Single-pot synthesis of zinc nanoparticles, borane (BH3) and closo-dodecaborate (B12H12)2- using LiBH 4 under mild conditions

Article abstract of DOI:10.1039/c3dt00092c

Reduction of ZnCl₂ using LiBH₄ in mesitylene yielded zinc nanoparticles (Zn-NPs), borane (BH₃) and closo-dodecaborate (B₁₂H₁₂)^2-. The BH₃ evolved gas was trapped as Ph₃

Full text of DOI:10.1039/c3dt00092c

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Single-pot synthesis of zinc nanoparticles, borane (BH3)
and closo-dodecaborate (B12H12)2− using LiBH4 under
mild conditions†
Cite this: Dalton Trans., 2013, 42, 8420
Sekher Reddy Ghanta,a Muddamarri Hanumantha Raoa and
Krishnamurthi Muralidharan*a,b
Received 10th January 2013,
Accepted 25th March 2013
Reduction of ZnCl2 using LiBH4 in mesitylene yielded zinc nanoparticles (Zn-NPs), borane (BH3) and closo-
DOI: 10.1039/c3dt00092c
2−
dodecaborate (B12H12
was extracted by methanol and characterized from spectral data.
) )
2−. The BH3 evolved gas was trapped as Ph3P:BH3 adduct while closo-(B12H12
www.rsc.org/dalton
lithium borohydride (LiBH4) in mesitylene as solvent. The
results obtained from this reaction are described in this paper.
Introduction
The chemical syntheses of metal nanoparticles (NPs) generally
provides a less energy consuming alternative route to physical
methods. Carefully chosen reducing and stabilizing molecules Results and discussion
help to synthesize NPs, whose purity is the same as that
obtained through chemical vapour deposition (CVD) or any
Zinc nanoparticles (Zn-NPs)
other physical methods.1 Further, if easily removable, volatile
and synthetically useful side products are produced during
NPs synthesis, it will embellish the interest for further
advancement. In this paper we report an enticing method of
chemical synthesis for zinc nanoparticles (Zn-NPs) and simul-
taneous production of borane (BH3) and closo-dodecaborate
In a typical synthetic procedure (Scheme 1), the ZnCl2 was
reduced by LiBH4 in deoxygenated mesitylene. The reaction
mixture was refluxed under a static nitrogen atmosphere. Gas
evolution was observed during the reaction. When the gas evol-
ution ceased and increasing amounts of a black-grey residue
was observed (∼24 h) the reaction mixture was allowed to cool
to room temperature. The solid residue obtained was filtered
through a glass frit and washed with 20–30 mL portions of ice
cold dry methanol to remove LiCl and other side products of
the reaction. The grey coloured residue was dried under
vacuum and finally designated as Zn-NPs (yield: 80–90%).
In recent reports, it was described that when LiBH4 and
NaBH4 were thermally decomposed they yielded lithium and
sodium salts of (B12H12)2−, respectively.6 It is also well known
that the oxidation of NaBH4 by I2 would evolve the BH3 gas,
and this method has been well utilized in organic synthesis.7
Further, the same reaction was modified8 to prepare various
salts of (B12H12)2− with good yields. In this context, it is inter-
esting to find the possible boron-containing product from a
reaction wherein LiBH4 is used as the reducing agent. Also, it
would be useful if we could isolate one or more boranes from
such a reaction involving LiBH4 and ZnCl2. Since such boron-
containing compounds were anticipated to be side products in
(B12H12)
2− in a single reaction.
The Zn-NPs and its composites are expected to find wide
applications in several areas including biomedical, antibacter-
ial coatings, cosmetology, catalysis, lubrication, and solar
cells.2 They were also tested as an ingredient to shorten the
ignition delay time of explosive materials, and as a catalyst for
the decomposition of ammonium perchlorate in a propellant
composition.3 Few physical methods, including laser ablation
and electrodeposition, are known to produce Zn-NPs.4 Still
chemical synthetic methods are appealing because of the
possibility of obtaining nanoparticles as a solution or suspen-
sion.5 Therefore, with an intention of finding a new synthetic
route to Zn-NPs, we chose reducing zinc chloride (ZnCl2) using
aAdvanced Centre of Research in High Energy Materials (ACRHEM), University of
Hyderabad, Hyderabad 500046, AP, India
bSchool of Chemistry, University of Hyderabad, Hyderabad 500046, AP, India.
