Synthesis, solid-state structures, and EPR spectroscopic studies on polycrystalline and single-crystal samples of α-diimine cobalt(II) complexes

Article abstract of DOI:10.1002/ejic.200600448

Cobalt compounds of the general formula [CoX₂(α-diimine)], where X = Cl or I and the α-diimines are 1,4-diaryl-2,3-dimethyl-1,4- diaza-1,3-butadiene (Ar-DAB) and bis(aryl)-acenaphthenequinonediimine (Ar-BIAN) were synthesized by the direct reaction of the anhydrous cobalt salts CoCl ₂ or CoI₂ and the corresponding α-diimine ligand in dried CH₂Cl₂. The synthesized compounds are [Co(Ph-DAB)Cl₂] (1a), [Co(o,o′,p-Me₃C ₆H₂-DAB)Cl₂] (1b), and [Co(o,o′-iPr ₂C₆H₃-DAB)Cl₂] (1c) with the ligands Ar-DAB, and also [Co(o,o′,p-Me₃C₆H ₂-BIAN)I₂] (2′b) with the ligand Ar-BIAN. The crystal structures of all the compounds were solved by single-crystal X-ray diffraction. In all cases the cobalt atom is in a distorted tetrahedron, which is built up of two halide atoms and two nitrogen atoms of the α-diimine ligand. X-band EPR measurements of polycrystalline samples performed on compounds 1b, 1c, and 2′b indicate a high-spin Co^II ion (S = 3/2) in an axially distorted environment. Single-crystal EPR experiments on compounds 1b and 1c allowed us to evaluate the orientation of the g tensor in the molecular frame. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200600448

FULL PAPER
DOI: 10.1002/ejic.200600448
Synthesis, Solid-State Structures, and EPR Spectroscopic Studies on
Polycrystalline and Single-Crystal Samples of α-Diimine Cobalt(II) Complexes
Vítor Rosa,[a] Pablo J. Gonzalez,[a] Teresa Avilés,*[a] Pedro T. Gomes,[b] Richard Welter,*[c]
Alberto C. Rizzi,[d] Mario C. G. Passeggi,[d,e] and Carlos D. Brondino*[d]
Keywords: Cobalt(II) complexes / α-Diimine ligands / Single-crystal X-ray diffraction / EPR spectroscopy
Cobalt compounds of the general formula [CoX2(α-diimine)],
where X = Cl or I and the α-diimines are 1,4-diaryl-2,3-di-
methyl-1,4-diaza-1,3-butadiene (Ar-DAB) and bis(aryl)-
acenaphthenequinonediimine (Ar-BIAN) were synthesized
by the direct reaction of the anhydrous cobalt salts CoCl2 or
CoI2 and the corresponding α-diimine ligand in dried
CH2Cl2. The synthesized compounds are [Co(Ph-DAB)Cl2]
(1a), [Co(o,oЈ,p-Me3C6H2-DAB)Cl2] (1b), and [Co(o,oЈ-
iPr2C6H3-DAB)Cl2] (1c) with the ligands Ar-DAB, and also
[Co(o,oЈ,p-Me3C6H2-BIAN)I2] (2Јb) with the ligand Ar-BIAN.
The crystal structures of all the compounds were solved by
single-crystal X-ray diffraction. In all cases the cobalt atom
is in a distorted tetrahedron, which is built up of two halide
atoms and two nitrogen atoms of the α-diimine ligand. X-
band EPR measurements of polycrystalline samples per-
formed on compounds 1b, 1c, and 2Јb indicate a high-spin
CoII ion (S = 3/2) in an axially distorted environment. Single-
crystal EPR experiments on compounds 1b and 1c allowed
us to evaluate the orientation of the g tensor in the molecular
frame.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
Less information exists about the magnetic properties of
these types of compounds. The CoII ions can be present in
two different spin configurations (high, S = 3/2 or low, S =
1/2), but in general they adopt the high-spin configuration
when they are in distorted coordination environments. A
large amount of work has focused on the study of the mag-
netic properties of CoII compounds in different coordina-
tion environments, some of them analyzing CoII in metall-
oproteins and compounds of biological interest.[13–19] Most
of them have involved EPR spectroscopic studies on pow-
der or frozen solutions and the evaluation of thermo-
dynamic properties such as magnetic susceptibility and
magnetization. In contrast, only a small amount of work
related to high-spin CoII compounds having a low sym-
metry has been devoted to the study of the correlation
between the principal values and directions of the g ten-
sor.[20–24] This type of information, which can be only ob-
tained from oriented single-crystal EPR spectroscopic ex-
periments, is essential to understand the electronic and
magnetic properties of the systems containing CoII ion
complexes.
