Supramolecular hydrogen-bonded structure of a 1:2 adduct of melamine with boric acid

Article abstract of DOI:10.1016/S0022-2860(02)00128-X

Boric acid (B) forms a 2:1 hydrogen-bond adduct, M.2B, with melamine (M), wherein, a finite tetrameric ribbon of boric acid forms hydrogen bonds with the nitrogen donor sites of M. Hydrogen-bonded layers of B and M in the bc-plane are connected by boric acid units along the a-axis to give a three dimensional structure. Interestingly, one of the amino groups of M does not participate in hydrogen bonding unlike all the hydroxyl groups of B.

Full text of DOI:10.1016/S0022-2860(02)00128-X

Journal of Molecular Structure 613 (2002) 61–66
www.elsevier.com/locate/molstruc
Supramolecular hydrogen-bonded structure of a 1:2 adduct of
melamine with boric acid
Abhijit Roya, Amitava Choudhurya,b, C.N.R. Raoa,b,
*
aChemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur PO, Bangalore 560 064, India
bSolid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India
Received 24 January 2002; revised 28 February 2002; accepted 28 February 2002
Abstract
Boric acid (B) forms a 2:1 hydrogen-bond adduct, M·2B, with melamine (M), wherein, a finite tetrameric ribbon of boric acid
forms hydrogen bonds with the nitrogen donor sites of M. Hydrogen-bonded layers of B and M in the bc-plane are connected by
boric acid units along the a-axis to give a three dimensional structure. Interestingly, one of the amino groups of M does not
participate in hydrogen bonding unlike all the hydroxyl groups of B. q 2002 Elsevier Science B.V. All rights reserved.
Keywords: Boric acid; Melamine; Hydrogen bonding; Supramolecular chemistry; Hydrothermal synthesis
1. Introduction
interesting supramolecular hydrogen-bonded archi-
tectures involving tetrahydroxy pentaborate and boric
Boric acid, B(OH)3, would be expected to be a
good building block for supramolecular hydrogen
bonded structures by virtue of the presence of three –
OH groups oriented in a planar triangular fashion.
Boric acid is known to behave as a lewis acid with the
formation of the tetrahedral anion [B(OH)4]2 rather
than as a Brønsted acid with the formation of the
conjugate base anion [BO(OH)2]2. In dilute aqua
solution B(OH)3 exist as an equilibrated mixture of
the undissociated acid and [B(OH)4]2 anion. At
higher concentrations monomeric borate species
undergo condensation to give a variety of oligomeric
hydroxo (oxo) borate anions [1]. Accordingly, several
acid incorporating
a tertiary or quarternary
ammonium ion have been isolated and characterized
[2–5]. Hydrogen-bonded structures involving only
monomeric borate species is, however rare. In (CH3)4-
Nþ(BO(OH)2)2·2(NH2)2CO·H2O and (NEt4)2[-
BO(OH)2]2·B(OH)3·5H2O, [BO(OH)2]2 is present in
the host lattice [6,7]. [(PPh3)2Nþ·Cl2]·B(OH)3 and
[(C2H5)4Nþ]2CO322·(NH2)2CO·2B(OH)3·H2O seems
to be the only two structures known to date containing
the undissociated B(OH)3 unit [7,8]. We wanted to
study supramolecular hydrogen-bonded assemblies
formed by boric acid with organic amines and such
compounds (Scheme 1) under aqueous conditions. We
have found that melamine (M) which is well known to
yield excellent hydrogen-bonded structures also forms
a stable 1:2 adduct with boric acid (M·2B). Impor-
tantly, this adduct is a precursor for making BN
nanotubes [9–11]. We have carried out a study of
*
Corresponding author. Address: Chemistry and Physics of
Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific
Research, Jakkur PO, Bangalore 560 064, India. Tel.: þ91-80-846-
2762; fax: þ91-80-846-2766.
E-mail address: cnrrao@jncasr.ac.in (C.N.R. Rao).
0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(02)00128-X