E-mail: kmsc@uohyd.ernet.in; Fax: +91 40 2301 2460; Tel: +91 40 2313 4819
†Electronic supplementary information (ESI) available: Details about synthetic
procedure, materials and instrumentation; TEM, SEM and FESEM pictures;
NMR and FT-IR spectra. CCDC 926150. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c3dt00092c
Scheme 1 Reduction of zinc chloride by lithium borohydride (the reaction is
not balanced for either stoichiometry or charge).
8420 | Dalton Trans., 2013, 42, 8420–8425
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our reaction, it could be also possible to monitor the reaction
and products by 11B NMR spectral analysis. As expected, a few
boron-containing compounds were also formed in the above
reaction, as observed from the 11B{1H} NMR spectrum of the
methanol extracts. However, we were able to isolate only BH3
gas and [B12H12]2− from the above reaction (vide infra).
The large surface to volume ratio makes the metal nanopar-
ticles highly susceptible to surface oxidation and aggregation.9
Therefore, some surfactants or polymers are widely used to
protect and stabilize metal nanoparticles. In order to circum-
vent both problems we intended to protect the Zn-NPs using
poly(vinylpyrrolidone) (PVP). It is also interesting to note at
this juncture that the polymer stabilized Zn-NPs were tested as
an antibacterial coating and in corrosion protection.10 To
obtain polymer protected nanoparticles, we have conducted
the above synthesis of Zn-NPs in the presence of PVP also.
This reaction yielded Zn-NPs stabilized in polymer
(Zn-PVP-NPs). The reactions were performed aiming at a 1 : 0.5
weight ratio of Zn-NPs to PVP. However, the actual ratio would
vary slightly based on the extent of the completeness of the
reduction reaction and possible loss of polymer during
washing. It is believed that PVP stabilizes the nanoparticles by
donating its lone pair of electrons to the metal atoms on the
surface of the particles.11,12
Fig. 2 DTA curve of Zn-NPs and Zn-PVP-NPs.
salt having such a high melting point was possible from our
reaction, the data confirmed the formation of lithium chloride
as one of the side products (Fig. S2, ESI†). In the FT-IR
spectrum (Fig. S3, ESI†) of the Zn-PVP-NPs, the carbonyl
stretching frequency of PVP was observed. A slight shift in the
frequency from pure PVP was observed (1660 cm−1 was shifted
to 1650 cm−1), which can be ascribed to the coordination
of the oxygen atoms of PVP to atoms on the surface of the
Zn-NPs.14
Characterization of Zn-NPs
The PXRD patterns (Fig. 1) obtained for the Zn-NPs, with and
without PVP, were compared with that of JCPDS (#87-0713)
and were found to be in good agreement with the reported
PXRD pattern of hexagonal Zn. Thermogravimetric (TG–DTA)
analyses (Fig. 2 and Fig. S1, ESI†) of both Zn-NPs and
Zn-PVP-NPs were also carried out. In both cases, sharp
endothermic peaks were observed around 415 and 417.5 °C,
respectively, in the DTA curve, corresponding to the melting
point of zinc. Such a small difference in the melting points of
free nanoparticles and nanoparticles in a matrix has been
thoroughly studied and was explained based on Gibbs–
Thomson equation.13 In the DTA curve of the side products, a
sharp endothermic peak corresponding to the melting point
of lithium chloride was observed. Because no other inorganic
Microscopic investigations
Investigation of the NPs by FESEM (Fig. 3) showed that most
of the uncoated Zn-NPs were cubic, while Zn-PVP-NPs were
irregular flakes. The FESEM picture of the Zn-NPs obtained
immediately after the reaction showed smaller particles
(50–100 nm), while bigger particles were observed (750 nm–1
µm) in pictures taken after a few days (Fig. S4, ESI†). This may
be due to the kinetic instability of the metal nanoparticles,
leading to aggregation which is a common phenomenon
in the absence of any surfactant.15 However, the sizes of the
Zn-PVP-NPs were in the range of 50–100 nm, indicating the
prevention of aggregation by a polymer. The prevention of
the particle size growth might have been due to the atoms on
the surface of Zn-NPs being coordinated by lone pair electrons
of the oxygen and nitrogen atoms present in PVP. This coordi-
nation on the surface was evidenced from the FT-IR spectrum,
Fig. 3 FESEM image of (a) Zn-NPs with scale bar 1 µm. (b) Zn-PVP-NPs with
Fig. 1 PXRD pattern of Zn-NPs in polymer (PVP) matrix.
scale bar 1 µm.
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Fig. 4 TEM and HRTEM images of the Zn-PVP-NPs with scale bars 100 nm and
5 nm, respectively. FFT of a single nano crystal is also shown.
as discussed earlier. This phenomenon could have been the
reason for smaller sized particles compared to free nanoparti-
cles.16 The advantage of stabilization of the nanoparticles
using a polymer is that it can protect them from oxidation and
also from aggregation.