Introduction
Cobalt is a 3d transition metal element that has been
extensively studied over the last decades. Interest in cobalt
compounds arises from their participation in many different
processes, from industrial and technological applications as
catalysts to the essential role in several biological systems
of central importance in nature.[1–5]
A large number of late transition metal complexes bear-
ing α-diimine[6] ligands have been extensively studied in the
commercial production of polyolefin from ethene and pro-
pene.[7,8] However, reports on cobalt complexes containing
this type of ligands are still scarce.[9] Recently, some results
from the insertion polymerization of ethylene have been re-
ported using tetrahedral CoII complexes with related li-
gands.[10] α-Diimine ligands have also been the subject of
different studies regarding the relative binding strengths,[11]
as well as their reactivity with main group metals.[12]
[a] REQUIMTE/CQFB, Departamento de Química, FCT, Uni-
versidade Nova de Lisboa,
Caparica 2829-516, Portugal
E-mail: tap@dq.fct.unl.pt
We report herein the synthesis and characterization of a
series of CoII complexes of general formula [CoX2(α-di-
imine)], where X = Cl or I and the α-diimines are 1,4-diaryl-
2,3-dimethyl-1,4-diaza-1,3-butadiene (Ar-DAB) or bis-
(aryl)acenaphthenequinonediimine (Ar-BIAN), namely,
[Co(Ph-DAB)Cl2] (1a), [Co(o,oЈ,p-Me3C6H2-DAB)Cl2] (1b),
[b] Centro de Química Estrutural, Instituto Superior Técnico,
Av. Rovisco Pais, 1049-001 Lisboa, Portugal
[c] Laboratoire DECOMET, ILB Université Louis Pasteur
4, rue Blaise Pascal, 67000 Strasbourg, France
E-mail: welter@chimie.u-strasbg.fr
[d] Departamento de Física, Facultad de Bioquímica y Ciencias
Biológicas, Universidad Nacional del Litoral,
C.C. 242, 3000 Santa Fe, Argentina
E-mail: brondino@fbcb.unl.edu.ar
[e] INTEC, Universidad Nacional del Litoral,
Güemes 3450, 3000 Santa Fe, Argentina
[Co(o,oЈ-iPr2C6H3-DAB)Cl2]
(1c),
and
[Co(o,oЈ,p-
Me3C6H2-BIAN)I2] (2Јb). Their crystal structures were
solved by single-crystal X-ray diffraction. Powder samples
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T. Avilés, R. Welter, C. D. Brondino et al.
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of compounds 1b, 1c, and 2Јb were studied by EPR spec- ligand were observed in the region ν = 1674–1633 cm–1. The
˜
troscopy. Compounds 1b and 1c were also studied by single- bands in the complexes are shifted to lower wavenumbers,
crystal EPR spectroscopy. The EPR spectroscopic results which is a criterion for the coordination of both diimine
obtained from both powder and single-crystal samples were nitrogen atoms of the α-diimine ligands to the cobalt(II)
correlated with the structures of the compounds.
ion. Selected IR absorption bands are displayed in Table 2.
Table 2. Selected IR data in Nujol mulls (values of ν given in cm–1).
˜
Ar-DAB[a]
1
Ar-BIAN[a]
2Ј
Results and Discussion
a
b
c
1633
1637
1639
1643
1635
1641
General Characterization of the Compounds
1674, 1650
1621
The reaction of equimolar quantities of CoX2, where X
= Cl or I, and α-diimine ligands in dried CH2Cl2 at room
temperature yields crystalline solids of compounds of the
general formula [Co(α-diimine)X2] in c.a. 75 to 85% yield
(Scheme 1).
[a] Free ligands.
Crystal Structure Descriptions
Elemental analyses, mass spectrometry, IR, X-ray, and
EPR spectroscopy were used to characterize all of the syn-
thesized compounds. The analytical data of the synthesized
compounds are shown in Table 1.
Complex 1a
The complex 1a crystallizes in an orthorhombic space
group P212121. An ATOMS[25] view of the asymmetric unit
The IR spectra of the cobalt complexes recorded as Nu- of the crystal structure is shown in Figure 1. The cobalt
jol mulls display medium absorption bands in the region ν atom is in a deformed tetrahedron, which is built up of two
˜
= 1643–1621 cm–1, this is the absorption region for ν(C=N). chloride atoms and two nitrogen atoms of the ligand [Co–
Bands assigned to C=N stretching vibrations of the free Cl1: 2.214(1) Å; Co–Cl2: 2.210(1) Å; Co–N1: 2.043(4) Å;
Scheme 1. Synthesized complexes.
Table 1. Analytical data.
