Synthesis and electrical properties of some new heterobimetallic coordination polymers MPb(SCN)4 (M = Co, Ni) and their reaction products with Lewis bases

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SYNTHESIS AND ELECTRICAL PROPERTIES OF SOME
NEW HETEROBIMETALLIC COORDINATION POLYMERS
MPb(SCN)4 (M = Co, Ni) AND THEIR REACTION
PRODUCTS WITH LEWIS BASES
Nanhai Singh a & Vinay Kumar Singh a
a Department of Chemistry, Faculty of Science , Banaras Hindu University , Varanasi, 221
005, India
Published online: 15 Feb 2007.
To cite this article: Nanhai Singh & Vinay Kumar Singh (2001) SYNTHESIS AND ELECTRICAL PROPERTIES OF SOME NEW
HETEROBIMETALLIC COORDINATION POLYMERS MPb(SCN)4 (M = Co, Ni) AND THEIR REACTION PRODUCTS WITH LEWIS BASES,
Synthesis and Reactivity in Inorganic and Metal-Organic Chemistry, 31:10, 1743-1757, DOI: 10.1081/SIM-100108259
To link to this article: http://dx.doi.org/10.1081/SIM-100108259
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SYNTH. REACT. INORG. MET.-ORG. CHEM., 31(10), 1743–1757 (2001)
SYNTHESIS AND ELECTRICAL
PROPERTIES OF SOME NEW
HETEROBIMETALLIC COORDINATION
POLYMERS MPb(SCN)4 (M ¼ Co, Ni) AND
THEIR REACTION PRODUCTS WITH
LEWIS BASES
Nanhai Singh* and Vinay Kumar Singh
Department of Chemistry, Faculty of Science,
Banaras Hindu University, Varanasi 221 005, India
ABSTRACT
This paper describes a study of the electrical conducting beha-
viour of some new heterobimetallics of the composition
MPb(SCN)4 (M ¼ Co(II), Ni(II)) and their Lewis base products
of the forms MPb(SCN)4L2 (L ¼ pyridine (py) or triphenyl-
phosphine (PPh3)) and [M(phen)3][Pb(SCN)4] (M ¼ Co(II) or
Ni(II); phen ¼ 1,10-phenanthroline). The properties of these
were investigated by using spectroscopic (IR, UV-Visible)
methods as well as powder XRD and conductivity techniques.
The IR spectra showed the bridging nature of SCNÀ in
MPb(SCN)4 and MPb(SCN)4(py)2, both terminal and bridging
SCNÀ in NiPb(SCN)4(PPh3)2 and only terminal SCNÀ
groups in [M(phen)3][Pb(SCN)4]. The magnetic data and elec-
tronic absorptions are also compatible with these proposed
*Corresponding author. E-mail: nsingh@banaras.ernet.in
1743
Copyright # 2001 by Marcel Dekker, Inc.
www.dekker.com

1744
SINGH AND SINGH
arrangements. Based on these studies, polymeric, monomeric
and ionic structures have been proposed for these compounds.
All of the compounds exhibited solid state srt values in the
10À12 to 10À6 S cmÀ1 range, and all showed semiconductivity
from 303 to 363 K. A notable increase in conductivity, 1027103
increase, at higher temperatures suggested some type of phase
transition/packing equilibrium changes which produceeffective
M. . .S/Pb. . .S/S. . .S intermolecular
NiPb(SCN)4 and [M(phen)3][Pb(SCN)4].
interactions
in
INTRODUCTION
The field of inorganic complex-based conductors is now very well
advanced175. Many charge transfer salts173, 6710 of TTF (tetrathiafulvalene)
and BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene) with anionic com-
plexes of: thiocyanate (SCNÀ) or dithiolates of transition metals e.g.,
K(BEDT-TTF)2[Cu(NCS)2], (BEDT-TTF)2X[M(SCN)4] (X ¼ NH4þ, Tlþ,
Naþ, Kþ or Rbþ; M ¼ Co(II), Zn(II), Cd(II) or Hg(II)), [(BEDT-TTF)n]
[Re2(NCS)10]ÁC6H5CN=CH2Cl2
(n ¼ 2
or
3),
[(BEDT-
TTF)2][Cr(NCS)4(NH3)2], [TTF][Ni(dmit)2]2 (dmit2À ¼ 1, 3-dithiol-2-thione-
4, 5-dithiolate) and heterobimetallic complexes M[Ag2(ecda)2]4,
[NBu4 ]2[M(mnt)2Cu4I4]5 (M ¼ Ni, Pd or Pt; ecda2À ¼ 1-ethoxycarbonyl-1-
n
cyanoethylene-2,2-dithiolate; mnt2À ¼ maleonitriledithiolate), showed semi-
conducting, metallic or superconducting behaviour. In these systems, the
conducting properties have been mainly attributed to S. . .S=M. . .S inter-
molecular contacts present in the solid-state. Also, some related complexes
are important MOCVD precursors for semiconducting materials11.
Since the introduction of CoHg(SCN)4 as a well known Gouy stan-
dard in magnetic measurements, an extensive amount of work has been
done on homo- and heteropolynuclear thiocyanato complexes12718. This
extremely versatile ambidentate ligand is capable of bridging two similar or
dissimilar metals providing diverse and unusual15 electronic properties that
depend on the bonding arrangements of the S and N donors. For example,
the tetranuclear complex, [Cu(oxpn)Ni(m-NCS)H2O(tmen)]2(ClO4)2
(oxpn ¼ N,N0-bis(3-aminopropyl)oxamide,
tmen ¼ N,N,N0,N0-tetra-
methylethylenediamine) possess a Cu-Ni bond as well as a Cu-SCN-Ni
grouping, and it displays ferro- and antiferromagnetic alternations16. While,
in the case of [Cu2Pb(SCN)3L3] (L ¼ 2-dimethylaminoethanol)16, where di-
merisation has occurred through bridging NCSÀ groups the hexanuclear
units so formed are linked by Pb. . .S interactions between adjacent units to
form layered supramolecular assemblies.

HETEROBIMETALLIC COORDINATION POLYMERS
1745
Keeping with the well-pronounced conducting properties of the chal-
cogen-rich ligand complexes and: their applications to material science, we
herein present the syntheses, characterizations and solid state conductivities
of some new heterobimetallic coordination polymers, CoPb(SCN)4 and
NiPb(SCN)4 and their reaction products with Lewis bases.
EXPERIMENTAL
Materials
Nickel nitrate, potassium thiocyanate, pyridine (py) and 1,10-phe-
nanthroline (phen), all British Drug House products, were manufactured by
E. Merck India Ltd., and triphenylphosphine (PPh3) was obtained from
Aldrich Chemical Co., Inc. These were used without further purification.
KSCN was dried before use. The starting compounds Co(NCS)2 or
Ni(NCS)2 were prepared by reacting equimolar-quantities of Co(NO3)2 or
Ni(NO3)2 and KSCN in ethanol while Pb(SCN)2 was obtained by reacting
Pb(NO3)2 and KSCN in a 1:2 molar ratio in water according to the known
methods19,20. Ethanol was dried over calcium oxide and distilled before use.
All experiments were carried out in air.
Synthesis of the Heterobimetallic Complexes
CoPb(SCN)4 and NiPb(SCN)4
A suspension of 2 mmol of Co(NCS)2 (0.350 g) or Ni(NCS)2 (0.349 g)
and Pb(SCN)2 (0.646 g) in dichloromethane (25mL) was stirred at room
temperature (30 ꢀC) for about 72 h. Alternatively, these were also prepared
by refluxing equimolar quantities of the constituent metal thiocyanates in
dichloromethane. The resulting bluish-green=yellowish-green powders were
filtered out and dried in vacuo over CaCl2; the yields were, 0.976 g (98%) for
CoPb(SCN)4 and 0.915g (92%) for NiPb(SCN) 4.
MPb(SCN)4L2 and MPb(SCN)4(phen)3 (M ¼ Co(II)) or Ni(II);
L ¼ py or PPh3)
A solution of py (0.2 mL, ꢁ2 mmol), PPh3 (0.524 g, 2 mmol) or phen
(0.595 g, 3 mmol) in dichloromethane (5 mL), was added to 20 mL of
a suspension of 1 mmol of CoPb(SCN)4 (0.498 g) or NiPb(SCN)4 (0.497 g) in
the same solvent. In each case, the reaction mixture was stirred for about

