Probing the effect of isomeric -OH group in heteroleptic Copper(I)-triphenylphosphine appended dithiocarbamates on their electrocatalytic properties for homogeneous hydrogen evolution reactions

Due to increasing interest in alternative cost-effective electrocatalysts containing transition metals, three new heteroleptic triphenylphenylphosphine appended copper(I) dithiocarbamate complexes [Cu(S₂CN(CH₂C₆H₅)(CH₂C₆H₄-o-OH))(PPh₃)₂] (Cu-o-OH), [Cu(S₂CN(CH₂C₆H₅)(CH₂C₆H₄-m-OH))(PPh₃)₂] (Cu-m-OH) and [Cu(S₂CN(CH₂C₆H₅)(CH₂C₆H₄-p-OH))(PPh₃)₂] (Cu-p-OH), have been synthesized, characterized spectroscopically and by single crystal X-ray diffraction. The single crystal X-ray studies suggest that for Cu-o-OH and Cu-m-OH, copper(I) possess a distorted P₂S₂ coordinate tetrahedral geometry. The solid-state framework of Cu-o-OH has been found to be majorly stabilized by O-H···S intermolecular interactions, while that of Cu-m-OH is sustained by C-H···C and O-H···S interactions, whose nature has been assessed by Hirshfeld surface analysis. The homogeneous electrocatalytic properties of the complexes for proton reduction reveal that the i_cat/i_Pc values of complexes are greater than 28, while utilizing trifluoroacetic acid (TFA) as a proton source. In all complexes, the ratio i_c/i_wc have been found to be greater than 1, indicating that these complexes can effectively catalyze proton reduction to hydrogen in acetonitrile. Amongst all the three complexes, Cu-p-OH has been found to be the best electrocatalyst with low overpotential (η) of 0.876 V and a highest turnover frequency (TOF) of 539.60 s⁻¹ on adding 70 mM TFA. The best catalytic activity of Cu-p-OH has been attributed to its larger HOMO-LUMO energy gap and a more positive charge on copper(I) compared to other isomeric complexes that assists the complex to attract more electron density that could have readily enhanced its electrocatalytic performance.