Stabilizing Terminal Ni(III)-Hydroxide Complex Using NNN-Pincer Ligands: Synthesis and Characterization

Article abstract of DOI:10.1021/acs.inorgchem.9b00466

The reaction of [Ni(COD)₂] (COD; cyclooctadiene) in THF with the NNN-pincer ligand bis(imino)pyridyl (L₁) reveals a susceptibility to oxidation in an inert atmosphere ([O₂] level <0.5 ppm), resulting in a transient Ni:dioxygen adduct. This reactive intermediate abstracts a hydrogen atom from THF and stabilizes an uncommon Ni(III) complex. The complex is crystallographically characterized by a molecular formula of [Ni^III(L₁··)^2-(OH)] (1). Various isotopically labeled experiments (¹⁶O/¹⁸O) assertively endorse the origin of terminal oxygen based ligand in 1 due to the activation of molecular dioxygen. The presence of proton bound to the terminal oxygen in 1 is well supported by NMR, IR spectroscopy, DFT calculations, and hydrogen atom transfer (HAT) reactions promoted by 1. The observation of shakeup satellite peaks for the primary photoelectron lines of Ni(2p) in the X-ray photoelectron spectroscopy (XPS) unambiguously confirms the paramagnetic signature associated with the distorted square planar nickel ion, which is consistent with the trivalent oxidation state assigned for the nickel ion in 1. The variable temperature magnetic susceptibility data of 1 shows dominant antiferromagnetic interactions exist among the paramagnetic centers, resulting in an overall S = 1/2 ground state. Variable temperature X-band EPR studies performed on 1 show evidence for the S = 1/2 ground state, which is consistent with magnetic data. The unusual g-tensor extracted for the ground state S = 1/2 is analyzed under a strong exchange limit of spin-coupled centers. The electronic structure predicted for 1 is in good agreement with theoretical calculations.

Full text of DOI:10.1021/acs.inorgchem.9b00466

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Stabilizing Terminal Ni(III)−Hydroxide Complex Using NNN-Pincer
Ligands: Synthesis and Characterization
Jitendrasingh Rajpurohit,† Pragya Shukla,† Pardeep Kumar,† Chinmoy Das,† Shefali Vaidya,†
Mahesh Sundararajan,§,∥ Muralidharan Shanmugam,*,‡ and Maheswaran Shanmugam*,†
†Department of Chemistry, Indian Institute of Technology Bombay, Powai − 400076, Mumbai, Maharashtra, India
‡Manchester Institute of Biotechnology, The University of Manchester, 131 Princes Street, Manchester − M1 7DN, U.K.
§Theoretical Chemistry Section, Bhabha Atomic Research Centre, Mumbai − 400 085, India
S
* Supporting Information
ABSTRACT: The reaction of [Ni(COD)2] (COD; cyclo-
octadiene) in THF with the NNN-pincer ligand bis(imino)-
pyridyl (L1) reveals a susceptibility to oxidation in an inert
atmosphere ([O2] level <0.5 ppm), resulting in a transient
Ni:dioxygen adduct. This reactive intermediate abstracts a
hydrogen atom from THF and stabilizes an uncommon
Ni(III) complex. The complex is crystallographically charac-
III
2−
··
terized by a molecular formula of [Ni (L1 ) (OH)] (1).
Various isotopically labeled experiments (16O/18O) assertively
endorse the origin of terminal oxygen based ligand in 1 due to
the activation of molecular dioxygen. The presence of proton
bound to the terminal oxygen in 1 is well supported by NMR,
IR spectroscopy, DFT calculations, and hydrogen atom transfer (HAT) reactions promoted by 1. The observation of shakeup
satellite peaks for the primary photoelectron lines of Ni(2p) in the X-ray photoelectron spectroscopy (XPS) unambiguously
confirms the paramagnetic signature associated with the distorted square planar nickel ion, which is consistent with the trivalent
oxidation state assigned for the nickel ion in 1. The variable temperature magnetic susceptibility data of 1 shows dominant
antiferromagnetic interactions exist among the paramagnetic centers, resulting in an overall S = 1/2 ground state. Variable
temperature X-band EPR studies performed on 1 show evidence for the S = 1/2 ground state, which is consistent with magnetic
data. The unusual g-tensor extracted for the ground state S = 1/2 is analyzed under a strong exchange limit of spin-coupled
centers. The electronic structure predicted for 1 is in good agreement with theoretical calculations.
catalyst is exemplified by Hillhouse and co-workers in hydrogen
atom transfer (HAT) reactions.6
INTRODUCTION
■
Nickel-containing enzymes catalyze various reactions such as
the hydrolysis of urea, the disproportionation of the toxic
superoxide radical, and the reversible oxidation of carbon
monoxide, etc.1 During the catalytic processes, especially with
redox-active enzymes such as superoxide dismutase (SOD) and
[NiFe]-hydrogenase, the Ni(III) ion is believed to be the
reactive species responsible for the catalytic activity.1 It has
been shown by Jaun and co-workers that the Ni(III) species of
the methanogenesis enzyme is involved in the activation of a
C−H bond of CH4 under anaerobic conditions.2 In the recent
past, selective mononuclear nickel complexes have been used in
a number of organometallic transformation reactions, such as
Negishi, Kumuda, and Suzuki cross-coupling reactions, where
the Ni(III) ion was proposed as an intermediate.3 Mirica and
his co-workers have exemplarily revealed the involvement of
the Ni(III) ion as the reactive species for various cross-coupling
and oxidative bond formation reactions.4 The importance of the
Ni(III) ion in halogen photoelimination was shown by Nocera
and co-workers,5 and further its significance as a reactive
In general, to stabilize the Ni(III) oxidation state, ligands
with hard donor atoms have predominantly been employed.7
However, in the majority of cases, the Ni(III) valence-state was
mainly accessed, through either chemical/electrochemical
oxidation or pulse radiolysis methods. It has been found that
only a small number of Ni(III) complexes have been
crystallographically characterized due to its high reactivity/
sensitivity and short-lived nature.7d,8
On the contrary, such an unusual oxidation state is easily
accessed under biological conditions through aerial or O2
oxidation due to the low NiIII/NiII redox potential (E0), while
for the pertinent inorganic mimics the E0 value is not in the
accessible range to utilize molecular oxygen as an oxidant;
hence, stronger oxidizing agents or electrochemical methods
are essential. It has been proposed that the presence of radicals
in metalloenzymes alters the redox potential of the reactive
Received: February 19, 2019
© XXXX American Chemical Society
A
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Scheme 1. General Synthetic Scheme Followed for the Synthesis of Complex 1
metal center significantly,9 in addition to the formation of a
strong in-plane metal−ligand interaction and the degree of
ligand unsaturation.8a,10
performed using an Easy Spin toolbox (5.2.18) for the Matlab program
package.14
The X-ray photoelectron spectra were obtained on an axis supra
model of a Kratos analytical electron spectrometer. The samples were
mounted on carbon tape such that the sample cylinder was completely
covered. The Al Kα X-ray line (1486.6 eV) was used for photoelectron
excitation. The C 1s peak from surface adsorbed carbon is used for
internal calibration and corrected to be at 284.7 eV.
