Effective catalytic disproportionation of aqueous H2O 2 with di- and mono-nuclear manganese(ii) complexes containing pyridine alcohol ligands

Article abstract of DOI:10.1039/c3dt53288g

The two novel manganese(ii) complexes with 2-hydroxymethylpyridine (2-CH₂OHpy) {[Mn₂(μ-Cl)₂(2-CH ₂OHpy)₄]Cl₂·2H₂O (1)} and 2-hydroxyethylpyridine (2-(CH₂)₂O

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Effective catalytic disproportionation of aqueous
H2O2 with di- and mono-nuclear manganese(II)
complexes containing pyridine alcohols ligands
Cite this: DOI: 10.1039/x0xx00000x
M. Zienkiewicz,a A. Jabłońska-Wawrzycka,a J. Szlachetko,b,c Y. Kayser,b K.
Stadnicka,d W. Sawka-Dobrowolska,e J. Jezierska,e B. Barszcz,*a and J.
Sá*b
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
The two novel manganese(II) complexes with 2-hydroxymethylpyridine (2-CH2OHpy) {[Mn2(
Cl)2(2-CH2OHpy)4]Cl2 2H2O )} and 2-hydroxyethylpyridine (2-(CH2)2OHpy) {[Mn(2-
(CH2)2OHpy)2(NCS)2] ( )} were synthesized and characterized by means of X-ray diffraction, IR,
μ-
∙
(1
2
EPR, HF EPR spectroscopy, magnetic and TG/DTG. The complexes show catalase-like activity
in neutral aqueous solution since they were able to disproportionate H2O2 to harmless H2O and
O2. Both complexes act as true catalysts since they reverted to their original form after depleting
all the H2O2, as suggested by the operando resonant inelastic X-ray spectroscopy (RIXS)
measurements.
hydroxymethylpyridine)8 could be a good low molecular
weight model for manganese catalase because it was able to
disproportionate H2O2 to H2O and O2 in neutral aqueous
conditions, which is an uncommon case among synthetic
antioxidants, known to display high activity in organic
solvents9 but extremely low reactivity or even none in aqueous
Introduction
Catalases are enzymes that protect cells from deleterious
effects caused by hydrogen peroxide. Besides heme-type
catalases1 and particular enzymes such as chloroperoxidase2
that disproportionate hydrogen peroxide in addition to their
peroxidase activity, there is a class of manganese catalases that
can be found in bacterial organisms such as Lactobacillus
plantarum3, Thermus thermophilus4, and Thermoleophilum
album5. These enzymes are an extremely effective defence
against ‘oxidative stress’ since they convert hydrogen peroxide
into water and oxygen. They have been the motif of earlier
structural and functional model research6. There is a need for
low molecular weight enzymes models because natural
enzymes revealed low effectiveness in clinical trials allegedly
due to their low cell membrane permeability.
Manganese complexes are an important class of synthetic
antioxidants6h-i,7, inspired by natural enzymatic systems that
contain this metal ion, which are able to neutralize reactive
oxygen species (ROS). Previous reports were focused on the
synthesis of the dimmeric manganese complexes containing
manganese ions in +II and +III oxidation states as catalase
mimics systems6h-i,7. Our work concerns the synthesis of
structurally different dimmeric, monomeric and polymeric
compounds, containing manganese ions in the +II oxidation
state. We have found earlier that the complex [Mn(2-
solutions with
a pH < 8 (physiological pH values).
Furthermore, the manganese(II) polymeric complex [Mn(2-
CH2OHpy)(SO4)(H2O)]n was found to be stable and catalytic by
means of resonant inelastic scattering spectroscopy (RIXS).
There is an appreciable interest in pyridine-based chelating
alcohols that are widely used in coordination chemistry due to
the presence of ligands containing N,O-donor atoms, which can
be adopted to modeling coordination environments of synthetic
analogues that mimic both the structure and function of the
active sites of metal enzymes. Based on the results above our
approach was to design a highly active structurally various
manganese(II) complexes with homologous of pyridine
alcohols with different counter-ions and compare their catalytic
activity.
Herein, we report the synthesis and full physical
characterization of two novel manganese(II) complexes with 2-
hydroxymethylpyridine
CH2OHpy)4]Cl2∙2H2O (
(CH2)2OHpy) {[Mn(2-(CH2)2OHpy)2(NCS)2]
(2-CH2OHpy)
)} and 2-hydroxyethylpyridine (2-
)}. The
{[Mn2(μ-Cl)2(2-
1
(2
catalytic activity of the complexes in the presence of H2O2 in
aqueous solution was evaluated and the evolution of molecular
CH2OHpy)(SO4)(H2O)]n
(2-CH2OHpy
=
2-
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oxygen was monitored by a quadrupole mass-spectrometer structural stabilisation (Table 4, Fig. 3). The Cg01-Cg01(-x+1, -
(QMS). RIXS measurements depicted the involvement of y, -z+1) centroid distance is approximately 3.654 Å. This
manganese +II and +III oxidation states in the reaction. The distance is slightly shorter than that for a cadmium complex
complexes reverted to their original form after H2O2 depletion, (3.796 Å) with the same ligand13.
which confirms these systems as truly catalytic process. In
addition, the presented catalytic activity of Mn-model
complexes may also provide valuable scientific background
information for green chemistry10 and cancer therapy due to the
fact that cellular levels of H2O2 directly or indirectly play a key
role in malignant transformation and sensitize cancer cells to
death11.
