Synthesis, structural and physicochemical properties of a series of manganese(II) complexes with a novel N5 tripodal-amidate ligand and their potential use as water oxidation catalysts

Article abstract of DOI:10.1016/j.poly.2021.115260

Water oxidation plays a crucial role in both natural and artificial photosynthesis, which is an attractive solution to the depletion of fossil fuels as energy sources due to the increasing consumption. Thus, the search for oxygen evolution reaction cataly

Full text of DOI:10.1016/j.poly.2021.115260

Polyhedron 204 (2021) 115260
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Synthesis, structural and physicochemical properties of a series of
manganese(II) complexes with a novel N5 tripodal-amidate ligand and
their potential use as water oxidation catalysts q
d,4,
Michael G. Papanikolaou a, Sofia Hadjithoma b, John K. Gallos c,5, , Haralampos N. Miras
⇑
⇑
,
John C. Plakatouras a,e,3, , Anastasios D. Keramidas b,2, , Themistoklis A. Kabanos a,1,
⇑
⇑
⇑
a Section of Inorganic and Analytical Chemistry, Department of Chemistry, University of Ioannina, GR 45110 Ioannina, Greece
b Department of Chemistry, University of Cyprus, 2109 Nicosia, Cyprus
c Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki, GR 541 24, Greece
d WestCHEM, School of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK
e Institute of Materials Science and Computing, University Research Center of Ioannina, Ioannina GR-45110, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
Water oxidation plays a crucial role in both natural and artificial photosynthesis, which is an attractive
solution to the depletion of fossil fuels as energy sources due to the increasing consumption. Thus, the
search for oxygen evolution reaction catalysts is a hot topic of research. Reaction of the N5-tripodal ami-
date ligand, N-{2-[(bis(pyridine-2-ylmethyl)amino)methyl]phenyl}picolinamide (Htrip) with MIIX2
(X = Cl-, Br-, I-), in anhydrous ethyl alcohol, and C2H5ONa yields the complexes [MnII(trip)Cl] (1),
[MnII(trip)Br] (2), and [MnII(trip)I] (3). Single crystal X-ray structure analysis of 1–3 revealed that the
manganese(II) atom in the three manganese compounds occupies the center of a distorted octahedral
coordination sphere consisting of two pyridine, one picoline and one amino nitrogen atoms on the equa-
torial plane, while the axial positions are occupied by one amido nitrogen atom and the halogen anion.
The three manganese(II) complexes 1–3 constitute the first examples of mononuclear {MnII(N5trip)X} spe-
cies to be reported. Magnetic susceptibility measurements showed that these complexes are high-spin d5
systems. Cyclic voltametric study of 1–3 revealed an unexpected two electron, electrochemically rever-
sible redox process, assigned to the oxidation of MnII to MnIV. The electrochemical properties for oxygen
evolution reaction of complexes 1–3 showed that the oxidized trip- ligand is responsible for the electro-
catalytic oxidation of water to dioxygen.
Received 5 April 2021
Accepted 3 May 2021
Available online 7 May 2021
Keywords:
Manganese
Electrochemistry
X-ray structure
Ó 2021 Elsevier Ltd. All rights reserved.
1. Introduction
systems. For example, manganese-based active sites can be found
in superoxide dismutase [2], Mn-catalase [2] and the oxygen-
Manganese is one of the most abundant first-row transition
elements in nature [1] and plays a significant role in biological
evolving complex of Photosystem II [3]. The manganese cores are
critical for the activity of these enzymes and take part in important
proton-coupled electron-transfer (PCET) reactions [4]. Moreover,
manganese-based molecular catalysts have attracted a lot of the
scientific interest, since manganese can cycle through the oxida-
tion states of II, III, IV, VII and even the less stable V and VI oxida-
tion states. Many manganese-based catalysts have been developed
for challenging chemical reactions like the photocatalytic reduc-
tion of CO2 to CO [5], dioxygen activation [6] and various organic
oxidative transformations [7]. It has been proven that the coordi-
nation of deprotonated amides helps the isolation of manganese
complexes in these important higher oxidation states by providing
stability to the metal center through their negative charge [8]. By
using deprotonated amido-ligands, complexes of Mn(III) [9], Mn
q
Part of the special issue dedicated to the element manganese, entitled
Manganese: A Tribute to Chemical Diversity.
⇑
Corresponding authors at: Section of Inorganic and Analytical Chemistry,
Department of Chemistry, University of Ioannina, GR 45110 Ioannina, Greece (T.A.
Kabanos).
E-mail addresses: igallos@chem.auth.gr (J.K. Gallos), Charalampos.Moiras@
glasgow.ac.uk (H.N. Miras), iplakatu@uoi.gr (J.C. Plakatouras), akeramid@ucy.ac.cy
(A.D. Keramidas), tkampano@uoi.gr (T.A. Kabanos).
1
ORCID: 0000-0001-6944-5153.
ORCID: 0000-0002-0446-8220.
ORCID: 0000-0003-4665-6593.
ORCID: 0000-00020086-5173.
2
3
4
5
ORCID: 0000-0002-3732-918X.
https://doi.org/10.1016/j.poly.2021.115260
0277-5387/Ó 2021 Elsevier Ltd. All rights reserved.

