Synthesis and characterization of dimetallic oxorhenium(V) and dioxorhenium(VII) compounds, and a study of stoichiometric and catalytic reactions.

Abstract

Chelating dithiolate ligands--e.g., mtp from 2-(mercaptomethyl)thiophenol, edt from 1,2-ethanedithiol, and pdt from 1,3-propanedithiol--stabilize high-valent oxorhenium(V) against hydrolytic and oxidative decomposition. In addition to the dithiolate chelating to a single rhenium, one sulfur forms a coordinate bond to the other rhenium. In one arrangement this gives a dimer with a nearly planar diamond core with different internal Re-S distances. The new compounds are [MeReO(edt)](2) (2) and [MeReO(pdt)](2) (3), which can be compared to the previously known [MeReO(mtp)](2) (1). Another mode of synthesis leads to [ReO](2)(mtp)(3) (5) and [ReO](2)(edt)(3) (6). They, too, have similar Re(2)S(2) cores that involve donor atoms from two of the dithiolate ligands; the third dithiolate chelates one of the rhenium atoms. Gentle hydrolysis of 1 affords [Bu(n)4][[MeReO(mtp)](2)(mu-OH)] (7) in low yield. It appears to be the first example of this structural type for rhenium. The use of dithioerythritol as a starting material allowed the synthesis of a dioxorhenium(VII) compound, [MeReO(2)](2)(dte) (8). Its importance lies in understanding the role such compounds are believed to play as intermediates in oxygen atom catalysis. Ligation of the dimers 1-3 converts them into monomeric compounds, MeReO(dithiolate)L. These reactions go essentially to completion for L = PPh(3), but reach an equilibrium for L = NC(5)H(4)R. With R = 4-Ph, the values of K/10(3) L mol(-1) for the reactions (1-3) + 2L = 2MeReO(dithiolate)L are identical within 3 sigma: 1.15(3) (1), 1.24(4) (2), and 1.03(16) (3). The rates of monomer formation follow the rate law -d ln [dimer]/dt = k(a)[L] + k(b)[L](2). These trends were found: (1) phosphines are slow to react compared to pyridines, (2) the edt dimer 2 reacts much more rapidly than 1 and 3. Dimer 1 and MeReO(mtp)PPh(3) both catalyze oxygen atom transfer: PicO + PPh(3) --> Pic + Ph(3)PO. Compound 1 is ca. 90 times more reactive, which can be attributed to its lability toward small ligands as opposed to the low rate of displacement of PPh(3) from the mononuclear catalyst. The kinetics of this reaction follows the rate law -d[PicO]/dt = k[PicO][1]/[1 + kappa[PPh(3)]], with k = 5.8 x 10(6) L mol(-1) s(-1) and kappa = 3.5 x 10(2) L mol(-1) at 23 degrees C in benzene. A mechanism has been proposed to account for these findings.