A new ONO(3-) 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(CH2)O]H3 (1) ligand and its W(vi)-alkylidene and alkylidyne complexes. The alkylidyne complex [ONH(CH2)O]W[triple bond, length as m-dash]C(t)Bu(O(t)Bu) (2) forms as a result of alcoholysis of 1 with ((t)BuO)3W[triple bond, length as m-dash]C(t)Bu. Complex 2 evolves to [ON(CH2)O]W[double bond, length as m-dash]CH(t)Bu(O(t)Bu) (3) through proton migration from the N atom of the pincer ligand to the W[triple bond, length as m-dash]Cα bond. Deprotonation of 2 or 3 with Ph3PCH2 affords the anionic alkylidyne {CH3PPh3}{[ON(CH2)O]W[triple bond, length as m-dash]C(t)Bu(O(t)Bu)} (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(CH2)O]W(CH2(t)Bu)(O(t)Bu)(Cl) (5). MeOTf or TMSOTf addition to 4 generates the dianionic pincer ligated alkylidynes [ONR(CH2)O]W[triple bond, length as m-dash]C(t)Bu(O(t)Bu) (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(CH2)N(CH2)O](3-) ligands. DFT computations permit comparison of the inorganic enamine effect within alkylidynes supported by all three trianionic-pincer type ONO ligands.