62
A. Roy et al. / Journal of Molecular Structure 613 (2002) 61–66
Table 1
Crystal data and structural refinement parameters for M·2B
Empirical formula
Crystal system
Space group
C3H12B2N6O6
Monoclinic
P21/c
Crystal size (mm)
˚
a (A)
˚
b (A)
0.33 £ 0.10 £ 0.08
3.6015 (2)
20.1400 (13)
14.1279 (9)
90.0
˚
c (A)
Scheme 1.
a (8)
b (8)
92.134 (2)
90.0
M·2B prepared by the hydrothermal technique, to
examine the features of this supramolecularly hydro-
gen-bonded structure.
g (8)
Volume (A )
3
˚
1024.05 (11)
4
Z
Formula mass
r (g cm23
249.81
)
1.620
˚
l (Mo Ka) (A)
m (mm21
0.71073
2. Experimental
)
0.144
Temperature of
measurement (K)
u range (8)
293 (2)
0.1295 g of H3BO3 was dissolved in 2 ml
distilled water. To this solution, 0.1317 g of
melamine was added with stirring. The stirring
was continued for another 30 min to get a
homogenous solution. The reaction mixture with
a composition of 2H3BO3/melamine/100H2O was
then sealed in a 23 ml capacity teflon-lined stainless
steel autoclave and heated at 150 8C for 2 d. It was
subjected to natural cooling (,4 h). The product
containing rod-shaped crystals suitable for single
crystal X-ray diffraction was filtered and washed with
chilled de-ionized water. The initial pH of the mixture
was seven and there was no appreciable change in the
pH of the supernatant in the final mixture. Heating the
reaction mixture for a longer time (3 d) makes the
supernatant highly alkaline (pH , 10.5), probably
due to the evolution of the ammonia gas which is
produced due to the decomposition of melamine
under hydrothermal condition as indicated by the
smell of ammonia upon opening the autoclave. The
product was subsequently characterized by powder X-
ray diffraction (PXRD), single crystal X-ray diffrac-
tion, thermogravimetric analysis (TGA) and IR
spectroscopy. The PXRD immediately revealed that
the product is a 1:2 (M·2B) adduct of melamine and
boric acid [9]. Our attempts to synthesize 1:1 adduct
under similar condition led to recrystallization of
melamine. On the other hand attempts to synthesize a
composition of 1:3 under similar condition gives back
the 1:2 adduct of melamine and boric acid.
1.76–23.32
Total data collected
Index ranges
4170
23 # h # 3,
220 # k # 22,
211 # l # 15
1446
Unique data
Observed data [I . 2s(I )]
Rint
923
0.0529
Refinement method
R indices [I . 2s(I )]
R (all data)
Full-matrix least-squares on F 2
R1 ¼ 0.0490, wR2 ¼ 0.1014a
R1 ¼ 0.0910, wR2 ¼ 0.1214
1.004
Goodness of fit (S)
Number of variables
Largest difference map peak
202
0.186 and 20.286
23
˚
and hole e A
a
R1 ¼ kF0l2lFck=lF0l; wR2 ¼ {½wðF022Fc Þ ꢀ=½wðF0 Þ ꢀ}1=2
;
2
2
2 2
w ¼ 1=½s2ðF0Þ þ ðaPÞ þ bPꢀ;
P ¼ ½maxðF02; 0Þ þ 2ðFcÞ ꢀ=3;
2
2
2
where a ¼ 0.0596 and b ¼ 0.0000 for m.2B.
selected under a polarizing microscope and glued to a
thin glass fiber with cyanoacrylate (super glue)
adhesive. Single crystal structure determination by
X-ray diffraction was performed with a Siemens
Smart-CCD diffractometer equipped with a normal
focus, 2.4 kW sealed tube X-ray source (Mo Ka
ꢀ
radiation, l ¼ 0:71073 A) operating at 50 kV and
40 mA. A hemisphere of intensity data were collected
using SMART software [12] at room temperature in
1321 frames with v scans (width of 0.308 and
exposure time of 20 s per frame) in the 2u range 3–
46.58. The total collected data were reduced using
SAINT [12] program and the orientation matrix along
with the detector and the cell parameters were refined
A suitable single crystal of M·2B was carefully

