Solid-state hydrogen-bonding self-assemblies and keto-enol tautomerism of 1,3-dipyrrolyl-1,3-propanediones

Article abstract of DOI:10.1080/10610278.2010.521834

Single-crystal X-ray analyses of various 1,3-dipyrrolyl-1,3-propanedione derivatives confirmed the formation of keto-based 1D NZH· · ·O=C hydrogen-bonding chains and cis-enol-based hydrogen-bonding chains. The preferences for keto and enol tautomers in the solid state were found to depend significantly on the positions of substituents at pyrrole rings. α-Aryl-substituted derivatives afford keto forms, while β-alkyl- and β-aryl-substituted derivatives provide cis-enol forms.

Full text of DOI:10.1080/10610278.2010.521834

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Solid-state hydrogen-bonding self-assemblies
and keto–enol tautomerism of 1,3-dipyrrolyl-1,3-
propanediones
Yohei Haketa a , Nazuki Eifuku a , Yuya Bando a , Ippei Yamada a , Ayano Hagihara a
Hiromitsu Maeda a b
&
a College of Pharmaceutical Sciences, Institute of Science and Engineering, Ritsumeikan
University, Kusatsu, 525-8577, Japan
b PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, 332-0012, Japan
Published online: 13 Apr 2011.
To cite this article: Yohei Haketa , Nazuki Eifuku , Yuya Bando , Ippei Yamada , Ayano Hagihara & Hiromitsu Maeda (2011)
Solid-state hydrogen-bonding self-assemblies and keto–enol tautomerism of 1,3-dipyrrolyl-1,3-propanediones, Supramolecular
Chemistry, 23:03-04, 209-217, DOI: 10.1080/10610278.2010.521834
To link to this article: http://dx.doi.org/10.1080/10610278.2010.521834
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Supramolecular Chemistry
Vol. 23, Nos. 3–4, March–April 2011, 209–217
Solid-state hydrogen-bonding self-assemblies and keto–enol tautomerism of
1,3-dipyrrolyl-1,3-propanediones
Yohei Haketaa, Nazuki Eifukua, Yuya Bandoa, Ippei Yamadaa, Ayano Hagiharaa and Hiromitsu Maedaab*
aCollege of Pharmaceutical Sciences, Institute of Science and Engineering, Ritsumeikan University, Kusatsu 525-8577, Japan;
bPRESTO, Japan Science and Technology Agency (JST), Kawaguchi 332-0012, Japan
(Received 6 July 2010; final version received 25 August 2010)
Single-crystal X-ray analyses of various 1,3-dipyrrolyl-1,3-propanedione derivatives confirmed the formation of keto-based
1D NZH· · ·OvC hydrogen-bonding chains and cis-enol-based hydrogen-bonding chains. The preferences for keto and enol
tautomers in the solid state were found to depend significantly on the positions of substituents at pyrrole rings. a-Aryl-
substituted derivatives afford keto forms, while b-alkyl- and b-aryl-substituted derivatives provide cis-enol forms.
Keywords: hydrogen bonding; pyrrole; self-assembly; supramolecular chemistry; tautomerism
1. Introduction
(dipyrrolyldiketones; e.g. 1a; Figure 1(a)) (7), which are
the essential precursors of anion-responsive boron com-
plexes(8,9)andpyrazolederivatives(10)(Figure1(b)),form
1D NZH· · ·OvC hydrogen-bonding chains and organised
structures in the solid state (11). In this case, pyrrole NH acts
as a hydrogen-bonding donor that is coplanar and faces the
same side as the proximal carbonyl unit. A previous study
reported that various pyrrole rings can be introduced in the
core 1,3-propanedione spacer (11). Similar to other 1,3-
propanedione derivatives, keto–enol equilibria were
observed in the solution state for these dipyrrolyldiketones.
Control of keto–enol equilibria in solution and in the solid
state enables the tuning of electronic and optical properties
for fabrication of functional materials. However, enol
tautomers of dipyrrolyldiketones have not been observed in
the solid state thus far, presumably due to the formation of
more stable intermolecular hydrogen bonding using keto
carbonyl moieties. These initial findings have been used as a
basis for the control of solid-state assembled structures and
keto–enol tautomerism in an attempt to modify pyrrole
rings.
Hydrogen bonding is an essential non-covalent interaction
that allows for the fabrication of biomacromolecular systems
such as DNA double helices and protein folding structures
and artificial supramolecular assemblies, including both
crystals and soft materials (1, 2). In biotic and artificial
systems, appropriately designing and arranging the hydro-
gen-bonding donor and acceptor sites in building com-
ponents are crucial. One way to control hydrogen-bonding
sites is through controlling tautomerism, the equilibrium
between two or more states by intramolecular proton
transfer. The equilibrium between a keto form and an enol
form is a classic example of tautomerism and has been well
studied (3). In particular, keto–enol tautomerism in linear
1,3-propanediones exhibits transition between mainly
flexible keto forms and fairly rigid cis-enol forms with
intramolecular hydrogen bonding. In the keto form, carbonyl
oxygen moietiescan behave as hydrogen-bondingacceptors,
whereas, in the cis-enol form, hydroxy oxygen moieties
contribute to the hydrogen-bonding interaction, but to a
lesser extent.
The component of supramolecular assemblies, pyrrole,
which is a p-conjugated heterocycle composed of functional
biotic dyes such as haem and chlorophyll (4), affords various
types of cyclic and acyclic anion receptors consisting of
pyrrole ring(s) on the basis of the hydrogen-bonding donor
NH site (5). Furthermore, self-assembled supramolecular
networks of acyclic pyrrole derivatives with carbonyl groups
at a-positions have been observed in the solid state as well as
in the solution state (6). Therefore, increasing hydrogen-
bonding sites, e.g. by the addition of pyrrole to 1,3-
propanediones, would afford various supramolecular assem-
blies. For example, 1,3-dipyrrolyl-1,3-propanediones
2. Results and discussion
2.1 Solid-state hydrogen-bonding assemblies of a-aryl
derivatives
As reported in a previous study, alkyl substitution at pyrrole
a-positionsasseenin1b(Figure2)provides sheet-likesolid-
stateorganisedstructuresforaliphaticchains(n ¼ 10,12,14,
16) (11). In addition to alkyl chains, aryl moieties can extend
p-planes and, essentially, behave as platforms to connect
various substituents to the dipyrrolyldiketone core. We
obtained single crystals of phenyl 2a, 2,6-dimethylphenyl 2b
*Corresponding author. Email: maedahir@ph.ritsumei.ac.jp
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2011 Taylor & Francis
DOI: 10.1080/10610278.2010.521834
http://www.informaworld.com

