A novel 3D coordination polymer containing pentacoordinate cobalt(II) dimers with antiferromagnetic ordering

Article abstract of DOI:10.1016/j.inoche.2004.05.008

A novel three dimensional (3D) polymer, [Co₂(hypa) ₂(4,4^′-bipy)] (1) (H₂hypa = hydroxy-phenyl-acetic acid, 4,4^′-bipy = 4,4^′- bipyridine), featuring pentacoordinate, dimeric cobalt(II) as subunits have been prepared, which shows low-dimensional antiferromagnetic ordering blow 15.2 K (T_N).

Full text of DOI:10.1016/j.inoche.2004.05.008

Inorganic Chemistry Communications 7 (2004) 864–867
www.elsevier.com/locate/inoche
A novel 3D coordination polymer containing
pentacoordinate cobalt(II) dimers with antiferromagnetic ordering
b
a,
*
Ming-Hua Zeng a,c, Song Gao , Xiao-Ming Chen
a
School of Chemistry and Chemical Engineering, Sun Yat-Sen University, 135 Xingang Rd. W., Guangzhou 510275, PR China
State Key Laboratory of Rare-Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering,
Peking University Beijing 100871, PR China
b
c
Research Institute of Bioinorganic Chemistry, Department of Chemistry and Chemical Engineering, GuangXi Normal University,
Guilin 541004, PR China
Received 29 March 2004; accepted 9 May 2004
Available online 7 June 2004
Abstract
A novel three dimensional (3D) polymer, [Co2(hypa)2(4,40-bipy)] (1) (H2hypa ¼ hydroxy-phenyl-acetic acid, 4,40-bipy ¼ 4,40-
bipyridine), featuring pentacoordinate, dimeric cobalt(II) as subunits have been prepared, which shows low-dimensional antifer-
romagnetic ordering blow 15.2 K (TN).
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Pentacoordination; Antiferromagnetism; Cobalt(II); Hydrothermal synthesis
Judicious choice of appropriate molecular building
blocks provides the means to create a rich variety of new
materials with interesting structural and magnetic
properties [1]. Typically, these compounds consist of
paramagnetic transition metal ions interlinked together
by bridging ligands into polymeric arrays. The magne-
tism of high-spin (HS) cobalt(II) complexes is a chal-
lenging topic because the orbital angular momentum
causes difficulties in the magnetic analysis [2]. In general,
the orbital angular momentum is totally or partially
quenched in a ligand field of a certain symmetry [3]; the
effect of the orbital angular momentum is highly de-
pendent on the symmetry around the metal ion. Al-
though abundant cases of the magnetic properties of the
HS cobalt(II) coordination polymers under an octahe-
dral symmetry (Oh) or a tetrahedral symmetry (Td) have
been investigated [1c,1e,4], to our knowledge, there are
only few cases of one- or two dimensional (2D) coor-
dination polymers with HS cobalt(II) under trigonal–
bipyramidal symmetry (TBP) or a square–pyramidal
symmetry (SP) [5], but no three dimensional (3D)
framework structure featuring uniquely pentacoordinate
cobalt(II). The magnetism of HS cobalt(II) complexes of
a TBP or SP symmetry can be classified into two cate-
gories: (1) in a normal (or slightly distorted) SP or TBP
symmetry, the ground term does not have an orbital
angular momentum, thus a spin-only treatment is valid
for them; (2) in a highly distorted lower symmetrical
case, the ground term possesses an orbital angular mo-
mentum due to the admixture with higher states, thus a
spin–orbit coupling treatment is valid for them [2,3]. In
this communication, we report the synthesis, crystal
structure and magnetic properties of a 3D coordination
polymer comprising l-alkoxo-bridged, penta-coordinate
CoII dimeric subunits, namely [Co2(hypa)2(4,40-bipy)]
(1) (H2hypa ¼ hydroxy-phenyl-acetic acid, 4,40-bipy ¼
4,40-bipyridine), which was generated through hydro-
thermal treatment using cobalt(II) ions and chiral
* Corresponding author. Tel.: +86-20-84113986; fax: +86-20-
84112245.
E-mail address: cescxm@zsu.edu.cn (X.-M. Chen).
H2hypa (D- or L-isomers, or the racemate) as the reac-
1387-7003/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2004.05.008

