HETEROCYCLES

Communication | Special issue | Vol. 90, No. 1, 2015, pp. 104-107
Received, 6th March, 2014, Accepted, 24th March, 2014, Published online, 24th March, 2014.
DOI: 10.3987/COM-14-S(K)17

■ Crystal Structures of 3-Methylpyrrolo[2,3-b]tropone and Its Copper(II) Complex

Kanji Kubo,* Taisuke Matsumoto, Keiko Ideta, and Akira Mori

Department of Life Science and Technology, Faculty of Engineering, Hokkai-Gakuen Univiersity, Sapporo, Hokkaido 062-8605, Japan

Abstract

The crystal structures of 3-methylpyrrolo[2,3-b]tropone (1) and its Cu(II) complex (12∙Cu) were analyzed by X-ray crystallography. The structure of 1 exists in the crystal in its keto form rathar than in the enol form (1’). Pyrrolotropone (1) formed a 2:1 complex with Cu(II). The Cu(II) ion has a tetragonal environment formed by two tropone O atoms and two pyrrole N atoms.

Pyrroles represent an important class of heterocycles in organic chemistry. They are structural units in many natural products such as porphyrin, which form the complexes with Fe(II) and Cu(II) ions.1 Troponoids, being a remarkable class of non-benzenoid π conjugated systems, form complexes with metal salts such as CuCl2, ZnCl2, CdCl2, CoCl2, MnCl2, and HgCl2.2 Pyrrolo[2,3-b]tropones (cyclohepta[b]pyrrol-8(1H)-one), being isoelectronic with 8-quinolinols, are interesting compounds biochemically.3 They give coloration when treated with metal ion such as Fe(III) and Cu(II). The color fades upon addition of mineral acid.3,4 This suggests that pyrrolo[2,3-b]tropones can be used as an analytical reagent for metal cations. However the crystal structures of the metal complex of pyrrolo[2,3-b]tropone have not been elucidated. We now report the structures of 3-methylpyrrolo[2,3-b]tropone (1) and its Cu(II) complex (12∙Cu).

Compound (1)5 was prepared by condensation of 2-hydrazinotropone with 1-propanal accompanied by cyclization as reported in a previous paper.3 Single crystals of 1 were grown in a mixture of chloroform-methanol (1:1 v/v) at room temperature. An ORTEP drawing6 of molecule of 1 is shown in Figure 1a. The structure of 1 contains two crystallographically independent molecules (1a) and (1b). The pyrrolotropone moiety of 1 is approximately planar; the deviations from the least-squares plane defined by C1-C9/O1/N1 do not exceed 0.1 Å. The C-C bond lengths of seven-membered ring show bond alternation typical of tropone; the C1-O1 bond length (1.248(3) Å) is similar to that observed in the structure of unsubstituted tropone (1.259 Å).7 The C-C and C-N bond lengths in the pyrrole ring are close to those found in unsubstituted pyrrole.8 The objective location of the H1 atom bond to N1, rather than O1, as well as the tropone-like bond-length distribution, makes assignment of the keto form (1) rather than the enol form (1’), unambiguous.

The pyrrole NH group participates in an intermolecular hydrogen bond of the N-H∙∙∙O type as shown in Figure 1b and Table 2. An intermolecular π∙∙∙π interaction is observed. The distance between intermolecular pyrrolotropone planes is 3.424(4) Å for C9∙∙∙C13, which is within the range associated with π∙∙∙π interactions.4,9 There is an intermolecular C-H∙∙∙π interaction with distance for this type of interaction (2.8–3.1 Å).4,10 The combination of intermolecular N-H···O, π∙∙∙π, and C-H···π interactions in 1 builds up a three-dimensional network.
Single crystals of Cu(II) complex of 1 (12∙Cu) were grown in a mixture of chloroform-methanol (1:1 v/v) containing of 1 (2 eq.) and copper(II) acetate (1eq.) at room temperature. An ORTEP drawing11 of molecule of 12∙Cu is shown in Figure 2a. The pyrrolotropone (1) forms 2:1 complex with Cu(II) ion. The complex has a tetragonal planar Cu(II). The Cu atom is positioned at the center of symmetry, coordinated with two N atoms of the pyrrole ring and two O atoms of the tropone ring. The Cu-O bond distance (1.9860(19) Å) is shorter than that (2.036 Å)12 of bis(2-acetylpyrrolate)copper(II) and longer than that (1.913, 1.915 Å)13 of bis(tropolonato)copper(II). The Cu-N bond distance is similar to that (1.912 Å)13 of bis(2-acetylpyrrolate)copper(II). The C-O bond length is longer than that of 1 and agree with those (1.27-1.31 Å)13,14 in other metal tropolonates.
Intermolecular π∙∙∙π interactions in the crystal structure of 12∙Cu are observed in Figure 2b. The distance between intermolecular pyrrolotropone planes is 3.454(4) Å for C1∙∙∙C8iv (symmetry codes: (iv) x, y-1, z), which is within the range associated with π∙∙∙π interactions.4,9 Intermolecular C-H∙∙∙π and C-H∙∙∙O interactions (Figure 2b and Table 3) are observed with distances typical for these type of interactions:

C-H∙∙∙π=2.8–3.1 Å and C-H∙∙∙O=2.5–2.7 Å.4,10 The combination of intermolecular C-H∙∙∙π and C-H∙∙∙O interactions and π∙∙∙π stacking interactions in 12∙Cu build up a three-dimensional network.

ACKNOWLEDGEMENTS
This work was performed under the Cooperative Research Program of "Network Joint Research Center for Materials and Devices".

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