-B(OH)2 versus -OH in supramolecular synthesis: Molecular complexes of 4-hydroxyphenylboronic acid with aza-donor compounds

Article abstract of DOI:10.1016/j.tetlet.2010.10.126

-B(OH)₂ moiety forms interactions with aza-donor compounds as much as -OH does, as derived from the supramolecular assemblies of 4-hydroxyphenylboronic acid with aza-donor compounds demonstrating for the first time, a comparative study of -B(OH)₂ versus -OH.

Full text of DOI:10.1016/j.tetlet.2010.10.126

Tetrahedron Letters 51 (2010) 6901–6905
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
–B(OH)2 versus –OH in supramolecular synthesis: molecular complexes
of 4-hydroxyphenylboronic acid with aza-donor compounds
⇑
Mayura Talwelkar, V. R. Pedireddi
School of Basic Sciences, Indian Institute of Technology Bhubaneswar, Bhubaneswar 751 013, India
a r t i c l e i n f o
a b s t r a c t
Article history:
–B(OH)2 moiety forms interactions with aza-donor compounds as much as –OH does, as derived from the
supramolecular assemblies of 4-hydroxyphenylboronic acid with aza-donor compounds demonstrating
for the first time, a comparative study of –B(OH)2 versus –OH.
Received 28 September 2010
Revised 18 October 2010
Accepted 24 October 2010
Available online 30 October 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Boronic acid
Co-crystallization
Molecular recognition
Hydrogen bonds
In supramolecular synthesis, recognition between the compli-
mentary functional groups is a key factor for the evaluation of
influence of noncovalent interactions in the formation of specific
architectures.1–4 In this process, in recent times, boronic acids,
with general formula R–B(OH)2, have been well considered to be
of potential co-crystal formers,5,6 as much as other well known or-
ganic entities with functional groups like –COOH, –CONH2, –OH,
etc.,7–9 especially due to the ability of the –B(OH)2 functionality
H
B
H
H
B
O
O
H
O
O
O
N
O
O
H
O
N
O
H
O
H
syn-anti
B
H
anti-anti
B
syn-syn
H
H
O
O
H
O
O
H
H
H
(d)
(e)
(a)
(c)
(b)
Scheme 1.
reactants products
1 + (a) 1a
N
N
HO
B(OH)2
N
N
4-hydroxyphenylboronic acid (1)
1,10-phenanthroline (b) 1 + (b) 1b
1 + (c) 1c
phenazine (a)
N
N
1 + (d) 1d
N
N
N
N
1 + (e) 1e
&
4,4'-bipyridine (d)
1e'
4,7-phenanthroline (c)
1,2-bis(4-pyridyl)ethene (e)
Chart 1.
⇑
Corresponding author. Tel.: +91 0674 2306235; fax: +91 0674 2301983.
E-mail address: vr.pedireddi@iitbbs.ac.in (V.R. Pedireddi).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.10.126

