On the structure of unsolvated free-base 5,10,15,20-tetra(3-pyridyl)porphyrin Rüdiger W. Seidel a,⇑ , Richard Goddard b , Constantin Hoch c , Jürgen Breidung b , Iris M. Oppel d a Lehrstuhl für Analytische Chemie, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum, Germany b Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany c Max-Planck-Institut für Festkörperforschung, Heisenberg Straße 1, 70569 Stuttgart, Germany d Institut für Anorganische Chemie, Rheinisch-Westfälische Technische Hochschule Aachen, Landoltweg 1, 52074 Aachen, Germany article info Article history: Received 1 October 2010 Received in revised form 2 November 2010 Accepted 2 November 2010 Available online 8 December 2010 Keywords: Porphyrin Crystal structure DFT calculation NMR spectroscopy abstract A combined X-ray and density functional theory (DFT) study on the structure of 5,10,15,20-tetra(3-pyr- idyl)porphyrin (1) has been undertaken. Single-crystal X-ray diffraction revealed molecules of 1 in the aabb conformation with molecular and crystallographic C i point group symmetry. The porphyrin macro- cycle shows no marked deviation from planarity. Positional disorder of the pyrrole NH atoms is discussed. The crystal packing of 1 is achieved through common off-set stacking of the porphyrin entities. Bulk material of 1 was further investigated by scanning electron microscopy (SEM) and powder X-ray diffrac- tion (PXRD). The crystal and molecular structure of 1 is compared to the known structures of the unsol- vated free-base 2- and 4-pyridyl isomers. The molecular structure of 1 in solution has been investigated by variable temperature 1 H NMR spectroscopy in CDCl 3 . Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Functionalized porphyrins have attracted considerable interest as building blocks in supramolecular chemistry [1–6], crystal engi- neering [7,8] and for the synthesis of nanomaterials [9], because of their relatively rigid planar structure and interesting photophysical properties (i.e. intense light absorption in the visible region and fluorescence). 5,10,15,20-Tetra(4-pyridyl)porphyrin (4TPyP) [10] and its metalloderivatives have been widely used for the construc- tion of coordination polymers [11]. The title compound, 5,10,15,20-tetra(3-pyridyl)porphyrin (3TPyP) (1) (Scheme 1a), has also been used similarly but less frequently. Polynuclear metal- losupramolecular complexes [12,13], two coordination polymers [14,15] and trigonal prismatic shaped coordination cages [16–18] bearing either free-base 1 or its Zn 2+ core metalated derivative have been reported. Most recently, Lipstman and Goldberg re- ported a series of coordination polymers based on 3TPyP and var- ious metal ions [19,20]. Crystallographic studies of unsolvated free-base tetrapyridyl- porphyrins are rather rare. In 2008, Gross and co-workers reported a crystal structure of 5,10,15,20-tetra(2-pyridyl)porphyrin (2TPyP) [21]. Single-crystalline 4TPyP rectangular nanotubes synthesized from the vapor phase, the crystal structure (space group C2/c) of which was determined, were reported by Yoon et al. [22]. Shortly afterwards, Lipstman and Goldberg published the crystal structure of a second polymorph of this compound (space group Cc) [23]. Comparison of these structures reveals significant structural differ- ences in the molecules. Whereas the porphyrin macrocycle is approximately planar in the C2/c polymorph of 4TPyP, in the other and in the crystal structure of 2TPyP, it shows ruf [24] deformation. 2TPyP, and indeed 1, have the additional property that the free rotation of the pyridyl groups in solution at room temperature can lead to four major possible conformational isomers in the so- lid-state (Scheme 1b). In the reported structure, 2TPyP adopts the abab [25] conformation [21]. Clearly, detailed structural infor- mation about 1 is important for a better understanding of the fac- tors that determine these differences. To our knowledge, 1 was first mentioned in the literature in 1975 [26]. A while ago, we published a preliminary crystal struc- ture determination from weak data resulting in an isotropically re- fined structural model [27]. In the meantime, while this work was still in progress, another structure determination of unsolvated free-base 1 has been reported, where the crystal structure was de- scribed briefly [19]. Here, we report an improved structural model based on new diffraction data. Furthermore, a density functional theory study (DFT), powder X-ray diffraction analysis (PXRD) and scanning electron microscopy (SEM) have been undertaken. Struc- tural features and intermolecular interactions of the title com- pound in the solid-state are discussed. Conformational preferences of 1 in solution have been investigated by tempera- ture-dependent 1 H NMR spectroscopy. 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.11.011 ⇑ Corresponding author. Tel.: +49 234 32 25464; fax: +49 234 32 14420. E-mail address: Ruediger.Seidel@rub.de (R.W. Seidel). Journal of Molecular Structure 985 (2011) 307–315 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc