Unusually stable tungstenacyclobutadienes featuring an ONO trianionic pincer-type ligand

This report presents the synthesis of the first neutral trianionic ONO pincer-type tungsten alkylidyne complex, [CF(3)-ONO]W≡C((t)Bu)(OEt(2)) (5) {where CF(3)-ONO = (MeC(6)H(3)[C(CF(3))(2)O])(2)N(3-)}. Treating 5 with 1-phenylpropyne, 4,4-dimethyl-2-pentyne, and cyclooctyne yields the corresponding tungstenacyclobutadiene complexes [CF(3)-ONO]W[κ(2)-C((t)Bu)C(Me)C(Ph)] (6), [CF(3)-ONO]W[κ(2)-C((t)Bu)C(Me)C((t)Bu)] (7), and [CF(3)-ONO]W[κ(2)-C((t)Bu)C(CH(2))(6)C] (8). Complexes 6, 7, and 8 do not undergo retro-[2 + 2]-cycloaddition even at 200 °C or in the presence of PMe(3). DFT methods to elucidate the electronic structure of complexes 5 and 6 reveal important electronic factors that contribute to the lack of reactivity for the tungstenacyclobutadienes. An important bonding combination between the pincer N-atom lone pair and the W[triple bond, length as m-dash]C bond within 5, termed an inorganic enamine, provides an explanation for the lack of retro-[2 + 2]-cycloaddition from 6, 7, and 8. (15)N NMR spectroscopy was used to confirm the computational finding of an inorganic enamine bonding combination. Single crystal X-ray analysis of 5, 6, 7, and 8 provides insight into possible steric inadequacies within the CF(3)-ONO(3-) ligand to promote catalytic metathesis.