Mononuclear and dinuclear manganese compounds stabilized by supramolecular interactions

Article abstract of DOI:10.1039/c2dt31060k

New manganese compounds [Mn(HphpzMe)₂(H₂phpzMe) (HCO₂)] (1), [Mn₂(phpzMe)₂(HphpzMe) ₂(OCH₃)]·2CH₃OH (2), Na{[Mn(HphpzPh)(phpzPh)(MeOH)₂]₂}(HCO₂) (3), [Mn(HphpzPh)₂(EtOH)₂]ClO₄·2EtOH (4) and [Mn(HphpzPh)₂N₃] (5) were synthesized and characterized with various techniques. 1, 4 and 5 are mononuclear manganese(iii) compounds, 2 is a mixed-valence dinuclear manganese(iii/iv) compound, and 3 is a trinuclear compound containing two manganese(iii) ions and a sodium(i) ion. A remarkable feature is the spontaneous formation of the formate ion as a result of the methanol or methoxide oxidation in compounds 1 and 3. Using ethanol precludes the formation of the formate and compound 4 is obtained. The molecular structure of all compounds is stabilized by supramolecular interactions, including strong hydrogen bonding and π-π interactions.

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PAPER
Mononuclear and dinuclear manganese compounds stabilized by
supramolecular interactions†
Marta Viciano-Chumillas,a,b Mónica Giménez-Marqués,a Stefania Tanase,‡a Ilpo Mutikainen,c
Urho Turpeinen,c Jan M. M. Smits,d René de Gelder,d L. Jos de Jongh*b and Jan Reedijk*a,e
Received 16th May 2012, Accepted 12th June 2012
DOI: 10.1039/c2dt31060k
New manganese compounds [Mn(HphpzMe)2(H2phpzMe)(HCO2)] (1),
[Mn2(phpzMe)2(HphpzMe)2(OCH3)]·2CH3OH (2), Na{[Mn(HphpzPh)(phpzPh)(MeOH)2]2}(HCO2) (3),
[Mn(HphpzPh)2(EtOH)2]ClO4·2EtOH (4) and [Mn(HphpzPh)2N3] (5) were synthesized and characterized
with various techniques. 1, 4 and 5 are mononuclear manganese(III) compounds, 2 is a mixed-valence
dinuclear manganese(III/IV) compound, and 3 is a trinuclear compound containing two manganese(III) ions
and a sodium(I) ion. A remarkable feature is the spontaneous formation of the formate ion as a result of
the methanol or methoxide oxidation in compounds 1 and 3. Using ethanol precludes the formation of the
formate and compound 4 is obtained. The molecular structure of all compounds is stabilized by
supramolecular interactions, including strong hydrogen bonding and π–π interactions.
In recent years, we developed a new category of phenol–pyra-
Introduction
zole ligands (H2phpzR = 3(5)-(2-hydroxyphenyl)-5(3)-R-pyra-
zole; R = H, Me, Et, Ph). Our goal is to study the impact of the
ligand steric effects on the structure of the manganese com-
pounds and their relevance with respect to the magnetic
properties.7–12 We discovered that the reaction conditions, i.e.
the solvent, the metal/ligand ratio, etc., lead to a variety of com-
pounds having different nuclearities.7–12 It is clear that the
amount of base determines the product. For example, octanuclear
manganese(III) compounds [Mn8(μ4-O)4(phpzR)8] (R = H, Me, Et)
are formed by using an excess of triethylamine.7,8 Mononuclear
compounds [Mn(HphpzR)2X] (R = H, Me, Et, Ph; X = Cl, Br) are
isolated in good yield when using a small quantity of triethyl-
amine.9 The type of base and the solvent employed are also impor-
tant. Thus, we obtained trinuclear manganese(III) complexes by
using sodium methoxide as a base and methanol as a solvent.10–12
We discuss here new reaction conditions, including different
metal to ligand molar ratios, various manganese(II) salts, and
different alcoholic solvents. We also investigated the ability of
nBu4NMnO4 to generate manganese(III) and/or manganese(IV)
ions, by using a comproportionation reaction.13,14 Finally, the
impact of an additional bridging ligand is also described. These
studies led to the isolation of five novel compounds. Their syn-
thesis, molecular structure and magnetic properties are presented
here in detail.
Manganese polynuclear complexes have received great attention
in the last decades. They are interesting model systems for
metalloenzymes, including manganese catalases and the oxygen-
evolving complex (OEC) in photosystem II.1–3 They also often
have exciting magnetic properties.4–6 Designing new manganese
complexes is a challenging task. This is because the selection of
the organic ligand is very important. It plays a crucial role in
stabilizing the oxidation state of the manganese ion. It also deter-
mines the nuclearity and the structure type of the final
compound.
aGorlaeus Laboratories, Leiden Institute of Chemistry, Leiden
University, PO Box 9502, 2300 RA, Leiden, The Netherlands.
E-mail: reedijk@chem.leidenuniv.nl; Fax: +31-71-527-4671;
Tel: +31-71-527-4459
bKamerlingh Onnes Laboratory, Leiden Institute of Physics, Leiden
University, PO Box 9504, 2300 RA, Leiden, The Netherlands.
E-mail: jongh@physics.leidenuniv.nl; Fax: +31-71-527-5404;
Tel: +31-71-527-5466
cLaboratory of Inorganic Chemistry, Department of Chemistry,
University of Helsinki, PO Box 55 (A.I. Virtasen aukio 1), 00014
Helsinki, Finland
dSolid State Chemistry, Institute for Molecules and Materials, Radboud
University Nijmegen, Heijendaalseweg 135, 6525 AJ, Nijmegen,
The Netherlands
eDepartment of Chemistry, King Saud University, PO Box 2455, Riyadh
11451, Saudi Arabia
†Electronic supplementary information (ESI) available: Figure of the
3-D network for compounds 1, 4 and 5; table of π–π interactions of com-
pounds 1 and 5; and magnetic data of compounds 1 and 3–5. CCDC
861014 (1), 858616 (2), 858617 (3), 858618 (4) and 858619 (5). For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt31060k
‡Current address: Van’t Hoff Institute for Molecular Sciences, Univer-
sity of Amsterdam, Science Park 904, 1098 XH Amsterdam, The
Netherlands.