Results and discussion
We synthesised two novel manganese(II) complexes that
can mimic catalytic properties of manganese catalase, namely
[Mn2(µ-Cl)2(2-CH2OHpy)4]Cl2∙2H2O (
1) (Fig. 1) and [Mn(2-
(CH2)2OHpy)2(NCS)2] ( ) (Fig. 2), and characterized using X-
2
ray diffraction, IR, EPR, HF EPR spectroscopy, magnetic and
TG/DTA data. The catalytic activity of synthesized complexes
was evaluated by monitoring the oxygen evolution with a QMS,
and the changes in the catalysts metal centre were determined
by RIXS.
Fig. 1 Molecular structure with the atom numbering scheme and coordination
polyhedron of complex
1.
Manganese(II) complexes with pyridine alcohols
characterization
Table 1 crystal data and structure refinement for 1 and 2
Crystal and refinement data of compounds
in Table 1. According to the crystallographic analysis, the
molecular structure of [Mn2(µ-Cl)2(2-CH2OHpy)4]Cl2∙2H2O (
1 and 2 are listed
1
2
Empirical formula
Formula weight
C24H32N4O6Cl4Mn2
724.22
C16H18N4O2S2Mn
417.40
1
)
consists of two centro-symmetrically related Mn(II) centres Temperature (K)
293(2)
triclinic, P1
100(2)
monoclinic, P21/c
bridged by two chloride ions. Mn(1)-Cl(1)-Mn(1)i-Cl(1)i atoms
Crystal system, space
group
form a square with a centre of inversion (Fig. 1). The Mn(II)
Unit cell dimensions
ion is surrounded by two chelating 2-CH2OHpy molecules
bonded through N and O atoms (Mn(1)-N(2A) = 2.2309(1) Å;
Mn(1)-N(2B) = 2.2273(1) Å; Mn(1)-O(8A) = 2.2143(1) Å;
Mn(1)-O(8B) = 2.1912(1) Å). The two coordinated bridging
chlorides, with distances of 2.5065(5) and 2.5280(5) Å, are
nearly symmetrical, and the Mn-Cl-Mn bridging angle is
92.81°. The shape of the polyhedra is irregular and does not
assume octahedral geometry because possesses only the
identity symmetry. The other two chloride ions act as counter-
ions. The Mn∙∙∙Mn distance is equal to 3.646(5) Å, which is
much shorter than that for Cu centres in the dimmeric structure
of another 2-CH2OHpy complex12. Selected geometrical data
a (Å)
b (Å)
c (Å)
α (˚)
β (˚)
γ (˚)
7.8970(2)
9.7655(3)
11.4184(4)
73.570(2)
70.964(2)
77.267(2)
790.41(4)
1
0.40 × 0.23 × 0.15
3.08-27.50
1.521
-10 ≤ h ≤ 9, -12 ≤ k
≤ 12, -14 ≤ l ≤ 11
1.179
8.778(1)
14.298(2)
7.848(1)
90.00
112.71(1)
90.00
908.68(2)
2
0.23 × 0.21 × 0.15
2.89-27.98
1.526
-10 ≤ h ≤ 11, -18 ≤ k
≤ 18, -10 ≤ l ≤ 10
0.973
Cell volume (Å3)
Z
Crystal size (mm)
θ range (°)
Calculated density (gcm-3)
Limiting indices
Absorption coefficient
(mm-1)
Absorption correction
Semi-empirical from Analytical
equivalents
0.8429 and 0.6429
3551/2986
for
1
are given in Table 2.
The crystal structure of
1
is stabilised by two types of
Tmax and Tmin
Reflections
collected/unique/observed
[I > 2sigma(I)]
0.901 and 0.846
2166/1669
hydrogen bonds: O-H···O and O-H···Cl (Fig. 3, Table 3).
Intramolecular interactions involve the oxygen atoms O(8A)
and O(8B) of the hydroxymethyl group of 2-CH2OHpy as
donors and the oxygen atom O(1W) of water molecule or a
non-coordinated chloride ion Cl(2) as an acceptor, respectively.
Two non-coordinated chloride ions involved in the formation of
H-bonds with two water molecules form a system with a centre
of inversion within the unit cell. The intermolecular π∙∙∙π
interactions between the parallel-displaced pyridine rings of
two neighbouring complex molecules contribute also to the
[Rint = 0.0191]
3551/194
0.875
R1 = 0.0270,
wR2 = 0.0695
R1 = 0.0356,
wR2 = 0.0736
0.250 and -0.263
[Rint = 0.0310]
1669/127
0.936
R1 = 0.0386,
wR2 = 0.0920
R1 = 0.0553,
wR2 = 0.0966
0.465 and -0.296
Data/parameters
Goodness-of-fit on F2
Final R indices [I
>2sigma(I)]
R indices (all data)
Largest differences in
peak and hole (e Å3)
2 | J. Name., 2012, 00, 1-3
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Fig. 2 Molecular structure with the atom numbering scheme and coordination
polyhedron of complex
2.
Fig. 3 Crystal packing of
π∙∙∙π interactions viewed along [1 0 0] direction.
