Facile N═N Bond Cleavage of Cis-Azobenzene with Bis-silylenes

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(tBuN)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.