Results and discussion
Synthesis and spectroscopic properties
Reacting Mn(ClO4)2·6H2O and H2phpzR (R = Me, Ph) in a
1 : 2 molar ratio in the presence of sodium methoxide in metha-
nol affords 1 and 3. Compound 3 is also obtained by employing
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 10249–10257 | 10249

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different manganese(II) salts (MnX2·nH2O where X = Cl−,
AcO−, PhCOO−, BF4−) in a ratio 1 : 1. The formation of formate
in 1 and 3 is proven by X-ray analysis. Most likely, the formate
is formed by the oxidation of the MeOH solvent or/and
methoxide.15–17 Using ethanol instead of methanol affords com-
ligands. This indicates the coordination of the phenol–pyrazole
ligands to the manganese ions. We assign the weak peaks
observed in the region of 3057–2800 cm−1 to the stretching
vibrations of the aromatic rings.18 In addition, 1 exhibits two
sharp bands at 3383 and 3279 cm−1 that belong to the νO–H and
νN–H stretching from the H2phpzMe ligand. In 4, the broad band
at 3372 cm−1 is assigned to the ethanol molecules. The presence
of the perchlorate counter ion is indicated by the bands at 1063,
n
pound 4. We observed that Bu4NMnO4 is necessary to obtain
the mixed-valence compound 2, irrespective of the manganese(II)
salt used, e.g. Mn(OAc)2·4H2O or Mn(ClO4)2·6H2O. Reacting
MnBr2·4H2O and H2phpzPh in the presence of a small quantity
of triethylamine and sodium azide affords compound 5. The
same compound is also obtained by reaction of [Mn-
(HphpzPh)2Br] with sodium azide. In this case, after isolation of
5, secondary compounds crystallize from the filtrate. We did not
study in detail these compounds, because they are formed in low
yield.
940 and 623 cm−1 19
The infrared spectrum of 5 indicates the
.
presence of the azide: a strong sharp band at 2068 cm−1. A small
sharp band at 3348 cm−1 is assigned to the νN–H stretching
vibrations.18
The diffuse reflectance spectra of 1 and 3–5 exhibit a broad
band with a maximum in the range of 630–719 nm; this band is
assigned to the d–d transitions of the manganese(III) ion.20 The
intense band at 332–459 nm belongs to the LMCT transitions.20
Repeated chemical analyses of 2 showed that the solvent mo-
lecules are substituted by water molecules upon air exposure.
Thus, [Mn2(phpzMe)2(HphpzMe)2(OCH3)]·4H2O (2a) is
formed. Thermogravimetric analysis (TGA) of 2a shows a mass
loss of 8% in the range 25–200 °C. This is in agreement with the
loss of four water molecules.
In Table 1 some characteristics of compounds 1–5 are shown.
The infrared spectra of 1–5 show a shift of the νCvN and νC–O
stretching vibration to lower energies as compared to the free
Table 1 Some characteristics of the compounds 1–5
Isolated
Compound yield (%)
d–d band χMTa
(nm)
ν (cm−1
)
(cm3 K mol−1
)
1
2
3
4
5
34
59
60
22
64
—
—
—
684
563
630
662
719
2.96 (3.0)
2.82 (4.9)
5.65 (6.0)
2.98 (3.0)
2.79 (3.0)
Description of the molecular structures
−
1063 (ClO4
2068 (N3
)
Crystal and refinement data of compounds 1–5 are in Table 2.
−
)
Fig.
1
shows the molecular structure of [Mn-
a Values in brackets are calculated based on the free ion approximation.
(HphpzMe)2(H2phpzMe)(HCO2)] (1). The manganese(III) ion is
surrounded by two monodeprotonated HphpzMe− ligands that
Table 2 Crystal data and structure refinements for [Mn(HphpzMe)2(H2phpzMe)(HCO2)] (1), [Mn2(phpzMe)2(HphpzMe)2(OCH3)]·2CH3OH (2),
Na{[Mn(HphpzPh)(phpzPh)(MeOH)2]2}(HCO2) (3), [Mn(HphpzPh)2(EtOH)2](ClO4)·2EtOH (4) and [Mn(HphpzPh)2N3] (5)
1
2
3
4
5
C31H29MnN6O5
620.54
Monoclinic
P21/n
11.1408(4)
13.807(4)
18.780(7)
90
C41H37Mn2N8O5, 2(CH4O) C65H60Mn2N8NaO10 C38H46ClMnN4O10 C30H22MnN7O2
Formula weight (g mol−1
Crystal system
Space group
a (Å)
b (Å)
c (Å)
)
895.75
Monoclinic
P21/n
1246.08
Monoclinic
C2/c
34.527(6)
7.828(2)
23.528(5)
90
809.18
Monoclinic
C2/c
19.8610
11.252(2)
18.074(3)
90
567.49
Monoclinic
P21/c
8.6920(10)
12.8650(10)
22.539(2)
90
10.6470(10)
21.768(4)
18.897(4)
90
α (°)
β (°)
105.958(13)
90
2777.4(13)
4
92.19(3)
90
4376.4(13)
4
114.71(3)
90
5777(3)
4
102.35(3)
90
3945.6(11)
4
98.320(10)
90
2493.8(4)
4
γ (°)
V (Å3)
Z
D
calc (g cm−3
)
1.484
1.360
1.433
1.362
1.512
Crystal size
0.24 × 0.09 × 0.07 0.20 × 0.20 × 0.20
0.3 × 0.3 × 0.4
38 466(3889)
0.03 × 0.17 × 0.2
27 407(4066)
0.18 × 0.20 × 0.22
23 888(5697)
Number of collected
reflections (unique)
Number of observed
reflections (Io > 2σ(Io))
Internal R factor
24 761(5620)
54 196(9444)
3285
6012
4421
2545
4468
0.047
0.067
0.0445
0.072
0.032
Number of parameters
392
1.05
548
1.09
409
1.238
248
1.05
361
1.04
Goodness-of-fit S on F2
Largest peak and hole in final
−0.72 and 0.44
−0.54 and 0.95
0.622 and −0.515
1.20 and −1.09
−0.40 and 0.31
difference Fourier map (e Å−3
)
μ (mm−1
)
0.529
0.0667
0.1115
208
0.635
0.0589
0.1871
173
0.514
0.0726
0.1777
173
0.463
0.0647
0.1907
173
0.574
0.0413
0.1110
173
R1 a [I > 2.0σ(I)]
wR2 b [all data]
T (K)
2 2
2 2
a R1 = ∑kFo| − |Fck/∑|Fo|. b wR2 = {∑[w(Fo2 − Fc ) ]/∑w(Fo ) }1/2
.
