Cobalt Tetrabutano- and Tetrabenzotetraarylporphyrin Complexes: Effect of Substituents on the Electrochemical Properties and Catalytic Activity of Oxygen Reduction Reactions

Abstract

Three series of cobalt tetraarylporphyrins were synthesized and characterized by electrochemistry and spectroelectrochemistry. The investigated compounds have the general formula (TpYPP)Co, butano(TpYPP)Co, and benzo(TpYPP)Co, where TpYPP represents the dianion of the meso-substituted porphyrin, Y is a CH, H, or Cl substituent on the para position of the four phenyl rings, and butano and benzo are respectively the β- and β'-substituted groups on the four pyrrole rings of the compound. Each porphyrin undergoes one or two reductions depending upon the meso substituent and solvent utilized. Two irreversible reductions are observed for (TpYPP)Co and butano(TpYPP)Co in CHCl containing 0.1 M tetra-n-butylammonium perchlorate; the first leads to the formation of a highly reactive cobalt(I) porphyrin, which can then rapidly react with a solvent to give a CoCHCl as the product. Only one reversible reduction is seen for benzo(TpYPP)Co under the same solution conditions, and the one-electron-reduction product is assigned as a cobalt(II) porphyrin π-anion radical. Three oxidations can be observed for each examined compound in CHCl. The first oxidation is metal-centered for the (TpYPP)Co and benzo(TpYPP)Co derivatives, leading to generation of a cobalt(III) porphyrin with an intact π-ring system, but this redox process is ring-centered in the case of butano(TpYPP)Co and gives a Co π-cation radical product. Each porphyrin was also examined as a catalyst for oxygen reduction reactions (ORRs) when adsorbed on a graphite electrode in 1.0 M HClO. The number of electrons transferred (n) during ORRs is 2.0 for the butano(TpYPP)Co derivatives, consistent with only HO being produced as a product for the reaction with O. However, the reduction of O using the cobalt benzoporphyrins as catalysts gave n values between 2.6 and 3.1 under the same solution conditions, thus producing a mixture of HO and HO as the reduction product. This result indicates that the β and β' substituents have a significant effect on the catalytic properties of the cobalt porphyrins for ORRs in acid media.

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

“…8D and Table 2, row 4). 37,38 However, when stabilisation of this intermediate via H-bonding or bonding with a distal metal atom is included along with the ET site (e.g. FeFc 4 and FeCu-phenol) there is a distinct improvement in selectivity ( Fig.…”

Significantly improved electrocatalytic oxygen reduction by an asymmetrical Pacman dinuclear cobalt(ii) porphyrin–porphyrin dyad

“…4, [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] In addition to biologically relevant Fe porphyrins, Co porphyrins have also received great attention because of their high activity and stability. [19][20][21][22][23][24][25][26][32][33][34] However, mononuclear Co porphyrins are much less selective than Fe analogues to catalyze the 4e reduction of O 2 to water. Unlike Fe complexes, Co complexes in general catalyze the 2e reduction of O 2 because the peroxo intermediates of Co are challenging to undergo heterolytic O-O bond cleavage to generate terminal Co-oxo species that have large d electron counts and thus are high in energy.…”

“…[35][36][37] Several molecular design strategies have been reported to improve the selectivity for the 4e ORR, including (1) formation of intramolecular hydrogen bonding interactions to stabilize O 2 adducts, 27,30,38,39 (2) appending acid groups to assist proton delivery, 19,28,29,40 and (3) introduction of functional groups to ensure rapid electron transfer between catalyst molecules and electrodes. 34,[41][42][43][44] Additionally, for complexes of late-transition metals, such as Co, it is found that dinuclear complexes are typically more efficient than mononuclear analogues to catalyze the 4e reduction of O 2 . [20][21][22][23][24][25][26][45][46][47][48][49][50][51][52][53] This improvement is likely caused by the formation of peroxo-bridged dinuclear intermediates, 45,47 whose subsequent reduction can result in the 4e ORR.…”

Oxygen Reduction Reaction at Penta-Coordinated Co Phthalocyanines

“…The ORR catalyzed at electrodes modified with CoN4 complexes is of major interest because of the ease and the convenience of the synthesis of those redox complexes and because of the different catalytic properties of the various complexes which were synthetized to optimize the catalysis. For example ORR catalyzed by unsubstituted CoPc or by CoPc with the addition of one or more electron-donating or electronwithdrawing substituents adsorbed directly on graphite proceeds via a 2-electron transfer process (Zagal et al, 1992;Sun et al, 2014;Zagal and Koper, 2016;Ye et al, 2017;Abarca et al, 2019) whereas complexes with pendant residues that increase the electron back-bonding would promote a direct 4-e process as in Equation 2 (Steiger and Anson, 1995;Tse et al, 1997;Riquelme et al, 2018a,b). Co-cofacial bimetallic complexes and dimeric Co porphyrins that bind simultaneously both dioxygen atoms would also transfer directly 4 electrons to O 2 (Collman et al, 1980;Durand et al, 1983;Le Mest et al, 1997;Peljo et al, 2012;Swavey and Eder, 2013;Wada et al, 2013;Dey et al, 2017;Oldacre et al, 2018).…”