New cadmium thio- and selenocyanato coordination compounds: Structural snapshots on the reaction pathway to more condensed anionic networks

Article abstract of DOI:10.1039/c1dt11377a

In this contribution several new coordination compounds on the basis of cadmium(ii) thio- and selenocyanate with pyrimidine as co-ligand were prepared and investigated for their structural, thermal and spectroscopic properties. The reaction of cadmium(ii) thiocyanate with pyrimidine leads to the formation of four compounds, which from a structural point of view are closely related. In the most pyrimidine-rich 1:2 compound [Cd(NCS)₂(pyrimidine) ₂]_n (1A) (1:2 = ratio between metal salt and the co-ligand pyrimidine) the Cd cations are linked by the pyrimidine ligands into layers and are additionally coordinated by two terminal N-bonded anions. In the 2:3 compound {[Cd(NCS)₂]₂(pyrimidine)₃}_n (1B) the Cd cations are linked into chains by μ-1,3 bridging thiocyanato anions, which are connected into layers by only half of the pyrimidine ligands, whereas the other co-ligands are only terminal coordinated. Further reduction of the pyrimidine content leads to the formation of the 1:1 2D compound [Cd(NCS)₂(pyrimidine)]_n (1CI) in which the terminal N-bonded thiocyanato anions become bridging. Surprisingly, crystallization experiments lead to the formation of an additional pyrimidine-deficient intermediate of composition {[Cd(NCS)₂]₃(pyrimidine) ₂}_n (1D), in which some of the μ-1,3 coordinated anions transform into μ-1,1,3 bridging thiocyanato anions. Consequently the four structures can be used as snapshots of intermediates on the way to a more condensed thiocyanato coordination network. In contrast, with cadmium selenocyanate only two different compounds were obtained. The 1:2 compound [Cd(NCSe)₂(pyrimidine)₂]_n (2A) is not isotypic to 1A and shows a completely different coordination topology whereas the pyrimidine-deficient 1:1 compound (2B) shows a more condensed network with μ-1,3 coordinating selenocyanato anions. On heating, the 1:2 compound 1A decomposes into Cd(NCS)₂via a new polymorphic modification (1CII) as intermediate which is metastable, whereas the 1:2 selenocyanato compound 2A transforms into the 1:1 compound 2B on heating which cannot be obtained phase pure under these conditions. If faster heating rates are used, there are indications for the formation of a 3:2 compound, which is amorphous to X-rays. The results are compared with those obtained for related thio- and selenocyanato coordination polymers with pyridine, pyridazine and pyrazine as co-ligand. Moreover, their impact on the structures and thermal reactivity of analogous paramagnetic compounds is discussed in detail. Based on the structural data of compound 1D the unknown structures of two intermediates were determined, which are formed in the thermal decomposition reaction of the Mn and Fe thiocyanato pyrimidine coordination polymers, reported recently. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1dt11377a

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PAPER
New cadmium thio- and selenocyanato coordination compounds: Structural
snapshots on the reaction pathway to more condensed anionic networks†
Inke Jeß, Jan Boeckmann and Christian N a¨ ther*
Received 21st July 2011, Accepted 22nd September 2011
DOI: 10.1039/c1dt11377a
In this contribution several new coordination compounds on the basis of cadmium(II) thio- and
selenocyanate with pyrimidine as co-ligand were prepared and investigated for their structural, thermal
and spectroscopic properties. The reaction of cadmium(II) thiocyanate with pyrimidine leads to the
formation of four compounds, which from a structural point of view are closely related. In the most
pyrimidine-rich 1 : 2 compound [Cd(NCS)
2
2 n
(pyrimidine) ] (1A) (1 : 2 = ratio between metal salt and the
co-ligand pyrimidine) the Cd cations are linked by the pyrimidine ligands into layers and are
additionally coordinated by two terminal N-bonded anions. In the 2 : 3 compound
{
[Cd(NCS)
2
]
2
(pyrimidine)
3
}
n
(1B) the Cd cations are linked into chains by m-1,3 bridging thiocyanato
anions, which are connected into layers by only half of the pyrimidine ligands, whereas the other
co-ligands are only terminal coordinated. Further reduction of the pyrimidine content leads to the
formation of the 1 : 1 2D compound [Cd(NCS)
thiocyanato anions become bridging. Surprisingly, crystallization experiments lead to the formation of
(pyrimidine) (1D), in
2
n
(pyrimidine)] (1CI) in which the terminal N-bonded
an additional pyrimidine-deficient intermediate of composition {[Cd(NCS)
2
]
3
2 n
}
which some of the m-1,3 coordinated anions transform into m-1,1,3 bridging thiocyanato anions.
