The supramolecular frameworks and electrocatalytic properties of two new structurally diverse tertiary phosphane-appended nickel(ii) and copper(i) thiosquarates

Abstract

In this study, two new heteroleptic complexes with compositions [Ni(mtsq)_1.85Cl_0.15(dppe)](CH₂Cl₂)_0.06(CH₃OH)_0.09 (Ni-mtsq) and [Cu(mtsq)(PPh₃)₂] (Cu-mtsq) (dppe = 1,2-bis-(diphenylphosphino)ethane; PPh₃ = triphenylphosphane and mtsq = 3-ethoxycyclobutenedione-4-thiolate) have been synthesized and characterized by spectroscopy analyses as well as by single crystal X-ray diffraction technique. X-ray analyses reveal square planar geometry around Ni(II) in Ni-mtsq, where Ni(II) coordinates with two sulfur centres of two mtsq ligands with monodentate coordination and two phosphorus centres of the dppe ligand in chelating mode. Unlike Ni-mtsq, Cu(I) in Cu-mtsq adopts a tetrahedral geometry, which is satisfied by sulfur and oxygen centres of one mtsq ligand with a bidentate coordination mode and two phosphorus centres of two PPh₃ ligands. The supramolecular architecture of both Ni-mtsq and Cu-mtsq is sustained by intermolecular C–H⋯π and O⋯H interactions. Also, Ni-mtsq exhibits intriguing intramolecular semicoordination Ni⋯O and π⋯π non-covalent interactions. The nature of these interactions has been addressed with the aid of Hirshfeld surface analysis, density functional theory and quantum theory of atoms in molecules (QTAIM) analyses and has been visualized with the help of non-covalent interactions reduced density gradient (NCI-RDG). Further, the emissive nature of Cu-mtsq in a solution and solid phase suggests aggregation-induced emission with a bathochromic shift in the solid phase as compared to the solution phase. Both complexes have been used as electrocatalysts for oxygen and hydrogen evolution reactions, which suggests that Cu-mtsq offers better catalysis due to the presence of a redox active, less hindered Cu(I) centre with an η¹⁰ of 600 mV and a Tafel slope of 228 mV dec⁻¹ in the OER, while in HER too, Cu-mtsq exhibited the best onset potential of −0.36 V with an η¹⁰ of 647 mV. Also, homogeneous electrocatalysis for the HER for both complexes was investigated using trifluoroacetic acid as the hydrogen source, which suggests a better electrocatalytic performance of Cu-mtsq.