Synthetic Metallodithiolato Ligands as Pendant Bases in [FeIFeI], [FeI[Fe(NO)]II], and [(-H)FeIIFeII] Complexes.
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abstract
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 aza-dithiolate linker in the active site of [FeFe]-H2ase has inspired the development of synthetic analogues that utilize ligands which serve as conventional 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 bi- and monodentate ligation. Herein, synthetic efforts have led to the isolation and characterization of three heterotrimetallics that employ metallodithiolato ligand binding to di-iron scaffolds in three redox levels, (-pdt)[Fe(CO)3]2, (-pdt)[Fe(CO)3][(Fe(NO))II(IMe)(CO)]+, and (-pdt)(-H)[FeII(CO)2(PMe3)]2+ to generate (-pdt)[(FeI(CO)3][FeI(CO)2NiN2S2] (1), (-pdt)[FeI(CO)3][(Fe(NO))II(IMe)(CO)]+ (2), and (-pdt)(-H)[FeII(CO)2(PMe3)][FeII(CO)(PMe3)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-+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.
