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)

Abstract

The diamagnetic dirhenium(II) complexes Re₂(pic)Cl₃(μ-dppm)₂ (2), Re₂(dipic)Cl₂(μ-dppm)₂ (3), Re₂(HnicO)₂Cl₂(μ-dppm)₂ (4) and Re₂(picO)₂(μ-dppm)₂ (5) that are formed by the reactions of cis-Re₂(μ-O₂CCH₃)₂Cl₂(μ-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(μ-dppm)₂. Compound 3, which retains the same cis,cis coordination of μ-dppm ligands that is present in 1, is the third structural isomer of Re₂(dipic)Cl₂(μ-dppm)₂; the other two, which are prepared from Re₂Cl₄(μ-dppm)₂, contain trans,trans and trans,cis ligation by the pair of μ-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 μ-HnicO does not coordinate, whereas the μ-picO ligands in 5 are bound in a tridentate fashion and, consequently, the two axial Re–Cl bonds of 1 have been replaced by axial Re–O (carboxylate) coordination.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.