A. Roy et al. / Journal of Molecular Structure 613 (2002) 61–66
63
Table 2
Atomic coordinates ( £ 104) and equivalent isotropic displacement
Table 3
Selected bond distances and bond angles for M·2B
2
3
˚
parameters (A £ 10 ) for, M·2B
˚
Distance (A)
Moiety
Moiety
Angle (8)
Atom
x
y
z
U(eq)a
B(1)–O(1)
B(1)–O(2)
B(1)–O(3)
B(2)–O(4)
B(2)–O(5)
B(2)–O(6)
C(1)–N(4)
C(1)–N(1)
C(1)–N(3)
N(1)–C(2)
C(2)–N(5)
C(2)–N(2)
N(2)–C(3)
C(3)–N(3)
C(3)–N(6)
1.370 (4)
1.372 (4)
1.349 (4)
1.352 (4)
1.356 (4)
1.373 (4)
1.339 (4)
1.339 (4)
1.353 (4)
1.344 (4)
1.338 (4)
1.348 (4)
1.338 (4)
1.339 (4)
1.351 (4)
O(3)–B(1)–O(1)
O(3)–B(1)–O (2)
O(1)–B(1)–O(2)
O(4)–B(2)–O(5)
O(4)–B(2)–O(6)
O(5)–B(2)–O(6)
N(4)–C(1)–N(1)
N(4)–C(1)–N(3)
N(1)–C(1)–N(3)
C(1)–N(1)–C(2)
N(5)–C(2)–N(1)
N(5)–C(2)–N(2)
N(1)–C(2)–N(2)
C(3)–N(2)–C(2)
N(2)–C(3)–N(3)
N(2)–C(3)–N(6)
N(3)–C(3)–N(6)
C(3)–N(3)–C(1)
120.0 (3)
120.1 (3)
119.9 (3)
120.5 (3)
120.3 (3)
119.2 (3)
118.3 (3)
116.6 (3)
125.0 (3)
115.0 (3)
117.4 (3)
117.4 (3)
125.1 (3)
114.1 (3)
126.3 (3)
117.0 (3)
116.6 (3)
114.1 (3)
B(1)
B(2)
O(1)
O(2)
O(3)
O(4)
O(5)
O(6)
C(1)
N(1)
C(2)
N(2)
C(3)
N(3)
N(4)
N (5)
N(6)
H(1)
H(2)
H(3)
H(4)
H(5)
H(6)
H(7)
H(8)
H(9)
H(10)
H(11)
H(12)
4507 (11)
3380 (11)
3264 (7)
5998 (7)
4251 (7)
2641 (7)
2512 (7)
5003 (7)
8379 (8)
9621 (7)
10305 (9)
9997 (7)
8648 (8)
7729 (7)
7727 (9)
11429 (9)
8262 (9)
7323 (93)
3673 (2)
4891 (2)
3640 (1)
4253 (1)
3137 (1)
4377 (1)
4852 (1)
5459 (1)
3726 (2)
3728 (1)
3125 (2)
2546 (1)
2606 (2)
3174 (1)
4309 (2)
3098 (2)
2043 (2)
1673 (3)
6371 (3)
32 (1)
35 (1)
37 (1)
40 (1)
41 (1)
48 (1)
43 (1)
44 (1)
29 (1)
29 (1)
29 (1)
30 (1)
30 (1)
30 (1)
41 (1)
38 (1)
43 (1)
52 (11)
73 (14)
34 (10)
43 (11)
70 (14)
59 (13)
52 (12)
72 (14)
79 (16)
69 (13)
62 (14)
63 (13)
2576 (2)
1348 (2)
1102 (2)
5776 (2)
7295 (2)
6045 (2)
21110 (2)
21992 (2)
22354 (2)
21885 (2)
21020 (2)
2595 (2)
2695 (2)
23245 (2)
2514 (3)
254 (26)
4299 (17)
4696 (22)
3485 (18)
2700 (18)
1642 (22)
2069 (19)
3272 (19)
4539 (22)
3173 (19)
4455 (19)
4505 (21)
5719 (21)
7977 (105)
11804 (82)
12162 (88)
8396 (107)
7406 (109)
2254 (100)
6361 (108)
5274 (117)
3309 (103)
1466 (105)
5675 (100)
21059 (27)
23544 (22)
23500 (22)
2825 (29)
31 (28)
The last cycles of refinement included atomic
positions for all the atoms, anisotropic thermal
parameters for all the non-hydrogen atoms and
isotropic thermal parameters for all the hydrogen
atoms. Full-matrix-least-squares structure refinement
against lF 2l was carried out using SHELXTL-PLUS
[16] package of programs. Details of the final
refinements are given in Table 1. The atomic
coordinates, bond distances and angles for M·2B is
presented in Tables 2 and 3. The experimental powder
pattern of the bulk sample is in good agreement with
the simulated pattern generated from single crystal
data.
2740 (26)
1816 (28)
531 (32)
5161 (28)
7486 (25)
6522 (26)
a
U(eq) is defined as one third of the trace of the orthogonalized
Uij tensor.
for every 40 frames on all the measured reflections.
Pertinent experimental details for the structure
determinations are presented in Table 1.
FT-IR spectroscopy of M·2B on KBr disc indicates
the presence of asymmetric and symmetric –NH2
stretching frequency at 3495 and 3415 cm21, respect-
ively. Sharp band around 1607 and 1249 cm21 is due
to CN and B–O stretching modes. The O–B–O
The structure of M·2B was solved by direct
methods using SHELXS-86 [13,14] which readily
established the position of the light atoms (C, N, O, B
and H) from difference Fourier maps. An empirical
absorption correction based on symmetry equivalent
reflections was applied using the SADABS [15]
program. Other effects, such as absorption by glass
fiber etc. were simultaneously corrected. All the
hydrogen positions for the melamine and boric acid
molecules were initially located in the difference
Fourier maps, and for the final refinement, the
hydrogen atoms were placed in the located position.
bending modes are observed at 803 and 652 cm21
,
respectively. TGA was carried out in a nitrogen
atmosphere in the 25–800 8C range. The TGA curve
indicates that there is a sharp weight loss around
140 8C due the loss of three molecules of water
(calc. ¼ 21.6%, obs. ¼ 22.5%) which is followed by a
continuous weight loss till 800 8C, mainly due to
the decomposition of the amine moiety. It has
been observed earlier by Hagio et al. [9] using

