Transient silenes and their dimethyl-d6 ether donor complexes from the gas-phase pyrolysis of siletanes

The silenes H2Si=CH2 (2), MeHSi=CH2 (7), and Me2Si=CH2 (8) have been prepared by low-pressure flow pyrolysis from the corresponding siletanes H2SiCH2CH2CH2 (1), MeHSiCH2-CH2CH2 (5), and Me2SiCH2CH2CH2 (6). 2, 7, and 8 were identified by residual gas analysis of the pyrolysis stream and by low-temperature NMR spectroscopy of their complexes 2.O(CD3)(2) (16), 7.O(CD3)(2) (17), and 8.O(CD3)(2) (18). The complexes 16-18 are stable to temperatures between -140 and -100 degrees C. Above these temperatures rapid polymerization occurs. The complexation with dimethyl ether obviously stabilizes the silenes and allows their spectroscopic investigation. The Si-29 chemical shifts of the complexes are -25.2 (16), -1.8 (17) and 16.8 ppm (18). The experimental results are supported by ab initio and density functional calculations of geometries and magnetic properties of the silenes 2, 7, and 8 and the dimethyl ether complexes 16-18. The calculations predict a relatively short Si-O separation of about 1.7 Angstrom in the corresponding silene-ether adducts. Natural bond analysis of the donor-acceptor complex 16 reveals that the interaction between the ether molecule and the silene is predominantly ionic, despite the relatively short Si-O distance.