Monomeric, trimeric, and tetrameric transition metal complexes (Mn, Fe, Co) containing N,N-bis(2-pyridylmethyl)-2-aminoethanol/-ate: preparation, crystal structure, molecular magnetism and oxidation catalysis

Article abstract of DOI:10.1039/c1dt10028a

The reaction of N,N-bis(2-pyridylmethyl)-2-aminoethanol (bpaeOH), NaSCN/NaN₃, and metal (M) ions [M = Mn(ii), Fe(ii/iii), Co(ii)] in MeOH, leads to the isolation of a series of monomeric, trimeric, and tetrameric metal complexes, namely [Mn(bpaeOH)(NCS)₂] (1), [Mn(bpaeO)(N ₃)₂] (2), [Fe(bpaeOH)(NCS)₂] (3), [Fe ₄(bpaeO)₂(CH₃O)₂(N₃) ₈] (4), [Co(bpaeOH)(NCS)₂] (5), and [Co ₃(bpaeO)₂(NO₃)(N₃) ₄](NO₃) (6). These compounds have been investigated by single crystal X-ray diffractometry and magnetochemistry. In complex 1 the Mn(ii) is bonded to one bpaeOH and two thiocyanate ions, while in complex 2 it is coordinated to a deprotonated bpaeO^- and two azide ions. The oxidation states of manganese ions are 2+ for 1 and 3+ for 2, respectively, indicating that the different oxidation states depend on the type of binding anions. The structures of monomeric iron(ii) and cobalt(ii) complexes 3 and 5 with two thiocyanate ions are isomorphous to that of 1. Compounds 1, 2, 3, and 5 exhibit high-spin states in the temperature range 5 to 300 K. 4 contains two different iron(iii) ions in an asymmetric unit, one is coordinated to a deprotonated bpaeO^-, an azide ion, and a methoxy group, and the other is bonded to three azide ions and two oxygens from bpaeO^- and a methoxy group. Two independent iron(iii) ions in 4 form a tetranuclear complex by symmetry. 4 displays both ferromagnetic and antiferromagnetic couplings (J = 9.8 and -14.3 cm^-1) between the iron(iii) ions. 6 is a mixed-valence trinuclear cobalt complex, which is formulated as Co^III(S = 0)-Co^II(S = 3/2)-Co^III(S = 0). The effective magnetic moment at room temperature corresponds to the high-spin cobalt(ii) ion (～4.27 μ_B). Interestingly, 6 showed efficient catalytic activities toward various olefins and alcohols with modest to excellent yields, and it has been proposed that a high-valent Co^V-oxo species might be responsible for oxygen atom transfer in the olefin epoxidation and alcohol oxidation reactions. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10028a

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PAPER
Monomeric, trimeric, and tetrameric transition metal complexes (Mn, Fe, Co)
containing N,N-bis(2-pyridylmethyl)-2-aminoethanol/-ate: preparation,
crystal structure, molecular magnetism and oxidation catalysis†
Jong Won Shin,a Sankara Rao Rowthu,a Min Young Hyun,b Young Joo Song,b Cheal Kim,*b Bong Gon Kimc
and Kil Sik Min*a
Received 7th January 2011, Accepted 25th March 2011
DOI: 10.1039/c1dt10028a
The reaction of N,N–bis(2-pyridylmethyl)-2-aminoethanol (bpaeOH), NaSCN/NaN3, and metal (M)
ions [M = Mn(II), Fe(II/III), Co(II)] in MeOH, leads to the isolation of a series of monomeric, trimeric,
and tetrameric metal complexes, namely [Mn(bpaeOH)(NCS)2] (1), [Mn(bpaeO)(N3)2] (2),
[Fe(bpaeOH)(NCS)2] (3), [Fe4(bpaeO)2(CH3O)2(N3)8] (4), [Co(bpaeOH)(NCS)2] (5), and
[Co3(bpaeO)2(NO3)(N3)4](NO3) (6). These compounds have been investigated by single crystal X-ray
diffractometry and magnetochemistry. In complex 1 the Mn(II) is bonded to one bpaeOH and two
thiocyanate ions, while in complex 2 it is coordinated to a deprotonated bpaeO- and two azide ions. The
oxidation states of manganese ions are 2+ for 1 and 3+ for 2, respectively, indicating that the different
oxidation states depend on the type of binding anions. The structures of monomeric iron(II) and
cobalt(II) complexes 3 and 5 with two thiocyanate ions are isomorphous to that of 1. Compounds 1, 2,
3, and 5 exhibit high-spin states in the temperature range 5 to 300 K. 4 contains two different iron(III)
ions in an asymmetric unit, one is coordinated to a deprotonated bpaeO-, an azide ion, and a methoxy
group, and the other is bonded to three azide ions and two oxygens from bpaeO- and a methoxy group.
Two independent iron(III) ions in 4 form a tetranuclear complex by symmetry. 4 displays both
ferromagnetic and antiferromagnetic couplings (J = 9.8 and -14.3 cm-1) between the iron(III) ions. 6 is a
mixed-valence trinuclear cobalt complex, which is formulated as CoIII(S = 0)–CoII(S = 3/2)–CoIII(S =
0). The effective magnetic moment at room temperature corresponds to the high-spin cobalt(II) ion
(~4.27 mB). Interestingly, 6 showed efficient catalytic activities toward various olefins and alcohols with
modest to excellent yields, and it has been proposed that a high-valent CoV–oxo species might be
responsible for oxygen atom transfer in the olefin epoxidation and alcohol oxidation reactions.
and size.2 Recently, a zigzag tetranuclear iron(III) complex with
bridged azide and methoxy ions and N,N-bis(2-pyridylmethyl)-2-
Introduction
aminoethanolate ions has been reported.3 The complex has shown
interesting magnetic properties, indicative of both ferromagnetic
and antiferromagnetic interactions. The tetradentate N,N-bis(2-
pyridylmethyl)-2-aminoethanolate (bpaeO-) is a multifunctional
ligand, in which it has a potential N3 donor as bis(picoly)amine
and an oxygen atom acting as proton donor/acceptor. As relating
to bpaeO-, studies on the metal complexes with organic deriva-
tives including N,N-bis(2-pyridylmethyl)-2-aminoethanolate moi-
ety have been performed with objectives such as sequence-specific
cleavage of DNA, synthesis of biodegradable polymers, and the
development of quadruplex DNA binders.4 Furthermore, the
tetradentate ligand as a capping molecule has been used in the
formation of a bis(m-alkoxo)-bridged dinuclear iron(III) complex
as a biomimetic model for the intradiol-cleaving dioxygenase
enzymes and the synthesis of oxorhenium(V) and copper(II) com-
plexes for mechanistic studies and cleaving an activated phosphate
diester.5
The design and preparation of mono- and polynuclear transition
metal complexes has enabled the understanding and development
of molecule-based magnetic materials and the structural eluci-
dation of metalloenzymes in biological systems, as well as the
development of catalysts for hydrocarbon oxidations.1 It has been
recognized that the metal centers and the binding ligands as anions
or capping molecules may play important roles in the formation
of desirable compounds, due to the molecular shape, charge,
aDepartment of Chemistry Graduate School, Kyungpook National Univer-
sity, Daegu, 702-701, Republic of Korea. E-mail: minks@knu.ac.kr
bDepartment of Fine Chemistry, Seoul National University of Science
and Technology, Seoul, 139-743, Republic of Korea. E-mail: chealkim@
snut.ac.kr
cDepartment of Chemistry Education, Gyeongsang National University,
Jinju, 660-701, Republic of Korea
† Electronic supplementary information (ESI) available. CCDC reference
numbers 749014, 806664–806668. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1dt10028a
5762 | Dalton Trans., 2011, 40, 5762–5773
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The versatile azide ion for dimers, clusters, and polymers
exhibiting significant magnetic properties such as ferro- and
antiferromagnetic interactions has been extensively used because it
may induce interesting magnetic couplings by two different bond-
ing modes, i.e. end-on (m-1,1, ferromagnetic) and end-to-end (m-
1,3, antiferromagnetic).6 Additionally, mixed-valence compounds
have been studied in relation to the characterization of magnetic
exchange interactions and valence tautomerism.7 For example,
the cobalt-based [(TPyA)CoIII(DBQ∑3-)CoIII(TPyA)](BF4)3 com-
plex is identified as [(TPyA)CoII(DBQ2-)CoIII(TPyA)](BF4)3 over
end, terminal, bridging)9 was used to obtain transition metal
-
complexes (1, 3, 5). It has been reported that the azide (N3 )
ion is a stronger oxidizing agent than the thiocyanate (NCS-)
ion in aqueous and mixed medium.10 Monomeric metal(II)
octahedral complexes (1, 3, 5) are formed using the NCS- ion,
while polynuclear complexes (4, 6) including the mononuclear
-
Mn(III) compound 2 are obtained using the N3 ion. Herein we
report the synthesis, structure and magnetic properties of 1–6. In
addition, the catalytic activities toward hydrocarbon oxidation of
mixed-valence complex 6 will be also described.
room temperature [TPyA = tris(2-pyridylmethyl)amine; DBQ∑3-
=
2,5-di-tert-butyl-3,6-dihydroxy-1,4-benzoquinonate].8 The mixed-
valence cobalt compound is obtained through electron transfer
depending on the temperature. Likewise, such compounds can
potentially also be used as catalysts for oxidation.