Molecular mass [gmol–1];
Molecular formula
Microanalysis (calcd.) [%]
Color
green
MS(FAB) m/z[a]
C
H
N
1a
366.15;
C16H16CoCl2N2
[L1 + H+] 237 (77%);
[L1CoL1]2+ 265.5 (100%);
[CoL1Cl]+ 330 (36%);
[L1CoL1Cl]+ 566 (71%)
[L2CoCl]+ 414.2 (97%)
52.09
(52.48)
4.34
(4.40)
7.63
(7.65)
1b
1c
450.31;
C22H28CoCl2N2
576.94;
C28H40CoCl2N2·1/2CH2Cl2
729.30;
58.46
(58.68)
60.63
6.42
(6.27)
7.30
6.34
(6.22)
5.16
(4.86)
3.65
(3.84)
green
green
brown
[CoL3Cl]+ 498.3 (96%)
[L4 + H+] 417.3 (16%);
[L4Co I]+ 602.1 (30%)
(59.33)
49.26
(49.41)
(7.16)
4.16
(3.87)
2Јb
C30H28CoI2N2
[a] Ligand 1,4-diaryl-2,3-dimethyl-1,4-diaza-1,3-butadiene (Ar-DAB); L1: Ar = Ph; L2: Ar = o,oЈ,p-Me3C6H2; L3: Ar = o,oЈ-iPr2C6H3;
Ligand bis(aryl)acenaphthenequinonediimine (Ar-BIAN); L4: Ar = o,oЈ,p-Me3C6H2N.
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Polycrystalline and Single-Crystal α-Diimine Cobalt(II) Complexes
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Co–N2: 2.034(4) Å; Cl1–Co–Cl2: 119(1)°; N1–Co–N2: tetrahedral site around the Co atom is characterized by the
80(1)°]. The [Co–N1–C1–C3–N2] plane is strictly planar following distances and angles: Co–Cl1: 2.197(2) Å; Co–
[maximum deviation from least-squares plane is 0.007(5) Å] Cl2: 2.201(1) Å; Co–N: 2.050(3) Å Cl1–Co–Cl2: 113(1)°;
and makes an angle of 56(1)° with both phenyl groups N1–Co–N2: 80(1)°. Compared to that observed in both the
([C5–C10] and [C11–C16]). No hydrogen bonds have been previous structures, the plane containing the Co atom is not
found in this crystal structure.
well fitted by the least-squares planes calculation
(SHELXL-97)[27] with a maximum deviation from the plane
of 0.112(3) Å for both nitrogen atoms. This pseudo plane
makes an angle of 87(1)° with both phenyl groups (identical
by symmetry), the latter are tilted with an angle of 15(1)°,
one compared to the other. At least two nonclassical intra-
molecular hydrogen bonds have been detected (PLATON
software) between C9 and N [H9–N: 2.43(1) Å; C9–N:
2.94(1) Å; C9–H9–N: 111(1)°] and C12 and N [H12–N:
2.38(1) Å; C12–N: 2.88(1) Å; C12–H12–N: 110(1)°].
Figure 1. Molecular structure of complex 1a (ATOMS view) with
partial labeling scheme. The hydrogen atoms are omitted for clarity.
The thermal ellipsoids enclose 50% of the electronic density.
Complex 1b
The complex 1b crystallizes in a monoclinic space group
P21/c. An ATOMS view of the asymmetric unit of the crys-
tal structure is shown in Figure 2. In this case again, the
cobalt atom is in a deformed tetrahedron [Co–Cl1:
2.224(1) Å; Co–Cl2: 2.212(1) Å; Co–N1: 2.050(3) Å; Co–
N2: 2.051(3) Å; Cl1–Co–Cl2: 113(1)°; N1–Co–N2: 80(1)°].
Two nonclassical intermolecular hydrogen bonds have been
detected (PLATON software[26]) between C3 and Cl2
[H3B–CL2: 2.75(1) Å; C3–Cl2: 3.68(1) Å; C3–H3B–Cl2:
160(1)°] and C3 and Cl1 [H3C–CL1: 2.81(1) Å; C3–Cl1:
3.78(1) Å; C3–H3C–Cl1: 169(1)°]. The angle between both
phenyl groups is 14(1)° and these groups are perpendicular
to the central plane [Co–N1–C1–C2–N2] [maximum devia-
tion from least-squares plane is 0.035(2) Å] with angles of
88(1)° and 90(1)°, respectively.
Figure 3. Molecular structure of complex 1c (ATOMS view) with
partial labeling scheme. The hydrogen atoms are omitted for clarity.
The thermal ellipsoids enclose 50% of the electronic density. Oper-
ators for generating equivalent atoms: x, y, –z+3/2.
Complex 2Јb
Complex 1c
The complex 2Јb crystallizes in an orthorhombic space
The complex 1c crystallizes in an orthorhombic space group Pcca. An ATOMS view of the asymmetric unit of
group Pnma. An ATOMS view of the asymmetric unit of the crystal structure is shown in Figure 4. The deformed
the crystal structure is shown in Figure 3. The deformed tetrahedral site around the Co atom is characterized by the
Figure 2. Molecular structure of complex 1b (ATOMS view) with partial labeling scheme. The hydrogen atoms and the solvent molecules
(CH2Cl2) are omitted for clarity. The thermal ellipsoids enclose 50% of the electronic density.
