Self-assembled metallacyles and cages formed via coordination chemistry have been used as catalysts to enforce 4H^(+)/4e^(-)reduction of oxygen to water with an emphasis on attenuating the formation of hydrogen peroxi...Self-assembled metallacyles and cages formed via coordination chemistry have been used as catalysts to enforce 4H^(+)/4e^(-)reduction of oxygen to water with an emphasis on attenuating the formation of hydrogen peroxide.That said,the kinetically favored 2H^(+)/2e^(-)reduction to H_(2)O_(2) is critically important to industry.In this work we report the synthesis,characterization,and electrochemical benchmarking of a hexaporphyrin cube which catalyses the electrochemical reduction of molecular oxygen to hydrogen peroxide.An established sub-component self-assembly approach was used to synthesize the cubic freebase porphryin topologies from 2-pyridinecarboxaldehyde,tetra-4-aminophenylporphryin(TAPP),and Fe(OTf)_(2)(OTf^(-)=trifluoromethansulfonate).Then,a tandem metalation/transmetallation was used to introduce Co(Ⅱ)into the porphyrin faces of the cube,and exchange with the Fe(Ⅱ)cations at the vertices,furnishing a tetrakaideca cobalt cage.Electron paramagnetic resonance studies on a Cu(Ⅱ)/Fe(Ⅱ)analogue probed radical interactions which inform on electronic structure.The efficacy and selectivity of the CoCo-cube as a catalyst for hydrogen peroxide generation was investigated using hydrodynamic voltammetry,revealing a higher selectivity than that of a mononuclear Co(Ⅱ)porphyrin(83% versus ~50%)with orders of magnitude enhancement in standard rate constant(k_(s)=2.2×10^(2)M^(-1)s^(-1)versus k_(s)=3×100 M^(-1)s^(-1)).This work expands the use of coordination-driven self-assembly beyond ORR to water by exploiting post-synthetic modification and structural control that is associated with this synthetic method.展开更多
Bis-iron(Ⅲ)-μ-oxo porphyrins are known electrocatalysts for Oxygen Reduction Reaction(ORR)that favor four-electron four-proton chemistry.The facile formation of theμ-oxo motif makes forming cofacial structures stra...Bis-iron(Ⅲ)-μ-oxo porphyrins are known electrocatalysts for Oxygen Reduction Reaction(ORR)that favor four-electron four-proton chemistry.The facile formation of theμ-oxo motif makes forming cofacial structures straightforward unlike for other metalloporphyrins where the face-to-face geometry must be enforced by tethering groups.We have pioneered the use of coordination chemistry to obtain cofacial porphyrin ORR catalysts containing Co(II)ions.Here,we adapt our use of molecular clips for the post-synthetic modification ofμ-oxo porphyrin dimers rather than as self-assembly building blocks.Although the cofacial geometry is inherent to theμ-oxo core,under ORR catalysis conditions,the dimer is rapidly cleaved,resulting in reactivity differences between traditional unclipped bis-iron(Ⅲ)-μ-oxo porphyrins and our post-synthetically tethered architectures.We demonstrate our approach using two molecular clips that differ in length.The bis-iron(Ⅲ)-μ-oxo precatalysts were characterized by ^(1)H NMR,ESI-MS,and molecular modeling to support the formation of cofacial prisms.Catalytic activity was studied electrochemically using cyclic voltammetry and hydrodynamic voltammetry.Whereas untethered bis-iron(Ⅲ)-μ-oxo porphyrins are limited to heterogenous catalysis so that the dimeric structure is not lost upon the removal of the oxo bridge,our clipped architectures show significant catalytic current response under homogeneous conditions.Furthermore,when a shorter molecular clip is used as a post-synthetic tether,the selectivity under heterogeneous conditions is significantly enhanced:monomeric Fe(Ⅲ)tetraphenylporphyrin(TPhP)generates 64.3%H_(2)O_(2).When this same porphyrin is templated into a cofacial environment(Fe_(2)O TPhP)the selectivity improves to 15.8%H_(2)O_(2).When a tetrapyridyl porphyrin analogue is post-synthetically tethered through a oxalate-bridged Rh_(2) molecular clip,the selectivity improves further to 7.2%H_(2)O_(2).In contrast,when a longer bis-hydroxybenzoquinato bridge is used in the clip,the selectivity drops back to 14.5%H_(2)O_(2).Our most selective system also shows the highest current response and therefore our post-synthetic modification provides a kinetic enhancement as well.These results establish that straightforward coordination chemistry can be used for post-synthetic tuning of molecular ORR catalysts and when proper molecular clips are selected,marked enhancements to both selectivity and activity may be realized.展开更多
