Crystal Packing Induced-Carbon-Carbon Double-Triple Bond Isomerization in a Zirconocene Complex

We present a combined theoretical and experimental analysis of the carboncarbon bond character in two prototypical zirconocene complexes. The two cyclic seven-membered ring zirconium compounds 2a and 2b differ by the substitution of a tert-butyl by a trimethylsilyl group. Due to the coordination of the ?-system to the metal atom, a formally forbidden (eta(2)-allenyl)/enamido-Zr to (eta(2)-alkyne)/?N-imine-Zr complex isomerization is feasible. State-of-the-art dispersion-corrected density functional theory (DFT-D3) is used in both the solid and condensed phase to examine and quantify the experimental structures (X-ray diffraction) and C-13 NMR magnetic shielding. The complementary investigations demonstrate the importance of nonlocal London dispersion interactions. Both X-ray structures agree excellently with the results of the solid-state DFT-D3 calculations. Interestingly, 2b exhibits a mixed allenealkyne form in the solid state, while its gas phase structure has a strong allene character. The substitution leading to 2a prevents this isomerization in the solid state by the intramolecular stabilization of the allene structure. NMR solid and liquid phase measurements confirm the theoretically proposed transition. By combining the experimental and theoretical information, the rather unusual triple/single to double/double-bond transition is attributed to an intermolecular London dispersion induced crystal packing effect.