Experimental
General
All chemicals used in the synthesis and oxidation reactions were of
reagent grade and used without further purification. N,N-bis(2-
pyridylmethyl)-2-aminoethanol (bpaeOH) was prepared accord-
ing to the literature procedure.11 UV-Vis absorption spectra were
recorded with a SCINCO S-2100 spectrophotometer. Infrared
spectra were recorded with a Thermo Fisher Scientific IR200
spectrophotometer ( 1 cm-1) using KBr disks. Elemental analyses
were carried out using a Fissons/Carlo Erba EA1108 instrument.
Magnetic susceptibilities were measured in an applied field of
In
a
continuation of our previous work on the
tetranuclear Fe(III) complex with N,N-bis(2-pyridylmethyl)-2-
aminoethanolate and azide ions, we have prepared a series of novel
transition metal complexes with Mn/Fe/Co and bpaeOH as well
as NCS-/N3 ions; [Mn(bpaeOH)(NCS)2] (1), [Mn(bpaeO)(N3)2]
(2), [Fe(bpaeOH)(NCS)2] (3), [Fe4(bpaeO)2(CH3O)2(N3)8] (4),
[Co(bpaeOH)(NCS)2] (5), and [Co3(bpaeO)2(NO3)(N3)4](NO3)
(6) (Scheme 1). For comparison to the azide ion, the thiocyanate
(NCS-) ion which has various bonding modes (end-on, end-to-
-
Scheme 1 Schematic presentation of the formation of transition metal complexes 1–6.
The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 5762–5773 | 5763
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5000 Oe between 2 or 5 and 300 K on a Quantum Design
MPMS superconducting quantum interference device (SQUID)
magnetometer. Diamagnetic corrections were made [226.8 (1),
258.3 (2), 219.3 (3), 724.8 (4), 224.2 (5), and 542.6 ¥ 10-6 (6) emu
mol-1] by using Pascal’s constants. Susceptibility data of 1–6 were
simulated with the julX program package for magnetic models.12
The zero-field Mo¨ssbauer spectrum of 4 was recorded at room
temperature. The spectrometer was calibrated by collecting the
Mo¨ssbauer spectra of a standard a-Fe foil at room temperature.
The Mo¨ssbauer parameters were obtained by a least-square fitting
program assuming Lorentzian line shapes. Product analysis for the
oxidation reaction was performed on either a Hewlett–Packard
5890 II Plus gas chromatograph interfaced with a Hewlett–
Packard Model 5989B mass spectrometer or a Donam Systems
6200 gas chromatograph equipped with an FID detector using a
30-m capillary column (Hewlett–Packard, HP-1, HP-5, and Ultra
2).
3426, 3248; n(SCN-) 2077, 2059; n(C–H) 2924; n(C–O) 1052, 1018;
n(C–N) 1604, 1346, 1320, 1290; n(C–C) 1442.
[Fe4(bpaeO)2(CH3O)2(N3)8] (4). To an MeOH solution
(10 mL) of Fe(NO3)3·9H2O (994 mg, 2.46 mmol) was added
an MeOH solution (10 mL) of N,N-bis(2-pyridylmethyl)-2-
aminoethanol (bpaeOH, 300 mg, 1.23 mmol) and an MeOH
solution (10 mL) of sodium azide (560 mg, 8.61 mmol). The
mixture was stirred for 1 h at room temperature. The color
turned reddish–brown and then reddish–brown solid formed. The
precipitate was collected by filtration and washed with methanol
and diethyl ether and dried in air (yield: 524 mg, 77%). Reddish–
brown needle-shaped crystals of 4 suitable for X-ray crystal
analysis were obtained by layering of the MeOH solution of
Fe(NO3)3·9H2O on the MeOH solution of bpaeOH with NaN3 for
several days. Anal. calcd for C30H42Fe4N30O6: C, 31.55; H, 3.71;
N, 36.79. Found: C, 31.77; H, 3.59; N, 37.00. FT–IR (KBr, cm-1):
-
n(N3 ) 2077, 2061, 2028; n(C–H) 3082, 2924; n(C–O) 1048, 1026;
n(C–N) 1606, 1346, 1320, 1290; n(C–C) 1435, 1384.
Syntheses of compounds
[Co(bpaeOH)(NCS)2] (5). To an MeOH solution (5 mL) of
Co(NO3)2·6H2O (60 mg, 0.21 mmol) was added an MeOH solution
(5 mL) of bpaeOH (50 mg, 0.21 mmol) and an MeOH solution
(4 mL) of sodium thiocyanate (33 mg, 0.41 mmol). The mixture
was stirred for 1 h at room temperature, and subsequently heated to
reflux for 30 min. After cooling to room temperature, the resulting
mixture was allowed to stand for 2 days. Purple crystals were
formed and collected by filtration and washed with methanol and
diethyl ether and dried in air (yield: 31 mg, 35%). Anal. calcd
for C16H17CoN5OS2: C, 45.93; H, 4.10; N, 16.74; S, 15.33. Found:
C, 45.70; H, 4.09; N, 17.00; S, 14.67. FT–IR (KBr, cm-1): n(OH)
3436, 3230; n(SCN-) 2087, 2071; n(C–H) 2905; n(C–O) 1051, 1020;
n(C–N) 1605, 1287; n(C–C) 1443.
[Mn(bpaeOH)(NCS)2] (1). To an MeOH solution (5 mL) of
Mn(NO3)2·4H2O (104 mg, 0.41 mmol) was added an MeOH
solution (5 mL) of bpaeOH (100 mg, 0.41 mmol) and an MeOH
solution (5 mL) of sodium thiocyanate (67 mg, 0.82 mmol).
The mixture was stirred for 1 h at room temperature. The
resulting solution was allowed to stand at ambient temperature
until pale yellow crystals had formed, which were collected by
filtration, washed with diethyl ether, and air dried (yield: 107
mg, 63%). Anal. calcd for C16H17MnN5OS2: C, 46.37; H, 4.13;
N, 16.90; S, 15.48. Found: C, 46.34; H, 4.13; N, 16.92; S, 14.90.
FT–IR (KBr, cm-1): n(OH) 3423, 3253; n(SCN-) 2068, 2056;
n(C–H) 2962, 2899; n(C–O) 1060, 1016; n(C–N) 1603; n(C–C)
1440.
[Co3(bpaeO)2(NO3)(N3)4](NO3) (6). To an MeOH solution
(5 mL) of Co(NO3)2·6H2O (120 mg, 0.41 mmol) was added an
MeOH solution (5 mL) of bpaeOH (50 mg, 0.21 mmol) and an
MeOH solution (7 mL) of sodium azide (54 mg, 0.82 mmol). The
mixture was stirred for 1 h at room temperature. The color of
the mixture became a dark red and then a dark red solid formed.
