Design and Synthesis of a Molecular Rotary Motor Incorporating Insulating Fragments

1. Design and Synthesis of a Molecular Rotary Motor Incorporating Insulating Fragments Guillaume Vives, Alexandre Carella, Gwénaël Rapenne, Jean-Pierre Launay NanoSciences Group, CEMES UPR 8011 CNRS, 29 rue Jeanne Marvig, F-31055 Toulouse, France • Introduction A molecular motor is a molecule which can transform energy into a unidirectional movement. In the field of nanosciences, the conception and the construction of a molecular motor is still a conceptual and synthetic challenge.[1] The rotative movement of a single molecule has already been observed under the tip of a STM [2] :Hexa-tert-butyl decacyclene behaves on a surface like a non-directional rotor. When the molecule is locked by neighbouring molecules, six bright spots corresponding to the 6 tert-butyl groups (top figure) can be seen. After desincarceration by translation of the central molecule by the tip of the STM, the molecule freely rotates, or more precisely oscillates in a random fashion. The average position of the bulky tert-butyl groups gives a toroïdal shape (bottom figure). Our aim is to design and prepare a molecule able to work as a single molecule for which we have a control on the direction of the rotation. Motionless In rotation ESQC Calculations Design and Requirements Concept The molecule, which must be electrically neutral to ease its deposition, has three parts:  The rotor, a cyclopentadienyl ligand with five rigid arms terminated by electroactive groups (EG). The rotor must be free to rotate / stator and the EG must be connected to the rotor by rigid and linear spacers. The spacers must also ‘insulate’ the EG groups in order to prevent intramolecular electron transfer which could lead to electron crossing through the molecule without rotation.  The stator will be a hydrotris(indazolyl) borate ligand which is functionalized in order to be anchored to the surface.  The joint between the two parts of the motor will be a ruthenium(II) center. • Our concept is based on the use of electroactive groups (EG) which have the ability to transport electrons between the two electrodes of a nanojunction. The rotation will result from the electrostatic repulsion of the oxidized EG.  The control of the unidirectio- nality will be ensured by the dissymmetry of the environment. •  The molecule is designed to be studied by an Atomic Force Microscope (AFM). Step 2: The oxidized form (in red) is pushed back by electrostatic repulsion. A rotation of one fifth of a turn occurs. Step 1: Oxidation of the EG (ferrocene) closest to the anode. Step 3: The ferricinium closest to the cathode is reduced while a new ferricinium is generated at the anode. Step 4: A new rotation of a fifth of a turn occurs after the electrostatic repulsion of the ferricinium cation. Desired behavior: Electron transport by the rotation of the rotor. Unwanted mechanism: Electron transport by intramolecular transfer without movement of the rotor Insulating Role of Platinum Synthesis and Characterisation  Synthesis of the Ruthenium center: Experimental study: 1 • Electrochemistry  Synthesis of the Platinum fragment:  Connection of the two fragments: 2 • Spectroelectrochemisty Hush Formula : Electronic coupling parameter : 1 : Vab = 0,036 eV 2 : Vab = 0,025 eV Spectrum of the electrolysis of 1 and 2 (CH2Cl2, Bu4NBF4 0,1 M)  NMR : Electrochemistry : • CV : CH2Cl2, nBu4PF6 0.1 M, 100 mV.s-1 Theoretical study: 195Pt Geometry optimisation by DFT (BP86/6-31G**) 31P JPt-P(trans) = 2374 Hz The parameter of electronic coupling is determined using the ‘dimer splitting method’. The molecular orbitals are obtained by an extend Huckel calculation on the DFT optimised geometry. 1H • No metal to metal charge transfer transition observed by spectro-electrochemistry experiment • Robustness of the complex proved by electrolysis Vab/3 with Platinum The rotation of the rotor is free down to -90 °C in CD2Cl2 Conclusion References The study of the role of platinum in electronic coupling shows an insulating trend in comparison to conjugated connexion. Therefore, we have synthesised the active part of a molecular motor incorporating five insulating platinum fragments in order to prevent intramolecular electron transfer from an oxidized ferricinium to the ferrocenes centers. Work is underway to introduce ester groups to anchor this motor onto an oxide surface. [1] C.P. Mandl, B. König, Angew. Chem., Int. Ed.2004, 43, 1622-1624 and references therein. [2] J. K. Gimzewski, C. Joachim, R.R. Schlitter, V. Langlais, H. Tang, Science1998, 281, 531-533. [3] A. Carella, G. Rapenne, J.-P. Launay, New J. Chem. 2005, 29, 288-290. [4] G. Vives, A. Carella, G. Rapenne, J.-P. Launay, Manuscript in preparation Acknowledgements GV wants to thank the French Ministry of National Education and the Ecole Normale Supérieure de Lyon for a Ph. D. Fellowship. AC wants to thank the French Ministry of National Education and the Institut Universitaire de France for a Ph. D. Fellowship. The European Science Foundation is also acknowledged for a financial support to attend this conference.