Synthesis and Characterization of Tungsten(VI) Alkylidene Complexes Supported by an [OCO]3− Trianionic Pincer Ligand: Progress towards the [tBuOCO]WCC(CH3)3 Fragment

The synthesis and characterization of trianionic [tBuOCO]3− pincer-supported tungsten alkylidene and alkylidyne complexes are described. The reaction of an equimolar ratio of (tBuO)3WCC(CH3)3 (where tBuO = tert-butoxide) with [tBuOCO]H3 (9) and 2,6-diisopropylphenol affords the alkylidene [tBuOCO]WCHC(CH3)3(O-2,6-C6H3-iPr2) (10), with a five-coordinate tungsten center that adopts a distorted square-pyramidal geometry. Treatment of (Np)3WCC(CH3)3 (11) with 9 provides an equilibrium mixture of the two isomeric alkylidenes with the general formula [(tBuOCO)WCHC(CH3)3-tBuOCHO)WCHC(CH3)3(tBuOCO)] (12kin and 12therm). Single crystals of the two isomers cocrystallize and were amenable to X-ray diffraction studies, which revealed subtle differences in their molecular structures, most notably the orientation of the bridging ligand. The complexes are each comprised of two square-pyramidal tungsten ions linked by the diphenolate form of the OCO ligand. Isomer 12kin could not be isolated independently; however, adding PMe2Ph to the metalation between 11 and 9 provided 12therm exclusively, thus enabling a full set of characterization techniques including 2-D NMR spectroscopy. Salt metathesis between [tBuOCHO]K2(THF)2 (15) and (DME)Cl3WCC(CH3)3 (16) in diethyl ether produces the alkylidyne (tBuOCHO)WCC(CH3)3Cl (17) as the major product along with 12kin and other unidentified decomposition products. As a consequence, characterization of 17 was limited to 1H NMR spectroscopy and mass spectrometry.