Consequently the four structures can be used as snapshots of intermediates on the way to a more
condensed thiocyanato coordination network. In contrast, with cadmium selenocyanate only two
different compounds were obtained. The 1 : 2 compound [Cd(NCSe)
2
2 n
(pyrimidine) ] (2A) is not isotypic
to 1A and shows a completely different coordination topology whereas the pyrimidine-deficient 1 : 1
compound (2B) shows a more condensed network with m-1,3 coordinating selenocyanato anions. On
heating, the 1 : 2 compound 1A decomposes into Cd(NCS)
as intermediate which is metastable, whereas the 1 : 2 selenocyanato compound 2A transforms into the
: 1 compound 2B on heating which cannot be obtained phase pure under these conditions. If faster
2
via a new polymorphic modification (1CII)
1
heating rates are used, there are indications for the formation of a 3 : 2 compound, which is amorphous
to X-rays. The results are compared with those obtained for related thio- and selenocyanato
coordination polymers with pyridine, pyridazine and pyrazine as co-ligand. Moreover, their impact on
the structures and thermal reactivity of analogous paramagnetic compounds is discussed in detail.
Based on the structural data of compound 1D the unknown structures of two intermediates were
determined, which are formed in the thermal decomposition reaction of the Mn and Fe thiocyanato
pyrimidine coordination polymers, reported recently.
Introduction
important goal in this area is the preparation of new materials
with desired physical properties. This approach, called “Crystal
Engineering”, must include a more rational approach to design
structures and the systematic investigations of structure-property
Investigations on the synthesis, structures and properties of
coordination polymers including metal–organic frameworks or
inorganic–organic hybrid compounds are an important field in
2
relationships in solids. Moreover, methods are needed that allow
1
chemical research and there are excellent reviews in this field. One
the preparation of a large amount of a desired compound phase
pure and in preferably high yields. Concerning the properties or
even future applications of such compounds, coordination poly-
mers with useful magnetic properties have become of increasing
Institut f u¨ r Anorganische Chemie, Universit a¨ t zu Kiel, Max-Eyth-Straße
2
8
, 24098, Kiel, Germany. E-mail: cnaether@ac.uni-kiel.de; Fax: +49-431-
801520; Tel: +49-431-8802092
3
interest. Consequently a large amount of different antiferro-,
†
Electronic supplementary information (ESI) available. CCDC reference
ferro- or ferrimagnetic materials has been reported which also
includes metamagnetic or SCM compounds (SCM = single chain
numbers 836232–836237. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt11377a
2
28 | Dalton Trans., 2012, 41, 228–236
This journal is © The Royal Society of Chemistry 2012

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4
magnetism). In most compounds paramagnetic transition metals
for 2 d. C10
H
8
CdN
6
S
2
(388.8): calcd. C 30.9, H 2.1, N 21.6, S
are linked by small-sized anions that can mediate magnetic
exchange interactions.
16.5; found C 30.7, H 2.0, N 22.3, S 16.7. Single crystals were
prepared by the reaction of Cd(NCS) (28.5 mg, 0.125 mmol) and
2
To add one piece of knowledge to the field, we have started
systematic investigations on compounds in which transition metal
cations are linked by thio- or selenocyanato anions. In some cases
this is a great challenge because especially paramagnetic cations
like e.g. Fe, Co or Ni prefer terminal N-bonding to such anions.
This is a pity because coordination compounds with m-1,3 bridging
anions might show cooperative magnetic phenomena. However, in
some cases the m-1,3 bridging compounds can simply be prepared
using conventional solution techniques but very often only the
compounds with terminal bonded anions are obtained. To close
this gap we have developed an alternative method which is based
on thermal decomposition of ligand-rich precursor compounds
pyrimidine (21 mL, 0.25 mmol) in 2 mL of ethanol in a closed glass
tube at 130 C for 1 week. The precipitate consists of a mixture of
single crystals of compounds 1A, 1B and 1CI.
◦
Synthesis of {[Cd(NCS) ] (pyrimidine) } (1B)
2
2
3 n
Compound 1B cannot be obtained as a phase pure powder from
solution. Single crystals were selected from a mixture which was
prepared as described for compound 1A.
Synthesis of [Cd(NCS)
2
(pyrimidine)]
n
(1CI)
Cd(NCS) (114 mg, 0.50 mmol) and pyrimidine (21 mL, 0.25 mmol)
2
(
ligand-rich = rich on neutral co-ligands) with terminal bonded
were stirred in 2 mL of methanol in a snap cap vial at RT for 3
d. If stoichiometric ratios of the reactants were used, compound
thio- or selenocyanato anions and additional neutral co-ligands
5–7
that can easily be extended to other anionic ligands. If such com-
pounds are heated usually ligand-deficient (ligand-deficient = lack-
ing neutral co-ligands) compounds are obtained as intermediates,
in which the anions become m-1,3 bridging. With this approach
we have prepared e.g. 1D and 2D compounds that show a slow
1
A formed. C
6
H
4
CdN
4
S (308.65): calcd. C 23.3, H 1.3, N 18.5,
2
S 20.7; found C 23.0, H 1.2, N 18.1, S 20.8. Single crystals were
selected from a mixture prepared as described in the synthesis for
1A.