210
Y. Haketa et al.
The diketones 2a,b and 3a–e form 1D intermolecular
hydrogen-bonding assemblies using pyrrole NH and
diketone CO moieties (Figure 4). The distances between
˚
N(H) and (C)O are 2.92 and 2.95 A for 2a, 2.84 and 2.85 A
˚
˚
for 2b, 2.94 A for 3a, 3.00 A for 3b, 2.87 and 2.93 A for 3c,
˚
˚
˚
˚
2.91 and 3.04 A for 3d and 2.95 A for 3e. Some of the
diketones show weak intermolecular hydrogen bonding
between aryl o-CH and carbonyl oxygen with the
˚
˚
C(ZH)· · ·O distances of 3.35 and 3.36 A for 2a, 3.21 A
˚
for 3b, 3.41 and 3.42 A for 3c, 3.25 and 3.31 A for 3d and
˚
˚
3.43 A for 3e. The values of distances, as well as angles, of
Figure 1. (a) Keto-enol tautomerism of dipyrrolyldiketone 1a
as the parent structure and (b) dipyrrolyldiketone-based BF2
complex and pyrazole derivative.
hydrogen-bonding assemblies are essential for exhibiting
the versatility of the assembled structures depending on
the pyrrole substituents. Regularly assembled structures
are repeated by each two molecules in 1a; the diketone 1a,
with chirality due to the twisted methylene (CH2) unit,
interacts with neighbouring enantiomers to afford racemic
strands consisting of the RSRS· · · configuration. Similar to
1a, a-aryl-substituted 2a,b and 3b–e also form racemic
wires of the RSRS· · · configuration. In contrast, 3a exhibits
the formation of the equimolar chiral columns RRRR· · ·
and SSSS· · ·, wherein o-methoxy moieties are located at
the valley zone constructed by p-conjugated moieties.
Hydrogen-bonding planes of 3a comprising NH and CO
with the bridging pyrrole a-C are intermolecularly
distorted at 50.688, which is in sharp contrast to the
completely parallel arrangements in 2a and 3b–e, where
the parallel distances of the corresponding planes are 0.81,
(9e), o,m,p-methoxyphenyl 3a–c and m,p-octyloxyphenyl
3d,e (9m). These diketones were synthesised from the
corresponding pyrrole derivatives and malonyl chloride in
CH2Cl2. As with the single-crystal structure of 1a (9a, 11),
those of a-aryl derivatives 2a,b and 3a–e exhibit keto forms
in the solid state (Figure 3). The intramolecular dihedral
angles between two planes consisting of five atoms (NH and
CO with the bridging pyrrole a-C) were estimated to be
64.188, 99.988, 50.688, 85.608, 74.078, 58.368 and 53.328 for
2a,b and 3a–e, respectively; these values are smaller than
those of 1a at 109.018 and 102.678. Between two pyrrole
rings, 2a,b and 3a–e had almost the same values to each
angle of the above planes at 61.928, 100.328, 49.838, 84.378,
79.228, 54.038 and 53.218, respectively. Furthermore, the
peripheralarylringsof2aare almostplanartothe connecting
pyrrole ring at 3.078 and 5.518. In contrast, the 2,6-dimethyl-
phenyl rings of 2b show significant distortion from the
pyrrole planes at 84.448 and 76.168; this is possible due to the
steric hindrance between o-CH3 and the pyrrole NH or
pyrrole b-CH unit. The corresponding values for 3a–e are
22.498 for 3a, 17.198 for 3b, 2.578 and 7.448 for 3c, 2.028 and
8.938 for 3d and 25.458 for 3e. o-Methoxy moieties of 3a
facing the pyrrole NH site exhibit intramolecular hydrogen-
bonding NZH· · ·O interactions.
˚
0.74, 0.20, 0.29 and 0.028 A, respectively. In addition, the
planes are almost parallel in 2b with the dihedral angle of
3.608. Due to the distortion around the sp3-CH2 unit
between two pyrrole moieties, hydrogen-bonding chains
for 2a,b and 3b–e show alternating crinkled structures.
For m-octyloxyphenyl 3d, alkyl chains of each hydrogen-
bonding chain are almost completely interdigitated to form
lamella-like structures; in contrast, p-octyloxyphenyl 3e
exhibits layer structures assisted by partial interactions
between alkyl chains.
Figure 2. a-Aryl-substituted dipyrrolyldiketones 2a,b and 3a–e along with parent 1a and a-alkyl 1b.

