Experimental and Theoretical Investigations of Catalytic Alkyne Cross-Metathesis with Imidazolin-2-iminato Tungsten Alkylidyne Complexes

The imidazolin-2-iminato tungsten alkylidyne complexes [Me3CC≡W(NImR)(OR′)2] (4a: R = tBu, R′ = CMe3; 4b: R = Dipp, R′ = CMe3; 5a: R = tBu, R′ = CMe(CF3)2; 5b: R = Dipp, R′ = CMe(CF3)2 have been prepared from [Me3CC≡W(OCMe3)3] (2) and [Me3CC≡W{OCMe(CF3)2}3(dme)] (3, dme = 1,2-dimethoxyethane) by reaction with the lithium reagents Li(NIm tBu) and Li(NImDipp), generated with MeLi from 1,3-di-tert-butylimidazolin-2-imine (Im tBuNH) or 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-imine (ImDippNH), respectively. Reaction of 3 with Li[N(tBu)Ar]·OEt2 (Ar = 3,5-dimethylphenyl) afforded the amido complex [Me3CC≡W{N(tBu)Ar}{OCMe(CF3)2}2] (6). Addition of Li[OCPh(CF3)2] to [Me3CC≡WCl3(dme)] (1) produced the dme-free complex [Me3CC≡W{OCPh(CF3)2}3] (7), which, upon treatment with Li(NIm tBu), gave the alkylidene complex 8, presumably formed by activation and addition of an ortho-C−H bond across the W≡C bond in the intermediate alkylidyne complex. Treatment of 1 with Li[OC(CF3)3] led to the substitution of only two chloride ligands and formation of cis-[Me3CC≡WCl{OC(CF3)3}2(dme)] (9), which exhibits long-range through-space 19F−19F coupling between the fluorine atoms of the two OC(CF3)3 ligands. Reaction of 9 with Li(NIm tBu) resulted in partial cleavage of the Im tBuN ligand and ligand redistribution to afford the dinuclear tungsten alkylidyne complex 10. The propylidyne complex [EtC≡W(NIm tBu){OCMe(CF3)2}2] (12) was obtained by treatment of 5a with 3-hexyne, which proceeded via the metallacyclobutadiene complex 11. Complex 5a is able to rapidly catalyze alkyne cross-metathesis of 3-heptyne to give a statistical 1:2:1 mixture of 3-hexyne, 3-heptyne and 4-octyne. The catalytic homodimerization of 1-phenylpropyne under vacuum-driven conditions was studied for 5a, 5b and 6 at 30 and 80 °C. The molecular structures of complexes 2, 3, 4a, 4b, 5b, 8, 9, 10 and 12 were determined by single crystal X-ray diffraction. High-level DFT calculations employing the B3LYP functional have been carried out for a series of experimentally studied and other alkylidyne complexes by choosing alkyne metathesis of 2-butyne as the model reaction.