Synthesis, Structure and Reactivity of some 2,6-Disubstituted Dimethylsilylbenzenes

Theoretical Study of the Catalytic Hydrogenation of Alkenes by a Disilaferracyclic Complex: Can the Fe-Si σ-Bond-Assisted Activation of H-H Bonds Allow Development of a Catalysis of Iron?

The mechanisms associated with the hydrogenation of alkenes catalyzed by the iron complex Fe(cis-CO)₂{o-(SiMe₂)₂C₆H₄}₂(H)₂ (1) were investigated by DFT calculations. The complex 1 has a structure in which the iron center is bonded to four silicon atoms and two hydrides. Secondary Si···H···Si interactions were also observed. The exchange of a 1,2-bis(dimethylsilyl)benzene ligand with ethylene and hydrogen gives a disilaferracycle bearing η²-(CH₂═CH₂) and η²-H₂ ligands. The catalytic cycle initiated from the disilaferracycle involves cleavage of a H-H linkage assisted by an Fe-Si bond to form Fe-H and η¹-(H-Si) moieties (step 1), hydrogen migration from the Fe-H group to the η²-(CH₂═CH₂) ligand which accomplishes the insertion of ethylene into the Fe-H bond (step 2), and reaction of the resulting β-agostic ethyl moiety with the η²-(H-Si) group to form ethane on the iron atom (step 3). The octahedral geometry of 1 as well as the presence of π-acidic CO ligands and Fe-Si σ-bonds contributes to all of the catalytic intermediates and the transition states being in the low-spin state. Steps 1 and 3 correspond to the σ-complex-assisted metathesis (σ-CAM) mechanisms proposed by Perutz and Sabo-Etienne, suggesting that these mechanisms can assist in the design of iron-based hydrogenation catalysts operating under mild conditions.

The Nature of Hydrogen Bonding Involving the Siloxane Group

Variation of the Si–O–Si angle in siloxane compounds is a way to tune their basicity from highly hydrophobic systems at linear geometry to hydrophilic systems at small angles. This has great potential in the design of new siloxane materials with properties distinct from those of known silicones. We investigate hydrogen bonds with the siloxane linkage as an acceptor in a large range of Si–O–Si angles for the two hydrogen-bonded complexes disiloxane⋯silanol [(H3Si)2O⋯HOSiH3] and disiloxane⋯water [(H3Si)2O⋯HOH] with free disiloxane [H3SiOSiH3] as reference in a quantum-mechanical ab-initio study. Geometry, electron density, and the electron localizability indicator provide several complementary indicators of hydrogen bonding which show how Si–O–Si angle variation affects the nature and strength of these unusual hydrogen bonds.