Electrocatalytic Hydrogen Evolution Of Triaryl Corroles Bearing Hydroxyethyl Amino Groups And Their Cobalt Complexes

Water electrolysis is one of the traditional and yet superior methods of hydrogen production.However the energy of producing hydrogen is very high in the absence of catalyst.The electrocatalysis of metal-corrole is a frontier topic of contemporary porphyrin macrocyclic chemistry.In this work,we wish to report the electrocatalytic hydrogen evolution reaction?HER?by four cobalt corroles and their corrole ligands.Possible catalytic pathways of corrole complexes and the effects of different proton sources on their catalytic pathways are discussed.And we also investigate that whether the method of introducing polyhydroxy group into hydrophobic corrole macrocycle can improve their catalytic efficiency.In addition,we also explore the electrocatalytic activity of four free-base corroles and their possible catalytic pathways for catalyzing proton reduction.The specific research content is as follows:5,10,15-tris?pentafluorophenyl?corrole?TPFC?,5,15-bis?pentafluorophenyl?-10-?phenyl?corrole?BPFC?,5,10,15-tris?4-?2-hydroxyethyl?-amino-2,3,5,6-tetrafluoro-phenyl?corrole?TPFC-OH?,5,15-bis?4-?2-hydroxyethyl?-amino-2,3,5,6-tetrafluorophenyl?-10-phenyl)corrole?BPFC-OH?and their cobalt complexes TPFC-Co,BPFC-Co,TPFC-OH-Co and BPFC-OH-Co were synthesized and characterized by ultraviolet-visible absorption spectroscopy,high resolution mass spectroscopy,nuclear magnetic resonance spectroscopy and infrared spectroscopy.Single crystal structures of BPFC,BPFC-OH and BPFC-Co were measured using X-ray single crystal diffractometer.Four cobalt corroles TPFC-Co,TPFC-OH-Co,BPFC-Co and BPFC-OH-Co were used as homogeneous electrocatalysts for hydrogen evolution using acetic acid,trifluoroacetic acid and water as proton sources,respectively.Electrochemical tests showed that the four cobalt corroles could keep stable and durable activity during catalyzing proton reduction in three proton sources.Cobalt corrole bearing hydroxyethyl amino groups TPFC-OH-Co was the best than other three cobalt corroles,the catalytic performance of four cobalt corroles followed an order of TPFC-OH-Co>TPFC-Co>BPFC-OH-Co>BPFC-Co.The types of proton sources had a prominent influence on the electrochemical behavior of the four cobalt corroles.When acetic acid was used as proton source,Co^II-H was the active species for four cobalt corroles;With using trifluoroacetic acid as proton source,Co^II-H was the mainly active species for TPFC-Co and BPFC-Co,Co^III-H for hydroxylated cobalt corrole TPFC-OH-Co and BPFC-OH-Co;Interestingly,the HER in water was a proton coupled electron transfer process.Both Co^III-H and triphenylphosphine that the axial ligand of cobalt corroles may be involved in catalyzing HER.These results suggested that the method of introducing hydroxyethyl amino groups by nucleophilic substitution could significantly improve the electrocatalytic efficiency of hydrophobic TPFC-Co and BPFC-Co,and the hydroxyethyl amino groups played an important role in proton electrophilic attack.The investigation of electrocatalytic activity for catalyzing HER by four free-base corrles was carried out in organic DMF using trifluoroacetic acid as proton source.As far as we know,this was the first time that free-base corrles had been used as electrocatalyst for HER.Relevant electrochemical results showed that four free-base corroles had catalytic activity for proton reduction,but the stability and durability of the catalysts were not high enough.These corroles degraded after electrolysis about 5 h.The catalytic efficiency of TPFC-OH was the best of the four free-base corroles,and their catalytic activity order was TPFC-OH>BPFC-OH>TPFC?BPFC.Based on the literatures and experimental data,it was implied that the active species for electrocatalytic proton reduction may be?^·Cor?H₄ and[?^·Cor?H₃]^-for TPFC-OH and BPFC-OH?Cor is the corrole macrocycle?.And for TPFC and BPFC,the active species involved in the electrocatalysis may be mainly[?^·Cor?H₃]^-.