A High Spin Mn(IV)-Oxo Complex Generated via Stepwise Proton and Electron Transfer from Mn(III)-Hydroxo Precursor: Characterization and C-H Bond Cleavage Reactivity

Article abstract of DOI:10.1021/acs.inorgchem.9b00579

The oxomanganese(IV) complex [(dpaq)Mn^IV(O)]⁺-Mⁿ⁺ (1-Mⁿ⁺, Mⁿ⁺ = redox-inactive metal ion, H-dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-ylacetamide), generated in the reaction of the precursor hydroxomanganese(III) complex 1 with iodosylbenzene (PhIO) in the presence of redox-inactive metal triflates, has recently been reported. Herein the generation of the same oxomanganese(IV) species from 1 using various combinations of protic acids and oxidants at 293 K is reported. The reaction of 1 with triflic acid and the one-electron-oxidizing agent [Ru^III(bpy)₃]³⁺ leads to the formation of the oxomanganese(IV) complex. The putative species has been identified as a mononuclear high-spin (S = 3/2) nonheme oxomanganese(IV) complex (1-O) on the basis of mass spectrometry, Raman spectroscopy, EPR spectroscopy, and DFT studies. The optical absorption spectrum is well reproduced by theoretical calculations on an S = 3/2 ground spin state of the complex. Isotope labeling studies confirm that the oxygen atom in the oxomanganese(IV) complex originates from the Mn^III-OH precursor and not from water. A mechanistic investigation reveals an initial protonation step forming the Mn^III-OH₂ complex, which then undergoes one-electron oxidation and subsequent deprotonations to form the oxomanganese(IV) transient, avoiding the requirements of either oxo-transfer agents or redox-inactive metal ions. The Mn^IV-oxo complex cleaves the C-H bonds of xanthene (k₂ = 5.5 M^-1 s^-1), 9,10-DHA (k₂ = 3.9 M^-1 s^-1), 1,4-CHD (k₂ = 0.25 M^-1 s^-1), and fluorene (k₂ = 0.11 M^-1 s^-1) at 293 K. The electrophilic character of the nonheme Mn^IV-oxo complex is demonstrated by a large negative ρ value of 2.5 in the oxidation of para-substituted thioanisoles. The complex emerges as the "most reactive" among the existing Mn^IV/V-oxo complexes bearing anionic ligands.

Full text of DOI:10.1021/acs.inorgchem.9b00579

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
A High Spin Mn(IV)-Oxo Complex Generated via Stepwise Proton
and Electron Transfer from Mn(III)−Hydroxo Precursor:
Characterization and C−H Bond Cleavage Reactivity
Sachidulal Biswas,†,⊥ Amritaa Mitra,‡,⊥ Sridhar Banerjee,§ Reena Singh,§ Abhishek Das,§
Tapan Kanti Paine,*,§ Pinaki Bandyopadhyay,‡ Satadal Paul,*,∥ and Achintesh N. Biswas*,†
†Department of Chemistry, National Institute of Technology Sikkim, Ravangla, South Sikkim 737139, India
‡Department of Chemistry, University of North Bengal, Raja Rammohunpur, Siliguri 734013, India
§School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A and 2B Raja S.C. Mullick Road, Jadavpur,
Kolkata 700032, India
∥Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34−36, 45470 Mu
̈
lheim an der Ruhr, Germany
S
* Supporting Information
ABSTRACT: The oxomanganese(IV) complex [(dpaq)MnIV(O)]+-Mn+ (1-Mn+, Mn+ = redox-inactive metal ion, H-dpaq = 2-
[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-ylacetamide), generated in the reaction of the precursor hydroxomanganese(III)
complex 1 with iodosylbenzene (PhIO) in the presence of redox-inactive metal triflates, has recently been reported. Herein the
generation of the same oxomanganese(IV) species from 1 using various combinations of protic acids and oxidants at 293 K is
reported. The reaction of 1 with triflic acid and the one-electron-oxidizing agent [RuIII(bpy)3]3+ leads to the formation of the
oxomanganese(IV) complex. The putative species has been identified as a mononuclear high-spin (S = 3/2) nonheme
oxomanganese(IV) complex (1-O) on the basis of mass spectrometry, Raman spectroscopy, EPR spectroscopy, and DFT
studies. The optical absorption spectrum is well reproduced by theoretical calculations on an S = 3/2 ground spin state of the
complex. Isotope labeling studies confirm that the oxygen atom in the oxomanganese(IV) complex originates from the MnIII−
OH precursor and not from water. A mechanistic investigation reveals an initial protonation step forming the MnIII−OH2
complex, which then undergoes one-electron oxidation and subsequent deprotonations to form the oxomanganese(IV)
transient, avoiding the requirements of either oxo-transfer agents or redox-inactive metal ions. The MnIV−oxo complex cleaves
the C−H bonds of xanthene (k2 = 5.5 M−1 s−1), 9,10-DHA (k2 = 3.9 M−1 s−1), 1,4-CHD (k2 = 0.25 M−1 s−1), and fluorene (k2
= 0.11 M−1 s−1) at 293 K. The electrophilic character of the nonheme MnIV−oxo complex is demonstrated by a large negative ρ
value of 2.5 in the oxidation of para-substituted thioanisoles. The complex emerges as the “most reactive” among the existing
MnIV/V−oxo complexes bearing anionic ligands.
