cis-Dioxorhenium(V/VI) Complexes Supported by Neutral Tetradentate N 4 Ligands. Synthesis, Characterization, and Spectroscopy.

A series of cis-dioxorhenium(V) complexes containing chiral tetradentate N ₄ ligands, including cis-[Re ^V (O) ₂ (pyxn)] ⁺ (1; pyxn = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)cyclohexane-1,2-diamine), cis-[Re ^V (O) ₂ (6-Me ₂ pyxn)] ⁺ (cis-2), cis-[Re ^V (O) ₂ (R,R-pdp)] ⁺ (3; R,R-pdp = 1,1'-bis((R,R)-2-pyridinylmethyl)-2,2'-bipyrrolidine), cis-[Re ^V (O) ₂ (R,R-6-Me ₂ pdp)] ⁺ (4), and cis-[Re ^V (O) ₂ (bqcn)] ⁺ (5; bqcn = N,N'-dimethyl-N,N'-di(quinolin-8-yl)cyclohexane-1,2-diamine), were synthesized. Their structures were established by X-ray crystallography, showing Re-O distances in the range of 1.740(3)-1.769(8) Å and O-Re-O angles of 121.4(2)-124.8(4)°. Their cyclic voltammograms in MeCN (0.1 M [NBu ₄ ]PF ₆ ) display a reversible Re ^VI/V couple at E _1/2 = 0.39-0.49 V vs SCE. In aqueous media, three proton-coupled electron transfer reactions corresponding to Re ^VI/V , Re ^V/III , and Re ^III/II couples were observed at pH 1. The Pourbaix diagrams of 1·OTf, 3·OTf, and 5·OTf have been examined. The electronic absorption spectra of the cis-dioxorhenium(V) complexes show three absorption bands at around 800 nm (600-1730 dm ³ mol ^-1 cm ^-1 ), 580 nm (1700-5580 dm ³ mol ^-1 cm ^-1 ), and 462-523 nm (3170-6000 dm ³ mol ^-1 cm ^-1 ). Reaction of 1 with Lewis acids (or protic acids) gave cis-[Re ^V (O)(OH)(pyxn)] ²⁺ (1·H ⁺ ), in which the Re-O distances are lengthened to 1.788(5) Å. Complex cis-2 resulted from isomerization of trans-2 at elevated temperature. cis-[Re ^VI (O) ₂ (pyxn)](PF ₆ ) ₂ (1'·(PF ₆ ) ₂ ) was obtained by constant-potential electrolysis of 1·PF ₆ in MeCN (0.1 M [NBu ₄ ]PF ₆ ) at 0.56 V vs SCE; it displays shorter Re-O distances (1.722(4), 1.726(4) Å) and a smaller O-Re-O angle (114.88(18)°) relative to 1 and shows a d-d transition absorption band at 591 nm (ε = 77 dm ³ mol ^-1 cm ^-1 ). With a driving force of ca. 75 kcal mol ^-1 , 1' oxidizes hydrocarbons with weak C-H bonds (75.5-76.3 kcal mol ^-1 ) via hydrogen atom abstraction. DFT and TDDFT calculations on the electronic structures and spectroscopic properties of the cis-dioxorhenium(V/VI) complexes were performed.