Linear and Bent Cp*₂ Si: Reversible Phase Transition of a Key Molecule.

Area of Science:

• Organometallic chemistry
• Solid-state chemistry
• Crystallography

Background:

• Decamethylsilicocene (Cp*2Si) typically displays a bent molecular structure, unlike heavier analogues.
• Previous studies noted an unusual co-existence of bent and linear Cp*2Si molecules in the same unit cell, considered an exception.
• The structural anomaly lacked a clear explanation, with electronic or packing effects being speculative.

Purpose of the Study:

• To resolve the structural conundrum of decamethylsilicocene (Cp*2Si).
• To investigate the existence and nature of a low-temperature phase in Cp*2Si.
• To provide a definitive explanation for the presence of linear molecules in the solid-state structure.

Main Methods:

• Single-crystal X-ray diffraction at variable temperatures.
• Differential scanning calorimetry to identify phase transitions.
• Crystallographic analysis of molecular structures and packing.

Main Results:

• A reversible enantiotropic phase transition was identified in Cp*2Si between 80 K and 130 K.
• In the low-temperature phase, all symmetrically independent molecules adopt a bent conformation.
• The transition provides a clear explanation for the previously observed linear molecules in the higher-temperature phase.

Conclusions:

• The presence of linear decamethylsilicocene (Cp*2Si) molecules is attributed to a higher-entropy state above 130 K.
• The low-temperature phase, with all bent molecules, is the thermodynamically favored state.
• This finding clarifies the structural behavior of Cp*2Si and its heavier congeners.