the formation of alkyl peroxides.8−12 Spectroscopic and
INTRODUCTION
■
computational studies revealed the importance of such species
in the oxidized state of manganese−superoxide dismutase
(Mn-SOD).13 However, arguably the most important physio-
logical role played by the hydroxo−manganese species is its
involvement in the oxygen evolving complex (OEC) during
photosynthesis.5−7 The supremacy of these compounds in
catalyzing myriads of important biological reactions has
stimulated the synthesis of high-valent manganese−oxygen
High-valent manganese−oxygen species play diverse biological
functions in several manganese enzymes.1−4 In particular,
oxo−manganese species have been invoked as key inter-
mediates in the oxidation of organic substrates by manganese
enzymes and in water oxidation by the oxygen-evolving
complex (OEC) in Photosystem II (PS-II).5−7 Albeit few,
hydroxo−manganese species have also been envisaged as
important intermediates in biology. For instance, in mono-
nuclear nonheme lipoxygenases, the hydroxo−manganese
species acts as the active species that abstracts hydrogen
atoms from 1,4-pentadiene subunits of fatty acids, leading to
Received: March 4, 2019
© XXXX American Chemical Society
A
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species mimicking the active site structure and function of
metalloenzymes.14−18
nese complexes bearing either anionic tripodal ligands or
porhyrinoids have been reported.39−46 However, in the
majority of these cases, the MnIVO species have been
obtained by O atom transfer agents:viz., m-CPBA, PhIO, etc.
In contrast, PCET conversion of the Mn−OH2/Mn−OH
complexes to the corresponding oxo−manganese species
remains largely unknown.
In the present study, we have investigated the synthesis and
spectroscopic characterization of a high-spin oxomanganese-
(IV) species (1-O) bearing the ionic N5 ligand dpaq (dpaq =
2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-ylacetamide)
(Scheme 1). The anionic carboxamido unit of ligand (dpaq) at
Formation of oxo−metal transients in O2-activating enzymes
is accomplished via activation of dioxygen through two-
electron reduction and protonation (eq 1).4,19−21 However, an
alternative process for the generation of oxo−metal transients
has been proposed during water splitting by OEC in natural
photosynthesis.22 In this case, the putative manganese(V)−oxo
is formed by stepwise one-electron oxidation of the
corresponding MnIII−OH2 and MnIV−OH species (eqs 2
and 3).
M
n+ + O2 + 2e− + 2H+ → M(n+2)+(O) + H2O
(1)
(2)
(3)
Scheme 1. (A) Pentadentate Ligand dpaq-H and (B)
Geometry Optimized Structure of [MnIV(O)(dpaq)]+ (1-
Mn3+(OH2) − e− − H+ → Mn4+(OH)
Mn4+(OH) − e− − H+ → Mn5+(O)
a
O)
Combined spectroscopic and theoretical investigations have
revealed that in the transition from the S2 to S3 state of Kok
cycle, the tetramanganese cluster of the oxygen evolving
complex (OEC) undergoes hydrogen atom abstraction from
hydroxo/aqua−manganese(III) prior to generation of an
electrophilic oxomanganese(IV) transient through oxidation
by a nearby tyrosyl radical (YzO•).23 This process is followed
by another PCET to reach the final stage (S4) of O−O bond
formation. In the past decades, efforts have been made to
mimic the activity of OEC toward the development of suitable
solar-driven water oxidation catalysts for hydrogen generation,
which is considered to be the cleanest C-neutral source of
fuel.24−29 In order to achieve this goal, formation of M(n+2)+

O through stepwise oxidation of the corresponding low-valent
metal−hydroxo or −aqua complex has attracted increasing
attention in recent years.30−35 Lassalle-Kaiser et al. reported
sequential electrochemical oxidation of an octahedral MnII−
aqua complex to a high-valent MnIVO species.30 Recently,
Nishida et al. have elegantly shown the generation of
[FeIV(O)(N4Py)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-
bis(2-pyridyl)methylamine), via the oxidation of [FeIII(OH)-
(N4Py)]2+ in the presence of various proton acceptors (PA)
and the one-electron-oxidizing agent [RuIII(bpy)3]3+.31
a
Purple, red, blue, gray, and cyan atoms refer to Mn, O, N, C, and H,
respectively.
the position trans to the potential oxo site is expected to
support generation of a reactive oxomanganese transient.
Moreover, the presence of the quinoline moiety is likely to
provide some steric crowding around the oxo ligand. Very
recently, Sankaralingam et al. reported the formation of a high-
spin redox-inactive metal ion bound MnIV−oxo complex of the
H-dpaq ligand upon oxidation of the precursor
hydroxomanganese(III) complex with PhIO in the presence
of metal triflates in acetonitrile at 253 K.47 Herein, we wish to
report that the high-valent oxomanganese(IV) transient can be
generated using a combination of a proton donor and a one-
electron oxidant at room temperature (293 K), avoiding the
requirement of both an O atom transfer agent and a redox-
inactive metal ion. The oxomanganese(IV) complex
[MnIV(O)(dpaq)]+ (1-O), generated from the corresponding
manganese(III)−hydroxo complex [MnIII(OH)(dpaq)]+ (1)
via PCET, was characterized using optical spectroscopy, mass
spectrometry, Raman spectroscopy, electron paramagnetic
resonance spectroscopy (EPR), and density functional theory
(DFT) calculations. Moreover, the reactivity of 1-O toward
hydrogen atom abstraction from substrates with weak C−H
bonds with bond dissociation energies (BDEs) in the range of
72−80 kcal/mol48 and its ability in oxygen atom transfer to
various para-substituted thioanisoles are reported.
