Synthesis and characterization of para-nitrophenoxides of zirconium(IV)

Article abstract of DOI:10.1081/SIM-120037525

Compounds of the composition ZrCl₃(OC₆H ₄NO₂-p) and ZrCl₂(OC₆H₄ NO₂-p)₂ have been synthesized from the reaction of ZrCl₄ with equimolar and bimolar amounts of the trimethylsilyl derivative of p-nitrophenol in benzene under reflux. Attempts to isolate the tetrakis derivative, Zr(OC₆H₄NO₂-p) ₄, even by heating the components in high-boiling solvents were unsuccessful. Based upon analytical data, conductance, cryoscopic, IR and ¹H NMR studies, dimeric structures with bridging through phenoxo groups have been proposed for these compounds. The thermal decomposition of these complexes is a combination of both exothermic and endothermic processes, as is inferred from TG/DT studies. Reactions of these compounds with nitrogen bases viz., pyridine, 1,10-phenanthroline and 2,2′-bipyridyl results in breakdown of the bridged structure and formation of simple addition compounds, which have been assigned octahedral structures.

Full text of DOI:10.1081/SIM-120037525

SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 34, No. 6, pp. 1031–1040, 2004
Synthesis and Characterization of
para-Nitrophenoxides of Zirconium(IV)
*
S. C. Chaudhry, Champa Verma, S. S. Bhatt,
and Neeraj Sharma
Department of Chemistry, Himachal Pradesh University, Shimla, India
ABSTRACT
Compounds of the composition ZrCl3(OC6H4NO2-p) and ZrCl2(OC6H4
NO2-p)2 have been synthesized from the reaction of ZrCl4 with equimolar
and bimolar amounts of the trimethylsilyl derivative of p-nitrophenol
in benzene under reflux. Attempts to isolate the tetrakis derivative,
Zr(OC6H4NO2-p)4, even by heating the components in high-boiling
solvents were unsuccessful. Based upon analytical data, conductance,
cryoscopic, IR and 1H NMR studies, dimeric structures with bridging
through phenoxo groups have been proposed for these compounds. The
thermal decomposition of these complexes is a combination of both
exothermic and endothermic processes, as is inferred from TG/DT
studies. Reactions of these compounds with nitrogen bases viz., pyridine,
1,10-phenanthroline and 2,20-bipyridyl results in breakdown of the bridged
*
Correspondence: S. C. Chaudhry, Department of Chemistry, Himachal Pradesh
University, Summer Hill, Shimla 171005, India; E-mail: s.c.chaudhry@sancharnet.in.
1031
DOI: 10.1081/SIM-120037525
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Chaudhry et al.
structure and formation of simple addition compounds, which have been
assigned octahedral structures.
Key Words: para-Nitrophenoxides; Exothermic process; Thermal
decomposition; Dimeric structures.
INTRODUCTION
Zirconium alkoxides and their derivatives[1–6] have well emerged as
precursors for the preparation of zirconia thin films by sol–gel and MOCVD
processes involving the hydrolysis of M–OR bonds. However, little effort seems
to have been made to synthesize the closely related zirconium aryloxides.[7–10]
The synthesis of such complexes containing M–OAr bonds, in fact, are likely
to change their properties due to the possibility of incorporation of different
substituents in the phenol ring. The catalytic properties of zirconium and titanium
chelating alkoxide[11] and phenoxide[12] complexes in ethylene polymerization
recently have been demonstrated. In the present investigation, therefore, an
attempt has been made to synthesize and characterize some new zirconium(IV)
aryloxides using ZrCl4 and p-nitrophenol, which may find potential utility of
diverse nature.
RESULTS AND DISCUSSION
Formation of compounds of composition ZrCl3(OC6H4NO2-p) and ZrCl2-
(OC6H4NO2-p)2 by the trimethylsilyl method maybe rationalized in terms of
the following reaction:
C6H6
ZrCl4 þ nðMe3SiOC6H4NO2-pÞ ꢀꢀꢀ! ZrCl4ꢀnðOC6H4NO2-pÞn
reflux
þ nMe3SiCl n ¼ 1 or 2
A compound of composition ZrCl(OC6H4NO2-p)3 however, could not be pre-
pared by the above reaction. A direct reaction of ZrCl4 with p-nitrophenol in
benzene or toluene under reflux, however, did not yield the desired com-
pounds. It is also pertinent to mention here, that attempts to isolate the tetra-
kis derivative, Zr(OC6H4NO2-p)4, by the reaction of ZrCl4 with excess of
p-nitrophenol or its trimethylsilyl derivative in benzene or in high-boiling
solvents (toluene or xylene), even under prolonged refluxing was unsuccessful
and did not yield the desired product. This reaction, at no stage, appeared to

