Synthesis and Characterization of Cobalt-Based Complexes and Metal-Organic Frameworks for Hydrogen Evolution Catalysis Under Acidic Conditions

The synthesis of two cobalt(II) complexes: the novel complex [Co(phen)₃]₂[Co(pytc)₂]•16.25H₂O 1 and the previously reported [Co(bpy)₃](OH)₂•7H₂O 2, along with a cobalt(II/III)-containing MOF, Co-PPF2, previously reported as well, was achieved. These complexes were characterized by elemental analysis, X-ray crystallography, microscopy, conductivity measurements, TGA, SEM, CV , and UV-visible , and FTIR spectroscopies. From the CV scan rates, diffusion coefficients for complexes 1 and 2 were determined to be 1.0 × 10−6 cm2 s−1 and 4.8 × 10−7 cm2 s−1. Electrocatalytic studies with complexes 1 and 2 revealed insights into the mechanistic aspects of hydrogen evolution and the influence of acid strength on catalytic performance utilizing p-cyanoanilinium tetrafluoroborate, p-toluenesulfonic acid monohydrate, trifluoroacetic acid, and acetic acid by using DMSO and DMF:H2O (90:10, v/v). Under homogeneous photocatalytic conditions, the maximum turnover frequency (TOFmax) for complex 1 was 1118 min-1 in DMF:H2O (90:10, v/v) and 1038 min-1 in DMF:H2O (50:50, v/v), with turnover numbers (TON) of 41 and 16, respectively. Meanwhile, under heterogeneous photocatalytic conditions, Co-PPF2 exhibited a TOFmax of 2361 min-1 and a TON of 60 in DMF:H2O (50:50, v/v). The reduction of sodium bromate (NaBrO3) by [Co(dmgBF2)2(OH2)2] (where dmgBF2 = difluoroboryldimethylglyoximato) was investigated in hydrochloric and nitric acids. Analytical studies were carried out via gravimetric analysis and UV-visible spectrophotometry. In the gravimetric analysis, a 0.100 M AgNO3 solution was added to the HCl-acidified reaction mixture, forming a precipitate of AgX (X = halide). The gravimetric analysis proved that sodium bromate was reduced to from quantitative amounts of sodium bromide in the presence of hydrochloric acid. Through simulated stoichiometric calculations, the amount of AgBr that precipitated gave a yield of 98 ± 1%. Biochar was used to sequester the complex from the reaction mixture and to neutralize the acidic media. The UV-visible data confirmed the sequestration of the complex from the reaction mixture after exposure to the biochar. The collected pH data proved a direct relationship between pH and the amount of biochar used in the form of a titration curve. These findings demonstrate the efficiency of using biochar in the sequestration and neutralization process.