In the past decades, several ligand frameworks having
varying denticities have been successfully utilized to access the
high-valent oxo−metal transients.5 Recently, polydentate
anionic ligands containing higher electron density have
emerged as potential ligand scaffolds providing easy access to
these elusive oxo transients.32,36−38 Borovik et al. successfully
utilized the strategy and reported a series of monomeric Mn−
oxo complexes [MnnH3 buea(O)]m (n = 3+ to 5+; m = 2− to
0) based on the strong anionic ligand [H3buea]3−([H3buea]3−
= tris[(N′-tert-butylureaylato)-N-ethylene]aminato).32−35 The
high-valent oxomanganese species (n = +4, + 5) were prepared
from the precursor [MnIIIH3buea(O)]2− complex using
ferrocenium as the oxidant.35 Brudvig et al. explored the
reactivity of a manganese(III) complex bearing a deprotonated
carboxamido ligand, PaPy3 (HPaPy3 = N,N-bis(2-pyridyl-
methyl)-amine-N-ethyl-2-pyridine-2-carboxamide), in catalytic
water oxidation using either oxone or H2O2.37 The presence of
the carboxamido unit of metal-coordinated PaPy3 at the
position trans to the oxo site was envisioned to favor a reactive
species by weakening the Mn−oxo bond. This is well
demonstrated by the fact that, although the complex was
shown to evolve O2, the high-valent oxomanganese transients
could not be detected. So far, several high-valent oxomanga-
RESULTS AND DISCUSSION
■
Generation and Characterization of MnIV−O Com-
plex. The manganese(III) complex [(dpaq)MnIII(OH)]-
(ClO4) (1) was isolated from the reaction of Mn(ClO4)2·
B
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6H2O with the pentadentate ligand 2-[bis(pyridin-2-
ylmethyl)]amino-N-quinolin-8-ylacetamide (H-dpaq) in ace-
tonitrile at room temperature in air followed by the addition of
a base, triethylamine (see the Experimental Section for details).
Complex 1 was previously obtained from the reaction of the
corresponding Mn(II) complex with O2 at room temper-
ature.49 The UV−visible spectrum of 1 in acetonitrile exhibited
characteristics absorptions at 550 and 780 nm (black line in
Figure 1 and Figure S1), consistent with the previous report.49
The complex was further characterized by ESI-MS (Figure S2)
and elemental analysis (Experimental Section).
combinations of additives and oxidants. When strong acids, viz.
perchloric acid (HClO4) and triflic acid (HOTf), were
separately added to complex 1 in acetonitrile, formation of a
new species displaying absorption bands at 370 and 410 nm
was observed. The species has recently been characterized as
the mononuclear MnIII−aqua complex by Sankaralingam et
al.50 Subsequent addition of m-CPBA to the MnIII−aqua
species led to the accumulation of the same oxo transient
reported by Sankaralingam et al., as was evident from the
appearance of the 700 and 500 nm bands (Figure 2, green
Figure 2. Electronic spectra of the oxomanganese complex 1-O
generated upon addition of different protic acids and oxidants to 1:
final spectra obtained after the addition of [Ru(bpy)3]3+ (0.5 mM)
and HOTf (0.5 mM) (purple), m-CPBA (0.6 mM) and Sc(OTf)3
(0.5 mM) (blue), m-CPBA (0.6 mM) and HClO4 (0.5 mM) (green),
and CAN (0.5 mM) and HOTf (0.5 mM) (red) to a solution of 1
(0.5 mM) in acetonitrile at 293 K. The inset shows the formation of
[MnIII(dpaq(OH2))]2+ (red line) upon addition of HOTf (0.2 mM)
to 0.2 mM complex 1 (black line) in acetonitrile at 293 K. Addition of
m-CPBA (0.25 mM) to this solution forms the oxomanganese(IV)
complex 1-O (blue line).
Figure 1. (top) Generation of 1-O upon the addition of m-CPBA (0.6
mM) followed by HOTf (0.5 mM) into 1 (0.5 mM) in acetonitrile at
293 K. The inset shows the decay pattern of the in situ generated
high-valent MnIV(O) intermediate (1-O). (bottom) Computed
optical absorption spectrum of 1-O along with the donor and
acceptor orbitals participating in electronic transitions. Red sticks
imply individual transitions contributing to Gaussian peaks.
line).47 In contrast, slow and incomplete accumulation of the
intermediate was observed upon addition of m-CPBA to a
solution of p-toluenesulfonic acid (p-TsOH) and 1 in
acetonitrile (Figure S4). Considering the much weaker acidity
of p-TsOH in comparison to HClO4 and HOTf in acetonitrile,
the results suggest that the initial protonation of the
hydroxomanganese(III) complex (1) is critical in the
formation of the oxomanganese(IV) complex 1-O. The results
further demonstrate that the generation of the putative species
[MnIV(O)(dpaq)]+ does not require addition of redox-inactive
metal triflates as claimed by Sankaralingam et al.47 In fact, on
the basis of the generation of the putative species having
characteristic bands at 700 and 500 nm using acid additives
and m-CPBA, we argue that the oxo transient can be better
formulated as the “unbound” oxomanganese(IV) complex 1-O.
On the basis of the above observations, we reasoned that the
formation of the oxo transient could be achieved using a
combination of a one-electron oxidant and a strong protic acid.
Thus, reactions were performed by replacing the terminal O-
donor (m-CPBA) with the one-electron-oxidizing agent
[Ru(bpy)3]3+ (Figure 2). Control reactions using the oxidant
alone did not result in the formation of 1-O. However,
When 1.2 equiv of m-CPBA was added to a solution of 1
(0.5 mM) containing triflic acid (HOTf) (1.0 equiv) in
acetonitrile at 293 K, a dark green solution exhibiting broad
absorptions at 700 nm and a shoulder at 500 nm appeared
(Figure 1), indicating the formation of a new intermediate (1-
O). In fact, the electronic spectrum of the dark green species is
identical with that of the reported [MnIV(O)(dpaq)]+-Mn+
(Mn+ = Sc3+) species (Figure S3).47 The intermediate is
moderately stable at 293 K with a t1/2 value of 8 h.