6902
M. Talwelkar, V. R. Pedireddi / Tetrahedron Letters 51 (2010) 6901–6905
to form a variety of hydrogen bonds through different conforma-
tions, identified as syn–anti, syn–syn, and anti–anti, as depicted in
Scheme 1.
Also, competitive nature of –B(OH)2, in the presence of func-
tional groups like –COOH, was well explored.6a–c However, studies
related to the efficacy of –B(OH)2 versus –OH, in supramolecular
synthesis are apparently not known although –B(OH)2 is known
to have affinity toward pyridyl N-atoms, as similar to –OH moiety,
with the formation of O–Hꢀ ꢀ ꢀN hydrogen bonds, as observed in
some co-crystals of boronic acids with aza compounds.5a,10 Thus,
we considered co-crystallization studies of 4-hydroxyphenylbo-
ronic acid, which have both –OH and –B(OH)2 moieties, with some
aza-donor compounds, as listed in Chart 1, having pyridyl N-atoms
at different positions and we report, herein, the salient features of
these structural assemblies.
Co-crystallization of 4-hydroxyphenylboronic acid, 1, with
phenazine gave a 1:1 complex, 1a,11 in the form of a layered struc-
ture. Within a typical layer, both the co-crystal formers interact
with each other through O–Hꢀ ꢀ ꢀN hydrogen bonds (Hꢀ ꢀ ꢀN, 1.81
Å), established between –OH group of 1 and pyridyl N-atoms of
phenazine, which is further complimented by C–Hꢀ ꢀ ꢀN (Hꢀ ꢀ ꢀN,
2.70 Å) hydrogen bonds, yielding a quartet network, as shown in
Figure 1a. Thus, –B(OH)2 moiety did not interact directly with
phenazine, but formed O–Hꢀ ꢀ ꢀO (Hꢀ ꢀ ꢀO, 1.91 Å) hydrogen bonds,
as shown in Figure 1b, connecting the adjacent ensembles of quar-
tets shown in Figure 1a, in the three dimensional packing (see the
Fig. 1c).
Nevertheless, in the co-crystals of 1 with 1,10-phenanthro-
line,11 the complimentary molecules recognize each other through
the formation of O–Hꢀ ꢀ ꢀN hydrogen bonds (Hꢀ ꢀ ꢀN, 1.92 Å) exclu-
sively formed by –B(OH)2 moiety with hetero N–atom of phenan-
throline. Indeed, the boronic acid, 1 adopts syn–syn conformation
and glue to both the N-atoms on phenanthroline (Fig. 2a).
Further, as observed for –B(OH)2 in complex 1a, the –OH group
in complex 1b is involved in the formation of O–Hꢀ ꢀ ꢀO (Hꢀ ꢀ ꢀO, 1.89
Å) hydrogen bonds that are involved in the aggregation of the
ensembles as shown in Figure 2b in the form of crinkled tapes,
which, in turn, are packed in three dimensional arrangement such
that phenanthroline molecules are sandwiched between the layers
of molecule 1.
Figure 1. (a) Recognition pattern between the co-crystal formers 4-hydrox-
yphenylboronic acid and phenazine in 1a; (b) O–Hꢀ ꢀ ꢀO hydrogen bonds formed
by –B(OH)2 moieties; (c) packing of molecules in three dimensional arrangement in
the complex 1a.
Figure 2. (a) Recognition pattern observed between 1,10-phenanthroline and the
molecules of 1, in the complex 1b; (b) infinite tapes of boronic acid 1 held together
by O–Hꢀ ꢀ ꢀO hydrogen bonds in the co-crystals of 1b; (b) Sandwich structure in the
crystal lattice of complex, 1b.
Figure 3. (a) Recognition pattern observed between, the boronic acid, 1 and 4,7-
phenanthroline in the co-crystal, 1c; (b) intermolecular interactions formed by –
B(OH)2 with the neighboring molecules; (c) packing of molecules in crystal lattice.

M. Talwelkar, V. R. Pedireddi / Tetrahedron Letters 51 (2010) 6901–6905
6903
Thus, it is apparent that –B(OH)2 and –OH on 1 show exactly
opposite trends in the formation of complexes of 1 with phenazine
