Reactions of the triply-bonded complex cis-Re2(µ-O2CCH3)2Cl2(µ-dppm)2 with pyridine carboxylic acids. The isolation and structural characterization of a third structural isomer of Re2(dipic)Cl2(µ-dppm)2(dipic = pyridine-2,6-dicarboxylate)

The diamagnetic dirhenium(II) complexes Re2(pic)Cl3((mu)-dppm)2(2), Re2(dipic)Cl2((mu)-dppm)2(3), Re2(HnicO)2Cl2((mu)-dppm)2(4) and Re2(picO)2((mu)-dppm)2(5) that are formed by the reactions of cis-Re2((mu)-O2CCH3)2Cl2((mu)-dppm)2(1) with picolinic acid (Hpic), dipicolinic acid (H2dipic), 2-hydroxynicotinic acid (HnicOH) and 6-hydroxypicolinic acid (HpicOH) represent four different types of reaction products from the displacement of the cis acetate groups of 1. All four compounds have been characterized by X-ray crystallography. Compound 2 can be prepared more logically and in higher yield from Re2Cl4((mu)-dppm)2. Compound 3, which retains the same cis,cis coordination of (mu)-dppm ligands that is present in 1, is the third structural isomer of Re2(dipic)Cl2((mu)-dppm)2; the other two, which are prepared from Re2Cl4((mu)-dppm)2, contain trans,trans and trans,cis ligation by the pair of (mu)-dppm ligands. Compounds 4 and 5 are the first dirhenium(II) compounds that contain bridging 2-pyridonate ligands; in 4 the carboxylic acid group of (mu)-HnicO does not coordinate, whereas the (mu)-picO ligands in 5 are bound in a tridentate fashion and, consequently, the two axial Re-Cl bonds of 1 have been replaced by axial Re-O (carboxylate) coordination. Complexes 3-5 have quite accessible oxidation chemistry as shown by cyclic voltammetric measurements.