Synthesis, Structure, and Electrocatalysis of Diiron C-Functionalized Propanedithiolate (PDT) Complexes Related to the Active Site of [FeFe]-Hydrogenases

New C-functionalized propanedithiolate-type model complexes (1−8) have been synthesized by functional transformation reactions of the known complex [(μ-SCH₂)₂CH(OH)]Fe₂(CO)₆ (A). Treatment of A with the acylating agents PhC(O)Cl, 4-pyridinecarboxylic acid chloride, 2-furancarbonyl chloride, and 2-thiophenecarbonyl chloride in the presence of Et₃N affords the expected C-functionalized complexes [(μ-SCH₂)₂CHO₂CPh]Fe₂(CO)₆ (1), [(μ-SCH₂)₂CHO₂CC₅H₄N-4]Fe₂(CO)₆ (2), [(μ-SCH₂)₂CHO₂CC₄H₃O-2]Fe₂(CO)₆ (3), and [(μ-SCH₂)₂CHO₂CC₄H₃S-2]Fe₂(CO)₆ (4). However, when A is treated with the phosphatizing agents Ph₂PCl, PCl₃ and PBr₃, both C- and Fe-functionalized complexes [(μ-SCH₂)₂CHOPPh₂-η¹]Fe₂(CO)₅ (5), [(μ-SCH₂)₂CHOPCl₂-η¹]Fe₂(CO)₅ (6), and [(μ-SCH₂)₂CHOPBr₂-η¹]Fe₂(CO)₅ (7) are unexpectedly obtained via intramolecular CO substitution by P atoms of the initially formed phosphite complexes. The simplest C-functionalized model complex [(μ-SCH₂)₂CO]Fe₂(CO)₆ (8) can be produced by oxidation of A with Dess−Martin reagent. While 8 is found to be an electrocatalyst for proton reduction to hydrogen, starting complex A can be prepared by another method involving the reaction of HC(OH)(CH₂Br)₂ with the in situ generated (μ-LiS)₂Fe₂(CO)₆. X-ray crystallographic studies reveal that the bridgehead C atom of 8 is double-bonded to an O atom to form a ketone functionality, whereas the bridgehead C atoms of A, 1, 3, and 4 are equatorially-bonded to their functionalities and those of 5−7 axially-bonded to their functionalities due to formation of the corresponding P−Fe bond-containing heterocycles.