Cobalt complexation with unsymmetrical tripodal ligands

Article abstract of DOI:10.1515/znb-2005-0610

The reaction of the aliphatic unsymmetrical tripod [N(CH₂CH ₂NH₂)₂(CH₂CH₂OH)], H ₅-1, with cobalt(II) chloride in THF yields after aerial oxidation the dinuclear complex [(H₄-1)Co^III(μ-OH)Co ^III(H₄-1)](Co^IICl₄)Cl, [5](CoCl ₄)Cl. The trianion 5^3- contains two cobalt atoms triply bridged by two alkoxo groups of the singly deprotonated ligand (H ₄-1)^- and a hydroxo group. The new ligand [N(CH ₂CH₂CH₂NH₂)(CH₂CH ₂OH)₂], H₄-3, providing an N₂O ₂ donor set reacts with cobalt(II) chloride to give after aerial oxidation the hexanuclear complex [Co^III₄(H ₂-3)₄Co^II₂(HOMe)₂Cl ₂(μ-OH)₄], [6]Cl₂, containing an unprecedented mixed-valent Co^III₄Co^II ₂ core.

Full text of DOI:10.1515/znb-2005-0610

Cobalt Complexation with Unsymmetrical Tripodal Ligands
Christoph Jocher, Tania Pape, and F. Ekkehardt Hahn
Institut fu¨r Anorganische und Analytische Chemie und NRW Graduate School of Chemistry,
Westfa¨lische Wilhelms-Universita¨t Mu¨nster, Wilhelm-Klemm-Straße 8, D-48149 Mu¨nster, Germany
Reprint requests to Prof. Dr. F. E. Hahn. E-mail: fehahn@uni-muenster.de
Z. Naturforsch. 60b, 667 – 672 (2005); received February 16, 2005
The reaction of the aliphatic unsymmetrical tripod [N(CH2CH2NH2)2(CH2CH2OH)], H5-1, with
cobalt(II) chloride in THF yields after aerial oxidation the dinuclear complex [(H4-1)CoIII(µ-
OH)CoIII(H4-1)](CoIICl4)Cl, [5](CoCl4)Cl. The trianion 53− contains two cobalt atoms triply
bridged by two alkoxo groups of the singly deprotonated ligand (H4-1)− and a hydroxo group.
The new ligand [N(CH2CH2CH2NH2)(CH2CH2OH)2], H4-3, providing an N2O2 donor set re-
III
acts with cobalt(II) chloride to give after aerial oxidation the hexanuclear complex [Co 4(H2-
3)4CoII2(HOMe)2Cl2(µ-OH)4], [6]Cl2, containing an unprecedented mixed-valent CoIII4CoII2 core.
Key words: Cobalt, Tripodal Ligands, Aminoalcohols, Crystal Structure
Introduction
Until recently, unsymmetrically built or substituted
aliphatic tripodal ligands have received little attention.
The main reason for this situation is the cumbersome,
often time consuming synthesis of such derivatives [1].
We have initiated a program to synthesize and evalu-
ate the coordination chemistry of N-centered unsym-
metrical aliphatic tripodal ligands. Initially, aliphatic
tetramine tripods with an unsymmetrical topology [2]
were studied. Later alcohol functions were incorpo-
rated into the tripodal ligands [3]. We reported on
aliphatic tripods providing an NOS2 donor set [4] and
described the preparation and coordination chemistry
of a totally unsymmetric N-centered ligand with NH2,
OH and SH donor groups [5].
Scheme 1. Unsymmetrical tripodal ligands.
NN2O ligands H5-A [8], H5-B [8] and H5-1 [3b, 9] re-
act with zinc salts with formation of mono- or dinu-
clear zinc complexes.
In this contribution, we describe the synthesis of
the unsymmetrical NN2O ligand H4-3. In addition, the
products of the reaction of H5-1 [3b] with cobalt(II)
chloride and of H4-3 with cobalt(II) chloride hexahy-
drate, both followed by aerial oxidation, are presented.