1746
SINGH AND SINGH
24 h. The resulting solid products, were filtered and dried in vacuo over
CaCl2; the yields were 0.636 g (97%) for M ¼ Co(II), L ¼ py; 0.955 g (92%)
for M ¼ Co(II), L ¼ phen; 0.950 g (93%) for M ¼ Ni(II), L ¼ PPh3 0.965g
(93%) for M ¼ Ni(II), L ¼ phen; 0.623 g (95%) for M ¼ Ni(II), L ¼ py.
Analysis and Physical Measurements
Microanalyses of C, H and N were performed on a model 1108 Carlo
Erba elemental analyser at the Central Drug Research Institute, Lucknow.
Sulfur was determined as BaSO4. Conductivities were measured in 10À3 M
DMSO solutions on a WTW Leitfahigkeitsmesser conductivity-meter, and
the melting points were obtained in open capillaries and are uncorrected.
Powder XRD patterns of the samples were recorded at room temperature
on a Rigaku (Japan) Rotaflex M/S diffractometer, using Cu-Ka radiation at
l ¼ 1.5405 A. IR spectra (40007400 cmÀ1) were recorded in KBr pellets with
a JASCO FTIR-5300 spectrophotometer. Electronic spectra were recorded
on a Cary-14 or 23 spectrophotometer as Nujol mulls by following the
techniques of Lee21. Room temperature magnetic susceptibilities were
measured on a Cahn Faraday electrobalance using CoHg(SCN)4 as the
standard. The diamagnetic corrections were evaluated from Pascal’s
constants.
RESULTS AND DISCUSSION
The elemental analyses (Table I) are consistent with the formation of
heterobimetallic thiocyanato complexes of the type, MPb(SCN)4,
MPb(SCN)4L2 and MPb(SCN)4(phen)3 (M ¼ Co(II) or Ni(II); L ¼ py or
PPh3), and these have been characterized by magnetic susceptibility mea-
surements, IR and UV-Visible spectroscopy and powder XRD analysis.
These solids are stable at room temperature, and decompose in the
1957295 ꢀC temperature range. In the case of the py adducts, there are
colour changes above 70 ꢀC. The compounds are insoluble in water and
common organic solvents like ethanol, methanol and acetone, but they are
soluble in DMSO and DMF, with characteristic changes in their colours i.e.,
from bluish-green to pink for the cobalt and yellowish-green/sky-blue to
green for the nickel complexes. The electrical conductivities of
> 66.6 OÀ1 cm2 moleÀ1 in 10À3 M DMSO solution are close to those
expected for 2:2 electrolytes22, and this is consistent with their substantial
decomposition as shown below.