64
A. Roy et al. / Journal of Molecular Structure 613 (2002) 61–66
Fig. 1. The ORTEP plot of the asymmetric unit of M·2B. The thermal ellipsoids are given at 50% probability.
TG-mass spectrometry that (M·2B) decomposes with
the evolution of NH3, CO2 and H2NCN. The annealed
product remaining at 800 8C was amorphous and
weakly diffracting and yielded BN nanotubes on
further treatment at higher temperatures [10].
3. Results and discussion
The asymmetric unit of M·2B contains two
crystallographically distinct boric acid molecules
and one melamine molecule as shown in Fig. 1. The
planar triangular boric acid molecules and the
melamine molecule form a three-dimensional hydro-
gen-bonded supramolecular structure involving O–
H· · ·O, N–H· · ·O and O–H· · ·N hydrogen bonds. The
four B(OH)3 units are linked to one another to form a
finite tetrameric ribbon through strong O–H· · ·O
˚
(d(O· · ·O)av ¼ 2.716 A) hydrogen bonding with the
formation of three eight-membered cyclic [R22(8)]
rings [17] as shown in Fig. 2(a). In the tetramer,
B(2)(OH)3 forms a dimer in the middle while
B(1)(OH)3 is located at the two ends of the dimer.
The B(1)(OH)3 unit interacts with three melamines
and one boric acid, B(2)(OH)3. The B(2)(OH)3 unit,
on the other hand, interacts with two boric acid
molecules, B(1)(OH)3 and B(2)(OH)3, as well as one
melamine molecule through O–H· · ·O and O–H· · ·N
interactions, utilizing all the donor and acceptor sites.
Of the six oxygens in the two boric acid units,
Fig. 2. (a) The finite tetrameric ribbon of boric acid and the
environment of each crystallographically distinct boric acid unit. A,
D and E denotes the three eight-membered [R22(8)] ring. (b) The
environment around a melamine unit has been shown, F, G and I
denotes the three eight-membered [R22(8)] ring.
O(6) attached to B(2) does not participate in
Hydrogen bonding with the melamine molecule
either as donor or an acceptor. The average B–O
bond lengths in the two distinct boric acids are

