E/Z-isomerization dynamics in molybdenum and tungsten η1-ketone complexes

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).2OEt(2) (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 eta(1)-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 H-1 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 <-- O = C transition state. The vacant d pi orbital of the d(4) metal and the versatility of the alkyne pi perpendicular to donation into the d pi-orbital enhances accessibility of the linear transition state for E/Z-isomerization.