Synthesis and characterization of a new series of 1D cobalt(II) complexes of pyridine derivative ligands with a single azido bridge and X-ray crystal structure of catena-(μ-N3)[Co(4-methylpyridine)4](PF6)

Article abstract of DOI:10.1016/S0277-5387(99)00127-8

A new series of polymeric cobalt(II) azido complex cations of the formula [CoL₄(N₃)]PF₆-nH₂O (n=0 or 1), for L=some pyridine derivative ligands, have been prepared and characterized. Solid state electronic spectra suggest six-coordinated cobalt(II) in all complexes. IR spectral results reveal the existence of symmetrical azido ligand, except for complexes of L=3-Etpy and 4-Etpy(red) which contain asymmetrical azides, as well as PF₆^- counter ions in all complexes. X-ray crystal structure determination has shown the complex for L=4-methylpyridine to contain polymeric [CoL₄(N₃)]⁺ cation and PF₆^- anions. In the cation, each cobalt(II) atom is coordinated by four nitrogen atoms from the pyridine ligands and two nitrogen atoms each from a μ-(1,3)-bridging azide giving rise to polymeric chains (1D) of six-coordinate Co(II) polyhedra. The azide ligand is symmetrical [N₁-N₂=N₂-N₃=1.170(3) A] and linear within experimental error. Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(99)00127-8

Polyhedron 18 (1999) 2339–2344
Synthesis and characterization of a new series of 1D cobalt(II) complexes
of pyridine derivative ligands with a single azido bridge and X-ray crystal
structure of catena-(m-N3)[Co(4-methylpyridine)4](PF6)
b
Mohamed A.S. Gohera , , Franz A. Mautner
*
aDepartment of Chemistry, Faculty of Science, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait
bInstitut fur Physikalische und Theoretische Chemie, Technische Universitat Graz, A-8010 Graz, Austria
Received 6 February 1999; accepted 15 April 1999
Abstract
A new series of polymeric cobalt(II) azido complex cations of the formula [CoL4(N3)]PF6 –nH2O (n50 or 1), for L5some pyridine
derivative ligands, have been prepared and characterized. Solid state electronic spectra suggest six-coordinated cobalt(II) in all complexes.
IR spectral results reveal the existence of symmetrical azido ligand, except for complexes of L53-Etpy and 4-Etpy(red) which contain
asymmetrical azides, as well as PF26 counter ions in all complexes. X-ray crystal structure determination has shown the complex for
L54-methylpyridine to contain polymeric [CoL4(N3)]1 cation and PF26 anions. In the cation, each cobalt(II) atom is coordinated by four
nitrogen atoms from the pyridine ligands and two nitrogen atoms each from a m-(1,3)-bridging azide giving rise to polymeric chains (1D)
˚
of six-coordinate Co(II) polyhedra. The azide ligand is symmetrical [N1 –N25N2 –N351.170(3) A] and linear within experimental error.
1999 Elsevier Science Ltd. All rights reserved.
Keywords: 1D Cobalt(II) azido complexes; Pyridine derivative complexes; Preparation of pyridine derivative complexes; Crystal structure; Spectra
1. Introduction
formula [CoL4(N3)]PF6 –nH2O (n50 or 1). This paper
reports the synthesis and characterization of such a series
of complexes along with the X-ray structure analysis of
The azide ion N23 , is a versatile ligand which can bind
atoms in a variable manner giving rise to terminal, end-on
bridging and end-to-end bridging modes described some-
where else [1–8]. This versatility of the azide ligand not
only enriches the stereochemistries of transition metal ions
but also their spectroscopic and magnetic properties [9–
16].
catena-(m-N3)[Co(4-methylpyridine)4](PF6),
a
typical
member of this series.
2. Experimental
Despite the fact that a substantial array of copper(II)
azido complexes [17–23] of pyridine derivative ligands
and very recently the corresponding manganese(II) azido
complexes [24,25] have been synthesized, those of cobal-
t(II) azide have received a little attention. Thus, in the
early 1980s, some cobalt(II) azido complexes with
pyridine derivative ligands of the general formula
CoL2(N3)2 have been reported [26,27] but none of them
were determined by X-ray crystal diffraction. In a trial to
synthesize polynuclear cobalt(II) complexes containing
bridging azides of such pyridine ligands, we succeeded in
the preparation of a new series of complexes of the general
Pyridine derivative ligands were purchased from the
Aldrich or BDH companies and other chemicals are of
analytical grade quality.
