Synthesis and Determination of Polypyrazolylborates:
K[HB(3,5-C5H7N2)3] and HB(3,5-C5H7N2)3Cu(CO)
Author: Adam Capriola
CHM 2521 Section 151, Department of Chemistry, Saint Joseph’s University
Date Submitted: March 4, 2010
Abstract
The reaction of KBH4 heated with 3,5-dimethylpyrazole produces potassium tris(3,5dimethylpyrazolyl)hydroborate at an unknown percent yield. 1H NMR spectroscopy of
K[HB(3,5-C5H7N2)3] shows singlets at δ 1.79 and 2.06 indicative of –CH3 whereas the same
spectroscopy of 3,5-dimethylpyrazole shows only one singlet in the same area. IR spectroscopy
of K[HB(3,5-C5H7N2)3] shows a B-H stretch at 2420 cm-1, which is absent from the IR spectrum
of 3,5-dimethylpyrazole. The reaction of K[HB(3,5-C5H7N2)3] with CuI and CO gives rise to
HB(3,5-C5H7N2)3Cu(CO) at 265% yield.
13
C NMR spectroscopy of HB(3,5-C5H7N2)3Cu(CO)
produces a singlet at δ 172.4 indicative of C-O bonding, which does not appear on the 13C NMR
spectrum of K[HB(3,5-C5H7N2)3]. IR spectroscopy of HB(3,5-C5H7N2)3Cu(CO) produces a C-O
stretch at 2053 cm-1, which is absent from the IR spectrum of K[HB(3,5-C5H7N2)3]. The analysis
of these spectra seems to validate the supposed products from the reactions.
Introduction
The reaction of KBH4 with 3,5-dimethylpyrazole yields tris(3,5dimethylpyrazolyl)hydroborate. The reaction specifically takes place in the following manner:
This is compound of interest because it a polypyrazolylborate, or scorpionate, formed
from a binary born hydride, which are difficult to handle.1,2 In order to determine the structure
of said substance from a 1H NMR and IR spectrum, the peaks and stretches must be compared to
the same spectra for 3,5-dimethylpyrazole to look for indications of boron in the structure and
differentiation in methyl groups. The spectra for the two compounds should be similar save for
those two main differentiations.
Metal complex HB(3,5-C5H7N2)3Cu(CO) can be synthesized from the following reaction:
The identity of this product can be confirmed by comparing its 13C NMR and IR spectra to the
same spectra of the reagent K[HB(3,5-C5H7N2)3]. In this case, the spectra are compared to look
for the presence of C-O bonding. The spectra should appear similar aside from peaks and
stretches indicative that bond. The systematic addition of functional groups to these two
compounds is what makes them comparable and identifiable to and from one another in their 1H
NMR, 13C NMR, and IR spectra.
Experimental
All syntheses were carried out in air and the reagents and solvents were purchased from
commercial sources and used as received unless otherwise noted. The synthesis of K[HB(3,5C5H7N2)3] (1) and HB(3,5-C5H7N2)3Cu(CO) (2) were based on reports published previously.1
K[HB(3,5-C5H7N2)3] (1). KBH4 (1.028 g, 19.1 mmol) and 3,5-dimethylpyrazole (7.002
g, 72.8 mmol) were added subsequently to a 100 mL round-bottom flask along with a small
magnetic stir bar. A cold water condenser with greased joint was inserted into the round-bottom
flask containing the solution. This connection was further secured with a keck clip. A silicone
oil bath was constructed with a glass dish containing a paper clip as a stirring instrument. The
bath was placed on a hot plate and the round bottom flask was placed in the oil bath. The cold
water condenser was not connected to a cold water source; it was used to allow air to circulate.
