Strong Intramolecular Secondary Si⋅⋅⋅N Bonds in Trifluorosilylhydrazines

The simple silylhydrazines F 3 SiN(Me)NMe 2 ( 1 ), F 2 Si(N(Me)NMe 2 ) 2 ( 2 ), and F 3 SiN(SiMe 3 )NMe 2 ( 3 ) have been prepared by reaction of SiF 4 with LiN(Me)NMe 2 and LiN(SiMe 3 )NMe 2 , while F 3 SiN(SnMe 3 )NMe 2 ( 4 ) was prepared from SiF 4 and (Me 3 Sn) 2 NNMe 2 ( 5 ). The compounds were characterized by gas‐phase IR and multinuclear NMR spectroscopy ( 1 H, 13 C, 14/15 N, 19 F, 29 Si, 119 Sn), as well as by mass spectrometry. The crystal structures of compounds 1 – 5 were determined by X‐ray crystallography. The structures of free molecules 1 and 3 were determined by gas‐phase electron diffraction. The structures of 1 , 2 , and 4 were also determined by ab initio calculations at the MP2/6‐311+G** level of theory. These structural studies constitute the first experimental proof for the presence of strong Si⋅⋅⋅N β‐donor–acceptor bonds between the SiF 3 and geminal NMe 2 groups in silylhydrazines. The strength of these non‐classical Si⋅⋅⋅N interactions is strongly dependent on the nature of the substituent at the α‐nitrogen atom of the SiNN unit, and has the order 3 > 4 > 1 . The valence angles at these extremely deformed α‐nitrogen atoms, and the Si⋅⋅⋅N distances are (crystal/gas): 1 104.2(1)/106.5(4)°, 2.438(1)/2.510(6) Å; 3 83.6(1)/84.9(4)°, 2.102(1)/2.135(9) Å; 4 89.6(1)°, 2.204(2) Å.