Cobalt complexes of aliphatic amine ligands have
been studied for their potential in hydrolyzing phos-
phate esters [6]. We intended to prepare cobalt com-
plexes of tripods providing an NN2O or NNO2 donor
set and to investigate their catalytic activity in the hy-
drolysis of phosphate esters. However, substitution of
an alkylamine donor in tris(2-aminoethyl)amine (tren)
or tris(3-aminopropyl)amine(trpn) for an alkoxy donor
generates a ligand with a much higher tendency to
bridge metal centers with the O-donor function thereby
enhancing the tendency to form polynuclear com-
plexes. For example, the reaction of iron salts with the
NNO2 ligand H4-D (Scheme 1) leads to cubane clus-
Experimental Section
Chemicals and most solvents are purchased from Aldrich,
Acros and Merck. THF and diethyl ether are distilled prior
to use from sodium/benzophenone under argon. NMR spec-
tra are recorded on a Bruker AC 200 NMR spectrometer. IR
ters [7], while the NNO2 ligand H4-C [3b] and the spectra are measured on a Bruker Vector 22 FT spectrome-
c
0932–0776 / 05 / 0600–0667 $ 06.00 ꢀ 2005 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
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668
C. Jocher et al. · Cobalt Complexation with Unsymmetrical Tripodal Ligands
ter as KBr pellets. MALDI mass spectra are obtained on a (NCH2CH2CH2NH2), 40.1 (NCH2CH2CH2NH2), 29.4
Bruker Reflex IV spectrometer. Elemental analyses are per- (NCH2CH2CH2NH2). – IR (KBr, cm−1): ν = 3430 (s, OH),
˜
formed with a Vario EL III CHNS-O Elemental Analyzer 3354 (s, NH), 3281 (s, NH), 2942 (s), 2868 (s), 1600 (m),
at the Institut fu¨r Anorganische und Analytische Chemie, 1462 (m), 1366 (m), 1151 (m), 1044 (s). – MALDI MS
Westfa¨lische Wilhelms-Universita¨t Mu¨nster. Ligand H5-1 is (337.0 nm, 3 ns): m/z (%) = 163.0 (100) [MH]+, 106.1
prepared as described previously [3b].
(12) [H2N(CH2CH2OH)2]+. – C7H18N2O2 (162.23): calcd.
C 51.88, H 11.18, N 17.27; found C 51.50, H 11.55, N 16.95.
N,N-Bis(2-hydroxyethyl)-3-aminopropionitrile H2-2
[(H4-1)CoIII(µ-OH)CoIII(H4-1)](CoIICl4)Cl,
[5] (CoIICl4)Cl
Freshly distilled diethanolamine (57.11 g, 0.543 mol)
is placed in a Schlenk flask under argon and cooled to
−5 ◦C. To this is added dropwise acrylonitrile (28.83 g,
0.543 mmol). The reaction mixture is stirred over night at
room temperature. Subsequently volatiles are removed un-
der reduced pressure leaving analytically pure H2-2 as a col-
orless liquid in essentially quantitative yield. Yield 85.9 g
(0.54 mol, 99%). – 1H NMR (200.1 MHz, CDCl3): δ =
3.80 (s, 2 H, OH), 3.53 (t, 4 H, CH2OH), 2.82 (t, 2 H,
NCH2CH2CN), 2.61 (t, 4 H, NCH2CH2OH), 2.49 (t, 2 H,
Anhydrous cobalt(II) chloride (195 mg, 1.5 mmol) is sus-
pended in THF (20 ml). Ligand H5-1 (220 mg, 1.5 mmol)
is added and the reaction mixture is stirred for 5 days. The
amount of cobalt(II) chloride decreases and a small residue
of blue powder is removed by filtration. Diffusion of diethyl
ether into the resulting blue solution gives blue air sensitive
crystals, presumably of complex 4. These crystals are iso-
lated by filtration, dissolved in methanol and exposed to air.
The color changes from blue to red. Slow diffusion of di-
ethyl ether into the red solution gives dark red single crystals
of [5](CoIICl4)Cl which are suitable for an X-ray diffraction
study. Yield 90 mg (0.13 mmol, 18%). – IR (KBr, cm−1):
1
NCH2CH2CN). – 13C { H} NMR (CDCl3, 50.3 MHz):
δ = 119.2 (CN), 60.1 (CH2OH), 55.8 (NCH2CH2OH), 50.6
(NCH2CH2CN), 16.3 (CH2CN).
