Cooperative association of pyrazoles and phenols: A versatile binary system

Article abstract of DOI:10.1039/b317104c

The ability of self-complementary H-bond donor/acceptor phenol (OH/O) and pyrazole (NH/N) molecules for cooperative association allowed the developing of a range of binary compounds, in which the molecular counterparts are integrated by hydrogen bonding. Complexes involving either monofunctional 3,5-dimethylpyrazole and phenol and polyfunctional 3,3′,5,5′- tetramethyl-4,4′-bipyrazole, hydroquinone and phloroglucinol are based on mixed phenol-pyrazole chains. They reveal common features allowing high level of control over the entire connectivity and dimensionality. Unprecedented NH...π pyrazole hydrogen bonding was essential for the structure of the resorcinol complex.

Full text of DOI:10.1039/b317104c

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P A P E R
Cooperative association of pyrazoles and phenols: A versatile
binary systemy
Ishtvan Boldog,a Eduard B. Rusanov,a Joachim Sielerb and Konstantin V. Domasevitch*a
a
Inorganic Chemistry Department, Kiev University, Volodimirska Street 64, Kiev 01033,
Ukraine. E-mail: dk@anorgchemie.univ.kiev.ua
b
´
Institut fu¨r Anorganische Chemie, Universita¨t Leipzig, Linnestraße 3, D-04103,
Leipzig, Deutschland
Received (in Durham, UK) 5th January 2004, Accepted 3rd March 2004
First published as an Advance Article on the web 23rd April 2004
The ability of self-complementary H-bond donor/acceptor phenol (OH/O) and pyrazole (NH/N) molecules for
cooperative association allowed the developing of a range of binary compounds, in which the molecular
counterparts are integrated by hydrogen bonding. Complexes involving either monofunctional
3,5-dimethylpyrazole and phenol and polyfunctional 3,30,5,50-tetramethyl-4,40-bipyrazole, hydroquinone
and phloroglucinol are based on mixed phenol–pyrazole chains. They reveal common features allowing high
level of control over the entire connectivity and dimensionality. Unprecedented NHꢀ ꢀ ꢀp pyrazole hydrogen
bonding was essential for the structure of the resorcinol complex.
Introduction
Developing of multicomponent organic systems is an outlook
of solid state organic chemistry1 that offers a significant poten-
tial for construction of materials in which single molecular
functionalities are rationally integrated and may also present
a major contribution to the better understanding of complex
supramolecular relations, dynamics and reactivity in chemistry
and biology.2,3 Proper handling of binary organic/organic
relations is important also from a synthetic perspective as
the existence of stable and concomitant molecular combina-
tions can significantly complicate or, vice versa, simplify many
isolation and purification strategies.
The ability of certain complementary donor and acceptor
Scheme 1 Illustration of how an additional self-complementary
component can be incorporated into the structure without alteration
of the connectivity.
organic molecules to afford binary complexes is well known
and widely used for supramolecular synthesis.1–4 A special,
highly illustrative example of a binary organic system may arise
if one achieves a cooperative association of different self-com-
plementary molecules, i.e. the species combining both donor
and acceptor binding sites in equal proportions.5 In this case
incorporation of an extra component does not effect an altera-
tion of a given connectivity (for example, topology of inter-
molecular hydrogen bonding) and it assumes the existence of
the classes of binary compounds based upon the complemen-
tary combination of the H-bonding donating/accepting sites:
Results and discussion
The binary composite (pyrazole)n(phenol)m consists of molecu-
lar components with maximum one/one H-bond donor/accep-
tor; therefore a completely interconnected structure may
contain cyclic or 1D chain motifs. Control over the structure
dimensionality is feasible by evolution from mono- to poly-
functional species considering either pyrazole and phenol
counterparts. As an especially suitable bipyrazole tecton we
have examined 3,30,5,50-tetramethyl-4,40-bipyrazole Me4bpz.
This conformationally flexible molecule is easily adaptable to
the crystal environment demands and also minimizes effects
of the pꢀ ꢀ ꢀp stacking interactions owing to its non-planar
structure.11 Me4bpz and its monofuctional analogue 3,5-
dimethylpyrazole Me2pz easily co-crystallize with a representa-
tive range of species: phenol, hydroquinone, resorcinol and
phloroglucinol, yielding binary molecular complexes by a set
of DHꢀ ꢀ ꢀA (D,A ¼ O,N) interactions.
