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(μ-^tBuOCHO)WCHC(CH₃)₃(^tBuOCO)] (12_kin and 12_therm). 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 12_kin could not be isolated independently; however, adding PMe₂Ph to the metalation between 11 and 9 provided 12_therm 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 12_kin and other unidentified decomposition products. As a consequence, characterization of 17 was limited to ¹H NMR spectroscopy and mass spectrometry.