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Solid State Sciences 14 (2012) 495e500

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Solid State Sciences

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Preparation, characterization and thermal behaviour of polymeric complex ofcadmium hexamethylenetetramine nitrateq

Dinesh Kumar a, I.P.S. Kapoor a, Gurdip Singh a,*, Nidhi Goel b, Udai P. Singh b

aDepartment of Chemistry, DDU Gorakhpur University, Gorakhpur, 297-Mohaddipur, Uttar Pradesh 273 009, IndiabDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee e 247 667, India

a r t i c l e i n f o

Article history:Received 11 May 2011Received in revised form12 November 2011Accepted 26 January 2012Available online 9 February 2012

Keywords:HexamethylenetetramineIgnition delayIsothermal TGKineticsX-ray CrystallographyCHNC (cadmium hexamethylenetetraminenitrate complex)

q Part 80.* Corresponding author. Tel.: þ91 551 2200745

fax: þ91 551 2340459.E-mail address: gsingh4us@yahoo.com (G. Singh).

1293-2558/$ e see front matter � 2012 Elsevier Masdoi:10.1016/j.solidstatesciences.2012.01.021

a b s t r a c t

The preparation of cadmium nitrate complex with bridged hexamethylenetetramine-[{Cd(HMTA)(-NO3)2(H2O)2}n], CHNC, of polymeric nature has been reported here for the first time. It was characterizedby X-ray crystallography, 1H NMR, FT-IR and elemental analysis. The crystal structure is stabilized byhydrogen bonding between the oxygen atom of the nitrate group with the methyl hydrogen of HMTA andhydrogen of the coordinated water molecule via CeH/O and OeH/O interactions. The thermolysis ofthis complex was investigated by TG-DSC and ignition delay measurements. The model-free iso-conversional and model-fitting kinetic approaches have been applied to isothermal TG data for kineticsinvestigation of thermal decomposition of the complex. At higher temperatures, the complex gets ignitedto produce highly thermally stable residue most closely resembles CdO proved by XRD.

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1. Introduction

Chemically and thermally stable explosive compounds areneeded for military applications. Many complexes of transitionmetals having hexamethylenetetramine (HMTA) [1e4] and NO3

� orClO4

� ions are well known for their exothermic autocatalyticthermal decomposition. Thus, they are of interest as rocketpropellants [5,6] and explosives [7,8]. In addition, these complexesare good source of ultra-fine metal oxides. Frazer and Hicks [9,10]proposed the thermal ignition model where heat of reaction istaken to be a function of temperature. The heat liberated due toexothermic reaction leads to deflagration. It is well known thatwhen a solid material deflagrates, a steep temperature gradient isproduced at the surface [11]. The surface region can be thought of asa thin film of material where heat and mass transfer are driven byphysico-chemical changes. The reaction zone in the condensedphase (which may be a solid phase) is thin, transient and non-isothermal. Reactions in the condensed phase liberate gaseousproducts for ignition. In our earlier publications [12e14], it has been

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possible to gain insight into the mechanism of pre-ignition reac-tions by using tube furnace (TF) and thermogravimetric (TG)technique where deflagration conditions are simulated.

Hexamethylenetetramine (HMTA) known as urotropine ortetraazaadamantane (taad) having four nitrogen atoms at cornersof a tetrahedron, is a ligand of polycyclicpolydentate type. Inmany complexes, it acts as a monodentate [15] or bidentate ligand[1] and shows nonchelating behavior [16] (in low valent organ-ometallic complexes). In addition, it is useful in the production ofantibacterial agents, adhesives, coatings, dye fixatives, anticor-rosive agents as well as powerful explosives, e.g., 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane(HMX), 1,3,5-trinitro-1,3,5-triazacyclohexane(RDX) and dinitropentamethylenetetramine(DPT) [17,18]. The mechanism of nitration of HMTA was alsostudied in detail [17]. Synthesis of two new 1D and 3D networksof Cu(II) and Co(II) using malonate and HMTA as bridging ligandshas also been reported by S. Konar et al. [3]. The crystal structureof a novel copper (I) cyanide complex with HMTA of themolecularformula (CuCN)3(C6H12N4)2 was reported by F.B. Stocker [19].Simultaneous determination of Pt and Rh by catalytic adsorptivestripping voltametry, using HMTA as a complexing agent wasinvestigated by A.A. Dalvi et al. [4].