5
relaxation of the magnetization. In this context it is mentioned
Synthesis of {[Cd(NCS)
2
]
3
(pyrimidine)
2
}
n
(1D)
that also the corresponding compounds based on diamagnetic
Cd(II) cations are of extraordinary importance for our project,
because in contrast to the paramagnetic analogue they prefer a
bridging coordination. Therefore, the Cd compounds with m-1,3
bridging anions can easily be crystallized from solution. In those
cases where they are isotypic to their paramagnetic analogue the
Single crystals were prepared bythe reaction of Cd(NCS)
0.50 mmol) and pyrimidine (21 mL, 0.25 mmol) in 2 mL of
acetonitrile in a closed glass tube at 130 C for 1 week. The
precipitate consists of a mixture of single crystals of compound
2
(114 mg,
◦
1CI and 1D.
8
structures of the latter can be determined by Rietveld refinement.
Moreover, the thermal decomposition of the paramagnetic pre-
cursors lead in most cases to only one intermediate. In this context
the question arises whether additional e.g. metastable interme-
diates might exist. From previous investigations on compounds
based on manganese and iron thiocyanates with pyrimidine we had
strong indications that additional ligand-deficient intermediates
Synthesis of [Cd(NCSe) (pyrimidine) ] (2A)
2
2 n
Cd(NO
.50 mmol) were stirred together in 1 mL of ethanol at RT in
a 3 mL snap cap vial. The resulting colorless precipitate of KNO
3
)
2
·4H
2
O (77.1 mg, 0.25 mmol) and KNCSe (72.0 mg,
0
3
was filtered off and pyrimidine (2 mL, 25.0 mmol) was added
to the filtrate. After stirring the reactants for 2 d the light beige
crystalline powder was filtered off and the purity was checked
9
occur but we were not able to identify them. Because of the
higher chalcophilicity of Cd it might be that such intermediates
can be prepared in solution which would allow conclusions on the
identity or structure of their paramagnetic intermediates. This is
one additional reason why we have investigated cadmium thio-
and selenocyanates with pyrimidine as co-ligand. Here we report
on our investigations.
by XRPD. C10
found C 24.3, H 1.5, N 17.3. Single crystals were prepared by
the reaction of Cd(NO O (77.1 mg, 0.25 mmol), KNCSe
·4H
136.9 mg, 0.95 mmol) and pyrimidine (2 mL, 25.0 mmol) in 1 mL
H
8
CdN
6
Se
2
(482.54): calcd. C 24.9, H 1.7, N 17.4;
3
)
2
2
(
of water at RT in a 3 mL snap cap vial without stirring. After 3 d
colorless well shaped single-crystals were obtained.
Experimental section
Synthesis of [Cd(NCSe)
2
(pyrimidine)]
n
(2B)
Cd(NCS)
2
Single crystals were prepared by the reaction of Cd(NO
3
)
2
·4H
2
O
(
77.1 mg, 0.25 mmol) and KNCSe (72.0 mg, 0.50 mmol) in 1 mL
of ethanol at RT in a 3 mL snap cap vial. The resulting colorless
precipitate of KNO was filtered off and pyrimidine (19.7 mL,
.25 mmol) was added to the filtrate. After 3 d the colorless single-
Ba(NCS)
2
·3H
2
O (3.0755 g, 10 mmol) and CdSO
4
·8H
2
O (3.5258 g,
1
0 mmol) were stirred in water (100 mL). The colorless precipitate
3
of BaSO
4
was filtered off and the water was removed from the
0
◦
filtrate by heating. The final product was dried by 80 C. The
homogeneity of the product was investigated by X-ray powder
diffraction.
crystals were obtained in a mixture with unknown phases.
Spectroscopy
Fourier transform IR spectra were recorded on a Genesis series
FTIR spectrometer from ATI Mattson in KBr pellets as well on
an Alpha IR spectrometer from Bruker equipped with a Platinum
Synthesis of [Cd(NCS)
2
(pyrimidine)
2
]
n
(1A)
Cd(NCS) (228 mg, 1 mmol) and pyrimidine (170 mL, 2.00 mmol)
2
TM
-1
were stirred together in 3 mL of water at RT in a snap cap vial
ATR QuickSnap sampling module between 4000–375 cm .