M.G. Papanikolaou, S. Hadjithoma, J.K. Gallos et al.
Polyhedron 204 (2021) 115260
(IV) [10] and Mn(V) [11] have been successfully isolated and struc-
turally characterized.
Tripodal ligands have found application in various scientific
fields including biomimetic synthesis [12], small molecule cataly-
sis [13], synthesis of sensors [14] and synthesis of NO releasing
molecules to treat cancer [15,16]. In the field of catalysis, tripodal
ligands, are frequently used due to their ability to occupy most of
the coordination sites around the metal leaving only one axial posi-
tion available for the substrate (Scheme 1). This is inspired by the
way enzymes work in nature, where they achieve extraordinary
reactivity and selectivity by creating a substrate-specific active site
for the coordination and activation of the substrate [17].
More specifically, water oxidation catalysis has attracted a lot of
the scientific interest in the recent years. Water oxidation plays a
crucial role in both natural and artificial photosynthesis [18],
which is an attractive solution to the depletion of fossil fuels as
energy sources due to the increasing consumption. Through artifi-
cial photosynthesis the sun’s light energy can be converted and
stored in chemical bonds and can be used as fuel (hydrogen gener-
ation from water splitting Eqs. (1) to (3)) [19]. However, the effi-
ciency of artificial photosynthesis, is greatly hindered by the
slow kinetics of the water oxidation reaction. The most efficient
water oxidation catalyst in nature is the oxygen-evolving complex
(OEC) in photosystem II, which contains a multinuclear manganese
core (CaMn4O5) and can achieve a turnover frequency (TOF) of
400 sꢀ1 [20]. Inspired by this system, many multinuclear
manganese complexes have been developed as catalysts for water
oxidation and many of them showed efficient results, such as
Scheme 1. Schematic representation of the coordination of the substrate to the
‘’active site’’ of the Mn(II) complexes.
O
N
H
N
N
N
N
[OH2(terpy)Mn(O)2Mn(terpy)OH2]
[21a],
[Mn4O4L6]
[21b],
[Mn4V4O17(OAc)3] [21c] and the newly reported Mn12 clusters
bearing –OH groups [21d]. The high efficiency of these complexes
is attributed to the multinuclear sites which are believed to play a
significant role to the OAO formation of the water oxidation
reaction [22]. However some mononuclear manganese complexes
(for example [(Py2N(tBu)2)Mn(H2O)2] [21e]) have also showed
promising results for water oxidation.
Scheme 2. The tripodal ligand (Htrip) used in this study.
2.2. Preparations
2.2.1. Synthesis of N-(2-((bis(pyridine-2-ylmethyl)amino)methyl)
phenyl)picolinamide, (Htrip) (Scheme 3)
2H2OðlÞ ! 4HþðaqÞ þ O2ðgÞ þ 4eꢀ
4HþðaqÞ þ 4eꢀ ! 2H2ðgÞ
ð1Þ
ð2Þ
ð3Þ
2-Nitrobenzaldehyde (1.00 g, 6.6 mmol) was added to a stirred
solution of bis(2-pyridylmethyl)amine (1.31 g, 6.6 mmol) in dry
CH2Cl2 (20 ml). After 30 min of stirring at room temperature
(20 °C), sodium triacetoxyborohydride (2.23 g, 10.5 mmol) was
added and the resulting mixture was stirred overnight at room
temperature (20 °C). 5% aqueous NaHCO3 (20 ml) was then added,
and the mixture was extracted with CH2Cl2 (2 ꢁ 15 ml). The
organic layer was separated, dried over magnesium sulfate and
the solvent was removed under vacuum. The resulting product
was purified by column chromatography and was dissolved in
50 ml of MeOH. 0.35 g of hydrogenation catalyst (10% Pd on acti-
vated carbon) was added to this solution and pure hydrogen was
admitted for 24 h under magnetic stirring. The mixture was fil-
tered, and the filtrate was evaporated to dryness to yield a yellow
oil. The oil was dissolved in dry THF and pyridine-2-carbonyl chlo-
ride hydrochloride (1.25 g, 7 mmol) and triethylamine (2.1 ml,
15 mmol) were added to it. The mixture was stirred overnight at
room temperature (20 °C) and the solvent was removed under vac-
uum. 5% aqueous NaHCO3 (20 ml) was added to the solid, and the
mixture was extracted with CH2Cl2 (2 ꢁ 15 ml). The organic layer
was separated, dried over magnesium sulfate and the solvent was
removed under vacuum to give a light-yellow oil. The product was
purified by recrystallisation from diethyl ether to give the final
product as a white powder. Yield (1.16 g, 43% based on 2-nitroben-
zaldehyde). Anal. calcd for C25H23N5O: C, 73.33; H, 5.66; N, 17.10.
Found: C, 73.26; H, 5.74; N, 17.04. ESI(+) HRMS: calcd for
[M + H]+: m/z 410.1975. Found: 410.1958 (100%). 1H and 13C
NMR and UV–vis spectra of the organic molecule Htrip are shown
in Figs. S5–S7 respectively.