Eur. J. Inorg. Chem. 2006, 4761–4769
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following distances and angles: Co–I: 2.509(1) Å; Co–N:
2.05(1) Å; I–Co–IЈ: 111(1)°; N–Co–NЈ: 82(1)°. The angle
between both phenyl groups is 14(1)° and these groups
make an angle of 85(1)° with the central plane [Co–N–C1–
C1Ј-NЈ] [maximum deviation from least-squares plane is
0.001(4) Å]. It is important to note that the molecule of
complex 2Јb is placed on a crystallographic mirror i.e. the
following atoms are on the mirror (see Figure 4): Co, N,
C8, C1, C2, C4, C5, C6, C7 and the corresponding equiva-
lent atoms by the following symmetry operator: –x, y, –z +
1/2. No H bond has been detected in the case of this crystal
structure. No H bond has been detected in the case of this
molecular structure.
Figure 5. EPR spectra of powdered samples of compounds 1b, 1c,
and 2Јb together with simulations. The EPR parameters obtained
from simulation are given in Table 3. For compound 1c, the peak
at Ϸ3300 Gauss corresponds to CuII impurities whereas the peak
at Ϸ500 Gauss corresponds to a cavity background.
Table 3. EPR parameters obtained from powder and single-crystal
samples for 1b, 1c, and 2Јb. Linewidths (∆B) are in Gauss. The
components of the crystal g tensors are expressed in the abc* (c*
= a×b) and abc coordinates system for compounds 1b and 1c,
respectively. They were obtained by least-squares analyses of the
single-crystal data.
Figure 4. Molecular structure of complex 2Јb (ATOMS view) with
partial labeling scheme. The hydrogen atoms and the solvent mole-
cules (CH2Cl2) are omitted for clarity. The thermal ellipsoids en-
close 50% of the electronic density. Operators for generating equiv-
alent atoms: –x, y, –z+1/2.
Compound
1b
1c
2Јb
Powder sample
g1
g2
g3
∆B1
∆B2
∆B3
4.83(5)
4.37(5)
2.58(9)
580(20)
680(20)
900(20)
4.85(5)
4.35(5)
3.12(9)
280(20)
280(20)
600(20)
4.87(5)
4.71(5)
3.08(9)
580(20)
480(20)
800(20)
EPR Measurements
The X-band EPR spectra of polycrystalline samples of
compounds 1b, 1c, and 2Јb and their simulations are shown
in Figure 5. The peaks broaden at higher T and disappear
at ca. 50 K. The hyperfine structure expected for the 100%
abundant 59Co isotope (59I = 7/2) is not observed in any of
the spectra of the powder samples. The values of the g fac-
tors g1, g2, and g3 are given in Table 3 and are typical for
CoII ions with a high spin S = 3/2 configuration.[17] The
EPR spectrum obtained for compound 1a (data not shown)
is also compatible with a CoII ion in a high-spin configura-
tion, however, a broad signal detected in the range 500–
8000 Gauss and overlapping the CoII ion transitions pre-
cluded the interpretation.
Single crystal
gaa
gbb
gcc
gac
gab
gcb
2.66(2)
4.83(1)
4.32(1)
1.51(4)
0
4.47(1)
3.12(2)
4.71(1)
0
0
0
–
–
–
–
–
–
0
gʈ
gЌ
E/D
2.58(2)
2.30(1)
0.03
3.12(2)
2.30(1)
0.04
3.08(9)
2.40(5)
0.01
to be substantially populated.[28] The high anisotropy ob-
In a purely tetrahedral environment, CoII has the orbital served is also expected for CoII compounds with a distorted
singlet A2 at lowest energy,[13,14] and since S = 3/2, there is tetrahedral coordination.[29,30] Since the structural data do
a fourfold spin degeneracy. As for compounds 1b, 1c, and not show relevant chemical paths connecting the metal sites
2Јb the sites are distorted, the fourfold spin degeneracy is that could efficiently transmit exchange interactions, the
lifted into two Kramers doublets. If the distortion is high principal g values obtained from the powder spectra
enough (ϾϾkT) to neatly separate both doublets and the (Table 3) can be considered as corresponding to isolated
experiments are performed at low temperatures, then the CoII ions, in which the resonance lines may be broadened
observed spectra can be assigned to transitions between the by dipolar interactions. The crystal lattice of all compounds
states of the lowest SЈ = 1/2 spin doublet, which is expected consists of a 3D network of cobalt(II) ions, with closest
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Polycrystalline and Single-Crystal α-Diimine Cobalt(II) Complexes
FULL PAPER
metal center distances in the range 6.4–8.4 Å. This could be in the experimental section. In all the planes and for the
the cause of the unobserved hyperfine structure in the EPR two compounds we observed one resonance line for all
powder spectra.