and 1,10-phenanthroline. In contrast, in the complex of 1 and 4,7-
phenanthroline (an analog of 1,10-phenanthroline with just chang-
ing positions of N-atoms), 1c, both –OH and –B(OH)2 are involved
in the formation of recognition patterns with the aza-donor
molecules.
Surprisingly, in co-crystals, 1d,11 of 4,40-bipyridine (bpy) with 1,
an intriguing asymmetric unit is observed with a total of fifteen
molecules comprising of three symmetry independent units of
co-crystal formers in 3:2 ratio. It is noteworthy to mention that
molecules of 1 exist in syn–syn as well as syn–anti conformations.
Interestingly, three distinct ensembles of six molecular units, with
each being formed by association of each of three molecules of aza
and 1, are held together by O–Hꢀ ꢀ ꢀN and C–Hꢀ ꢀ ꢀO hydrogen bonds
(Fig. 4a). Complete characteristics of the hydrogen bonds are listed
in Table S1, ESI . In a detailed analysis of recognition features, it is
apparent that in each ensemble both the conformations of –B(OH)2
and –OH moiety show affinity toward bpy simultaneously. For
example, –B(OH)2 in syn–syn conformation forms two O–Hꢀ ꢀ ꢀN
hydrogen bonds with two bpy molecules while –B(OH)2 in syn–anti
conformation forms one O–Hꢀ ꢀ ꢀN hydrogen bond with another bpy
molecule. Similarly –OH moiety interacts with those aza-donor
molecules hold by 1 through –B(OH)2, through single O–Hꢀ ꢀ ꢀN
hydrogen bond. Ultimately, the ensembles are held together by
the O–Hꢀ ꢀ ꢀO and C–H. . .O hydrogen bonds formed between –
B(OH)2 and –OH moieties, leading to the formation of a ladder
structure, as shown in Figure 4b.
In complex, 1c,11 although recognition between the constitu-
ents is through O–Hꢀ ꢀ ꢀN hydrogen bonds, as noted in 1a and 1b,
but they are formed due to the interaction of both –B(OH)2
(Hꢀ ꢀ ꢀN, 1.97 Å) and –OH (Hꢀ ꢀ ꢀN, 1.74 Å) with the aza-donor com-
pounds, as shown in Figure 3a. Further, –B(OH)2 exist in syn–anti
conformation as observed in 1a. Such ensembles, however, further
interact with each other through O–Hꢀ ꢀ ꢀO hydrogen bonds (Hꢀ ꢀ ꢀO,
1.96 Å), formed between –B(OH)2 and –OH moieties (Fig. 3b), ex-
actly as observed in 1a and 1b. Such an aggregation ultimately
packs, in the crystal lattice, in the form of ladders, with rods being
formed by 1 while phenanthroline molecules act as rungs (Fig. 3c).
It is apparent from the structural analysis of 1a–1c that, both –
B(OH)2 and –OH show predominance in the formation of a specific
assembly, depending upon the position of N-atoms on the aza-do-
nors. Since, only the rigid aza-donors have been used in 1a–1c, the
study is extended to other pyridyl ligands with conformational
flexibility, to explore its impact on the affinity of –B(OH)2 and –
OH toward the aza-donors. Thus, co-crystallization experiments
of 1, carried out with 4,40-bipyridine and 1,2-bis(4-pyridyl)ethene,
are discussed, herein.
However, an exciting feature is observed in the co-crystals of 1
with 1,2-bis(4-pyridyl)ethene, as two different types of crystals
were obtained, depending upon the composition of the co-crystal
formers in the reaction mixture. The crystals, thus, obtained are la-
beled 1e and 1e’ for the complexes of 1:2 and 1:1, respectively.11
Structure determination reveals that both the complexes are
Figure 4. (a) Recognition pattern of –B(OH)2 and –OH moieties with bpy molecules, in the complex 1d. (b) Arrangement of molecules in the form of ladder-type architecture
in complex, 1d.