The solid was collected by filtration and washed with methanol
and diethyl ether and dried in air (yield: 115 mg, 63%). Dark red
needle-shaped crystals of 6 suitable for X-ray crystal analysis were
obtained by layering of the MeOH solution of Co(NO3)2·6H2O on
the MeOH solution of bpaeOH with NaN3 for several days. Anal.
calcd for C28H32Co3O8N20: C, 35.25; H, 3.38; N, 29.39. Found: C,
[Mn(bpaeO)(N3)2] (2). To an MeOH solution (5 mL) of
Mn(NO3)2·4H2O (104 mg, 0.41 mmol) was added an MeOH
solution (5 mL) of bpaeOH (100 mg, 0.41 mmol) and an MeOH
solution (5 mL) of sodium azide (54 mg, 0.82 mmol). The resulting
solution was stirred for 1 h at room temperature and then allowed
to stand in a refrigerator for 3 days. Dark gray crystals were formed
and collected by filtration and washed with cold methanol and
diethyl ether and dried in air (yield: 70 mg, 45%). Anal. calcd for
C14H16MnN9O: C, 44.11; H, 4.23; N, 33.06. Found: C, 43.93; H,
4.21; N, 32.69. FT–IR (KBr, cm-1): n(N3 ) 2066, 2044; n(C–H)
-
3052, 2957, 2837; n(C–O) 1062, 1040; n(C–N) 1601, 1286; n(C–C)
1444.
35.14; H, 3.38; N, 29.53. FT–IR (KBr, cm-1): n(N3 ) 2068, 2029;
-
-
n(NO3 ) 1384, 1363; n(C–H) 3089, 3042, 2858; n(C–O) 1063, 1036;
n(C–N) 1612, 1282; n(C–C) 1447.
[Fe(bpaeOH)(NCS)2] (3). To an MeOH solution (4 mL) of
Fe(BF4)2·6H2O (69 mg, 0.21 mmol) was added an MeOH solution
(4 mL) of bpaeOH (50 mg, 0.21 mmol) and an MeOH solution
(4 mL) of sodium thiocyanate (33 mg, 0.41 mmol). The mixture
was stirred for 1 h at room temperature. The color of the mixture
turned orange–yellow and then orange–yellow solid formed. The
solid was collected by filtration and washed with methanol and
diethyl ether and dried in air (yield: 38 mg, 44%). Orange–yellow
crystals of 3 suitable for X-ray crystal analysis were obtained by
diffusion of diethyl ether into the MeOH solution of 3. Anal. calcd
for C16H19FeN5O2S2: C, 44.35; H, 4.42; N, 16.16; S, 14.80. Found:
C, 44.21; H, 3.90; N, 16.18; S, 14.55. FT–IR (KBr, cm-1): n(OH)
X-Ray crystallographic data collection and refinement
R
R
Crystals of 1–6 were mounted on a CryoLoopꢀ with Paratoneꢀ
oil. Intensity data for all structures were collected with a
Bruker APEX CCD-based diffractometer (Korea Basic Science
Institute, Chonju Branch) and using Mo-Ka radiation (l =
˚
0.71073 A, graphite monochromator) at 173(2)–293(2) K. The
raw data were processed to give structure factors using the Bruker
SAINT program and corrected for Lorentz and polarization
effects.13 The intensity data of 2–3 and 5–6 were corrected for
absorption using the SADABS program with multi-scan data
5764 | Dalton Trans., 2011, 40, 5762–5773
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(Tmin/Tmax = 0.861 for 2, 0.805 for 3, 0.925 for 5, and 0.756 for 6).14
The crystal structures were solved by direct methods,15 and refined
by full-matrix least-squares refinement using the SHELXL-97
computer program.16 The positions of all non-hydrogen atoms
were refined with anisotropic displacement factors. All hydrogen
atoms were placed using a riding model, and their positions were
constrained relative to their parent atoms using the appropriate
HFIX command in SHELXL-97, except the hydrogen of hydroxyl
mmol), deuterated benzyl alcohol (0.12 mmol), cobalt complex
6 (0.001 mmol), and solvent (CH3CN, 1 mL) was added m-
chloroperoxybenzoic acid (MCPBA, 0.035 mmol). The mixture
was stirred for 10 min at room temperature. The reaction was
monitored by GC–MS analysis of 20 mL aliquots withdrawn
periodically from the reaction mixture. All reactions were run at
least in triplicate and the average KIE values are presented.
-
groups in 3 and 5. Due to the disorder, one of two NO3 anions
Competitive reactions of styrene and para-substituted styrenes for
for 6 was treated with the SQUEEZE routine in the program
PLATON17 and was omitted from the final refinements as atomic
contributions, but are included in the computation of intensive
properties. The crystallographic data and the result of refinements
of 1–6 are summarized in Table 1.
Hammett plot
To a mixture of styrene (0.02 mmol) and para(X)-substituted
styrene (0.02 mmol, X = –OCH3, –CH3, –Cl, –F, and –CN),
cobalt complex 6 (0.001 mmol), and solvent (CH3CN, 1 mL) was
added m-chloroperoxybenzoic acid (MCPBA, 0.035 mmol). The
mixture was stirred for 10 min at room temperature. The amounts
of styrenes before and after reactions were determined by GC. The
relative reactivities were determined using the following equation:
kx/ky = log(Xf /Xi)/log(Yf /Yi) where Xi and Xf are the initial
and final concentrations of substituted styrenes and Yi and Yf are
the initial and final concentrations of styrene.18
Catalytic hydrocarbon oxidations upon treatment of 6 with
m-chloroperoxybenzoic acid
To a mixture of substrate (0.1 mmol), cobalt complex 6
(0.001 mmol), and solvent (CH3CN, 1 mL) was added m-
chloroperoxybenzoic acid (MCPBA, 0.035 mmol). The mixture
was stirred for 10 min at room temperature. Each reaction
was monitored by GC–MS analysis with 20 mL aliquots being
withdrawn periodically from the reaction mixture. All reactions
were run at least in triplicate and the average product yields
are presented. The product yields are based on MCPBA. In
the competitive reaction of cis-2-hexene and trans-2-hexene, the
amount of each substrate was 0.05 mmol.
Results and discussion
Synthesis and characterization
The reaction of one equivalent of bpaeOH and 2 equiv of
NaSCN with M(NO3)2·xH2O or Fe(BF4)2·6H2O (M = Mn
(x = 4), Co (x = 6)) in MeOH solution at room tempera-
ture affords the mononuclear complexes, [MnII(bpaeOH)(NCS)2]
(1, pale yellow), [FeII(bpaeOH)(NCS)2] (3, orange–yellow), and
[CoII(bpaeOH)(NCS)2] (5, purple), in good yield, respectively. In
the infrared (IR) spectra of 1, 3, and 5, broad bands in the range
of 3253–3423, 3248–3426, and 3230–3436 cm-1, respectively, are
displayed in which these correspond to O–H peaks. The strong
peaks in the 2056–2068 (1), 2059–2077 (3), and 2071–2087 cm-1 (5)
Kinetic isotope effect study for the oxidation of benzyl alcohol by 6
and m-chloroperoxybenzoic acid
In order to improve the accuracy for measuring the amount
of deuterated benzyl alcohol product, a 1 : 6 mixture of benzyl
alcohol and deuterated benzyl alcohol was used. The reaction
conditions were as follows: To a mixture of benzyl alcohol (0.02
Table 1 Summary of the crystallographic data for 1–6
1
2
3
4c
5
6
Formula
Mr
C16H17MnN5OS2
414.41
C14H16MnN9O
381.30
C16H17FeN5OS2
415.32
C30H38Fe4N30O4
1106.30
C16H17CoN5OS2
418.40
C28H32Co3N20O8
953.52
Crystal system
Space group
Monoclinic
P21
Monoclinic
P21/c
Monoclinic
P21/n
Orthorhombic
Pbca
13.780(4)
13.921(4)
22.471(7)
Monoclinic
P21/n
Monoclinic
P2/n
˚
a/A
8.3632(12)
11.6074(17)
9.7721(14)
93.997(3)
946.3(2)
2
1.454
173(2)
0.71073
0.932
5965
4078
0.0594
0.0696
0.1315
0.05(3)
806668
8.6742(8)
13.3698(13)
13.6701(12)
98.150(2)
1569.3(3)
4
1.614
293(2)
0.71073
0.867
11303
3870
0.0488
0.0436
0.0888
none
806667
8.4629(9)
11.5128(12)
19.611(2)
94.807(3)
1904.0(3)
4
1.449
195(2)
0.71073
1.025
13975
4719
0.0813
0.0691
0.1521
none
806666
8.4373(7)
11.4202(9)
19.3899(16)
94.095(2)
1863.6(3)
4
1.491
200(2)
0.71073
1.159
13476
4616
0.0690
0.0615
0.1303
none
806665
8.544(3)
18.177(7)
12.538(5)
101.358(8)
1908.9(13)
2
1.659
173(2)
0.71073
1.365
13681
4737
0.0836
0.0766
0.1787
none
806664
˚
b/A
˚
c/A
b/◦
3
˚
V/A
4311(2)
4
Z
Dc/g cm-3
T/K
1.705
173(2)
0.71073
1.396
30470
5362
0.1362
0.0481
0.0950
none
˚
l/A
m/mm-1
Reflections collected
Independent reflections
Rint
a
R1 (4s data)
b
wR2 (4s data)
Flack parameter
CCDC no.