Abbreviations for the pyridine ligands are given as a
footnote under Table 1.
2.1. Preparation of complexes
2.1.1. [Co(4-Mepy)4(N3 )]PF6 (1)
This complex was prepared by mixing an aqueous
solution of Co(NO3)2 –6H2O (2.0 mmol) dissolved in 10
cm3 water and 4-Mepy (4.0 mmol) in ca. 10 cm3 ethanol,
followed by KPF6 (2.4 mmol, 3 cm3 water) and NaN3(2.0
mmol) in ca. 3 cm3 water. The final clear solution mixture
*Corresponding author. Fax: 1965-481-6482.
E-mail address: goher@ku01.kuniv.edu.kw (M.A.S. Goher)
0277-5387/99/$ – see front matter
PII: S0277-5387(99)00127-8
1999 Elsevier Science Ltd. All rights reserved.

2340
M.A.S. Goher, F.A. Mautner / Polyhedron 18 (1999) 2339–2344
Table 1
Analytical dataa
Complex
Color
Analytical: Found (required)
C
H
N
Co
[Co(py)4(N3)]PF6
[Co(4-Mepy)4(N3)]PF6
[Co(4-Etpy)4(N3)]PF6
[Co(4-Etpy)4(N3)]PF6
[Co(3-Etpy)4(N3)]PF6
[Co(4-t-but-py)4(N3)]PF6?H2O
[Co(4-Bzpy)4(N3)]PF6?H2O
[Co(3-Bzpy)4(N3)]PF6?H2O
Violet crystals
Violet-red crystals
Pink powder
Red crystals
Violet powder
Pink powder
Violet
42.3 (42.7)
46.1 (46.6)
49.2 (49.8)
49.6 (49.8)
50.0 (49.8)
53.2 (53.7)
57.1 (57.8)
57.0 (57.8)
3.8 (3.6)
4.8 (4.6)
5.6 (5.4)
5.3 (5.4)
5.6 (5.4)
6.7 (6.8)
3.9 (3.8)
3.7 (3.8)
17.8 (17.4)
15.5 (15.8)
14.2 (14.5)
14.6 (14.5)
14.8 (14.5)
12.5 (12.2)
10.0 (9.8)
9.5 (9.8)
10.0 (10.5)
9.3 (9.6)
8.4 (8.7)
8.3 (8.7)
8.9 (8.7)
7.6 (7.3)
6.2 (5.9)
5.4 (5.9)
b
Red
a Abbreviations: py, pyridine; Mepy, methylpyridine; Etpy, ethylpyridine; t-but-py, t-butylpyridine; Bzpy, benzoylpyridine.
b Two types of crystals: red and deep violet both have similar analytical results.
was allowed to stand in an open beaker over several days
until good quality violet crystals of the complex deposited.
2.3. X-ray crystallography
A modified STOE four-circle diffractometer was used.
Orientation matrix and lattice parameters were obtained by
least-squares refinement of the diffraction data from 50
reflections in the u range 8.8–14.48. Data were collected at
298(2) K using graphite crystal-monochromatized MoKa
2.1.2. [Co(py)4(N3 )]PF6
This complex was prepared by mixing Co(NO3)2 –
6H2O, pyridine, NaN3 and KPF6 in the molar ratio of
1:8:1:1.2, respectively, in ca. 30 cm3 aqueous–ethanol
medium. The final clear solution mixture was allowed to
stand over several days to produce the complex as red
crystals along with a violet blue microcrystalline powder.
˚
radiation (l50.71069 A) and the v scan technique. The
intensities were collected for Lorentz-polarization effects,
for absorption and for intensity decay (intensities of three
standard reflections dropped during data collection by 7%).
