An isolable, crystalline complex of square-planar silicon(IV)

doi:10.1016/j.chempr.2021.05.002

Chem

Volume 7, Issue 8, 12 August 2021, Pages 2151-2159

https://doi.org/10.1016/j.chempr.2021.05.002 Get rights and content

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Highlights

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The first square-planar silicon(IV) is isolated
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A low-lying LUMO provokes CH-agostic interaction and visible-light absorption
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Transition metals are mimicked without leaving the natural oxidation state

The bigger picture

Tetrahedral silicon(IV) compounds are the building blocks of our Earth’s crust. Here, we describe the first species of silicon(IV) with a square-planar configuration. The structural deformation has substantial consequences for the compounds’ physicochemical properties and imparts features usually associated with transition metals. Upon planarization, the frontier molecular orbital gap shrinks by more than 50% and enables ligand-element charge transfer, CH-bond agostic interactions, and spontaneous reactivity with inert bonds. Small frontier molecular orbital gaps are critical for bond-activation reactivity, catalysis, and photochemistry with transition metals. Traditional approaches to mimic these characteristics with the more abundant p-block elements rely on unusual valence or oxidation states. With the realization of square-planar silicon(IV), these peculiarities start reaching p-block elements in their natural oxidation states.

Summary

The structure and reactivity of silicon(IV), the second most abundant element in our Earth's crust, is determined by its invariant tetrahedral coordination geometry. Silicon(IV) with a square-planar configuration (ptSi^IV) represents a transition state. Quantum theory supported the feasibility of stabilizing ptSi^IV by structural constraint, but its isolation has not been achieved yet. Here, we present the synthesis and full characterization of the first square-planar coordinated silicon(IV). The planarity provokes an extremely low-lying unoccupied molecular orbital that induces unusual silicon redox chemistry and CH-agostic interactions. The small separation of the frontier molecular orbitals enables visible-light ligand-element charge transfer and bond-activation reactivity. Previously, such characteristics have been reserved for d-block metals or low-valent p-block elements. Planarization transfers them, for the first time, to a p-block element in the normal valence state.

Graphical abstract

Keywords

silicon

planar

p-block element

structural constraint

agostic interaction

bond activation

ligand-element charge transfer

element-ligand cooperativity

UN Sustainable Development Goals

SDG7: Affordable and clean energy

SDG12: Responsible consumption and production

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