Investigations on the reactivity of arylboronic acid with phenolic pyrazole

Article abstract of DOI:10.1016/j.ica.2011.01.104

Reaction of phenylboronic acid with phenolic pyrazole was carried out in 1:1 stochiometry using toluene as a solvent. Depending on the steric bulk of the group present on the pyrazole ring, PhB (HPhPzPh)(OEt) 1, [(PhB)(PhPzHt-Bu)(OH) ][(PhB)₂(PhPzt-Bu)₂(O)] 2 and (PhBPhPz)₂ 3 were isolated. Compound 3 is an example of cis-isomer of pyrazabole crystallized in a boat conformation for the B₂N₄ heterocycle.

Full text of DOI:10.1016/j.ica.2011.01.104

Inorganica Chimica Acta 372 (2011) 321–326
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Investigations on the reactivity of arylboronic acid with phenolic pyrazole
⇑
Pilli V.V.N. Kishore, Ananda Kumar Jami, Viswanathan Baskar
School of Chemistry, University of Hyderabad, Hyderabad 500046, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 4 February 2011
Reaction of phenylboronic acid with phenolic pyrazole was carried out in 1:1 stochiometry using toluene
as a solvent. Depending on the steric bulk of the group present on the pyrazole ring, PhB (HPhPzPh)(OEt)
1, [(PhB)(PhPzHt-Bu)(OH)][(PhB)2(PhPzt-Bu)2(O)] 2 and (PhBPhPz)2 3 were isolated. Compound 3 is an
example of cis-isomer of pyrazabole crystallized in a boat conformation for the B2N4 heterocycle.
Ó 2011 Elsevier B.V. All rights reserved.
Dedicated to Prof. S.S. Krishnamurthy.
Keywords:
Organoboron compounds
Phenolic pyrazoles
Pyrazabole
Structural elucidation
1. Introduction
with symmetrical 4,8-disubstitutions at the boron atom have been
synthesized by the reaction of amine complexes of borane or
Organoboron compounds have been an area of active research
for several decades now due to the innumerable applications of
organoboron compounds in the fields ranging from organic synthe-
sis [1], organometallic and inorganic coordination chemistry [2–9]
to materials chemistry [10]. The coordination chemistry of
poly(pyrazolyl)borates have been investigated in detail and several
reviews have appeared describing the rich coordination chemistry
arising out of these systems [2,11–18]. The ease with which boron
forms coordinate bonds with donor atoms like N and O have been
exploited and a series of interesting organoboron complexes and
macrocycles have been isolated by reaction of arylboronic acids
with salen-type ligands and aminophenols [19–26]. Another inter-
esting class of boron compound is the pyrazabole, a boron nitrogen
containing heterocycle [27,28]. Ever since the isolation of pyraza-
bole was reported in 1966 by Trofimenko along with the poly(pyr-
azolyl)borate ion and their free acids, several pyrazaboles with
substitution both at the boron atoms and the carbon atoms have
been reported [28–34]. Pyrazaboles are relatively more air and
moisture stable compared to other boron-nitrogen heterocycle
and hence a variety of organic transformations have been carried
out resulting in a large number of derivatives bearing various func-
tional groups. Importantly, pyrazaboles have been shown to be
useful as building block for discotic liquid crystals [35], linkers of
ansa ferrocenes to form active container molecules for supramolec-
ular applications [36,37] and for synthesizing luminescent poly-
mers [38,39,10]. Though pyrazaboles mainly crystallize with the
boat conformation for the B2N4 ring, planar and chair like conform-
ers have also been reported [30,32,40,41]. In general, pyrazaboles
organoborane reagents with pyrazole under reflux conditions in
toluene [27,28]. The other synthetic route employed is by function-
alizing the hydrogen coordinated to boron of pyrazabole by other
groups [42,43]. In the case of gem boron disubstituted or asymmet-
rically boron monosubstituted materials, the synthesis is challeng-
ing due to unpredictable reaction of the bis(pyrazolyl)borate salt
with the amine complexes of the boranes bearing good leaving
group [44,45].
On the other hand, phenolic pyrazoles (N2O-type ligand) have
been used as ligands for synthesizing multinuclear transition metal
clusters; some of which possess interesting magnetic properties
[46–51]. But the ligating ability of phenolic pyrazoles (O,N,N-do-
nors) towards main group metals were not known. Recently we
have shown the utility of phenolic pyrazoles as ligands for synthe-
sizing organoantimony oxo clusters by carrying out reactions with
organostibonic acids [52]. Depending on the steric bulk of the
group present in the pyrazole ring, either one or two nitrogen
atoms coordinate to the antimony metal leading to the formation
of novel tetranuclear clusters. Considering the various applications
of organoboron complexes the complexing ability of phenolic pyr-
azoles towards organoboronic acids have been investigated. The
syntheses and structural elucidation of the products are presented
in this manuscript.
