A side-on Mn(iii)-peroxo supported by a non-heme pentadentate N3Py2ligand: Synthesis, characterization and reactivity studies

Article abstract of DOI:10.1039/d0dt03706k

A mononuclear manganese(iii)-peroxo complex [MnIII(N3Py2)(O2)]+ (1a) bearing a non-heme N,N′-dimethyl-N-(2-(methyl(pyridin-2-ylmethyl)amino)ethyl)-N′-(pyridin-2-ylmethyl)ethane-1,2-diamine (N3Py2) ligand was synthesized by the reaction of [Mn(N3Py2)(H2O)](ClO4)2 (1) with hydrogen peroxide and triethylamine in CH3CN at 25 °C. The reactivity of 1a in aldehyde deformylation using 2-phenyl propionaldehyde (2-PPA) was studied and the reaction kinetics was monitored by UV-visible spectroscopy. A kinetic isotope effect (KIE) = 1.7 was obtained in the reaction of 1a with 2-PPA and α-[D1]-PPA, suggesting nucleophilic character of 1a. The activation parameters ΔH? and ΔS? were determined using the Eyring plot while Ea was obtained from the Arrhenius equation by performing the reaction between 288 and 303 K. Hammett constants (σp) of para-substituted benzaldehydes p-X-Ph-CHO (X = Cl, F, H, and Me) were linear with a slope (ρ) = 3.0. Computational study suggested that the side-on structure of 1a is more favored over the end-on structure and facilitates the reactivity of 1a.

Full text of DOI:10.1039/d0dt03706k

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
A side-on Mn(III)–peroxo supported by a non-
heme pentadentate N3Py2 ligand: synthesis,
characterization and reactivity studies†
Cite this: Dalton Trans., 2021, 50,
2824
d
Dattaprasad D. Narulkar,a,b Azaj Ansari, c Anil Kumar Vardhaman,
e
Sarvesh S. Harmalkar,a Giribabu Lingamallu, d Vishal M. Dhavale,
Muniyandi Sankaralingam, f Sandip Das, g Pankaj Kumar g and
a
Sunder N. Dhuri
*
A
mononuclear manganese(III)–peroxo complex [MnIII(N3Py2)(O2)]+ (1a) bearing
a
non-heme
N,N’-dimethyl-N-(2-(methyl(pyridin-2-ylmethyl)amino)ethyl)-N’-(pyridin-2-ylmethyl)ethane-1,2-diamine
(N3Py2) ligand was synthesized by the reaction of [Mn(N3Py2)(H2O)](ClO4)2 (1) with hydrogen peroxide and
triethylamine in CH3CN at 25 °C. The reactivity of 1a in aldehyde deformylation using 2-phenyl propional-
dehyde (2-PPA) was studied and the reaction kinetics was monitored by UV-visible spectroscopy. A
kinetic isotope effect (KIE) = 1.7 was obtained in the reaction of 1a with 2-PPA and α-[D1]-PPA, suggesting
nucleophilic character of 1a. The activation parameters ΔH‡ and ΔS‡ were determined using the Eyring
plot while Ea was obtained from the Arrhenius equation by performing the reaction between 288 and
303 K. Hammett constants (σp) of para-substituted benzaldehydes p-X-Ph-CHO (X = Cl, F, H, and Me)
were linear with a slope (ρ) = 3.0. Computational study suggested that the side-on structure of 1a is more
favored over the end-on structure and facilitates the reactivity of 1a.
Received 27th October 2020,
Accepted 6th January 2021
DOI: 10.1039/d0dt03706k
rsc.li/dalton
oxygen-evolving complexes of photosystem II.6 The manga-
nese–peroxo adduct in Mn-SOD is the only structurally charac-
Introduction
Metal-dioxygen compounds are key intermediates formed terized intermediate in biological systems where the peroxide
during the dioxygen activation by various metal-containing binds to the manganese by side-on binding mode instead of
enzymes.1 In the 3d metal series, manganese plays a vital role the end-on way.7 Many manganese(III)–peroxo compounds
in the active sites of several metalloenzymes, offering different bearing flexible and rigid ligand environments have been now
oxidation states in redox reactions.2 The manganese–peroxo reported and characterized by either spectroscopic methods or
intermediates have been detected as the reactive species in by X-ray crystallography (Scheme 1, ESI Table S1†).8–13 The first
several enzymes like manganese superoxide dismutase (Mn-
SOD),3 manganese ribonucleotide reductase,4 manganese
homoprotocatechuate 2,3-dioxygenase (Mn-HPCD),5 and
aSchool of Chemical Sciences, Goa University, Goa-403206, India.
E-mail: sndhuri@unigoa.ac.in; Tel: +918669609172
bDepartment of Chemistry, Dnyanprassarak Mandal’s College and Research Centre,
Assagao, Goa-403507, India
cDepartment of Chemistry, Central University of Haryana, Mahendergarh-123031,
Haryana, India
dPolymers & Functional Materials Division, CSIR-Indian Institute of Chemical
Technology, Uppal Road, Tarnaka, Hyderabad-500007, India
eCSIR-Central Electrochemical Research Institute, CSIR Madras Complex, Taramani,
Chennai-600 113, India
fBioinspired & Biomimetic Inorganic Chemistry Lab, Department of Chemistry,
National Institute of Technology Calicut, Kozhikode, Kerala 673601, India
gIndian Institute of Science Education and Research (IISER), Tirupati-517507, India
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0dt03706k
Scheme 1 Structurally characterized Mn(III)-O22− intermediates.13
2824 | Dalton Trans., 2021, 50, 2824–2831
This journal is © The Royal Society of Chemistry 2021

View Article Online
Dalton Transactions
Paper
2−
crystal structure of a side-on manganese(III)–peroxo species sup- reactions, Mn(III)–O2
species have nucleophilic character.
