μ-Chlorido-bridged dimanganese(II) complexes of the schiff base derived from [2+2] condensation of 2,6-diformyl-4-methylphenol and 1,3-bis(3- aminopropyl)tetramethyldisiloxane: Structure, magnetism, electrochemical behaviour, and catalytic oxidation of secondary alcohols

Article abstract of DOI:10.1002/ejic.201300969

The reaction of 2,6-diformyl-4-methylphenol with 1,3-bis(3-aminopropyl) tetramethyldisiloxane in the presence of MnCl₂ in a 1:1:2 molar ratio in methanol afforded a dinuclear μ-chlorido-bridged manganese(II) complex of the macrocyclic [2+2] condensation product (H₂L), namely, [Mn ₂Cl₂(H₂L)(HL)]Cl·3H₂O (1). The latter afforded a new compound, namely, [Mn₂Cl₂(H ₂L)₂][MnCl₄]·4CH₃CN·0. 5CHCl₃·0.4H₂O (2), after recrystallisation from 1:1 CHCl₃/CH₃CN. The co-existence of the free and complexed azomethine groups, phenolato donors, μ-chlorido bridges, and the disiloxane unit were well evidenced by ESI mass spectrometry and FTIR spectroscopy and confirmed by X-ray crystallography. The magnetic measurements revealed an antiferromagnetic interaction between the two high-spin (S = 5/2, g = 2) manganese(II) ions through the μ-chlorido bridging ligands. The electrochemical behaviour of 1 and 2 has been studied, and details of their redox properties are reported. Both compounds act as catalysts or catalyst precursors in the solvent-free low-power microwave-assisted oxidation of selected secondary alcohols, for example, 1-phenylethanol, cyclohexanol, 2- and 3-octanol, to the corresponding ketones in the absence of solvent. The highest yield of 72 % was achieved for 1-phenylethanol by using a maximum of 1 % molar ratio of catalyst relative to substrate. Copyright

Full text of DOI:10.1002/ejic.201300969

FULL PAPER
DOI:10.1002/ejic.201300969
μ-Chlorido-Bridged Dimanganese(II) Complexes of the
Schiff Base Derived from [2+2] Condensation of 2,6-Di-
formyl-4-methylphenol and 1,3-Bis(3-aminopropyl)tetra-
methyldisiloxane: Structure, Magnetism, Electrochemical
Behaviour, and Catalytic Oxidation of Secondary Alcohols
Mihaela Alexandru,[a] Maria Cazacu,[a] Adina Arvinte,[b]
Sergiu Shova,[a] Constantin Turta,[a] Bogdan C. Simionescu,*[a,c]
Anatolie Dobrov,[d] Elisabete C. B. A. Alegria,[e,f]
Luísa M. D. R. S. Martins,[e,f] Armando J. L. Pombeiro,*[e] and
Vladimir B. Arion*[d]
Keywords: Manganese / Template synthesis / Macrocycles / Schiff bases / Oxidation
The reaction of 2,6-diformyl-4-methylphenol with 1,3-bis(3-
aminopropyl)tetramethyldisiloxane in the presence of MnCl2
in a 1:1:2 molar ratio in methanol afforded a dinuclear μ-
magnetic measurements revealed an antiferromagnetic in-
teraction between the two high-spin (S = 5/2, g = 2) manga-
nese(II) ions through the μ-chlorido bridging ligands. The
chlorido-bridged manganese(II) complex of the macrocyclic electrochemical behaviour of 1 and 2 has been studied, and
[2+2] condensation product (H2L), namely, [Mn2Cl2(H2L)-
(HL)]Cl·3H2O (1). The latter afforded a new compound,
namely, [Mn2Cl2(H2L)2][MnCl4]·4CH3CN·0.5CHCl3·0.4H2O
(2), after recrystallisation from 1:1 CHCl3/CH3CN. The co-
existence of the free and complexed azomethine groups,
phenolato donors, μ-chlorido bridges, and the disiloxane unit
were well evidenced by ESI mass spectrometry and FTIR
spectroscopy and confirmed by X-ray crystallography. The
details of their redox properties are reported. Both com-
pounds act as catalysts or catalyst precursors in the solvent-
free low-power microwave-assisted oxidation of selected sec-
ondary alcohols, for example, 1-phenylethanol, cyclohexa-
nol, 2- and 3-octanol, to the corresponding ketones in the
absence of solvent. The highest yield of 72% was achieved
for 1-phenylethanol by using a maximum of 1% molar ratio
of catalyst relative to substrate.
Introduction
searchers owing to the structural diversity of the resulting
complexes.[1] Dinuclear manganese(II) complexes are of
special interest because of their magnetic properties and as
biologically relevant small-molecule model compounds; for
example, a Mn catalase[2] has applications in materials sci-
ence and industrial homogeneous catalysis.[3,4] The interest
The coordination chemistry of manganese with N- and/
or O-containing ligands has received attention from re-
[a] “Petru Poni” Institute of Macromolecular Chemistry,
Aleea Gr. Ghica Voda 41 A, Iasi 700487, Romania
http://www.icmpp.ro/staff/bogdansimionescu.html
[b] “Petru Poni” Institute of Macromolecular Chemistry, Centre of in bridged polynuclear complexes stems from their signifi-
Advanced Research in Bionanoconjugates and Biopolymers,
cance for understanding the mechanisms of magnetic inter-
Aleea Gr. Ghica Voda 41 A, Iasi 700487, Romania
actions between metal ions and the synthesis of single-mo-
lecule magnets (SMMs).[5] The behaviour of these com-
plexes is mainly dependent on the coordination geometry
and binding mode of the ligands as well as on the oxidation
state of the manganese ion.[1] Therefore, a large variety of
ligands were designed and prepared that can maintain the
manganese centres in close proximity or separate them
markedly.[6] A number of Schiff bases derived from 2,6-di-
formyl-4-methylphenol (dfmp) and various amines with
NNOO, NON and NOO binding sites have been re-
ported.[7–18] The phenolate-containing ligands are useful
models for biological metal binding sites and have the ca-
pacity to form metal complexes with interesting magnetic
exchange, redox and catalytic properties.[1f,1g,19] Phenolates
are also of interest in the design of compartmental ligands
[c] “Gh. Asachi” Technical University of Iasi, Department of
Natural and Synthetic Polymers,
Bd. Mangeron 71A, 700050 Iasi, Romania
E-mail: bcsimion@icmp.ro
http://www.icmp.ro/staff/bogdansimionescu.html
[d] Institute of Inorganic Chemistry of the University of Vienna,
Währinger Strasse 42, 1090 Vienna, Austria
E-mail: vladimir.arion@univie.ac.at
http://anorg-chemie.univie.ac.at/magnoliaPublic/Research/
Bioinorganic-chemistry/Researchers/Arion.html
[e] Centro de Química Estrutural, Instituto Superior Técnico,
Universidade de Lisboa,
Lisbon, Portugal
E-mail: pombeiro@ist.utl.pt
http://cqe.ist.utl.pt/personal_pages/pages/armando_pombeiro.php
[f] Chemical Engeneering Departmental Area, ISEL,
R. Conselheiro Emídio Navarro,
1959-007 Lisboa, Portugal
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300969.
