Elucidating the Mechanism of the Halide-Induced Ligand Rearrangement Reaction

The formation of heteroligated RhI complexes
containing two different hemilabile phosphinoalkyl ligands, (κ2-Ph2PCH2CH2S-Aryl)­(κ1-Ph2PCH2CH2O-C6H5)­RhCl, through a halide-induced ligand rearrangement
(HILR) reaction has been studied mechanistically. The half-life of
this rearrangement reaction depends heavily on the RhI 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)­(κ1-Ph2PCH2CH2O-C6H5)2RhCl (nbd
= norbornadiene), converts to (nbd)­(κ1-Ph2PCH2CH2O-C6H5)­RhCl resulting
in a free PO ligand. The free PO ligand can then react with a homoligated
PS complex [(κ2-Ph2PCH2CH2S-Aryl)2Rh]+Cl– producing the heteroligated product. The PS ligand generated during
the reaction pathway can be trapped by the monoligated PO complex
(nbd)­(κ1-Ph2PCH2CH2O-C6H5)­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.