OXYGEN ATOM TRANSFER-REACTIONS OF CATIONIC RHENIUM(III), RHENIUM(V), AND RHENIUM(VII) TRIAZACYCLONONANE COMPLEXES

Re(O)CI3(Me2S)(OPPh3) reacts readily with 1,4,7-trimethyltriazacyclononane (Me3tacn) to form the rhenium(V) oxo cation [Re(O)Cl2(Me3tacn)] + (1) in good yield. With the unsubstituted triazacyclononanc (tacn), however, both [Re(O)Cl2(tacn)]+ (2) and [Re(O)3(tacn)]+ (3) are formed, even under anaerobic conditions. Oxidation of 2 to 3 [Re(V) → Re(VII)] can be easily accomplished with a variety of mild oxidizing agents such as Me2SO and I2, but the oxidation of 1 requires over a month at 80 °C in aqueous nitric acid. Complex 1 is reduced [Re(V) → Re(III)] by oxygen atom transfer to phosphines, forming [Re- (OPR3)Cl2(Mc3tacn)]+ (R = Ph, 4; Me, 5). The OPPh3 ligand in 4 is easily displaced by other neutral ligands such as acetonitrile or acetone. The acetone complex [Re(O=CMe2)Cl2(Me3tacn)]+ (7) is readily oxidized back to 1 [Re(III) → Re(V)] by the oxygen atom donors nBuNCO, OAsPh3, Me2SO, ethylene oxide, pyridine N-oxide, and N2O. These reactions require an open coordination site at the rhcnium(III) center. Surprisingly, it is not substantially easier to oxidize the rhenium(III) complex 7 than the rhenium(V) species 2. On the basis of these reactions, simple thcrmochemical cycles arc used to estimate the rhenium–oxo bond strength in 1 to be 141 ± 9 kcal/mol. © 1990, American Chemical Society. All rights reserved.