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able to bind two identical or different metal ions in close oxidation of secondary alcohols to ketones with tert-butyl
proximity.[20–25] Dinuclear μ-chlorido-bridged manga- hydroperoxide as oxidant at 80 °C in the absence of solvent
nese(II) compounds with N/O donor ligands are relatively with microwave (MW) irradiation with 1 and/or 2 as cata-
rare and only a few six-coordinate manganese(II) com- lysts or catalyst precursors.
pounds have been structurally investigated.[26–29] Only some
of these compounds have also been magnetically characteri-
sed.[1,26,29b] Catalytic studies on manganese complexes are
Results and Discussion
also scarce.[30]
Quite recently, we reported a new salen-type Schiff base
derived from 1,3-bis(3-aminopropyl)tetramethyldisiloxane
and substituted 2-hydroxybenzaldehydes. This Schiff base
forms copper(II) complexes with a large 12-membered cen-
tral chelate ring and exhibits catalytic activity in the oxi-
dation of primary and secondary alcohols in the presence
of air as oxidant.[31] The choice of the siloxane-containing
amine is based on the high flexibility and extremely low
polarity of the siloxane moiety.[32] The difference in the po-
larity between the tetramethyldisiloxane moieties and the
amine groups or the azomethine groups that emerge in con-
densation reactions confers somewhat amphiphilic charac-
ter to Schiff bases formed and to their metal complexes.
Herein, we report the synthesis of two new μ-chlorido-
bridged dimanganese(II) complexes with macrocyclic Schiff
bases that resulted from the [2+2] condensation of 2,6-di-
formyl-4-methylphenol with 1,3-bis(3-aminopropyl)tet-
ramethyldisiloxane (H2L in Scheme 1) in the presence of
MnCl2·4H2O, namely, [Mn2Cl2(H2L)(HL)]Cl·3H2O (1) and
Synthesis and Characterisation of Metal Complexes
The reaction of 2,6-diformyl-4-methylphenol with 1,3-
bis(3-aminopropyl)tetramethyldisiloxane and MnCl2·4H2O
in a molar ratio of 1:1:2 in methanol afforded a dimanga-
nese(II) complex with macrocyclic [2+2] ligands and two μ-
chlorido bridging ligands as shown in Scheme 1. The
recrystallisation of [Mn2Cl2(H2L)(HL)]Cl·3H2O (1) from
1:1 chloroform/acetonitrile produced another complex
[Mn2Cl2(H2L)2][MnCl4]·4CH3CN·0.5CHCl3·0.4H2O (2) in
47% yield with 1 as the limiting reagent. By recrystallisation
of 1 to form 2, a marked change of the Mn/ligand/Cl ratio
from 2:2:3 in 1 to 3:2:6 in 2 occurs. In addition, the trans-
formation of 1 into 2 is accompanied by a change of the
protonation state of one of the ligands from monodepro-
tonated to neutral.
The formation of a macrocyclic ligand was confirmed by
the positive-ion ESI mass spectra of 1 and 2, which showed
the presence of a peak with m/z = 1686 owing to the
[Mn2Cl2(H2L)(HL)]+ ion. The isotopic pattern of this peak
fits well the theoretical isotopic distribution expected for
this ion. Other signals observed at m/z = 842, 806 and 403.5
could be attributed to the ions [MnCl(H2L)]+, [Mn(HL)]+
and [Mn(H2L)]2+, respectively. In line with these data, in
the FTIR spectra of both 1 and 2, strong absorption bands
at 1540 and 1601 cm–1 owing to aromatic C=C and C=N
stretching vibrations were observed, and no C=O signals at
ca. 1700 cm–1 were observed.[33] A broad band at ca.
3400 cm–1 indicates the presence of cocrystallised water mo-
lecules and OH···N groups. The presence of the dimethyl-
[Mn2Cl2(H2L)2][MnCl4]·4CH3CN·0.5CHCl3·0.4H2O
(2),
their solid-state magnetic behaviour and redox properties.
Being interested in the metal-catalysed mild oxidative func-
tionalisation of alkanes and alcohols, we also report the
Scheme 1. The reaction to form the dinuclear manganese(II) Schiff
base complex [Mn2Cl2(H2L)(HL)]Cl·3H2O (1); in complex 2 the/
N* atom is protonated and, therefore, the overall charge of the
complex cation is 2+.
Figure 1. UV/Vis spectra for 1 (blue trace) and 2 (magenta trace)
in chloroform.
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siloxane unit is evidenced by strong absorptions at 2953 and dinate Mn2+ ions joined into a dinuclear complex by two
2928 cm–1, which correspond to νas and νs (C–H from Si– μ-chlorido bridging ligands and has a Mn1···Mn2 separa-
CH3), respectively. The bands at 446 and 498 cm–1 in 1 tion of 3.786(1) Å. The four Mn–Cl bond lengths for the
might be assigned to ν(Mn–N) and ν(Mn–O), respec- bridging μ-chlorido groups fall within the range 2.599(2)–
tively.[4,34,35] The band of ν(Mn–Cl) was observed at 2.618(2) Å, which is in a good agreement with that found
341 cm–1 and is characteristic of μ-chlorido-bridged dinu- for similar bis(μ-chlorido) dimanganese(II) complexes
clear complexes.[34] The electronic spectrum of 1 exhibited (2.406–2.661 Å).[1,26,28,39–44] Each manganese atom displays
intense bands at 256 and 429 nm and a shoulder at 274 nm a distorted octahedral cis geometry (Table 1) and is coordi-
(Figure 1). We assign the strong absorption band at 256 nm nated by the neutral H2L (for Mn1) or monodeprotonated
to the π–π* transition of the phenol rings, whereas that at HL– (for Mn2) as tetradentate macrocyclic ligands. A per-
429 nm is attributed to a combination of π–π* transitions of spective view of the Mn1 coordination site showing the con-
the azomethine chromophore and a ligand-to-metal charge- formation of H2L is depicted in Figure 3. The coordination
transfer transition.[21,36,37] The charge transfer may be from environment of Mn1 is formed by the N2O2Cl2 donor
the p orbital of the phenolic oxygen atom to the metal d atoms. The other two imine nitrogen atoms from the H2L
orbitals.[35,38] Analogously, the UV/Vis absorption spectrum ligand are not coordinated to the metal centre and act as
of 2 shows intense bands at 256 and 429 nm and a shoulder proton donors in hydrogen bonding with the phenolato
at 274 nm (Figure 1).
