Facile N═N Bond Cleavage of <i>Cis</i> ‐Azobenzene with Bis‐silylenes

Abstract

Abstract The very different features of cooperative disilicon(II)‐mediated N═N bond activation of trans ‐ vs. cis ‐azobenzene are reported, employing two bis‐silylenes with distinct intramolecular Si···Si distances, PhN(LSi:)₂ 1 (L = PhC( t BuN)₂, Si···Si: 2.9 Å) and XT(LSi:)₂ 2 (XT = 9,9‐dimethyl‐xanthene‐4,5‐diyl, Si···Si: 4.3 Å). While trans ‐azobenzene reacts with both bis‐silylenes to form C─H and N═N π bond activation products, the cis ‐isomer undergoes only N═N bond scission. Thus, the reaction of 1 with cis ‐azobenzene at room temperature affords the unprecedented N═N bond cleavage product 4 , featuring a bis‐silaimine with terminal and bridging Si═N moieties. In contrast, the reaction of 2 with cis ‐azobenzene at –30 °C in THF allows for the isolation of the [1+2] cycloaddition intermediate 6 , containing a three‐membered SiN₂ ring (siladiazirane), which rearranges to the N═N bond cleavage product 8 at room temperature. Compound 6 reacts with one additional equivalent of cis ‐azobenzene to form bis‐silaazirane 7 with two SiN₂ rings. Density functional theory (DFT) calculations support stepwise Si(II)···Si(II) cooperative activation mechanisms and provide insights into the role of bis‐silylenes for selective N═N cleavage reactions.