The structure was solved by direct methods. Anisotropic
displacement parameters were only applied to the non-
hydrogen atoms in full-matrix least-squares refinement
based on F2. The H atoms were included in the final
refinement cycles on calculated positions. The disorder of
PF26 counterions was taken into account by refining two
split orientations with occupation factors of 0.676(14) and
0.324(14), respectively, and applying P–F distance re-
straints. The program DIFABS [29] and the SHELXTL/
PC [30] program package were used for computations.
C24H28CoF6N7P, (FW5618.43), monoclinic, space group
2.1.3. [CoL4(N3 )]PF6?n(H2O) (n50 or 1) for L53-,4-
Bzpy, 4-t-butpy and 3-Etpy
These complexes were prepared in powder form just
upon mixing the components as given in the procedure in
Section 2.1.1. The filtrates in the case of 3-, and 4-Bzpy,
however, deposit crystalline compounds of the 1:2 or 1:4
cobalt(II) azide complexes rather than the desired ones
containing PF26 ions.
2.1.4. [Co(4-Etpy)4(N3 )]PF6 (pink form)
˚
P21 /C, a510.406(3), b511.183(3), c524.630(8) A, b5
This complex was prepared according to the procedure
in Section 2.1.3 which produces the complex as a pink
powder. The filtrate deposits long pink crystals of the
complex upon standing in open air over several days.
3
21
˚
90.51(3)8, V52866(2) A , Z54, m(MoKa)50.720 mm
,
Dcalc /Dobs51.433/1.43(2) g cm23, crystal size50.503
0.4030.28 mm, 5048 reflections, 414 parameters, R5
0.0466, Rint50.0315.
2.1.5. [Co(4-Etpy)4(N3 )]PF6 (red form)
This complex was prepared by mixing the components
according to the procedure in Section 2.1.2. Filtration of
the immediately formed precipitate and allowing the
filtrate to stand over several days led to separation of two
types of crystals; deep violet and red, of the same complex
as shown by their elemental analysis and IR spectra.
The elemental analyses of the isolated complexes are
collected in Table 1.
3. Results and discussion
3.1. Preparation of the complexes
The reactions between cobalt(II) ions, pyridine deriva-
tive ligands, azide and PF26 ions in aqueous–ethanol
medium resulted in the isolation of cobalt(II) azido
complexes given in Table 1. While mixing of cobalt(II) ion
and pyridine derivative ligand, among other components,
in the 1:2 molar ratio yielded [CoL4(N3)]PF6 complexes
with almost all given ligands, this ratio in case of pyridine
itself led to the isolation of a 1:2 cobalt(II) azido complex
2.2. Physical measurements
The experimental procedures and instruments used for
physical measurements were described previously [28].

M.A.S. Goher, F.A. Mautner / Polyhedron 18 (1999) 2339–2344
2341
rather than the desired complex containing PF26 anion. In
order to obtain the pyridine complex, a large excess of
pyridine (8 folds) with respect to cobalt (see Section 2)
must be added. In the case of 4-Etpy, we isolated a pink
form and two other forms; red and deep violet. The IR
spectra of these later forms could not be distinguished
from each other.
The isolated complexes, except those for L53-, and
4-Bzpy, are soluble in one or the other of the following
solvents, water, MeOH, EtOH, acetone, DMF and DMSO.
Complexes for L53-, and 4-Bzpy are soluble only in DMF
and DMSO with a change of their colors to blue. Aqueous
solutions of these complexes, except for L53-, and 4-
Bzpy, behave as 1:1 electrolytes giving rise to 170–190
V cm mol21. This result suggests that they are ionic:
[CoL4(N3)]1?PF62 in aqueous solutions.