Once secure, the hot plate was turned on to 230 °C and the stirring instruments were spun
at a moderate speed. A thermometer was inserted into oil bath to monitor the temperature, which
fluctuated between 230 °C and 250 °C during the experiment. The solution was allowed to heat
for 1 h. After this time, the round-bottom flask was taken off the oil bath and the condenser was
removed. A white solid precipitate submerged in liquid remained and was allowed to cool to
90 °C, again using the thermometer to measure temperature. 50 mL of toluene was added to the
flask and the solution was vacuum filtered with a 60 mL frit. A total of about 100 mL more
toluene was added to wash the resulting white solid precipitate. The precipitate was washed a
final time with 50 mL of diethyl ether. The precipitate was then vacuum dried for 0.33 h. The
precipitate was a powdery white substance 1. 1H NMR (D2O): δ 1.79 (s, -CH3), 2.06 (s, -CH3),
5.82 (s, C-H).
13
C NMR (D2O): δ 11.05 (s, -CH3), 12.44 (s, -CH3), 104.9 (s, C-H), 146.1 (s, C-
CH3), 148.9 (s, C-CH3). FTIR (ATR) ν(C=N) 1560 cm-1 (s, pyrazolyl), ν(B-H) 2420 cm-1 (s, B-H
linkage), ν(C-H) 2950 cm-1 (br, C-H linkage).
HB(3,5-C5H7N2)3Cu(CO) (2). CuI powder (0.394 g, 2.07 mmol) and acetone (35 mL)
were subsequently added to a 100 mL round-bottom flask along with a small magnetic stir bar.
A septum was attached to the flask. Previously synthesized 1 (0.191 g, 0.567 mmol) was
dissolved in a minimal amount of acetone (3 to 5 mL). CO gas was bubbled into the roundbottom flask for about 5 min. At this time, the solution of 1 and acetone was injected into the
round-bottom flask, and CO gas was allowed bubbled in for another few minutes. A yellowish
liquid resulted and the flask was put on ice for 1 h to allow for recrystallization. The solution
was then roto-vaporized for 5 to 10 minutes to make up for inadequate recrystallization.
A grayish, greenish powder remained in the round-bottom flask, which was scraped out
using a spatula and determined to be 2 (0.584 g, 265% yield based on the amount of 1 used). 1H
NMR (CDCl3): δ 2.30 (s, -CH3), 2.50 (s, B-H), 5.68 (s, C-H).
13
C NMR (CDCl3): δ 12.50 (s, -
CH3), 13.92 (s, -CH3), 104.37 (s, C=C), 143.60 (s, C=N), 147.42 (s, C-N), 172.4 (s, C-O). FTIR
(ATR) ν(C=N) 1543 cm-1 (s, pyrazolyl), ν(C-O) 2053 cm-1 (s, carbonyl), ν(B-H) 2499 cm-1 (s, B-H
linkage), ν(C-H) 2921 cm-1 (br, C-H linkage).
C5H8N2 (3). The 1H NMR and IR spectra of (3) were obtained from Sigma Aldrich.3,4
1
H NMR (CDCl3): δ 2.25 (s, -CH3), 5.8 (s, C-H). FTIR (ATR) ν(C-N) 1030 cm-1 (s, pyrazolyl),
ν(C-H) 2860 cm-1 (br, C-H linkage), ν(C-H) 2930 cm-1 (br, C-H linkage).
Results
The reaction of KBH4 and 3,5-dimethylpyrazole was not measured for yield of the
product, K[HB(3,5-C5H7N2)3], but theoretical yield would be 19.1 mmol. Theoretical yield of H2
gas, though not measured, was 57.3 mmol, based on the amount of KBH4 used, which was the
limiting reagent. KBH4 reacts to form H2 in a 1:3 ratio, and 19.1 mmol of KBH4 was used to
start, so that proportion was taken into account when calculating the theoretical yield. 1H and
13
C NMR spectroscopy of the product yielded several peaks. The 1H NMR spectrum presented a
two singlets found at δ 1.79 and 2.06, representative of methyl groups. A singlet found at δ 5.82
was indicative of the hydrogen attached directly to pyrazolyl ring. The 13C NMR spectrum
yielded a pair of singlets found at δ 11.05 and 12.44, which was suggestive of methyls attached
to the pyrazolyl ring. A singlet found at δ 104.9 was from the C-H bond on the ring, and two
final singlets found at δ 146.1 and 148.9 were from the carbons on the ring attached to the
methyl groups. The IR spectrum showed a sharp peak around 1560 cm-1 indicative of a C=N
bond forming the pyrazolyl ring, a sharp peak around 2420 cm-1 indicative of B-H linkage, and
finally a broad peak near 2950 cm-1 suggestive of C-H bonding.