groups to support the binding mode of the ligand. The geo-
metric parameters of these H bonds are given in the caption
to Figure 2. The coordination of a tetradentate N2O2 Schiff
base ligand results in the formation of two six-membered
chelate rings and two 16-membered metallocycles each con-
taining a disiloxane unit. The coordination environment of
the Mn2 atom (Figure 2) is very similar to that for Mn1,
X-ray Crystallography
An X-ray diffraction study has revealed that the crystal
structure of 1 consists of a [Mn2Cl2(H2L)(HL)]+ dinuclear
cationic species, Cl– counteranions and cocrystallised water
molecules in
a
1:1:3 ratio. The structure of
[Mn2Cl2(H2L)(HL)]+ (Figure 2) is built up of two six-coor-
Table 1. Selected bond lengths [Å] and angles [°] for 1 and 2.
Atom1–Atom2
1
2
Mn1–O1
Mn1–O2
Mn1–N2
Mn1–N3
Mn1–Cl1
Mn1–Cl2
Mn2–O5
Mn2–O6
Mn2–N6
Mn2–N7
Mn2–Cl1
Mn2–Cl2
Si1–O3
Si2–O3
Si3–O4
Si4–O4
Si5–O8
Si6–O8
Si7–O7
Si8–O7
2.084(5)
2.095(5)
2.280(5)
2.301(4)
2.599(2)
2.606(2)
2.093(4)
2.086(4)
2.284(4)
2.274(4)
2.617(2)
2.618(2)
1.592(6)
1.640(6)
1.635(7)
1.655(7)
1.638(5)
1.629(5)
1.666(5)
1.641(5)
2.089(4)
2.095(4)
2.292(5)
2.273(5)
2.558(2)
2.570(2)
2.078(4)
2.088(4)
2.262(5)
2.273(5)
2.604(2)
2.595(2)
1.634(6)
1.627(6)
1.627(6)
1.622(6)
1.619(6)
1.635(6)
1.638(7)
1.637(7)
Atom1–Atom2–Atom3
O1–Mn1–O2
O1–Mn1–N2
O1–Mn1–N3
O2–Mn1–N3
N2–Mn1–N3
O5–Mn2–O6
O6–Mn2–N7
O5–Mn2–N7
O6–Mn2–N6
O5–Mn2–N6
N7–Mn2–N6
Si1–O3–Si2
178.6(2)
176.0(2)
80.8(2)
95.0(2)
81.0(2)
85.4(2)
177.7(2)
96.7(2)
81.3(2)
83.0(2)
97.9(2)
86.5(2)
149.0(4)
152.3(4)
147.8(6)
146.2(4)
81.3(2)
99.0(2)
80.5(2)
84.8(2)
178.0(2)
96.2(2)
82.3(2)
81.3(2)
97.3(2)
89.2(2)
157.6(4)
145(1)
Figure 2. X-ray diffraction structure of a dinuclear cationic com-
plex [Mn2Cl2(H2L)(HL)]+ in the crystal structure of 1. Three intra-
molecular H bonds N1–H···O1 [N1–H 0.88 Å, H···O1 1.93 Å,
N1···O1 2.601(7) Å, N1–H···O1 131.9°], N4–H···O2 [N4–H 0.88 Å,
H···O2 1.91 Å, N4···O2 2.585(7) Å, N4–H···O2 132.1°] and N8–
H···O5 [N8–H 0.88 Å, H···O5 1.91 Å, N8···O5 2.596(6) Å, N8–
H···O5 133.1°] are also shown.
Si3–O4–Si4
Si7–O7–Si8
Si5–O8–Si6
147.7(3)
155.9(3)
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but the Schiff base ligand is monodeprotonated (HL–), the tonated azomethine groups act as proton donors in H
phenolic proton migrates to the noncoordinated azo- bonds with phenolate oxygen atoms. The parameters of the
methine group, as observed for H2L and analogous ligands corresponding H bonds are given in the caption to Figure 4.
in related systems.[45–49] The protons attached to the C=N The doubly positive charge of the dicationic complex is
groups in both H2L and HL– were located from a difference counterbalanced by the anion [MnCl4]2–. The average Mn–
Fourier map, and their positional parameters were con- Cl distances and Cl–Mn–Cl angles in the dianionic units
strained accordingly. Thus, the charge balance in 1 corre- are 2.359(2) Å and 109.45(8)°, respectively, which fall
sponds to the formulation [Mn2Cl2(H2L)(HL)]Cl. Four within the parameters found in related compounds.[50–53]
negative charges provided by three chlorido ligands and one
monodeprotonated ligand HL– are counterbalanced by the
four positive charges of two Mn2+ ions. The geometrical
features of the coordinated ligands confirm the azomethine
character of the C–N interatomic bonds in the 2,6-diformyl-
4-methylphenol moieties [C–Nav 1.299(8) Å] when com-
pared with those from the 1,3-bis(3-aminopropyl)-
tetramethyldisiloxane moiety [C–Nav 1.462(8) Å]. These pa-
rameters along with the C–O bond lengths [C–Oav
1.282(7) Å] are in line with those reported for chemically
related protonated (neutral) Schiff base ligands.[45–49]
Figure 4. X-ray diffraction structure of a dinuclear complex
[Mn2Cl2(H2L)2]2+ in the crystal structure of 2. Four intramolecular
H bonds N1–H···O1 [N1–H 0.88 Å, H···O1 1.93 Å, N1···O1
2.601(7) Å, N1–H···O1 131.9°], N4–H···O2 [N4–H 0.88 Å, H···O2
1.91 Å, N4···O2 2.585(7) Å, N4–H···O2 132.1°], N5ϪH···O6
[N5ϪH 0.88 Å, H···O6 1.904 Å, N5···O6 2.588(6) Å], N8–H···O5
[N8–H 0.88 Å, H···O5 1.91 Å, N8···O5 2.596(6) Å, N8–H···O5
133.1°] are also shown.