1
with marked H-bonds O-H···O and O-H···Cl type and
Table 2 Selected bond lengths (Å) and valence angles (°) for 1 and 2
Compound 1
Bond lengths (Å)
Table 3 Hydrogen bonds for complexes 1 and 2 (Å) and (°)
Mn(1)-N(2A)
Mn(1)-O(8A)
Mn(1)-Cl(1)
2.2309(1)
2.2143(1)
2.5065(5)
Mn(1)-N(2B)
Mn(1)-O(8B)
Mn(1)-Cl(1)i
2.2273(1)
2.1912(1)
2.5280(5)
Compound 1
D-H···A
d(D-H) d(H···A)
0.85(3) 2.33(1)
0.85(3) 2.32(1)
0.85(3) 1.77(1)
0.85(3) 2.15(1)
d(D···A)
3.155(2)
3.153(2)
2.614(2)
3.002(1)
<DHA
163(3)
169(3)
173(2)
175(2)
O(1W)-H(1W1)···Cl(2)i
O(1W)-H(1W2)···Cl(2)
O(8A)-H(8A)···O(1W)ii
O(8B)-H(8B)···Cl(2)
Compound 2
Valance angles (°)
O(8B)-Mn(1)-O(8A) 91.82(5)
O(8B)-Mn(1)-N(2B) 72.50(5)
O(8A)-Mn(1)-N(2B) 92.64(5)
O(8B)-Mn(1)-N(2A) 90.63(5)
N(2B)-Mn(1)-N(2A) 158.26(5)
O(8B)-Mn(1)-Cl(1)
O(8A)-Mn(1)-Cl(1)
O(8A)-Mn(1)-N(2A) 73.85(5)
O(8B)-Mn(1)-Cl(1)i 172.94(3)
O(8A)-Mn(1)-Cl(1)i 87.68(4)
N(2B)-Mn(1)-Cl(1)i 100.48(4)
N(2A)-Mn(1)-Cl(1)i 95.98(4)
N(2B)-Mn(1)-Cl(1) 98.97(4)
Cl(1)-Mn(1)-Cl(1)i 87.191(1)
N(2A)-Mn(1)-Cl(1) 95.91(4)
D-H···A
d(D-H) d(H···A)
0.76(3) 2.43(3)
d(D···A)
3. 1612(2)
3.356(3)
<DHA
162(3)
138
O(1)-H(1)···S(1)i
C(7)-H(7)···N(2)ii
94.58(4)
168.00(3)
0.97
2.57
Symmetry codes: 1 (i) –x+2, -y+1, -z+1, (ii) x-1, y, z; 2 (i) x, -y+0.5, z+0.5,
(ii) –x, -y, -z.
Compound 2
Bond lengths (Å)
Mn(1)-O(1)
Mn(1)-N(1)
2.2020(16)
2.2463(18)
Mn(1)-N(2)
2.2069(19)
Valance angles (°)
O(1)i-Mn(1)-O(1)
O(1)-Mn(1)-N(2)i
O(1)-Mn(1)-N(2)
N(2)-Mn(1)-N(2)i
180.00
N(2)-Mn(1)-N(1)i
O(1)-Mn(1)-N(1)
N(2)-Mn(1)-N(1)
N(1)-Mn(1)-N(1)i
94.18(7)
85.35(6)
85.82(7)
180.00
91.08(7)
88.92(7)
180.00
Symmetry codes: 1 (i)–x+1,- y, -z+2, 2 (i) –x, -y, -z.
Molecular structure of
2 is best represented by the formula
[Mn(2-(CH2)2OHpy)2(NCS)2], and structure of the complex is
shown in Fig. 2. Selected bond distances and bond angles
values are listed in Table 2. In the symmetric manganese
complex [Mn(2-(CH2)2OHpy)2(NCS)2] a MnII ion is located on
the centre of symmetry and the octahedral environment
provided by one 2-(CH2)2OHpy, one SCN- and their symmetry
related counterparts. Thus, the Mn center is surrounded by two
nitrogen from the SCN- anions (Mn(1)-N(2) 2.2069(19) Å) and
two nitrogen and two oxygen from the chelating 2-(CH2)2OHpy
ligands (Mn(1)-N(1) 2.2463(18) Å, Mn(1)-O(1) 2.2020(16) Å)
that form an almost perfect octahedron with angular deviations
smaller than 5°. Distances and angles within the coordination
environment of MnII are in agreement with the distances found
in the literature for similar compound14.
Fig. 4 View of a 1D supramolecular network of complex 2. View along [1 0 0]
direction.
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Table 4 π···π interactions for 1 and 2 (Å)
0,45
9
8
7
6
5
4
3
2
1
0
d(Cg···Cg) (Å)
offset
0,4
π···π
1
2
Cg01-Cg01(-x+1, -y, -z+1)
Cg-Cg(-x+1, -y, -z)
3.654
3.598(2)
1.435
0,35
1.105
0,3
0,25
0,2
Compound 1: Cg01, N(2A)-C(6A), compound 2: Cg, N(1)-C(6)
Analysis of the crystal structure at the supramolecular level
reveals that the complex molecules are held together by one
intermolecular O(1)-H(1)∙∙∙S(1)i hydrogen bond and one weak
C(7)-H(7)∙∙∙N(2)ii intramolecular interaction. (Table 3). These
interactions define a 1D structure running along [1 0 0] (Fig 4).
and intermolecular π∙∙∙π stacking interactions (Table 4) between
centrosymmetric molecules of pyridine ring. The mean
intercentroid and interplanar distances between aromatic rings
(3.598(2) Å and 3.424(2) Å; symmetry code –x+1, -y, -z)
corresponds well with the relative parameter found in structures
of metal complexes with aromatic nitrogen-containing ligands.
