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).
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