Silylene complexes stabilized by sulphur substituents; a structure and reactivity study

Abstract

A new base-stabilized silylene (silanediyl) complex (t-BuS) 2(HMPA)SiFe(CO) 4 ( 9b) has been prepared from Na 2Fe(CO) 4 and (t-BuS) 2SiCl 2 ( 2b) in the presence of HMPA (hexamethylphosphoric triamide). An X-ray structure analysis of 9b shows the silylene coordinated to the iron atom [Fe—Si = 2.278(1) Å]. The HMPA is bound via its oxygen atom to the unsaturated silicon [O—Si 1.734(2) Å], which adopts a distorted tetrahedral coordination geometry. A comparison of 9b with the analogous oxo compound (t-BuO) 2(HMPA)Si Fe(CO) 4 ( 9a) indicates an almost identical stabilizing effect of the electron-deficient silicon atom in 9b by π-mesomeric and polarization effects of the t-BuS substituents. The conformation of the substituents found at the silicon atom is reproduced by a force field calculation. A base-free silylene complex (t-BuS) 2SiFe(CO) 4 (10) has been observed directly by NMR spectroscopy ( 1H NMR: δ 1.70, s; 29Si NMR: δ 83.2, s) and was further characterized by a derivatization reaction with HMPA to form 9b. The HMPA adduct 9b is a valuable model compound for mechanistic investigations: photolysis of 9b in the presence of 2,3-dimethylbutadiene as trapping reagent yields 1,4-addition of the silylene to the butadiene. In the absence of any trapping reagents, polysilanes are formed. The experiments provide evidence for a silylene cleavage mechanism.