Unusual reactivity of the {Re^VO}³⁺ core: Syntheses and characterization of novel rhenium halide complexes with N-methyl-o- diaminobenzene

The reactions of 1 or 2 equiv of N-methyl-o-diaminobenzene with trans-[ReOX₃(PPh₃)₂] (X = Cl, Br) in refluxing chloroform gave oxo-free rhenium complexes [Re^VIX₄(NC ₆H₄NHCH₃)(OPPh₃)] (X = Cl, 3; X = Br, 6), [Re^VX₂Y-(NC₆H₄NHCH ₃)(PPh₃)₂] (X, Y = Cl, 4; X = Br, Y = Cl, 7), [Re^IVCl₂(NHC₆H₄NCH₃) ₂] (5), and [Re^IVBr₃-(NHC₆H ₄NCH₃)(PPh₃)] (8). All complexes were characterized by elemental analysis, ¹H NMR and IR spectroscopy, cyclic voltammetry, EPR spectroscopy, and X-ray crystallography. The complexes all display distorted octahedral coordination geometry. For Re(IV) complexes 5 and 8, the ligands coordinate in the benzosemiquinone diimine form. In Re(VI) complexes 3 and 6 and the Re(V) complexes 4 and 7, the ligands coordinate in the dianionic monodentate imido form. The EPR spectra of Re(VI) species 3 and 6 in dichloromethane solution at room temperature exhibit the characteristic hyperfine pattern of six lines, with evidence of strong second-order effects. The IR spectra of the complexes are characterized by Re=N and Re-N stretching bands at ca. 1090 and 540 cm-1, respectively. The Re(IV) and Re(V) complexes display well-resolved NMR spectra, while the Re(VI) complexes exhibit no observable spectra, due to paramagnetism. The cyclic voltammograms of complexes 3 and 6 display Re(VII)/ Re(VI) and Re(VI)/Re(V) processes, those of 4 and 7 exhibit Re(VI)/Re(V) and Re(V)/Re(IV) couples, and those of 5 and 8 are characterized by Re(V)/Re(IV) and Re(IV)/Re(III) processes.