Three series of cobalt(III) corroles were tested as catalysts for the electroreduction of dioxygen to water. One was a simple monocorrole represented as (Me(4)Ph(5)Cor)Co, one a face-to-face biscorrole linked by an anthracene (A), biphenylene (B), 9,9-dimethylxanthene (X), dibenzofuran (O) or dibenzothiophene (S) bridge, (BCY)Co(2) (with Y = A, B, X, O or S), and one a face-to-face bismacrocyclic complex, (PCY)Co(2), containing a Co(II) porphyrin and a Co(III) corrole also linked by one of the above rigid spacers (Y = A, B, X, or O). Cyclic voltammetry and rotating ring-disk electrode voltammetry were both used to examine the catalytic activity of the cobalt complexes in acid media. The mixed valent Co(II)/Co(III) complexes, (PCY)Co(2), and the biscorrole complexes, (BCY)Co(2), which contain two Co(III) ions in their air-stable forms, all provide a direct four-electron pathway for the reduction of O(2) to H(2)O in aqueous acidic electrolyte when adsorbed on a graphite electrode, with the most efficient process being observed in the case of the complexes having an anthracene spacer. A relatively small amount of hydrogen peroxide was detected at the ring electrode in the vicinity of E(1/2) which was located at 0.47 V vs SCE for (PCA)Co(2) and 0.39 V vs SCE for (BCA)Co(2). The cobalt(III) monocorrole (Me(4)Ph(5)Cor)Co also catalyzes the electroreduction of dioxygen at E(1/2) = 0.38 V with the final products being an approximate 50% mixture of H(2)O(2) and H(2)O.

Co(III) corroles were investigated as efficient catalysts for the reduction of dioxygen in the presence of perchloric acid in both heterogeneous and homogeneous systems. The investigated compounds are (5,10,15-tris(pentafluorophenyl)corrole)cobalt (TPFCor)Co, (10-pentafluorophenyl-5,15-dimesitylcorrole)cobalt (F 5PhMes 2Cor)Co, and (5,10,15-trismesitylcorrole)cobalt (Mes 3Cor)Co, all of which contain bulky substituents at the three meso positions of the corrole macrocycle. Cyclic voltammetry and rotating ring-disk electrode voltammetry were used to examine the catalytic activity of the compounds when adsorbed on the surface of a graphite electrode in the presence of 1.0 M perchloric acid, and this data is compared to results for the homogeneous catalytic reduction of O 2 in benzonitrile containing 10 (-2) M HClO 4. The corroles were also investigated as to their redox properties in nonaqueous media. A reversible one-electron oxidation occurs at E 1/2 values between 0.42 and 0.89 V versus SCE depending upon the solvent and number of fluorine substituents on the compounds, and this is followed by a second reversible one-electron abstraction at E 1/2 = 0.86 to 1.18 V in CH 2Cl 2, THF, or PhCN. Two reductions of each corrole are also observed in the three solvents. A linear relationship is observed between E 1/2 for oxidation or reduction and the number of electron-withdrawing fluorine groups on the compounds, and the magnitude of the substituent effect is compared to what is observed in the case of tetraphenylporphyrins containing meso -substituted C 6F 5 substituents. The electrochemically generated forms of the corrole can exist with Co(I), Co(II), or Co(IV) central metal ions, and the site of the electron-transfer in each oxidation or reduction of the initial Co(III) complex was examined by UV-vis spectroelectrochemistry. ESR characterization was also used to characterize singly oxidized (F 5PhMes 2Cor)Co, which is unambiguously assigned as a Co(III) radical cation rather than the expected Co(IV) corrole with an unoxidized macrocyclic ring.

A series of biscobalt cofacial porphyrin-corrole dyads bearing mesityl substituents at the meso positions of the corrole ring were investigated as to their electrochemistry, spectroelectrochemistry, and CO binding properties in nonaqueous media and then applied to the surface of a graphite electrode and tested as electrocatalysts for the reduction of dioxygen to water or hydrogen peroxide in air-saturated aqueous solutions containing 1 M HClO(4). The catalytic reduction of O(2) with the same dyads was also investigated in the homogeneous phase using 1,1'-dimethylferrocene as a reductant in PhCN containing HClO(4). The examined compounds are represented as (PMes(2)CY)Co(2), where P = a porphyrin dianion, Mes(2)C = a corrole trianion with two mesityl groups in trans meso-positions of the macrocycle, and Y is one of three bridging groups separating the two metallomacrocycles in a face-to-face arrangement, either with 9,9-dimethylxanthene, dibenzofuran, or diphenylether as linkers. Cyclic voltammetry and rotating disk electrode voltammetry revealed that the examined compounds are all catalytically active toward the electroreduction of dioxygen in acid media giving H(2)O(2) or H(2)O depending upon the type of linkage (Y) and the initial site of electron transfer which, in nonaqueous media, could be switched between the corrole and the porphyrin metal center by variations of substituents on the corrole macrocycle or the gas above the solution. The homogeneous reduction of dioxygen via a two- or four-electron transfer process was also investigated using 1,1'-dimethylferrocene as reductant in PhCN containing HClO(4).

