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Polyhedron 25 (2006) 3006–3016

Two new hydrothermally synthesised L–M–L type cobalt(II)hydrogen bonded co-ordination complexes with mixed

ligands: Characterisation and magneto-structural correlation

Joy Chakraborty a, Brajagopal Samanta a, G. Rosair b, Volker Gramlich c,M. Salah El Fallah d, Joan Ribas d, T. Matsushita e, Samiran Mitra a,*

a Department of Chemistry, Jadavpur University, Kolkata-700 032, West Bengal, Indiab Department of Chemistry, Heriot-Watt University, Edinburgh, EH 14 4AS, UK

c Laboratorium fur Kristallographie ETH, Eidgenossische Technische Hochschule, Zurich, CH-8092 Zurich, Switzerlandd Departamento de Quımica Inorganica, Universitat de Barcelona, Martı i Franquees, 1-11, 08028-Barcelona, Spain

e Department of Materials Chemistry, Faculty of Science and Technology, Ryukoku University, Seta, Otsu 520-2194, Japan

Received 4 April 2006; accepted 4 May 2006Available online 11 May 2006

Abstract

Two new hydrogen bonded polymeric L–M–L type Co(II) metal co-ordination complexes [{Co(hmt)2(NCS)2(H2O)2}{Co(NCS)2-(H2O)4}(2H2O)] (1) and [Co(hmt)(NNN)2(H2O)2]n (2) have been synthesised under controlled hydrothermal condition and characterisedby elemental analyses, FT-IR, UV/vis spectroscopy and thermal analyses. Here 1,3,5,7-tetraazatricyclo [3.3.1] decane [or hexamethylene-tetramine (hmt)] has been used as a neutral organic bidentate spacer molecule. The structures of the complexes have been confirmedunequivocally from single crystal X-ray diffraction studies which reveals the presence of weak covalent and H-bonding interactionsbetween octahedral Co(II) complexes. Magneto-structural correlations have been drawn from cryo-magnetic susceptibility measurementsover a wide range of temperature (2–300 K) under 0.5 T magnetic fields. A weak antiferromagnetic interaction of J = �0.77 cm�1 foundis as expected from X-ray structure determination. The high dimensionality of the structures is probably a manifestation of extensiveweak covalent interactions and H-bondings.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Hydrothermal synthesis; L–M–L type co-ordination polymers; Cobalt(II); Hmt; Thiocyanate; Azide

1. Introduction

In recent years, inorganic co-ordination chemistry has gota new face for the remarkable development in the area ofcrystal engineering [1,2]. Considerable research effort hasalready been focused in recent years on the designing andsynthesis of supramolecular architectures analogous toimportant minerals such as zeolites and other nanoporousmaterials [3–7] due to their important applications in separa-

0277-5387/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.poly.2006.05.001

* Corresponding author. Tel.: +91 033 2414 6666x2505; fax: +91 0332414 6414/6210.

E-mail address: smitra_2002@yahoo.com (S. Mitra).

tion processes and catalysis [8]. Such co-ordination polymersof high dimensionality are usually organised by weak co-ordinate covalent or hydrogen bondings. This can beachieved by using bi- or multidentate bridging ligands to jointhe metal centers together. Use of symmetrically-bridgingneutral organic spacer ligands (i.e. 2-connectors, trigonal3-connectors, tetrahedral or square-planar 4-connectors)[3,9–13] along with other anionic ligands (e.g. thiocyanate,azide [14–16] or aromatic polycarboxylic acid anions[17–20], etc.) in combination, is one of the several popularstrategies that could be employed successfully in the synthe-sis of frameworks of different dimensionalities. Explorationof this combination effect [21] is less frequent in the case of

J. Chakraborty et al. / Polyhedron 25 (2006) 3006–3016 3007

first row transition metals. Hmt, a spacer molecule which hasbeen using to prepare different co-ordination polymers fordecades can connect through two, three or all four of itstertiary nitrogen atoms to furnish a variety of polymersbearing interesting topologies [22]. Different co-ordinationpolymers are already reported in this area using it as a simplebidentate, tridentate or tetradentate spacer along withseveral bridging or non-bridging coligands as it seems to bequite efficient for the self-assembly of networks resulting indifferent topological motifs in the two or three-dimensionalnets involving transition [12,23,24] and non-transition metal[25,26] centers. The self-assembly of these metallo-organicframeworks is vulnerable to many factors such as solventsystem, template effect, pH of the solutions, steric parame-ters of the counter ions and above all the imposed reactionenvironment [24]. We have given our effort to study thecombination effect of hmt, a polycyclic tertiary amine alongwith thiocyanate and azide as anionic ligands in two differentcases with the same metal ion that ends in a construction oftwo new Co(II) metallo-organic clusters of high dimension-ality containing hmt as a neutral organic spacer. In complex1, hexamine being attached terminally to the Co(II) center inone co-ordination group is acting as an efficient hydrogenbond carrier throughout the chain with its different donorsites. It helps to bind the adjacent metal centers toincorporate a high dimensionality inside the system byseveral inter-chain (O–H� � �N, O–H� � �S) and intra-chain(C–H� � �O, C–H� � �N, C–H� � �S) hydrogen bondings. Incomplex 2, the same spacer ligand joins the adjacent Co(II)centers in a l-N,N 0 fashion by simple covalent bonds in a1-D polymeric chain. Thiocyanate and azide are acting asterminal ligands in the respective complexes and efficientlyinvolved in H-bondings with lattice water molecules to resultin an infinite 2-D polymeric network finally.

We have reported herein the controlled hydrothermalsynthesis of two extensively hydrogen bonded Co(II) L–M–L type polymeric co-ordination complexes. They havebeen systematically characterised by elemental analysis,FT-IR, UV–vis spectroscopy and thermal analyses. Struc-tures have been established from X-ray single crystal dif-fraction data. Magneto-structural correlations have beendrawn for both the complexes. Although there are plentyof interesting co-ordination complexes involving hmt asspacer, but their systematic characterisation, spectral andmagneto-structural correlation studies especially for Co(II)is not a well explored field.

2. Experimental

2.1. Materials

All the chemicals and solvents used for this synthesiswere of AR grade. Cobalt(II) chloride, hexahydrate; cobal-t(II) acetate, tetrahydrate; potassium thiocyanate, sodiumazide and hexamethylenetetramine were obtained fromAldrich Chemical Co. Inc. and used as received withoutfurther purification.

2.2. Physical measurements

The Fourier Transform infrared spectra (400–4000 cm�1) of the complexes were recorded on a Perkin–Elmer Spectrum RX I FT-IR system with KBr disc. Theelectronic spectra were recorded on a Perkin–Elmer k-40UV/vis-spectrometer using HPLC grade methanol as sol-vent. C, H, N analyses were carried out using a Perkin–Elmer 2400 II elemental analyser. Thermal studies weredone at a heating rate of 10 �C min�1 using a SchimadzuTGA-50 and DTA-50 thermal analyser system in adynamic atmosphere of N2 (flow rate 80 ml/min) in an alu-mina crucible.

Cryo-magnetic susceptibility measurements for the com-plexes were carried out at the Servei de Magnetoquımica ofthe Universitat de Barcelona, Spain with a QuantumDesigned SQUID MPMS-XL susceptometer apparatuswhich works in the range 2–300 K under magnetic fieldof approximately 0.5 T. Diamagnetic corrections were esti-mated from Pascal’s tables.

2.3. Synthesis of the complexes

2.3.1. [{Co(hmt)2(NCS)2(H2O)2}{Co(NCS)2(H2O)4}(2H2O)] (1)

The complex was prepared under controlled hydro-thermal condition. 0.299 g (1.03 mmol) CoCl2, 6H2O,0.217 g (2.237 mmol) KSCN, 0.067 g (0.484 mmol) hmtand 10 ml of water were loaded into a 23 cm3 Teflon-lined stainless steel vessel, which was then sealed andplaced in a computer programmable furnace. The result-ing mixture was heated at the rate of 100 �C/h up to160 �C, kept for 10 h, then cooled to 140 �C at the rateof 4 �C/h, held for 5 h, followed by further cooling atthe same rate to room temperature to give an approxi-mately 85.24% yield based on Co(II), as rosy pink col-ored prismatic single-crystals which were later collectedby mechanical isolation and washed with water–ethanol(2:1 v/v) mixed solvent, then dried in an oven keepingthem at 40 �C for 10 min.

Anal. Calc. for [C16H40Co2N12O8S4]: C, 24.80; H, 5.20;N, 21.70. Found: C, 24.79; H, 5.21; N, 21.69%.