The IR spectra confirmed the results of the X-ray studies
with respect to the N, O-donor type of coordination of both
ligands. The electron – donating capabilities of the homologous
pyridine alcohols: 2-CH2OHpy and 2-(CH2)2OHpy can be
estimated by comparing the OH stretching vibration of the –
CH2OH and –(CH2)2OH groups and the C=N frequencies of the
0,15
0,1
0,05
0
0
50
100
150
T(K)
M (
200
250
300
350
Fig. 5 The temperature dependence of
lines result from the best least-squares fit using the parameters described in the
text.
χ
•) and
χ
MT (○) for
1
. The solid red
magnetic susceptibility χM as well as χMT shown in Fig. 5 was
analyzed according to equation 1:
Eq. 1
where x= J/kT.
The best least-squares fitting of the experimental magnetic data
using this equation leads to g = 2,00 and J = -1.63 cm-1 with
very good agreement factor R = 2.8 x 10-4. The negative J value
indicates weak but distinct antiferromagnetic coupling in
pyridine ring of MnII complexes (
1, 2) with those of the free
ligands, respectively. Both organic molecules exhibit broad
absorption bands at 3300-3500 cm-1, which are assigned to
intramolecular hydrogen bond ν(O-H∙∙∙N) vibration14a,15. The
complexes formation leads to hampering this proton exchange,
which results in one absorption band for each complex shifted
towards higher energies. On the coordination of homologous
pyridine alcohols with MnII ions the δC−O bands are shifted to
compound 1.
The magnetic interactions in Mn(II) compounds with
double-chloride bridges are very sensitive to the structural
parameters (distances and angles) of {Mn2(µ-Cl)2}2+ core17,
which is directly related to the space group of monocrystals
collected in Table 5. For these complexes, which are
antiferromagnetically or ferromagnetically coupled, we try to
find some magneto-structural correlation (Fig. S1). The
correlation between J values and Mn-Cl-Mn angles presented in
Fig. S1a indicates that the ferromagnetic interactions between
two manganese(II) ions increases with the increase of the Mn-
Cl-Mn angle. Furthermore, the linear regression in Fig. S1a
calculated from the geometrical data predicts a change from
antiferromagnetic to ferromagnetic behaviour at angle value of
95°. On the other hand the relationship between the measured
Mn∙∙∙Mn distance values and their magnetic exchange coupling
constant (J) predicts the crossover from anti- to ferromagnetic
interactions at about 3.76 Å (Fig. S1b).
Basing on plots in Fig. S1, and despite the existing
correlation between J and the Mn-Cl-Mn angle (R = 0.8125 or
including only dimers: R = 0.8848) and Mn∙∙∙Mn distance (R =
0.7830), the complexes I and II exhibit antiferromagnetic
exchange whereas complex X exhibit ferromagnetic exchange,
when their geometrical parameters are taken into account.
These deviations suggest that some differences between the
theoretical predictions and the experimental findings occur. In
addition, the structural dependence of magnetic interaction
between MnII centres may also be referred to donor properties
and the structure of chelating ligands around the metal ion. The
higher wavenumbers compared to free ligands (Δ = 5 cm-1 for
and Δ = 9 cm-1 for
), which confirms the expected M−O
coordination. Additionally, the νC=N frequencies of the
pyridine ring are shifted by 12 cm-1 and 7 cm-1 for
and 2,
1
2
1
respectively. These shifts are correlated with the binding mode
of these ligands, which are sensitive to manganese coordination
through the nitrogen atom from pyridine ring and also via the
oxygen atom of the –CH2OH or –(CH2)2OH groups. Moreover,
the IR spectra of
1 and 2 show new bands that are attributed to
chloride and thiocyanate ions in the coordination sphere of the
complexes. Two new bands for νCl- at 686 and 644 cm-1 in
complex
1 indicate the presence of chlorides in the structure.
The strong new band at 2079 cm-1 in
2 can be assigned to the
νNCS- of the thiocyanate ion.
The room temperature magnetic moment of powdered
sample of monomeric complex is 5.58 B.M. The calculated
2
value of μeff was similar to those observed for the five
uncoupled spins (S = 5/2, 5.92 B.M.) of Mn(II)16. This contrasts
to the dinuclear complex
1, in which magnetic moments
decreased when temperature was lowered, especially below 50
K. This behavior was interpreted in terms of HDVV
Hamiltonian, H = -J Ŝ1·Ŝ2, where S1 = S2 = 5/217b,e,h,18. The
temperature dependence (in the range 1.8 – 300 K) of the molar
4 | J. Name., 2012, 00, 1-3
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Table 5 Comparison of the structural parameters (distances (Å) an angles (°)) and magnetic interaction (J (cm-1), based on the Spin Hamiltonian -J Ŝ1·Ŝ2) for
dinuclear and polynuclear complexes with a {Mn2(µ-Cl)2}2+ core
No
I
Compounda
Space
group
C2/c
α(Mn-Cl-Mn) (°)
93.36(2)
93.53(4)
95.68(7)
96.06(6)
95.52(2)
94.137(2)
93.5
d(Mn-Cl) (Å)
d(Mn∙∙∙Mn) (Å)
3.664(4)
3.740
J (cm-1)
+0.42
Interactions
Ref.