˜
ν = 3363 (m), 3238 (m), 3192 (m), 3102 (m), 2963 (m,
N,N-bis(2-hydroxyethyl)-propylenediamine H4-3
CH), 2880 (m, CH), 1605 (m), 1472 (m), 1309 (m), 1242 (s),
1153 (s), 1065 (w), 1038 (s), 984 (s). – MALDI MS (330 nm,
3 ns, DHB): m/z (%) = 366 (100) [Co(H4-1)2O]+.
Lithium aluminium hydride (4.00 g, 0.105 mol) is sus-
pended in THF (120 ml) and placed in a 250 ml Schlenk
flask. After stirring for 30 minutes while cooling to −5 C,
◦
concentrated sulfuric acid (3.85 g, 0.038 mol) is added
carefully dropwise (Caution: this is a strongly exother-
mic reaction with hydrogen evolution). The suspension
is stirred for 60 minutes and N,N-bis(2-hydroxyethyl)-3-
aminopropionitrile H2-2 (3.09 g, 0.02 mol) dissolved in THF
(30 ml) is slowly added. The reaction mixture should not
be allowed to boil during addition of the nitrile. The re-
action mixture is stirred at room temperature for 8 h and
then quenched by slow addition of degassed water (6.2 g,
[CoIII4(H2-3)4CoII2(HOMe)2Cl2(µ-OH)4]Cl2, [6]Cl2
A solution of H4-3 (330 mg, 2.0 mmol) in methanol
(10 ml) is added to a solution of CoCl2 · 6H2O (470 mg;
2.0 mmol) in methanol (15 ml). To this is added triethy-
lamine (400 mg, 4.0 mmol) and the resulting red brown so-
lution is stirred for 5 days. Diffusion of diethyl ether into
the reaction mixture gives a precipitate of colorless needles
(HNEt3Cl) which are covered by a dark red amorphous ma-
terial. Several recrystallization steps allow to remove the tri-
ethylammonium chloride and the isolation of dark red crys-
tals of [6]Cl2 ·4CH3OH·H2O. Single crystals suitable for X-
◦
0.34 mol) at 0 C. The resulting suspension is filtered un-
der argon. The filtrate contains H4-3 and side products, but
most of the ligand is found in the solid residue. It is iso-
lated by continuous extraction with methanol for one to three
days. The methanol extract and the original filtrate are com-
bined and dried over sodium sulfate. Removal of the solvent
yields a colorless to yellow residue, which is suspended in
dichloromethane and dried over sodium sulfate. The drying
agent and precipitated lithium salts are separated by filtra-
◦
ray diffraction are obtained by this method at 4 C. Yield
150 mg (0.11 mmol). – MALDI MS (330 nm, 3 ns, DHB)
m/z (%) = 102 (100) [HNEt3]+, 163 (30) [H5-3]+.
X-ray structure determinations
R
tion t◦hrough Celiteꢀ. Distillation under reduced pressure
Suitable crystals of [5](CoCl4)Cl and [6]Cl2 · 4 CH3OH
(180 C, 0.05 mbar) gives H4-3 as a colorless viscous oil. · H2O were mounted on a Bruker AXS 2000 CCD diffrac-
Yield 1.03 g (6.35 mmol, 32%). – 1H NMR (200.1 MHz, tometer equipped with a rotating molybdenum anode (λ =
˚
CDCl3): δ = 3.51 (t, 4 H, NCH2CH2OH), 3.25 (s br, 4 H, 0.71073 A) and a CCD area detector. Diffraction data
OH +NH2), 2.76 (t, 2 H, NCH2CH2CH2NH2), 2.54 (t, 4 H, were measured at 153(2) K in the range 3.4 ≤ 2θ ≤
NCH2CH2OH), 2.52 (t, 2 H, NCH2CH2CH2NH2), 1.56 (qi, 55.0◦ for [5](CoCl4)Cl and 2.8 ≤ 2θ ≤ 50.0◦ for [6]Cl2 ·
1
2 H, NCH2CH2CH2NH2). – 13C { H} NMR (50.3 MHz, 4CH3OH·H2O. Structure solution [10] and refinement [11]
CDCl3): δ = 59.4 (CH2OH), 56.1 (NCH2CH2OH), 52.3 were achieved with standard Patterson and Fourier tech-
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C. Jocher et al. · Cobalt Complexation with Unsymmetrical Tripodal Ligands
669
niques. All non-hydrogen atoms with the exception of three
of the four non-coordinated methanol molecules and the
water molecule in [6]Cl2 · 4CH3OHH2O were refined with
anisotropic displacement parameters. The positional param-
eters of the atoms of these solvent molecules, which are par-
tially disordered, were refined with isotropic displacement Scheme 2. Synthesis of the tripodal ligand H4-3.