Thermogravimetric Analysis
Thermogravimetric studies on 1 and 2 were performed in
the 25 to 900 °C temperature range under a nitrogen atmo-
Figure 3. ORTEP plot with thermal ellipsoids at the 40% prob-
ability level showing the coordination of H2L to Mn1 in the crystal
structure of 1. Irrelevant hydrogen atoms are omitted for clarity.
sphere (Figure S1). The initial weight loss of 10% in the
temperature range 120–300 °C is attributed to the loss of
water and solvent molecules. At higher temperature, a fur-
A view of the dinuclear dicationic complex in 2 is shown ther weight loss of ca 40% was observed to 450 °C and is
in Figure 4. The structure of the complex cation resembles attributed to the pyrolysis of the ligand. The large amount
that of the dimanganese(II) species found in 1. One feature of residue is presumably caused by the formation of metal
of note is that both macrocyclic tetradentate ligands are oxides (MnO and SiO2).[54] The analysis confirmed the
coordinated to Mn1 and Mn2 as neutral ligands H2L to presence of cocrystallised solvent molecules and indicated
form the dicationic complex [Mn2Cl2(H2L)2]2+. All the pro- the optimal temperature for its removal.
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Magnetic Measurements
of total spin S* with the following energies E: S*(E) = 0(0),
1(–J), 2(–3J), 3(–6J), 4(–10J), 5(–15J). The least-squares fit
of the experimental data for 1 and 2 gave the following set
of parameters: g = 2.018(2), J = –0.453(5) cm–1 for 1, and g
= 2.074(1), J = –0.708(3) cm–1 for 2. The agreement factors
Σ(χTcalc – χTobs)2/Σ(χTobs)2 are then 5.3ϫ 10–4 and 8ϫ 10–5
for 1 and 2, respectively.
The values of exchange magnetic parameters J =
–0.453(5) cm–1 for 1 and J = –0.708(3) cm–1 for 2 are com-
parable to those calculated for [Mn2(2-pyridinemethanol)4-
(μ-Cl)2Cl2][44] with J = –0.36 cm–1 and [Mn2(tacud)2(μ-
Cl)2Cl2] (tacud = 1,4,8-triazacycloundecane)[56] with J =
–1.81 cm–1.
The magnetic susceptibility for a powdered sample of 1
(MnII2) was measured in the temperature range 2–300 K
under an applied field of 0.1 T. The value of the χT product
is 8.70 cm3 mol–1 K at 300 K and corresponds to two uncou-
pled high-spin (S1 = S2 = 5/2, g = 2) manganese(II) ions
(Figure 5).[55] This value remains almost unchanged until
ca. 100 K and than continuously decreases with tempera-
ture and reaches a value of 2.09 cm3 mol–1 K at 2 K. The
temperature dependence of χT indicates an antiferromag-
netic interaction between the manganese(II) ions mainly
through the μ-chlorido bridging ions. The χT value for 2
(MnII + MnII) is ca. 13.81 cm3 mol–1 K at 300 K and has
2
similar temperature dependence: at 2 K χT is
5.54 cm3 mol–1 K, which indicates dominant antiferromag-
netic MnII–MnII interactions within the dinuclear entity of
2 (Figure 5). This temperature dependence of the magnetic
properties for both complexes is in good agreement with
their crystal structures discussed above. The molar magnetic
susceptibility was computed by using the expression derived
from the spin-only isotropic exchange Hamiltonian: H =
–JS1S2 (S1 = S2 = 5/2) and fitted to the experimental data
[Equation (1)]. No correction was made for temperature-in-
dependent paramagnetism or paramagnetic impurities.
Electrochemical Behaviour of 1 and 2
The redox properties of 1 and 2 as well as, for compara-
tive purposes, [Me4N]2[MnCl4] and benzyltriethylammo-
nium chloride have been investigated by cyclic voltammetry
(CV) at a Pt electrode (d = 1 mm) in a 0.2 m [nBu4N][BF4]/
CH3CN solution at 25 °C.
Cyclic voltammograms of the di-μ-chlorido-bridged di-
manganese(II) complexes 1 and 2 exhibit one two-electron
I
ox
irreversible oxidation process (Figure 6, wave Iox) at Ep
= 0.74 and 0.78 V versus the standard calomel electrode
(SCE) for 1 and 2, respectively. These are followed, at
higher potential, by a second overall three-electron oxi-
II
ox
dation (Figure 6, wave IIox) at
E
= 1.13 and 1.18 V
1/2
(1)
versus SCE for 1 and 2, respectively. For 2, a third one-
For 2, the additive value for magnetic susceptibility is electron irreversible oxidation wave (Figure 6, wave IIIox for
ox
assumed [Equation (2)].
2) is observed at IIIEp = 1.63 V versus SCE.
For both 1 and 2, the first two-electron anodic process
χ(2) = χd + χ(MnII)
(2)
(Iox) is believed to correspond to the MnII/MnII ǞMnIII
/
MnIII oxidation of the two MnII centres (Scheme 2).[57]
Exhaustive controlled-potential electrolyses to measure
the number of electrons involved in each redox process were
not possible owing to fast electrode passivation. However,
the involvement of two electrons in the first anodic process
(Iox) was deduced from the observed ratio (2.8 = n3/2; n is
the number of electrons involved, i.e., two in this case) of
the current functions ipC–1v–1/2 (ip = peak current, C = con-
centration, v = scan rate) calculated for Iox of 2 (or 1) and
for the first one-electron MnII ǞMnIII anodic process of
[Me4N]2[MnCl4] under the same experimental conditions.
The cyclic voltammogram of [Me4N]2[MnCl4] (see below)
displays two one-electron oxidation waves, the first of which
is partially reversible, attributed to the consecutive
I
ox
MnII ǞMnIII ǞMnIV processes at E1/2 = 1.19 and IIEp
= 1.61 V versus SCE. The observed second overall three-
electron partially reversible oxidation wave (IIox) in 1 at
Figure 5. Temperature dependence of the molar values of χT for 1
ox = 1.13 V versus the SCE results from the overlap of
II
E
1/2
{MnII2} and 2 {MnII + MnII}. The solid line is the best least-
2
two different oxidation waves, one assigned to the one-elec-
tron irreversible oxidation of the chloride counterion and
squares fit to the experimental magnetic data.
