OrganoSilicon Compounds Peter H.M. Budzelaar

OrganoSilicon Compounds 2 Organosilicon compounds Silicon is just like carbon, but: Larger –five- and six-coordinate structures positive More electropositive –easier attack by nucleophiles Does not want to form multiple bonds –SiO 2 vs CO 2 Organo Ge etc... compounds: slow transition to more typical metal chemistry; M II becomes more stable for Pb

OrganoSilicon Compounds 3 Bonds to silicon Si-Cbond covalent, unreactive Si-O, Si-Fvery ("abnormally") strong Si-Cl, Si-Br, Si-Ireactive Si-Siweak (!) Si=Si and Si=Cweak, very reactive

OrganoSilicon Compounds 4 Silicones !

OrganoSilicon Compounds 5 Silicones Based on strong -Si-O- backbone –a pure -Si-Si- backbone would be weak "Slick" Viscosity change little with temperature –silicone oil Easy chemical modification and/or attachment to a surface –Silan –Silylation of GC column surfaces

OrganoSilicon Compounds 6 The Si-O-Si bond Si-O-Si flexible Is also seen in structures of SiO 2 No d  -p  bond !!!! Ph 3 COCPh 3  COC = 128° Ph 3 SiOSiPh 3  SiOSi = 159°

OrganoSilicon Compounds 7 Chain folding - silicones vs alkanes At low temperatures, both alkanes and silicones prefer to form extended chains At higher temperature: For alkanes, the main change is formation of "gauche defects", i.e. partially folded chains. –Reduces contacts between chains, lowers viscosity For silicones, a wider range of Si-O-Si angles will occur, mainly around 180°, resulting in more extended chains –Increases contacts between chains –Counteracts the "normal" viscosity decrease, so viscosity stays nearly constant –Very useful for high-temperature synthetic lubricants

OrganoSilicon Compounds 8 Synthesis Direct synthesis –Si is not electropositive enough for unassisted direct synthesis –An Si/Cu alloy works better: Metathesis Hydrosilylation Catalyzed/initiated by light, free radicals, many transition metal compounds.

OrganoSilicon Compounds 9 Reactivity Nucleophilic attack Electrophilic attack (on C!) With halogens R 3 Si often behaves like a "large hydrogen"

OrganoSilicon Compounds 10 Sterics down group IV Ph 4 CPh 4 SiPh 4 Ge Ph 4 SnPh 4 Pb

OrganoSilicon Compounds 11 Higher coordination numbers Ph 4 Si 4-coordinate Ph 3 SiF coordinate Ph 2 Si(tropolonate) 2 6-coordinate

OrganoSilicon Compounds 12 Maybe not useful, but certainly beautiful C 6 (SiMe 2 OMe) 6