magnetic field orientations, except around the b axis in the
Single-crystal EPR spectroscopic measurements were orthorhombic compound 1c. In the planes ab and cb the
only performed on compounds 1b and 1c as the size of the signals become very broad around 90°, presumably because
crystals of compounds 1a and 2Јb was not appropriate for of a nonresolved hyperfine structure and therefore the posi-
the EPR spectroscopic experiments. In order to understand tion of the resonances in this zone could not be precisely
the single-crystal EPR spectroscopic experiment, it is im- evaluated. The position of the resonance line was obtained
portant to first analyze the orientation of the Co sites as by least-squares fitting to a single Lorentzian line shape.
well as the symmetry operations relating them within the Figure 7 shows the angular variation of the g factor as a
unit cell. The unit cell of compound 1b contains four iden- function of the magnetic field orientation for both com-
tical Co molecules, which are related by the symmetry oper- pounds. The data in Figure 7 were least-squares fitted to a
ations of the space group P21/c (Figure 6a). Co1 at the ge- second rank tensor g2(θ,φ) = h.g.g.h, in which h = sinθcosφ,
neral position (x,y,z) is related to Co2 by a C2b rotation, as sinθsinφ, cosθ, is the magnetic field orientation, and g is the
well as Co3 to Co4. Co1 and Co2 are related to Co3 and crystal g tensor defined as the average of the molecular g
Co4, respectively, by an inversion. From the magnetic point tensors of the nonequivalent CoII ions in the crystal lattice.
of view, this compound can be assumed as having two mag- The results are given in Table 3 and were used to obtain the
netically nonequivalent molecules per unit cell, because the solid lines in Figure 7. The overall symmetry of the evalu-
Co sites related by an inversion are indistinguishable for ated g tensors follows the symmetry determined by the
EPR spectroscopy.
space group of each compound.
Figure 6. (a) Projection along a of the four CoII ions and their li-
gands in the unit cell for compound 1b. (b) Projection along b of
the four CoII ions and their ligands in the unit cell for compound
1c.
Figure 7. Angular variation of the g factor obtained from oriented
single-crystal EPR spectroscopic measurements of compounds 1b
and 1c at 9.65 GHz and T = 4.6 K. The solid lines in both plots
were calculated with the components of the g2 tensor given in
Table 3.
Compound 1c also has four identical Co molecules in
the unit cell, related by the symmetry operations of the
space group Pnma (Figure 6b). Co1 at the special position
As discussed above, the angular variation of the g factor
(x,1/4,z) is related to Co2, Co3, and Co4 by C2b, C2a, and in the orthorhombic compound 1c can be associated with
C2c rotations, respectively. These operations determine that a single CoII ion in the planes ab and cb. By contrast, in
there are two CoII ion pairs magnetically equivalent per the ca plane we would expect two resonance lines corre-
crystal plane, and also that the four Co sites are magneti- sponding to the pairs Co1–Co2 and Co3–Co4. However,
cally equivalent along the crystal a,b,c axes. The Co mole- only one signal with a nearly isotropic angular variation
cule has a mirror plane defined by the CoII ion and the two was registered in this plane, which corresponds to the mir-
Cl ligands, which is parallel to the ca plane. This particular ror Cl–Co–Cl plane. This fact can be due to either collapse
local symmetry together with the space orientation of the by exchange interactions or resonance lines with similar g
Co molecules in the unit cell determines that the nonequiva- factors. Taking into account that from the structural data
lent pairs of CoII ions are equivalent in the planes ab and we can discard the first possibility, the observed behavior
cb.
can only be explained assuming two resonance lines with
Single-crystal EPR spectra were recorded with the mag- similar g factors, which cannot be resolved by the EPR
netic field in three orthogonal crystal planes as explained spectroscopic experiment at the X-band.