749014
2
a R1 = RꢀFo| - |Fcꢀ/R|Fo|. b wR2 = [Rw(Fo - Fc2)2/Rw(Fo2)2]1/2
.
c ref. 3
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are assigned to coordinated thiocyanate ions.19 These complexes
all have divalent metal ions such as MnII, CoII, and FeII, indicative
of weak coordination ability of the thiocyanate ion (vide infra).
Dark gray crystals of [MnIII(bpaeO)(N3)2] (2) were obtained in
45% yield from a solution of bpaeOH, Mn(NO3)2·4H2O (1 : 1) and
NaN3 (2 eq) inmethanolatroomtemperature inthe presence of air.
The sharp and strong peaks at 2044–2066 cm-1 for 2 are assigned
to coordinated azide ions.19 Interestingly, the hydroxyl group of
bpaeOH ligand is deprotonated and the manganese ion is oxidized
to 3+ as the azide ions are coordinated to the manganese ion.
That is, two manganese complexes 1 and 2 are formed in different
oxidation states, e.g. 2+ and 3+, depending on the coordinated
anions. From a solution of bpaeOH, Fe(NO3)3·9H2O, and NaN3
(1 : 2 : 6) in methanol at ambient temperature in the presence of
air, a reddish–brown precipitate of 4 was obtained in 77% yield.
Compound 4 in the IR spectra shows strong bands at 2077, 2061,
and 2028 cm-1 which is where the nas bands for coordinated
azide ions appear.19 Contrary to 3, very interestingly, a zigzag
tetranuclear Fe(III) complex 4 is obtained. From a solution of
bpaeOH, Co(NO3)2·6H2O, and NaN3 (1 : 2 : 4) in methanol at
ambient temperature in the presence of air a dark red precipitate
of [Co3(bpaeO)2(NO3)(N3)4](NO3) (6) was obtained in 63% yield.
Compound 6 in the IR spectra shows very sharp and strong peaks
at 2029–2068 cm-1 which are assigned to coordinated azide ions.19
Strong bands observed at 1364–1384 cm-1 indicate the nitrate
anions. Contrary to 5, the cobalt(II) ions in 6 are oxidized partially
by using the azide ion, which is a more oxidizing agent compared
to the thiocyanate ion. As a result, mixed-valence trinuclear
cobalt complex 6 is obtained. The composition and structure of
all compounds were determined by elemental analysis, infrared
spectrum, solid state magnetochemistry, and single crystal X-ray
diffraction analysis.
Fig. 1 Structure of the neutral complex [MnII(bpaeOH)(NCS)2] in
crystals of 1. Those of the neutral complexes in crystals 3 (Fe) and 5 (Co)
are similar and not shown. The atoms are represented by 50% probable
thermal ellipsoids. The hydrogen atom on the hydroxyl group is shown as
a small open circle; all other H-atoms are omitted for clarity.
The coordination geometry around the Mn(II) can be described as
distorted octahedral, consisting of the three N atoms and one O
atom from the bpaeOH ligand, and two N atoms of thiocyanate
˚
anions. The average Mn–LbpaeOH bond length is 2.287(2) A and
the Mn–Nthiocyanato bond lengths are in the range of 2.099(5) to
˚
2.184(5) A. The Mn–Nthiocyanato bond lengths are comparable to
those of [MnII(TPyA)(NCS)2]·CH3CN (i.e. 2.110(4) and 2.171(4)
A) (TPyA = tris(2-pyridylmethyl)amine).20 From the bond lengths,
˚
the spin state of the manganese(II) ion is believed to be high-
spin (d5, S = 5/2).21 The bond angles relating to the manganese
atom lie from 73.91(17) to 168.69(17)◦. The thiocyanate ions
are nearly linear and show bent coordination modes with the
manganese atom (∠N4C15S1; 179.0(6)◦, ∠N5C16S2; 179.0(5)◦,
∠Mn1N4C15; 172.3(5)◦, ∠Mn1N5C16; 142.0(5)◦). The hydroxyl
group of 1 is involved in a hydrogen-bonding interaction with
the sulfur atom of thiocyanate ion belonging to an adjacent
Description of crystal structures
Structure of 1. Compound 1 crystallizes in the monoclinic
P21 space group, and the unit cell includes two mononuclear
complexes, and the ORTEP drawing of 1 is shown in Fig. 1,
and the selected bond lengths and angles are listed in Table 2.
˚
manganese atom: O1 ◊ ◊ ◊ S2 (x + 1, y, z) 3.207(4) A, ∠O1H1AS2
167.4◦. Due to the interactions, compound 1 is linked to form a
one-dimensional chain extending along the a axis as shown in Fig.
2. Additionally, the pyridyl groups of 1 are involved in extensive p–
p stacking interactions with offset face-to-face p–p interactions.22
Between the monomers, the pyridine ring involving N1 undergoes
the offset p–p interaction with the pyridine group involving N3
(-x, y - 0.5, -z), which is positioned at the neighboring monomer.
The inter-planar separation of the pyridine rings is 3.63(1)–4.09(2)
◦
˚
Table 2 Selected bond distances (A) and angles ( ) for 1, 2, 3 and 5
1
2
3
5
M1–N1
M1–N2
M1–N3
M1–N4
M1–N5
M1–O1
M1–N7
2.267(5)
2.300(4)
2.269(5)
2.099(5)
2.184(5)
2.298(4)
2.232(3)
2.188(3)
2.240(3)
1.994(3)
2.182(5)
2.214(4)
2.180(5)
2.124(6)
2.017(5)
2.256(4)
2.138(4)
2.149(4)
2.129(4)
2.092(4)
2.013(4)
2.205(3)
1.856(2)
2.010(3)
O1–M1–N1
O1–M1–N2
O1–M1–N3
O1–M1–N4
O1–M1–N5
O1–M1–N7
N1–M1–N2
N1–M1–N3
N2–M1–N3
N4–M1–N5
N4–M1–N7
90.92(16)
75.54(15)
83.63(17)
89.2(2)
94.24(11)
83.19(11)
89.97(11)
175.37(13)
85.79(17)
77.68(16)
86.25(17)
173.40(18)
89.56(18)
86.21(15)
80.44(14)
88.15(15)
89.88(16)
174.40(17)
168.69(17)
90.09(12)
75.55(11)
151.45(11)
76.95(11)
73.91(17)
148.76(18)
74.94(17)
99.4(2)
75.74(19)
151.74(19)
76.08(18)
96.10(20)
78.52(16)
156.56(16)
78.11(15)
94.69(18)
Fig. 2 A one-dimensional chain of 1 formed by hydrogen-bonding
interactions between the hydroxyl group and the sulfur atom of thiocyanate
ion. Hydrogen bonds are indicated by dotted lines.
93.98(14)
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˚
˚
A (centroid ◊ ◊ ◊ centroid, 4.08 A) and the dihedral angle between
the pyridine ring planes is 5.9(1)◦. Due to the inter-monomer offset
face-to-face p–p interactions, 1 forms a 2-D network.
Structure of 2. Compound 2 crystallizes in the monoclinic
P21/c space group, and the unit cell includes four mononuclear
complexes, and the ORTEP drawing of 2 is shown in Fig. 3, and
the selected bond lengths and angles are listed in Table 2. The
manganese(III) ion is coordinated by three N atoms and one O
atom from bpaeO- ligand, and two N atoms of the azide anions.