Keeping in mind all these factors, we sought to unveil the
transient nature of the putative Ni(III) intermediate by
employing the redox active ligand, bis(imino)pyridyl
(C43H47N3, L1), which can act as a reservoir to accommodate
a maximum of three electrons.11 It is expected that the strong
σ-donating capability and the extended π-cloud of L1 could
potentially alter the electronic structure of the nickel complex,
which should theoretically help to stabilize the Ni(III)
oxidation state under mild conditions. This article reports the
isolation and multiple spectroscopic characterizations of a
trivalent nickel complex stabilized by a doubly reduced L1
(Scheme 1) via the activation of molecular oxygen. Although
NCN [C6H3(CH2NMe2)2-2,6] and other modified pincer type
ligands have been employed in the past toward the isolation of
Ni(III) complexes, the trivalent oxidation state was accessed
either by chemical or electrochemical oxidation exclusively.7d,12
The stabilization of Ni(III) ion in 1 using redox-active NNN
pincer ligand (L1), however, is unprecedented in the literature.
Computational Details. The electronic structure of 1 is further
probed through density functional theory (DFT) calculations15 using a
Jaguar electronic structure package.16 Optimized geometries were
obtained with a B3LYP-D3 functional17 and 6-31G** basis set. For Ni,
a Los Alamos LACVP basis set was used18 which includes effective
core potentials. The energies of the optimized structures were
readdressed by single-point calculations using Dunning’s correlation
consistent triple-ζ basis set cc-pVTZ(-f) basis set.19 Ni ion is treated
with the modified version of LACVP, denoted LACV3P. Vibrational
frequency calculations were conducted at the same calculating theory
level as the geometry optimization calculations. We obtained solvation
energies using the optimized gas phase structures from self-consistent
reaction field (SCRF) calculations with dielectric constants ε = 2.38
(toluene). Time-dependent (TD) DFT calculations are performed
using the ω-B97X functional20 in conjunction with the def2-TZVP
basis set21 using toluene as a solvent to obtain the lowest 30 doublet
states performed in the ORCA 4.0.1 electronic structure package.22
III
Synthesis of [Ni (L1 )2−(OH)] (1). In an argon filled glovebox, 50
EXPERIMENTAL SECTION
··
■
mg of L1 (0.082 mmol) and 23 mg of [Ni(COD)2] (0. 082 mmol) was
weighed and transferred into a 25 mL screw cap fitted Schlenk tube in
which 10 mL of dry THF was added, which was cooled to −40 °C (liq.
N2 + CH3CN). 0.2 mL of 16O2/18O2 gas was injected into this solution
with the help of a gastight syringe, and slowly solution was allowed to
warm up to room temperature (∼30−40 min). The color of the
solution was changed slowly from yellow to dark brown on stirring at
room temperature, and the reaction mixture was allowed to stir for 12
h. The resulting maroon solution was evaporated to dryness and
extracted into hexane (5 mL), and the filtrate was concentrated to 3−4
mL, which was kept for crystallization at room temperature. Within 2
days, brown color block-shaped crystals were grown from the filtrate
upon slow evaporation. Yield (based on nickel): 23 mg (47%). Anal.
calcd for (C43H48NiN3O): C = 75.77; H = 7.09; N = 6.16. Found: C =
75.56; H = 7.15; N = 6.19. The compound obtained was further
characterized by MALDI spectrum to check the incorporation of 18O
labeled atom. The m/z value of 681 along with 679 of equal intensity
suggests the 18O atom from 18O2 gas (vide infra, also see Figure S1).
[We have also observed that the reaction works very well even if we
do not inject the 16O2/18O2 into the reaction mixture externally; that
is, the ppm level of oxygen gas present in the glovebox is sufficient to
isolate 1.]
Materials and Methods. Unless otherwise specified, all the
reactions were carried out under anaerobic condition using Schlenk
techniques and/or a glovebox. All the chemicals were purchased from
commercially available sources, which were used without further
purification. The ligand L1 was synthesized according to the reported
procedure.13 Single crystal data were collected on a Rigaku Saturn
CCD diffractometer using a graphite monochromator (Mo Kα, λ =
0.71073 Å). The selected crystals were mounted on the tip of a glass
pin using mineral oil and placed in the cold flow produced with an
Oxford Cryo-cooling device. Complete hemispheres of data were
collected using ω- and ϕ-scans (0.3°, 16 s per frame). Integrated
intensities were obtained with Rigaku Crystal Clear-SM Expert 2.1
software, and they were corrected for absorption correction. Structure
solution and refinement were performed with the SHELX-package.
