Isomer Formation and Other Issues in the Substitution Reactions of Oxorhenium(V) Complexes of 2,2‘-Bipyridine and Related Ligands

Two new oxorhenium(V) compounds were prepared and characterized:  MeReO(mtp)(Me2Bpy) and MeReO(mtp)(dppb), where mtpH2 is 2-(mercaptomethyl)thiophenol, Me2Bpy is 4,4‘-dimethyl-2,2‘-bipyridine, and dppb
is 1,2-(Ph2P)2C6H4. The more stable geometric isomer of MeReO(mtp)X forms MeReO(mtp)Y (X, Y = PR3,
NC5H4R) in two steps, both of which show a first-order dependence on [Y], proceeding through the metastable
geometric isomer MeReO(mtp)Y*. When Y = PR3, no MeReO(mtp)Y* was detected at equilibrium; with NC5H4R,
however, both isomers were detected. The values of KPy*Py were 8.5−9.8, largely irrespective of R; for NC5H5,
ΔH° = −4.47 ± 0.29 kJ and ΔS° = 3.9 ± 1.0 J K-1. For the more symmetric edt ligand, geometric isomers do
not exist, but enantiomers do. The rate of racemization of MeReO(edt)(NC5H4R) was proportional to [Py]. Values
of krac for 16 compounds span the range 135−370 L mol-1 s-1 in C6H6 at 25 °C (ρ = −0.39 ± 0.07). In toluene-d8, krac for 4-picoline has ΔH⧧ = 28.9 ± 0.4 kJ, ΔS⧧ = −103.6 ± 0.9 J K-1. A common mechanism applies to
ligand substitution (mtp) and racemization (edt). MeReO(dithiolate)Py complexes react with Bpy, Me2Bpy, Phen,
and Me2Phen to form six-coordinate chelates, with rate constants 0.024−0.74 L mol-1 s-1 at 25 °C, some 103
times smaller than with pyridines, no doubt owing to the bulk of the bidentates. Values of ΔS⧧ are −86 to −138
J K-1, reflecting substantial orientational barriers as well as the inherent contribution of the associative mechanism.
The product is MeReO(mtp)(Me2Bpy)*. The formation of the metastable isomer is consistent with the mechanism
assigned to the ligand substitution and racemization reactions. Such compounds, once formed, no longer participate
in ligand substitution reactions at reasonable rates. The formation of the metastable isomer is consistent with the
mechanism assigned to the ligand substitution and racemization reactions.