ported by a heme ligand tetraphenylporphyrin (TPP) was Interestingly, in recent reports, it was shown that the deformy-
[MnIII(O2)(TPP)]− (structure a, Scheme 1).9 The structural charac- lation of aldehydes by a Mn(III)–O22− species initially occurs via
terization and reactivity of Mn(III)–peroxo species bearing non- H-atom abstraction at the α-position to the –C(H)vO group in
heme tetramethylated cyclam ligands namely, 12-TMC, 13-TMC, an electrophilic manner instead of nucleophilic attack on the
and 14-TMC were for the first time reported by Nam and co- carbonyl group of aldehyde.13f,g,i Since there are only a few
workers (structures b–d, Scheme 1).13a,b,c,15c The structures of reports on the electrophilic nature of an electron-rich manga-
manganese(III)–peroxo species of trispyrazolyl based ligands nese–peroxo species, it is important to investigate more such
such as (hydrotris(3,5-diisopropylpyrazol-1-yl)borate (Tpipr2) and reactions using different ligands that stabilize manganese–
tris(3,5-diphenylpyrazol)hydroborate (TpPh2) are also investi- peroxo intermediates. Influenced by the dichotomic reactivity
gated (structures e–g, Scheme 1).10 Studies on a large number of of Mn(III)–peroxo intermediates (nucleophilic or electrophilic)
Mn–peroxo intermediates have provided in-depth knowledge of proposed by the Nam and Sastri groups, herein we report the
their molecular and electronic structures and such intrinsic pro- synthesis, characterization and reactivity studies of a new
perties have been correlated with the reactivity of active sites of manganese(III)–peroxo stabilized by a non-heme pentadentate
manganese-based enzymes.8 The manganese containing ligand N,N′-dimethyl-N-(2-(methyl(pyridin-2-ylmethyl)-amino)
enzymes usually react with oxidants like dioxygen, superoxides, ethyl)-N′-(pyridin-2-ylmethyl-)ethane-1,2-diamine (N3Py2) at
or peroxides to form Mn-peroxide intermediates.11 Several of room temperature. Our literature studies show that very few
the reported Mn(III)–peroxo species were prepared by reacting a peroxo intermediates are kinetically stable at room tempera-
Mn(II)-precursor compound with potassium superoxide,9,12 ture, and others are spectroscopically detected at low tempera-
hydrogen peroxide,10 hydrogen peroxide in the presence of a tures (Table S1†).10,13b,f Many high valent metal–oxygen inter-
mild base,12c–e,13 or O2
generated by electrochemical mediates of pentadentate ligands similar to our ligand N3Py2
•−
methods14 or O2 (see ESI Table S1†). It has been well docu- are stable at room temperature.16a,17
mented that in the Mn–O22− species structure, the peroxo anion
15
can exist in two binding modes viz side-on (η2) or end-on (η1).8–13
The Mn(III)–peroxo intermediates bearing tetradentate ligands
Experimental
Materials and methods
bind to Mn(III) in a side-on (η2) peroxo species fashion,8 while
on the other hand, both side-on (η2) and end-on (η1) coordi-
nation modes are observed in Mn(III)–peroxo species bearing All solid chemicals used in this work were purchased and used
pentadentate ligands.13c,e,13f,h–j Nam and co-workers have well as received without recrystallization. The solvents were dried
demonstrated the conversion of a side-on Mn(III)–peroxo species and distilled under a N2 atmosphere before their use. N3Py2
to end-on peroxo bearing a tetradentate TMC ligand on the and [Mn(N3Py2)(H2O)](ClO4)2 (1) were prepared according to
addition of axial ligands at the fifth coordination site.13c our recent report.18,19 The deuterated 2-phenyl propionalde-
However, the computational studies have shown that Mn(III)– hyde was synthesized with a slight modification of the litera-
2
2
peroxo species with pentadentate N5 ligands (mL5 and imL5 , ture method.20 A dry powder of sodium hydride (0.3 g) was
N4Py, and Pro3Py) with a bispidine backbone can exist in only taken in DMSO-d6 maintained under an Ar atmosphere, and
side-on (η2) binding mode (Scheme 2).13e,f,13h–j
then the temperature was lowered to 0 °C. 2-Phenyl propion-
Apart from these two coordination modes (side-on (η2) or aldehyde (2-PPA) (1.335 mL) was then slowly added (this is an
end-on (η1)), the formation of bis(μ-oxo)dimanganese (III, IV) exothermic reaction and hence precautions are required), and
wherein pentadentate ligands N4py and dpaq are bound to Mn the temperature of the reaction mixture was then slowly
in an unusual tetradentate manner is also reported.16
increased to 25 °C. The solution was kept under stirring for
The reactivity study of Mn(III)–peroxo species in the defor- 8 h and then neutralized with D2O, followed by extraction with
mylation of aldehydes is well documented13a–e and in these ethyl acetate. Purity of >99% deuteration in the product was
1
confirmed by H NMR spectroscopy (ESI, Fig. S1†). UV-visible
spectra and kinetic data were collected on an Agilent diode
array 8453 UV-visible spectrophotometer equipped with a cir-
culating water bath. Electrospray ionization mass spectra
(ESI-MS) of 1 and 1a were recorded on an Agilent mass spectro-
meter (6200 series TOF/6500 series Q-TOF B.08.00), by infusing
samples directly into the source using a manual method.
1HNMR spectra were measured with a Bruker model Ascend
400 FT-NMR spectrometer. The electron paramagnetic reso-
nance (EPR) spectra were recorded using an X-band Bruker
EMX-plus spectrometer equipped with a dual-mode cavity (ER
4116DM). Low temperatures were attained by using an Oxford
Instruments ESR900 liquid He quartz cryostat with an Oxford
Instruments ITC503 temperature and gas flow controller. The
Scheme 2 List of N5 pentadentate ligand supported Mn(III) peroxo
structures.