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 228–236 | 229

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Table 1 Selected crystal data and details on the structure determinations from single crystal data (*Please note, compound 1CI was twinned and refined
using the HKLF5 option. Thus, the number of independent reflections is artificially too high)
Compound
1A
1B
1CI
1D
2A
2B
Formula
C
388.74
1 : 2
10
H
8
CdN
6
S
2
C
697.40
2 : 3
16
H
12Cd
2
N
10
S
4
C
308.65
1 : 1
6
H
4
CdN
4
S
2
C
845.86
3 : 2
14
H
8
Cd
3
N
10
S
6
C
482.54
1 : 2
10
H
8
CdN
6
Se
2
12 8 2 8 4
C H Cd N Se
804.90
1 : 1
MW g- mol-1
1
Ratio M:L
Crystal system
Orthorhombic
Cmca
10.0015(6)
15.6680(7)
8.6696(5)
90
Orthorhombic
Monoclinic
Triclinic
P1
6.9128(4)
9.2461(6)
10.6609(7)
108.162(5)
91.352(5)
103.857(5)
625.04(7)
293(2)
Monoclinic
Monoclinic
C 2/c
¯
Space group
P 2
1
2
1
2
P 2 /m
1
P 2 /c
1
a/A˚
11.2398(6)
14.8326(7)
7.3449(4)
90
5.6358(9)
12.4946(15)
7.1516(13)
90
8.566(2)
6.0220(10)
13.829(4)
90
8.6778(3)
16.7503(5)
29.2877(8)
90
92.158(2)
90
4254.1(2)
293(2)
8
2.513
8.857
0.2326/0.3771
26.81
23168
b/A˚
c/A˚
◦
a ( )
◦
b ( )
90
90
90
90
105.634(14)
90
484.96(13)
293(2)
2
2.114
2.637
—
25.89
3936
6520*
5132
92.07(3)
90
712.9(3)
293(2)
2
2.248
6.632
0.0441/0.1144
27.98
5395
1667
1347
◦
g ( )
V/A˚
T/K
3
1358.56(13)
293(2)
4
1.901
1.908
0.6235/0.8408
29.21
7848
967
795
1224.51(11)
293(2)
2
1.891
2.103
0.7001/0.7900
25.10
6607
2180
1896
Z
1
-3
D
c
/g cm
m/mm
min/max transmission
max ( )
Measured reflections
Unique reflections
2.247
3.055
0.6103/0.6741
27.99
7240
2980
2761
155
-1
◦
q
4500
4092
237
Reflections [F
Parameter
0
> 4s(F
0
)]
51
146
65
88
R
int
b]
0.0816
0.0485
0.0895
1.213
0.0456
0.0414
0.0647
1.148
0.0296
0.0355
0.0636
1.065
0.0171
0.0190
0.0426
1.077
0.0393
0.0272
0.0666
1.047
0.0285
0.0318
0.0663
1.110
[
R
1
[F
0
> 4s(F
[all data]
0
)]
[
c]
wR
2
GOF
Drmax, Drmin/e A˚ -
3
0.653, -0.568
0.627, -0.629
0.397, -0.770
0.411, -0.963
0.441, -0.719
1.332, -1.074
Raman-spectra were recorded using a Bruker ISF66 FRA106
between 3500–100 cm .
with idealized geometry and were refined with fixed isotropic
displacement parameters [Uiso(H) = -1.2·Ueq(C)] using a riding
model. Details of the structure determinations are given in Table
1. The crystal of compound 1CI was non-merohedrically twinned.
Both individuals were indexed separately, the twin law was
calculated and finally the structure was refined using the HKLF5
option. If the overlapping reflections are omitted a completeness
-
1
Elemental analysis
CHNS analysis was performed using an EURO EA elemental an-
alyzer, fabricated by EURO VECTOR Instruments and Software.
of only 69.4% is reached but the R value drops down to 2.98%
for the observed reflections. However, to refine against all data the
HKLF-5 option was used.
1
Elemental analysis of the residue obtained in the thermal
decomposition
CCDC-836235 (1A), CCDC-836234 (1B), CCDC-836233 (1CI),
CCDC-836232 (1D), CCDC-836237 (2A) and CCDC-836236 (2B)
contain the supplementary crystallographic data for this paper.†
These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/.
Isolated in first heating step of compound 1A: calculated for a 1 : 1
compound: C 23.3, H 1.3, N 18.5, S 20.7; found C 23.7, H 1.2, N
18.4, S 22.6.
X-ray powder diffraction (XRPD)
XRPD experiments were performed using a Stoe Transmission
Powder Diffraction System (STADI P) with Cu-Ka-radiation (l =
Differential thermal analysis and thermogravimetry
The DTA-TG measurements were performed in nitrogen at-
1
54.0598 pm) that is equipped with a linear position-sensitive
◦
mosphere (purity: 5.0) in Al
2
O
3
crucibles using a STA-409CD
detector (Delta 2 Theta = 6.5–7 simultaneous; scan range overall =
◦
thermobalance from Netzsch. All measurements were performed
2–130 ) from STOE & CIE and an Image Plate Detector (scan
-
1
◦
with a flow rate of 75 mL min and were corrected for buoyancy
and current effects. The instrument was calibrated using standard
reference materials.
range overall = 0–127 ).
