Magneto-structural correlation for binuclear octahedral vanadium(iv)-oxo complexes. Synthesis, structure and magnetic properties of a VIVO2+ complex with a new ligand derived from glycine

Article abstract of DOI:10.1039/a909846a

The synthesis of a new ligand, H₃BBAC (N,N-bis(2-hydroxybenzyl)aminoacetic acid) and its first V^IVO²⁺ complex, [Et₃NH]₂[O=V(BBAC)]2 (1), are presented in order to model the active site of vanadium tra

Full text of DOI:10.1039/a909846a

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Magneto-structural correlation for binuclear octahedral
vanadium(IV)–oxo complexes. Synthesis, structure and magnetic
properties of a VIVO2؉ complex with a new ligand derived from
glycine
Augusto S. Ceccato,*a Ademir Neves,a Marcos A. de Brito,a Sueli M. Drechsel,b
Antonio S. Mangrich,b Rüdiger Werner,c Wolfgang Haasec and Adailton J. Bortoluzzid
a Departamento de Química, Universidade Federal de Santa Catarina, 88040-900-Florianópolis,
SC, Brasil. E-mail: ceccato@qmc.ufsc.br
b Departamento de Química, Universidade Federal do Paraná, Curitiba, PR, Brasil
c Institut für Physikalische Chemie, Technische Hochschule, Darmstadt, D-6100 Darmstadt,
Germany
d Departamento de Química, Universidade Federal de Santa Maria, Santa Maria, RS, Brasil
Received 7th December 1999, Accepted 24th March 2000
Published on the Web 27th April 2000
The synthesis of a new ligand, H3BBAC (N,N-bis(2-hydroxybenzyl)aminoacetic acid) and its first VIVO2ϩ complex,
[Et NH] [O᎐V(BBAC)] (1), are presented in order to model the active site of vanadium transferrins. Spectroscopic
᎐
3
2
2
(EPR, UV-VIS), magnetic properties and crystal data of 1 are discussed and a new magneto-structural correlation
turns out to be relevant for binuclear RO-bridging octahedral vanadium()–oxo complexes.
solid precipitated which was washed with 6 × 50 ml of water
Introduction
and dried at 80 ЊC. Yield 64% (23 g of H2BAC). The precursor
ligand was purified (water ≈ 90 ЊC) and presents mp ≈ 222–
223 ЊC. IR (cmϪ1): νasym(COOϪ), 1570; νsym(COOϪ), 1460;
Magnetic properties of molecular compounds are important in
bioinorganic chemistry when one tries to correlate them with
structural parameters.1–4 For some dicopper() d9–d9 complexes
it is possible to establish a quantitative correlation between
magnetic and structural data,1 but for binuclear octahedral
vanadium()–oxo complexes such data are rare.5 In the oxo-
vanadium case the dxy orbitals are magnetically relevant to
couple the VIV centers in the binuclear unit when compared
ϩ
ν(NH) NH2 , combination bands centered at 2500; δ(OH), 1378
cmϪ1. Elemental analysis calculated for C9H11NO3: C, 59.96; H,
6.12; N, 7.73. Found: C, 59.53; H, 5.87; N, 7.88%.
[Et3NH][H2BBAC] ؍ Triethylammonium N,N-bis(2-hydroxy-
benzyl)aminoacetate. To a solution of 2-bromomethyl phenyl
acetate (11.0 g, 48 mmol) in 100 ml of THF was added H2BAC
(8.1 g, 45 mmol). To the resulting solution was added, drop by
drop, 14.4 ml (105 mmol) of Et3N, and the reaction was carried
out during 24 h, under stirring. The white precipitate of
[Et3NH][H2BBAC]ؒHBrؒ2H2O was filtered off washed with
THF and diethyl ether and dried in air. Yield 21.8 g (89%),
mp ≈ 135–136 ЊC. IR (cmϪ1): ν(C᎐O) ester 1768; ν(NH) Et NHϩ
2 Ϫ y2
to binuclear copper complexes in which the dx
are the
magnetic orbitals. A recent structural study of duck ovotrans-
ferrin shows two iron() centers coordinated to oxygen
residues from tyrosines, a carboxylic oxygen from aspartate and
a guanidinium group that interacts with CO32Ϫ.6 Transferrins
are also able to coordinate to other transition metal ions such
as vanadium in vanadium transferrins ([VIV(tf)]),7,8 therefore
octahedral vanadium()–oxo compounds that present oxygen
(from phenolate and carboxylic) and nitrogen donor groups are
important to model the coordination sphere of modified trans-
ferrin.9 Furthermore, d1 (V()) systems are interesting tools for
EPR studies10 when correlated to other magnetic and structural
studies.