This journal is © The Royal Society of Chemistry 2021
Dalton Trans., 2021, 50, 2824–2831 | 2825

View Article Online
Paper
Dalton Transactions
experimental parameters for EPR spectra were as follows: Reactivity studies of 1a
microwave frequency = 9.648 GHz, microwave power = 1.0 mW,
The reactivity of [Mn(N3Py2)(16O2)]+ 1a was investigated in
deformylation of aldehydes such as 2-phenylpropionaldehyde
(2-PPA) and cyclohexanecarboxyaldehyde (CCA). The reactions
were performed in a 1 cm quartz cuvette and the kinetics of
the reactions were followed by monitoring the decay of the
572 nm band of 1a using a UV-visible spectrophotometer. Rate
constants were determined under pseudo-first-order con-
ditions (i.e., [substrate]/[1a] > 10), by fitting the changes in the
absorbance of the decay of the peak at 572 nm.
modulation amplitude = 10 G, gain = 1 × 104, modulation fre-
quency = 100 kHz, time constant = 40.96 ms, conversion time
= 85.00 ms and measurement temperature = 77 K. The product
analysis was performed using a Shimadzu GC 2014 gas chro-
matograph equipped an HP capillary column (30 m × 0.25 mm
× 2.5 μM) using an FID detector. The nitrogen gas was used as
a carrier gas. The retention time and peak areas of the pro-
ducts were compared with those of authentic samples using
decane as an internal standard.
Computational techniques
Results and discussion
Generation and characterization of 1a
The formation of [Mn(N3Py2)(16O2)]+ 1a intermediate in
CH3CN was monitored by UV-visible spectroscopy (Fig. 1a).
The UV-Vis spectrum of 1 in CH3CN exhibits bands only in the
All the geometry optimization was performed using Gaussian
09, a program suite.21 In a similar work, several functionals
such as B3LYP, B3LYP-D, wB97XD, B97D, M06-2X, OLYP,
TPPSh, and MP2 were employed and B3LYP, B3LYP-D2, and
wB97XD were advocated to correctly predict the spin ground
state of the reactant and intermediates compared to experi-
mental data.22 Our calculations were restricted with two func-
tionals, B3LYP and a dispersion corrected functional B3LYP-D2,
for the geometry optimization of the Mn(III)–peroxo species
1a.23 Here, two different basis sets, LanL2DZ for Mn24 and a
6-31G basis set, were used for the C, H, N, and O atoms.25 The
optimized geometries were then used to perform single-point
energy calculations using a TZVP26 basis set on all atoms. The
quoted DFT energies are B3LYP-D2 solvation, including free-
energy corrections with a TZVP basis set at a temperature of
298.15 K. The optimized geometries were verified by animating
frequencies by using Chemcraft software. The solvation energies
were computed at the B3LYP-D level by using a polarizable con-
tinuum model (PCM) with acetonitrile as a solvent.
Synthesis of [Mn(N3Py2)(H2O)](ClO4)2 (1)
Compound 1 was prepared according to our recent work.18
N3Py2 (0.452 g, 1.38 mmol) in CH3CN (2 mL) was added to Mn
(ClO4)2·6H2O (0.5 g, 1.38 mmol) (2 mL CH3CN) at RT under a
N2 atmosphere. The reaction mixture upon stirring for 12 h
resulted in a brown solution. Addition of diethyl ether
afforded a white-buff powder which on work-up gave yield =
0.68 (82%). Calc. for C19H31N5Cl2O9Mn: C, 38.08; H, 4.91; N,
11.69%. Found C, 38.19; H, 4.91; N, 11.59%. IR (KBr, cm−1):
3412 ν(O–H); 1093, 621 ν(ClO4). Magnetic moment, μeff = 5.97
BM. ESI-MS: m/z = 191.18 (Calc. m/z = 191.21): [Mn(N3Py2)]2+
and m/z = 481.08 (Calc. m/z = 481.86): [Mn(N3Py2)(ClO4)]+.
Generation of a Mn(III)–peroxo intermediate (1a)
A light purple manganese(III)–peroxo complex [Mn(N3Py2)(O2)]+
(1a) was generated by reacting Mn(II)-complex 1 (1 mM, 2 mL
CH3CN) with 10 equiv. of H2O2 (30% v/v) and 5 equiv. of tri-
ethylamine (TEA) at 25 °C. The formation of 1a and its stability Fig. 1 (a) UV-visible spectral changes after the addition of 10 equiv.
H2O2 in the presence of 5 eq. of TEA at 15 °C. The inset shows a time
trace monitored at 572. (b) ESI-MS spectrum of 1a recorded in CH3CN
showing a mass peak at m/z = 414.17 corresponding to [Mn(N3Py2)
were analyzed by monitoring the spectral changes of the result-
ing solution with a UV-visible spectrophotometer. λmax, CH3CN/
nm (ε/L mol−1 cm−1): 572(254), 412(120); magnetic moment, μeff
(
16O2)]+ species. The insets show the observed distribution patterns
which correspond to the [Mn(N3Py2)(16O2)]+ 1a in black and [Mn(N3Py2)
18O2)]+ in red. A peak m/z = 164.62 corresponds to [N3Py2H2]2+
= 2.91 BM, ESI-MS: m/z = 414.17 (Calc. 414.17) for [Mn(N3Py2)
(16O2)]+ and m/z = 418.17 (Calc. 418.17) for [Mn(N3Py2)(18O2)]+.
(
.
2826 | Dalton Trans., 2021, 50, 2824–2831
This journal is © The Royal Society of Chemistry 2021

View Article Online
Dalton Transactions
Paper
UV region which are associated with intra-ligand transitions.
