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

Reaction of propiolic acid (HCCCOOH) with dicobalt octacarbonyl (Co2(CO)8) and tetracobalt dodecacarbonyl (Co4(CO)12) leads to the organometallic carboxylic acids (CO)6Co2HCCCOOH (1) and (CO)10Co4HCC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.