Syntheses and characterization of 1- and 2-hydroxypyren-coordinated Sn(IV) porphyrins: Transmission-like motion of alcoholato-coordinated Sn (IV) porphyrin Md. Moshiur Rahman, Kamrul Hassan, Kazunori Hirabayashi, Soichi Sato, Toshio Shimizu, Ken-ichi Sugiura ⇑ Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachi-Oji, Tokyo 192-0397, Japan article info Article history: Received 27 April 2019 Accepted 3 July 2019 Available online 17 July 2019 Keywords: Tin(IV)porphyrin Pyrene p-Expanded ligand Axial ligand Molecular transmission abstract Treatments of dihydroxyporphyrinato Sn(IV) with 1- and 2-hydroxypyrenols afforded 1- and 2-hydrox- ypyrene-coordinated Sn(IV) porphyrins. Because of their stability, these complexes enable conventional chemical manipulations, such as SiO 2 column chromatography. Single-crystal diffraction studies revealed that the dihedral angles between the porphyrin and the pyrenolato ligand are small. The large high-field shifts of 1 H NMR signals attributable to the pyrenolato ligand support the parallel alignment of the por- phyrin and the pyrenolato ligand. Singlet signals of the b-hydrogens of porphyrin suggest the free rota- tion of the pyrenolato ligand around the Sn-O coordination bond. In addition, low-field shifts were observed for these b-hydrogens. These phenomena suggest the free rotation of the pyrenolato ligand around the pyrenolato O-C bond. Because these two rotation axes are positioned at an angle of 120°, this moiety could be regarded to show the transmission-like motion in molecular machines. Ó 2019 Elsevier Ltd. All rights reserved. 1. Introduction The synthesis and characterization of porphyrins is one of the most important research areas in contemporary interdisciplinary chemistry and/or science [1]. The research started from the charac- terization of naturally occurring porphyrins, such as hemes and chlorophylls [2], and shifted to advanced materials science because the porphyrins’ narrow HOMO–LUMO gap and perfect coordina- tion ability to most elements are useful to improve the properties of advanced materials, e.g., conductivity [3], magnetism [4], and photo-induced electron and/or energy transfer, thereby leading to artificial photosynthesis [5]. To improve the properties of porphyrins, thousands of chemical modifications of the porphyrin nucleus have been conducted [1]. Along with those efforts, complex formation reactions with most elements have been examined. However, studies of the modifica- tion of the axial ligand of metalloporphyrins are limited compared to extensive research of the modification of the porphyrin ligand. The primary reason is the weak coordination bond between the coordination atom of the axial ligand and the incorporated metal. For example, Zn(II) porphyrin, a widely used metalloporphyrin, coordinates with the N-atom of pyridine, amine, and imidazole derivatives [6]. However, the association constants of these coordi- nation bonds do not exceed 10 4 , and significant dissociation is observed in solution. Other metalloporphyrins have a similar nat- ure; exceptions are, for example, Ru(II), Si(IV), and P(V) porphyrins, which form a strong metal–ligand coordination bond comparable to a covalent bond [7]. However, Ru is too expensive for large-scale experiments and P(V) produces ionic salt such as [P(V)Por] + X  which prevents the chromatography. Considering these circumstances, we focused on Sn(IV) por- phyrins (Sn(IV)Pors) [8,9] and their ligand design. The stable oxida- tion state of Sn is +4 and the formal charge of the porphyrin ligand is 2. Therefore, two negative charges should be supplied by two mono-anionic ligands (Ls), such as chloro (Cl  ) or hydroxo (OH  ) producing [Sn(IV)Por]L 2 . It should be emphasized that the Sn bond with Cl  or OH  is as stable as a covalent bond. The hydroxo ligand is replaceable with an alcoholato or a carboxylato, such as pheno- lato (PhO  ) and benzoato (PhCO 2  ), respectively, and this nature could enhance the potential of this class of materials modifying the foregoing Ph group. For example, Ravikanth and co-workers successfully prepared multiporphyrin arrays of Sn(IV)Por with hydroxy-substituted meso-tetraphenylporphyrins (TPPs) [9–11]. Unique molecular structures are created by bulky axial ligands https://doi.org/10.1016/j.poly.2019.07.004 0277-5387/Ó 2019 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Fax: +81 42 677 2525. E-mail address: sugiura@porphyrin.jp (K.-i. Sugiura). Polyhedron 171 (2019) 128–136 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly