Reversible proton-coupled Re^VII–Re^VI and Re^VI–Re^V couples and crystal structure of [Re^VO₂(OH₂)(Me₃tacn)]BPh ₄ (Me₃tacn = 1,4,7-trimethyl-1,4,7- triazacyclononane)

Abstract

The electrochemistry of [Re^VIIO₃(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 -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^VO₂(OH₂)(Me₃tacn)]BPh ₄ 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, β = 91.38(63)°, 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)°.

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