Supramolecular Assemblies Involving Organometallic Free Acids

Reactions between W(η2-Ph2C2)Cl4 and salicylic acid derivatives generate analytically pure free acids of formula W(η2-Ph2C2)Cl3(Hsal-R) (1) in high yields (Hsal = a salicylate monoanion). The products exist as hydrogen-bonded dimers in the solid state. The acid functionality on one molecule hydrogen bonds to one of the cis-chloride ligands of an adjacent complex at a 3.03 Å distance. The more electron-rich tungsten center renders these acetylene complexes less acidic than their oxo and arylimido analogs. As a result, W(η2-Ph2C2)Cl3(Hsal) exhibits partial dimerization in solution and have relatively weak hydrogen bonds to nitrogen- and oxygen-containing organic molecules. Among a range of possible phenol−phenoxide complexes of the W(η2-Ph2C2)Cl3 subunit, only the ether adduct of the catecholate, W(η2-Ph2C2)Cl3(Hcat···OEt2) (2), has been isolated and structurally characterized. The weaker hydrogen bond strength of larger chelating bis(phenolates) evidently destabilize the phenol−phenoxide structures in favor of simple chelating bis(phenoxides). The salicylate free acids form various supramolecular complexes in solution and the solid state, including [W(η2-Ph2C2)Cl3(Hsal)]4(18-crown-6) (5), one of a family of tetranuclear systems organized around hydrogen bonding to an 18-crown-6 template. This structure is characterized by π-stacking of the Hsal ligands between confacial free acid complexes and the steric screening of the two non-hydrogen-bonded 18-crown-6 oxygens by pairs of antarafacial W(Ph2C2) units.