The structures were solved by direct methods and completed by
iterative cycles of ΔF syntheses and full-matrix least-squares
refinement against F2. CCDC number: 1498088. UV−vis−NIR
measurement is performed on a Shimadzu (Japan) UV-vis-NIR-
3600. The magnetic susceptibility measurements were performed on
an MPMS-XL SQUID magnetometer equipped with 70 kOe magnet
using toluene solution of 1. The magnetic data were corrected for the
1
sample holder and diamagnetic contributions. H NMR spectra were
Control Experiment with H218O: To probe the possible origin of
the −OH group in 1, a similar synthetic procedure was followed as
above, but 18O labeled water (H218O; 1.7 μL) was used in place of
18O2 gas. A MALDI spectrum was recorded for the crystals obtained in
this reaction (see Figure S1 and the related discussion given in the
Results and Discussion section).
Synthetic Procedure Followed for the in Situ HAT Reaction.
In an argon-filled glovebox, 25 mg of ligand, L1 (0.0415 mmol), and
12.5 mg of [Ni(COD)2] (0.0415 mmol) were weighed and added into
recorded on a Bruker Avance III 500. Mass spectra were obtained from
MALDI-TOF (matrix-assisted laser desorption/ionization time of
flight) to determine the solution stability of 1. The OTOF control
analysis software was used to model the isotopic distribution patterns
in mass spectra. Electron Paramagnetic Resonance (EPR) measure-
ments were carried out using a Bruker EMX plus X-band EPR
spectrometer operating in continuous wave (CW) mode, equipped
with an Oxford variable-temperature unit and ESR900 cryostat with
Super High-Q resonator. Simulations of the CW-EPR spectra were
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a 25 mL Schlenk tube. Into this Schlenk tube, 5 mL of dry THF was
added. This sealed tube was connected to a Schlenk line and cooled
down to −78 °C (methanol/liquid nitrogen bath) and stirred for 5
min. After 5 min, 81 mg of 2,4-di-tert-butyl phenol (0.415 mmol) was
added into the Schlenk tube. To this reaction mixture, the required
amount of dioxygen gas (0.5, 1.0, 2.0, and 4 mL) was injected with the
help of a gastight syringe. This entire assembly remains in the
methanol/liquid nitrogen bath, which was slowly allowed to attain
room temperature with constant stirring. The reaction mixture was
continued to stir for 6 more hours at room temperature. An aliquot of
the reaction mixture was taken for quantitative analysis through GC
using decane as an external reference.
presence of the bulky substituents at the ortho-positions of the
phenyl ring on the amine.
The precursor [Ni(COD)2] employed in the reaction
reduces the ligand (L1) by two electrons, and the evidence
for the radical dianion formation directly comes from the
crystal structure of 1, where the bond length of the imine group
(C14−N12 = 1.337(1) Å) is shorter than a single bond (1.465
Å) and longer than a double bond (1.279 Å). The observed
bond length is consistent with the other literature reports.23
The fourth coordination site of the nickel ion is occupied by
an oxygen atom in 1, with a bond length of 1.824(5) Å. The
observed Ni1−O11 bond length is considerably shorter than
Ni(II)−H2O (2.044(4)−2.107(3) Å) and longer than Ni-oxo
(1.790(2) Å; note: there is no terminal Ni-oxo complex
structurally characterized in the literature to date, but an oxo-
Note: The substrate is introduced into the reaction mixture after 5
min; the rationale for this is described in the main text below (vide
infra).
HAT Reaction Promoted by 1. In an argon-filled glovebox, 10
mg of 1 (1.47 μmol) and 1.5 mL of THF was added into a 10 mL
Schlenk tube. Into that 4 mg (22 μmol) of 9,10-dihydroanthracene or
2 μL (22 μmol) of 1,4-cyclohexadiene was added. The reaction
mixture was stirred for 24 h at room temperature. After completion of
the reaction, 3 μL of n-decane was added as an internal standard. The
resultant solution was subjected to GC-MS analysis to confirm the
formation of the product. The formation of product via HAT reaction
was quantified with an internal standard by the means of GC, and
conversions of 11% and 13% were found for 9,10-dihydroanthracene
and 1,4-cyclohexadiene, respectively.
bridged Ni(III) complex was structurally characterized23b
)
complexes reported in the literature.24 The oxidation state of
nickel ion and the overall electronic structure of 1 depend on
the nature of the terminal ligand i.e., whether it is water or
hydroxo or oxo (Scheme 1). Therefore, several analytical
techniques and isotopic labeling experiments have been
employed to determine the identity of the terminal ligand.
The single crystal X-ray diffraction confirms the presence of
an oxygen donor bound to nickel ion which is firmly supported
by the matrix-assisted laser desorption/ionization (MALDI)
spectrum recorded for 1 (Figure 2). Figure 2 shows an m/z
RESULTS AND DISCUSSION
■
The reaction of bis(cyclooctadiene)nickel(0) with L1 in the
presence of a trace amount of oxygen under an argon
atmosphere in THF (tetrahydrofuran) yielded brown crystals,
which were suitable for single crystal X-ray diffraction
measurements. The crystal structure analysis revealed the
formation of a mononuclear nickel complex (1), which was
crystallized in the monoclinic space group, C2/c (Table S1 of
ESI). The asymmetric unit consists of only half of the molecule,
and the complete structure is generated by rotation/inversion
symmetry, with the rotational axis passing through O11, Ni1,
N11, and C13 (Figure 1). The nickel ion exists in a distorted
square-planar geometry and is surrounded by three nitrogen
atoms of the reduced L1 ligand, which complete three of the
four coordination sites. The average Ni1−N bond length is
found to be 1.8622(3) Å, with the Ni1−N11(pyridine) bond
being the shortest (1.8078(2) Å). This is likely due to the
Figure 2. (A−B) MALDI spectra of 1 showing the molecular ion (blue
trace, panel A) and its first fragmentation isotopic distribution patterns
(blue trace, panel B), respectively. Figure 2A Inset: MALDI spectrum
of the reaction mixture of 1 carried out in the presence of 18O labeled
18O2 gas.
peak at 679 (Figure 2A) and its first fragmentation peak
observed at 663 (Figure 2B). The difference of 16 atomic mass
units between the M+ and its first fragmentation peak
unequivocally confirms the presence of an oxygen-based ligand
bound to the Ni ion in 1 (Figure 2); however, the protonation
state of this donor atom remains elusive.