᎐
3
and NHϩ ≈ 2500 from combination bands; ν(OH) PhOH and
H2O, 3394 cmϪ1. Elemental analysis calculated for C24H38-
N2O7Br: C, 52.85; H, 7.09; N, 5.14. Found: C, 53.19; H, 7.09; N,
5.35%.
The obtained ester was hydrolyzed by adding 21.8 g (40
mmol) of [Et3NH][H2BBAC]ؒHBrؒ2H2O to 300 ml of a meth-
anolic solution of KOH (4.7 g, 84 mmol). The mixture was
placed under stirring during 1 h, evaporated and a white solid
was washed with 2-propanol (2 × 50 ml) and diethyl ether. Yield
15.6 g of [Et3NH][H2BBAC] that decomposes at ≈ 255 ЊC. IR
(cmϪ1): ν(OH) PhOH, 3358; ν(NH) Et3NHϩ, 2600. Elemental
analysis calculated for C22H32N2O4: C, 68.01; H, 8.30; N, 7.21.
Found: C, 67.59; H, 7.91; N, 7.03%.
Experimental
Syntheses
H2BAC ؍ N-(2-Hydroxybenzyl)aminoacetic acid. To a sus-
pension of glycine (15.0 g, 200 mmol) in 250 ml of methanol,
was added LiOHؒH2O (8.4 g, 210 mmol). To this solution
was added under stirring 24 ml (200 mmol) of salicylaldehyde,
which resulted in a yellow solid. The resulting imine was
H3BBAC ؍ N,N-bis(2-hydroxybenzyl)aminoacetic acid. The
title ligand was isolated by adding HCl 1 M to 350 ml of an
aqueous solution of 15.6 g of [Et3NH][H2BBAC] until the pH
reached 4.0. A white solid was filtered off and washed 4 times
with 40 ml of water and dried in the oven (≈100 ЊC). Yield 6.6 g
(58% relative to [Et3NH][H2BBAC]ؒHBrؒ2H2O), mp ≈ 210–
211 ЊC. IR (cmϪ1): νasym(COOϪ), 1622; νsym(COOϪ), 1404; ν(OH)
analyzed by IR spectroscopy: ν(C᎐N), 1656; νasym(COOϪ) 1620;
᎐
νsym(COOϪ) 1394 cmϪ1. The imine was dissolved in methanol
and reduced to the corresponding amine with NaBH4 (3.7 g,
100 mmol) during 15 minutes under stirring. The resulting pale
yellow solution was concentrated under reduced pressure and
dissolved in water. The pH was adjusted to ≈6.0 and a white
DOI: 10.1039/a909846a
J. Chem. Soc., Dalton Trans., 2000, 1573–1577
This journal is © The Royal Society of Chemistry 2000
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at 2500–3000, from combination bands; δ(OH), 1362. 1H NMR
(D2O): δ 3.2 (2H, CH2OAc); 3.8 (4H, CH2Ar) and 6.8–7.3 (8H,
ArH). No protons from OH groups were observed until 10
ppm. Elemental analysis calculated for C16H17NO4: C, 66.87; H,
5.89; N, 4.88. Found: C, 66.71; H, 5.94; N, 4.74%.
Crystal structure determination
A violet single crystal of the 1ؒ4CH2Cl2 complex was isolated in
inert oil, mounted on a glass fiber and quickly placed on an
Enraf-Nonius CAD4 diffractometer equipped with a low tem-
perature system. These crystals are very sensitive to solvent loss.