The reaction of CuI, K[HB(3,5-C5H7N2)3], CO, and acetone yielded 0.584 g of product,
HB(3,5-C5H7N2)3Cu(CO). This translated to 1.502 mmol, and thus was a 265% yield. 1H and
13
C NMR spectroscopy of the product yielded several peaks. The 1H NMR spectrum contained a
singlet found at δ 2.30, representative of methyl groups. A singlet found at δ 2.50 was indicative
of the hydrogen bonded to boron. A third singlet found at δ 5.68 was from protons bonded to the
pyrazolyl ring. The 13C NMR spectrum produced a two singlets found at δ 12.50 and 13.92,
which suggested methyls carbons. A singlet found at δ 104.37 was from double bonded carbons,
a singlet found at δ 143.60 was from carbon double bonded to nitrogen, another singlet found at
δ 147.42 was from carbon singly bonded to nitrogen, and one final singlet at δ 172.4 was
representative of carbon bonded to oxygen. The IR spectrum yielded a sharp peak around 1543
cm-1 indicative of a C=N bond forming the pyrazolyl ring, a sharp peak around 2053 cm-1
indicative of C-O bonding, a sharp peak around 2499 cm-1 indicative of B-H linkage, and finally
a broad peak near 2921 cm-1 suggestive of C-H bonding.
Discussion
The percent yield of K[HB(3,5-C5H7N2)3] was not able to be determined. The weight of
this product was either never obtained or the figure was lost during the experiment. Percent error,
though not measured, could possibly have been affected from heating the KBH4 and 3,5dimethylpyrazole solution at too high a temperature, as it went above the 230 °C limit specified
by the experimental guidelines.1 The solution was heated for only 1 h, when the suggested time
was 1 to 1.5 h, which means the reagents may not have completely reacted. When the solution
was taken off the oil bath and allowed to cool, it cooled more quickly than expected, and
dropped to 90 °C or lower before adding the 50 mL of toluene when 100 °C was specified the
addition temperature.1 There was some confusion as far as the protocol at this point, so the
solution with the toluene added was allowed to cool for a short while, when the guidelines asked
for the residue to be filtered and washed hot. The solution was still warm when filtered and
washed, but not nearly as hot as it could have been. These types of errors would have resulted in
loss of potential product and negatively affected the percent yield, had it been measured.
The product did seem pure, as it was a clean white color, and its 1H NMR, 13C NMR, and
IR spectra yielded clear readings. The 1H spectrum shows methyl peaks at δ 1.79 and 2.06
whereas the 1H spectrum for the reagent in the reaction, 3,5-dimethylpyrazole, shows only one
methyl peak at δ 2.25. This seems to validate that addition of the boron to the molecule, as it
would cause make each methyl group slightly different from the other. Polypyrazolylborates
produce sharp a B-H stretch in their IR spectra, and this is evident in the IR spectrum reading for
K[HB(3,5-C5H7N2)3].2 A sharp peak is noted at 2420 cm-1, whereas the IR spectrum for 3,5dimethylpyrazole does not contain said stretch, again supporting the claim for addition of boron
to the molecule.
The percent yield for HB(3,5-C5H7N2)3Cu(CO) was not accurate. The product obtained
from the roto-vaporization was not washed, so it is suspected that the other product of the
reaction, KI, was mixed in with the desired product. It is believed that the greenish powder was
HB(3,5-C5H7N2)3Cu(CO) while the greyish powder was KI. This is why the percent yield was
above 100%. Aside from that inaccuracy, the only 0.567 mmol of K[HB(3,5-C5H7N2)3] was
used, when the protocol called for 2 mmol to be used.1 This would not affect the percent yield,
as the amount of K[HB(3,5-C5H7N2)3] used would still be the limiting reagent, but could have
affected the NMR and IR spectrums. However, it was intuitively noted that the greenish powder
was the desired product, and an effort was made to extract only that powder from the product for
the spectroscopy determinations.
