Extended triple-bridged Ni(II)- and Co(II)-hydroxamate trinuclear complexes: Synthesis, crystal structures, and magnetic properties

Article abstract of DOI:10.1021/ic800264y

Crystal structures of new trinuclear complexes [Ni₃(μ- OAc_F)₄(μ-AA)₂(tmen)₂], [Ni ₃(μ-OAc_F)₄(μ-BA)₂(tmen) ₂], and [Co₃(μ-OAc_F

Full text of DOI:10.1021/ic800264y

Inorg. Chem. 2008, 47, 6956-6963
Extended Triple-Bridged Ni(II)- and Co(II)-Hydroxamate Trinuclear
Complexes: Synthesis, Crystal Structures, and Magnetic Properties
Z. Tomkowicz,† S. Ostrovsky,‡ H. Mu¨ller-Bunz,§ A. J. Hussein Eltmimi,§ M. Rams,† D. A. Brown,§
and W. Haase*,|
Institute of Physics, Jagiellonian UniVersity, Reymonta 4, 30-059 Krako´w, Poland, Institute of
Applied Physics, Academy of Sciences of MoldoVa, Academy str. 5, MD-2028 Chisinau, MoldoVa,
Department of Chemistry, Centre for Synthesis and Chemical Biology, ConVay Institute of
Biomolecular and Biomedical Research, UniVersity College Dublin, Belfield, Dublin 4, Ireland,
and Eduard-Zintl-Institute of Inorganic and Physical Chemistry, Darmstadt UniVersity of
Technology, Petersenstrasse 20, 64287 Darmstadt, Germany
Received March 17, 2008
Crystal structures of new trinuclear complexes [Ni3(µ-OAcF)4(µ-AA)2(tmen)2], [Ni3(µ-OAcF)4(µ-BA)2(tmen)2], and [Co3(µ-
OAcF)4(µ-BA)2(tmen)2] have been determined (OAcF ) CF3COO-, AA ) acetohydroxamate anion, BA )
benzohydroxamate anion, tmen ) N,N,N′,N′-tetramethylethylenediamine). In each structure, the metal ions have
distorted octahedral coordination and are triply bridged by one hydroxamate and two trifluoroacetate bridges. Magnetic
properties of these compounds and of relative [Co3(µ-OAcF)4(µ-AA)2(tmen)2] were studied by susceptibility and
magnetization measurements. It was shown that for nickel trimers the intramolecular magnetic coupling is weak
ferromagnetic in the case of the complex with the AA group, and there is nearly no coupling in the case with BA
group. Rather large zero field splitting was obtained for the distorted octahedral environments of the terminal nickel
ions. The cobalt trimers were additionally studied by magnetic circular dichroism (MCD) measurements. The exchange
interaction of the cobalt complexes is antiferromagnetic.
1. Introduction
The complexes [M2(µ-O2CR)2(O2CR)2(µ-H2O)(tmen)2] (M
) Ni, Co, Mn; R ) CH3; tmen ) N,N,N′,N′-tetramethyl-
ethylenediamine) consist of a dinuclear metal center with
bridging carboxylates and a bridging water, making them
ideal candidates for structurally modeling the dinuclear
metallohydrolases. We have shown that hydroxamic acids
react with a range of these dinuclear complexes to form µ2-
η1-η2 bridged hydroxamates with very similar structures to
these shown by the inhibited ureases3 and provide good
biomimetic complexes. However, when R ) CF3, reaction
of [M2(µ-OAcF)2(OAcF)2(µ-H2O)(tmen)2] (M ) Co(II), Ni(II))
with aceto- and benzohydroxamic acids (AHA, BHA) gave
the trinuclear complexes, [M3(µ-OAcF)4(µ-RA)2(tmen)2], M
) Co(II), RA ) AA (abbreviation CoAA); M ) Co(II), RA
) BA (abbr. CoBA); M ) Ni(II), RA ) AA (abbr. NiAA,
Dinuclear metal hydrolases, which catalyze the hydrolysis
of a range of peptide and phosphate ester bonds, contain a
dinuclear metal site featuring Zn(II), Ni(II), Co(II), and
Mn(II) with carboxylate bridges.1 Hydroxamic acids are
known inhibitors of the metallohydrolases2 and feature a
novel µ2-η1-η2 hydroxamate with the deprotonated hydroxyl
oxygen of the hydroxamate bridging the two metals of the
dinuclear site and the carbonyl oxygen bonding one metal
atom only as observed in the acetohydroxamate inhibited
Klebsiella aerogenes urease and Bacillus pasteurii urease3
and the model complex [Ni2(HShi)(H2Shi)(pyr)4(OAc)].4
* To whom correspondence should be addressed. E-mail: haase@
chemie.tu-darmstadt.de.
†
Jagiellonian University.
Academy of Sciences of Moldova.
University College Dublin.
Darmstadt University of Technology.
‡
(3) (a) Pearson, M. A.; Nickel, L. O.; Hausinger, R. P.; Karplus, P. A.
Biochemistry 1997, 36, 8164. (b) Ciurli, S.; Benini, S.; Rypniewski,
W. R.; Wilson, K. S.; Miletti, S.; Mangani, S. Coord. Chem. ReV.
1999, 331, 190.
§
|
(1) Wilcox, D. E. Chem. ReV. 1996, 96, 2435.
(2) Muri, E. M. F.; Nieto, M. J.; Sindelar, R. D.; Willliamson, J. S. Curr.
Med. Chem. 2002, 9, 1631.
(4) Stemmler, A. J.; Kampf, J. W.; Kirk, M. L.; Pecoraro, V. L. J. Am.
Chem. Soc. 1995, 117, 6368.