Supramolecular Chemistry
211
Figure 3. ORTEP drawings (top and side views) of single-crystal X-ray structures of a-aryl-substituted dipyrrolyldiketones: (a) 2a,
(b) 2b, (c) 3a, (d) 3b, (e) 3c, (f) 3d and (g) 3e. Thermal ellipsoids are scaled to the 50% probability level. Solvent molecules are omitted
for clarity for 2b.

212
Y. Haketa et al.
Figure 4. 1D hydrogen-bonding chains of (a) 2a, (b) 2b, (c) 3a, (d) 3b, (e) 3c, (f) 3d and (g) 3e. Solvent molecules are omitted for clarity
for 2b. Atom colour code: brown, blue, pink and red represent carbon, nitrogen, hydrogen and oxygen, respectively.
˚
2.2 Solid-state keto–enol equilibria ofb-alkyland b-aryl
derivatives
2.40/2.42, 2.43, 2.48 and 2.42 A, respectively. The
intramolecular dihedral angles between two planes consist-
ing of five atoms (NH and CO with the bridging pyrrole a-C)
were estimated to be 6.128/7.148, 17.308, 4.618 and 20.028 for
4a–d, respectively. Those between two pyrrole rings had
almost the same values to each angle of the above planes at
4.998/5.368, 27.458, 2.218 and 33.498 for 4a–d, respectively.
In solutions, the equilibrium between keto and enol forms
was observed in 4a–d as well as in the other derivatives 1a,
2a,b and 3a–d; the correlations between the solid and
solution states cannot be discussed at present and will be
reported elsewhere.
Modifications of the molecular structures are crucial for
controlling assembled structures and their electronic and
optical properties. By focusing on the pyrrole rings,
b-substituents can also be effective for the fabrication and
controloffunctionallyorganisedstructures(9r).Weobtained
single crystals of b-methyl- and b-ethyl-substituted 4a–c
(9h,l), and b-phenyl-substituted 4d(Figure 5). The diketones
4a,b,dweresynthesisedfromthecorrespondingpyrrolesand
malonyl chloride in CH2Cl2, whereas a-iodo-substituted 4c
waspreparedbyremovalofaboronmoietyfortheprecursory
bisiodo-substituted BF2 complex (9h) by treatment with
AlCl3 and tert-butyl alcohol. In contrast to the single-crystal
structures of 1a, 2a,b and 3a–d, those of b-alkyl and b-aryl
derivatives 4a–d form enol tautomers in the solid state
(Figure 6). The distances between two oxygens of hydrogen-
bonding cyclic enol-type 1,3-diketones of 4a–d are
b-Alkyl- and b-aryl-substituted 4a–d in cis-enol forms
also provide solid-state assembled structures through
hydrogen bonding between pyrrole NH and carbonyl or
hydroxy oxygen (Figure 7): the distances between N(H) and
˚
carbonyl oxygen in 4a–d are 2.87, 2.91, 2.95 and 2.83A,
respectively, whereas those for hydroxy oxygen in 4a,b,d are