Single-crystal structure analysis
All investigations were performed with an imaging plate diffrac-
tion system (IPDS-1 for 1A, 2A/IPDS-2 for 1B, 1CI, 1D and 2B)
with Mo-Ka-radiation from STOE & CIE. The structure solution
Results and discussion
Investigations on Cd thiocyanato coordination compounds
10
was performed with direct methods using SHELXS-97, and
2
structure refinements were performed against F using SHELXL-
Structural investigations. The reaction of cadmium thio-
cyanate with an excess of pyrimidine leads to the phase
pure formation of the ligand-rich precursor compound
11
9
7. All non-hydrogen atoms were refined with anisotropic
displacement parameters. The hydrogen atoms were positioned
2
30 | Dalton Trans., 2012, 41, 228–236
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[Cd(NCS)
2
(pyrimidine)
2
n
] (1A; Fig. S1 in the ESI†). Compound
1A crystallizes in the orthorhombic space group Cmca with 4
formula units in the unit cell. The asymmetric unit consists of
one Cd cation that occupies position 2/m, one thiocyanato anion
located on a crystallographic mirror plane and one pyrimidine
ligand which is situated on a 2-fold rotation axis. The Cd
cations are coordinated by two N-bonded symmetry equivalent
thiocyanato anions and four symmetry related pyrimidine ligands
within slightly distorted octahedra (Fig. 1, Fig. S2 and Table S1
in the ESI†).
Fig. 2 Crystal structure of compound 1B with view along the crystal-
lographic c axis (black = cadmium, dark-grey = sulphur, grey = carbon,
light-grey = nitrogen) (an Ortep plot of 1B is shown in Figure S3 in the
ESI†).
Fig. 1 Crystal structure of compound 1A with view along the crystal-
lographic b-axis (black = cadmium, dark-grey = sulphur, grey = carbon,
light-grey = nitrogen) (an Ortep plot of 1A is shown in Fig. S2 in the ESI†).
in ratio 2 : 1 (Fig. S4 in the ESI†). The compound crystallizes in the
monoclinic space group P2 /m with 2 formula units in the unit cell.
1
The asymmetric unit consists of one Cd cation located on a centre
of inversion, one thiocyanato anion in a general position and one
pyrimidine ligand located on a mirror plane. The Cd cations are
surrounded by two N and two S atoms of four symmetry equivalent
thiocyanato anions and two N atoms of two pyrimidine ligands
related by symmetry (Fig. 3, Fig. S5 and Table S3 in the ESI†).
In the crystal structure the Cd cations are connected via the
pyrimidine ligands into layers that are located in the ac plane
(
Fig. 1). These layers are stacked in the direction of the c
axis. It must be noted that the topology of the coordination
network is similar to that of the corresponding ligand-rich 1 : 2
coordination compounds of composition [M(X)
2
2 n
(pyrazine) ] (X =
-
-
12
NCS , NCSe ; M = Zn, Cd) reported recently.
Compound 1B was obtained as single crystals in a mixture
with compound 1A and 1CI by the reaction of Cd(NCS)
and pyrimidine under solvothermal conditions. The 2 : 3 com-
pound {[Cd(NCS) (pyrimidine) (1B) crystallizes in the non-
centrosymmetric orthorhombic space group P2 2 with 2 formula
2
2
]
2
3 n
}
2
1 1
units in the unit cell. The asymmetric unit consists of one Cd
cation, two crystallographically independent thiocyanato anions
and one pyrimidine ligand all in general positions, as well as of
one pyrimidine ligand that is located on a 2-fold rotation axis. The
Cd cations are surrounded by two N and two S atoms of two pairs
of symmetry equivalent thiocyanato anions and two N atoms of
two crystallographically independent pyrimidine ligands within
slightly distorted octahedral polyhedron (Fig. 2, Fig. S3 and Table
S2 in the ESI†).
The Cd cations are linked by m-1,3 bridging thiocyanato anions
into chains that elongate in the direction of the a axis. These
chains are further linked into corrugated layers by pyrimidine
ligands (Fig. 2). Interestingly only half of the co-ligands act as
bridging ligands whereas the others are only terminal bonded. It
must be noted that this structural motif was never observed before
in thiocyanato coordination polymers.
Fig. 3 Crystal structure of compound 1CI with view along (101) (black =
cadmium, dark-grey = sulphur, grey = carbon, light-grey = nitrogen) (an
Ortep plot of 1CI is shown in Figure S5 in the ESI†).
The Cd cations are linked via m-1,3 bridging thiocyanato anions
into chains that elongate in the direction of the crystallographic
a axis. These chains are further linked by bridging pyrimidine
ligands into layers, which is a common motif for 1 : 1 thiocyanato
The 1 : 1 compound [Cd(NCS)
2
(pyrimidine)]
n
(1CI) can be
7,9
prepared phase pure by the reaction of Cd(NCS)
2
and pyrimidine
coordination polymers with bidentate co-ligands.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 228–236 | 231

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The 3:2 compound {[Cd(NCS)
2
]
3
(pyrimidine)
2
}
n
(1D) can only
be obtained as single crystals in a mixture with compound 1CI
(
Fig. S6 in the ESI†). This compound crystallizes in the triclinic
¯
centrosymmetric space group P1 with Z = 1 formula unit in the
unit cell. The asymmetric unit consists of three crystallographically
independent thiocyanato anions and one pyrimidine ligand in gen-
eral positions as well as of three crystallographically independent
Cd cations that are located on a centre of inversion (Fig. 4 and
Fig. S7 in the ESI†).