6956 Inorganic Chemistry, Vol. 47, No. 15, 2008
10.1021/ic800264y CCC: $40.75  2008 American Chemical Society
Published on Web 07/02/2008

Ni(II)- and Co(II)-Hydroxamate Trinuclear Complexes
Table 1. Microanalytical Data for NiAA, CoBA, and NiBA
reorientation of grains in magnetic field. All temperature depend-
encies were measured in a magnetic field of 1000 Oe. Magnetization
curves M(H) were measured up to H ) 50 kOe at the constant
temperature of 2 K. The data were corrected for the diamagnetism
of the sample holder and for the core diamagnetic contribution.
MCD spectra were measured for cobalt complexes using a
JASCO J-810 spectropolarimeter, interfaced with an Oxford Instru-
ments cryostat Spectromag SM 4000-9T equipped with a split-
coil superconducting magnet. The samples, thoroughly milled, were
mixed with Nujol and placed between two thin quartz plates in a
copper sample holder that was screwed to the lower end of the
sample probe. The temperature was regulated by adjusting the
helium gas flow rate from the main bath through the heat exchanger
and by using two pairs of heater and thermometer mounted at the
heat exchanger and at the sample probe near the sample. The
accuracy of the temperature control was better than 0.3 K. The
cryostat windows and the sample holder were checked to make
sure that they were not giving MCD signals. Data acquisition was
achieved using JASCO spectra manager software. The obtained
MCD spectra were corrected by subtracting the small zero-field
MCD signal.
compound
abbr.
%C %H %N %M
[Ni3(µ-OAcF)4(µ-AA)2(tmen)2] × NiAA calcd 28.58 4.00 8.33 17.46
C4H10O
found 29.14 4.06 8.07 16.89
[Co3(µ-OAcF)4(µ-BA)2(tmen)2]
[Ni3(µ-OAcF)4(µ-BA)2(tmen)2]
CoBA calcd 36.03 3.91 7.41 15.60
found 36.21 3.83 7.38 14.70
NiBA calcd 36.05 3.92 7.42 15.54
found 36.80 3.79 7.39 15.21
including solvent); M ) Ni(II), RA ) BA (abbr. NiBA) in
which each hydroxamate bridges two metal centers in the
µ2-η1-η2 mode together with the doubly protonated salt
[(tmen) · 2H][OAcF]2.5
The structure of CoAA has been already described,5 and
in the present communication the structures of CoBA, NiAA,
and NiBA are given together with the magnetic properties
of all complexes, including CoAA.
Synthesis. Complexes NiAA, NiBA, CoAA, and CoBA
were prepared similarly using appropriate reactants. The
procedure for CoBA is as follows:
[Co2(µ-OAcF)2(OAcF)2(µ-H2O)(tmen)2] (0.5 mmol) was
dissolved under nitrogen in methanol. A solution of 1 mmol
of benzohydroxamic acid in methanol was then added and
the mixture stirred for 1 hour. After filtration, the solvent
was removed under vacuum to give a solid. Diethylether was
added to the resulting solid, giving a white precipitate and a
pink solution. The precipitate was dissolved in acetonitrile,
and vapor diffusion of diethylether into the solution led to
colorless needles of [tmen · 2H][OAcF]2, suitable for X-ray
crystallography. Yield: 0.26 mmol, 26%.
3. Crystal and Molecular Structures
The general crystallographic data are given in Table 2.
NiAA crystallizes in the orthorhombic space group Aba2
(No. 41) (this complex crystallizes with solvent C4H10O; we
preserve the abbreviation NiAA for the full composition),
whereas NiBA and CoBA crystallize in the monoclinic group
P21/n (No. 14). The nickel complexes show no internal
symmetry, whereas in CoBA, as in the earlier published5
j
CoAA(triclinic, P1, No. 2), the central cobalt atom occupies
Pink crystals of [Co3(µ-OAcF)4(µ-BA)2(tmen)2], CoBA,
suitable for crystallography formed from the above pink
filtrate on standing at room temperature. Yield: 0.22 mmol,
66%. (microanalytical data, Table 1)
an inversion center. Molecular structures of NiAA, NiBA,
and CoBA are shown in Figures 1, 2, and 3, respectively.
In all complexes, including CoAA, the environments of all
three metal ions have distorted octahedral geometry, for
example, for NiAA the angle O(1)-Ni(2)-O(6) is 91.0° and
O(3)-Ni(3)-O(10) is 91.2°. The coordination of the central
metal atom is O6, and coordination of terminal metal atoms
is N2O4. The main difference between the cobalt and the
nickel complexes is the relative orientation of the bridging
ligands: in the cobalt complexes, the hydroxamate groups
are on exactly opposite sides of the Co-Co-Co line due to
the inversion center, but in the nickel complexes the angle
between them (given as the angle between the two average
planes through the hydroxamate groups) are only 50° (NiBA)
and 47° (NiAA).
2. Experimental Section
Crystal data were collected using a Bruker SMART APEX CCD
area detector diffractometer. A full sphere of reciprocal space was
scanned by phi-omega scans. Pseudoempirical absorption correction
based on redundant reflections was performed by the program
SADABS.6 The structures were solved by direct methods using
SHELXS-977 and refined by full matrix least-squares on F2 for all
data using SHELXL-97.8 Hydrogen atoms attached to nitrogen were
located in the difference Fourier map and allowed to refine freely.
All other hydrogen atoms were added at calculated positions and
refined using a riding model. Their isotropic temperature factors
were fixed to 1.2 times (1.5 times for methyl groups) the equivalent
isotropic displacement parameters of the parent carbon atom.
Anisotropic thermal displacement parameters were used for all non-
hydrogen atoms.
