Oxidorhenium(V) Complexes of a Family of Bipyridine-Like Ligands Including Pyridyltriazines and Pyrazinyltriazine: Oxygen-Atom Transfer, Metal Redox and Correlations

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 ReVO moiety by varying X or L facilitates oxygen‐atom transfer. Thus, the rates follow the orders Br < Cl; mbpy < bpy < phen < pbpy