The second term in Equation (2) describes the magnetic the second to the two-electron MnIIIMnIII/MnIVMnIV oxi-
susceptibility of the (MnIICl4)2– ion, which can be expressed dation process (Scheme 2).
by the Curie–Weiss equation (S = 5/2). The coupling be-
The involvement of the chloride ion oxidation in wave
tween the two paramagnetic S = 5/2 ions generates six states IIox for 1 is supported by the independently measured value
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Figure 6. Cyclic voltammogram initiated by the anodic sweep at a Pt electrode of a solution of (a) 1 (1 mm) and (b) 2 (1.4 mm) in 0.2 m
[nBu4N][BF4]/MeCN (v = 200 mV/s).
Scheme 2. Oxidation pathways for 1.
(Epox = 1.1 V versus SCE) of the irreversible oxidation wave
The occurrence of both (MnII/MnIII) oxidations at iden-
of benzyltriethylammonium chloride under the same experi- tical potentials at the oxidation wave Iox, that is, without
mental conditions and confirmed by the increase of the cur- differentiation of the potentials of the MnIIMnII/MnIIMnIII
rent intensity of the wave IIox upon the addition of this and MnIIMnIII/MnIIIMnIII redox pairs, indicates that the
chloride salt to a solution of 1.
mixed-valence MnIIMnIII species is rather unstable and the
For 2, the observed second overall three-electron par- μ-chlorido bridging ligands, in our complexes, isolate the
tially reversible oxidation wave (Figure 6, IIox) also con- metal atoms from each other electronically, which results in
cerns the involvement of two different oxidation processes, an undetectable (by CV) electronic interaction. Similarly, a
the single electron oxidation of the counterion [MnCl4]2– rather weak Mn–Mn interaction was proposed for the μ-
(see above) and the MnIIIMnIII/MnIVMnIV two-electron dichlorido-bridged dimanganese(II) compound [{MnCl-
oxidation.
(bpea)}2(μ-Cl)2] [bpea = N,N-bis(2-pyridylmethyl)ethyl-
The irreversible third anodic process observed for 2 (Fig- amine].[30] However, stronger interactions have been re-
ure 6, b, IIIox) is attributed to the MnIII ǞMnIV oxidation ported for other μ-chlorido-bridged species such as
of the counterion [MnCl4]2– (vide supra). Moreover, the in- [{MnCl(dipa)}2(μ-Cl)2] [dipa = dipyridylmethylamine][58]
volvement of the oxidation of [MnCl4]2– in waves IIox and and (Et3NH)2[{Mn(TPA)}2(μ-Cl)2](ClO4)4,[57] which exhi-
IIIox for 2 was confirmed by the addition of [NMe4]2- bit distinct MnIIMnII/MnIIMnIII and MnIIMnIII
[MnCl4], which resulted in the increase of such oxidation MnIIIMnIII oxidations waves.
waves. The difference between the oxidation potentials of
/
ox
I
ox
the IIox and Iox waves in 1 and 2 (IIE1/2 – Ep ≈ 0.4 V)
is comparable to that (0.45 V) observed in (Et3NH)2-
[{Mn(TPA)}2(μ-Cl)2](ClO4)4 [TPA = tris(2-pyridylmethyl)-
amine] for the same redox pairs MnIVMnIV/MnIIIMnIII and
MnIIIMnIII/MnIIMnII.[57]
Catalytic Oxidation of Secondary Alcohols
Complexes 1 and 2 have been tested as catalysts (or cata-
lyst precursors) for the oxidation of common secondary
alcohols (mainly 1-phenylethanol) to the respective ketones
with tert-butyl hydroperoxide (tBuOOH, TBHP; 2 equiv.)
as oxidising agent under typical conditions of 80 °C, micro-
wave (MW) irradiation, 3 h reaction time and in the absence
of any added solvent (Scheme 3 for the oxidation of 1-phen-
ylethanol). Selected results are summarised in Table 2.
Upon scan reversal, after the first oxidation wave Iox (for
1 and 2), one irreversible reduction process (Ired) is detected
red
at Ep ≈ –1.4 V versus SCE and is conceivably caused by
the reduction of a new species, denoted by (MnIIIMnIII)Ј in
Scheme 2, formed at the anodic wave (Iox). This indicates
that the dinuclear MnIII/MnIII species, formed at the first
anodic process in 1 and 2, is unstable. The partial reversibil-
ity of the second oxidation wave (IIox) is preserved even at
low scan rates, and the current function ipC–1v–1/2 does not
vary appreciably within the studied scan rate range
(50 mVs–1 to 4 Vs–1), which is consistent with the involve-
ment of a constant number of electrons.
Scheme 3. Solvent-free oxidation of 1-phenylethanol to aceto-
phenone.
Eur. J. Inorg. Chem. 2014, 120–131
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Table 2. Oxidation of selected secondary alcohols with 1 or 2 as catalyst precursors.[a]
Entry
Catalyst
Substrate
Catalyst amount
[mol-% vs. substrate]
TON[b]
Yield[c]
[%]
1
2
3
4
5
6
7
8
1
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
cyclohexanol
0.02
0.04
0.1
0.2
0.4
0.8
1.0
1.4
0.4
0.4
0.1
0.1
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.02
0.1
0.2
0.4
0.8
0.8
238
185
122
113
82
78
72
66
83
13
46
99
5
121
51
94
107
21
89
44
310
99
5
8
1
1
12
22
36
66
72
74
30
5
1
1
1
1
1
9[d]
10[e]
11[f]
12[g]
13[h]
14[i]
15[j]
16[k]
17[l]
18
1
1
1
15
38
2
1
1
1
57
34
20
42
39
28
15
6
10
18
37
12
7
1
1
1
1
19
20
21
22
1
2-octanol
3-octanol
1
2
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenyethanol
1-phenylethanol
1-phenylethanol
2
23
2
187
81
15
24
2
25[m]
26[m]
[Me4N]2[MnCl4]
MnCl2
9
[a] Reaction conditions: 5 mmol of substrate, 1–70 μmol of catalyst (0.02–1.4 mol-% vs. substrate), 10 mmol of TBHP (2 equiv.), 80 °C,
3 h reaction time, microwave irradiation (10 W). [b] Turnover number = number of mol of product per mol of catalyst. [c] Mol of ketone
product per mol of alcohol. [d] 20 mmol of TBHP (4 equiv.). [e] H2O2 30% aqueous solution instead of TBHP. [f] T = 50 °C. [g] T =
90 °C. [h] In the presence of Ph2NH (10 mmol). [i] In the presence of TEMPO (5 mol-% vs. substrate). [j] In MeCN. [k] In H2O. [l] In
K2CO3 aqueous solution (1 m). [m] Included for comparative purposes.