The fact that identical electronic features are observed for
the products of the reactions of 1 with PhIO/Mn+ and m-
CPBA/HOTf is striking and indicates a role of additives
different from that proposed.47 These apparently contradicting
results prompted us to investigate the mechanistic pathways
leading to the formation of the oxo transient. To explore the
role of additives, reactions were performed with different
C
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addition of 1.0 equiv of [Ru(bpy)3]3+ to a solution of the
MnIII−aqua complex, generated in situ from 1 (0.5 mM) and
HOTf (0.5 mM), resulted in the rise of the absorption band at
690 nm (Figure 2, purple line). Addition of 10 equiv of
thioanisole as an oxygen atom acceptor to this solution and
nonheme oxomanganese(IV) complexes: viz., [MnIV(H3
buea)(O)]− (νMnO = 737 cm−1)39 and [MnIV(BQCN)-
(O)]2+(νMnO = 707 cm−1; BQCN = N,N′-dimethyl-N,N′-
bis(8-quinolyl)cyclohexanediamine).51
The formation of the mononuclear S = 3/2 oxomanganese-
(IV) complex [(dpaq)MnIV(O)]+ (1-O) was further supported
by EPR spectroscopy. In contrast to the starting MnIII−OH
complex (1), which is EPR silent in the perpendicular mode,
an X-band EPR spectrum of 1-O at 77 K showed one strong
transition at g = 2.08 and a less intense feature at lower field, g
= 4.88 (Figure 4). Similar EPR transitions have previously
1
subsequent product identification by H NMR indicated the
formation of methyl phenyl sulfoxide as the sole product in ca.
95% yield based on 1, confirming the generation of an
oxomanganese species in the present case (Figure S5).
Electrospray ionization mass spectrometry (ESI-MS) of the
species further supported the formulation of [MnIV(O)-
(dpaq)]+, displaying an ion peak at m/z 453.1 with the
expected isotope distribution pattern (Figure S6). The absence
of the UV−vis feature at 500 nm in this case may be
rationalized in terms of the formation of the one-electron-
reduced product [Ru(bpy)3]2+, which is known to absorb
strongly in that region. The formation of 1-O could also be
achieved using ceric ammonium nitrate (CAN) as the one-
electron oxidant and HOTf as the proton donor. As shown in
Figure 2 (red line), addition of 1 equiv of CAN to an
acetonitrile solution of 1 in the presence of an equimolar
amount of HOTf at 293 K led to the accumulation of the dark
green chromophore with broad absorptions at 700 nm and a
shoulder at 500 nm.
Isotope labeling studies were performed to confirm the
source of oxygen in the oxo unit of [MnIV(O)(dpaq)]+. Efforts
to generate [MnIII(18OH)(dpaq)]+ (1-18OH) from the
corresponding Mn(II) complex using 16O2 and H218O resulted
in no 18O incorporation, indicating O2 (and not water) as the
source of oxygen in the hydroxo ligand of [MnIII(OH)-
(dpaq)]+ (1) (Figure S7). Additionally, the ESI-MS spectrum
of the mixture obtained after the reaction of 1 with triflic acid
and [Ru(bpy)3]3+ in the presence of H218O exhibited an ion
peak at m/z 354.1 consistent with the hydroxomanganese(III)
complex [MnIII(16OH)(dpaq)]+, formed presumably due to
the rapid decay of the oxo species in the presence of water.
Indeed, the rate of decay of 1-O in acetonitrile was accelerated
almost 20 times upon addition of water (Figure S8). The
synthesis of 1-18OH was eventually accomplished by the
reaction of 18O2 and [Mn(dpaq)](ClO4) in acetonitrile.
Isotopically labeled 1-18O was synthesized from the in situ
generated 1-18OH using [Ru(bpy)3]3+ as the oxidant. The
Raman spectra showed an isotope-sensitive feature at 682 cm−1
for 1-16O, which shifted to 653 cm−1 in the case of 1-18O
(Figure 3). The observed shift of 29 cm−1 upon 18O
substitution agrees with that expected on the basis of Hooke’s
law for a Mn−O diatomic harmonic oscillator. The observed
Mn−O frequency at 682 cm−1 is lower than those reported in
Figure 4. Experimental X-band EPR spectrum of 1-O (black line) and
the simulated EPR spectrum (green line) obtained using Easy Spin.
been reported for other S = 3/2 Mn(IV) complexes,52−54
though precise ground-state spin Hamiltonian parameters have
not been reported frequently for mononuclear Mn(IV)
compounds.47,55 This is mainly because the axial zero field
splitting parameter (D) is often of the same magnitude as the
microwave frequency, 9.4 GHz or 0.3 cm−1, of the most
conventional X-band EPR.56 Simulation of the experimental X-
band EPR spectrum of 1-O was performed using the Easy Spin
package.57
A comparison between experimental and simulated EPR
(Figure 4, green line) provides additional support regarding
the identity of the oxo−manganese(IV) transient. In the
noninteger quartet spin state, the width of the spectrum is
defined by the axial component of the zero field splitting (zfs)
parameter (D), which describes the magnetic splitting(s) of
the complex at zero applied magnetic field. The exact shape of
the signal reflects the symmetry of the fine structure tensor,
described by the rhombicity parameter. It is typically reported
as the dimensionless parameter E/D, with a unique range of 0
≤ E/D ≤ 0.33. For this simulation, the zfs |D| value of ca. 0.08
cm−1 was required along with the rhombicity parameter (E/D)
of 0.40. As no significant hyperfine structure was observed, it
can be inferred that the hyperfine constant for the Mn ion is
less than the fitted line width (70 mT). Since the energy gap
(∼1750 G) between two Kramers doublets (ms = 1/2 and
3/2) corresponds to 2|D|, the magnitude of |D| can be
estimated as 0.08 cm−1, which is the same zfs value used in the
simulation. Such a small zfs value also explains the high-spin S
4
= 3/2 ground state of the molecule, since the high-spin A2g
ground state makes very little contribution toward zfs (D < 0.3
cm−1, microwave energy of X-band EPR) because of the
approximately spherically symmetric half-filled t2g orbital set
Figure 3. Raman spectra of 1-16O (5 mM, green line) and 1-18O
(wine red line) in acetonitrile at 293 K upon 532 nm excitation.
