Twist of a Silicon–Silicon Double Bond: Selective Anti-Addition of Hydrogen to an Iminodisilene

Abstract

Hydrogenation of alkenes with C═C bonds is a ubiquitous reaction in organic chemistry. However, this transformation remains unknown for heavier counterparts, disilenes with Si═Si bonds. Here we report the isolation of (Z)-diiminodisilyldisilene 2 featuring a highly trans-bent and twisted structure and the longest silicon-silicon double bond reported to date. In silico studies suggested that the Si═Si bond in 2 is described as very weak double donor-acceptor bond. We utilized the remarkable electronic and structural features of this product to achieve the first demonstration of hydrogen activation by a multiply bonded silicon compound under ambient conditions. Interestingly, NMR and X-ray analysis gave exclusively racemic (RR/SS)-1,2-disilane 3a, indicating a stereospecific trans-hydrogenation of the Si═Si bond. In-depth calculations revealed that in strong contrast to the reactivity of C═C bonds, a concerted anti-addition pathway was favored due to the twisted structure of 2.

Precise Activation of Ammonia and Carbon Dioxide by an Iminodisilene

“…The 1 J (Si‐H) of 174 Hz of 2 a , however, is in fact comparable with the coupling constant of IIb′ (187 Hz) and as expected smaller than the one of silane 2 b (202 Hz). In analogy to the stereospecific hydrogenation, the crystallographic data of 2 a (Figure ) elucidated a racemic mixture of ( RR / SS )‐1‐aminodisilane with a typical Si−Si single bond length (Si1−Si6 2.3984(6) Å) that is slightly shorter than that of the respective hydrogen activation product (2.4142(7) Å) . Both silanes exhibit short Si−N NHI bond lengths ( 2 a Si1−N1 1.6694(15) Å, Si6−N4 1.6605(16) Å; 2 b Si1−N1 1.6350(17) Å) and quite obtuse SiN NHI C angles ( 2 a Si1‐N1‐C1 177.51(14)°, Si6‐N4‐C21 167.43(15)°; 2 b Si1‐N1‐C1 177.51(15)°), indicating an increased s‐character of the orbital on the nitrogen centers .…”

“…To explain the controllable product formation depending on the reaction conditions, we performed DFT calculations at the B3LYP‐D3(BJ)/def2‐SVP level of theory. In agreement with our reported trans ‐hydrogenation, an analogous anti ‐transition state (TS1) was found with a low energy barrier involving two ammonia molecules (7.7 kcal mol −1 ) (Scheme ; see the Supporting Information). This activation energy is significantly smaller than the value of the congeneric anti ‐addition of H 2 (15.6 kcal mol −1 ) and hence explains the proceeding of the NH 3 reaction at −78 °C.…”

The Road Travelled: After Main‐Group Elements as Transition Metals

“…However, reactivity towards small molecule activation remains scarce in comparison, although a few examples have shown NH 3 activation . The first reported H 2 activation by a multiply bonded Si compound was accounted by our group in 2017 . Use of a bulky NHI ligand, with t Bu substituents rather than aryl, prevents formation of the silepin and yields a trans ‐bent disilene compound ( 40 ) with a twisted double bond.…”

Functional Disilenes in Synthesis

“…The realization that peripheral functionality is nonetheless well tolerated has considerably expanded the potential of disilene chemistry to include, for instance, the deliberate incorporation into conjugated π‐electron systems Such systems have even been shown to be active emitting layers in yellow OLEDs . Moreover, the relatively small HOMO–LUMO gap in conjugated Si=Si bonds leads to spectacular new reactivity patterns because of the very recently published article about the selective anti‐addition of hydrogen to a disilene . Similarly, the facile [2+2] cycloaddition of Si=Si bonds may be exploited for the preparation of organic‐inorganic hybrid polymers: proof‐of‐concept for this approach was provided by the synthesis of a σ‐π‐conjugated material …”

“…[124] Moreover, the relatively small HOMO-LUMOg ap in conjugated Si=Si bonds leads to spectacular new reactivity patterns because of the very recently published article about the selective anti-addition of hydrogen to ad isilene. [125] Similarly,t he facile [2+ +2] cycloaddition of Si=Si bonds may be exploited for the preparation of organicinorganic hybrid polymers:p roof-of-concept for this approach was provided by the synthesis of a s-p-conjugated material. [126] In the emerging field of small unsaturated siliconoid clusters, the other facet of unsaturated silicon species becomes more and more apparent;i na greementw ith the relative weakness of the Si=Si bond more compact,c lustera rrangements are often more stable.…”