Syntheses and oxidation of methyloxorhenium(V) complexes with tridentate ligands

Four new methyloxorhenium(V) compounds were synthesized with these tridentate chelating ligands: 2-mercaptoethyl sulfide (abbreviated HSSSH), 2-mercaptoethyl ether (HSOSH), thioldiglycolic acid (HOSOH), and 2-(salicylideneamino)benzoic acid (HONOH). Their reactions with MeReO(3) under suitable conditions led to these products: MeReO(SSS), 1, MeReO(SOS), 2, MeReO(OSO)(PAr(3)), 3, and MeReO(ONO)(PPh(3)), 4. These compounds were characterized spectroscopically and crystallographically. Compounds 1 and 2 have a five-coordinate distorted square pyramidal geometry about rhenium, whereas 3 and 4 are six-coordinate compounds with distorted octahedral structures. The kinetics of oxidation of 2 and 3 in chloroform with pyridine N-oxides follow different patterns. The oxidation of 2 shows first-order dependences on the concentrations of 2 and the ring-substituted pyridine N-oxide. The Hammett analysis of the rate constants gives a remarkably large and negative reaction constant, rho = -4.6. The rate of oxidation of 3 does not depend on the concentration or the identity of the pyridine N-oxide, but it is directly proportional to the concentration of water, both an accidental and then a deliberate cosolvent. The mechanistic differences have been interpreted as reflecting the different steric demands of five- and six-coordinate rhenium compounds.