Lattice constants were determined from 25 reflections in the 2θ
range 15–27Њ. Cell parameter determinations and data collec-
tion were performed using CAD-4 software.13 All data were
corrected for Lorentz, polarization effects and for linear decay
(4.1%). Empirical absorption corrections based on the azi-
muthal scans of 5 reflections were applied with transmission
factors 0.9143–0.9999.14 The structure was solved by direct
methods and refined by full-matrix least-squares methods using
SHELXS9715 and SHELXL9716 programs, respectively. All
non-hydrogen atoms were refined anisotropically. An independ-
ent dichloromethane molecule was found to be disordered. Two
alternative positions for each chlorine (Cl52 and Cl53) were
assigned at 50% occupancy, while the carbon atom (C(51))
showed a single site occupancy. The best model for the dis-
ordered molecule was obtained by restraining C–Cl and Cl–Cl
distances, isotropic and anisotropic thermal parameters. Hydro-
gen atoms were not located in this disordered solvent. Other
hydrogens were placed at their idealized positions (C–H =
0.93 Å) with the thermal isotropic parameter fixed at 1.2 (CH,
CH2) or 1.3 (CH3) times Ueq of the preceding atom, except for
H1NA which was found from the difference Fourier map and
refined as a free isotropic atom. The plot of the molecular struc-
ture of 1ؒ4CH2Cl2, as shown in Fig. 1, used the program
ZORTEP.17 Crystal data: formula V2Cl8O10N4C48H68; M =
Scheme 1 shows the synthetic route to obtain the title ligand.
¯
1246.06; triclinic; space group P1; a = 10.638(2); b = 11.380(2);
c = 13.512(2) Å; α = 98.11(3); β = 102.00(3); γ = 110.17(3)Њ;
V = 1461.0(4) Å3; Z = 1; T = Ϫ70 ЊC; µ(Mo-Kα) 7.41 cmϪ1
;
5432 reflections collected; 5133 unique [Rint = 0.0274]; final
indices: R(F) = 0.0406, wR(F2) = 0.1105 and GOF(F2) = 1.066.
CCDC reference number 186/1911.
Scheme 1
[Et NH] [O᎐V(BBAC)] (1). The complex was prepared from
᎐
3
2
2
the reaction of 0.255 g (1 mmol) of [O᎐V(acac) ], dissolved in
See http://www.rsc.org/suppdata/dt/a9/a909846a/ for crystal-
lographic files in .cif format.
᎐
2
15 ml of THF, and a solution of H3BBAC (0.287 g, 1 mmol)
and 0.83 ml (6 mmol) of Et3N in 20 ml of THF, under argon.
After 20 minutes at ambient temperature and constant stirring
the green solution changed to a violet color, which was concen-
trated to ≈6 ml. The solution was cooled and the precipitate
was filtered off and washed with acetonitrile and diethyl ether.
Yield 0.28 g (31%). Elemental analysis calculated for C44H60-
N4O10V2: C, 58.27; H, 6.67; N, 6.18. Found: C, 57.93; H, 6.59;
N, 6.12%. Crystals of 1ؒ4CH2Cl2 suitable for single crystal
X-ray analysis were obtained by slow evaporation from a dry
diethyl ether/CH2Cl2 (1:1) solution at room temperature and
under an N2 atmosphere.
Results and discussion
A new tetradentate ligand, H3BBAC, containing biologically
relevant donor groups, and its first complex, [Et NH] [O᎐
᎐
3
2
V(BBAC)]2 1 were prepared and fully characterized. The
electronic spectrum of 1 in CH2Cl2 under argon showed two
d–d transitions (λmax, 567 nm/ε = 73 MϪ1 cmϪ1 and λmax, 787 nm/
ε = 68 MϪ1 cmϪ1). This transition changes to λmax 563 nm/
ε ≈ 2100 MϪ1 cmϪ1, after 2 h when the solution has been in
contact with the air, characteristic of V()–oxo complexes.