Thus, the coordination geometry of 2 can be regarded as being
same as that of 1. The average Mn–LbpaeO bond length is 2.064(1)
Fig. 4 A one-dimensional chain of 2 formed by offset face-to-face
p–p interactions between the pyridine rings of the bpaeO- ligand. The
interactions are indicated by dotted lines.
and one O atom from the bpaeOH ligand, and two N atoms
from thiocyanate anions. The average Fe–LbpaeOH bond length
˚
is 2.214(2) A and the Fe–Nthiocyanato bond lengths are in the
˚
A and the Mn–Nazido bond lengths are in the range of 1.994(3)
˚
range of 2.017(5) to 2.124(6) A. Both the average Fe–Npyridine
˚
˚
to 2.010(3) A. The Mn–N bond lengths are longer than 2.00 A
in accordance with a high-spin MnIII complex (S = 2).23 Bond
valence sum (BVS) calculations give the valence sums of 2.09
for Mn(II) in 1 and 3.06 for Mn(III) in 2.24 The results indicate
that the manganese ions in 1 and 2 are divalent and trivalent,
respectively. The bond angles relating to the manganese(III) ion
lie from 75.55(11) to 175.37(13)◦. The azide ions are nearly
linear and show bent coordination modes with the manganese
atom (∠N4N5N6; 176.9(4)◦, ∠N7N8N9; 177.7(4)◦, ∠Mn1N4N5;
125.7(3)◦, ∠Mn1N7N8; 118.1(3)◦). The pyridyl groups of 2 are
involved in extensive p–p stacking interactions with offset face-
to-face p–p interactions.22 Between the monomers, the pyridine
ring involving N1 undergoes the offset p–p interaction with the
pyridine group involving N3 (x - 1, y, z), which is positioned
at the neighboring monomer. The inter-planar separation of the
˚
˚
distance (2.181(4) A) and the Fe–Namine distance (2.214(4) A) in
3 are comparable to those found in the N4-coordinate high-spin
iron(II) complex, [(LBzl2)FeIICl2]·H2O (LBzl2 = N,N¢-bisbenzyl-
N,N¢-bis(2-pyridylmethyl)ethane-1,2-diamine).25 The bond angles
relating to the iron atom lie from 75.74(19) to 173.40(18)◦. The
thiocyanate ions are nearly linear and show a bent coordination
mode with the iron(II) ion (∠N4C15S1; 179.3(6)◦, ∠N5C16S2;
179.2(6)◦, ∠Fe1N4C15; 152.1(5)◦, ∠Fe1N5C16 169.1(5)◦). The
hydroxyl group of 3 is involved in a hydrogen-bonding interaction
with the sulfur atom of the thiocyanate ion belonging to an
˚
adjacent iron(II) ion: O1 ◊ ◊ ◊ S1 (x - 1, y, z) 3.190(5) A, ∠O1H1AS1
156(7)◦. Due to the interactions, compound 3 is linked to form
a one-dimensional chain extending along a axis as shown in
Fig. 5. Furthermore, between the 1-D chains, the sulfur atom of
coordinated thiocyanate ion undergoes a sulfur–sulfur interaction,
which leads to a ladder-like chain extending along the a axis (Fig.
˚
˚
pyridine rings is 3.07(1)–3.36(1) A (centroid ◊ ◊ ◊ centroid, 3.70 A)
and the dihedral angle between the pyridine ring planes is 7.2(2)◦.
Due to the inter-monomer offset face-to-face p–p interactions, 2
forms a 1-D chain structure along the a axis (Fig. 4).
˚
5). The distance between the sulfur atoms is 3.441(3) A, which is
26
˚
shorter than the van der Waals radii (3.60 A).
Fig. 5 Perspective view of 3 showing the ladder-like 1-D chains. That of
5 is similar and not shown. The hydrogen bonding interactions and the
sulfur–sulfur interactions are indicated as
and , respectively.
Fig. 3 Structure of the neutral complex [MnIII(bpaeO)(N3)2] in crystals
of 2. The atoms are represented by 50% probable thermal ellipsoids. All
hydrogen atoms are omitted for clarity.
Structure of 4. Compound 4 crystallizes in the orthorhombic
Pbca space group, and the unit cell includes four tetranuclear
complex molecules, and the ORTEP drawing of 4 is shown in Fig.
◦
3
˚
Structure of 3. The structure of compound 3 is similar to that
of 1. Compound 3 crystallizes in the monoclinic P21/n space
group, and the unit cell includes four mononuclear complexes,
and the ORTEP drawing of 3 is shown in Fig. 1, and the
selected bond lengths and angles are listed in Table 2. The
coordination geometry around the iron(II) ion can be described
as a distorted octahedron, consisting of the three N atoms
6. Table 3 exhibits selected bond distances (A) and angles ( ) for 4.
The asymmetric unit of 4 contains two different iron(III) ions, one
(Fe1) is coordinated with a deprotonated bpaeO- ligand, an azide
ion, and a methoxy group, and the other (Fe2) is bonded to four
azide ions and two oxygens from the bpaeO- ligand and a methoxy
group. That is, a Fe2(bpaeO)(CH3O)(N3)4 unit results from the
bridging of two iron(III) ions through one m2-O bridging CH3O-
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were identified as 3+, as demonstrated by the powder zero-field
Mo¨ssbauer spectrum (isomer shifts of 0.21 and 0.28 mm s-1 and
quadrupole-splitting parameters of 0.85 and 0.38 mm s-1 for six-
coordination of Fe1 and Fe2).3 Furthermore, the average N–N
˚
bond distance of the azide ions is 1.177(2) A, indicative of the
delocalization of the azide ions. The intradinuclear Fe1 ◊ ◊ ◊ Fe2
and interdinuclear Fe2 ◊ ◊ ◊ Fe2¢ (1 - x, -y, 1 - z) distances between
˚
adjacent iron centers are 3.185(1) and 3.416(2) A, respectively.
Structure of 5. Compound 5 crystallizes in the monoclinic
P21/n space group, and the unit cell includes four mononuclear
complexes, and the ORTEP drawing of 5 is shown in Fig. 1, and
the selected bond lengths and angles are listed in Table 2. The
coordination geometry around the cobalt(II) ion can be described
as a distorted octahedron. Three nitrogen atoms and an oxygen
atom of the bpaeOH ligand constitute an N3O donor set around
the cobalt(II) ion and the remaining coordination positions are
occupied by the thiocyanate anions. The coordination configura-
tion of bpaeOH around the cobalt(II) ion apparently differs from
that of the analogous N4-ligand, LBzl2.25 The Co–LbpaeOH bond
Fig.
6
Molecular structure of 4 with thermal ellipsoids at 30%
probability.3 All hydrogen atoms are omitted for clarity. Symmetry code:
(¢) 1 - x, -y, 1 - z.
◦
˚
Table 3 Selected bond distances (A) and angles ( ) for 4
˚
length is 2.131(2) A and the Co–Nthiocyanato bond lengths are in the
Fe1–O1
Fe1–O2
Fe1–N1
Fe1–N2
Fe1–N3
Fe1–N13
2.001(3)
1.936(3)
2.141(3)
2.215(3)
2.115(3)
2.005(4)
Fe2–O1
Fe2–O2
Fe2–N4
Fe2–N7
Fe2–N10
Fe2–N4i
1.979(3)
2.087(3)
2.100(3)
1.997(4)
2.018(4)
2.155(3)
˚
range of 2.013(4) to 2.092(4) A. The bond angles relating to the
cobalt(II) ion lie from 78.11(15) to 174.40(17)◦. The thiocyanate
ions are nearly linear and show a bent coordination mode with
the cobalt(II) ion (∠N4C15S1; 179.6(5)◦, ∠N5C16S2; 179.3(5)◦,
∠Co1N4C15; 151.3(4)◦, ∠Co1N5C16; 170.5(4)◦). The hydroxyl
group of 5 is involved in a hydrogen-bonding interaction with
the sulfur atom of the thiocyanate ion belonging to an adjacent
N1–Fe1–O1
N1–Fe1–O2
N1–Fe1–N2
N1–Fe1–N3
N1–Fe1–N13
N2–Fe1–O1
Fe1–O1–Fe2
Fe2–N4–Fe2i
94.67(12)
106.19(13)
75.06(13)
152.25(14)
87.17(14)
80.70(12)
106.27(12)
106.78(15)
O1–Fe2–O2
O1–Fe2–N4
O1–Fe2–N4i
O1–Fe2–N7
O1–Fe2–N10
O2–Fe2–N4
Fe1–O2–Fe2
73.13(11)
159.59(13)
90.28(12)
89.91(14)
104.94(15)
94.52(13)
104.60(12)
◦
˚
cobalt(II) ion: O1 ◊ ◊ ◊ S1 (x - 1, y, z) 3.196(4) A, ∠O1H1S1 162(6) .
