2 and H3L. The Schlenk
tube was then placed in an oil bath and heated to reflux for 4 h under
air.The initial yellow color of the solution gradually changes to bluish
green, and a green colored precipitate was obtained. The crude mass,
obtained by evaporation of solvent in vacuum, was purified by
fractional crystallization from dichloromethane and hexane solvent
mixture. The precipitate was crystallized by slow evaporation of a
dichloromethane solution of the compound into hexane. Its yield and
characterization data are as follows: Yield: 90%, ESI-MS, m/z 452 [2]+,
UV/vis: 240(18841), 276 (10938, s), 325 (5969, s), 845 (22724), IR
(KBr cm−1) 3438, 3317, 3053, Anal. Calcd for C24H22N6Ni: C 63.61,
1
H 4.89, N 18.55; Found: C 63.65; H 4.93; N 18.51. H NMR (500
MHz, CDCl3): 7.46 (d, 2H, J = 9.5 Hz); 7.32 (t, 2H, J = 9.5 Hz); 6.93
(m, 4H), 6.45 (m, 6H); 6.46 (d, 2H, J = 10 Hz); 6.37 (s (b), 2H
(−NH)); 3.91 (s(b), 4H (−NH2)).
EPR Spectral Studies. The one-electron reduced complex [2]−
was generated by exhaustive electrolysis of 2 at −0.9 V in CH2Cl2/0.1
M TBAP and was immediately dipped into liquid nitrogen, and the
resulting frozen solution was used for the EPR measurements at 77 K.
During deprotonation studies, EPR spectral measurements were
carried out immediately after the addition of NaOH to an acetonitrile
solution of [1]2+ in air at 10 K.
Catalytic Alcohol Oxidation. Under aerobic conditions, the
catalyst [1](ClO4)2 (8 mol %) and the respective alcohols (0.3 mmol)
were added to a flame-dried Schlenk tube. Subsequently NaOH (0.45
mmol, 1.5 equiv) and 10 mL of dry toluene was added, and the
Schlenk tube was placed in an oil bath and heated to 70 °C for 24 h.
After the reaction was completed, the resulting mixture was
concentrated, and the residue was purified by column chromatography
to get the products (3a−g). For catalyst 2, using the same procedure
described above, reactions were found to be completed in 16 h. The
1
pure products were identified by H NMR.
X-ray Crystallography. Suitable X-ray quality crystals of
complexes [1](ClO4)2 and 2 were obtained by the slow evaporation
of a dichloromethane/methanol and dichloromethane/hexane solution
of the complexes, respectively. The X-ray single crystal data of both
complexes [1](ClO4)2 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 150(2) K. The
crystals were positioned at 40 mm from the CCD, and the spots were
measured using 5 s and 10 s counting time, respectively. Data
reduction was carried out using the SAINT-NT software package.17
Multiscan absorption correction was applied to all intensity data using
H
DOI: 10.1021/acs.inorgchem.6b00646
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the SADABS program.18 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.19 All non-hydrogen atoms were refined with anisotropic thermal
displacements. The N15 atom in complex 2 was refined over two
positions with refined occupancies of x (connected to C14) and 1 − x
(connected to C10), x being equal to 0.717. The C−H hydrogen
atoms were included at calculated positions and refined with isotropic
parameters equivalent 1.2 times those of the atom to which they are
attached. Furthermore, the hydrogen atoms bound to nitrogen atoms
(except for disordered N15 in NP4 compound) were unequivocally
located from the last calculated difference Fourier maps. Molecular
diagram were drawn with the Olex220 and platon21 software package.
The crystal data together with refinement details are given in Table 7.
Computational Details. All DFT calculations presented herein
were carried out using the Gaussian 09 program package.22 Geometry
optimizations were performed without imposing geometric con-
straints. The vibrational frequency calculations were performed to
ensure that the optimized geometries represent the local minima and
that there are only positive Eigen values. All calculations utilized the
B3LYP hybrid functional in conjunction with the LANL2DZ on Ni;
and 6-31G* on C, H, N atoms.23 Mulliken spin densities were used for
analysis of spin populations on ligand and metal centers.24
1588. (d) Dzik, W. I.; van der Vlugt, J. I.; Reek, J. N. H.; de Bruin, B.