M.-H. Zeng et al. / Inorganic Chemistry Communications 7 (2004) 864–867
865
Fig. 1. Perspective view of the dimeric cobalt(II) subunit in 1.
tants. Obviously, the chiral H2hypa becomes racematic
1
during the hydrothermal procedure.
Fig. 2. Perspective view of the layer constructed by the metal ions and
l3-hypa-O,O0,O00 ligands in 1. Hydrogen atoms and phenyl groups are
omitted for clarity.
2
X-ray crystallography shows that 1 has a centro-
symmetrical dimeric Co2(hypa)2 subunit (Fig. 1). The
Co(1) ion is coordinated by one 4,40-bipy nitrogen atom
ꢀ
(Co(1)–N(1) 2.075(2) A), two hypa carboxy oxygen at-
II
ꢀ
bridges (Coꢀ ꢀ ꢀCo 11.191(1) A) to connect the Co ions
in the adjacent layers into a 3D pillared-layer structure
(Fig. 3), in which the pyridyl groups of each 4,40-bipy is
significantly twisted with a dihedral angle of ca. 50°. It
should be pointed out that there exist different types of
aromatic p–p stacking interactions, providing extra in-
tramolecular forces for 1, namely very strong face-to-
face p–p stacking between vicinity phenyl groups of
ꢀ
oms (Co(1)–O(2b) 1.982(2) A and Co(1)–O(1) 2.192(1)
ꢀ
ꢀ
A), and two l-alkoxo groups (Co(1)–O(3a) 2.000(1) A,
ꢀ
Co(1)–O(3) 1.947(1) A). The bond angles around the
CoII ion are in the range of 77.65(5)° to 156.30(5)°, which
have been analyzed with PLATON [6] to be an inter-
mediate between a TBP symmetry and an SP symmetry
with a s value of 0.51 (s ¼ 0:00 for SP and 1.00 for TBP).
It should be noted the chiral hypa ligands are race-
matic, each acts in a tridentate-O,O0,O00 mode. The
Co2(hypa)2 dimer with a very short Coꢀ ꢀ ꢀCo distance of
0
ꢀ
hypa and pyridyl groups of 4,4 -bipy (3.27 A) and strong
edge-to-face interactions between vicinity bipy pyridyl
ꢀ
groups of (3.59 A), as well as weak edge-to-face inter-
ꢀ
actions between hypa phenyl groups (3.72–3.97 A) [7].
ꢀ
3.021(1) A is sustained by a pair of l-alkoxo groups,
In words, 1 represents a new 3D framework structure
featuring uniquely pentacoordinate cobalt(II) centers.
The magnetic susceptibility of 1 from 1.89 to 300 K was
measured at 10 kOe (Fig. 4). The vMT value is 5.630
K molꢁ1 per CoII dimer at 300 K, corresponding to a
moment of 4.745 lB per CoII ion, which is in the range of
the value of an HS pentacoordinate CoII ion (4.3–4.8 lB)
under the strong influence of the spin–orbital coupling [8].
The vMT decreases continuously with decreasing tem-
perature and reaches a minimum of 0.14 cm3 molꢁ1 K
near 1.89 K. The magnetic susceptibility above 50 K
obeys the Curie–Weiss law with a Weiss constant h of
which is further interconnected by the syn–anti car-
boxylate bridges into neutral 2D layers about the (1 0 0)
planes (Fig. 2), with the adjacent interdimer Coꢀ ꢀ ꢀCo
0
ꢀ
distance of 5.206(1) A. The 4,4 -bipy ligands act as
1
In a typical experiment, H2hypa (0.152 g, 1 mmol) in an aqueous
solution (6 mL) of NaOH (0.080 g, 2 mmol) was mixed with 4,40-bipy
in EtOH (2 mL). The resulting solution was then added to an aqueous
solution (2 mL) of Co(NO3)2 ꢀ 6H2O (0.291 g, 1 mmol). The mixture
was placed in a 23-mL Teflon-lined autoclave and heated at 160 °C for
160 h. The autoclave was cooled over a period of 12 h at a rate of 5
°C hꢁ1, and 1 as dark-purple crystals was collected by filtration,
washed with water, and dried in air. Pure product was obtained by
manual separation (Final yield: 106 mg, 37% based on Co). Elemental
analysis (%) Calc. for 1 (C13H10CoNO3): C, 54.37, H, 3.51, N, 4.88;
Found: C, 54.34, H, 3.52, N, 4.86%. IR data (m, cmꢁ1) for 1: 3431.5 m,
3048.4 w, 3021.2 w, 1609.9 m, 1553.0 vs, 1416.1 vs, 1104.5 s, 1065.9 m,
806.0 m, 753.3 m, 699.7 m, 538.8 m, 498.3 w, 432.0 w.
2
Crystal data for C13H10CoNO3 1: Mr ¼ 287:16, monoclinic, space
ꢀ
group C2=c (No. 15); a ¼ 27:077ð1Þ, b ¼ 9:887ð1Þ, c ¼ 9:217ð1Þ A,
b ¼ 108:85ð1Þ°, V ¼ 2335:1ð4Þ A and Z ¼ 8, qcalcd: ¼ 1:645 g cmꢁ3
,
3
ꢀ
l ¼ 1:474 mmꢁ1. The data collections were carried out on a Siemens
ꢀ
R3m diffractometer using Mo Ka radiation (h ¼ 0:71073 A) at 293(2)
K. Final R1 value of 0.0294 for 164 parameters and 2289 unique
reflections with I P 2rðIÞ and wR2 of 0.0816 for all 2338 reflections.
Crystallographic data for the structural analysis have been deposited
with the Cambridge Crystallographic Data Center, CCDC-211373.
Fig. 3. Perspective view of the 3D framework of 1.