2.3.2. [Co(hmt)(N3)2(H2O)2]n (2)

The complex was prepared following the same proce-dure as described above for the complex 1 exceptCo(CH3COO)2 Æ 4H2O, NaN3 and hexamethylenetetra-mine were taken in the proportion of 0.262 g (1.05 mmol),0.143 g (2.20 mmol) and 0.133 g (0.95 mmol), respectively.On slow cooling to ambient temperature purple prismaticsingle crystals were obtained with a yield of 80.04% basedon Co(II). Suitable crystals were later collected bymechanical isolation and washed with water–ethanol(2:1 v/v) mixed solvent, dried in an oven keeping themat 40 �C for 10 min.

3008 J. Chakraborty et al. / Polyhedron 25 (2006) 3006–3016

Anal. Calc. for [C6H16CoN10O2]: C, 22.57; H, 5.05; N,43.87. Found: C, 22.56; H, 5.04; N, 43.85%.

2.4. Crystal structure determination

The X-ray diffraction experiment was carried out at293 K on a rosy pink needle shaped single crystal block(0.09 mm · 0.08 mm · 0.07 mm) of complex 1 using graph-ite monochromatised Mo-Ka radiation (k = 0.71073 A).The data were collected on a CAD4 diffractometer withthe x-scan technique. Measuring standard reflections atfixed intervals during the data collection checked the stabil-ity of the crystal. However, no significant loss of intensitywas noted for the crystals. The data were corrected for Lor-entz and polarisation effects. The structures were solved bydirect methods using the SHELXLTL-PLUS and SHELXS-97 sys-tem and refined to convergence by full-matrix least-squarestechniques based on F2 using SHELXL-97 [27]. The non-hydrogen atoms were refined with anisotropic thermalparameters in all the cases. The hydrogen atoms of thehexamethylenetetramine entity were positioned geometri-cally and allowed to ride on the parent C atoms;Uiso(H) = 1.2Ueq(C).

The data for the complex 2 were collected at 293 K on aRigaku AFC7R diffractometer using graphite monochro-matised Mo-Ka radiation (k = 0.71073 A) and a rotating

Table 1Crystal parameters of 1 and 2

Parameters 1 2

Empirical formula C16H40Co2N12O8S4 C6H16CoN10O2

Formula weight 774.74 319.22Crystal system triclinic monoclinicSpace group P�1 (No. 2) C2/c (No. 15)a (A) 7.916(16) 9.232(3)b (A) 9.024(18) 11.317(18)c (A) 12.880(3) 12.626(2)a (�) 94.20(3) 90b (�) 96.75(3) 108.19(17)c (�) 115.35(3) 90V (A3) 817.9(4) 1253.3(5)Z 1 4T (K) 293 293kMo Ka (A) 0.71073 0.71073Dc (g cm�3) 1.573 1.692l (mm�1) 1.327 1.388F(000) 402 660h (�) 2.5–22.5 2.9–27.50Total data 2253 1532Unique data 2137 1445Observed data [I > 2r(I)] 1872 1198Nref, Npar 2137, 228 1445, 101Ra 0.0388 0.0418Rw

b 0.1081 0.1354s 1.07 1.08Rint 0.023 0.035Dqmax (e� A�3) 0.49 0.76Dqmin (e� A�3) �0.86 �0.60

a R =P

(|Fo � Fc|)/P

|Fo|.b Rw = {

P[w(|Fo � Fc|)

2]/P

[w|Fo|2]}1/2.

anode generator for a prismatic purple shaped single crys-tal block (0.50 mm · 0.20 mm · 0.10 mm). Cell constantsand an orientation matrix for data collection were obtainedfrom least-squares refinement using the setting angles of 12carefully centered reflections in the range of 20� < 2h < 22�.Structure was solved by direct methods with SHELXS-97 andrefined to convergence on F2 using SHELXL-97 [27]. Lorentz-polarisation corrections were applied. Hydrogen atomsattached to the oxygen atoms were located in the Fouriermap and isotropically refined. All calculations were per-formed using SHELXS-97 and the structure was generatedusing TEXSAN crystallographic software package [27] fromMolecular Structure Corporation. Other remaining mea-surement parameters are exactly the same as in the complex1. Selected crystallographic data, experimental conditionsand some features of the structural refinements of boththe complexes are summarised in Table 1.

