Incomplete Electrocyclization of a Sterically Hindered 1,4-Diphosphabutadiene

Area of Science:

• Organophosphorus Chemistry
• Main Group Chemistry
• Supramolecular Chemistry

Background:

• Butadiene typically remains stable and avoids electrocyclization.
• Congeners with heavier elements often undergo facile electrocyclization to cyclobutene systems.
• The stability and reactivity of π-conjugated systems with heavier elements remain an active area of research.

Purpose of the Study:

• To investigate the electrocyclization behavior of a sterically encumbered 1,4-diphosphabutadiene.
• To explore the influence of temperature on the solid-state structure and reactivity of this system.
• To characterize the formation and reversibility of a 1,2-dihydrodiphosphete intermediate.

Main Methods:

• Synthesis and characterization of 2,3-bis(phosphanylidene)-1,4-disilinane.
• Solution-state Nuclear Magnetic Resonance (NMR) spectroscopy, specifically 31P NMR.
• Solid-state 31P Cross-Polarization Magic-Angle Spinning (CP-MAS) NMR spectroscopy.
• Single-crystal X-ray diffraction analysis at variable temperatures.

Main Results:

• The 1,4-diphosphabutadiene skeleton undergoes "incomplete electrocyclization" to form a 1,2-dihydrodiphosphete (3,4-diphosphacyclobutene) in the solid state upon cooling.
• Solution NMR indicates a predominant open-ring P=C-C=P structure, while solid-state NMR shows the P=C-C=P structure is dominant.
• Variable-temperature single-crystal X-ray analysis confirmed the reversible formation of the 1,2-dihydrodiphosphete system with decreasing temperature and its disappearance upon warming.

Conclusions:

• The study demonstrates a unique, reversible solid-state electrocyclization reaction in a sterically encumbered diphosphabutadiene.
• Temperature plays a crucial role in controlling the equilibrium between the open-ring diphosphabutadiene and the closed-ring dihydrodiphosphete in the crystalline state.
• This finding provides new insights into the reactivity and structural dynamics of π-conjugated systems containing heavier elements.