Benzvalene-like Carboranes: Genuine Rule Breakers and Good Kinetic Stability

Area of Science:

• Computational Chemistry
• Inorganic Chemistry
• Materials Science

Background:

• Benzvalene-like structures are important valence isomers, studied in electron-neutral and electron-rich systems.
• Stable benzvalene-like examples with electron-deficient skeletons are rare due to complex bonding.
• The Wade-Mingos rule governs the structures of boranes and carboranes, but exceptions are sought.

Purpose of the Study:

• To investigate the lowest-lying structures of six-vertex dicarboranes (C2B4R6) using computational methods.
• To identify potential 'rule breaker' structures that challenge the Wade-Mingos rule.
• To explore the stability and synthetic potential of novel carborane structures.

Main Methods:

• Extensive structural search using Density Functional Theory (DFT) and CBS-QB3 computational levels.
• Analysis of thermodynamic and kinetic stabilities of various substituted dicarborane structures.
• Comparison of calculated structures with predictions from the Wade-Mingos rule.

Main Results:

• Previously reported benzvalene-like structures (III) were found not to be the lowest-lying.
• Several substituted benzvalene-like structures (III') were identified as genuine lowest-lying isomers.
• These 'benzvalene-like carboranes' represent the fourth class of lowest-lying structures for six-vertex dicarboranes.

Conclusions:

• Benzvalene-like structures are confirmed as true 'rule breakers' of the Wade-Mingos rule in dicarboranes.
• The identified substituted benzvalene-like carboranes exhibit good kinetic stability.
• This discovery opens avenues for synthesizing novel carboranes beyond the scope of the Wade-Mingos rule.