The TEM images of the Zn-PVP-NPs (Fig. 4 and Fig. S5,
ESI†) also confirmed the formation of irregularly shaped flakes
with sizes ranging 50–100 nm. Fast Fourier Transformations
(FFT) of HRTEM showed the orientation of the (0 0 2) plane
matching with PXRD. The restriction of the growth of the
particles by the polymer chains could be the probable reason
for the irregular shape of the nanoparticles, otherwise they
could have grown to a particular shape.
The elemental composition of both Zn-NPs and Zn-
PVP-NPs specimens were determined by EDX (energy disper-
sive X-ray spectrometry) on SEM, FESEM and TEM. The EDX
spectra (Fig. S7 and S8, ESI†) showed a strong peak corre-
sponding to zinc. The weight percentage of zinc in all the EDX
spectra was consistent. Due to surface oxidation, a peak corre-
sponding to the oxygen appeared in the EDX spectrum of
Zn-NPs without a polymer coating, indicating the requirement
for surface protection.
Fig. 5 ORTEP diagram showing crystal structure of the Ph3P:BH3 complex.
Phenyl hydrogens are omitted for clarity. Selected bond lengths [Å] and bond
angles [°]: P1–B1 = 1.924(3), P1–C1 = 1.814(3), P1–C7 = 1.84(3), P1–C14 = 1.811(3),
B1–H1A = 0.960, B1–H1B = 0.960, B1–H1C = 0.960, C7–P1–B1 = 112.261(16),
C14–P1–B1 = 113.88(16), C1–P1–B1 = 111.83(15), C14–P1–C7 = 104.85(12),
C7–P1–C1
= 106.62(12), C14–P1–C1 = 106.86(12), P1–B1–H1A = 109.5,
P1–B1–H1B = 109.5, P1–B1–H1C = 109.5, H1A–B1–H1C = 109.5, H1A–B1–H1B =
109.5, H1B–B1–H1C = 109.5.
presence of PPh3. After reaction, the mesitylene was evaporated
under vacuum and the product was extracted with benzene. It
was further recrystallized from benzene under low temperature
and the crystals were analysed by spectroscopy. Formation of
phosphine–borane adduct Ph3P:BH3 was primarily confirmed
from multinuclear NMR spectral data.7c,20,21 Further, pure
crystals were obtained by recrystallization from benzene solu-
tion and characterized by single crystal X-ray crystallography.
However, the structure had already been reported in 1982.22
The molecular structure of the adduct is shown in the Fig. 5,
which provided unambiguous proof of the nature of the
material obtained from this reaction. The crystal data and
structural refinement details are provided in the ESI.†
Isolation of BH3
It is worth mentioning that BH3, while being one of the
major reagents in organic synthesis, has only a few methods
known so far for convenient laboratory scale preparation.7 The
Lithium borohydride (LiBH4) and lithium aluminium hydride
(LiAlH4) are well known hydride sources for use as reducing
agents in synthetic chemistry. Studying the fate of a reagent
used in the synthesis and side products formed from such
reagent, assists in understanding the reaction pathway. For
example, in recent reports, when AlCl3 was reduced by LiAlH4,
lithium chloride and alane formation was observed.17 Alane
further decomposed at higher temperatures to yield alu-
minium nanoparticles.18 However, the fate of boron from
LiBH4 in a reduction reaction has not been understood much.
Since there was a probability for the formation of one or
more boranes in our reactions, it is essential to identify each
reaction described here provided
a single-pot synthetic
method for Zn-NPs and BH3 gas under mild conditions. There-
fore, our method is expected to also be a useful addition to
synthetic chemistry for the production of BH3. The clean
PXRD pattern of the Zn-NPs obtained from this reaction
suggested that addition of triphenylphosphine to the reaction
mixture did not affect the formation of the Zn-NPs.
