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

The imidazolin-2-iminato tungsten alkylidyne complexes [Me<sub>3</sub>CC≡W(NIm<sup>R</sup>)(OR′)<sub>2</sub>] (<b>4a:</b> R = <i>t</i>Bu, R′ = CMe<sub>3</sub>; <b>4b:</b> R = Dipp, R′ = CMe<sub>3</sub>; <b>5a:</b> R = <i>t</i>Bu, R′ = CMe(CF<sub>3</sub>)<sub>2</sub>; <b>5b:</b> R = Dipp, R′ = CMe(CF<sub>3</sub>)<sub>2</sub> have been prepared from [Me<sub>3</sub>CC≡W(OCMe<sub>3</sub>)<sub>3</sub>] (<b>2</b>) and [Me<sub>3</sub>CC≡W{OCMe(CF<sub>3</sub>)<sub>2</sub>}<sub>3</sub>(dme)] (<b>3</b>, dme = 1,2-dimethoxyethane) by reaction with the lithium reagents Li(NIm<sup><i>t</i>Bu</sup>) and Li(NIm<sup>Dipp</sup>), generated with MeLi from 1,3-di-<i>tert</i>-butylimidazolin-2-imine (Im<sup><i>t</i>Bu</sup>NH) or 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-imine (Im<sup>Dipp</sup>NH), respectively. Reaction of <b>3</b> with Li[N(<i>t</i>Bu)Ar]·OEt<sub>2</sub> (Ar = 3,5-dimethylphenyl) afforded the amido complex [Me<sub>3</sub>CC≡W{N(<i>t</i>Bu)Ar}{OCMe(CF<sub>3</sub>)<sub>2</sub>}<sub>2</sub>] (<b>6</b>). Addition of Li[OCPh(CF<sub>3</sub>)<sub>2</sub>] to [Me<sub>3</sub>CC≡WCl<sub>3</sub>(dme)] (<b>1</b>) produced the dme-free complex [Me<sub>3</sub>CC≡W{OCPh(CF<sub>3</sub>)<sub>2</sub>}<sub>3</sub>] (<b>7</b>), which, upon treatment with Li(NIm<sup><i>t</i>Bu</sup>), gave the alkylidene complex <b>8</b>, presumably formed by activation and addition of an <i>ortho</i>-C−H bond across the W≡C bond in the intermediate alkylidyne complex. Treatment of <b>1</b> with Li[OC(CF<sub>3</sub>)<sub>3</sub>] led to the substitution of only two chloride ligands and formation of <i>cis-</i>[Me<sub>3</sub>CC≡WCl{OC(CF<sub>3</sub>)<sub>3</sub>}<sub>2</sub>(dme)] (<b>9</b>), which exhibits long-range through-space <sup>19</sup>F−<sup>19</sup>F coupling between the fluorine atoms of the two OC(CF<sub>3</sub>)<sub>3</sub> ligands. Reaction of <b>9</b> with Li(NIm<sup><i>t</i>Bu</sup>) resulted in partial cleavage of the Im<sup><i>t</i>Bu</sup>N ligand and ligand redistribution to afford the dinuclear tungsten alkylidyne complex <b>10</b>. The propylidyne complex [EtC≡W(NIm<sup><i>t</i>Bu</sup>){OCMe(CF<sub>3</sub>)<sub>2</sub>}<sub>2</sub>] (<b>12</b>) was obtained by treatment of <b>5a</b> with 3-hexyne, which proceeded via the metallacyclobutadiene complex <b>11</b>. Complex <b>5a</b> 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 <b>5a</b>, <b>5b</b> and <b>6</b> at 30 and 80 °C. The molecular structures of complexes <b>2, 3</b>, <b>4a</b>, <b>4b</b>, <b>5b</b>, <b>8</b>, <b>9</b>, <b>10</b> and <b>12</b> 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.