Angew. Chem., Int. Ed. 2011, 50, 3356−3358. (e) Chaudhuri, P.;
Wieghardt, K. Prog. Inorg. Chem. 2002, 50, 151−216. (f) Stubbe, J.;
Van der Donk, W. A. Chem. Rev. 1998, 98, 705−762. (g) Jazdzewski,
B. A.; Tolman, W. B. Coord. Chem. Rev. 2000, 200, 633−685.
(h) Pierpont, C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331−
341.
(2) (a) Luca, O. R.; Crabtree, R. H. Chem. Soc. Rev. 2013, 42, 1440−
1459. (b) Lyaskovskyy, V.; de Bruin, B. ACS Catal. 2012, 2, 270−279.
(c) O’Reilly, M. E.; Veige, A. S. Chem. Soc. Rev. 2014, 43, 6325−6369.
(d) Dzik, W. I.; Zhang, X. P.; de Bruin, B. Inorg. Chem. 2011, 50,
9896−9903. (e) de Bruin, B.; Gualco, P.; Paul, N. D. In Ligand Design
In Metal Chemistry: Reactivity and Catalysis; Stradiotto, M., Lundgren,
R., Eds.; Wiley: New York, 2016, in press. (f) Nadif, S. S.; O’Reilly, M.
E.; Ghiviriga, I.; Abboud, K. A.; Veige, A. S. Angew. Chem., Int. Ed.
2015, 54, 15138−15142.
(3) (a) Herebian, D.; Wieghardt, K. E.; Neese, F. J. Am. Chem. Soc.
2003, 125, 10997−11005. (b) Herebian, D.; Bothe, E.; Neese, F.;
Weyhermuller, T.; Wieghardt, K. J. Am. Chem. Soc. 2003, 125, 9116−
̈
9128. (c) Metcalfe, R. A.; Lever, A. B. P. Inorg. Chem. 1997, 36, 4762−
4771. (d) Metcalfe, R. A.; Dodsworth, E. S.; Fielder, S. S.; Stufkens, D.
J.; Lever, A. B. P.; Pietro, W. J. Inorg. Chem. 1996, 35, 7741−7750.
(4) (a) Poddel’sky, A. I.; Cherkasov, V. K.; Abakumov, G. A. Coord.
Chem. Rev. 2009, 253, 291−324. (b) Kokatam, S.; Weyhermuller, T.;
̈
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b00646.
Bothe, E.; Chaudhuri, P.; Wieghardt, K. Inorg. Chem. 2005, 44, 3709−
■
S
3717. (c) Chun, H.; Bill, E.; Bothe, E.; Weyhermuller, T.; Wieghardt,
̈
K. Inorg. Chem. 2002, 41, 5091−5099. (d) Mukherjee, M.;
Weyhermuller, T.; Bill, E.; Wieghardt, K.; Chaudhuri, P. Inorg.
̈
Chem. 2005, 44, 7099−7108.
(5) (a) Nguyen, A. I.; Blackmore, K. J.; Carter, S. M.; Zarkesh, R. A.;
Heyduk, A. F. J. Am. Chem. Soc. 2009, 131, 3307−3316. (b) Heyduk,
A. F.; Zarkesh, R. A.; Nguyen, A. I. Inorg. Chem. 2011, 50, 9849−9863.
Experimental and characterization details, NMR spectral
data (PDF)
X-ray crystallographic files (CIF1 and CIF2)
́
(c) Munha, R. F.; Zarkesh, R. A.; Heyduk, A. F. Inorg. Chem. 2013, 52,
11244−11255. (d) Szigethy, G.; Shaffer, D. W.; Heyduk, A. F. Inorg.
Chem. 2012, 51, 12606−12618. (e) Zarkesh, R. A.; Heyduk, A. F.
Organometallics 2011, 30, 4890−4898. (f) Broere, D. L. J.; de Bruin,
B.; Reek, J. N. H.; Lutz, M.; Dechert, S.; van der Vlugt, J. I. J. Am.
Chem. Soc. 2014, 136, 11574−11577. (g) Ren, P.; Vechorkin, O.;
Allmen, K.; Scopelliti, R.; Hu, X. J. Am. Chem. Soc. 2011, 133, 7084−
7095. (h) Perez Garcia, P. M.; Di Franco, T.; Orsino, A.; Ren, P.; Hu,
X. Org. Lett. 2012, 14, 4286−4289. (i) Breitenfeld, J.; Ruiz, J.;
Wodrich, M. D.; Hu, X. J. Am. Chem. Soc. 2013, 135, 12004−12012.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ndpaul@gmail.com.