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p-Nitrophenoxides of Zirconium(IV)
1033
proceed beyond bis-substitution by phenoxo groups and gave ZrCl2(OC6H4-
NO2-p)2 as the product.
The stoichiometric composition of the solids isolated was established by
elemental analyses given in Table 1. These compounds are bright yellow solids
and change their color on exposure to moist air. They are insoluble in most of
the common organic solvents, partially soluble in nitrobenzene and nitromethane
but are soluble in methanol. Molar conductance values of millimolar solutions
of these compounds in nitrobenzene suggest them to be non-electrolytes.
Molecular weight determinations of these compounds at three different concen-
trations in nitrobenzene suggest their dimeric nature in this solvent.
1H NMR Spectra
The 1H NMR spectra of the compounds under study do not show any signal
which could be assigned to phenolic OH groups, suggesting their formation via
1
deprotonation of this group. A perusal of H NMR spectra indicates that the
signals observed due to o- and m-ring protons at d 8.0 and d 6.7 ppm in ZrCl2
(OC6H4NO2-p)2 and at d 8.2 and d 6.9 ppm in ZrCl3(OC6H4NO2-p) are slightly
downfield in contrast to those observed in pure p-nitrophenol or its trimethylsilyl
derivative known to occur at d 7.8 and d 6.5 ppm, respectively. These shifts have
been attributed to the delocalization of electrons from aromatic ring to zirconium
resulting in the formation of bonding of the type ArO .... Zr. A similar viewpoint
has been advanced by Clark et al.[13] in their reports on tantalum(V) phenoxides.
IR Spectra
Formation of the compounds isolated in the present studies has also been
inferred from their IR spectra. The bands observed in the 1270–1160 cm21
region attributed to nring(CO) modes are, comparatively, at a lower region
than those in pure p-nitrophenol or its trimethylsilyl derivative and this shift
to lower frequency has been rationalized in terms of bonding from oxygen
of the phenol to zirconium, which is also supported by the appearance of
entirely new bands in the 590–460 cm21 region assigned to zirconium–
oxygen modes[10,14] in ZrCl3(OC6H4NO2-p) at 485 and 468 cm21; in ZrCl2
(OC6H4NO2-p)2 at 588, 560, 490, and 472 cm21. The bands observed at
560 and 588 cm21 tentatively have been assigned to n(Zr–O) modes while
those in the 490–460 cm21 region have been assigned to bridging

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Chaudhry et al.

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p-Nitrophenoxides of Zirconium(IV)
1035
modes, because it is well known that bridging modes are expected to appear at
lower frequency than those due to the terminal modes. Apart from these, bands
observed in the 360–340 cm21 region have been assigned to n(Zr–Cl)
modes.[10] Interestingly, there is no splitting of bands due to the n(Zr–Cl)
mode, thus indicating that chlorine atoms tend to occupy trans positions in
these structures. Based upon limited analytical, conductance, molecular
weight determinations, 1H NMR and IR spectral studies, the following dimeric
structure bridging through phenoxo groups tentatively maybe proposed for
these compounds (Fig. 1).
Thermal Studies
The thermal behavior of these complexes has been studied using TG/DTA.
The thermograms of ZrCl2(OC6H4NO2-p)2 (Fig. 2) and ZrCl3(OC6H4NO2-p)
(Fig. 3) reveal that they are stable up to 44.28C and 51.08C, respectively,
after which temperature they display single stage decompositions. Although
the final residue in both of these compounds is ZrO2, as is indicated by the
overall weight loss of about 72% and 63.33% for the respective complexes, it
is difficult to comment precisely on the type of intermediates being formed
during the course of thermal decomposition. A careful examination of the
DTA curves indicates that the entire decomposition of the complexes is a
combination of both exothermic and endothermic processes.
Figure 1. Proposed structures of zirconium p-nitrophenoxides.

