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

We present a combined theoretical
and experimental analysis of
the carbon–carbon 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 π-system to the metal atom, a formally forbidden (η2-allenyl)/enamido-Zr to (η2-alkyne)/κN-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 13C 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 allene–alkyne 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.