The metal ions in CoAA and NiAA are triply bridged by
a single acetohydroxamate bridge and two trifluoroacetates.
Benzohydroxamate bridges instead of acetohydroxamate
bridges are present in CoBA and NiBA. All structures are
additionally stabilized by the intramolecular hydrogen bond-
ing as shown in figures by the dashed line.
DC magnetization measurements for polycrystalline samples
were done with a Quantum Design SQUID magnetometer, model
MPMS 5XL. The samples were pressed into pellets to avoid
The bonding M-O-M angles in NiBA are the same and
equal to 117.2°, but they are different in NiAA: the angle
Ni2-O1-Ni1 is equal 116.7° and angle Ni1-O3-Ni3 is
equal 118.1°. The M-O-M angles in CoAA are equal to
119.3° and in CoBA equal to 118.5°. The M-M-M angle
in NiAA is 167.5° and 167.9° in NiBA and 180° in cobalt
complexes.
(5) Brown, D. A.; Clarkson, G. J.; Fitzpatrick, N. J.; Glass, W. K.; Hussein,
A. J.; Kemp, T. J.; Mu¨ller-Bunz, H. Inorg. Chem. Commun. 2004, 7,
495.
(6) Sheldrick, G. M. SADABS; Bruker AXS Inc.: Madison, WI, 2000.
(7) Sheldrick, G. M. SHELXS-97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(8) Sheldrick, G. M. SHELXL-97-2; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
Inorganic Chemistry, Vol. 47, No. 15, 2008 6957

Tomkowicz et al.
Table 2. Crystallographic Data for CoBA, NiAA, and NiBA
identification code
CoBA
NiAA
NiBA
molecular formula
fw
C34H44N6O12F12Co3
1133.54
(C24H40N6O12F12Ni3)2 × C4H10O
C34H44N6O12F12Ni3
1132.88
2091.62
cryst syst
space group
a
b
monoclinic
P21/n (No. 14)
12.822(4) Å
orthorhombic
monoclinic
P21/n (No. 14)
12.340(2) Å
Aba2 (No. 41)
16.0444(15) Å
36.222(3) Å
12.952(4) Å
30.197(5) Å
c
14.699(4) Å
15.2264(14) Å
90°
12.986(2) Å
R
90°
90°
ꢁ
105.954(4)°
90°
92.645(3)°
γ
90°
90°
90°
V
2346.9(12) Å3
2
8848.9(14) Å3
4
4834.2(14) Å3
4
Z
Dcalcd
1.604 g/cm3
1.570 g/cm3
1.557 g/cm3
absorption coefficient
F(000)
cryst size
1.157 mm-1
1.372 mm-1
1.262 mm-1
1150
4280
2312
0.81 × 0.76 × 0.64 mm3
-16, 16, -16, 16, -18, 18
37 872
0.50 × 0.40 × 0.40 mm3
-20, 20, -46, 47, -19, 19
36 970
0.80 × 0.50 × 0.20 mm3
-14, 14, -29, 34, -14, 13
21 973
h k l ranges
Reflections collected
independent reflns
absorption correction
max. and min. transmission
GOF on F2
5112 [R(int) ) 0.0168]
semiempirical from equivalents
0.5247 and 0.4139
1.019
10 223 [R(int) ) 0.0240]
semiempirical from equivalents
0.6098 and 0.5470
1.010
7489 [R(int) ) 0.0239]
semiempirical from equivalents
0.7864 and 0.6184
1.020
final R indices [I > 2σ(I)]
R indices (all data)
largest diff. peak and hole
R1 ) 0.0393, wR2 ) 0.1080
R1 ) 0.0424, wR2 ) 0.1113
0.897 and -0.467 e ·Å-3
R1 ) 0.0402, wR2 ) 0.0946
R1 ) 0.0522, wR2 ) 0.1014
0.647 and -0.266 e ·Å-3
R1 ) 0.0472, wR2 ) 0.1160
R1 ) 0.0578, wR2 ) 0.1232
0.625 and -0.369 e ·Å-3
The shortest M-M distances are 3.48 Å in both nickel
complexes and 3.55 Å in cobalt complexes. Metal-ligand
interatomic distances are given in Table 3.
4. Magnetic properties
4.1. Nickel Trimers. Temperature dependence of the ꢀT
product (ꢀ-magnetic susceptibility) is shown in Figure 4 and
magnetization M vs magnetic field H dependence (at T ) 2
K) is shown in Figure 5 for both studied nickel compounds.
From first glance at the figures, it seems that the nickel trimer
in NiAA is internally ferromagnetically coupled, whereas
the coupling in NiBA is antiferromagnetic. The data analysis
will show that it is not obvious in the case of NiBA.
The ground state of the d8 ion in an octahedral crystal
field is an orbital singlet 3A2g. Further splitting of spin levels
is possible in the field of lower symmetry. It is so-called
zero field splitting (ZFS). The detailed analysis of interactions
for the nickel trimer was carried out using the following spin
Hamiltonian:
Figure 1. Molecular structure of [Ni3(µ-OAcF)4(µ-AA)2(tmen)2], NiAA.
