Ferrocenyl Dithiophosphonate Ag(I) Complexes: Synthesis, Structures, Luminescence, and Electrocatalytic Water Splitting Tuned by Nuclearity and Ligands

The heterometallic [Ag(I)/Fe(II)] molecular electrocatalysts for hydrogen production were introduced here to recognize the mutual role of metallic nuclearity and ligand engineering. A series of ferrocenyl dithiophosphonate stabilized mononuclear [Ag(PPh3)(2){S(2)PFc(OR)}] {where R=Me (1), Et (2), nPr (3), iPr (4), iAmyl (5); Fc=Fe (eta(5)-C5H4) (eta(5)-C5H5)} and dinuclear [Ag(PPh3){S(2)PFc(OR}](2) {where R=Et (2 a), and nPr (3 a)} complexes were synthesized and characterized by SCXRD, NMR (P-31 and H-1), ESI-MS, UV-Vis, and FT-IR spectroscopy. The comparative electrocatalytic HER behavior of 1-5 and 2 a-3 a showed effective current density of 1 mA/cm(2) with overpotentials ranging from 772 to 991 mV, demonstrating the influence of extended and branched carbon chains in dithiophosphonates and metallic (mono-/di-) nuclearity, which correlates with documented tetra-nuclear [Ag-4(S(2)PFc(OnPr)(4)], 6. DFT study suggests the coordinated (mu(1)-S) site of ligands is the reactivity center and the adsorption energy of intermediate [H*-SM] varies with the engineering of ligand and nuclearity. A catalytic mechanism using mononuclear (1) and di-nuclear (2 a) was proposed with the assistance of DFT. Each complex, being the first example of Ag(I) dithiophosphonates, exhibits intense photoluminescence with high quantum yields ranging from 33 % to 67 %. These results link the lower nuclearity structures to their physical and catalytic properties.