3. Results and discussion

Both the complexes have been synthesised under con-trolled hydrothermal condition by continuously varyingthe hmt concentration in different sets while keeping theconcentration of other added reagents constant. Surpris-ingly, it was observed that the single crystal of the complex1 was found as the sole product specifically in the concen-tration range of Co-salt:hmt:KSCN = 1:0.47:2.2, while sin-gle crystals were successfully obtained in case of complex 2

only when the relative concentrations were maintained atcobalt-salt:hmt:NaN3 = 1:0.90:2.1.

3.1. FT-IR spectra

The solid state Fourier Transform infrared spectra ofboth the complexes, recorded on a FT-IR spectrophotom-eter for the range of 400–4000 cm�1 are fully consistentwith their structures later confirmed from single crystalX-ray diffraction studies. The samples were studied as pow-der dispersed in KBr pellets. The spectral region for boththe complexes are more or less similar due to their similarco-ordination modes except for the terminal ligands whichis NCS� in the first case and N3

� in the later one. Thewhole spectrum region can be classified into three distinc-tive zones with the characteristic absorptions [28,29].

1. Low energy range. dOH: 1639 cm�1; dCH2: 1460 cm�1 and

1382 cm�1. dCN: 1225 cm�1, 1019 cm�1 and 1001 cm�1;mCS: 1255 cm�1.

2. Mid energy range. Shows characteristic absorption rangefor terminal NCS� and N3

� groups at 2110 cm�1 and2117 cm�1, respectively.

3. High energy range. mCH2: at 2920 cm�1 reflects the fre-

quency for methylene groups of hexamethylenetetra-mine molecules and mOH2

: >3400 cm�1 as a distinctivebroad peak probably corresponding to different sur-roundings of methylene groups in hmt.

J. Chakraborty et al. / Polyhedron 25 (2006) 3006–3016 3009

The peaks obtained in the different spectral regions cor-relate well with the observed values in the case of similarkind of systems found in the literature [25,26].

3.2. Electronic spectra

The UV–vis spectra of both the complexes were recordedin methanol HPLC grade solvent. They show a few strongand broad absorption bands at 8800, 12 000, 21000 cm�1,e = 6 L mol�1 cm�1, except m2 (very weak). Those absorp-tion peaks can be tentatively assigned as typical d–d transi-tions of an octahedral Co(II) ion [30–32]. The speculatedtransitions may be due to the following: 4T1g(F)! 4T2g –m1, 4T1g(F)! 4A2g – m2, 4T1g(F)! 4T1g(P) – m3. The d–dtransition energy levels for the octahedral Co(II) ion thusfound experimentally are in well accordance with thosefound in the literature. The hydrogen bonding networkstructure in both the cases imparts apparently no effect onelectronic absorption spectra.

3.3. Description of the crystal structures

3.3.1. [{Co(hmt)2(NCS)2(H2O)2}{Co(NCS)2(H2O)4}(2H2O)] (1)

The X-ray structure of 1 reveals that it containstwo independent neutral co-ordination groups:[Co(hmt)2(NCS)2(H2O)2] and [Co(NCS)2(H2O)4] in thesame asymmetric unit along with two lattice waters. Inthe first group Co(II) enjoys a perfectly octahedral environ-ment with an equatorial base formed by two short distantCo(2)–N(2) bonds [2.043(4) A] coming from NCS� termi-nal ligands and two comparatively longer Co(2)–O(3W)bonds [2.095(3) A] from co-ordinated water moleculeswhile two hmt molecules occupying the apical positionswith two Co(2)–N(3) bonds [2.325(3) A]. An ORTEP

Fig. 1. ORTEP view of 1 with

drawing of the complex with atom labeling scheme hasbeen provided in Fig. 1. Co(II) follows an ideal octahedralsituation except a slight deviation in bond lengths betweenCo–N and Co–O, forming the equatorial base. ApicalCo(2)–N(3) bonds are also a little bit longer by an amountof 0.265(3) A than the average equatorial bonds. It may bedue to the effect of slight distortion. The co-ordinationgroup possesses an inversion center. The second co-ordina-tion group contains one octahedral Co(II) center bearing aCoN2O4 chromophore where four oxygen atoms from fourco-ordinated water molecules form an equatorial basewhile two terminal NCS� groups occupy apical positions[Co(1)–O(1W) and Co(1)–O(2W): 2.065(3) A and Co(1)–N(1): 2.096(4) A, respectively]. The most interesting featureis that these independent co-ordination fragments areinvolved in extensive intra-chain hydrogen bondings[C(3)–H(3A)� � �O(3W), C(6)–H(6A)� � �N(2) and C(6)–H(6B)� � �O(3W)] among themselves or with lattice watersto shape it in an infinite 1-D polymeric chain that runsparallel to bc-plane. Such adjacent parallel 1-D chains lyingon the same plane are further locked by several inter-chain hydrogen bonds [C(4)–H(4A)� � �S(2), C(9)–H(9A)� � �O(2W)] and many others involving the co-ordinated andlattice water molecules with the terminal NCS� ligands toframe an infinite 2-D neutral polymeric sheet structure(Figs. 2 and 3) running along crystallographic a-axis. Allthe selected bond distances, bond angles and H-bondingparameters for complex 1 are listed in Tables 2 and 3,respectively.