Unfortunately we were not able to isolate the corresponding
oxidation product. The reflectance spectrum of 1 (KBr pellets)
showed the same transition mode when compared to the spec-
trum in solution. The IR spectrum of 1, when compared to the
Physical measurements
IR spectra were obtained on a Perkin-Elmer FTIR model I16
PC spectrometer as KBr disks. Elemental analyses were per-
formed on a Perkin-Elmer model 2400. The 1H NMR spectrum
of H3BBAC was obtained on a Bruker 200 MHz, model
AC200F in D2O. Magnetic susceptibility data were obtained
spectrum of H3BBAC, shows ν(V᎐O) at 963 cmϪ1 that is charac-
᎐
teristic of V()–oxo compounds, and, as expected, lacks δ(OH)
at 1362 cmϪ1. We were not able to observe any activity of the
title complex on the time scale of the cyclic voltammetry
experiments employed.
on a polycrystalline sample of [Et NH] [O᎐V(BBAC)] over a
᎐
3
2
2
temperature range from 4.5 to 300.0 K with a Faraday-type
magnetometer; details of the apparatus have been described
elsewhere.11 Diamagnetic corrections were applied in the usual
manner with the use of tabulated Pascal constants.12 Visible
spectra were recorded in CH2Cl2 under argon with a Perkin-
Elmer Lambda 19 spectrometer. Cyclic voltammetric experi-
ments were performed with a PAR 273 (Princeton Applied
Research) in CH2Cl2 under argon at room temperature with
0.1 M [Bu4N][PF6] as the supporting electrolyte. The EPR spec-
trum was obtained at 77 K in X-band (9.7 GHz) on a Bruker
ESP 300E spectrometer, using 100 kHz modulation frequency.
The spectrum of the frozen solution was recorded using a
quartz tube.
Description of the molecular structure of 1ؒ4CH2Cl2
The crystal structure of 1ؒ4CH2Cl2 was determined by single-
crystal X-ray diffraction methods. The asymmetric unit consists
of half [O᎐V(BBAC) ]2Ϫ, a triethylammonium cation and two
᎐
2
dichloromethane solvent molecules arranged in the space group
¯
P1. The molecular structure of 1ؒ4CH2Cl2, and selected bond
lengths and angles are shown in Fig. 1.
The [O᎐V(BBAC) ]2Ϫ unit shows a centrosymmetric structure
᎐
2
and has two V() centers bridged by two phenolate groups. The
metallic ions are in pseudo-octahedral environments (Fig. 1),
where O(2) of the carboxylate group occupies the trans position
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J. Chem. Soc., Dalton Trans., 2000, 1573–1577

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Fig. 1 A view of the anion of 1ؒ4CH2Cl2 with numbering scheme.
Thermal ellipsoids are shown at the 50% probability level. Hydrogen
atoms are omitted for clarity. Selected bond lengths [Å] and angles [Њ]:
V1–O1 1.610(2), V1–O10 1.948(2), V1–O20A* 2.032(2), V1–O20
2.087(2), V1–N1 2.147(2), V1–O2 2.170(2); O1–V1–O1 99.77(8), O1–
V1–O20A* 100.85(7), O10–V1–O20A* 95.92(7), O1–V1–O20 96.45(8),
O10–V1–O20 163.77(7), O20A*–V1–O20 81.38(7), O1–V1–N1
90.94(8), O10–V1–N1 86.11(7), O20A*–V1–N1 167.48(7), O20–V1–N1
93.22(7), O1–V1–O2 166.44(7), O10–V1–O2 84.49(7), O20A*–V1–O2
91.43(6), O20–V1–O2 79.60(7), N1–V1–O2 76.44(7), V1A–O20–V1
98.62(7). Symmetry operation: * Ϫx ϩ 1, Ϫy ϩ 2, Ϫz ϩ 1.
with respect to the terminal oxo group. In this arrangement
nitrogen and oxygen atoms from phenolates occupy equatorial
positions. The terminal oxo groups are anti and the vanadium
ion is 0.251(1) Å from the equatorial plane with respect to
the terminal oxo group. The complex shows a V ؒ ؒ ؒ V distance
of 3.125, which is 0.1 Å longer than similar V–O–V
complexes.13–15 The V(1)–O(2) (2.170 Å) bond length is 0.12 Å
shorter than the same bond observed in Na[VO(EHGS)]18
(EHGS = N-[2-(o-salicylideneamino)ethyl](o-hydroxyphenyl)-
glycine) where the carboxylate group is trans to the V᎐O group,
᎐
but is 0.17 Å longer than that found in the complex
NH4[VO(EHPG)],18 (EHPG = ethylenebis[(o-hydroxyphenyl)-
glycine] where the carboxylate group is in the cis position with
respect to the vanadyl group. These data indicate that the ter-
minal oxo and carboxylate groups present a trans effect between
them. This trans effect is also observed in the equatorial plane
where O(20) and O(20A) are trans to O(10) and O(10A) from
phenolates, and also with respect to N(1) and N(1A), respec-
tively. This arrangement results in unsymmetric µ-phenoxo
bridges. The rings C(11)–C(16) and C(21)–C(26) are planar and
the angle between the planes is 46.24(13)Њ.