10250 | Dalton Trans., 2012, 41, 10249–10257
This journal is © The Royal Society of Chemistry 2012

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from the pyrazole rings acts as a donor and the oxygen atom
from the phenoxide groups as an acceptor. Another interaction
involves the N2C–H and the oxygen atoms from the same ligand
(O1C) and from the neighbouring HphpzMe− ligand (O1A), as
well as the oxygen (O1C) and the oxygen atom (O13) of the
formate from the other molecule. Furthermore, intermolecular
hydrogen bonding interactions are present between the oxygen
atoms (O11 and O13) of the formate ligand and the N–H group
from one of the HphpzMe− ligands of two neighbouring mo-
lecules (see Fig. 2). Thus, the molecules arranged in an alternat-
ing fashion are bound by strong hydrogen bonds forming 1-D
chains. The planes formed by the manganese(III) ions that
compose the chain are arranged in an alternating manner: parallel
or forming an angle of 17.72° with the other manganese(III)
chains (see Fig. S1† for details). The crystal lattice is further
stabilized by π–π stacking interactions involving the pyrazole
and the phenol ring (see Table S1 in the ESI†). The shortest
Mn⋯Mn intermolecular distance is 5.68 Å.
The dinuclear mixed-valence manganese(III/IV) compound 2
has the two manganese ions hexacoordinated (see Fig. 3 for
details). Each manganese ion is coordinated by one HphpzMe−
ligand and two phpzMe2− ligands, both binding in a N,O-chelat-
ing manner. The two manganese ions are linked by two pyrazo-
lato bridges and by one methoxide ligand. The Mn⋯Mn
separation is 3.268 Å. Selected distances and angles are listed in
Table 3. The Mn1–O1–Mn2 angle has a value of 112.87°. The
bond distances and the charge-valence considerations indicate a
Fig. 1 Platon projection of the molecular structure of
[Mn(HphpzMe)2(H2phpzMe)(HCO2)] (1). Hydrogen atoms are omitted
for clarity. Colour code: green, manganese; blue, nitrogen; red, oxygen;
grey, carbon.
Table 3 Selected bond lengths (Å) and angles (°) for [Mn-
(HphpzMe)2(H2phpzMe)(HCO2)] (1) and for [Mn2(phpzMe)2-
(HphpzMe)2(OCH3)]·2CH3OH (2)
Compound 1
Bond lengths
Mn1–O1A
Mn1–O11
Mn1–O2A
1.850(2)
2.290(3)
2.003(3)
Mn1–O1B
Mn1–N1C
Mn1–N2B
1.847(2)
2.307(3)
1.990(3)
Bond angles
O1A–Mn1–O1B
O1A–Mn1–N1C
O1A–Mn1–N2B
178.22(12)
86.85(12)
88.66(12)
O1A–Mn1–O11
O1A–Mn1–N2A
92.04(11)
89.78(12)
Compound 2
Bond lengths
Mn1–O1
Mn1–O70
Mn1–N45
Mn2–O1
Mn2–O50
Mn2–N41
Mn⋯Mn
2.018(3)
1.909(3)
2.179(3)
1.904(3)
1.902(3)
1.956(3)
3.268
Mn1–O10
Mn1–N21
Mn1–N81
Mn2–O30
Mn2–N25
Mn2–N61
1.900(3)
1.980(3)
2.179(3)
1.861(3)
1.998(3)
1.994(3)
Fig. 2 View
of
the
hydrogen
bonding
interactions
in
[Mn(HphpzMe)2(H2phpzMe)(HCO2)] (1). Hydrogen atoms that are not
involved in hydrogen bonding are omitted for clarity. Colour code:
green, manganese; blue, nitrogen; red, oxygen; grey, carbon.