文摘Self-assembled metallacyles and cages formed via coordination chemistry have been used as catalysts to enforce 4H^(+)/4e^(-)reduction of oxygen to water with an emphasis on attenuating the formation of hydrogen peroxide.That said,the kinetically favored 2H^(+)/2e^(-)reduction to H_(2)O_(2) is critically important to industry.In this work we report the synthesis,characterization,and electrochemical benchmarking of a hexaporphyrin cube which catalyses the electrochemical reduction of molecular oxygen to hydrogen peroxide.An established sub-component self-assembly approach was used to synthesize the cubic freebase porphryin topologies from 2-pyridinecarboxaldehyde,tetra-4-aminophenylporphryin(TAPP),and Fe(OTf)_(2)(OTf^(-)=trifluoromethansulfonate).Then,a tandem metalation/transmetallation was used to introduce Co(Ⅱ)into the porphyrin faces of the cube,and exchange with the Fe(Ⅱ)cations at the vertices,furnishing a tetrakaideca cobalt cage.Electron paramagnetic resonance studies on a Cu(Ⅱ)/Fe(Ⅱ)analogue probed radical interactions which inform on electronic structure.The efficacy and selectivity of the CoCo-cube as a catalyst for hydrogen peroxide generation was investigated using hydrodynamic voltammetry,revealing a higher selectivity than that of a mononuclear Co(Ⅱ)porphyrin(83% versus ~50%)with orders of magnitude enhancement in standard rate constant(k_(s)=2.2×10^(2)M^(-1)s^(-1)versus k_(s)=3×100 M^(-1)s^(-1)).This work expands the use of coordination-driven self-assembly beyond ORR to water by exploiting post-synthetic modification and structural control that is associated with this synthetic method.
文摘Bis-iron(Ⅲ)-μ-oxo porphyrins are known electrocatalysts for Oxygen Reduction Reaction(ORR)that favor four-electron four-proton chemistry.The facile formation of theμ-oxo motif makes forming cofacial structures straightforward unlike for other metalloporphyrins where the face-to-face geometry must be enforced by tethering groups.We have pioneered the use of coordination chemistry to obtain cofacial porphyrin ORR catalysts containing Co(II)ions.Here,we adapt our use of molecular clips for the post-synthetic modification ofμ-oxo porphyrin dimers rather than as self-assembly building blocks.Although the cofacial geometry is inherent to theμ-oxo core,under ORR catalysis conditions,the dimer is rapidly cleaved,resulting in reactivity differences between traditional unclipped bis-iron(Ⅲ)-μ-oxo porphyrins and our post-synthetically tethered architectures.We demonstrate our approach using two molecular clips that differ in length.The bis-iron(Ⅲ)-μ-oxo precatalysts were characterized by ^(1)H NMR,ESI-MS,and molecular modeling to support the formation of cofacial prisms.Catalytic activity was studied electrochemically using cyclic voltammetry and hydrodynamic voltammetry.Whereas untethered bis-iron(Ⅲ)-μ-oxo porphyrins are limited to heterogenous catalysis so that the dimeric structure is not lost upon the removal of the oxo bridge,our clipped architectures show significant catalytic current response under homogeneous conditions.Furthermore,when a shorter molecular clip is used as a post-synthetic tether,the selectivity under heterogeneous conditions is significantly enhanced:monomeric Fe(Ⅲ)tetraphenylporphyrin(TPhP)generates 64.3%H_(2)O_(2).When this same porphyrin is templated into a cofacial environment(Fe_(2)O TPhP)the selectivity improves to 15.8%H_(2)O_(2).When a tetrapyridyl porphyrin analogue is post-synthetically tethered through a oxalate-bridged Rh_(2) molecular clip,the selectivity improves further to 7.2%H_(2)O_(2).In contrast,when a longer bis-hydroxybenzoquinato bridge is used in the clip,the selectivity drops back to 14.5%H_(2)O_(2).Our most selective system also shows the highest current response and therefore our post-synthetic modification provides a kinetic enhancement as well.These results establish that straightforward coordination chemistry can be used for post-synthetic tuning of molecular ORR catalysts and when proper molecular clips are selected,marked enhancements to both selectivity and activity may be realized.