Supramolecular Chemistry
213
propanedione moiety, in4a,c,d are 38.298/38.318, 42.318 and
0.978, respectively. Furthermore, b-methyl-substituted 4a
exhibits infinitely stacking structures of the p-planes at a
˚
distance of 3.49A, which is defined as the average distance
of stacking tetramers each comprising 15 core atoms, while
˚
4cformsparallelstackingdimersatadistanceof3.45A. This
Figure 5. b-Alkyl- and b-aryl-substituted dipyrrolyldiketones
4a–d.
is in sharp contrast to 4b,d, which have no significant
stacking structures. These observations can be derived from
the differences in the substituents, which affect the
appropriate packing modes.
˚
2.92, 2.89and 2.88A, respectively. Amongthesederivatives,
a-iodo-substituted 4c lacks interaction of pyrrole NH with
hydroxy oxygen; instead, it forms N(ZH)· · ·I hydrogen
˚
bonding with the distance of 3.79A. In these cases, as
3. Experimental section
discussed in Section 1, building components are fairly planar
due to intramolecular hydrogen bonding. Therefore,
hydrogen-bonding chains exhibit tape-like structures,
wherein molecules are arranged in a completely parallel
3.1 General procedures
Starting materials were purchased from Wako Pure
Chemical Industries, Ltd. (Osaka, Japan), Nacalai Tesque
Inc. (Kyoto, Japan), Sigma-Aldrich Co. (St Louis, MO)
and used without further purification unless otherwise
stated. NMR spectra used in the characterisation of
products were recorded on a JEOL ECA-600 600 MHz
spectrometer. All NMR spectra were referenced to solvent.
Matrix-assisted laser desorption ionisation time-of-flight
mass spectrometries (MALDI-TOF-MS) were recorded on
˚
and step-like form in4bwiththe distances of1.34and 1.97A
between the neighbouring hydrogen-bonded planar moieties
comprising NH, CO and the bridging pyrrole a-C; they are
almost parallel in 4d and are fairly distorted in 4a,c. The
dihedral angles between hydrogen-bonding molecules,
defined as 15 core atoms of two pyrrole rings and 1,3-
Figure 6. ORTEP drawings (top and side views) of single-crystal X-ray structures of b-alkyl- and b-aryl-substituted
dipyrrolyldiketones: (a) 4a (one of the two independent structures), (b) 4b, (c) 4c and (d) 4d. Thermal ellipsoids are scaled to the
50% probability level. Solvent molecules are omitted for clarity for 4d. The structure of 4b, which has been reported as a keto form (6),
was examined again and solved as a cis-enol form.