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Chaudhry et al.
Figure 2. TGA/DTA curves of ZrCl2(OC6H4NO2-p)2.
Reactions with Nitrogen Bases
The accepter properties of ZrCl3(OC6H4NO2-p) and ZrCl2(OC6H4NO2-p)2
compared to ZrCl4 are expected to undergo a change due to replacement of chlor-
ine by phenoxo groups in ZrCl4. It is, therefore, worthwhile to explore the reac-
tion of these phenoxides with some nitrogen bases to establish their Lewis acid
character. The bases used in the present studies include pyridine, 1,10-phenan-
throline, and 2,20-bipyridyl. The addition compounds, which have been isolated,
are listed in Table 1. These compounds are moisture-sensitive solids but are stable
in a dry atmosphere. Molar conductance values of the millimolar solutions of
these compounds in nitrobenzene are too low to be attributed to 1 : 1 electrolytes.
Molecular weight determinations of some of these addition compounds in nitro-
benzene, where the solubility permitted indicate that they exist as monomers.
Information on the structure of these adducts has been obtained by scanning
their IR spectra. The most interesting feature of the infrared spectra of the adducts
is the disappearance of bands in the 480–460 cm21 region due to bridging
modes in parent complexes and is thus indicative of a rupture of the phenoxo
bridged structure on adduct formation. The coordination of bases to
zirconium has been confirmed by the appearance of entirely new bands in

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p-Nitrophenoxides of Zirconium(IV)
1037
Figure 3. TGA/DTA curves of ZrCl3(OC6H4NO2-p).
the 290–270cm21 region assigned to n(Zr–N) modes.[10] Apart from this, bands
due to n(C55C) and (C55N) and C–H out-of-plane deformation modes of the
free bases have been found to shift towards higher spectral region by about
20–30cm21. These observations suggest the coordination from both nitrogen
atoms of the ligand, in agreement with the previously reported results of
2,20-bipyridyl and 1,10-phenanthroline complexes of metal halides.[10,15]
Coordination of pyridine has been confirmed similarly from the shifts to higher
frequency of bands in the 1600–1400 cm21 region due to C55C and C55N and
those at 605 and 405 cm21 due to ring deformation vibrations.[16]
On the basis of these limited analytical, conductance, molecular weight deter-
minations, and infrared spectral data, an octahedral structure may tentatively be
assigned for these addition compounds (Fig. 4). It may also be concluded that
ZrCl3(OC6H4NO2-p) and ZrCl2(OC6H4NO2-p)2 are fairly good Lewis acids.
Figure 4. Structures of ZrCl3(OC6H4NO2-p)B and ZrCl2(OC6H4NO2-p)2(A)2 (where
B ¼ 1,10-phenanthroline, 2,20-bipyridyl or py ¼ pyridine).

