cis-Dioxorhenium(V/VI) Complexes Supported by Neutral Tetradentate N4 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) angstrom and O Re-O angles of 121.4(2)-124.8(4)degrees. Their cyclic voltammograms in MeCN (0.1 M [NBu4]PF6) 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 center dot OTf, 3 center dot OTf, and 5 center dot 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 center dot H+), in which the Re-O distances are lengthened to 1.788(5) angstrom. Complex cis-2 resulted from isomerization of trans -2 at elevated temperature. cis-[Re-VI(O)(2)(pyxn)](PF6)(2) (1'center dot(PF6)(2)) was obtained by constant-potential electrolysis of 1 center dot 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) angstrom) and a smaller O-Re-O angle (114.88(18)degrees) relative to 1 and shows a d-d transition absorption band at 591 nm (epsilon = 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.