Bond angles
O1–Mn1–O10
O1–Mn1–N21
O1–Mn1–N81
N21–Mn1–O70
O1–Mn2–O50
O1–Mn2–N41
O50–Mn2–N41
Mn1–O1–Mn2
170.78(13)
83.55(13)
87.01(12)
179.64(13)
94.86(13)
87.13(13)
177.66(13)
112.87(14)
O1–Mn1–O70
O1–Mn1–N45
N21–Mn1–O70
O1–Mn2–O30
O1–Mn2–N25
O1–Mn2–N61
N25–Mn2–N61
96.81(13)
81.24(12)
179.64(13)
175.70(12)
86.40(13)
89.90(13)
175.82(14)
bind in a N,O-chelating manner in the equatorial plane. One
neutral H2phpzMe ligand and a formate ligand are at the axial
positions (Mn1–O11 = 2.290(3) Å and Mn1–N1C = 2.307(3)
Å). Selected bond lengths and angles are listed in Table 3. The
Mn–Oeq and Mn–Neq average bond lengths are 1.85 Å and
2.00 Å, respectively. The crystal lattice is highly stabilized by
strong intramolecular and intermolecular hydrogen bonds (see
Fig. 2 and Table 5 for details). Intramolecular hydrogen bonds
exist between two HphpzMe− ligands, in which the N–H group
Fig. 3 Platon projection of the molecular structure of
[Mn2(phpzMe)2(HphpzMe)2(OCH3)]·2CH3OH (2). Hydrogen atoms not
involved in hydrogen bonding are omitted for clarity. Colour code:
green, manganese; blue, nitrogen; red, oxygen; grey, carbon.
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dinuclear manganese(III)/manganese(IV) compound. The Mn2 is
assigned as the manganese(IV) ion in view of the shorter Mn–O
and Mn–N distances. The manganese(III) ion can be assigned to
Mn1, because of the presence of the Jahn–Teller axis (N45–Mn1
and N81–Mn1 = 2.179 Å). The bending angles between the
bridging pyrazole least-squares planes and the Mn1–N21–N25–
Mn2 and Mn1–N45–N41–Mn2 coordination planes are 6.59°
and 6.76°, respectively. The dihedral angle between the planes of
the two bridging pyrazole rings is 86.17°; the dihedral angle
between the Mn1–pz–Mn2 planes is 74.31°. Hydrogen bonds
are present between the lattice methanol molecules and the
oxygen atoms (O50 and O70) of the phenoxide moiety and the
N–H of the HphpzMe− ligands (see Fig. 3 and Table 5 for
details). The shortest intermolecular Mn⋯Mn distance is
8.339 Å.
Compound 3 is composed of two [Mn(HphpzPh)(phpzPh)-
(MeOH)2] units bridged by a sodium(I) ion (see Fig. 4). The
sodium(I) ion is coordinated by four oxygen atoms from four
phenolate bridging ligands (O10 and O40 from each [Mn-
(HphpzPh)(phpzPh)(MeOH)2] moiety), and by a formate ion.
Selected bond lengths and angles are listed in Table 4. The Mn
coordination geometry is octahedral as expected for a high-spin
manganese(III) ion. The HphpzPh− and phpzPh2− ligands
occupy the equatorial plane and two methanol molecules are at
the axial positions. The Mn–N bond distances are 1.978(4) and
2.001(4) Å and the Mn–O bond distances are 1.889(4) and
1.906(4) Å for phpzPh2− and HphpzPh− ligands, respectively.
The Mn⋯Na and Mn⋯Mn distances are 3.276 and 6.446 Å,
respectively, similar to those observed for other Mn2IIINa com-
plexes.21,22 The angle spanned by the atoms that form the Jahn–
Teller axes (O1–Mn1–O2) has a value of 31.90°. Table 5
includes details of the hydrogen bonding. Intramolecular hydro-
gen bonds are present between the N55–H group of the
HphpzPh− ligand and N25 of the phpzPh2− ligand; and between
the oxygen atom O1 of one methanol molecule with the oxygen
atoms of the disordered formate. Additionally, intermolecular
hydrogen bonding involves the oxygen atoms from a formate
ligand and the methanol O2 of two neighbouring molecules. The
shortest intermolecular Mn⋯Mn distance is 7.828 Å.
In 4, the manganese(III) is surrounded by two HphpzPh−
ligands binding in a N,O-bidentate chelate mode and two
ethanol molecules at the axial positions (see Fig. 5). In addition,
the crystal lattice contains a perchlorate anion to balance the
charge and two ethanol molecules. Table 4 gives a selection of
bond lengths and angles. The coordination geometry of the
manganese(III) is a distorted octahedron. The elongated axial
axes are defined by the two coordinated ethanol molecules with
Mn–O bond distances of 2.247(3) Å. The Mn–Neq and the Mn–
Oeq bond distances are 2.001(3) and 1.860(3) Å, respectively.
The Oeq–Mn–Neq average angles are 90° and the manganese
atom is located in the N2O2 plane. The HphpzPh− ligands
deviate from planarity. The dihedral angles between the plane of
the pyrazole ring, and the plane of phenyl and the phenolate
rings are 3.97° and 9.57°, respectively. Intramolecular hydrogen
bonds are present between the N25–H and the O10 from the
HphpzPh− ligands. Intermolecular hydrogen bonds involve the
oxygen atoms of the coordinated ethanol O1, the lattice ethanol
Fig. 4 Platon projection of the molecular structure of Na{[Mn-
(HphpzPh)(phpzPh)(MeOH)2]2}(HCO2) (3). Hydrogen atoms are
omitted for clarity. Colour code: green, manganese; yellow, sodium;
blue, nitrogen; red, oxygen; grey, carbon.