Thermal ellipsoids are drawn on the 15% probability level; disorder
neglected for clarity. The solvent molecule is not shown.
i)3
i)3
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
gˆ · S
i i
H ) -2J (S · S + S · S ) +
S · D · S + µ H ·
∑
∑
ex
2
1
1
3
i
i
i
B
i)1
i)1
(1)
The first term represents the isotropic magnetic exchange
between the nearest neighbor Ni(II) ions. The second term
represents ZFS. The last term in eq 1 is the Zeeman
interaction. It consists of spin contribution only, with a Si
value equal to 1; µB is the Bohr magneton. Zero field splitting
ˆ
Di and spectroscopic gˆi tensors, expressed in their local
principal axes coordinate systems, have the following
diagonal forms,
-1 ⁄ 3Di + Ei
0
0
0
ˆ loc
Di
-1 ⁄ 3Di - Ei
)
0
0
,
[
]
2 ⁄ 3Di
0
gi⊥
0
0
gi⊥
0
0
gˆlioc
)
(2)
Figure 2. Molecular structure of [Ni3(µ-OAcF)4(µ-BA)2(tmen)2], NiBA. Thermal
[
]
g|
0
0
ellipsoids are drawn on the 50% probability level; disorder neglected for clarity.
6958 Inorganic Chemistry, Vol. 47, No. 15, 2008

Ni(II)- and Co(II)-Hydroxamate Trinuclear Complexes
Table 3. Metal-Ligand Interatomic Distances (Angstroms) in NiAA,
NiBA, and CoBA
NiAA
NiBA
CoBA
Ni1-O1 2.049
Ni1-O3 2.045
Ni1-O5 2.069
Ni1-O7 2.047
Ni1-O9 2.06
Ni1-O11 2.051
Ni2-O1 2.037
Ni2-O2 2.043
Ni2-O6 2.128
Ni2-O8 2.040
Ni2-N3 2.147
Ni2-N4 2.130
Ni3-O3 2.023
Ni3-O4 2.054
Ni3-O10 2.111
Ni3-O12 2.027
Ni3-N5 2.162
Ni3-N6 2.133
Ni1-O1 2.052
Ni1-O3 2.044
Ni1-O5 2.040
Ni1-O7 2.070
Ni1-O10 2.084
Ni1-O12 2.049
Ni2-O1 2.027
Ni2-O2 2.042
Ni2-O6 2.041
Ni2-O8 2.127
Ni2-N3 2.135
Ni2-N4 2.131
Ni3-O3 2.043
Ni3-O4 2.038
Ni3-O9 2.109
Ni3-O11 2.054
Ni3-N5 2.139
Ni3-N6 2.133
Co1-O1 2.054
Co1-O3 2.155
Co1-O6 2.101
Co2-O1 2.086
Co2-O2 2.093
Co2-O4 2.096
Co2-O5 2.096
Co2-N2 2.185
Co2-N3 2.191
Figure 3. Crystal structure of [Co3(µ-OAcF)4(µ-BA)2(tmen)2], CoBA.
Thermal ellipsoids are drawn on the 15% probability level; disorder
neglected for clarity.
where Di are the axial and Ei rhombic ZFS parameters. It
difference in geometry of the coordination polyhedrons of
NiBA and NiAA. It was believed that, in this way, at least
the signs of D can be predicted. We used again the point
charge model and calculated the components of the crystal
field gradient tensor to estimate the axial and rhombic
distortion. This time, eigenvalues were considered. The
results, expressed in the form of δ and ꢀ parameters, are
given in Table 4. The inequality δ < 0, obtained for the
central ion, is equivalent to the elongation of its O6
octahedron. The axial distortion for the terminal nickel ions
is of the opposite sign and a little greater than that of the
was assumed that the principal axes of Diloc and gˆiloc tensors
ˆ
coincide.
Hamiltonian (eq 1) is written in the common molecular
ˆ
system. Using orthogonal transformation matrices νi, the Di
and gˆi tensors in Hamiltonian (eq 1) can be expressed by
the local tensors and Hamiltonian (eq 1) is transformed to
the form
i)3
loc
ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
-1 ˆ
ˆ
H ) -2J (S · S + S · S ) +
S · ν Di νi · Si +
i i
∑
ex
2
1
1
3
i)1
i)3
µ H ·
ν -1gˆliocνi · Si (3)
i
ˆ
∑
B
i)1
It is not easy to predict the directions of principal axes of
the local tensors when the coordination octahedrons are
distorted. We used the point charge model to find the principal
axes by the diagonalization of the crystal electric field gradient
tensors calculated at the nickel sites. The matrices build from
the eigenvectors of the crystal field gradient tensors are our
transformation matrices νi. These matrices were calculated for
the various relations of the nitrogen to oxygen charge (in the
limits from 1 to 0.75). For calculation details of νi matrices,
see the Supporting Information.
The parameters Jex, Di, Ei, gi were obtained from simul-
taneous fits of susceptibility vs T and magnetization vs H
data. Such procedure allows determination of the sign of the
D parameter from the powder data.9 The full-matrix numer-
ical diagonalization of the Hamiltonian was performed by a
computer program. The calculated data of ꢀ and M were
averaged over many orientations.
Figure 4. Temperature dependence of the ꢀT product for two nickel
complexes. Solid lines are fits (text for details).
The ferromagnetic coupling for NiAA can be relatively
easily and unambiguously confirmed by fitting. Because
anisotropy of gi did not affect the fits, only the average value
of gi was retained. The large number of solutions was found
with the NiBA data and they were of the worse quality than
that for NiAA. They differed, first off all, with values and
signs of D parameters; the absolute values of D were large.
For that reason, we would like to check in detail the
Figure 5. Magnetization M vs H curves for two nickel complexes. Solid
lines are fits (text for details). The dashed line is the Brillouin curve for
three free nickel spins calculated with g ) 2.
(9) Bocˇa, R.; Dlhan, L.; Haase, W.; Herchel, R.; Maslejova, A.; Pa-
pankova, B. Chem. Phys. Lett. 2003, 373, 402.
Inorganic Chemistry, Vol. 47, No. 15, 2008 6959

Tomkowicz et al.