2H2OðlÞ ! 2H2ðgÞ þ O2ðgÞ
Herein, it is reported the synthesis, structural and physico-
chemical characterization of three manganese(II) complexes of
the general formula [MnII(trip)X] (X = Cl-, Br-, I-) with the novel,
tripodal-amidate ligand N-{2-[(bis(pyridine-2-ylmethyl)amino)
methyl]phenyl}picolinamide (Htrip) (Scheme 2). Moreover, the
potential application of these manganese(II) complexes as water
oxidation catalysts is also reported.
2. Experimental
2.1. Materials, general methods
All chemicals and solvents were purchased from Sigma-Aldrich
and Merck, were of reagent grade, and were used without further
purification. C, H, and N analyses were conducted by the microan-
alytical service of the School of Chemistry, the University of Glas-
gow. FT-IR transmission spectra of the compounds, in KBr pellets,
were acquired using a JASCO Model 460 spectrophotometer in
the 4000–400 cmꢀ1 range. Ethyl alcohol was dried over magne-
sium(II) ethoxide and distilled just prior to its use. Merck silica
gel 60 F254 TLC plates were used for thin layer chromatography.
The UV–Vis spectra were performed in a SHIMADZU UV–Vis 2600.
2

M.G. Papanikolaou, S. Hadjithoma, J.K. Gallos et al.
Polyhedron 204 (2021) 115260
2.2.2. Synthesis of (Chlorido) {N-{2-[(bis(pyridine-2-ylmethyl)amino)
methyl]phenyl}picolinamido-Npy,Npy,Nam,Nami,Npic}manganese(II),
[MnII(trip)Cl] (1)
2.2.4. Synthesis of (Iodido) {N-{2-[(bis(pyridine-2-ylmethyl)amino)
methyl]phenyl}picolinamido-Npy,Npy,Nam,Nami,Npic}manganese(II),
[MnII(trip)I] (3)
MnCl2ꢂ4H2O (73 mg, 0.37 mmol) was dissolved in dry ethyl
alcohol (10 ml) under high purity argon and magnetic stirring.
Solid Htrip (150 mg, 0.37 mmol) was added to the stirred solution
in one portion and its color turned to light yellow. Then, sodium
ethoxide (30 mg, 0.44 mmol) was added to it and its color changed
to orange and a small amount of white precipitate (NaCl) was
formed. The mixture was stirred at room temperature overnight,
the solvent was evaporated to dryness under vacuum, and the resi-
due was extracted with CHCl3 (25 ml) and filtered to remove insol-
uble salts. The volume of the filtrate was reduced to 5 ml under
vacuum and diethyl ether (30 ml) was added dropwise under con-
stant stirring to get 0.135 g of a yellow solid. Yield: (73%, based on
Htrip). Anal. calcd for (C25H22ClN5OMn Mr = 498.87): C, 60.19; H,
4.45; N, 14.04. Found: C, 60.23; H, 4.59; N, 13.94. leff = 5.81 BM.
MnI2 (113 mg, 0.37 mmol) was dissolved in dry ethyl alcohol
(10 ml) under high purity argon and magnetic stirring. Solid Htrip
(150 mg, 0.37 mmol) was added to the stirred solution in one por-
tion and its color turned to orange yellow and then, sodium ethox-
ide (30 mg, 0.44 mmol) was added to it and the color of the
solution changed to light yellow. The mixture was stirred at room
temperature overnight, filtered and layered with diethyl ether to
obtain X-ray quality, 0.042 g of orange crystals of 3. Yield: (19%,
based on Htrip). Anal. calcd for C25H22IN5OMn (Mr = 590.33): C,
50.87; H, 3.76; N, 11.86. Found: C, 50.79; H, 3.84; N, 11.94.
leff = 5.79 BM.
2.3. Crystal data collection and refinement
Crystals of
1 suitable for X-ray diffraction analysis were
Suitable crystals were glued to
a thin glass fibber with
obtained by layering diethyl ether into a concentrated dichloro-
cyanoacrylate (super glue) adhesive and placed on the goniometer
head. Diffraction data were collected on a Bruker D8 Quest Eco
diffractometer, equipped with a Photon II detector and a TRIUMPH
(curved graphite) monochromator utilizing Mo Ka radiation
(k = 0.71073 Å) using the APEX 3 software package [23]. The col-
lected frames were integrated with the Bruker SAINT software
using a narrow-frame algorithm. Data were corrected for absorp-
tion effects using the Multi-Scan method (SADABS) [24]. The struc-
tures were solved using the Bruker SHELXT Software Package and
refined by full-matrix least squares techniques on F2 (SHELXL
2018/3) [25] via the ShelXle interface [26]. The non-H atoms were
treated anisotropically, whereas the organic H atoms were placed
in calculated, ideal positions and refined as riding on their respec-
tive carbon atoms. All compounds were refined as inversion twins;
Flack parameters, x, are included in Table 1. PLATON [27] was used
methane solution of 1.
2.2.3. Synthesis (Bromido) {N-{2-[(bis(pyridine-2-ylmethyl)amino)
methyl]phenyl}picolinamido-Npy,Npy,Nam,Nami,Npic}manganese(II),
[MnII(trip)Br] (2)
The compound 2 was prepared (in 69% yield, based on Htrip) in
the same way as compound 1 except that MnBr2ꢂ4H2O (105 mg,
0.37 mmol) was used instead of MnCl2ꢂ4H2O. The colour of the
compound 2 was also yellow. Anal. calcd for C25H22 BrN5OMn
(Mr = 543.33): C, 55.27; H, 4.08; N, 12.89. Found: C, 55.33; H,
4.09; N, 12.81. leff = 5.97 BM.