Fig. 5 DTG, TG and DTA curves for compound 1A. Given are the mass
◦
Fig. 4 Crystal structure of compound 1D with view along the a axis
loss in % and the peak temperatures in C.
(
black = cadmium, dark-grey = sulphur, grey = carbon, light-grey =
that calculated for 1CI from single crystal structure analysis it is
obvious that a different modification (1CII) has formed (Fig. S9
in the ESI†). This is in agreement with the results of an elemental
analysis, which is in perfect agreement with that expected for a
1 : 1 compound. Moreover, information on the coordination mode
of the thiocyanato anions can be retrieved by IR- and Raman
nitrogen) (an Ortep plot of 1D is shown in Fig. S7 in the ESI†).
Two of the cations are each coordinated by two N and two
S atoms of m-1,3 bridging thiocyanato anions and two N atoms
of two bridging pyrimidine ligands within a distorted octahedral
coordination geometry (Table S4 in the ESI†). The remaining
Cd cations are coordinated by two N and four S atoms of m-
13
-1
spectroscopy. For form 1CI nas(CN) = 2080 cm whereas for the
-
1
1
,3 bridging thiocyanato anions and the coordination polyhedra
new form 1CII nas(CN) = 2079 cm , which clearly shows that in
both modifications the coordination mode must be similar (Fig.
S10 and S11 in the ESI†). It must be noted that the new form
1CII is not isotypic to its selenocyanato analogue (see below).
Unfortunately all attempts to determine the structure of this new
form from X-ray powder data fail.
can also be described as a slightly distorted octahedron. Two of
the three Cd cations are bridged by alternating m-1,3 (N, S) and
m-1,1,3 (S, S, N) bridging thiocyanato anions into chains, which
are further connected into layers by additional Cd cations that
are coordinated to the S atoms of the m-1,1,3 bridging anions
(
Fig. 4). This Cd cation is additionally connected to the other two
To prove which of the two 1 : 1 modifications is the thermo-
dynamically most stable form at room temperature a mixture
of both forms were stirred in a saturated acetonitrile solution
at room temperature. Finally, the product formed after one week
was investigated by X-ray powder diffraction. These investigations
clearly show that the new modification transforms into form 1CI,
which proves that this modification represents the most stable form
at room-temperature (Fig. 6). Additional DSC measurements gave
no hints for a transformation of 1CI into a further modification
and thus, it is not proven if both forms behave enantiotropically
or monotropically.
independent Cd cations via m-1,3 bridging anions (Fig. 4).
Thermoanalytical investigations. To prove which of the ligand-
deficient intermediates can be obtained on thermal decomposition
of the most ligand-rich precursor 1A, this compound was investi-
gated by DTA-TG measurements (Fig. 5).
On heating two mass steps are observed, which are accompanied
◦
with two endothermic events in the DTA curve at T
P
= 182 C
◦
and 218 C. From the DTG curve it is obvious that both steps
are not well resolved. The experimental mass loss of 20.1%
and 20.4% is in good agreement with that calculated for the
loss of one pyrimidine ligand in each step (Dmcalc = 20.6%).
Thus, it can be assumed that in the first step a 1 : 1 compound
Investigations on Cd selenocyanato coordination compounds
of composition [Cd(NCS)
second step transforms into Cd(NCS)
further heating. Heating rate dependent measurements show that
the resolution cannot be improved (Fig. S8 in the ESI†). However,
to check if the 1 : 1 compound 1CI has formed in the first step,
2
(pyrimidine)]
n
is obtained that in the
, which decomposes on
Crystal structures. In contrast to the cadmium thiocyanato
coordination polymers mentioned above, with cadmium seleno-
cyanate only two different compounds were obtained. The reaction
2
of Cd(NO
pyrimidine leads to the phase pure formation of the most ligand-
rich compound [Cd(NCSe) (pyrimidine) (2A) (Fig. S12 in the
3
)
2
·4H
2
O and KNCSe in ethanol with a large excess of
◦
the residue formed at about 178 C was isolated and investigated
2
2 n
]
by XRPD measurements. If the powder pattern is compared with
ESI†). Compound 2A crystallizes in the monoclinic space group
2
32 | Dalton Trans., 2012, 41, 228–236
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Fig. 6 Experimental X-ray powder pattern of 1CII (A), of the mixture of
CI and 1CII (B), after stirring the mixture for one week (C) and X-ray
powder pattern for form 1CI calculated from single crystal data (D).
1
P2 /c with Z = 2 formula units in the unit cell. The structure
1
consists of Cd cations which are coordinated by two N and
two Se atoms of the m-1,3 bridging selenocyanato anions and
by two N atoms of the pyrimidine ligands (Fig. S13 and Table
S5 in the ESI†). This structure exhibits the same coordination
topology as that of 1CI, but in contrast the neutral co-ligands are
terminal bonded. The Cd cations are connected into chains via
m-1,3 bridging selenocyanato anions (Fig. 7).