D
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(Figure 5a).58−60 In this situation, i.e. when D < 0.3 cm−1, the
g ≈ 2 signal dominates over the weak and broad lower field
unusually high intensity of d−d transitions in the present case
can be attributed to the inherent asymmetry imposed by the
nonequivalent nature of equatorial nitrogen donor ends, as is
apparent from the shorter Mn−N1 bond length (1.99 Å) in
comparison to those of manganese to other equatorial
nitrogens (ca. 2.03 Å) (Figure 1 and Table S1). The shoulder
near 500 nm is assigned to electronic excitation from a ligand-
based doubly degenerate set of π orbitals (metal dxz and dyz
orbitals in bonding combination with oxygen px and py
orbitals) to a metal-based nonbonding dxy orbital. The single
occupancy of the nonbonding dxy orbital only allows the spin-
down (β) of ligand-based π orbitals to be promoted at this
level, which leaves one unpaired electron in the oxygen-based
orbital, giving it an oxyl radical character. The implication for
such a transition leading to oxyl character is important, since
the appearance of such states is thought to enhance the
reactivity of the compound manifold.64−66
The conversion from MnIII−OH to MnIV−oxo is further
substantiated through comparing the spectral feature of MnIV−
oxo species with computed optical absorption spectra of
MnIII−OH species. The optimized structure shows a Mn−O
bond distance of 1.83 Å, in analogy with the experimental value
of 1.81 Å obtained through an X-ray diffraction study in a
similar compound.49 Comparison of the optical absorption
spectra of MnIII−OH and MnIV−oxo mirrors the experimental
trend (Figure S9), in the sense that the spectra for MnIII−OH
have intensity much lower than that for MnIV−oxo. This
spectral feature can be understood by the electronic
configuration (Figure 5b) of the triplet ground spin state of
the compound, which is 8 kcal mol−1 below the lowest lying
quintet excited state. The double occupancy of the non-
bonding orbital, η, in triplet MnIII−OH hinders the π−η
transition in the ∼400−500 nm range, in contrast to the case
of an MnIV−oxo compound. However, the peaks in this range
for MnIII−OH compound are attributed to d−d transitions
(η−σ* and π*−σ1*) (Figure S10). Thus, the DFT and TD-
DFT analysis clearly favor the formation of the unbound
[MnIV(O)(dpaq)]+.
Taken together, the spectroscopic data suggest that the
intermediate contains a terminal oxo ligand and the oxygen
atom in the oxo−manganese(IV) complex evolves from the
MnIII−OH precursor and not from water. On the basis of the
available data, formation of the oxo transient can be explained
by considering initial proton transfer to 1 to form the
corresponding MnIII−aqua complex, followed by its oxidation
to form a reactive MnIV−aqua complex, which finally
undergoes rapid deprotonation owing to the higher Lewis
acidity of the water-bound Mn(IV) center. To the best of our
knowledge, the present work represents the first report of
generation of a high-valent Mn(O) transient from the Mn−
OH precursor using [Ru(bpy)3]3+ as a one-electron-oxidizing
agent and a protic acid.
Figure 5. DFT-calculated MO splitting diagrams of (a) 1-O and (b)
1.
signal at g ≈ 4.8, as can be seen in the experimental
spectrum.61−63 The small value of zfs parameter enables all the
allowed transitions (Δms = 1) among magnetic sublevels,
which include intradoublet (within the ms = 1/2 level) as
well as interdoublet transitions (ms = −3/2 to ms = −1/2 and
ms = 3/2 to ms = 1/2).
DFT Studies. Additional support regarding the formation
of the oxo-transient has been obtained from density functional
theory (DFT) calculations. The initial geometry of the
compound was taken from the crystal structure of its precursor
[(dpaq)MnIII(OH)](ClO4)49 and was fully optimized. To
confirm the ground spin state of the molecule, optimization
was performed in both the 3/2 and 1/2 spin states. The
quartet spin state was found stable (ca. 5 kcal mol−1) over the
doublet, and hence all the subsequent calculations were carried
out on the quartet ground spin state. The optimized Mn−O
bond length of 1.69 Å is quite similar to that obtained for
[MnIV(O)(dpaq)]+-Sc3+ by EXAFS analysis (1.68 Å).47 The
striking similarity between the Mn−O bond length obtained by
the EXAFS analysis of [MnIV(O)(dpaq)]+-Sc3+ and that
calculated on the basis of our DFT optimization reinforces
the fact that the high-valent oxomanganese(IV) transient (1-
O) is an “unbound” MnIV−oxo species. However, there may
exist a possibility of Sc3+ binding, as indicated by the
appearance of a Sc3+ scatterer at 3.99 Å in the EXAFS
analysis.47
The DFT-derived molecular orbital (MO) splitting (Figure
5) reproduces the classic pattern for a metal−oxo species in a
tetragonal environment. The t2g-derived orbitals are split into
metal nonbonding η (dxy) and π* (metal dxz and dyz orbitals in
an antibonding combination with oxygen px and py orbitals)
orbitals. Each of these MOs is singly occupied and gives rise to
a quartet ground state. The optical absorption spectra obtained
through TD-DFT nicely resemble the shape and position of
experimental peaks and hence help to assign the absorption
peaks to certain electronic transitions (Figure 1, bottom
panel). The 700 nm peak is attributed to the d−d transition
from doubly degenerate π* orbitals to the empty metal-based
Reactivity of 1-O toward HAT and OAT. It is pertinent
to note here that other reported oxo−metal transients bearing
nonheme anionic ligand exhibit diverse reactivity toward C−H
bonds.32−38,47 These previous findings encouraged us to
explore the reactivity of [MnIV(dpaq)(O)]+ (1-O) toward
C−H bonds. The complex was found to react with substrates
with C−H bond dissociation energies of less than 80 kcal
mol−1. Addition of these substrates to a solution of 1-O in
acetonitrile at 293 K resulted in the decay of its characteristic
bands at 700 and 500 nm (Figure 6).