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Chaudhry et al.
EXPERIMENTAL
Materials
Zirconium tetrachloride (Fluka) was purified by sublimation in a vacuum
while p-nitrophenol (Sisco) was recrystallized from benzene and its purity was
checked by its melting point. Pyridine (AR Grade) was fractionally distilled
before use. 1,10-Phenanthroline was kept in an oven at 100 8C for an hour
and then placed over P2O5 in a vacuum desiccator before use, m.p.: 117 8C
(lit.[17] 114–117 8C). 2,20-Bipyridyl (Sisco) was recrystallized from ethanol and
was then sublimed in vacuo, m.p.: 73 8C (lit.[18] 70–738C). The trimethylsilyl
derivative of p-nitrophenol, Me3SiOC6H4NO2-p, was prepared by the well-
established procedure involving the reaction of Me3SiCl with p-nitrophenol in
the presence of triethylamine.
Analysis and Physical Measurements
Zirconium in the complexes was determined gravimetrically as ZrO2 and
chlorine by Volhard’s method. Carbon and hydrogen microanalyses were per-
formed on a Coleman CHN analyzer. The IR spectra (4000–200 cm21) were
recorded in KBr discs and Nujol mulls on a Beckman IR 4250 spectropho-
1
tometer and H NMR spectra on a Jeol JNM PMX 60 SI spectrometer using
CDCl3/TFA as solvent. Molecular weights were determined cryoscopically in
nitrobenzene using a Beckmann thermometer while conductance measurements
in the same solvent were made on an Elico conductivity bridge (CM Type 82 T)
using 1023 M solutions at 258C. Thermograms were recorded on a Shimadzu
simultaneous DT-40 DT-TG thermal analyzer, heating rate: 208C min21
sample size 5–10mg, reference Al2O3 thermocouple: Pt/Pt–Rh 10%.
,
Preparation of ZrCl2(OC6H4NO2-p)2 and ZrCl3(OC6H4NO2-p)
In a typical reaction, to a suspension of zirconium tetrachloride (1.93 g,
8.2 mmol) in dry benzene (40 mL), two equivalents of the trimethylsilyl deri-
vative of p-nitrophenol, Me3SiOArNO2-p, (3.50 g, 16.5 mmol) in the same
solvent (20 mL) were added. The reaction mixture was initially stirred for
1 hr followed by heating under reflux for 3–4 hr whereupon the formation
of a solid resulted. The solid was separated by filtration under anhydrous con-
ditions, washed repeatedly with hot benzene and finally with petroleum ether
(b.p. 60 8C) and dried in vacuo. The compound obtained was recrystallized

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p-Nitrophenoxides of Zirconium(IV)
1039
from MeOH. Yield of ZrCl2(OC6H4NO2-p)2, 2.1 g (60%). A similar procedure
was adopted for the synthesis of ZrCl3(OC6H4NO2-p).
Preparation of Addition Compounds of ZrCl2(OC6H4NO2-p)2
and ZrCl3(OC6H4NO2-p) with Nitrogen Bases
A typical synthetic method for the preparation of addition compounds of
ZrCl3(OC6H4NO2-p) involves the reaction of ZrCl3(OC6H4NO2-p) (1.20 g,
3.6 mmol) suspended in benzene (40 mL) with an equimolar amount of
1,10-phenanthroline (0.64 g, 3.6 mmol) or 2,20-bipyridyl (0.56 g, 3.6 mmol)
and bimolar amount of pyridine (0.56 g, 7.2 mmol) dissolved separately in
the same solvent (40 mL) with continuous stirring. The reactions were slightly
exothermic. The formation of addition compounds was indicated by dissol-
ution of the parent phenoxide and change of color of the resulting solution.
After stirring the solution overnight, solid compounds separated out. These
compounds were separated by filtration, washed with petroleum ether (b.p.:
.
60 8C), and finally dried in vacuo. Yield of ZrCl3(OC6H4NO2-p) phen,
.
1.18 g (65%); ZrCl3(OC6H4NO2-p) bipy, yield, 1.41 g (80%); ZrCl3(OC6H4
.
NO2-p) 2py, yield 0.59 g (72%). A similar procedure was employed for
the preparation of addition compounds of ZrCl2(OC6H4NO2-p)2 with 1,10-
phenanthroline, 2,20-bipyridyl and pyridine.
REFERENCES
1. Jones, A.C. Molecular design of improved precursors for the MOCVD of
electroceramic oxides. J. Mater. Chem. 2002, 12, 2576–2590.
2. Jones, A.C.; Leedham, T.J.; Wright, P.J.; Crosbie, M.J.; Williams, D.J.;
Fleeting, K.A.; Davies, H.O.; Otway, D.J.; O’Brien, P. Liquid injection
MOCVD of zirconium dioxide using a novel mixed-ligand zirconium pre-
cursor. Chem. Vap. Dep. 1998, 4, 197–201.
3. Fallon, G.D.; Spiccia, L.; West, B.O.; Zhang, Q. Chemical synthesis studies
on PbZr alkoxides: a new Pb–Zr structural type, Pb3Zr7(m4-O)4(m3-O)4(m-
O2CCH3)8(m-OiPr)6-(OiPr)4. J. Sol–Gel Sci. Technol. 1999, 16, 119–128.
4. Kawasaki, K.; Okazaki, Y. Synthesis of zirconia balloons from metal
alkoxide. J. Ceram. Soc. Jap. 2002, 110, 748–754.
5. Zechmann, C.A.; Huffman, J.C.; Folting, K.; Caulton, K.G. Controlled
stoichiometric incorporation of Ti(IV) into a Zr(IV) pinacolate. Inorg.
Chem. 1998, 37, 5856–5861.
6. Evan, J.W.; Ansari, M.A.; Ziller, W.J. Synthesis of zirconium aryloxide
complexes containing pendent vinyl groups. Inorg. Chem. 1999, 38, 1160.