17a
17b
17c
17d
17e
17f
[MnCl(pyz)(H2O)]n
[Mn(µ-Cl)(biz)2]2Cl2
[Mn2(dpp)2(H2O)2Cl4]2H2O
[Mn(µ-Cl)2(phen)]n
[(µ-Cl)2MnL2(PF6)]2
[Mn(µ-Cl)2(mppma)]n
[Mn(4-CNpy)2Cl2]
[TPA2Mn2(µ-Cl)2]2+
[Mn(µ-Cl)2(bpy)]n
[{MnCl((-)-L)}2(µ-Cl)2]
[{Mn(mpbpa)Cl}2(µ-Cl)2]
[{Mn(bpea)Cl}2(µ-Cl)2]
1
2.4837(8)
2.5513(8)
2.5757(1)
2.5578(1)
2.523(2)
weak ferromagnetic
weak ferromagnetic
weak ferromagnetic
weak ferromagnetic
II
+0.33(1)
+0.11
P1
III
IV
V
P21/c
C2/c
P21/n
P21/c
P21/c
P21/c
I2/a
3.805(2)
3.817
2.610(2)
2.4821(2)
2.6499(2)
2.4915(6)
2.5408(6)
2.5811(6)
2.5265(6)
2.525(2)
+0.017
+1.49
3.726
ferromagnetic/
antiferromagnetic
weak
antiferromagnetic
antiferromagnetic
VI
VII
VIII
IX
X
3.7398(4)
3.700
-0.255(0)
-0.792(6)
-8.8
17g
17h
17i-j
17k
17l
2.555(2)
<90
2.4056(9)
2.6286(1)
2.4813(1)
2.6616(1)
2.5772(1)
2.4356(1)
2.5684(6)
2.5950(6)
2.5374(9)
2.5683(7)
2.507(2)
3.521(2)
3.8352(7)
3.736
antiferromagnetic
96.4
+0.19
weak ferromagnetic
P21
95.91(5)
96.426(2)
95.72(2)
92.81(2)
-0.22
weak
antiferromagnetic
ferromagnetic
XI
XII
P21/c
P21/c
P1
3.8500(6)
3.786
+1.10
+0.68
ferromagnetic
17m
3.646(5)
-1.63
antiferromagnetic
this
work
2.528(4)
aAbreviations: pyz = 2-pyrazinecarboxylate, biz = 2,2-bi-imidazoline, dpp = 2,3-bis(2-pyridyl)pyrazine, phen = 1,10-phenanthroline, L2 = N,N’-bis(2-
pyridinylmethylene)propanediamine, mppma = N-(3-methoxypropyl)-N-(pyridin-2-ylmethyl)amine, 4-CNpy = 4-cyanopyridine, TPA = tris(2-
pyridylmethyl)amine, bpy = 2,2’-bipyridine, (-)-L = (-)-pinene[5,6]bipyridine, mpbpa = N-(3-methoxypropyl)-N,N-bis(pyridin-2-ylmethyl)amine, bpea = N,N-
bis(2-pyridylmethyl)ethylamine
Table 6 The calculated singly occupied natural orbitals of 1 for S1 = S2 = 5/2
Orbital
Overlap
(occup. number)
152
155
153
154
156
0.05590
0.01455
0.02727
0.02526
0.01290
153
154
152
preparation and study of additional systems of this category Table 6; the calculated magnetic orbitals at separate MnII ions
will play an instrumental role in confirming the trends observed suggest that dx2-y2 orbital in 153 and dz2 orbital in 154 and 152
here.
The value of exchange integral was also calculated at DFT
level by using the “broken symmetry” method. The predicted J at 77 K are shown in Fig. 6. For monomeric complex
are dominant.
The X-band EPR spectra of powdered compounds
1
and
2
2
the
value is -3.4 cm-1 that satisfactory reproduces the experimental spectrum exhibits fine structure signals resulting from ZFS of
value. The quality of the DFT prediction of J value is in our the spin states for the high-spin d5 metal ion. It can be
opinion reasonable, taking into account that theoretically parameterized using the following Spin Hamiltonian:
predicted exchange integrals of small values are usually twice
H = βSg B + D[Sz2 - 1/3S(S+1)] + E(Sx2-Sy2) Eq. 2
greater than experimental19. It seems to be specially important
that the result of DFT calculations theoretically supports an
antiferromagnetic character of the exchange interactions. They
are illustrated by the diagrams of three from five MnII d-orbitals
participating in weak metal-metal interactions are shown in
where S = 5/2 and D and E are ZFS parameters for individual
MnII ion representing the axial and rhombic parts, respectively.
The simulation of the experimental spectrum for 2, using a
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The TG curve of complex
1
exhibits seven decomposition
steps (Fig. S2a). The two molecules of lattice water (4.24%) are
released in the temperature range of 70 of 140 °C. Endothermic
effects (max. 93 °C) are observed in the DSC curve (Fig. S2c).
In the second step, compound
1 loses fragments of organic
ligands and a Cl2 molecule (from non-coordinated chloride
ions). The next four steps over the temperature range of 195 to
500 °C correspond to the loss of parts of the four pyridine rings
(37.80%). The last stage is described by a mass loss of 12.99%
and is accompanied by the final fragmentation of organic ligand
molecules containing N-donor atoms and a Cl2 molecule from
1 and 2 at 77 K, together the double chloride bridges. A residue of MnO2 remains.