The absorption spectrum of 1 in CH3CN does not show any
band in the visible region,18 which is the typical characteristic
feature of high spin Mn(II) centers due to spin forbidden d–d
transitions as observed in similar Mn(II) complexes.27 On
addition of 10 equiv. of H2O2 to the 1 mM solution of 1 in
CH3CN in the presence of 5 equiv. of triethylamine (TEA) at
25 °C, the formation of a purple-colored intermediate (1a) was
observed (ESI, Fig. S2†) with the appearance of a new band at
572 nm (254 ε/L mol−1 cm−1) and a shoulder peak at 412 nm
(120 ε/L mol−1 cm−1) (Fig. 1a). The intermediate (1a) was
stable for a sufficient time which allowed us to characterize
and perform the reactions. The species (1a) was characterized
by ESI-MS (Fig. 1b) and EPR studies (ESI, Fig. S3†). The ESI-MS
of 1a in CH3CN shows a peak at m/z = 414.17 whose mass and
isotopic distribution pattern corresponded to [Mn(N3Py2)(O2)]+
species (Calc. m/z = 414.17). When H218O2 was used in the
generation of 1a, the peak due to Mn-16O2 shifted by two units
to m/z = 418.17. Unlike the EPR spectrum of 1 which showed
six-line hyperfine signals (ESI, Fig. S3a†),18 the X-band EPR
spectrum of 1a was devoid of any active signals, suggesting the
EPR silent nature of 1a (ESI, Fig. S3b†). These EPR data thus
suggest that the Mn in 1a is Mn(III) with a d4 electron configur-
ation.13c Furthermore, we have determined the magnetic
moment of 1a, using Evans’ method,28 and found it to be 2.91
BM (ESI, Fig. S4b†), in agreement with our assumption of a
low spin Mn3+ complex compared to high spin Mn(II) (ESI,
Fig. S4a†). The ESI-MS, EPR and Evans method characteriz-
ation results unambiguously support the formation of Mn(III)–
peroxo intermediate 1a. On confirming the formula of Mn(III)–
peroxo species, the binding mode of 1a was elucidated by
using computational study (vide infra).
Fig. 2 (a) UV-visible spectral changes of 1a (1 mM) upon addition of 10
equiv. of 2-PPA in acetonitrile. The inset shows the time course of the
reaction monitored at 572 nm. (b) A plot of kobs against the concen-
tration of 2-PPA and 2-PPA-d to determine the second-order rate
constant.
The product analysis of the reaction mixture by the gas
chromatographic (GC) method revealed acetophenone as the
predominant product (88% ( 2)) based on 1a 29b,c (Scheme 3).
We have also investigated the reactivity of cyclohexanecarbox-
aldehyde (CCA) with 1a; however, this reaction was very fast to
determine kinetic data using a UV-Vis spectrometer. The
product analysis of the final reaction mixture gave cyclohexene
in quantitative yields (75% ( 4)) based on the amount of 1a
used.13b,c,29b,c The activation parameters for the aldehyde
deformylation reaction of 2-PPA were obtained from the Eyring
plot to give ΔH‡ and ΔS‡ by determining pseudo-first-order
rate constants from 288 to 303 K (Fig. 3a). The rates of reac-
tions were dependent on the temperature, and a linear Eyring
plot was obtained to give the activation parameters of ΔH‡ =
42 ( 2) kJ mol−1 and ΔS‡ = −139 ( 3) J mol−1 K−1. The acti-
vation energy (Ea) = 42 ( 1) kJ mol−1 was obtained from the
Arrhenius equation from the plot of ln Kobs versus 1/T (ESI,
Fig. S5†).
Reactivity of 1a in aldehyde deformylation
The reactivity of 1a was investigated by using different sub-
strates. The electrophilic character of 1a was initially tested in
the oxidation of triphenylphosphine (PPh3), thioanisole
(PhSCH3), cyclohexene and xanthene. However, we did not
observe any spectral changes upon the addition of these sub-
strates to the CH3CN solution of 1a and the band at 572 nm
remained the same. The product analysis of these reactions
did not show the formation of any oxygenated products. This
indicated to us that 1a does not show electrophilic character in
the above oxidation reactions. This is also in agreement with
the peroxo species of other first-row transition metals reported
earlier.29 The reactivity of 1a was then investigated in aldehyde
deformylation as the precedent study of first-row transition
metal–peroxo species showed the reactivity of Mn(III)–peroxo
species with aldehydes. Upon addition of 10 equiv. of 2-PPA to
the solution of 1a, the UV-visible band at 572 nm decayed with
a pseudo-first-order rate constant, kobs = 1.6 × 10−3 s−1, and
showed the isosbestic points at 501 and 778 nm (Fig. 2a).
Upon increasing the concentration of 2-PPA, the pseudo-first-
order rate constants increased proportionally, allowing us to
determine a second-order rate constant (k2) of 1.6 × 10−1 M−1
S−1 for the reaction at 25 °C (Fig. 2b).
It is reported that the proposed mechanism of aldehyde
deformylation by metal–peroxo species involves a nucleophilic
attack of peroxo species on the carbonyl group of an aldehyde
forming peroxyhemiacetal intermediate.13b,c,29 The O–O bond
homolytic cleavage of this intermediate then yielded deformy-
Scheme 3 Upon deformylation of 2-PPA, acetophenone is obtained as
the major product.
This journal is © The Royal Society of Chemistry 2021
Dalton Trans., 2021, 50, 2824–2831 | 2827

View Article Online
Paper
Dalton Transactions
notion of nucleophilicity of 1a, we measured the reactivity of
1a with different para-substituted benzaldehydes, p-X-Ph-CHO
(where X = Cl, F, H, Me, and OMe) substrates. Our results
suggest that the electron withdrawing para-substituents on the
benzaldehyde react at a faster rate with 1a compared to the
electron donating para-substituted aldehydes, following the
reactivity trend Cl > F > H > Me > OMe. The plot of log kobs
versus Hammett constants (σpþ) was found to be linear, giving a
slope (ρ) = 3.0 (Fig. 3b). These data thus provide additional evi-
dence for the nucleophilic reactivity of 1a with 2-PPA.