Table 4 Selected bond lengths (Å) and angles (°) for Na{[Mn(HphpzPh)(phpzPh)(MeOH)2]2}(HCO2) (3), [Mn(HphpzPh)2(EtOH)2](ClO4)·2EtOH
(4), and [Mn(HphpzPh)2N3] (5)
Compound 3
Bond lengths
Mn1–O10
Mn1–O2
Mn1⋯Na1
Bond angles
O10–Mn1–O40
O40–Mn1–N51
N21–Mn1–Na1
1.889(4)
2.265(5)
3.2761
Mn1–O40
Mn1–O1
Mn1⋯Mn1
1.906(4)
2.341(5)
6.446
Mn1–N21
O10–Na1
1.978(4)
2.389(4)
Mn1–N51
O4–Na1
2.001(4)
2.257(9)
87.75(17)
87.74(16)
133.19(13)
O10–Mn1–N21
N21–Mn1–N51
N51–Mn1–Na1
89.38(17)
95.12(16)
128.84(12)
O40–Mn1–N21
O10–Mn1–Na1
O2–Mn1–Na1
177.13(18)
46.03(11)
101.39(13)
O10–Mn1–N51
O40–Mn1–Na1
O1–Mn1–Na1
174.66(18)
44.04(11)
75.75(11)
Compound 4
Bond lengths
Mn1–O1
2.247(3)
Mn1–O10
1.860(3)
Mn1–N21
2.001(3)
Bond angles
O1–Mn1–O10
89.85(13)
O10–Mn1–N21
90.02(14)
O10–Mn1–N21B
89.98(14)
O1–Mn1–O1B
Mn1–N11
180.00
Compound 5
Bond lengths
Mn1–O118
1.8396(16)
1.9885(18)
Mn1–O218
1.8666(16)
Mn1–N1
2.085(2)
1.9812(17)
Mn1–N21
Bond angles
O118–Mn1–O218
O218–Mn1–N1
O218–Mn1–N21
163.99(7)
98.54(8)
88.69(7)
O118–Mn1–N1
N1–Mn1–N11
N11–Mn1–N21
97.16(8)
97.89(7)
157.84(7)
O118–Mn1–N11
N1–Mn1–N21
88.28(7)
104.22(7)
O118–Mn1–N21
O218–Mn1–N11
90.46(7)
86.52(7)
10252 | Dalton Trans., 2012, 41, 10249–10257
This journal is © The Royal Society of Chemistry 2012

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Table 5 Hydrogen bond details (distances (Å) and angles (°)) for 1–5
Donor–H⋯acceptor
D–H
H⋯A
D⋯A
D–H⋯A
Compound 1
N1A–H1A⋯O1B
N1A–H1A⋯O13
N1B–H1B⋯O1A
N1B–H1B⋯O11
O1C–H1C⋯O13
N2C–H2⋯O1A
N2C–H2⋯O1C
Compound 2
0.87
0.87
0.87
0.87
0.83
0.87
0.87
2.51
2.07
2.43
2.20
1.77
2.31
2.02
2.910(4)
2.868(4)
2.825(4)
2.959(4)
2.583(4)
2.837(4)
2.590(5)
108
151
108
146
167
119
122
Fig. 6 View
of
the
hydrogen
bonding
interactions
in
[Mn(HphpzPh)2(EtOH)2](ClO4)·2EtOH (4). Hydrogen atoms that are
not involved in hydrogen bonding are omitted for clarity. Colour code:
green, manganese; yellow, chloride; blue, nitrogen; red, oxygen; grey,
carbon.
O2–H2⋯O50
0.8406
0.8396
0.8790
0.8800
1.9008
2.0828
2.0227
1.9385
2.738(4)
2.873(4)
2.869(5)
2.784(5)
173.64
156.72
161.19
160.61
O3–H3⋯O70
N65–H65A⋯O3
N85–H85A⋯O2
Compound 3
N55–H55A⋯N21
N55–H55A⋯N25
O1–H1⋯O4
0.8797
0.8797
0.8406
0.8406
0.8408
2.6227
1.9183
1.9618
1.7364
1.7082
3.057(6)
2.584(6)
2.786(9)
2.466(10)
2.543(11)
111.52
131.19
166.47
143.98
171.30
O1–H1⋯O6
O2–H2⋯O6
Compound 4
O1–H1⋯O2
O2–H2D⋯O3
N25–H25A⋯O10
Compound 5
0.8277
0.8401
0.8804
1.9832
2.0581
2.4078
2.659(5)
2.854(9)
2.832(5)
138.37
157.83
109.97
N12–H12A⋯O218
N22–H22A⋯O118
N22–H22A⋯N3
0.8798
0.8804
0.8804
2.2453
2.5287
2.1215
2.690(2)
2.880(2)
2.905(3)
111.04
104.60
147.93
Fig. 7 Platon projection of the molecular structure of
[Mn(HphpzPh)2N3] (5). Hydrogen atoms are omitted for clarity. Colour
code: green, manganese; blue, nitrogen; red, oxygen; grey, carbon.
of the Mn(III) ion is square-pyramidal with a value for τ of
0.10.23 Two monodeprotonated HphpzPh− ligands are at the
equatorial position whilst the azide ligand is at the apical pos-
ition. The manganese(III) ion is 0.318 Å above the N2O2 plane.
This type of structure is commonly observed for manganese(III)
compounds containing azide.24,25 However, such compounds are
often polymeric chains (with azide bridges between the manga-
nese(III) ions),26–32 dinuclear manganese(III) compounds (with
the azide ligand at the axial positions)27,31,33–35 or mononuclear
hexacoordinated manganese(III) compounds.26,32,36–38 In our
case, the strong coordination of the azide to the manganese(III)
ion (the Mn–Nazide distance is 2.085(2) Å) gives a shorter bond,
compared to what is observed in the literature
(2.18–2.47 Å).26–32,36–38 Intermolecular hydrogen bonds are
formed between the N–H of the pyrazole ligand and the azide
nitrogen (N22–H22A⋯N3 = 2.905(3) Å). Fig. 8 illustrates the
intermolecular hydrogen bonding interactions between two
mononuclear compounds that are oriented face-to-face. In
addition, intramolecular interactions involve the acid N–H group
as a donor and the phenoxide group as an acceptor. The shortest
Mn⋯Mn distance is 6.338 Å, between mononuclear manganese(III)
species with opposite faces. Weak π–π stacking is also present
between the aromatic rings (see Table S1 and Fig. S3†
for details).
