Reactions of the Doubly Bridged Bis(cyclopentadienyl) Dinuclear Molybdenum Complex (Me 2 C)(Me 2 Si)[(η 5 ‐C 5 H 3 )Mo(CO) 3 ] 2 with a Carboxylate‐Substituted Allene

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Reaction of the doubly bridged bis(cyclopentadienyl) dinuclear molybdenum complex (Me2C)(Me2Si)[(η5-C5H3)Mo(CO)3]2 (1) with equivalent molar amount of allene H2C=C=CHCO2Me in refluxing toluene gave four products: complex (Me2C)(Me2Si)[(η5-C5H3)2Mo2(CO)4(μ-η2-η2-HC≡CCH2CO2Me)] (3) (10 %) with a crosswise substituted alkyne bridge, η2-η2-HC≡CCH2CO2Me derived from a 1,3-hydrogen shift of H2C=C=CHCO2Me, complex (Me2C)(Me2Si)[(η5-C5H3)2Mo2(CO)2(O)2{μ-η1-η3-C(CO2Me)CHCH2}] (6) (16 %) with a bridging η1,η3-C(CO2Me)CHCH2 allylic group, and allene C–C coupled complexes (Me2C)(Me2Si)[(η5-C5H3)2Mo2(CO)4{μ-η3-η3-{MeO2CCH(CH2)C}(CHCHCHCO2Me)}](4) (7 %) and (Me2C)(Me2Si)[(η5-C5H3)2Mo2(CO)4{μ-η3-η3-{MeO2CCH(CH2)C}2}] (5) (13 %), in which the two allene molecules were coupled in head-to-center and center-to-center coupling modes, respectively. When 1 was treated with an excess amount of H2C=C=CHCO2Me, only C–C coupled products 4 (10 %) and 5 (22 %) were obtained. In comparison, reaction of the singly bridged bis(cyclopentadienyl) dinuclear molybdenum complex (Me2C)[(η5-C5H4)Mo(CO)3]2 (2) with H2C=C=CHCO2Me only gave the η2-η2 coordinated complex (Me2C)[(η5-C5H4)Mo2(CO)4(μ-η2-η2-H2CCCHCO2Me)] (7) (21 %). These results marked the different reactivities of doubly bridged bis(cyclopentadienyl) dinuclear molybdenum complex 1 and the corresponding singly bridged analogue 2. The formation of these complexes was discussed and the molecular structures of 3–7 were determined by X-ray diffraction. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)