{[–X1H–]} ꢁ {[–X2H–]}){[–X1H–]n[–X2H–]m},
a meccano-
like principle (Scheme 1).6 Thus both the components possess
a parity of structural functions and hence an equally rich and
overlapping potential from the design perspective for the motif
propagation and control over the structure dimensionality.
Herein we report a family of binary compounds formed by
the most paradigmatic and general self-complementary species,
pyrazoles7,8 and phenols,9,10 that combine NH/N and OH/O
hydrogen bond donor/acceptor functionalities.
The simplest binary composite sustained with two mono-
functional tectons, Me2pzꢀPhOH 1, affords a 1D zig-zag chain
of alternating components and it is a prototype for cooperative
assembly of pyrazoles and phenols (Fig. 1, A). The geometry
y Electronic supplementary information (ESI) available: Details for
crystal structure determination and refinement and geometry of hydro-
gen bonding in structures. See http://www.rsc.org/suppdata/nj/b3/
b317104c/
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
756
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 5 6 – 7 5 9

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Fig. 1 Crystal structures of Me2pzꢀPhOH 1 (A), Me2pzꢀ(p-C6H4(OH)2) 2 (B), Me4bpzꢀ2PhOH 3 (C), Me4bpzꢀ(p-C6H4(OH)2) 4 (D). 2–4 can be
viewed as propagation of H-bonded pyrazole–phenol motif of 1 (marked with dotted borders) by means of doubling of the molecular functionality.
Numeric parameters here and below indicate Dꢀ ꢀ ꢀA (D,A ¼ N,O) separations of the hydrogen bonding.
of the intermolecular bonding is characteristic for hydrogen
˚
chains in 1, fused by the analogous [p-C6H4(OH)2]n thread
ꢂ
˚
bonds of the types NHꢀ ꢀ ꢀO (Oꢀ ꢀ ꢀN 2.916(3) A cNHꢀ ꢀ ꢀO
(Fig. 1B) (Oꢀ ꢀ ꢀO 2.763(3) A, cOHꢀ ꢀ ꢀO 168(3) ). A similar
feature of both structures of 2 and 4 is the important role of
[p-C6H4(OH)2]n thread that runs angle-wise to the basic
phenol–pyrazole chain and the para-disposition of OH func-
tionalities can be thought as a main reason of such inheritance.
The only previously known catameric phenol/pyrazole adduct,
Me2pzꢀ1,1-bis(p-hydroxyphenyl)cyclohexane15 is an analogue
of Me2pzꢀ(p-C6H4(OH)2) 2 in the sense of functionalities ratio.
Both structures comprise clearly related 2D sheets consisting
of [Me2pzꢀR(OH)2]n chains. It is rather surprising that self-
173(3)ꢂ) and OHꢀ ꢀ ꢀN (Oꢀ ꢀ ꢀN 2.700(3) A, cOHꢀ ꢀ ꢀN
˚
170(2)ꢂ), the latter were observed for adducts of phenols and
pyridines.12
The binary complex of bifunctional pyrazole Me4bpzꢀ
2PhOH 3 possesses the same pyrazole/phenol ratio and its
structure can be thought as a result of simple propagation of
the array in a second direction. Partial rearrangement of the
chain motif yields AA(BBAA)n connectivity mode that enables
the satisfaction of steric demands by longer (PhOH)2 bridge
˚
(Oꢀ ꢀ ꢀO 2.647(3) A). In that sense, the structure 3 is also built
complementary
4-(p-hydroxyphenyl)-3,5-dimethylpyrazole
up from zigzag chains, but with doubled [(pyrazole)2(PhOH)2]
unique fragment. Chains are connected by means of carbon–
carbon bonds within a frame of bipyrazole molecules in 2D
displays no direct interaction between principal pyrazole and
phenol functions in the crystal.16
The above observations are applicable also for the more
complicated binary complex 2Me4bpzꢀ(1,3,5-C6H3(OH)3)ꢀ
H2O 6, containing trifunctional phloroglucinol as the pheno-
lic component: phloroglucinol molecules form helices that
are encompassed by eight characteristic (Me4bpz)n threads
(Fig. 2). Such a high number of neighboring threads is pro-