Figure 1. X-ray structure of 1. Hydrogen atoms and side chain carbon
atoms are removed for clarity. Selected bond lengths: Ni1−N11 =
1.808(4) Å; Ni1−N12 = 1.889(3) Å; Ni1−O11 = 1.824(5) Å. Selected
bond angles: ∠O1−1Ni1−N11 = 180°; ∠N12−Ni1−N12# =
165.3(2)°; ∠N12−Ni1−N11 = 82.67(11)°; ∠O11−Ni1−N12 =
97.33(11)°.
Before probing the protonation state of the OHx ligand in 1
(where x = 2 or 1 or 0), to shed light on the origin of oxygen-
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based ligand in 1, 18O-isotopic labeling experiments were
performed under the identical reaction conditions used to
isolate 1. The reactions were carried out in the presence of (i)
18O-labeled 18O2-gas and (ii) 18OH2 (see the Experimental
Section for details).
In the former case, the reaction did not proceed well,
presumably due to the decomposition of the radical anionic
ligand, as 18O2 gas was introduced into the reaction mixture at
room temperature. In contrast, a stable complex formation of 1
is noticed, if 18O2 gas is introduced into the reaction mixture at
low temperature (−78 to −40 °C) and subsequently warmed
up to room temperature. The MALDI spectrum recorded for
the solution unveils an almost equal intensity m/z peak of 679
and 681 (see the inset of Figure 2A). In contrast, the MALDI
spectrum of 1 isolated using 18O labeled water does not show a
m/z peak correspond to 18O-incorporation and the MALDI
spectrum resembles that shown in Figure 2A (Figure S1).
These two 18O-labeling experiments unequivocally confirm that
the origin of the terminal oxygen based ligand (−OHx) in 1 is
due to molecular O2 activation and not due to the (trace) water
in dry THF solvent.
In an effort to identify the protonation state of the OHx (x =
0, 1, or 2) ligand in 1, the infrared (IR) spectrum was recorded
for 1 in Nujol mulls.
The IR spectrum of 1 markedly shows a sharp peak at 3605
cm−1 (Figure 3), which is assigned to the ν(−OH) stretching
1
Figure 4. (A) H NMR (400 MHz in C6D6) spectra of 1 and 2
recorded at room temperature in C6D6 solvent (# = grease, * = THF,
$ = toluene, @ = residual C6D6). Inset: expanded region of 5.2−6.0
ppm. (B) O2 concentration-dependent biphenol product formation in
the presence of [Ni(COD)2] and L1. The values SE reported here
are an average value of three independent experiments carried out
under identical experimental conditions.
1
The H−13C HSQC spectrum of 1 shows correlation peaks
correspond to the isopropyl and the aromatic groups in the
respective chemical shift range. In contrast, no correlation peak
Figure 3. IR spectrum of 1 recorded in Nujol mulls.
1
is observed for the broad signal at 5.56 ppm in the H−13C
spectrum (Figure S2). This scenario evidently confirms our
assignment that the NMR signal observed at 5.56 ppm
corresponds to a proton, which is bound to a heteroatom
(such as oxygen). Although the IR spectrum of 1 signals the
presence of a hydroxyl group, due to the broadening of the 1H
NMR signal and the absence of an absolute molecular ion peak
in MALDI, the number of protons bound to the oxygen donor
cannot be determined accurately. However, using X-ray
photoelectron spectroscopy we did identify the oxidation
state of the terminal ligand bound to nickel ion and
subsequently the oxidation state of nickel ion in 1 (vide infra).
Next, we turned our attention to answer the question what is
the source of a proton(s) which is(are) bound to the terminal
oxygen ligand? In order to shed light on the origin of the
proton, the following two experiments were performed; (i)
deuterium (d8-THF) labeling experiments and (ii) in-situ HAT
reaction which is detailed below.
frequency. In addition, routine −C−H, −CC, and −CN
stretching frequencies are observed in the usual fingerprint
regions. IR experiment irrefutably reveals that a proton(s)
is(are) bound to the oxygen donor atom in 1. In general, a
water molecule (i.e. OH2) bound to the metal ion is expected
to show a broad −OH band centered at 3000 cm−1 in the IR
spectrum. The absence of such a broad band and the presence
of a sharp peak at 3605 cm−1 hint that the peak presumably
originates from a −OHx (x = 1) group. A hydroxyl ligand
terminally bound to a Cu(III) ion reported elsewhere25 shows a
sharp peak in the same region as shown by 1, further adding
confidence to our notion.
To further probe the protonation state of the OHx (x = 1 or
2), 1H NMR and 1H−13C HSQC (heteronuclear single
quantum coherence) spectra were recorded for 1 in dry
C6D6. The 1H NMR signals observed in the range of 0.99−3.30
ppm and 6.45−8.50 ppm are assigned to the isopropyl and the
Deuterium Labeling Experiments. As the reaction was
performed in THF for the isolation of 1, we hypothesize that
THF solvent could possibly be the source of a proton. To test
this, the reaction was performed under identical conditions
1
aromatic groups, respectively. In addition, a broad H NMR
signal is observed at 5.56 ppm, which is tentatively assigned to
the OHx ligand of 1 (bluish-green trace of Figure 4A and inset).