A. Roy et al. / Journal of Molecular Structure 613 (2002) 61–66
65
˚
comparable (d(B(1)–O)avg ¼ 1.363 and 1.360 A).
However, the individual B–O distance in each boric
acid moiety varies considerably depending on their
participation in hydrogen bonding.
In contrast to the boric acid moieties, the melamine
unit does not utilize all its donor and acceptor sites.
Melamine interacts with four boric acid units through
the utilization of two N donor sites (N4 and N5) and
all the acceptor sites as shown in Fig. 2(b). Out of the
four boric acid units, three form cyclic eight-
membered [R22(8)] rings [17]. It is interesting that
the N(6) of melamine unit does not involve in
hydrogen bonding. When viewed along the a-axis,
M·2B forms a layer like structure in the bc-plane as
shown in Fig. 3(a). Within the layer, four melamine
units and four boric acid units are linked alternatively
to form an elliptical ring which is bisected along one
of the axis by the B(2)(OH)3 dimer into two 22-
membered [R76(22)] rings [17]. The layers are
connected along the a-axis in AAAA fashion through
B(1)(OH)3 units as shown in Fig. 3(b) to give the
three-dimensional structure. The details of the hydro-
gen bonding interactions in M·2B are given in Table 4.
There are many interesting features in the M·2B
adduct. Besides being a novel precursor for the BN
nanotubes, it represents a rather rare hydrogen-bonded
solid containing boric acid. The tetrameric ribbon in
M·2B is akin to a neutral ribbon of boric acid [18]. A
wide variety of supramolecular adducts with mela-
mine, where the amine uses all its donor and acceptor
sites, are reported in the literature [19–21], a well-
known example being the melamine–cyanuric acid
adduct with the hydrogen-bonded rosette structure
Fig. 3. (a) A projection of a section of the layer-like architecture into
the bc-plane has been shown. J and K denotes the 22- and eight-
membered [R76(22)] and [R22(8)] ring, respectively. (b) The boric
acid unit which connects the layers along the a-axis from top (A)
and bottom (E).
Table 4
Selected Hydrogen bond interactions in compound M·2B
D–H· · ·A
D–H
H· · ·A
D· · ·A
D–H· · ·A
N(4)–H(1)· · ·O(2)
N(4)–H(2)· · ·O(2)
N(5)–H(3)· · ·O(4)
N(5)–H(4)· · ·O(3)
O(1)–H(7)· · ·N(2)
O(2)–H(8)· · ·O(5)
O(3)–H(9)· · ·N(3)
O(4)–H(10)· · ·O(6)
O(5)–H(11)· · ·N(1)
O(6)–H(12)· · ·O(1)
0.93 (4)
0.95 (4)
0.90 (3)
0.92 (4)
0.86 (4)
0.89 (4)
0.90 (4)
0.92 (4)
0.84 (4)
0.88 (4)
2.05 (4)
2.57 (4)
2.07 (3)
1.94 (4)
1.92 (4)
1.79 (4)
1.85 (4)
1.84 (4)
1.87 (4)
1.84 (4)
2.977 (4)
3.308 (5)
2.964 (5)
2.853 (4)
2.781 (4)
2.671 (4)
2.747 (4)
2.759 (4)
2.702 (4)
2.718 (4)
175 (3)
135 (3)
180 (3)
172 (3)
180 (4)
174 (4)
174 (4)
174 (3)
174 (4)
172 (4)

66
A. Roy et al. / Journal of Molecular Structure 613 (2002) 61–66
[4] C.C. Freyhardt, M. Wiebcke, J. Felsche, G. Engelhardt,
J. Inclusion Phenom. Macrocycl. Chem. 18 (1994) 161.
[5] Q. Li, T.C.W. Mak, Supramol. Chem. 8 (1997) 147.
[6] C.C. Freyhardt, M. Weibcke, J. Chem. Soc., Chem. Commun.
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[21]. Surprisingly in the present case, one of the NH2
groups does not participate in hydrogen bonding.
There is no proton transfer between the amine and
boric acid.
In the hydrogen bonded structures formed by boric
acid or by the monomeric borate species reported in
the literature, infinite ribbon of B(OH)3 and BO(OH)22
[6], a rosette-like structure of B(OH)3–CO232–urea
linkages [7] and head-to-head dimer of B(OH)3 [8] are
known to occur. Thus, the hydrogen bonded structure
in M·2B is quite different, having a finite tetrameric
ribbon of B(OH)3.
[7] Q. Li, F. Xue, T.C.W. Mak, Inorg. Chem. 38 (1999) 4142.
[8] S.J. Andrews, D.A. Robb, A.J. Welch, Acta Crystallogr. C39
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(2001).
[12] Siemens Users Manual, Siemens Analytical X-ray Instru-
ments, Madison, WI, 1995.
[13] G.M. Sheldrick, SHELXS-86 Program for Crystal Structure
¨
Determination, University of Gottingen, Germany, 1986.
Acknowledgment
[14] G.M. Sheldrick, Acta Crystallogr. A35 (1990) 467.
[15] G.M. Sheldrick, SADABS Siemens Area Detector Absorption
¨
Correction Program, University of Gottingen, Germany, 1994.
One of the authors (AR) thanks University of North
Bengal, Darjeeling for a leave of absence.
[16] G.M. Sheldrick, SHELXTL-PLUS Program for Crystal
¨
Structure Solution and Refinement, University of Gottingen,
Germany, 1993.
[17] M.C. Etter, J.C. MacDonald, Acta Crystallogr. B46 (1990)
256.
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