New tertiary phosphane appended Ag^Ⅰ 3-ethoxycyclobutene-4-thiolates: Insights into phosphane denticity effect on electrocatalytic water splitting properties

Three new heteroleptic Ag^Ⅰ dithiolate complexes [Ag (PPh₃)₂ mtsq] (Ag–P), [Ag₂(dppe)₂ (mtsq)₂] (Ag-E) and [Ag₂(dppf)₂(mtsq)₂] (Ag–F), (PPh₃ = triphenylphosphane, dppe = 1,2-bis-(diphenylphosphino)ethane, dppf = 1,1′-bis-(diphenylphosphino)ferrocene, mstq = 3-ethoxycyclobutenedione-4-thiolate) have been synthesized and characterized. The single crystal X-ray diffraction study for Ag–P reveals a trigonal planar geometry around the Ag^Ⅰ center satisfied by sulfur of mtsq and phosphorus centers of two PPh₃ ligands. Ag–F is a centrosymmetric dimer, wherein the distorted tetrahedral geometry around the two Ag^Ⅰ centers is satisfied by two sulfur centers of two mtsq ligands in bridging mode and two phosphorus centers of dppf ligands each. Furthermore, these complexes have been used as electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in 1.0 M KOH and 0.5 M H₂SO₄, respectively. Amongst all three complexes Ag–P has the best electrocatalytic performance. In the OER, Ag–P exhibits an onset potential of 1.56 V with η¹⁰ of 520 mV and a Tafel slope of 108 mV∙dec⁻¹, while in HER it displays an onset potential of −0.56 V with η¹⁰ of −660 mV and a Tafel slope of 137 mV∙dec⁻¹. MEP surface analysis shows that all three complexes exhibit superior OER activity than their corresponding HER performance. Density of states and adsorption energy calculations further reveal that the Ag–P possess a narrower band gap and higher adsorption energies for the key intermediates than Ag-E and Ag–F and hence accounting for its relatively enhanced electrocatalytic activity.