The disappearance of the stable slightly-bent isomer of germasilaallenes and the appearance of its cyclic isomer. (1st December 2020)

• Record Type: Journal Article
• Authors: Xi, Hong-Wei; Bedoura, Sultana; Sk, Mahasin Alam; Lim, Kok Hwa
• Is Part Of: Polyhedron. Volume 192(2021)
• Publication Date: 2020-12-01
• Subjects: Germasilaallene -- Highly-bent isomer -- Cyclic isomer -- DFT -- NBO -- Natural steric analysis; Chemistry, Inorganic -- Periodicals; Chimie inorganique -- Périodiques; Organometaalverbindingen; Anorganische chemie; 546.05
• DOI: 10.1016/j.poly.2020.114821
• Languages: English
• ISSNs: 0277-5387

Abstract

Graphical abstract: The disappearance of their cyclic isomers of 1, 3-digermasilaallene and 2-germadisilaallene are due to higher ring strain at cyclic isomers. The stronger π orbital delocalization effect and aromatic effect with the germasilaallenic bending resulted into decreased potential energy so that there is no transition barrier to stabilize the slightly-bent isomer. Highlights: Disappearance of "bond-stretch" isomerism are due to slightly higher ring strain. π-Orbital delocalization and aromaticity let slightly-bent germasilaallene vanish. Donor-acceptor interaction stabilize a slightly-bent geometry of germasilaallenes. Abstract: Systematic density functional theory (DFT) calculations and natural bond orbital (NBO) analyses were carried out to understand the reason behind the disappearance of linear and slightly-bent isomer of germasilaallenes and the bond-stretch phenomenon of their cyclic isomer, unusual electronic properties of highly-bent isomers of germasilaallenes. Our results show that in contrast to trisilaallene and trigermaallene, 1, 3-digermasilaallene and 1-germadisilaallene do not have stable cyclic isomers, due to increased ring strains. DFT calculations and NBO analyses indicate that the stability of highly-bent isomers of germasilaallenes arise from three parts: p(terminal) → p(central) donor–acceptor interaction, p(terminal)-p(terminal) partial overlapping and the aromatic delocalization along the ring. Different from mono- and di-germasilaallene, DFT and CCSD calculations indicate that there is no stable minimum along the first half, from 180° to 90.0°, of the potential energy surface of trigermasilaallene. For trigermasilaallene, for bending angle α > 120.0°, π-π* orbital delocalization effect dominates the stability and when α < 120.0°, aromatic delocalization of the conformer becomes the main factor. With the bending angle lighter, the stable energy becomes larger. Thus, the germasilaallenes gradually become more stable along the bending PES from 180° to 90.0° and no barrier stabilizes the slightly-bent isomer. DFT calculations indicate that strong donor effect from substituents attached to terminal Si (Ge) to central Si (Ge) can stabilize the slightly-bent isomer of germasilaallenes.