Direct Observation of the “Pac-Man” Effect from Dibenzofuran-Bridged Cofacial Bisporphyrins

Deng, Yongqi; Chang, and Christopher J.; Nocera, Daniel G.

doi:10.1021/ja992955r

Cited by 137 publications

(125 citation statements)

References 37 publications

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“…The Pd···Pd separation varies between 3.544 and 4.120 , and, in contrast to Pacman diporphyrinic analogues in which the M···M separation can vary between 3.5 and 7.8 (this range is dependant on the nature of the spacer in the Pacman ligand), [34] represents a small and rigidly constrained vertical translation. Furthermore, the X-ray crystal structures of the dipalladium Pacman bis-porphyrin complexes [Pd 2 …”

Section: Structural Characterisation Of [Pd 2 (L)]mentioning

confidence: 93%

Design and Synthesis of Binucleating Macrocyclic Clefts Derived from Schiff‐Base Calixpyrroles

Givaja

Volpe

Leeland

et al. 2007

Chemistry A European J

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The syntheses, characterisation and complexation reactions of a series of binucleating Schiff-base calixpyrrole macrocycles are described. The acid-templated [2+2] condensations between meso-disubstituted diformyldipyrromethanes and o-phenylenediamines generate the Schiff-base pyrrolic macrocycles H(4)L(1) to H(4)L(6) upon basic workup. The single-crystal X-ray structures of both H(4)L(3).2 EtOH and H(4)L(6).H2O confirm that [2+2] cyclisation has occurred, with either EtOH or H2O hydrogen-bonded within the macrocyclic cleft. A series of complexation reactions generate the dipalladium [Pd2(L)] (L=L(1) to L(5)), dinickel [Ni2(L(1))] and dicopper [Cu2(L)] (L=L(1) to L(3)) complexes. All of these complexes have been structurally characterised in the solid state and are found to adopt wedged structures that are enforced by the rigidity of the aryl backbone to give a cleft reminiscent of the structures of Pacman porphyrins. The binuclear nickel complexes [Ni2(mu-OMe)2Cl2(HOMe)2(H(4)L(1))] and [Ni2(mu-OH)2Cl2(HOMe)(H(4)L(5))] have also been prepared, although in these cases the solid-state structures show that the macrocyclic ligand remains protonated at the pyrrolic nitrogen atoms, and the Ni(II) cations are therefore co-ordinated by the imine nitrogen atoms only to give an open conformation for the complex. The dicopper complex [Cu2(L(3))] was crystallised in the presence of pyridine to form the adduct [Cu2(py)(L(3))], in which, in the solid state, the pyridine ligand is bound within the binuclear molecular cleft. Reaction between H(4)L(1) and [Mn(thf){N(SiMe(3))2}2] results in clean formation of the dimanganese complex [Mn2(L(1))], which, upon crystallisation, formed the mixed-valent complex [Mn2(mu-OH)(L(1))] in which the hydroxo ligand bridges the metal centres within the molecular cleft.

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Section: Structural Characterisation Of [Pd 2 (L)]mentioning

confidence: 93%

Design and Synthesis of Binucleating Macrocyclic Clefts Derived from Schiff‐Base Calixpyrroles

Givaja

Volpe

Leeland

et al. 2007

Chemistry A European J

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“…18 We note, however, that the DPD framework itself is extremely flexible and is able to span large vertical dimension. The metal-metal distances of DPD can range from 3.504 Å for Fe 2 O(DPD), which clamps its Pacman bite around a bridging oxo ligand, to 7.775 Å for Zn 2 -(DPD), 19 which allows the DPD platform to attain its natural splayed conformation. The spring action of the Fe 2 (III,III) µ-oxo Pacman clefts 20,21 has been exploited for the selective photochemical oxidation of olefins 22 and hydrocarbons.…”

Section: Xanthene-and Dibenzofuran-bridged Pacman Assembliesmentioning

confidence: 99%

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

Rosenthal

Nocera

2007

ChemInform

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The selective reduction of oxygen to water requires four electrons and four protons. The design of catalysts that promote oxygen reduction therefore requires the management of both electron and proton inventories. Pacman and Hangman porphyrins provide a cleft for oxygen binding, a redox shuttle for oxygen reduction, and functionality for tuning the acid-base properties of bound oxygen and its intermediates. With proper control of the proton-coupled electron transfer events, O-O bond breaking of oxygen, and more generally oxygenated substrates, may be achieved with high efficiencies. The rule set developed for oxygen reduction may be applied to a variety of other small molecule activation reactions of consequence to energy conversion.

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“…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 = 7.775 Å, d Ct-Ct = 7.587 Å). 12 The Co(II) cores adopt an approximate square-pyramidal geometry, as the N-Co-N bond angles are 90 ± 1. (2) is essentially flat (average deviation 0.0307 Å).…”

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confidence: 99%

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

et al. 2000

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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 = ...

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Direct Observation of the “Pac-Man” Effect from Dibenzofuran-Bridged Cofacial Bisporphyrins

Cited by 137 publications

References 37 publications

Design and Synthesis of Binucleating Macrocyclic Clefts Derived from Schiff‐Base Calixpyrroles

Design and Synthesis of Binucleating Macrocyclic Clefts Derived from Schiff‐Base Calixpyrroles

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

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

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