Table 4. Electric Field Gradient Components at the Nickel Sites,
Calculated in Their Local Principal Axes Systems, Using a Point Charge
Model for NiAA and NiBAa
NiAA
Ni2
Ni1
Ni3
qN/qO ) 1
δi
ꢀi
0.13
0.01
-0.09
-0.02
0.12
0.02
qN/qO ) 0.8
δi
0.17
0.02
Ni2
-0.09
0.16
0.02
Ni3
ꢀi
-0.02
Ni1
NiBA
qN/qO ) 1
δi
ꢀi
0.14
0.01
-0.10
-0.02
0.11
0.01
qN/qO ) 0.8
-0.10
-0.02
δi
0.17
0.02
0.15
0.01
Figure 6. ꢀT vs T for CoAA and CoBA complexes measured at H )
1000 Oe. Solid lines are fits (text).
ꢀi
a δi is a measure of the axial distortion, and ꢀi is a measure of the rhombic
distortion (in arbitrary units).
Table 5. Best Fit Parameters for Nickel Trimers Obtained from
Magnetic Dataa
NiAA
NiBA
Jex(cm-1
g1
)
0.56 ( 0.05
2.25 ( 0.04
2.23 ( 0.03
4.3 ( 0.5
∼0 ( 0.05
2.19 ( 0.03
2.18 ( 0.03
3.9 ( 0.6
g2 ) g3
D1 (cm-1
)
D2 ) D3 (cm-1
)
-7.0 ( 0.6
-13.9 ( 1.5
TIP (emu/mol)b
(188 ( 30) × 10-6
(697 ( 60) × 10-6
a Uncertainties reflect the influence of the starting parameters and of the
way of the minimization on the final results. b TIP stands for temperature-
independent paramagnetism
Figure 7. Magnetization vs field for CoAA and CoBA complexes measured
central nickel atom, assuming the same charge of all
neighboring ions. It becomes nearly two times greater by
the relation of nitrogen to oxygen charge equal to 0.8.
Now, assuming approximately D ∝-δ, E ∝-ꢀ, some
conclusions can be drawn. This is a rough approximation
because of covalency, which was not taken into account in
the point charge model, but when successfully tested for one
complex, it can be successfully used for the second one.
Because the calculated corresponding components of the
electric-field gradient tensor are of comparable magnitude
for both compounds, the values of Di parameters for NiBA
are expected to be comparable to those of NiAA. In turn,
the values of rhombicity parameters are expected to be small
(D/E ) ∼10 for terminal ions and D/E ) ∼5 for the central
ions, as follows from the relations of corresponding δ and ꢀ
quantities in Table 4). During the fit, conditions D2 ) D3
for the terminal ions and Ei ) 0 for all ions were assumed.
We were looking for the solution with D1 > 0 (elongation)10
and D2 ) D3 < 0. There were together six parameters to fit.
The best-fit parameters obtained for NiAA are given in Table
5. As seen, the signs of Di values for NiAA agree with the
signs of -δi in Table 4. Therefore, it was reasonable to use
this method to find the solution for NiBA. The obtained
parameters are given in Table 5. Fits obtained for both
complexes are shown in Figures 4 and 5. The large value of
|D2,3| for NiBA is at the limit of values reported for Ni(II)
complexes.10 It is responsible for the lower magnetization
values of NiBA with respect to NiAA at 2 K (Figure 5).
The obtained value of the TIP parameter, much larger than
at 2 K. Solid lines are fits (text).
for NiAA, is responsible for decreasing of the ꢀT product
with decreasing temperature in the high temperature range
(Figure 4). The reason for the large TIP can be a small
amount of some impurity. There is nearly no exchange
coupling for the NiBA complex.
It should be noted that the solutions found for both
complexes seem to be unique, as confirmed by fits of the
equally good quality. This statement was also confirmed by
Monte Carlo searches in the configurational parameter space.
In the Supporting Information, exemplary ꢀT(T) and M(H)
curves are presented calculated for different sets of D and J
parameters, thus demonstrating sensitivity of fit on changes
of signs and values of these parameters.
4.2. Cobalt Trimers. Temperature dependence of the ꢀT
product and M versus H dependence (at 2 K) obtained for
cobalt trimers are presented in Figures 6 and 7, respectively.
The Hamiltonian of the studied cobalt trimers includes
spin-orbit coupling, low-symmetry crystal field term, magnetic
exchange between cobalt ions, and Zeeman perturbation:
ˆ
ˆ
ˆ
ˆ
ˆ
H ) HSO + HCr + HEx + HZe
(4)
For each cobalt ion, the spin-orbit interaction within the
4T1g triplet can be written as:
3
2
ˆ
ˆ ˆ
i i
HSO ) -
κ λS · L
(5)
∑
i
i
(10) Bocˇa, R. Coord. Chem. ReV. 2004, 248, 757.
6960 Inorganic Chemistry, Vol. 47, No. 15, 2008
where λ ) -170 cm-1 is the spin-orbit coupling parameter

Ni(II)- and Co(II)-Hydroxamate Trinuclear Complexes
Table 6. The Best Fit Parameters for Cobalt Trimers Obtained from
for the Co(II) ion and κ is the orbital reduction factor that
takes into account both the covalency of the cobalt ligand
bonds and the mixture of 4T1g states originated from 4F and
4P terms.11
The low-symmetry (noncubic) crystal field term takes into
account the distortions of the local surroundings and, in
general, can be written as:
Magnetic Dataa
CoAA
CoBA
∆1 (cm-1
)
645 ( 5
372 ( 5
∆2 ) ∆3 (cm-1
)
-642 ( 5
-775 ( 5
κ1
0.92 ( 0.005
0.90 ( 0.005
-6.4 ( 0.05
0.82 ( 0.005
0.88 ( 0.005
-3.1 ( 0.05
κ2 ) κ3
Jex (cm-1
)
a Uncertainties reflect the influence of the starting parameters and of the
way of the minimization on the final results.