2
2
dx −y orbital. Another d−d transition from manganese dxy to
2
2
dx −y orbital is identified near the 400 nm region. The
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[MnIIIH3buea(O)]2− and 1 order of magnitude higher than
that for [MnIVH3buea(O)]− (Table 1).33 It is noteworthy that
the other oxo−manganese complexes bearing anionic ligands
do not react at all with DHA.11 As is evident from Table 1, the
oxo−manganese(IV) complex [MnIV(O)(N4Py)]2+ shows
reactivity comparable to that of 1-O, with a k2 value of 8.9
M−1 s−1 at 298 K. A similar trend in reactivity is also found in
the case of 1,4-cyclohexadiene (1,4-CHD) (Figure S13) and
fluorene (Table 1 and Table S3).
The second-order rate constant for oxidation of 1,4-CHD
(0.25 M−1 s−1) at 293 K agrees well with that reported for
redox-inactive metal-bound [MnIV(dpaq)(O)]+ at 253 K (1.6
× 10−1 to 1.3 × 10−1 M−1 s−1),47 considering the temperature
correction. To the best of our knowledge, complex 1-O
exhibits the highest HAT reactivity among the oxo−
manganese(IV) complexes bearing anionic ligands. The
normalized second-order rate constants k2′ (k2′ = k2/(number
of equivalent target C−H bonds)), measured for the reactions
of 1-O with the substrates, were then plotted as a function of
C−H bond dissociation energy (BDEC−H) (Figure 7). A linear
Figure 6. UV−visible spectral changes for 1-O upon addition of 50
equiv of xanthene at 293 K. The inset shows the pseudo-first-order
decay pattern of 1-O after addition of xanthene.
The first-order rate constants (kobs), determined by pseudo-
first-order fitting of the kinetic data for the decay of 1-O,
increased linearly with increasing substrate concentration
(inset to Figure 6 and Figures S11−S13), allowing us to
determine the second-order rate constants (k2) (Table 1).
When xanthene was employed as a substrate, a second-order
rate constant (k2) of 5.5 M−1 s−1 at 293 K was obtained
(Figure S11).
It is noteworthy that the precursor complex 1 has previously
been reported to cleave C−H bonds of xanthene with a
pseudo-first-order rate constant of 8 × 10−4 s−1 at 50 °C.49 The
reactivity of 1-O toward xanthene is, therefore, several orders
of magnitude higher than that of the corresponding hydroxo-
manganese complex (1). Complex 1-O oxidized 9,10-
dihydroanthracene (DHA) to yield anthracene (60%) as the
major product. In this case, a second-order rate constant (k2)
of 3.9 M−1 s−1 at 293 K was obtained (Figure S12). The X-
band EPR spectrum of the reaction mixture, after 1,4-
cyclohexadiene (1,4-CHD) oxidation, was silent, suggesting a
Mn(III) end product, formed upon H abstraction from the
substrate by 1-O. Given the fact that the HAT reactivity of 1 is
several orders of magnitude lower than that of 1-O,
accumulation of the MnIII−OH complex after the substrate
oxidation is expected. The result is also consistent with the
already established fact that C−H bond activation by nonheme
metal−oxo complexes does not occur via an oxygen rebound
mechanism.67 The DHA oxidation rate found for 1-O at 293 K
is 2 orders of magnitude higher than the k2 value at 293 K for
Figure 7. Plot of log k2′ of 1-O vs the C−H BDE of substrates (BDE
of substrates: xanthene, 75.5 kcal mol−1; 9,10-DHA, 77 kcal mol−1;
1,4-CHD, 78 kcal mol−1; fluorene, 80 kcal mol−1).
correlation between the reaction rates and C−H bond
dissociation energies of the substrates was obtained, indicating
that H atom abstraction by 1-O is the rate-determining step in
the C−H bond activation reaction. Moreover, the decay
feature of the 700 nm absorption band in the reaction with the
hydrocarbons (Figure 6) is very similar to the gradually
developing feature of the same peak in a triplet MnIII−OH to
quartet MnIV−oxo transition (Figure 1). Hence, it is
Table 1. Second-Order Rate Constants (k2) for Oxidation Reactions of Oxomanganese Complexes
k2 (M−1 s−1
)
complex
xanthene
9,10-DHA
1,4-CHD
ref
[(dpaq)MnIV(O)]+ (1-O)
[(dpaq)MnIV(O)]+-Sc3+ (1-Sc3+)
[(N4Py)MnIV(O)]2+
[MnIV(O)(N4Py)]2+-Sc3+
[MnIV(O)(N4Py)]2+-Sc3+
[(N4Py)MnIV(O)]2+-(HOTf)2
[MnIVH3buea(O)]−
5.5 (20 °C)
0.49 (−20 °C)
3.9 (20 °C)
0.25 (−20 °C)
8.9 (25 °C)
0.25 (20 °C)
1.6 × 10−2 (−20 °C)
6.2 (25 °C)
1.2 (25 °C)
0.035 (0 °C)
0.92 (25 °C)
this work
47
70
71
71
72
34
34
73
0.026 (20 °C)
0.48 (20 °C)
0.015 (15 °C)
[MnIIIH3buea(O)]2−
MnIV(Me2EBC)(O)2
0.048 (15 °C)
0.016 (15 °C)
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reasonable to believe that the electron from the reactant
hydrocarbons prefers to be accommodated in the lower-lying
nonbonding η orbital of the 1-O complex rather than the
1 with PhIO/Sc3+, m-CPBA/H+, [Ru(bpy)3]3+/H+, and CAN/
H+ strongly points to the formulation of an “unbound”
oxomanganese(IV) complex. The formulation is further
supported by detailed spectroscopic characterization of the
chromophore. The present work also raises some questions
regarding the role of acids in generating the oxo transient.