Crystals of
2 suitable for X-ray diffraction analysis were
obtained by layering diethyl ether into a concentrated dichloro-
methane solution of 2.
Scheme 3. Synthesis of the ligand Htrip.
3

M.G. Papanikolaou, S. Hadjithoma, J.K. Gallos et al.
Polyhedron 204 (2021) 115260
Table 1
Crystal data and details of the structure determination and refinement for the manganese(II) compounds 1, 2, and 3.
Compound
1
2
3
Empirical formula
Formula weight
C
25H22ClMnN5O
C25H22BrMnN5O
543.32
C25H22IMnN5O
590.31
498.86
Crystal system
monoclinic
monoclinic
monoclinic
Space group
P21
P21
P21
Unit cell dimensions a, b, c (Å), b (o)
8.6669(7), 15.6858(13),
9.2712(7), 113.570(3)
1155.24(16)
1.434
0.715
514
8.6950(4), 15.6878(7),
9.2885(4), 113.388(2)
1162.90(9)
1.552
2.313
8.7948(3), 15.7519(6),
9.3991(4), 112.746(1)
1200.84(8)
1.633
1.862
Volume (Å3)
Density (calculated) (g/cm3)
Absorption coefficient (mmꢀ1
F(000)
)
550
586
h range for data collection
Reflections collected
Independent reflections
Data/restraints/parameters
Flack parameter, x
2.721 to 26.369°
15,867
4720 [Rint = 0.0824]
4720/1/299
2.552 to 26.993°
41,346
5059 [Rint = 0.1076]
5059/1/299
2.586 to 24.999°
24,729
4212 [Rint = 0.0308]
4212/1/298
0.44(5)
0.23(2)
–
Goodness-of-fit
1.076
1.051
1.114
Final R indices [I > 2
r
(I)]
Robs = 0.0748, wRobs = 0.1422
Rall = 0.1047, wRall = 0.1528
0.675 and ꢀ0.732
Robs = 0.0602, wRobs = 0.1775
Rall = 0.0691, wRall = 0.1835
1.321 and ꢀ1.286
Robs = 0.0145, wRobs = 0.0384
Rall = 0.0152, wRall = 0.0384
0.254 and ꢀ0.378
R indices [all data]
Largest diff. peak and hole (e/Å3)
R =
R||Fo|-|Fc||/R|Fo|, wR = {R[w(|Fo|2 - |Fc|2)2]/R[w(|Fo|4)]}1/2 and w = 1/[r2(Fo2)+(aP)2 + bP] where P=(Fo2 + 2Fc2)/3. 1: a = 0.0668, b = 0.1209; 2: a = 0.1305, b = 0.9700;
3: a = 0.0200, b = 0.0844.
for geometric calculations, and Diamond [28] for molecular
graphics.
Table 2
Interatomic distances (Å) and angles (°) relevant to the manganese(II) coordination
sphere for the manganese(II) compounds 1, 2 and 3.
Details on data collection and refinement are presented in
Table 1. Full details on the structures can be found in the CIF files
in the ESI. CCDC 2067257-2067259 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif.
Parameter
1
2
3
Mn(1) – Xa
Mn(1) – N(1)
Mn(1) – N(2)
Mn(1) – N(3)
Mn(1) – N(4)
Mn(1) – N(5)
X – Mn(1) – N(1)
X – Mn(1) – N(2)
X – Mn(1) – N(3)
X – Mn(1) – N(4)
X – Mn(1) – N(5)
N(1) – Mn(1) – N(2)
N(1) – Mn(1) – N(3)
N(1) – Mn(1) – N(4)
N(1) – Mn(1) – N(5)
N(2) – Mn(1) – N(3)
N(2) – Mn(1) – N(4)
N(2) – Mn(1) – N(5)
N(3) – Mn(1) – N(4)
N(3) – Mn(1) – N(5)
N(4) – Mn(1) – N(5)
2.436(3)
2.231(9)
2.205(9)
2.294(8)
2.325(6)
2.265(8)
96.2(2)
165.3(2)
104.9(2)
93.1(2)
93.1(2)
73.6(3)
151.7(3)
88.1(3)
124.3(3)
88.4(3)
97.1(3)
85.3(3)
72.4(3)
74.1(3)
146.3(3)
2.551(2)
2.238(8)
2.199(7)
2.298(7)
2.269(7)
2.325(5)
95.4(2)
165.2(2)
104.3(2)
92.4(2)
93.0(2)
74.1(3)
152.8(3)
124.5(3)
88.2(3)
89.2(3)
85.5(3)
96.8(3)
73.9(3)
72.4(3)
146.2(3)
2.8404(7)
2.236(3)
2.193(3)
2.290(3)
2.323(2)
2.278(3)
95.52(8)
165.53(7)
102.75(7)
91.60(7)
90.89(7)
74.8(1)
154.1(1)
89.1(1)
124.0(1)
90.03(9)
98.8(1)
2.4. Electrochemistry
Electrochemical measurements were performed under an argon
atmosphere, at room temperature, in either dichloromethane or
dimethylformamide (DMF) solutions containing 0.1 M Et4NBF4 as
a supporting electrolyte, and 1 mM of the manganese complex or
the ligand Htrip in a CHI650E potentiostat equipped with a
three-electrode cell. The scan rate was 100 mV sꢀ1 unless other-
wise stated. The working electrode was a glassy carbon disk, and
a platinum wire was used as a counter electrode. The potential
was measured against a Ag/AgNO3 reference and converted to
the normal hydrogen electrode (NHE) potential by recording the
CVs of DMF or CH2Cl2 solutions of ferrocene (0.65 V vs NHE). The
potential values are reported against NHE throughout the manu-
script. Exhaustive electrolysis was performed in CH2Cl2 solution
using platinum mesh as a working electrode, a Ag/AgNO3 as a ref-
erence electrode and a platinum wire separated with a Teflon frit
from the solution as auxiliary electrode. But4NBF4 0.2 M was used
as electrolyte. The solvents CH2Cl2 and DMF were dried over CaH2
and distilled just prior to their use. DMF was distilled under
reduced pressure. The simulation of CVs was performed using
the program CVSim https://sourceforge.net/projects/cvsim/.