The fact that recrystallization did not seem take place as detailed1 and that a rotovaporizer had to be used to dry the product most likely did not help the yield of product either.
Product may have been lost during this process. If the powder had been washed with acetone, a
more accurate percent yield would have been obtained because the KI would have been washed
away, but this was not extremely necessary for the purposes of this experiment. The product
obtained did give clear 1H NMR, 13C NMR, and IR spectra, meaning it was fairly pure. The
addition of the CO to the molecule from K[HB(3,5-C5H7N2)3] is evident in the 13C NMR and IR
spectra. There is a distinct peak at δ 172.4 on the 13C NMR spectrum which is not noted on the
same spectrum for K[HB(3,5-C5H7N2)3]. The IR spectrum of HB(3,5-C5H7N2)3Cu(CO) shows a
tall sharp stretch at 2053 cm-1 distinctive of C-O bonding; the IR spectrum of K[HB(3,5C5H7N2)3] shows no such stretch. Peaks and stretches for the spectra were labeled with the help
of colleagues. An acknowledgement is made that are more than likely downfield or upfield
shifts of some of the peaks from one product to another because of changes in chemical structure,
but these postulates were not explored. The oxidation state and electron count of Cu in HB(3,5C5H7N2)3Cu(CO) are +1 and 10 electrons, so it is a 18 electron complex.
Conclusion
The main purpose of the experiment was to decipher the structural changes from 3,5dimethylpyrazole to potassium tris(3,5-dimethylpyrazolyl)hydroborate to a copper complex of
potassium tris(3,5-dimethylpyrazolyl)hydroborate through 1H NMR, 13C NMR, and IR spectra.
Addition of boron to 3,5-dimethylpyrazole was apparent in the 1H NMR and IR spectra of the
first product. The 1H spectrum shows methyl peaks at δ 1.79 and 2.06 whereas the 1H spectrum
for the reagent in the reaction, 3,5-dimethylpyrazole, shows only one methyl peak at δ 2.25,
seemingly confirming the addition of boron as this would make each methyl group differentiable.
The IR spectrum of this product showed a sharp stretch around 2420 cm-1, indicative of B-H
bonding, which is absent in the IR spectrum for 3,5-dimethylpyrazole. All of these finding seem
to validate K[HB(3,5-C5H7N2)3] as being the product of the reaction.
The 1H NMR, 13C NMR, and IR spectra of the second product also seem to confirm its
expected structure. The 13C NMR spectrum for the second product shows a peak at δ 172.4,
which is an area suggestive of C-O bonding. The 13C NMR spectrum of K[HB(3,5-C5H7N2)3]
contains no peak in this area. The IR spectrum of the second product shows a sharp stretch
around 2053 cm-1, which is also indicative of C-O bonding. The IR spectrum of K[HB(3,5C5H7N2)3] contains no stretch in this area. These noted findings on the spectra all point towards
to product being HB(3,5-C5H7N2)3Cu(CO).
The percent yield for the first reaction was not monitored, but would have been aversely
affected by factors such as poor temperature control, short reaction time, and more prompt
washing technique. The percent yield for the second reaction was poor, but could have been
improved by washing the product with acetone and by allowing for a longer recrystallization
period.
References
(1) Bochmann, M. Preparation and Complexation of Tris(3,5-dimethylpyrazoyl)hydroborate. pp
33-35.
(2) Trofimenko, S. Polypyrazolylborates: Scorpionates. Journal of Chemical Education. 2005,
82, 1715-1720.
(3) http://www.sigmaaldrich.com/spectra/fnmr/FNMR010068.PDF
(4) http://www.sigmaaldrich.com/spectra/ftir/FTIR007818.PDF