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The orientation of the molecular g tensor for compound
Structural data for compound 1b show Co sites with co-
1c in the molecular frame can be evaluated from both the ordination environments and space orientations similar to
analysis of the geometry of coordination of the metal site that of 1c, but the local symmetry is lower (the local mirror
and the EPR spectroscopic results. One eigenvector must plane cannot be defined in this case) and the Cl–Co–Cl
correspond to the b crystal axis (0, 1, 0) because, as ex- plane is twisted by nearly 10° from the cb crystal plane (Fig-
plained above, the planes ab and cb are associated with a ure 6a and b). The structural similarities are reflected in
single CoII ion and the angular variation of the g factor is Figure 7: a nearly isotropic g factor was obtained in the
symmetric around the b crystal axis (g3 = 3.12). Thus, the plane c*b, and the lowest g value corresponds to a direction
two remaining eigenvectors must be in the Cl–Co–Cl plane. close to the normal of the Cl–Co–Cl plane. However, hav-
The Co molecule can be considered as having a local ing nonresolved lines in the ab and cb planes precludes de-
pseudo-C2 symmetry axis (–0.760, 0, 0.650) determined by termining the g tensor associated with single CoII ions. De-
the intersection of the Cl–Co–Cl plane (parallel to the ca spite its lower symmetry, the coordination around CoII ions
crystal plane) and the N–Co–N plane. This C2 symmetry in both compounds shows no significant differences and we
axis must correspond to another eigenvector. The direction expect a g tensor orientation similar to that shown in Fig-
of the third eigenvector is straightforward (0.650, 0, 0.760). ure 8 for 1b. Although we were not able to perform single-
The orientation of the g tensor is shown in Figure 8. This crystal EPR experiments on compounds 2Јb, the EPR spec-
experiment allowed us to identify the eigenvalue along the troscopic data from powdered samples are similar for the
b crystal axis, but not the remaining two eigenvalues be- three compounds. Consequently, we also expect similar g
cause the single EPR line observed in the ca plane corre- tensor orientation for the compounds in which I substitutes
sponds to two overlapping resonance lines associated with for Cl, but single-crystal experiments are necessary to con-
the pairs Co1–Co2 or Co3–Co4. However, they should firm this hypothesis.
correspond to the highest g values (4.35–4.85) of the g ten-
After this phenomenological assignment has been made,
sor obtained from the EPR powder spectra. Because of the one wonders whether it might be possible to relate the
above discussed nonequivalence of the cobalt ions in the ca highly anisotropic g factors corresponding to the effective
plane both eigenvalues can be assigned to any of the g1 and SЈ = 1/2 of the lowest Kramers doublet with the distortions
g2 eigenvectors. The g-tensor orientation of compound 1c affecting the overall state of S = 3/2. The appropriate spin
is similar to that determined in the pseudo-tetrahedral Hamiltonian within the S = 3/2 can be written as [Equation
dichlorobis(triphenylphosphaneoxide)cobalt(II)
com- (1)]
pound.[21] This compound shows rhombic EPR spectra (g1
= 5.67, g2 = 3.69, g3 = 2.16) with the two lowest g values
approximately included in the Cl–Co–Cl plane. Compound
1c presents a nearly axial signal, with the lowest g value (g3)
lying along the normal to the Cl–Co–Cl plane. The main
structural difference between both compounds is that 1c
contains the stronger N ligands instead of O ligands. On
the other hand, EPR spectroscopic studies performed on a
square-pyramidal CoII complex showed that small devia-
tions of the ideal C4 site symmetry lead to important
changes in the EPR spectroscopic properties of high-spin
CoII ions.[22] Clearly, additional experimental work on a
larger number of high spin CoII compounds with different
structural characteristics is necessary to clarify how the dif-
ferent EPR spectroscopic properties can be correlated with
differences in the coordination environment of the metal
ion.
where D and E are the axial and rhombic zero-field split-
ting parameters, respectively. The quantization axis z is as-
sumed to be along the axial distortion. In principle, it is not
required to assume that g will have its principal axes along
any of these directions.
An effective Hamiltonian within the lowest SЈ = 1/2 (cor-
responding to either the originals |3/2, 3/2Ͼ or the |3/2,
1/2Ͼ states) Kramers doublet can be obtained under the
form[31–34]
Heff = µBBgЈSЈ
If the ground state is that derived from to the original
|3/2, 1/2Ͼ doublet and if one assumes now that the princi-
pal axes of g coincide with that of the axial distortion such
that gz = gʈ and gx = gy = gЌ, one gets gЈz = gʈ while gЈx =
3 E
2gЌ(1 – · ) and gЈy = 2gЌ(1 + · ). Note that gЈx, gЈy,
2 D 2 D
3 E
and gЈz correspond to g2, g1, and g3, respectively, given in
Table 3. The experimental data are not consistent with as-
signing the ground state to that derived from the original
|3/2, 3/2Ͼ doublet, in which gЈz = 3gʈ while gЈx Ϸ 0 and
gЈy Ϸ 0, which indicates a positive sign for the zero-field
splitting parameter D. EPR data obtained from Cs3CoCl5
and Cs3CoBr5 complexes have been analyzed assuming a
negative value for the D parameter,[35] and EPR spectro-
scopic results obtained from bis[dihydrobis(1-pyrazolyl)-
Figure 8. Orientation of the principal axes of the g tensor for the
CoII site in compound 1c. The direction cosines referring to the
crystal axes are: (–0.760, 0, 0.650), (0.650, 0, 0.760), and (0, 1, 0)
for g1, g2, and g3 = 3.12, respectively. g1 and g2 can take either the
values 4.35 and 4.85, respectively, or vice versa.
