The metal alkoxides M(OR)x, (M = a metal
valency, x; R = alkyl or aryl
group) are excellent precursors for the deposition of metal oxides, and current
interest in using metal oxides in optoelectronics, high-T, superconductors, and
ceramics has led to a resurgence of interest in the chemistry of these
compounds. Over
the years, an ever-increasing research interest in chemistry of niobium has
aroused not only from the striking structural novelties and complexities
exhibited by niobium compounds but also from their unique applications as
catalysts [1-9] and in material science [10-16]. Most of the earlier work
reported the synthesis of mono-, di- or trivalent metal niobates viz. lead
magnesium niobate (PMN) and niobium doped lead zirconate titanate (PNZT) as
ferro- or piezoelectric resonator materials. The rare earth niobates and
tantalates are useful class of materials as photocatalysts, host lattices for
phosphors and ion conductors [17]. The synthesis of lithium niobate has drawn much
attention in recent years owing to its excellent piezoelectric, pyroelectric,
electro-optic, photo-refractive properties.
The niobium(V) catalysts bearing di- and
tri-phenolate ligands have been demonstrated to polymerize ethylene in the
presence of an organoaluminium cocatalyst [18]. Owing to the utility of niobium
oxides and mixed oxides as effective catalysts in many processes, the synthesis
of metal-organic precursors affording these oxides has assumed remarkable
importance. In this context, phenomenal interest in niobium chemistry has centred
on complexes derived from alkoxo and aryloxo ligands which can be easily
modified in steric bulk by substitution. Furthermore, niobium alkoxides,
chloroalkoxides, heterometallic alkoxides and aryloxides have been extensively
studied as precursors for the preparation of mixed metal oxides with potential
utility as electro-optic materials [19-22]. In view of our continuing interest
on niobium aryloxides [23-25], we report herein thermal studies of
niobium(V)isopropylphenoxides with an objective to study their thermal behaviour,
formation of intermediate if any and ultimate thermolytic product. There is
also the description of kinetic parameters evaluated from TG data using
Coats-Redfern equation [26-27].
EXPERIMENTAL:
Synthesis
of Niobium(V)4-isopropylphenoxides [NbCl5-n(OC6H4CH(CH3)2-2)n](n
= 1→5)
A suspension of niobium pentachloride (2.0
g, 0.007 mol) in CCl4 (20 mL) was treated with a solution of one,
two, three, four and five equivalents of 2-isopropylphenol (1.0 g, 0.007 mol; 1.90
g, 0.014 mol; 2.86 g, 0.021 mol; 3.81 g, 0.028 mol and 4.76 g, 0.035 mol) in
dry carbon tetrachloride (15 mL) in separate experiments. The mixing of
reactants resulted in an immediate colour change from yellow to dark orange
with the evolution of HCl gas. The reaction contents were initially stirred for
3-4 h and were then refluxed till the evolution of hydrogen chloride gas ceased
which ensured the completion of the reaction. No separation of any solid was
observed during the course of the reaction in the mixture solution. It was
filtered and the filtrate was then concentrated by distilling off the solvent.
The resultant concentrated solution was treated with petroleum ether whereupon
the formation of solids was observed. It was dried under vacuum.
Thermograms of complexes were recorded on
simultaneous TG-DTA SHIMADZU DT-60 thermal analyzer in air at a heating rate of
20oC/min using platinum crucible. Thermocouple used was
Pt/Pt-Rh (10%). The sample size for TG-DTA analysis was 5-7 mg. The temperature
range of the instrument was from room temperature to 1300 oC. IR
spectra were recorded as (KBr pellets) on Nicolet-5700 FTIR spectrometer. The
kinetic and thermodynamic parameters viz. energy of activation, frequency
factor, entropy of activation etc. have been computed using Coats-Redfern
equation from mathematical analysis of TG data.
RESULTS
AND DISCUSSION:
Complexes of composition [NbCl5-n(OC6H4CH(CH3)2-2)n]
(where n = 1→5) have been synthesized according to the scheme 1
The complexes are yellow to dark brown
solids and have been thoroughly characterized by physicochemical, IR, 1H
and 13C NMR, UV-vis and mass spectral studies [28].