Fig. 6 X-band EPR spectra of powdered compounds
with the spectrum for
2 (sim 2) simulated-with the spin Hamiltonian parameters
Decomposition process of
2 proceeded in three main stages
given in the text.
over the temperature range of 90 to 1000 °C (Fig. S2b). The
first step begins at 90 °C and is attributed to the elimination of
the 2-(CH2)2OHpy molecules fragments (44.15%). In the
second stage, up to 255 °C, a CS2 molecule from thiocyanate
ligand is released (18.24%). The successive mass loss (16.79%)
from 405 to 1000 °C may be attributed to the final
decomposition of two organic ligands and thiocyanate ions,
leading to MnO2 formation.
computer program SPIN20 gave the approximate spin
Hamiltonian parameters, g = 2, D= 0.089 cm-1 and E = 0.023
cm-1. The parameters are close to those estimated from the EPR
spectra with similar fine structure signals for other MnII
complexes forming the distorted polyhedra21, especially for
octahedral MnII and seven-coordinated MnII with N3O4 donor
set (D = 0.086 cm-1, E = 0.026 cm-1)22.
Dinuclear compound
with geff = 2.03, what precludes finding of the zero field
splitting parameters. Hence, we carried out the measurements at
1 exhibits a characteristic broad line
Catalase-like activity of complexes 1 and 2 and RIXS
measurements
The synthesized complexes catalytic activity was tested in
H2O2 disproportionation. According to catalytic activity of this
metalloenzyme, hydrogen peroxide is destroyed resulting in the
formation of oxygen and water in the reaction (Eq. 3):
224 GHz and 5 K for dinuclear compound 1. Unfortunately, the
interpretation of the obtained spectrum (Fig. 7), consisting of
large amount of the signals from the resonance transitions
within most likely very complicated system of energy levels,
appeared to be practically impossible. Only the Ddip
contribution to D12 in MnII dimer due to the dipole-dipole
interaction of unpaired electrons was estimated. It equals -
2 H2O2 → 2 H2O + O2 Eq. 3
The oxygen evolution for both compounds is depicted in Fig. 8.
The average oxygen evolution achieved with dimmeric
3g2μB /r2 giving -0.107(2) cm-1 for r(Mn-Mn) = 3.646 Å.
2
complex
significantly higher than with monomeric complex
cm3(min*molcat)-1). Even if the results are normalized to Mn
content, the complex is still roughly eight times more active
than . The result suggests that the monomeric complex needs
1
was ca. 414.8 cm3(min*molcat)-1, which is
The thermal data correlates well with the molecular
structure of both compounds. The fragments of ligands
containing N, O-donor atoms decomposed during the last
stages. The detailed results of the TG/DTG analyses of
are listed in Fig. S2.
2
(ca. 35.6
1 and 2
1
2
2
to find a second complex in order to react, similarly to what
was published elsewhere8, and catalytic activity increases if
they are in close proximity like in the case of dimmeric
complex. It should be mentioned that both complexes are
significantly more active than the one publish by Zienkiewicz
et al.8 since reaction was completed within 2-3 h.
Mn K-edge RIXS maps were collected to determine the
electronic structure of the metal center, in particular the p-
orbital contribution to the unoccupied orbitals23, which are
involved in chemical reaction24. The RIXS maps of complex
1
prior, during and after reaction are depicted in Fig. 9. The RIXS
map of in water is dominated strong absorption around 6551-
1
6552 eV, which is similar to the RIXS map of solid complex.
Therefore the signal can be ascribed to MnII 25. The addition of
H2O2 led to a significant increase in the absorption, i.e.,
decrease in the occupancy of the p-states that is consistent with
Fig. 7 The EPR spectrum of the powdered compound
inset the amplified signals on left and right from the asterisks are presented.
1 at 224 GHz and 5 K; in
increase of oxidation state from +II to +III. The RIXS map of
after reaction completion was similar to complex in water.
1
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involved in the catalytic cycle and the system is in fact
catalytic. RIXS experiments are indicate that both complexes
follow the mechanism proposed elsewhere8, suggesting that Mn
(+II) is not only the final oxidation state but also necessary for a
new catalytic cycle to take place.
Fig. 8 Oxygen evolution from the disproportionation of H2O2 in the presence of
(blue) and (red) in water at 22 °C.
1
2
Fig. 10 RIXS maps of
reaction
2 a) in water; b) during reaction (water + H2O2); and c) after
.
Conclusions
In conclusion, we reported two novel manganese(II) complexes
that show promising catalase-like properties. RIXS measurements
revealed that the manganese redox chemistry is a key for the process,
and the complexes are not stoichiometric catalysts, i.e., truly
catalytic system. Despite being far from catalase turnover
frequencies, which are among the highest of all the enzymes, the
reported complexes are active in neutral aqueous conditions
(physiological conditions), uncommon for catalase-mimics, making
these low molecular systems a promising class of materials.
Fig. 9 RIXS maps of
reaction.
1 a) in water; b) during reaction (water + H2O2); and c) after
The RIXS results suggest that Mn redox chemistry is directly
involved in the catalytic reaction, and the reaction with
dimmeric complex
Similar strategy was use to evaluate Mn oxidation state on
complex . The RIXS maps of prior, during and after reaction
are depicted in Fig. 10. As with dimmeric complex , the RIXS
1 is in fact catalytic not stoichiometric.
