Functionalized cobalt clusters.: Properties of carboxylate clusters (CO)6Co2HCC-COOH and (CO)10Co4HCC-COON as related to their structures

Reaction of propiolic acid (HCCCOOH) with dicobalt octacarbonyl (Co-2(CO)(8)) and tetracobalt dodecacarbonyl (Co-4(CO)(12)) leads to the organometallic carboxylic acids (CO)(6)Co2HCC-COOH (1) and (CO)(10)Co4HCC-COOH (2) in good yield. Both organometallic compounds show a cobalt carbonyl core bonded to a carboxylate function. The structure of the acetylene-carboxylic group in both clusters deviates from that of ethylene. The C(1)-C(2)-C(3) fragment is half way between acrylic and acetylene-carboxylic acid. The comparative acidity of the carboxylic group measured in methanol reveals that (1) is a stronger acid than (2), but less acidic than propiolic acid. Both organometallic carboxylic acids are thermally decomposed into phases with high metal content at relatively low temperatures. Fenske-Hall calculations on this series of cobaltocarbonyl cluster carboxylic acids confirm that the Co(Co)(3) donates electron density to the HCC-COOH fragment, thus decreasing the acidity of the carboxylic function. The redox properties and the electronic spectra are also well correlated to the HOMO-LUMO gap thus calculated by this non-parametric SCF method. (C) 2002 Elsevier Science B.V. All rights reserved.