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used to isolate 1, except the THF solvent was replaced by d8-
THF (99.95%). The single crystals obtained from the d8-THF
reaction exhibit the same unit cell as 1. The 1H NMR spectrum
recorded on this sample in C6D6 reveals the disappearance of
the peak at 5.56 ppm (wine-red trace in Figure 4A and its
inset), while the remaining spectral features exactly overlap with
X-ray Photoelectron Spectroscopy (XPS). The Ni(2p)
X-ray photoelectron spectrum recorded for 1 (Figure 5) reveals
1
the H NMR of 1 isolated from regular THF (bluish-green
trace in Figure 4A and its inset). This experiment apparently
demonstrates that THF solvent acts as the source of the proton.
In-Situ HAT Reaction. The objectives of conducting these
O2 concentration-dependent HAT experiments are 2-fold: (i)
to show that the O2 molecule gets activated at the nickel site
and produces a transient nickel:dioxygen adduct and (ii) to
show that hydrogen atom abstraction by this adduct facilitates
the formation of 1.
In 1, we have shown in the earlier section that the source of
oxygen donor is an O2 molecule. We surmise that O2 molecule
that gets activated at the nickel site results in the formation of a
transient nickel:dioxygen adduct whose identity (3-dimensional
structure) is currently unknown. This adduct could conceivably
Figure 5. X-ray photoelectron spectrum of the Ni(2p) binding energy
region for 1.
the presence of two main peaks with the binding energy of
853.87 and 871.77 eV which are attributed to the primary
photoelectron lines of 2p3/2 and 2p1/2, respectively. In addition,
there are two shakeup satellite peaks associated with the 2p3/2
and 2p1/2 lines respectively, observed at 860.12 and 878.27 eV.
The latter two peaks which are approximately 6 eV higher than
the primary lines observed for 1. It is very well exemplified in
the literature that just by looking at the binding energy of
Ni(2p) primary lines alone one cannot determine the oxidation
state of the metal, as the binding energy of the metal ion
depends on the charge on the metal ion.27 The charge on the
metal ion is governed by the nature of the coordinating ligand
bound to the metal ion or in other words how well the ligands
are able to alter/remove the charge on the metal ions.
Circumstances that lead to increase the charge on metal ion, for
example, strongly electron donating ligand such as nitrogen (as
in case 1) or arsenic etc., binding energy of the primary lines
will shift to lower energy, while a vice versa scenario is observed
if the ligand bound to the metal tends to remove the charge on
the metal ion (example, halide ion bound to a metal).
The photoelectron line of 2p3/2 observed at 853.87 eV for
nickel ion in 1 is significantly lower than the binding energy
observed for Ni(III) ions (855.1 0.1 to 857.5 0.2) reported
in the literature.28 This is attributed to the strong electron
donating nature of nitrogen donors of L1, which consequently
increases the charge on Ni ion. It is a well-accepted mechanism
among the community and has been used widely to explain the
unusual results of various metal complexes; for example, the
binding energies of the [Ni(diars)Cl2](ClO4)2 complex were
observed at 854.7 and 872.6 eV, respectively, for the
2−
be stabilized in a high valent (could be NiIVO or NiIVO2
or NiIIIO2.−) oxidation state. To prove the hydrogen atom
abstraction ability of the transient species and the oxygen
tolerance of the radical containing the nickel:dioxygen adduct,
O2 concentration-dependent HAT experiments were per-
formed using 2,4-di(tert-butyl)phenol substrate (see Exper-
imental Section for details and also see Figure S3). Here, we
would like to point out that the HAT reaction is promoted by
[Ni(COD)2] itself with a phenolic substrate (2,4-di(tert-
butyl)phenol); however, to avoid the involvement of [Ni-
(COD)2] in HAT reactions, the substrate was added after 5
min. By this time [Ni(COD)2] is absolutely consumed by L1
(before the addition of substrate) as evident by the UV−vis
spectrum (Figure S3).
Into the reaction mixture of [Ni(COD)2] and L1, various
amounts of dioxygen gas (0.5, 1.0, 2.0, and 4.0 mL) were
injected externally in the presence of 2,4-di(tert-butyl)phenol at
low temperature (−78 to −40 °C). The hydrogen atom
abstraction promoted by the transient nickel:dioxygen adduct is
quantified by the formation of 3,3′,5,5′-tetra-tert-butyl-(1,1′-
biphenyl)-2,2′-diol (Figure 4B and Figure S4).26a−d From
Figure 4B, it is evident that the biphenolic product formation
increases with an increase in concentration of molecular
oxygen. This unambiguously confirms that the O2 becomes
activated at the nickel center and the resultant adduct is capable
of promoting hydrogen atom abstraction. The reaction was
therefore performed in the absence of a phenolic substrate in
THF, results in transient Ni:dioxygen adduct abstracting a
hydrogen from THF, thus accounting for the stabilization of 1.
1
Through MALDI, NMR (both 1D H NMR and 2D NMR
(1H−13C HSQC)) and isotopic (18O/2H) labeled experiments,
respectively, we have proven unequivocally that (i) an oxygen
donor atom bound to nickel ion, (ii) proton is bound to oxygen
atom, and (iii) the proton is originated from THF solvent
(through hydrogen atom abstraction) in 1. The characteristic
(shakeup satellite peaks) XPS spectrum observed for 1 helped
us to assign the oxidation state of the nickel ion in 1 and
provide indirect support for the protonation state of the
terminal −OHx (x = 1) ligand (vide infra). The oxidation state
of the nickel ion in 1 is further decisively substantiated by
catalytic and spectroscopic investigations (superconducting
quantum interference device (SQUID) measurements and
electron paramagnetic resonance (EPR) spectroscopy).
photoelectron lines 2p3/2 and 2p1/2
.
