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Synthesis of Molybdenum Nitrido Complexes for Triple-Bond Metathesis of Alkynes and Nitriles

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journal contribution
posted on 04.07.2011, 00:00 by Eric S. Wiedner, Kimberley J. Gallagher, Marc J. A. Johnson, Jeff W. Kampf
Complexes of the type NMo(OR)3 (R = tertiary alkyl, tertiary silyl, bulky aryl) have been synthesized in the search for molybdenum-based nitrile–alkyne cross-metathesis (NACM) catalysts. Protonolysis of known NMo(NMe2)3 led to the formation of NMo(O-2,6-iPr2C6H3)3(NHMe2) (12), NMo(OSiPh3)3(NHMe2) (5-NHMe2), and NMo(OCPh2Me)3(NHMe2) (17-NHMe2). The X-ray structure of 12 revealed an NHMe2 ligand bound cis to the nitrido ligand, while 5-NHMe2 possessed an NHMe2 bound trans to the nitride ligand. Consequently, 17-NHMe2 readily lost its amine ligand to form NMo(OCPh2Me)3 (17), while 12 and 5-NHMe2 retained their amine ligands in solution. Starting from bulkier tris-anilide complexes, NMo(N[R]Ar)3 (R = isopropyl, tert-butyl; Ar = 3,5-dimethylphenyl) allowed for the formation of base-free complexes NMo(OSiPh3)3 (5) and NMo(OSiPh2tBu)3 (16). Achievement of a NACM cycle requires the nitride complex to react with alkynes to form alkylidyne complexes; therefore the alkyne cross-metathesis (ACM) activity of the complexes was tested. Complex 5 was found to be an efficient catalyst for the ACM of 1-phenyl-1-butyne at room temperature. Complexes 12 and 5-NHMe2 were also active for ACM at 75 °C, while 17-NHMe2 and 16 did not show ACM activity. Only 5 proved to be active for the NACM of anisonitrile, which is a reactive substrate in NACM catalyzed by tungsten. NACM with 5 required a reaction temperature of 180 °C in order to initiate the requisite alkylidyne-to-nitride conversion, with slightly more than two turnovers achieved prior to catalyst deactivation. Known molybdenum nitrido complexes were screened for NACM activity under similar conditions, and only NMo(OSiPh3)3(py) (5-py) displayed any trace of NACM activity.