Investigations of rotamers in diaxial Sn(IV)porphyrin phenolatesdtowards a molecular timepiece Sheshanath V. Bhosale, Connie Chong, Craig Forsyth, Steven J. Langford * , Clint P. Woodward School of Chemistry, Monash University, Clayton Victoria 3800, Australia article info Article history: Received 10 March 2008 Received in revised form 21 May 2008 Accepted 30 May 2008 Available online 5 June 2008 Keywords: Porphyrin Coordination Tin(IV) Molecular dynamics Rotamers Supramolecular chemistry Clock abstract An approach to the formation of molecular timepieces is outlined based on differentiating between rotamers in diaxial Sn(IV) porphyrin phenolates. Two models are explored in detail. The first explores how the rates of rotation of the diaxial ligands is discriminated based on steric hindrance of the two porphyrin macrocycle faces at low temperature. The second model explores a ‘stopwatch’ function based on the ligation of Ag(I) ions to a 5,15-dipyridylporphyrinato tin(IV) complex bearing 3-hydroxypyridine ligands. The complexation inhibits rotation of the axial ligand, a result, which can be reversed by pre- cipitation of Ag(I) using tetraethylammonium bromide. X-ray crystallography has also been used to characterize two Ag(I) 5,15-dipyridylporphyrinato tin(IV)complexes. The two isoforms differ in their supramolecular organization. One structure is formed through a cofacial stack linking each porphyrin by Ag(I) coordination. The other displays a sheet-like coordination polymer structure. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Mechanical devices that function as a result of the rotary motion of some component are common within our macroscopic world. Some familiar examples are helicopters, propellers, internal com- bustion engines, ATP synthase and flagellae to name but a few. In these cases, the rotation leads to mechanical work. However, rota- tional mechanical devices can also be used for measurement, typi- cally as a result of (random) changes in rotary motion as derived from environmental stimuli including thermal energy. Examples here include gyroscopes, wind vanes and wind turbines, and indeed many macroscopic rotors have been demonstrated at a molecular level. 1,2 One class of driven rotor is the analogue clock, stopwatch or timepiecedinstruments that measure and record time with a dial. 3 In chemical terms, a timepiece is an entity that kinetically iso- merizes through 86,400 dependent rotational isomers in a 24 h period. The dependency lies in the defined transition from one isomer to the next. Each of these rotational isomers, or rotamers, is measurable and hence instructive. Molecular rotamers, particularly displaying two states have been demonstrated elegantly in the fields of supramolecular chemistry. 4 Three example classes are the fluorescent molecular rotors, 5 the rotary motors of Feringa 6 and the catenanes of Sauvage 7 and Stoddart. 8 The latter system being used in a unique memory element for molecular based computing. 9 In each case, stimuli (e.g., electronic or photonic) produce a change in the system leading to the predominance of a new and different rotamer, which is addressable by spectroscopic means. It occurred to us that the metalloporphyrin macrocycle, func- tionalized at all 12 outer peripheral positions could lead to a series of rotamers that mimic the basis of an analogue timepiece (Fig. 1). In this simple model, the four meso positions can be likened to the 3, 6, 9 and 12 positions of the timepiece. The eight remaining 12 3 6 9 1 2 4 5 7 8 10 11 Figure 1. Metalloporphyrins bearing two axial ligands could conceivably act as a mo- lecular timepiece in which the 12 positions around the porphyrin’s outer periphery represent the 12 hourly positions on an analogue clock face. * Corresponding author. Tel.: þ61 3 9905 4569; fax: þ61 3 9905 4597. E-mail address: steven.langford@sci.monash.edu.au (S.J. Langford). Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2008.05.127 Tetrahedron 64 (2008) 8394–8401