parameters. Hydrogen atoms were added to the structure
models in calculated positions with two exceptions: (i) no
hydrogen positions were determined for disordered solvent
molecules and (ii) the positional parameters for the pro-
ton at the bridging hydroxyl group in [5]3− were identified
and refined. The asymmetric unit of [6]Cl2 · 4CH3OHH2O
contains two independent halves of the cation [6]2+, each
of which is related to the second half by an inversion
center.
Selected crystallographic details for [5](CoCl4)Cl
Formula C12H33N6Cl5Co3O3, M = 663.48, red crystal
0.11 × 0.05 × 0.04 mm, a = 22.2845(9), b = 14.3459(6),
◦
3
˚
˚
c = 14.9396(6) A, β = 92.603(1) , V = 4771.1(3) A ,
ρcalcd = 1.847 g cm−3, µ = 2.648 mm−1, empirical ab-
sorption correction (0.7594 ≤ T ≤ 0.9015), Z = 8, mono-
clinic, space group C2/c, 23049 intensities collected ( h,
k, l), 5477 independent (Rint = 0.034) and 4878 ob-
served intensities [I ≥ 2σ(I)], 266 refined parameters, R =
0.0315, wR2 = 0.0697, max. residual electron density 0.771
Scheme 3. Preparation of complex [(H4-1)CoIII(µ-OH)
CoIII(H4-1)](CoIICl4)Cl, [5](CoCl4)Cl.
and H4-3 (Scheme 2). Michael addition of acrlonitrile
to diethanolamine [12] gave H2-2 in nearly quantita-
tive yield. The nitrile group of H2-2 was reduced with
aluminium hydride, which was prepared from LiAlH4
and concentrated sulfuric acid [13].
−3
˚
(−0.486) e A
.
Selected crystallographic details for [6]Cl2 ·4CH3OH·H2O
Formula C34H94N8Cl4Co6O19, M = 1414.55, red crys-
tal 0.10×0.09×0.06 mm, a = 11.3675(5), b = 14.6965(7),
Complex [5](CoCl4)Cl was isolated from the reac-
tion of ligand H5-1 with cobalt(II) chloride followed
by aerial oxidation (Scheme 3). The reaction mix-
ture of H5-1 and anhydrous cobalt(II) chloride in THF
remained a suspension with a decreasing amount of
cobalt(II) chloride upon stirring for several days. Fil-
tration of this suspension afforded a dark blue solution.
Diffusion of diethyl ether into this filtrate under anaer-
obic conditions precipitated dark blue single crystals
of a cobalt(II) species (presumably 4). These crystals
dissolved in methanol. Upon aerial oxidation the blue
methanol solution turned to red immediately. From this
red solution dark red crystals of [5](CoCl4)Cl precipi-
tated upon addition of diethyl ether.
˚
c = 16.6642(8) A, α = 89.2920(10), β = 89.1960(10), γ =
81.1320(10) , V = 2750.3(2) A , ρcalcd = 1.711 g cm−3
,
◦
3
˚
µ = 2.036 cm−1, empirical correction (0.8228 ≤ T ≤
¯
0.8879), Z = 2, triclinic, space group P1, 22354 inten-
sities collected ( h, k, l), 9703 independent (Rint
=
0.036) and 7906 observed intensities [I ≥ 2σ(I)], 615 re-
fined parameters, R = 0.0481, wR2 = 0.12−932, max. resid-
˚
ual electron density 3.107 (−1.387) e A
(near disor-
dered solvent molecules). Crystallographic data (exclud-
ing structure factors) have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary pub-
lication no. CCDC-257323 for [5](CoCl4)Cl and CCDC-
257324 for [6]Cl2 · 4CH3OH·H2O. Copies of the data can
be obtained free of charge on application to The Director,
CCDC, 12 Union road, CambridgeCB2 1EZ, UK [fax: int.
code +44(1223)3 36-033, e-mail: deposit@ccdc.cam.ac.uk].