structure determination (described later), but also for the
other complexes. For the 3-Etpy complex, two very strong
bands at 2110 and 2077 cm21 [nas (N3)] and a weak to
medium band at 1344 cm21 [ns (N3)] are observed. These
results suggest the existence of two azido ligands;
symmetrical and asymmetrical m-(1,3) bridges. Azide
bridges of the type m-(1,1) usually exhibit a strong band
around 1280 cm21 [17–23]. Thus we may assume dimeric
structure for this complex containing di-m-(1,3) azido
ligands as well as octahedral cobalt(II) environment. In
case of the red form of the 4-Etpy complex, two bands are
observed at 2099 and 2055 cm21 [nas (N3)] and two weak
to medium bands at 1340 and 1289 cm21 [ns (N3)]. These
latter bands are not observed in the infrared spectrum of
the pink form, in addition to the 2085 cm21 band [ns
(N3)]. As the spectrum of the pink form is very similar to
that of 4-pic, therefore, this form has a polymeric cation
[Co(4-Etpy)4(N3)]1 with a m(1,3) azide bridge. For the
red form, the appearance of the 1289 cm21 band suggests
that it contains two different azido ligands, m(1,3) and
m(1,1) bridges, both, however, are asymmetric. We may
assume that this red compound also is a dimer but contains
simultaneous m(1,3) and m(1,1) bridging azides connecting
the same two cobalt atoms. Simultaneous m(1,3) and
m(1,1) azides linking the same two metal atoms were
previously observed in the structure of [Cu(py)2(N3)2]n
[32].
3.2. IR spectra
The IR spectra (Table 2) of the isolated complexes
suggest monodentate pyridine derivative ligands in all
complexes as indicated by the systematic shifts of the
vibrations related to pyridine nucleus and the appearance
of the carbonyl frequencies at the same positions in spectra
of both free and coordinated ligands. These spectra exhibit
bands that can be readily assigned to the PF26 anions [31]
as follows; 550–560 s (n4), 740–745 w (n1) and 840–845
vs,br (n3) (all values in cm21). These results must meet the
criteria for non-coordinated PF26 anions as described
previously [31] and are consistent with the structure of
complex (1). The spectra of all complexes, except those
for L53-Etpy and 4-Etpy (red form) show a single very
strong band in the 2055–2100 cm21 range, due to the nas
(N3) mode. However, we could not locate any absorption
band in the 1360–1270 cm21 range, due to the ns (N3)
mode in these spectra. The disappearance of the latter band
is strong evidence of the symmetrical nature of the azido
bridge not only in complex (1) as shown by the X-ray
Lattice water molecules are indicated by the appearance
of a broad band of medium intensity centered around 3400
cm21 in the infrared spectra of the respective complexes.
In the far IR region we tentatively assigned [33,34] the
bands in the 290–250 cm21 and 240–210 cm21 ranges to
nCo–N(azide) and nCo–N(L) vibrations, respectively.
3.3. Electronic spectra
The room temperature magnetic moments and the results
Table 2
a
IR spectral data (cm21
)
Complex
Azide vibrations
PF26 vibrations
nCo–N3
nCo–N
nasN3
nsN3
dN3
[Co(py)4(N3)]PF6
[Co(4-Mepy)4(N3)]PF6
2101 vs
2075 vs
–
–
601 wm
620 w
605 w
614 w
660 w
623 w
610 w
844 vs, 750 m, 5595
845 vs,br, 745 wm, 550 m
290 wm
275 wm
240 m
235 w
b
[Co(4-Etpy)4(N3)]PF6
[Co(4-Etpy)4(N3)]PF6
2085 vs
2099 vs
2055 vs
2089 vs
2073 vs
2056 vs
2101 vs
2077 vs
–
844 vs,br, 740 w, 558 s
846 vs, 740 w, 557 s
278 w
228 wm
222 wm
c
1340 wm
1289 m
–
–
282 wm
275 sh
285 w
272 wm
265 wm
280 w
[Co(4-t-but-py)4(N3)]PF6?H2O
[Co(4-Bzpy)4(N3)]PF6?H2O
[Co(3-Bzpy)4(N3)]PF6?H2O
[Co(3-Etpy)4(N3)]PF6
842 vs, 724 w, 558 m
840 s, 742 wm, 558 m
845 vs, 743 wm, 558 m
844 vs,br, 757 s
232 w
225 wm
233 wm
229 wm
–
610 w
614 w
1344 wm
260 w
a v, very; s, strong; m, medium; w, weak; br, broad.
b Pink form.
c Red form.