Fig. 8 Crystal structure of compound 2B with view along the a-axis
(
black = cadmium, dark-grey = selenium, grey = nitrogen, light-grey =
carbon, white = hydrogen) (an Ortep plot of 2B is shown in Figure S14 in
the ESI†).
coordination polyhedron can be described as a slightly distorted
octahedron. Surprisingly, the third Cd cation is only coordinated
by N atoms. Four of them originate from two pairs of symmetry
related m-1,3 bridging selenocyanato anions and the other two N
atoms belong to symmetry equivalent and bridging pyrimidine
ligands. The Cd cations are linked by the selenocyanato anions
and the pyrimidine ligands into a three-dimensional coordination
network, which is different from that of its thiocyanato analogue
1CI and 1CII and which is without any precedence in the literature
(
Fig. 8).
Thermoanalytical investigations. If the most ligand-rich com-
pound [Cd(NCSe)
ance at least three low resolved mass steps are observed, which
are accompanied with endothermic events in the DTA curve
2
2 n
(pyrimidine) ] (2A) is heated in a thermobal-
Fig. 7 Crystal structure of compound 2A with view along the c axis
(
black = cadmium, dark-grey = selenium, grey = nitrogen, light-grey =
carbon, white = hydrogen) (an Ortep plot of 2A is shown in Figure S13 in
the ESI†).
(
Fig. 9, left). The mass loss in the first and second TG step is
in reasonable agreement with that calculated for the removal of
one pyrimidine ligand in each step (Dmcalc = 16.6%), leading in the
first mass step to a ligand-deficient intermediate of composition
Using slightly different reaction conditions, a second compound
of composition [Cd(NCSe)
2
n
(pyrimidine)] (2B) was obtained. The
1
: 1 compound 2B crystallizes in the monoclinic space group C2/c
2
n
[Cd(NCSe) (pyrimidine)] which decomposes on further heating.
with Z = 8 formula units in the unit cell. The asymmetric unit
consists of four crystallographically independent selenocyanato
anions, two pyrimidine ligands and three Cd cations, whereby
two of them are located on special positions. One Cd cation is
coordinated by two N and two Se atoms of four crystallograph-
ically independent and m-1,3 bridging selenocyanato anions and
two independent and bridging pyrimidine ligands within a slightly
distorted octahedron (Fig. 8, Fig. S14 and Table S6 in the ESI†).
The second Cd cation is coordinated by four Se atoms of two pairs
of symmetry related m-1,3 bridging selenocyanato anions and two
N atoms of two symmetry related pyrimidine ligands and the
In order to prove if the 1 : 1 compound 2B has formed in the
first step, the residue formed at about 130 C was isolated and
◦
investigated by XRPD measurements. Unfortunately the residue
formed is always a mixture of 2A and 2B. Therefore heating
rate dependent measurements were performed to investigate if
the resolution of the thermal event can be improved. These
measurements showed, that on faster heating the mass loss of
the first step increases to a value calculated for a 3 : 2 compound
(Fig. 9, right). Unfortunately, if the TG experiment is stopped
after this mass step only a dark brown residue is obtained which
is amorphous to X-rays and therefore was not further analyzed.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 228–236 | 233

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◦
-1
◦
-1
Fig. 9 DTG, TG and DTA curves for compound 2A with heating rate of 1 C min (left) and 8 C min (right). Given are the mass loss in % and the
peak temperatures in C.
◦
Impact on the paramagnetic counterparts
Table 2 Selected crystal data of the 3 : 2 intermediates of the ligand-rich
compounds [M(NCS)
2
(pyrimidine)
2 n
] (M = Mn, Fe)
As mentioned in the introduction one important reason for the
present investigations on the diamagnetic cadmium thio- and
selenocyanato coordination compounds is to extract structural in-
formation on intermediates of their analogue paramagnetic com-
pounds, in the case that they cannot be crystallized from solution.
In this context we have previously reported on manganese and
iron thiocyanato coordination compounds with pyrimidine as co-
{
[Mn(NCS)
(pyrimidine)
2
]
3
-
n
{[Fe(NCS)
2
]
3
-
}
Compound
2
}
(pyrimidine)
2
n
MW g-1 mol-1
Crystal system
673.50
Triclinic
P1
6.8633(3)
9.1548(3)
10.4201(4)
108.538(1)
91.106(1)
103.736(1)
599.79(4)
676.22
Triclinic
¯
¯
Space group
P1
a/A˚
6.7050(6)
9.0752(6)
10.3754(7)
108.842(2)
91.204(2)
104.148(2)
575.93(7)
b/A˚
9
ligand. We have reported that on heating the ligand-rich 1 : 2 com-
˚
c/A
◦
pounds [Mn(NCS)
a transformation into ligand-deficient 1 : 1 compounds of com-
position [M(NCS) (pyrimidine)] (M = Mn, Fe) is observed.