The presence of ligand 1n− in the red crystals
can be concluded from an IR spectrum. The MALDI
mass spectrum suggests a chemical composition of one
cobalt atom and two molecules of ligand (H4-1)−. The
Results and Discussion
A procedure similar to the one published for the X-ray diffraction analysis of the red crystals shows the
preparation of H5-1 [3b] can be used to synthesize lig- composition to be [5](CoCl4)Cl. Both CoIII and CoII
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670
C. Jocher et al. · Cobalt Complexation with Unsymmetrical Tripodal Ligands
Fig. 2. Molecular structure of one of the two hexanuclear
Fig. 1. Molecular structure of the dinuclear cation [5]3+
.
cations [6]2+ in the unit cell. Hydrogen atoms are omit-
˚
Hydrogen atoms are omitted. Selected bond lengths [A]
and angles [◦]: Co1-Co2 2.5242(4), Co1-O1 1.915(2), Co1-
O10 1.909(2), Co1-O11 1.919(2), Co1-N1 1.919(2), Co1-
N2 1.925(2), Co1-N3 1.943(2), Co2-O1 1.923(2), Co2-
O10 1.922(2), Co2-O11 1.909(2), Co2-N4 1.917(2), Co2-N5
1.932(2), Co2-N6 1.918(2); O1-Co1-O10 79.34(7), O1-Co1-
O11 83.34(7), O1-Co1-N1 93.60(8), O1-Co1-N2 169.41(8),
O1-Co1-N3 95.04(8), O10-Co1-O11 81.47(7), O10-Co1-N1
88.28(7), O10-Co1-N2 90.28(8), O10-Co1-N3 173.39(8),
O11-Co1-N1 169.68(8), O11-Co1-N2 93.10(8), O11-Co1-
N3 101.49(8), N1-Co1-N2 88.15(8), N1-Co1-N3 88.58(8),
N2-Co1-N3 93.44(8), O1-Co2-O10 78.83(7), O1-Co2-O11
83.38(7), O1-Co2-N4 99.81(8), O1-Co2-N5 90.70(8), O1-
Co2-N6 169.92(8), O10-Co2-O11 81.40(7), O10-Co2-N4
169.33(8), O10-Co2-N5 102.07(8), O10-Co2-N6 92.32(8),
O11-Co2-N4 87.93(8), O11-Co2-N5 172.47(8), O11-Co2-
N6 90.59(8), N4-Co2-N5 88.50(9), N4-Co2-N6 87.99(9),
N5-Co2-N6 95.91(9), Co1-O1-Co2 82.24(7), Co1-O10-Co2
82.44(6), Co1-O11-Co2 82.51(6).
ted. Starred atoms represent coordinates related by an in-
◦
˚
version center. Selected bond lengths [A] and angles [ ]:
Co1-O1 1.891(3), Co1-O2 1.911(3), Co1-O5 1.924(3), Co1-
O6 1.943(3), Co1-N1 1.960(4), Co1-N2 1.941(4), Co2-
O3 1.922(3), Co2-O4 1.867(3), Co2-O5 1.920(3), Co2-
O6 1.927(3), Co2-N3 1.944(4), Co2-N4 1.939(4), Co3-
Cl1 2.4108(12), Co3-O1 1.989(3), Co3-O3∗ 2.047(3), Co3-
O4∗ 2.070(3), Co3-O40 2.071(3), Co1-Co2 2.8941(8), Co1-
Co3 3.5293(8), Co2-Co3∗ 2.7832(8); Co1-O1-Co3 130.9(2),
Co2-O3-Co3∗ 88.98(12), Co2-O4-Co3∗ 89.81(12), Co1-O5-
Co2 97.67(13), Co1-O6-Co2 98.82(13).