vided by water molecules that extend the H-bonding bridge
for two of these threads. An extra hydrogen bond donating
OH (H2O) group is not involved in the entire strong H-
bonding linkage but unprecedentedly utilizes a neighboring
pyrazole cycle as a p-acceptor system (Fig. 3) thus uniting
two bipyrazole threads. Such a p-accepting function is also
known for imidazole compounds.17 Interconnection of
pyrazole and OH functions in a AA(BBAA)n fashion yields a
large cycle involving eight pyrazole, six phenol and two
water molecules. In contrast, the structure of the resor-
cinol complex Me4bpzꢀ(m-C6H4(OH)2) 5 displays no homo-
molecular association and in this sense it does not follow the
general scheme. It is based upon cyclic H-bonded fragments
with two phenol and two pyrazole molecules that afford a
puckered 2D sheet (Fig. 4). Steric demands of the packing
are essential for the structure and they result in surprising
‘‘p-confusion’’ of the connectivity involving NHꢀ ꢀ ꢀp bonding
pyrazole/resorcinol, instead of the anticipated NHꢀ ꢀ ꢀO
scheme. This clear and illustrative p-hydrogen bonding
pattern is unique for pyrazoles, despite of its significant role
for related heterocycles,2,18 particularly for pyrrole.19 In
5 the NHꢀ ꢀ ꢀp interacting pyrazole and resorcinol cycles
are actually orthogonal (dihedral angle 102.6ꢂ) and the
distance between N(2) and the plane of the resorcinol
1
2
˚
puckered sheets (Fig. 1c) that are separated at b (ca. 7.3 A)
by protruding phenyl groups. The dimensionality of the archi-
tecture apparently grows by one after two pyrazole functions
are collected within a single molecule. X-Ray data clearly
suggest the possibility of solid state tautomerism of the com-
pound as all NH and OH protons were disordered over two
possible positions, which is a sign for solid state proton trans-
fer. Both phenomena are typical for cyclic pyrazole patterns,
in contrast to catameric structures where the hydrogen atoms
in NHꢀ ꢀ ꢀN bridges are ordered and immobile.13 Inclusion of
the strongly binding phenolic O–H group enhances the
strength of the partial bonding contributors. This effect,
known as s-bond cooperativity or polarization enhanced
hydrogen bonding,14 can in principle promote the rather pro-
mising proton dynamics in catamer patterns: NHꢀ ꢀ ꢀOHꢀ ꢀ ꢀN
to Nꢀ ꢀ ꢀHOꢀ ꢀ ꢀHN, and thought as a design tool.
Considering the parity of structural roles of pyrazole and
phenol counterparts, doubling of the phenolic function has
the same impact for the motif expansion. Me2pzꢀ(p-
C6H4(OH)2)
2
and Me4bpzꢀ(p-C6H4(OH)2)
4 comprising
hydroquinone as the phenolic component are an evolution of
parent adducts 1 and 3. The connection is most clear in the
3D structure of 4, where [C6H4(OH)2]2 bridges, functionally
analogous to [PhOH]2 bridges in 3, are the part of the H-
bonded hydroquinone thread that fuses the pack of 2D sheets,
similar to the main topology of 3 (Fig. 1D). In other words,
two protruding phenyl groups of prototypal 2D sheets are co-
inciding by means of the common ring of hydroquinone that
increases the dimensionality of the structure (2D ! 3D). 2D
structure 2 is composed from extended chains of [(Me2pz)-
(C6H4(OH)2)2]n those are resemblant to the [(Me2pz)(PhOH)]n
˚
molecule, 3.27 A, is shorter than the closest Nꢀ ꢀ ꢀC contact at
˚
3.38 A.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 5 6 – 7 5 9
757

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Fig. 4 2D sheets constituting the structure of Me4bpzꢀ(m-C6H4-
(OH)2) 5 showing alteration of the molecular components.