2. Experimental
2.1. Reagents and general procedure
Phenylboronic acid, solvents and other common reagents were
purchased from commercial sources. Ligands were synthesized
using literature procedures [53,54].
⇑
Corresponding author. Tel.: +91 40 66794825; fax: +91 40 23012460.
E-mail address: vbsc@uohyd.ernet.in (V. Baskar).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.01.104

322
P.V.V.N. Kishore et al. / Inorganica Chimica Acta 372 (2011) 321–326
2.2. Instrumentation
3. Results and discussion
Infrared spectra were recorded on a JASCO-5300 FT-IR spec-
trometer as KBr pellets. The 1H, 13C, and 11B solution NMR spectra
were recorded on a Bruker DRX 400 instrument. Elemental analysis
were performed on a Flash EA series 1112 CHNS analyzer. Single
crystal X-ray data collection was carried at 100 k on a Bruker smart
General synthetic method used is as follows. Phenylboronic acid
and phenolic pyrazole (H2PhPzR, where R = Ph 1, t-Bu 2, H 3) were
either stirred at room temperature (for 1) or refluxed in toluene for
6 h (2 and 3) until a clear solution was obtained. When reflux con-
dition was employed, the water formed during the reaction was re-
moved as an azeotropic mixture using a Dean Stark apparatus.
Compound 1 dissolved in high polar solvents like ethanol and
methanol whereas 2 and 3 dissolved in a wide range of solvents
like dichloromethane, chloroform and toluene. Compounds 1–3
were characterized using standard spectroscopic and analytical
techniques. IR spectrum of 2 shows the presence of a broad peak
around 3067 cmÀ1 which can be attributed to the presence of OH
group. 11B NMR for 1–3 (in DMSO for 1, CDCl3 for 2 and 3) shows
single resonance at 5.0, 4.8 and 3.6 ppm, respectively, which is in
similar range as reported in literature for analogous organoboron
compounds. The products were crystallized in ethanol (1) and
chloroform (2 and 3) to isolate crystals suitable for single
crystal X-ray diffraction analysis. Crystallographic data and refine-
ment details for 1, 2 and 3 are provided in Table 1. Structural elu-
cidation revealed the formation of PhB(HPhPzPh)(OEt) 1,
[(PhB)(PhPzHt-Bu)(OH)][(PhB)2(PhPzt-Bu)2(O)] 2 and (PhBPhPz)2
3 (Scheme 1).
Apex CCD area detector system (k(Mo Ka) = 0.71073 Å), with a
graphite monochromotor. The data was reduced using SAINT PLUS
and the structure was solved using SHELXS-97 [55] and refined
SHELXL-97 [56]. All non-hydrogens are refined anisotopically.
2.3. Synthesis
2.3.1. Synthesis of 1
Phenylboronic acid (0.15 g, 1.26 mmol), H2PhPzPh (0.30 g,
1.26 mmol) were taken in 50 ml of toluene and the solution was
stirred for 6 h. The white solid which precipitated was filtered
and dissolved in ethanol for crystallization. White crystals suitable
for X-ray studies were grown over night by slow evaporation.
(Yield: 0.40 g, 44%, based on weight of phenylboronic acid). m.p:
260 °C. Anal. Calc. for 1: C23H21O2BN2 : C, 75.01; H, 5.74; N, 7.60.
Found C, 75.12; H, 5.89; N, 7.75%. IR (KBr, cmÀ1) 2964(w),
1614(s), 1574(s), 1485(m), 1460(w), 1431(w), 1319(s), 1267(m),
1167(w), 1107(s), 974(w), 835(w), 748(m), 692(m). 1H NMR
(CDCl3): d 7.65 (t, 3H); 7.39 (q, 6H); 6.9 (d, 5H); 6.86 (d, 2H);
3.68 (q, 2H); 1.14 (t, 3H). 13C NMR (CDCl3): d 146.89; 145.82;
133.23; 131.74; 131.40; 130.72; 130.06; 129.49; 128.98; 127.87;
127.38; 126.73; 125.73; 125.15; 119.44; 118.78; 97.54 .11B NMR
(DMSO): d 5.0 ppm.