plexes are closely related to [5]3+. An N3O3 donor set
like in [5]3+ is found in the dinuclear CoIII complexes
with triazacyclononane (tacn) [17c] and dipropylene
triamine [17d]. In these cases the CoIII centers are
bridged by three hydroxo ligands. This leads to slightly
˚
˚
longer Co-Co distances [2.565(1) A and 2.579(1) A]
compared to [5]3+
are present in the crystals. CoII is found in the tetrahe-
dral [CoCl4]2− dianions which together with Cl− con-
stitute the anions in the crystal structure. The cation
[5]3+ is a triply bridged dinuclear CoIII complex with
(H4-1)− as a tetradentate ligand (Fig. 1). Only the alco-
hol functions of the ligands are deprotonated. They act
as bridges between two CoIII centers. The third bridge
is formed by a hydroxo group.
.
We propose a dinuclear CoII intermediate 4
(Scheme 3) for the formation of [5]3+. In contrast
to CoIII, CoII is often found four- or five-coordinate.
However, the intermediate was not further character-
ized and therefore a mononuclear CoII intermediate
cannot be ruled out.
Ligand H4-3 was added to CoCl2 ·6H2O in methanol
All three Co-O-Co angles measure about 82◦. The in the presence of two equivalents of triethylamine.
six Co-O distances fall in a narrow range from 1.909(2) The color of the solution turned brownish red upon
˚
to 1.923(2) A. The Co-N distances are also very similar stirring in air for 5 days. Colorless triethylammonium
[1.917(2) to 1.943(2) A]. The distance between cobalt chloride and dark red prisms precipitated after slow ad-
centers measures 2.5242(4)˜A. Triply bridged dinu- dition of diethyl ether. A MALDI mass spectrum of a
˚
˚
clear CoII and CoIII complexes are known with acac mixture of both crystals shows the protonated ligand
(acac = 2,4-pentadionate) [14], acetate [15], phosphine (H5-3)+ and the triethylammonium cation. Mechanical
oxide [16] and alkoxide or hydroxide anions [16, 17] separation of triethylammonium chloride allowed the
as bridging ligands. In most cases cobalt is coordi- isolation of the red crystals. They were recrystallized
nated by six oxygen atoms. Two of the described com- from methanol and found by X-ray diffraction to have
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C. Jocher et al. · Cobalt Complexation with Unsymmetrical Tripodal Ligands
671
the composition [CoIII4(H2-3)4CoII2(HOMe)2Cl2(µ- bly deprotonated with one alkoxo function bridging
two metal centers and the other one coordinating to
only one metal. EPR experiments established that the
tetranuclear complex retains its structure in methanol
or DMF solution.
OH)4]Cl24CH3OH·H2O [6]Cl2 ·4CH3OH·H2O.
Complex [6]2+ is a dicationic hexanuclear CoIII
4
CoII cluster (Fig. 2). Six cobalt atoms, four lig-
2
and molecules, four bridging hydroxo anions, two
In summary, coordination of both H5-1 or H4-3 pro-
methanol molecules and two chloride ligands form
motes the oxidation of CoII to CoIII. Further variations
a centrosymmetric wheel [CoIICl(CH3OH)(µ-H2-
of aliphatic tripodal ligands may provide more inter-
3)CoIII(µ-OH)2CoIII(µ-H2-3)]22+. One (Co1-Co3) or
esting structural arrangements in cobalt coordination
both (Co2-Co3∗) of the alkoxo groups of each coor-
chemistry, because the ligand topology is one govern-
dinated ligand act as bridges between CoIII and CoII
ing force in the cluster formation aside from the nature
centers. The assignment of cobalt oxidation states is
of the metal site or the solubility of the resulting coor-
consistent with the observed Co-O bond lengths, which
II
dination compounds.
are longer for Co (average: 2.04 A) than for CoIII (av-
˚
˚
erage 1.91 A). Similar values are found in literature
[14– 17].
Acknowledgements
The hexanuclear arrangement of the cobalt atoms
in [6]2+ is unique. A tetranuclear CuII2CoIII ring is This research was supported by the Deutsche Forschungsge-
2
meinschaft. C. J. thanks the International Graduate School of
Chemisty NRW for a 3 year scholarship.
known with diethanolamine as a ligand [18]. In this
molecule diethanolamine coordinated to CoIII is dou-
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