Isolation of closo-(B12H12)2−
of the products from the reaction mentioned in the Scheme 1. The 11B{1H} NMR spectrum of the methanol extracts from the
Indeed, when we probed the evolved gas from our reaction we reduction reactions showed a predominant peak correspond-
were able to identify the BH3. It was shown that triphenylpho- ing to (B12H12)2− and also a few more boron containing pro-
sphine (PPh3) easily reacts with borane to form a triphenylpho- ducts (Fig. S9, ESI†). In order to isolate any borane, after
sphine–borane adduct.19 Hence, in order to confirm the filtering the Zn-NPs from the reaction mixture, methanol
formation of BH3, the same reaction was performed in the extracts were dried to obtain a solid product. This methanol
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solution also contained lithium chloride. The possibility The structure was solved by direct methods and refined by full-
existed that closo-(B12H12)2− could be present as a lithium salt, matrix least-squares cycles on F2. All non-hydrogen atoms were
as well as a zinc salt in the same solution. Hence, the product refined anisotropically.24 The TG-DTA analyses were conducted
was dissolved again in deionized water and triethylamine was on a Ta Q600 SDT instrument with a standard heating rate
added to convert closo-(B12H12)2− into (B12H12)(HNEt3)2. Since 10 °C min−1 from room temperature to 600 °C under a
this triethylamine salt was not soluble in water, it was isolated dynamic nitrogen atmosphere.
easily by filtration (isolated yield: 4–5%). Its formation was
The SEM analyses were performed on Philips XL-30 SEM
confirmed by multinuclear NMR, FT-IR and High Resolution operating at 20 kV. The FESEM analyses were performed on
Mass spectral data.8,23 The attempted isolation of other ultra 55 of Carl Zeiss. Samples for SEM and FESEM were pre-
boranes failed owing to their trace quantity and also probably pared on a glass surface by drop casting. The samples were
due to their volatility. It should be mentioned that a number suspended in suitable solvents and sonicated for 1–2 min.
of lower boranes are volatile. Despite the stability of closo- Specimens for EDS (energy dispersive X-ray spectrometry)
(B12H12)
2−, there has been only one viable laboratory scale analysis were prepared by dusting the compounds on carbon
synthetic procedure for the preparation of its salt.8 Hence the tape. The TEM analyses were conducted on FEI technai G2 20
method presented here is worthwhile in that context but STEM with a 200 kV acceleration voltage. TEM specimens were
has to be optimized for use as an alternative method for the prepared on carbon coated copper grids with 200 meshes.
synthesis of higher boranes.
Samples were suspended in suitable solvents and ultra soni-
cated for 1–2 min.
Synthesis of Zn-NPs
Conclusion
The reactants (ZnCl2, 10 mmol; LiBH4, 20 mmol) were added
to the mesitylene (20 mL) which was already placed in a
100 mL two-necked round-bottom flask equipped with a mag-
netic stirring bar. A reflux condenser with a nitrogen inlet was
fitted on the reaction flask. The reaction mixture was refluxed
for 24 hours under static nitrogen atmosphere. Gas evolution
was observed during the reaction. On completion of the reac-
tion the reaction mixture was allowed to cool to room tempera-
ture. The solid residue was filtered through the glass frit and
washed with 20–30 mL portions of ice cold dry methanol to
remove the LiCl and other side products of the reaction.
Further, the black-grey colored residue was dried under
vacuum and characterized as Zn-NPs (yield: 80–90%).
In conclusion, reduction of zinc(II)chloride yielded three pro-
ducts; viz. Zn-NPs, BH3 and (B12H12)2−. The reaction provided
an attractive chemical synthetic route to Zn-NPs and an
alternative method to produce BH3 and (B12H12)2−. The ease
with which BH3 and (B12H12)2− were produced in the presence
of ZnCl2 from LiBH4 is anticipated to be a valuable addition to
synthetic chemistry.
Experimental
Materials and synthesis
Zinc(II) chloride, poly(vinylpyrollidone) (molecular weight
10 000), and mesitylene were purchased from the Aldrich and
lithium borohydride was purchased from the Acros chemicals.
Triphenylphosphine was purchased from the SRL, India.
All the chemicals were used as received. Mesitylene was
thoroughly deoxygenated using a freeze–pump–thaw method
before use.
Characterization of (B12H12)(HNEt3)2
After removing the Zn-NPs from the above reaction mixture the
methanol extracts were dried to obtain a solid product. The
solid was dissolved again in deionized water and then an
excess amount of triethylamine was added drop by drop with
continuous stirring to maintain the pH around 9–10 at ice
cold temperature. Immediately a voluminous precipitate was
obtained. To complete the precipitation, the stirring was con-
Instruments and sample preparation
The pure samples obtained after removing the LiCl completely tinued at room temperature overnight. The formation of a col-
were used for the characterization. The X-ray diffraction orless solid was observed. This colorless solid was filtered and
patterns were recorded on Bruker D8 X-ray diffract meter (Cu washed with a small amount of distilled water and dried.
