Dipyrrolyl Precursors to Bisalkoxide Molybdenum Olefin Metathesis Catalysts

Addition of 2 equiv of lithium pyrrolide to Mo(NR)(CHCMe2R‘)(OTf)2(DME) (OTf = OSO2CF3; R
= 2,6-i-Pr2C6H3, 1-adamantyl, or 2,6-Br2-4-MeC6H2; R‘ = Me or Ph) produces Mo(NR)(CHCMe2R‘)(NC4H4)2
complexes in good yield. All compounds can be recrystallized readily from toluene or mixtures of pentane
and ether and are sensitive to air and moisture. An X-ray structure of a 2,6-diisopropylphenylimido species
shows it to be an unsymmetric dimer, {Mo(NAr)(syn-CHCMe2Ph)(η5-NC4H4)(η1-NC4H4)}{Mo(NAr)(syn-CHCMe2Ph)(η1-NC4H4)2}, in which the nitrogen in the η5-pyrrolyl bound to one Mo behaves as a donor to
the other Mo. All complexes are fluxional on the NMR time scale at room temperature, with one symmetric
species being observed on the NMR time scale at 50 °C in toluene-d8. The dimers react with PMe3 (at Mo)
or B(C6F5)3 (at a η5-NC4H4 nitrogen) to give monomeric products in high yield. They also react rapidly with
2 equiv of monoalcohols (e.g., Me3COH or (CF3)2MeCOH) or 1 equiv of a biphenol or binaphthol to give 2
equiv of pyrrole and bisalkoxide or diolate complexes in ∼100% yield.