HYDROGEN-BONDING IN TUNGSTEN(IV) SALICYLATE FREE ACIDS

Hydrogen-bonded free acid dimers of the formula [W(X)Cl-3(Hsal-R)](2) (where X = O (1), NC6H3-2,6-Me-2 (2), Ph2C2 and Hsal-R = substituted s alicylate monoanion) are prepared through reactions between W(X)Cl-4 p recursors and functionalized salicylic acids (H(2)sal-R). [GRAPHICS] T he free acids are stable at ambient temperature and exist as dimers in solution, although the structure of the dimer is not known. Spectrosc opic studies show that the electronic characteristics of the pi-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 pi donat ing: O greater than or equal to NC6H3-2,6-Me-2 > Ph2C2. In other words , the oxo and aryl imido species are more acidic than the diphenylacet ylene compounds in the Bronsted sense owing to the higher Lewis acidit y of their tungsten centers. Salicylate adducts of the type W(= X)Cl-3 (Hsal-R)... OR2 (X = O (3), NC6H3-2,6-Me-2 (4)) and W(=X)Cl-3(Hsal-R). .. NEt3 (X = O (5), NC6H3-2,6-Me-2 (6)) have been isolated and charact erized. A comparison of two structures, W(=NC6H3-2,6-Me-2)Cl-3(Hsal-3- Me ... L) where L = OEt2 (4c) and NEt3 (6c), shows that 6c has more ch arge localization on both the carboxylate group and tungsten center th an 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 banding exhibited b y the free acids leads to the formation of supramolecular complexes or ganized around poly(ether) templates, including dimethoxyethane (in 3e ) and 18-crown-6. (C) 1998 Elsevier Science S.A. All rights reserved.