Reactions of the dirhenium(II) complex Re2Cl4(μ-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)(2)N](+) 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 Re-2(eta(2)-N,O)Cl-3(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. Structure determinations have shown that 1, 3, and 4 possess similar structures in which there is an axial Re-O (carboxylate) bond (collinear with the RedropRe bond) and the mu-dppm ligands are bound in a trans,cis fashion to the two Re atoms which have the ligand atom arrangement [P2NOClReReCl2P2] The tridentate dianionic pyridine-2,6-dicarboxylate ligand (dipic) reacts with Re2Cl4(mu-dppm)(2) in ethanol at room temperature to give a compound Re-2(dipic)Cl-2(mu-dppm)(2) (6) in which the dipic ligand is bound in a symmetrical eta(3)-(O,N,O), fashion to one Re atom, with the N atom in an axial position (collinear with the RedropRe bond) and with preservation of the same trans,trans coordination of the mu-dppm ligands that is present in Re2Cl4(mu-dppm)(2). Under reflux conditions, this kinetic product isomerizes to the thermodynamically favored isomer 5 with an unsymmetrical structure in which the dipic ligand chelates to one Re atom (as in 1, 3, and 4) and uses its other carboxylate group to bridge to the second Re atom. The isomerization of 6 to 5, which also results in a change in the coordination of the pair of mu-dppm ligand to trans,cis, is believed to occur by a partial "merry-go-round" process, a mechanism that probably explains the structures of the thermodynamic products 1, 3, and 4. The reaction of Re2Cl4(mu-dppm)(2) with pyridine-3-carboxylate gives the trans isomer of Re-2(mu:eta(2)-O2C-3-C5H4N)(2)-Cl-2(mu-dppm)(2) (2) in which a pair of carboxylate bridges are present and the pyridine N atom is not coordinated. Single-crystal X-ray structural details are reported for 1-6.