Propelling single molecules in a controlled manner along an unmodified surface remains extremely challenging because it requires molecules that can use light, chemical or electrical energy to modulate their interaction with the surface in a way that generates motion. Nature's motor proteins have mastered the art of converting conformational changes into directed motion, and have inspired the design of artificial systems such as DNA walkers and light- and redox-driven molecular motors. But although controlled movement of single molecules along a surface has been reported, the molecules in these examples act as passive elements that either diffuse along a preferential direction with equal probability for forward and backward movement or are dragged by an STM tip. Here we present a molecule with four functional units--our previously reported rotary motors--that undergo continuous and defined conformational changes upon sequential electronic and vibrational excitation. Scanning tunnelling microscopy confirms that activation of the conformational changes of the rotors through inelastic electron tunnelling propels the molecule unidirectionally across a Cu(111) surface. The system can be adapted to follow either linear or random surface trajectories or to remain stationary, by tuning the chirality of the individual motor units. Our design provides a starting point for the exploration of more sophisticated molecular mechanical systems with directionally controlled motion.

The design, synthesis, stability, and catalytic activity of nitro-substituted Hoveyda-Grubbs metathesis catalysts are described. The highly active and stable meta- and para-substituted complexes are attractive from a practical point of view. These catalysts operate in very mild conditions and can be successfully applied in various types of metathesis [ring-closing metathesis, cross-metathesis (CM), and enyne metathesis]. Although the presence of a NO(2) group leads to catalysts that are dramatically more active than both the second-generation Grubbs's catalyst and the phosphine-free Hoveyda's carbene, enhancement of reactivity is somewhat lower than that observed for a sterically activated Hoveyda-Grubbs catalyst. Attempts to combine two modes of activation, steric and electronic, result in severely decreasing a catalyst's stability. The present findings illustrate that different Ru catalysts turned out to be optimal for different applications. Whereas phosphine-free carbenes are catalysts of choice for CM of various electron-deficient substrates, they exhibit lower reactivity in the formation of tetrasubstituted double bonds. This demonstrates that no single catalyst outperforms all others in all possible applications.

and obtained her degree in Pharmacology in 1999. She has received her Ph.D. degree in Chemistry from the A. N. Nesmeyanov Institute of Organoelement Compounds, Moscow, Russia, with Professor Yuri N. Belokon'. She had a predoctoral stay at the Institute of Organic Chemistry, Polish Academy of Sciences with Dr. C. Grela. After receiving her Ph.D. in 2003, she joined the group of Professor Ben L. Feringa at the University of Groningen as a postdoc. Her work was focused on enantioselective catalysis, mechanistic studies, and total synthesis, in particular using asymmetric conjugate addition. In 2007 she obtained the position of research scientist at Tibotec BVBA, a division of Johnson & Johnson in Belgium. Tim den Hartog was born in 1982, and he studied at the University of Amsterdam, The Netherlands, to obtain his degree in Organic Chemistry in 2005. He is currently performing his graduate studies in the Feringa Group at the University of Groningen in the field of asymmetric catalysis. His research focuses primarily on asymmetric conjugate addition. Koen Geurts was born in 1975, and he studied at the University of Groningen, The Netherlands, and obtained his degree in Organic Chemistry in 2002. He is currently finishing his graduate studies in the Feringa Group at the University of Groningen in the field of asymmetric catalysis. His thesis focuses primarily on asymmetric allylic alkylation. He has recently accepted a position as a chemical development engineer within SABIC Europe.

The development of accessible metathesis catalysts that combine high activity with excellent tolerance to a variety of functional groups has been key to the widespread application of olefin metathesis in organic synthesis. In spite of the general superb application profile of the ruthenium carbene 1 a, its limited thermal stability and the low activity towards substituted double bonds are major drawbacks. [1] Specifically, the preparation of substituted olefins with electron-withdrawing functionality (such as a,b-unsaturated carbonyl compounds, nitriles, sulfones, etc.) through cross metathesis (CM) with terminal alkenes remains a difficult task. The newly introduced highly active ruthenium alkylidene complexes with sterically demanding N-heterocyclic carbene (NHC) [3] ligands have dramatically alleviated this limitation. [2] Compounds of type 1 b and 1 c were found to be efficient catalysts in the reactions of previously metathesisinactive substrates, including a,b-unsaturated olefins (Scheme 1). [2,4] Hoveyda and co-workers have recently established 2 as a remarkably robust complex, which promotes olefin metathesis by a ™release±return∫ mechanism. [5] Despite the fact that phosphane-free catalyst 2 was found to be more sluggish than 1 b, [6] it has a superior general reactivity toward electrondeficient olefins. [7] The fact that the ruthenium carbene 2 is air-stable and can be easily purified by standard silica-gel chromatography and recycled after the reaction is a particularly appealing facet of this chemistry. [5,8] Blechert and Wakamatsu have shown recently that the replacement of the isopropoxystyrene ™ligand∫ in 2 by binolor biphenyl-based styrene results in a large improvement in the activity of the catalyst, as complexes 4 and 5 are much more reactive than both 2 and the ™second-generation∫ Grubbs catalyst 1 b. [9] During our project aimed at the preparation of the immobilized metathesis catalyst, we prepared the bromo analogue 3 of Hoveyda©s catalyst 2. [10] Although the reactivity patterns of complexes 2 and 3 were in general similar, the latter system was visibly less reactive in some model reactions. [4b] This result once again shows that even a small variation in the isopropoxystyrene ™ligand∫ can result in a change in the activity of the catalyst. Impressed by results published recently by Blechert and Wakamatsu, [9] we decided to investigate the electronic effects in the isopropoxystyrene ™ligand∫ sphere of complex 2 which are not fully understood. [11] At first, we decided to test whether a decrease in the electron density of the styrene part of 2 would result in increased catalyst reactivity.As illustrated in Scheme 2, we used commercially available 6 as a starting material for preparation of the corresponding ruthenium carbene 9. The green microcrystalline complex 9 was easily obtained in good yield (83 %) by the reaction of 1 b (1 equiv) and CuCl (1 equiv) with styrene 8 (1 equiv), followed by routine flash chromatography. Having secured an efficient method for the preparation of complex...