Certain properties of ligands such as steric properties can
2– 34
affect the nucleophilic reactivity of Mn(III)–O2
.
In our case
we feel that the flexible ligand N3Py2 may be tangling
around the Mn(III)–O22−, making the initial H-atom abstraction
process more difficult, thus following the nucleophilic
pathway. To gain more understanding of the mechanism
of deformylation of aldehydes and influence of ligand struc-
tures on reactivity, our efforts are underway to prepare new
2−
ligands which can stabilize M(III)–O2
species at room
temperature.
Computational studies
Fig. 3 (a) Plot of ln (Kobs/T) against 1/T to determine the activation
parameters for the reaction of 1a and 2-PPA. (b) Hammett plot for the
oxidation of para-substituted benzaldehydes p-X-Ph-CHO (X = Cl, F, H,
and Me) by 1a in CH3CN at 25 °C.
The experimental characterization gave a formula of 1a as
[Mn(N3Py2)(η2-O2)]+. However, two isomeric structures for 1a
are possible, end-on [Mn(N3Py2)(η1-O2)]+ (1a1) and side-on
[Mn(N3Py2)(η2-O2)]+ (1a2) (Scheme 4). To gain further insight
into the stability of the binding mode of the peroxo ligand
lated products.29c The ferric-peroxo porphyrin intermediate in with Mn(III), we then performed a computational study on
cytochrome P450 progesterone 17α-hydroxylase-17,20-lyase
(CYP 45017α) is reported to attack the carbonyl of progester-
Mn(III)–peroxo intermediate 1a.
End-on [Mn(N3Py2)(η1-O2)]+ (1a1). All the possible spin states
one, leading to the formation of androstenedione and of the end-on [Mn(N3Py2)(η1-O2)]+ (1a1) species are optimized
acetate.30 Very recently, Sastri and co-workers have demon-
using the B3LYP-D2 functional. Our DFT calculations reveal
strated that the deformylation of an aldehyde such as 2-PPA by
that the quintet state (high spin) is computed as the ground
a nonheme Mn(III)–peroxo complex occurs via initial H-atom state, and this is consistent with the earlier reports on similar
abstraction.13f Such an electrophilic abstraction was supported
based on evidence such as the observation of a large kinetic
isotope effect (KIE = 5.4) compared to KIE = 1 for the reaction
of Mn(III)–peroxo with 2-PPA/α-[D1]-PPA and computational
studies.13f,g,13i They also reported that Mn(III)–peroxo species
showed no reactivity with 2-methyl-2-phenyl propionaldehyde,
which lacks α-hydrogen atoms. In the present, when we per-
formed the reaction of 1a with α-[D1]-PPA, we obtained a
second-order rate constant (k2) of 9.3 × 10−2 M−1 S−1, giving us
KIE = 1.7 (Fig. 2b), and this value is slightly higher than the
expected KIE = 1 for the nucleophilic reactions.31 A slight
increase in the value of KIE in our work could be attributed to
the presence of a water molecule in the reaction mixture which
is generated through the release from the parent compound
[Mn(N3Py2)(H2O)]2+ 1 on addition of H2O2. Such a wet reaction
in the presence of a trace amount of water is earlier reported
by Tolman and coworkers.32 Thus in comparison with the pre-
vious study, which showed a high kinetic effect (KIE = 5.4) for
the deformylation of 2-PPA via initial H-atom abstraction, in
our work we propose that the mechanism could still proceed
via nucleophilic pathway as reported in several other 3d metal
ligand architectures.35 The triplet state lies at 107.6 kJ mol−1
higher in energy, while a low-spin singlet lies at 164.4 kJ mol−1
Scheme 4 Probable structure of the reactive intermediate 1a which
(III)–peroxo intermediates.13b,e,29b,c,33 Further to support our can exist as side-on (η2) or end-on (η1) peroxo.
2828 | Dalton Trans., 2021, 50, 2824–2831
This journal is © The Royal Society of Chemistry 2021

View Article Online
Dalton Transactions
Paper
higher in energy. The optimized structure of the high spin this is due to the presence of two ligated oxygen molecules.
end-on [Mn(N3Py2)(η1-O2)]+ (1a1) species is shown in the ESI, A significant spin density on proximal oxygen (ESI, Fig. S6d†)
Fig. S6a.† The Mn–O1 and O1–O2 bond lengths of the ground facilitates the reactivity of 1a. Since 1a is relatively stable at
state of the Mn(III)–peroxo species are computed to be 2.004 Å room temperature, our efforts are underway to grow the
and 1.392 Å. The Mn–N(aq) bond length is found to be single crystal of 1a.
2.401 Å. The O1–Mn–N3 bond angle is computed to be 157.3°.