Experimental section
2
2
General remarks
1
map shows a strong absorption at 6551-6552 eV, which was MnCl2·4H2O and NH4SCN were purchased from the P.O.Ch.
also present in the solid form of the complex. Therefore the Gliwice, 2-hydroxymethylpyridine (Aldrich, 99%), 2-(2-
i
oxidation state of Mn is +II prior to reaction. The addition of hydroxyethyl)pyridine (Aldrich, 99%), methanol, and PrOH
H2O2 resulted in a significant increase in the absorption (P.O.Ch. Gliwice) were used as received.
intensity, reflecting an increase of the unoccupied p-states,
Synthesis of the Mn(II) complexes
which is synonymous of increase of Mn oxidation state from
+II to +III. RIXS map after reaction is similar to the one in
[Mn2(µ-Cl)2(2-CH2OHpy)4]Cl2∙2H2O (1)
water (prior to reaction). Therefore, as with the case of
1 RIXS
MnCl2∙4H2O (0.2774 g, 1.4 mmol) dissolved in 25 cm3 of
maps confirmed that redox chemistry of Mn are directly
i
analytically pure PrOH was added into a 25 cm3 solution of 2-
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CH2OHpy (0.4589 g, 4.2 mmol) in iPrOH. The molar ratio M:L program and refined by full-matrix least-squares methods on all
was 1:3. The mixture was stirred for 5h and then left to F2 data using the SHELXL-9729 program. All non-hydrogen
crystallize at room temperature. After four weeks, light olive atoms were refined with anisotropic thermal parameters. All H
crystals of
product was filtered and dried under vacuum. The yield of attached to O atom was refined with isotropic thermal
compound was about 28%. Anal. Calcd. for [Mn2(µ-Cl)2(2- parameter. Other H atoms were treated as riding with isotropic
1, suitable for X-ray analysis were obtained. The atoms in 2 were found in the difference-Fourier map. H atom
1
CH2OHpy)4]Cl2 ∙ 2H2O (C24H32N4O6Cl4Mn2): C, 39.78; H, thermal parameters.
4.46; N, 7.73. Found: C, 39.74; H, 4.41; N, 7.64. IR (νmax/cm-1): A summary of the condition for the data collection and the
3388(br), 3357(br), 3206 (br), 3047(m), 3025(m), 2834(m), structure refinement parameters are given in Table 1. The
1606(s), 1568(m), 1491(m), 1481(m), 1456(m), 1429(m), figures
were
made
using
DIAMOND31 software.
1363(m), 1294(m), 1271(m), 1233(m), 1155(m), 1108(w), Crystallographic data (excluding structure factors) for the
1062(s), 1043(vs), 1017(s), 995(m), 971(w), 911(w), 893(w), structure reported here have been deposited with the Cambridge
818(m), 785(s), 773(vs), 724(s), 686(s), 644(s), 624(s), 595(vs), Crystallographic Data Centre, as supplementary publication
589(vs).
number CCDC 902422 (
be obtained
https://www.ccdc.cam.ac.uk/services/structure_deposit/
1
), 972967 (
2). Copies of the data can
free of
charge
via
[Mn(2-(CH2)2OHpy)2(NCS)2] (2)
(or
The complex was synthesized by reacting MnCl2∙4H2O (0.4801 from the CCDC, 12 Union Road, Cambridge CB2 1EZ,UK;
g, 2.4 mmol), 2-(CH2)2OHpy (0.2988 g, 2,4 mmol) and fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk).
NH4SCN (0.1850 g, 2.4 mmol) in methanolic solution. The
Infrared (IR) analysis
mixture was refluxed for 1.5 h. After this time, the volume of
the solvent was reduced to about 30 cm3 and the solution left Infrared (IR) spectra were recorded on a Nicolet 380 FT-IR
out to slow evaporation. After few days, colorless crystals of type spectrophotometer in the spectral range 4000-500 cm-1
the complex were obtained. The compound was filtered off and using the ATR-diffusive reflection method.
dried under vacuum box. Yield 72%. Anal. Calcd. for [Mn(2-
EPR analysis
(CH2)2OHpy)2(NCS)2] (C16H18N4O2S2Mn) C, 46.04; H, 4.35;
N, 13.42; S, 15.36. Found: C, 45.87; H, 3.96; N, 13.43; S, High-field, high-frequency EPR spectra were recorded during
14.96. IR (νmax/cm-1): 3223(br), 2077(vs), 1603(m), 1568(w), professor Julia Jezierska visit in 2011 in National High
1483(m), 1470(w), 1443(m), 1432(w), 1417(m), 1383(w), Magnetic Field Laboratory, Florida State University,
1354(w), 1329(m), 1311(m), 1151(m), 1101(m), 1066(m), Tallahassee, on a home-built spectrometer at the EPR facility
1025(m), 966(w), 854(m), 785(m), 766(m), 640(m), 615(w), of NHMFL32. X-band (9.8 GHz) were recorded on a Bruker
594(m), 569(m), 559(m), 544(m), 536(m), 520(m), 509(m).
ElexSys E500 instrument equipped with an NMR teslameter
ER 036TM and a frequency counter E 41 FC (Chemistry,
Wroclaw University).
Methods
Chemical analysis
Computational details
Elemental analysis was run on a Model Elemental Vario EL III
CHN Analyzer.
All calculations for
1 were performed with the ORCA program
package33. The program can be downloaded free of charge from
http://www.thch.uni-bonn.de/tc/orca. The calculations were
Crystal structure determination
based on the X-Ray structure of 1. The exchange integral J was
computed at the DFT level by employing the “broken
symmetry” method using the B3LYP34 functional combined
Diffraction intensity data for
1 was collected at room
temperature on a KappaCCD (Nonius) diffractometer with
graphite-monochromated MoKα radiation (λ = 0.71073 Å).
Corrections for Lorentz, polarization and absorption effects26,27
were applied. The structures were solved by direct methods
using SIR-92 program package28 and refined using a full-matrix
least square procedure on F2 using SHELXL-9729. Anisotropic
displacement parameters for all non-hydrogen atoms and
isotropic temperature factors for hydrogen atoms were
introduced.