Fig. 5 Platon projection of the molecular structure of
[Mn(HphpzPh)2(EtOH)2](ClO4)·2EtOH (4). Hydrogen atoms are
omitted for clarity. Colour code: green, manganese; yellow, chloride;
blue, nitrogen; red, oxygen; grey, carbon.
O2 and the perchlorate counter ion O3. The result is a chain
having Mn⋯Mn distances of 11.912 Å (see Fig. 6 and Table 5).
The manganese(III) units in the chain have different orientations.
The dihedral angle between the N2O2 equatorial planes is
81.85°. The chains formed due to hydrogen bonding are further
stabilized by π–π ring stacking (3.731(3) Å). This stacking is
between the phenol and the pyrazole rings of neighbouring
manganese(III) units of two different chains. The shortest inter-
molecular Mn⋯Mn distance is 10.645 Å (see Fig. S2 in the
ESI†).
Fig. 7 shows the molecular structure of 5 and Table 4 gives a
selection of bond lengths and angles. The coordination geometry
This journal is © The Royal Society of Chemistry 2012
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Fig. 9 Plot of χMT vs. T for 2a in the range 1.8 to 300 K in a 0.1 T
applied field. Inset: molar magnetization measured at 2 K.
Fig. 8 Intermolecular and intramolecular hydrogen bonds in [Mn-
(HphpzPh)2N3] (5). Hydrogen atoms that are not involved in hydrogen
bonding are omitted for clarity. Colour code: green, manganese; blue,
nitrogen; red, oxygen; grey, carbon.
The experimental magnetic susceptibility data were fitted to
the spin Hamiltonian:6
Ĥ ¼ ꢀ2JŜ1Ŝ2
ð1Þ
Magnetic properties
Magnetic susceptibilities of compounds 1–5 were measured
under a 0.1 T applied field in the 2–300 K temperature range.
For compounds 1, 4 and 5 the χMT values are ca. 2.9 cm3 K mol−1
at 300 K, i.e. around the value expected for one manganese(III)
ion with g = 2. The data are presented in the ESI (see Fig. S4†).
By lowering the temperature, the χMT value increases slightly
for 1. This small increase might be due to a very weak ferromag-
netic interaction between the two closest manganese(III) ions that
are forming the chain as a result of hydrogen bonding (Mn⋯Mn
= 5.680 Å and Mn⋯Mn = 7.637 Å).39,40 For 4 and 5, the χMT
values remain constant. At about 25 K, the χMT products for 1, 4
and 5 decrease considerably due to the zero-field splitting of
manganese(III) ions and possible weak intermolecular antiferro-
magnetic interactions.
The magnetic susceptibility of 2a was measured under a 0.1 T
field in the 2–300 K temperature range (Fig. 9). The χMT value
at 300 K is 2.82 cm3 K mol−1, much lower than the expected
value of 4.87 cm3 K mol−1 for a non-interacting manganese(III)
ion and a manganese(IV) ion (considering a g-value of 2). A
gradual decrease is observed upon cooling and the χMT reaches a
value of 1.6 cm3 K mol−1 at 60 K. Below this temperature, it
decreases steeply to 0.7 cm3 K mol−1 at 2 K. This value is
higher than the characteristic one for an antiferromagnetic coup-
ling between manganese(III) and manganese(IV) ions with a
ground state of ST = 1/2 with g = 2.0 (0.375 cm3 K mol−1).
Field-dependent magnetization was measured at 2 K. The inset
of Fig. 9 shows that the molar magnetization reaches almost 1
Nβ at 5 T. This value would correspond with the (saturated) ferri-
magnetic moment of a dimer with S1 = 3/2 and S2 = 2. As can
be seen the magnetization is still increasing at 5 T and is far
from the non-interacting saturation value for one manganese(III)
and one manganese(IV) ion (7 Nβ). This clearly indicates the
presence of a substantial antiferromagnetic intradimer coupling,
in agreement with the lowering of the χMT value at 300 K far
below the non-interacting limit.
However, unsatisfactory fits were obtained. Some observations
can be made.
Compound 2 is the first example of a dinuclear manganese(III/IV)
compound bridged by one methoxide and two pyrazole ligands,
although numerous mixed-valence manganese(III/IV) compounds
are found in the literature.41–72 The most common bridging
ligands between the two manganese ions are double
oxido-bridges,41,43–46,49–52,54,55,58,63,66–68,72 or double oxido-
bridges with additional bridging ligands, such as carboxylates,
etc.42,60,61,64–67,71,73 In this type of compound, the strength of the
magnetic exchange interaction depends mainly on two factors: the
type of bridging ligand and the bridge angle, Mn–O(R)–Mn (R =
H, Me). In dinuclear manganese(IV) compounds, a decrease of the
antiferromagnetic interaction occurs in the order oxido > hydroxido
> alkoxido bridges.48,74 The Mn–O distance plays also a key role
in the strength of the magnetic interaction.45,48,75 Compound 2a
contains a methoxide and two pyrazole ligands bridging the two
metal ions. Consequently, one would expect weaker antiferromag-
netic interactions than those observed for dinuclear oxido-bridged
compounds. In addition, the Mn–O–Mn angle has a value of 113°,
being far from the orthogonality, and the distance between the
manganese ions is 3.268 Å and it is in the order of other dinuclear
compounds.48,56 All of these compounds have a weak antiferro-
magnetic exchange (2 < J < −10 cm−1). Thus, it is clear that a
weak antiferromagnetic interaction should indeed exist between the
manganese(III/IV) ions, in spite of the unsatisfactory fit obtained.
The difficulty in the fitting procedure originates from the compe-
tition between the weak antiferromagnetic exchange and the crystal
field anisotropy of the manganese ions. The crystal field axes of
the manganese ions are not parallel, and therefore the anisotropy
value is comparable with the antiferromagnetic exchange, giving
rise to a complicated system.