Thermal studies:
The thermal decomposition behaviour
of complexes, [NbCl4OC6H4CH(CH3)2-2)]
(I),
[NbCl3(OC6H4CH(CH3)2-2)2]
(II), [NbCl2(OC6H4CH(CH3)2-2)3]
(III), [NbCl(OC6H4CH(CH3)2-2)4]
(IV) and [Nb(OC6H4CH(CH3)2-2)5]
(V) has been studied by thermogravimetric and differential thermal analysis
techniques in air. The TG-DTA curves of niobium(V)2-isopropylphenoxides are
illustrated in Figs 1-4 and thermal data is presented in Table-1.
Table
1 Thermal data of niobium(V)2-isopropylphenoxides
Complex
|
Sample mass/
mg
|
IDT/oC
|
TGA
|
DTA
|
Stages of decomp.
|
Decomp. Range/oC
|
Wt. Loss/%
|
Decomp.
product
|
Peak temp./oC
|
Peak
nature
|
NbCl4
(OC6H4CH(CH3)2-2)
(I)
|
5.25
|
39.20
|
Single
|
39.2-516.5
|
42.33
|
NbOCl3
|
474.02ºC
89.98oC
|
Exothermic
Endothermic
|
NbCl3
(OC6H4CH(CH3)2-2)2
(II)
|
6.36
|
55.00
|
Single
|
55-600.03
|
37.31
|
NbO(OC6H5)3
|
---
|
---
|
NbCl2
(OC6H4CH(CH3)2-2)3
(III)
|
6.40
|
56.63
|
Single
|
56.63-600.03
|
48.66
|
NbO2(OC6H5)
|
---
|
---
|
NbCl
(OC6H4CH(CH3)2-2)4
(IV)
|
6.77
|
53.86
|
Two
|
53.86-500.66
500.66-600.66
|
60.10
18.99
|
NbO2
(OC6H5CH(CH3)2
Nb2O5
|
475.00ºC
550.00oC
|
Exothermic
|
NbCl5-n(OC6H4CH(CH3)2-2)n]
(n = 1→4)
The TG curve of [NbCl4(OC6H4CH(CH3)2-2)]
(Fig. 1) has shown it to be thermally stable upto 39.20ºC after which temperature,
the complex seems to decompose in a continuous manner in single step as is
indicated by the non-observance of any plateau in TG curve thereby excluding
the possibility of formation of any intermediate. The observed total weight loss
of 42.33% has accounted for the formation of NbOCl3 as the ultimate
product of decomposition according to the equation:
The
DTA curve of complex [NbCl4(OC6H4CH(CH3)2-2)]
has shown both endothermic and exothermic peaks at 89.98oC and 474.02oC
respectively.
Fig 1. TG-DTA curve of NbCl4(OC6H4CH(CH3)2-2)
The TG curves of [NbCl3(OC6H4CH(CH3)2-2)2]
and [NbCl2(OC6H4CH(CH3)2-2)3]
in Figs. 2 and 3 respectively have shown their initial temperature of
decomposition as 55.0ºC and 56.63ºC respectively. The single step decomposition
pattern and the weight loss of 37.31% and 48.66% in respective complexes
accounted for the formation of NbO(OC6H5)3 and
NbO2(OC6H5) as the respective final products
of decomposition. The following scheme of decomposition for these complexes may
be written as:
Fig 2. TG-DTA curve of NbCl3(OC6H4CH(CH3)2-2)2
Fig 3. TG-DTA curve of NbCl2(OC6H4CH(CH3)2-2)3
Strikingly, the DTA curve of [NbCl3(OC6H4CH(CH3)2-2)2]
and [NbCl2(OC6H4CH(CH3)2-2)3]
did not display any physical change corresponding to thermal
decomposition in TGA.
