Second Sphere Control of Redox Catalysis: Selective Reduction of O₂ to O₂^– or H₂O by an Iron Porphyrin Catalyst

Abstract

"Click" reaction has been utilized to synthesize porphyrin ligands possessing distal superstructures functionalized with ferrocenes, carboxylic acid esters, and phenols. Both structural and spectroscopic evidence indicate that hydrogen bonding interaction between the triazole residues resulting from the "click" reaction promotes axial ligand binding into the sterically demanding distal pocket in preference to the open proximal side. An iron porphyrin complex with four ferrocene groups is found to bind O(2) and quantitatively reduce it by one electron to O(2)(-) in apolar organic solvents. However the same complex electro-catalytically reduces O(2) by four electrons to H(2)O in aqueous medium under fast, moderate, and slow electron fluxes. This selectivity for O(2) reduction is governed by the reduction potential of the electron transfer site (i.e., ferrocene) which in turn is governed by the solvent. This catalyst mimics control of catalysis of an enzyme active site by a second sphere electron transfer residue which is often encountered in naturally occurring metallo-enzymes.

Influence of the distal guanidine group on the rate and selectivity of O2 reduction by iron porphyrin

“…These include the "push" effect of thiolate, presence of 2 nd sphere electron transfer and hydrogen bonding effects. 15,26,35,36 Previously the best results were obtained with synthetic mimics of CcO which includes a redox active Cu and phenol ( Fig. 8B and Table 2 row 2).…”

“…8B and Table 2, row 2). 11,35 The rate, on the other hand, depends solely on the extent of activation of the O-O bond of the bound peroxide. The activation of the O-O bond can be achieved by a distal metal like Cu (Fig.…”

Small-molecule activation with iron porphyrins using electrons, photons and protons: some recent advances and future strategies

“…1B), they analyzed the role of the secondary coordination sphere effect for facilitating ORR catalysis. 21,28,29 They found that the α 4 -FeFc 4 complex could act both as a homogeneous catalyst (in an organic solvent in the presence of acid) and as a heterogeneous catalyst (in an aqueous medium, pH 1-9) for ORR, and proposed that the triazole residues offer an efficient proton-transfer pathway into the active site.…”

Efficient Solar‐Assisted O2 Reduction Using a Cofacial Iron Porphyrin Dimer Catalyst Integrated into a p‐CuBi2O4 Photocathode

“…Due to their relevance in biological ORR systems, mononuclear Fe porphyrins have received extensive attention to understand the ORR mechanism of biological enzymes and to develop efficient non‐precious metal‐based catalysts for fuel cells . Dinuclear Fe porphyrin dimers, however, have not yet been investigated for the ORR, although their dinuclear Co analogues had been extensively investigated, in which two Co ions capture O 2 through coordination and facilitate O−O bond cleavage, enabling catalytic electrochemical 4 e − reduction of O 2 to H 2 O .…”

“…Due to their relevance in biological ORR systems, mononuclear Fe porphyrins have received extensivea ttention to understandt he ORR mechanism of biological enzymes and to develop efficient non-precious metal-based catalystsf or fuel cells. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] Dinuclear Fe porphyrin dimers, however,h ave not yet been investigatedfor the ORR, although their dinuclear Co analogues had been extensively investigated,i nw hich two Co ions captureO 2 through coordinationa nd facilitateO ÀOb ond cleavage, enabling catalytic electrochemical4e À reduction of O 2 to H 2 O. [28][29][30][31][32] Recently,acofaciald inuclear Fe porphyrinphthalocyanine heterodimerl inked by flexible rotaxane showed efficient electrocatalytic ORR starting at ap ositive onsetp otential (0.78 V) compared to that obtained from its Fe phthalocyanine (0.68)o rF ep orphyrin (0.59)m oieties,i ndicating the importance of the dimer for efficient ORR catalysis.…”