In summary, we believe that the common features of 1–5 are
applicable for preparation of a wide variety of binary pyra-
zole/phenol composites and may be categorized as follows:
combination of two monofunctional molecules affords an
alternating 1D chain; bifunctional molecules themselves form
homomolecular threads that are interlinked by bridges of
monofunctional molecules forming a 2D structure, while a
combination of two bifunctional molecules yields a 3D inter-
linked ensemble of two sorts of homomolecular threads, and
both A(BA)n and AA(BBAA)n motifs of pyrazole–phenol
interactions are feasible. In that sense the structural functions
of the two components are equal. The existence of such alter-
nating patterns clearly demonstrates the prospect for develop-
ing multidimensional systems with H-bonding properties
presumably intermediate between the ones for individual com-
ponents, with a special impact for supramolecular devices with
mobile protons. The Me4bpzꢀ2PhOH 3 complex provides a
possible and encouraging prototype of infinite NHꢀ ꢀ ꢀOHꢀ ꢀ ꢀN
architecture with disordered protons and potential for long
range proton transfer.
Fig. 2 2Me4bpzꢀ(1,3,5-C6H3(OH)3)ꢀH2O 6 adduct: A–phloroglucinol
helix associates with linear chains of bipyrazoles (part of the chains
are represented with hatched lines for clarity), all running along c axis.
B–View down c axis showing eight closest neighbors.
Experimental
Starting compounds were materials of reagent grade used with-
out further purification. 3,30,5,50-tetramethyl-4,40-bipyrazole
Me4bpz was synthesized according to the literature method.20
Binary compounds Me2pzꢀPhOH 1, Me2pzꢀ(p-C6H4(OH)2)
2, Me4bpzꢀ2PhOH 3, Me4bpzꢀ(p-C6H4(OH)2) 4, 2Me4bpzꢀ
(1,3,5-C6H3(OH)3)ꢀH2O 6 were prepared from methanolic
solutions of molecular components given in 1:1 molar ratio,
the yields of colorless large block shaped crystals were 30–
60%. Bipyrazole and resorcinol do not form adducts under
these conditions, but readily co-crystallize from chloroform
as 1:1 complex Me4bpzꢀ(m-C6H4(OH)2) 5.
Crystal data are presented in Table 1.z X-ray diffraction
data for 1 were collected using a Stoe STADI-4 diffractometer;
psi-scan based semi-empirical absorption correction was
applied. The data for 2–6 were collected using a Siemens
SMART CCD diffractometer (absorption correction using
SADABS21) (graphite monochromated Mo–Ka radiation,
˚
l ¼ 0.71073 A). The structures were solved by direct methods
and refined in the anisotropic approximation using SHELX-
97.22 In structure 3 the orientation of two unique phenol
molecules (dihedral angle 4.5ꢂ) forming unequal hydrogen
˚
bonds (Nꢀ ꢀ ꢀO 2.681(4) and 2.887(3) A) lowers the entire sym-
metry and eliminates a possible center of inversion. The refine-
ment in centrosymmetric space group C2/c led only to poor
convergence at R1 ¼ 0.12 and large atomic thermal parameters.
Fig. 3 p-Hydrogen bonding involving pyrazole nuclei either as donor
or acceptor seen in binary compounds 5 (ꢂA) and 6 (B). Separations
z CCDC reference numbers 22877–22782. See http://www.rsc.org/
suppdata/nj/b3/b317104c/ for crystallographic data in .cif or other
electronic format.
ꢂ
˚
˚
Hꢀ ꢀ ꢀp and angles D–Hꢀ ꢀ ꢀp are 2.65 A, 149 (5) and 2.61 A, 157 (6).
