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Too Persistent to Give Up: Aromaticity in Boron Clusters Survives Radical Structural Changes.
Jordi Poater1,2, Clara Viñas3, Ines Bennour3
1Departament de Quı́mica Inorgànica i Orgànica & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain.
Journal of the American Chemical Society
|April 24, 2020
Summary
The stability of carboranes like meta-C2B10H12 is linked to carbon atom placement, not aromaticity. Aromaticity in boron clusters persists through structural changes, even in nido-carboranes.
Area of Science:
- Inorganic Chemistry
- Computational Chemistry
- Materials Science
Background:
- Carboranes (C2B10H12) exhibit isomerism, with meta-carborane (m-C2B10H12) being more stable than ortho-carborane (o-C2B10H12) due to easier deboronation of the latter.
- Closo-boranes and carboranes are recognized for their aromatic properties, a characteristic often linked to their stability.
- Traditional views equate sandwich metallocenes with sandwich metallabis(dicarbollides), suggesting similar electronic and structural properties.
Purpose of the Study:
- To investigate the relationship between deboronation difficulty, thermodynamic stability, and aromaticity in carborane isomers.
- To explore the persistence of aromaticity in boron clusters, including nido-carboranes, and compare it with traditional aromatic systems.
- To differentiate the aromaticity characteristics of sandwich metallocenes and metallabis(dicarbollides).
Main Methods:
- Experimental studies involving heating o-C2B10H12 to induce isomerization to m-C2B10H12 and deboronation.
- Density Functional Theory (DFT) calculations to analyze thermodynamic stability and electronic structures.
- Nuclear Magnetic Resonance (NMR) spectroscopy (1H and 11B) to probe electronic currents and aromaticity in nido-carboranes.
Main Results:
- The enhanced thermodynamic stability of m-C2B10H12 is attributed to the specific arrangement of carbon atoms, not differences in aromaticity.
- Aromaticity is demonstrated to be present in nido-carboranes, indicating its resilience to significant structural modifications within boron clusters.
- Metallabis(dicarbollides) exhibit global aromaticity, contrasting with the local aromaticity observed in the ligands of sandwich metallocenes.
- Experimental evidence of diatropic currents in nido-[C2B9H12]- using NMR spectroscopy confirms aromaticity in these clusters.
Conclusions:
- Aromaticity in boron clusters is a robust feature that survives substantial structural changes, including the transition from closo to nido frameworks.
- The stability of carboranes is primarily governed by atomic positioning rather than solely by aromaticity.
- A clear distinction in aromaticity (global vs. local) exists between metallabis(dicarbollides) and metallocenes.
- The electron requirements paradigm for clusters is supported, explaining the aromatic nature of both closo and nido species.
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