ORDER
REPRINTS
1040
Chaudhry et al.
7. Malhotra, K.C.; Mehrotra, G.; Chaudhry, S.C. Phenoxides of zirconium.
Natl. Acad. Sci. Lett. 1980, 3, 21–24.
8. Shah, A.; Singh, A.; Mehrotra, R.C. Synthesis and characterization of
alkoxo- and chloro-aryloxo derivatives of titanium and zirconium. Ind.
J. Chem. 1993, 32(A), 632–635.
9. Latesky, S.L.; Keddington, J.; McMullen, A.K.; Rothwell, I.P.;
Huffman, J.C. Synthesis, structure, spectroscopic properties, and electro-
chemical behavior of group IV metal derivatives containing bulky aryl-
oxide ligands. Inorg. Chem. 1985, 24, 995–1001.
10. Shah, A.; Singh, A.; Mehrotra, R.C. Synthesis, characterization, and adduct
formation of chloro 2,4,6-trimethylphenoxide derivatives of titanium and
zirconium. Polyhedron 1986, 5, 1285–1290.
11. Froese, R.D.J.; Musaev, D.G.; Matsubara, T.; Morokuma, K. Theoretical
studies of ethylene polymerization reactions catalyzed by zirconium and
titanium chelating alkoxide complexes. J. Am. Chem. Soc. 1997, 119,
7190–7196.
12. Linden, A. van der; Schaverien, C.J.; Meijboom, N.; Ganter, C.;
Orpen, A.G. Polymerization of a-olefins and butadiene and catalytic
cyclotrimerization of 1-alkynes by a new class of group IV catalysts. Con-
trol of molecular weight and polymer microstructure via ligand tuning in
sterically hindered chelating phenoxide titanium and zirconium species.
J. Am. Chem. Soc. 1995, 117, 3008–3021.
13. Clark, G.R.; Nielson, A.J.; Rickard, C.E.F. Sterically congested tris(phen-
oxide) complexes of tantalum(V). The x-ray crystal structures of [TaCl2(2,6-
di-t-butylphenoxide)3] and [Ta2Cl(m-Cl)2(2,6-di-isopropylphenoxide)5
(m-O)]. Polyhedron 1987, 6, 1765–1774.
14. Barraclough, C.G.; Bradley, D.C.; Lewis, J.; Thomas, I.M. The infrared
spectra of some metal alkoxides, trialkylsilyloxides, and related silanols.
J. Chem. Soc. 1961, 2601–2605.
15. Malhotra, K.C.; Balkrishan. Nature of compounds of Tl(III) chloride with
oxygen and nitrogen bases. Ind. J. Chem. 1976, 14(A), 869–873.
16. Greenwood, N.N.; Wade, K. Complexes of boron trichloride with pyri-
dine and piperidine. J. Chem. Soc. 1960, 1130–1141.
17. Aldrich Handbook of Fine Chemicals and Laboratory Equipments; Aldrich
Chemical Company, Inc.: USA, P13, 2000–2001; Vol. 137-7, 1289.
18. Aldrich Handbook of Fine Chemicals and Laboratory Equipments; Aldrich
Chemical Company, Inc.: USA, D21, 2000–2001; Vol. 630-5; 708.
Received May 22, 2003
Accepted February 24, 2004
Referee I: J.U. Mondal
Referee II: S.M. Socol

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