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 N4 ligands, including cis-[ReV (O)2 (pyxn)]+ (1; pyxn = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)cyclohexane-1,2-diamine), cis-[ReV (O)2 (6-Me2 pyxn)]+ (cis-2), cis-[ReV (O)2 (R,R-pdp)]+ (3; R,R-pdp = 1,1'-bis((R,R)-2-pyridinylmethyl)-2,2'-bipyrrolidine), cis-[ReV (O)2 (R,R-6-Me2 pdp)]+ (4), and cis-[ReV (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 [NBu4 ]PF6 ) display a reversible ReVI/V couple at E1/2 = 0.39-0.49 V vs SCE. In aqueous media, three proton-coupled electron transfer reactions corresponding to ReVI/V , ReV/III , and ReIII/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 dm3 mol-1 cm-1 ), 580 nm (1700-5580 dm3 mol-1 cm-1 ), and 462-523 nm (3170-6000 dm3 mol-1 cm-1 ). Reaction of 1 with Lewis acids (or protic acids) gave cis-[ReV (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-[ReVI (O)2 (pyxn)](PF6 )2 (1'·(PF6 )2 ) was obtained by constant-potential electrolysis of 1·PF6 in MeCN (0.1 M [NBu4 ]PF6 ) 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 dm3 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.