Unusually stable tungstenacyclobutadienes featuring an ONO trianionic pincer-type ligand

Abstract

This report presents the synthesis of the first neutral trianionic ONO pincer-type tungsten alkylidyne complex, [CF₃–ONO]WC(^tBu)(OEt₂) (5) {where CF₃–ONO = (MeC₆H₃[C(CF₃)₂O])₂N³⁻}. Treating 5 with 1-phenylpropyne, 4,4-dimethyl-2-pentyne, and cyclooctyne yields the corresponding tungstenacyclobutadiene complexes [CF₃–ONO]W[κ²-C(^tBu)C(Me)C(Ph)] (6), [CF₃–ONO]W[κ²-C(^tBu)C(Me)C(^tBu)] (7), and [CF₃–ONO]W[κ²-C(^tBu)C(CH₂)₆C] (8). Complexes 6, 7, and 8 do not undergo retro-[2 + 2]-cycloaddition even at 200 °C or in the presence of PMe₃. 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 WC bond within 5, termed an inorganic enamine, provides an explanation for the lack of retro-[2 + 2]-cycloaddition from 6, 7, and 8. ¹⁵N 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 CF₃–ONO³⁻ ligand to promote catalytic metathesis.