Ahlrichs-type basis set TZVPP34 for the Mn and Cl
with
atoms while for all other atoms with SVP functions34.
The
DFT calculations of ZFS were performed with the BP8636 and
B3LYP34 functionals by using
basis
TZVPP35
for the Mn and
Zn atoms and TZVP functions35 for all other atoms.
For the
calculations of spin-orbit (SOC) contribution to D1, the coupled
approach (CP) was used37.
The crystallographic measurements for
2 were performed on a Thermal analysis
Kuma KM4CCD automated four-circle diffractometer with the
graphite-monochromatized MoKα radiation. The data for the
The thermal analyses (TG/DTG) of complexes 1, and 2 were
carried out using a TG/SDTA 851e METTLER-TOLEDO
thermobalance. The experiments were performed in nitrogen
atmosphere at a heating rate of 5 °Cmin-1 in the temperature
crystals were collected at 100(2)
K using the Oxford
Cryosystem cooler. The data were analytical corrected for
absorption with the used CrysAlis RED30 program. The
structure was solved by direct methods using the SHELXL-9729
range of 25–800 °C
1 and 25–1000 °C 2, using Al2O3 crucible.
The sample sizes are ranged in mass from 3.45 to 3.91 mg. The
8 | J. Name., 2012, 00, 1-3
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thermoanalytical curves were obtained using STARe System The authors are specially grateful Dr A. Ożarowski (National
METTLER-TOLEDO software. DSC analysis for was carried High Magnetic Field Laboratory, Florida State University, 1800
out using DSC METTLER-TOLEDO Instrument in standard E. Paul Dirac Drive, Tallahassee, Florida, 32310 USA) for
1
,
closed sample pans, static air atmosphere, and heating rate of 5 experimental and theoretical support connected with HF EPR
°Cmin-1.
and DFT calculations of the dinuclear complex (1).
We thank the Paul Scherrer Institute for providing access to the
SuperXAS (X10DA) beam line at the Swiss Light Source.
Magnetic measurement
Magnetic measurements were carried out on a magnetic
susceptibility balance (Sherwood Scientific) at room
temperature by Gouy’s method using Hg[Co(NCS)4] as a
calibrant.
Notes and references
aInstitute of Chemistry, Jan Kochanowski University in Kielce, 15G
Świętokrzyska Str., 25-406 Kielce, Poland. E-mail:
barbara.barszcz@ujk.edu.pl
Magnetic susceptibility data of powdered samples
1 were
bPaul Scherrer Institute (PSI), 5232 Villigen, Switzerland. E-mail:
jacinto.sa@psi.ch; Tel: +41563102910
measured with a SQUID magnetometer (Quantum Design
MPMSXL-5, Faculty of Chemistry, Wroclaw University) over
the temperature range 1.8−300 K at the magnetic induction of
0.5 T. Corrections for the sample holders were applied.
cInstitute of Physics, Jan Kochanowski University in Kielce, 25-406 Kielce,
Poland
dFaculty of Chemistry, Jagiellonian University, 3 Ingardena Str., 30-060
Kraków, Poland
Diamagnetic corrections for
constants.
1 were determined from Pascal’s
eFaculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie Str., 50-383
Wrocław, Poland
Operando resonant inelastic X-ray spectroscopy (RIXS) and
catalase-like activity
†Electronic Supplementary Information (ESI) available: Plots of J (cm-1)
versus various parameters. (a) J (cm-1) versus the Mn-Cl-Mn angle (deg).
(b) J (cm-1) versus the Mn∙∙∙Mn distance (Å) (Fig. S1) and TG/DTG of
The Mn K-edge RIXS maps were performed at the SuperXAS
beamline at the Swiss Light Source, Paul Scherrer Institute,
Switzerland. The X-ray beam delivered by the 2.9 Tesla super-
cooled bending magnet was collimated by an spherically bent
Rh mirror. The collimated X-rays were monochromatized by
means of a double Si(111) crystal monochromator and focused
by a toroidal bent Rh mirror. On the sample the photon flux
was 7–8 Å~ 1011 photons.s-1 with an energy resolution of ΔE/E
≈ 1.4 Å~ 10−4 and a spot size of 100 μm2. A 4 μm thick Mn foil
was used to calibrate the energy. X-ray detection was
performed using a wavelength-dispersive spectrometer, in the
von Hamos geometry, which prevents scanning of components
during the acquisition38. The X-rays emitted from the sample
were diffracted by a Ge(660) cylindrically bent crystal with a
radius of curvature of 25.4 cm, and detected via a 2D Pilatus
detector. The spectrometer was operated in the vertical
scattering geometry.
The experiments were performed in a sealed semi-batch
polyvinyl container, which was connected to a quadrupole mass
spectrometer (QMS) from Hiden Analytical in order to monitor
the oxygen evolution. 175 mg of the catalyst was charged into
the reactor containing 20 cm3 of Milli-Q quality water, and
continuously flushed with 30 cm3min-1 of argon. The reaction
was started, after the stabilization of the mass spectrometer, by
adding 20 cm3 of H2O2 (30% w/w from Merck) through a
spectrum with a syringe. The reaction mixture was stirred
continuously to avoid beam damage, and to limit problems with
mass transfer. The presented O2 evolution profiles were
corrected for QMS signal drifts with tracer (Ar m/q=36, natural
abundance 0.3%), and by uncatalyzed signal, which was
estimated using blank experiments (without catalyst).
complex
1 (a), 2 (b), and DSC curves of complex 1 (c) (Fig. S2). See
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