ˆ
ˆ
ˆ
ˆ
i i
H )
L · ∆ · L
(6)
∑
Cr
i
i
tions are required. The relatively strong spin-orbit coupling
results in the ground doublet on each cobalt ion. This
Kramers doublet is well separated from the excited levels.
The effect of the low-symmetry crystal field does not change
this conclusion (we neglect the case of very strong positive
∆, where the orbital angular momentum is completely
quenched). As a result, at low temperatures only these ground
Kramers doublets of each cobalt ion are thermally populated,
and magnetization at T ) 2 K can be well simulated with
the use of perturbation theory.
4
ˆ
where the ∆i tensor describes the splitting of the T1g term
in the local crystal field. We suppose that in the local
coordinate systems these tensors are diagonal for each of
the cobalt ions and can be written in the similar way as the
ZFS tensors for nickel ion (eq 2). In the molecular coordinate
system, these tensors can be written as:
ˆ
-1 ˆ loc
∆i ) νi ∆i νi
(7)
where orthogonal transformation matrices νi were described
earlier.
The Zeeman interaction consists of both spin and orbital
contribution and is
To obtain the key parameters of the studied complex the
simultaneous fits of ꢀ(T) and M(H) were performed. The
results are shown in Figures 6 and 7. The best fit parameters
are given in Table 6.
3
2
The exchange interaction in the studied complex is found
to be antiferromagnetic. The κ values are typical for this kind
of complexes. The ∆i parameters obtained in the best fit
procedure are in agreement with the structural data. The
ground state of the Co(II) ion in the octahedral surrounding
ˆ
ˆ
ˆ
i i
H )
µ H · (g S - κ L )
(8)
∑
Ze
B
0
i
i
where g0 is the spin Lande factor.
As long as the ground-state of Co(II) is orbitally degener-
ate, the exchange interaction between cobalt ions should
contain, in general, both orbital and spin contributions. Some
ab initio calculations performed for the binuclear chlorine-
bridged Co(II) complexes demonstrated that the energy
pattern due to the exchange interaction can not be described
by the simple Heisenberg scheme.12 The exchange Hamil-
tonian deduced for the corner shared bioctahedral Co(II) of
D4h symmetry on the base of microscopic approach13
demonstrates that the exchange interaction for this complex
is essentially anisotropic. However in many cases the
approach of Lines14 that exchange between cobalt centers
contains only an isotropic part operating with the real spins
leads to reasonable explanation of the properties of corre-
sponding complexes and the exchange part of the Hamilto-
nian can be written as the first term of eq 1.
4
represents a superposition of two T1g states, namely
3 2
|t52g(2T2g)eg2(3A2), 4T1g〉 and |t42g(3T1g)eg ( Eg), 4T1g〉. The coef-
ficients in this superposition depend on the strength of the
crystal field; however, in any case the contribution of t52ge2g
configuration is bigger. So, it can be assumed that the
behavior of the high-spin Co(II) ion with symmetry lowering
is determined by the behavior of this electronic configuration.
The axial distortion along local z axis leads to the splitting
of eg and t2g states. The compression leads to the stabilization
of dxy orbital, whereas for elongation dxz and dyz are the
lowest in energy. The obtained states can be filled in with 7
d-electrons, and one finds that for t52geg2 configuration the
compression results in the doubly degenerate ground-state,
whereas for the elongation there is no orbital degeneracy.
In terms of eq 6, it means that the compression and the
elongation of the local surrounding can be described with
the negative and positive values of ∆i parameters, respec-
tively. So, the positive value of ∆1 and negative values of
We start the analysis of the cobalt trimers with the CoAA
complex. Because of the symmetry of the system, it was
assumed that κ2 ) κ3 and ∆2 ) ∆3, where ∆i is the axial
parameter of the ∆iloc tensor. The powder averaged magnetic
ˆ
∆
) ∆3 parameters agree with the conclusions about
2
susceptibility can be calculated as ꢀav ) (ꢀx + ꢀy + ꢀz)/3,
whereas for the calculation of magnetization the averaging
over all possible orientations of the external magnetic field
should be performed. As distinguished from nickel trimers,
in the case of cobalt complex this averaging procedure is
time-consuming due to the large size of the matrices to be
diagonalized (1728 × 1728 matrices) and some simplifica-
distortions around the central and terminal ions made on the
analysis of the crystal-field gradient tensor (part 4.1).
The experimental MCD spectra of CoAA are presented
in Figure 8. They are dominated by temperature-dependent
contributions. The strong signal at 324 nm corresponds to
the metal-ligand charge transfer, whereas bands at 495 and
510 nm are typical for d-d transitions. To construct the
saturation magnetization curves, we recorded signal intensi-
ties at the maxima of all three bands for temperatures up to
7 K. The results are shown in Figure 9. There are two
different types of behavior. The signal at 324 nm is
(11) Kahn, O. Molecular Magnetism; VCH Publishers: New York, 1993.
(12) Fink, K.; Wang, C.; Staemmler, V. Inorg. Chem. 1999, 38, 3847.
(13) Palii, A. V.; Tsukerblat, B. S.; Coronado, E.; Clemente-Juan, J. M.;
Borras-Almenar, J. J. J. Chem. Phys. 2003, 118, 5566.
(14) Lines; M.E., J. Chem. Phys. 1971, 55, 2977.
Inorganic Chemistry, Vol. 47, No. 15, 2008 6961

Tomkowicz et al.