Due to the interactions, compound 5 is linked to form a one-
dimensional chain extending along the a axis as shown in Fig.
5. Furthermore, between the 1-D chains, the sulfur atom of
coordinated thiocyanate ion undergoes a sulfur–sulfur interaction,
which gives rise to a ladder-like chain extending along the a axis
Symmetry transformations used to generate equivalent atoms: (i) -x +
1,-y,-z + 1.
˚
(Fig. 5). The distance between the sulfur atoms is 3.417(3) A, which
is shorter than the van der Waals radii (3.60 A). The structure
26
˚
of 5 is isomorphous to those of 1 and 3. Furthermore, the BVS
calculations give the valence sums of 2.07 for Fe(II) in 3 and 2.11
for Co(II) in 5.24 The values indicate that the iron and cobalt ions
in 3 and 5, respectively, are divalent.
ligand and the terminal Oalkoxo of the tetradentate bpaeO- ligand.
The two Fe2(bpaeO)(CH3O)(N3)4 units are related by symmetry
through an inversion center situated at the barycenter of the
complex. That is, this new type of a zigzag tetranuclear iron(III)
structure results from the bridging of two Fe2(bpaeO)(CH3O)(N3)4
dinuclear moieties through two m2-N-azido anions. The amine
nitrogen and the bridged ethoxyl oxygen atoms lie in the plane
of the Fe2O2 core. The pyridine nitrogen atoms occupy two axial
positions in the octahedron. The distorted octahedral coordina-
tion sphere of the internal iron (Fe2) ion involves a N4O2 donor
Structure of 6. Compound 6 crystallizes in the monoclinic
P2/n space group, and the unit cell includes two trinuclear com-
plexes and two nitrate counteranions, and the ORTEP drawing of 6
is shown in Fig. 7. The selected bond lengths and angles are listed
in Table 4. The core structure of 6 is composed of one mixed-
valence CoIIICoIICoIII trimer and one nitrate anion. The central
cobalt(II) ion is bonded to four oxygen atoms, Co–O(av.) = 1.999(3)
-
-
set including: two N(N3 ), two m2-N(N3 ), m2-Oalkoxo(bpaeO-), and
m2-Oalkoxo(CH3O-). The coordination environment of the external
iron (Fe1) ion shows a N4O2 donor set including two Npy(bpaeO-),
˚
˚
A and to two bridged nitrogen atoms, Co–N(av.) = 2.211(4) A.
The terminal cobalt(III) ions on the other hand are bonded to five
Namine(bpaeO-), N(N3 ), m2-Oalkoxo(bpaeO-), and m2-Oalkoxo(CH3O-).
nitrogens, Co–N(av.) = 1.946(2) A and to one bridged methoxy
-
˚
˚
Complex 4 has the average Fe1–N, Fe1–O, Fe2–N, and Fe2–O
group, Co–O = 1.889(4) A. The latter distances compare well with
˚
bond distances of 2.133(2), 1.969(2), 2.084(2), and 2.033(2) A,
those previously reported for cobalt(III) ions,8 while the former are
much longer and suggest a cobalt(II) ion. In fact the difference in
the average Co–N/O distance of the central and external cobalt
respectively. Interestingly, the average Fe1–N/O and Fe2–N/O
˚
bond lengths are very similar to each other (2.051(1) A for Fe1
˚
˚
and 2.059(1) A for Fe2), and indicate that the oxidation states
ions (~0.15 A) corresponds well to the difference in octahedral
and electronic structures of the iron ions are 3+ and high spin,
respectively. Additionally, by analysis of BVS calculations for
4 (2.95 (Fe1) and 3.03 (Fe2)), both iron ions correspond to
the 3+ valence states.24 In 4, the oxidation states of iron ions
ionic radii of cobalt(II) and cobalt(III) ions.26 In 6 the cation
is formulated as [(bpaeO)(N3)CoIII(m-1,1-N3)CoII(h -NO3 )(m-1,1-
N3)CoIII(N3)(bpaeO)]+. Two bpaeOHs are deprotonated by coor-
dination to the cobalt ion. This result is in line with the observed
2
-
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◦
˚
Table 4 Selected bond distances (A) and angles ( ) for 6
ples of 1–6 were investigated. Complex 1 has a room-temperature
effective moment, meff [= (8 cT)1/2], of 5.88 mB, indicative of a
high spin (S = 5/2) configuration. This value is slightly lower
than the expected spin-only value of 5.92 mB. On lowering the
temperature, the meff values are almost constant and maintain
the high-spin state (5.70 mB at 3 K) (Fig. 8). The best fit has
g = 1.985, |D| = 0.01 cm-1, and q = -0.14 K, which is an
S = 5/2 ground state.12,28 This result is consistent with those
displayed by several high-spin manganese(II) complexes such as
[MnII(en)3]Br2 and [MnII([9]aneN2S)2](ClO4)2 (en = ethylenedi-
amine, [9]aneN2S = 1-thia-4,7-diazacyclononane).29 The value of
zero-field splitting (zfs) parameter (|D|) in 1 is in good agreement
with [MnII([9]aneN3)2](ClO4)2 (|D| = 0.09 cm-1, [9]aneN3 = 1,4,7-
triazacyclononane) and all known MnII protein species (|D| <
0.5 cm-1).29,30 In complex 2, the effective moment, meff [(= (8 cT)1/2],
at 300 K is 4.91 mB. This value is a little higher than the expected
spin-only value of 4.89 mB, indicative of a high spin MnIII (S =
2) configuration. On lowering the temperature, the meff values are
nearly constant and maintain the high-spin state (4.74 mB at 5 K).
The best fit has g = 2.014, and |D| = 1.999 cm-1 (Fig. 8).12 The
zero-field splitting (zfs) parameter of 2 is very similar those found
for MnIII high-spin complexes.31
Co1–O1
Co1–N1
Co1–N2
Co1–N3
Co1–N4
1.889(4)
1.948(5)
1.955(6)
1.932(6)
1.954(6)
Co1–N7
Co2–O1
Co2–O2
Co2–N4
1.942(5)
1.965(4)
2.136(8)
2.211(5)
O1–Co1–N1
O1–Co1–N2
O1–Co1–N3
O1–Co1–N7
N1–Co1–N2
N1–Co1–N3
N1–Co1–N7
92.5(2)
87.0(2)
90.5(2)
179.3(2)
85.1(2)
168.1(2)
86.9(2)
N2–Co1–N3
N2–Co1–N7
N3–Co1–N7
N4–Co1–N7
O1–Co2–O1ii
O2–Co2–O2ii
N4–Co2–N4ii
83.5(3)
93.4(2)
90.1(2)
97.7(2)
122.6(3)
58.5(5)
156.5(3)
Symmetry transformations used to generate equivalent atoms: (ii) -x +
3/2, y, -z + 3/2.
Fig. 7 Molecular structure of 6 with thermal ellipsoids at 30% probability.
All hydrogen atoms and non-coordinated nitrate anion are omitted for
clarity.
distance between oxygen and the terminal cobalt ion, which is
slightly shorter than that observed in 5. The distorted octahedral
coordination sphere of the central cobalt (Co2) involves an
N2O4 donor set including: two m2-N(N3 ), two m2-Oalkoxo(bpaeO-),
-
Fig. 8 Temperature dependence of the effective magnetic moment, meff(T),
2
-
and two O(h -NO3 ). The octahedral coordination sphere of
of complexes 1–6 [1 ( ), 2 (ꢀ), 3 (᭛), 4 (x), 5 (+), 6 (ꢀ)]. Solid lines
᭺
the external cobalt (Co1) shows an N5O donor set including
represent best fits by using parameters given in the text.
