Reversible proton-coupled ReVII-ReVI and ReVI-ReV couples and crystal structure of [(RevO2(OH2)(Me3tacn)]BPh4 (Me3tacn=1,4,7-trimethyl-1,4,7-triazacyclononane)

Abstract

The electrochemistry of [ReVIIO3(Me3tacn)]PF6 (Me3tacn = 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 20.14 and II at 20.36 V vs. saturated calomel electrode. Constant-potential coulometry at 20.50 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 E2¯ 1 of couple I shifts cathodically by 60 mV per pH unit. For couple II there are two straight-line segments with slopes of 2118 mV (0.9 < pH < 4.1) and 260 mV (pH > 4.1) per pH unit. The complex [ReVO2(OH2)(Me3tacn)]BPh4 was prepared and structurally characterized by X-ray crystal analysis: monoclinic, space group P21/n (no. 14), a = 10.387(9), b = 21.176(4), c = 15.452(2) Å, b = 91.38(63)8, Z = 4. The Re]OH2 distance is 2.10(2) Å and the Re]] O 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)8.