■
Author Contributions
#R.S. and S.S. contributed equally to this work.
Notes
The authors declare no competing financial interest.
̀ ̌
(6) (a) Paretzki, A.; Bubrin, M.; Fiedler, J.; Zalis, S.; Kaim, W. Chem. -
Eur. J. 2014, 20, 5414−5422. (b) Hubner, R.; Weber, S.; Strobel, S.;
̈
ACKNOWLEDGMENTS
̀ ̌
Sarkar, B.; Zalis, S.; Kaim, W. Organometallics 2011, 30, 1414−1418.
■
(c) Ye, S.; Sarkar, B.; Lissner, F.; Schleid, Th.; van Slageren, J.; Fiedler,
J.; Kaim, W. Angew. Chem., Int. Ed. 2005, 44, 2103−2106.
The research was supported by CSIR, New Delhi (Project:
01(5234)/15//EMR-II) and DST (Project:YSS/2015/
001552). R.S. thanks RGNF and S.S. thanks IIESTS for
fellowship support. Financial Assistance from IIESTS is
acknowledged. P.B. thanks CICECO-Aveiro Institute of
Materials, POCI-01-0145-FEDER-007679 (FCT ref. UID/
CTM/50011/2013), financed by national funds through the
FCT/MEC and when appropriate cofinanced by FEDER under
the PT2020 Partnership Agreement”. B.dB. thanks The
Netherlands Organization for Scientific Research (NWO-CW
VICI Project 016.122.613) for financial support
(7) (a) Steves, J. E.; Stahl, S. S. J. Am. Chem. Soc. 2013, 135, 15742−
15745. (b) Hoover, J. M.; Stahl, S. S. J. Am. Chem. Soc. 2011, 133,
16901−16910. (c) Song, H.; Kang, B.; Hong, S. H. ACS Catal. 2014,
4, 2889−2895. (d) Zhang, G.; Vasudevan, K. V.; Scott, B. L.; Hanson,
S. K. J. Am. Chem. Soc. 2013, 135, 8668−8681. (e) Weiss, C. J.; Das,
P.; Miller, D. L.; Helm, M. L.; Appel, A. M. ACS Catal. 2014, 4, 2951−
́
2958. (f) Tejel, C.; Asensio, L.; Pilar del Río, M.; de Bruin, B.; Lopez,
J. A.; Ciriano, M. A. Eur. J. Inorg. Chem. 2012, 3, 512−519. (g) Tejel,
C.; Asensio, L.; del Río, M. P.; de Bruin, B.; Lopez, J. A.; Ciriano, M. A.
́
Angew. Chem., Int. Ed. 2011, 50, 8839−8843. (h) Tejel, C.; del Río, M.
P.; Asensio, L.; van den Bruele, F. J.; Ciriano, M. A.; Tsichlis i Spithas,
N.; Hetterscheid, D. G. H.; de Bruin, B. Inorg. Chem. 2011, 50, 7524−
7534. (i) Dzik, W. I.; Calvo, S. E.; Reek, J. N. H.; Lutz, M.; Ciriano, M.
A.; Tejel, C.; Hetterscheid, D. G. H.; de Bruin, B. Organometallics
2011, 30, 372−374. (j) Tejel, C.; del Río, M. P.; Ciriano, M. A.;
DEDICATION
■
Dedicated to Prof. Sreebrata Goswami on the occasion of his
60th birthday.
́ ̌
Reijerse, E. J.; Hartl, F.; Zalis, S.; Hetterscheid, D. G. H.; Tsichlis i
REFERENCES
Spithas, N.; de Bruin, B. Chem. - Eur. J. 2009, 15, 11878−11889.
(k) Tejel, C.; Ciriano, M. A.; del Rio, M. P.; Hetterscheid, D. G. H.;
Tsichlis i Spithas, N.; Smits, J. M. M; de Bruin, B. Chem. - Eur. J. 2008,
14, 10932−10936. (l) Tejel, C.; Ciriano, M. A.; del Río, M. P.; van den
Bruele, F. J.; Hetterscheid, D. G. H.; Tsichlis i Spithas, N.; de Bruin, B.
