Elucidating the mechanism of the halide-induced ligand rearrangement reaction

The formation of heteroligated Rh ^I complexes containing two different hemilabile phosphinoalkyl ligands, (κ ²-Ph ₂PCH ₂CH ₂S-Aryl)(κ ¹-Ph ₂PCH ₂CH ₂O-C ₆H ₅)RhCl, through a halide-induced ligand rearrangement (HILR) reaction has been studied mechanistically. The half-life of this rearrangement reaction depends heavily on the Rh ^I precursor used and the chelating ability of the phosphinoalkyl thioether (PS) ligand, while the chelating ability of the phosphinoalkyl ether (PO) ligand has less of an effect. An intermediate complex which contains two PO ligands, (nbd)(κ ¹-Ph ₂PCH ₂CH ₂O-C ₆H ₅) ₂RhCl (nbd = norbornadiene), converts to (nbd)(κ ¹-Ph ₂PCH ₂CH ₂O-C ₆H ₅)RhCl resulting in a free PO ligand. The free PO ligand can then react with a homoligated PS complex [(κ ²-Ph ₂PCH ₂CH ₂S-Aryl) ₂Rh] ⁺Cl ^- producing the heteroligated product. The PS ligand generated during the reaction pathway can be trapped by the monoligated PO complex (nbd)(κ ¹-Ph ₂PCH ₂CH ₂O-C ₆H ₅)RhCl, leading to the formation of the same heteroligated product. In this study, some of the key intermediates and reaction steps underlying the HILR reaction have been identified by variable temperature ³¹P{ ¹H} NMR spectroscopy and in two cases by single-crystal X-ray diffraction studies. Significantly, this work provides mechanistic insight into the HILR process, which is a key reaction used to prepare a large class of highly sophisticated three-dimensional metallosupramolecular architectures and allosteric catalysts.