Electrocatalytic four-electron reduction of oxygen to water by a highly flexible cofacial cobalt bisporphyrin

Abstract

Dicobalt(II) cofacial bisporphyrins anchored by dibenzofuran (DPD) and xanthene (DPX) are efficient electrocatalysts for the four-electron reduction of oxygen to water despite their ca. 4 Å difference in metal-metal distances, suggesting that the considerable longitudinal 'Pac-Man' flexibility of the pillared platforms is the origin for the similar catalytic reactivity of these structurally disparate systems.Enzymatic systems are remarkable in their ability to accommodate the large range of motion required for the binding and catalysis of small molecules. In many cases, the kinetic steps of the processes involved are ultimately predicated on conformational changes of the active site upon substrate binding, activation, and/or product release. An outstanding example is the binding and biological reduction of dioxygen to water by cytochrome c oxidase (CcO). 1 The critical O-O bond cleavage chemistry is mediated by a flexible, dinuclear iron-heme/ copper (Fe a3 /Cu B ) assembly. 1,2 Nevertheless, the pursuit of structural and functional models for O 2 activation have emphasized, for the most part, bimetallic reaction centers poised within well-defined, rigid pockets. [3][4][5][6][7] For example, pillared cofacial dicobalt bisporphyrins bridged by anthracene (DPA) and biphenylene (DPB) 8-11 impair ring slippage, and as a result, these complexes efficiently electrocatalyze the direct fourelectron reduction of oxygen to water (as opposed to the twoelectron pathway involving peroxide) with little structural reorganization of juxtaposed subunits. Can efficient oxygenactivation chemistry be preserved when this cofacial structural motif exhibits a large range of motion? To address this issue, we have developed methods for the facile assembly of new cofacial bisporphyrins, incorporating dibenzofuran (DPD) 12 or xanthene (DPX) 13 pillars that exhibit variable pocket sizes with minimal lateral displacements. Herein, we report that dicobalt(II) complexes of both DPD and DPX efficiently mediate the direct four-electron reduction of oxygen to water despite a ca. 4 Å difference in their metal-metal distances (as determined from their X-ray crystal structures), suggesting that the longitudinal 'Pac-Man' flexibility of these molecular clefts allows the designed binding pocket to structurally accommodate reaction intermediates during multielectron catalysis.Co 2 (DPD) 1 was obtained in excellent yield (91%) from reaction of the corresponding free base bisporphyrin with CoCl 2 and 2,6-lutidine. † Crystals suitable for X-ray diffraction studies were grown from dichloromethane-methanol solutions. ‡ The structure of 1 (Fig. 1) shows that two methanol solvent molecules are coordinated inside the bisporphyrin pocket to the two cobalt(II) centers. In order to accommodate the two exogeneous ligands, the DPD framework opens its 'bite' considerably. The interplanar angle between the two macrocycles is 56.5°, resulting in metal-metal (8.624 Å) and centerto-center (8.874 Å) distances that are markedly larger than found in Zn 2 (DPD) (d Zn-Zn = ...

Role of Proton‐Coupled Electron Transfer in O—O Bond Activation

“…Dicobalt(II) Pacman complexes of both DPX and DPD are selective catalysts for the direct reduction of O 2 to H 2 O over H 2 O 2 with 72% and 80% selectivities, respectively. 33,34 This selectivity is markedly attenuated by the installation of a methoxyaryl group onto the Pacman motif. Co 2 (DPXM) catalyzes the reduction of oxygen at a positive potential of +0.24 V (vs Ag/AgCl); however, only 52% of the reaction proceeds along the 4e -/4H + pathway to produce water, while DPDM catalyzes oxygen reduction at a potential of +0.25 V (vs Ag/AgCl) with 46% going directly to water.…”

“…The resting state for oxygen binding is the Co 2 (II,III) superoxo species; this is experimentally observed by EPR spectra, which show a symmetric 17-line spectrum for a symmetrically bound superoxide within the mixed-valence cleft of the Pacman porphyrin. 33 on the porphyrin π system with no electron density observed on the superoxo. Moreover, Mulliken population analysis reveals that the oxygen atoms of the superoxo within the DPX cleft are ∼20% more negatively charged than when in the DPXM cleft.…”

“…The splitting of the oxidation processes arises from strong interactions between two proximate porphyrin π systems. 33 The redox properties of the parent DPX and DPD systems are not significantly perturbed by introduction of the sterically demanding methoxyaryl group trans to the spacer. The mixed-valence behavior of Co 2 (DPX) is displayed by Co 2 (DPXM) at comparable potentials (E°) +0.14 and +0.33 V vs Ag/AgCl).…”

Proton Pump for O2 Reduction Catalyzed by 5,10,15,20‐Tetraphenylporphyrinatocobalt(II)

“…[13][14][15] In most cases, monomeric cobalt porphyrins catalyze the electroreduction of O 2 to hydrogen peroxide (H 2 O 2 ), whereas dimeric cofacial cobalt porphyrins demonstrate the catalysis of four-electron reduction of O 2 to water. [3,7,8,10,12,15,16,[24][25][26][27] Employing ferrocene derivatives as electron donors, O 2 reduction catalyzed by various cobalt porphyrins has been investigated by Fukuzumi et al in organic media in the presence of HClO 4 . [13][14][15] In the reaction scheme, the steps of electron coordination to form a superoxide adduct and its reduction by ferrocene derivatives to produce H 2 O 2 /H 2 O and Co III are fast, and that of reducing Co III by ferrocene derivatives is slow and rate limiting.…”

Kinematic molecular manufacturing machines

“…For example, a diporphyrin containing a dibenzofuran linker 7, which led the two rings to be angled at 56.5 o with respect to each other on average, catalyzed the conversion of 80% of the O 2 reactants to water [23].…”