17
be simulated with the use of eq 9:
∆ε(ϑ, φ) ∝ Mxy cosϑ〈Lz〉T + Mxz sinϑ sinφ〈Ly〉T +
Myz sinϑ cosφ〈Lx〉T (9)
where 〈Lk〉T is thermally and orientationally averaged com-
ponent of the orbital angular momentum within the ground
state:
1
Z
〈Lk〉T )
〈g|Lk|g〉 exp(-E ⁄ k T), k ) x, y, z (10)
B
∑
g
g
where Z is a partition function, and ϑ and φ describe the
orientation of the magnetic field with respect to the molecule-
fixed coordinate system. To obtain the signal from the
randomly oriented molecules in frozen Nujol, eq 9 is
integrated over all magnetic field orientations.
Figure 8. Experimental MCD spectrum of CoAA recorded at 3 K by fields
up to 80 kOe.
To check if these equations are also applicable to the case
of complexes of exchange coupled ions with orbitally
degenerate ground states, we use them for simulation of
MCD behavior of the CoAA trimer. The result is shown in
Figure 9. The theoretical curves are calculated at the
assumption Mxz ) Myz ) 0 (λ ) 495 nm) and Mxy ) 0, Mxz
) Myz (λ ) 324 nm). There is reasonable agreement between
the experimental data and theoretical curves. This agreement
confirms the applicability of the model to the theoretical
explanation of the magneto-optical behavior of the investi-
gated cobalt trimer. Small differences between the theory
and experiment can be caused by the fact that the Lines
approach neglects the anisotropy of the exchange interaction.
However, this anisotropy could play a role at low temper-
atures.
At the next stage of investigations, the model presented above
was applied to the explanation of behavior of the CoBA
complex. The experimental ꢀT(T) and M(H) dependences, as
well as theoretical fits are presented in Figures 6 and 7. The
best-fit parameters are given in Table 6. One can see that for
both complexes the obtained single ion parameters are similar.
The only essential difference is in the magnitude of the exchange
interaction. In the CoAA complex, this interaction is stronger
due to the shorter distances between metal ions and bridging
oxygen. Like for CoAA complex, the experimental MCD
spectrum of the CoBA trimer consists of three bands located
at 324, 495, and 510 nm. As distinguished from the previous
case, now the saturation curves are temperature dependent not
only for the metal-ligand charge transfer (324 nm) but for the
d-d transitions (495 and 510 nm) as well (compare Figures 9,
10). Nevertheless, the signal behavior at 495 and 510 nm can
be well reproduced at the assumption of xy polarization of the
corresponding transitions just like as in the CoAA system. As
for the magnetization curves at 324 nm, the computer simulation
with the use of eqs 9 and 10 demonstrates that the transitions
of all polarizations contribute to this MCD signal.
Figure 9. Comparison of calculated and measured magnetization curves
for the CoAA complex at 324 nm (top, calculations were done at the
assumptions Mxy ) 0, Mxz ) Myz) and 495 nm (bottom, calculations were
done at the assumptions Mxz ) Myz ) 0).
temperature dependent, whereas for 495 and 510 nm satura-
tion curves recorded at different temperatures practically
coincide. The different behavior of the signal at different
wavelength is typical for the complexes where orbital angular
momentum is partially or completely quenched15–17 and can
be regarded as a confirmation of the relatively large values
of low-symmetry crystal-field parameters obtained earlier.
The magnetization curves in Figure 9 look similar to those
from the magnetic measurements presented in Figure 7.
However, the temperature and field dependence is different
because MCD signal depends both on the ground and excited
states and, as a consequence, on the polarization of the
corresponding optical transitions. Even if this polarization
is unknown from the experiment, MCD curves provide some
additional information and allow one to determine the key
parameters of the system more accurate.
(15) Ostrovsky, S. M.; Falk, K.; Pelikan, J.; Brown, D. A.; Tomkowicz,
Z.; Haase, W. Inorg. Chem. 2006, 45, 688.
In the case of mononuclear complexes with the orbitally
degenerate ground-state, MCD saturation behavior depends
on the polarization of the corresponding transitions and can
(16) Neese, F.; Solomon, E. I. Inorg. Chem. 1999, 38, 1847.
(17) Oganesyan, V. S.; George, S. J.; Cheesman, M. R.; Thomson, A. J.
J. Chem. Phys. 1999, 110, 762.
6962 Inorganic Chemistry, Vol. 47, No. 15, 2008

Ni(II)- and Co(II)-Hydroxamate Trinuclear Complexes
Jex values in the range -1.9 to –3.5 cm-1. There exist, however,
(µ-oxo)bis(µ-carboxylato) dinuclear nickel complexes with
Ni-O-Ni angle greater than 97° and ferromagnetic coupling.
We can quote one case,27 where the Ni-O-Ni angle is 113.6°
and Jex) +2 cm-1. The complex NiAA, studied in the present
work, is a new complex with ferromagnetic Ni-Ni coupling
by a relatively large Ni-O-Ni angle equal to 117°. The
essential difference with respect to the case of ref 27 is that the
methyl group of the acetate group is substituted by CF3. One
can conclude that the acetate bridge contributes ferromagnetic
coupling and that the coupling through the trifluoro-acetate
bridge is stronger than through the acetate bridge.
In NiBA, there is nearly no coupling, likely because of
compensation of the three bridges contributions. The large value
of D parameter, obtained for this complex, is near the limit of
highest values reported for the Ni(II) ion in a distorted octahedral
surrounding.9 This can be caused by the aromatic ring attached
to the hydroxamate bond.