85.91(9)
72.42(9)
74.32(9)
146.4(1)
a
X corresponds to Cl(1), Br(1) and I(1) for compounds 1, 2 and 3 respectively.
3. Results and discussion
3.1. Description of the structures
Interatomic distances and bond angles relevant to the man-
ganese(II) coordination sphere are listed in Table 2. The molecular
structure of compound 1 is presented in Fig. 1, and the structures
of 2 and 3 in Fig. 2. Compounds 1–3 crystallize in the monoclinic
space group P21. The origin of the chirality of the compounds is
located on the 2-methylenephenyl moiety which connects the
amino and the amido parts of the ligand, and upon coordination
Fig. 1. A thermal ellipsoid plot (50% probability level) of the molecular structure of
1 with a partial labeling scheme.
4

M.G. Papanikolaou, S. Hadjithoma, J.K. Gallos et al.
Polyhedron 204 (2021) 115260
Fig. 2. Thermal ellipsoid plots (50% probability level) of the molecular structures of 2 and 3.
can be turned to different sites. That can be easily seen in Fig. 2,
comparing the position of the linker in the structures of com-
pounds 2 and 3. They are isostructural with minor differences
induced by the presence of a different halogen in each compound.
It is noteworthy that the lattice dimensions and consequently the
volume of the unit cell, though very similar, appear to increase sys-
tematically with the size of halogens from Clꢀ in 1 to I- in 3. Due to
the similarity of the structures, only the structure of 1 will be
described in detail.
The manganese(II) atom in 1 occupies the center of a distorted
octahedral coordination sphere consisting of two pyridine, one
picoline and one amino nitrogen atoms on the equatorial plane
while the axial positions are occupied by one amido nitrogen atom
and the halogen anion.
Compound 1 belongs to a small group of manganese(II) com-
plexes coordinated to tripodal-amidate ligands, which contain five
nitrogen donor atoms: one amino, one amido, and three hetero-
cyclic nitrogen atoms [29]. There are only a few similar Mn(II)
complexes and these can be separated in two distinct groups; a)
there are six complexes where the sixth donor atom comes from
a coordinated NO and leads to low spin diamagnetic species [30–
32]. These complexes are considered rather as {MnI(N5trip)NO}+
species. and b) there is only one compound where the sixth coor-
dination position of the octahedron is a conventional donor, specif-
ically an oxygen atom belonging to the N-coordinated amido
moiety of an adjacent complex [33]. The complexes presented in
this study belong to the second group, and their structural charac-
teristics can only be compared to those of 1D-polymer-[MnII(-
dpaq2Me)](OTf) (where dpaq2Me is 2-[bis(pyridin-2-ylmethyl)]
amino-N-2-methyl-quinolin-8-yl-acetamidate and OTf is the tri-
fluoromethanesulfonate anion) [33] (Scheme 4). Thus, the three
manganese(II) complexes 1–3 constitute the first examples of
mononuclear {MnII(N5trip)X} species to be reported.
Scheme 4. The ligand Hdpaq2Me
.
tions appear to be larger than literature’s example [33], and we
believe that this is due to the presence of a 6-membered chelate
ring (instead of a 5-membered one) in the coordination sphere of
MnII in 1, with respect to the change of the ‘‘hinge” connecting
the amino with the amido nitrogen atoms (a 2-methylenephenyl
group in our case, a methylenecarbonyl group in the case of [MnII(-
dpaq2Me)]+). Additionally, there is an intramolecular
p – p interac-
tion between the phenyl ring and one of the pyridine rings
(centroid distance: 3.8 Å, angle between least square planes:
34.55°, distance of one ring’s centroid to the least square plane of
the other ring: 3.6 Å) which eventually hauls the picolinamide
moiety, leading to two very different Npicoline – MnII – Npyridine
angles. (88.13 and 124.32° compared to the corresponding
102.85 and 107.03° literature values) [33].