EPR
The X-band EPR powder spectrum (Fig. 2a) of 1 recorded at
room temperature shows broad structured bands with peaks
at 2900 and 4200 G, and a relatively low half-field absorption
(∆mS = 2). This spectrum is consistent with an S = 1 spin state
of a dimer with low rhombicity (D ≈ 0.1 cmϪ1). The weak struc-
tured feature around 3400 G is attributed to a hyperfine pattern
of the S = ¹ spin state, which was also detected by the magnetic
2ϩ
Fig. 2 EPR powder spectra of [O᎐V(BBAC)]2 where (a) was
᎐
recorded at 300 K and (b) at 77 K. EPR spectrum from a frozen solu-
tion in dichloromethane at 77 K (c) and (d) (–– experimental, ··· simu-
lation). Simulated spectrum obtained using WINEPR SIMFONIA
program.27 Note the different scales.
CH2Cl2 solution of 1 (Fig. 2d) shows only the S = ¹ component.
¯
²
¯
²
data (fit with 3.27% of paramagnetic impurity). On cooling
the powder to 77 K (Fig. 2b) the strong feature of the S = 1
component of the spectrum diminishes in relative intensity com-
pared to the S = ¹ component. The temperature dependence of
The parameters gx = 1.9750, gy = 1.9820, gz = 1.9457 and
Ax = 58.00, Ay = 57.00, Az = 164.50, A0,calc. = 92.97 (×10Ϫ4 cmϪ1
)
could be derived from this spectrum. From this information it
seems reasonable to conclude that the structure of the dimer is
not maintained in CH2Cl2 solutions. The Hamiltonian for the
solid solution is indicative of a monomeric vanadium() center
showing that the dimer dissociates in solution. The parameters
of the monomeric species obtained in solution are similar to
those of the human vanadium()–transferrin ([VIV(tfh)]):
gx = 1.973, gy = 1.973, gz = 1.938 and Ax = 56.6, Ay = 56.6,
Az = 168, A0,calc. = 93.7 (×10Ϫ4 cmϪ1),19 where A0,calc. = (Ax ϩ
Ay ϩ Az)/3. These data indicate that 1, and the monomeric
¯
²
the spectra is, in this case, also in agreement with the magnetic
result of a S = 0 ground state as expected from the results of the
magnetic study which shows an antiferromagnetic coupling for
the dimer. The spectrum of a freshly prepared frozen CH2Cl2
solution of 1 (Fig. 2c), under argon atmosphere, shows the
same hyperfine pattern S = ¹ and the large signal of the S = 1
¯
²
component, in a similar way to the powder spectrum at 77 K.
However, after five days, the spectrum of the same frozen
J. Chem. Soc., Dalton Trans., 2000, 1573–1577
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Table 1 Magnetic and structural parameters for binuclear octahedral oxovanadium() compounds
Compound
J/cmϪ1 (T/K)
V ؒ ؒ ؒ V/Å
V–O–V/Њ
τ/Њ
Ref.
[Et3NH]2[(VO)2(BBAC)2]ؒ4CH2Cl2 (1)
[(VO)2(L1)(OCH3)(DMSO)]a (2)
[(VO)2(L2)(OH)2]I2ؒ4H2Ob (3)
[(VO)2(L3)(OH)2]Br2 c (4)
Ϫ167.9 (5–300)
Ϫ122 (80–300)
Ϫ150 (103–293)
Ϫ177 (98–298)
ϩ3.1 (4–250)
3.125
3.026
2.965
3.033
—
98.6
94.3; 101.8
98.1
101.2
107.0
180
This work
131.1
175.7
180
22
23
24
5
[(VO)2(L4)2]d (5)
0.0
a L1 = 2,6-Bis(salicylideneaminomethyl)-4-methylphenol. b L2 = N,N,NЈ,NЈ-Tetrakis(2-pyridylmethyl)ethylenediamine. c L3 = 1,4,7-Triazacyclonon-
ane. d L4 = N-Salicylidene-2-(bis(2-hydroxyethyl)amino)ethylamine.
