New Alkylidyne Complexes Featuring a Flexible Trianionic ONO^3– Pincer-Type Ligand: Inorganic Enamine Effect versus Sterics in Electrophilic Additions

Metal–carbon multiple bonds exhibit enhanced nucleophilicity at the α carbon within ONO trianionic pincer alkylidyne complexes. Defined as the inorganic enamine effect, this phenomenon is a result of the overlap of the N atom lone pair within the ONO3– ligand and a π bond from the metal–carbon multiple bond. Treating the proligand [OCH2NCH2O]­H3 (2) with (tBuO)3WCR (where R = Et, tBu) results in the formation of the dianionic pincer complexes [OCH2NHCH2O]­WCR­(OtBu) (where R = Et (3-Et), tBu (3-tBuanti)). Deprotonation of 3-tBuanti by treatment with Ph3PCH2 forms the anionic alkylidyne complex {[OCH2NCH2O]­WCtBu­(OtBu)}­{CH3PPh3} (4-tBu). DFT calculations modeling 4-tBu reveal overlap of the N atom lone pair with a WC π bond. However, 4-tBu reacts with electrophiles preferentially at the pincer N atom as opposed to the WCα group. Multinuclear NMR spectroscopy, combustion analysis, and single-crystal X-ray crystallography are employed to characterize complexes 3-Et, 3-tBuanti, and 4-tBu.