In 1, the MnII-N bond distances span the range 2.205–2.325 Å,
and they are in good agreement with the literature data. The short-
est distance belongs to the MnII – Namido bond, which becomes
even shorter as the coordinated halogen in trans position changes
from Cl- to Br- to I-. So, the corresponding MnII – Namido and MnII – X
pairs of distances are: 2.205(8) and 2.436(3) Å for 1, 2.199(7) and
2.552(2) for 2, and 2.192(3) and 2.838(1) for 3, concurring with
the trans influence of the halides.
The deviation of 1 from the ideal octahedral geometry, apart
from the bond distances, can easily be seen from both the cis
and trans angles of the coordination sphere which span the ranges
72.37 – 124.32 and 146.30 – 165.30° respectively. These distor-
To quantify the distortions from the ideal octahedral geometry
the program OctaDist [34] was utilized, which calculates the
parameters f,
of six unique Μn–donor atom bond lengths from the average
value; is the sum of the deviation of 12 unique cis donor
atom–metal–donor atom angles from 90°; and is defined as
R and H. f is the average of the sum of the deviation
R
H
the degree of trigonal distortion of the coordination geometry from
an octahedron towards a trigonal prism, it is the sum of the devi-
ation of 24 unique torsional angles between the ligand atoms on
opposite triangular faces of the octahedron viewed along the
pseudo-threefold axis from 60°. In an ideal octahedron all those
parameters should be equal to zero. In our case their values are:
5

M.G. Papanikolaou, S. Hadjithoma, J.K. Gallos et al.
Polyhedron 204 (2021) 115260
f = 0.354 Å,
R = 125.8° and H = 482.7° showing a severe distortion
from the ideal polyhedron.
3.2. Infrared spectra
Table 3 depicts the assignments of some diagnostic bands for
the ligand Htrip and its manganese(II) complexes. Complexes 1–3
have almost the same infrared bands and therefore only complex
1 will be discussed. The medium-intensity band at 3332 cmꢀ1 in
the spectra of the ligand was assigned to the (NAH)amide stretching
vibration,
complex 1, the
v(NH). Due to the amide deprotonation of the ligand in
v
(NH)amide band is absent from the spectra of the
complex as expected. The bands at 1533 cmꢀ1 and 1291 cmꢀ1 in
the spectrum of the free ligand were assigned to the amide II and
amide III bands respectively and are caused by the coupling of
the v(CN)amide and d(NH)amide modes. In the spectra of the com-
plexes the amide II and amide III bands are replaced by a strong
band at 1367 cmꢀ1. This replacement might be expected as the
removal of the amide proton produces a pure CAN stretch [35].
The
1670 cmꢀ1 in the spectra of Htrip (Fig. 3). This band appears at
lower wavenumbers in the spectra of the complexes (1605 cmꢀ1
(Fig. 3). This shift to lower wavenumbers for the (C@O), is
expected because the amide deprotonation and the N-coordination
to the metal cause a decrease of the carbonyl double bond
character.
v(C@O) band (amide I) appears as a strong band at
Fig. 4. Cyclic voltammogram of the ligand Htrip in DMF solution.
)
v
the redox process at 0.645 V was assigned to the oxidation of man-
ganese(II). The CV for compound 1 in DMF (Fig. 5) also shows a
redox couple at 0.598 V (DE = 73 mV), in addition to the two irre-
versible ligand centered redox peaks (not shown in Fig. 5). The CV
for compound 2 (Fig. 6) revealed a very similar behavior with 1 in
both solvents and thus, it will not be further discussed. The CV for
compound 3 in DMF (in CH2Cl2 this compound is not soluble) is
shown in Fig. S1. Compound 3 also showed a redox couple at
0.613 V assigned to the oxidation of manganese(II).
The number of electrons of the metal centered redox processes
at ~ 0.60 V was measured by exhaustive electrolysis of the man-
ganese(II) complexes at 0.70 V. The number of electrons at 95%
of the initial electrolysis current was 1.85. The CVs of the CH2Cl2
3.3. Electrochemistry
The cyclic voltammograms (CVs) for the ligand Htrip and the
manganese(II) compounds 1 and 2 are illustrated in Figs. 4–6
respectively. The CV for the ligand Htrip revealed two strong mul-
tielectron irreversible redox peaks at 1.157 and 1.318 V (Fig. 4).
The CV for compound 1 in CH2Cl2 (Fig. 5) shows a well-defined
redox couple at 0.645 V (DE = 76 mV), in addition to the two irre-
versible ligand centered redox peaks (not shown in Fig. 5). Thus,
Table 3
Diagnostic infrared bands [cmꢀ1] of the ligand Htrip and its manganese(II) compounds.
m(ΝΗ)
m
(CO)a
amide IIb
v
(CN)amide
amide IIIb
Htrip
3332
1670
1605
1605
1603
1533
–
1291
1
2
3
–
–
–
–
–
–
1367
1366
1368
–
–
–
a
In secondary amides the
In secondary amides, these bands arise from coupled
m
(C@O) vibration is called amide I.
(CN) and d(NH) modes.
b
m
Fig. 3. Comparison of the IR spectra of the ligand and compound 1.
6

M.G. Papanikolaou, S. Hadjithoma, J.K. Gallos et al.
Polyhedron 204 (2021) 115260
Fig. 5. Cyclic voltammograms of the manganese(II) complex 1 in CH2Cl2 and in DMF solutions.