−1
The plots χMT vs. T and χM vs. T of 3 are also presented
in the ESI (see Fig. S5†). At 300 K, the χMT product is
5.65 cm3 K mol−1, slightly lower than the expected value of
10254 | Dalton Trans., 2012, 41, 10249–10257
This journal is © The Royal Society of Chemistry 2012

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6 cm3 K mol−1 for two non-interacting manganese(III) ions
(assuming g = 2). From 300 to 71 K, the χMT product remains
almost constant and then decreases sharply to 2.98 cm3 K mol−1
at 2 K. The decrease at low temperature is attributed to the zero-
field-splitting of the manganese(III) ions and to the presence of
small intermolecular interactions. The magnetic susceptibility
data were fitted to the Hamiltonian of an antiferromagnetic
dinuclear compound with two spins S = 2.6 The best fitting of
the experimental data above 20 K gave J = −0.19 cm−1 and g =
1.95. We expect weak intra- and intermolecular magnetic
exchange interaction in view of the long distance between the
manganese ions (the Mn⋯Mn distance is 6.446 Å via the dia-
magnetic bridging atom, Na+; the shortest intermolecular
Mn⋯Mn separation is 7.828 Å).
1456(vs), 1436(m), 1392(s), 1339(m), 1279(s), 1260(s), 1245
(vs), 1186(m), 1174(s), 1116(vs), 1073(m), 1046(m), 1014(s),
988(w), 966(w), 938(w), 866(vs), 812(m), 794(s), 782(s), 752
(vs), 720(s), 691(s), 668(m), 655(s), 638(m), 608(m), 571(m),
538(w), 468(w), 398(m), 366(s), 350(m). ESI-MS (m/z): 442
[Mn(HphpzMe)2(CH3CN) + H+]+, 401 [Mn(HphpzMe)2 + H+]+,
216 [HphpzMe(CH3CN) + H+]+. UV/VIS (λmax/nm): 278
(π–π ligand), 391 (LMCT), 443 (LMCT) and 684 (d–d band).
[Mn2(phpzMe)2(HphpzMe)2(OCH3)]·2CH3OH (2)
A solution of sodium methoxide (242 mg, 4 mmol) and
H2phpzMe (392 mg, 2.24 mmol) in methanol was added to a
solution of Mn(OAc)2·4H2O (138 mg, 0.56 mmol) in methanol
and stirred, followed by the addition of nBu4NMnO4 (81 mg,
0.22 mmol) in methanol. The solution mixture resulted in the
formation of a grey precipitate, which was filtered off and dis-
carded. The filtrate was allowed to evaporate slowly affording
black crystals within a few days. Repeated analysis showed that
compound 2 rapidly exchanged lattice methanol molecules
by water molecules upon air exposure to form
[Mn2(phpzMe)2(HphpzMe)2(OCH3)]·4H2O (2a). Yield: 42%
(150 mg, 0.17 mmol). Anal. calcd for 2a (C41H45Mn2N8O9): C,
54.49; H, 5.02; N, 12.40. Found: C, 54.00; H, 4.19; N, 12.57. IR
(νmax/cm−1): 3057(br), 2924(w), 1597(m), 1575(w), 1558(m),
1463(s), 1456(vs), 1331(w), 1297(m), 1248(s), 1121(m), 1064
(w), 1034(m), 982(w), 850(s), 799(w), 755(vs), 726(m), 668(m),
618(m), 581(w), 524(s), 451(s), 427(s). UV/VIS (λmax/nm): 275
(π–π ligand), 383 (LMCT) and 563 (d–d band).
Conclusions
We presented the synthesis, crystal structure and magnetic pro-
perties of four new manganese(III) compounds and one mixed-
valence manganese(III/IV) compound. A remarkable feature is the
spontaneous formation of the formate ion, due to the oxidation
of methanol and/or methoxide in compounds 1 and 3. All com-
pounds are stabilized by hydrogen bonding and π–π interactions.
1, 4 and 5 are mononuclear manganese(III) compounds which
show very weak magnetic interactions. In compound 2 antiferro-
magnetic interactions are present between the manganese(III/IV)
ions. Despite the unsatisfactory fits, the exchange strength is
expected to be weak due to the geometrical parameters. Com-
pound 3 behaves as a paramagnet due to the diamagnetic sodium
ion that precludes significant interactions between the manga-
nese(III) ions.
Na{[Mn(HphpzPh)(phpzPh)(MeOH)2]2}(HCO2) (3)
The synthesis of compound 1 was repeated using H2phpzPh
(191 mg, 0.81 mmol) instead of the ligand H2phphzMe. This
results in the formation of a green crystalline solid which was
collected by filtration, washed with methanol and diethyl ether
and dried under air (153 mg, 0.123 mmol). Yield: 60%. Crystals
suitable for X-ray diffraction were obtained by slow evaporation
of the filtrate. Anal. calcd for 3 (C65H59Mn2N8NaO10): C, 62.70;
Experimental section
General remarks
Starting materials were purchased from Aldrich. All manipula-
13
tions were performed using materials as received. nBu4NMnO4
and the ligands76 3(5)-(2-hydroxyphenyl)-5(3)-methylpyrazole
(H2phpzMe) and 3(5)-(2-hydroxyphenyl)-5(3)-phenylpyrazole
(H2phpzPh) have been synthesized according to reported pro-
cedures. Safety note: Perchlorate salts and permanganate salts are
potentially explosive. Such compounds should be used in small
quantities and should be treated with utmost care at all times.