2342
M.A.S. Goher, F.A. Mautner / Polyhedron 18 (1999) 2339–2344
of electronic spectral measurements of the isolated com-
plexes are collected in Table 4. The magnetic moments of
4.8–5.1 B.M. given for those complexes fall in the range
4.8–5.6 B.M. expected [35] for high-spin six-coordinate
cobalt(II) complexes and above the ranges associated with
tetrahedral or high-spin five-coordinate cobalt(II). These
values, however, do not indicate whether the complexes
display the metal–metal interaction expected in complexes
containing m-(1,3) bridging azides. In fact, similar nic-
kel(II) complexes with a single m-(1,3) azide bridge have
been found to exhibit strong antiferromagnetic coupling
between nickel(II) ions through the N23 bridge [36,37].
Nevertheless, a detailed magnetic susceptibility at variable
temperature should be done in order to obtain a better
understanding of this effect.
The electronic spectra (Table 3) of solid complexes are
typical of cobalt(II) in a weak octahedral field and show
the T1g(F)→4T1g(P), transition as a band of moderate
4
intensity around 19 300 cm21 with a shoulder at high
frequency and sometimes another shoulder at lower fre-
quency. For the series of complexes derived from 4-
substituted pyridine ligands the position of the main band
is shifted to lower energy in the order 4-t-but-py.py.4-
Etpy.4-Mepy.4-Bzpy. In methanol, the main band is
shifted to lower wave numbers or two separate bands are
observed.
Fig. 1. [Co(4-methylpyridine)4(N3)] PF6, 1: Molecular geometry and
atom labeling scheme. For clarity only disordered PF26 counter ion with
occupancy of 0.676(14) is given (atoms represented as spheres with
arbitrary size).
pyridine ligands are in the same plane (maximum deviation
˚
from plane50.025(1) A); the coordination of the m(1,3)
azido bridge is almost symmetrical; the two Co–N–N
angles are 135.3(3) and 133.0(3)8, respectively, and the
3.4. Structure of [Co(4-Mepy)4(N3 )](PF6 ) (1)
˚
torsion angle Co–NNN–Co is 299.2(3) A. The four Co–
˚
N(L) distances are variable from 2.135(3) to 2.201(3) A,
The principal structural features of the complex (1) are
illustrated in Figs. 1 and 2, and selected bond distances and
angles are given in Table 4. The structure of 1 consists of
1D cobalt-azido chains, isolated by PF26 anions, which are
found in the interchain space. In the chain structure each
cobalt(II) atom is coordinated by four nitrogen atoms from
the four pyridine ligands and two azido ligands in an
octahedral trans arrangement. The four N atoms of the
whereas the Co–N(N3) bond lengths are 2.120(3) and
˚
2.135(3) A. The azido ligand itself is symmetrical [N1 –
˚
N25N2 –N351.170(4) A] and linear [,NNN5177(4) 8]
within the experimental error.
A single m(1,3) azido bridge has previously been found
in the cation in [Ni2(Me4Cyclam)2(N3)3]I [38,39]. Fur-
thermore, X-ray analyses of Mn(acac)2N3 has established
Table 3
Electronic spectral data and magnetic moments
Complex
Solvent
lmax (cm21
)
meff (B.M.)a
[Co(py)4(N3)]PF6
NMb
MeOH
NM
21739 sh, 19607, 17857 sh
21186, 18348
20833 sh, 18975, 18181 sh
26315, 19607
4.82
[Co(4-Mepy)4(N3)]PF6
[Co(4-Etpy)4(N3)]PF6
4.80
4.90
c
NM
MeOH
NM
NM
MeOH
NM
NM
20833
d
[Co(4-Etpy)4(N3)]PF6
21700 sh, 19047
19880, 18181 sh, 16501
16500
20000 sh, 18867, 17857 sh
18510
4.84
5.10
[Co(4-t-but-py)4(N3)]PF6?H2O
[Co(4-Bzpy)4(N3)]PF6?H2O
[Co(3-Bzpy)4(N3)]PF6?H2O
[Co(3-Etpy)4(N3)]PF6
5.00
4.96
4.80
NM
MeOH
19417, 17857 sh
16666
a Measured at room temperature.
b NM, Nujol mull.
c Pink form.
d Red form.

M.A.S. Goher, F.A. Mautner / Polyhedron 18 (1999) 2339–2344
2343
Fig. 3. Thermal decomposition of [Co(4-methylpyridine)4(N3)] PF6.
˚
are longer (2.165 and 2.172 A) than the four Ni–
˚
N(Cyclam) distances (2.059 to 2.073 A).
