Preparation and structure of the first ansa-molybdenocene and tungstenocene derivatives. Reaction of [O(SiMe₂C₅H₄)₂]MoH₂ with methyl methacrylate to probe the olefin attacking site

Bridged-bis(cyclopentadienyl) molybdenum and tungsten dihydrides and their derivatives have been synthesized for the first time. [(1,1,3,3-Tetramethyldisiloxane-1,3-diyl)bis(η⁵-cyclopentadienyl)lmolybdenum dihydride (1a) and tungsten dihydride (1b) were prepared by sodium borohydride reduction of [O(SiMe₂C₅H₄)₂]MCl₂ (2a: M Mo, 2b: M W) which was obtained by the reaction of disodium 1,1,3,3-tetramethyl-1,3-bis(cyclopentadienyl)disiloxane with MCl₄(thf)₂ (M Mo, W). The dichlorides 2 reacted with ZnMe₂ and NaSEt to afford [O(SiMe₂C₅H₄)₂]MMe₂ (3a: M Mo, 3b: M W) and [O(SiMe₂C₅H₄)₂]M(SEt)₂ (4a: M Mo, 4b: M W), respectively. Photolysis of 1b in benzene gave [O(SiMe₂C₅H₄)₂]WH(C₆H₅) (5b). An X-ray crystal analysis of 3a has proved that the bridging chain blocks one of the side positions of the complex, the siloxy unit being present at one side of the MeMoMe bisector. Consistent with side position attack by olefins, the reaction rate of 1a with methyl methacrylate was found to be nearly one half that of the parent Cp₂MoH₂ (6a).