214
Y. Haketa et al.
Figure 7. 1D hydrogen-bonding chains of (a) 4a, (b) 4b, (c) 4c and (d) 4d. Solvent molecules are omitted for clarity for 4d. Atom colour
code: purple represents iodine.
a Shimadzu Axima-CFR plus using negative mode. TLC
analyses were carried out on aluminium sheets coated with
silica gel 60 (Merck 5554). Column chromatography was
performed on Wakogel C-300 and Merck silica gel 60H.
0.25 mmol) at room temperature and stirred for 1.5 h at
the same temperature. After the consumption of the
starting pyrrole confirmed by TLC analysis, the mixture
was washed with saturated aqueous Na2CO3 and water,
dried over anhydrous Na2SO4, filtered and evaporated to
dryness. The residue was then chromatographed over a
silica gel column (Wakogel C-300, 2% MeOH–CH2Cl2)
and crystallisation from CH2Cl2–hexane afforded 4d
(88.9 mg, 71%) as a pale yellow solid. Rf ¼ 0.37 (3%
MeOH–CH2Cl2). 1H NMR (600 MHz, CDCl3, 208C;
diketone 4d was obtained as a mixture of keto and enol
tautomers in the ratio of 1:0.35): d (ppm) keto form 9.46 (s,
2H, NH), 7.23–7.11/7.06–6.98 (m, 20H, Ph-H), 7.17 (d,
J ¼ 3.6 Hz, 2H, pyrrole-H), 3.19 (s, 2H, CH2); enol form d
(ppm) 16.78 (s, 1H, enol-OH), 9.03 (s, 2H, NH), 7.23–
7.11/7.06–6.98 (m, 20H, Ph-H), 7.08 (d, J ¼ 3.0 Hz, 2H,
pyrrole-H), 5.41 (s, 1H, CH). MALDI-TOF-MS: m/z (%
intensity): 505.2 (100), 506.2 (79). Calcd for C35H25N2O2
([M 2 H]2): 505.19.
3.2 1,3-Bis-(3,4-diethyl-5-iodopyrrol-2-yl)-1,3-
propanedione (4c)
A solution of the BF2 complex of 4c (100 mg, 0.16 mmol)
in dry CH2Cl2 (68 ml) was treated with AlCl3 (129.5 mg,
0.97 mmol) and stirred for 5–10 min at reflux temperature.
To the mixture was added t-BuOH (4.83 ml, 0.05 mmol)
and stirred for 5–10 min at room temperature. To the
mixture was added water and stirred for 17 h at the same
temperature. The mixture was partitioned between water
and CH2Cl2, and the combined extracts were dried over
anhydrous MgSO4 and evaporated. The residue was then
purified by silica gel chromatography (Wakogel C-300,
CH2Cl2:hexane ¼ 2:1) to give 4c (34.4 mg, 0.06 mmol,
38%) as a yellow solid. Rf ¼ 0.32 (CH2Cl2:hexane ¼ 2:1).
1H NMR (600 MHz, CDCl3, 208C; diketone 4c was
obtained as a mixture of keto and enol tautomers in the
ratio of 0.19:1): keto form d (ppm) 9.88 (s, 2H, NH), 4.17
(s, 2H, CH2), 2.83 (q, J ¼ 7.8 Hz, 4H, CH2), 2.37 (q,
J ¼ 7.8 Hz, 4H, CH2), 1.23 (m, 6H, CH3), 1.10 (m, 6H,
CH3); enol form d (ppm) 17.40 (s, 1H, enol-OH), 9.10 (s,
2H, NH), 6.15 (s, 1H, CH), 2.75 (q, J ¼ 7.8 Hz, 4H, CH2),
2.40 (q, J ¼ 7.8 Hz, 4H, CH2), 1.23 (m, 6H, CH3), 1.10 (m,
6H, CH3). MALDI-TOF-MS: m/z (% intensity): 565.0
(100). Calcd for C19H23I2N2O2 ([M 2 H]2): 564.98.
3.4 Single-crystal X-ray analysis
Crystallographic data for dipyrrolyldiketones are summar-
ised in Table 1. All the data were collected at 123 K on a
Rigaku RAXIS-RAPID diffractometer with graphite mono-
˚
chromated Mo Ka radiation (l ¼ 0.71075 A); the structure
was solved by a direct method. A single crystal of 2a was
obtained by vapour diffusion of hexane into a CH2Cl2
solution. The data crystal was a colourless prism of
approximate dimensions, 0.40mm £ 0.20mm £ 0.10mm.
A single crystal of 2b was obtained by vapour diffusion of
hexaneintoaCHCl3 solutionwith asmall amountoftoluene.
The data crystal was a colourless prism of approximate
dimensions, 0.40mm £ 0.30mm £ 0.30mm. A single crys-
tal of 3a was obtained by vapour diffusion of hexane into a
CH2Cl2 solution. The data crystal was a yellow prism of
approximate dimensions, 0.45mm £ 0.20mm £ 0.20mm.
A single crystal of 3b was obtained by vapour diffusion of
3.3 1,3-Bis-(3,4-diphenylpyrrol-2-yl)-1,3-propanedione
(4d)
Following the literature procedure (9a), to a CH2Cl2
(7.0 ml) solution of 3,4-diphenylpyrrole (12) (108.5 mg,
0.49 mmol) was added malonyl chloride (34.7 mg,

