Hydrophilic Quaternary Ammonium‐Group‐Containing [FeFe]‐Hydrogenase Models: Synthesis, Structures, and Electrocatalytic Hydrogen Production

Abstract

Abstract The first quaternary ammonium‐group‐containing [FeFe]‐hydrogenase models [(μ‐PDT)Fe 2 (CO) 4 {κ 2 ‐(Ph 2 P) 2 N(CH 2 ) 2 NMe 2 BzBr}] ( 2 ; PDT=propanedithiolate) and [(μ‐PDT)Fe 2 (CO) 4 {μ‐(Ph 2 P) 2 N(CH 2 ) 2 NMe 2 BzBr}] ( 4 ) have been prepared by the quaternization of their precursors [(μ‐PDT)Fe 2 (CO) 4 {κ 2 ‐(Ph 2 P) 2 N(CH 2 ) 2 NMe 2 }] ( 1 ) and [(μ‐PDT)Fe 2 (CO) 4 {μ‐(Ph 2 P) 2 N(CH 2 ) 2 NMe 2 }] ( 3 ) with benzyl bromide in high yields. Although new complexes 1 – 4 have been fully characterized by spectroscopic and X‐ray crystallographic studies, the chelated complexes 1 and 2 converted into their bridged isomers 3 and 4 at higher temperatures, thus demonstrating that these bridged isomers are thermodynamically favorable. An electrochemical study on hydrophilic models 2 and 4 in MeCN and MeCN/H 2 O as solvents indicates that the reduction potentials are shifted to less‐negative potentials as the water content increases. This outcome implies that both 2 and 4 are more easily reduced in the mixed MeCN/H 2 O solvent than in MeCN. In addition, hydrophilic models 2 and 4 act as electrocatalysts and achieve higher i cat / i p values and turnover numbers (TONs) in MeCN/H 2 O as a solvent than in MeCN for the production of hydrogen from the weak acid HOAc.