Compound 1 crystallizes in monoclinic space group P2(1)/n
(Fig. 1). The ligand H2PhPzPh binds to the boron atom through
Table 1
Crystal data and structure refinement details for 1, 2 and 3.
2.3.2. Synthesis of 2
1
2
3
Phenylboronic acid (0.20 g, 1.64 mmol), H2PhPztBu (0.35 g,
1.64 mmol) were taken in toluene (50 ml). The solution was re-
fluxed for 6 h using a Dean stark apparatus to remove the water
eliminated in the reaction as an azeotropic mixture. The clear solu-
tion then formed was cooled to room temperature, filtered and
evaporated under reduced pressure to yield a colorless solid. Crys-
tals suitable for X-ray studies were grown from chloroform solu-
tion by direct evaporation after a week time. (Yield: 0.45 g,
29.6%, based on the weight of phenylboronic acid). m.p: 192 °C.
Anal. Calc. for 2: C57H61N6B3O5 : C, 72.63; H, 6.52; N, 8.91. Found
C, 72.45; H, 6.59; N, 9.03%. IR (KBr, cmÀ1): 3460(w), 3136(w),
2964(w), 1616(s), 1564(m), 1491(s), 1369(m), 1311(m), 1259(w),
1192(m), 1105(m), 951(m), 856(w), 810(w), 754(s), 704(s),
667(w), 553(w). 1H NMR (CDCl3): d 7.32 (d, 1H); 7.17 (d, 5H);
7.06 (d, 2H); 6.98 (d, 1H); 6.73 (t, 3H); 0.92 (s, 9H). 13C NMR
Formula
Formula
weight
Crystal system
Crystal size
(mm)
Space group
a (Å)
b (Å)
C
23H21BN2O2
C59H63B3Cl6N6O5
1181.28
C30H22B2N4O2
492.14
368.23
monoclinic
orthorhombic
monoclinic
0.22 Â 0.16 Â 0.10 0.26 Â 0.18 Â 0.14 0.24 Â 0.18 Â 0.12
P2(1)/n
12.248(9)
11.880(9)
12.844(9)
90
95.452(10)
90
1860.6(2)
4
1.315
100(2)
0.084
776
pbca
P2(1)/n
17.970(17)
24.943(2)
26.735(2)
90
90
90
11983.5(17)
8
1.310
100(2)
0.340
12.302(7)
10.243(6)
19.898(12)
90
104.812(10)
90
2424.1(2)
4
1.348
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (mg mÀ3
T (K)
)
100(2)
0.085
1024
l
(mmÀ1
)
(CDCl3):
d 157.60; 155.95; 144.86; 134.22; 132.21; 130.95;
F(0 0 0)
4928
127.58; 126.96; 126.70; 125.00; 119.72; 118.59; 114.31; 96.64;
h (°)
Index ranges
2.20–24.99
À14 6 h 6 14
À14 6 k 6 14
À15 6 l 6 15
16 900
1.52–25.03
À21 6 h 6 21
À29 6 k 6 29
À31 6 l 6 31
108 902
1.77–25.09
À14 6 h 6 14
À12 6 k 6 12
À23 6 l 6 23
22 718
31.48, 30.94, 29.47. 11B NMR (CDCl3): d 4.8 ppm.
Number of
reflections
collected
Completeness
to hmax (%)
Number of
independent
reflections
Goodness-of-fit 1.076
(GOF)
2.3.3. Synthesis of 3
Synthetic procedure is similar to 2. Amount and stochiometry of
the reactants taken are phenylboronic acid (0.15 g, 1.24 mmol),
H2PhPzH (0.20 g, 1.24 mmol). (Yield: 0.25 g 41%, based on the
weight of phenylboronic acid). m.p: 270 °C. Anal. Calc. for 3:
99.9
99.2
99.8
3266
10 513
4308
C30H22N4B2O2 : C, 73.21; H, 4.50; N, 11.38. Found: C, 73.11; H,
4.55; N, 11.45%. IR (KBr, cmÀ1): 3117(w), 3067(w), 2964(w),
1614(s), 1574(s), 1537(m), 1502(m), 1448(m), 1412(m), 1365(w),
1313(s), 1249(s), 1205(m), 1124(w), 1051(m), 908(w), 856(w),
835(w), 744(m), 692(m), 646(m), 488(w). 1H NMR (CDCl3): d 8.28
(d, 2H); 7.60 (d, 2H); 7.27 (t, 2H); 7.02 (d, 2H); 6.92 (m, 4H);
6.78 (d, 6H); 6.70 (d, 2H). 13C NMR (CDCl3): d 153.65; 143.39;
134.58; 131.73; 130.79; 126.87; 126.81; 125.27; 120.06; 119.65;
115.69; 101.19. 11B NMR (CDCl3): d 3.6 ppm.