866
M.-H. Zeng et al. / Inorganic Chemistry Communications 7 (2004) 864–867
The magnetic properties of molecules, whether they
interact with one another or not, depend on the local
geometry and the chemical links between them. Com-
pared with the structures of other CoII coordination
polymers [1c,1e,4,11] the orderly magnetic behavior in 1
may be attributed mainly to the unusual CoII coordi-
nation geometry and the connection between Co ions,
which has an implication of a two-sublattice collinear
structure by the offset arrangement of the 2D nets into a
3D framework (Fig. 3).
The antiferromagnetism observed for 1 can be at-
tributed to the contribution of the orbital angular mo-
mentum due to the spin–orbit coupling as well as the
magnetic couplings between the CoII ions within the 3D
framework. According to the superexchange mechanism
[12] and structural parameters, three possible magnetic
exchange pathways can be defined in 1. The first and
strongest is the exchange between CoII ions in the di-
meric subunits featuring the shortest metal-ligand bond
distance with the exchange angle Co(1)–O(3)–Co(1B) at
99.89(6)°. Such a larger (compared with 90°) superex-
change angle usually leads to strong antiferromagnetic
interaction [11a], consistent with the observed negative
Weiss temperature (h ¼ ꢁ25:01 K) and the moderate
Neel constant (TN ¼ 15:2 K) in 1. The second in the
hierarchy is the exchange between the binuclear subunits
via the syn–anti O–C–O bridges at a CoIIꢀ ꢀ ꢀCoII dis-
Fig. 4. Temperature dependence of vM (j) and vMT (D) values per
CoI2I for 1.
)25.01 K and a Curie constant C of 5.94 cm3 molꢁ1 K,
suggesting a strong antiferromagnetic interaction among
the CoII ions. As the temperature is decreased from 50 K,
the magnetic susceptibility slowly increases up to a local
maximum of 0.114 cm3 molꢁ1 at 19.9 K, then the vM
quickly decreases and tends to a constant value of 0.076
cm3 molꢁ1 below ca. 5 K, which is 2/3 of the vM value
(0.114 cm3 molꢁ1) at Tmax. This behavior is expected for a
polycrystalline two-sublattice collinear antiferromagnet
[2b,9]. and is in agreement with the equation
(vpowder ¼ hvi ¼ ðv þ 2v Þ=3 and so hvi at 0 K is 2/3 of
jj
?
ꢀ
tance of 5.206(1) A, which is weak and could be anti-
the value at TN) in molecular field theory of antiferro-
magnetism [2b]. The critical temperature TN ¼ 15:2 K is
determined by the peak of d(vT)/dT. The field dependence
of the magnetization for 1 at different temperatures below
and around TN (15.2 K) all shows a highly linear slow
increase, as shown in Fig. 5, and no spin flop state was
observed. The magnetization under 70 kOe at 1.8 K is
only 0.92 Nb per dimeric CoII, also far from the satura-
tion value of ca. 2.5 Nb expected for a CoII with
Seff ¼ 1=2, and g ¼ 5:0 [10]. This being further evidence
supporting the antiferromagnetic ordering of 1.
ferromagnetic [4b,13]. The last is that between the layers
involving the non-coplanar 4,40-bipy pillars with the
longest CoIIꢀ ꢀ ꢀCoII distances (11.19 A), which should
ꢀ
therefore be much weaker. Therefore, the antiferro-
magnet possess obviously 2D characteristics, as the ratio
of TN=Tmax 0.76, is quite low.
In summary, we have successfully constructed a new
3D polymer featuring pentacoordinate cobalt(II) dimeric
subunits, which is a low-dimensional antiferromagnet.
Acknowledgements
This work was supported by NSFC (No. 20131020)
and Ministry of Education of China.
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