Protolytic behavior of axially coordinated hydroxy groups of Tin(IV) porphyrins as promising molecular catalysts for water oxidation

Add time:08/03/2019    Source:sciencedirect.com

A series of four Tin(IV)porphyrins with different substituents on meso-phenyl groups, trans-dihydroxy-5,10,15,20-tetra(p-tolyl)porphyrinatetin(IV) (Sn(IV)TTP(OH)2), trans-dihydroxy-tetra(2,4,6-trimethyl)phenylporphyrinatetin(IV) (Sn(IV)TMP(OH)2), trans-dihydroxy-5,10,15,20-tetra(p-carboxyphenyl)porphyrinatetin(IV) (Sn(IV)TCPP), and trans-dihydroxy-5,10,15,20-tetra(N-methyl-4′-pyridiniumyl)porphyrinatetin(IV) (Sn(IV)TMPyP(OH)2) have been prepared and characterized focusing on their protolytic behavior of the axial ligands, hydroxy groups, both in the ground and excites states as well as their electrochemical oxidation behavior. The two axially ligating hydroxy groups on each Tin(IV)-porphyrins exhibit protonations and deprotpnations in five steps. The pKa value of each protolytic equilibrium in the ground states was spectrophotometrically determined, while fluorescence decay time profiles indicate that the protolytic processes are sufficiently slower than the intrinsic fluorescence decay rates to lead to non-protolysis in the excited states. The oxidation peak potentials were determined by cyclic voltammetric analysis. Pourbaix diagram of each protolytic species of Sn(IV)TMPyP in water indicated that the one-electron oxidation of fully deprotonated Sn(IV)TMPyP(O−)2 is not a proton-coupled process, while the doubly hydroxy coordinated species Sn(IV)TMPyP(OH)2 has a proton-coupled one. Tin(IV)-porphyrins exhibit efficient catalytic currents on the first oxidation wave in their cyclic voltammetry and the saturated catalytic currents were high enough in a wide range of pH to be promising as water oxidation catalysts. This is the first detailed study on Tin(IV)-porphyrins to examine water oxidation ability of the one-electron oxidized form against twenty kinds of species (five protolytic species for each of the four Tin(IV)-porphyrins).

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