Oxidorhenium(V) complexes of a family of bipyridine-like ligands including pyridyltriazines and pyrazinyltriazine : oxygen-atom transfer, metal redox and correlations

Abstract

The title ligands (general abbreviation L) are bipyridine (bpy), its dimethyl (mbpy) and diphenyl (pbpy) derivatives, phenanthroline (phen), 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine (ppyt) and its dimethyl (mpyt) and pyrazinyl (ppzt) analogues. The concerned oxido complexes are [ReOCl₃(L)],[ReOBr₃(ppyt)] and [ReOBr₃(ppzt)]. The chloro and bromo complexes of ppyt and ppzt were prepared by reacting these ligands with [ReOX₃(AsPh₃)₂] (X = Cl, Br). The X-ray structures of [ReOCl₃(ppyt)] and [ReOCl₃(ppzt)] reveal that the ReCl₃ fragment is meridionally disposed and that the L ligand is N,N-coordinated such that the pyridine/pyrazine nitrogen lies trans to the oxido oxygen atom. The Re-O lengths [1.656(10)/1.625(9) Å] correspond to approximate triple bonding. The rate of oxygen-atom transfer from [ReOX₃(L)] to triphenylphosphane in solution follows second-order kinetics and is associated with a large and negative entropy of activation (approx. -30 cal K^-1 mol^-1). The initial attack is believed to involve the phosphane lone pair and Re≡O π-orbitals. Electron withdrawal from the Re^VO moiety by varying X or L facilitates oxygen-atom transfer. Thus, the rates follow the orders Br < Cl; mbpy < bpy < phen < pbpy << mpyt < ppyt < ppzt. The reduction potential of the quasireversible Re^VIO/Re^VO couple displays similar trends and the logarithmic rate constant of oxygen-atom transfer is found to correlate linearly with the reduction potential.