Both cobalt trimers are explained in the same model with
similar single ion parameters. The difference in the magneto-
optical behavior of these systems is mainly caused by the
magnitude of the magnetic exchange between Co(II) ions. For
comparison, some isolated dimeric and trimeric Co(II) com-
plexes were magnetically studied recently,28–30 and in all
complexes the exchange interaction was antiferromagnetic. The
typical value of this exchange for water-bridged complexes is
about –0.5 to –0.7 cm-1, whereas for hydroxamate bridged ones
the value of this interaction reaches several wave numbers (for
example, in the [Co2(µ-OAc)2(µ-AA)(urea)(tmen)2] dimer Jex
) -3.6 cm-1). In the present work, we obtained the similar
values of the magnetic exchange for acetohydroxamate and
benzohydroxamate bridged compounds. It should be mentioned
once again that in general this exchange could be anisotropic.
However, the approach of Lines14 used in our investigations
gives a good explanation of the experimental results.
Figure 10. Comparison of calculated and measured magnetization curves
for CoBA at 324 nm (top, calculations were done at the assumptions Mxy
) 3Mxz ) 3Myz) and 495 nm (bottom, calculations were done at the
assumptions Mxz ) Myz ) 0).
5. Discussion
Three new trinuclear complexes [Ni3(µ-OAcF)4(µ-AA)2-
(tmen)2],[Ni3(µ-OAcF)4(µ-BA)2(tmen)2],and[Co3(µ-OAcF)4(µ-BA)2-
(tmen)2] have been synthesized and their crystal structures
were determined. These complexes and the related [Co3(µ-
OAcF)4(µ-AA)2(tmen)2] were magnetically characterized.
Metal ions in all studied compounds have distorted
octahedral coordination. The central M ion has coordination
O6. The terminal M ions are in the N2O4 coordination
environment. Each pair of the nearest neighbors M ions is
triply bridged by one hydroxamate bridge and two trifluo-
roacetate bridges. The dominating magnetic exchange is
through M-O-M path of the hydroxamate bridge (super-
exchange through one intermediary).
For the nickel case, the sign of the exchange integral Jex
depends on the Ni-O-Ni angle. The linear relationship was
found between the value of Jex and this bridging angle, for which
a limit value of 97° is anticipated18 for the transition from fer-
romagnetic (angle < 97°) to antiferromagnetic coupling. There
are known some dinuclear, related to urease, nickel complexes
with Ni-O-Ni bonding angle below 97 and they have
ferromagnetic Ni-Ni coupling.19–21 There were also reported
(µ-oxo)bis(µ-carboxylato) dinuclear nickel complexes22–26 with
a Ni-O-Ni bonding angle above 97°, that is, in the range
111.9-118.0°. They all have antiferromagnetic coupling with
Acknowledgment. Z.T., W.H., and S.O. thank DFG for
the financial support.
Supporting Information Available: Listings of crystallographic
data in CIF format. Derivation of νi matrices. Two figures demon-
strating sensitivity of the calculated ꢀT and magnetization curves
to the signs of D and to the value of J parameters. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC800264Y
(22) Volkmer, D.; Ho¨rstmann, A.; Griesar, K.; Haase, W.; Krebs, B. Inorg.
Chem. 1996, 35, 35.
(23) Buchanan, R. M.; Mashuta, M. S.; Oberhausen, K. J.; Richardson,
J. F. J. Am. Chem. Soc. 1989, 111, 4497.
(24) Greatti, A.; de Brito, M. A.; Bortoluzzi, A. J.; Ceccato, A. S. J. Mol.
Struct. 2004, 688, 185.
(25) Adams, H.; Bradshaw, D.; Fenton, D. E. Inorg. Chim. Acta 2002, 332,
195.
(26) Greatti, A.; Scarpellini, M.; Peralta, R. A.; Casselato, A.; Bortoluzzi,
A. J.; de Brito, M. A.; Szpoganicz, B.; Tomkowicz, Z.; Haase, W.;
Neves, A. Inorg. Chem. 2008, 47, 1107.
(18) Nanda, K. K.; Thompson, L. K.; Bridson, J. N.; Nag, K. J. J. Chem.
Soc., Chem. Commun. 1994, 1337.
(27) Eremenko, I. L.; Nefedow, S. E.; Sidorov, A. A.; Golubnichaya, M. A.;
Danilov, P. V.; Ikorskii, V. N.; Shvedenkov, Y. G.; Novotortsev,
V. M.; Moiseev, I. I. Inorg. Chem. 1999, 38, 3764.
(28) Calvo-Perez, V.; Ostrovsky, S.; Vega, A.; Pelikan, J.; Spodine, E.;
Haase, W. Inorg. Chem. 2006, 45, 644.
(19) Arnold, M.; Brown, D. A.; Deeg, O.; Errington, W.; Haase, W.;
Herlihy, K.; Kemp, T. J.; Nimir, H.; Werner, R. Inorg. Chem. 1998,
46, 2920.
(20) Paital, A. R.; Wong, W. T.; Aromi, G.; Ray, D. Inorg. Chem. 2007,
46, 5727.
(29) Brown, D. A.; Errington, W.; Glass, W. K.; Haase, W.; Kemp, T. J.;
Nimir, H.; Ostrovsky, S. M.; Werner, R. Inorg. Chem. 2001, 40, 5962.
(30) Ostrovsky, S. M.; Werner, R.; Brown, D. A.; Haase, W. Chem. Phys.
Lett. 2002, 353, 290.
(21) Brown, D. A.; Glass, W. K.; Fitzpatrick, N. J.; Kemp, T. J.; Errington,
W.; Clarkson, G. J.; Haase, W.; Falk, K.; Mahdy, A. H. Inorg. Chim.
Acta 2004, 357, 1411.
Inorganic Chemistry, Vol. 47, No. 15, 2008 6963