1748
SINGH AND SINGH
2À
MPbðSCNÞ ½MðDMSOÞ Š2þ þ ½PbðSCNÞ4Š
ð1Þ
*
)
4
6
However, in the case of the phen complexes the colour is unchanged on
dissolution. Their LM values of 44.80 and 42.38 OÀ1 cm2 moleÀ1 [Co-Pb, Ni-
Pb] are somewhat low for 2:2 electrolytes, and this is consistent with extensive
ion-pairing in DMSO, a solvent of relatively low dielectric constant, and the
presence of highly charged species as shown in the following equilibria:
2þ
*
MPbðSCNÞ ðphenÞ ½MðphenÞ Š
)
4
3
3
2À
2þ
2À
*
þ ½PbðSCNÞ4Š
½MðphenÞ Š ½PbðSCNÞ4Š
ð2Þ
)
3
Magnetic and Spectroscopic Studies
In the IR spectra of the compounds, the bands are highly character-
istic12714,23 for bridging and terminal, coordinated SCNÀ ligands (Table II).
A very strong band at 2100 cmÀ1 and a shoulder at 2170 cmÀ1 due to
n(CꢂN) in CoPb(SCN)4 and NiPb(SCN)4, correspond to bridging SCNÀ
groups. NiPb(SCN)4(PPh3)2 exhibits distinct bands at 2170 and 2090 cmÀ1
,
which are indicative of both bridging and terminal SCNÀ groups. In the case
of their py adducts, the occurrence of bands at around 2100 cmÀ1 is con-
sistent with the conclusion that the coordination of py does not affect the
bridging mode of these bimetallic tetrathiocyanates whereas their trisphen
groups rupture the bridges between the two metal centres resulting in only
terminal SCNÀ groups as indicated by the IR bands near 2080 cmÀ1. All of
the compounds show weak absorptions around 760 and 850 cmÀ1 char-
acteristic of the n(C-S) frequency of coordinated SCNÀ moieties. Absorp-
tions at around 430 and 640 cmÀ1 indicate mono=bidentate coordination of
the py and phen groups. In the far-IR region (Fig. 1), NiPb(SCN)4Á2PPh3
consists of bands at 307 and 206 cmÀ1 which correspond to n(Ni-NCS) and
À1
n(Pb-SCN) modes12,24, respectively. The bands at 175and 154 cm
are
characteristic24 of n(Ni-P) vibrations. These IR bands, and the established25
bulkiness of the ligand PPh3 suggest a monomeric formulation of this
compound.
Both the effective magnetic moments and the electronic spectral bands
(Table I and II) are compatible with a symmetrical tetrahedral geometry
about the Co(II) centre in CoPb(SCN)4 and octahedral geometries26 around
Co(II) in its py and phen complexes. However, the features of the absorp-
tions for the two nitrogen-containing base adducts are somewhat different
because of changes in the environment about the metal centre. The dia-

HETEROBIMETALLIC COORDINATION POLYMERS
1749

1750
SINGH AND SINGH
Figure 1. Far-IR (5007100 cmÀ1) spectrum of NiPb(SCN)4(PPh3)2.
magnetic behaviour together with the UV-Visible bands are suggestive of
square-planar geometry26,27 about Ni(II) in the orange coloured
NiPb(SCN)4(PPh3)2. Magnetic moment values and electronic spectral bands
suggest octahedral geometries26 for nickel(II) in NiPb(SCN)4 and its base
adducts with py and phen. In the case of NiPb(SCN)4, octahedral geometry
around Ni(II) is possibly achieved, probably because of Ni. . .S axial inter-
actions in a layered polymeric array of this compound.
Powder X-Ray Diffraction Spectra
Powder X-ray diffraction patterns show the crystalline nature of the
compounds. A comparison of powder patterns (Fig. 2) of the starting
compounds, e.g., Co(NCS)2, Ni(NCS)2 and Pb(SCN)2 with the bimetallic
complexes CoPb(SCN)4 and NiPb(SCN)4, clearly reveals that the hetero-
bimetallics are new species and not mixtures of the constituent metal thio-
cyanates. Further, the powder patterns, which are largely dependent on the
lattice geometries of the crystal structures, show that the compounds are not
isostructural.
Inspite of all efforts, the general insolubility of these compounds
prevented their crystallization. Based on the foregoing studies and known
structures12, a polymeric structure for MPb(SCN)4 and MPb(SCN)4(py)2, a
monomeric structure for NiPb(SCN)4(PPh3)2, and an ionic structure for
MPb(SCN)4(phen)3 (Fig. 3) have been proposed.