The TG curve of [NbCl(OC6H4CH(CH3)2-2)4]
in Fig. 4 has indicated it to be thermally stable upto 53.86ºC after which
temperature, it has been observed to decompose in two steps. The weight loss of
61.10% in first step of decomposition corresponds to the formation of NbO2(OC6H5CH(CH3)2-2)
as the probable intermediate. The weight loss of 18.99% in second step has been
attributed to the oxidative decomposition of the intermediate to yield Nb2O5
as the final product of decomposition in accordance with the equation:
Fig 4. TG-DTA curve of NbCl(OC6H4CH(CH3)2-2)4
The DTA curve of [NbCl(OC6H4CH(CH3)2-2)4]
exhibited two exothermic peaks at 475.0ºC and 550.0oC.
Kinetic
Parameters:
The evaluation of kinetic parameters such
as rate constant, apparent activation energies, reaction order and
pre-exponential factors has been one of the widespread quantitative
applications of thermogravimetry. Compared to many reported methods for the
evaluation of kinetic parameters from TG data, Coats-Redfern equation has been
described to be less tedious, giving satisfactory kinetic analysis of
thermogravimetric data. Hence, the mathematical analysis of TG data was computed
using Coats-Redfern equation [29,30] as:
where Wα
= mass loss at the completion of the reaction, W = mass loss at time t, Φ
= linear rate of heating.
The plots of log{ln[Wα/(Wα-W)/T2]} vs 1/T were drawn and straight lines were obtained. The activation
energy was calculated from the slope –E*/2.303R.
The kinetic parameters viz. entropy (S*)
[31], free energy (G*) [32] and
enthalpy of activation (H*) have been
calculated from the relations:
where
‘k’ is Boltzmann’s constant, ‘h’ is Planck’s constant, ‘Z’ is frequency factor, ‘Ts’ is the peak temperature computed
from DTG and kr is specific reaction rate constant calculated from
the relation
From
the perusal of data (Table-2), the negative values of ‘S*’ indicate that the activated complex has a more ordered
structure than the reactants [33]. The low values of ‘Z’ suggest that the reactions are slower than normal [34,35]. The
non-observance of any correlation between the kinetic parameters and order of
thermal stability may be ascribed to the fact that decisive criteria in
kinetics are different from those determining thermal stability.
Table 2
Kinetic parameters of Niobium(V)2-isopropylphenoxides
Complex
|
Activation
Energy
E*/kJmol-1
|
Frequency
Factor
Z/s-1
|
Entropy
ΔS*/kJmol-1K-1
|
Free
Energy
G*/kJmol-1K-1
|
Enthalpy
H*/kJmol-1
|
NbCl4
(OC6H4CH(CH3)2-2)
(I)
|
20.43
|
2.93×10-6
|
-357.03
|
24.46×104
|
20.43
|
NbCl3
(OC6H4CH(CH3)2-2)2
(II)
|
5.93
|
1.31×10-4
|
--
|
--
|
5.93
|
NbCl2(
OC6H4CH(CH3)2-2)3
(III)
|
6.35
|
1.07×10-4
|
--
|
--
|
6.35
|
NbCl
(OC6H4CH(CH3)2-2)4
(IV)
|
6.77
|
8.95×10-5
|
--
|
--
|
6.77
|
|
30.99
|
--
|
-691.31
|
74.41×104
|
30.99
|
CONCLUSION:
The TG-DTA studies of newly synthesized
niobium(V)2-isopropylphenoxides have demonstrated these to undergo decomposition
in single step. The thermogravimetric analysis of [NbCl4(OC6H4CH(CH3)2-2)]
(I) gave NbOCl3 as the ultimate product of decomposition authenticated
by its colour and IR spectra while thermal study of complexes of composition [NbCl3(OC6H4CH(CH3)2-2)2]
(II) and [NbCl2(OC6H4CH(CH3)2-2)3]
(III complexes yielded NbO(OC6H5)3 and NbO2(OC6H5)
as the respective final product of thermal decomposition and [NbCl(OC6H4CH(CH3)2-2)4]
has been observed to decompose in two steps. In first step of decomposition
corresponds to the formation of NbO2(OC6H5CH(CH3)2-2)
as the probable intermediate which has been attributed to the oxidative
decomposition of the intermediate to yield Nb2O5. [NbCl(OC6H4CH(CH3)2-2)4]
(IV) yielded Nb2O5 as the final decompositional
product suggesting these to be potential precursors of niobium pentoxide. The
complex of composition [Nb(OC6H4CH(CH3)2-2)5]
has been observed to be least thermally stable. The kinetic and thermodynamic
parameters have been computed from TG data using Coats-Redfern equation.