Three face-to-face linked porphyrin-corrole dyads were investigated as to their electrochemistry, spectroelectrochemistry, and chloride-binding properties in dichloromethane or benzonitrile. The same three compounds were also investigated as to their ability to catalyze the electroreduction of dioxygen in aqueous 1 M HClO4 or HCl when adsorbed on a graphite electrode. The characterized compounds are represented as (PCY)H2Co, where P = a porphyrin dianion; C = a corrole trianion; and Y = a biphenylenyl, 9,9-dimethylxanthenyl, or anthracenyl spacer, which links the two macrocycles in a face-to-face arrangement. An axial binding of one or two Cl- ligands to the cobalt center of the corrole is observed for singly and doubly oxidized (PCY)H2Co, with the exact stoichiometry of the reaction depending upon the spacer size and the concentration of Cl- added to solution. No Cl- binding occurs for the neutral or reduced forms of the dyad, which contrasts with what is seen for the monocorrole, (Me4Ph5Cor)Co, where a single Cl- ligand is added to the Co(III) corrole in PhCN. The Co(III) form of the corrole in (PCY)H2Co also appears to be the catalytically active species in the electroreduction of dioxygen, which occurs at potentials associated with the Co(IV)/Co(III) reaction, that is, 0.35 V in 1 M HClO4 as compared to 0.31-0.42 V for the same three dyads in PhCN and 0.1 M TBAP. The potential for the catalytic electroreduction of O2 in HCl shifts negatively by 60 to 70 mV as compared to E(1/2) values in 1 M HClO4, consistent with the binding of Cl- to the Co(IV) form of the corrole and its rapid dissociation after electroreduction to Co(III) at the electrode surface.

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The copper(II) binding properties of the macrobicyclic diamide 1,9,12,18,22-pentaazatricyclo[7.6.6.1(3,7)]docosa-3,5,7(22)-triene-13,19-dione (L1) have been fully investigated by spectroscopic (IR, UV-vis, EPR, MALDI-TOF MS), X-ray diffraction, potentiometric, electrochemical, and spectroelectrochemical methods. This constrained receptor possesses a hemispherical cavity created by cross-bridging the 1 and 8 positions of trans-dioxocyclam (1,4,8,11-tetraazacyclotetradecane-5,12-dione, L2) with a 2,6-pyridyl strap. Treatment of L1 with a copper salt in methanol produces a red complex of [Cu(L1H(-1))]+ formula in which the copper atom is embedded in a 13-membered ring and coordinated by both amines as well as the pyridine and one deprotonated amide nitrogen atoms. Infrared spectroscopy provides evidence for protonation of the carbonyl oxygen atom belonging to the copper-bound amide of [Cu(L1H(-1))]+ under strongly acidic conditions. The resulting conversion of the amidate into an iminol group highlights the inert character of the corresponding complexes, which do not dissociate at low pH values. In contrast, both secondary amides of L1 deprotonate in the presence of a weak base, thus affording a blue pentacoordinated [Cu(L1H(-2))] compound where the copper atom sits in the center of the 14-membered dioxocyclam fragment. In aqueous solution, both complexes undergo a pH-driven (pK(a) = 8.73(2)) molecular reorganization, which is reminiscent of a glider motion. The copper(II) cation switches rapidly and reversibly from a four-coordinate flattened tetrahedral arrangement of the donor atoms in the red species to a five-coordinate environment in the blue species, which is intermediate between a square pyramid and a trigonal bipyramid. Conversion of the red to the blue form was also demonstrated to occur upon reduction of [Cu(L1H(-1))]+ by cyclic voltammetry (E(pc) = -0.64 V/SCE in CH(3)CN).

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