Fig. 3 Plots of molar magnetic susceptibility per vanadium χm (᭹)
and effective magnetic moment per vanadium µeff (Њ) vs. temperature
for 1.
Fig. 4 Plot of the J value vs. the dihedral angle between the equatorial
planes, τ, for binuclear RO-bridging octahedral oxovanadium()
complexes.
species obtained from the dissociation of the dimer, are good
models to simulate the active site structure of the enzyme.
Therefore, based on the structure of the model complex, we can
suggest NO3 as donor atoms in the equatorial position of the
coordination sphere of the protein, where it should be possible
to find two oxygen atoms from tyrosine, and a nitrogen atom
from histidine.
binuclear octahedral complexes generally resides in a dxy
orbital, with the oxo group oriented along the z direction,
and this allows five magnetic interactions between the corres-
ponding orbitals.5 According to Plass5 the configurations are
classified in relation to the orientation of the V᎐O group with
᎐
respect to the plane defined by the two vanadium centers
and the two bridging oxygen atoms (orthogonal, coplanar and
Magnetic properties
twist) and the orientation of the two V᎐O groups (syn or anti).
᎐
The variable-temperature magnetic data for complex 1 in the
solid state, collected in the temperature range 4.5–300.0 K, are
shown in Fig. 3 and indicate the presence of a strong anti-
ferromagnetic coupling process.
The room temperature magnetic moment of 1 is 1.82 µB per
binuclear molecule, which is significantly lower than the
spin-only value (2.45 µB) for two uncoupled S = 1/2 spins, and
gradually decreases on decreasing the temperature, reaching
0.31 µB at 4.5 K. The data were fitted by using eqn. (1)12 for
From this classification there are the following possibilities:
anti-orthogonal, syn-orthogonal, anti-coplanar, syn-coplanar
and twist. For the orthogonal configurations a direct inter-
action or a superexchange mechanism between the magnetic
orbitals is possible, implying a strong antiferromagnetic
coupling process like those observed in complexes 1 and 4. On
the other hand for anti-coplanar and twist configurations5 in
which a direct interaction and superexchange mechanism is not
effective one usually expects a ferromagnetic interaction.5,20,21
We present in Table 1 magnetic data and structural parameters
for binuclear RO-bridging octahedral V()–oxo compounds
with phenoxo, alkoxo and hydroxo bridging ligands.5,22–24 This
series of compounds shows that J is independent of the V–O–V
angle, and the V ؒ ؒ ؒ V and the V ؒ ؒ ؒ O distances. A plot of
coupling constants (J ) vs. the dihedral angle (τ) between the two
equatorial coordination planes is shown in Fig. 4.
2Ng2β2
χm = (1 Ϫ Ximp
)
ͩ
(3 ϩ exp(ϪJ/kT ))Ϫ1
ϩ
ͪ
kT
Ximp(3/(8T )) ϩ TIP = Ϫ2JS1S2 (S1 = S2 = 1/2) (1)
molar susceptibility versus temperature derived from the
spin-exchange Hamiltonian H.
For a satisfactory fit, the following parameters: g = 1.945,
temperature independent magnetism (TIP) = 83 × 10Ϫ6 cm3
molϪ1, paramagnetic impurity (Ximp) = 3.27%, θ = Ϫ3.7 K, J =
Ϫ167.9(9) cmϪ1 were used. The antiferromagnetic coupling
constant of Ϫ167.9 cmϪ1 lies in the range of values found for
other binuclear vanadyl complexes containing µ-OR bridges
and dihedral angles varying between 175.7 to 180Њ as shown in
Table 1. A more accurate analysis of the relationship between
the magnetic coupling and the dihedral angle formed between
the two equatorial coordination planes (τ) is presented in this
paper. In fact, a qualitative analysis of magnetic interaction for
dimers of transition metal ions could be centered at the mag-
netic orbitals.20 In this view the electron in oxovanadium()
When τ is 180Њ (anti- and syn-orthogonal configurations
respectively for complexes 1 and 4) the interaction between the
magnetic orbitals is highest and consequently the correspond-
ing antiferromagnetic coupling is strongest. It is important to
note that the syn-orthogonal configurations are favored in
complexes 2 and 3 which exhibit steric hindrance due to the
ligand and/or the number of bridges between the vanadium
centers. The J values become less, relative to τ and in the limit
of 90Њ where we could expect an orthogonal situation between
the magnetic orbitals, a twist5 or an anti-coplanar configur-
ation5,10 can appear. In both cases ferromagnetic or small anti-
ferromagnetic coupling can be detected. Therefore, Fig. 4 shows
a linear dependence between the J values and τ. Furthermore,
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4 K. K. Nanda, L. K. Thompson, J. N. Bridson and K. Nag, J. Chem.