The effect of the amount of catalyst 1 was studied for
the 1-phenylethanol oxidation (Table 2, Entries 1–8; Fig-
ure 7). An increase from 1 (0.02 mol-% vs. substrate) to
70 μmol (1.4 mol-% vs. substrate) resulted in a yield en-
hancement from 5 to 74%. However, beyond 50 μmol of
catalyst, the yield remained almost unchanged. As ex-
pected, the increase of the catalyst amount resulted in a
decrease of the turnover number (TON; mol of product/
mol of catalyst) from 238 to 66 as the amount of catalyst
changed from 0.02 to 1.4 mol-% versus substrate (Table 2,
Entries 1 and 8). The use of more oxidant does not lead to
a better conversion (Table 2, Entries 5 and 9). Blank tests
(in the absence of any catalyst) were performed under com-
mon reaction conditions and no significant conversion was
observed (Ͻ0.5%).
Microwave irradiation (MW) can provide a more ef-
ficient synthetic method than conventional heating and al-
Figure 7. Effect of the amount of catalyst 1 (0.02–1.4 mol-% vs.
substrate) on the yield and TON for the oxidation of 1-phenyl-
lows the attainment of similar yields in shorter times, im-
ethanol to acetophenone, 80 °C, 3 h.
proved yields and/or selectivities.[59–63] A favourable effect
of MW is also observed in this study, even with the low
power of 10 W, as reported for other systems.[60–62] Hence, yield enhancement, as once the desired temperature is
for example, only 6% of product yield was obtained after achieved, the power decreases automatically to values be-
3 h reaction under the same conditions as those of Table 2, low 10 W.
Entry 5 (36% yield) with conventional heating (oil bath).
The use of hydrogen peroxide (30% aqueous solution)
After 15 h reaction, 58 and 17% yields were obtained for instead of TBHP results in a large decrease in the yield from
MW and conventional heating, respectively. Higher micro- 36 to 5% (Table 2, Entries 5 and 10), in accord with the
wave power (from 10 to 40 W) does not show a significant expected decomposition of H2O2 under the reaction condi-
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tions (80 °C). The temperature is an important factor, as the at 80 °C in the presence of the same metal molar amount
reaction proceeds more efficiently at higher temperatures. (40 μmol of Mn) of the Mn compound led to much lower
Attempts to perform the oxidation of 1-phenylethanol in yields of acetophenone after 3 h in the cases of [Me4N]2-
the presence of 1 at room temperature failed, whereas the [MnCl4] and MnCl2 (12 and 7%, respectively; Table 2, En-
reaction conducted at 50 °C resulted in a marked acetophen- tries 25 and 26) than for 1 and 2 (36 and 37%, Table 2,
one yield drop relative to that at 80 °C (from 36% at 80 °C Entries 5 and 24). The similar results obtained for 1 and 2
to 15% at 50 °C; Table 2, Entries 5 and 11). The ketone and the lower performance of [Me4N]2[MnCl4] show that
yield does not increase significantly above 80 °C (from 36% the [MnCl4]2– counterion present in 2 does not have a domi-
at 80 °C to 38% at 90 °C; Table 2, Entries 5 and 12).
nant influence on the catalytic activity.
Performing the reaction in acetonitrile (5 mL) does not
The addition to the reaction mixture of Ph2NH or
change significantly the yield, for example, the 1-phenyl- CBrCl3, well known oxygen- or carbon-radical traps,
ethanol oxidation in the presence of 1 (Table 2, Entries 5 respectively,[66,67] led to a large yield drop of over 90%,
and 15), whereas the addition of the same volume of water compared to the reaction under the same conditions
results in a significant yield reduction from 36 to 20% (20 μmol, 80 °C, MW, 3 h) in the absence of a radical trap.
(Table 2, Entry 16; Figure 8) under the same reaction condi- This result suggests the generation of oxygen and carbon
tions. On the contrary, the use of a basic 1 m solution of radicals in the reaction, which are trapped by those radical
K2CO3 (Table 2, Entry 17; Figure 8) results in a significant scavengers. A possible mechanism[64h,64i,65a,65d] for this sys-
increase of the conversion of the alcohol to the ketone com- tem may involve coordination of the alcohol PhCH(OH)-
pared to the reaction in water (from 20% in water only to Me (with deprotonation to form the alkoxide ligand) and
42% in basic solution). The role of basic additives, which 2,2,6,6-tetramethylpiperidyl-1-oxyl (TEMPO) radical, fol-
facilitate the deprotonation of the alcohol was demon- lowed by H transfer from the former to the latter to form
strated previously.[64,65]
the O-ligated radical PhC·(O)Me– and TEMPOH. Intramo-
lecular electron-transfer from coordinated PhC·(O)Me– to
the MnII ion leads to the formation of the ketone PhC(O)-
Me and MnI ion, which is reoxidised to MnII by O2/
tBuOOH. The TEMPO radical is also regenerated upon
oxidation of TEMPOH.
To increase the activity of 1 in solvent-free MW-assisted
peroxidative oxidation of 1-phenylethanol, we have investi-
gated the influence of TEMPO, a nitroxyl radical that pro-
motes the oxidation catalysis of alcohols.[60,65a,65c,65d,68–71]
Recently, some of us reported on several efficient systems
involving copper(II) triazapentadienate,[60,63] bis- and tris-
pyridyl amino and imino thioether Cu and Fe complexes[61]
for the MW-assisted oxidation of secondary alcohols to the
corresponding ketones, as well as the in situ generated cop-
per(II)-diimine complexes toward the TEMPO-mediated
oxidation of benzylic alcohols in aqueous media[65a] and
CuII complexes containing arylhydrazones from methylene-
active nitriles toward the selective oxidation of primary and
secondary alcohols to the corresponding carbonyl com-
pounds.[60] Other manganese-based systems were applied
Figure 8. Influence of different solvents (MeCN, H2O) and addi-
tives (1m K2CO3, TEMPO, radical traps) on the yield of aceto-
phenone from oxidation of 1-phenylethanol.