1.112
1.004
R1 (F) (I > 2
wR2
Largest
difference
r
(I)) 0.0556
0.0984
0.1971
0.0364
0.0912
0.1211
0.228 and À0.202 0.731 and À0.553 0. 247 and À0.239
in peak and
hole (e ÅÀ3
)

P.V.V.N. Kishore et al. / Inorganica Chimica Acta 372 (2011) 321–326
323
H2PhPzPh
crystallised from EtOH
1a
O
N
O
N
H
B
O
N
N
H
B
H
N
O
HO
H2PhPzt-Bu
1b
B
PhB(OH)2
O
B
N
N
H
N
O
R
OH
HN
N
1a : R =Ph
H2PhPzR
1b: R = t-Bu
1c : R = H
O
N
N
N
Ligand
H2PhPzH
1c
B
B
O
N
Scheme 1.
the pyrazole N atom and the phenolic oxygen while the other
nitrogen of the pyrazole group bearing the hydrogen atom remains
a spectator. The remaining two coordinations of the tetrahedral
boron atom come from the phenyl carbon and oxygen of ethoxy
group. Since ethanol was used for crystallization, the unreacted
OH group from the phenylboronic acid is present as the ethoxy
derivative in 1. The crude product was not characterized by 1H
NMR as they were not isolated, but the crystals formed from etha-
nol shows the peaks corresponding to the ethyl groups present in 1
(Section 2). Selected bond lengths (Å) and angles (deg) for 1 are gi-
ven in the figure caption 1. The B–O bond distances in 1 are
1.477(3) Å (B1–O1) and 1.459(3) Å (B1–O2). The phenolic oxygen
to boron is slightly longer than the B–Oethoxy group. The B–N bond
distance is 1.590(3) Å, which falls in the range as reported in liter-
ature for tetrahedral boron atoms containing B–N bonds [57]. The
% THC [58] of boron in 1 was found to be 99.73 which confirms that
the boron atom is in a perfect tetrahedral site. It is of interest to
mention here that monometallic boron compounds are a rarity
when compared to the other elements of group 13 like aluminum,
gallium and indium which readily forms monometallic complexes
even with flexible salen-type N2O2 ligands. The isolation of a
monometallic near perfect tetrahedral organoboron complex is
probably due to the chelating O,N-donor atoms of the phenolic
pyrazole.
Fig. 1. ORTEP view of 1. Thermal ellipsoids have been set at the 30% probability
level. Selected bond lengths (in Å) and bond angles (in deg) in 1: B1–O1 = 1.477(3),
B1–O2 = 1.459(3), B1–N1 = 1.590(3), B1–C16 = 1.600(4), N1–N2 = 1.348(2), O1–B1–
C16 = 111.42(19), O1–B1–N1 = 104.76(18), O2–B1–N1 = 109.40(18), O1–B1–
O2 = 110.05(19), O2–B1–C16 = 109.27(18), B1–N1–N2 = 124.40(17).

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P.V.V.N. Kishore et al. / Inorganica Chimica Acta 372 (2011) 321–326
Organoboron compound 2 crystallizes in orthorhombic space
the B–OH group combines with a similar moiety of a neighboring
boron atom forming an oxo bridged dimer eliminating a water
molecule in the process. Similar observations have been reported
earlier for organoboron compounds involving other ligand systems
wherein crystallization from chlorinated solvents or acetonitrile
have led to the formation of oxo-bridged dimers. Moreover, in
the crystal lattice of 2, a organoboron phenolic pyrazolyl complex
similar to 1 co-crystallizes with the dimer. Hence we propose that
a monometallic compound is formed initially which subsequently
dimerises by water elimination leading to the formation of an oxo
bridged dimer. The dimer and monomer are held together by
hydrogen bonds. Selected bond lengths and angles for 2 are given
in the figure caption 2. The B–O distances are similar to that of 1.