3.3.2. [Co(hmt)(N3)2(H2O)2]n (2)

The X-ray analysis reveals that the complex 2 containsoctahedral Co(II) center co-ordinated by two terminalN3� groups, two water molecules and two neutral hmt

molecules (Fig. 4). The two nitrogen atoms N(1) and its

atom numbering scheme.

Fig. 2. 2D sheet view of 1, showing the hydrogen-bonding interactions.

Fig. 3. Packing view along a-axis for 1.

3010 J. Chakraborty et al. / Polyhedron 25 (2006) 3006–3016

symmetry related counter part N(1**) of terminal azidegroups [Co(1)�N(1) (azide): 2.044(3) A] and the two oxy-gen atoms O(1) and O(1**) of coordinated water molecules[Co(1)–O(1)(H2O): 2.099(2) A], form the equatorial planearound each Co(II) and the trans-axial sites are occupied

by the two nitrogen atoms N(4) and N(4**) from pendentbridging hmt molecules [Co(1)–N(4)(Hmt): 2.354(2) A] toprovide each Co(II) a CoN2ðhmtÞN2ðN3

�ÞO2ðH2OÞ chro-mophore (Fig. 4). The Co(II) atoms are bridged by neutralhmt molecule in a l-N,N 0 fashion with the occurrence of a

Table 2Selected bond lengths and bond angles for 1 and 2

Bond lengths (A) Bond angles (�)

1

Co(1)–O(1W) 2.065(3) Co(1)–N(1)–C(1) 177.4(3)Co(1)–O(2W) 2.102(3) N(3)–Co(2)–N(3) 180(4)Co(1)–N(1) 2.096(4) Co(2)–N(2)–C(2) 168.52(13)Co(2)–O(3W) 2.095(3)Co(2)–N(2) 2.043(4)Co(2)–N(3) 2.325(3)

2

Co(1)–O(1) 2.099(2) O(1)–Co(1)–N(1) 89.56(5)Co(1)–N(1) 2.044(3) Co(1)–N(1)–N(2) 165.6(3)Co(1)–N(4) 2.354(2) N(4)–Co(1)–N(1) 90.15(12)

O(1)–Co(1)–N(4) 88.12(9)

Table 3Hydrogen bonding parameters for 1 and 2

D–H� � �A d(D–H) (A) d(H� � �A) (A) d(D� � �A) (A)

1

O4W–H4WB� � �N4 0.80(4) 2.03(4) 2.821(5)O4W–H4WA� � �S2 0.79(5) 2.61(4) 3.341(4)O3W–H3WA� � �S2 0.79(5) 2.85(5) 3.578(4)O2W–H2WA� � �S1 0.79(5) 2.55(5) 3.300(4)O1W–H1WB� � �N6 0.79(4) 2.07(4) 2.847(5)O2W–H2WB� � �N5 0.79(3) 2.02(4) 2.800(4)C3–H3A� � �O3W 0.97 2.60 3.040(5)C4–H4A� � �S2 0.97 2.85 3.770(4)C6–H6A� � �N2 0.97 2.61 3.179(5)C6–H6B� � �O3W 0.97 2.60 3.155(5)C9–H9A� � �O2W 0.97 2.60 3.557(5)

2

O1–H1W� � �N5 0.90(5) 1.91(5) 2.782(4)O1–H2W� � �N3 0.74(5) 2.09(5) 2.772(4)C3–H3B� � �N1 0.97 2.55 3.175(5)C3–H3A� � �O3W 0.93(2) 2.448(19) 2.901(2)

J. Chakraborty et al. / Polyhedron 25 (2006) 3006–3016 3011

1-D infinite polymeric chain, further carrying into a 2-Dnet with the help of several weak inter-chain hydrogen bon-dings. ORTEP and PLATON [42] drawings of the chain