Soc., Chem. Commun., 1994, 1337.
we can infer that τ is the principal factor affecting J which
suggests that in binuclear RO-bridging octahedral V()–oxo
compounds the most effective mechanism for magnetic inter-
actions is a direct interaction between dxy orbitals. Interestingly
the [HB(pz)3VO(OH)2]2 complex (HB(pz)3 = hydrotris(pyr-
azolyl)borate) shows two RO-bridges with an anti-orthogonal
configuration (τ = 180Њ) and therefore it should be strongly anti-
ferromagnetically coupled.25 However, it is clear that the
coupling in [HB(pz)3VO(OH)2]2 is anomalously low (i.e., Ϫ40
vs. ca. –150(20) cmϪ1) compared to that seen in the related com-
plexes.25 Therefore, based on the magnetic structural correlation
presented for the family of binuclear RO-bridging octahedral
V()–oxo complexes we could make a prediction about the
dihedral angle between the equatorial planes when it is not
possible to obtain crystals suitable for X-ray analysis. For
example [{VO(Hhebab)}2] (H3hebab = 1,1-bis(2-hydroxyethyl)-
4-(2-hydroxybenzyl)-1,4-diazabutane) shows a coupling con-
5 W. Plass, Angew. Chem., Int. Ed. Engl., 1996, 35, 627.
6 A. Rawas, H. Muirhead and J. Williams, Acta Crystallogr., Sect. D,
1996, 52, 631.
7 E. Sabbioni and E. Marafante, Bioinorg. Chem., 1978, 9, 389.
8 W. R. Harris and C. J. Carrano, J. Inorg. Biochem., 1984, 22, 201.
9 A. Neves, A. S. Ceccato, C. Erasmus-Buhr, S. Gehring, W. Haase,
H. Paulus, O. R. Nascimento and A. A. Batista, J. Chem. Soc.,
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17 L. Zsolnai, ZORTEP, An Interactive ORTEP Program, University
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stant, J = Ϫ170 cmϪ1 25
, which corresponds to τ ≈ 180Њ, similar
to that found in complexes 1 and 4. This prevision is in agree-
ment with that based on spectroscopic data and functional
density calculations.26
18 P. E. Riley, V. L. Pecoraro, C. J. Carrano, J. A. Bonadies and K. N.
Raymond, Inorg. Chem., 1986, 25, 154.
19 N. D. Chasteen, Biological Magnetic Resonance, ed. L. Berliner and
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20 A. P. Ginsberg, Inorg. Chim. Acta Rev., 1971, 5, 45.
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22 M. Mikuriya and M. Fukuya, Bull. Chem. Soc. Jpn., 1996, 69,
679.
Acknowledgements
We acknowledge grants from CAPES-Projeto PROBRAL,
CNPQ (PIBIC), DLR (Germany). We thank Professor Dr
J. Strähle (Institut für Anorganische Chemie der Universität
Tübingen, Germany) for X-ray diffraction facilities.
23 A. Neves and K. Wieghardt, Inorg. Chim. Acta, 1988, 150, 183.
24 K. Wieghardt, U. Bossek, K. Volckmar, W. Swiridoff and J. Weiss,
Inorg. Chem., 1984, 23, 1387.
25 N. S. Dean, M. R. Bond, C. J. O’Connor and C. J. Carrano, Inorg.
Chem., 1996, 35, 7643.
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