There is growing interest in understanding the thermolysis ofsome transition metal nitrate and perchlorate complexes of HMTA

Fig. 1. Crystal structure of CHNC.

D. Kumar et al. / Solid State Sciences 14 (2012) 495e500496

with H-bonding network [1,2]. This paper describes for the firsttime, preparation, characterization and thermal behaviour ofenergetic polymeric complex of cadmium nitrate with bridgedHMTA (CHNC).

2. Materials and method

2.1. Materials

Cadmium nitrate was obtained from s.d.fine and HMTA fromLancaster and these were used without any further purification.

2.2. Preparation and characterization

Solid cadmium nitrate and solid HMTA into 2:1 weight ratiowere mixed together at room temperature. After mixing

Fig. 2. C-H.O and O-H.O intermo

thoroughly few drops of water were added into the solid content.During addition of water reaction between cadmium nitrate andHMTA starts which results slight increase in the temperature ofreaction container. Then more water was added with constantstirring to dissolve the contents. The solution was filtered, and thefiltrate was kept in vacuum for about 4 h to afford crystals.Recrystallization from water yielded colorless crystals. Differentdata obtained from various techniques are listed as; [1H NMR (dppm) (300MHz, DMSO) (s, d¼ 3.4, t, d¼ 4.5); FT-IR cm�1 (403, 504,1006, 1234, 1373, 1466 & 2941) and elemental analysis Calculated:C, 17.5; H, 3.8; N, 20.3. Found; C, 17.9; H, 3.9; N, 20.4%]. Caution! Thecomplex ignites when heated rapidly at higher temperatures.Therefore, due care was taken in handling the sample.

2.3. X-ray structural determination

A crystal of [{Cd(HMTA)(NO3)2(H2O)2}n] of size0.27 � 0.23 � 0.19 mm was grown at room temperature overa period of 10 h in evaporation from the mother liquor water . TheX-ray data collection was performed on Brucker Kappa Apex-CCDdiffractometer by using graphite monochromated Mo-Ka radia-tion (l ¼ 0.71073 Å) at 296 K. The structure was straightforwardlysolved by direct methods. SHELXTL software [20,21] was used forstructure solutions, refinement and data output. Hydrogen atomswere placed in geometrically calculated positions by using a rigidmodel. Image was created with the DIAMOND and MERCURYprogram [22,23]. Refinement with anisotropic thermal parametersfor non-hydrogen atoms led to Final R indices value of 0.0313. Partof the zig-zag coordination network and various hydrogen bondinginteractions between the adjacent layers are depicted in Figs. 1 and2 respectively. The crystallographic data, structure refinementparameters, bond length, and bond angles are given, respectively, inTables 1 and 2 [CCDC number 765844]. The selected hydrogenbonding parameters are summarized in Table 3.

NMR spectroscopy studies conducted with high resolution NMRspectrometer (Bruker, 300 MHz); FTIR on Nicolet Impact I-410spectrophotometer and elemental analysis on Analytic Gena.

lecular interactions in CHNC.

Table 3Selected hydrogen bond parameters (Å and o) for CHNC.

D-H$$$A d(D-H) d(H-A) d(D-A) <(DHA)>

[{Cd(HMTA)(NO3)2(H2O)2}n] (1)

O2-H2B$$$O11#1 0.789 2.539(33) 3.252 151.1C12-H12B$$$O3#2 0.970 2.607(4) 3.288 127.4

Symmetry transformations were used to generate equivalent atoms: #1 2 � x, �y,1�z #2 �1/2 þ x, �1/2 þ y, z.

Table 1Crystal parameters and refinement parameters for cadmium hexamethylenetetra-amine nitrate complex (CHNC).

