Finally! A Room-Temperature Stable Silicon Carbonyl Complex

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For some years, we have successfully prepared group 15 element compounds with unusual bonding situations such as gallapnictenes with gallium-pnictogen double bonds, homo- and heteroleptic pnictogen-centered radicals as well as reactive intermediates such as carbene-stabilized stibinidenes. While extending these reactions to other main group element compounds, we became particularly interested in silicon chemistry.

Following a preliminary report by R. Fischer et al., we reacted DDPGa {DDP = HC[C(Me)N(2,6-i-Pr2-C6H3)]2} with silicon tetrabromide. Our first aim was the preparation of the DDPGa-coordinated, electron-rich homoleptic silylene [DDP(Br)Ga]2Si: using the same synthetic approach that was established for the synthesis of carbene-stabilized stibinidenes. However, the reaction with SiBr4 did not yield the expected silylene species, even when trapping agents such as carbenes (IDipp) were used. Instead, we isolated compound 3; a product resulting from the C–C bond activation of one DDP ligand which alongside points to the intermediate formation of the highly reactive silylene [DDP(Br)Ga]2Si:. To confirm this hypothesis, additional experiments were carried out.

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Figure 1: Synthesis of 16 and exemplary reactions of 5; Ar is 2,6-i-Pr2-C6H3.

CO/CO2 activation is one of the prime targets of main group element chemistry. Consequently, we carried out our first experiments with CO2 from which we were able to isolate crystals after a short period of time. Surprisingly, our crystallographer identified the isolated product as a silicon-carbonyl compound! This has never been seen before since reactions of silylenes with CO or CO2 typically proceed with deoxygenative homocoupling to disilylketenes or with reductive C–C coupling. This is an astonishing discovery that we first couldn't believe. A room temperature stable main group carbonyl? We had to prove it to ourselves and to our readers and immediately recorded the compound’s IR spectrum, which showed a strong and characteristic vibration at 1945 cm–1, hence confirming, together with the accompanying X-ray and NMR analysis, the synthesis of [DDP(Br)Ga]2SiCO 5. Subsequently, 5 was prepared in better yields by directly reacting the silylene intermediate with carbon monoxide. 

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Figure 2: A) ATR-IR spectrum of [L(Br)Ga]2Si:-CO 5 and B) molecular structure of 5 as derived from single-crystal X-ray diffraction. Thermal ellipsoids in the structures represent the 30% probability level. For clarity, hydrogen atoms were removed.

The electronic nature of the intermediate silylene [DDP(Br)Ga]2Si: as well as of the carbonyl complex 5 was examined more precisely by quantum chemical computations utilizing density functional theory (DFT) with and without dispersion corrections included. The silylene [DDP(Br)Ga]2Si: shows strong electron-donating properties, which originate from the electropositive DDP(Br)Ga substituents and the wide Ga–Si–Ga bond angle. London dispersion also contributes to the stability of 5 through mutual interactions of the isopropyl groups with each other and the aryl moieties.

We tested whether the silicon-carbonyl compound could also be used as a masked silylene. Reactions with hydrogen or phosphorus tribromide confirmed this application. In addition, we were able to synthesize compound 6 in a CO exchange reaction with cyclohexyl isocyanide. We are looking forward to new insights into the reactivity of this remarkable carbonyl complex and others that can be derived from this novel concept!

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Stephan Schulz

Profesor, University of Duisburg-Essen

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