Supramolecular Chemistry
215
s
s
a
b
g
r
m
l

216
Y. Haketa et al.
hexane into a CH2Cl2 solution. The data crystal was a yellow
prism of approximate dimensions, 0.60 mm £ 0.60
mm £ 0.50mm. A single crystal of 3c was obtained by
vapour diffusion of hexane into a CH2Cl2 solution. The data
crystal was a red block of approximate dimensions,
0.50mm £ 0.20mm £ 0.20mm. A single crystal of 3d was
obtained by vapour diffusion of MeOH into a CDCl3
solution. The data crystal was a yellow prism of approximate
dimensions, 0.80mm £ 0.30mm £ 0.10mm. A single crys-
tal of 3e was obtained by vapour diffusion of MeOH into a
CHCl3 solution. The data crystal was a yellow prism of
approximate dimensions, 0.40mm £ 0.20mm £ 0.05mm.
A single crystal of 4a was obtained by vapour diffusion of
hexane into a CH2Cl2 solution. The data crystal was a yellow
prism of approximate dimensions, 0.60 mm £ 0.40
mm £ 0.30mm. A single crystal of 4b was obtained by
vapour diffusion of hexane into a CH2Cl2 solution. The data
crystal was a yellow prism of approximate dimensions,
0.30mm £ 0.20mm £ 0.20mm. A single crystal of 4c was
obtained by vapour diffusion of hexane into a CH2Cl2
solution. The data crystal was a yellow prism of approximate
dimensions, 0.30mm £ 0.10mm £ 0.05mm. A single crys-
tal of 4d was obtained by vapour diffusion of hexane into a
CH2Cl2 solution with a small amount of toluene. The data
crystal was a yellow prism of approximate dimensions,
0.40mm £ 0.20mm £ 0.20mm. In each case, the non-
hydrogen atoms were refined anisotropically. The calcu-
lations were performed using the Crystal Structure crystal-
lographic software package of Molecular Structure
Corporation. CIF files (CCDC-626137 for 4b (a revised
structure), 664473 and 664474 for 2a and 2b, respectively
and 782949–782956 for 3a–e and 4a,c,d) can be obtained
free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif
Acknowledgements
This work was supported by Ritsumeikan Global Innovation
Research Organization (R-GIRO) Project (2008-2013). The
authors thank Prof. Atsuhiro Osuka, Dr Shigeki Mori, Dr Shohei
Saito, Mr Eiji Tsurumaki, Mr Taro Koide and Mr Takayuki
Tanaka, Kyoto University, for single-crystal X-ray analyses and
Prof. Hitoshi Tamiaki, Ritsumeikan University, for various
measurements. Y.H. thanks JSPS for a Research Fellowship for
Young Scientists.
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4. Summary
As observed in a parent structure, a-aryl-substituted 1,3-
dipyrrolyl-1,3-propanedione (dipyrrolyldiketone) deriva-
tives prefer keto tautomers and form solid-state 1D
hydrogen-bonding chain structures, whose geometries
differ according to the substituents. In contrast, b-alkyl-
and b-aryl-substituted derivatives stabilise the cis-enol
forms in the solid state, resulting in the formation of 1D
hydrogen-bonding chains. At present, although it is not
easy to discuss the details of the correlation of pyrrole
substituents with tautomeric forms in the solid state, the
existence of b-substituents seems to influence the packing
structures and, as a result, preferred tautomeric forms.
A series of dipyrrolyldiketones, the building subunits for
molecular assemblies, afford complexes with not only
boron but also various metal cations that exhibit
fascinating electronic and optical properties along with
the formation of functional materials. Further investigation
to prepare various metal complexes is under way.
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