The phenolic oxygen to the boron atom bond distances are slightly
group pbca (Fig. 2). Despite several attempts the structure
refinement for 2 was poor due to the inferior quality of the crystals
obtained. However the data was sufficiently good to arrive at the
core structure. The asymmetric unit consists of an oxo bridged bor-
on dimer and a organoboron monomer. The mode of binding of the
ligand H2PhPzt-Bu in 2 is similar to that of H2PhPzPh in 1. The only
difference observed is that the fourth coordination of boron in 1
was from an ethoxy group whereas in 2 it is from a
l2-bridging
oxo group. Since the compound was crystallized from chloroform,
longer than the boron to oxygen atoms of the
the terminal OH groups. The B–O (phenolic) distances fall in the
range 1.493(6) to 1.504(6) Å; the B–O 2-bridging) being
l2-bridging oxo and
(
l
1.427(6) and 1.446(6) Å and B–O (terminal OH) distance being
1.439(6) Å, respectively. The B–N bond distances fall in the range
1.591(6) to 1.612(7) Å, which is in the range as reported in litera-
ture for monometallic tetrahedral organoboron compounds con-
taining B–N bonds indicating the coordinative nature of the bond
[57]. The % THC of boron in 2 are 99.11 (B1), 98.67 (B2) and
99.50 (B3) which confirms that the boron atoms in 2 are also in a
near perfect tetrahedral site. The B2–O2–B3 bond angle is
128.6(4)° which again falls in the range as reported earlier in liter-
ature for oxo-bridged boron dimmers [23].
Compound 3 crystallizes in monoclinic space group P2(1)/n
(Fig. 3). Structural elucidation reveals the stabilization of the
B2N4 heterocycle, pyrazabole. The ligand chelates to the boron
atom through the O,N-end while the other nitrogen of the pyrazole
coordinates to the second boron atom and subsequent dimeriza-
tion leads to the formation of the cis-pyrabazole with the boat
conformation for the B2N4 ring. The B–O distances for B1–O1 and
B2–O2 are 1.458(2) and 1.449(2) Å, respectively, and the B–N dis-
tances fall in the range 1.570(1) to 1.575(2) Å. Selected bond
lengths and angles for 3 are given in the figure caption 3. The iso-
lation of pyrazabole by reaction of arylboronic acid with phenolic
pyrazole is interesting since it provides an easy access to this het-
erocycle bearing substitutions at both the boron and carbon atoms.
Compared to earlier reports where pyrazabole has been synthe-
sized either by reactions of the highly reactive amine complexes
Fig. 2. ORTEP view of 2. Thermal ellipsoids have been set at the 30% probability
level. Selected bond lengths (in Å) and bond angles (in deg) in 2: B1–O1 = 1.493(6),
B1–O5 = 1.439(6), B1–N1 = 1.612(7), B1–C14 = 1.615(7), B2–O2 = 1.446(6), B2–O3 =
1.495(6), B2–N3 = 1.591(6), B2–C33 = 1.622(7), B3–O2 = 1.427(6), B3–O4 = 1.504(6),
B3–N5 = 1.591(6), B3–C52 = 1.628(7), O1–B1–N1 = 102.2(4), O5–B1–N1 = 110.6(4),
O5–B1–C14 = 114.2(4), O1–B1–C14 = 110.6(4), O1–B1–O5 = 109.3(4), O2–B2–O3 =
105.2(4), O3–B2–N3 = 103.6(3), N3–B2–C33 = 108.9(4), O2–B2–C33 = 110.5(4),
C33–B2–O3 = 110.5(4), O2–B3–O4 = 110.9(4), O4–B3–N5 = 104.0(3), N5–B3–
C52 = 108.5(3), O2–B3–C52 = 112.5(4), N5–B3–O2 = 109.6(4), B2–O2–B3 = 128.6(4).
Fig. 3. ORTEP view of 3. Thermal ellipsoids have been set at the 30% probability level. Selected bond lengths (in Å) and bond angles (in deg) in 3a and 3b are O1–
B1 = 1.458(18), B1–N1 = 1.575(18), B1–N4 = 1.570(18), B1–C19 = 1.609(2), O2–B2 = 1.449(17), B2–N2 = 1.570(18), B2–N3 = 1.571(17), B2–C25 = 1.607(2), O1–B1–
N4 = 108.06(11), N4–B1–C19 = 112.11(11), O1–B1–C19 = 113.54(11), O1–B1–N4 = 108.06(11), O2–B2–N2 = 108.51(11), O2–B2–N3 = 107.23(10), N2–B2–N3 = 102.51(10),
O2–B2–C25 = 112.52(11), N2–B2–C25 = 111.31(11), N3–B2–C25 = 114.15(11), N1–B1–C19 = 113.16(11).

P.V.V.N. Kishore et al. / Inorganica Chimica Acta 372 (2011) 321–326
325
of borane or air and moisture sensitive organoborane reagents with
References
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Acknowledgements
V.B. thanks DST for funding under the SERC-Fast track Scheme.
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