Bis-tropolonate complexes of tungsten: scaffolds for selective side-on binding of nitriles, imines and ketones

Abstract

The chiral bis-tropolonate tungsten(II) tricarbonyl compound, (trop)₂W(CO)₃ (1), has been synthesized and structurally characterized. This seven-coordinate compound readily loses two carbonyl ligands to preferentially bind a series of π-bonding substrates to form six-coordinate complexes of the type (trop)₂W(CO)(L). Alkynes coordinate strongly to form (trop)₂W(CO)(η²-RCCR) (2) in which spectroscopic data is consistent with the alkyne serving as a 4-electron donor. Compound 1 will also preferentially coordinate organic nitriles in a side-on fashion through the CN triple bond. A dramatic shift in the nitrile carbon signals to greater than 210 ppm in the ¹³C NMR confirms the nitriles are coordinated in an η² 4-electron donating capacity. Aldehydes, ketones, and imines also react with 1 to form 4-electron donor η² adducts. The imine adduct (trop)₂W(CO)(η²-MeNC(H)(tol)) (5) was characterized crystallographically and the short 1.91 A W–N bond distance supports the postulation of 4-electron donation from the imine through CN π-bonding and N lone pair donation. Side-on coordination of ligands of this type is rare and may provide a means towards asymmetric functionalization of these substrates. All of the tropolonate compounds are prone to oxidation in air and the alkyne compounds will oxidize to stable W^IV oxo alkyne species, (trop)₂W(O)(η²-RCCR) (6). This causes a 90° rotation of the alkyne ligand and a reduction in alkyne donation to approximately 3 electrons, to maintain an optimal 18 electron configuration.