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Polycrystalline and Single-Crystal α-Diimine Cobalt(II) Complexes
FULL PAPER
borato]CoII have been interpreted assuming transitions
|3/2, 3/2Ͼ.[36] In contrast, our EPR spectroscopic data
indicate a |3/2, 1/2Ͼ ground state, in line with both theo-
retical and experimental results obtained from other pseu-
dotetrahedral CoII compounds.[20,21]
According to the previous discussion, adopting a coordi-
nate system in which the z axis lies along the g3 direction (in
compound 1c this coincides with the crystal b axis), leads to
gЈz = gʈ. One can also estimate gЌ by using the other two
ter using a rectangular cavity with 100 kHz field modulation, and
equipped with an Oxford continuous flow cryostat. The EPR pa-
rameters of powdered samples were obtained from spectral simula-
tions using the program Simfonia (v. 1.25, Bruker Instruments
Inc.). Powder samples for EPR spectroscopy were obtained by
grinding single crystals.
The samples for the single-crystal EPR spectroscopic measure-
ments were oriented by gluing the cb and ca faces of 1b and 1c,
respectively, to a cleaved KCl cubic holder, which defines a set of
orthogonal laboratory axes. The habit of the crystals was deter-
mined by measuring the angles between crystal faces using a gonio-
metric microscope. The sample holder was introduced into a 4 mm
OD quartz tube, and positioned in the center of the microwave
cavity (see ref.[38] for details). The tubes were attached to a gonio-
meter and the sample was rotated in steps of 10° with the magnetic
field in three crystal planes.
eigenvalues obtained from the powder spectra, e.g. gЌ
Х
(gЈx + gЈy)/4. On the other hand, under the same assump-
tions, one gets
Synthesis of Complexes
Table 3 shows these parameters obtained for the three
compounds using this model. The results indicate a weak
rhombic distortion for all the compounds.
Synthesis of [Co(o,oЈ,p-Me3C6H2-DAB)Cl2] (1b): A suspension of
CoCl2 (0.26 g, 2 mmol) in CH2Cl2 (20 mL) was treated with a yel-
low solution of o,oЈ,p-Me3C6H2-DAB, (0.64 g, 2 mmol) in CH2Cl2
(30 mL), a color change was observed almost immediately from the
blue of the initial suspension to green. The mixture was left stirring
at room temperature for about 2 h until everything was dissolved.
The solution was filtered, and concentrated by vacuum removal of
the solvent. A green crystalline solid precipitated, which was sepa-
rated by filtration, washed with diethyl ether (2×10 mL) and petro-
leum ether (2×10 mL), and then recrystallized from hot CH2Cl2.
1b (0.76 g) was obtained in 85% yield.
Conclusions
A series of cobalt(II) complexes of general formula
[Co(α-diimine)X2], 1a, 1b, 1c, and 2Јb, have been synthe-
sized by the direct reaction of equimolar quantities of the
corresponding cobalt dihalide and the α-diimine ligand in
dried CH2Cl2 in c.a. 75 to 85% yield. Single-crystal X-ray
structural data for all compounds show in all cases that the
cobalt atom is in a distorted tetrahedral coordination,
which is built by two halide atoms and two nitrogen atoms
of the α-diimine ligand. X-band EPR measurements in
polycrystalline samples performed on 1b, 1c, and 2Јb indi-
cate high-spin CoII (S = 3/2) in an axially distorted environ-
ment. The single-crystal EPR spectroscopic experiment al-
lowed us to determine the g-tensor orientation for a low
symmetry CoII compound. Structural and EPR spectro-
scopic results suggest similar g-tensor orientations for all
the compounds and the electronic properties of the CoII
ions seem to be independent of the type of halide coordi-
nated to the metal site.
Compounds 1a and 1c were obtained by this method. Yields ob-
tained were between 75 and 85%. Compounds 1a, 1b, and 1c are
moisture sensitive.
Synthesis of [Co(o,oЈ,p-Me3C6H2-BIAN)I2] (2Јb): A green suspen-
sion of CoI2 (0.16 g, 0.5 mmol) in CH2Cl2 (20 mL) was treated with
a red solution of o,oЈ,p-Me3C6H2-BIAN (0.21 g, 0.5 mmol) in
CH2Cl2 (30 mL). The mixture quickly turned to a red-wine color,
and it was stirred overnight until everything was dissolved. After
filtration of the solution the solvent was partially removed leaving
a dark-red solid, which was separated by filtration, washed with
petroleum ether (2×10 mL), and then recrystallised from CH2Cl2/
petroleum ether. 2Јb (0.29 g) was obtained in 78% yield.
