Synthesis and reactivity of doubly bridged mu-eta(5):eta(7)-cyclopentadienylcycloheptatrienyl dimolybdenum complexes

The reaction of tropylium tetrafluoroborate, (C7H7)BF4, with lithium cyclop entadienides 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)(3)Mo(CO)(3)], oxidation with iodine, and hydride abstraction using triphenylcarbenium tet rafluoroborate, (Ph3C)BF4, resulted in the formation of the intensely color ed sesquifulvalene complexes [(eta (5)-2,4-t-Bu2C5H2-C7H6)Mo(CO)(3)I]BF4 (2 a), [(eta (5)-3-t-BuC5H3-C7H6)Mo(CO)(3)I]BF4 (2b), and [(eta (5)-C5H4-C7H6) Mo(CO)3I]BF4 (2c), Because bf the poor solubility and stability of 2c, the arene exchange reaction with [(eta (6)-p-xylene)Mo(CO)(3)] and complexation of the cycloheptatrienyl moiety could only be carried out with 2a,b, givin g bimetallic [mu-eta (5):eta (7)-2,4-t-Bu2C5H2- C7H6)Mo-2(CO)(6)I]BF4 (3a) and [(mu-eta (5):eta (7)-3-t-BuC5H3-C7H6)MO2(CO)(6)I]BF4 (3b), in which the five- and seven-membered rings are coordinated in eta (5) and eta (7) mode s, respectively. 3a,b were thermally converted into Syn-facial [(mu-eta (5) :eta (7)-2,4-t-Bu2C5H2-C7H6)Mo-2(CO)(5)(mu -I)]- BF4 (4a) and [mu-eta (5):e ta (7)-3-t-BuC5H3-C7H6)Mo-2(CO)(5)(mu -I)]BF4 (4b) by intramolecular CO sub stitution and formation of an additional Mo-I-Mo bridge. Reaction with trip henylphosphine lead to cleavage of the newly formed Mo-I bond and addition to the dicarbonylcycloheptatrienylmolybdenum unit to yield [mu-eta (5):eta (7)-2,4-t-Bu2C5H2-C7H6)Mo-2(CO)(5) (5a) and [mu-eta (5):eta (7)-3-t-BuC5H3- C7H6)Mo-2(CO)(5)I(PPh3)]BF4 (5b). Similarly, addition of NaI to 4a gave neu trally charged [mu-eta (5):eta (7)-2,4-t-Bu2C5H2-C7H6)Mo-2(CO)(5)I-2] (5) I n addition, the dihydrosesquifulvalene la has been employed in the synthese s of the methyl derivatives [(mu-eta (5):eta (7)-2,4-t-Bu2C5H2-C7H6)Mo(CO)( 3)(CH3)]BF4 (8) and [(mu-eta (5):eta (7)-2,4-t-Bu2C5H3-C7H6)Mo-2-(CO)(6)(CH 3)]BF4 (9) following a protocol similar to that described for complexes 2 a nd 3. Attempts to convert 9 into a methyl-bridged complex via addition of N aI and Ag(CF3SO3) unexpectedly gave the corresponding triflate [(mu-eta (5) :eta (7)-2,4-t-Bu2C5H2-C7H6)Mo-2(CO)(6)CF3SO3 (10) only. The X-ray crystal structures of 4a . CH2Cl2, 5a . CH2Cl2, and 10 . CH2Cl2 a re reported. Attempts to obtain sesquifulvalene complexes containing Mo-Mo bonds proved to be unsuccessful.