J. Am. Chem. Soc. 2008, 130, 5844−5845.
■
(1) (a) Forum on Redox-Active Ligands. Inorg. Chem. 2011, 50,
9737−9740.10.1021/ic201881k (b) Special Issue: Cooperative and
Redox Non-Innocent Ligands in Directing Organometallic Reactivity.
Eur. J. Inorg. Chem. 2012, 2012340−342.10.1002/ejic.201290001
(c) Kaim, W.; Schwederski, B. Coord. Chem. Rev. 2010, 254, 1580−
I
DOI: 10.1021/acs.inorgchem.6b00646
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(8) Jones, M. B.; MacBeth, C. E. Inorg. Chem. 2007, 46, 8117−8119.
(9) Tsvetkov, N. P.; Chen, C. H.; Andino, J. G.; Lord, R. L.; Pink, M.;
Buell, R. W.; Caulton, K. G. Inorg. Chem. 2013, 52, 9511−9521.
(10) CSD Cambridge database, 1.17, 2014.
(11) (a) Chłopek, K.; Bothe, E.; Neese, F.; Weyhermuller, T.;
̈
Wieghardt, K. Inorg. Chem. 2006, 45, 6298−6307. (b) Chłopek, K.;
Bill, E.; Weyhermuller, T.; Wieghardt, K. Inorg. Chem. 2005, 44, 7087−
̈
7098. (c) Chaudhuri, P.; Verani, C. N.; Bill, E.; Bothe, E.;
Weyhermuller, T.; Wieghardt, K. J. Am. Chem. Soc. 2001, 123,
̈
2213−2223. (d) Bachler, V.; Olbrich, G.; Neese, F.; Wieghardt, K.
Inorg. Chem. 2002, 41, 4179−4193. (e) Abe, M. Chem. Rev. 2013, 113,
7011−7088. (f) Ali, A.; Barman, S. K.; Mukherjee, R. Inorg. Chem.
2015, 54, 5182−5194. (g) Rajput, A.; Sharma, A. K.; Barman, S. K.;
Koley, D.; Steinert, M.; Mukherjee, R. Inorg. Chem. 2014, 53, 36−48.
(12) (a) Brown, S. N. Inorg. Chem. 2012, 51, 1251−1260.
(b) Bhattacharya, S.; Gupta, P.; Basuli, F.; Pierpont, C. G. Inorg.
Chem. 2002, 41, 5810−5816.
(13) (a) Bill, E.; Bothe, E.; Chaudhuri, P.; Chlopek, K.; Herebian, D.;
Kokatam, S.; Ray, K.; Weyhermuller, T.; Neese, F.; Wieghardt, K.
̈
Chem. - Eur. J. 2005, 11, 204−224. (b) Mukherjee, A.; Mukherjee, R.
Indian J. Chem. 2011, 50A, 484−490.
(14) (a) Sengupta, D.; Ghosh, P.; Chatterjee, T.; Datta, H.; Paul, N.
D.; Goswami, S. Inorg. Chem. 2014, 53, 12002−12013. (b) Buriez, O.;
Moretto, L. M.; Ugo, P. Electrochim. Acta 2006, 52, 958−964.
(15) Paul, N. D.; Samanta, S.; Mondal, T. K.; Goswami, S. Inorg.
Chem. 2011, 50, 7886−7893.
(16) Goswami, S.; Mukherjee, R. N.; Chakravorty, A. Inorg. Chem.
1983, 22, 2825−2832.
(17) Bruker SAINT-plus; Bruker AXS Inc.: Madison, Wisconsin, USA,
2007.
(18) Sheldrick, G. M. SADABS; University of Gottingen: Germany,
̈
1996.
(19) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr.
2008, 64, 112−122.
(20) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.;
Puschmann, H. J. Appl. Crystallogr. 2009, 42, 339−341.
(21) Spek, A. L. Acta Crystallogr., Sect. D: Biol. Crystallogr. 2009, 65,
148−155.
(22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J.
B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J.
J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 09, Revision
A.02; Gaussian, Inc.: Wallingford, CT, 2010.
(23) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (b) Lee,
C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter Mater. Phys.
1988, 37, 785−789. (c) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys.
1980, 58, 1200−1211. (d) Stephens, P. J.; Devlin, F. J.; Chabalowski,
C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623−11627.
(24) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833−1840.
J
DOI: 10.1021/acs.inorgchem.6b00646
Inorg. Chem. XXXX, XXX, XXX−XXX