Crystallography: Single crystals of complexes 1a, 1b, 1c, and 2Јb
were mounted on a Nonius Kappa-CCD area detector dif-
fractometer (Mo-Kα λ = 0.71073 Å). The complete conditions of
data collection (DENZO software) and structure refinements are
given below. The cell parameters were determined from reflections
taken from one set of 10 frames (1.0° steps in phi angle), each at
20 s exposure. The structures were solved by direct methods
(SHELXS97) and refined against F2 using the SHELXL97 soft-
ware.[27] The absorption was not corrected. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were generated
according to stereochemistry and refined using a riding model in
SHELXL97. Some disorders have been fixed for solvent molecules
in the case of 1b and 2Јb complexes (for details, see the correspond-
ing cif files). The fully-labeled ORTEP views have been deposited
as supplementary materials. CCDC-297433–297436 contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Experimental Section
General Procedures and Materials
All reactions and manipulations of solutions were performed under
an argon atmosphere using Schlenk techniques. Solvents were rea-
gent grade and were dried according to literature methods. CoCl2
and CoI2 were purchased from Aldrich and the α-diimine ligands
were synthesized as described elsewhere.[37]
Physical Methods
Infrared spectra were recorded as Nujol mulls on NaCl plates using
a Mattson Satellite FTIR spectrometer. Elemental analyses were
performed at the Analytical Services of the Laboratory of
REQUIMTE. Mass spectra were recorded using an HP298S GC/
MS system by the Mass Spectra Service of the Universitat Autò-
noma de Barcelona, Spain.
1a: Green single crystal, dimension: 0.10×0.10×0.10 mm.
C16H16Cl2CoN2, M = 366.14 gmol–1; orthorhombic; space group
P212121; a = 10.5880(4) Å; b = 8.2160(3) Å; c = 19.1880(9) Å; Z =
X-band CW EPR spectra of polycrystalline samples and single-
crystal samples were taken at 4.6 K with a Bruker EMX spectrome-
Eur. J. Inorg. Chem. 2006, 4761–4769
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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T. Avilés, R. Welter, C. D. Brondino et al.
FULL PAPER
4; Dcalcd. = 1.457 gcm–3; µ(Mo-Kα) = 1.342 mm–1; a total of 14084
reflections; 2.20° Ͻ θ Ͻ 30.00°, 4806 independent reflections with
3211 having IϾ2σ(I); 191 parameters; final results: R1 = 0.1149;
wR2 = 0.1256, Gof = 1.179, Flack x; 0.00(7), maximum residual
electronic density = 0.614 eÅ–3.
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1b: Green single crystal, dimension: 0.10×0.08×0.03 mm.
C22H28Cl2CoN2, CH2CL2, M = 535.22 gmol–1; monoclinic; space
group P21/c; a = 12.686(5) Å; b = 14.514(5) Å; c = 14.180(5) Å;
β
= 90.76(5)°; Z = 4; Dcalcd. = =
1.362 gcm–3; µ(Mo-Kα)
1.079 mm–1; a total of 9800 reflections; 2.13° Ͻ θ Ͻ 29.98°, 7555
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1c: Green single crystal, dimension: 0.13×0.10×0.08 mm.
C28H40Cl2 CoN2, M = 534.45 gmol–1; orthorhombic; space group
Pnma; a = 10.4110(10) Å; b = 12.707(2) Å; c = 21.311(4) Å; Z = 4;
Dcalcd. = 1.259 gcm–3; µ(Mo-Kα) = 0.816 mm–1; a total of 33608
reflections; 1.91° Ͻ θ Ͻ 29.83°, 3965 independent reflections with
2385 having IϾ2σ(I); 154 parameters; final results: R1 = 0.0614;
wR2 = 0.2027, Gof = 0.825, maximum residual electronic density
= 0.324 eÅ–3.
2Јb: Brown single crystal, dimension: 0.08×0.07×0.06 mm.
C30H28CoI2N2, 0.2(C4H10O), M = 744.10 gmol–1; orthorhombic;
space group Pcca;
a = 16.769(3) Å; b = 11.676(2) Å; c =
18.313(3) Å; Z = 4; Dcalcd. = 1.378 gcm–3; µ(Mo-Kα) = 2.219 mm–1;
a total of 9862 reflections; 1.74° Ͻ θ Ͻ 30.04°, 5230 independent
reflections with 3769 having IϾ2σ(I); 168 parameters; Final re-
sults: R1 = 0.0710; wR2 = 0.2249, Gof = 1.049, maximum residual
electronic density = 1.424 eÅ–3.
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Acknowledgments
Work was supported by Fundação para a Ciência e Tecnologia
(Project POCI/QUI/55519/2004) in Portugal and SEPCYT:PICT
2003-06-13872, CONICET PIP 02559/2000, and CAI+D-UNL in
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a Ph.D. grant (SFRH/BD/13777/2003). T. A. and R. W. thank CPU
(France) and CRUP (Portugal) for the Portuguese-French Inte-
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Polycrystalline and Single-Crystal α-Diimine Cobalt(II) Complexes
FULL PAPER
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Received: May 12, 2006
Published Online: October 5, 2006
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