1H NMR study of paramagnetic rhenium(III) monomers with nitrogen heterocycles. Crystal structures of ReCl3L2(PPh3) complexes (L = pyridine, 3-picoline, and 1-methylimidazole)

Abstract

Rhenium(III) monomers of the type ReCl3Ln(PPh3)3−n, where n = 1,2, and 3, were prepared for simple nitrogen heterocyclic ligands L and characterized mainly by 1H NMR and IR spectroscopies. Most of the 1H NMR signals of these paramagnetic compounds were assigned by using methylated pyridines. A mer-cis octahedral geometry slightly distorted by triphenylphosphine was determined by X-ray diffraction for three ReCl3L2(PPh3) complexes: L = pyridine, triclinic, [Formula: see text], a = 11.379, b = 13.532, c = 18.160 Ǻ, α = 80.16°, β = 89.57, γ = 86.27°, Z = 4, R = 0.076; L = 3-picoline, triclinic, [Formula: see text], a = 10.557, b = 12.580, c = 13.153 Ǻ, α = 66.80°, β = 67.46°, γ = 75.89°, Z = 2, R = 0.021; L = 1-methylimidazole, monoclinic, P21/c, α = 15.437, b = 9.436, c = 18.607 Ǻ, β = 92.23°, Z = 4, R = 0.046. The same structure was deduced from 1H NMR data for these compounds in solution, whereas the other types of complexes were found to exist as the mer,trans-ReCl3L(PPh3)2 and mer-ReCl3L3 isomers. In contrast with these complexes, which contain the mer-ReCl3 core, the starting material ReCl3(benzil)(PPh3) is found to be the fac isomer in solution. The typical low-field shifted pattern of 1H NMR signals for PPh3 is not greatly perturbed by changes in the number or arrangement of the remaining ligands in the coordination sphere, but the signals for protons on conjugated systems directly bonded to the metal atoms are much more affected by the unpaired electrons. Keywords: paramagnetic NMR, crystal structures, rhenium(III) complexes, imidazole, pyridine, triphenylphoshine.