-
-
two Npy(bpaeO-), Namine(bpaeO-), N(N3 ), m2-N(N3 ), and m2-
Oalkoxo(bpaeO-). In particular, the distortion from ideal octahedral
geometry of the central cobalt(II) ion is due to the coordination
Compound 3 has a 300 K effective magnetic moment of 5.40
mB that decreases monotonically with decreasing temperature to
4.72 mB at 5 K. This is higher than 4.89 mB, expected for g = 2
S = 2 FeII ion. On lowering the temperature, the meff values are
almost constant and decrease rapidly on further cooling below
50 K, indicative of zero-field splitting and/or intermolecular
interactions. The best fit from magnetic susceptibility has g =
2.219 and |D| = 6.45 cm-1 (Fig. 8).12 The zero-field splitting (zfs)
parameter has a typically large value as a non-Kramers (integer
spin) ion and is in good agreement with high-spin FeII zero-field
splitting parameters (5–20 cm-1).32 As reported earlier,3 complex
4 has a room-temperature moment of 9.54 mB/Fe4 in accord with
the moments of teteranuclear complexes containing high-spin FeIII
ions. meff(T) decreases monotonically with decreasing temperature
to 1.39 mB/Fe4 at 2 K. It should be noted that the continuous
2
-
bond of the nitrate ion (h -NO3 ), which forms a four-membered
ring with cobalt ion (∠O2Co2O2(-x + 1.5, y, -z + 1.5) = 58.5(5)◦).
˚
The average N–N bond distance of azide ions is 1.188(4) A and the
bonds are delocalized. The CoIII–CoII distance within the trimeric
˚
˚
cation is 3.105(1) A, lower than those (3.7121(16) and 3.828(9) A)
found in the cases of CoII–CoII systems.27 The BVS calculation for
Co1 (3.46) agreed with the 3+ valence state, whereas that for Co2
(2.12) indicated the 2+ valence state in 6.24 That is, this strongly
demonstrates the +2 oxidation state for central cobalt ion and the
+3 oxidation states for terminal cobalt ions.
Magnetic properties. Variable-temperature 2 or 5 to 300 K
magnetic susceptibility, c, measurements on polycrystalline sam-
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decrease in meff(T) with decreasing temperature from 300 to 2 K
is suggestive of an antiferromagnetic interaction in 4. However, as
modelled, this could be only due to a stronger antiferromagnetic
coupling than the ferromagnetic one in a system having both
ferromagnetic and antiferromagnetic interactions. A satisfactory
fit of data has been obtained using the spin-Hamiltonian operator
for a zigzag tetranuclear system of four high-spin iron(III) with spin
Sa = Sb = Sc = Sd = 5/2 (H = -2J1(Sa·Sb + Sc·Sd) - 2J2(Sb·Sc)).12
The best fit had J1 = -14.3 cm-1, J2 = 9.8 cm-1, g = 2.00, and the spin
impurity, r = 0.05 (Fig. 8). The coupling constant (J1) through the
bridging oxygen atoms is expected to be antiferromagnetic and
is consistent with value for [Fe2(salam)(OH)2]·2H2O·2py (salam =
N,N¢-ethylenebis(salicylamine), J = -10.4 cm-1).33 Contrary to J1,
the ferromagnetic coupling constant (J2) via the end-on bridging
mode in 4 is almost twice that reported for the [Fe2(N3)10]4-
anion (J = 4.8 cm-1), even◦ though the angles of Fe–N–Fe
are similar (106.0 and 106.8 ).34 Interestingly, 4 exists clearly
both ferromagnetic and antiferromagnetic couplings within the
tetranuclear unit, which is different to reported linear tetranuclear
iron(II) compounds.35
As shown in Fig. 8, the effective moment, meff [= (8 cT)1/2],
value of 5 at 300 K is 4.94 mB, suggesting that the cobalt(II) ion
is in the high-spin state. This value is larger than the spin-only
value (3.87 mB) for a high-spin CoII, indicative of the strong spin–
orbit coupling. On lowering the temperature, meff(T) decreases
gradually with decreasing temperature to 3.72 mB/Co at 5 K. The
decrease of the magnetic moment at low-temperature (<150 K) is
most likely due to the strong spin–orbit coupling of CoII. The
spin–orbit coupling contributes to the zero-field splitting. The
best fit from the data has g = 2.580, |D| = 87.9 cm-1 and q =
-0.575 K (TIP = 100 ¥ 10-6 emu mol-1). As shown in Fig. 8,
the fit of the observed meff(T) for 5 displays a very large D-value.
The zero-field splitting parameter between the 1/2 and 3/2
Kramers doublets is comparable to that for [Co(Me6tren)Cl]Cl
(|D| = 52.8 cm-1, Me6tren = tris[2-(dimethylamino)ethyl]amine).36
The trinuclear compound 6 is composed of two low-spin CoIII as
wingtip ions and one high-spin CoII as the central ion, i.e. CoIII(S =
0)–CoII(S = 3/2)–CoIII(S = 0). The wingtip cobalt(III) ions are
diamagnetic, while the central cobalt(II) ion is paramagnetic (S =
3/2). The effective magnetic moment at 300 K of the trinuclear
complex 6 is measured to be 4.27 mB (3.58 mB at 5 K), which
is larger than that of the spin-only value (3.87 mB) for a high-
spin CoII and corresponds to the strong spin–orbit coupling.
The decrease of the magnetic moment at low-temperature (<40
K) is most likely due to the strong spin–orbit coupling of CoII.
The best fit of the magnetic data has g = 2.212 and |D| =
18.2 cm-1 (Fig. 8). The zero-field splitting parameter is comparable
to those for [Co(salophen)(2-methylimidazole)] (|D| = 22.6 cm-1)
and CoCl2(PPh3)2 (|D| = 14.8 cm-1) (salophen = N,N¢-phenylene-
bis(salicylaldiminato) dianion, Ph = phenyl).37
hydroperoxide and m-chloroperoxybenzoic acid (MCPBA). Only
the combination of cobalt complex 6 and MCPBA was effective
for the olefin epoxidation and alcohol oxidation and CH3CN was
the best solvent found for the oxidation reactions. To a mixture of
substrate (0.1 mmol), cobalt complex 6 (0.001 mmol), and solvent
(1 mL; CH3CN) was added MCPBA (0.035 mmol). This mixture
was stirred for 10 min at room temperature. We confirmed that
direct substrate oxidation by MCPBA was negligible (occurring
over a period of 10–60 min) whereas oxygen transfer reactions
catalyzed by the cobalt complex 6 occur in less than one minute
after the addition of the peroxybenzoic acid. Addition of a further
portion of MCPBA regenerated the catalytic cycle with the same
yield as the first cycle, indicating that the catalyst is robust under
the conditions used.
