Attractive and repulsive forces in a crystal of [Rb(18-crown-6)][SbCl6] under high pressure.

The compression behavior of [Rb(18-crown-6)][SbCl6] crystal under pressure up to 2.16 (3) GPa was investigated in a diamond anvil cell (DAC) using a mixture of pentane-isopentane (1:4) as the pressure-transmitting fluid. The compound crystallizes in trigonal space group R3 and no phase transition was observed in the indicated pressure range. The low value of pressure bulk modulus [9.1 (5) GPa] found in this crystal is a characteristic of soft materials with predominant dispersive and electrostatic interaction forces. The nonlinear relationship between unit-cell parameters under high pressure is attributed to the influence of reduced intermolecular H...Cl contacts under pressure over 0.73 GPa. It also explains the high compression efficiency of [Rb(18-crown-6)][SbCl6] crystals at relatively low pressures, resulting in a significant shift of the Rb atom to the center of the crown ether cavity. At pressures above 0.9 GPa, steric repulsion forces begin to play a remarkable role, since an increasing number of interatomic H...Cl and H...H contacts become shorter than the sum of their van der Waals (vdW) radii. Below 0.9 GPa, both unit-cell parameter dependences (P-a and P-c) exhibit hysteresis upon pressure release, demonstrating their influence on the disordered model of Rb atoms. The void reduction under pressure also demonstrates two linear sections with the inflection point at 0.9 GPa. Compression of the crystal is accompanied by a significant decrease in the volume of the voids, leading to the rapid approach of Rb atoms to the center of the crown ether cavity. For the Rb atom to penetrate into the center of the crown ether cavity in [Rb(18-crown-6)][SbCl6], it is necessary to apply a pressure of about 2.5 GPa to disrupt the balance of atomic forces in the crystal. This sample serves as a compression model demonstrating the influence of both attractive and repulsive forces on the change in unit-cell parameters under pressure.

A low-valent dinuclear ruthenium diazadiene complex catalyzes the oxidation of dihydrogen and reversible hydrogenation of quinones.

The dinuclear ruthenium complex [Ru2H(µ-H)(Me2dad)(dbcot)2] contains a 1,4-dimethyl-diazabuta-1,3-diene (Me2dad) as a non-innocent bridging ligand between the metal centers to give a [Ru2(Me2dad)] core. In addition, each ruthenium is bound to one dibenzo[a,e]cyclooctatetraene (dbcot) ligand. This Ru dimer converts H2 to protons and electrons. It also catalyzes reversibly under mild conditions the selective hydrogenation of vitamins K2 and K3 to their corresponding hydroquinone equivalents without affecting the C[double bond, length as m-dash]C double bonds. Mechanistic studies suggest that the [Ru2(Me2dad)] moiety, like hydrogenases, reacts with H2 and releases electrons and protons stepwise.

From 0 to II in One-Electron Steps: A Series of Ruthenium Complexes Supported by TropPPh2.

We report the synthesis of a series of ruthenium complexes supported by the phosphine olefin ligand tropPPh2 (trop=5-H-dibenzo-[a,d]cyclohepten-5-yl) in the oxidation states 0, +I, and +II, formed via successive one-electron oxidization steps from Ru(0) (tropPPh2 )2 . The bidentate character of the tropPPh2 ligand and its steric hindrance force the complexes to adopt uncommon geometries, which were investigated by X-ray diffraction analysis. EPR data of the mononuclear Ru(I) complex reveal couplings of the unpaired spin with the ruthenium and two phosphorus nuclei, as well as the olefinic protons which show that the spin is mainly localized on the Ru(I) center.