Although the formal oxidation state of the nickel ion in the
above molecular formula represents +4, the experimentally
observed binding energy is in the range observed for the +2
oxidation state of nickel ion.27a Significantly low binding energy
in this complex is attributed to the electron donating nature of
arsenic ligand bound to the nickel ion.27a In contrast, the
presence of a shakeup satellite peak gives invaluable
information not only about the geometry of the metal ion
but also about the nature of the magnetic property associated
with the metal ion, i.e. whether the nickel ion is paramagnetic
or diamagnetic. The origin and the selection rule for observing
shakeup satellite peaks are described in detail elsewhere.29 To
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Figure 6. (A) Packing diagram of 1. Panel B shows the intermolecular H-bonding between the two molecules in the crystal lattice of 1.
summarize the literature reports, shakeup satellite peaks are
observed only for the paramagnetic nickel ion (example
octahedral nickel(II) or tetrahedral Ni(II)), and the same will
not be observed in diamagnetic square planar nickel(II)
complexes due to the forbidden nature of the shakeup
lines.28,29b The square planar geometry around the nickel ion
and the two-electron reduction of L1 are discernible directly
from the X-ray diffraction. Therefore, the overall oxidation state
of nickel ion in the complex is exclusively determined by the
oxidation state of the terminal ligand coordinated to the nickel
ion. Based on the terminal ligand oxidation state, one can safely
assume the following three possible oxidation states for the
are relatively shorter than other trivalent nickel complexes
reported in the literature.4a,b,4,7a,c,d,12a This is because there is
no distortion in the plane formed by N11, N12, O11, or N12#
(where # represents the symmetrically generated atom, dihedral
angle N11, N12, O11, N12# = 0°) and also the Ni(III) ion
resides at the center of the plane. The short bond distances
(Ni−N and Ni−O) in 1 have a significant influence on the
electronic structure of 1 (vide infra).
As stated above, the majority of the trivalent nickel
complexes have been accessed either by chemical or electro-
chemical oxidation. Therefore, 1 is a rare example, where
molecular dioxygen acts as an oxidant, which facilitates to
stabilize the unusual oxidation state for nickel, a unique method
which is distinctly different from the other routes reported in
the literature. In an unrelated report, it has been pointed out
the importance of radical(s) in metalloproteins, which facilitate
the stabilization of unusual metal ion oxidation states (by
altering the redox potential) to carry out the complicated
catalytic process with ease and elegance.1,30 It is likely that the
presence of the two radicals along with the strong σ-donor
atoms in 1 considerably lowers the oxidation potential of the
nickel ion, allowing the isolation of 1 with trivalent oxidation
state using a mild oxidant such as O2.
With the protonation state of the terminal ligand and the
oxidation state of the nickel ion in 1 proven, the crystal packing
diagram of 1 was analyzed carefully to understand the
supramolecular interactions in the crystal lattice (Figure 6).
The closest Ni···Ni distance was found to be 8.644(8) Å
(Figure 6). The crystal packing diagram evidently shows an
array of molecules of 1 oriented along the b-axis. A similar
scenario was witnessed for the adjacent array too; however, the
adjacent array of molecules is correlated by an inversion
symmetry (Figure 6). This unique packing diagram of the
molecule in the crystal lattice leads to intermolecular H-
bonding (Figure 6B; magenta trace) between the terminal
−OH ligand of one molecule and the para-proton of the
pyridinic ring of the adjacent molecule (O11···H13 = 2.185(2)
Å; see Figure 6B). Such supramolecular interaction might have
a dramatic influence in the electronic structure of the
molecules, particularly at a lower temperature (vide infra).
Direct Current (dc) Magnetic Susceptibility Studies of
1. Variable temperature (2−300 K) dc magnetic susceptibility
measurements were performed on a toluene solution of 1, using
an applied magnetic field of 10 kOe (Figure 7). The observed
square planar nickel ion, namely [NiIV(L1 )2−(O)] (1a),
..
[NiIII(L1 )2−(OH)] (1), [NiIII(L1 )2−(O·)−] (1b), and
..
..
[NiII(L1 )2−(H2O)] (1c). As nickel ion in both 1a (low spin
..
3d6 configuration) and 1c (low spin 3d8 configuration) exists in
a square planar geometry, all the valence electrons are paired
up, resulting in the diamagnetic signature. As stated above,
diamagnetic nickel ion does not show shakeup satellite peaks,
but this contradicts the experimental observation; therefore,
one can safely neglect the possible models of 1a and 1c. The
observation of a shakeup satellite peak for both the primary
photoelectron peaks apparently reveals that nickel ion must be
trivalent and paramagnetic (low spin Ni3+ = 3d7 configuration)
in nature. This situation leaves only two possible models now,
i.e. 1 and 1b. But both 1H NMR and IR experiments performed
on 1 evidently suggest that a proton is bound to the oxygen
atom, which is absolutely satisfied by the model 1, not 1b. This
scenario is consistent with the experimental EPR spectra
recorded for 1, which resembles that of a noninteger spin state
rather than an integer spin state (vide infra). Further, a
noninteger spin state for the model 1b is not possible as it
contains even number (total four) of unpaired electrons (two
from L1, one from nickel, and one from oxyl radical). This is
further firmly supported by the room temperature χMT value
(where χM is molar magnetic susceptibility) which substantiates
the presence of three unpaired electrons (vide infra). By
employing various analytical techniques, the molecular formula
of 1 is unambiguously determined to be [NiIII(L..)2−(OH)].
The multiple spectroscopic characterizations (X-ray, XPS,
NMR, and MALDI) of 1 confirm that the nickel ion exists in a
trivalent oxidation state. This is highly unusual, and the Ni(III)
state stabilized by a redox active ligand, such as L1, is extremely
scarce in the literature. The Ni−N bond lengths observed in 1
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Article
Figure 7. Temperature-dependent magnetic susceptibility measure-
ment performed on the toluene solution of 1 with 10 kOe applied field
in the temperature range of 2−300 K.