Triflic acid has previously been shown to bind nonheme
metal−oxo complexes, which in turn influences their reactivity
in OAT and HAT reactions.72 Acids seem to play a completely
different role in the formation of 1-O from 1. Protonation of
the hydroxo−manganese(III) complex 1 is shown to be
critical, as addition of oxidant alone does not generate the
putative oxo−manganese(IV) complex. Moreover, Brønsted
acids and Lewis acids seem to play similar roles in the
generation of the oxo complex.
The presence of an anionic amidate trans to the oxo moiety
in 1-O has been found to increase its reactivity significantly
toward C−H bonds. The oxomanganese(IV) complex 1-O
exhibits excellent reactivity in hydrogen atom abstraction
reactions and sulfide oxidation. In fact, the HAT reactivity of
1-O has been found to be comparable with that of the most
reactive oxo−manganese(IV) complexes reported so far (Table
1). Moreover, the complex exhibits the highest reactivity
among the reported nonheme MnIV−O complexes bearing
anionic ligands. In contrast to the commonly employed
method of generation of the oxo−metal transients employing
O atom transfer agents, the present work also demonstrates
that oxo−manganese complexes can be synthesized via
stepwise proton and electron transfer from the corresponding
Mn−hydroxo complexes. We believe that the results reported
herein will stimulate research in PCET formation of oxo−
metal transients and help in the design of synthetic first-row
metal catalysts for C−H functionalization and water oxidation.
This study illustrates the complexity associated with the
manganese−dioxygen chemistry, and further reactivity studies
are actively being pursued in our laboratories to establish a
generic mechanistic path for the PCET-based formation of
oxomanganese(IV) transients.
2
2
higher-lying empty metal-based dx −y orbital (Figure 5), which
is also supported by the higher energy of the quintet spin state
in comparison to the triplet ground spin state of MnIII−OH.
Theoretical studies have identified two main reaction pathways
in C−H activation by nonheme iron(IV) complexes.68,69 In the
quintet channel, the electron of the substrate is transferred into
the σ*(FeO) orbital that requires a vertical approach of the
substrate (σ reaction channel). In the triplet channel, in
contrast, the electron of the substrate is accepted by the
π*(FeO) orbital requiring a horizontal approach of the
substrate (π reaction channel). Our finding, therefore,
indicates the π reaction channel of hydrogen atom
abstraction,68,69 which of course can be confirmed only after
more rigorous experimental and theoretical studies.
The oxygen-atom transfer (OAT) reactivity of 1-O was also
investigated. Addition of 10 equiv of thioanisole to a
preformed solution of 1-O in acetonitrile at 293 K resulted
in rapid decay of the 700 and 500 nm absorption bands
(Figure S14). Product analysis at the end of the reaction by 1H
NMR and GC revealed almost quantitative formation of
methyl phenyl sulfoxide. The fate of the oxo transient (1-O)
after the reaction with thioanisole was assessed by EPR. In
contrast to C−H cleavage reactions, 1-O was shown to be
converted into the Mn(II) complex (Figure S15), indicating
that the oxidation of thioanisole by 1-O occurs via a two-
electron-oxidation process. The reaction of 1-O with
thioanisole was so fast that the second-order rate constant
for the reaction could not be precisely obtained. Instead, the
second-order rate constant was determined in the reaction
between 1-O and p-nitrothioanisole at 293 K and the decay of
the MnIV−oxo species was monitored by optical spectroscopy.
From the plot of kobs versus substrate concentration, k2 was
evaluated (5.0 × 10−2 M−1 s−1 at 293 K). The reactions
between 1-O and para-substituted thioanisoles were also
followed kinetically. The electrophilic character of 1-O is well
demonstrated by a large negative ρ value of −2.5 (Figure S16).
EXPERIMENTAL SECTION
■
SUMMARY AND CONCLUSION
■
General Methods. All reagents and solvents were purchased from
commercial sources and were used without further purification, unless
otherwise noted. The ligand H-dpaq was synthesized according to the
procedure reported in the literature.38 [Ru(bpy)3](ClO4)3 was
prepared by a slight modification of the reported procedure.84
[Ru(bpy)3](ClO4)2 was prepared by reacting an aqueous solution of
In summary, we have prepared and characterized a
mononuclear high-spin (S = 3/2) nonheme oxo−manganese-
(IV) complex bearing an anionic N5 ligand (dpaq). The
complex was prepared from the corresponding MnIII−OH
complex using various combinations of electron acceptors and
proton donors. The oxo−manganese complex 1-O thus
generated parallels the chemistry of oxygen evolving complex
(OEC) in PS-II, wherein stepwise PCET oxidation of the
tetramanganese cluster occurs during turnovers. The present
results offer an alternative mechanism for the formation of the
oxomanganese(IV) transient reported recently by Sankaralin-
gam et al.47 Whereas the synthesis of the oxo transient
reported by them relied on the use of a two-electron oxo-
transfer agent (PhIO) and redox-inactive metal triflates, we
propose that the protonation of the MnIII−OH complex (1)
followed by one-electron oxidation of the Mn(III) center and
subsequent deprotonations yield [(dpaq)MnIV(O)]+. Consid-
ering the available evidence in the literature, it is quite evident
that binding of redox-inactive metal ions or HOTf to high-
valent oxo transients results in substantial changes in their
UV−vis spectra.71,72 In contrast, the observation of identical
electronic spectral features for the products of the reactions of
[Ru(bpy)3]Cl2 with NaClO4 according to the reported procedure.84
A
10 mM solution of [Ru(bpy)3](ClO4)2 was prepared by dissolving the
solid in acetonitrile containing 10 mM HOTf. To this solution was
added excess PbO2, and the solution was stirred, which led to the
formation of [Ru(bpy)3]3+. The formation of [Ru(bpy)3]3+ was
confirmed by electronic absorption spectroscopy. [Mn(dpaq)]ClO4
was synthesized under an inert atmosphere in a glovebox as reported
previously.49 Although no problem was encountered during the synthesis
of the complexes, perchlorate salts are potentially explosive and should be
handled with the utmost care.
