Synthesis and Reactivity of Doubly Bridged μ-η⁵:η⁷-Cyclopentadienylcycloheptatrienyl Dimolybdenum Complexes

Abstract

The reaction of tropylium tetrafluoroborate, (C7H7)BF4, with lithium cyclopentadienides afforded isomeric mixtures of dihydrosesquifulvalenes ((1,3,5-cycloheptatrien-7-yl)cyclopentadienes) of the type C5H3RR‘−C7H7 (1a, R = R‘ = tert-butyl; 1b, R = tert-butyl, R‘ = H; 1c, R = R‘ = H). Lithiation of the hydrocarbons 1a−c followed by reaction with [(CH3CN)3Mo(CO)3], oxidation with iodine, and hydride abstraction using triphenylcarbenium tetrafluoroborate, (Ph3C)BF4, resulted in the formation of the intensely colored sesquifulvalene complexes [(η5-2,4-t-Bu2C5H2−C7H6)Mo(CO)3I]BF4 (2a), [(η5-3-t-BuC5H3−C7H6)Mo(CO)3I]BF4 (2b), and [(η5-C5H4−C7H6)Mo(CO)3I]BF4 (2c). Because of the poor solubility and stability of 2c, the arene exchange reaction with [(η6-p-xylene)Mo(CO)3] and complexation of the cycloheptatrienyl moiety could only be carried out with 2a,b, giving bimetallic [(μ-η5:η7-2,4-t-Bu2C5H2−C7H6)Mo2(CO)6I]BF4 (3a) and [(μ-η5:η7-3-t-BuC5H3−C7H6)Mo2(CO)6I]BF4 (3b), in which the five- and seven-membered rings are coordinated in η5 and η7 modes, respectively. 3a,b were thermally converted into syn-facial [(μ-η5:η7-2,4-t-Bu2C5H2−C7H6)Mo2(CO)5(μ-I)]BF4 (4a) and [(μ-η5:η7-3-t-BuC5H3−C7H6)Mo2(CO)5(μ-I)]BF4 (4b) by intramolecular CO substitution and formation of an additional Mo−I−Mo bridge. Reaction with triphenylphosphine lead to cleavage of the newly formed Mo−I bond and addition to the dicarbonylcycloheptatrienylmolybdenum unit to yield [(μ-η5:η7-2,4-t-Bu2C5H2−C7H6)Mo2(CO)5I(PPh3)]BF4 (5a) and [(μ-η5:η7-3-t-BuC5H3−C7H6)Mo2(CO)5I(PPh3)]BF4 (5b). Similarly, addition of NaI to 4a gave neutrally charged [(μ-η5:η7-2,4-t-Bu2C5H2−C7H6)Mo2(CO)5I2] (5). In addition, the dihydrosesquifulvalene 1a has been employed in the syntheses of the methyl derivatives [(μ-η5:η7-2,4-t-Bu2C5H2−C7H6)Mo(CO)3(CH3)]BF4 (8) and [(μ-η5:η7-2,4-t-Bu2C5H2−C7H6)Mo2(CO)6(CH3)]BF4 (9) following a protocol similar to that described for complexes 2 and 3. Attempts to convert 9 into a methyl-bridged complex via addition of NaI and Ag(CF3SO3) unexpectedly gave the corresponding triflate [(μ-η5:η7-2,4-t-Bu2C5H2−C7H6)Mo2(CO)6(CH3)]CF3SO3 (10) only. The X-ray crystal structures of 4a·CH2Cl2, 5a·CH2Cl2, and 10·CH2Cl2 are reported. Attempts to obtain sesquifulvalene complexes containing Mo−Mo bonds proved to be unsuccessful.