Other secondary alcohols were also tested, in particular for alcohol oxidation, namely, silica-supported manganese
cyclohexanol, and similar results were obtained, that is, the dioxide (MnO2) in the oxidation of benzyl alcohol under
oxidation of cyclohexanol yielded 39% of cyclohexanone solvent-free conditions,[72] which yielded 88% of aceto-
(Table 2, Entry 18), which is comparable to the 36% yield phenone under MW for 20 s. However, this system required
(Table 2, Entry 5) obtained for 1-phenylethanol under the excess MnO2 relative to the substrate (5:1 molar ratio),
same reaction conditions (20 μmol, 80 °C, MW, 3 h).
whereas in the present study we have achieved 72% yield
Linear aliphatic alcohols, namely 2-octanol and 3-oc- by using a maximum of 1% molar ratio of catalyst relatively
tanol, lead to lower yields under similar reaction condi- to substrate. Furthermore, mixed Mn–Cu or Mn–Co
tions, as reported in other cases.[60,63,65a] Thus, the oxi- nitrates[70] and heterogeneous Cu–Mn mixed oxides[71] in
dation of 2-octanol and 3-octanol yield 28 and 18% of the combination with TEMPO have been employed for the se-
respective ketones, 2-octanone and 3-octanone, in 3 h.
lective aerobic oxidation of a variety of alcohols to the cor-
The relevance of the H2L and HL ligands on the catalytic responding aldehydes and ketones under mild conditions.
activity of 1 and 2 is shown by the catalytic performances In our case, a significant yield increase was observed for the
of [Me4N]2[MnCl4] and MnCl2 in the oxidation of 1-phen- 1-phenylethanol oxidation in the presence of 1 (from 36%
ylethanol compared with those of 1 and 2 under the same in the absence of TEMPO to 57% in its presence; Table 2,
reaction conditions. The oxidation of that alcohol (5 mmol) Entries 5 and 14).
Eur. J. Inorg. Chem. 2014, 120–131
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(10 mL). Orange crystals suitable for X-ray diffraction data collec-
tion formed in one week, yield 0.28 g, 47.0% (with 1 as the limiting
reagent). The product was collected by filtration and dried in vacuo
at 125 °C for 4 h to give solvent-free [Mn2Cl2(H2L)2][MnCl4].
C76H128Cl6Mn3N8O8Si8 (1884.09): calcd. C 48.45, H 6.85, N 5.95;
Conclusions
Two new μ-chlorido-bridged dimanganese(II) complexes
of the Schiff base derived from 2,6-diformyl-4-meth-
ylphenol and 1,3-bis(3-aminopropyl)tetramethyldisiloxane
in the presence of MnCl2, [Mn2Cl2(H2L)(HL)]Cl·3H2O (1)
crystallised from methanol or [Mn2Cl2(H2L)2][MnCl4]·
4CH3CN·0.5CHCl3·0.4H2O (2) crystallised from 1 in a
chloroform/acetonitrile mixture, have been obtained. The
complexes were well-characterised by elemental analysis,
ESI mass spectrometry and spectroscopic methods. The
FTIR spectroscopy and ESI mass spectra emphasise the co-
existence of the free and complexed azomethine and phen-
olato groups, μ-chlorido bridges and siloxane unit. The
structures of the complexes have been determined by single-
crystal X-ray diffraction. The magnetic measurements re-
vealed two high-spin (S = 5/2, g = 2) MnII ions and an
found C 48.10, H 6.98, N 5.71. FTIR (KBr pellet): ν = 3437 (m),
˜
2953 (s), 2924 (m), 1653 (vs), 1628 (s), 1609 (m), 1541 (vs), 1495
(s), 1456 (m), 1449 (m), 1412 (w), 1385 (w), 1385 (s), 1362 (m),
1329 (w), 1254 (s), 1231 (m), 1182 (m), 1065 (s), 988 (m), 878 (m),
837 (s), 820 (s), 781 (s), 745 (w), 708 (w), 679 (w), 669 (w), 565 (w),
498 (w), 474 (w), 446 (w) cm–1. UV/Vis (CHCl3): λmax (ε, m–1cm–1)
= 256 (8.03ϫ 104), 274 (6.76ϫ 104), 429 (4.45ϫ 104) nm.
Physical Measurements: FTIR spectra were recorded with a Bruker
Vertex 70 FTIR spectrometer in transmission mode at room tem-
perature with a resolution of 2 cm–1 and 32 scans in the window
400–4000 cm–1 for mid-IR (MIR) and with a resolution of 2 cm–1
and 64 scans in the region 180–670 cm–1 for far-IR (FIR). UV/Vis
absorption spectra were recorded with an Analytik Jena SPE-
antiferromagnetic interaction between them through bridg- CORD 200 spectrophotometer by using a quartz cuvette with a
1 cm path length. Elemental analyses (carbon, hydrogen, nitrogen)
were performed by using a Perkin–Elmer CHNS 2400 II elemental
analyser. Magnetic measurements were performed on microcrystal-
line samples of 1 and 2 with a Quantum Design superconducting
quantum interference device (SQUID) magnetometer (MPMS-
XL). Variable-temperature (2–300 K) direct current (dc) magnetic
susceptibility was measured under an applied magnetic field of
0.1 T. All data were corrected for the contribution of the sample
holder, and diamagnetism of the complexes was estimated from
Pascal’s constants.[73]
ing μ-chlorido ligands. Complexes 1 and 2 act as catalysts
or catalyst precursors for the oxidation of selected second-
ary alcohols, such as 1-phenylethanol, cyclohexanol, 2- and
3-octanol, to the respective ketones with tert-butyl hydro-
peroxide as oxidant at 80 °C with low-power microwave ir-
radiation and a moderate reaction time. The effect of dif-
ferent factors (catalyst and/or oxidant amount, tempera-
ture, solvent, presence of a base, etc.) on the efficiency of
catalytic conversion of 1-phenylethanol into acetophenone
has been elucidated. A maximum yield of 72% was
achieved with a 1% molar ratio of 1 relative to substrate.
Crystallographic Structure Determination: The X-ray diffraction
data for 1 and 2 were collected with an Oxford Diffraction XCALI-
BUR E diffractometer equipped with an Eos CCD detector with
graphite-monochromated Mo-Kα radiation. The crystals were
placed at 40 mm from the CCD detector. The unit-cell determi-
nation and data integration were performed with the CrysAlis
package of Oxford Diffraction.[74] All structures were solved by
direct methods by using SHELXS-97 and refined by full-matrix
least-squares on Fo2 with SHELXL-97[75] with anisotropic displace-
ment parameters for non-hydrogen atoms. All H atoms attached to
carbon atoms were inserted in idealised positions (dCH = 0.96 Å) by
using the riding model with their isotropic displacement parameters
fixed at 120% of that of their riding atom. Positional parameters
of the H atoms attached to N atoms were obtained from difference
Fourier syntheses and verified by the geometric parameters of the
corresponding hydrogen bonds. Most of the atoms from disiloxane
moieties in the two structures as well as the counteranions Cl– in 1
showed quite large thermal ellipsoids; therefore, disorder models in
combination with the available tools (PART, DFIX, and SADI) of
SHELXL-97 were applied to better fit the electron density. The
Si, O and N atoms with fractional site occupancies were refined
isotropically. The main crystallographic data together with refine-
ment details are summarised in Table 3.