Fig. 4. ORTEP view of 2 with atom numbering scheme (symmetry code (

with atom labeling scheme are shown in Figs. 5 and 6,respectively. The degrees of distortion from the ideal octa-hedral geometry are reflected in cisoid and transoid angles,all resembling the ideal values for an octahedral arrange-ment. The shortest Co–Co distance along the same chainis 6.313(3) A. The N3

� groups show almost linearity withN–N–N angle of 179.3(4)�. The azide ligand shows a slightdeviation from linearity in connection with Co(II)[Co–N(1)–N(2): 165.6(3)�]. Each of the hmt molecule actsas a neutral organic bidentate spacer molecule joining theadjacent metal centers in l-N,N 0 fashion, while the Co(II)centers acting as a linear spacer to generate a 1-D zigzagpolymeric chain. The hmt molecule bridges two consecu-tive metal centers using only two out of its four possibledonor sites to link the adjacent units together. All the watermolecules and uncoordinated N atoms are involved inintermolecular H-bondings (Fig. 5) making hmt a good tet-rahedral template which is the ultimate requirement factorto establish an ordered supramolecular structure formed byself-assembly. Each of the infinite 1-D chains results in a 2-D polymeric network (Figs. 5 and 6) through pronouncedinter-chain O–H� � �N hydrogen bonding between the co-ordinated water molecules to the Co(II) in one chain andN atom of the hmt in the other chain. Further inter-chainO–H� � �N hydrogen bonding is found between the co-ordi-nated water molecules of the Co(II) in one chain and the Natoms from terminal azide ligands present in an adjacentchain. Apart from this, there are also three different typesof intra-chain hydrogen bonds present in the system (C–H� � �O, C–H� � �N, etc.). Another noteworthy point is thatthe water molecules co-ordinated to the alternative Co(II)centers along the same chain are not behaving identicallyin the hydrogen bonding mode to the acceptor sites presentin the adjacent chain (Fig. 5). Such motif of H-bonding canbe described in Etter’s graph set notation as R2

2ð12Þ [33]. Asa result of these two types of hydrogen bonds the systemhas a high degree of thermal stability which is reflectedthrough the thermal study. The co-ordinated water mole-cules are lost at a much higher temperature than what is

0) = x, �y, z + 1/2; (*) = 2 � x, y, 1/2 � z; (**) = 2 � x, 1 � y, 1 � z).

Fig. 5. 2D sheet view of 2, showing the hydrogen-bonding interactions.

3012 J. Chakraborty et al. / Polyhedron 25 (2006) 3006–3016

normally observed. All the selected bond distances, bondangles and H-bonding parameters for complex 2 are givenin Tables 2 and 3, respectively.

3.4. Cryo-magnetic susceptibility studies

The magnetic properties of both the complexes are moreor less same. So only a single plot of vMT versus T (vM isthe molar magnetic susceptibility for one Co(II) ion),where T varies from 2 K to 300 K, has been provided forcomplex 2 (Fig. 7). An exactly similar curve correspondingto the first complex has been provided in Fig. 10 as Supple-mentary material. The value of vMT at 300 K is2.685 cm3 K mol�1, which are larger than that expectedfor the spin-only case (vMT = 1.87 cm3 mol�1 K, S = 3/2)indicates that an important orbital contribution isinvolved. The vMT value continuously decreases fromroom temperature to 1.238 cm3 K mol�1 at 1.99 K. Theglobal feature is characteristic of weak antiferromagneticinteractions. The vM curve is less indicative at room tem-perature. It starts from 0.0089 cm3 mol�1 and increases ina uniform way to 0.619 cm3 mol�1 at 1.99 K. The absenceof maxima in these vM curves may indicate that the possi-ble antiferromagnetic coupling is very weak.