3.5. Thermal analysis of (1)
Fig. 2. [Co(4-methylpyridine)4(N3)] PF6, 1: Packing plot with chains of
polyhedra oriented along the b-axis of unit cell.
Fig. 3 represents the TGA and DTA curves for the
thermal decomposition of complex (1). It is clear that the
complex is stable till ca. 1208C at which it starts decompo-
sition which takes place in the 120–2608C range with an
experimental loss of weight of 72.8%. This value corre-
spond to a loss of four 4-pic molecules, an azide and two F
atoms (Calcd.: 73.4%). The IR spectrum at 2508C shows
that the residue contains traces of 4-pic but no azide at all.
It also shows a little change in the appearance and position
(not intensities) of the bands at 840 and 558 cm21 and a
disappearance of the 720 cm21 (weak) band, all of them
are related to the PF6 group. The former bands remain and
appear in the spectrum of the residue at 4008C. This step
does not reflect a simple one-stage process and the DTA
curve shows a weak endotherm (Tmax51708C and DH52
59.7 kJ mol21), followed by a narrow exotherm at Tmax5
202.48C (DH5154.6 kJ mol21) and then two successive
endotherms with Tmax5231.8 and 2498C and DH5218.5
kJ mol21 and 212.3 kJ mol21, respectively.
the presence of single m(1,3) azide bridging which links
adjacent metal atoms to form polymeric chains of six-
coordinate Mn(III) polyhedra [36]. The structure of the
present complex resembles that of catena-m(N3)-
[Ni(Cyclam)]ClO4?H2O
(Cyclam51,4,8,11-tetraazo-
cyclotetradecane) (2) [33] which features six-coordinate
Ni(II), trans arrangement, 1D nickel azide chains and
ClO24 anions. Complex (2) differs in the asymmetric
arrangement of the azido bridge; the two Ni–N–N angles
are 140.7(3) and 128.2(3)8, the azide ligand itself is
˚
asymmetric (N1 –N251.178(5), N2 –N351.138(5) A), as
well as the bond length of the two Ni–N(azide) distances
Table 4
a
˚
Selected bond lengths (A) and angles (8) for (1)
Co(1)–N(13a)
Co(1)–N(4)
Co(1)–N(1)
N(11)–N(12)
2.130(3) Co(1)–N(11)
2.158(3) Co(1)–N(2)
2.200(3) Co(1)–N(3)
1.168(3) N(12)–N(13)
2.137(3)
2.194(3)
2.201(3)
Supplementary data
Further details of the crystal structure investigation are
available on request from the Cambridge Crystallographic
Centre, 12 Union Road, Cambridge CB2 7EZ, UK, on
request, quoting the deposition number CCDC 111534.
1.170(3)
N(13a)–Co(1)–N(11) 176.01(11) N(13a)–Co(1)–N(4)
93.24(11)
91.82(10)
85.46(10)
90.78(10)
168.95(10)
86.45(10)
95.76(10)
N(11)–Co(1)–N(4)
N(11)–Co(1)–N(2)
N(13a)–Co(1)–N(1)
N(4)–Co(1)–N(1)
N(13a)–Co(1)–N(3)
N(4)–Co(1)–N(3)
N(1)–Co(1)–N(3)
90.37(11) N(13a)–Co(1)–N(2)
90.16(10) N(4)–Co(1)–N(2)
87.94(10) N(11)–Co(1)–N(1)
83.52(10) N(2)–Co(1)–N(3)
89.90(10) N(11)–Co(1)–N(3)
167.59(10) N(2)–Co(1)–N(3)
Acknowledgements
95.28(10) N(12)–N(11)–Co(1) 135.4(2)
N(11)–N(12)–N(13) 132.9(2)
N(11)–N(12)–N(13) 177.5(3)
Financial support by the Kuwait University Research
Administration Project (SC077) and the Department of
Chemistry General Facility Projects (Analab) are gratefully
a Symmetry code: (a) 2x11, y11/2, 2z13/2; (b) 2x11, y21/2,
2z13/2.

2344
M.A.S. Goher, F.A. Mautner / Polyhedron 18 (1999) 2339–2344
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