A new ONO3− trianionic pincer ligand with intermediate flexibility and its tungsten alkylidene and alkylidyne complexes

Abstract

This report details the synthesis and characterization of the semi-flexible [ON^CH2O]H₃ (1) ligand and its W(VI)-alkylidene and alkylidyne complexes. The alkylidyne complex [ONH^CH2O]WC^tBu(O^tBu) (2) forms as a result of alcoholysis of 1 with (^tBuO)₃WC^tBu. Complex 2 evolves to [ON^CH2O]WCH^tBu(O^tBu) (3) through proton migration from the N atom of the pincer ligand to the WC_α bond. Deprotonation of 2 or 3 with Ph₃PCH₂ affords the anionic alkylidyne {CH₃PPh₃}{[ON^CH2O]WC^tBu(O^tBu)} (4). Complex 4 exhibits pincer-ligand-centered reactivity with electrophiles (H⁺, Me⁺, and TMS⁺), in spite of its enhanced inorganic enamine interaction. Addition of 2 equiv. of HCl to 4 yields the W(VI)-neopentyl complex [ON^CH2O]W(CH₂^tBu)(O^tBu)(Cl) (5). MeOTf or TMSOTf addition to 4 generates the dianionic pincer ligated alkylidynes [ONR^CH2O]WC^tBu(O^tBu) (R = Me (6-Me); TMS (6-TMS)). Complexes 2–5 were characterized by multinuclear NMR spectroscopy, and combustion analysis. Complexes 4 and 5 were also characterized by single crystal X-ray diffraction. This work bridges the gap in the series involving W(VI)-alkylidynes ligated to the rigid [CF₃-ONO]³⁻, and the flexible [O^CH2N^CH2O]³⁻ ligands. DFT computations permit comparison of the inorganic enamine effect within alkylidynes supported by all three trianionic-pincer type ONO ligands.