Empirical formula C6 H16 Cd N6 O8

Color ColorlessFormula weight 412.66Temp/K 296(2) Kl/Å 0.71073 ÅCrystal system MonoclinicSpace group C2/cUnit cell dimensions a ¼ 8.8386(2), b ¼ 12.1007(3), c ¼ 12.9485(3) Å

a ¼ 90.00, b ¼ 109.1910(10), g ¼ 90.00Volume 1307.92(5) Å3

Molecules per unit cell, Z 4Calculated density 2.096 Mg m�3

Absorption coefficient 1.724 mm�1

Crystal size 0.27 � 0.23 � 0.19 mmRefinement method Full-matrix least-squares on F2

q range for data collection 2.96e40.62�

Diffractometer used Bruker Kappa ApexGoodness-off-fit on F2 1.142Final R indices [I > 2s(I)] 0.0313Absorption coefficient 1.724 mm�1

Radiation Mo KaCCDC No. 765844

D. Kumar et al. / Solid State Sciences 14 (2012) 495e500 497

2.4. Thermal analysis

Thermogravimetry and differential scanning calorimetry (TG-DSC) were performed on Perkin Elmer (Pyris Diamond) instrumentanalyzer. 2.8 mg of sample was used and heating rate was 10 �C/min. In addition, an N2 flow rate of 200ml/ minwas present as inertatmosphere. Traces are shown in Fig. 3. To verify the residue ob-tained from TG-DSC experiment, X-ray diffraction (XRD)measurement was performed on XPERTPro PANalytical instrumentbetween the 2q ranges of 10e90� in the interval of 1�/min (Fig. 4).Isothermal TG experiments were performed at appropriatetemperatures in static air (mass w20 mg) with an indigenouslyfabricated TG apparatus [24]. Gold crucible was used as a sampleholder in all isothermal TG experiments and traces are shown inFig. 5.

2.5. Kinetic analysis of isothermal TG data

Kinetic analysis of thermal decomposition of a solid is usuallybased on a single-step kinetic equation (6) [25]

da=dt ¼ kðTÞf ðaÞ (1)

where t is the time, T is the temperature, a is the extent ofconversion (0 < a < 1), k(T) is the rate constant, and f(a) is the

Table 2Bond lengths and bond angles for CHNC.

Bond lengths Bond angles

Cd1-O2 2.283(13)Cd1-O3 2.312(3)Cd1-N5 2.4051(12)Cd1dO20 2.283(1)Cd1dO30 2.312(4)Cd1dN50 2.405(2)

O2dCd1dO20 180.00(5)O2dCd1dO30 78.44(6)O20dCd1dO30 101.56(6)O2dCd1dO3 101.56(6)O20dCd1dO3 78.44(6)O2dCd1dN5 90.94(5)O30dCd1dN5 91.40(6)N50dCd1dN5 179.99(4)O30dCd1dO3 180.00(11)O2dCd1dN50 89.06(5)O20dCd1dN50 90.94(5)O30dCd1dN50 88.60(6)O3dCd1dN50 91.40(6)O20dCd1dN5 89.06(5)O3dCd1dN5 88.60(6)

reaction model [25], which describes the dependence of the reac-tion rate on the extent of reactions. The value of a is experimentallyderived from the global mass loss in TG experiments. The reactionmodel may take various forms. The temperature dependence ofk(T) can be satisfactorily described by the Arrhenius eq., which aftersubstitution into equation (1) yields

da=dt ¼ A exp ð�E=RTÞ$f ðaÞ (2)

where, A is pre-exponential factor, E activation energy and R the gasconstant.

2.6. Model fitting method

Rearrangement and integration of equation (1) for isothermalconditions gives

gjðaÞ ¼ kjðTÞt (3)

where g(a) ¼ 0!a[f(a)] � 1 da is the integrated form of the reactionmodel . The subscript j has been introduced to emphasize thatsubstituting a particular reaction model in equation (3) results inevaluating the corresponding rate constant, which is determinedfrom the slope of a plot of gj(a) verses t. For each reaction modelselected, the rate constants are evaluated at several temperatures Tiand Arrhenius parameters are determined using the Arrheniusequation (4) in its logarithmic form

ln kj�Tj� ¼ ln Aj � Ej=RTi (4)

Arrhenius parameters were evaluated from isothermal experi-mental data by the model fitting method.

2.7. Isoconversional method

This method allows the activation energy to be evaluatedwithout making any assumptions about the reaction model. Addi-tionally, the method evaluates the effective activation energy asa function of the extent of conversion (Fig. 6) which allows one toexplore multistep kinetics.

Fig. 3. TG-DSC thermograms of CHNC in nitrogen atmosphere.

Fig. 6. A plot of ln Di vs. 1/T for CHNC.

Fig. 4. XRD pattern of Cadmium oxide (CdO).

