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Demasiado persistente para renunciar: la aromaticidad en los grupos de boro sobrevive a cambios estructurales
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
Resumen
La estabilidad de los carboranos como el meta-C2B10H12 está relacionada con la ubicación del átomo de carbono, no con la aromaticidad. La aromaticidad en los grupos de boro persiste a través de cambios estructurales, incluso en los nidocarboranos.
Área de la Ciencia:
- Química inorgánica
- Química computacional
- Ciencias de los materiales
Sus antecedentes:
- Los carboranos (C2B10H12) exhiben isomerismo, siendo el metacarborano (m-C2B10H12) más estable que el ortocarborano (o-C2B10H12) debido a la desboronación más fácil de este último.
- Los closoboranos y los carboranos son reconocidos por sus propiedades aromáticas, una característica a menudo vinculada a su estabilidad.
- Las opiniones tradicionales equiparan los metallocenos sándwich con los metallabis sándwich, lo que sugiere propiedades electrónicas y estructurales similares.
Objetivo del estudio:
- Investigar la relación entre la dificultad de desboronación, la estabilidad termodinámica y la aromaticidad en los isómeros de carborano.
- Explorar la persistencia de la aromaticidad en los grupos de boro, incluidos los nidocarboranos, y compararlo con los sistemas aromáticos tradicionales.
- Para diferenciar las características de aromaticidad de los metallocenos sándwich y los metallabis (dicarboluros).
Principales métodos:
- Estudios experimentales que incluyen el calentamiento de o-C2B10H12 para inducir la isomerización a m-C2B10H12 y la desboronación.
- Cálculos de la Teoría Funcional de Densidad (DFT) para analizar la estabilidad termodinámica y las estructuras electrónicas.
- Espectroscopia de resonancia magnética nuclear (1H y 11B) para sondear las corrientes electrónicas y la aromaticidad en los nidocarboranos.
Principales resultados:
- La estabilidad termodinámica mejorada de m-C2B10H12 se atribuye a la disposición específica de los átomos de carbono, no a las diferencias en la aromaticidad.
- Se ha demostrado que la aromatización está presente en los nidocarboranos, lo que indica su resistencia a las modificaciones estructurales significativas dentro de los grupos de boro.
- Metallabis ((dicarbollidos) exhiben una aromaticidad global, en contraste con la aromaticidad local observada en los ligandos de los metallocenos sándwich.
- La evidencia experimental de las corrientes diatrópicas en nido-[C2B9H12]- utilizando espectroscopia de RMN confirma la aromaticidad en estos grupos.
Conclusiones:
- La aromaticidad en los grupos de boro es una característica robusta que sobrevive a cambios estructurales sustanciales, incluida la transición de los marcos de closo a nido.
- La estabilidad de los carboranos se rige principalmente por el posicionamiento atómico en lugar de únicamente por la aromaticidad.
- Existe una clara distinción en la aromaticidad (global frente a local) entre los metallabis (dicarboluros) y los metallocenos.
- Se apoya el paradigma de los requisitos de electrones para los grupos, explicando la naturaleza aromática de las especies de closo y de nido.
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