Reactions of the Dirhenium(II) Complex Re2Cl4((mu)-dppm)2 with Pyridinecarboxylic Acids. Examples of Bidentate O,O versus N,O Coordination and Tridentate O,N,O Coordination and Structural Isomers That Contain the Pyridine-2,6-dicarboxylate Ligand

Pyridine-2-carboxylic acid, pyridine-2,3-dicarboxylic acid, and pyridine-2,4-dicarboxylic acid or their [(Ph3P)2N]+ salts react with the triply bonded dirhenium(II) complex Re2Cl4((mu)-dppm)2 (dppm = Ph2PCH2PPh2) in refluxing ethanol to afford unsymmetrical substitution products of the type Re2((eta)^2-N,O)Cl3((mu)-dppm)2, where N,O represents a chelating pyridine-2-carboxylate ligand (N,O = O2C-2-C5H4N (1), O2C-2-C5H3N(-3-CO2Et) (3), or O2C-2-C5H3N(-4-CO2H) (4)). The carboxylate groups in the 3- and 4- positions are not bound to the metal centers; in the case of 3 this group undergoes esterification in the refluxing ethanol solvent.