H, 4.78; N, 9.00. Found: C, 62.50; H, 4.54; N, 9.18. IR (νmax
/
cm−1): 3057(w), 2797(w), 1600(m), 1564(m), 1558(m), 1506(w),
1472(s), 1452(s), 1435(m), 1382(m), 1362(m), 1296(m), 1267
(m), 1241(s), 1127(s), 1042(m), 1024(m), 1002(m), 932(w), 854
(vs), 805(m), 767(s), 760(s), 748(vs), 739(vs), 706(vs), 688(vs),
663(w), 658(s), 622(vs), 586(m), 541(w), 521(w), 490(w),
472(w), 452 (s), 408(w), 374(m), 348(s), 323(m). ESI-MS (m/z):
H+]+,
762
[Mn(HphpzMe)2(H2phpzMe)(HCO2)] (1)
566
[Mn(HphpzPh)(phpzPh)(CH3CN)
+
[Mn(HphpzPh)3 + H+]+. UV/VIS (λmax/nm): 264 (π–π* ligand),
391 (LMCT) and 630 (d–d band).
A solution of H2phpzMe (142 mg, 0.81 mmol) and sodium
methoxide (90 mg, 1.66 mmol) in methanol was slowly added to
a solution of Mn(ClO4)2·6H2O (148 mg, 0.41 mmol) in metha-
nol. The resulting green solution was stirred for 10 min to give a
green precipitate. The green solid (87 mg, 0.14 mmol) was col-
lected by filtration, washed with methanol and diethyl ether and
dried under air. Yield: 34%. Slow evaporation of the filtrate
affords single-crystals for X-ray determination. Anal. calcd for 1
(C31H29MnN6O5): C, 60.00; H, 4.71; N, 13.54. Found: C,
59.36; H, 5.19; N, 13.56. IR (νmax/cm−1): 3383(m), 3279(w),
2974(w), 1600(m), 1564(m), 1558(vs), 1540(m), 1506(w),
[Mn(HphpzPh)2(EtOH)2]ClO4·2EtOH (4)
Reaction of Mn(ClO4)2·6H2O (144 mg, 0.40 mmol) in ethanol
with a solution of H2phpzPh (192 mg, 0.81 mmol) and sodium
methoxide (92 mg, 1.70 mmol) in ethanol affords a green pre-
cipitate that was collected by filtration, washed with ethanol and
diethyl ether and dried under air (73 mg, 0.09 mmol). Yield:
22%. Suitable crystals for X-ray analysis were obtained by
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 10249–10257 | 10255

View Online
diffusion of diethyl ether into the ethanolic solution of 4. Anal.
calcd for 4 (C38H46ClMnN4O10): C, 56.66; H, 5.28; N, 7.34.
Found: C, 56.28; H, 5.27; N, 7.31. IR (νmax/cm−1): 3372(m),
1602(m), 1562(m), 1558(vs), 1512(w), 1490(m), 1476(vs), 1456
(vs), 1418(m), 1398(w), 1304(m), 1285(m), 1271(m), 1243(vs),
1192(s), 1125(vs), 1063(vs), 1040(vs), 995(s), 940(w), 863(vs),
821(m), 751(vs), 709(m), 668(vs), 650(m), 623(s), 583(m),
550(m), 466(w), 452(m), 374(m). ESI-MS (m/z): 525
[Mn(HphpzPh)2]+, 566 [Mn(HphpzPh)2(CH3CN)]+, 762 [Mn-
(HphpzPh)3 + H+]+. UV/VIS (λmax/nm): 264 (π–π ligand), 332
(LMCT), 459 (LMCT) and 662 (d–d band).
diffractometer. Crystal and refinement data for 1, 2, 3, 4 and 5
are collected in Table 2. The intensity data were corrected for
Lorentz and polarization effects, and for absorption (multiscan
absorption correction).77 The structures were solved by Patterson
methods.78 The programs EvalCCD,79,80 DIRDIF96,81
SHELXS-9782 and SHELXL-9783 were used for data reduction,
structure solution and refinement, respectively. All non-hydrogen
atoms were refined with anisotropic displacement parameters.
All hydrogen atoms were placed at calculated positions and were
refined riding on the parent atoms. Geometric calculations and
molecular graphics were performed with the PLATON
package.84 CCDC 861014 (1), 858616 (2), 858617 (3), 858618
(4) and 858619 (5) contain the supplementary crystallographic
data for this paper.
[Mn(HphpzPh)2N3] (5)
A solution of H2phpzPh (133 mg, 0.56 mmol) and triethylamine
(14 mg, 0.14 mmol) in acetonitrile was added to a solution of
MnBr2·4H2O (80 mg, 0.28 mmol) in acetonitrile. Then sodium
azide (64 mg, 0.98 mmol) dissolved in methanol was added to
the reaction mixture. Slow evaporation of the resulting solution
afforded green crystals that were collected by filtration, washed
with Et2O and dried under vacuum. Yield: 64%. Anal. calcd for
5 (C30H22MnN7O2): C, 63.50; H, 3.91; N, 17.28. Found: C,
63.45; H, 4.64; N, 17.44. IR (νmax/cm−1): 3349(m), 3054(w),
2068(vs), 1600(s), 1563(m), 1554(s), 1510(m), 1495(m), 1487(m),
1472(vs), 1456(vs), 1418(s), 1353(m), 1305(s), 1279(m),
1244(vs), 1193(s), 1126(vs), 1068(w), 1052(s), 1036(s), 992(s),
916(m), 864(vs), 808(s), 798(m), 756(vs), 708(s), 692 (vs),
681(vs), 664(vs), 648(s), 618(w), 582(vs), 510(m), 451(s),
398(m), 373(m), 333(s). ESI-MS (m/z): 525 [Mn(HphpzPh)2]+,
Acknowledgements
This work was financially supported by the EC-RTN
“QuEMolNa” (No. MRTN-CT-2003-504880) and the ECNet-
work of Excellence “MAGMANet” (No. 515767-2). We also
acknowledge J. M. Martínez-Agudo (Universitat de València) for
the TGA measurements.
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10256 | Dalton Trans., 2012, 41, 10249–10257
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