Under the optimized conditions, reactions with olefins in
the presence of 6 resulted predominantly in the formation of
epoxides as shown in Table 5. Cyclic olefins such as cyclopentene,
cycloheptene, and cyclooctene were oxidized to the corresponding
epoxides in good yields (60–90%; entries 1–3). The terminal olefin
1-hexene, which is notoriously resistant to oxidation, was also
oxidized to 1-hexene oxide with moderate yield (48.5%, entry
4) under these conditions. The reaction of cyclohexene produced
mainly the epoxidation product (65.0%), along with cyclohexen-
1-one (yield 4.8%) and cyclohexenol (yield 5.8%) as the minor
allylic hydrogen abstraction products (entry 5), indicating that
free radical oxidation reactions were almost not involved in the
olefin epoxidation reactions.39 cis-2-Hexene was used to probe
the stereochemistry of the reaction, with cis-2-hexene oxide being
produced as the major species (74.5%) along with some amounts
of trans-2-hexene oxide (17.0%) (entry 6). This result indicates that
the catalytic epoxidation reaction occurs with approximately 63%
stereochemical retention. trans-2-Hexene was oxidized exclusively
to trans-2-hexene oxide (entry 7). In the competitive epoxidation
of cis- and trans-2-octene, the ratio of cis- to trans-2-octene oxide
Table 5 Hydrocarbon oxidations by MCPBA with cobalt catalyst 6 in
CH3CN at room temperaturea
Entry Substrate
Product
Yield (%)b TNc
1
2
3
4
5
Cyclopentene
Epoxide
Epoxide
Epoxide
Epoxide
59.5 1.4
90.4 3.3
78.1 3.7
48.5 0.5
65.0 1.7
5.8 0.4
4.8 0.6
74.5 2.1
17.0 0.6
84.2 1.2
1.7
21.0
31.5
27.3
17.2
22.8
2.1
1.7
26.3
6.0
29.4
—
22.4
3.9
Cycloheptene
Cyclooctene
1-Hexene
Cyclohexene
Epoxide
2-Cyclohexene-1-ol
2-Cyclohexenone
cis-Oxide
trans-Oxide
trans-Oxide
cis-/trans-Oxide
Epoxide
6
cis-2-Hexene
7
8
9
trans-2-Hexene
cis-/trans-2-Octene
styrene
64.3 0.3
11.6 0.1
Benzaldehyde
Phenylacetaldehyde 5.6 0.2
2.0
10
11
12
Cyclohexanol
2-Hexanol
Benzyl alcohol
Cyclohexanone
2-Hexanone
Benzaldehyde
benzoic acid
kH/kD
55.7 0.8
70.7 1.7
76.9 5.8
trace
19.6
24.9
27.0
—
Hydrocarbon oxidations with the cobalt complex 6
Since manganese, iron and cobalt have continuously gained
prominence as catalysts for numerous transformations such as
olefin epoxidation, alcohol oxidation, and etc.,38 the reactivity of
the complexes 1–6 as catalysts was examined in the epoxidation
reaction and alcohol oxidation of a wide range of olefin and
alcohol substrates with various oxidants such as H2O2, tert-butyl
13
benzyl
alcohol/benzyl
alcohol-d7
3.1 0.5
—
a See Experimental section for details. b Based on MCPBA. c Turnover
numbers.
5770 | Dalton Trans., 2011, 40, 5762–5773
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©

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was determined to be 1.7 (entry 8), indicating the preference of cis-
olefin epoxidation to trans-olefin epoxidation by the intermediate
generated in the reaction of 6 and MCPBA.
we propose that the reactive species responsible for the alcohol
oxidation in our catalytic system might be a similar high-valent
cobalt intermediate that is produced following heterolytic O–O
bond cleavage of acylperoxo intermediate Co–OOC(O)R. In the
present study, we report for the first time that a cobalt trimer 6
catalyzes the olefin epoxidation and alcohol oxidation by MCPBA
via a non-radical type of oxidation reaction, while the structure
of the Co catalyst in its active state during the catalysis is not
clear yet. Nevertheless, we are speculating that it might be the
high-valent CoV–oxo for oxygen atom transfer in the epoxidation
reaction. Future studies will focus on attempts to understand the
exact nature of the reactive intermediate.
With styrene, the dominant reaction involved the formation of
the epoxide, yielding styrene oxide (64.3%) with some amounts
of benzaldehyde (11.6%) and phenylacetaldehyde (5.6%; entry 9).
Product distribution of this aromatic olefin suggests that either the
peroxyl radical or oxocobalt(IV) is partly involved as the epoxidiz-
ing agent since these species would oxidize cis-aromatic olefins
to non-stereospecific or radical-induced rearranged products.40
In the literature, cobalt(IV)–oxo species41 and, to a lesser extent,
cobalt(V)–oxo species are commonly proposed to be the active
intermediates for oxygen transfer in reactions mediated by cobalt
compounds with oxidants such as MCPBA and PhIO,42 while the
reactions of cobalt complexes with hydroperoxides often proceed
via free-radical type of oxidation reactions.43 Moreover, several
groups have proposed a CoV O intermediate as a reactive species
responsible for olefin epoxidation,42 whereas a CoIV O complex
was ascribed radical-type oxidations to account for the product
distribution in the CoIII-catalyzed epoxidation of a radical probe.41
Therefore, a minimal portion of free radical oxidation reactions,
a high degree of stereospecificity, and the formation of some
aldehydes from the epoxidation reaction of cis-aromatic olefin
observed in our catalytic systems imply that two different oxidants,
CoV O and CoIV O, might be produced in these catalytic
reactions. We propose that the CoIV O complex would be
responsible for the non-stereoretentive portion of the epoxidation
reaction. Importantly, to the best of our knowledge, 6 is the most
effective cobalt catalyst that affords high epoxide yields, small
amounts of allylic oxidation products, and a high stereospecificity
in the epoxidation of olefins by MCPBA.41–43
Conclusions
We have prepared
a series of monomeric, trimeric, and
tetrameric metal complexes from the reaction of metal
(Mn, Fe, Co) ions, N,N-bis(2-pyridylmethyl)-2-aminoethanol
(bpaeOH), and NaSCN/NaN3 in MeOH: [Mn(bpaeOH)(NCS)2]
(1),
[Mn(bpaeO)(N3)2]
(2),
[Fe(bpaeOH)(NCS)2]
(3),
[Fe4(bpaeO)2(CH3O)2(N3)8] (4), [Co(bpaeOH)(NCS)2] (5),
and [Co3(bpaeO)2(NO3)(N3)4](NO3) (6). These are characterized
by X-ray crystallography and magnetic data. Compounds 1,
3, and 5 are isomorphous structures as distorted octahedral
geometries. The structure of 2 is similar to that of 1, however,
the oxidation state of 2 is +3 and different to that of 1 (+2).
Compound 4 is a zigzag tetranuclear complex possessing high-
spin iron(III) ions and shows both ferromagnetic (via end-on
azide ion) and antiferromagnetic (via oxygen atoms of methoxy
and hydroxo groups) couplings between the iron(III) ions within
the tetranuclear unit. The compound 6 is a mixed-valence
trinuclear cobalt complex, CoIII(S = 0)–CoII(S = 3/2)–CoIII(S =
0) and displays the magnetic moment of a cobalt(II) ion in a
high-spin state. In addition, the reactivity study with the complex
6 describes a robust cobalt catalyst that epoxidizes a wide range
of olefins and alcohols with modest to excellent yields, and that
represents a new class of inexpensive catalysts for epoxidation
and alcohol oxidation reactions. Moreover, we have shown for
the first time that the cobalt trimer catalyzes a non-radical type
of oxidation reactions with MCPBA as an oxidant. We suggest
that a high-valent cobalt–oxo complex might be generated as
a reactive oxidizing intermediate in the reaction of the cobalt
complex and MCPBA.
To obtain more information about the nature of the reactive
intermediates we studied the influence of the substituent electronic
effects on the rate of epoxidation using styrene and five para-
substituted styrenes. The rate data gave a good linear Hammett
plot against s+ with a r- value of -0.8 (Fig. S1 in the Support-
ing Information, SI†). This confirms the expected electrophilic
character of the oxidant and the value compares well with those
reported for the epoxidation of styrenes using manganese(III)
tetraphenylporphyrin (r = -0.41),44 and manganese(III) salen (r =
-0.3)45 systems. The latter two systems are believed to involve
MnV O species as the active oxidant.
Alcohols were also converted efficiently to the corresponding
carbonyl compounds (entries 10–12). 2-Hexanol was oxidized to
2-hexanone (70.7%) and benzyl alcohol to benzaldehyde (77%).
For solutions containing a low concentration of benzyl alcohol,
the product benzaldehyde underwent further oxidation to benzoic
acid (data not shown). Kinetic isotope effect (KIE) studies
have been used as a mechanistic probe in alkane hydroxylation
reactions catalyzed by both iron-containing enzymes and model
complexes.46 To gain insight into the nature of the oxidizing species
in the alcohol oxidation reactions catalyzed by 6, we performed
intermolecular competition reactions involving benzyl alcohol and
benzyl alcohol-d7. The kinetic isotope effect for benzaldehyde
formation by 6 and MCPBA was determined to be 3.1 0.5 (entry
13), which is nearly identical to that of a hydrogen abstraction
reaction catalyzed by a high-valent iron–oxo (3.2 0.3).1h This
value strongly indicates that cleavage of the (benzylic) C–H bond
is involved in the rate-determining step. Based on this result,
Acknowledgements
This work was supported by the Korea Research Foundation
(KRF) grant funded by the Korea government (MEST) (No. 2009-
0073897).
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