Fig. 6. Cyclic voltammograms of the manganese(II) complex 2 in CH2Cl2 and in DMF solutions.
solution of 2 after electrolysis remain the same and this presum-
ably mean that that this redox process is fully reversible (Fig. 7).
The brown color of the CH2Cl2 solution of 2 after electrolysis chan-
ged to green and its UV–Vis spectrum (Fig. 8) exhibits two peaks at
727 (
= 610 Mꢀ1 cmꢀ1) and 374 nm ( = 1181 Mꢀ1 cmꢀ1) and is
like the spectrum of [MnIVClsalen] [36]. Apparently, the metal cen-
e
e
7

M.G. Papanikolaou, S. Hadjithoma, J.K. Gallos et al.
Polyhedron 204 (2021) 115260
a DMF solution of ligand Htrip, the CVs show a decrease of the ano-
dic current of the ligand’s oxidation peaks (Fig. 9). This decrease
might be attributed to either the interaction of the ligand with
water, i.e., hydrogen bonding, or to an associated chemical reaction
or both. A cathodic peak at ꢀ0.50 V (Fig. 9) associated with the oxi-
dation peaks of the ligand (Fig. S2), which appears upon addition of
water to the DMF solution, was assigned to the reduction of dioxy-
gen to H2O. The current of this cathodic peak is increasing by
increasing the quantity of water to the DMF solution (Fig. 9). The
origin of the cathodic peak was confirmed by spike experiments,
after addition to the DMF solution of either O2 or H2O2.
The electrochemical properties for OER of the manganese(II)
complex 1 have been also investigated in DMF solution containing
1 (1.0 mM, Fig. 10). Upon gradual addition of water to a DMF solu-
tion of 1, the CVs also show a decrease of the anodic current of the
ligand’s oxidation peaks at 1.155 and 1.326 V (Fig. 10). The current
decrease of these two peaks in 1 upon gradual addition of water to
DMF solution is larger than the corresponding decrease of ligand’s
oxidation peaks with the same concentration of water (Fig. 9).
Moreover, upon the gradual addition of water to the DMF solution
a negative shift of the ligand’s oxidation peak at 1.155 V and a pos-
itive shift of the ligand’s oxidation peak at 1.326 V occurs. When
the concentration of water to the DMF solution becomes higher
than 10 mM, the ligand’s oxidation peaks diminish dramatically.
At this point, it is worth noting that the wave at 0.60 V, due to
manganese oxidation, remains intact during all the electrochemi-
cal measurements and thus, it is reasonable to assume that 1
retains its integrity.
Fig. 7. CVs of a CH2Cl2 solution (1 mM) of 2 at time 0, 1400 and 2600 s (0%, 65% and
87% of the initial current respectively) after the commencement of the exhaustive
electrolysis at 0.70 V.
At voltage higher than 1.369 V the current (Fig. 10) is increasing
upon gradual addition of water and this increase was assigned to
the catalytic oxidation of water to dioxygen. This assignment was
supported by the normalized catalytic currents (icat/u1/2, u = scan
rate), which they decreased by increasing the scan rates (Fig. 11).
The two anodic peaks which correspond to the oxidation of the
ligand are associated with the cathodic peak at ꢀ0.50 V assigned to
the reduction of O2 to H2O. This is supported from the CVs at var-
ious switching potentials, showing that the cathodic peak at ꢀ0.5 V
emerges in the switching potentials ranging from 1.2 to 1.8 V
(Fig. 12). Thus, the experiment supports that the oxidized species
of the ligand react and oxidize H2O. Photoinduced or electro-catal-
ysed oxidation of water from organic molecules or ligands in metal
organic complexes have been reported previously [42,43]. A com-
Fig. 8. UV–Vis of a CH2Cl2 solution (1 mM) of 2 after the end of exhaustive
electrolysis at 0.70 V.
mon element in these molecules is the presence of delocalized
p-
tred reversible redox processes of complexes 1 – 3 are of 2e- and
were assigned to MnII-MnIV redox couple. At this point, it is worth
noting that mononuclear or multinuclear manganese(IV) com-
pounds are stabilized by either ligands containing nitrogen donor
atoms [37–40] or ligands with mixed nitrogen and oxygen donor
atoms [41]. Based on Hard-Soft Acid-Base (HSAB) theory someone
expects harder donor atoms such as oxygen donor atoms to stabi-
lize better MnIV, which is a hard acid, than nitrogen donor atoms.
On the other hand, the splitting of the d orbitals by the nitrogen
donor atoms is larger than oxygen donor atoms, and thus, nitrogen
atoms can stabilize MnIV by crystal field stabilization energy
(CFSE). This stabilization is expected to be significant for metal ions
having d3 electronic configuration such as MnIV. Therefore, it is
crystal clear that the stabilization of MnIV by coordination of the
ligand trip- might be a result of the ligation of the nitrogen atoms
to MnIV, through CFSE, and the strong coordination of the deproto-
nated amide nitrogen which satisfies the electron needs of MnIV.
systems. Thus, the electrocatalytic oxidation of water from trip-
in 1–3 might be attributed to the presence of the extended p-sys-
tems of the organic ligand.