UV−vis absorption spectra were recorded in acetonitrile (CH3CN)
on Agilent 8453 spectrometers. Electrospray ionization mass spectra
were recorded with a Waters QTOF Micro YA263 instrument. X-
band EPR measurements were performed on a JEOL JES-FA 200
instrument. NMR spectra were collected on a Bruker Avance 500
MHz spectrometer. Raman spectra were measured with a Triple
Raman spectrometer (Model T64000, Make J-Y Horiba) equipped
with 1800 grooves/mm gratings, a TE cooled Synapse CCD detector,
and an open stage Olympus microscope with a 50× objective. The
G
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samples were excited with a 532 nm wavelength (0.2 mW power, 20 s
integration time, and 10 accumulations) laser from a DPSS Nd:YAG
laser (Make: Spectra Physics). Labeling experiments were carried out
with 18O2 gas (99 atom %) or H218O(98 atom %) purchased from
Icon Services Inc., USA. Samples for recording Raman spectra were
prepared by drop-casting an acetonitrile solution of the complex on a
glass slide. Product analyses were done with a PerkinElmer Clarus-580
GC with FID apparatus (Elite-I, polysiloxane, 30 m column).
Synthesis of [MnIII(OH)(dpaq)](ClO4) (1). To a solution of H-
dpaq (0.26 mmol) in CH3CN (2 mL) was added a solution of
Mn(ClO4)2·6H2O (0.11 g, 0.31 mmol) in CH3CN (1 mL) at room
temperature, followed by the dropwise addition of triethylamine. The
mixture was stirred for 2 h. The light beige precipitate was collected
and washed with diethyl ether followed by drying in vacuo. The solid
was then dissolved in acetonitrile and stirred in the presence of air,
yielding a golden brown solution. The solution was evaporated to
dryness, and the solid product was recrystallized using acetonitrile/
Et2O. The product was obtained as a brown solid in 81% yield (0.12
g). UV/vis (CH3CN): λmax [nm] (ε in M−1 cm−1): 486 (252), 743
(108). ESI MS: m/z 454.04. Anal. Calcd: C, 49.97; H, 3.82; N, 12.67.
Found: C, 49.26; H, 3.95; N, 12.47.
polarizable continuum model (CPCM).80,81 To obtain optical
absorption spectra of the compound, time-dependent density
functional theory (TDDFT) was employed with a long-range
corrected CAM-B3LYP functional82 with all other settings unchanged
from those above. All of the calculations were implemented in ORCA
(version 4.0).83
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00579.
Spectroscopic data for 1 and 1-O, reactivity data for 1
and 1-O, and DFT calculations (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail for T.K.P.: ictkp@iacs.res.in.
*E-mail for S.P.: satadal2008@gmail.com.
*E-mail for A.N.B.: achintesh@nitsikkim.ac.in.
■
[MnIII(18OH)(dpaq)](ClO4) was prepared by treating a 10 mM
[MnII(dpaq)](ClO4) solution in dry acetonitrile with 18O2 gas. The
formation of [MnIII(18OH)(dpaq)](ClO4) was confirmed by UV−vis
spectroscopy.
ORCID
Tapan Kanti Paine: 0000-0002-4234-1909
Achintesh N. Biswas: 0000-0002-5828-7230
Preparation of [MnIV(O)(dpaq)]ClO4 (1-O). [MnIV(O)(dpaq)]-
ClO4 (1-O) was generated at 293 K from the reaction of
[MnIII(OH)(dpaq)](ClO4) (1) (0.5 mM solution in acetonitrile)
with 0.6 mM m-CPBA (1.2 equiv) in the presence of 0.5 mM triflic
acid (HOTf) or 0.5 mM Sc(OTf)3. The formation of [MnIV(O)-
(dpaq)]ClO4 (1-O) was monitored by UV−vis spectroscopy by
following the increase in the absorption features at 700 and 500 nm.
Alternatively, [MnIV(O)(dpaq)]ClO4 (1-O) was generated at 293
K from the reaction of [MnIII(OH)(dpaq)](ClO4) (1) (0.5 mM
solution in acetonitrile) with a mixture of [RuIII(bpy)3]3+ and HOTf
(0.5 mM each in acetonitrile).
Author Contributions
⊥S.B. and A.M. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by grants from the Science and
Industrial research Board (SERB, Project No. YSS/2015/
000829 to A.N.B.) & Council of Scientific and Industrial
Research (CSIR, Project No. 01(2862)/EMR-II to A.N.B. and
Project No. 01(2926)/18/EMR-II to T.K.P.). A.M. and S.B.
thank University Grants Commission (UGC) & NIT Sikkim,
respectively, for research fellowships. S.P. thanks the project
“MANGAN (03EK3545)” fellowship funded by the Bundes-
[MnIV(18O)(dpaq)]ClO4 (1-18O) was generated from 1-18OH
using [Ru(bpy)3]3+ as an oxidant in the presence of HOTf in
acetonitrile at 293 K.
Kinetic Measurements and Product Analysis. Kinetic
measurements of substrate oxidations were performed in a 1 cm
UV cuvette at 293 K by following the UV−vis spectral changes of
reaction solutions. Rate constants were determined under pseudo-
first-order conditions by fitting the changes in absorbance at 700 nm
for 1-O, generated in situ by the addition of 1.0 equiv of Sc(OTf)3
and 1.2 equiv of m-CPBA. Fitting the decay of 1-O to a single-
̈
ministeriums fur Bildung und Forschung (BMBF).
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