Stretching-Induced Conductance Increase in a Spin-Crossover Molecule.

We investigate transport through mechanically triggered single-molecule switches that are based on the coordination sphere-dependent spin state of Fe(II)-species. In these molecules, in certain junction configurations the relative arrangement of two terpyridine ligands within homoleptic Fe(II)-complexes can be mechanically controlled. Mechanical pulling may thus distort the Fe(II) coordination sphere and eventually modify their spin state. Using the movable nanoelectrodes in a mechanically controlled break-junction at low temperature, current-voltage measurements at cryogenic temperatures support the hypothesized switching mechanism based on the spin-crossover behavior. A large fraction of molecular junctions formed with the spin-crossover-active Fe(II)-complex displays a conductance increase for increasing electrode separation and this increase can reach 1-2 orders of magnitude. Theoretical calculations predict a stretching-induced spin transition in the Fe(II)-complex and a larger transmission for the high-spin configuration.

Single-Molecule Spin Switch Based on Voltage-Triggered Distortion of the Coordination Sphere.

Here, we report on a new single-molecule-switching concept based on the coordination-sphere-dependent spin state of Fe(II) species. The perpendicular arrangement of two terpyridine (tpy) ligands within heteroleptic complexes is distorted by the applied electric field. Whereas one ligand fixes the complex in the junction, the second one exhibits an intrinsic dipole moment which senses the E field and causes the distortion of the Fe(II) coordination sphere triggering the alteration of its spin state. A series of complexes with different dipole moments have been synthesized and their transport features were investigated via mechanically controlled break-junctions. Statistical analyses support the hypothesized switching mechanism with increasing numbers of junctions displaying voltage-dependent bistabilities upon increasing the Fe(II) complexes' intrinsic dipole moments. A constant threshold value of the E field required for switching corroborates the mechanism.

Josiphos-catalyzed asymmetric homodimerization of ketoketenes.

In this paper the development of a chiral phosphine-catalyzed homodimerization of ketoketenes that provides access to a variety of highly substituted ketoketene dimer ß-lactones (11 examples) is reported. The Josiphos catalytic system displays good to excellent enantioselectivity (up to 96% ee). Ring-opening reactions of the enantioenriched ketoketene dimers were also carried out to access 1,3-diketones, enol esters, and ß-hydroxyketones with good diastereoselectivity.

Organocatalytic dimerization of ketoketenes.

A general method for the catalytic dimerization of ketoketenes is described. Tri-n-butylphosphine was found to be the optimal organocatalyst for the racemic reaction. When lithium iodide was used as an additive, the reaction was rendered selective for dimer formation (dimer/trimer > or = 16:1). Ring-opening reactions of the ketoketene dimers as well as preliminary studies toward the development of an asymmetric variant are also reported.