REFERENCES:
1. Ziolek M. Niobium-containing
catalysts−the state of the art. Catalysis Today. 2003; 78:47-64.
2. Tanabe K. Catalytic application of
niobium compounds. Catalysis Today. 2003; 78:65-77.
3. Tanabe K, Okazaki S. Various reactions
catalysed by niobium compounds and materials. Applied Catalysis A: General.
1995; 133:191-218.
4. Wachs IE, Jehng JM, Deo G, Hu H, Arora
N. Redox properties of niobium oxide catalysts. Catalysis Today. 1996;
28:199-205.
5. Andrade KZ, Carlos, Rocha, Rafael.
Recent Applications of Niobium Catalysts in Organic Synthesis. Reviews in
Organic Chemistry. 2006; 3:271-80.
6. Nowak I, Ziolek M. Niobium compounds:
preparation, characterization and application in heterogeneous catalysis.
Chemical Reviews. 1999; 99:3603-24.
7. Tanabe K. Application of niobium oxides
as catalysts. Catalysis Today. 1990; 8:1-11.
8. Ichikuni N, Shirai M and Iwasawa.
Surface structures and catalytic properties of supported niobium oxides.
Catalysis Today. 1996; 28:49-58.
9. Mal NK, Bhaumik A, Kumar P, Fujiwara M,
Matsukata M. Novel organic-inorganic hybrid and organic-free meso-porous
niobium oxophosphate synthesized in the presence of an anionic surfactant.
Microporous and Mesoporous Materials. 2006; 93:40-45.
10. Nowak I, Jaroniec M. Three-Dimentional
Cubic Mesoporous Molecular Sieves of FDU-1 containing Niobium: Dependence of
Niobium source on Structural Properties. Langmuir. 2005; 21:755-60.
11. McKarns PJ, Heeg MJ, Winter CH.
Synthesis, Structure, Hydrolysis and Film Deposition Studies of Complexes of
the Formula [NbCl4(S2R2)2][NbCl6]. Inorg Chem. 1998; 37:4743-47.
12. Filipek E, Piz M. The reactivity of SbVO5
with T-Nb2O5 in solid state in air. J Therm Anal Calorim. 2010; 101:447-53.
13. Tabero P. The formation and properties of
new Al8V10W16O85 and Fe8-xAlxV10W16O85 phases with the M-Nb2O5 structure. J
Therm Anal Calorim. 2010; 101:561-66.
14. Czeppe T. Mechanism and kinetics of
nano-crystallization of the thermally stable NiNb(ZrTi)Al metallic glasses. J
Therm Anal Calorim. 2010; 101:615-22.
15. Mansurova AN, Gulyaeva RI, Chumarev VM,
Marevich VP. Thermochemical properties of MnNb2O6. J Therm Anal Calorim. 2010;
101:45-47.
16. Ivanov MG, Shmakov AN, Drebushchak VA,
Podyacheva OY. Two mechanisms of thermal expansion in perovskite
SrCo0.6Fe0.2Nb0.2O3-z. J Therm Anal Calorim. 2010; 100:79-82.
17. Nyman M, Rodriguez MA, Alam TM, Anderson
TM, Ambrosini A. Aqueous Synthesis and Structural Comparison of Rare Earth
Niobates and Tantalates: (La, K)2Nb2O7-x(OH)2 and Ln2Ta2O7(OH)2 (Ln = La-Sm).
Chem Mater. 2009; 21:2201-08.
18. Redshaw C, Homden DM, Rowan MA, Elsegood
MRJ. Niobium-based ethylene polymerization procatalysts bearing di- and
triphenolate ligands. Inorganica Chimica Acta. 2005; 358:4067-74.
19. Turevskaya EP, Turova NY, Korolev AV,
Yanovsky AI, Struchkov YT. Bimetallic alkoxides of niobium. Polyhedron. 1995;
14:1531-42.