In order to understand the mechanisms of the experimental
voltammogram (Fig. 10), we simulated the CVs considering a sim-
plified model of three redox reactions of the species s1, s2, s3
(s3 = Htrip, s1 and s2 the oxidized species of Htrip at approxi-
mately 1.15 and 1.33 V respectively, s4, s5 (s5 = H2O and s4 oxi-
dised H2O species) and the chemical reactions of the species s4
(conversion of s4 to s6 = O2) and of the reactions of s5 with s1
and s2 to give s4. To successfully simulate the large decrease of
the ligand’s oxidation peaks current upon gradual addition of
H2O to the DMF solution of 1 we had to consider a fast reaction
of the ligand’s oxidized species s2 and s3 with s5 (H2O). In addition,
the simulated spectra (Fig. 13) show the same shift for the peaks at
1.155 and 1.326 V with the addition of s5 as in the experimental
data (Fig. 10).
Experimental and simulation data clearly reveal that the free
and the bound to the manganese complex oxidized ligand oxidizes
H2O to O2. The MnII complexes are more effective in oxidizing H2O
than the free ligand (Fig. 9). This might be due to the higher ther-
modynamic stability of the ligand trip- in 1–3 in comparison with
Htrip.
3.4. Water oxidation by the ligand Htrip, 1 and 2
The electrochemical properties for oxygen evolution reaction
(OER) of the ligand Htrip have been investigated in a DMF solution
containing 1.0 mM Htrip (Fig. 9). Upon gradual addition of water to
8

M.G. Papanikolaou, S. Hadjithoma, J.K. Gallos et al.
Polyhedron 204 (2021) 115260
Fig. 9. CVs of the ligand Htrip in DMF solution upon gradual addition of H2O.
Fig. 10. CVs of the manganese(II) complex 1 in DMF solution upon gradual addition of H2O. Scan rate 25 mV sꢀ1
.
9

M.G. Papanikolaou, S. Hadjithoma, J.K. Gallos et al.
Polyhedron 204 (2021) 115260
Fig. 11. Normalized CVs of the manganese(II) complex 1 in DMF containing water (10.0 and 20.0 mM) at two scan rates 25 and 100 mV/s for both concentrations of water.
Fig. 12. CVs of the manganese(II) complex 1 (1 mM) in DMF containing water
(20.0 mM) at different switching potentials. Scan rate 25 mV/s.
Fig. 13. Simulated voltammograms considering three electrochemical and three
chemical reactions. s2 + 1e -> s1 E = 1.326 V, s1 + 1e -> s3 E = 1.155 V, s4 + 1e -> s5
E = 1.75 V, s4 -> s6 k = 0.5 sꢀ1, s1 + s5 -> s4 k = 500 s-1, s2 + s5 -> s4 k = 500 sꢀ1, E
redox potentials and k rate constants. In the simulated spectra the concentration of
s1 was 0.001 M and the concentration of s5 ranged from 0.0005 to 0.010 M.
3.5. UV–vis spectroscopy
The UV–vis spectra of 1 and 2 in CH2Cl2 are shown in Fig. 14. In
Table 4 are depicted the peaks with the absorption coefficients of 1,
2, and 3. All complexes show a peak in the UV region assigned to
the intra-ligand and ligand to metal electronic transitions. The
absorption obeys the Lambert-Beer law for various concentrations
of the complexes suggesting that the complexes remain stable in
both CH2Cl2 and DMF solutions. In addition, the stability of the
DMF solutions of the complexes after the addition of H2O were also
examined by UV–Vis. The UV–Vis spectra of the complexes were
identical in 0 and 10% of H2O/DMF solutions suggesting the com-
plexes remain intact at these conditions (Fig. S8).
4. Conclusion
In conclusion, three manganese(II) complexes with the new N5
tripodal-amidate ligand Htrip were synthesized, structurally and
physicochemically characterized. These complexes constitute the
first mononuclear MnII high-spin complexes of the general formula
{MnII(N5trip)X} to be reported. Single crystal X-ray structure analy-
sis revealed that the manganese(II) atom in 1–3 occupies the cen-
ter of a severely distorted octahedron.
10

M.G. Papanikolaou, S. Hadjithoma, J.K. Gallos et al.
Polyhedron 204 (2021) 115260
istry, University of Ioannina for helping in recording FTIR spectra
and the Unit ORBITRAP-LS-MS of the University of Glasgow for
supporting this work. This work was co-funded by the European
Regional Development Fund and the Republic of Cyprus through
the Research and Innovation Foundation (Project: EXCEL-
LENCE/1216/0515). The Single-Crystal X-ray Diffraction Unit of
the Network of Research Supporting Laboratories, University of
Ioannina, financed by the Regional Operational Programme (ROP)
of the Epirus Region General Secretariat is also acknowledged.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.poly.2021.115260.
Fig. 14. UV–Vis spectra of 1 and 2 in CH2Cl2.
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CRediT authorship contribution statement
Michael G. Papanikolaou: Data curation. Sofia Hadjithoma:
Data curation. John K. Gallos: Methodology, Writing - Original
Draft, Writing - Review & Editing. Haralampos N. Miras: Method-
ology, Writing - Original Draft, Writing - Review & Editing. John C.
Plakatouras: Methodology, Writing - Original Draft, Writing -
Review & Editing. Anastasios D. Keramidas: Supervision, Soft-
ware, Resources, Writing - Original Draft, Writing - Review & Edit-
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