Inorganic Chemistry, Vol.59, No.6, 3753-3763, 2020
Synthetic Metallodithiolato Ligands as Pendant Bases in [(FeFeI)-Fe-I], [Fe-I[Fe(NO)](II)], and [(mu-H)(FeFeII)-Fe-II] Complexes
The development of ligands with specific stereo- and electrochemical requirements that are necessary for catalyst design challenges synthetic chemists in academia and industry. The crucial azadithiolate linker in the active site of [FeFe]-H(2)ase has inspired the development of synthetic analogues that utilize ligands which serve as conventional sigma donors with pendant base features for H+ binding and delivery. Several MN2S2 complexes (M = Ni2+, [Fe(NO)](2+), [Co(NO)](2+), etc.) utilize these cis-dithiolates to bind low valent metals and also demonstrate the useful property of hemilability, i.e., alternate between biand monodentate ligation. Herein, synthetic efforts have led to the isolation and characterization of three heterotrimetallics that employ metal-lodithiolato ligand binding to di-iron scaffolds in three redox levels, (mu-pdt)[Fe(CO)(3)](2), (mu-pdt)[Fe(CO)(3)][(Fe(NO))(II)(IMe)(CO)](+), and (mu-pdt)(mu-H)[Fe-II (CO)(2) (PMe3)](2)(+) to generate (mu-pdt)[(Fe-I(CO)(3)][Fe-I(CO)(2)center dot NiN2S2] (1), (mu-pdt)[Fe-I(CO)(3)][(Fe(NO))(II)(IMe)-(CO)](+) (2), and (mu-pdt)(mu-H)[Fe-II(CO)(2) (PMe3)][Fe-II(CO)(PMe3)center dot NiN2S2](+) (3) complexes (pdt = 1,3-propanedithiolate, IMe = 1,3-dimethylimidazole-2-ylidene, NiN2S2 = [N,N'-bis(2-mercaptidoethyl)-1,4-diazacycloheptane] nickel(II)). These complexes display efficient metallodithiolato binding to the di-iron scaffold with one thiolate-S, which allows the free unbound thiolate to potentially serve as a built-in pendant base to direct proton binding, promoting a possible Fe-H-center dot center dot center dot H+-S coupling mechanism for the electrocatalytic hydrogen evolution reaction (HER) in the presence of acids. Ligand substitution studies on 1 indicate an associative/dissociative type reaction mechanism for the replacement of the NiN2S2 ligand, providing insight into the Fe-S bond strength.