The analysis of magnetic data for cobalt complexes isusually complicated by the fact that single-ion effects, suchas spin–orbit coupling, distortion from regular stereochem-istry, electron delocalisation, crystal field mixing of excited

states into the ground state, affect the magnetic propertiesin addition to possible magnetic exchange interactions[34a]. Considering the spin–orbit coupling due to the 4T1g

ground state for octahedral Co(II) complexes [34b,34c,34d,34e,34f], exact calculations for deriving the J parameterfrom experimental data in all the temperature range isnot possible unless for dinuclear complexes [35,36]. Othersmall polynuclear systems can also be fitted throughsophisticated computer programs, based on full diagonali-sation methods at low temperature region (where the effec-tive spin S 0 = 1/2) [37]. One-dimensional systems of Co(II)are frequently associated with anisotropic Ising systems,and they can be fitted in the low temperature zone assum-ing again an effective spin S 0 = 1/2 [38]. More recently,Rueff et al. [38c,39] have proposed a phenomenologicalapproach for some low-dimensional Co(II) systems thatallows to gain an estimate of the strength of the antiferro-magnetic exchange interactions. They postulate the phe-nomenological equation:

vMT ¼ A expð�E1=kT Þ þ B expð�E2=kT Þ ð1Þ

in which, A + B equals to the Curie constant (�2.8–3.4 cm3 mol�1 K for octahedral cobalt(II) ions), and E1,E2 represent the ‘‘activation energies’’ corresponding tothe spin–orbit coupling and the antiferromagneticexchange interaction, respectively. The E1/k which is theeffect of spin–orbit coupling and site distortion is of the

Fig. 6. Packing view along a-axis for 2.

0 50 100 150 200 250 300

1.2

1.6

2.0

2.4

2.8

χ MT

/ cm

3·K

·mol

-1

T / K

Fig. 7. Plot of vMT vs. T for 2.

J. Chakraborty et al. / Polyhedron 25 (2006) 3006–3016 3013

order of +100 K [38c–40]. Eq. (1) adequately describes thespin–orbit coupling, which results in a splitting betweendiscrete levels, and the exponential low-temperature diver-

gence of the susceptibility. Very good results have been re-ported in one- and two-dimensional Co(II) complexes[38c,39].

An important experimental feature obtained in theseoctahedral cobalt(II) complexes is that the vMT (or leff)values at room temperature are greater than those expectedfor one isolated spin-only ion (vMT = 1.87 cm3 mol�1 Kfor a S = 3/2 ion), indicating that an important orbital con-tribution is involved [34]. Typical values of vMT (or leff) are2.75–3.4 cm3 mol�1 K (4.7–5.2 lb) [34,41]. Lower values atroom temperature indicate perturbation from ideal octahe-dral geometry [41]. Based on Eq. (1), commented above,which is suitable for any temperature greater than the pos-sible Tc [38c,39], an estimated J value for our complexeshas been calculated. The fit values from this procedureare: A + B = 2.87 cm3 mol�1 K, which perfectly agreeswith those given in the literature for the Curie constant(C � 2.8–3.4 cm3 mol�1 K), [38c,39] E1/k = 56.14 K is ofthe same magnitude than those reported by Rueff et al.for several one- and two-dimensional cobalt(II) complexes[39]. As for the value found for the antiferromagneticexchange interaction, it is very weak (E2/k = 0.55 K),

0 50 100 150 200 250 300

1.2

1.6

2.0

2.4

2.8

χ MT

/ cm

3·K

·mol

-1

T / K

Fig. 10. Plot of vMT vs. T for 1.

3014 J. Chakraborty et al. / Polyhedron 25 (2006) 3006–3016

corresponding to J = �1.1 K (=�0.77 cm�1), according tothe Ising chain approximation, vMT � exp(J/2kT).

These weak magnetic exchange couplings can be under-stood in both the cases considering the hexamethylenetet-ramine which is separating the Co(II) atoms with largedistances (�6.313 A) either through long distant covalentbondings or through weak hydrogen bondings, leading inthis way to almost negligible coupling.

The attempts carried out to fit these complexes, assum-ing isolated Co(II) ions with perfect octahedral geometry[34] did not give good fit. Thus, we can conclude that thesecomplexes are magnetically behaving as one-dimensionalsystem, which are weakly coupled by hexamethylenetetra-mine in both the cases.

The reduced molar magnetisation (M/Nb) per Co(II)attends to 1.88 (Fig. 8 for complex 2, Fig. 11 correspondingto the first complex has been placed as Supplementarymaterial). These values are less than that expected for anisolated Co(II) ion not coupled (2.5–3 Nb). [39] This fea-

Fig. 9. Thermal stability curve for 1.

0 10000 20000 30000 40000 50000

0.0

0.5

1.0

1.5

2.0

M /

Nβ

H / G

Fig. 8. Plot of the reduced magnetisation M/Nb vs. applied field H at 2 Kfor 2.

