Issues of Microscopic Reversibility and an Isomeric Intermediate in Ligand Substitution Reactions of Five-Coordinate Oxorhenium(V) Dithiolate Complexes

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Abstract
Ligand substitution reactions between five-coordinate oxorhenium(V) dithiolates, [CH3ReO(SCH2C6H4S)X], or MeReO(mtp)X, and entering ligands Y have been studied; Y is a phosphine and X is a phosphine (usually) or a pyridine. Many of them occur in two distinct stages, and other two-stage reactions merge to a single kinetic term when the successive rate constants are quite different in value. An intermediate can be detected directly by electronic and NMR spectroscopy. Just for phosphines, the range of rate constants is remarkably large; in the first stage, k spans the range 10-4−101 L mol-1 s-1 at 25 °C in benzene; in the second, which also shows a first-order dependence on the concentration of the entering ligand, the range is 10-4−103 L mol-1 s-1. Spectroscopic evidence shows that the intermediate has the same composition as the product; the metastable form is designated as MeReO(mtp)Y*. The structures of all the isolated products MeReO(mtp)Y have a single stereochemistry: Me and −SCH2 lie in trans positions, as do Y and −SAr. This structure is believed to be reversed in the transient, Y and −SCH2 occupying trans positions. Further support for this assignment comes from the 31P splitting of the 1H NMR spectrum, where additional coupling indicates unusual four-bond coupling from a W-pattern of the hydrogen and phosphorus atoms. The intermediate does not undergo an intramolecular rearrangement to the final product; instead, it reacts with a ligand of the same type in an intermolecular reaction leading to rearrangement. The activation parameters were determined for selected reactions, and the results support a mechanism with considerable associative character; ΔS⧧ values are ca. −125 J K-1 mol-1. Because ligand Y must enter the coordination sphere from the vacant coordination position trans to the ReO group, a means must be devised for the leaving group X to gain that position. To account for the intervention of the isomer while honoring the principle of microscopic reversibility, two mechanisms are proposed. One involves a C3 (“turnstile”) rotation of a specific group of three ligands in the six-coordinate transition state. Turnstile rotation of the groups X, Me, and Y can accomplish the needed transposition; the transition state passes through an approximate trigonal prismatic configuration, giving rise to a different and less stable isomer. The alternative mechanism, which may more easily accommodate data for Y = Me2bpy, involves rearrangement of the common octahedral intermediate to a pentagonal pyramid. The arrangement of ligands in the intermediate, governed by their sizes, determines that isomerization accompanies product formation. Following either rearrangement, a second reaction, between MeReO(SCH2C6H4S)Y* and Y, then ensues by the same mechanism. The second rearrangement process then generates the more stable isomer of the product. Results are also presented from a study of monomerization of the dimeric rhenium species, {MeReO(mtp)}2, with phosphines(X) of various size and basicity. The results support a mechanism with two intermediates on the pathway to MeReO(mtp)X.

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