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

The first quaternary ammonium-group-containing [FeFe]-hydrogenase models [(mu-PDT)Fe-2(CO)(4){kappa(2)-(Ph2P)(2)N(CH2)(2)NMe(2)BzBr}] (2; PDT=propanedithiolate) and [(mu-PDT)Fe-2(CO)(4){m-(Ph2P)(2)N(CH2)(2)NMe(2)BzBr}] (4) have been prepared by the quaternization of their precursors [(mu-PDT)Fe-2(CO)(4){kappa(2)-(Ph2P)(2)N(CH2)(2)NMe2}] (1) and [(mu-PDT)Fe-2(CO)(4){mu-(Ph2P)(2)N(CH2)(2)NMe2}] (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/H2O 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/H2O 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/H2O as a solvent than in MeCN for the production of hydrogen from the weak acid HOAc.