Experimental Section
Materials: All chemicals were obtained from commercial sources
and used as received. 2,6-Diformyl-4-methylphenol (99% purity,
m.p. 129–131 °C) was purchased from Polivalent-95. 1,3-Bis(3-
aminopropyl)tetramethyldisiloxane was received from Alfa Aesar,
and MnCl2·4H2O was purchased from Sigma–Aldrich.
Synthesis of Complexes
[Mn2Cl2(H2L)(HL)]Cl·3H2O (1):
A solution of 1,3-bis(3-ami-
nopropyl)tetramethyldisiloxane (0.61 g, 2.45 mmol) in methanol
(5 mL) was added dropwise to a solution of 2,6-diformyl-4-meth-
ylphenol (0.40 g, 2.44 mmol) in methanol (7.5 mL) and dichloro-
methane (2 mL), and the resulting mixture was heated to reflux for
2 h. Then, a solution of MnCl2·4H2O (0.97 g, 4.9 mmol) in meth-
anol (9.5 mL) was added, and the mixture was heated to reflux for
24 h. The solution was filtered, and the resulting solution was al-
lowed to stand at room temperature to produce orange crystals,
which were separated after 3 d, washed with cold methanol and
dried in air, yield 0.75 g, 17.0%. C76H133Cl3Mn2N8O11Si8
(1775.84): calcd. C 51.40, H 7.55, N 6.31; found C 51.29, H 7.37,
CCDC-942438 (for 1) and -942439 (for 2) contain the supplemen-
N 6.10. FTIR (KBr): ν = 3441 (s), 2953 (s), 2870 (m), 1653 (vs),
tary crystallographic data for this paper. These data can be ob-
˜
1623 (s), 1609 (m), 1540 (vs), 1493 (s), 1456 (m), 1409 (m), 1362 tained free of charge from The Cambridge Crystallographic Data
(m), 1254 (s), 1231 (m), 1184 (m), 1069 (s), 987 (m), 838 (s), 783 Centre via www.ccdc.cam.ac.uk/data_request/cif.
(s), 707 (m), 629 (w), 590 (w), 566 (m), 552 (w), 544 (w), 536 (w),
Thermogravimetric measurements (TGA) were performed with a
528 (w), 521 (w), 498 (m), 491 (m), 482 (m), 474 (m), 458 (w), 429
Mettler Toledo TGASDTA851e derivatograph in a 20 mLmin–1 ni-
(w), 421 (w), 414(w) cm–1. UV/Vis (CHCl3): λmax (ε, m–1cm–1) =
trogen stream in the temperature range 25–900 °C and at a heating
256 (6.63ϫ 104), 274 (5.42ϫ 104), 429 (3.43ϫ 104) nm.
rate of 10 Kmin–1. The operational parameters were kept constant
[Mn2Cl2(H2L)2][MnCl4]·4CH3CN·0.5CHCl3·0.4H2O (2): Complex for both samples to obtaining comparable data. The magnetic
1 (0.5 g, 0.28 mmol) was dissolved in a 1:1 CHCl3/CH3CN mixture
susceptibility (χ) of samples 1 (MnII2) and 2 (MnII + MnII) has
2
Eur. J. Inorg. Chem. 2014, 120–131
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flame ionisation detector (FID) and a capillary column (DB-WAX,
column length: 30 m; internal diameter: 0.32 mm). The injection
temperature was 240 °C. The initial temperature of the column was
maintained at 120 °C for 1 min, then increased by 10 °C/min to
200 °C and held at this temperature for 1 min. Helium was used as
the carrier gas.
Table 3. Crystallographic data, details of data collection and struc-
ture refinement for 1 and 2.
1
2
Empirical formula C76H133Cl3Mn2N8O11Si8 C84.5H141.3Cl7.5Mn3N12O8.4Si8
Fw
Space group
a [Å]
b [Å]
c [Å]
1775.85
P21/n
2115.21
P21/c
Supporting Information (see footnote on the first page of this arti-
cle): TGA curves for 1 and 2.
16.359(5)
24.271(5)
28.770(5)
106.611(5)
10946(4)
4
17.5399(8)
29.9176(9)
25.0021(6)
92.122(3)
13110.9(8)
4
β [°]
V [Å3]
Z
Acknowledgments
ρcalcd. [gcm–3]
1.078
1.072
This research was financially supported by the European Regional
Development Fund, Sectoral Operational Programme “Increase of
Economic Competitiveness”, Priority Axis 2 (SOP IEC-A2-O2.1.2-
2009-2, ID 570, COD SMIS-CSNR: 12473, Contract 129/2010-PO-
LISILMET) and by the Foundation for Science and Technology
(FCT), Portugal, Project Pest-OE/QUI/UI0100/2013. One of the
authors (A. A.) acknowledges the financial support of the Euro-
pean Social Fund (Cristofor I. Simionescu Postdoctoral Fellowship
Programme, ID POSDRU/89/1.5/S/55216), Sectoral Operational
Programme Human Resources Development 2007Ϫ2013.
Crystal size [mm] 0.25ϫ 0.25ϫ 0.10
0.30ϫ 0.10ϫ 0.10
150
0.555
T [K]
200
μ [mm–1]
0.439
θmin/θ max [°]
R1[a] [IϾ2σ(I)]
wR2[b] (all data)
GOF[c]
1.83/25.00
0.0937
0.3001
1.035
3.00/25.03
0.0988
0.2952
1.096
2
2 2
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR2 = {Σ[w(Fo – Fc ) ]/Σ[w-
(Fo2)2]}1/2. [c] GOF = {Σ[w(Fo – Fc ) ]/(n – p)}1/2; n is the number
of reflections and p is the total number of parameters refined.
2
2 2
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been measured in the 2–300 K range with a SQUID magneto-
meter.
The electrochemical experiments were performed with an EG&G
PAR 273A potentiostat/galvanostat connected to a personal com-
puter through a GPIB interface. Cyclic voltammetry (CV) studies
were undertaken in 0.2 m [nBu4N][BF4]/CH3CN at a platinum disc
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Received: July 28, 2013
Published Online: November 22, 2013
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