HETEROBIMETALLIC COORDINATION POLYMERS
1751
Figure 2. Powder XRD patterns of the complexes.

1752
SINGH AND SINGH
Figure 3. Proposed structures for the complexes.

HETEROBIMETALLIC COORDINATION POLYMERS
Electrical Conductivity
1753
The temperature (3037363 K) dependence of the compressed pellet
conductivities were studied by using a conventional two-probe technique
on a Keithley 236 source unit. The pellets were prepared at a load of
20 KN; silver paint was used to adhere the contacts to the pellet surfaces.
A sequence of increasing voltages were passed through the leads while the
current across the current leads was measured. Resistance was calculated
from the slope of I/V curves. Resistivities were calculated from the re-
sistance measurements using known dimensions of the sample pellets. All
of the compounds show srt values in the 10À12 to l0À6 S cmÀ1 range, and
they are semiconductors (Fig. 4) over the above temperature range. Ex-
cept for CoPb(SCN)4(py)2 where a deviation from a straight line is ob-
served, the remaining compounds provide plots of log s versus TÀ1 that
are nearly linear which produce activation energies, Ea, in the 0.06 to
0.76 eV range. In the cases of NiPb(SCN)4 and the complex bimetallic
salts [M(phen)3][Pb(SCN)4], a remarkable increase in conductivity is ob-
served, i.e., on the order of 1027103, with increasing temperature which
may be the result of some type of phase transformations or changes in
packing equilibria. In the majority of cases, the temperature dependence
of the conductivity is reproducible, but in the case of the py adducts the
behaviour is almost similar to their parent Lewis acids, CoPb(SCN)4 and
NiPb(SCN)4, which strongly suggests that at higher temperatures the py
molecules are lost. The lower conductivities of these materials, as com-
pared to anionic thiocyanato compounds6710 with dominant S. . .S inter-
molecular stacking containing BEDT-TTF as counter cation, may be
attributed to weaker S. . .S, Pb. . .S and M. . .S intermolecular contacts16.
However, in the absence of single crystal X-ray structures, definite con-
clusions about the conductivity mechanisms cannot be drawn.
CONCLUSION
All of the heterobimetallic complexes reported herein exhibit semi-
conducting properties, and in a few cases remarkable increases in con-
ductivity at higher temperatures which are attributed to effective
intermolecular stackings. In conclusion, unlike CoHg(SCN)4 and
NiHg(SCN)4, which can be prepared in water as well as in methanol,
ethanol or acetone by reaction of the constituent metal thiocyanates or by
replacement of Kþ in K2[Hg(SCN)4] by Co(II)/Ni(II), analogous bimetallic
compounds containing Pb(II) could not be synthesized in either of the polar
solvents, instead they could be synthesized by reaction of monometallic

1754
SINGH AND SINGH
Figure 4. Temperature dependence of the electrical conductivities of the complexes.

HETEROBIMETALLIC COORDINATION POLYMERS
1755
thiocyanates in highly non-polar solvents like CH2Cl2/CCl4 in which they
are insoluble. This difference in synthetic methodology may be attributed to
the borderline character of both Co(II)/Ni(II) and Pb(II) in contrast to the
soft behaviour of Hg(II) in accordance with Pearson’s HSAB principle28.
Due to the same reasons, the strength of bridging in CoPb(SCN)4/
NiPb(SCN)4 is weaker than that of the CoHg(SCN)4, coordination polymer.
It is observed that the use of non-polar solvents greatly influence the
hard=soft nature of metal ions and promotes even less favourable interac-
tions to occur.
ACKNOWLEDGMENT
One of the authors, Vinay Kumar Singh, acknowledges Banaras
Hindu University for financial assistance in the form of a scholarship.
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Received July 25, 2000
Accepted August 21, 2001
Referee I: K. Moedritzer
Referee II: B. N. Storhoff