The computed structural parameters of the quintet, triplet,
and singlet spin states of the Mn(III)–peroxo species are shown
in Table S2.† The ground state’s spin density plot is also
Conclusions
shown in Fig. S6b† and suggests the presence of four unpaired We have described the synthesis and characterization of a new
electrons in the d-orbital of Mn(III)–peroxo species (Table S3†).
mononuclear non-heme Mn(III)peroxo species stabilized by a
Side-on [Mn(N3Py2)(η2-O2)]+ (1a2). The possible spin states of pentadentate N3Py2 ligand. The species is metastable at room
the side-on [Mn(N3Py2)(η2-O2)]+ (1a2) species are also opti- temperature. The Mn(III)peroxo species depicts its reactivity in
mized. The B3LYP-D2 computed results show that the triplet aldehyde deformylation on reacting with 2-PPA. The kinetics
and the singlet lie at 99.6 and 152.6 kJ mol−1, respectively. The of reactions was monitored by following the decay of the peak
optimized structure of the side-on [Mn(N3Py2)(η2-O2)]+ (1a2) corresponding to Mn(III)–peroxo. The activation parameters
species is shown in Fig. S6c.† The computed Mn–O1, Mn–O2, ΔH‡ and ΔS‡ for the aldehyde deformylation reaction were
and O1–O2 bond lengths of the ground state are 2.037 Å, obtained from the Eyring plot and activation energy Ea from
2.044 Å, and 1.443 Å. The axial Mn–N bond length is computed the Arrhenius equation by performing the reactions at a
to be 2.458 Å. The O1–Mn–N3 and O2–Mn–N3 bond angles are different temperature ranging from 288 to 303 K. A KIE = 1.7
found to be 154.0° and 150.3°. The computed structural para- was obtained on carrying out the reaction with α-[D1]-PPA.
meters of side-on [Mn(N3Py2)(η2-O2)]+ (1a2) species are shown KIE = 1.7 and a positive (ρ) value of 3.0 suggest that the defor-
in Table S2.† The spin density plot of the ground state is mylation reaction proceeds by nucleophilic attack on the
shown in Fig. S6d† and the spin density data is given in carbonyl group. DFT calculations were performed to provide
Table S3.†
evidence for the side-on binding mode of the (O2)2− ligand to
DFT calculations predicted that the side-on [Mn(N3Py2)(η2- Mn(III).
O2)]+ (1a2) species is found to be the lowest in energy by
14.6 kJ mol−1 compared with the end-on [Mn(N3Py2)(η1-O2)]+
(1a1) species. The B3LYP functional was also employed for
the optimization of the structural parameter (Table S4†),
Conflicts of interest
which further suggests the side-on [Mn(N3Py2)(η2-O2)]+ (1a2) There are no conflicts to declare.
as the more stable intermediate than the end-on [Mn(N3Py2)
(η1-O2)]+ (1a1) species. The DFT optimized structure of 1a is
shown in Fig. 4, and DFT results are in accordance with the
Acknowledgements
literature report wherein side-on (η2-O2) binding mode is
observed in Mn(III)–peroxo species bearing a pentadentate N5 SND gratefully acknowledges the Council of Scientific and
ligand showing seven coordination around Mn(III).28–31,33 In Industrial Research (CSIR), New Delhi, India (No. 01(2923)/18/
some other cases, to form the side-on peroxo complex, the EMR-II), for financial support, University Grants Commission
one arm of the pentadentate ligand is free; thus, a six-coordi- (UGC) and Department of Science and Technology (DST), New
nate Mn(III) complex is formed,34 unlike the present case. The Delhi, India, for the support to School of Chemical Sciences,
axial Mn–N bond length of the side-on [Mn(N3Py2)(η2-O2)]+ Goa University, through their SAP and FIST programs. D.D.N
(1a2) species is slightly longer than that of the six-coordinate thanks UGC, New Delhi, for SRF Fellowship (UGC-BSR). AKV
end-on [Mn(N3Py2)(η1-O2)]+ (1a1) species (Table S2†), and acknowledges SERB, India (PDF/2016/001158). MS sincerely
acknowledges the DST for the award of the DST-Inspire Faculty
research grant (IFA-17-CH286). SND thanks IISER Pune for
providing the ESI-MS and NMR facility.
Notes and references
1 (a) P. R. O. De Montellano, Cytochrome P450: structure,
mechanism, and biochemistry, Springer Science & Business
Media, 2005; (b) J. Lawrence Que, W. B. Tolman,
J. McCleverty and T. J. Meyer, in Comprehensive
Coordination Chemistry II: From Biology to Nanotechnology,
Fig. 4 DFT optimized structure of [Mn(N3Py2)(O2)]+ 1a.
Elsevier, Oxford, 2004; (c) B. Meunier, Biomimetic oxidations
This journal is © The Royal Society of Chemistry 2021
Dalton Trans., 2021, 50, 2824–2831 | 2829

View Article Online
Paper
catalyzed by transition metal complexes, World Scientific,
Dalton Transactions
D. Kumar, C. V. Sastri and S. P. de Visser, J. Am. Chem. Soc.,
2017, 139, 18328–18338; (h) J. Du, C. Miao, C. Xia and
W. Sun, Chin. Chem. Lett., 2018, 29, 1869–1871;
(i) P. Barman, F. G. Cantú Reinhard, U. K. Bagha,
D. Kumar, C. V. Sastri and S. P. de Visser, Angew. Chem.,
Int. Ed., 2019, 58, 10639–10643; ( j) R. A. Geiger, D. F. Leto,
S. Chattopadhyay, P. Dorlet, E. Anxolabéhère-Mallart and
T. A. Jackson, Inorg. Chem., 2011, 50, 10190–10203.
2000.
2 E. J. Larson and V. J. Pecoraro, Manganese redox enzymes,
VCH Publishers, New York, 1992.
3 (a) C. Bull, E. C. Niederhoffer, T. Yoshida and J. A. Fee,
J. Am. Chem. Soc., 1991, 113, 4069–4076; (b) A. S. Hearn,
C. Tu, H. S. Nick and D. N. Silverman, J. Biol. Chem., 1999,
274, 24457–24460.
4 J. A. Cotruvo Jr., T. A. Stich, R. D. Britt and J. Stubbe, J. Am. 14 S. El Ghachtouli, H. Y. Vincent Ching, B. Lassalle-Kaiser,
Chem. Soc., 2013, 135, 4027–4039.