Figure 8. Experimental (black trace) and simulated (red trace) EPR
spectra of 1 measured as a KBr pellet at 10 K. Conditions; microwave
power 22 dB (1.262 mW), modulation amplitude 1 G, sweep time 40
s, average microwave frequency 9.3829 GHz.
room temperature χMT value of 1.22 cm3 K mol−1 is in close
agreement with the expected value of three magnetically
uncoupled spins (1.16 cm3 K mol−1) with average gNi = 2.1 and
grad = 2.0023 values. This apparently confirms the presence of
three unpaired electrons and supports the trivalent oxidation
state of nickel [two radicals on L1 and an unpaired electron
from low spin Ni(III)]. Upon decreasing the temperature, the
χMT value decreases steeply until 70 K before increasing below
this temperature and reaching a maximum of 1.06 cm3 K mol−1
at 18 K.
Below 18 K, the χMT value abruptly decreases, reaching a
value of 0.308 cm3 K mol−1 at 1.99 K. The overall trend
observed in the χMT = f(T) plot (up to 70 K) reveals that the
unpaired electrons are coupled antiferromagnetically, which
presumably stabilizes an overall ground state of S = 1/2. The
variable temperature X-band EPR spectra recorded for this
exchange coupled system are in good agreement with the
magnetic susceptibility data recorded for 1 (vide infra).
X-Band Electron Paramagnetic Resonance (EPR) of 1.
To probe the detailed electronic structure of 1, variable
temperature, X-band EPR spectra were recorded as a pure
polycrystalline powder and also in a dilution matrix of KBr
pellet/toluene solution from 80 to 5 K. At room temperature,
the EPR spectrum of the polycrystalline powder sample of 1
shows a broad, single line EPR signal around the g = 2 region,
due to the population of all possible spin states; however, at the
temperature ≤80 K, the EPR spectrum is nicely resolved, with
features centered around 3250−3450 G. The intensity of all the
observed EPR signals increases as the temperature is lowered
and reached a maximum intensity at 5 K (Figure S5). The
spectra showed no evidence for EPR signals appearing at low
magnetic fields in the temperature range of 80−5 K. The lack of
EPR signals at low magnetic fields and the shape of the EPR
features in the field range of 3250−3450 G suggest that the
EPR transitions originate from an S = 1/2 spin state.
exchange-coupled spin system. In the case of the frozen
toluene solution, some of the EPR signals (3348 G and 3390 G;
Figure S7) showed maximum intensity at 20 K and then drop
significantly below this temperature. Another EPR signal shows
maximum intensity only at 5 K (3328 G; Figure S7). The above
trend indicates both depopulations of excited states and
population of low-lying ground states take place at the same
time. This behavior is consistent with the variable temperature
magnetic susceptibility measurements performed on the frozen
toluene solution of 1. It is noteworthy to mention here that in
all three cases, the EPR spectra showed no evidence for the
presence of a large spin ground state i.e. S ≥ 1, and the
observed EPR signals are consistent with a half-integer spin-
system only, specifically S = 1/2, although the χMT value (1.06
cm3 K mol−1) of 1 at 18 K indicates the population of a triplet
state (S = 1), which is not consistent with the EPR observation
at low temperature. In fact, observation of integer spin as a
ground state is not quite possible for 1, as it contains an odd
number of electrons. Nevertheless, the observed χMT at low
temperature can be accounted, presumably due to the equal
population of two distinct and nearly degenerate S = 1/2 spin
states at this temperature with very weak ferromagnetic
coupling between them.
The EPR spectra of 1 were simulated with the following spin-
Hamiltonian parameters: 1 as a KBr pellet; g = [2.0123, 2.0031,
1.9764] (giso = 1.9972); Lorentzian line width of 14.5 G was
used. The experimental and simulated EPR spectra of 1 as a
polycrystalline powder and as a frozen solution are given in
Figure S8 along with the simulated spin-Hamiltonian
parameters. The observed g-tensors significantly deviate from
both the radical as well as Ni(III) single ion g-tensor values,
which is rationalized below.
The magnetic susceptibility data of 1 show dominant
antiferromagnetic interactions and the nonzero χMT value at
1.99 K (0.31 cm3 K mol−1) is consistent with an overall S = 1/2
ground state. Considering the crystal structure of 1, the
When the EPR experiments of 1 were performed on either a
KBr pellet or frozen toluene solution, the observed trend in the
temperature dependent behavior was different. As in the case of
a KBr pellet, the maximum intensity is observed at 30 K and the
intensity drops abruptly at 20 K (Figure 8 and Figure S6).
Below this temperature, the intensities remain constant within
experimental error (3332 G, 3352 G, and 3393 G). The abrupt
change in intensity at this temperature, 30 K−20 K is plausibly
due to either the depopulation of excited states or
intermolecular antiferromagnetic interactions, as 1 is an
1
2
1
2
unpaired electron spins
S
(
= S2
=
and S3
=
could
)
1rad
rad
Ni
be treated as a triangle. Under this scenario, the total-ground
state of S = 1/2 can arise from two states, namely, |S12, S⟩ = |0,
1/2⟩, and |S12, S⟩ = |1, 1/2⟩. Under the strong exchange limit, the
G
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g-value of these two different spin states is given by |0, /2⟩
Article
1
To obtain further insight into the electronic structure of 1,
DFT calculations are carried out. Both ferromagnetic and
antiferromagnetic S = 1/2 spin states are explored. The
antiferromagnetic spin state is the most stable species by 1.6
kcal/mol, which is consistent with the dc magnetic suscepti-
bility measured on 1. The optimized structure of 1 shows the
Ni−OH bond length of 1.82 Å, which is good agreement with
the single crystal X-ray structure (Figure S9). The computed
O−H stretching of 3667 cm−1 is in qualitative agreement with
the experimental data of 3605 cm−1 which further emphasizes
the presence of terminal hydroxo species bound to Ni(III) ion.