D. Kumar et al. / Solid State Sciences 14 (2012) 495e500498

The basic assumption of the isoconversional method [26] is thatthe reaction model as defined in equation (1) is not dependent ontemperature or heating rate. Under isothermal conditions, aftercombining equations (3) and (4) we get,

�ln ta;i ¼ ln ½Aa=gðaÞ� � Ea=RTi (5)

where Ea is evaluated from the slope of the plot of �ln ta,i againstTi�1.The ignition delay (Di) measurements were made on 20 mg

samples by using a tube furnace technique [13] in the temperaturerange 310e390 �C. The accuracy of the temperature of tube furnacewas �1 �C. Each run was repeated five times and mean ignitiondelay times (Di) data, reported in Table 4, were fitted in the equa-tion [27e29].

Di ¼ A expE*=RT (6)

where E* is the activation energy for ignition and T is the absolutetemperature. A plot of ln Di vs. 1/T is apparent in Fig. 7.

3. Results and discussion

The structure analysis of the complex showed the monoclinicspace group of C2/c with (Z ¼ 4) (Table 1). In the crystal structurethe cadmium atom is hexacoordinated, with the coordinationpolyhedron possessing a distorted octahedral geometry (Fig. 1).Octahedral geometry is achieved by coordination of four oxygenatoms (two of nitrate ions and two of water molecule) and twonitrogen atoms of two HMTA to the cadmium metal. Two nitrogenatoms (N5) of one HMTA are coordinated with two cadmium

Fig. 5. Isothermal TG of CHNC in static air.

metals and forms a bridge in such a way that leads to a zig-zagchain like polymer with formulation of [{Cd(HMTA)(NO3)2(-H2O)2}n]. A perspective drawing is depicted in Fig. 2. The twocoordinated water ligands are trans with respect to the O2N2 plane.The crystals are held together by hydrogen bonds between nitrateions, eCH2 group and solvent water molecules. The water mole-cules also form a hydrogen bond to oxygen atom of the nitrateanion. In addition there exist interactions between eCH2 of HMTAand oxygen atom of nitrate anion. The CdeN bond distance is2.405 Å (Table 2). The CdeO bond distances fall in the range of2.283(13) to 2.312(4) Å, where Cd1eO2 bond distance is smallerthan Cd1-O3. Table 2 also points out that this complex possesseshydrogen bonding and some selected hydrogen bond parametersare listed in Table 3. 1H NMR and IR spectral data [30] have alsobeen given in Supplementary data. The observed percentages of C,H and N were 17.9, 3.9 and N, 20.4% which is consistent with theempirical formula of the complex as C6H16CdN6O8.

TG-DSC curves (Fig. 3) confirm beyond doubt that the complexdecomposes in two steps. Step I in the 170e300 �C temperaturerange corresponds to mass loss of 42.6% which is due to removal oftwo water molecules and HMTA moiety to yield cadmium nitrate.In the second step w26.1% mass loss is attributed to cadmiumnitrate decomposition in the range of 400e600 �C to yield CdO [31].XRD pattern presented in Fig. 4, having (111) maximum intensitypeak at 2q equal to 32.9 undoubtly suggest that the residue is CdO.This XRD pattern of CdOmatches with the JCPDS Card No. 78-0653.

The DSC traces (Fig. 3) also verifies the results obtained from TG;first endotherm is due to removal of water molecules and secondexothermic peak is due to exothermic decomposition of thecomplex leading to ignition to yield finally CdO as a residue (Fig. 4).Based on thermoanalytical observations and XRD pattern ofresidue, it is inferred that during the thermal decomposition ofCHNC, oxidationereduction reactions between fuel (HMTA) andoxidizer (NO3

�) are taking place and cadmium oxide is left asa residue after thermolysis.

To calculate the Ea values for thermal decomposition of CHNC,a set of reaction models [25,32] were used to analyze isothermal TGdata in the temperature range of 260e300 �C (Fig. 5). In the modelfitting method, the kinetics is analyzed by choosing a ‘best fit’model based on the values of correlation coefficient ‘r’ close to 1.

Table 4Ignition delay, activation energy for ignition delay (E*) and correlation coefficient (r)for CHNC.

Di/s at temperature/�C E*/kcal mol�1 r

310 � 1 330 � 1 350 � 1 370 � 1 390 � 1

131 117 108 90 82 7.6 � 1 0.9915

Fig. 7. A plot of Ea vs a for CHNC.