Kα = 1.54 Å) with a scan rate of 6° min−1. The 11B, 31P, 1H, Analysis of this solid by spectroscopy confirmed the formation
NMR spectra were recorded using a Bruker Avance 400 MHz of (B12H12)(HNEt3)2 salt. NMR: 11B{1H} (CD3CN) δ −15.31 ppm
instrument using BF3-ether, 85% H3PO4, and SiMe4, respect- (s) (Fig. S10, ESI†); 11B −15.33, −16.30 (d, J = 124.5 Hz)
ively, as the standard reference. The Fourier transform infrared (Fig. S10, ESI†); 1H (D2O) δ 3.10 (q, 12H, –CH2–, J = 7.4 Hz), 1.18
(FT-IR) spectra (KBr pellet) were recorded on a Jasco 5300 or (t, 18H, –CH3, J = 7.2 Hz), 0.56 to 1.70 [12H, broad poorly
Nicolet 380 spectrophotometer. Single-crystal X-ray data collec- resolved, (B12H12)2−] (Fig. S11, ESI†); FT-IR: 2482 cm−1 (strong,
tion for the triphenylphosphine borane adduct was carried out B–H stretching) (Fig. S12, ESI†). HRMS: (M + 23)+ = 369.41
at 298(2) K on a Bruker Smart Apex CCD area detector system (Fig. S13, ESI†). Elem. Anal.: Calcd (found) for C12H44B12N2: C,
[λ(Mo Kα) = 0.71073 Å] with graphite monochromator. The 41.63 (41.52); H, 12.81 (12.76); N, 8.09 (8.16) (Fig. S14, ESI†).
data were reduced using SAINTPLUS, and the structure was Attempted isolation of other boranes failed owing to their
solved using SHELXS-97 and refined using SHELXL-97. trace quantity.
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Synthesis of Zn-PVP-NPs
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(0.3269 g). The reactions were performed aiming at a 1 : 0.5
ratio of Zn-NPs to PVP. After 24 hours, the reaction mixture
was allowed to cool to room temperature. The solid residue
was filtered through the glass frit and washed with 20–30 mL
portions of ice cold dry methanol to remove the LiCl and other
side products of the reaction. Further, the black-grey colored
residue was dried under vacuum and characterized as
Zn-PVP-NPs.
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Formation of the phosphine–borane complex
The above reduction reaction was carried out in the presence
of PPh3 (20 mmol). After the reaction time, mesitylene was
evaporated under vacuum. Then the Ph3P:BH3 adduct was
extracted with 25 mL of benzene. It was further recrystallized
from benzene under low temperature. The formation of the
phosphine–borane complex was primarily confirmed by NMR
spectral data. NMR: 11B{1H} (CDCl3), broad doublet at
δ −38.019 and −38.442 ppm (JB–P = 54.3 Hz); 11B{1H} (THF-d8)
broad doublet at δ −39.64 and −40.07 ppm (JB–P = 55.2 Hz),
(Fig. S15, ESI†); 11B (CDCl3) δ −37.94 ppm (m) (Fig. S16, ESI†);
31P{1H} (CDCl3) broad, merged lines at δ 20.85 and 20.43 ppm
(JP–B = 68.0 Hz); 31P{1H} (THF-d8) broad, merged lines 20.90
and 20.52 (61.5 Hz) (Fig. S17, ESI†). The dissimilarities in the
coupling constants are probably due to the line-broadening
in the 31P NMR spectra, which might be induced by the quad-
rupolar interaction of the two nuclear isotopes (11B, I = 3/2,
80.42% naturally abundant and 10B, I = 3, 19.58% naturally
abundant) of the adjacent boron leading to poorly resolved
lines in 31P NMR spectra complicating accurate J measure-
ment. Hence it will be reasonable to use the value obtained
from the 11B NMR spectra.20a,b 1H (CDCl3), δ 7.62–7.28 (15H,
Ph, three set of multiplets), 1.58–1.01, (3H, BH3, poorly
resolved multiplet) (Fig. S18, ESI†). FT-IR: 2245.4 cm−1 (weak,
B–H asymmetric), 2377.6 cm−1 (strong B–H symmetric),
604.41 cm−1 (strong B–P stretching) (Fig. S19, ESI†). Elem.
Anal.: Calcd (found) for C18H18BP: C, 78.30 (78.32); H, 6.57
(6.41); (Fig. S20, ESI†).
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Acknowledgements
The authors gratefully acknowledge the funding from DRDO,
India, in the form of a research grant to ACRHEM and School
of Chemistry, University of Hyderabad for infrastructure. The
authors acknowledge Mr Anand Kumar Jami, School of Chemi-
stry, University of Hyderabad for his kind help regarding
crystal structure solution.
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