Design and synthesis of highly-active group 6 metal catalysts for use in triple -bond metathesis.

Several novel group 6 metal complexes were synthesized and their use in catalytic triple bond metathesis was investigated. Application of these complexes to C≡N and C≡C containing compounds led to reactivity of the Mo≡N bond, the first conversion of a metal-nitride species to a metal-alkylidyne, a facile four-step synthesis of highly-active alkyne metathesis catalysts, and a new class of compounds that catalyze alkyne-nitrile cross metathesis. Three new molybdenum-nitride complexes were synthesized, N≡Mo(OC(CH 3)2CF3)3 (1), N≡Mo(OC(CF 3)2CH3)3(NCMe)x (x = 0, 1) (2), and N≡Mo(OC(CF3)3)3 (NCMe) (3). Exposure of 2 and 3 to 15NCMe led to degenerate N-atom exchange, determined through 15N NMR spectroscopic studies, and the realization that the Mo≡N bond was metathesis active. Compounds 2 and 3 also catalytically scrambled N-atoms among nitriles in solution. The reactions were influenced by the solvent, proceeding more rapidly in THF- d8 than in CD2Cl2. Treatment of 1-3 with asymmetric alkynes led to the formation of alkyne metathesis products. Studies of 1-3 in the presence of symmetrical alkynes and 1,2-dimethoxyethane (DME) revealed that, when heated, the nitride species were transformed irreversibly to their alkylidyne analogues. Furthermore, the omission of DME from the reaction mixture reduced the reaction time from 52 hours to 15 hours. The product EtC≡Mo(OC(CF3) 2CH3)3(DME) (4) was recovered in yields greater than 55% on scales larger than 2.5 grams. Treatment of 4 with three equiv of BCl3 and 9.4 equiv of DME led to the formation of EtC≡MoCl3(DME) (5). This species proved to be a versatile precursor to various alkylidyne complexes, leading to the first isolation of EtC≡Mo(OC(CF3)3) 3(DME) (6). Three new and one previously known tungsten alkylidyne complexes were synthesized via a novel method. Both N≡W(OC(CF3)2CH 3)3(DME) (7) and [Li(DME)2][N≡W(OC(CF 3)2CH3)4] (8) were found to be active in nitrile-alkyne cross metathesis. The process progressed more rapidly with 7 than with 8, though a method was found to interconvert the two. Successful metathesis reactions were observed with nitriles containing aryl and alkyl halides, ethers and thiophenes at temperatures ranging between 65°C and 95°C. The metathesis reaction was also observed for N≡W(OC(CH3)2CF3) 3 (9), though it proceeded at a slower rate. No cross-metathesis was observed for N≡W(OC(CF3)3)3(DME) (10).PhDInorganic chemistryOrganic chemistryPolymer chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126028/2/3224881.pd