Hydrogen bonding in tungsten(VI) salicylate free acids

Abstract

Hydrogen-bonded free acid dimers of the formula [W(X)Cl 3(Hsal–R)] 2 (where X=O ( 1), NC 6H 3-2,6-Me 2 ( 2), Ph 2C 2 and Hsal–R=substituted salicylate monoanion) are prepared through reactions between W(X)Cl 4 precursors and functionalized salicylic acids (H 2sal–R). The free acids are stable at ambient temperature and exist as dimers in solution, although the structure of the dimer is not known. Spectroscopic studies show that the electronic characteristics of the π-donor ligands directly affect the electronic environments of the carboxylate functionality. This influence is observed in weaker binding constants of diethyl ether ( K X) as the X ligand becomes more strongly π donating: O⩾NC 6H 3–2,6-Me 2>Ph 2C 2. In other words, the oxo and aryl imido species are more acidic than the diphenylacetylene compounds in the Brønsted sense owing to the higher Lewis acidity of their tungsten centers. Salicylate adducts of the type W(X)Cl 3(Hsal–R)⋯OR 2′ (X=O ( 3), NC 6H 3–2,6-Me 2 ( 4)) and W(X)Cl 3(Hsal–R)⋯NEt 3 (X=O ( 5), NC 6H 3–2,6-Me 2 ( 6)) have been isolated and characterized. A comparison of two structures, W(NC 6H 3–2,6-Me 2)Cl 3(Hsal–3-Me⋯L) where L=OEt 2 ( 4c) and NEt 3 ( 6c), shows that 6c has more charge localization on both the carboxylate group and tungsten center than 4c. The charge separation in amine adducts (i.e. salts) contributes to association of these salts with the free acid (i.e. W(X)Cl 3(Hsal–R⋯L)⋯[W(X)Cl 3(Hsal–R)]). The strong hydrogen bonding exhibited by the free acids leads to the formation of supramolecular complexes organized around poly(ether) templates, including dimethoxyethane (in 3e) and 18-crown-6.