As expected, the unpaired electron spins on reduced L1 are
fully delocalized; however, maximum spin density is found on
imino and pyridinic carbon atoms due to extensive delocaliza-
tion (Figure 10). Similarly, spin density localized on nickel ion
(g = 0. g1 + 0. g2 + 1. g3 )
a n d | 1 ,
rad
rad
Ni
1/2⟩(g = (2/3). g + (2/3). g2 − (1/3). g3 ). By using
1rad
rad
Ni
the typical gra1d = 2.0023 and gNi = 2.1, the effective g-value of
1
the state |0, /2⟩ = 2.1 and |1, /2⟩ = 1.97 is observed. The
analysis based on a strong exchange limit indicates that the S =
1
1/2 ground state is likely to originate from the |1, /2⟩ rather
1
than the |0, /2⟩ state as the calculated effective g-value is in
good agreement with the experimentally observed giso value
(1.9972). Observation of unusual g-tensors for a strongly
exchange coupled system is witnessed already in the literature.
For example, Collison and co-workers have rationalized the
unusual g-value (geff = 2.48) observed for a Ni2Cr triangle using
multifrequency high-field EPR techniques elegantly.31 Overall,
variable temperature EPR experiments not only rationalize the
origin of unusual g-values but also confirm the uncommon
trivalent oxidation state observed for nickel ion in 1.
is found to be 1.4 e− with a major contribution from the dx −y
2
2
orbital, which indeed corroborates the Ni(III) center having
one unpaired electron. Likewise, the spin densities on the −OH
anion and the two electrons reduced L1 ligand are found to be
0.16 e− and 1.44 e−, respectively. The significantly lower spin
density on the ligated atoms suggests that spin density is
delocalized onto the nickel center which rationalizes the
increased spin density observed on the nickel site as compared
to the expected value for an unpaired electron. The small
radical character in the −OH moiety is sufficient to catalyze C−
H activation through the radical mechanism, which rationalizes
why 1 is promoting HAT reactions efficiently.
UV−vis−NIR Data and Theoretical Calculation of 1.
The UV−vis-NIR spectrum of 1 was recorded in toluene at
room temperature in a sealed cuvette. The spectrum shows four
distinct peaks, an intense band at 447 nm along with a shoulder
at 548 nm: a relatively less intense peak at 717 nm and
broadband with low intensity centered at 975 nm (Figure 9).
The observation of a band in the near IR region is a
characteristic signature of a Ni(III) ion d−d transition, which is
well documented in the literature.8b,30
Finally, time dependent-DFT calculations were performed
on the optimized structure of 1. The computed spectrum also
shows four distinct peaks (358, 397, 680, and 1099 nm), and
the observed transition energies are in line with the
experimental values (Figure 9 and Figure S10). The calculated
transitions are rather strongly mixed. The computed weak d-d
transition is predicted at 1099 and 680 nm. Overall, the
calculations support the findings from the experiments that the
oxidation state of the nickel ion is trivalent.
Chemical Reactivity of 1. Tolman and co-workers and
other research groups have elegantly shown the hydrogen atom
abstraction ability of various transition metal complexes (for
example LCu(III)−OH, where L = NNN pincer ligand) from
hydrocarbons.16,17a,25,26,32,33 As 1 is structurally similar to the
copper(III) complex, HAT reactions were performed on
hydrocarbon substrates, such as dihydroanthracene (DHA)
and dihydrobenzene (DHB) in the presence of a THF solution
of 1 at room temperature (Scheme 2).
Not surprisingly, 1 promotes the HAT reaction efficiently
and results in the formation of anthracene and benzene,
respectively (Figures S11−S12). The HAT reaction proceeds
smoothly in these cases without any added external oxidant,
Figure 9. UV−visible spectrum of 1 measured in toluene (5.88 × 10−4
M) at room temperature (red trace). Inset: the broad, weak near IR
transition of 1. Green sticks represent the computed UV−vis spectrum
from TD-DFT calculations (refer to Figure S10 for A−D details).
Figure 10. Computed Mulliken spin density on 1 at the B3LYP-D3/LAC3VP**/cc-pVTZ(-f) level.
H
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Article
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
Scheme 2. Schematic Representation of HAT Reaction
Promoted by 1 at Room Temperature
AUTHOR INFORMATION
Corresponding Author
*E-mail for Maheswaran Shanmugam: eswar@chem.iitb.ac.in.
■
ORCID
Maheswaran Shanmugam: 0000-0002-9012-743X
Present Address
∥Department of Chemistry, Korea Advanced Institute of
Science and Technology (KAIST), Daejeon 34141, Republic
of Korea. Center for Catalytic Hydrocarbon Functionalizations,
Institute for Basic Science (IBS), Daejeon 34141, Republic of
Korea.
implying the striking reactivity of 1. The chemical reactivity of
1, similar to that of a terminal Cu(III)−OH complex,
undisputedly reveals that a −OH group bound to the nickel
ion and rules out the possibility of the terminal −OH2 group in
1.
Notes
CONCLUSIONS
The authors declare no competing financial interest.
■
The reaction of zerovalent nickel precursor and L1 in the
presence of trace amount of oxygen led to the formation of
mononuclear nickel complex, which is crystallographically
ACKNOWLEDGMENTS
■
MS thanks the funding agencies (SERB (EMR/2015/000592),
INSA (SP/YSP/119/2015/1264)) and CSIR (01(2933)/18/
EMR-II) for financial assistance and the central facility IRCC,
IIT Bombay for the access of ESCA, MALDI, and EPR. MS
acknowledges The University of Manchester and MIB for
financial support.
III
2−
··
characterized with the molecular formula of [Ni (L1 ) (OH)]
(1). XPS measurement performed on 1 unambiguously
confirms the paramagnetic signature of the nickel ion, which
helped to rationalize the nature of the fourth coordinating
ligand as a terminal −OH group. The origin of −OH ligand in
1 is confirmed by 18O2/2H (d8-THF) labeled experiment upon
activation of molecular oxygen followed by hydrogen atom
abstraction from THF solvent. The appearance and disappear-
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00466.
■
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CCDC 1498088 contains the supplementary crystallographic
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I
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