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M. Talwelkar, V. R. Pedireddi / Tetrahedron Letters 51 (2010) 6901–6905
Figure 5. Network of intermolecular interactions formed by –B(OH)2 and –OH in (a) 1e and (b) 1e0.
hydrates.12 –B(OH)2 moiety adopts syn–syn and syn–anti confor-
mations in 1e and 1e0, respectively. Thus, as observed in 1b, the
syn–syn mediated –B(OH)2 moiety in 1e directly interacts with
the aza-donor compounds through O–Hꢀ ꢀ ꢀN (Hꢀ ꢀ ꢀN, 2.06, 2.01 Å)
hydrogen bonds. However, the –OH moiety is involved in the for-
mation of O–Hꢀ ꢀ ꢀO (Hꢀ ꢀ ꢀO, 2.34 Å) hydrogen bonds with water
molecules, which in turn interact with aza-donor molecules that
are being connected to the molecules of 1 through –B(OH)2 moiety.
Similarly, in the complex 1e0, wherein the –B(OH)2 moiety
shows syn–anti conformation, the –OH moiety is connected to
aza-donor through O–Hꢀ ꢀ ꢀN (Hꢀ ꢀ ꢀN, 1.83 Å) hydrogen bond, while
–B(OH)2 moiety establish interaction with water molecule, which
are further connected to the aza-donors as shown in Figure 5b.
It is noteworthy to mention that the involvement of water with
–OH and –B(OH)2 in 1e and 1e0, respectively, as shown in Figure 5,
clearly demonstrates the unique role being played by –B(OH)2 and
–OH, independent of each other, in the formation of molecular
complexes 1e and 1e0. Thus, 1e and 1e0 may be grouped along with
1b and 1a, respectively.
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In conclusion, the study of assemblies, 1a–1e and 1e0 reveals
that –B(OH)2 influence the formation of distinct co-crystals, as
much as –OH does, despite the fact that the hydrogen bonds
formed by –OH are relatively stronger than the ones being formed
by –B(OH)2. In fact, formation of different molecular complexes, 1e
and 1e0 demonstrate the real competitive nature of the –B(OH)2.
Thus, utilization of –B(OH)2 in the supramolecular synthesis does
not limit to the mimic of –COOH and –CONH2 but also can be em-
ployed as equivalent of –OH moiety as well.
Acknowledgment
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We thank the Department of Science and Technology (DST) for
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.10.126.
References and notes
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M. Talwelkar, V. R. Pedireddi / Tetrahedron Letters 51 (2010) 6901–6905
Cryst. Growth Des. 2008, 8, 4533–4545; (i) Biradha, K.; Zaworotko, M. J. J. Am. wR2 = 0.0738 for 2858 reflections (I > 2
6905
r
(I)), CCDC 742752. Compound (1d)
ꢀ
Chem. Soc. 1998, 120, 6431–6432; (j) Papaefstathiou, G. S.; MacGillivray, L. R.
Org. Lett. 2001, 3, 3835–3837.
C
42H38B2N6O6, P1, a = 13.960(9), b = 17.186(11), c = 25.222(16) Å,
b = 100.65(1),
= 111.18(1)°, V = 5503.0(6) Å3, Z = 2, calcd = 1.346 g cmꢁ3
= 0.091 mmꢁ1
2hmax = 51.20, k(Mo ) = 0.71073 Å, T = 298(2) K, 53346
total reflections, R1 = 0.0962 and wR2 = 0.2674 for 19893 reflections
(I > 2 (I)), CCDC 742751. Compound (1e) C30H31BN4O5, orthorhombic, Pbca,
a = 14.153(5), b = 18.023(6), c = 22.549(6) Å, Z = 8,
V = 5751.0(3) Å3,
calcd = 1.224 g cmꢁ3 = 0.085 mmꢁ1
2hmax = 46.52, k(Mo ) = 0.71073 Å,
T = 293(2) K, 11295 total reflections, R1 = 0.1631 and wR2 = 0.2348 for 4066
a = 92.64(1),
c
q
,
10. (a) Shivachev, B.; Petrova, R.; Naydenova, E. Acta Crystallogr., Sect. E 2006, 62,
o3887–o3889; (b) Aakeröy, C. B.; Desper, J.; Levin, B.; Salmon, D. J. Trans. Am.
Crystallogr. Assoc. 2004, 39, 123.
l
,
Ka
r
11. Crystal data (1a) C18H15BN2O3, monoclinic, P21/n, a = 8.405(5), b = 23.225(12),
c = 9.102(5) Å, b = 115.18(9)°, V = 1607.9(2) Å3, Z = 4,
q
calcd = 1.341 g cmꢁ3
,
q
,
l
,
Ka
l
= 0.089 mmꢁ1, 2hmax = 56.84, k(Mo K
reflections R1 = 0.0613, and wR2 = 0.1386 for 3699 reflections (I > 2
742753. Compound (1b) 18H15BN2O3, monoclinic, P21/c, a = 12.435(3),
b = 10.941(3), c = 12.177(3) Å, b = 108.18(1)°, V = 1574.0(7) Å3, Z = 4,
calcd = 1.342 g cmꢁ3 = 0.091 mmꢁ1
2hmax = 50.04, k(Mo ) = 0.71073 Å,
T = 293(2) K, 10432 total reflections R1 = 0.0525 and wR2 = 0.1648 for 2772
reflections (I > 2 (I)), CCDC 742750. Compound (1c) C18H15BN2O3, P21/n,
a = 7.229(1), b = 20.718(4), c = 11.036(2) Å, b = 107.58(5)°, V = 1575.7(5) Å3,
Z = 4, 2hmax = 55.00, k(Mo
calcd = 1.341 g cmꢁ3 = 0.091 mmꢁ1
) = 0.71073 Å, T = 293(2) K, 11590 total reflections, R1 = 0.0660 and
a
) = 0.71073 Å, T = 273(2) K, 9414 total
ꢀ
r(I)), CCDC
reflections (I > 2
a = 9.269(6), b = 9.322(7), c = 11.591(8) Å,
= 112.48(2)°, V = 887.2(1) Å3, Z = 2, calcd = 1.266 g cmꢁ3
2hmax = 50.56, k(Mo ) = 0.71073 Å, T = 293(2) K, 6675 total reflections,
r
(I)), CCDC 745865. Compound (1e0) C18H18BN2O4, triclinic, P1,
= 90.80(2), b = 104.90(2),
= 0.089 mmꢁ1
C
a
c
q
,
l
,
q
,
l
,
Ka
Ka
R1 = 0.1642 and wR2 = 0.3867 for 3194 reflections (I > 2r(I)), CCDC 745866.
r
12. The R-factors for 1e and 1e0 are found to be higher than the other structures,
1a–1d, but the authenticity of the homogeneity is established by powder
patterns as given in Supplementary data (Fig. S1, ESI )
q
,
l
,
Ka