Metal–organic frameworks based on tetraphenylpyrazine-derived tetracarboxylic acid for electrocatalytic hydrogen evolution reaction and NAC sensing

Abstract

A series of metal–organic frameworks (MOFs), namely {[Cd(H₂TCPP)]·2H₂O}_n (1), {[Pb₂(TCPP)]}_n (2), {[La(H₂TCPP)]·3.7H₂O}_n (3), {[Sr₃(HTCPP)₂]·6H₂O}_n (4), {[Ce(HTCPP)]·H₂O}_n (5) and {[Mn₂(TCPP)]·2.13H₂O}_n (6), are prepared from 2,3,5,6-(4-carboxyl-tetraphenyl)pyrazine (H₄TCPP) with various metal salts under hydrothermal conditions and are well characterized. The structural analysis shows that compound 1 presents a 2-fold interpenetrating structure, which exhibits a uninodal 4-c net with a {6⁶}{6·6·6·6(2)·6(2)·6(2)} topology. Compound 2 shows a 3-nodal (5,6,11)-connected 3D framework with a point symbol of {4¹⁰}{4¹⁴·6}{4³⁴·6²¹}. Compound 3 features a (4,8)-connected 3D network built from a binuclear La cluster with a point symbol of {4¹⁶·6¹²}{4⁴·6²}₂. Compound 4 demonstrates a 5-nodal (4,5,6,7,8)-connected network with a {4¹⁰·6⁵}{4¹²·6⁹}{4¹⁸·6¹⁰}{4⁶}{4⁷·6³} topology. Compound 5 exhibits a 3D interlocked eight-connected 20-c net with a point symbol of {3⁷⁸·4⁹⁸·5¹⁴}, while compound 6 forms a 3,4,4,5,17-connected net with a point symbol of {4·5²}₂{4³²·5⁶·6⁸⁴·7⁴·8¹⁰}{4⁵·5}{4⁶}{4⁸·5²}. In addition, almost all the MOFs display significant fluorescence quenching behaviors by the addition of various nitro-aromatic compounds (NACs), especially TNP. More importantly, six new MOFs have been further investigated to evaluate their electrocatalytic activities for the hydrogen evolution reaction (HER). The results demonstrate that all the MOFs exhibit improved performance compared to ligand H₄TCPP for the HER with the lowest onset overpotential of 244 mV and the smallest Tafel slope of 248 mV dec⁻¹, respectively.