E/Z-Isomerization Dynamics in Molybdenum and Tungsten η1-Ketone Complexes

Abstract

A series of tungsten(II) and molybdenum(II) iodide complexes of the type {Tp}M(CO)(RC⋮CR‘)I ({Tp} = Tp, Tp‘; M = Mo, W; R, R‘ = Ph, Me) (5a−g) have been synthesized by utilizing hydridotris(1-pyrazolyl)borate (Tp) and hydridotris(3,5-dimethylpyrazolyl)borate (Tp‘) ligands and 1-phenyl-1-propyne, 2-butyne, and diphenylacetylene alkynes. Reacting the iodide complexes 5a−g with either LiCuMe2 or Me2Mg formed the corresponding methyl complexes {Tp}M(CO)(RC⋮CR‘)Me (7a−g), which serve as Lewis acid precursors in these systems. Protonation of the methyl complexes with HBAr‘4·2OEt2 (BAr‘4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), loss of methane, and addition of a ketone (acetone, 2-butanone, 3-methyl-2-butanone, acetophenone, or pinacolone) gave the η1-ketone complexes. The ketone complexes exhibit different conformational preferences about the M−O bond. The E/Z-isomerization barriers of the ketone complexes were calculated using coalescence temperatures from low-temperature 1H NMR spectra. Differences in E/Z-isomerization barriers between Tp and Tp‘ tungsten and molybdenum systems are analyzed in terms of an isomerization mechanism involving a linear M←OC transition state. The vacant dπ orbital of the d4 metal and the versatility of the alkyne π⊥ donation into the dπ-orbital enhances accessibility of the linear transition state for E/Z-isomerization.