0 10000 20000 30000 40000 50000

0.0

0.5

1.0

1.5

2.0

M /

Nβ

H / G

Fig. 11. Plot of the reduced magnetisation M/Nb vs. applied field H at2 K for 1.

ture agrees to the weak antiferromagnetic coupling withinthe Co(II) ions reported in the literature [38c]. The shapeof the curves at low fields (sigmoid shape) is also indicativethat this antiferromagnetic coupling occurs in thesecomplexes.

Experimental values of susceptibilities and other rele-vant magnetic parameters for the complexes 1 and 2 arelisted in Tables 4 and 5.

Table 4Experimental values of the susceptibility

Parameters Complexes

1 2

vMTr.t. (cm3 K mol�1) 2.685 (at 300 K) 2.684 (at 300 K)vMTlow (cm3 K mol�1) 1.238 (at 1.99 K) 1.236 (at 1.99 K)vM r.t. (cm3 mol�1) 0.0089 (at 300 K) 0.0088 (at 300 K)vM low (cm3 mol�1) 0.619 (at 1.99 K) 0.617 (at 1.99 K)

Table 5Different magnetic constants and parameters

Parameters Complexes

1 2

A 1.26845 1.26843B 1.60351 1.60351A + B 2.87196 2.87194E1/k (K) 56.14565 56.14564E2/k (K) 0.55539 0.55540Approximately J = �E2/k * 2 * 0.965 (cm�1) �0.772 �0.772

vMT = Aexp(�E1/kT) + Bexp(�E2/kT).

J. Chakraborty et al. / Polyhedron 25 (2006) 3006–3016 3015

3.5. Thermal analyses

Thermal analysis was done only in the case of complex1. Complex 2 was avoided as it contains azide which maycause explosion under thermal treatment. The curve shownin Fig. 9 for thermal studies clearly depicts the decomposi-tion of 1 in three consecutive steps. The weight losses areapproximately 10.5% (205 �C); 15.0% (230 �C) and about32% (234–345 �C) showing through three consecutiveendothermal breaks. Weight loss at 205 �C is very closeto the calculated water content for two co-ordinated H2Omolecules. So it can be regarded as an evidence of deaqua-tion. This temperature is rather higher compared to thesimilar co-ordination polymers. It suggests the higher ther-modynamic stability of the water molecules in the poly-meric complex reported herein [43]. It may be due to thestrong and extensive inter- and intra-chain hydrogen bond-ing network. The weight loss at 230 �C may be correlatedto the decomposition of the SCN groups. The final stepleads to the formation of a stable residue.

4. Conclusion

In this paper we have reported the controlled hydrother-mal synthesis of two new hydrogen bonded metallo-organicco-ordination polymers [{Co(hmt)2(NCS)2(H2O)2}{Co-(NCS)2(H2O)4}(2H2O)] (1) and [Co(hmt)(NNN)2(H2O)2]n(2) using hexamethylenetetramine as a neutral organicspacer or terminal ligand. Thiocyanate or azide is co-ordinated as terminal coligands in either case. Both thestructures show an infinite robust 2-D network with severalinter- and intra-chain hydrogen bondings which may be aprobable cause for the stability of the system. The factsare well supported by several instrumental analyses andalso firmly established from cryo-magnetic susceptibilitystudies.

Acknowledgements

We acknowledge the support to Joy Chakraborty fromthe University Grants Commission and also DefenceResearch and Development Organisation, Ministry ofDefence, New Delhi, Government of India. M. Salah ElFallah acknowledges the financial support of the Ministe-rio de Educacion y Ciencia (Programa Ramon y Cajal).

Also the grant from Spanish Government (GrantBQU2003/00539) is acknowledged.

Appendix A. Supplementary data

X-ray crystallographic data in the ‘CIF’ format corre-sponding to the complexes reported in this paper have beendeposited with the Cambridge Crystallographic DataCenter and supplementary crystallographic data for thispaper can be obtained free of charge on request atwww.ccdc.cam.ac.uk/conts/retrieving.html [or from theCambridge Crystallographic Data Centre (CCDC), 12Union Road, Cambridge CB2 1EZ, UK; fax: +44 (0)1223 336033; email: deposit@ccdc.cam.ac.uk], quotingthe CCDC number 293386 and 293385 for complexes 1

and 2, respectively. vMT versus T and M/Nb versus Hcurves corresponding to the complex 1 are also depositedas supplementary materials. Supplementary data associ-ated with this article can be found, in the online version,at doi:10.1016/j.poly.2006.05.001.

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