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
758
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 5 6 – 7 5 9

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Table 1 Crystal data for Me2pzꢀPhOH 1, Me2pzꢀ(p-C6H4(OH)2) 2, Me4bpzꢀ2PhOH 3, Me4bpzꢀ(p-C6H4(OH)2) 4, Me4bpzꢀ(m-C6H4(OH)2) 5, and
2Me4bpzꢀ(1,3,5-C6H3(OH)3)ꢀH2O 6
1
2
3
4
5
6
Formula
C11H14N2O
190.24
C11H14N2O2
206.24
C22H26N4O2
378.47
C16H20N4O2
300.36
C16H20N4O2
300.36
C26H36N8O4
524.63
Formula weight
Temperature, K
Crystal system
Space group, Z
293
223
Orthorhombic
Pbca, 8
223
Monoclinic
Cc, 4
223
Triclinic
¯
P1, 4
223
Orthorhombic
Pbca, 8
223
Monoclinic
P21/c, 4
8.043(1)
15.116(1)
8.934(1)
Orthorhombic
Fdd2, 16
29.564(3)
46.197(6)
8.554(1)
˚
a/A
11.901(2)
9.902(2)
18.667(4)
19.065(2)
14.725(2)
8.583(1)
7.3717(7)
14.192(1)
16.436(2)
98.782(2)
94.460(2)
103.055(2)
1644.1(3)
0.83
18.270(3)
8.528(1)
21.054(4)
˚
b/A
˚
c/A
a/ꢂ
b/ꢂ
g/ꢂ
96.650(12)
114.80(2)
3
˚
U/A
1078.9(2)
0.77
52.0
2199.8(7)
0.87
54.6
2187.3(5)
0.75
51.0
3280.4(9)
0.83
51.4
11 683(2)
0.83
52.7
m(Mo-Ka), cmꢃ1
ꢂ
2ymax
/
51.4
9068/6141
0.021
Meas/Unique reflns
2377/1344
0.025
12 807/2465
0.023
5869/2838
0.024
17 024/3119
0.044
15 462/5317
0.039
Rint
R1, wR2 [I > 2s(I)]a
R1, wR2 (all data)
0.036, 0.090
0.079, 0.103
0.036, 0.091
0.044, 0.096
0.042, 0.119
0.048, 0.126
0.051, 0.135
0.089, 0.151
0.049, 0.128
0.072, 0.141
0.043, 0.106
0.055, 0.111
a
All unique reflections were used in the refinements.
7
8
9
C. Foces-Foces, I. Alkorta and J. Elguero, Acta Crystallogr. Sect.
B, 2000, 56, 1018; A. L. Llamas-Saiz, C. Foces-Foces, F. H. Cano,
P. Jimenez, J. Laynez, W. Meutermans, J. Elguero, H.-H.
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I. Alkorta, J. Elguero, B. Donnadieu, M. Etienne, J. Jaffart, D.
Schagen and H.-H. Limbach, New J. Chem., 1999, 23, 1231;
J. Elguero, F. H. Cano, C. Foces-Foces, A. L. Llamas-Saiz,
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In 3 all NH and OH hydrogen atoms were located to be
disordered over two positions. Attempted refinement led to
unreasonable elongation of N–H and O–H bonds and high
thermal values for hydrogen atoms that may be suggestive of
dynamic character of proton disordering. For this reason,
the hydrogen atoms were fixed and the partial occupancies
were set at 0.5. Other structures did not show any sign of solid
state tautomerism, and all NH and OH hydrogen atoms were
located and refined isotropically. CH hydrogen atoms were
constrained geometrically and considered in riding model with
isotropic U values at 1.2–1.5 times the equivalent isotropic U
value of the atoms to which they are attached. Disordering
of CH3 hydrogen atoms was relevant for refinement of 1 and
2 and was accounted using a symmetric binary model.
R. Perrin, R. Lamartine, M. Perrin and A. Thozet, Solid state
chemistry of phenols and possible industrial applications, in
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Amsterdam, 1987, 271–329.
10 Recent examples for use of phenol molecules in crystal
engineering P. I. Coupar, C. Glidewell and G. Ferguson, Acta
Crystallogr. Sect. B, 1997, 3, 521; P. Vishweshwar, A. Nangia
and V. M. Lynch, Cryst. Eng. Comm., 2003, 164.
11 I. Boldog, E. B. Rusanov, A. N. Chernega, J. Sieler and K. V.
Domasevitch, Angew. Chem., Int. Ed., 2001, 40, 3435; E. B.
Rusanov, V. V. Ponomarova, V. V. Komarchuk, H. Stoeckli-
Evans, E. Fernandez-Iban˜ez, F. Stoeckli, J. Sieler and K. V.
Domasevitch, Angew. Chem., Int. Ed., 2003, 42, 2499.
12 E. Corradi, S. V. Mielle, M. T. Messina, P. Metrangolo and
G. Resnati, Angew. Chem., Int. Ed., 2000, 39, 1782; L. R.
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Acknowledgements
The work was in part supported by a grant from Deutsche
Forschungsgemeinschaft (JS and KVD) and INTAS Fellow-
ship grants YSF 2002-130 (IB) and YSF 2002-310 (KVD).
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C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 7 5 6 – 7 5 9
759