Competition between Hydrogen and Anagostic Bonds in Ruthenocene Phases under High Pressure

High pressure favors anagostic bonds CH center dot center dot center dot Ru, which leads to a new polymorph of ruthenocene RuCp2, where Cp denotes the cyclopentadienyl ring [C5H5](-). Ruthenocene can be isothermally compressed in its ambient-pressure phase a up to 3.9 GPa, when it transforms to the new phase, beta. The transition between phases a and beta proceeds with an exceptionally wide hysteresis extending between 0.7 and 3.9 GPa. The transition destroys single crystals of RuCp2, but a single crystal of polymorph beta can be obtained by high-pressure nucleation (above 3.9 GPa) and its isochoric growth continued above 1.0 GPa. On decompression, phase beta transforms back to phase a at 0.7 GPa. The structures of phases a (space group Pnma) and beta (space group Pcmb) have been determined by X-ray diffraction in the pressure range from 0.1 MPa up to 3.9 GPa. The eclipsed conformation of alpha-RuCp2 is retained in phase beta, which contrasts with the conformational phase transitions in ferrocene and nickelocene. The lattice dimensions and molecular orientations in ruthenocene polymorphs are clearly related, which leads to the characteristic twinning-like oriented growth of the crystals of concomitant phases.

Automated summary

High pressure promotes the formation of anagostic bonds CH center dot center dot center dot Ru in the polymorphs of ruthenocene RuCp2. The transition between different phases of ruthenocene under pressure exhibits a wide hysteresis and is accompanied by twinning-like oriented growth of crystals.