Reversible proton-coupled ReVII–ReVI and ReVI–ReV couples and crystal structure of [ReVO2(OH2)(Me3tacn)]BPh 4 (Me3tacn = 1,4,7-trimethyl-1,4,7- triazacyclononane)

Abstract

The electrochemistry of [Re ^VII O ₃ (Me ₃ tacn)]PF ₆ (Me ₃ tacn = 1,4,7-trimethyl-1,4,7- triazacyclononane) in aqueous solution has been studied. At pH 1 it shows two quasi-reversible couples I at -0.14 and II at -0.36 V vs. saturated calomel electrode. Constant-potential coulometry at -0.50 V shows that the total number of electrons transferred for these two couples is two. The Pourbaix diagram over the range pH 0.9–12.2 shows that E _½ of couple I shifts cathodically by 60 mV per pH unit. For couple II there are two straight-line segments with slopes of -118 mV (0.9 < pH < 4.1) and -60 mV (pH > 4.1) per pH unit. The complex [Re ^V O ₂ (OH ₂ )(Me ₃ tacn)]BPh ₄ was prepared and structurally characterized by X-ray crystal analysis: monoclinic, space group P2 ₁ /n (no. 14), a = 10.387(9), b = 21.176(4), c = 15.452(2) Å, β = 91.38(63)°, Z = 4. The Re–OH ₂ distance is 2.10(2) Å and the ReO distances are 1.78(1) and 1.82(1) Å. The two oxo groups are cis to each other with an angle of 106.7(5)°.