20. Boulmaaz S, Papiernik R, Hubert-Pfalzgraf
LG, Septe B, Vaissermann J. Chemical routes to oxides: alkoxide vs.
alkoxide-acetate routes: synthesis, characterization, reactivity and
polycondensation of MNb2(OAc)2(OPri)10 (M = Mg, Cd, Pb) species. J Mater Chem.
1997; 7:2053-61.
21. Sobota P, Utko J, Szafert S. Synthesis
and Molecular Structures of the Magnesium and Aluminium Adducts of a
Niobium-Oxo Complex. X-ray Crystal Structures of [{NbOCl4(THF)}2Mg(THF)4] and
of [{NbOCl4(THF)}2AlCl(THF)3]. Inorg Chem. 1997; 36:2227-29.
22. Goel SC, Hollingsworth JA, Beatty AM,
Robinson KD, Buhro WE. Preparation of volatile molecular lithium-niobium
alkoxides. Crystal structures of [Nb(µ-OCH2SiMe3)(OCH2SiMe3)4]2 and
[LiNb(µ3-OCH2SiMe3)-(µ2-OCH2SiMe3)2(OCH2SiMe3)]2. Polyhedron. 1998; 17:781-90.
23. Sharma N, Sharma M, Kumari M, Chaudhry
SC. Synthesis, Characterization and Thermal Studies of Niobium(V) Complexes of
2-tert-Butylphenol. Polish J Chem. 2009; 83:1265-76.
24. Sharma N, Sharma M, Kumari M, Chaudhry
SC. Synthesis, Characterization and Reactivity of
Niobium(V)-2-tert-butylphenoxides. Polish J Chem. 2009; 83:1565-73.
25. Sharma N, Sharma M, Bhatt SS, Chaudhry
SC. Synthesis, characterization and acceptor behaviour of
dichlorotris(2-t-butylphenoxo)niobium(V). J Coord Chem. 2010; 63:680-87.
26. Sharma N, Pathania A, Sharma M.
Thermoanalytical investigations of niobium(V) complexes of 4-isopropylphenol. J
Thermal Analysis and Calorimetry. 2012; 107:149
27. Sharma M, Sharma M, Sharma N.
Physicochemical, quantum mechanical and thermoanalytical investigations of
newly synthesized pentakis(2,4-dimethylphenoxo) niobium (V) as potential
precursor of Nb2O5. Arabian Journal of Chemistry. 2019; 12 5268-5277.
28. Sharma M. Synthesis, spectroscopic
studies and reactivity of monochlorotetrakis(2-/4- isopropylphenoxo)niobium(V)
complexes. International Journal of Current Advanced Research.2018;
7:11777-11783.
29. Coats AW, Redfern JP. Kinetic parameters
from thermogravimetric data. Nature. 1964; 201:68-9.
30. Coats AW, Redfern JP. Kinetic parameters
from thermogravimetric data. II. J Polym Sci Polymer Lett. 1965; 3:917-20.
31. Zsako J, Varhelyl Cs, Kekedy E. Kinetics
and mechanism of substitution reactions of complexes—III : Thermal
decomposition of complexes of the type [Co(DH)2Am2]X. J Inorg Nucl Chem. 1966;
28:2637-46.
32. Khadikar PV, Ali SM, Heda B. Kinetics of
thermal dehydration of some bis-(4-aminosalicylato)-diaquo complexes of
transition metal ions. Thermochim Acta. 1984; 82:253-61.
33. Frost AA and Pearson RG. Kinetic and
mechanism. Wiley: New York; 1961.
34. Sawhney SS, Bansal AK. Kinetics of the
non-isothermal decomposition of some metal derivatives of 8-quinolinol and its
dihalo derivatives from DTG/DTA curves. Thermochim Acta. 1983; 66:347-50.
35. Aravindakshan KK, Muraleedharan K.
Thermal decomposition kinetics of polymeric complexes of nickel(II), zinc(II)
and cadmium(II) with N,N '-bis(dithiocarboxy)piperazine. Thermochim Acta. 1989;
140:325-35.