Unusually stable tungstenacyclobutadienes featuring an ONO trianionic pincer-type ligand?

Dalton Transactions Pub Date: 2012-12-18 DOI: 10.1039/C2DT32653A

Abstract

This report presents the synthesis of the first neutral trianionic ONO pincer-type tungsten alkylidyne complex, [CF3–ONO]W[triple bond, length as m-dash]C(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 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. 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.

Graphical abstract: Unusually stable tungstenacyclobutadienes featuring an ONO trianionic pincer-type ligand
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