D. Kumar et al. / Solid State Sciences 14 (2012) 495e500 499

Among the various values of ‘r’, calculated using different models,the highest value of ‘r’ corresponds to model 5, Parabolic law (ratecontrolling process being as one-dimensional diffusion) withr ¼ 0.9964. The corresponding value of Ea is 6.6 � 1 kcal mol�1.

Isoconversional kinetic analysis [26,33e35] has also beencarried out which concerns with the estimation of the apparentactivation energy independent of the model corresponding to theextent of conversion of the sample. Additionally the method eval-uates the effective activation energy as a function of conversionwhich allows one to explore multistep kinetics. According to Fig. 6,each activation energy has a separate value at different a’s for thiscomplex. Kinetic analysis performed by the isoconversionalmethod on thermogravimetric data has shown (Fig. 6) that thermaldecomposition of CHNC complex has an initial overall activationenergy of 115 kcal mol�1. This value decreases with the extent ofconversion to about 72 kcal mol�1 at the end of this reaction.Though model fitting method using a set of reaction model appliedto isothermal data but model-free approach (isoconversionalmethod) is a better method of obtaining reliable and consistentkinetic information.

It turns out that activation energies, calculated under differenttemperature regimes, for isothermal kinetics and explosion delaymeasurements are different. The thermolysis of an energetic co-ordination complex often involves a concert of bond-breakingand bond-forming steps under condensed or gas-phase reactions,with destruction of initial crystal lattice; formation of crystal latticeof the solid products (metal oxides), absorption/desorption ofgaseous products, diffusion of gaseous products and heat transfer.Solid-phase interactions to chain e like or even branching chain e

like processes having strongly exothermic reactions cause anignition.

Although this complex is stable at room temperature, butignited when subjected to sudden high temperature. Activationenergy for ignitionwas found to be 7.6� 1 kcal mol�1 (Table 4). Theignition delay depends exponentially on temperature. The processof ignition [36] can never be treated as steady-state since it isa transient process prior to sustained combustion. Freeman andGordon [28] have suggested the following heat balance equation inorder to evaluate the pre-ignition reactions.

dQH=dt ¼ dH=dt � dq=dt (7)

where dQH/dt is the net rate of heat grain in the system. dH/dt is therate of heat produced by pre-ignition reactions and dq/dt is the rateof heat dissipation.

Ignition will occur when

QH ¼ H0 (8)

where H0 ¼ minimum amount of heat required to raise thetemperature of the system to the point of ignition. From equation(7), it turns out that

QH ¼ H � q (9)

where H ¼ total heat produced by pre-ignition reactions. Theignition would occur only if H �q � H0.

The total heat produced by the pre-ignition reactions must begreater than H0 by the amount of heat dissipated. Thus

H ¼ H0 þ q (10)

The following equationwas derived by Freeman andGordon [28].

tid ¼ AeDH*=RT (11)

where DH* is the heat of activation and is approximately equal toactivation energy (Ea). If the activities of the reactants do notchange significantly during pre-ignition reactions, the log of thetime of ignition (Di or tid) should be a linear function of the recip-rocal of the absolute temperature and the relation comes out to beas given in equation (6) (Fig. 7).

4. Conclusions

The crystal structure analysis of the complex revealed that thecomplex is zig-zag polymeric network in which the Cd(II) ions arelinked via nitrogen atom of the HMTA ligand. TG-DSC studiesshowed two-step decomposition of the CHNC. Theoxidationereduction reaction between oxidizer (NO3

�) and fuel(HMTA) leads to ignition yielding CdO as an end product. The oxideresidue was confirmed by the XRD pattern. The complex hasa network of hydrogen bonds. For isothermal TG data, use of theisoconversional method is an effective means of unmaskingcomplex kinetics.

Acknowledgements

Thanks are due to Head, Chemistry Department DDU GorakhpurUniversity, Gorakhpur for lab facilities and CSIR for EmeritusScientist to Dr. Gurdip Singh & SRF to Dinesh Kumar.

The authors are also thankful to Alok M. Tripathi and BharatChaubey for helping in IR and NMR spectra. Chairman, Departmentof materials Engineering is also acknowledged for providing XRDfacility.

Appendix. Supplementary material

Supplementary material associated with this article can befound, in the online version, at doi:10.1016/j.solidstatesciences.2012.01.021.

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