R. Guillot, D. F. Leto, S. Chattopadhyay, T. A. Jackson,
P. Dorlet and E. Anxolabéhère-Mallart, Chem. Commun.,
2013, 49, 5696–5698.
5 W. A. Gunderson, A. I. Zatsman, J. P. Emerson,
E. R. Farquhar, L. Que, J. D. Lipscomb and M. P. Hendrich,
J. Am. Chem. Soc., 2008, 130, 14465–14467.
6 J. P. McEvoy and G. W. Brudvig, Chem. Rev., 2006, 106,
4455–4483.
7 J. Porta, A. Vahedi-Faridi and G. E. O. Borgstahl, J. Mol.
Biol., 2010, 399, 377–384.
8 D. F. Leto and T. A. Jackson, J. Biol. Inorg. Chem., 2014, 19,
1–15.
15 (a) R. L. Shook, W. A. Gunderson, J. Greaves, J. W. Ziller,
M. P. Hendrich and A. S. Borovik, J. Am. Chem. Soc., 2008,
130, 8888–8889; (b) R. L. Shook and A. S. Borovik, Inorg.
Chem., 2010, 49, 3646–3660; (c) M. Sankaralingam,
Y.-M. Lee, S. H. Jeon, M. S. Seo, K.-B. Cho and W. Nam,
Chem. Commun., 2018, 54, 1209–1212.
16 (a) D. F. Leto, R. Ingram, V. W. Day and T. A. Jackson,
9 R. B. VanAtta, C. E. Strouse, L. K. Hanson and
J. S. Valentine, J. Am. Chem. Soc., 1987, 109, 1425–1434.
10 (a) N. Kitajima, H. Komatsuzaki, S. Hikichi, M. Osawa and
Y. Moro-oka, J. Am. Chem. Soc., 1994, 116, 11596–11597;
Chem.
Commun.,
2013,
49,
5378–5380;
(b) M. Sankaralingam, S. H. Jeon, Y.-M. Lee, M. S. Seo,
K. Ohkubo, S. Fukuzumi and W. Nam, Dalton Trans., 2016,
45, 376–383.
(b) U. P. Singh, A. K. Sharma, S. Hikichi, H. Komatsuzaki, 17 (a) X. Wu, M. S. Seo, K. M. Davis, Y.-M. Lee, J. Chen,
Y. Moro-oka and M. Akita, Inorg. Chim. Acta, 2006, 359,
4407–4411.
11 (a) S. Sahu and D. P. Goldberg, J. Am. Chem. Soc., 2016,
138, 11410–11428; (b) M. Guo, Y.-M. Lee, R. Gupta,
M. S. Seo, T. Ohta, H.-H. Wang, H.-Y. Liu, S. N. Dhuri,
K.-B. Cho, Y. N. Pushkar and W. Nam, J. Am. Chem. Soc.,
2011, 133, 20088–20091; (b) J. Chen, Y.-M. Lee, K. M. Davis,
X. Wu, M. S. Seo, K.-B. Cho, H. Yoon, Y. J. Park,
S. Fukuzumi, Y. N. Pushkar and W. Nam, J. Am. Chem. Soc.,
2013, 135, 6388–6391.
R. Sarangi, S. Fukuzumi and W. Nam, J. Am. Chem. Soc., 18 D. D. Narulkar, K. Devulapally, A. K. Undrajavarapu,
2017, 139, 15858–15867; (c) D. B. Rice, A. A. Massie and
T. A. Jackson, Acc. Chem. Res., 2017, 50, 2706–2717.
S. N. Dhuri, V. M. Dhavale, A. K. Vardhaman and
L. Giribabu, Sustainable Energy Fuels, 2020, 4, 2656–2660.
12 (a) S. Groni, G. Blain, R. Guillot, C. Policar and 19 D. D. Narulkar, A. K. Srivastava, R. J. Butcher, K. M. Ansy
E. Anxolabéhère-Mallart, Inorg. Chem., 2007, 46, 1951– and S. N. Dhuri, Inorg. Chim. Acta, 2017, 467, 405–414.
1953; (b) H. E. Colmer, A. W. Howcroft and T. A. Jackson, 20 (a) C. R. Goldsmith, R. T. Jonas and T. D. P. Stack, J. Am.
Inorg. Chem., 2016, 55, 2055–2069; (c) R. A. Geiger,
S. Chattopadhyay, V. W. Day and T. A. Jackson, J. Am. Chem.
Soc., 2010, 132, 2821–2831; (d) H. E. Colmer, R. A. Geiger,
D. F. Leto, G. B. Wijeratne, V. W. Day and T. A. Jackson,
Dalton Trans., 2014, 43, 17949–17963; (e) R. A. Geiger,
Chem. Soc., 2002, 124, 83–96; (b) C. Arunkumar, Y.-M. Lee,
J. Y. Lee, S. Fukuzumi and W. Nam, Chem. – Eur. J., 2009,
15, 11482–11489; (c) S. N. Dhuri, Y.-M. Lee, M. S. Seo,
J. Cho, D. D. Narulkar, S. Fukuzumi and W. Nam, Dalton
Trans., 2015, 44, 7634–7642.
S. Chattopadhyay, V. W. Day and T. A. Jackson, Dalton 21 M. Frisch, G. Trucks, H. B. Schlegel, G. E. Scuseria,
Trans., 2011, 40, 1707–1715.