2
(pyrimidine)
2
]
n
and [Fe(NCS)
2
(pyrimidine)
2
]
n
a ( )
◦
b ( )
◦
g ( )
2
n
V/A˚
3
However, from temperature dependent X-ray powder diffraction
and DTA-TG measurements we had strong hints that the overall
reaction is much more complicated in order that a second
intermediate of composition {[M(NCS)
2
]
3
(pyrimidine)
2
}
n
(M =
Mn, Fe) is formed. These intermediates can only be obtained as a
microcrystalline powder on thermal decomposition and we were
not able to determine their crystal structure. However, if the X-
ray powder pattern of these intermediates is compared with that
calculated for the 3 : 2 compound {[Cd(NCS)
2
]
3
(pyrimidine)
2 n
} it
is obvious that all compounds are isotypic (Fig. 10).
To prove this assumption we isolated the 3 : 2 intermediates
obtained by thermal decomposition of the ligand-rich compounds
[
M(NCS)
2
2 n
(pyrimidine) ] (M = Mn, Fe) and determined its
structure by Rietveld refinement (Table 2 and Fig. S16–17†).
Fig. 10 X-ray powder pattern of the 3 : 2 intermediate isolated in the
Conclusions
thermal decomposition reaction of [Mn(NCS)
Fe(NCS) (pyrimidine) (middle) as well as calculated pattern for the
: 2 compound {[Cd(NCS) (pyrimidine) (1D) (bottom).
2 2 n
(pyrimidine) ] (top) and
[
2
2
]
n
In this contribution we reported on new Cd(II) thio- and se-
lenocyanato coordination polymers with pyrimidine as co-ligand.
3
2
]
3
2 n
}
2
34 | Dalton Trans., 2012, 41, 228–236
This journal is © The Royal Society of Chemistry 2012

View Article Online
With cadmium thiocyanate and pyrimidine we obtained a ligand-
rich precursor that on heating forms an intermediate which is
a polymorphic modification of the 1 : 1 compound prepared in
solution. IR- and Raman spectroscopic investigations clearly
prove that the coordination mode of the thiocyanato anions is
identical and therefore, the coordination topology of both forms
should be identical. Solvent mediated conversion experiments
reveal that form 1CI represents the thermodynamic most stable
2 G. R. Desiraju, Crystal Engineering. The Design of Organic Solids,
1
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form at room-temperature. However, if Cd(NCS) and pyrimidine
2
are reacted under different conditions in solution four different
compounds were obtained and their structures are strongly related.
Therefore, our results indicate that on the way from coordination
compounds with terminal N-bonded thiocyanato anions to more
condensed networks with m-1,3 bridging anions a large amount of
structural intermediates might be observed.
4
6, 5349–5351; S. Hu, J.-L. Liu, Z.-S. Meng, Y.-Z. Zheng, Y. Lan,
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Moreover, based on the structure of the 3 : 2 compound we have
determined the structures of two ligand-deficient intermediates
4
R. Georges, J. J. Borr a´ s-Almenar, E. Coronado, J. Cur e´ ly and M.
Drillon, in Magnetism: Molecules to Materials, ed. J. S. Miller and
M. Drillon, WILEY-VCH, Weinheim, Editon edn, 2001, pp. 1–43C.
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Related Phenomena, ed. R. Winpenny, Springer Berlin, Heidelberg,
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Sun, A. V. Prosvirin, H.-H. Zhao, J.-G. Mao and K. R. Dunbar, J.
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C.-H. Yan, Inorg. Chem., 2006, 45, 6694–6705; X.-J. Li, X.-Y. Wang,
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9
with Mn and Fe reported recently.
In contrast, with cadmium selenocyanate we obtained only two
different compounds that are not isotypic to their thiocyanate
analogue. Such observations might be of interest in those cases,
in which thermal decomposition reactions lead to the formation
of a new modification of a metal thiocyanate, which might be
isotypic to its selenocyanate analogue. Even if this is not the
case here, because the new modification 1CI is not isotypic to
2B, we have recently reported on an example in which we have
determined the structure of an metastable modification of a metal
thiocyanato coordination polymer based on the structural data of
12
its thermodynamically more stable selenocyanate analog.
Summarizing, the exchange of the paramagnetic metal cations
like e.g. Mn(II), Fe(II), Co(II) or Ni(II) by cadmium seems to
be a promising approach to retrieve structural information on
the paramagnetic analogues and to get a deeper insight into the
structural aspects of thermal decomposition reactions on the way
to more condensed coordination networks based on transition
metal thio- or selenocyanates. The former is of special interest,
because structural information is really needed to understand their
magnetic properties in more detail.
2
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Acknowledgements
3
661–3665; R. Gheorghe, A. M. Madalan, J.-P. Costes, W. Wernsdorfer
This project was supported by the Deutsche Forschungsgemein-
schaft (project: NA 720/3-1) and the State of Schleswig-Holstein.
We thank Professor Dr Wolfgang Bensch for access to his
experimental facility.
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