13 (a) H. Kang, J. Cho, K.-B. Cho, T. Nomura, T. Ogura and
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnernberg, M. Hada, M. Ehara, K. Toyata,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, T. Vreven, J. A. Montgonery Jr.,
J. E. Paralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers,
K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand,
K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Khox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas,
W. Nam, Chem.
– Eur. J., 2013, 19, 14119–14125;
(b) M. S. Seo, J. Y. Kim, J. Annaraj, Y. Kim, Y. M. Lee,
S. J. Kim, J. Kim and W. Nam, Angew. Chem., Int. Ed., 2007,
46, 377–380; (c) J. Annaraj, J. Cho, Y.-M. Lee, S. Y. Kim,
R. Latifi, S. P. de Visser and W. Nam, Angew. Chem., Int.
Ed., 2009, 48, 4150–4153; (d) R. A. Geiger, G. B. Wijeratne,
V. W. Day and T. A. Jackson, Eur. J. Inorg. Chem., 2012,
2012, 1598–1608; (e) J. Du, D. Xu, C. Zhang, C. Xia, Y. Wang
and W. Sun, Dalton Trans., 2016, 45, 10131–10135;
(f) P. Barman, P. Upadhyay, A. S. Faponle, J. Kumar,
S. S. Nag, D. Kumar, C. V. Sastri and S. P. de Visser, Angew.
Chem., Int. Ed., 2016, 55, 11091–11095; (g) F. G. Cantú
Reinhard, P. Barman, G. Mukherjee, J. Kumar, D. Kumar,
2830 | Dalton Trans., 2021, 50, 2824–2831
This journal is © The Royal Society of Chemistry 2021

View Article Online
Dalton Transactions
Paper
J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox,
Gaussian 09 (Revision A.1), Gaussion, Inc., Wallingford CT,
2009.
2010, 132, 16977–16986; (b) J. Cho, R. Sarangi, J. Annaraj,
S. Y. Kim, M. Kubo, T. Ogura, E. I. Solomon and W. Nam,
Nat. Chem., 2009, 1, 568–572; (c) Y. Jo, J. Annaraj, M. S. Seo,
Y.-M. Lee, S. Y. Kim, J. Cho and W. Nam, J. Inorg. Biochem.,
2008, 102, 2155–2159.
22 A. Ansari, A. Kaushik and G. Rajaraman, J. Am. Chem. Soc.,
2013, 135, 4235–4249.
23 S. Grimme, J. Comput. Chem., 2006, 27, 1787–1799.
24 (a) T. H. Dunning Jr. and P. J. Hay, in J Methods of
30 M. Akhtar, D. Corina, S. Miller, A. Z. Shyadehi and
J. N. Wright, Biochemistry, 1994, 33, 4410–4418.
Electronic Structure Theory, ed. H. F. Schaefer III, Plenum, 31 S. H. Bae, X.-X. Li, M. S. Seo, Y.-M. Lee, S. Fukuzumi and
New York, 1976, pp. 1; (b) P. J. Hay and W. R. Wadt, W. Nam, J. Am. Chem. Soc., 2019, 141, 7675–7679.
J. Chem. Phys., 1985, 82, 270–283; (c) W. R. Wadt and 32 W. D. Bailey, N. L. Gagnon, C. E. Elwell, A. C. Cramblitt,
P. J. Hay, J. Chem. Phys., 1985, 82, 284–298; (d) P. J. Hay and
W. R. Wadt, J. Chem. Phys., 1985, 82, 299–310.
C. J. Bouchey and W. B. Tolman, Inorg. Chem., 2019, 58,
4706–4711.
25 R. Ditchfield, W. J. Hehre and J. A. Pople, J. Chem. Phys., 33 (a) D. L. Wertz and J. S. Valentine, in Metal-Oxo and Metal-
1971, 54, 724–728.
Peroxo Species in Catalytic Oxidations, ed. B. Meunier,
Springer Berlin Heidelberg, Berlin, Heidelberg, 2000, pp.
37–60, DOI: 10.1007/3-540-46592-8_2; (b) J. Annaraj, Y. Suh,
M. S. Seo, S. O. Kim and W. Nam, Chem. Commun., 2005,
4529–4531; (c) X. Hu, I. Castro-Rodriguez and K. Meyer,
J. Am. Chem. Soc., 2004, 126, 13464–13473; (d) J. Cho,
R. Sarangi and W. Nam, Acc. Chem. Res., 2012, 45, 1321–
1330; (e) J. Cho, H. Y. Kang, L. V. Liu, R. Sarangi,
E. I. Solomon and W. Nam, Chem. Sci., 2013, 4, 1502–1508.
26 (a) A. Schäfer, H. Horn and R. Ahlrichs, J. Chem. Phys.,
1992, 97, 2571–2577; (b) A. Schäfer, C. Huber and
R. Ahlrichs, J. Chem. Phys., 1994, 100, 5829–5835.
27 N. Saravanan, M. Sankaralingam and M. Palaniandavar,
RSC Adv., 2014, 4, 12000–12011.
28 (a) D. F. Evans, J. Chem. Soc., 1959, 2003–2005, DOI:
10.1039/jr9590002003; (b) S. Das, Kulbir, S. Ghosh,
S. C. Sahoo and P. Kumar, Chem. Sci., 2020, 11, 5037–5042;
(c) M. Yenuganti, S. Das, Kulbir, S. Ghosh, P. Bhardwaj, 34 J. Kim, B. Shin, H. Kim, J. Lee, J. Kang, S. Yanagisawa,
S. S. Pawar, S. C. Sahoo and P. Kumar, Inorg. Chem. Front.,
2020, 7, 4872–4882.
T. Ogura, H. Masuda, T. Ozawa and J. Cho, Inorg. Chem.,
2015, 54, 6176–6183.
29 (a) J. Cho, R. Sarangi, H. Y. Kang, J. Y. Lee, M. Kubo, 35 A. Ansari, P. Jayapal and G. Rajaraman, Angew. Chem., Int.
T. Ogura, E. I. Solomon and W. Nam, J. Am. Chem. Soc.,
Ed., 2015, 54, 564–568.
This journal is © The Royal Society of Chemistry 2021
Dalton Trans., 2021, 50, 2824–2831 | 2831